Article pubs.acs.org/joc
Cite This: J. Org. Chem. 2018, 83, 1259−1277
Synthesis of Double-Modified Xyloside Analogues for Probing the β4GalT7 Active Site Daniel Willén, Dennis Bengtsson, Sebastian Clementson, Emil Tykesson, Sophie Manner, and Ulf Ellervik* Centre for Analysis and Synthesis, Centre for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden S Supporting Information *
ABSTRACT: Monosubstituted naphthoxylosides have been shown to function as substrates for, and inhibitors of, the enzyme β4GalT7, a key enzyme in the biosynthetic pathway leading to glycosaminoglycans and proteoglycans. In this article, we explore the synthesis of 16 xyloside analogues, modified at two different positions, as well as their function as inhibitors of and/or substrates for the enzyme. Seemingly simple compounds turned out to require complex synthetic pathways. A meta-analysis of the synthetic work shows that, regardless of the abundance of methods available for carbohydrate synthesis, even simple modifications can turn out to be problematic, and double modifications present additional challenges due to conformational, steric, and stereoelectronic effects.
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INTRODUCTION Among the three most important classes of biopolymers, i.e., proteins, nucleic acids, and carbohydrates, the latter is the least explored and also the most complicated to sequence and synthesize. However, the last decades of research have emphasized the important roles of carbohydrates for cell−cell interactions as well as interactions with bacteria, fungi, and viruses. Proteoglycans (PG) are large macromolecules that consist of a core protein decorated by large linear, negatively charged, carbohydrate chains called glycosaminoglycans (GAG). The PGs are found on the cell surface as well as in the extracellular matrix (ECM) where they have important roles in the regulation of growth factor signaling, inflammation, angiogenesis, and cell−cell interactions.1−3 The carbohydrate structures of the ECM and the cell surfaces are constantly changing, on a time scale of minutes, depending on the cells’ requirements, e.g., growth signals. β-1,4-Galactosyltransferase 7 (β4GalT7) is a key enzyme in the synthesis of two different classes of GAG chains, i.e., heparan sulfate (HS) and chondroitin/dermatan sulfate (CS/ DS). The first two steps in the synthesis of these GAG chains are xylosylation of a suitable serine residue of a proteoglycan core protein by xylosyltransferase (XT), followed by galactosylation by β4GalT7 (Figure 1a).4 By stepwise additions of another galactose unit and a glucuronic acid (by the enzymes β3GalT6 and GlcAT-I, respectively), a tetrasaccharide linker is formed, which is a branching point for the biosynthesis of HS and CS/DS. It is well-known that simple xylosides, such as 2-naphthyl βD-xylopyranoside (XylNap), can initiate the biosynthesis of © 2017 American Chemical Society
Figure 1. (a) Biosynthesis of the linker tetrasaccharide of HS and CS/ DS PGs. (b) Galactosylation of XylNap (5) to form GalXylNap.
