Straightforward synthesis of thiodisaccharides by ring-opening of sugar epoxides

Epoxide-Mediated Trans-Thioglycosylation and Application to the Synthesis of Oligosaccharides Related to the Capsular Polysaccharides of C. jejuni HS:4

The enzyme-resistant thioglycosides are highly valuable immunogens because of their enhanced metabolic stability. We report the first synthesis of a family of thiooligosaccharides related to the capsular polysaccharides (CPS) of Campylobacter jejuni HS:4 for potential use in conjugate vaccines. The native CPS structures of the pathogen consist of a challenging repeating disaccharide formed with β(1→4)-linked 6-deoxy-β-D-ido-heptopyranoside and N-acetyl-D-glucosamine; the rare 6-deoxy-ido-heptopyranosyl backbone and β-anomeric configuration of the former monosaccharide makes the synthesis of this family of antigens very challenging. So far, no synthesis of the thioanalogs of the CPS antigens have been reported. The unprecedented synthesis presented in this work is built on an elegant approach by using β-glycosylthiolate as a glycosyl donor to open the 2,3-epoxide functionality of pre-designed 6-deoxy-β-D-talo-heptopyranosides. Our results illustrated that this key trans-thioglycosylation can be designed in a modular and regio and stereo-selective manner. Built on the success of this novel approach, we succeeded the synthesis of a family of thiooligosaccharides including a thiohexasaccharide which is considered to be the desired antigen length and complexity for immunizations. We also report the first direct conversion of base-stable but acid-labile 2-trimethylsilylethyl glycosides to glycosyl-1-thioacetates in a one-pot manner.

Synthesis of thiodisaccharides related to 4-thiolactose. Specific structural modifications increase the inhibitory activity against E. coli β-galactosidase

In the search for new glycosidase inhibitors, a set of benzyl β-D-Gal-S-(1→4)-3-deoxy-4-thio-α-D-hexopyranosides was synthesized. Diverse configurations were installed at C-2 and C-4 of the glucose residue. The benzyl glycosidic group was kept intact or substituted by an electron-donating or electron-withdrawing group that could also participate in hydrogen bonding. All thiodisaccharides were found to be inhibitors of E. coli β-galactosidase. In general, benzyl thiodisaccharides were better inhibitors than those substituted (NO₂ or NH₂) on the benzyl ring. Thiodisaccharides containing a hexopyranoside, instead of a pentopyranoside, showed a weaker inhibitory activity, except for those having the α-D-xylo configuration, which exhibited inhibition constants of the same order of magnitude. These and previous results indicated that the inhibition process by thiodisaccharides is strongly dependent on the configuration of the 3-deoxy-4-thiopyranoside, as well as its substitution pattern (such as the presence of a benzyl glycoside). The enzyme-inhibitor interaction during the hydrolysis process involves a conformational selection resulting from rotation around the thioglycosidic bond and the flexibility of the terminal six-membered ring. Thus, the mentioned structural features of the inhibitor could give rise to favorable ground state conformations for the interaction with the enzyme, similar to those found for selected thiodisaccharides in the bound state. These studies demonstrated that the performance of thiodisaccharides as enzyme inhibitors could be increased by selecting the appropriate configuration and substitution of the hexopyranoside replacing the glucose moiety of 4-thiolactose.

Photooxidation of thiosaccharides mediated by sensitizers in aerobic and environmentally friendly conditions

A series of β-d-glucopyranosyl derivates have been synthesized and evaluated in photooxidation reactions promoted by visible light and mediated by organic dyes under aerobic conditions. Among the different photocatalysts employed, tetra-O-acetyl riboflavin afforded chemoselectively the respective sulfoxides, without over-oxidation to sulfones, in good to excellent yields and short reaction times. This new methodology for the preparation of synthetically useful glycosyl sulfoxides constitutes a catalytic, efficient, economical, and environmentally friendly oxidation process not reported so far for carbohydrates.

Synthetic Studies on the Incorporation of N-Acetylallosamine in Hyaluronic Acid-Inspired Thiodisaccharides

Two approaches for the synthesis of the thiodisaccharide β-S-GlcA(1→3)β-S-AllNAc are described here. The target disaccharide was a C-3 epimer and thio-analogue of the hyaluronic acid repetitive unit, tuned with a thiopropargyl anomeric group for further click conjugation. Thus, we analysed and tested two convenient sequences, combining the two key steps required to introduce the thioglycosidic bonds and consequently reach the target molecule: the SN2 substitution of a good leaving group (triflate) present at C-3 of a GlcNAc derivative and the introduction of the anomeric thiopropargyl substituent. The use of a 2-azido precursor showed to be a convenient substrate for the SN2 step. Nevertheless, further protecting group manipulation and the introduction of the thiopropargyl anomeric residue were then required. This approach showed to provide access to a variety of thiodisaccharide derivatives as interesting building blocks for the construction of neoglycoconjugates.

