Catalytic Glycosylations in Oligosaccharide Synthesis - Chemical

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

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Catalytic Glycosylations in Oligosaccharide Synthesis Michael Martin Nielsen and Christian Marcus Pedersen*

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Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark ABSTRACT: Catalytic glycosylation has been a central reaction in carbohydrate chemistry since its introduction by Fischer 125 years ago, but it is only in the past three to four decades that catalytic methods for synthesizing oligosaccharides have appeared. Despite the development of numerous elegant and ingenious catalytic glycosylation methods, only a few are in general use. This review covers all methods of catalytic glycosylation with the focus on the development and application in oligosaccharide synthesis and provide an overview of the scope and limitations of these. The review also includes relevant mechanistic studies of catalytic glycosylations. The future of catalytic glycosylation chemistry is discussed, including specific, upcoming methods and possible directions for the field of research in general.

CONTENTS 1. Introduction 2. Glycosyl Imidate Donors 2.1. (N-Methyl)acetimidates 2.2. Trichloroacetimidates Early Approaches and Modern Variations Cooperative and Acid/Base Catalysis Metal Triflate Catalysts Counter Ion Effect Glycosylations in Ionic Liquids Solid Acid Catalysts Transition Metal Catalysis Chiral Catalysts for TCA Activation Photo Acid Catalysts Trichloroacetimidates on Solid Phase General Remarks for Catalytic Activation of TCA Donors 2.3. (N-Phenyl)trifluoroacetimidates General Remarks Regarding the Use of PTFA Donors 2.4. Thioimidate Glycosyl Donors 2.5. Other Imidate-Like Glycosyl Donors 3. Glycosyl Halides 3.1. Glycosyl Fluorides 3.2. Glycosyl Bromides and Chlorides 3.3. Glycosyl Iodides 4. Alkyne-Based Glycosyl Donors 5. Alkene-Based Glycosyl Donors 6. Alkyl and Silyl Glycosides as Glycosyl Donors Aryl Glycosides As Glycosyl Donors 7. Glycosyl Esters As Donors 8. Orthoesters as Glycosyl Donors 9. Glycals as Donors Nitroglycals 10. Anhydro Sugars As Donors 1,2-Cyclopropanyl Glycosyl Donors © XXXX American Chemical Society

11. Phosphorus-Based Glycosyl Donors 11.1. Phosphate-Derived Glycosyl Donors 11.2. Glycosyl Phosphite Donors 12. Oxazoline Glycosyl Donors Formation of Oxazoline Glycosyl Donors Activation of Oxazoline Glycosyl Donors Enzyme-Catalyzed Glycosylations with Oxazoline Glycosyl Donors 13. Glycosyl Carbonate and Carbamate Donors 14. Thioglycosides 15. Thiocyanates 16. Other Glycosyl Donor Types 17. Conclusion and Future Direction Conclusion and Future Direction Author Information Corresponding Author ORCID Notes Biographies References

A B B B B E H I J K K M P Q R R T T U W W Y Z Z AD AE AH AH AM AN AQ AR AT

AT AT AV AW AW AX AY AY AZ BA BB BB BB BC BC BC BC BC BC

1. INTRODUCTION One-hundred and forty years of research has gone into to the synthesis of saccharides since the pioneering work by Arthur Michael1 and Emil Fischer,2 who were the first to perform chemical glycosylations. Since then, countless researchers have worked on improving the methods for glycoside-bond formation. Thus, it might seem astonishing that the acidcatalyzed glycosylation developed by Fischer in 1893, regularly referred to as a “Fischer glycosylation”, is still the preferred Special Issue: Carbohydrate Chemistry Received: March 5, 2018

A

DOI: 10.1021/acs.chemrev.8b00144 Chem. Rev. XXXX, XXX, XXX−XXX

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Figure 1. Impact of three successful glycosylation methods7−9 in the years 1981−2017.

has since been developed further into a catalytic glycosylation strategy. The key focus points for evaluating the various procedures are chemical yield and selectivity but also the operational convenience will be considered. All methods for catalytic glycosylations developed before 2018 will be presented (Figure 2).

reaction for the formation of simple glycosides. But since the Fischer glycosylation is rarely useful for oligosaccharide synthesis, many novel methods of catalytic glycosylation have been developed ever since. The synthesis of oligosaccharides is intrinsically challenging since every glycosylation leads to the formation of a pair of diastereoisomers. This not only decreases the yield of the desired glycoside but also makes it virtually impossible to do multiple couplings without purification. Despite these challenges, many methods have been developed for high-yielding and stereoselective construction of natural products, both by nonautomated3,4 and automated approaches.5,6 Many glycosylation strategies involve the use of one or multiple equivalents of a promoter, and sometimes an acid scavenger in excess, for the glycosylations to proceed in acceptable yields. Seen from an environmental and industrial perspective, catalytic glycosylation protocols that do not require this addition of excess reagents present a much better alternative. Historically, especially three methods of catalytic glycosylation have proven particularly popular (see Figure 1). The trichloroacetimidate method developed by Schmidt,7 the glycosyl fluoride method developed by Mukaiyama8 and the N(phenyl)trifluoroacetimidate method developed by Yu9 are all examples of glycosyl donors that have gained significant attention over many years. One key reason for this success might be that these procedures have been easily adopted by various groups since they require very little optimization and involve chemicals that are usually commercially available. Furthermore, the three research groups have put a lot of effort into assessing and expanding the scope of each particular procedure. The current catalogue of catalytic glycosylations involves everything from simple acid-catalyzed activation of an anomeric leaving group to more complex systems requiring precomplexation of multiple species for efficient activation. This review will focus on the development of novel catalytic methods for oligosaccharide synthesis and later modifications of these protocols. Hence not every single example of using a standard method will be included. Catalytic glycosylations are defined in this review as glycosylation protocols that function without the addition of equivalent amounts of promoter or scavengers. Some glycosylation methods violating this definition have been included primarily for two reasons: either due to the similarity to catalytic methods presented in the review or if the method

Figure 2. Overview of proven and some selected upcoming methods of catalytic glycosylation. The alkyne-based glycosyl donors developed by Yu and Hotha are discussed in chapter 4, while the recent work by Toshima on boronic acid catalysis is presented in chapter 10.

2. GLYCOSYL IMIDATE DONORS 2.1. (N-Methyl)acetimidates

In 1976, Sinaÿ and co-workers reported a general synthesis of glycosyl imidates from glycosyl halides.10 This discovery soon led to the use of glycosyl imidates as glycosyl donors providing an alternative method for synthesizing disaccharides without the use of heavy metal salts.11 These glycosyl donors were activated by a stoichiometric amount of p-toluenesulfonic acid (TsOH) with a wide range of different donors and sugar acceptors (Scheme 1).11−13 2.2. Trichloroacetimidates

Early Approaches and Modern Variations. The (Nmethyl)acetimidate donor developed by the Sinaÿ group did not gain general popularity, mainly due to the fact that the (Nmethyl)acetimidates had to be synthesized from a glycosyl halide, which in itself functions as a glycosyl donor, as well as their low reactivity.14,15 The previous work by the Sinaÿ group and the knowledge that ketenimines and nitriles were commonly known to form imidates when reacting with alcohols,16 inspired the development of the trichloroacetimidate (TCA) and ketenimidate B


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Scheme 1. First Reported Glycosylation Using an Imidate Donor by Sinaÿ and Coworkers

Scheme 2. Formation of the Perbenzylated β-Ketenimidate and α-Trichloroacetimidates

Scheme 3. First Catalytic Glycosylations with TCA Donors

donors in 1980 by Michel and Schmidt (Scheme 2).7 The TCA donors have since become the most popular glycosyl donor type for catalytic activation. The first part of this chapter will focus on the early development of the TCA methodology as well as highlight a few interesting examples from the literature that do not fit into the following subsections of the chapter. Reaction conditions for the formation of TCA donor resembled the first account of the base-catalyzed formation of imidates by Nef,17 which has since been used in imidate synthesis.18,19 The specific reaction between an alcohol and trichloroacetonitrile was first reported by Steinkopf.20 In the first report by Schmidt and Michel,7 the TCA donors were shown to be catalytically activated (Scheme 3) using both TsOH and BF3·OEt2. The β-ketenimidate was not reported to be catalytically activated. The formation of TCA donors was investigated in 1984 by Michel and Schmidt.21 They reported that the β-TCA was formed more rapidly than the α-TCA donor due to the higher nucleophilicity of the anomeric β-alkoxide. The increased nucleophilicity was reportedly due to free orbital repulsion between the β-OH and the endocyclic oxygen making the lone pair more accessible, thus more reactive. On the basis of this, K2CO3 was found ideal for the formation of the β-TCA donor, whereas NaH would primarily give rise to the α-TCA donor (Scheme 4). NaH was also shown to convert TCA donors from β- to α-configuration, a reversibility previously reported for imidates.22 DBU,23 Cs2CO3,24 and NaOH under phase transfer conditions25 have also been used in the formation of α-TCA donors. Generally, stronger bases and prolonged reaction times will favor the formation of the α-TCA, whereas weaker bases will enable the isolation of primarily the β-TCA donors. In an early report, Grundler and Schmidt showed that the stereoselectivity of a glycosylation could be affected by the choice of catalyst (Scheme 5).26 They found varying stereoselectivity when using either TMSOTf or BF3·OEt2, but these

Scheme 4. Base-Catalyzed Formation of Trichloroacetimidates

results were likely influenced by the different anomeric configuration of the two TCA donors investigated (Scheme 5). TMSOTf has been reported to silylate acceptors, leading to lowered glycosylation yields, which can be limited by using a bulkier trialkylsilyl triflate or simply another catalyst.27,28 TBDMSOTf has been used as a catalyst by the Schmidt group, giving rise to very comparable yield and selectivity as with TMSOTf.29 Krepinsky and co-workers have investigated dibutylboron triflate as a catalyst in 1993 to counteract the problems with TMS triflate catalysts.27 Glycosylations using 0.1 equiv of the dibutylboron catalyst were completed on solid support and in solution with 6-OH, 4-OH, and 3-OH glycosyl acceptors at −45 °C in yields of 50−85%. This has, however, not been used by others since. The formation of an undesired orthoester byproduct has been observed from peracetylated glycosyl donors.30,31 Urban and co-workers found that using ZnBr2 instead of BF3·OEt2, would increase the yield of the desired glycoside,24 thus not requiring bulky pivaloyl protecting groups at C-2, which is a C


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Scheme 5. Lewis Acid-Dependent Selectivity Reported by Grunder and Schmidt

Scheme 6. Proposed Mechanism by Qiao et al

Scheme 7. Proposed Cyclic Intermediate of an α-TCA Donor and the “ABCH” Phosphate in Red

resulted in yields below 50%. The two bistriflimide-catalysts, TMSNTf2 and HNTf2, were reinvestigated as catalysts in the activation of TCA donors by Qiao et al. in 2016 during a mechanistic study on the glycosylation reaction.42 It was found that unlike the more standard catalysts in TCA activation, TMSOTf and BF3·OEt2 that often gives rise to a triflate and a fluoride intermediate respectively,38,40 the TMSNTf2 catalyst would, at low temperatures, leave the glycosidic bond intact until the attack of a better nucleophile (Scheme 6). Thus, the TCA donor would be activated in a high-energy complex, which was stable at low temperatures, which led to a more SN2like transition state for the glycosylation. This concept was further developed by Kowalska and Pedersen in 2017 who reported a stereospecific glycosylation of gluco-, manno-, and galactosyl TCA donors.43 In this study, it was found that 0.1 equiv TMSNTf2 or HNTf2 would activate the TCA donors and to some degree allow for the nucleophile to eject the TCA leaving group with inversion of the anomeric configuration. Thus, an α-TCA donor would lead to βselectivity, and vice versa. A range of glycosylations were performed, generally leading to yields in excess of 70%, with the exception of a 2-azido galactosyl TCA donor which only led to a 50% yield. HNTf2 was later found by Elferink and Pedersen44 to not facilitate stereospecific glycosylations on the more reactive rhamnosyl TCA donors. Selectivity could however be obtained by utilizing solvent effects. NOBF4 was introduced as a novel catalyst for TCA activation in 2012 by Sau et al.45 It was shown that 0.3 equiv of the catalyst would facilitate the formation of disaccharides in yields of 72−78% with both 6-OH, 4-OH, 3-OH, and 2-OH sugar acceptors from various gluco-, galacto-, manno-, and rhamnosyl TCA donors thus underlining a wide scope for the method. In all the glycosylations, only one stereoisomer of the glycosides formed were reported, either obtained as 1,2-trans-glycosides by neighboring group participation or as 1,2-cis-glycosides by solvent effects in 1:1 Et2O/CH2Cl2. This method has since

more common way of eliminating orthoester formation. ZnBr2 had also previously been employed by Schmidt and Hoffmann for C-glycoside formation.32 MgBr2·OEt2 was also investigated by Urban and co-workers, but although this Lewis acid did activate the TCA donor, it proved to be less capable of transforming the orthoester byproduct into the desired glycoside. The use of pyridinium p-toluenesulfonate (PPTS) was introduced by the Nicolaou group during the total synthesis of Amphotericin B.33 Mild conditions were required since the synthesis involved a reactive deoxy sugar and an acid sensitive acceptor, thus a weak Brønsted acid as PPTS was essential, leading to a 40% yield. Schmidt and Grundler have activated a perbenzylated TCA donor with a weak acid such as acetic acid, underlining the high reactivity of these donors.26 Even methanol has been found to react with a perbenzylated TCA donor in absence of a catalyst.34 These findings were in trend with the contemporary awareness in the 1980’s that ether-protected donors were highly reactive.14,26,35,36 TMSOTf and BF3·OEt2 quickly became the two standard-catalysts for TCA activation. Since then, glycosyl fluorides have been reported as byproducts37 and intermediates38 in glycosylations catalyzed by BF3·OEt2 and other boron fluoride Lewis acids. Modern, low-temperature NMR techniques39 have paved the way for a much more thorough understanding of intermediates and mechanisms of glycosylations in recent years. Thus, glycosyl triflates have also been reported as intermediates in the TMSOTf-catalyzed activation of TCA donors.40 The Lewis acid TMSNTf2 and the Brønsted acid NHTf2 were introduced as novel catalysts in the activation of TCA donors in 2010 by Zandanel et al.41 They demonstrated that these two catalysts were capable of catalytic activation of a permethacrylated glycosyl donor. Stereoselectivity was only obtained by neighboring group participation and TMSNTf2 led to sluggish reactions. 0.5 equiv of this catalyst was necessary to facilitate a glycosylation which D


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Scheme 8. In situ Formation of an A−B−C−H Compound (In Blue/Red) and Activation of a TCA Donor

been used in the synthesis of a wide range of natural products by the Misra group46−53 and Mandal54,55 to selectively obtain the axial glycosidic bonds as the only stereoisomer in all the above-mentioned examples, seemingly only requiring changes in the ratio of the Et2O/CH2Cl2 solvent mixture. The formation of the undesired 1,2-trans-glycoside was reported as a 5% byproduct in two cases.56,57 Thus, NOBF4 appears to be an robust catalyst for axial glycosidic bond-formation in ethereal solvents but is yet to be utilized by other research groups. The selectivity of the glycosylations is not discussed in detail but generally seems to be determined from isolated material. Cooperative and Acid/Base Catalysis. An interesting new concept of catalysis was introduced by the Schmidt group58 when investigating the well-known15,59,60 reaction between a phosphate and a TCA-donor. For the phosphateactivation of a TCA-donor and following stereospecific glycosylation, they suggested a cyclic intermediate resulting in the inversion of the anomeric stereochemistry (Scheme 7). A novel activation system with structural similarity was envisaged. They proposed that an ABCH system would form an A−B−C−H-like product upon reaction with an electrophile as described in Scheme 7. A method of in situ formation of the A−B−C−H system was presented (Scheme 8).58 A range of different carbonyl compounds were tested, but the most promising results were obtained from chloral, which was able to catalyze glycosylations using only 0.1 equiv of the aldehyde in the presence of a sugar acceptor (Table 1). Unsurprisingly, the selectivity dropped with increasing temperature, although the β-glycoside was still the major product, presumably due to the participation of the nitrile solvent.10,61,62

The concept of complexation of acceptor and catalyst was given no attention for more than 20 years until the Schmidt group took it up for reinvestigation.37 After a screening of candidates with the correct configuration, a range of boron fluoride compounds were employed as catalysts for TCA activation by making the oxygen of the acceptor alcohol more nucleophilic as well as making the hydrogen more acidic (Scheme 9). Scheme 9. Proposed Reversible A−B−C−H Formation by Boron Fluoride Compounds and an Alcohol Nucleophile

The following list of requirements to the activation by precomplexation was stated: (a) Fast and reversible formation of A−B−C−H from the acceptor, A−H. (b) Higher acidity of the A−H proton (consequence of a). (c) No activation of the glycosyl donor by the catalyst in absence of an acceptor. (d) Increased nucleophilicity of the alcohol during precomplexation. The commonly used Lewis acids, TMSOTf and BF3·OEt2, led to decomposition of the donor in absence of an acceptor, this failing criteria (c). PhBF2, however, gave promising results since no decomposition of the donor was observed even after 2 days at room temperature in the absence of an acceptor. Furthermore, fast adduct-formation between the acceptor and catalyst was observed by 1H NMR. 0.1 equiv PhBF2 was shown to lead to di- and trisaccharide formation in gluco- and galactosylations, typically within 30 min at −78 °C, with β-selectivity (1:3−1:24 α/β) in CH2Cl2 without neighboring group participation. Ph2BF was also shown as an effective catalyst leading to similar results, and it was found that although the yields were similar to that of TMSOTf and BF3·OEt2, the β-selectivity was much lower for these more conventional catalysts. The catalytic system was to some degree stereospecific, leading to the above-mentioned β-selectivity from an α-TCA donor, whereas the opposite was only possible with simple alcohol acceptors.37 Shortly after, it was found by the Schmidt group63 that sulfur nucleophiles would react with TCA donors in a similar manner. An acceptor with both a thioland hydroxy functionality, 4-mercaptobutanol, was investigated under the PhBF2-catalyzed conditions, which only yielded Oglycosides, underlining the oxygen affinity of the catalyst. An SN2-like pathway involving a cyclic intermediate was proposed to account for the high stereoselectivity and chemoselectivity of the catalytic system (Scheme 10). Three silicon fluorides were later introduced as novel catalysts for acid/base catalysis by the Schmidt group (Figure 3).64 The three catalysts were all weaker Lewis acids than PhBF2 but could prove to have interesting properties since the Si−F···

Table 1. Reaction of the Acceptor (1.5 equiv), α-TCA Donor, and Chloral in Acetonitrile

α/β ratio entry

cat (mol %)

temp (°C)

time (h)

yield (%)

α

β

1 2 3 4 5 6 7

100 100 100 100 100 30 10

80 rt −10 −20 −40 rt rt

3 2 3 3 18 1 10

91 94 83 84 79 91 76

25 15 6 5 0 20 15

75 85 94 >95 100 80 85 E


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Scheme 10. Oxygen-Affinity of the PhBF2 Acid/Base Catalyst

stereospecific reaction pathway than from the corresponding βTCA. The activation of TCA donors via acid/base catalysis is undoubtedly very elegant. There is, however, a need for further development of this glycosylation method, since it has only been able to afford 1,2-trans-glycosides in a consistent manner with a limited acceptor scope, which is routinely achievable by neighboring group participation and to some extent by solvent effects. If a method can be developed to successfully and reliably synthesize 1,2-cis-glycosides via a stereospecific reaction course, this will be of great value, thus solving what is arguably the main challenge in carbohydrate chemistry. In 2013, a new concept of cooperative catalysis was introduced by the Schmidt group67 in which a thiourea cocatalyst 4 (Scheme 12) was shown to increase reaction rate and β-selectivity in glycosylations catalyzed by various carboxylic acids and phosphoric acids. Gluco-, galacto-, and xylosyl TCA donors were employed as glycosyl donors. In the absence of the thiourea cocatalyst, the formation of an ester byproduct was observed, formed by nucleophilic attack by the carboxylate/phosphate. This indicated that the Brønsted acid alone was only functioning as a very weak activator of the donor in the absence of cocatalyst 4. 5 was selected as the best candidate for cocatalysis with thiourea 4 and a series of glycosylations were undertaken, revealing a clear preference for the formation of β-glycosides in CH2Cl2 at rt. With sugar acceptors, the solvent was changed to MeCN which, perhaps unsurprisingly, led to β-selective glycosylations in 78−87% yields. To justify these results, a number of mechanistic experiments were undertaken. It was ruled out that the glycosyl phosphate was acting as the key intermediate in the reaction since the activation of this, as an isolated glycosyl donor, reduced the selectivity and yield. 1H NMR experiments revealed interactions not only between the donor, catalyst, and acceptor, but also gave clear evidence that the protons of the cocatalyst, 4, were involved in precomplexation (Scheme 13). The proposed mechanism justifies the loss of selectivity, when applying larger acceptors, since this would lead to a more crowded activated complex, but β-selectivity was still observed with 6-OH and 4-OH glycosyl acceptors in MeCN, likely due to participation from the solvent.

Figure 3. Novel silicon fluoride acid/base-catalysts investigated by Schmidt and co-workers.

H hydrogen bonds were reported in the literature,65,66 thus possibly fitting into the concept and the proposed SN2-like reaction mechanism. Catalyst 2 was found to be too weakly Lewis acidic to efficiently coordinate to the acceptor and activate the TCA donor. Both 1 and 3 were not Lewis acidic enough to activate the glycosyl donor without an acceptor but were reactive when an acceptor was added. Catalyst 1 reacted at a much higher rate than 3 but at the price of lowered selectivity. It was found that catalyst 3 gave rise to very similar reactions as the PhBF2-catalyst previously introduced by the Schmidt group. Notably, the stereochemical outcome was similar in CH2Cl2, MeCN, and toluene. The inversion of the anomeric configuration of the donor (i.e., going from α-TCA to βglycoside) was very efficient (Scheme 11), although the degree of inversion deteriorated when using less nucleophilic acceptors such as secondary sugar alcohols.64 It is reasonable to assume that the less reactive α-TCA donor is more capable of sustaining the cyclic intermediate without loss of the TCA leaving group, thus resulting in a more Scheme 11. Glycosylation of Simple Acceptors with Silicon Fluoride Catalyst 3

F


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Scheme 12. Phosphoric Acid/Thiourea-Catalyzed Glycosylations by the Schmidt Group

Scheme 13. Proposed Mechanism for the Selective Formation of β-Glycosides

Scheme 14. Comparison of the Reaction Conditions and Products from Glycosylations Performed by Roy et al.69 and Peng and Schmidt.70

In 2017, Shaw et al.68 published a very similar system for cooperative catalysis involving a pyridinium salt and the same thiourea catalyst, 4 (Scheme 12 and Scheme 13), as employed by Schmidt and co-workers.67 Generally, this method suffers from relatively low yields, even with a primary sugar acceptor and low stereoselectivity in the case of a galactosyl TCA donor. The mechanism was studied by 1H NMR, which supported a mechanism similar to that reported by the Schmidt group (Scheme 13) but without leading to significant inversion of the anomeric configuration of the TCA donor in practice. In 2015, the use of AuCl3 and AuCl3 combined with phenyl acetylene as novel catalysts for TCA activation was reported by Roy et al.69 A catalytic amount of AuCl3 was capable of

activating an armed or disarmed TCA donor, but the optimal procedure involved the addition of phenyl acetylene in equimolar amounts to the 3 mol % of AuCl3 . The glycosylations were done on both glucose- and galactosederived donors and a range of different acceptors, including 4OH and 6-OH sugar acceptors. 60−86% yields of disaccharides were obtained but with no notable stereoselectivity. Shortly after the discovery of AuCl3-activation by Roy et al.,69 Peng and Schmidt published their results from using only AuCl3 as the catalyst in TCA activation.70 They reported that AuCl3 in itself was incapable of activating a TCA donor at low temperature but in contrast had a very high affinity for the acceptor alcohol, a conclusion that was supported by various G


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Scheme 15. Proposed Mechanism for the Activation of a TCA Donor by AuCl3

that α-TCA donors were indeed activated in the absence of an acceptor. Clearly, there has been a lot of interest and development in the field of both cooperative and acid/base catalysis for TCA activation. This field of research has also been recently discussed in detail by Peng and Schmidt.73 Metal Triflate Catalysts. Sn(OTf)2 was introduced as a catalytic activator of TCA donors in 1995 by Castro-Palomino and Schmidt.74 This new catalyst gave rise to high yields of 76− 91% of di- and trisaccharides within 15 min at room temperature on a disarmed glycosyl donor. The actual amount of added catalyst was however not reported but just stated as catalytic. Sn(OTf)2 has since been used in glycosylations. Bartek et al. have experienced possible limitations to the efficiency of the Sn(OTf)2 catalyst who could only obtain 20− 41% yield even when adding up to 1.3 equiv of Sn(OTf)2.75 In 1997, a publication by Drouillat et al. introduced LiOTf as a mild activator for TCA donors76 as a means to avoid problems with decomposition of the TCA donor. Previously, other researchers had also looked into milder activation conditions; Krepinsky77 had used a stoichiometric amount of AgOTf, Waldmann78 utilized a 1 M LiClO4 solution, and Jensen used a 0.09 M LiClO4 solution,79 for activating TCA donors under virtually neutral conditions. These preceding accounts did not seem appealing due to the use of AgOTf and LiClO4, claimed by Drouillat to be toxic and explosive in large amounts, respectively. It was found that 5 mol % of LiOTf was sufficient for activation of a perbenzylated TCA donor in CH2Cl2, resulting in a yield of 60% with a 3-OH sugar acceptor but with no stereoselectivity. It was also shown that the catalyst was severely deactivated in MeCN, and that the LiOTf was not capable of activating a peracetylated TCA donor. The first instance of TCA activation by a catalytic amount of AgOTf was reported by the Yamada group in 2000.80 Other groups had previously investigated AgOTf as an activator77,81,82 but not performed the glycosylations with substoichiometric amounts of the catalyst. AgOTf was shown to activate armed and disarmed glycosyl donors to facilitate glycosides in moderate to excellent yields at rt with a catalyst loading ranging from 10 to 15 mol % leading to glycosidic linkages between a disaccharide acceptor and disaccharide TCA donor in 49−67% yields, apparently with α-selectivity. In 2003, a more thorough investigation of AgOTf as a catalyst in glycosylations was published by Wei et al.83 It was reported that an armed donor without neighboring group participation and 0.3 equiv of

low-temperature NMR experiments. The high affinity for the acceptor led to an increase in the acidity of the alcohol proton while making the oxygen even more nucleophilic. This facilitated a catalytic system that was dependent on the presence of an acceptor, much in line with the findings by the Schmidt group37,63,64,67 that have been described earlier in this section. Interestingly, the results obtained from Roy et al. and Peng and Schmidt were quite different in regard to yield and stereoselectivity as shown in Scheme 14. Importantly, the glycosylations presented by Peng and Schmidt were carried out at much lower temperatures, which generally favor the formation of the β-glycoside.71 This could provide part of an explanation to the large difference in the stereochemical outcome. It also seems that AuCl3 is quite a strong Lewis acid, shown by Peng and Schmidt to result in decomposition of the donor at room temperature.70 Peng and Schmidt were attentive to the findings of Roy et al. and hence experimented with adding phenyl acetylene to their reactions as an additive but also performed some initial glycosylations in CH3CN, which should be capable of similar complexation to the AuCl3 catalyst. These experiments revealed that the order in which the reactants were mixed had a big impact on the reaction course. If the AuCl3 and the additive were allowed to be premixed with the acceptor before the addition of a glycosyl donor, the reaction would not yield any product. If, however, the catalyst was added last, the reaction would give rise to a 70% yield of the glycosylation product as a 1:1 mixture of α/β within 10 min. Thus, an unreactive adduct formation between the catalyst, additive, and acceptor could explain this. It also seems reasonable that the phenyl acetylene is important to add when performing the glycosylations at rt since this may help reduce the apparent high reactivity of the AuCl3 catalyst and inhibit the decomposition of the donor, thus leading to higher yields. Peng and Schmidt carried out a range of glycosylations at −70 °C in CH2Cl2 2-OH, 3-OH, 4-OH, and 6-OH sugar acceptors, all resulting in yields in excess of 80% with βstereoselectivity of α/β 1:>4. Thus, an efficient method of converting a perbenzylated α-glucosyl TCA donor into a disaccharide with almost exclusive 1,2-trans selectivity without neighboring group participation. A proposed mechanism for the AuCl3-catalyzed glycosylations is shown in Scheme 15. The Sasaki group recently employed the use of AuCl3 as the catalyst in the stereoselective β-mannosylations on 2,6lactones72 but reported, in contrast to Peng and Schmidt, H


