Direct Synthesis of Glycosyl Chlorides from Thioglycosides.

Reported herein is a new method for the direct synthesis of glycosyl chlorides from thioglycosides using sulfuryl chloride at rt. A variety of thioglycosides and thioimidates could be used as substrates. Both acid- and base-sensitive protecting groups were found compatible with these reaction conditions. Preliminary investigation of the reaction mechanism indicates chlorination of the leaving group at the anomeric sulfur as the key step of the reaction.

Photosensitizer-thioglycosides enhance photodynamic therapy by augmenting cellular uptake.

Photodynamic therapy (PDT) uses photosensitizing agents along with light to ablate tissue, including cancers. Such light-driven localized delivery of free-radical oxygen to kill target tissue depends on photosensitizer cell penetration efficacy. While the attachment of monosaccharides and disaccharides to photosensitizers has been shown to potentially provide improved photosensitizer delivery, the range of glycan entities tested thus far is limited. We sought to expand such knowledge by coupling N-acetylglucosamine (GlcNAc) to pyropheophorbides as thioglycosides, and then testing photosensitizer efficacy. To this end, GlcNAc was conjugated to both pyropheophorbide-a and methyl pyropheophorbide-a. Among the entities tested, the conjugation of N-acetylglucosamine to methyl pyropheophorbide-a ('PSe') as thioglycoside enhanced cell uptake both in the presence and absence of human serum proteins, relative to other compounds tested. The enhanced PSe penetrance into cells resulted in higher cell death upon illumination with 665 nm light. While acting as a potent photosensitizer, PSe did not affect cellular carbohydrate profiles. Overall, the study presents a new pyropheophorbide glycoconjugate with strong in vitro PDT efficacy.

Iodosylbenzene-Promoted Glycosylation with Selenoglycosides: Application in One-Pot Glycosylation.

A novel method for the glycosylation of selenoglycosides activated by iodosylbenzene was developed. The glycosylation reaction conditions were mild, fast, and efficient, with a high tolerance to diverse protecting groups and a wide substrate scope, which is advantageous for synthesizing complex glycosides. In addition, selenoglycosides were shown to be orthogonal to thioglycosides under the promotion of iodosylbenzene. Notably, a high yield of the poorly reactive glucuronidation reaction product was obtained by acetyl-protected selenoglycoside. Finally, the orthogonal one-pot synthesis of ß-(1→6) oligoglucans demonstrated the usefulness of this method in oligosaccharide synthesis.

Circumventing aglycon transfer en route to the synthesis of pentasaccharide thioglycoside donor for the chain extension of Plesiomonas shigelloides strain 302-73 (serotype O1) repeating unit.

Herein we report a chemical synthesis of a pentasaccharide thioglycoside repeating unit of Plesiomonas shigelloides Strain 302-73 (Serotype O1), as a chain extension unit. In our synthetic endeavor we encountered multiple aglycon transfer reactions during glycosylations. This problem was obviated by employing a PMP group as a transient protecting group.

Thioglycosides Act as Metabolic Inhibitors of Bacterial Glycan Biosynthesis.

Glycans that coat the surface of bacteria are compelling antibiotic targets because they contain distinct monosaccharides that are linked to pathogenesis and are absent in human cells. Disrupting glycan biosynthesis presents a path to inhibiting the ability of a bacterium to infect the host. We previously demonstrated that O-glycosides act as metabolic inhibitors and disrupt bacterial glycan biosynthesis. Inspired by a recent study which showed that thioglycosides (S-glycosides) are 10 times more effective than O-glycosides at inhibiting glycan biosynthesis in mammalian cells, we crafted a panel of S-glycosides based on rare bacterial monosaccharides. The novel thioglycosides altered glycan biosynthesis and fitness in pathogenic bacteria but had no notable effect on glycosylation or growth in beneficial bacteria or mammalian cells. In contrast to findings in mammalian cells, S-glycosides and O-glycosides exhibited comparable potency in bacteria. However, S-glycosides exhibited enhanced selectivity relative to O-glycosides. These novel metabolic inhibitors will allow selective perturbation of the bacterial glycocalyx for functional studies and set the stage to expand our antibiotic arsenal.