Photoinitiated Thiol-ene Reactions of Enoses: A Powerful Tool for Stereoselective Synthesis of Glycomimetics with Challenging Glycosidic Linkages

Thioglycosides and C-glycosides represent pharmacologically useful classes of glycomimetics that possess a high degree of biological stability. One emerging tool for the stereoselective synthesis of thioglycosides is the photoinitiated addition of thiols to unsaturated sugars. Moreover, thiyl radical-mediated reactions of exo-glycals and 1-substituted endo-glycals offer facile routes to β-C-glycosidic structures. This Concept article summarizes the thiol-ene coupling strategies developed recently by our group and Somsák's group for the synthesis of several kinds of glycomimetics which are difficult to synthesize by conventional methods. One unusual characteristic of the thiol-ene reactions of endo-glycals is that heating inhibits, whereas cooling promotes the reaction. This unique temperature dependence as well as the effects of the enose structures and thiol configurations on the efficacy and stereoselectivity of the reactions are also discussed.

Synthesis of (1→3) Thiodisaccharides of GlcNAc and the Serendipitous Formation of 2,3-Dideoxy-(1→2)-thiodisaccharides through a Vinyl Azide Intermediate

The syntheses of β-S-GlcA(1→3)GlcNAc and β-S-Gal(1→3)GlcNAc thiodisaccharides, which can be considered mimetics of the repeating units of hyaluronan and keratan respectively, were achieved by S_N2 displacement of a triflate group allocated at the 3-position of a convenient 2-azido-4,6-O-benzylidene-2-deoxy-β-d-allopyranose precursor by the corresponding nucleophilic suitable protected thioaldoses derived from glucuronic acid (GlcA) and galactose (Gal). The study of the reaction led to the finding that the vinyl azide formed by competitive E2 reaction of the mentioned triflate was an interesting precursor of a new kind of 2,3-dideoxy-2-azido-(1→2) thiodisaccharides through an addition reaction. Determination of the stereochemistry of the new stereocenter at C-2 was achieved by NOESY experiments. Final protecting group manipulation of the (1→3) thiodisaccharides led to a family of derivatives that could be used as building blocks for the synthesis of complex glycomimetics.

Internal asymmetric induction by the C-6 substituent on the oxidation reaction of interglycosidic sulfur atom of thiodisaccharides

Complete diastereoselectivity was archived in the oxidation of 3-deoxy-S-(1-4)-disaccharides due to anchimeric assistance of the terminal CH₂OH group.

Synthesis of galactofuranosyl-(1 → 5)-thiodisaccharide glycomimetics as inhibitors of a β-d-galactofuranosidase

Description of the synthesis, molecular modeling and inhibitory properties of furanosyl thiodisaccharides that are mimetics of the motif β-d-Galf-(1 → 5)-d-Galf, found in glycoconjugates of pathogenic microorganisms.

Synthesis and reactivity of 4'-deoxypentenosyl disaccharides

4-Deoxypentenosides (4-DPs) are versatile synthons for rare or higher-order pyranosides, and they provide an entry for structural diversification at the C5 position. Previous studies have shown that 4-DPs undergo stereocontrolled DMDO oxidation; subsequent epoxide ring-openings with various nucleophiles can proceed with both anti or syn selectivity. Here, we report the synthesis of α- and β-linked 4'-deoxypentenosyl (4'-DP) disaccharides, and we investigate their post-glycosylational C5' additions using the DMDO oxidation/ring-opening sequence. The α-linked 4'-DP disaccharides were synthesized by coupling thiophenyl 4-DP donors with glycosyl acceptors using BSP/Tf2O activation, whereas β-linked 4'-DP disaccharides were generated by the decarboxylative elimination of glucuronyl disaccharides under microwave conditions. Both α- and β-linked 4'-DP disaccharides could be epoxidized with high stereoselectivity using DMDO. In some cases, the α-epoxypentenosides could be successfully converted into terminal l-iduronic acids via the syn addition of 2-furylzinc bromide. These studies support a novel approach to oligosaccharide synthesis, in which the stereochemical configuration of the terminal 4'-DP unit is established at a post-glycosylative stage.

Tri-isopropylsilyl thioglycosides as masked glycosyl thiol nucleophiles for the synthesis of S-linked glycosides and glyco-conjugates

Tri-isopropylsilyl thio-glycosides (TIPS S-glycosides), readily synthesized from glycosyl halides, glycosyl acetates, or p-methoxyphenyl glycosides, were in one-pot de-silylated and S-alkylated, -acylated, or -glycosylated in high yields and short time.

Expedient synthesis of an α-S-(1→6)-linked pentaglucosyl thiol

An α-S-(1→6)-linked pentaglucosyl thiol (1) is synthesized via the longest linear sequence of eleven steps from an α-glucosyl thiol (7).