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AgOTf would favor the formation of the β-glycoside, yielding a 1:9 mixture of α/β anomers at −42 °C. In the following years, various groups continued the search for milder activating conditions for TCA donors. In 2000, Adinolfi et al. reported the use of Sm(OTf)3 as a catalytic activator of perbenzylated TCA donors (Scheme 16).84

and it is arguably not surprising that adding more acid to the reaction mixture resulted in a more efficient activation of the acid-labile TCA donors. The Iadonisi group later reported that 4 Å acid washed mol sieves were able to activate TCA donors in the absence of a Lewis acid.103 The absence of Yb(OTf)3, when compared to their earlier report,94 resulted in a lowered reaction rate at higher temperatures ranging from rt to 70 °C rather than rt or lower when a catalytic amount of Yb(OTf)3 was added. No significant stereoselectivity was observed when using glycosyl donors without participation from neighboring groups. Another metal triflate, Bi(OTf)3, was introduced as a catalytic TCA activator by the Iadonisi group in 2006.104 5 mol % Bi(OTf)3 catalyzed 1,2-trans-glycosylations from various esterprotected TCA donors, giving disaccharides in 85−90% yields with 1,2-dichloroethane as the solvent. 1,2-Orthoesters were formed quickly and observed from TLC, but these were converted into the desired glycosides by Bi(OTf)3 under the reaction conditions. Acetonitrile was, however, found to deactivate the Bi(OTf)3 and hence nitrile solvents were not compatible with this catalyst. This was explained by the common observation that Bi(III) forms complexes with nitriles.105 Despite the struggles in nitrile solvents, Bi(OTf)3 was arguably a more potent catalyst, although less robust, than Yb(OTf)3 previously investigated by the same group.94 In a later publication by the Iadonisi group in 2008, the synthesis of a pentasaccharide was completed in three steps starting from monosaccharide building blocks via Yb(OTf)3- and Bi(OTf)3 catalysis of both TCA and N-phenyl trifluoroacetimidate (PTFA) donors.106 Mattson et al.107 reported the use of In(III) salts as a catalyst for TCA activation in 2012. During their investigation, InCl3, InBr3, and In(OTf)3 were studied as novel catalysts in glycosylations. It was found that in general, the reactivity of the three In(III) salts investigated was In(OTf)3 > InBr3 > InCl3. No sugar acceptors were investigated, but despite this, the glycosylations resulted in quite low yields of less than 70% and byproducts formed via Friedel−Crafts alkylation to the anomeric carbon was observed with benzyl-protecting groups. The In(III) salts have also been used as a catalyst for the peracetylation of sugars,108 InCl3 as a catalyst in thioglycosylations,109 and the use of InCl3 to activate peracetylated TCA donors had previously been reported by Ghosh et al. in 2003.110 Counter Ion Effect. The Mukaiyama group introduced HB(C6F5)4 as a novel catalyst for TCA activation in 2001111 following an interest in related catalysts over a number of years for catalytic aldol reactions,112,113 activation of glycosyl fluoride donors,111,114−117 and Fischer glycosylations.118,119 The HB(C6F5)4 catalyst, in a 5:1 mixture of trifluorotoluene (BTF) and tBuCN as solvent, would activate an armed β-

Scheme 16. Activation of Armed Glycosyl Donor 6 by Sm(OTf)3a

Conditions: (a) CH3CN, −25 °C, 88%, α/β 1:10, (b) 4:1 toluene/ THF, −25 °C, 80%, α/β 1:2, (c) 4:1 toluene/dioxane, −25 °C, 73%, α/β 1:1, and (d) 4:1 Et2O/THF, −25 °C, 76%, α/β 1:1.

a

Although being a highly hygroscopic compound that requires an intensive dehydrating procedure prior to use,85 Sm(OTf)3 is stable in air, thus giving it a long shelf life. 2−10 mol % Sm(OTf)3 activated a perbenzylated TCA donor with sugar acceptors yielding disaccharides in 47−88%. In most cases, they observed β-selectivity, but the selectivity was highly influenced by the choice of solvent in accordance with solvent effects.61,62,86−91 Activation of a disarmed donor (Scheme 17) using a mild Lewis acid was reported by Adinolfi et al. when continuing their work on lanthanide triflates as catalytic activators of TCA donors.92,93 Sm(OTf)3, Tb(OTf)3, and Yb(OTf)3 were found as efficient catalysts, whereas Sc(OTf)3 proved to be very unreactive towards the disarmed donor. On the basis of these findings, the group decided on doing further experiments with the cheaper Lewis acid, Yb(OTf)3. This was shown to catalyze the formation of 1,2-trans-glycosides with 5− 50 mol % loading resulting in 35−85% yield but required higher reaction temperatures when decreasing the catalyst loading (Scheme 17). Much better results were reported for the Yb(OTf)3-catalyzed glycosylations of the more reactive, perbenzylated TCA donors, leading to 77−97% yields of disaccharides.94 Large amounts of 1,2-orthoester byproducts were produced from ester-protected TCA donors. Although that 1,2-orthoesters can usually be rearranged into the 1,2-transglycoside by Brønsted- or Lewis acids,95−99 this was not achievable with the lanthanide triflates investigated by Adinolfi et al.92 It was also shown that acid washed 4 Å mol sieves improved the yield of the Yb(OTf)3-catalyzed glycosylation.94 The use of acid-washed molecular sieves in glycosylations had previously been reported sporadically in the literature,100−102

Scheme 17. (a) CH3CN, 50 mol % Yb(OTf)3, 0 °C rt, 70%, (b) toluene, 50 mol % Yb(OTf)3, rt, 64%, (c) CH3CN, 15 mol % Yb(OTf)3, 50 °C, 61%, (d) toluene, 15 mol % Yb(OTf)3, 50 °C, 58%, (e) CH3CN, 5 mol % Yb(OTf)3, 80 °C, 85%, (f) toluene, 5 mol % Yb(OTf)3, 80 °C, 51%

I


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Scheme 18. One-Pot Solution Phase Synthesis of Trisaccharide Derivatives LeX and LeA Using 0.3 mol % HClO4

Figure 4. Illustration of the on-column setup used by Mukhopadhyay and Field.

acceptors, including sugar alcohols, generally in low-tomoderate yields. It was also noted that deprotection of the C-2 O-protecting group of the glycosyl donor was a common side reaction as well as the formation of unexpected methyl glycosides. The Poletti group has done a thorough investigation of the activation of TCA donors in ILs.126−128 They have shown that using [emim]OTf as solvent results in enhanced βselectivity, arguably due to the interaction of the triflate counterion in the reaction intermediate.127 It was also shown that the solvent in itself had the ability to activate the TCA donors, even in the absence of a catalyst, giving an indication of the IL to act as a Lewis acid in itself.126,127 [bmim]PF6 and [emim]BF4 were investigated and found to facilitate inversion of the anomeric stereochemistry during glycosylation in excellent yields when using TMSOTf as the catalyst. Also, during the investigation by the Poletti group, an unusual byproduct was observed, namely, that the β-TCA donor was observed to anomerize to the α-O-TCA donor.127 This highly unusual behavior has also been confirmed by NMR experiments.128 Several groups have been investigating the use of ILs as solvents for glycosylations on a range of different glycosyl donors.129−133 And although a versatile field of research exists for the use of ILs in oligosaccharide synthesis,134 it seems that there are some reasons for the methods to not really gain more attention in the scientific community. Certain challenges are reported, including issues with removing either the active catalyst from the solvent before reuse126 or the accumulation of byproducts in the IL.135 Thus, there is no doubt that ILs have the potential to be an environmentally benign, integrated, and important part of oligosaccharide synthesis, but there is still

TCA donor with a catalyst loading of 20 mol % giving rise to a yield of 95% and excellent β-selectivity leading to a 1:9 mixture of α/β glycosides. This result was compared to an analogous experiment employing the use of HClO4 as the catalyst, but using Et2O as the solvent, which gave rise to a 99% yield, as a 9:1 mixture of α/β glycosides, thus reversing the stereochemical outcome depending on the choice of solvent and catalyst. Furthermore, when exchanging the solvents, thus having the HB(C6F5)4 catalyst in Et2O, and vice versa, both catalytic systems lost their selectivity providing some insight into both a solvent- and an anion-effect being determinants of the stereochemical outcome of the glycosylations. HB(C6F5)4 has been used for the activation of TCA donors in recent years, notably by the Li group when synthesizing glycoalkaloids for achieving excellent 1,2-trans-selectivity without having to rely on neighboring group participation.120,121 The findings by the Mukaiyama group later inspired the Fukase group to introduce TMSB(C6F5)4 as a catalyst in Nphenyltrifluoroacetimidate (PTFA) activation.122 Likewise, the Gin group has investigated the use of HB(C6F5)4 in glycosylations but only got decomposition of their glycosyl donor. B(C6F5)3 turned out to be a more useful catalyst in glycosylations, favoring the formation of 1,2-trans-glycosides in glycosylations where neighboring group participation was not possible.123,124 Glycosylations in Ionic Liquids. The use of ionic liquids (ILs) as the solvent in glycosylation reactions from TCA donors was introduced by Pakulski in 2003.125 During these studies, an ester-protected TCA donor was activated by a catalytic amount in a solvent mixture of [bmim]PF6 and 20− 40% Et2O to yield glycosides with a range of different J


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combining LiClO4 or SiO2 with the common TMSOTf-catalyst did not lead to similar yields or selectivity. Besides serving as an efficient catalyst in TCA activation, HClO4−SiO2 has found relevance beyond glycosylation chemistry,147,148 been used in a catalytic, solvent-free peracetylation of sugars,141 selective deacetylation of the anomeric position,149 as a cocatalyst or catalyst in the activation of thioglycosides and phosphate glycosides,150−152 in conversions of pyranosides to furanosides,151 and in regioselective openings of benzylidine protecting groups.153 Sulfated zirconia (SO4/ZrO2 or “SZ”), a commercially available, solid superacid was investigated as a novel catalyst for TCA activation by the Nishimura group in 2006 using supercritical CO2 (“scCO2”) as the solvent.154 This was an environmental friendly approach since it involved no organic solvents and a reusable catalyst. Both a peracetylated- and a more reactive, perbenzylated galactosyl donor were investigated and gave rise to yields of 51−81% of disaccharides albeit with no significant stereoselectivity (α/β 32:68 to 37:63) without neighboring group participation. A β-glycosyl fluoride donor was also shown to be activated under similar conditions. Although the performance of the glycosylations was not ideal, it does seem like an appealing approach for large-scale glycosylations due to the environmentally benign conditions. This method has since been employed in a more advanced setup under microwave irradiation of the reaction mixture, which showed an increase in yield with aliphatic acceptors.155 The Nishimura group also investigated the use of the SZ superacid doped with CaCl2 as a catalyst in glycosylations using CH2Cl2 as the solvent.156 This, however, only resulted in moderate yields (ca. 50−70%), with low stereoselectivity. Other salt additives were also investigated, but calcium proved to be the most efficient, perhaps due to increasing the Lewis acidity of the SZ, while making it allegedly almost non-Brønsted acidic. The Nishimura group have also done microwave-assisted glycosylations in scCO2 but not with glycosyl acceptors.155 Transition Metal Catalysis. In 2009, the Nguyen group utilized their experience with palladium chemistry157,158 and activated a glucosyl α-TCA donor with a Pd-complex, facilitating excellent β-selectivity in the absence of neighboring group participation and in a nonparticipating solvent.159 Pd(PhCN)2(BF4)2 was initially shown to facilitate the activation and glycosylation of perbenzylated glucosyl α-TCA donor resulting in β-selectivity, but it was decided to pursue the use of another catalyst since the Pd(PhCN)2(BF4)2 catalyst was very expensive. Pd(PhCN)2(OTf)2 formed in situ from the more affordable Pd(PhCN)2Cl2, and 2 equiv AgOTf was instead employed (Scheme 20). This decreased the selectivity compared to Pd(PhCN)2(BF4)2, yielding a 1:1 mixture of anomers at room temperature, but a 1:10 mixture of α/β glycosides was obtained with a 1% loading of the Pd(II)-catalyst at −78 °C giving

more exploration to do in terms of reactivity and the formation of undesired byproducts. Solid Acid Catalysts. HClO4 immobilized on silica was used as a catalytic TCA activator in 2005 by Mukhopadhyay and Field et al.136 who had previously reported HClO4 immobilized on silica as a cocatalyst for the activation of thioglycosides and in protecting group chemistry.137,138 It was shown that the presence of approximately 0.3 mol % of HClO4 was sufficient for many glycosylations with glycosyl acceptors and successfully completed a one-pot synthesis of the Lex and LeA trisaccharide resulting in a yield of 62% and 59%, respectively, over two steps (Scheme 18). The catalyst was shown to be applicable for on-column glycosylations, thus making immediate purification after the synthesis possible. A flash column was added 20 g of silica gel and topped off with a layer of 5 g of HClO4−SiO2. To this, a solution of TCA donor in CH2Cl2 (1.3 equiv with respect to the acceptor) and acceptor was added and allowed to react at room temperature for 30 min (Figure 4). Afterwards, the column was eluded in accordance to regular purification by flash column chromatography to yield disaccharides in yields comparable to the same reactions carried out in solution phase. The on-column synthesis did, however, involve approximately 25 mol % of the catalysts with respect to the donor as compared to 0.3 mol % in solution. In 2006, the use of HClO4 immobilized on silica was continued by Du et al.139 who drew inspiration from various other reactions involving this catalyst,138,140−144 to improve the synthesis of avermectin B1α and avoid large-scale use of the more expensive AgOTf (Scheme 19). Scheme 19. General Outline for the Reaction Conditions and Scope of Glycosylations Using HClO4−SiO2 as the Catalyst

After initial experiments, the optimal ratio was found to be 6 mol % of catalyst resulting in the shortest reaction times and highest yields. Interestingly, they also accomplished a large scale glycosylation with 100 g of TCA donor, resulting in a 77% yield of the desired glycoside from which the catalyst could easily be filtered off. However, attempting to reuse the catalyst from this glycosylation resulted in yields of 10:1.

disaccharides in 85−87% yield. Similar results were obtained with an acid scavenger present, thus excluding the presence of TfOH in the reaction mixture. Pd(PhCN)2Cl2 and AgOTf alone could not activate the donor. A wide range of different sugar acceptors (6-OH and 3-OH) and various TCA donors were employed, all giving rise to yields of 62−97% and α/β selectivity of 1:>6, with a 1 mol % catalyst loading. On the basis of control experiments with a TIPS-protected 2-OH or a 2deoxy TCA donor, both giving much lower stereoselectivity, the authors proposed a seven-membered, cyclic intermediate, which would lead to the high 1,2-trans-selectivity. If R* is a bulky group (Scheme 21) then the SN1-pathway was claimed to dominate, which could explain the lowered selectivity. A related cationic Pd(II)-catalyst, Pd(CH3CN)4(BF4)2, has also been investigated by the Nguyen group.160 During these studies, both manno-, rhamno-, and glucosyl α-TCA donors were investigated under catalysis by this air- and moisture stable Pd(II) species. Interestingly, a very high 1,2-trans stereoselectivity was reported, enabling the synthesis of various disaccharides in 67−96% yields independent of the parent glycosyl donor. The presence of an acid scavenger did not lead to diminished yields or selectivity, thus signifying that the reactions are not Brønsted-catalyzed. Also, a control experiment using a neutral Pd(II) salt, Pd(CH3CN)2Cl2, was unable to give even a 5% yield, thus underlining the fact that the cationic character of the catalyst was essential to catalyze the reaction. The fact that both the α-L-rhamno- and α-D-mannosyl TCA donors gave rise to 1,2-trans-glycosides was interesting because of the opposite configuration at the C-2 position. Thus, the authors argue that a different mechanism leading to the 1,2trans selectivity of rhamno- and mannosyl donors must occur via an alternative mechanism with more cationic character (SN1 L


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Scheme 23. Initial Step of the Synthesis of Mycothiol by the Nguyen Group

Scheme 24. Example of an FeCl3-Catalyzed Glycosylation by Mukherjee et al.

Figure 5. Novel catalysts for TCA activation investigated by Miller and co-workers.

A mechanism explaining selectivity of the glycosylations with the new Ni(II) catalyst was suggested (Scheme 22). It includes a seven-membered ring as an intermediate reacting with the acceptor alcohol to facilitate the 1,2-cis-glycoside, which is in contrast to the 1,2-trans selectivity reported for a very similar mechanism (Scheme 21). A control experiment revealed that the TCA donor would slowly rearrange to the trichloroacetamide in absence of an acceptor alcohol, indicating participation from the acceptor in the activation step. A wide range of sugar acceptors and other common nucleophiles were investigated in a substrate scope leading to yields of 72−97% with 1,2-cis selectivity of α/β > 10:1. The same Ni(II) catalyst was used later, also by the Nguyen group, to inhibit the intermolecular transfer of the sulfide group of a thioglycoside acceptor, while having a 2amino TCA donor.167 A handful of other metal catalysts, not previously described for TCA activation, Cu(OTf)2, Rh(COD)2OTf, Ir(COD)2OTf, Ph3PAuOTf, and Fe(OTf)2, were also investigated as catalysts for TCA activation but gave low yields with reduced selectivity. The Ni(4-F-PhCN)4(OTf)2 catalyst has since been employed by the Nguyen group,168−170 notably in the synthesis of mycothiol which had previously proved challenging since the required 1,2-cis couplings are difficult to synthesize (Scheme 23). During these studies it was confirmed that the β-TCA donor was unreactive toward the Ni(II) catalyst as previously reported,171 whereas only the α-TCA donor facilitated the formation of a glycoside with excellent α-selectivity. Ni catalysts were also reported by the Nguyen group to catalyze the formation of trichloroacetamide from TCA donors in order to obtain N-glycosides with high α-selectivity.172

Inspired by the Pd-catalyzed formation of 1,2-trans-glycosides by the Nguyen group, the use of FeCl3 as a novel, 1,2trans-selective catalyst was reported by Mukherjee et al. in 2016 (Scheme 24).173 10 mol % of FeCl3 was shown to lead to inversion of the anomeric configuration from both perbenzylated glucosyl and galactosyl α-TCA donors in yields of 85− 96% with 6-OH, 4-OH, 3-OH, and 2-OH glycosyl acceptors, all leading to exclusive isolation of the β-glycosides in the reported yields. The procedure was also found to tolerate ester protecting groups, similarly leading to 1,2-trans-glycosides in 88−91% yields. A 4-OH sugar acceptor was glycosylated with a mannopyranosyl TCA donor in 87% yield with a 1,2-transglycosidic linkage. It was furthermore shown that the FeCl3 was a good cocatalyst in the activation of thioglycosides, which encouraged the authors to do an orthogonal one-pot sequential synthesis of a trisaccharide in 87% yield. Subsequently, no other research groups have reported use of this catalytic system, but it seems that there is a lot of potential for FeCl3, especially for large-scale oligosaccharide synthesis when considering the low cost of this catalyst. The first example of activation of a TCA donor by a gold catalyst was published on 2009 by the Kunz group.174 In this paper, AuCl was reported as a mild, catalytic activator for TCA donors. Only one disaccharide was synthesized with 5 mol % of AuCl, which led to a 60% yield and a > 30:1 ratio of α/βanomers. Chiral Catalysts for TCA Activation. In an attempt to gain further understanding of the activation of TCA donors, the Miller group have examined a range of hydrogen bonddonating catalysts 6−9 (Figure 5).175 M


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the catalysts investigated provide inversion of the anomeric configuration during the glycosylation since the glycosyl donor was a mixture of anomers, but generally, no significant preference for either product was found. Two chiral Co(III) triflate complexes “(salen)Co” were introduced as a catalyst for TCA activation in 2015 by Medina et al.177 The reactions were slow, unselective, and resulted in 74% and 89% yields which led to investigate oligomers of the catalysts with different lengths of bridges (Figure 7). Ten mol % of the oligomer resulted in shorter reaction times, leading to formation of disaccharides from a 6-OH sugar acceptor, in yields of >50%. Many common solvents were tolerated with short reaction times of around 1 h at rt, although no significant stereoselectivity was observed. β-Selective gluco- and fucosylations were possible in MeCN with 10 mol % of the catalyst with 6-OH sugar acceptors, while mannosylations led to αselectivity. The yield of the glycosylations ranged between 50 and 90%. Catalytic activation of a TCA donor with a chiral catalyst was reported in 2010 by the Fairbanks group178 inspired by similar work on noncarbohydrate trichloroacetimidates by the Toste group.179 Their method encompassed the use of a BINOL-derived phosphoric acid catalyst (Figure 8) known as the AkiyamaTerada catalyst, 180−182 which was used to activate a perbenzylated galactosyl TCA donor. TMSOTf was chosen as the reference catalyst. The yield and stereochemical outcome is summarized in Table 2. A lowered stereoselectivity was reported in more polar solvents, which could indicate the presence of a close ion-pair intermediate that will be more solvated and thus further apart in the more polar solvents. The stereochemical outcome was not influenced by lowering the concentration of the donor and acceptor. (S)-12 achieved the best β-stereoselectivity (α/β = 1:70, 88% and 1:4.9, 73%) in glycosylations with 6-OH and 3OH sugar acceptors, respectively, indicating a drop in selectivity and yield with decreasing nucleophilicity of the acceptor. Thus, it was shown that the stereochemical outcome of a

They found that 0.1 equiv of 6, 7, or 8 could efficiently activate TCA donors in the presence of cyclohexanol, but only sulfonamide catalyst 9 (0.15 equiv) was employed in glycosylations with glycosyl acceptors giving rise to a 67% yield with a 6-OH acceptor, which decreased to just 28% when serine was used as the acceptor. Both glycosylations were nonstereoselective. In an attempt to mimic the glycosylation mechanism in enzymes, the Miller group have also investigated a combination of Brønsted acids and Lewis acids to activate TCA donors.176 It was found that although benzoic acid was unable to activate the TCA donor, the tetrapeptide 11 (Figure 6) could,

Figure 6. Structures of the Brønsted acids, 10 and 11, employed by Gould et al.

resulting in a low yield of 32% with cyclohexanol as the acceptor. It was speculated that the carboxylate itself could instead act as an acceptor, resulting in the “rebounded” product (Scheme 25). MgBr2·OEt2 was added as an additive, inspired by the Mg2+-abundance in the human body and could act as a Lewis acidic cocatalyst, but this failed to increase the yield of the formed glycoside, although lowering the amount of rebounded product. On the basis of evidence from NMR experiments, a complex mechanistic pathway was suggested that incorporated all the observed byproducts. The mechanism suggested a Brønsted acid activation of the TCA leaving group, since Mg2+ was shown to be a very weak activator. A series of glycosylations using 0.1 equiv of the two carboxylic acids led to disaccharide yields of 32−58% after 24 h. It is not possible to tell whether

Scheme 25. Mechanism Proposed by Gould et al. for the Activation of TCA Donors Using Carboxylic Acid Catalystsa

a

The desired glycosides are highlighted in a rectangle. N


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Figure 7. Three chiral Co(III) complexes investigated as novel catalysts in glycosylation reactions.

and reactivity of the acceptor, which unfortunately limits the scope of the method. In 2013, the work on BINOL-derived Brønsted acid catalysts (R)-12 and (S)-12 was continued by the Toshima group when investigating not only the stereochemical outcome of a glycosylation depending on the stereochemistry of the catalyst but also whether the chirality of the catalyst would result in recognition of certain acceptors.183 In the initial screening, a series of glycosylations were carried out using a perbenzylated α-glucopyranosyl TCA donor. Furthermore, the reactivity of the two chiral Brønsted acids, (R)-12 and (S)-12, was compared to reactions using an achiral Brønsted acid catalyst and TMSOTf. The acceptor in their initial experiments was a racemic mixture of a chiral acceptor, which would make it possible to investigate the possible recognition of the two catalysts used in the glycosylations. It was found that (S)-12 could recognize the (R)-acceptor, resulting in a 93% yield with a high preference for the β-glycoside in Et2O. When carrying out the reaction in CH3CN, the yield and recognition dropped although β-selectivity was still observed. This could indicate interaction of the nitrile with the expected oxocarbenium ion intermediate, obstructing the precomplexation by the donor, catalyst, and acceptor. This was seemingly not the case with Et2O, although ether solvents are also known to influence the stereochemical outcome of a glycosylation reaction, often favoring the 1,2-cis couplings.86,87,89,91 A range of different chiral acceptor alcohols were investigated as racemates under optimized reaction conditions (Scheme 26) favoring the formation of the β(R)-glycosides. No isomerization of the glycosides was observed under the reaction conditions, but it was found that the β-TCA donor could not serve as a donor with the (S)-catalyst, which could indicate a reaction pathway necessitating an α-configuration of the donor for the

Figure 8. Structures of the two BINOL-derived enantiomers investigated by Fairbanks and co-workers.

Table 2. Conditions for the Initial Experiments Investigating the Reactivity of the Catalysts (R)-12 and (S)-12

ratio entry

catalyst

yield (%)

α

β

1 2 3 4

TMSOTf (R)-12 (S)-12 (PhO)2PO2H

98 88 80 19

55 33 12 34

45 67 88 66

glycosylation can differ according to the structure and chirality of the catalyst, but there is still a strong dependence on the size

Scheme 26. Optimized Reaction Conditions and General Trend in the Acceptor Recognition Reported by the Toshima Group

O


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Scheme 27. Synthesis of Flavan Glycoside Utilizing Chiral Recognition of (S)-12

Scheme 28. Synthesis of Disaccharide with Photo Acid Catalysts 13 and 14

Scheme 29. Concentration-Dependent Stereoselectivity of Glycosylations

and other chiral catalysts have furthermore been used for regioselective protection and functionalization of hydroxy groups in carbohydrates by various groups,188−195 which indicates a big interest for these catalysts in carbohydrate chemistry and perhaps indicates an unfulfilled potential of these catalysts in glycosylation reactions. There is undoubtedly a big potential for chiral catalysts in glycosylations, but the structure and nucleophilicity of the acceptor seems to have a significant effect on the success of the glycosylations. This is an obstacle since it is hard to imagine a simple system where one catalyst can facilitate glycosylations with a wide scope of acceptors in a stereoselective manner. It seems that a “match” of donor, acceptor, and catalyst is needed for the glycosylations to succeed, which, despite the complexity, could have a large potential in the future within glycosylation chemistry if a suitable library of fitting catalysts becomes available. Photo Acid Catalysts. Inspired by the interest in lightinduced reactions in carbohydrate chemistry,196−203 the Toshima group activated TCA donors with organic acids that are more acidic in the photoinduced excited state (Scheme 28).204 Initially, a range of glycosylations were carried out on a perbenzylated glucosyl donor primarily using simple alcohols as the acceptors. With a 6-OH glycosyl acceptor, yields of 85 and 87% were obtained in a 12 h reaction as a 1:1 mixture of

formation of an activated complex to be formed prior to glycosylation. To further expand the scope of the method, a flavan glycoside was synthesized. High stereo- and diastereoselectivity was achieved when using a racemic mixture of the acceptor (Scheme 27) (i.e., only the desired enantiomer of the acceptor was found to react under the reaction conditions). Generally, the results indicated that (S)-12 was the superior catalyst in regard to recognition of a chiral acceptor alcohol. A sugar acceptor has not been employed in a system similar to that developed by Kimura et al.183 The interactions of TCA donors and acceptors with chiral Brønsted acids has been described by Liu et al. when considering possible matches and mismatches between these species. It was found that one enantiomer of the BINOLderived catalyst would lead to α-selectivity, whereas the other enantiomer would lead to β-selectivity from the same 2-deoxy glucosyl TCA donor.184 Furthermore, it was reported recently by Tay et al. that a BINOL-based phosphoric acid catalyst would lead to regio- and stereoselective 6-deoxyglucosylations of macrolactones.185 A similar glycosylation was also performed with mannopyranosyl TCA donors, giving high α-stereoselectivity and lower degree of regioselectivity.186 Also, the use of chiral BINOL-derived catalysts in asymmetric functionalization of alcohols with TCA electrophiles has been reported recently by Yamada and Takasu.187 BINOL-derived catalysts P


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Scheme 30. Two Comparable Glycosylations Performed by Toshima and Co-Workers at 35 °C and Added 100 wt % 5 Å MS

Scheme 31. Solid Phase Glycosylations with TCA Donors by the Iadonisi Group

investigated further by Danishefsky210−213 and Kahne.214 In 1996, the Schmidt group were the first to do solid phase synthesis of oligosaccharides using TCA donors.215 During this investigation, a single glycosyl TCA donor was coupled to a linker via a thiol. This could be selectively deprotected to give the unprotected C-6 hydroxy group to serve as the glycosyl acceptor for another molecule of the same TCA donor, added in three equiv in each cycle under catalytic TMSOTf activation (0.2 equiv). The coupling/deprotection cycle was repeated five times before the glycosides were cleaved from the solid phase by dimethylmethylthiosulfonium tetrafluoroborate (DMTSB). This yielded a complex mixture of 32 diastereomeric pentasaccharides that were separable by HPLC, indicating some interesting possibilities for combinatorial synthesis. The di-, tri-, and tetrasaccharides were isolated from the pentasaccharides by column chromatography. At the same time as the Schmidt group, a report on the solid phase synthesis of oligosaccharides employing TCA donors by the Iadonisi group appeared.216 During this investigation, the glycosyl acceptors were coupled to the commercially available solid phase linker, Tentagel (TG). This linker could easily be cleaved by washing with conc. aq ammonia, thereby making it easy to remove the glycosylation product from the solid phase. TfOH or the Lewis acids TMSOTf, BF3·OEt2, and LiClO4 were used in the glycosylations to yield the desired glycoside in moderate 45−70% yields depending on the nucleophilicity acceptor (Scheme 31). An increase in yield of up to 55−90% was observed when the coupling reaction was repeated twice. During the glycosylations, 20 equiv of the donor was added, which can pose a challenge in regard to cost-efficiency. Since the introductory work by the Iadonisi group217,218 on applying TCA donors to solid phase chemistry, many other scientists have been involved in the oligosaccharide synthesis on a solid support. A wide range of different systems, also incorporating a range of different donor types, have been investigated,5,6,219

anomers. Galacto- and mannosyl TCA donors were also glycosylated with simple alcohols under the reaction conditions. In 2016, a continuation of the work on photoacid catalysis was published by the Toshima group.205 The study was quite similar to the previous; however, a thiourea photoacid, 4, was identified as a catalyst for TCA activation. It was also found that high concentrations of donor and acceptor would lead to a preferential formation of the β-glycoside, whereas more dilute conditions would favor the formation of the α-glycoside (Scheme 29). The catalyst was shown to activate glucosyl TCA donors with a primary alcohol acceptor without photo irradiation, but this only led to a very low yield of 31% at high concentration (1.0− 2.0 M of TCA donor), which increased to yields in excess of 80% and ca. 1:3 α/β-selectivity upon light irradiation (Scheme 30). This selectivity was evident, even in Et2O that usually favors the formation of axial glycosidic bonds. When switching to CH3CN as the solvent, the yield and selectivity increased further. With donor concentrations of 0.005−0.1 M α-selectivity was observed. An optimized procedure was investigated using a range of primary and secondary glycosyl acceptors giving rise to yields of 56−78% in Et2O (0.01 M, “low concentration”) resulting in a ∼4:1 α/β ratio and, conversely, giving yields of 83−89% in CH3CN (1.0 M, “high concentration”), resulting in a ∼1:10−1:3 α/β ratio of the desired glycosides. Galacto- and mannosyl donors were also investigated with a primary alcohol acceptor behaving in accordance with glucosyl TCA; however, only a 1:1 α/β mixture of mannosides was obtained under high concentration, underlining the difficulty of achieving β-selective mannosylations. Trichloroacetimidates on Solid Phase. A lot of early work on the solid phase synthesis of oligosaccharides was carried out by the Schuerch group206−208 and Excoffier et al.209 Later, the field of solid phase oligosaccharide synthesis was Q


