Regio- and Stereoselective Synthesis of 1,2-cis-Glycosides by

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Regio- and Stereoselective Synthesis of 1,2-cis-Glycosides by Anomeric O‑Alkylation with Organoboron Catalysis Sanae Izumi, Yusuke Kobayashi, and Yoshiji Takemoto* Graduate School of Pharmaceutical Sciences, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan

Org. Lett. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/11/19. For personal use only.

S Supporting Information *

ABSTRACT: Regio- and stereoselective synthesis of 1,2-cisglycosides has been achieved by catalytic anomeric O-alkylation using organoboron compounds. Modulating steric and electronic factors of both catalysts and substrates enables activation of the axially oriented anomeric oxygens of glucose-derived dialkoxyborates. The mild reaction conditions allow broad functionalgroup tolerance. This approach can be applied to the efficient sequential synthesis of oligosaccharides.

C

Scheme 1. Strategies for Stereoselective Synthesis of Glycosides

hemical synthesis of oligosaccharides has attracted much attention for medical applications because carbohydrates are common in natural products and play important roles in numerous biological processes such as cell−cell communication, pathogen recognition, and immune response.1 Because of the structural diversity and complexity of glycoconjugates, efficient access to a sufficient quantity of pure and structurally well-defined carbohydrates is crucial to investigate their biological functions.2 However, complete regio- and stereoselective glycosylation is still difficult because transient oxocarbenium intermediates often cause a severe decrease in the stereoselectivity of the resulting anomeric isomers.3 To date, various synthetic strategies to control the anomeric α/βconfiguration have been developed. The neighboring-group participation of 2-O-acyl groups is a reliable method for transselective glycosylation (Scheme 1a, R2 = Ac).4 In contrast, cisselective glycosylation remains challenging,5 and various strategies for cis-selectivity such as the use of chiral auxiliary groups at the C-2 position (Scheme 1a, R2 = CHPhCH2SPh),6 O-picoloyl groups,7 and O-(o-tosylamide)benzyl groups8 have been developed. Whereas a range of substrate-controlled methods for cis-selective glycosylation including intramolecular approaches9 and conformational restriction control10 have been reported, effort has also been devoted to developing new catalytic systems.11 For example, the regio- and cis-selective glycosylation of a 1,2-anhydroglycosyl donor has been achieved using glycosyl-acceptor-derived boronic12 or borinic13 ester catalysts. O-Alkylation of anomeric hydroxy groups, in which oxocarbenium cations are not formed, is another approach for stereoselective synthesis of glycosides (Scheme 1b). Based on the high nucleophilicity of β-oxide anions of hexoses, the reaction generally provides kinetically controlled products such as 1,2-trans-β-glucosides.14a,b,15 Therefore, it seems difficult to © XXXX American Chemical Society

apply this protocol to the synthesis of 1,2-cis-α-glucosides. Furthermore, catalytic anomeric O-alkylation to form oligosaccharides has not been achieved to date because of the requirement of a stoichiometric amount of a strong base15,16 or organotin reagent.14 Recently, boronic acids or borinic acids have been used as effective catalysts for not only stereo- or Received: November 29, 2018

A

DOI: 10.1021/acs.orglett.8b03823 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters regioselective protection of 1,2- and 1,3-diols17b−d,18,19 but also regioselective glycosylation.12,17a,e,f,20 Although the functionalization of equatorial OH groups in hexoses has been achieved with high selectivity, few reports describe the predominant alkylation of axial OH groups because of their weaker nucleophilicity than that of equatorial groups.12a Herein, we report a catalytic anomeric O-alkylation of 1,2-dihydroxyglucoses 1 via borate complexes with borinic acids, providing 1,2cis-α-glucosides through predominant activation of axially oriented anomeric oxygens (Scheme 1c). We initially examined the reaction of diol 1a and triflate 2a in the presence of 10 mol % of commercially available phenylboronic acid 5 and 4 Å molecular sieves in MeCN (Table 1).21 As expected, no 1,2-trans-product was detected,

afforded high regioselectivity (cis-3aa/4aa = 12, entry 9). In all cases, no trans-product was obtained irrespective of the organoboron catalyst used (entries 1−9). Optimization of the base revealed that i-Pr2NEt was the best choice; a less bulky base like Et3N completely inhibited the reaction because of its high nucleophilicity toward 2a (entry 10). Stronger bases such as DBU and Cs2CO3 produced trans-3aa, which led us to investigate the formation of trans-3aa as a major product (entries 13−16). Use of Cs2CO3 as the base and 1,2dichloroethane as the solvent in the absence of catalyst completely changed the selectivity, furnishing trans-3aa exclusively.16h These results clearly demonstrate the importance of borinic acids in regio- and stereoselective O-alkylation. Using the optimized reaction conditions, we investigated the substrate scope of glucose-derived diols 1b−h in the reaction with triflate 2a (Table 2). Although 6g showed excellent