soluble GAG-chains, i.e., structures not connected to a core protein (Figure 1b), and also inhibit the biosynthesis of the natural structures.5−7 We have recently developed assays for measurement of galactosylation by, as well as inhibition of, β4GalT7.8 Furthermore, from several series of xyloside analogues modified in the xylose moiety8,9 as well as by alterations in the aglycon,10 we have developed a pharmacophore model for β4GalT7 (Figure 2). In our previous studies, we concluded that most alterations concerning the hydroxyl groups of the xylose moiety resulted in compounds without the capability to function as substrates for Received: November 6, 2017 Published: December 28, 2017 1259
DOI: 10.1021/acs.joc.7b02809 J. Org. Chem. 2018, 83, 1259−1277
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The Journal of Organic Chemistry
are secondary and equatorial and consequently exhibit a similar nucleophilicity. Despite the similarities between the three hydroxyl groups, there are minor differences in reactivity. In general, for α-xylosides, hydroxyl group O-2 is the more reactive, followed by O-4 and O-3. For β-xylosides, O-4 is the more reactive followed by O-3 and O-2.12 There are, however, contradicting reports regarding the reactivity order.9 Furthermore, xylose exhibits conformational flexibility, mainly between the 4C1-, the 1C4-, and the 2SO-conformations, which not only complicates structure determination but also opens up for new synthetic strategies.8 Although a number of methods have been developed for selective introduction of protective groups such as esters, ethers, and acetals,12 the selectivity is still highly dependent on the anomeric configuration, the aglycon, and the substitution pattern. Furthermore, aglycon and substituents may alter the conformation, which in turn has a large impact on the reactivity, and enable creative protective group strategies.13 To summarize, despite the simplicity of xylose, it is difficult to generalize a synthetic protocol for the synthesis of xyloside analogues. In this project, we have focused on binary combinations of four different modifications of the xylose moiety, i.e., methylation, epimerization, deoxygenation, and fluorination. Methylation of any hydroxyl group does not significantly alter the stereoelectronics of the molecule or induce steric hindrance. Therefore, it can be performed at almost any time during the synthesis. However, deoxyfluorination consists of two distinctive steps, i.e., epimerization and fluorination, and must be carefully planned into the synthetic scheme. The relative stereochemistry of the hydroxyl groups is of outermost importance, and in general terms, cis diols allow for selective protective group manipulations, in contrast to trans diols (Figure 3). Furthermore, a participating equatorial
Figure 2. Pharmacophore model of β4GalT7.
β4GalT7. However, several analogues functioned as moderate inhibitors. Alterations of the aglycon did affect the priming ability, but did not increase inhibition.10 The xylose moiety is thus the prioritized target for design of efficient inhibitors. From our recent investigations,8,11 we conclude that position 3 is of high importance for the binding of the substrate to the active site, and modification of this position generally renders inactive compounds. However, exchange of the hydroxyl group at position 4 for fluoride as well as epimerization gave relatively strong (43 and 52%, respectively) inhibitors of galactosylation by β4GalT7. Highest inhibitory effect, i.e., 64%, was achieved by deoxygenation of position 2. Here we explore a set of seemingly simple xylose derivatives with double modifications (Chart 1) to fine-tune the Chart 1. Overview of Synthetic Targetsa
a
Xyloside analogues, modified in positions 2 and 4. Figure 3. Synthetic considerations.
pharmacophore model for β4GalT7. Our hypothesis is that the combination of modifications in positions 2 and 4 may generate strong inhibitors. Despite the intrinsically low degree of complexity of these monosaccharides, as well as the profound knowledge on carbohydrate synthesis and readily available starting materials with defined stereochemistry, it turned out to be a complicated and time-consuming process. We thus want to discuss the chemistry associated with synthetic modifications of monosaccharides and point to the continuous need for further development of synthetic strategies.
protective group in position 2 is crucial for the stereoselective introduction of aglycon. Thus, the aglycon should preferably be introduced early on in the synthesis. In addition, deoxygenation of position 2 generally renders acid labile compounds.9 Deoxygenation of position 2 alters the electronic properties of the molecule, which leads to increased electron density at the anomeric position and therefore higher rates of hydrolysis.14 Unpublished data from our group indicate that 2-deoxy naphthyl xylosides are also base labile. Alkali sensitivity of phenyl glucosides have been known for a long time,15 and the hydrolysis is envisioned to proceed through an elimination of the neighboring hydrogen, followed by rapid addition of solvent to the unsaturated sugar.