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notably by the Danishefsky,213,220 Schmidt,221,222 Nicolaou,223,224 Seeberger,225−228 Boons229 and the Demchenko groups.230 A detailed description of glycosylations on solid phase is outside the scope of this review but has been reviewed in recent years by Bennett.231 General Remarks for Catalytic Activation of TCA Donors. (1) Lower reaction temperatures and higher concentration generally favor formation of the kinetic glycosylation product, which involves an equatorial glycosidic bond. (2) Weak Lewis acids or weakly coordinating counterions favor an SN2-like reaction mechanism in nonpolar solvents. (3) Protocols involving precomplexation of donor, acceptor, and catalyst are very dependent on the size of the acceptor, generally resulting in a loss of stereoselectivity with increasing size. (4) TCA donors with an equatorial TCA-substituent (most often β-TCA’s) are more reactive than their axial counterparts, which limits the possibility of precomplexation and thus, generally, leads to lowered stereoselectivity in glycosylations. (5) The trichloroacetamide leaving group is nucleophilic and will, if the acceptor is a poor nucleophile, lead to formation of glycosyl trichloroacetamide as a byproduct which has no donor ability. Thus, low glycosylation yields are often encountered in these cases, or an excess of the TCA donor must be used. The rearrangement of trichloroacetimidates to the corresponding amides has been known for many years, even before the application in carbohydrate chemistry.232−237

base such as NaH, the equatorial PTFA donor is the major product.94 PTFA glycosyl donors were initially activated by 5 mol % of TMSOTf, yielding the desired glycosides in excellent yields of 86−99% from rhamnosyl- and glucosyl donors having primary sugar alcohols, cholesterol, and 1-adamantol as acceptors.9 In the absence of neighboring group participation, no significant stereoselectivity was observed, but the high yields persisted using both armed and disarmed donors. On the basis of a comparative study by the Ubukata group on TCA- and TFA donors (without the N-phenyl substituent), it has shown that the trifluoro donors were generally less reactive than the corresponding trichloro donors (Scheme 34).242 The Yu group synthesized the natural products dioscin, xeibai saponin, and lexagenin using the PTFA donors in excellent yields, underlining the scope of the PFTA method.241 Additionally, the donors were proven to be shelf stable, if kept in a refrigerator, showing no signs of decomposition over an extended period of time. During synthesis of the natural product Ginsenoside Ro by the Yu group, the use of TBSOTf as a catalytic activator (10−40 mol %) gave a 79% yield with a 2-OH glycosyl acceptor.28 TBSOTf was employed instead of TMSOTf to avoid the formation of a trimethylsilyl ether byproduct from TMS-protection of a free alcohol. Soon after the discovery of PTFA as a novel glycosyl donor, the Iadonisi group published a report on a new method of activation by I2/Et3SiH, which should function similarly for both TCA and PTFA donors.240 Interestingly, the PTFA donors gave much better results than corresponding TCA donors under I2/Et3SiH activation, resulting in an 85% yield with a sugar acceptor. This method, however, was probably not catalytic since lowering the iodine loading from stoichiometric amounts resulted in a dramatic decrease in yield. Encouraged by the performance of the PTFA donors compared to TCA donors, the Iadonisi group continued the search for other catalysts. In 2002, the group published the first account of using Yb(OTf)3 for PTFA activation.94 To investigate the glycosylation, a 4-OH model acceptor was reacted with both a TCA and a PTFA donor using 15 mol % Yb(OTf)3 as the catalyst. This revealed that although the reaction of the PTFA donor took place at a higher temperature than the TCA counterpart, the yields were higher for the PTFA donor. Through optimization of the reaction conditions, it was revealed that a 15 mol % Yb(OTf)3 in (CH2Cl)2/EtCN as a 4:1 mixture together with acid-washed AW300 molecular sieves gave the best results. Glycosylations of both armed and disarmed PTFA donors were performed on a range of different 2-OH, 3-OH, and 4-OH glycosyl acceptors in yields of 75−97%, but with low stereoselectivity. Also, the addition of the AW300 molecular sieves allowed lowered reaction temperatures, arguably due to the more Brønsted acidic reaction medium. It was also later shown that the sieves alone were able to activate the PTFA donors.243 The mild Yb(OTf)3 catalyst were later shown not to affect the very acid-labile dimethoxytrityl (DMT) protecting group.244 BF3·OEt2 was introduced as a catalytic PTFA activator by the Yu group in 2003, when synthesizing flavonoid 7-O-glycosides.245 It was shown that the PTFA donors facilitated glycosylation of the 7-OH of flavonoid acceptors, generally considered a very weak nucleophile, in 64−90% yields upon activation of disarmed, acyl protected glucosyl-, rhamnosyl-, and arabinosyl donors with 0.3 equiv of BF3·OEt2. Interestingly, better yields were generally observed in the absence of regular 4

2.3. (N-Phenyl)trifluoroacetimidates

The N-(phenyl)trifluoroacetimidate (PTFA) donors were introduced by the Yu group in 2001 as a novel donor inspired by the very similar TCA donors.9 The PTFA donor was prepared from a C-1 unprotected sugar and 2,2,2-trifluoro-N(phenyl)acetamidoyl chloride in the presence of a base (Scheme 33). The fluorinated acetimidoyl chloride, now made commercially available, can be made via a one-pot procedure in high yields from trifluoroacetic acid (Scheme 32).238 Scheme 32. One-Pot Formation of 2,2,2-Trifluoro-N(phenyl)acetamidoyl Chloride

The reaction of the acetamidoyl chloride and an alcohol was also known in the literature239 prior to the findings of the Yu group. Since the formation of the PTFA donor is an irreversible reaction, the anomeric configuration of the donor cannot be controlled by an equilibrium (Scheme 33). Only slight control of the anomeric configuration of the PTFA donors has been achieved,240 and the reaction has been shown to proceed at a lowered rate in very dry conditions.241 When using a strong Scheme 33. Formation of the PTFA Donors from a C-1 Unprotected Sugar

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Scheme 34. Selected Glycosylations from the First Report on PTFA Donors by the Yu Group

Scheme 35. General Procedure for the TMSB(C6F5)4 Catalyzed β-Selective Mannosylations by the Fukase Group

Scheme 36. One-Pot Synthesis of a Trisaccharide by the Iadonisi Group

The TMSB(C6F5)4 catalyst was also later shown by the Fukase group to be a valuable catalyst in the solid phase synthesis of an oligosaccharide, giving excellent β-selectivity from a mannosyl PTFA donor (3:97 α/β) in CH2Cl2247 and also applied in gram-scale β-mannosylations.248 However, it must be underlined that the 4,6-O-benzylidene protected mannopyranosyl donor is inherently β-selective in glycosylations,249,250 and thus the stereoselectivity of the glycosylation is not exclusively governed by the catalyst. An interesting property of the PTFA donors is their lower reactivity compared to the corresponding TCA donors.251 This feature was exploited by the Iadonisi group by activating a TCA donor selectively in the presence of a PTFA donor (Scheme 36).252 The remaining PTFA donor could then subsequently be activated by increasing the temperature to facilitate a one-pot synthesis of a trisaccharide in a 55% overall yield, although the product was reported to contain small amounts of impurities from α-linked products. The one-pot strategy was also shown to facilitate α-linkages with a mannosyl TCA donor and a mannosyl PTFA donor, resulting in an overall yield of 40% over two steps. Continuing the optimization of activation conditions of glycosyl trihaloimidate donors, the Iadonisi group introduced Bi(OTf)3 as a novel catalyst in 2006.104 During this investigation, glucosyl-, mannosyl-, fucosyl-, and galactosyl PTFA donors were employed using a range of primary and secondary sugar acceptors as well as an amino acid derivative. The reactions proceeded at much higher rates than the

Å molecular sieves, which possibly could be due to the ability of the molecular sieves to function as an acid scavenger,94,240 although this was not discussed by the authors. In 2005, the Fukase group reported novel catalysts for PTFA activation capable of high β-selectivity in mannosylation reactions.122 When activating a PTFA donor with TMSOTf at −78 °C, a mannoside was obtained with excellent βselectivity (7:93 α/β) in a 90% yield from a 4,6-benzylidene protected mannosyl PTFA donor. Encouraged by this promising result, the Fukase group investigated the effect of the counterion on the stereoselectivity of the glycosylation and found the investigations of the B(C6F5)4− counterion by the Mukaiyama group111,114,117−119,246 of particular interest. Thus, HB(C6F5)4 was added as a cocatalyst (20 mol %) in addition to the TMSOTf catalyst (15 mol %), resulting in the same level of selectivity (4:96 α/β) and a 89% yield, but at a much higher reaction rate. The glycosylation was complete within 20 min compared to 16 h without HB(C6 F 5) 4 (Scheme 35). Surprisingly, HB(C6F5)4 alone was not capable of activating the PTFA donor. On the basis of this finding, the group realized that the activating species was in fact TMSB(C6F5)4, which was synthesized and investigated as a novel catalyst in PTFA activation. This catalyst was shown to facilitate βselective mannosylations in excellent yields and short reaction times (20 min) resulting in almost exclusive formation of the βanomer (5:95 α/β) at low temperatures. This suggests that the novel catalyst could function both as a Lewis acid, but also, perhaps, as a participating counterion. S


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In certain cases, the use of PTFA donors over TCA donors has been beneficial, for instance in glycosylations of amides255 where the undesired rearranged trichloroacetamide from TCA donors can be very limiting as well as when employing weak nucleophiles as acceptors.122,167,245,256 The PTFA donors are also good alternatives in sialylation reactions,257 when using ketose donors,258−260 and in the glycosylation of hydroxamic acids.256 Furthermore, certain TCA donors can be so reactive that the donors simply decompose261,262 rather than facilitate the desired glycosylation. Also, in certain cases, PTFA donors have been shown to offer advantageous reactive behavior resulting in better yields than similar TCA donors.251,263−266 The more difficult and environmentally hazardous procedure of synthesizing the PTFA donors, incorporating, for example, CCl4, has been a limitation to the PTFA method, but the amidoyl chloride has since become commercially available. Also, there is no way of controlling the anomeric configuration of PTFA donors since the formation of the imidate is an irreversible reaction, leading to a mixture of anomers. General Remarks Regarding the Use of PTFA Donors. (1) The PTFA donors are in general less reactive than their respective TCA counterparts251 and can, consequently, be orthogonally activated in one pot. (2) The lower reactivity of the PTFA donors can be an advantage if a very reactive (for instance a 2-deoxy sugar-derivative) glycosyl donor is used, since rapid decomposition of the TCA analogue can happen. (3) PTFA donors eliminate the formation of a well-known amide byproduct from the back-reaction of the leaving group. This makes the PTFA donors a great alternative to the TCA method when glycosylating poor nucleophiles. (4) The formation of the PTFA donors is under kinetic control, thus resulting in an anomeric mixture of α- and β-donors.

previously developed Yb(OTf)3, introduced by the same group92,93 but did not express any particular stereoselectivity. Due to the high reactivity of Bi(OTf)3, the reactions were carried out at temperatures as low as −50 °C, still proceeding in high yields. Generally, the glycosylations were done with a catalyst loading of 5−10 mol %. Using nitrile solvents to get βselectivity in the glycosylations proved a challenge, since Bi(OTf)3 forms strong complexes with nitrile solvents, thus resulting in a very inactive catalyst and a very slow reaction. Furthermore, Bi(OTf)3 cannot be completely dried by heating,253 hence coevaporation with toluene was done to remove moisture. The Bi(OTf)3 was later proven to function in one-pot synthesis of oligosaccharides, also by the Iadonisi group, behaving in a very similar manner to the Yb(OTf)3 catalyzed procedure described earlier in this review.106 In 2012, the Nguyen group activated a PTFA donor with a catalytic amount of Ni(4-F-PhCN)4(OTf)2.167 This catalyst had previously been employed in glycosylations on TCA donors, but due to the formation of vast amounts of the undesired rearranged trichloroacetamide byproduct, the group turned its focus toward the PTFA donors instead. This study revealed that both the α- and the β-PTFA donors could be catalytically activated by 5−10 mol % of Ni(4-F-PhCN)4(OTf)2 with a broad scope of diverse parent sugar donors with arming or disarming protecting groups as well as a variety of sugar acceptors. The glycosylations, generally in yields on excess of 70%, showed a strong preference for the formation of αglycosidic bonds. Besides the Ni(4-F-PhCN)4(OTf)2, a range of other catalysts were also investigated. TMSOTf was proven unsuccessful, which led to the investigation of Cu(OTf)2, Rh(COD) 2 OTf, Ir(COD) 2 OTf, Pd(4-F-PhCN) 2 OTf, Ph3PAuOTf, and Fe(OTf)2 (all in 10 mol % loadings) as alternative catalysts. These were however unable to facilitate glycosylations in excess of 45% yield. It is of interest though that both the α- and β-PTFA donor (however with slightly lowered reaction rates for the β-anomer170) could be activated by the Ni catalyst. This is surprising, since only the α-TCA donor was sensitive to activation by the catalyst and the β-TCA donor was left unaffected when exposed to the reaction conditions. Thus, it seems likely that a different mechanism for the activation of the PTFA donors is at play, but this is yet to be investigated further. Ni(OTf)2 was introduced as an alternative, cheap catalyst for PTFA activation by the Nguyen group in 2016.254 Interestingly, the Ni(OTf)2 catalyst was able to facilitate glycosylations with a 15 mol % loading, expressing similar α-selectivity as the previously investigated Ni(4-F-PhCN)4(OTf)2 salt on 2-amino glycosyl PTFA donors to form 1,2-cis-glycosides. Ni(OTf)2 was formed in situ from NiCl2 and Ag(OTf). Due to the previous report of AgOTf activation of TCA donors, 77 it was investigated whether AgOTf was actually the activating species; however, activation by AgOTf alone was sluggish and unselective, while NiCl2 was unable to activate the PTFA donor. Besides Ni(OTf)2, a series of other metal triflates were also investigated, but neither Cu(OTf)2, Zn(OTf)2, Fe(OTf)3, Au(OTf)3, or In(OTf)3 were capable of achieving similarly satisfying results in regard to yield and selectivity, although Au(OTf)3 and Cu(OTf)2 did perform well. The Ni(OTf)2 catalyst was proven to function using a wide range of different acceptors, including secondary alcohols and 1-adamantanol, resulting generally in high α-selectivity (>11:1 α/β) and 58− 93% yield, in certain cases leading to better results than the Ni(4-F-PhCN)4(OTf)2 catalyst.

2.4. Thioimidate Glycosyl Donors

Glycosyl thioimidates have been employed as glycosyl donors for many decades. Since the introduction of 2-benzothiazolyl thioglycosyl donors by Mukaiyama and co-workers in 1979,267 many similar thioimidate glycosyl donors have been investigated. Hanessian and co-workers introduced the 2-pyridineand 2-pyrimidine-derived glycosyl donors268 that were later reinvestigated by the Mereyala group269 and the Kong group.270 Demchenko and co-workers have introduced a variety of thioimidate glycosyl donors, namely the Sbenzoxazolyl (SBox),271−274 S-thiazolyl (STaz),273,275−277 Sglycosyl O-methyl phenylcarbamothioates (SNea),278 and Sbenzimidazolyl (SBiz)279 glycosyl donors. However, neither of these glycosyl donors are catalytically activated. The first instance of a catalytically activated glycosyl thioanalogue to the PTFA donors was reported by the Zhu group in 2008.280 The new donor type was activated in glycosylations with a catalytic amount of BF3·OEt2, TMSOTf, or AgOTf, of which the former yielded the most promising results. Using a peracetylated glycosyl donor of various parent sugars, 0.1−0.25 equiv BF3·OEt2 was sufficient to give 1,6-disaccharides in yields in excess of 91%. One noteworthy advantage of the glycosyl thioimidate donors is the selective availability of either the α- or the β-glycosyl thiol281,282 when forming the trifluorothioacetimidate, thus yielding primarily one anomer, similarly to the formation of TCA donors by Schmidt and Michel.21 This is in contrast to the PTFA method developed by Yu,9,241 where the anomeric configuration of the PTFA donor was more challenging to control. T


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The N-phenyl trifluorothioacetimidate glycosyl donor has since been used in sialylations of 6-OH sugar acceptors by the Zhu group,283 resulting in yields of 86−96% with α-selectivity (Scheme 37). Scheme 37. Activation of Trifluorothioacetimidate Glycosyl Donor by the Zhu Group

generally achieving yields of 60−80% upon activation with various Brønsted- and Lewis acids; however, the exact amounts were not stated. It is, however, likely that the promoters were added in stoichiometric amounts. Other groups have investigated 2-pyridyl glycosyl donors289 and 2-pyridyl glycosyl donors with a fluorine or trifluoromethyl-substituent on the 5position of the leaving group.290 Neither of these have, however, been catalytically activated. Palme and Vasella introduced the 1-phenyltetrazol-5-ylglycosides as novel, catalytically activated glycosyl donors in 1994.291 This was found to be activated in the presence of 0.2− 0.5 equiv of Lewis acids or 0.5 equiv of a Brønsted acid (Scheme 39). In an optimized procedure, the glycosyl donor

2.5. Other Imidate-Like Glycosyl Donors

Scheme 39. 1-Phenyl-tetrazyl-5-yl Glycosyl Donor Developed by Palme and Vasella

In 1978, the Ishido group developed the glycosyl isoureas as novel glycosyl donors.284 The glycosyl donor itself was prepared by adding a dicyclohexylcarbodiimide to a C-1 unprotected sugar in the presence of a catalytic amount of CuCl. Aq ammonia was then used to wash out the catalyst before adding in the acceptor. The glycosylation proceeded without any further addition of catalyst (Scheme 38). No change in yields or selectivity was observed when diisopropylcarbodiimide or di-p-tolylcarbodiimide was used to form the glycosyl donor. The Ishido group later investigated the mechanism of the glycosylations of the urea donors but were not able to get a very detailed picture of the reaction pathway due to difficulties with NMR spectroscopy of the complex glycosyl donors.285 It was, however, stated that an SN2-displacement of the urea leaving group by the acceptor alcohol was affording β-stereoselectivity since the urea glycosyl donor preferably exists as the α-anomer. In 1979, 3,5-dinitro-2-pyridyl glycosides were introduced as a novel glycosyl donor by the Mukaiyama group. This has, however, only been activated with a stoichiometric amount of BF3·OEt2.286 A very related compound, a 3-nitro-2-pyridyl glycoside, was introduced in 1999 as a catalytically activated glycosyl donor by Yasukochi et al.287 This glycosyl donor was activated with 0.1 equiv of TMSOTf, yielding a disaccharide in 78%, exclusively as the 1,2-trans-glycoside was aided by neighboring group participation from a 2-O-acetyl protective group. Interestingly, a stereoisomer of the donor, 5-nitro-2pyridyl glycoside, was not activated even in the presence of 3 equiv of TMSOTf. The 3-nitro-2-pyridyl donor has not been employed by other researchers since its introduction. Very similar 2-pyridyl and 3-methoxy-2-pyridyl glycosides have been introduced as glycosyl donors by the Hanessian group in 2001.288 These donors were employed in glycosylations,

was activated with 0.2 equiv of TMSOTf with both 6-OH and 4-OH glycosyl acceptors to give disaccharides in 61−79% yield and relied on solvent effects to control the stereochemical outcome of the glycosylation. It was also reported that the anomeric configuration of the donor had no effect on the stereochemical outcome of the glycosylation, hence indicating a dominating SN1-like mechanism. This method of glycosylation did however not gain popularity and has not been reported in oligosaccharide synthesis since. Later, it was found that the tetrazol glycosides were good starting materials for the preparation glycosyl fluorides with HF-pyr.292 The Mukaiyama group introduced glycosyl p-trifluoromethylbenzylthio-p-trifluorophenyl formimidate as a novel glycosyl donor type in 2002 (Scheme 40).293 The donor was activated with catalytic amounts of various Brønsted- and Lewis acids to yield the desired disaccharides in 52−96% yield depending on the chosen catalyst. From the initial screening, TMSOTf, SnCl2−AgB(C6F5)4, HB(C6F5)4, and TfOH proved to be the most efficient catalysts. A range of different sugar acceptors were employed in glycosylations, notably resulting in a 96% yield using a 4-OH acceptor. This novel glycosyl donor type has since been further investigated by Mukaiyama and coworkers involving various acceptors, protecting groups, catalysts, additives, and solvents.294−296

Scheme 38. Isourea Glycosyl Donors Developed by Ishido and Co-Workers

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Scheme 40. p-Trifluoromethylbenzylthio-p-trifluorophenyl Formimidate Glycosyl Donor Developed by the Mukaiyama Group

atom, preferred to be positioned below the sugar component, which could indicate a slight conformational difference between the dichloro-cyanoacetimidate donors and the TCA donors. Other derivatives of the TCA donors have also been developed by the Schmidt group. A range of N-aryl-O-glycosyl haloacetimidates have been investigated as glycosyl donors.301 The main difference of these donors compared to their TCA counterparts is N-aryl substitution and either trichloro- trifluoro or dichloro acetimidates. Generally, the trifluoroacetimidates and other acetimidate-derivatives, developed by the Schmidt group, have not gained attention compared to the TCA and PTFA donors. In 2012, the glycosyl N-tosyl benzimidate was introduced as a novel glycosyl donor by the Li group.302 The donors were synthesized via a procedure similar to that employed by the Yu group during PTFA synthesis,9,241 coupling the hemiacetal and N-tosyl benzimidoyl chloride under basic conditions to yield the desired donor. Contrary to its PFTA counterpart, the Ntosyl benzimidate was not reported to be catalytically activated. In 2013, the Demchenko group developed an O-benzoxazolyl (OBox) imidate donor. Unlike the thio-analogue, SBox, these novel glycosyl donors could be activated by a catalytic amount of either Brønsted or Lewis acids giving comparable yields and selectivities to TCA donors. A range of OBox glycosyl donors from glucose, mannose, and galactose parent sugars were investigated in a series of glycosylations with 2-, 3-, 4-, and 6OH glycosyl acceptors. In general, yields of 75−97% were obtained using 10 mol % of either MeOTf, TfOH, TMSOTf, or SnCl4 as the catalyst in dichloroethane. The donors showed low selectivity in the absence of neighboring group participation. A catalytic amount of Cu(OTf)2, Bi(OTf)3, or AgOTf were also used as catalysts. To compare the reactivity of the OBox donors with the very similar TCA donors, a competition experiment was performed by having one equiv of each donor type, both obtained from perbenzoylated glucose, combined with one equiv of a 6-OH sugar acceptor. Upon activation with a catalytic amount of TMSOTf, Cu(OTf)2 or Bi(OTf)3, the OBox donor was shown to react faster than the TCA counterpart, thus the TCA donor was recovered in excess of 82%, while the desired glycoside was obtained in 90−96% yields. The OBox donors were also investigated as donors in a solid phase glycosylation HPLCsetup by the Demchenko group.303 Later, in 2014, a concept of regenerative glycosylations by employing a novel glycosyl donor, the 3,3-difluoro-3H-indol-2yl (OFox) glycosyl donor, was introduced by the Demchenko group. Although bearing strong resemblance to the PTFA donors developed by the Yu group, the OFox donors were

The Mukaiyama group continued investigating novel glycosyl donor types, which led to the discovery of the 6-nitro-2benzothiazoate glycosyl donor in 2003 (Scheme 41).297,298 This glycosyl donor proved to be very β-selective at low temperatures upon activation by 5 mol % of TfOH, even in a nonparticipating solvent like CH2Cl2. A similar glycosyl donor, without the nitro group, had also been synthesized, but proved to be very unstable. On the basis of the good β-selectivity of the 6-nitro-2benzothiazoate glycosyl donors, various mannosylations were carried out, resulting in a preference for β-mannosylations at −78 °C using HB(C6F5)4 (20 mol %) as the catalyst in CH2Cl2.298 However, the stereoselectivity diminished as the reactions were reproduced at 0 °C. It was also reported that in situ anomerization of the glycosyl donor took place upon activation of the glycosyl donor. When exposing the β-donor to the reaction conditions in the absence of an acceptor, the corresponding α-donor was present in a 47% yield after just 5 min. This also gives an explanation to why the anomeric ratio of the glycosyl donor was shown to have little effect on the stereochemical outcome of the reaction with an acceptor. Closely related derivates of TCA donors were already developed in the early 1980’s by the Schmidt group,299 including trifluoro-analogs, but were found not as practical as the TCA’s. Other donors, in which one chlorine atom is exchanged with a cyano group, were developed by the Schmidt group in 2007.300 By using dichloromalonitrile instead of trichloroacetonitrile, the novel dichloro-cyanoacetimidate glycosyl donor was obtained under basic conditions similar to TCA formation.7 In general, this glycosyl donor expressed similar reactivity compared to the TCA donors, although an interesting finding was made during X-ray crystallography. This revealed that the cyano group, being smaller than a chlorine Scheme 41. Glycosylations with 6-Nitro-2-benzothiazoate Donor

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Scheme 42. OFox Glycosyl Donor Developed by the Demchenko Group

BF3·OEt2 was introduced in 1983 as a stoichiometric activator of glycosyl fluorides by the Mukaiyama group.315 Under these conditions, anomerization of the α-glycosyl fluoride to the corresponding β-anomer was observed in just 10 min at room temperature, from which 1,2-cis glucosylations proceeded in good yields and with high stereoselectivity. It was shown by the Ishido group that catalytic activation with BF3· OEt2 was possible in C-glycosylations using silylated glycosyl acceptors that could scavenge the fluoride leaving group.316 By employing this method on β-ribofuranosyl fluoride donors, glycosylations in diethyl ether with 5 mol % of BF3·OEt2 proceeded in yields in excess of 76% and α-selectivity as great as 20:1 α/β. These results also showed that the preference for the α-glycoside increased as the amount of BF3·OEt2 decreased. The authors rationalized this by isomerization of the donor under the reaction conditions. Interestingly, the α-glycoside was still the major product when MeCN or CH2Cl2 was used as solvent instead of Et2O. Also, starting from the α-donor gave rise to a 20:1 mixture of α/β-glycosides, which could indicate anomerization of the α-glycosyl fluoride to the β-fluoride under the reaction conditions prior to the glycosylation since the configuration of the donor apparently had no effect on the stereoselectivity of the reaction. This anomerization to the βanomer has previously been noted by the Mukaiyama group.315 BF3·OEt2 was later used as a catalyst in O-, C-, S-, and Nglycosylations by the Nicolaou group.317,318 Additionally, it has been reported by Kunz and Sager, that addition of a stoichiometric amount of triethylamine was necessary, in BF3· OEt2-catalyzed glycosylations of glycosyl fluorides, to trap the equiv of hydrogen fluoride formed during glycosylations with protic acceptors.319 Dimethylgallium chloride has been investigated as a possible catalyst for glycosyl fluoride activation by Koide and Kobayashi.320 This was shown to lead to the formation of dimethylgallium fluoride, which in itself had no activity toward the glycosyl donors, thus imposing the need of a stoichiometric additive such as TMSCl for the reactions to run to completion. A range of rare earth perchlorate salts were introduced as novel catalysts for the coupling of silyl ethers and glycosyl fluorides by Shibasaki and co-workers.321 Previously, this research group had shown that these rare earth salts had to be used in stoichiometric amounts in glycosylations of protic acceptors.322 The formation of TMSF as a byproduct during the glycosylations was shown by 19F-NMR experiments. Thirty mol % of lanthanum(III)-, cerium(III)-, praseodymium(III)-, europium(III)-, and ytterbium(III)chloride were all investigated as catalysts in the glycosylations with cyclohexyl silyl ether as the acceptor, but all relied on neighboring group participation or solvent effects for a stereoselective glycosylation. In a similar type of reaction, cyanotrimethylsilane and

shown to have one important distinction; namely that the leaving group from the glycosyl donor can react with a glycosyl bromide to regenerate an active glycosyl donor under the glycosylation conditions (Scheme 42). The method of glycosylation involving nucleophilic catalysis made it possible to only have a small amount of the active glycosyl donor present at any given time, which could be efficient at countering problems with undesired formation of byproducts during glycosylations. Also, it is noteworthy that the glycosyl bromide donor seemingly reacts a lot slower with the acceptor than the OFox donor, leading to a glycosylation protocol only involving 10 mol % of HOFox giving the desired disaccharide in 84% yield within 3 h. The reaction time was shortened to 10 min by adding 1.0 equiv HOFox giving a 90% yield of the desired glycoside. The in situ formation of the OFox glycosyl donor from the glycosyl bromide under the reaction conditions was observed by NMR spectroscopy. A more thorough investigation of the donor properties of the OFox donors was performed by the Demchenko group in 2017.71 They found that although the stereochemical outcome could be controlled to some extent by changing solvent, protecting groups, and the reaction temperature, no intrinsic stereospecificity of the OFox donors was found. Also, it was realized that using TMSClO 4 as the catalyst in the glycosylations increased the α-selectivity, in good accordance with the work on counterion effects by the Mukaiyama group.111,115−119,304,305