Table 1. Screening of Reaction Conditions

Table 2. Scope of Diol 1a

entry

cat.

base

cis3aaa (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16d

5 6a 7 6b 6c 6d 6e 6f 6g 6e 6e 6e 6e 6e

i-Pr2NEt i-Pr2NEt i-Pr2NEt i-Pr2NEt i-Pr2NEt i-Pr2NEt i-Pr2NEt i-Pr2NEt i-Pr2NEt Et3N PMPe DTBMPf DBU Cs2CO3 Cs2CO3 Cs2CO3

21 60 50 18 42 58 82 89 92 2 79 4 10 72 8 0

trans3aaa (%)

4aaa (%)

cis-3aa/ (trans-3aa + 4aa)b

0 0 0 0 0 0 0 0 0 0 0 0 11 0.6 23 73

31 32 26 20 21 31 11 9 7 0 17 0 0 11 0 0

0.7 1.9 1.9 0.9 2.0 1.9 7.2 9.7 12c a

4.7 0.9 6.3 0.4

Isolated yields.

selectivity, we chose 6e to clarify the effect of protecting groups24 on regioselectivity. The mild catalytic conditions allowed broad functional-group tolerance, including trityl (1b), benzoyl (1c), acetal (1e−g), and silyl groups (1g and 1h). Moreover, the steric hindrance of 6-O-protecting groups dramatically affected the regioselectivity of the reaction. In fact, 6-O-trityl diol 1b increased the yield of byproduct 4ba compared with those obtained using 6-O-benzyl and 6-Obenzoyl diols 1a and 1c. However, 6-O-unprotected derivative 1d gave the desired cis-3da in 94% yield with complete regioselectivity, indicating that the reaction does not always require the protection of OH groups. In contrast, 3-Oprotecting groups should be bulky to achieve high regioselectivity. Although 3-O-unprotected glucose 1e provided 2-O-alkylated adduct 4ea as a major adduct rather than cis-3ea, 1g with a bulky TBDPS group exhibited complete regioselectivity, leading to cis-3ga in 95% yield. The reaction was also applicable to mannose-derived diol 1i and galactose-

a

Yields of isolated products. bDetermined from isolated yields. Determined from 1H NMR spectra of acetylated products. d1,2Dichloroethane, 40 °C, 24 h. ePMP: 1,2,2,6,6-pentamethylpiperidine. f DTBMP: 2,6-di-tert-butyl-4-methylpyridine. c

and the desired product cis-3aa was obtained, albeit in low yield (21%), together with 2-O-alkylated byproduct 4aa (entry 1). To improve the regioselectivity (cis-3aa/4aa), phenylborinic acid 6a and triarylborane 722 were screened; 6a showed catalytic performance superior to that of 7 (entries 2 and 3). Modification of 6a to ortho-substituted 6b23 and parasubstituted 6c and 6d did not improve the selectivity (entries 4−6). Further investigation revealed that tricyclic borinic acid 6e17f gave cis-3aa in 82% yield with high selectivity (cis-3aa/ 4aa = 7.2, entry 7). In addition, the electron-rich catalyst 6g B


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Organic Letters

configuration. Notably, while catalyst 6e afforded cis-3ag in only 53% yield, the same reaction with catalyst 6g raised the yield of cis-3ag to 86%. Except for disaccharide cis-3jg derived from 1,2-dihydroxygalactose 1j, branched products cis-3ag−aj and 3ig were obtained as single isomers in good yields. To clarify the reaction mechanism of the catalytic anomeric O-alkylation using borinic acids 6, control experiments were carried out with 2-deoxy and 2-O-protected glucoses 8a−c under the same conditions (eqs 1 and 2). In each case, no O-

derived diol 1j, giving the corresponding products cis-3ia and cis-3ja. As expected, excellent regioselectivity was observed in the case of mannose but not in the case of galactose. Further modification of the C-3 protecting group of 1,2-diol 1j as well as catalyst 6 would be needed to improve the selectivity in this case. As shown in Table 3, the reaction scope with triflates 2b−j was further examined with diols 1a, 1i, and 1j. Primary triflates Table 3. Scope of Triflate 2