Formation of ß-Configured Thioglycosides of d-Glucosamine and d-Galactosamine and Synthesis of Protected Human Milk Oligosaccharides.

We report on the stereoselective multigram scale preparation of cyclohexyl- and phenyl thioglycosides of 2-azido-2-deoxy-ß-d-gluco- and galactopyranosides from d-N-acetylglucosamine using a catalytic and solvent-free method. Two of the prepared building blocks were used as key intermediates for the synthesis of human milk oligosaccharides LNT and LNnT in their protected form.

Glycosyl benzoates as novel substrates for glycosynthases.

The development of a procedure for the one-pot synthesis of glycosyl benzoates directly from unprotected sugars in aqueous media using 2-chloro-1,3-dimethylimidazolium chloride (DMC), thiobenzoic acid, and triethylamine is reported. These glycosyl donors are excellent substrates for wild-type and mutant glycosidases. ß-Glucosyl benzoate was hydrolysed by the GH1 ß-glucosidase derived from Halothermothrix orenii (HorGH1). Subsequent use of this substrate in thioligase-mediated glycosylation of p-nitrothiophenol demonstrated their superiority as donors compared to their p-nitrophenol counterparts with excellent conversions. Using a series of arene nucleophiles, we also demonstrate good to excellent conversions (up to 94%) of ß-glucosyl benzoate to the corresponding p-nitrophenyl- and thioglycosides.

ß-Glycosylations with 2-Deoxy-2-(2,4-dinitrobenzenesulfonyl)-amino-glucosyl/galactosyl Selenoglycosides: Assembly of Partially N-Acetylated ß-(1 → 6)-Oligoglucosaminosides.

An efficient protocol has been established for ß-glycosylations with 2-deoxy-2-(2,4-dinitrobenzenesulfonyl)amino (2dDNsNH)-glucopyranosyl/galactopyranosyl selenoglycosides using PhSeCl/AgOTf as an activating system. The reaction features highly ß-selective glycosylation with a wide range of alcohol acceptors that are either sterically hindered or poorly nucleophilic. Thioglycoside- and selenoglycoside-based alcohols prove to be viable nucleophiles, opening up new opportunities for one-pot construction of oligosaccharides. The power of this approach is highlighted by the efficient assembly of tri-, hexa-, and nonasaccharides composed of ß-(1 → 6)-glucosaminosyl residues based on one-pot preparation of a triglucosaminosyl thioglycoside with DNs, phthaloyl, and 2,2,2-trichloroethoxycarbonyl as the protecting groups of amino groups. These glycans are potential antigens for developing glycoconjugate vaccines against microbial infections.

Activation of thioglycosides under mild alkylation conditions.

Reported herein is the development of a novel method for the activation of thioglycosides and thioimidates using benzyl trichloroacetimidate in the presence of catalytic triflic acid. Excellent yields have been achieved with reactive substrates, whereas efficiency of reactions with unreactive glycosyl donors and/or acceptors was modest.

Organocatalytic atroposelective synthesis of naphthoquinone thioglycosides from aryl-naphthoquinones and thiosugars.

In this study, we report an organocatalytic formal coupling strategy for aryl-naphthoquinones with thiosugars that provides straightforward access to the axially chiral naphthoquinone thioglycoside with excellent stereoselectivity. Mechanistic studies revealed the key role of H-bonding in stereochemical recognition. The reaction pathway involves the atroposelective addition, followed by stereoretentive oxidation of the hydroquinone intermediate.

Influence of Various Silyl Protecting Groups on Stereoselective 2-Deoxyrhamnosylation.