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RESULTS AND DISCUSSION Synthesis. Xylose is a pentopyranoside and thus lacks a primary hydroxyl group. The three remaining hydroxyl groups 1260
DOI: 10.1021/acs.joc.7b02809 J. Org. Chem. 2018, 83, 1259−1277
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The Journal of Organic Chemistry To summarize, in order to facilitate the synthesis of binary modified xyloside analogues, the aglycon should be introduced early on, an axial hydroxyl group in position 4 is favorable, and deoxygenations should be performed late in the synthesis. Generally, starting materials for modified glycosides are provided by the chiral pool. Of the eight aldopentoses, four are disregarded due to unfavorable stereochemistry. The remaining pentoses are D-xylose, D-lyxose, L-arabinose, and L-ribose. Further on, the need for a participating group in position 210 makes D-lyxose and L-ribose less favorable as starting materials. This leaves D-xylose and L-arabinose as the starting materials of choice. Interestingly, D-xylosides can easily be converted to Dlyxosides as well as L-arabinosides by an oxidation−reduction pathway.16 Apart from the chiral pool approach, a de novo synthesis might be practical for some compounds, mainly dideoxy analogues, but less suitable for most other modifications due to the need for asymmetric introduction of stereocenters and/or enantiomeric purification.17 The discussion is divided into sections, first describing the initial protection scheme followed by specific transformations, i.e., methylations, formation of xanthate esters, fluorinations, epimerizations, and finally deoxygenations. An overview of the syntheses can be found in Scheme S1 (Supporting Information). Synthesis of Suitably Protected Starting Materials. As mentioned earlier, there are only minor differences in reactivity between the three equatorial hydroxyl groups in xylopyranosides.12 For example, formation of the isopropylidene acetal by reaction with 2-methoxypropene and catalytic amounts of CSA gave a 10:3 distribution of the 2,3- and 3,4-protected compounds in 67% yield,16 while protection with 2,2,3,3tetramethoxybutane/BF3·OEt2 gave the 2,3- and 3,4-BDAprotected compounds in a 2:3 ratio in quantitative yield.9 Direct methylation of the unprotected xylopyranoside 5 using NaH and MeI gave a complex mixture of mono-, di-, and trisubstituted xylosides, with the starting material still present. An acetylation-separation-deacetylation sequence was required to isolate the final 2,4-methoxy product 2b in 9% yield over three steps (Scheme 1a). However, the simplicity as well as the readily available starting material justified the use of this method. In contrast, the cis diol in lyxose derivatives facilitates the subsequent protection scheme and the intermediate 7 was formed in 96% yield from 6 (Scheme 1b).9 Subsequent methylation to give 8, followed by deprotection using acetic acid, gave 4b. We then intended to use 4b as a starting material for further transformations. However, the introduction of a benzoyl group on the equatorial hydroxyl 3-OH in 4b, using a borinic acid catalyst, surprisingly yielded the 2-O-benzoyl ester 9 as the major product (Scheme 2).18 Benzoylation using the standard method, i.e., BzCl/pyridine, gave a mixture of 9 and the disubstituted 10 in a 1:3 ratio according to 1H NMR (Supporting Information). The reversed selectivity in the benzoyl protection indicates a conformational change during the reaction. Introduction of an orthoester to 4b, followed by mild acid hydrolysis formed 11 in 73%, accompanied by the regioisomer (21%). A crude NMR of the intermediate orthoester revealed a shift from the 4C1 to the 1 C4 conformation, making 3-OH the axial position. Several attempts to introduce an orthoester to unprotected 6 failed. The arabinoside 12 can be synthesized from isopropylideneprotected xyloside by Swern oxidation followed by in situ
Scheme 1. Protection of 2-Naphthyl D-Xyloside and 2Naphthyl D-Lyxosidea
Reagents and reaction conditions: (i) NaH, MeI, DMF, 0 °C to rt; (ii) 2,2-dimethoxypropane, p-TsOH (catalyzed), DMF, rt; (iii) NaH (60% dispersion in mineral oil), MeI, DMF, 0 °C to rt; (iv) 70% AcOH (aq), 70 °C, 0.5 h.
a
Scheme 2. Protection of 2-Naphthyl 4-Methoxy-L-lyxosidea
a
Reagents and reaction conditions: (i) BzCl, DIPEA, 2-aminoethyl diphenylborinate, CH3CN, rt; (ii) BzCl, pyridine, CH2Cl2, 0 °C; (iii) (1) triethylorthoacetate, p-TsOH (catalyzed), CH3CN, (2) 1 M HCl.
