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. 2024 Mar 4;9(10):11969–11975. doi: 10.1021/acsomega.3c09761

B(C6F5)3-Catalyzed Stereoselective 1,2-cis Arabinofuranosylation with a Conformationally Constrained Donor

Qian Xiao †, Sixian Fang †, Jiaming Ao †, Xiaoya Zhao †, Cai Huang †, Yuhua Liu §,*, Yichu Nie ‡,*, Akihiro Ishiwata ∥, Katsunori Tanaka ∥,⊥, Wenbin Deng †,*, Feiqing Ding †,*
PMCID: PMC10938590  PMID: 38497025

Abstract

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Compared with stereoselective glycosylation methods mainly addressed on the preparation of pyranose glycosides, the furanosylation has been more limited, especially for the 1,2-cis arabinofuranosylation. Herein, we report a novel stereoselective 1,2-cis-arabinofuranosylation strategy using a conformationally restricted 3,5-O-xylylene-protected arabinofuranosyl donor on activation with B(C6F5)3 for desired targets in moderate to excellent yields and β-stereoselectivity. The effectiveness of the 1,2-cis-arabinofuranosylation strategy was demonstrated successfully with various acceptors, including carbohydrate alcohols.

Introduction

Furanose-containing glycans are particularly important in Mycobacterium tuberculosis (TB), the organism that causes TB.1 The cell wall complex of mycobacteria is a unique assembly of lipids and carbohydrates, which consist largely of two furanose-rich polysaccharides, an arabinogalactan (AG) and a lipoarabinomannan (LAM).2 Common to both is an arabinan domain containing d-arabinofuranose (Araf) residues which are of interest to develop probes for understanding their biological roles and play key roles in the infectivity and pathogenicity of bacteria.3 There is a pressing need for the identification of new antibiotics, new drug targets, and vaccines of the bacterium. Thus, the synthetic cell wall glycans have been attracting particular attention for these purposes.4

Both AG and LAM have the hexasaccharide motif at their nonreducing terminal region, which has a branched structure with the two terminal β-Araf residues (Figure 1).5 The stereoselective formation of β-Araf linkage has been a long-standing synthetic problem because of its unique pseudoaxial alignment at C-2 position and neighboring group participation from 2-O-acyl functionality to result in predominantly 1,2-trans glycosides. To achieve the stereoselective construction of β-linked Araf glycoside, a variety of innovative glycosylation methodologies have been developed, including the use of intramolecular aglycone delivery,6 hydrogen-bond-mediated aglycone delivery,7 the glycosylation of 2,3-anhydro-d-lyxofuranosyl thioglycoside,8 the use of conformationally locked donors,9 and bis-thiourea hydrogen bond donors,10 among others.11 Of particular note, these studies have led to the successful synthesis of hexa- and larger saccharides.12 Among these methods, approaches based on the use of donors with certain cyclic protections, such as 3,5-O-di-t-butylsilylene-, 3,5-O-tetra-i-propyldisiloxanylidene (TIPDS)-, and 2,3-O-xylylene-protected thioglycoside donors, have given substantial β-selectivity.13

Figure 1.

Figure 1

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Structure of the Araf6 motif in both AG and LAM.

Most recently, our group discovered that zinc-directed stereocontrolled 1,2-cis-glucosylation/mannosylation (Figure 2a).14 The catalysts hypothesized that the zinc cation coordinating the two-position of the donors and serving as a directing agent controls stereoselectivity for glycosylation enhanced by the fixed conformation of the pyranose ring. While inspired by the strategy using conformationally constrained furanosyl donors (Figure 2b),13 we envisioned the extension of these strategies to furanose ring for 1,2-cis arabinofuranosylation reactions through the discovery of both appropriate catalysts and fixed conformation-controlled donors (Figure 2c).

Figure 2.

Figure 2

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Strategies of stereocontrolled 1,2-cis-arabinofuranosylation.