The influence of various silyl protecting groups on 2-deoxyrhamnosylation using 2-deoxyrhamnosyl acetates, thioglycosides, and (p-methoxyphenyl)vinylbenzoate (PMPVB) donors has been presented. C-Glycosylation reactions reveal that tert-butyldimethylsilyl (TBDMS), triisopropylsilyl (TIPS), and tert-butyldiphenylsilyl (TBDPS) silyl protected rhamnosyl oxocarbenium ions have no facial selectivity except for the conformationally (4H3) locked tetraisopropyldisiloxane (TIPDS) protected rhamnose donor, which provides complete α-selectivity. However, TBDPS protected rhamnosyl donors are found to be superior protecting groups for α-stereoselective O-glycosylation reactions with various acceptors. The observed results are found consistent across donors and donor activation conditions. Most importantly, the study was conducted at room temperature unlike the other energy-intensive low-temperature studies and was bound to have more practical utility. The outcomes have been explained using kinetic and thermodynamic analyses.

Metal-Free Synthesis of Acyl Glycosides and Application to Oligosaccharide Synthesis.

Oligosaccharides are involved in numerous biological processes. Achieving optimal anomeric selectivity and regioselectivity remains challenging. Herein, we present a method for the oxidative glycosylation of thioglycosides with hypervalent iodine reagents derived from carboxylic acid to form C-O bonds. The reaction of thioglycosides with various hypervalent iodine carboxylates afforded acyl O-glycosyl compounds. These acyl O-glycosyl compounds could react with thioglycoside acceptors to produce disaccharides; trisaccharides and tetrasaccharides could be synthesized by repeating this method.

Activation of Thioglycosides with Copper(II) Bromide.

Reported herein is a new protocol for glycosidation of alkyl and aryl thioglycosides in the presence of copper(II) bromide. While the activation with CuBr2 alone was proven suitable for reactive glycosyl donors, the activation of less reactive donors was more efficient in the presence of triflic acid as an additive. A variety of thioglycoside donors in reactions with different glycosyl acceptors were investigated to determine the initial scope of this reaction.

Novel triple mutant of an extremophilic glycosyl hydrolase enables the rapid synthesis of thioglycosides.

In order to expand the toolbox of enzymes available for thioglycoside synthesis, we describe here the first example of an extremophilic glycosyl hydrolase from Halothermothrix orenii (HorGH1) engineered towards thioglycosynthase activity with a novel combination of mutations. Using the triple mutant, HorGH1 M299R/E166A/E354G, a range of thioglycosides from glycosyl fluoride donors and aromatic thiols could be synthesised with exquisite stereoselectivity and good to excellent conversions (61-93%).

An Improved Protocol for the Stereoselective Synthesis of ß-d-Glycosyl Fluorides from 2-O-Acyl Thioglycosides.

A safe and operationally simple protocol for the preparation of ß-d-glycosyl fluorides is presented. We demonstrate that a precise combination of XtalFluor-M, N-bromosuccinimide, and Et3N·3HF can mediate facile, high-yielding, and diastereoselective conversions of 2-O-acyl thioglycosides to ß-d- and other 1,2-trans glycosyl fluorides. The key roles of these reagents are dissected in this work, as is the impact of their interplay on the fluorination stereoselectivity.

Design, Synthesis, Anticancer Activity and Molecular Docking of New 1,2,3-Triazole-Based Glycosides Bearing 1,3,4-Thiadiazolyl, Indolyl and Arylacetamide Scaffolds.