reduction using NaBH4 at 0 °C.16 12 can also be synthesized directly from readily available per-O-acetylated arabinose19 (Supporting Information). The cis diol in arabinosides facilitates selective protection. Thus, isopropylidination, using 2,2-dimethoxypropane/p-TsOH yielded the 3,4-protected compound 13 exclusively (Scheme 3), suitable for transformation of position 2-OH.20−22 In order to selectively protect arabinosides, several methods were tested. It is well-known that BDA-acetals preferably are formed over trans diols.9 To our surprise, BDA protection of 12 resulted in a 1:1 mix of the 2,3- and 3,4-protected compounds 14 and 15 in low yields (Supporting Information).23 Instead, attempts were made to selectively protect 3-OH using a borinic acid catalyst.18,24 This gave the 3-O-benzoylated compound 16 in 63% yield. Benzylation using the borinic acid methodology with Ag2O formed a 1:1 mixture of the 3- and 4-benzylated sugars 17 and 18 in modest yields. However, a metal-free version gave solely the 3-O-benzylated sugar 17. The 4-deoxy-L-threo-pentopyranoside 19 was synthesized from XylNap (5) as described before.9 Benzylation of 19 using the borinic acid methodology gave the two monobenzylated compounds 20 and 21 in a 1:1 ratio with no disubstitution (estimated by 1H NMR, Scheme 4, Supporting Information). Attempts to introduce a MOM-ether gave low yields, in spite of 1261
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The Journal of Organic Chemistry Scheme 3. Protection of 2-Naphthyl L-Arabinosidea
separations. This was solved by an acetylation-purificationdeacetylation sequence. The isopropylidene-protected arabinoside 13 was methylated to give 27 in 93% yield (Scheme 5). The isopropylidene acetal Scheme 5. Methylationsa
a
Reagents and reaction conditions: (i) (a) NaH (60% dispersion in mineral oil), DMF, 0 °C, 0.5 h, (b) MeI; (ii) 70% acetic acid in H2O, 60 °C; (iii) TBAB, MeI, CH2Cl2, 0.5 M NaOH.
was deprotected by mild acid hydrolysis in 70% AcOH to give 2d. Monomethylation of the D-threo-pentopyranoside 289 gave a complex mixture of 29, 1b, 30, and recovered starting material (21%). Finally, the 2-deoxy-2-fluoro-D-xyloside 319 was methylated under phase-transfer catalysis (PTC) to give the two regioisomers 32 and 3b, as well as recovered starting material (56%). Formation of Xanthate Esters. The introduction of a methyl xanthate ester on substrates with only one unprotected hydroxyl is generally straightforward and gives the desired product in high yields (33, 35) (Scheme 6). The isopropylidene acetals were then deprotected by mild acid hydrolysis in 70% AcOH to give 34 and 36. Formation of the xanthate ester in 3-OTBDMS-protected 23 gave both of the expected compounds 37 and 38 as an inseparable mixture (2:1) according to 1H NMR (Supporting Information). We speculate in that the unprotected deprotonated hydroxyl causes migration of the TBDMS group, followed by xanthate ester formation. Addition of carbon disulfide prior to deprotonation gave the desired product 37. Removal of the TBDMS group in 37 using alkaline conditions with TBAF in THF resulted in xanthate ester migration and a complex mixture of byproducts. Acidic deprotection with
a Reagents and reaction conditions: (i) 2,2-dimethoxypropane, pTsOH (catalyzed), DMF, rt, 1 h; (ii) 2,2,3,3-tetramethoxybutane, BF3· OEt2 (catalyzed), CH3CN; (iii) 2-aminoethyl diphenylborinate, DIPEA, BzCl, CH3CN, rt, 29 h; (iv) 2-aminoethyl diphenylborinate, BnBr, Ag2O, CH3CN, 40 °C, 30 h; (v) 2-aminoethyl diphenylborinate, BnBr, KI, K2CO3, CH3CN, 60 °C, 22 h.