3. GLYCOSYL HALIDES 3.1. Glycosyl Fluorides

In order to overcome difficulties with achieving high stereoselectivity in glycosylations with 2-benzothiazolyl glycosyl donors,267 the glycosyl fluorides were introduced as glycosyl donors by Mukaiyama and co-workers in 1981.8 The reasoning behind this was that the fluoride ion should serve as a weaker leaving group, thus enabling a more controlled, SN2-like glycosylation. Several other researchers had also previously investigated the properties of glycosyl fluorides, although without performing glycosylations with the fluorides as glycosyl donors.306,307 Initially, the glycosyl fluorides were found to be activated most efficiently by a stoichiometric amount of stannous chloride and silver perchlorate, giving rise to high yields and good α-stereoselectivity starting from β-glycosyl fluoride donors. Most notably, the glycosylation of a 4-OH glycosyl acceptor led to a 91% yield and a 80/20 ratio of α/β anomers. This novel donor type was immediately incorporated into total synthesis of carbohydrate-containing natural products by various groups308−310 and has since received considerable attention as an attractive glycosyl donor.3,311−314 W


Chemical Reviews

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Table 3. Overview of Comparable Glycosylations Performed by the Mukaiyama Group

catalyst

solvent

HClO4 TrClO4 HOTf HClO4 TrClO4 TrB(C6F5)4 HOTf TrB(C6F5)4 SnCl2−Ag(C6F5)4 TrB(C6F5)4 HNTf2 TrB(C6F5)4 HB(C6F5)4

Et2O Et2O Et2O BTF/t-BuCN 5:1 BTF/t-BuCN 5:1 CH2Cl2 BTF/t-BuCN 5:1 Et2O Et2O t-BuCN/CH2Cl2 5:1 BTF/t-BuCN 5:1 BTF/t-BuCN 5:1 BTF/t-BuCN 5:1

reaction time/temp 4 4 8 2 2 2 2 4 4 2 2 2 6

h/rt h/rt h/0 °C h/0 °C h/0 °C h/0 °C h/0 °C h/rt h/rt h/0 °C h/0 °C h/0 °C h/−20 °C

yield (%) (α/β)

ref

94 (92/8) 94 (93/7) 95 (89/11) >99 (60/40) 91 (58/42) 38 (50/50) 99 (49/51) 85 (47/53) 90 (43/57) 72 (10/90) 99 (9/91) 88 (4/96) 97 (4/96)

111 111 117 111 117 116 111 117 117 115 111 117 111

rise to β-selectivity in glycosylations when using these Brønsted acids as catalysts in nitrile solvents.111,117,329 It has also been shown that the addition of a catalytic amount of SnCl2 or SnCl4 can help to increase the yields of the glycosylations with disarmed glycosyl fluorides, both in the case of using the ClO4− or B(C6F5)4− counterions to direct the stereochemical outcome.328,334 Thus, it has been shown by the Mukaiyama group that certain counterions have the ability to amplify the solvent effects, thus giving rise to higher stereoselectivity in glycosylations. These results are summarized in Table 3, which gives an overview of the counterion- and solvent effects. Table 3 gives an overview of the counterion and solvent effect. Glycosylations with glycosyl fluorides in ionic liquids has been conducted by Toshima and co-workers.335 They found that using a solvent with a perchlorate counterion would increase the α-stereoselectivity, whereas a triflate counterion would instead favor the β-glycoside, the latter somewhat in contrast with the above-mentioned results from the Mukaiyama group. A disaccharide with a 6-OH glycosyl acceptor was synthesized in 99% yield as a 13:87 α/β-mixture, whereas a glycosylation involving a 4-OH acceptor could only achieve a 54% yield with no significant stereoselectivity. Furthermore, Schmid and Waldmann have used a 0.07 M solution of lithium perchlorate in CH2Cl2 to activate fucosyl fluorides with very high α-selectivity.336 Zirconocene bis(perfluorooctanesulfonate) has been shown by Qui et al. to be a catalytic activator of glycosyl fluorides with protic acceptors in CH2Cl2. Generally, these glycosylations gave rise to yields in excess of 80%, but only low stereoselectivity was observed.337 Toshima and co-workers have reported the use 20 wt % of a zirconium-based, solid acid, SO4/ZrO2 for activation of mannosyl fluorides.338 In an optimized procedure, α/β-selectivity of 97:3 and 98:2 was found for glycosyl acceptors in MeCN, in contrast with the more common reports of β-stereoselectivity of reactions in nitrile solvents.61,62,90 Conversely, higher β-selectivity of α/β 27:73 and 56:44 was found for mannosylations in Et2O for 6-OH and 4OH glycosyl acceptors, respectively. Other solid acids such as Nafion-H and montmorillonite K-10 were also investigated but not with glycosyl acceptors. The Toshima group205,338 and

allyltrimethylsilane were employed by the Ishido group to synthesize C-glycosides, enabling catalytic activation with BF3· OEt2.323,324 The Noyori group presented that TMSOTf and SiF4 could be used as catalysts in the glycosylation of glycosyl fluoride donors with silyl ethers or silyl amines as acceptors in various solvents giving rise to O- or N-glycosides, respectively.325,326 This procedure, however, was not stereoselective and solvent effects were used to increase the selectivity. SnF4 and TiF4 were investigated as catalysts by Kreuzer and Thiem.327 These glycosylations were also dependent on solvent effects to give stereoselective glycosylations but tolerated hydroxy groups as acceptors. The Mukaiyama group has investigated a wide range of different Lewis- and Brønsted acids as catalytic activators of glycosyl fluorides. TrB(C6F5)4 was introduced as a catalyst giving β-selective glycosylations in nitrile solvents.114 It was later shown, also by the Mukaiyama group, that the protic variant of this particular Lewis acid, HB(C6F5)4, also gave rise to β-stereoselectivity when used as a catalyst in glycosylations in nitrile solvents.111,115 Carrying out these glycosylations in ethereal solvents, however, led to a loss of selectivity. Other counter-cations such as silver(I)111,328 and bulky ammonium ions115 also led to β-stereoselective glycosylations with the B(C6F5)4−-ion, hinting at a possible stereochemically directing property of this anion, especially in nitrile solvents. It was also found that the B(C6F5)4−-derived catalysts had no catalytic activity toward other glycosyl halides.116 Interestingly, when Lewis acidic or Brønsted acidic perchlorate derivatives such as TrClO4 or HClO4 were used as catalytic activators of glycosyl fluorides in ethereal solvents, a clear preference for the formation of α-glycosides was found.111,117,328,329 Thus, an elegant way of utilizing the counterion of the Lewis- or Brønsted acid catalyst to amplify the stereoselectivity originating from solvent effects and remote protecting groups such as 5-diethylthiocarbamyl329−332 was found. TfOH and HOSO2C4F9 have been shown to be catalytic activators of glycosyl fluorides, generally leading to α-selectivity in ethereal solvents, but not with as high selectivity as that of perchlorate derivatives.111,117,329,333 HSbF6 and HNTf2 were found to give X


Chemical Reviews

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Scheme 43. Zink-Catalyzed Activation of Glycosyl Chloride

Scheme 44. Activation of Glycosyl Bromide with Supercritical CO2

others117,339 have reported that 5 Å molecular sieves effected both yields and selectivity of glycosylations, indicating that certain dehydrating agents can have an effect on a reaction outcome. Despite the fact that glycosyl fluorides have been thoroughly investigated as glycosyl donors over many years since being introduced by the Mukaiyama group,8 these donors have still received attention from various researchers in recent years. Hf(OTf)4 was investigated as a novel activator by Ito and Manabe.340 Although not catalytic, this method could be appealing for solid phase and solution phase synthesis since Hf(OTf)4 is stable in air. Glycosyl fluorides were also employed recently in a β-stereospecific glycosylation of a conformationally locked, highly reactive glycosyl donor by the Yamada group, utilizing in situ formation of the HB(C6F5)4 catalyst initially introduced by Mukaiyama and co-workers.341 Schepartz and Miller have recently shown that unprotected glycosyl fluorides serve as excellent glycosyl donors in glycosylations in an aqueous trimethylamine solution in the presence of calcium ions with unprotected sucrose as the acceptor.342 The ability of glycosyl fluorides to be feasible glycosyl donors in water highlights the good stability of these donors, which offers great versatility.

glycosylations. Despite the fact that this is often referred to as halide ion catalysis, this method is only applicable when equimolar amounts of an acid scavenger is present to circumvent the negative effects of having an increasingly more acidic reaction medium. Thus, catalytic activation of glycosyl bromides and glycosyl chlorides has generally been limited to only simple acceptors in a large excess, essentially solvolysis, ruling out oligosaccharide synthesis without having equimolar amounts of an acid scavenger present.311,352 There have been examples of catalytic glycosylations using glycosyl chlorides. Nishizawa and co-workers reported the use of various Lewis- and Brønsted acids as catalytic activators for glycosylations with rhamnosyl chloride as donor.353 Under optimized reaction conditions, a glycosylation of rhamnosyl chloride with a 6-OH glycosyl acceptor was carried out in refluxing CH2Cl2 with 0.1 equiv of Zn(acac)2 in a closed, inert Soxhlet setup with molecular sieves (Scheme 43). Nishizawa and co-workers reported that the addition of an acid scavenger in a model reaction would increase the yield but lower the stereoselectivity. The Nishizawa group has done some in-depth work on thermal activation of glycosyl chlorides, which has also enabled glycosylations in the absence of a catalyst; however, there is still a need for an acid scavenger or molecular sieves.354−357 Later, a modified version of the glycosylation was developed, but this relied on 22 equiv of an acid scavenger present, during glycosylations at rt.358,359 Another possible way of keeping the reaction mixture somewhat neutral is by evaporating off the HCl or HBr formed during the reaction, since both are highly volatile gases.360 Recently, the use of thiourea- and urea-derived catalysts to activate glycosyl chlorides and glycosyl bromides has been reported.361,362 Although these procedures still rely on the addition of an acid scavenger, these catalysts are good alternatives to the traditional use of metal salts and provide excellent stereoselectivity in glycosylations. In 2017, the use of supercritical CO2 (scCO2) as solvent and activator of glycosyl chlorides and bromides was reported by Cardona et al.,363 drawing inspiration from recent reports of scCO2 as an activator of carbon−halogen bonds364 and halogen−halogen bonds.365 It was found that glycosyl bromides were more reactive under the reaction conditions than glycosyl chlorides, and a disaccharide was obtained from a 2,3,4,6-tetraO-pivaloyl galactosyl bromide and a 6-OH glycosyl acceptor in 47% yield over 24 h (Scheme 44). The glycoside was obtained as a 1:4 mixture of α/β anomers, which was later shown to not be due to acid-catalyzed anomerization, thus the reaction was seemingly under kinetic

3.2. Glycosyl Bromides and Chlorides

Glycosyl bromides and glycosyl chlorides were first used as glycosyl donors by Koenigs and Knorr in 1901 under activation by silver carbonate,343 often referred to as the “Koenigs-Knorr glycosylation”. Helferich recognized that the formation of water during silver carbonate-promoted glycosylations was a weakness, since this led to the hydrolysis of the glycosyl halides, thus lowering the yield of the glycosylations.344,345 This led to the development of the “Helferich method” employing HgBr2 or Hg(CN)2 as the soluble, halophilic activator and an acid scavenger additive, which led to better yields and higher reactivity since no water was produced as a byproduct during glycosylations.346,347 Since the initial development of glycosyl chlorides and glycosyl bromides as glycosyl donors, many improvements of the original procedures have been developed, but they all have the formation of either HCl or HBr, strong Brønsted acids, in common, which makes this method of glycosylation unfavorable without adding at least one equiv of an acid scavenger to the reaction mixture. A method often referred to as “halide ion catalysis” was developed by the Lemieux group,348−350 utilizing the halide ion catalyzed anomerization of glycosyl bromides, an effect also investigated by Ishikawa and Fletcher,351 to enable stereoselective glycosylations with high α-selectivity in Y


Chemical Reviews

Review

Scheme 45. Propargyl Glycosyl Donors Activated under Gold Catalysis

Scheme 46. Propargyl Glycosides and Propargyl Orthoesters As Orthogonal Glycosyl Donors

demonstrated the low potency of the catalytic system (Scheme 45). Generally, the yields obtained with simple alcohol acceptors and primary sugar alcohols were good but with low anomeric selectivity. The gold-catalyzed activation of propargyl ethers was soon after followed up by a similar approach for catalyzing a Ferrier reaction by activation of a 3-O-propargyl group. This reaction could be performed in MeCN at lower temperatures (i.e., 0 °C to rt) but with slightly more catalyst (5 mol %) and longer reaction times. The method gave good yields of the corresponding 2,3-unsaturated O-glycosides with high αselectivity.382 The use of a propargyl ether as a leaving group in SN2′ type reactions was further demonstrated using C-2propargyloxymethyl glycals as the reactant under the same conditions as in the Ferrier-type reaction. The C-2 methylene glycosides were again obtained with high stereoselectivity toward the thermodynamically favored α-anomer. It was unclear whether the high selectivity is due to anomerization mediated by AuCl3 or another acid.383 Importantly, the functional group tolerance toward the catalyst was studied and alkenes, isopropylidenes, azides, and ethers were found to be stable under the standard conditions. Later work by Yadav et al. has shown that GaCl3 can be used as an alternative to AuCl3 for the activation of propargyl ethers, but so far this protocol has not been expanded beyond the synthesis of simple monosaccharides.384 To improve the stereoselectivity in the Au-catalyzed glycosylation, which could not be used directly when having an acyl group to perform neighboring group participation, Sureshkumar and Hotha found that the corresponding propargyl 1,2-orthoesters were able to give disaccharides with high trans selectivity.385 In these glycosylations, AuBr3 was found to be more efficient than AuCl3, AuCl, Au2O3, and HAuCl4, giving a faster reaction with a higher yield and less of the propargyl glycoside side product. The activation of the propargyl orthoesters was found to proceed much faster than the activation of the propargyl glycosides. By taking advantage of this difference in reactivity, Sureshkumar and Hotha glycosylated acceptors, having the propargyl moiety, using the perbenzoylated propargyl orthoesters as donor (Scheme 46). Even propargyl ethers of the more reactive furanosides were found to be stable under the reaction conditions (i.e., AuBr3 in CH2Cl2 with 4 Å MS at rt).386 The orthogonality between the propargyl glycosides and the propargyl 1,2-orthoesters was demonstrated in the synthesis of the tetrasaccharide motif found in Leishmania donovani lipophophoglycan.387

control. It was reported that the addition of an equiv of collidine to the reaction would not increase the yield of the desired glycoside but instead led to the formation of a 1,2orthoester byproduct instead. A Boc-protected serine acceptor was glycosylated with a peracetylated 2-deoxy-2-amino glucosyl chloride in 54% yield. The use of scCO2 as both the activator and solvent is very environmentally benign due to the absence of organic solvents and catalysts, but the process requires considerable optimization to just give rise to yields in excess of 50%. There could, however, be a potential for this type of reaction in the future with further development. 3.3. Glycosyl Iodides

Glycosyl iodides have since their discovery as glycosyl donors by Helferich and Gootz366 been thoroughly investigated as glycosyl donors. Although the high reactivity initially made the glycosyl iodides difficult to isolate and characterize,15,366−370 making in situ formation of the glycosyl iodide a route often preferred in glycosylations.15,371−377 Glycosyl iodides have certain advantages over the corresponding chlorides and bromides in regard to reactivity and selectivity, which has been reviewed in detail within the past decade.378,379 Glycosyl iodides can in essence be activated by a catalytic amount of either a Brønsted or Lewis acid. HI, a strong Brønsted acid, is the byproduct of the reaction with a protic nucleophile, and it became clear quite early that an acid scavenger in equivalent amounts is required in glycosylations, which has since become the standard approach.367,369,371,378,379 Alternatively, an anionic nucleophile can be used380 to enhance the yield and reaction progression and hence make oligosaccharide synthesis feasible. Early examples of direct alkylation with glycosyl iodides indicate that these glycosyl donors react in the absence of an activator, but these reactions have only been carried out with simple acceptors in large excess or as the solvent to counter the challenges of a continuously more acidic reaction medium.367−369 Thus, glycosylations using glycosyl iodide donors in the absence of an acid scavenger are not feasible for oligosaccharide synthesis.

4. ALKYNE-BASED GLYCOSYL DONORS Catalytic activation of an alkyne functionality is a relatively new research area in glycosylation chemistry. The first report came from Hotha and Kashyab in 2006, in which propargyl glycosides were shown to be activated by AuCl3 catalysis.381 It was found that only benzylated donors were transglycosylated under the catalytic conditions. Perbenzylated glucosyl, galactosyl, and mannosyl donors were activated by 3 mol % AuCl3 in MeCN at 60 °C for 3 to 15 h, which Z


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Review

Scheme 47. Proposed Mechanism of Gold-Catalyzed Glycosylations with Ethynylcyclohexyl Glycosides

Scheme 48. Catalytic Activation of Ethynylcyclohexyl Carbonate by Hotha and Co-Workers

reactivity of the donors, the Thorpe-Ingold effect393 was elegantly exploited. Different alkynyl aglycons were systematically studied, and the gem-disubstituted donors were found to be superior. The 1-ethynylcyclohexyl moiety was chosen for further optimization, using a simple tetrahydropyran as the sugar model and GC monitoring of the reaction. Interestingly, it was found that 1-ethynylcyclohexanol was the product formed when activating the donor with AuBr3 (Scheme 47). Different gold catalysts were studied as well as strong Brønsted acids, and it was disclosed that both Lewis and Brønsted acidity is required for the glycosylation. Using silver salts, such as AgSbF6 or AgOTf in combination with the gold(III) halides, was allowing the reactions to be carried out at room temperature in 4−8 h in contrast to the 60−70 °C giving the optimal yields in the original work. The importance of silver in the gold catalysis has recently been studied in detail by Shi and co-workers.394 The new optimized donor type together with the improved activation protocol was used to glycosylate a variety of acceptors including 4-OH and 6-OH sugar acceptors. The mannosyl donor gave high axial selectivity,395 whereas the perbenzylated glucosyl and galactosyl donors gave good to excellent yields and equatorial selectivity, due to the so-called nitrile solvent effect. Besides improvement by using a more reactive aglycon, taking advantage of the Thorpe-Ingold effect, the scope was still limited to reactive glycosyl donors with benzyl protective groups on the donor, which could not control the stereochemistry by neighboring group participation. If the reactivity of the donor was reduced, side reactions, such as glycosidic bond cleavage, competed with the glycosylation and hence disqualified the donor system for complex oligosaccharide synthesis. To overcome these issues, Hotha and co-workers introduced alkynyl glycosyl carbonates in combination with

The orthogonality of the propargyl-based donors was expanded to include n-pentenyl glycosides and n-pentenyl orthoesters. The propargyl orthoesters could be activated using AuBr3 and glycosylated on an acceptor containing the npentenyl moiety giving a disaccharide glycosyl donor. When using an n-pentenyl orthoester instead, it was possible to activate it in the presence of a propargyl glycoside using NIS and Yb(OTf)3.388 It was also found that n-pentenyl and propargyl glycosides were orthogonally activated, but that the more reactive n-pentenyl donors (benzyl protected) were also activated by AuBr3 at the elevated temperatures required for the propargyl activation (i.e., limiting the scope). The high oxophilicity of AuBr3 was found to be problematic in oligosaccharide synthesis at the elevated temperatures required for activation of the propargyl moiety. It was observed that glycosidic bonds were cleaved or anomerized, when using a more reactive benzylated disaccharide donor in combination with AuBr3.389 Even methyl glycosides, when used as 6-OH acceptors, were found to be activated under the conditions giving 1,6-anhydro sugars.390,391 This led to the discovery that fully protected methyl glycosides could be used directly for the synthesis of disaccharides in moderate to good yields but generally with low anomeric selectivity due to the lack of neighboring group participation in the “armed” donors. The mechanism of the gold-catalyzed activation of propargyl ethers was hypothesized in the first publication but has not been supported by experimental data. A central point in the suggested mechanism was the formation of cyclopropanone. This was however disproved in a later study by Kayastha and Hotha.392 In this, the conditions for the gold-catalyzed activation of the propargyl glycosides were optimized in order to lower the reaction temperature, which, as discussed above, has given rise to side reactions. In order to increase the AA


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Scheme 49. 2-Butynyl Glycoside As Glycosyl Donor

Scheme 50. Gem-Dimethyl-S-but-3-ynyl Thioglycosides As Donors

Scheme 51. Activation of o-Alkynylbenzoate Glycosyl Donor with PPh3AuOTf

using the ethynylcyclohexyl moiety (Scheme 48).396 A large number of reagents were screened as catalysts for the activation of the alkynyl glycosyl carbonate, and as in the studies with the propargyl-based donors, gold trihalides were found to be active at room temperature. The yields were however only moderate to good, and relative high loadings of the catalysts (15 mol %) were required. Changing to a highly alkynophilic gold phosphite complex in combination with silver triflate significantly improved the yields of reaction to nearly quantitative in just 15 min at rt. The high performance could be maintained when lowering the catalyst loading to 6 mol % of each but dropped fast below that point. The favorable combination of silver and gold reagents was surprising, and the authors suggested that the silver worked as a halide scavenger giving rise to the formation of a gold triflate complex, which was more reactive. Alternatively, the less reactive halidebridged gold complex dimers are avoided. Another aspect of the mechanism is the formation of a cyclic carbonate alkene from the activation of the alkynyl aglycon. No CO2 was released in contrast to many other carbonate-based donors, and the formation of an alcohol side product was avoided as well. The robustness of the method was furthermore illustrated by a synthesis of a tridecasaccharide mannose-capped arabinan with high glycosylation yields. Despite the dominance by the Hotha group in glycosylations using alkynes together with a gold catalyst, a few other groups have entered the area. Mamidyala and Finn showed that unprotected alkynyl donors could be used, for the glycosylation of simple protected sugar acceptors, when used in excess.397 The alkynyl donors could be synthesized via a Fischer glycosylation and hence used without further functionalization. The yields of the disaccharides are modest and the selectivity low, which showed the limited scope for oligosaccharide synthesis. Initially, the propargyl group was used, but it was found that a 2-butynyl was more reactive (Scheme 49); however, its synthesis by a BF3·OEt2-mediated glycosylation was less attractive. Disarmed, peracetylated donors were however still too unreactive to be activated by AuCl3, and the

stereoselectivity remained low when using unprotected or benzylated donors. Zhu and co-workers combined the alkyne and sulfide functionalities making S-but-3-ynyl398 and later gem-dimethyl S-but-3-ynyl thioglycosides and evaluated them as donors (Scheme 50).399 This donor type clearly showed some advantages over the propargyl glycosides, but their synthesis was less straightforward. From screening different gold catalysts, it was found that a combination of AuCl3 (10 mol %) and AgOTf (30 mol %) gave the highest yields at rt. As expected from a competition experiment, the gem-dimethyl Sbut-3-ynyl thioglycosides, taking advantage of the ThorpeIngold effect,393 were found to be more reactive and hence the preferred aglycon for the less reactive donor types. Leaving the alkyne functionality out significantly reduces the yield and the interaction of a cationic Au complex, this functionality seemed crucial for the reaction. Several disaccharides were synthesized and both benzoyls, benzyls, azides, and phthalimides were tolerated in the donor. Glycosylations were highly 1,2-trans selective when using participating protective groups but unselective when not. The mechanism was studied by NMR experiments showing that 4,4-dimethyl-2,3-dihydrothiophene was formed as the side product. Shortly after the first paper by the Hotha group, describing the use of AuCl3 to activate propargyl glycosides, Yu and coworkers introduced glycosyl ortho-alkynylbenzoates as donors using a gold(I) catalyst, PH3PAuOTf,400 which was earlier shown, by Umetsu and co-workers, to be a potent alkylating system.401 The glycosylation protocol using glycosyl orthohexynyl-benzoates as donors was shortly after used for the total synthesis of N,N,N-trimethyl-D-glucosamine-chitotriomycin using between 0.2 and 0.5 equiv of PPh3AuOTf as the catalyst.402 With this expansion of the donor scope, Yu and coworkers continued to study the generality and versatility of the protocol.403 From the study of the system, it was revealed that upon activating a peracetylated donor using AuCl3 (10 mol %), the orthoester was the major product. Using PPh3AuOTf instead, as the catalyst, the formed orthoester decomposed to AB


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Scheme 52. Mechanism of ortho-Alkynylbenzoate Activation

Scheme 53. β-Selective Glycosylation with 6-Deoxy-L-rhamnopyranosyl Glycosyl Donor

ligand reduced the rate significantly. By the addition of a vinyl ether to the reaction, the formation of a glycosyloxypyrylium intermediate could be trapped and hence its existence confirmed. Taking advantage of this, a stereospecific glycosylation (SN2 like) was attempted and resulted in a high degree of specificity under forced conditions (i.e., using 10 equiv acceptor). The highest stereospecificity was obtained when using the donor with an equatorial ortho-hexynylbenzoate. The formation of an isochromen-4-yl-gem-gold(I) complex was demonstrated and a crystal structure obtained. This intermediate was found to be inactive as a catalyst for the glycosylation reaction. The importance of the intermediates formed under the glycosylation conditions for the stereochemical outcome was demonstrated by both β-selective mannosylations406 and the even more challenging rhamnosylation,407 where the tunability of the donor system also was exploited. During the development of a β-selective mannosylation, it was found that activation of α-ortho-alkynylbenzoate donors was giving the β-mannosides, when the counterion of the gold catalyst was less nucleophilic. Starting from the β-donor, the selectivity was lost due to anomerization of the reactive intermediate, and hence, a preactivation/anomerization had to be performed before adding the acceptor. PPh3AuBAr4F was found to be the best catalyst in terms of β-selectivity. Using this activation system, 1α-glycosyloxy-isochromenylium-4-gold(I) was detected and characterized by NMR and hence the β-selectivity obtained could be rationalized. The protocol was importantly not restricted to 4,6-O-benzylidene protected mannosyl donors, and good β-selectivities could still be obtained with untethered, but otherwise disarmed, donors. By tuning the reactivity of the rhamnosyl donor, via the introduction of substituents in the aglycon, good β-selectivities could be obtained (Scheme 53). A p-NO2 substituent was found to give the best result. It was also found that low temperature and CH2Cl2 as the solvent gave the highest

give the glycoside. With the more reactive rhamnosyl donor, the synthesis of a protected saponin dioscin (trisaccharide) went smoothly and in an excellent yield. Several other catalysts were tried, but only the gold catalysts worked satisfactorily. The yields were generally very high (>90%), with a small drop when glycosylating the less reactive 4-OH acceptors (Scheme 51). Without participating groups on the 2-O-position, only a modest selectivity can be reached in Et2O, whereas CH2Cl2 resulted in no or a modest α-selectivity. When challenging the protocol by performing sialylations, it was found that the thermodynamically favored product, the β-glycoside, was predominantly formed. Using MeCN in order to improve the α-selectivity, an inefficient sialylation was the result and only a modest improvement in terms of selectivity could be achieved. Interestingly, it was found that using PPh3AuNTf2 as the catalyst instead of the corresponding triflate gave higher βselectivities in CH2Cl2. A central part of the proposed mechanism is the formation of an isochromen-4-yl-gold(I) complex as an intermediate, which upon a protodeauration reforms the active gold(I) catalyst.404 This complex was indeed isolated, characterized, and the importance of a sufficiently strong Brønsted acid for the catalytic cycle to be completed demonstrated in a pH study. Upon the addition of TfOH (0.1 equiv), the glycosylation still gave high yields when only using 0.001 equiv of PPh3AuOTf. The importance of the acidity was exploited by a difficult glycosylation of the 4-OH in a protected glucosamine, which previously required 0.5 equiv catalyst, and with the addition of TfOH (0.1 equiv), this could be lowered to 0.01 equiv. The mechanism was further studied, and from crossover experiments, an exogenous anomerization mechanism was revealed (Scheme 52).405 The rate of the anomerization was found to be dependent on the catalyst used, where PPh3AuOTf gave a faster reaction compared to PPh3AuNTf2 but with the same ratio at equilibrium. Also, the ligand was found to be important for the rate of anomerization, where (o-Tol)3P as AC


Chemical Reviews

Review

Scheme 54. Optimized Procedure for Glycosylations with Ortho-Hexynylbenzoate Glycosyl Donors