Results and Discussion

To this end, we commenced our investigation by testing the feasibility of performing glycosylation reactions of three typical arabinofuranosyl donors (1a–1c) with glucosyl acceptor 2a under previously developed Zinc(II) iodide-controlled 1,2-cis-glycosylation conditions (Scheme 1).14 Results showed that tri-O-benzyl protected donor (1a) afforded the desired product 3aa in good yield (80%), yet with poor stereoselectivity (α/β = 1:3), while the same reaction condition with known 3,5-O-TIPDS-protected donor (1b) gave the almost exclusively arabinofuranoside 3ab with poor yield (35%), presumably due to low reactivity of the donor under ZnI2 activation conditions. Interestingly, the moderate selectivity (α/β = 1:5) and acceptable yield (75%) of arabinofuranoside (3ac) was obtained using the donor (1c) equipped with the 3,5-O-xylylene-protected group, suggested that the donor (1c) is a potentially stereocontrolled donor properly favoring 1,2-cis-β-Araf products among the tested donors. Therefore, we then reoptimized the reaction conditions by screening a series of promotors, solvents, substrate concentrations, and reaction temperatures, employing the 3,5-O-xylylene-protected trichloroacetimidate donor 1c and the common glucosyl acceptor 2a. As shown in Table 1, results of the glycosylation reactions performed in diethyl ether (Et2O) solvent with a series of different promoters including Lewis acids or Brønsted acids (Tables 1, entries 1–4 and S1, entries 1–12) revealed that the B(C6F5)3 promoter led to the glycosylation of 1a with relatively high β-selectivity (Table 1, entry 4, α/β = 1:6) in excellent yield (92%). Encouraged by the previous reports15 on the B(C6F5)3 catalyst as the activation of Schmidt’s trichloroacetimidates donors for stereocontrolled glycosylation, further solvent screening with B(C6F5)3 employed as the promoter (Table 1, entries 4–8) revealed that glycosylation reactions performed to afford the glycoside products with even higher β-selectivity in toluene and DCM (Table 1, entries 5 and 6, α/β = 1:8) than the reactions performed in Et2O, whereas glycosylation reactions in acetonitrile showed no selectivity (Table 1, entry 7, α/β = 1:1) and THF affected to show even α-selectivity (Table 1, entry 8, α/β = 2:1). The glycosylation reaction promoted with 20 mol % of B(C6F5)3 at −40 °C (Table 1, entry 9) afforded the desired product with predominantly (α/β = 1:11), while, remarkably, at lower temperatures (−78 °C), glycosylation reactions afforded the glycoside product with the highest β-selectivity (Table 1, entries 10 and 11, α/β = 1:14). Importantly, pure β-glycoside could be easily separated from the reaction mixtures via thin-layer chromatography on silica gel with a 91% isolated yield (Table 1, entry 11). The stereochemistry of the anomeric center of arabinofuranosylated products was confirmed by δ C-1 and/or 3JH1–H2 values: for β-isomer, δ C-1 = 97–103 ppm and/or 3JH1–H2 = 4–5 Hz; for α-isomer, δ C-1 = 104–111 ppm and/or 3JH1–H2 = 1–3 Hz.16

Scheme 1. ZnI2-Mediated 1,2-cis-Arabinofuranosylation.

Scheme 1

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Table 1. Optimization of the Arabinofuranosylation.