elevated temperatures and long reaction times. TBDMS protection using TBDMSCl with DMAP/Et3N and microwave heating gave 23, protected in position 3, as the major product (48%), with minor amounts of 22, disubstituted 24, and unreacted starting material (21%). Finally, methylation of 19 using NaH and MeI gave the 2-methoxy product 2a in 42% yield, accompanied by 17% of 26 and 6% of 25. With a suitable set of protected sugars, we then initiated transformations into the target compounds. Methylations. As mentioned before, methylations can be performed at almost any time during the synthesis. The general method for methylation has been Williamson alkylations. However, multiple unprotected hydroxyl groups generally give selectivity problems, resulting in complex product mixtures. These compounds often coelute during chromatographic
Scheme 4. Protection of Naphthyl 4-Deoxy-L-threo-pentopyranosidea
Reagents and reaction conditions: (i) 2-aminoethyl diphenylborinate, BnBr, KI, K2CO3, CH3CN, N2, 60 °C, 12 h; (ii) TBDMSCl, DMAP, Et3N, CH2Cl2, MW 100 °C, 50 min; (iii) MeI, NaH (60% dispersion in mineral oil), DMF, 0 °C to rt, 4 days. a
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The Journal of Organic Chemistry Scheme 6. Formation of Xanthate Estersa
a Reagents and reaction conditions: (i) (1) NaH (60% dispersion in mineral oil), THF, 0 °C to rt, 0.5 h, (2) CS2, (3) MeI; (ii) 70% acetic acid in H2O, 60 °C; (iii) (1) CS2, THF, 0 °C, (2) NaH (60% dispersion in mineral oil), 0 °C to rt, 0.5 h, (3) MeI; (iv) 1 M NaOH, MeOH, CH2Cl2, rt, 1 h; (v) HCl (concentrated, 22 equiv), Pd/C (10 mol %), DMF, H2 (1 atm); (vi) TBAF, THF, 0 °C.
tri-, di-, and monoxanthate products depending on the reaction conditions. Attempts were also made to deliberately introduce a cyclic thionocarbonate, intended for radical deoxygenation. The same reagents as for methyl xanthate ester introduction were used, and a large excess of reagents together with unprotected 12 gave the thionocarbonate (cis only) in approximately 10% yield, in addition to all possible mono-, di-, and trixanthate ester products except for the 2-OH xanthate ester (Supporting Information). Fluorinations. We have previously performed deoxyfluorinations on each one of the three hydroxyl groups in xylosides, using a sequence of hydroxyl group inversion followed by fluorination by diethylaminosulfur trifluoride (DAST).9 The obvious starting material for the synthesis of compound 3d is a suitably protected riboside. The 3,4-isopropylideneprotected riboside 43 (Scheme 13) was found to reside in the 1 C4 conformation, and fluorination, using DAST, gave exclusive rearrangement to the anomeric fluoride 44 (Scheme 8) instead of 45. This kind of rearrangement has been observed earlier.30,31 TLC analysis indicated that the reaction intermediate formed at 0 °C, but rearranged at higher temperatures. No reaction was observed using the alternative reagent XtalFluor-M (activated by Et 3 N·3HF, Et 3 N·2HF, or DBU).32,33 The intermediate formed readily with PyFluor, but the reaction did not proceed into product. All attempts of fluorinations via the triflate were equally unsuccessful (Scheme 8). The relatively mild nucleophilic fluorination reagent triethylamine trihydrofluoride (TREAT) gave no reaction, and the more basic tetrabutylammonium fluoride (TBAF) gave 46 in trace amounts. Instead, 46 could be formed in a good yield using a non-nucleophilic base (DBU). Electrophilic fluorine (SelectFluor) gave only trace amounts of the wanted compound. The next approach was epoxidation of the arabinoside 13 (Scheme 9). Tosylation of the 2-OH in 13, with addition of ZnBr 2 ,34 gave 47 in 96% yield. Deprotection of the isopropylidene acetal gave 48, which was ring-closed to form the 2,3-anhydro-L-riboside 49.