Scheme 55. Latent and Active Donors by Boons and Isles

selectivities. Anomerization of the donor from β to α was again observed to take place. Selective α-rhamnosylation takes place under gold catalysis, when having neighboring group participation by, for example, benzoyl groups.408 The gold(I)-catalyzed glycosylation using ortho-hexynylbenzoate aglycon as the leaving group was further expanded to a 6deoxy-L-altropyranosyl donor (Scheme 54), for which the βglycosylation is challenging and comparable to the βrhamnosylation.409 To optimize the donor for a β-selective glycosylation, benzoyl protective groups were chosen for blocking the 3- and 4-OH’s. This lower reactivity of the donor favors the formation of the 1-α-altrosyloxy-isochromenylium-4-gold(I) intermediate over an oxocarbenium ion, which is destabilized. This intermediate will upon an SN2-like substitution give the β-glycoside. The 3-O-benzoyl can additionally shield the α-side of the donor by remote participation.410 From screening conditions, it was found that the PPh3AuBAr4F gave higher β-selectivity compared with having Tf2N− as the counterion but lower yields. Decreasing the temperature improved the selectivity but prolonged the reaction times. The best compromise was therefore to use PPh3AuNTf2 at −20 °C in CH2Cl2, which gave a 1:9.9 α:β ratio for the disaccharide formation. The selectivity gradually decreased as the oligosaccharide became longer and was only 1:2.5 for the last glycosylation giving the pentasaccharide. The oligosaccharide was carefully studied by NMR and molecular modeling showing that it adopted a left-handed helical structure. Yang and co-workers used the donor system for the βselective synthesis of kdo glycosides but only with one example using a glycosyl acceptor.411 The chemoselectivity in the glycosylation of acid alcohols, when using glycosyl orthohexynylbenzoates as glycosyl donors, was also studied. It was found that addition of bases such as DBU or 2,6-ditertbutylpyridine (2 equiv) together with BF3·OEt2, to the glycosylation catalyzed by Ph3PAuOTf (20 mol %), gave high selectivity for glycosylation of the carboxylic acid. When the BF3·OEt2 was left out, 1,2-orthoester formation became a major product.412 If the glycosylation protocol was carried out in THF without an acceptor present, Li and Yu noticed the formation of a gel.413 This was further investigated using PPh3AuNTf2 (instead of PPh3AuOTf) and found to be an effective cationic ring-opening polymerization, hence supporting the formation of an oxocarbeniumion intermediate in the activation. Another interesting outcome of this new glycosylation method was the formation of 1,2,4-orthoacetate when activating a per-acetylated glycopyranosyl ortho-hexynylben-

zoate, albeit as a minor product.414 This side reaction was studied, and it was found, by labeling experiments, that the orthoester was derived from the 4-O-acetyl. This example represents a rare direct evidence for the interaction of remote participation by acyl groups in glycosylations. The concept of activating an alkyne by gold catalysis followed by an intramolecular ring closure has been expanded to thioglycosides by Yu and co-workers.415 Ortho-alkynylphenyl thioglycosides were prepared, and upon screening catalysts it was found that [Btz-Au-PPh3]OTf (0.1 equiv) was superior to the catalysts used for the activation of ortho-hexynylbenzoates. The use of glycosyl ortho-alkynylbenzoates for natural glycoside synthesis416 and the use of gold catalyst for glycosylations in general have recently been reviewed.417

5. ALKENE-BASED GLYCOSYL DONORS The first examples of oligosaccharide using vinyl glycosides appeared in the early 1990’s. Schmidt and co-workers used an enolether derived from tetra-O-benzyl glucose and ethyl phenyl propiolate as the donor. The glycosylations were carried out as a part of studying the effect of acetonitrile as a solvent, and TMSOTf was used in a slight excess and hence the glycosylations were not catalytic.418 A slightly simpler aglycon was presented by the Sinaÿ group in 1992.419 Here, isoprenyl glycosides were studied as glycosyl donors in acetonitrile and TMSOTf as the promoter in equivalent amounts. The authors suggested that TMSOTf reacted with the acceptor alcohol and that the actual activator was TfOH. Alternatively, the isopropenyl group could be attached to the acceptor alcohol, which then reacts with the hemiacetal. The first example of catalytic activation of vinyl glycosides came shortly after from Boons and Isles.420 As part of developing “active-latent” glycosylation strategies, active vinyl glycosides were prepared from latent allyl glycosides. In this approach, only 0.1 equiv of TMSOTf was used to catalyze the reactions (Scheme 55). The solvent was again MeCN, giving high β-selectivity. The latent disaccharide formed could then be active via a rhodiumcatalyzed isomerization. In a follow up full paper, Boons and Isles suggested that an extra methyl substituent in their vinyl glycosides would stabilize the formation of a positive charge upon acid treatment, which was supposed to be the RDS, and hence a catalytic amount of acid should be sufficient.421 The Boons group studied the use of vinyl glycosides in the latent-active strategy further in a series of papers for the synthesis of increasingly complex oligosaccharides (e.g., with branching). In this work, it was also observed that there was no selectivity when performing the reactions in AD


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Review

Scheme 56. Catalytic Glycosylation with a Dihydrooxadiazine Glycosyl Donor

MeCN at rt.422 The use of vinyl glycosides were later expanded to 2-azido-2-deoxy and 2-phthalimido-2-deoxy glycosides still using catalytic activation with TMSOTf.423 Despite the work by Isles and Boons demonstrating the catalytic activation of vinyl glycosides, other contemporary groups seemed not to take notice and continued to use excess of acids in glycosylations using isoprenyl glycosides424,425 or ketene acetals.426 Osa et al. studied substituted enol ethers, somewhat similar to the donor types described by Schmidt. Attempts to lower the amounts of TMSOTf to stoichiometric amounts led to low yields (19%), and it was found that ca. 10 equiv was required for full conversion of the donor. Furthermore, it was found that BF3·OEt2 did not activate the system effectively. Fitzsimmons et al. showed that glycals could participate in a [4 + 2] cycloaddition with azodicarboxylates to give the corresponding dihydrooxadiazines.427 From the cycloadducts 2amino-2-deoxy-carbohydrates could be obtained by treatment with TsOH in MeOH followed by Raney Nickel reduction. The use of the cycloadduct in oligosaccharide synthesis was later demonstrated by the synthesis of both disaccharides and a trisaccharide.428 When less reactive secondary glycosyl acceptors were used, it was found that BF3·OEt2 gave higher yields compared with TsOH, both carried out in CH2Cl2. The exact amounts of the catalysts are not clear from the communications (Scheme 56). Marzabadi and Franck developed bicyclic vinyl glycosides prepared by a cycloaddition between glycals and 3-thiono-2,4pentanedione, followed by treatment with Nysteds or Tebbe’s reagents to transform the carbonyl into the methylene, which was found to be the better glycosyl donor (Scheme 57). The donors were initially activated by equivalent amounts of TfOH, giving glycosides with excellent 1,2-trans selectivity.429

The resulting glycosides could then be reduced using Raney nickel to give the corresponding 2-deoxy glycosides with control of anomeric selectivity. In a later full paper, a catalytic activation was suggested from the data, but it was emphasized that an equivalent amount of catalyst was required for optimal conditions. The role and positive influence of a triflate counterion for the thiaranium or oxocarbenium ions formed during the activation was also discussed, and the addition of Bu4NOTf was found to be beneficial and a way to lower the amount of acid used. Several other Brønsted and Lewis acids were also included in the study, and both strong Brønsted acids as well as BF3·OEt2 were found to activate the donors.430 The glucosyl and the mannosyl donors (Scheme 57 and Scheme 58) were found to behave differently in the glycosylations, with the glucosyl being reversible in O-glucosylations, whereas the mannosyl was not.431 Scheme 58. Glycosylation with Bicyclic Vinyl Glycosides

The interest in applying vinyl glycosides as glycosyl donors under catalytic conditions has not advanced much since the pioneering work by mainly the Boons group in the 1990’s. The same concept but on allyl glycosides have been investigated, but only by using equivalent amounts of electrophiles, such as NIS, occasionally in combination with TfOH.432,433

6. ALKYL AND SILYL GLYCOSIDES AS GLYCOSYL DONORS Using glycosides or the hemiacetals directly represents one of the simplest systems imaginable. Therefore, there have been some interest in using these compounds as glycosyl donors despite the obvious problems that one might face. First of all, the leaving group is by itself nucleophilic (i.e., water or a simple alcohol and hence often more reactive than the sugar acceptor). Second, the simple glycosides are very stable compounds and normally regarded as protective groups, which have to be removed under harsh conditions such as strong acids for longer time at elevated temperature. Using the hemiacetals seems somewhat simpler, and their reaction with simple alcohols under acidic conditions represent the most classic glycosylation one can think of, the Fischer glycosylation. Using it on more complex acceptors with a catalytic amount of acid and close to equivalent amounts of donor and acceptor is however much more demanding. The earliest example of synthesizing

Scheme 57. Formation of the Bicyclic Vinyl Glycosides by Marzabadi and Franck

AE


Chemical Reviews

Review

Scheme 59. Silyl Ether Glycosyl Donors by Glaudemans

Scheme 60. Silyl Ether Glycosyl Donors by Mukaiyama et al

Scheme 61. Catalytic Fischer Glycosylation with Removal of Water

disaccharides using simple glycosides came from Nashed and Glaudemans, who used a 1-O-TMS-protected glycoside as the donor and a 6-O-TBDPS protected acceptor under the catalysis of TMSOTf (ca. 0.5 equiv) (Scheme 59).434 Acetylated galactosyl- and glucosyl donors were coupled to the 6-O of a sugar acceptor with galacto stereochemistry (up to a trisaccharide). Acetyl, benzoyl, and p-phenylbenzoyl groups were tolerated under the relatively mild conditions (i.e., CH2Cl2 or C2H4Cl2 at low temperature, from −78 to −30 °C), giving good yields in the range of 47 to 76%. The reaction was not studied in greater detail and whether the catalyst loading could be lowered was not commented upon. Mukaiyama et al. came with a variation of Nashed and Glaudemans approach using the formation of silyl ethers as the driving force for the formation of the glycosidic linkage.435 Instead of only TMSOTf, a promoter system consisting of [1,2benzenediolato(2-)-O,O′]oxotitanium and TMSOTf was used in as low as 0.2 and 0.1 equiv, respectively. Both the donor and the acceptor are trimethylsilylated. There is only one example using a sugar acceptor, and here the loading of the promoters is increased by a factor of 4, questioning the catalytic ability of the system in oligosaccharide synthesis. A mechanism for the catalytic cycle is proposed in the paper. The reaction using the arabinofuranosyl donor results in remarkable high selectivity toward the usually very challenging β-product (1,2-cis selectivity) (Scheme 60). Despite these interesting initial results, this approach has never gained much popularity in oligosaccharide synthesis. The use of reducing sugars as the glycosyl donor has gained some attention during the past quarter of a century. The problems with these modified Fischer glycosylations is the formation of water during the reaction. If not continuously removed, an equilibrium toward the reactant side is obtained and hence not much of the oligosaccharide. To drive the reaction, drying reagents are normally required. Most of the methods apply Lewis acids, such as metal salts, often triflates and perchlorates, as the catalysts. Presumably, the corresponding Brønsted acid is formed and hence play a role in the

activation. This aspect is often not taken into account when suggesting the reaction mechanisms. Inanaga et al. used Yb(OTf)3 in combination with methoxyacetic acid as the catalytic system (Scheme 61).436 It was shown that 10 mol % of each gave the best results, but the amount could be lowered to 1 mol % and still give 40% yield of a simple glycoside. The reactions were carried out in refluxing CH2Cl2 or 1,2-dichloroethane with azeotropical removal of water using a column with 4 Å MS. Two donors were used, a perbenzylated ribofuranose and a perbenzylated glucose and three sugar acceptors. The disaccharides were formed in >90% yields and with β-selectivity for the ribofuranoside formation but low selectivity for the glucosides. Shimomura and Mukaiyama used the same perbenzylated ribofuranose as the donor as in the example by Inanaga and a 6OH sugar acceptor but with AgClO4 and Lawesson’s reagent (or Ph2SnS) as the catalytic promoter system.437 3 Å MS were used as the drying reagent, and the reactions were carried out at rt. The reaction was found to be β-selective and high yielding, but only one disaccharide was synthesized. The influence of perchlorate on the anomeric ratio in the glycosylation using the same donor and acceptor as above was also studied in a modified system with Sn(OTf)2 as the catalyst (1 mol %), (TMS)2O (10 mol %), and drierite in MeNO2.438 The anomeric ratio was strongly affected by the addition of LiClO4 (150 mol %) and could be shifted from α/β 5:95 without to 94:6 with the salt present. This ratio could be further improved by using Tf2NLi (150 mol %) instead of LiClO4 and with TrB(C6F5)4439 as the catalyst (10 mol %) in CH2Cl2.440 Hiroi and co-workers have later observed the same effect of adding LiClO4.441 In their system, the same donor was used, but CuCl2 was in combination with bis(diphenylphosphino)ferrocene (dppf) and AgClO4 as the catalytic system. Water was removed by using excess CaSO4 as a drying reagent. The scope of the direct glycosylation using the 1-hydroxy sugars was slightly widened to perbenzylated galactose in combination with Sn(OTf)2 and TMSCl (5% of each) in benzene containing drierite and LiClO4 (350 mol %).442 With a 6-OH glucoside acceptor, the yields were high (86−90%) and AF


Chemical Reviews

Review

Scheme 62. Alternating Stereoselectivity Depending on Protecting Group Pattern

Scheme 63. Synthesis of Trehalose Mimics by Yamanoi and Co-Workers

Scheme 64. Rhodium-Catalyzed Glycosylations by Ernst and Co-Workers

Scheme 65. Gold-Catalyzed Activation of Methyl Glycosides by Vidadala and Hotha

the glycosylations were α-selective. Glycosylation using a perbenzylated 2-deoxy glucose and TrB(C6F5)4 (5 mol %) gave mainly the α-glycosides, approximately in a 10:1 ratio and high yields.443 An interesting example of using a highly reactive olivose as the donor in combination with an acidic drying reagent was presented by Toshima and co-workers. 444 They used montmorillonite K-10 as the catalyst, although in 150 wt %. From systematically changing the O-protective groups (Ac, Bn, and TBS) on the 3 and 4 positions, it was found that having a 3-O-Ac-4-O-TBS-olivose gave α-selectivity, whereas the 3-OTBS-4-O-Ac-olivose gave β-selectivity (Scheme 62). Remote neighboring group participation was given as a plausible explanation of this change in selectivity. The method was improved by using a heteropolyacid, H4SiW12O40 (20 wt % in MeCN), which upon predrying worked as both the acid catalyst and the drying reagent.445 Several aldoses were used as donors, but only perbenzylated Lrhamnose and 2-deoxy-glucose were used together with a sugar acceptor. Yamanoi et al. used Bi(OTf)3 as the catalyst for the dehydration of the 1-hydroxy sugars. Using perbenzylated glucose glucosylation on the 6-OH of a benzylated methyl glucoside could be achieved in a moderate yield (52%) and selectivity (α/β 75:25). By using the same conditions, but two different aldoses, 1,1′-disaccharides could be obtained. Yields and selectivities were strongly dependent on the substrates chosen.446 By introducing a C-substituent at the anomeric position of the donor, the stability of an oxocarbenium

intermediate increased. This was studied by Yamanoi and coworkers who synthesized trehalose mimics using a perbenzylated 1-C-methyl-D-hexopyranose as the donor and different benzylated aldoses as the acceptors (Scheme 63).447 Bi(OTf)3 and Tf2NH were found to give the highest yields. The ketose derivatives, normally unnatural, generally performed better in the glycosylations both for the 1,1′ type and for glycosylation at the 6-OH in other glycosides.448 Ernst and co-workers used rhodium(III)-triphos catalysts in low loadings (0.5 mol %) to couple protected glucose derivatives (Scheme 64).449 When using the perbenzylated glucose, an anomeric product mixture was obtained, and hence, the reaction was under thermodynamic control. Installing an ester protective group, Ac or Bz, on the 2-O-position resulted in a high β-selectivity, when coupling to a 6-OH acceptor. Using a per benzoylated hemiacetal as donor hampered the reaction even when using a simple acceptor, and hence, the procedure seems to be limited to the more reactive donors. Using methyl or other simple alkyl glycosides as glycosyl donors could be an interesting alternative to other methods as methyl glycosides are readily available and normally very stable. One major problem in this approach is, however, that the methanol or methoxide formed is more nucleophilic than a sugar acceptor, and therefore, it has to be removed during the reaction. Mukaiyama and co-workers proposed a method using molecular sieves as the trap for methanol and a promoter system consisting of Sn(OTf)2 and TMSCl in tBuCN.450 5 Å MS was the preferred trap. Two examples of using a sugar acceptor and substoichiometric amounts of the promoter AG


Chemical Reviews

Review

Scheme 66. Early Glycosylations with Glycosyl Acetate Donors

Scheme 67. Procedure for Catalytic Activation of Glycosyl Acetate with a Silyl Ether Acceptor

system are given, both with reactive benzylated β-methyl glycosides as donors and deactivated benzoylated α-methyl glycosides as acceptors. The reported yields were excellent but the selectivities low, and presumably the reaction was under thermodynamic control. Whether the reaction is indeed catalytic remains unclear. More recently Vidadala and Hotha discovered that methyl glycosides were activated by AuCl3 at elevated temperatures.390 During the optimization of the conditions, it was found that AuBr3 gave higher yields and that the optimal temperature for the reaction in MeCN was 70 °C (Scheme 65). Using bigger aglycons or even allyl in the donor (instead of Me) lowered the yields. The glycosylation was tested on two primary sugar acceptors (benzoylated methyl glycosides) using benzylated methyl manno-, gluco-, and galactosides as the glycosyl donors. The anomeric orientation of the methyl aglycon in the donor and in the acceptor did not influence the outcome of the reaction significantly. The gold-catalyzed activation of methyl glycosides was also performed on a 2-C-branced donor giving high α-selectivities.451 The method has also been extended to benzylated methyl furanosides, which could be chemoselectively activated in the presence of benzoylated propargyl pyranosides.452

Helferich and Schmitz-Hillebrecht used the glycosyl acetates directly as glycosyl donors for the synthesis of phenol glycosides.456 The reaction was carried out under strong acidic conditions and is essentially the same used today for inserting anomeric protective groups, such as p-methoxy phenol. For the following half a century, glycosyl acetates were used as donors but always with at least stoichiometric amounts of the acid promoter. Things slowly started to change in the early 1980’s with Ogawa et al. introducing TMSOTf as an “efficient catalyst for glycoside synthesis”, albeit still used in equivalent amounts.457 Paulsen and Paal were the first to report a catalytic activation of glycosyl acetates for the synthesis of di- and trisaccharides.458 TMSOTf (∼0.1−0.5 equiv) was the preferred catalyst, whereas FeCl3 gave 30% of the disaccharide. The Lewis acids, BF3 and SnCl4, were found to result in dominant side reactions. The yields in the TMSOTf-catalyzed glycosylation were improved by adding 4 Å MS, which presumably works as an acid scavenger of the formed TfOH. With the optimized conditions, an 85% yield of the β-D-Gal-(1−3)-β-D-Gal disaccharide could be obtained. It was furthermore noticed that whereas the β-acetate was activated under the conditions, the α-acetate remained unconverted (i.e., suggesting anchimeric assistance from the 2-O-acetyl as important for the reaction to take place). The protocol was shown to work for glycosylations using peracylated galactose and a variety of carbohydrate-based acceptors for the synthesis of di- and trisaccharides. Impressively, a phthalimide-protected peracetylated glucosamine was also studied for the glycosylation of the notoriously difficult 4-OH on an N-acetyl glucosamine. Using the method, a 55% yield of the disaccharide could be obtained (Scheme 66). The glycosylation using acylated glucosamine derivatives was investigated further by Kiso and Anderson, varying the N-acyl group and the O-protective groups in the donor.459 FeCl3 was used as the promoter in excess, giving yields in the range of 30−60%. Mukaiyama and co-workers were the first to study the influence of substituents on the acetyl leaving group, finding that α-bromo and α-iodo substituents worked better in terms of selectivity and yield than the corresponding α-chloro acetyl.304 The promoter systems for the glycosylations were continuously improved over the years, and especially the Mukaiyama group refined the activation of glycosyl esters. As with many of the other donor systems studied by the group, salts were important additive to manipulate the stereochemical outcome. Also the acceptors were modified and silylated to avoid the buildup of

Aryl Glycosides As Glycosyl Donors

Jensen and co-workers developed a glycosylation method using an electron deficient phenol as the leaving group. The most successful of the tested aglycons was found to be methyl 2hydroxy-3,5-dinitrobenzoate, abbreviated DISAL.453 The DISAL donors were shown to be activated under neutral or basic conditions and hence without the aid of a catalyst. For the reaction to be effective, polar solvents, such as NMP, or the addition of LiClO4 was required.453 The glycosylation method using DISAL donors or equivalent donors were found to be promoted by Lewis acids, but catalytic amounts were found insufficient, presumably due to quenching by the phenol formed during the reaction.454,455

7. GLYCOSYL ESTERS AS DONORS Very early in the development of glycosylation chemistry, it was realized that an anomeric acetyl group has a unique reactivity in a peracetylated sugar. In the first reported glycosylation by Michael, the actual donor is prepared from a selective exchange of this acetyl by a chloride, which can then be substituted by an alkoxide.1 It, however, took more than half a century before AH


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Review

Scheme 68. Stereochemical Outcome of Alternative Reaction Conditions

Scheme 69. Synthesis of an Olivomycin Fragment with Glycosyl Acetate Donor

protected acceptors were used, the yields were high and the βselectivity excellent. Using a free alcohol lowered the yield from almost quantitative to ca. 30%. This could be solved by adding stoichiometric amounts of TMS2O, which silylated the acceptor in in situ. It was furthermore shown that the β-glycosides could be anomerized using SnCl4 and AgClO4 in MeNO2 at rt. A related catalyst, Yb(N(O2SC4F9)2)3, has been investigated by Yamanoi et al. for activation of perbenzylated glycosyl esters.469 In the glycosylation of a 6-OH acceptor, a 68% yield was obtained as a 1:1 mixture of anomers. Glycosyl fluorides, carbonates, and TCA donors were also subjected to this catalyst but only yielded 50−69% of the disaccharide with very low stereoselectivity. Hermans et al. used catalytic amounts of TMSOTf added in three portions to activate a ribofuranosyl acetate for the synthesis of protected 1-O-β-D-ribofuranosyl-D-ribitol building blocks.470 It was noticed that the quality of the catalyst was crucial for the amount needed. Later Verez-Bencomo used a 1,2-di-O-acetyl-3-O-allyl-5-O-benzyl-D-ribofuranose as a glycosyl donor in the synthesis of compounds related to the H. influenza type B oligosaccharides.471 When TMSOTf (56 mol %) was used in the glycosylation of a ribitol acceptor, 81% yield was obtained. With less reactive and more hindered acceptors acetyl migration became a problem and it was found that BF3· OEt2 in excess could prevent this side reaction maintaining good yields. The use of 1-O-acylated glycosyl donors for the synthesis of more complex natural products, or their derivatives, was initiated in the early nineties. An early example was by Scharf and co-workers, who used p-nitrobenzoyl as the leaving group and TMSOTf as the promoter (4.2 equiv) for the synthesis of the terminal AB unit of everninomicin antibiotics.472 Roush and Sebesta used a 2-deoxy-2-iodo-α-glycosyl acetate as the donor for the synthesis of a fragment of olivomycin A.473 The highly reactive donor was activated at −40 °C using TMSOTf as the catalyst (0.3 equiv). The reaction was found to be very αselective, which was explained by neighboring group participation by the 2-iodo group (Scheme 69).474 The method was expanded to the glycosylation reactions of 2-deoxy-2-iodo-α-mannopyranosyl acetates and the corresponding talo-donor giving excellent stereoselectivity for the 1,2-trans products and high yields. Besides using catalytic amounts of TMSOTf, TBSOTf was also introduced as a catalyst.475 This reduced a competing silyl exchange reaction, when having a TBS protective group in the donor, and hence reduced the formation of a trisaccharide byproduct. With the

Brønsted acids under the reaction conditions. SnCl4 and Sn(OTf)2 (20 mol % each) together with equivalent amounts of LiClO4 was found to give high yields in ribofuranosylations, albeit with a modest β-selectivity. Without LiClO4, the reaction was not working. In some cases, addition of NaIO4 (10 mol %) was found to improve the yields.460,461 Changing the system slightly to use a combination of AgClO4 and SnCl4 reduced the catalyst loading to 10 mol % without the need for stoichiometric amounts of LiClO4 added (Scheme 67).462 Other metal chlorides (based on Ge, Si, Ga, In, and Hf) were also found to catalyze the reaction in the presence of AgClO4 and gave similar results both in terms of yields and selectivities.463 The robustness of Mukaiyama’s SnCl4/ AgClO4 procedure has been exemplified by Petráková and Glaudemans, who synthesized isomalto-oligosaccharides deoxygenated on either the C3464 or C4465 position. With the access to a catalytic system, the ribofuranosylation was revisited with the focus on controlling the stereoselectivity using diphenyltin sulfide, silver salts, and LiClO4 as an additive.466 As the anomeric leaving group, iodoacetyl was chosen, based on the previous work by the research group. When the glycosylation was carried out in benzene, without addition of LiClO4, the β-product could be obtained as the major product. Adding 3 equiv of LiClO4 and changing the solvent to CH2Cl2 resulted in good α-selectivities (Scheme 68). Interestingly, solvents were screened and both Et2O and MeCN gave almost the same outcome (i.e., lower selectivity compared with benzene, CH2Cl2, and 1,2-dichloroethane). The influence of the silver salts used was also studied, and it was found that AgClO4, AgSbF6, and AgOTf did catalyze the reaction when combined with diphenyltin sulfide, whereas AgPF6 and AgBF4 did not. Similar results could be obtained by using Lawesson’s reagent instead of the diphenyltin sulfide.437 The influence of MeCN and Et2O was later studied using the related 1-O-2′-(2′methoxyethoxy)acetyl glucopyranoside as the donor. With SiCl4 and AgClO4 in MeCN, high β-selectivities could be obtained, whereas SnCl4 and AgClO4 in Et2O gave excellent αselectivity.467 The influence of remote neighboring group participation was studied using a 1-O-acetyl-5-O-benzyl-2deoxyribofuranoside, having different protective groups on the 3-O-group.330 The reaction was, however, not studied using carbohydrate-based acceptors. Matsubara and Kukaiyama simplified the catalytic system using metal triflates such as Sn(OTf)2 and Yb(OTf)3 for the synthesis of 2-amino-β-D-gluco or galactopyranosides.468 As Nprotective groups, Alloc or Troc were used. When TMSAI


Chemical Reviews

Review

Scheme 70. Catalytic, Stereoselective Glycosylation with Glycosyl Acetate Donor with a 3,4-O-Carbonate Tether

Scheme 71. Sc(OTf)3 as a Novel Catalyst in Glycosyl Acetate Activation

Scheme 72. Glycosylation with a Trichloroacetate Donor in Et2O

β-glycoside. When changing solvent to iPr2O, in the Dpsicofuranosylations, the selectivity slightly improved to 22:78 (α/β), whereas MeCN resulted in α-selectivity (67:33). The participation of the neighboring 3-O-benzoyl group is apparently not strong enough to ensure complete selectivity. Cai and co-workers introduced trichloroacetates as a leaving group in glycosylations.481−483 TMSOTf, used together with 3 Å MS, was found to catalyze the glycosylation, and adding an acid scavenger (2,6-ditert-butyl-4-methylpyridine) was initially found to improve the reaction. The reactions were however very slow at rt, with up to 4 days reaction time, although high yielding. In the synthesis of a complex hexasaccharide, the acid scavenger was left out and the reaction time shortened to hours instead of days.482 BF3·OEt2 was found to catalyze the glycosylation but less efficiently than TMSOTf.484 Kobayashi and co-workers have utilized the glycosyl trichloroacetyl donor for sequential one-pot glycosylations and, in connection with this, optimized the glycosylation conditions.485 As a model donor was chosen a per benzylated D-glucosyl trichloroacetate, which was reacted with methyl 2,3,4-tri-O-benzoyl-D-glucopyranoside in CH2Cl2 with 5 Å MS. TfOH, TMSOTf, TMSClO4, and SnCl4 were all found to catalyze the glycosylation, when used in 20 mol % at 0 °C, whereas Lewis acids such as AgOTf, Zn(OTf)2, Cu(OTf)2, and Sc(OTf)3 in excess did not activate the donor. The α-selectivity could be improved using Et2O at rt, which, though, resulted in longer reaction times of 24 h (Scheme 72). Using MeCN in order to increase the β-ratio resulted in decomposition and hence narrowed the scope of this donor type. Kim and co-workers used glycosyl p-bromophenyl phthalates in combination with TMSOTf for glycosylation of various carbohydrate-based acceptors.486 From an initial screening, it was found that the p-bromo derivative was more reactive than the corresponding fluoride or chloride. The p-nitrophenyl derivative was found to be highly unstable and could not be purified. The 4-bromophenyl was activated using 0.5−1.0 equiv TMSOTf in CH2Cl2 at low temperature. Glucosylations were practically unselective, whereas mannosylations gave moderate to excellent α-selectivity. What the actual catalyst is and how it

method, refined complex deoxy-oligosaccharides could be synthesized exemplified by the synthesis of the landomycin A hexasaccharide unit, using a combination of glycosyl acetates and trichlorocetimidates as donors and only 0.05 equiv of TBSOTf as the catalyst together with 4 Å MS in CH2Cl2.476 The stereochemistry could be effectively controlled by the 2iodo-substituent, which was removed as part of a final deprotection sequence. Motivated by the high selectivities obtained from glycosyl acetates with talo-, manno-, and glucostereochemistry, 2-deoxy-2-halo-galactopyranosyl donors were also investigated.477 Somewhat unexpected, the glycosylation resulted in low stereoselectivity with various protective group patterns, and it was suggested that the role of the halonium ion is less dominant in this system and hence an oxocarbenium ion intermediate could be the actual glycosylating agent. In an attempt to direct the glycosylation, a 3,4-O-tethering group was installed, which would restrict the conformational freedom toward a boatlike conformation of the oxocarbenium ion. A 3,4O-isopropylidene protective group resulted in some, albeit low, β-selectivity, whereas a cyclic 3,4-O-carbonate in combination with a 2-iodo-substituent gave higher selectivities (Scheme 70). Interestingly, Kirschning et al. later achieved high 1,2-trans selectivity using an nontethered galactosyl acetate donor activated with TMSOTf in CH2Cl2.478 The exact conditions are however not clear and maybe not directly comparable with the results from Roush. The glycosylation with the 2-iodoglycosyl acetates having talo-stereochemistry gave excellent αselectivity and high yields comparable with the results by Roush.479 Despite the simplicity of the glycosylations using the simple acetates as the leaving groups, these have only been sporadically used for oligosaccharide synthesis due to the harsh conditions and long reaction times. With the easier access to rare earth metal salts (e.g., triflates), new methods and improvements still appear, making the donors more attractive. Yamanoi et al. showed that Sc(OTf)3 catalyzed the glycosylation using Lfructo- and D-psicofuranosyl acetates as donors (Scheme 71).480 Only 5 mol % of the catalyst was required to give good yields, but generally, the selectivity was low with modest excess of the AJ