New 1,3,4-thiadiazole thioglycosides linked to a substituted arylidine system were synthesized via heterocyclization via click 1,3-dipolar cycloaddition. The click strategy was used for the synthesis of new 1,3,4-thiadiazole and 1,2,3-triazole hybrid glycoside-based indolyl systems as novel hybrid molecules by reacting azide derivatives with the corresponding acetylated glycosyl terminal acetylenes. The cytotoxic activities of the compounds were studied against HCT-116 (human colorectal carcinoma) and MCF-7 (human breast adenocarcinoma) cell lines using the MTT assay. The results showed that the key thiadiazolethione compounds, the triazole glycosides linked to p-methoxyarylidine derivatives and the free hydroxyl glycoside had potent activity comparable to the reference drug, doxorubicin, against MCF-7 human cancer cells. Docking simulation studies were performed to check the binding patterns of the synthesized compounds. Enzyme inhibition assay studies were also conducted for the epidermal growth factor receptor (EGFR), and the results explained the activity of a number of derivatives.

Structural mechanism of SGLT1 inhibitors.

Sodium glucose co-transporters (SGLT) harness the electrochemical gradient of sodium to drive the uphill transport of glucose across the plasma membrane. Human SGLT1 (hSGLT1) plays a key role in sugar uptake from food and its inhibitors show promise in the treatment of several diseases. However, the inhibition mechanism for hSGLT1 remains elusive. Here, we present the cryo-EM structure of the hSGLT1-MAP17 hetero-dimeric complex in the presence of the high-affinity inhibitor LX2761. LX2761 locks the transporter in an outward-open conformation by wedging inside the substrate-binding site and the extracellular vestibule of hSGLT1. LX2761 blocks the putative water permeation pathway of hSGLT1. The structure also uncovers the conformational changes of hSGLT1 during transitions from outward-open to inward-open states.

Concise synthesis of the tetrasaccharide repeating unit of the O-antigen from Escherichia coli O131 containing N-acetyl neuraminic acid.

Synthesis of the tetrasaccharide repeating unit of the O-antigen from E. coli O131 was accomplished through a linear strategy involving rationally protected monosaccharide units derived from commercially available sugars. The challenging α-glycosylation of N-acetyl neuraminic acid was achieved through activation of thioglycoside using NIS-mediated glycosylation strategy. The target tetrasaccharide in the form of its 3-aminopropyl glycoside may be used for further glycoconjugate formation without hampering the anomeric stereochemistry.

Synthesis and preliminary immunologic properties of di-/trisaccharide-conjugates related to Bacillus anthracis.

Herein, the di- and trisaccharide mimics of the hexasaccharide antigen related to Bacillus anthracis were synthesized and covalently coupled with carrier proteins, such as keyhole limpet hemocyanin (KLH) and bovine serum albumin (BSA), to form the corresponding glycoconjugates 1-6. 2,3,4,6-Tetra-O-benzyl thioglycoside and 2-deoxyl-2-phthalylamino-3,4,6-tri-O-benzyl thioglycoside were applied as glycosyl donors to guarantee α or ß-configuration of the newly formed glycosidic bonds. Glutaraldehyde was used as a homobifunctional cross-linker for high-efficiency coupling. The synthetic KLH-glycoconjugates 2, 4 and 6 were also used to vaccinate female Balb/c mice and the preliminary results of ELISA uncovered that all three KLH-conjugates could induce immune responses and generate oligosaccharide-specific total IgG antibodies. The trisaccharide 8, the glycosyl part of glycoconjugate 4, is of great immunogenicity.

Synthesis of the sialic acid-containing tetrasaccharide repeating unit corresponding to the cell wall O-antigen of Escherichia coli O131 strain.

A concise synthetic strategy have been developed for the synthesis of the sialic acid-containing tetrasaccharide repeating unit of the cell wall O-antigen of Escherichia coli (E. coli) O131 strain using regio- and stereoselective (2 â†’ 6)-α-glycosylations of judiciously protected sialic acid thioglycoside derivatives. Perchloric acid supported over silica (HClO4-SiO2) in combination with N-iodosuccinimide (NIS) has been used in the stereoselective activation of thioglycoside derivatives as well as used as a solid acid for the functional group modifications. Appropriate stereoselectivity was achieved in the glycosylation steps involved in the synthetic strategy.