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entrya promotor solvent temperature yield (%)b α/βc
1 TMSOTf Et2O r.t. 68 1:4
2 AuCl3 Et2O r.t. 72 1:1
3 BF3·OEt2 Et2O r.t. 91 3:1
4 B(C6F5)3 Et2O r.t. 92 1:6
5 B(C6F5)3 toluene r.t. 80 1:8
6 B(C6F5)3 DCM r.t. 81 1:8
7 B(C6F5)3 MeCN r.t. 54 1:1
8 B(C6F5)3 THF r.t. 40 2:1
9 B(C6F5)3 DCM –40 °C 90 1:11
10 B(C6F5)3 DCM –78 °C 86 1:14
11d B(C6F5)3 DCM –78 °C 91e 1:14
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a

Reaction conditions: donor 1a (1.5 equiv), acceptor 2a (1.0 equiv), promotor (0.1 equiv), 4 Å MS (100 mg/mL).

b

Combined yield of the anomeric mixture of corresponding glycoside.

c

Determined by the integration ratio obtained from 1H NMR of crude mixture.

d

0.2 equiv of promotor was used.

e

Isolated yield of β-glycoside.

Having identified optimal reaction conditions (Table 1, entry 10), the scope of the B(C6F5)3-catalyzed β-selective arabinofuranosylation reaction with the 3,5-O-xylylene-protected donor was explored (Scheme 2). Glycosylated with some simple alcohol acceptors, such as n-butanol (2b), n-hexanol (2c), cyclopentanol (2d), cyclohexanol (2e), i-propanol (2f), 3-pentanol (2g), and benzyl alcohol (2j) gave corresponding β-Araf glycosides (3b–3g and 3j) in excellent yields with high β-selectivity, whereas liner primarily alcohol with azido (3h) or benzyloxycarbonyl (Cbz) amino (3i) groups exhibited lower selectivity resulted in mixtures of isomers, probably due to the less nucleophilicity. Subsequently, protected amino acids (2k and 2L) and 1-adamantanol (2m) as well as naturally occurring monoterpene alcohol (2n) and steroids (2o and 2p), were examined as acceptors. In most cases, β-glucosylated products (3k–3p) were obtained in moderate to good yields predominantly. Glycosylation of a library of glycosyl acceptors including GlcO–6 (2a), GalO–6 (2q), GlcfO–3 (2r), GlcO–4 (2s), GlcO–3 (2t), GlcO–2 (2u and 2v), ArafO–2 (2w), as well as GalO–4-β-(1 → 4)-Glc16 (2x), with glucosyl donor 1c afforded 1,2-cis-linked disaccharides (3ac and 3q–3w) or trisaccharide (3x) in moderate to high yields with generally high 1,2-cis-stereoselectivity under the optimized conditions. However, the highly congested and weak nucleophiles (such as 2s and 2x) underwent glycosylation with a lower yield but good selectivity. Although disaccharide (3w) was assembled in moderate yield and 1,2-cis-stereoselectivity, it is important to synthesize a natural linkage of the mycobacterial cell wall. Notably, acid-labile groups such as Boc, TIPS, isopropylidene, and benzylidene acetal were well tolerant under both conditions despite in the absence of acid scavenger. To prove the utility of this methodology, a 1.0 mmol scale experiment with 3,5-O-xylylene-protected trichloroacetimidate donor 1c and glucosyl acceptor 2a was performed under the optimal conditions. 1,2-cis-arabinofurano-disaccharide (3ac) was obtained in a high yield of 85% and with the same level of stereoselectivity (α/β = 1:12). Then, the global deprotection of 3ac in the presence of Pd(OH)2/C under a H2 atmosphere to give methyl β-d-arabinofuranosyl-(1 → 6)-α-d-glucopyranoside (4) in 89% yield at one-pot (Scheme 3).

Scheme 2. Substrate Scope of B(C6F5)3-Directed 1,2-cis-Arabinofuranosylation.

Scheme 2

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Donor 1c (1.5 equiv), acceptor 2 (1.0 equiv), promotor (0.2 equiv), 4 Å MS (100 mg/mL) were used unless otherwise specified. Combined yields of the anomeric mixture of corresponding glycosides were shown. Stereoselectivity was determined by the isolated weight.

Scheme 3. 1.0 mmol Scale Synthesis and Global Deprotection.