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Scheme 73. Catalytic Activation of Glycosyl o-Methoxybenzoates Developed by the Jensen Group

Scheme 74. Microwave Irradiation-Assisted Glycosylation of Glycosyl Pivaloate Donor

Scheme 75. Benzyl Migration Encountered by the Jensen Group

donor was required to give high yields of the β-glycoside. It was also confirmed that the α-acetate was significantly less reactive compared to the β-anomer, which supports the need of anchimeric assistance from the N-acetyl group and hence an oxazolinium intermediate. The protocol was expanded by Jensen to the use of pivaloyaled GalNAc, which has the advantage of being synthesized in only 3 steps from the much cheaper GlcNAc.491 The key step is an intramolecular migration of a pivaloyl from the 3- to the 4-position with 4O-inversion of stereochemistry.492 It has later been found that other more commonly used acyl protective groups could be used for the synthesis of galactosamime donors via intramolecular inversion.493 Remarkably, the 1-O-pivalates could be activated with catalytic amounts of metal triflates and even strong Brønsted acids, such as TfOH. For the glycosylation of less reactive acceptors, it was found that the less reactive metal triflates, such as Cu(OTf)2 and Yb(OTf)3, performed better than the more reactive Bi(OTf)3 and Fe(OTf)3, when performed under microwave irradiation in a closed vial (Scheme 74). The importance of matching the reactivity of the catalysts with the acceptor reactivity was explained by a two-step activation of the donor. If the oxazoline is formed too fast, it decomposes before reacting with a slow acceptor, resulting in low yields and in addition, if the conditions are too acidic, anomerization. The mechanistic aspects of activating GlyNAc donors were studied in more detail by the Jensen group by comparing benzylated N-acetyl glucosamine donors having either an acetate or a thiophenyl leaving group.494 With the more “armed” GlcNAc donor, reaction times and temperatures could be reduced, but the yields, when glycosylating less reactive acceptors, were unsatisfactory. To get mechanistic insight, the conversion of the benzylated N-acetyl glucosamine acetate was monitored by NMR. This revealed a buildup of the oxazolinium intermediate according to the amount of, for example, TfOH added (i.e., confirming the previous hypothesis that a slow oxazolinium formation is desired when having slow acceptors). When using the corresponding thioglycoside, a very fast formation of the oxazolinium ion was observed, but this could be controlled by a slow addition of NIS. The fate of the oxazolinium ion formed, and thereby clues to why too fast activation gave lower yields, was also carefully studied. From

is regenerated is not described. Jensen and co-workers have recently introduced glycosyl o-methoxybenzoates as a glycosyl donor, which can be activated under catalytic conditions.487 Benzoates substituted with different patterns of CN, OMe, and unsubstituted, were tested but found to be less reactive. From these initial glycosylations, a chelation between the o-OMe and the ester-carbonyl was suggested as the mode of activation. Metal triflates were screened as catalysts, and it was found that Fe(OTf)3 and Bi(OTf)3 performed best in terms of reaction time, even when using down to 3 mol %. The effect of changing solvents was limited, and generally, the stereoselectivity was low. TfOH and TMSOTf worked equally well as catalysts and hence suggested that the actual catalyst was TfOH. However, Bi(OTf)3 was the preferred catalyst and a number of different donors were prepared and studied for the synthesis of disaccharides. The yields were generally high, and when using neighboring group participation, the 1,2-trans products were obtained with high selectivity. A β-selective mannosylation, using the 4,6-O-benzylidene protected mannosyl donor, was also attempted. In this case, TMSOTf was found to give a cleaner reaction and a moderate β-selectivity, when performed at 0 °C to rt, without using preactivation (Scheme 73).488 The limitation of the donor system seems to be their high stability and hence prolonged reaction times, which one might improve by altering the substituents on the benzoate. Furthermore, only the β-benzoate was reactive enough for glycosylation. Whereas general glycosylation using glycosyl acetates have had limited use, a more specialized area has received increasing interest (i.e., glycosylations using acetylated N-acetyl glucosamines and to some extend N-acetyl galactosamines). As mentioned above, Paal and Paulsen458 were the first to use Nprotected glycosyl acetates for glycosylation under catalytic conditions. Inspired by a procedure by Wittmann and Lennartz, who used stoichiometric amounts of CuCl2 for the activation of a peracetylated β-GlcNAc,489 Jensen and co-workers set out to find catalysts for this highly important glycosylation.490 From screening the commonly available rare earth metal triflates, Sc(OTf)3 was found to give the fastest reactions, but all catalysts tested were catalytically active. Using CH2Cl2 as a solvent was additionally found to give higher yields than when performing the glycosylation in MeCN, toluene, or THF. When carbohydrate-based acceptors were used, 2−4 equiv of the AK


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Scheme 76. Three Galactosylations on the Same Manno Acceptor

Scheme 77. Glycosylation with Preattached Glycosyl Donor- And Acceptor

was demonstrated by performing multiple glycosylations on a mannoside and thereby synthesizing structures resembling fragments of the core of complex N-glycans (Scheme 76).498 From two to four GlcNAc’s were added to a proper protected mannoside core in yields up to 84% albeit somewhat lower with three or four consecutive glycosylations (around 20%). Recently Urban and Beau exploited the use of the less reactive peracetylated α-D-glucosamine and the in situ anomerization of the obtained glycosides by using Cu(OTf)2 as the catalyst at 140 °C in a closed reactor.499 So far, the method has only been used for simple glycosides and not for disaccharides. Titz and co-workers also used Cu(OTf)2 for the activation of an anomeric mixture of acetates at high temperatures in 1,2-dichloroethane albeit only with simple noncarbohydrate acceptors.500 Related to glycosylation is the Ferrier reaction, which has been catalyzed by AuCl3 (0.5−2 mol %).501 Both peracetylated glucals and galactals have been used as substrates and gave moderate yields, when the nucleophile was a sugar 4- or 6-OH. The anomeric selectivity was moderate to good toward the αproduct. This could be improved by adding phenylacetylene to the catalyst system.502,503 This finding resulted in interesting Ferrier rearrangements and the synthesis of different disaccharide glycals, but maybe more importantly led the discovery that glycosyl acetates can be activated under the same catalytic conditions [i.e., AuCl3 and phenylacetylene (5 mol %)]. The glycosylations were still slow and required slightly elevated temperatures when using MeCN or CH2Cl2 as the solvents. No significant solvent effect was noticed. With more hindered and less reactive acceptors, the catalytic system could be further activated by using AgOTf and thereby giving shorter reaction times and higher yields. A conceptually interesting glycosylation method was introduced by Liu and Li.504 Instead of the classic glycosylation, where the acceptor reacts with an electrophilic donor, the two sugars are preattached by an ester linkage (Scheme 77). This is then partly reduced to give a hemiacetal, which upon treatment with nBu2BOTf cyclizes to give the disaccharide. Cu(OTf)2 or Zn(OTf)2 were also used in substoichiometric amounts but found to be less efficient even with simpler noncarbohydrate-

experiments without acceptor present, a migration of benzyl groups was revealed and mass spectrometry studies, using labeled benzyl groups, led to the conclusion that equal amounts of the 3-OBn and 4-OBn migrated to an activated donor (i.e., leading to a rapid consumption of donor instead of giving the desired glycosylation) (Scheme 75). This cleavage of benzyl ether protective groups under the relatively harsh conditions was already mentioned, but not studied in detail, by Paal and Paulsen in their pioneering work described above. Besides this side reaction, anomerization to the unreactive α-acetate was also found. Beau and Boyer used Fe(OTf)3 in combination with an acid scavenger (TTBP) to activate peracetylated β-D-N-acetylglucosamine.495,496 Elevated temperatures were again required, and microwave radiation was found to be a convenient method for slightly overheating the solvent (CH2Cl2). Several other catalysts were screened but found less efficient compared to the Fe(OTf)3 or its less hydroscopic DMSO complex Fe(OTf)3·6.2DMSO. The importance of TTBP for achieving high yield is somewhat contradicting the results by the Jensen group, who noticed a deterioration in reaction rate.494 The clear advantage of adding TTPS is a broader protective group scope, allowing the more acid-sensitive ones such as TBDPS. When using 2 equiv of the donor, good yield of disaccharides could be obtained, when primary acceptors were used. Shifting to the more demanding secondary sugar alcohol (4-OH) yields dropped to 20%. Doubling the amount of donor to 4 equiv only improved the yield slightly to 25%. From changing the Nprotective group, it was suggested that the amide functionality was involved in a precomplexation, which could surpass an oxazolinium ion as a glycosylating intermediate. The explanation, from the presented results, could also be related to the ability of anchimeric assistance by the amide derivative. The method was further adopted to flow chemistry, still involving heating by microwave irradiation, and it was found that TTBP did slow the reaction and resulted in lower yields and hence could be avoided as an additive.497 By this method, β-(1−6), β(1−2), and β-(1−3) glycosides could be effectively synthesized. The β-(1−4) still remained problematic. The strength of the method using readily available peracetylated β-D-glucosamine AL


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methane, giving the disaccharides in modest yields 51% and 64%, respectively. The scope and limitations of the glycosylation were studied with the synthesis of many different di- and trisaccharides containing both furanosides and pyranosides.96 It was found that the hindered and less reactive alcohols gave low yields and that a rearrangement of the orthoesters to the corresponding isomeric “simple” alkyl glycoside could be a dominant side reaction limiting the glycosylation yield. To overcome some of these limitations with the method, the orthoester moiety was modified, and it was found that using more bulky alcohols in the formation, such as tBuOH, gave more reactive glycosyl donors, which were less prone to rearrange to the undesired isomeric glycosides.506 The solvents system was also changed to 1,2-dichloroethane or chlorobenzene in combination with pyridinium or 2,6-dimethylpyridinium perchlorates. Even though the yields could be improved, it was still found to be an advantage to do the reaction over two steps: first a re-esterification with the acceptor and then the glycosylation/rearrangement. Wulff and Schmidt studied the competition between orthoester formation versus glycosylation.507 Interestingly, it was observed that when carrying out the reaction in THF as solvent, 4-bromobutyl β-glycosides were obtained as byproducts. This resembles a rare example of catching an intermediate oxonium ion, commonly referred to as the “ether effect”. Kochetkov et al. demonstrated the strength of the orthoester method for polymerization of the tricyclic orthoesters β-Larabinofuranose-1,2,5-ortho-benzoate into linear and branched arabinans.508 This conceptual new idea of using internal orthoesters was later investigated by Wong and co-workers for oligosaccharide synthesis, with the focus in α-mannosylations (Scheme 79).509 Using a 6-OH gluco acceptor in excess, the disaccharide could be obtained in modest to good yields. However, the formation of an undesired trisaccharide was a problem, as the 6-OH in the donor is liberated upon activation. This could to some extend be limited by using large excess of acceptor (5 equiv) in combination with TBSOTf (1.2 equiv), instead of TMSOTf as the catalyst (50 mol %), or changing to BF3·OEt2 (20 mol %), which gave the best yield (75%) and only trace of the trisaccharide. From the proposed mechanism, based on experimental evidence, it was clear that a dioxolenium ion was a key intermediate in the glycosylation using orthoesters. An alternative approach to form the dioxolenium ion was therefore developed by the Kochetkov group, using cyanoalkylidene donors and tritylated acceptors (Scheme 80).510 In a synthesis of gentiobiose, as the model reaction, it was observed that the reaction was solvent-dependent, whereas counterions from the trityl salts added did not seem to influence the reaction significantly; however, only BF4− and ClO4− were investigated. The best result was when using trityl perchlorate (5 mol %) in

based nucleophiles. Albeit not used in catalytic amounts, TfOH was found to give the best results in this ring-closing glycosylation. Despite the easy access to glycosyl esters and efforts to developed efficient glycosylation methods for oligosaccharide synthesis, their use is still very limited. Generally, the conditions are too harsh, involving strong acids at elevated temperatures, or the reactions are simply too slow. In most cases, solvent effects are not enough to control the stereochemical outcome and one has to rely on neighboring group participation.

8. ORTHOESTERS AS GLYCOSYL DONORS Orthoesters are well-known intermediates in glycosylations, when having a 2-O-acyl group, which upon neighboring group participation gives a dioxolenium ion. This can be trapped by the alcohol nucleophile to give the orthoesters, which then, if labile enough under the reaction conditions, rearranges to give mainly the 1,2-trans product. In many cases, the formation of orthoesters can be avoided, and it is now a standard procedure to use Piv or Bz protective groups for the 2-O-position, as these groups provide the desired neighboring group participation but gives less problems with orthoester formation. In some cases, the formation of an orthoester linkage between the glycosyl donor and acceptor has been used as step one in a two-step glycosylation.98 These are, however, normally not catalytic in the first step, during which a glycosyl bromide typically is activated and hence not within the scope of this review. Interestingly, the mechanism of the orthoester rearrangement has been studied by crossover experiments and found to give mixed products, which confirms a disproportionation rather than a rearrangement.97 The focus, in this review, will be on utilizing the orthoesters as the glycosyl donor for the synthesis of oligosaccharides.505 The first example of using orthoesters as glycosyl donor under catalytic conditions, came from Kochetkov et al.95 Using catalytic amounts of HgBr2 in CH2Cl2 resulted mainly in the formation orthoester, whereas more of the catalyst and nitromethane as the solvent gave the 1,2-trans-glycoside (Scheme 78). Two disaccharides were synthesized using two Scheme 78. HgBr2-Catalyzed Activation of an Orthoester by Kochetkov et al.

different orthoesters as donors [i.e., 3,4,6-tri-O-acetyl-1,2-(ethyl orthoacetyl)-α-D-glucopyranose and the corresponding galactoorthoester]. Both were glycosylated with 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose using 5 mol % HgBr2 in nitro-

Scheme 79. 1,2,5-Orthoester Employed in Glycosylations by the Wong Group

AM


Chemical Reviews

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Scheme 80. Cyanoalkylidene Glycosyl Donors Developed by the Kochetkov Group

Scheme 81. Methyl 1,2-Orthobenzoate Glycosyl Donor Investigated by Uriel et al.

CH2Cl2 at 45 °C, by which gentiobiose peracetate could be obtained in 64% yield. The cyanoalkylidene approach was studied in more detail, and a twostep mechanism was proposed, based on the relative reactivity of different trityl ethers used as acceptors, as well as the lack of correlation with the stereoselectivity observed in the glycosylations.511 It was also found that the trityl ethers do not participate in the rate-determining step (i.e., suggesting a stepwise mechanism). The rate of reaction was found to be dependent on the donor and the catalyst, being first-order in both.511,512 The more recent development in the use of orthoesters as glycosyl donors has led to electrochemical procedures, where the acid is generated electrochemically in situ from a supporting salt, which is present in a large excess.505 The procedure can therefore not be considered catalytic, in the same study; however, HClO4 and BF3·OEt2 were also tested in catalytic amounts but only very low yields were obtained, when glucosylating a simple acceptor like cyclohexanol. Uriel et al.99 reinvestigated95 the use of simple methyl 1,2ortho-acetates and benzoates and confirmed that they can be used as glycosyl donors under catalytic conditions. BF3·OEt2 was the preferred catalyst, albeit stoichiometric amounts were required to get yields above the fifties (Scheme 81). The glycosylation using methyl 1,2-orthoesters have later been improved in terms of yields, using acid-washed molecular sieves (15g/mmol) at rt or reflux.513 Vankar an co-workers used AuBr3 in combination with phenylacetylene to activate orthoesters.503 It was found that benzylated donors could be activated under the conditions, and that using CH2Cl2 as the solvent gave higher yields than MeCN. The yields, when using simple gluco 6-OH acceptors, were however modest (49 to 55%) and when using more hindered acceptors, like a 4-OH, only the methyl glycosides were obtained. AuBr3 alone was also shown to activate the orthoesters but gave yields in the thirties with the 6-OH gluco acceptors. Overall, the use of simple orthoesters in oligosaccharide synthesis seems limited by their low reactivity and the competing reaction with the alcohol formed from activating the donor. This has been solved by using cyanoalkylidenes, but then the acceptor has to be tritylated, which is also a limitation. The most promising approach is to use orthoesters with a functional side-group, which can be activated catalytically by forming a non-nucleophilic leaving group. This has, for example, been demonstrated with the use of alkynes,385,386,388 which is described in that part of this review.

9. GLYCALS AS DONORS The use of glycals for the direct synthesis of 2-deoxysugars,514−517 via an acid-mediated addition, seems straightforward. The reaction is however complicated by the competing allylic rearrangement of the intermediate oxocarbenium ion, giving rise to the formation of 2,3-unsaturated glycosides; these are normally not the desired products. This side reaction, commonly known as the Ferrier reaction, is favored under the strongly acidic conditions required to activate the glycal, and hence the reaction has traditionally not been used for oligosaccharide synthesis. The Ferrier reaction itself has been exploited and can normally be favored under Lewis acidic conditions as well as many Brønsted acidic conditions. The use of the Ferrier reaction in glycosylation has recently been thoroughly reviewed518 and is not within the scope of this review. The first example of a catalytic glycosylation using a glycal, with high selectivity for the 2-deoxy product, came from Bolitt et al. (Scheme 82).519 By activating protected glucals with Scheme 82. HBr-Catalyzed Formation of a 2-Deoxy Glycoside from a Glycal Donor by Bolitt et al

triphenylphosphine hydrobromide (TPHB, 5 mol %), 2deoxyglycoside were prepared using both 4-OH and 6-OH glycoside-derived acceptors. The reactions were highly αselective (>95:5 α/β), and the yields ca. 60%. Worth noticing is also that a 6-O-trityl group was stable under the reaction conditions. The involvement of a C1-phosphonium intermediate was excluded by preparing this independently, as it was found not glycosylate alcohols under the reaction conditions. The catalytic system using phosphine hydrobromide was immobilized on solid phase by Kirschning and co-workers, who studied polymer-assisted glycosylations.520 The polymer-bound dephenylphosphane hydrobromide was, like TPHB, found to suppress the undesired Ferrier reaction and to give high selectivity in the glycosylations using glycals. Wandzik and Bieg used TPHP for the glycosylation using L-rhamnals.521 The reactions were again found to be α-selective (5:1 α/β), which could be further enhanced (12:1 or better) by having a bulky AN


Chemical Reviews

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Scheme 83. CSA-Catalyzed Activation of L-Fucal Donor

Scheme 84. TsOH-Catalyzed Activation of a 3,4-Tethered Glucal Donor

Scheme 85. Activation of an exo-Glycal by Lin et al.

and the L-fucose glycal was activated using CSA giving the α-2deoxy-glycoside in high yields up to 95% (Scheme 83). The Ferrier reaction was not observed. Balmond et al. studied 3,4-trans-fused cyclic protective groups in combination with TsOH·H2O for the synthesis of 2-deoxyglycosides from D-glucals and L-rhamnals.526 Interestingly, it was observed that tethering the 3,4- or the 4,6-hydroxyl groups together hampered the undesired Ferrier reaction. It was furthermore demonstrated that the 3,4-O-tethering resulted in excellent α-selectivity using a range of different acceptors with 1 mol % of the catalysts present (Scheme 84). The glycosylation using 3,4-O-tethered rhamnals was slightly less selective but still high yielding. DFT calculation revealed that the glucal oxocarbenium ion adopted a 4H3 conformation, whereas the flipped 3H4 could not be reached due to the conformational restriction implied by the tethering. Attack on the 4H3 was preferred from the α-side, resulting in a stable chair conformation and not a high-energy twist boat, which would be the result from a β-attack. An interesting variation of Brønsted acid catalyzed Oglycosylation using glycals came from Thombal and Jadhav, who exploited the use of a magnetic solid acid catalyst (Glu− Fe3O4−SO3H).527 The catalyst was based on glucose, which was preloaded with iron salts, pyrolyzed and sulfonated. The glycosylation was tested on both glucals and galactals with diacetone glucose as the hindered acceptor. Except for the peracetylated glycals the yields were high (>80%) and the reactions α-selective. Lewis acids have been studied as catalyst for the synthesis of 2-deoxyglycosides, but the Ferrier reaction has been observed as a major side reaction. There seems to be a temperature dependence on this selectivity, with the direct addition taking place at higher temperatures, whereas the Ferrier reaction is dominant when cooling. This has been observed using Al(OTf)3 as the catalyst528 and under microwave heating in the presence of catalytic amounts of AlCl 3 . 529 The glycosylations were found to be α-selective, and it was

protective group on the 3-O-position and acetyl on the 4-Oposition. A variation of the catalytic system, using triphenylphosphine in combination with TMSI, was found by Cui et al.522 Several disaccharides were prepared from perbenzylated galactal and glucal, by using 5 mol % of the catalyst. The catalyst was furthermore found to outperform triphenylphosphine in combination with the acids: BF3·OEt2, TfOH, TMSOTf, and FeCl3 in product selectivity, hence avoiding the undesired Ferrier reaction. 2-Deoxy glycosyl iodides were suggested as the actual glycosyl donor under these conditions, which seems to be equivalent to the TPHP procedure described above. In parallel with the work by Bolitt et al.,519 Sabesan and Neira studied the use of cation-exchange resins and acids.523 The resin was dehydrated using MeCN, which caused it to shrink considerably. Surprisingly, the dehydrated resins did not activate the peracetylated glucal and neither the 2-deoxy glycoside nor the Ferrier product could be observed. However, the addition of LiBr was found to catalyze the reaction, whereas LiCl or LiI did not, which could be due to solubility problems. MeCN was found to be the best solvent, but acetone, THF, nitromethane, and chloroform did also work. The reactions were again found to be highly α-selective, and HBr was probably the actual catalyst (i.e., the reaction is similar to the one using TPHP). Excess of LiBr was added to the reactions, which is questioning whether the reaction indeed is catalytic. Pechamuthu and Vankar described the use of CAN (2 mol %) for the tetrahydropyranylation of alcohols.524 A single example of a disaccharide synthesis using a perbenzylated galactal and a 6-OH acceptor was included, but both the yield and the anomeric selectivity were modest (52%, α/β = 2:1). Nitric acid was suggested to be the actual catalyst. A somewhat different approach to the synthesis of 2-deoxy sugars came from McDonald and Wu, who synthesized a L-oliose trisaccharide via a combination of acid-catalyzed addition to glycals, followed by an alkynol cycloisomerization, to give another glycal ready for the next glycosylation.525 TBS groups used a protective group AO


Chemical Reviews

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Scheme 86. Rhenium(V)-Catalyzed Activation of Galactal by the Toste Group

Scheme 87. Mechanistic Investigation with a Deuterium-Labeled Glucal Donor

Scheme 88. Pd(II)-Catalyzed Activation by a Galactal Donor by the Galan Group

Scheme 89. Organocatalyzed Activation of Galactal Donor by Balmond et al.

found to be β-selective (∼4:1) under the reaction conditions, which were similar to the ones reported by Toste and coworkers (Scheme 86).532 Benzene was found to give slightly higher yield than toluene (92% vs 85%), whereas CH2Cl2 and MeCN resulted in low yields (21−31%) but slightly higher βselectivity (∼5:1). Galan and co-workers introduced Pd(II) in combination with monodentate phosphine ligands for the synthesis of 2-deoxy glycosides.534 As expected, perbenzylated D-galactal was giving high α-selectivities (>30:1 α/β) as well as good to excellent yields with sugar-based acceptors (73−84%). D-Glucals and Lrhamnals were also found to be highly α-selective when using bulky protective groups on the 3-O- and 4-O-positions. From screening ligands, N-phenyl-2-(di-tert-butylphosphino)pyrrole was found to perform best in combination with Pd(MeCN)2Cl2 as the catalyst (Scheme 88). In contrast, to the work by Toste, it was found that glycosylation with 2-d-3,4,6-tri-O-benzyl-Dgalactal resulted in an axial deuterium in the product, suggesting a concerted reaction, where the Pd first activates the acceptor, making it a stronger Brønsted acid, rather than a more classic Pd activation of the alkene. Sau and Galan noticed that when the same conditions were applied on glycals having a 3-O-acetyl protective group, the Ferrier reaction took over and the 2,3-unsaturated O-glycosides became the main products.535 Changing the catalytic system to Au(I)([(CF3Ph)3P]AuCl (3 mol %) in combination with AgOTf (6 mol %), the results were similar to the ones obtained with Pd (described above) but

suggested that the AlCl3 shielded the β-side by coordination to the protected hydroxyl groups in the glycal. Unnatural disaccharides have been synthesized in a similar fashion from the exoglycals, using either microwave heating or Lewis acids, by a Ferrier reaction in high yields (70−80%) and with excellent selectivity for the α-glycosides (Scheme 85).530,531 A breakthrough in the synthesis of 2-deoxy glycosides from glycals came with Toste’s introduction of rhenium(V)-catalyzed O-glycosylations (Scheme 86). 532 From a screening, [ReOCl3(SMe2)(Ph3PO)] was chosen as the preferred catalyst. One mol % was required for the glycosylation of glucals or galactals with benzyl, TIPS, acetyl, or acetal protective groups. The galactals were generally giving the highest selectivity toward the α-products, when glycosylating primary and secondary sugar alcohols. The glucals were also α-selective, albeit more dependent on the protective group pattern in the glucal, the more reactive ones gave higher percentage of the βproduct. The yields were in the range of 56−86%. The mechanism was studied by performing the glycosylation with 2-d-3,4,6-tri-O-benzyl-D-glucal. The reaction gave a 3.5:1 mixture of the α- and β-glycosides with a preference (2:1 ratio) for equatorial protonation in both anomers (Scheme 87). This suggested that the protonation of the olefin is not decisive for the selectivity, and the process is therefore not concerted. The method was later used by Zhu and co-workers for the catalytic and stereoselective synthesis of digitoxin and C1′-epidigitoxin.533 6-Deoxy D-allal was used as the glycosyl donor and AP


Chemical Reviews

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Scheme 90. Conditions for Activation of a Nitroglycal by the Schmidt Group

the nucleophile (i.e., the glycosylation is taking place under strongly basic conditions in contrast to most other glycosylation methods), which required acid catalysis. Lemieux et al. were the first to prepare a nitroglycal,541 but almost 30 years passed before this functionality was used for oligosaccharide synthesis by Das and Schmidt.542 As D-galactosamines are common in nature, but highly priced, the use of 2nitrogalactals would be very attractive in oligosaccharide synthesis of natural products and their mimics. It was therefore logical to choose the ready available 2-nitro-D-galactal as the model system. Initially, it was observed that strong bases like KN(SiMe3)2, in equimolar amounts, gave high selectivity for the α-product in approximately 80% yield, when using a 4-OH or 6-OH gluco acceptor. When using DBU as the base with the same reactants, the selectivity dropped and α/β mixtures were obtained. It is not clear from this first work whether the reaction indeed is catalytic. In a follow-up paper, discussing glycosylation of serine and threonine hydroxyl groups, it was, however, stated that 0.1 equiv of t-BuOK was used and resulted in excellent α-selectivity, albeit not in oligosaccharide synthesis.543 In the same context, it was furthermore noticed that the reactivity and stereocontrol in the glycosylation was significantly enhanced when a bulky protective group was installed on the 6-OH position in the nitro galactal.544 Later Schmidt et al. synthesized 1,1-linked oligosaccharides using anomeric O-unprotected sugars and 2-nitrogalactal together with equimolar amounts of t-BuOK (Scheme 90).545 The best yields and selectivities were obtained in a 1:1 mixture of toluene and n-heptane. It was furthermore demonstrated that both the reaction and the anomeric configuration of the nucleophile were under equilibrium control and that the selectivity in the product formation therefore would be time-dependent. Xue et al. found that catalyzing the Michael-type addition by DMAP or 4-(1-pyrrolidino)pyridine (PPY) gave high yield and selectivity toward the β-products (Scheme 91), making their

with shorter reaction times (30:1 α/β) using the (R)-enantiomer. The selectivity dropped to α/β 9:1, when changing to the (S)-enantiomer of the acid. The scope of the reaction was studied, and it was found that the perbenzylated glucal formed significant amounts of the Ferrier product under the conditions but gave the same high αselectivity. Syn addition was proposed to take place in a concerted manner via a cooperative Brønsted acid organocatalysis. Berkessel and co-workers introduced electron-deficient pyridinium salts in organocatalytic glycosylation of glycals.540 The catalysis was studied, and it was found to take place independent of eventual Brønsted acid released under the reaction conditions. The catalysis was instead involving pyridinium cations formed from an addition of the alcohol to the pyridinium salt. This complex was proposed to protonate the glycal, which could then be trapped by another alcohol. Reactions with both glucals and galactals are in most cases excellent, giving only the α-product and ∼90% yield. Generally, the acid-catalyzed addition of alcohols to glycals is an attractive approach for the synthesis of 2-deoxyglycosides. Lewis acids tend to give a higher degree of the undesired Ferrier reaction leading to 2,3-unsaturated glycosides, whereas the direct addition is favored when using Brønsted acids, organocatalysis, and metal catalysis. The reactions are always selective toward the axial product, and this is only moderately influenced by protective group manipulations and reaction conditions.