AcOH was not successful either. HCl (1%) in EtOH gave the desired product 3917 in 85% yield. The introduction of bulky substituents in position 2 of arabinosides often results in conformational changes from the 4 C1 conformation to the 1C4 conformation.25−27 Thus, protection of 2-OH as a xanthate (34) resulted in a ring-flip (Scheme 6). Subsequent benzoylation using the borinic acid methodology,gave a 1:5 mixture of the 3- and 4-O-benzoylated compounds 40 and 41 (Scheme 7), according to 1H NMR Scheme 7. Formation of 2-Xanthate-Protected LArabinosidesa
a
Reagents and reaction conditions: (i) BzCl, DIPEA, 2-aminoethyl diphenylborinate, CH3CN, rt; (ii) BzCl, pyridine, −35 °C, 4 h; (iii) triethyl orthoacetate, p-TsOH (catalyzed), CH3CN, rt, 1 h.
(Supporting Information). However, benzoylation using BzCl in pyridine at −42 °C26 gave 3-OBz 40 as the major product accompanied by recovered starting material (24%). However, migration of the benzoyl group to 4-OH occurred during purification by chromatography. Introduction of an orthoester followed by mild acidic conditions gave the 4-O-acetylated compound 42 (Supporting Information).28,29 Most attempts to introduce a single xanthate ester on a starting material with more than one free hydroxyl group gave either no or minor amounts of the target compound, accompanied by varying amounts of cyclic thionocarbonates, 1263
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The Journal of Organic Chemistry Scheme 8. Attempts to Synthesize 3d via Riboside Pathwaya
Scheme 10. Synthesis of 3d via 2-Deoxy-2-fluoroxyloside Pathwaya
a
Reagents and reaction conditions: (i) TBAHSO4, p-TsCl, CH2Cl2/ 5% NaOH; (ii) AcOH/DBU (12:6 equiv), toluene, 90 °C then 0.05 M NaOMe, MeOH, rt.
Scheme 11. Fluorination of Position 2a
Reagents and reaction conditions: (a) (i) DAST, CH2Cl2, 20 °C; (iii) PyFluor, toluene, rt; (iv) Tf2O, pyridine, −42 °C to rt; (v) Et3N·3HF, THF, rt to 40 °C; (vi) DBU, THF, reflux (DBU); (vii) SelectFluor, CH3CN, rt. a
Scheme 9. Attempts to Synthesize 3d via Arabinoside Pathwaya
Reagents and reaction conditions: (i) DAST, CH2Cl2, 40 °C;9 (ii) triethyl orthoacetate, p-TsOH (catalyzed), CH3CN, rt, 1.5 h, then DAST, CH2Cl2, 40 °C; (iii) DAST, CH2Cl2, sealed tube, 50 °C.
a
fluorination of the isopropylidene-protected 5516 gave no reaction, only rearrangement into the anomeric fluoride 56, according to 1H NMR (Supporting Information), whereas the analogous BDA-protected compound 57 gave 40% of the fluorinated compound 58.9 The fluorination reactions gave incomplete conversion, and starting materials were usually recovered. Due to the axial configuration of 4-OH in L-arabinosides, they could serve as starting materials for analogues with fluorine in position 4. Fluorination of the monoprotected arabinoside 16 or 17 gave the corresponding fluoro analogues 59 and 60 in modest yields, leaving 2-OH open for further transformations (Scheme 12). The rather low yields are justified by the synthetic simplicity. In the synthesis toward the 4-deoxy-4-fluoro analogue 1c, the xanthate group was used as a protective group39 that facilitated fluorination, similar to the fluorination of 36. Thus, fluorination of 40 gave the corresponding fluoro analogue 61 in 22% yield together with recovered 40 (47%). The 2-O-methylated arabinoside 2d was fluorinated without additional protecting groups to give the final product 2c in 10% yield. Fluorination of the L-erythropentopyranoside 1d (Scheme 15) with DAST led to fluorination solely in position 3 to give 62 in 31% yield. This suggests that the conformation of the starting material, under these reaction conditions, is skewed in favor of the axial 3-OH, leading to fluorination in position 3 due to better orbital positioning.
a Reagents and reaction conditions: (i) p-TsCl, ZnBr2, pyridine, −40 °C to rt; (ii) 70% AcOH (aq), 12 h; (iii) 0.05 M NaOMe, MeOH, rt; (iv) TBAF, p-TsOH (catalyzed), toluene, 80 °C.