alkylation occurred, and the starting materials 8a−c were fully recovered. These results strongly demonstrated the necessity of a 1,2-diol moiety to activate the axially oriented anomeric oxygen. The mild reaction conditions using organoborinic acids allowed broad functional-group tolerance, including a free OH group, which enabled the sequential synthesis of oligosaccharides (Scheme 2a).25 We first explored the transformation of glycoside cis-3da into α(1,6)-linked oligosaccharide 10. The triflation of cis-3da was followed by the borinic acid 6gcatalyzed O-alkylation of 1d to give trisaccharide 9 in 93% yield. One equivalent of diol 1d was sufficient to complete the reaction. The iterative exposure of 9 to the two-step sequence (triflation and O-alkylation) afforded the desired tetrasaccharide 10 as a single isomer. Compared with the previous work in which deprotection of 6-O-protecting groups is needed to prepare α(1,6)-linked glucosyl backbones,26 this mild and straightforward synthetic method represents an efficient alternative. We next applied the anomeric O-alkylation to 1,2,4,6tetrahydroxyglucose 11 to synthesize β(1,6)- and α(1,6)-linked trisaccharide 14 (Scheme 2b). The initial differentiation of 1,2and 4,6-diol units in 11, which was synthesized from commercially available 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose in two steps,27 was the key to the success of the anomeric O-alkylation. Although borinic acids are known to preferentially bind 1,2-diols over 1,3-diols,17d there has been no report on the site-selective protection of glucose-derived tetraols. We thus examined the boron-catalyzed O-alkylation of tetraol 11. The reaction of 11 with borinic acid 6g occurred at the 1-O-position over the 6-O-position to give the desired product 12 in 71% yield, whereas borinic acid 6e gave 12 in 59% yield. Moreover, the remaining 4,6-diol moiety of triol 12 can be functionalized regioselectively. Following Taylor’s procedure, the trans-selective glycosylation17a of triol 12 was carried out with glycosyl bromide 13 in the presence of the same borinic acid 6g to provide trisaccharide 14 in good yield, whereas the chemical yield of 14 decreased markedly without 6g or with phenylborinic acid. Finally, the three-step synthesis of tetrasaccharide 18 bearing β(1,2)- and α(1,6)-glycosyl bonds was attempted by combining anomeric O-alkylation and conventional glycosylation (Scheme 2c). The initial O-alkylation of diol 1g and

2 (2.0 equiv), catalyst (20 mol %), 60 °C, 72 h. b2 (2.0 equiv), 6g (20 mol %), rt, 48 h. c2 (2.0 equiv), 6g (20 mol %), 60 °C, 48 h.

a

2b−f with various kinds of protecting groups provided the desired products cis-3ab−3af, 3ib, and 3id in good to excellent yields. Furthermore, not only primary triflates but also secondary triflates 2g−j successfully underwent the anomeric O-alkylation with diol 1a in the presence of 20 mol % of catalyst 6g, resulting in the corresponding α(1,4)- and α(1,3)linked products cis-3ag−aj with complete inversion of C


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anomeric fluoride of product 16 remained intact under the reaction conditions. The obtained trisaccharide 16 was then converted into oligosaccharide 18 in 55% yield by treatment with alcohol 17 in the presence of Cp2HfCl2 and AgClO4.28 The reaction took place in an α-selective manner, isolating the target molecule as a single product in only three steps. In conclusion, we have developed a regio- and stereoselective synthetic route to 1,2-cis-glycosides by anomeric Oalkylation using tricyclic borinic acids. The novelty of this method is activating axially oriented anomeric oxygens diastereoselectively by modulating steric and electronic factors of both catalysts and substrates. The mild reaction conditions provide broad functional-group tolerance and enable efficient sequential synthesis of oligosaccharides. Studies on the further scope expansion and synthetic applications of this route are currently underway in our laboratory.

Scheme 2. Efficient Synthesis of Oligosaccharides

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03823. Experimental details; methods and results; characterization data (PDF) 1 H, 13C, and 2D NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yusuke Kobayashi: 0000-0003-3074-7378 Yoshiji Takemoto: 0000-0003-1375-3821 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant No. 16H06384. We gratefully acknowledge the JSPS Research Fellowship for Young Scientists (S.I., JSPS KAKENHI Grant No. 17J08174).

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REFERENCES

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Regio- and Stereoselective Synthesis of 1,2-cis-Glycosides by

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