Scheme 3

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Although the reaction mechanism is still uncertain, we have performed density functional theory (DFT) calculations to better understand the β-selectivity of this reaction and the catalytic system. On the basis of the DFT results and previous literature reports,15,17 the mechanistic hypothesis is proposed in Scheme 4. The mechanism of the transformation is likely to proceed through initial activation of the trichloroacetimidate leaving group. Both the unique electrophilic properties of B(C6F5)3 and the character of its association ability with Lewis bases provide the catalytic efficiencies of the reaction. Given that B(C6F5)3 is a kind of Lewis acid, it would prefer coordination to the more nucleophilic nitrogen atom on the leaving group other than the oxygen atom. We proposed that the catalyst B(C6F5)3 rapidly activate the leaving group of the glycosyl donor by associating with the Lewis basic nitrogen atom which directs the SN2 like nucleophilic attack of the acceptor hydroxyl at the α-face of glycosyl donor via transition state TS1 to afford the B-coordinated intermediate int2 and the intermediate int3 which immediately undergoes proton transfer via TS2 (Scheme 4, path a). The process of the rate-determining step nucleophilic attack requires a low activation barrier of 7.0 kcal/mol, which is feasible under the reaction condition (−78 °C). Finally, the active B(C6F5)3 catalyst was released to afford β-product concomitant with trichloroacetamide. The calculation result is consistent with the experimental observed β-selectivity.

Scheme 4. Proposed Catalytic Cycle of B(C6F5)3-Catalyzed β-Selective Arabinofuranosylation.

Scheme 4

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The bond lengths of the key transition states are given in Å. The energy given are in kcal/mol. Color code, C: dark gray, O: red, H: light gray, N: dark blue, B: pink, and Cl: light green. Irrelevant hydrogen atoms are omitted for clarity.

Besides, we proposed a B-chelation induced SN1 cleavage of glycosyl donor as show in Scheme 4 (path b). Calculations into the reaction indicated that the endocyclic cleavage mechanism is unfavorable under the reaction conditions with an activation barrier of 33.0 kcal/mol (TS1′) which is much higher in energy than the SN2 like nucleophilic attack mechanism in Scheme 4 (path a) (7.0 kcal/mol). Thus, we proposed that the Lewis acid-catalyzed SN2-like nucleophilic attack pathway predominates in this reaction. The acceptor such as methanol should react kinetically through this proposed mechanism. However, less nucleophilic acceptors such as 2h, 2i, and 2u afforded less selective β-arabinofuranosylation, probably through undesired anomerization of α-imidate or β-coordination of Int2 counteranion before the formation of β-glycoside.

Conclusions

In conclusion, a novel and versatile methodology toward β-selective arabinofuranosylation directed by B(C6F5)3 has been developed with highly stereoselectivity. The effectiveness of the 1,2-cis-arabinofuranosylation strategy was demonstrated successfully with various acceptors, including carbohydrate alcohols. Further experimental examinations toward the elucidation of the reaction mechanism and exploration of applications of this B(C6F5)3-mediated methodology for the synthesis of AG and LAM complex oligosaccharides containing β-Araf linkage(s) are the focus of our continuing investigation.

Acknowledgments

This work was supported partly by the Shenzhen Science and Technology Program (no. GJHZ20220913142602004 to F.D.) and the Province Natural Science Fund of Guangdong (no. 2021A1515010189 to F.D.). A.I. would like thank RIKEN Incentive Research Project 2023 support. The authors thank the NMR research group of Shenzhen Bay Laboratory for their continuous support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c09761.

  • Experimental procedures and copies of NMR spectra of all new compounds (PDF)

Author Contributions

Q.X., S.F., J.A., and X.Z. contributed equally. F.D. and Q.X. conducted most of the experiments and wrote the initial manuscript draft with A.I., K.T., and Y.N., and W.D., J.A., S.F., and C.H. performed part of the experiments. X.Z. and Y.L. contributed to the DFT computations. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao3c09761_si_001.pdf (12.3MB, pdf)

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