There are several nucleophilic fluoride reagents that have been used to open epoxides, such as TREAT,35 TBAF,36 and potassium hydrogen difluoride (KHF2).37,38 The epoxide 49 was quite inert, and neither fluorination nor naphthol cleavage occurred using treatment with TBAF, Et3N· 3HF, or Et3N·2HF. However, addition of p-TsOH to TBAF in toluene at 80 °C, opened the epoxide to give the 3-fluorinated L-xyloside 50 (Supporting Information). KHF2 at 130 °C in ethylene glycol gave a similar outcome, but with a decreased reaction rate. Finally, the 2-deoxy-2-fluoro xyloside 319 was used as a starting material (Scheme 10). Position 4 was selectively tosylated to give 51 and was subsequently inverted into 2deoxy-2-fluoro-L-arabinoside 52, which was immediately deprotected, to avoid acetate migration, to give 3d in 27% overall yield. Regarding the remaining 2-F compounds, fluorination using DAST on 3-acetyl-4-methoxy-lyxoside 11 gave only 15% of the desired product 53, whereas the analogous xanthate ester 36 gave 54 in 53% yield over two steps (Scheme 11). Interestingly, 1264
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The Journal of Organic Chemistry Scheme 12. Fluorination of Position 4a
a
inversion, i.e., the xylose configuration (64) according to 1H NMR (Supporting Information). Oxidation of 13 to the ketone using Dess−Martin periodinane followed by NaBH4 reduction gave 43 in 60% yield over two steps. Oxidation using TPAP with NMO in acetonitrile resulted in naphthol cleavage. A modified Albright−Goldman oxidation41 under microwave irradiation, followed by reduction by NaBH4, gave 43 in 75% yield over two steps. The isopropylidene acetal was cleaved off by mild acid hydrolysis in 70% AcOH 4d. Attempts to use the Albright−Goldman procedure for inversion of 2-OH of 4-fluorinated xyloside 59 gave an unexpected outcome. After reduction, the product obtained was not the expected 4-fluoro-3-O-benzoylated lyxoside, but rather the 4-deoxygenated compound 65. The Albright− Goldman oxidation is accomplished under alkaline conditions, and we speculate in that the reaction proceed through an E1cB mechanism with abstraction of the acidic α-proton and subsequent elimination of the fluorine to form the α,βunsaturated compound. 1H NMR of the crude reaction mixture revealed a vinylic proton that couples to the H-5eq and H-5ax. The acetate ester in compound 65 was cleaved by alkaline hydrolysis using NaOMe/MeOH to give 4a. A mild nucleophilic displacement of the 3-O-benzylated 60 (Scheme 14) using triflate formation followed by addition of AcOH/DBU in heated toluene42 gave 67% of fluorinated 66. The acetate ester was cleaved by alkaline hydrolysis using NaOMe/MeOH to give 67, and a subsequent debenzylation using a procedure with DMF/HCl to avoid saturation of the naphthyl moiety43 gave 4c. Fluorination of 67 gave the difluoro compound 68 in 61% yield. The benzyl group in 68 was then hydrogenolyzed in DMF/HCl to give 3c. The relatively high yield in this fluorination procedure, in comparison to the fluorinations in Scheme 14, indicates that an electron-withdrawing group in position 4 facilitates fluorination of position 2. Deoxygenations. The most common procedure for deoxygenation of a secondary alcohol is the Barton− McCombie method.44,39 Deoxygenation of methyl xanthates
Reagents and reaction conditions: (i) DAST, CH2Cl, 40 °C.