Scheme 91. β-Selective Glycosylations with Galactal and Glucal Donors

approach complementary to the α-selective procedure by Schmidt.546 Several disaccharides were synthesized using the 2nitrogalactal, as the Michael acceptor, and DMAP as the catalyst (0.15 equiv) in CH2Cl2 at rt. The yields were highest for primary sugar alcohols (87% and 89%), and the βselectivities in the range from 1:6.5 to 1:13. When using a more

Nitroglycals

Nitroglycals are good Michael acceptors and hence interesting for oligosaccharide synthesis, when a sugar alcoholate is used as AQ


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Scheme 92. Thiourea-Catalyst for Galactal Activation Developed by the Galan Group

Schmidt, nonpolar solvents improved the α-selectivity. Reactions with 6-O-unprotected glycosyl acceptors afforded disaccharides in modest to good yields (52−83%) and with high α-selectivity. Interestingly, it was observed that the addition of simple alcohols to the 2-nitro-D-glucal were βselective, suggesting that the 4-O-protective group is responsible for the high α-selectivity obtained with the 2-nitrogalactals. A mechanism was proposed but not studied in detail. 2-Nitroglycals have a great potential as glycosyl donors and especially for the synthesis of D-galactosamines or other less common 2-amino-2-deoxy sugars. Selective methods, have been developed, as discussed above, but several of them are limited, for use in oligosaccharide synthesis, by long reaction times and harsh conditions. The Michael-type addition on the 2nitroglycals have received somewhat more attention for the glycosylation of simpler glycosides and glycoconjugates.549,550 There is certainly still room for development in this relative young area in catalytic glycosylation chemistry.

hindered 4-OH glucoside, as the nucleophile, the selectivity shifted to complete α-selectivity, and the yield dropped to 69%. Applying the same conditions for the addition to 2-nitroglucal gave low yields, but the reaction could be improved by changing the base to PPY, which resulted in yields in the seventies and complete β-selectivity for glycosylations on 6-OH sugars. A mechanism explaining the high selectivities obtained was proposed but not studied further. Galan and co-workers547 introduced organocatalysis for the activation of 2-nitrogalactals as an alternative to the basic conditions used by the groups of Schmidt and Yu. To achieve both an activation of the nitro group as well as activation and directing the alcohol, bifunctional organocatalysts of the cinchona/thiourea type were screened. In refluxing CH2Cl2 (48 h), there was not much difference in the obtained selectivities. Of the various catalysts investigated, the high yielding ones all gave close to a 1:1 anomeric ratio. A thioureabased catalyst was however chosen, and it was found that using other solvents gave more of the α-product, with MeCN being the best (Scheme 92). A variety of sugar-based acceptors, with different protective groups, were glycosylated under the optimized conditions, and it was found that acetals, carbamates, esters, and ethers were tolerated. The yields and α-selectivities strongly depended on the nucleophiles used and ranged from 45% to 91% in the best case. The relative high loading of the catalyst and long reaction time in refluxing MeCN seems to be a limitation of the scope using this type of organocatalysts for addition to 2-nitrogalactals. Chen and co-workers recently developed a catalytic system based on N-heterocyclic carbenes for the selective glycosylation of 2-nitrogalactals.548 Several thiazolium and imidazolium salts were tested, and it was found that optimal yields and selectivities were obtained with the imidazolium salt with two bulky 2,6-diisopropylphenyl substituents (Scheme 93). Next, solvents were screened, and similar to the observations by

10. ANHYDRO SUGARS AS DONORS In the early 20th century, there already was interest in 1,2anhydro sugars as glycosylating agents. Peracetylated 1,2anhydro glucose, known as Brigl’s anhydride,551 was used for noncatalyzed, thermal glycosylations with simple alcohols as the acceptors.552−554 Later, Lemieux utilized this for the synthesis of maltose,555 trehalose,556 and sucrose.557 Anhydro sugars have also been employed as the starting material in acid-catalyzed polymerizations,558−562 but both the thermal, noncatalyzed glycosylations and the polymerization chemistry is outside the scope of this review. This section will instead focus on catalytic activation of anhydro sugars as glycosyl donors (Figure 9). Some examples of activation of 1,6-anhydro sugars for acetylprotection with TMSOTf563 and Sc(OTf)3564 and thioglycoside formation with ZnI2565 indicates potential, although these methods have not been employed for catalytic oligosaccharide synthesis. Solid acids have also been employed in glycosylations of 1,2-anhydro glucose with simple alcohols.566 Danishefsky and co-workers led the development of the 1,2-anhydro sugars as glycosyl donors promoted by ZnCl2,567−570 ZnBr2,571 or Zn(OTf)2,572,573 and Waldmann later used LiClO4 as a promoter in glycosylations of 1,2-anhydro sugars,336 but these methods have not yet been used for catalytic glycosylations in oligosaccharide synthesis. The first instance of catalytic activation of 1,2-anhydro sugars was reported by Hindsgaul and co-workers.574 They used 0.2 equiv of TfOH to activate a perbenzylated 1,2-anhydro glucose in the presence of a 6-OH sugar acceptor in THF, yielding the desired glycosides in 24−47% yield of the α- and β-anomer, respectively. BF3·OEt2575 and SiO2566 have also been reported as catalytic activators for 1,2-anhydro sugars but only with simple noncarbohydrate acceptors.

Scheme 93. Imidazolium Salt As Catalytic Activator of a Nitrogalactal

AR


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Figure 9. Scope of this section in blue.

reaction when secondary sugar acceptors were used. AgOTf and Ph3PAuCl were both found unsuitable as catalysts. The use of a boronic acid catalyst in 1,2-anhydro sugar activation was developed by the Toshima group in 2015.577 It was found that a boronic acid catalyst would react with a partially unprotected glycosyl acceptor, leading to formation of a boronic ester that would perform as the actual catalyst in the glycosylations. Furthermore, it was found that the glycosylations of this novel method would lead to a very high selectivity for 1,2-cis-glycosides as well as being highly regioselective for one of the alcohol functionalities of the acceptor depending on the parent sugar (Scheme 95). A series of glycosylations was performed with acceptors of gluco and manno stereochemistry yielding exclusively α(1,4)glycosides in yields of 72−92% as the products. Only with a glucosamine acceptor, 14% of the undesired α(1,6)-product was formed together with a 77% yield of the α(1,4)-glycoside. Using acceptors with galacto stereochemistry, the α(1,6)glycosides were the only products obtained. Other cis-diol acceptors such as a 2,3-unprotected mannosyl acceptor and a 3,4-unprotected galactosyl acceptor were also used in glycosylations, yielding only α(1,3)- and α(1,4) [albeit with 7% of the α(1,3) product] glycosides, respectively, in 65−70% yields, underlining the generality of the procedure. By introducing a p-nitro group instead of the methoxysubstituent in the boronic acid catalyst, the same principle of

The Yu group reported catalytic glycosylations on 1,2anhydro donors using an Au(I) catalyst with very high βselectivity using various sugar acceptors (Scheme 94).576 Scheme 94. Gold-Catalyzed Activation of Anhydro Sugar by the Yu Group

The Au(I) catalyst was also shown to provide superior yields to the previous ZnCl2- and LiClO4-promoted glycosylations and tolerated glucosamine acceptors. Both glucose- and galactose-derived 1,2-anhydro glycosyl donors were investigated, and it was found that the galatosyl donors, albeit generally favoring the formation of 1,2-trans-glycosides, expressed lower stereoselectivity. Ph3PAuNTf2 could also function as a catalyst, although this led to lower yields or no

Scheme 95. Boronic Acid-Catalyzed, Regio- And Stereoselective Glycosylation by the Toshima Group

AS


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regio- and stereoselective glycosylation was applied to βselective mannosylations by Nishi et al (Figure 10).578

furthermore been employed successfully by Aoyama581 and Taylor582−586 in the glycosylation of glycosyl halides and mesylates. Organoboron compounds were also employed recently to give high regio- and stereoselectivity with thioglycoside donors,587,588 which gives an indication of the big interest in this type of organocatalyst. 1,2-Cyclopropanyl Glycosyl Donors

1,2-Cyclopropanated sugars can be regarded as structurally equivalent to 1,2-anhydro sugars. These sugar derivatives are useful substrates for the synthesis of 2-C-functionalized sugars and oligosaccharides. In 1998, [Pt(C2H4)Cl2]2, known as Ziese’s dimer, was used as a catalyst for the activation of 1,2cyclopropanated sugars by Beyer and Madsen (Scheme 98).589 This method was then utilized for the synthesis of some disaccharides in 50−77% yield and high α-stereoselectivity.590 Similarly, the Shao group have done some thorough work on 1,2-cyclopropaneacetyl donors yielding 2-C-acetylmethyl glycosides.591−593 Twenty mol % of many Brønsted and Lewis acids have been employed in the catalytic activation of these glycosyl donors, yielding disaccharides and trisaccharides routinely in excess of 80% yield, thus presenting a very robust method for 2C-branched oligosaccharides. The synthesis of 2-C-functionalized carbohydrates has recently been reviewed by Yin and Linker.594

Figure 10. Stereoselective trends for two boronic acid catalysts investigated by the Toshima group.

It was found that MeCN served as the best solvent for these glycosylations with various acceptors, leading to yields of 75− 91% with various acceptors, again showing high stereo- and regioselectivity in the glycosylations, although lowered selectivity was encountered with a 3,4-cis diol of galacto stereochemistry (Scheme 96). Another variant of this glycosylation strategy has been used by Tanaka et al. in the synthesis of β-mannosides,579 which involved a modified boronic acid catalyst. This new procedure only involved mono-ol acceptors, and thus regioselectivity was not investigated. Various sugar acceptors were employed giving virtually quantitative yields with some primary acceptors, whereas more hindered sugar acceptors were found to be more difficult to glycosylate (Scheme 97). This was overcome by instead using a full equivalent of the boronic acid catalyst and 5 Å mol sieves at 0 °C, which increased the yield with the 3-OH glycosyl acceptor to 85% and to 61% with the 4-OH rhamnoside. Interestingly, this method does not require a pretreatment of the acceptor with the catalyst in refluxing toluene prior to glycosylation, thus arguably presenting a simpler protocol. However, a very significant solvent-effect was encountered during the optimization when it was found that using toluene or CH2Cl2 as the solvent would lead to exclusive formation of the undesired 1,2-trans-glycoside, suggesting a very important involvement of MeCN. The same reaction protocol was later used by Tanaka et al. to synthesize 1,2-cis-glycosides from 1,2-anhydro glucosyl and -galactosyl donors in THF.580 A wide variety of sugar acceptors were investigated and led to exclusive formation of the desired 1,2cis-glycosides in 75−99% yields, whereas two sterically demanding acceptors only yielded 38 and 59%, respectively. The ability to use donors of opposite C-2 configuration underlines the robustness of the procedure. Boronic acids have

11. PHOSPHORUS-BASED GLYCOSYL DONORS Over the years, a wide range of different phosphorus-based glycosyl donors have been developed. These donors can be split into two groups depending on the oxidation state of the central phosphorus atom: the phosphite glycosyl donors in oxidation state (III) and the phosphate glycosyl donors that are in oxidation state (V). Generally, phosphites are considered more reactive than glycosyl phosphates based on extensive experimental comparison of these donors.595−598 This section will go through both classes of donors and describe the many derivatives of each category. 11.1. Phosphate-Derived Glycosyl Donors

Glycosyl dimethylphosphinothioate donors were introduced by Inazu and co-workers in 1985,599 drawing inspiration from the use of this type of leaving group in peptide synthesis.600,601 It was concluded that silver perchlorate was an efficient activator of these novel glycosyl donors, leading to disaccharides in excess of 69%, although a stoichiometric amount of the promoter was essential for the glycosylations to take place. Inazu and co-workers have since investigated dimethylphos-

Scheme 96. Regioselective β-Mannosylations

AT


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Scheme 97. β-Selective Mannosylations with mono-ol Acceptors

activated phosphate derivatives. Examples of catalytic activation will be elucidated in the text. Catalytic glycosylations using phosphate-derived glycosyl donors have been reported in certain cases, where the acceptor itself is capable of participating in the catalytic cycle. Seeberger reported621 the use of TMS-protected glycosyl acceptors with a dibutylphosphate donor, resulting in a catalytic cycle with as little as 1 mol % TfOH to facilitate the glycosylation. The catalytic cycle was possible since TMSOTf would be liberated upon glycosylation. Wada and co-workers have also reported the use of boranophosphates as glycosyl donors626 in a very similar fashion by using a trityl ether as acceptor and 0.57 equiv of triflic acid as the catalyst, thus continuously leading to the formation of trityl cations as the active catalyst. Singh and co-workers introduced glycosyl propane-1,3-diyl phosphate as a novel glycosyl donor in 1999.622,627 During the synthesis of fucosidase substrates, it was shown that 0.2 equiv of TMSOTf was sufficient to activate the phosphate donor with an aromatic alcohol as the acceptor, whereas glycosylations with a 2-OH galactopyranose acceptor required 1.5 equiv of TMSOTf, which facilitated only the β-glycoside in 56% yield.627,628 The selectivity of this particular glycosylation was noteworthy since there was no participation from the 2-OH protecting group. Singh and Vankayalapati have investigated the effect of catalyst loading on the yield and stereoselectivity of mannosylations employing the propane-1,3-diyl phosphate donors.629 They found that using 1.5 equiv of TMSOTf would lead to higher yields but exclusive formation of the αmannoside, whereas 0.2 equiv would lead to a mixture of anomers, although primarily favoring the α-glycoside.629 It was also shown that a β-mannopyranoside would anomerize to the corresponding α-glycoside under the reaction conditions. The

Scheme 98. First Instance of Catalytic Activation of 1,2Cyclopropanated Glycosyl Donor by Beyer and Madsen

phinothioates very thoroughly as glycosyl donors and found that iodine in combination with trityl perchlorate602,603 was a very efficient activator for these glycosyl donors. Trityl salts such as TrClO4, TrBF4, and TrSnCl5 have since proven to be good catalysts for the activation of the dimethylphosphinothioate donors.602,604,605 Years after that, in 2006, Yamanoi and Inazu et al. found use of the dimethylphosphinothioate leaving group in glycosylations with 1-C alkylated glycosyl donors.606 These glycosyl donors were very reactive due to the formation of a more substituted cation upon departure of the leaving group, thus enabling 0.05 to 0.1 equiv of various trityl salts to catalyze glycosylations. 0.1 Equiv of TrClO4 was sufficient to catalyze the coupling of the glycosyl donor with a 6-OH glycosyl acceptor in 82% yield as a 7:3 ratio of α/β anomers. Since the introduction of phosphorus-derivatives as glycosyl donors by Inazu and co-workers,599 a wide range of different derivatives these donors have been developed (Table 4). Although none of the examples included in the table have been used in catalytic glycosylations of alcohols, there may be potential for this due to the similarity to the catalytically

Table 4. Overview of the Early Development of Phosphate Glycosyl Donors That Have Not Yet Been Reported As Catalytically Activated for the Glycosylation of Alcohols year

promoter(s)

ref

phosphorodithioate (multiple derivatives) P,P-diphenyl-N-(p-toluenesulfonyl)phosphinimidate N,N,N′,N′-tetramethylphosphoramidate N-phenyl-N′-diisopropyl-phosphoramidate dimethylthiophosphate diethylphosphordithioate

leaving group

Michalska et al. Ikegami et al.

author(s)

1988 1990

RO− TMSOTf, BF3·OEt2

607−609 610 and 611

Ikegami et al. Zhao, Landry et al. Yu, Hui et al. Plante, Seeberger

1991 1997 1998 1998

612−617 618 619 620 and 621

diphenylphosphinate difluorophosphate dibutylphosphate

Singh et al. Neda et al. Seeberger et al.

1998 1999 1999

TMSOTf, BF3·OEt2 TMSOTf, BF3·OEt2 HgCl2, NIS, MeOTf, AgOTf, TMSOTf NIS, NIS/TfOH, I(coll)2ClO4, AgF, AgClO4, MeOTf, DMTST TMSOTf AgClO4 TMSOTf

AU

622 and 623 624 625


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on 4,6-O-benzylidene protected 2-azido galactosyl donors by the same group,634 resulting in a lower yield of 63% and partial hydrolysis of the benzylidene, prompting the use of a full equivalent of TMSOTf instead to avoid this hydrolysis. In 2013, it was reported by the Schmidt group that a glycosyl bis(p-nitrophenyl) phosphate was catalytically activated by a combination of bis(p-nitrophenyl) phosphoric acid and a thiourea catalyst.635 However, only isopropyl alcohol was used as the acceptor, which gave rise to an approximately 90% yield but a 1:1 mixture of anomers.

same results were found for arabinofuranosyl donors, giving rise to higher yields and a preference for the thermodynamic products when using a stoichiometric amount of TMSOTf compared to when a catalytic amount was employed.630 The catalytic activation of the propane-1,3-diyl phosphate donor was also reported for the synthesis of C-glycosides.631 Catalytic activation of glycosyl phosphate donors was reported in 2008 by Hashimoto and co-workers, who claimed to be the first group to do so,152 although a single example of ZnCl2/AgClO4 in combination was reported as a catalyst for dimethylphosphate by the Watanabe group in 1997.632 They found that a catalytic amount of TMSOTf or HClO4 was sufficient to activate a glycosyl diphenylphosphate donor in the presence of a protic acceptor. Previously, other researchers had used stoichiometric amounts of activators when investigating the same donors.372,613,621,633 This new method of glycosylation proved very stereoselective for 1,2-cis-glucosides independent of the anomeric configuration of the glycosyl donor, typically with yields in excess of 90%. Since a 0.1 M solution of HClO4 in dioxane was commercially available and this catalyst gave promising results, the Hashimoto group decided to explore the scope of this procedure.152 Various sugar acceptors, including 4-OH and 6-OH, cholesterol, and adamantine were coupled with the phosphate donor using 0.05 equiv of HClO4 in a 1:1 dioxane/Et2O solvent mixture to yield the desired glycosides in excess of 80% yield in less than 30 min with approximately 90:10 α/β-stereoselectivity (Scheme 99).

11.2. Glycosyl Phosphite Donors

The phosphite glycosyl donors were introduced concurrently by the Schmidt636,637 and Wong595,596 groups in the early 1990’s. Both developed the phosphite leaving group for efficient sialylations under TMSOTf catalysis but with subtle differences: the Schmidt group reported the use of a diethyl phosphite,636,637 whereas the Wong group utilized dibenzyl phosphite.595,596 Schmidt and Martin formed a disaccharide with 0.1 equiv TMSOTf giving rise to a yield of 70% and 38% with 6-OH and 4-OH acceptors, respectively.636 The Wong group reported comparable sialylations with 78−80% yield595 and later reported the application of phosphite donors on glucosamine-, glucuronic acid-, fucose-, and lactose-derived glycosyl donors, albeit glycosylations requiring a higher TMSOTf-loading in certain cases.638,639 The Wong group also identified an unexpected phosphonate as a byproduct during the glycosylations with dibenzyl phosphite donors if the donor and catalyst was mixed before addition of the acceptor.639 The Schmidt group have developed a series of diverse sialyl phosphite donors, varying in P-substitution, including cyclic variants.597 From this screening, it was concluded that the diethyl phosphite was a superior glycosyl donor compared to the other analogues investigated. In a comparative study640 by the Schmidt group, phosphites were compared to the more popular trichloroacetimidate7 donors. It was found that both TMSOTf and BF3·OEt2 were suitable catalysts for the activation of phosphite donors, although it was concluded that the trichloroacetimidate method was superior in regard to glycosylation yields.640 This lower reactivity was, however, exploited with more reactive glycosyl donors such as deoxy sugars and ketose-derived donors that had previously not synergized well with trichloroacetimidate donors.258 Thus, reactive glycosyl donors derived from fructofuranose and 2deoxysugars were coupled with various sugar acceptors catalyzed by TMSOTf, BF3·OEt2 and Sn(OTf)2 at low temperatures. Notably, a 2-deoxyglucosyl donor was glycosylated with a 4-OH acceptor under Sn(OTf)2-catalysis in a 97% yield as a 3:1 α/β mixture of anomers. This outstanding performance with very reactive glycosyl donors was later confirmed by the Hashimoto group, who found this method very useful to obtain 2-deoxy-β-glycosidic linkages in high yields and selectivity.641 It was later established by the Hashimoto group that TMSOTf is generally a more active

Scheme 99. HClO4-Catalyzed Glycosylations with a Glycosyl Phosphate Donor

The optimized reaction conditions also facilitated glycosylations on galactosyl donors with equally promising results prompting the synthesis of galatosylceramide KRN7000, which proceeded with the desired stereoselectivity and a yield of 92%, although 0.5 equiv of the catalyst was needed for the ceramide coupling.152 Interestingly, it was investigated whether the strong α-selectivity from galacto- and glucosyl donors was due to anomerization of the glycoside. It was, however, established that the reaction was under kinetic control, and thus a βglycoside was found to not anomerize to the α-glycoside, even under reflux in the presence of a stoichiometric amount of HClO4. HClO4 has since been shown to be less efficient when

Figure 11. Variations of P-substitution of catalytically activated glycosyl phosphite donors. AV


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catalyst in the activation of diethylphosphite donors than BF3· OEt2.598,642 The Sulikowski group has shown that a one-pot synthesis of a 2-deoxy-β-oligosaccharide is possible by introducing a less reactive, cyclic pinacol phosphite glycosyl donor (Figure 11),643 in combination with a diethyl phosphite.644 In this procedure, a 2-deoxyglucosyl diethyl phosphite donor could be activated in the presence of the latent cyclic phosphite at −100 °C by 10 mol % of TMSOTf. After 10 min of stirring, a second acceptor would be added, leading to the desired trisaccharide with two βglycosidic bonds in a 50% yield. It has also been shown by the Veeneman group that a bicyclic phosphite donor derived from 1,2-cyclopentadiol could be catalytically activated by TfOH.645 This donor was used for sialylations; however, no sugar acceptors were employed. Dimethyl phosphite glycosyl donors were introduced by the Watanabe group in 1993646 and were shortly after shown to be catalytically activated by a 1:2 ratio of ZnCl2 and AgClO4 in the presence of a 4-OH sugar acceptor to yield a disaccharide in 86% yield.632,647 ZnCl2 had previously been used as a stoichiometric activator on the same system.646,648 The Watanabe showed that for reactive donors such as sialyl phosphites, a catalytic amount of ZnCl2/AgClO4 was sufficient, although a stoichiometric amount of the promoter was necessary in some glycosylations, depending on the reactivity of the donors. Solutions of a range of other metal perchlorate salts were later shown to be good activators of glycosyl phosphites by Schene and Waldmann; however, the stoichiometry of these glycosylations was not reported, but the perchlorate salts were very likely added in excess.649,650 The Toshima group has conducted a thorough investigation on the use of a heterogeneous solid acid, montmorillonite K-10, as a catalyst in glycosylations with glycosyl phosphites.651−654 Typically, between 20 and 100 wt % of the solid acid was added to the reaction mixture to facilitate β-selective glycosylations of 2-deoxysugars,651 glucopyranosyl donors,652 and mannopyranosyl donors653 in high yields. The solid acid could then be recovered and reused in multiple glycosylations, although this would usually result in a lowering in the yield of 6−24% after four recycles.129,652,654 Toshima and co-workers compared the reactivity and selectivity to a 0.3 mol % loading of the more common acids, TMSOTf and TfOH, and found it similar to the results obtained by using 50 wt % of montmorillonite K-10; however, a much higher degree of hydrolysis of the benzylidene protecting group was found in the case of the more common, homogeneous acid catalysts compared to montmorillonite.653 The Toshima group have also investigated the use of various protic acids such as HBF4, HOTf, and HNTf2 as catalytic activators of diethylphosphite donors in ionic liquids.129 It was found that HNTf2 was the superior catalyst in C6mim[NTf2] leading to glycosylations in yields of 84−99%, although a glycosylation with a 4-OH sugar acceptor only gave rise to a 63% yield and a loss of stereoselectivity. The C6mim[NTf2] ionic liquid was furthermore recycled 5 times, leading to just a 2% decrease in glycosylation yield.129

Formation of Oxazoline Glycosyl Donors

In 1957, Micheel and co-workers managed to isolate the hydrobromide salt of an N-benzoyl oxazoline-derivative obtained from the treatment of 1,3,4,6-tetra-O-acetyl-2benzoylamido-2-amino-2-deoxy-β-D-glucopyranoside with hydrogen bromide,655,656 followed by treatment with pyridine. Osawa also reported a range of oxazoline-derivatives with various N-substituents obtained from the chlorination of Nacyl-1,3,4,6-tetra-O-acetyl-β-D-glucosamines in the presence of either aluminum chloride, titanium chloride, or with HCl and acetic anhydride.657 Osawa noted that the more electronegative the substituent on nitrogen was, the faster the conversion into the corresponding glucosyl chloride, which indicated that these derivatives possessed more oxocarbenium ion character at C-1, which was essential for the formation of the glycosyl chloride. Also, it was recognized,657,658 that only the β-chloride and not the α-chloride would lead to the formation of an oxazoline intermediate, indicating anchimeric assistance from the 2-amido group. The formation of the oxazolinium ion-intermediate has been found to be even more pronounced in the case of glycosyl bromides, where the two compounds are in a solventdependent equilibrium.659 Many other methods of preparing oxazoline glycosyl donors has also been established; Wolfrom and Winkley transformed an N-acetyl ethyl 1-thioglucofuranoside into the corresponding 2-methyl oxazoline derivative by treatment with chlorine followed by silver carbonate.660 An excess of zinc chloride in acetic anhydride was found by Fletcher Jr. and co-workers to procedure the matching oxazoline derivatives from N-acyl gluco-, manno-, and galactopyranoses.661,662 Khorlin et al. have also published a general method of synthesizing oxazolines by converting a glycosyl chloride into the corresponding oxazoline by addition of a silver salt, typically silver nitrate, silver perchlorate, or silver tosylate, in the presence of a base such as pyridine or collidine.663,664 Stannic chloride has also been shown to facilitate a similar conversion.665 Lemieux and Driguez prepared oxazoline donors by a modified procedure349 in which the corresponding glycosyl chloride was subjected to halide ion catalysis348,666 in the presence of a base. 1-Propenyl glycosides have furthermore been effectively converted into the corresponding oxazolines by HgO and HgCl2 in acetonitrile by Anderson and co-workers.667 Kiso and Anderson subsequently reported the use of FeCl3 in the formation of oxazolines from anomeric acetates.668,669 Anderson and Kiso noted that the oxazoline donors were also activated under these conditions, but Seibel and co-workers670 have later recounted that FeCl3 could not activate a tetra-Oacetyl oxazoline donor in the presence of a serine acceptor under microwave heating, which indicates certain limitations to this procedure. Other procedures involve the reaction of either TMSOTf or TfOH with a peracetylated 2-amino-sugar,671 which yields the oxazoline glycosyl donor giving rise to yields in excess of 95% and reaction times as short as half an hour, showcasing the βacetate as a great substrate for oxazoline formation. De Castro and Marzabadi synthesized glucosamine-derived oxazoline donors from a glucal.672 This was accomplished by first preparing a 2-iodoamide and then treating it with sodium hydride, which yielded different ratios of N- and O-oxazolines, depending on temperature and the addition of 15-crown-5 as an additive.