Epimerizations. Previous work involving inversions of different positions of naphthoxylosides have involved the Lattrell−Dax reaction with nitrite on positions 3 and 4, triflate displacement with CsOAc on position 2,9 and Swern oxidation−reduction sequences.16 Several methods were attempted to invert the hydroxyl group in position 2 of the L-arabinoside 13 (Scheme 13). Using the same methodology as for inversion of position 2 in xylose, i.e., displacement of the triflate with CsOAc, gave only 11% of the inverted product 63. Attempts using Mitsunobu conditions were unsuccessful. The Lattrell−Dax reaction gave a modest increase of the yield of 43 (21%). These low yields can be attributed to the instability of the intermediate triflate. Interestingly, the Lattrell−Dax reaction has been used to make simultaneous inversions of multiple hydroxyl groups.40 Therefore, this method was attempted on 3-O-benzoylated Larabinoside 21. In our hands, the reaction only gave a single Scheme 13. Epimerizationsa
a Reagents and reaction conditions: (i) Tf2O, pyridine; (ii) CsOAc, DMF, 50 °C; (iii) TBANO2, DMF, 50 °C; (iv) Dess−Martin periodinane, NaHCO3, CH2Cl2; (v) DMSO, Ac2O, MW 80 °C, 5 min; (vi) NaBH4, MeOH, 0 °C; (vii) 70% acetic acid in H2O, 60 °C; (viii) 1 M NaOH, MeOH, CH2Cl2, rt, 1 h.
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The Journal of Organic Chemistry Scheme 14. Synthesis of Naphthyl 2,4-Difluoroxylosidea
Reagents and reaction conditions: (i) Tf2O, pyridine; (ii) AcOH/DBU (12:6 equiv), toluene, 85 °C; (iii) 1 M NaOMe, MeOH, rt; (iv) HCl (concentrated, 22 equiv), Pd/C (10 mol %), DMF, H2 (1 atm); (v) DAST, CH2Cl2, 40 °C.9
a
desired product 1a in 42% yield with no signs of glycosidic bond cleavage and only minor amounts of byproducts. Bols et al. have investigated the stereoelectronics of distant hydroxyl groups and found that there is a higher electron-withdrawal effect from an equatorial hydroxyl group than from an axial or, in this case, no hydroxyl group.13,45 The increased electron density on C-2 in 4-deoxy compounds can explain the observed higher propensity to acid degradation. Attempts to make a simultaneous double deoxygenation resulted in multiple byproducts and no 2,4-deoxy product (data not shown). Instead, 1a was synthesized using a previously published method, forming the monosubstituted xanthate ester 39 from 19, followed by deoxygenation using the same reagents as for other deoxygenations.17 Meta-Analysis of Synthetic Complexity. Regardless of the abundance of methods available for carbohydrate synthesis, even simple modifications can turn out to be problematic, and double modifications present additional challenges due to conformational, steric, and stereoelectronic effects. To emphasize the complexity of carbohydrate synthesis, we performed a meta-analysis of the synthetic efforts toward the 16 xyloside analogues (1a−4d). Despite the seemingly simple synthetic targets, and readily available starting materials, the development of a well-working synthetic scheme turned out to be far from uncomplicated. The data is presented in graphical form in Figure 4. The “number of experiments” is defined as the number of unique entries in the laboratory journals related to the compound, i.e., the number of experiments that were needed to establish the synthetic sequence leading to the final compound. The “number of synthetic steps” indicates the number of transformations in the optimized synthetic sequence of each compound. The “yield” is calculated by multiplication of individual yields in the final synthetic sequence. Data is presented as clustered columns and labeled from the perspective of modifications of naphthyl β-D-xylopyranoside. For example, ″4-epi, 2-deoxy” indicates compound 1d. Interestingly, most compounds can be synthesized via synthetic sequences of 5−6 steps. However, in order to find a working synthetic sequence, significantly more experiments have to be performed. Generally, methylations are straightforward as well as synthesis of xyloside analogues with a formal epimerization of position 4. The latter compounds are usually synthesized from the corresponding arabinosides, which open up for high-yielding synthetic pathways. In contrast, deoxygenation of position 2 and fluorinations are considerable more troublesome.
is normally performed using a hydride source in combination with a radical initiator. For environmental reasons, hypophosphorous acid with AIBN and Et3N was used in these deoxygenations, rather than the more common organic tin hydrides.9 The success of the radical deoxygenation with these compounds appears to be highly dependent on which position to be deoxygenated, the substituents present at other positions, the concentration and rate of addition of AIBN solution, and the pH of the reaction mixture. The use of a too weak AIBN solution (