12. OXAZOLINE GLYCOSYL DONORS Oxazoline derivatives of 2-amino-2-deoxysugars have been frequently observed as intermediates in glycosylations involving 2-amino-2-deoxyglycosyl donors. This section will solely focus on examples from the literature, in which the oxazoline derivatives have been isolated and used as glycosyl donors. AW


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Activation of Oxazoline Glycosyl Donors

This was in contrast to the typical 1,2-trans product observed from oxazoline donors due to neighboring group participation from the amide functionality, perhaps suggesting a more SN1like reaction or rapid anomerization of the glycoside. The findings by Khorlin and co-workers were also later confirmed by Jeanloz and co-workers.694,700 In 1975, it was found by Lemieux and Driguez, that triflic acid was a superior catalyst to p-toluenesulfonic acid, reportedly leading to approximately double as high yields (nearly 50% versus nearly 25%) in the glycosylation of a tetra-O-acetylglucose-derived oxazoline donor.349 It was also observed by Lemieux and Driguez that the formation of the undesired 1,2cis glycoside was increasingly significant at higher concentrations of triflic acid, thus indicating anomerization of the initial 1,2-trans glycoside. Warren and Jeanloz later reported difficulties in reproducing the good results with triflic acid, when p-toluenesulfonic acid performed much better in activating very similar oxazoline glycosyl donors, giving an insight into the perhaps unpredictable and complex behavior of different oxazoline donor/acceptor combinations.692 Ogawa et al. were the first14,701 to introduce trimethylsilyl triflate as a catalyst in glycoside synthesis, when activating both orthoesters, esters, and oxazoline donors in the presence of a catalytic amount of trimethylsilyl triflate.457 This reaction was carried out in the presence of 1,1,3,3-tetramethylurea as an acid scavenger to yield 1,2-trans glycosides under mild conditions. Thus, a disaccharide could be obtained from a peracetylated oxazoline donor and a 6-OH mannosyl acceptor in a 78% yield. The exact amount of the catalyst was, however, not reported by Ogawa at al.,457 but it was stated by the authors to be catalytic. Fe(OTf)3 as a DMSO complex has been shown in a few examples to catalyze the glycosylation of oxazoline donors under microwave heating, although not resulting in very promising yields.495,702 Other metal salts such as CuBr2 and CuCl2 have also been used in stoichiometric amounts to activate oxazoline donors.489 Crasto and Jones have shown that ytterbium triflate is a strong catalyst in the activation of oxazoline glycosyl donors.703 A very low catalyst loading of 5 mol % would lead to formation of the desired glycoside within hours, whereas the optimized procedure involved 30 mol % of the catalyst. This procedure yielded both O- and N-glycosides in 75−89% yields within 24 h in refluxing CH2Cl2. Interestingly, it was also found that addition of triethylamine as an acid scavenger would terminate the reaction, which indicated that the presence of triflic acid generated from the active catalyst might be of significance in the reaction mechanism. The Yb(OTf)3-catalyzed reaction proved to be very feasible for scale-up, facilitating synthesis on a 5 g scale.703 Various derivatives of the most common N-acetyl and Nbenzoyl oxazolines have been synthesized, with quite different N-substituents. Palacios Albarran and co-workers have reported the synthesis of 2-alkylamine- and 2-arylamine-substituted oxazolines704 by the stannic chloride procedure developed by Srivastava.665 2,2,2-Trichloroethyloxy- and isobutyloxy-derivatives of oxazoline donors have also been developed by Pertel and co-workers.678 Especially the trichloroethoxy-derivative proved to be an efficient glycosyl donor under mild conditions, leading to 1,2-trans disaccharides in yields of 57−70% using sym-collidinium perchlorate as the catalyst. A trichloromethyl oxazoline derivative was synthesized by Jacquinet and co-workers,705 drawing inspiration from the trichloroacetimidate7 method, but only a stoichiometric

It was established early by Micheel and co-workers that the hydrogen bromide salts of the oxazoline donors were efficient glycosyl donors in the presence of simple alcohols,656,658,673 a 6-OH sugar acceptor,674 or amino acids.658,673 Konstas et al. and others introduced dilute HCl as a catalyst in the activation of an N-benzoyl oxazoline glycosyl donor in the presence of methanol.675−677 It was also established in the same account by Konstas that addition of water under acidic conditions would cause the N-acetyl group to migrate to C-1, leaving the free ammonium ion on C-2, a reaction that has also been reported by others.678 p-Toluenesulfonic acid (TsOH) was introduced as a catalyst for an N-benzoyl oxazoline donor by Wolfrom and Winkley in 1966,660 leading to selective formation of the 1,2-trans glycoside under neat conditions at 110−120 °C. The use of a catalytic amount of TsOH became a very frequent method of activating oxazoline donors in the following years, typically at elevated temperatures in refluxing toluene or nitromethane.661−663,679−694 These conditions were generally applicable in many cases, but in the absence of an acceptor, the oxazoline donors have been reported to isomerize into 2acetamido-glycals by elimination of H-1.662 The elimination leading to the undesired byproduct can also be prevalent in glycosylations with poor nucleophiles.695 A catalytic amount of TsOH-like acids such as sulfuric acid679 and camphor sulfonic acid 696 (CSA) has also been shown to catalyze the glycosylations. It was found by Bundle and co-workers that while CSA and TsOH were good catalysts with lower, simple alcohol acceptors such as allyl alcohol and benzyl alcohol, more complicated and less nucleophilic alcohols typically required one equiv of the acid to avoid very slow reactions.696 Interestingly, it has been reported by Hashimoto and coworkers697 that even a stoichiometric amount of bis(trifluoromethane)sulfonimide (Tf2NH) was not capable of activating an oxazoline donor at −78 °C, although NMR-data revealed that the oxazoline donor was protonated under the reaction conditions. Salo and Fletcher Jr. reported the use of an acidic phosphate that acted both as the activator and acceptor in glycosylations during the synthesis of a sugar nucleotide from a mannopyranosyl donor.698 Similar experiments were carried out by Khorlin and co-workers on glucopyranosyl donors with diphenyl phosphate which yielded a 1,2-cis glucosyl phosphate (Scheme 100).699 Scheme 100. Formation of the Unexpected 1,2-cis-Glycosyl Phosphate from an Oxazoline Glycosyl Donor

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amount of TMSOTf was used in activating these donors.705,706 Nifantiev and co-workers have attempted glycosylations on similar oxazoline derivatives707 but also failed to activate these unless one equiv of TsOH was used as a promoter, yielding a trisaccharide in 66% yield from a peracetylated glycosyl donor. It was Donohoe and co-workers who first reported the catalytic activation of a trichloromethyl oxazoline derivative, when 20 mol % of TMSOTf was used as the catalyst in the presence of 1.2 equiv of a 2-OH, 4-OH, or 6-OH sugar acceptor to yield some rare disaccharides, all in excess of 80% yields.708 Various glycosyl donors were employed, ranging from peracetylated allo-, talo-, and altro stereochemistry, suggesting an attractive procedure for synthesis of 1,2-trans glycosides from these glycosyl donor types. Vauzeilles and co-workers recently reported the isolation of a trifluoromethyl oxazoline glycosyl donor that reacted with trimethylsilyl azide to give a glycosyl azide in 82% yield.709

13. GLYCOSYL CARBONATE AND CARBAMATE DONORS In 1998, a glycosyl phenyl carbonate was reported as a novel, catalytically activated glycosyl donor by the Mukaiyama group.720 Previously, the group had investigated this donor in a noncatalytic method for α-selective sialylations.721 For catalytic glycosylations, a methyl- and a phenyl-carbonate were investigated, the latter seemingly undergoing irreversible activation caused by the evolution of carbon dioxide during glycosylation.720 In this way, a series of glycosylations were conducted using 0.1 equiv of TrB(C5F6)4 as the catalyst in a 5:1 mixture of t-BuCN/CH2Cl2, resembling conditions previously described by the Mukaiyama group on other donors,111,114−117 to yield disaccharides with β-stereoselectivity better than α/β 6:94. This method was then employed in a one-pot synthesis of a trisaccharide by the Mukaiyama group,722 in which the catalytic glycosylation with the carbonate donor yielded a (1,6)disaccharide in 97% yield as a 2:98 α/β mixture (Scheme 101). During the one-pot glycosylation strategy, it was also found that using 4-chlorobenyl protection groups would increase the yield of the glycosylations.722 A perbenzoylated phenyl carbonate donor has been found too unreactive for activation with TrB(C5F6)4, thus no reaction took place at −40 °C, even with 30 mol % of the catalyst, necessitating room temperature for the glycosylation of a 6-OH sugar acceptor to be completed within 12 h.723 The solution to this was employing a very reactive, superarmed724 2-acyl-protected sugar with benzyl groups on the 3-, 4-, and 6-OH. This protection group pattern was later used with the phenylcarbonate donor in a one-pot synthesis of a mucin-related antigen by the Mukaiyama group.725 5 Mol % of TfOH has also been shown to activate the phenyl carbamate glycosyl donors, although this only gave rise to a 61% yield as a 72:28 mixture of α/β anomers of a (1,6)-disaccharide.333 In 2001, the Kiessling group introduced glycosyl sulfonyl carbamates as novel, catalytically activated glycosyl donors with tunable reactivity since the functionality of the sulfonamide functionality was manipulatable.726 Thus, four different Nalkylations on the sulfonamide were investigated, which gave rise to alternating reactivity. It was found that the methyl- and cyanomethyl-substituted carbamates were catalytically activated in the presence of 0.1 equiv TMSOTf, giving rise to yields of disaccharides of 53−82% and with α-selectivity of α/β 2:1 to 5:1. This is an attractive method of glycosylation since the reactivity of the donor can be fine-tuned depending on the synthetic requirements, but despite this, it has not been employed in the synthesis of oligosaccharides since. The Vankar group drew inspiration from Kiessling and coworkers and introduced trichloroacetylcarbamates as catalytically activated glycosyl donors in 2005.727 It was found that both gluco-, manno-, and galactosyl donors were catalytically activated by 0.1 equiv TMSOTf to give disaccharides with 3-

Enzyme-Catalyzed Glycosylations with Oxazoline Glycosyl Donors

Although oxazoline glycosyl donors have not received a lot of attention for chemical glycosylations in recent years, there has been considerable interest in employing oxazolines for enzymecatalyzed glycosylations (Figure 12).

Figure 12. General differences between chemical and enzymatic catalysis in oxazoline glycosyl donor activation.

An early report by Kobayashi showed an oxazoline-derived glycosyl donor being recognized by Chitinase from Bacillus sp. in the synthesis of artificial chitin710 and other, more general glycosylations711 with various glycosyl donors. Later, the use of bovine testicular hyaluronidase (BTH) and ovine testicular hyaluronidase (OTH) was reported by the same group712,713 to synthesize artificial hyaluronan and condroitin. Enzyme catalyzed glycosylations have since then gained significant interest and has, consequently, been reviewed in detail in recent years.714−719 It provides an easy and protecting group-free synthesis of very complex glycoproteins that are of great interest but is outside the scope of this particular review. Hence, we suggest the interested reader to investigate the reviews cited above for a thorough introduction to this field of research.

Scheme 101. β-Selective Glycosylation Using the Glycosyl Carbonate Method Developed by the Mukaiyama Group

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Scheme 102. Activation of a Thioglycoside with a Substoichiometric Amount of Activator

Scheme 103. Bismuth(V) as a Novel Activator of Thioglycosides

with the development of milder activation conditions, where especially the introduction of N-iodosuccinimide in combination with acid catalysts paved the way.738−740 Since this introduction of iodonium-based promoters, the field developed rapidly and several other promoter systems where introduced, which has recently been reviewed by Yu.741 With the increased use of thioglycosides for oligosaccharide synthesis, the interest in finding methods for a catalytic glycosylation increased as this could solve the problem with scalability and limit the use of strongly electrophilic reagents and heavy metals. Ito and Ogawa came up with a conceptually interesting idea for activating thioglycosides for O-glycoside bond formation.742 In their approach the acceptor carries a sulfonate ester, which is activated by the catalyst, TMSOTf; this then works as the promoter and activates the glycosyl donor. As the acceptor was present in a slight excess and reacts in a one-to-one ratio with the donor, the reaction is not considered catalytic. The first report claiming a catalytic glycosylation with thioglycosides came from Uchiro and Mukaiyama.743 In their method, TrB(C6F5)4 was used in combination with oxidants, where NaIO4 was the only one used in substoichiometric amounts, down to 30 mol % (Scheme 102). The preferred donors were methyl and ethyl thioglycosides. As this oxidant presumably can oxidize more than one sulfur, it can be questioned whether the reaction was in fact catalytic. Despite the impressive yields and selectivities obtained with perbenzylated glucosyl and galactosyl donors, the method has not found wide application in oligosaccharide synthesis. A variation of the method using DDQ instead of NaIO4 as the oxidant came shortly after.460 It was found that using 55 mol % DDQ in combination with 10 mol % I2 and 5 mol % TrB(C6F5)4 was the minimum requirement for a complete reaction, and hence, whether the system is catalytic is doubtful. It took more than a decade before the interest in finding a catalytic activation of thioglycosides was investigated again. Pohl and co-workers studied bismuth(V)-based reagents as the catalyst for the glycosylation.744 The rationale behind choosing bismuth was its resemblance with the thiophilic metals lead and mercury, which had already been used as promoters for activating thioglycosides since the very beginning. An advantage of bismuth should also be its lower toxicity and price. Surprisingly, there has been only a few reports of using bismuth-based reagents in carbohydrate chemistry and only one reagent [i.e., bismuth(III) triflate] had been used for glycosylation of a S-benzoxazolyl (SBox) donor.745,746 In these examples, it was found that the thioimidates (SBox) were activated in the presence of thioethers. To study the properties of bismuth(V) compounds as thiophilic reagents for glycosylations, Pohl and co-workers synthesized Ph3Bi(OTf)2, which turned out to be soluble in organic solvents and able to

OH, 4-OH, and 6-OH glycosyl acceptors in 57−94% yields, thus demonstrating a wide scope of both donors and acceptors. The selectivity was dependent on reaction temperature, with βglycosides generally favored at low temperatures, whereas αglycosides were predominant at rt. N-Alkylation of the donors, similar to the work by Kiessling,726 was found to lower the reactivity of the donors dramatically. O̅ mura and co-workers further developed the trichloroacetylcarbamate donors339 and demonstrated that 20 mol % TMSOTf in MeCN would lead to a β-stereoselective glycosylation, whereas 20 mol % TMSClO4 in Et2O would lead to an α-selective glycosylation, favoring the desired anomer in excess of 4:1. Very interestingly, it was found that these reactions would be noncatalytic in the absence of 5 Å molecular sieves, but the underlying reason for this was not investigated.339,728 5 Å Molecular sieves have also been reported as a superior additive in the activation of glycosyl fluorides338 and TCA donors.205 The trichloroacetylcarbamate donors have also been shown to function as donors in a onepot dehydrative approach in which the donor is formed in situ from a 1-OH sugar and activated in the presence of an acceptor without purification of the glycosyl donor.728 The trichloroacetylcarbamates were later used in one-pot sequential glycosylations; however, these reactions added a stoichiometric amount of catalyst.485 Glycosylations have also been done via a decarboxylative approach in which the glycosyl donor and acceptor are covalently tethered by a carbonate linker. Ishido established that these mixed carbonates can be selectively decarboxylated by heat,729,730 and Ikegami later advanced the reaction with various catalysts at lowered temperatures,731,732 to yield the desired glycosides. This concept has since gained some attention733 but is outside the scope of this review and will not be discussed in detail. Also, the O’Doherty group have investigated Pd(II)-catalyzed glycosylations with OBoc pyranone donors734−736 that will not be discussed in detail in this review. Generally, it seems that the use of both carbamate- and carbonate-derived glycosyl donors have received surprisingly little interest in regard to catalytic activation, especially when considering the structural similarity to the more widely used imidates and esters. Thus, this class of glycosyl donors could still house some potential for the future.

14. THIOGLYCOSIDES Thioglycosides are one of the most popular donor types for small-scale glycosylations. They were introduced as a glycosyl donor in the early 1970’s by Ferrier et al., who used mercury(II) salts as the promoter.737 It was however not until the 1990’s that this donor type got a break-through and became one of the most frequently used donors and intermediate to other donor types. The breakthrough came AZ


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Scheme 104. Activation of a Glycosyl Thiocyanate Donor

Scheme 105. Protective Group Influence on the Stereochemical Outcome of the Glycosylation with a Thiocyanate

activate a thioglycoside.744 A number of relatively reactive propyl thioglycosides were synthesized and exploited as donors, first with a stoichiometric amount of the bismuth reagent. Varying the amount of this revealed that 0.5 equiv was enough to maintain a high yield and full conversion of the donor. The method was also tested by using a more demanding 4-OH acceptor (Scheme 103). The activation using bismuth salts was studied in more detail, and it was found that only bismuth in oxidation state V resulted in an effective activation. Combinations of other bismuth(III) compounds and TfOH did lead to anomerization and hydrolysis upon standing but not an effective glycosylation of allyl alcohol. The activation of propyl thioglycosides was studied in more detail in a follow-up paper, and it was revealed that an anomerization of the β-propyl thioglycosides took place during the reaction course.747 The α-donor was however not observed to anomerize, and it was found that this reacted faster. On the basis of calculations, it was suggested that an oxocarbenium ion was the intermediate in this anomerization. Interestingly, some of the other bismuth promoters in combination with TfOH also led to anomerization as discussed above, and this could explain why substoichiometric amounts of reagent could be used. The catalytic cycle is not further discussed in the mechanistic paper747 and not clear from the mechanic proposal748 and hence remains somewhat unclear whether the Ph3Bi(OTf)2 or another species formed from it remains intact or is generated under the reaction conditions. Later work using this promoter was done with an excess.749 In 2016, a paper by Sureshan and co-workers described the catalytic activation of thioglycosides using a gold(III) catalyst.750 The protocol used 3−5 mol % Au(III)Cl3, or Au(III)Br3, without any copromoters and was illustrated with a wide substrate scope covering a range of donors, both tolyl, phenyl, and ethyl thioglycosides coupling to a wide variety of acceptors, including sugar-based. A mechanism for the catalytic cycle was proposed by the authors but not studied in detail. A part of the proposed mechanism was the formation of a disulfide from the thiols. This oxidation without an oxidant is not accounted for. Soon after the publication a correction appeared stating that 0.8 equiv of the gold(III) catalyst is in fact needed, putting into doubt whether the reaction indeed is catalytic or not.751 An interesting upcoming approach for the activation of thioglycosides is to use single electron transfer (SET). The methods are not strictly catalytic as light or electrochemical activation is required. An early and original example of using

light in combination with metal ligand charge complexes, in catalytic amounts, came from Bowers and co-workers.201 Interestingly, the reaction did not require stoichiometric amount of reagents or additive. The yields and scope for oligosaccharide synthesis was somewhat narrow as the yields and selectivities were low. The mechanism was studied in more detail, and it was found that hydrogen was formed during the reaction and that the conversion of the thioglycoside only took place when light was present. Several other approaches, albeit far from catalytic, as excess of other additive and electrolytes are required, have been presented over the years, and this area has recently been reviewed.752 With these few examples proposing catalytic activation of thioglycosides, this area remains underdeveloped, and at present, it seems like only substoichiometric amounts of the promoter has been found to be effective for glycosylation, whereas a real catalytic system is yet to be discovered.

15. THIOCYANATES Kochetkov et al. introduced thiocyanate as a leaving group in the late 1980’s.753 It was found that a protected glucosyl thiocyanate could be activated by trimethylphenylium perchlorate and reacted with tritylated glycosyl acceptors with a high degree of stereospecificity. The challenging 1,2-cisglucosylation was hence possible under catalytic conditions, when a nonparticipating group was installed on O-2. Interestingly, it was observed that having a participating group resulted in mixtures and hence suggested a direct substitution of the thiocyanate moiety, and the authors therefore proposed a SN2 type of reaction with tritylium as the catalyst. A side reaction under the reaction conditions was the rearrangement to the isothiocyanates, also in a stereospecific manner, to the 1,2-cis (Scheme 104).754 The method was also applied for mannosylation reactions but with no selectivity.755 The thiocyanate method was later modified to use TMSOTf (0.2 equiv) as the catalyst and alcohols as acceptors in the presence of 4 Å molecular sieves.756 The high degree of stereospecificity of the 1,2-cis selective glycosylation was demonstrated by a trityl-thiocyanate polycondensation to give α-1,6-D-glucan fragments.757 Despite the promising results using thiocynates, the method has not become widely applicable. One reason could be their synthesis via the corresponding glycosyl bromide and the use of tritylated acceptors, which limits their usefulness compared with modern catalytic glycosylation protocols. Demchenko and co-workers returned to thiocyanate donors and compared them with the corresponding thioimidates and thioglycosides, albeit using BA


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conditions. Not only is the environmental agenda an increasingly important matter but also as the field of carbohydrate research continues evolving, so does the industrial importance where cost efficiency is essential. These factors seem to be the driving force in the development of catalytic oligosaccharide synthesis and are likely to be the primary motivation for years to come. Incredibly creative methods of controlling glycosidic bond formation have been developed in the field of carbohydrate chemistry. Many new leaving groups and catalysts have been introduced, especially in recent years, often with the primary goal of developing a universal glycosylation protocol that fits most common glycosylation reactions. It seems that there still is significant interest from the scientific community for these versatile methods, despite 140 years of failure to even produce one that is universal. The ideal method of glycosidic bond formation has already been developed by nature, relying on selective enzymes to catalyze the formation of oligosaccharides in water without protective groups. A synthetic approach inspired by enzymatic glycosylations is not unrealistic but requires a tailor-made method optimized for every single type of glycosylation. With more specialized procedures developed, one could imagine a catalogue of suitable reaction conditions for a specific coupling rather than one solution for all. This could even be relatively simple methods utilizing common, commercially available chemicals, which would enable even nonspecialists to perform selective glycosylations. Still it seems that despite the development of such tailormade glycosylation protocols have not been a major focus in carbohydrate chemistry; chemists have to some extent, maybe unconsciously, worked in this direction. Examples of these more specialized procedures could be the most common donor for direct β-mannosylations is by introducing a 4,6-Obenzylidine protective group, which has been shown to be inherently β-selective.249,488 α-Galactosylations are routinely obtained by bulky 4,6-O-tethers, such as 4,6-di-tert-butylsilylene-tethering of the galactosyl donor.768,769 1,2-trans-Glycosylations are most commonly obtained by neighboring group participation. The use of solvent effects is also omnipresent in the literature. These methods have arguably gained popularity simply because they work, most often without the need for a prior optimization of various parameters that is not only timeconsuming but also typically requires a background as a carbohydrate chemist. The methods for catalytic glycosylation presented in this review still represent great value and are important contributions to the field of carbohydrate chemistry. Many of them express high yields and selectivities but often with a limited number of specific substrates. The above selected highlights of methods for selective glycosylation, that could be referred to as “privileged glycosylations”, might be worth more attention, thus

noncatalytic conditions in order to compare the donors inbetween.758 An interesting observation was that when using acceptors with different protective groups, the selectivity in the catalytic glycosylation with the thiocyanate donor dropped from α/β > 25:1 to 8.3:1 with acylated and benzylated acceptors, respectively. The observation that the presumably more nucleophilic benzylated acceptor gives a less stereospecific glycosylation does not support the earlier proposed SN2 type of mechanism (Scheme 105).

16. OTHER GLYCOSYL DONOR TYPES Diaziridines have been investigated in a catalyst-free glycosylation by Vasella and co-workers759−761 and have generally received considerable attention, but these donors are outside the scope of this review (Scheme 106). Scheme 106. Conceptual Outline of the Diaziridine Glycosyl Donor Developed by the Vasella Group

4,6-Dimethoxy-1,3,5-trazin-2-yl glycosides have been investigated as glycosyl donors by Tanaka et al.762 These were found to be catalytically activated in the presence of 0.1 equiv of various metal catalysts, generally leading to yields in excess of 90%. This procedure has however not been used in the preparation of oligosaccharides but only employed with simple alcohols as acceptors such as MeOH and EtOH that were also used as solvent for the reactions. This glycosyl donor has however received considerable attention within the field of enzymatic glycosylations.763−766 The Schmidt group have investigated a wide range of heteroaryl glycosides as glycosyl donors.767 During this screening, 11 novel glycosyl donors were investigated as catalytic glycosyl donors. As stated by the authors, a catalytic amount of either TMSOTf or BF3·OEt2 was used; however, the exact amount is not stated in the paper.767 The glycosyl donors were all compared to the corresponding TCA donors, and it was found that the tetrafluoropyridin-4-yl glycosyl donor (Scheme 107) was the most efficient of the new donors, rivaling the yields obtained from the TCA donor. This glycosyl donor has, however, not yet been employed in oligosaccharide synthesis.

17. CONCLUSION AND FUTURE DIRECTION Conclusion and Future Direction

Generally, it seems that the development of carbohydrate chemistry is moving gradually more towards catalytic activation

Scheme 107. Example of a Glycosylation with the Tetrafluoropyridin-4-yl Glycosyl Donor

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(6) Hsu, C.-H.; Hung, S.-C.; Wu, C.-Y.; Wong, C.-H. Toward Automated Oligosaccharide Synthesis. Angew. Chem., Int. Ed. 2011, 50, 11872−11923. (7) Schmidt, R. R.; Michel, J. Facile Synthesis of α- and β-O-Glycosyl Imidates; Preparation of Glycosides and Disaccharides. Angew. Chem., Int. Ed. Engl. 1980, 19, 731−732. (8) Mukaiyama, T.; Murai, Y.; Shoda, S. An Efficient Method for Glucosylation of Hydroxy Compounds Using Glucopyranosyl Fluoride. Chem. Lett. 1981, 10, 431−432. (9) Yu, B.; Tao, H. Glycosyl Trifluoroacetimidates. Part 1: Preparation and Application as New Glycosyl Donors. Tetrahedron Lett. 2001, 42, 2405−2407. (10) Pougny, J.-R.; Sinaÿ, P. Reaction D’imidates de Glucopyranosyle Avec L’acetonitrile. Applications Synthetiques. Tetrahedron Lett. 1976, 17, 4073−4076. (11) Pougny, J. R.; Jacquinet, J. C.; Nassr, M.; Duchet, D.; Milat, M. L.; Sinay, P. A Novel Synthesis of 1,2-Cis-Disaccharides. J. Am. Chem. Soc. 1977, 99, 6762−6763. (12) Szymoniak, J.; Sinay, P. A Novel Synthesis of 2′-Deoxy-αDisaccharides. Tetrahedron Lett. 1979, 20, 545−548. (13) Pougny, J.-R.; Nassr, M. A. M.; Naulet, N.; Sinaÿ, P. A Novel Glucosidation Reaction. Application to the Synthesis of α-Linked Disaccharides. Nouv. J. Chim. 1978, 2, 389. (14) Paulsen, H. Advances in Selective Chemical Syntheses of Complex Oligosaccharides. Angew. Chem., Int. Ed. Engl. 1982, 21, 155−173. (15) Schmidt, R. R. New Methods for the Synthesis of Glycosides and Oligosaccharides - Are There Alternatives to the Koenigs-Knorr Method? Angew. Chem., Int. Ed. Engl. 1986, 25, 212−235. (16) Roger, R.; Neilson, D. G. The Chemistry of Imidates. Chem. Rev. 1961, 61, 179−211. (17) Nef, J. U. Ueber Das Zweiwerthige Kohlenstoffatom. Die Chemie Des Cyans Und Des Isocyans. Liebigs Ann. 1895, 287, 265− 359. (18) Schaefer, F. C.; Peters, G. A. Base-Catalyzed Reaction of Nitriles with Alcohols. A Convenient Route to Imidates and Amidine Salts. J. Org. Chem. 1961, 26, 412−418. (19) Neilson, D. G. Imidates Including Cyclic Imidates. In Amidines and Imidates; Patai, S., Ed.; John Wiley & Sons, Ltd., 1975; pp 385− 489. (20) Steinkopf, W. Ü ber Trichloracetimido-Methyläther. Ber. Dtsch. Chem. Ges. 1907, 40, 1643−1646. (21) Schmidt, R. R.; Michel, J. Direct O-Glycosyl Trichloroacetimidate Formation, Nucleophilicity of the Anomeric Oxygen Atom. Tetrahedron Lett. 1984, 25, 821−824. (22) Hartigan, R. H.; Cloke, J. B. The Thermal and Hydrolytic Behavior of Imido and Thioimido Ester Salts1. J. Am. Chem. Soc. 1945, 67, 709−715. (23) Han, X.-B.; Jiang, Z.-H.; Schmidt, R. R. Glycosyl Imidates, 61. − Synthesis of the Hexasaccharide Moiety of the Saponin Holotoxin A. Liebigs Ann. 1993, 1993, 853−858. (24) Urban, F. J.; Moore, B. S.; Breitenbach, R. Synthesis of Tigogenyl β-O-Cellobioside Heptaacetate and Glycoside Tetraacetate via Schmidt’s Trichloroacetimidate Method; Some New Observatons. Tetrahedron Lett. 1990, 31, 4421−4424. (25) Patil, V. J. A Simple Access to Trichloroacetimidates. Tetrahedron Lett. 1996, 37, 1481−1484. (26) Grundler, G.; Schmidt, R. R. Glycosylimidate, 13. Anwendung Des Trichloracetimidat-Verfahrens Auf 2-Azidoglucose- Und 2Azidogalactose-Derivate. Liebigs Ann. 1984, 1984, 1826−1847. (27) Wang, Z.-G.; Douglas, S. P.; Krepinsky, J. J. Polymer-Supported Syntheses of Oligosaccharides: Using Dibutylboron Triflate to Promote Glycosylations with Glycosyl Trichloroacetimidates. Tetrahedron Lett. 1996, 37, 6985−6988. (28) Peng, W.; Sun, J.; Lin, F.; Han, X.; Yu, B. Facile Synthesis of Ginsenoside Ro. Synlett 2004, 2004, 259−262. (29) Wegmann, B.; Schmidt, R. R. The Application of the Trichloroacetimidate Method to the Synthesis of α-D-Gluco- and αD-Galactopyranosides. J. Carbohydr. Chem. 1987, 6, 357−375.

making these more obvious choices for other nonspecialist chemists. Thus, it seems that the field of carbohydrate chemistry is at a crossroad: either to pursue for a universal glycosylation strategy or instead start focusing more on ideal conditions for one particular type of coupling reaction. Candidates for such ideal conditions are presented throughout this review and generally in the literature but may be given too little attention due to the poor results when changing reactants. Perhaps such a catalogue of proven, easily accessible methods could already be constructed based on the current bank of knowledge found in the literature. No matter which direction the future takes us, catalysis will lead the way.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Christian Marcus Pedersen: 0000-0003-4661-4895 Notes

The authors declare no competing financial interest. Biographies Michael Martin Nielsen received his BSc from the University of Copenhagen under the supervision of Assoc. Prof. Christian Marcus Pedersen in 2016 for which he received an award for best bachelor’s thesis of 2016 at The Department of Chemistry. He is currently enrolled as a Ph.D. student with the same supervisor aiming to develop new methods of catalytic glycosylation and will obtain his Ph.D. degree in 2021. During his studies, he has conducted mechanistic studies on glycosylations using low-temperature NMR, worked with C−Si bond formation on carbohydrates and the synthesis of N-glycosides. He has a wide-ranging interest in organic chemistry and hopes to expand his current knowledge to a deeper understanding of silicon chemistry but is also keen on working with advanced automation for either mechanistic studies or upscaling, such as flow-techniques. Christian Marcus Pedersen received his Ph.D. in 2007 from Aarhus University under the supervision of Prof. Mikael Bols. During his Ph.D. studies, he was a visiting scientist at University of Illinois at Chicago (UIC) performing research with Professor David Crich. Postdoctoral studies were carried out at University of Konstanz with Professor Richard R. Schmidt working on the total synthesis of lipoteichoic acid from Streptococcus pneumoniae. He has received PIFI guest professorships at the CAS Institute in Taiyuan, China, in 2015 and 2018. He is currently associate professor at the University of Copenhagen, where his group is working on new methods for catalytic glycosylation, oligosaccharide synthesis, and the transformation of simple carbohydrates from biomass into industrial chemicals.

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