Selective Anomeric Acetylation of Unprotected Sugars with Acetic Anhydride in Water

Unprotected sugars are selectively acetylated simply by stirring in aqueous solution in the presence of acetic anhydride and a weak base such as sodium carbonate. The reaction is selective for acetylation of the anomeric hydroxyl group of mannose, 2-acetamido, and 2-deoxy sugars and can be performed on a large scale. Competitive intramolecular migration of the 1-O-acetate to the 2-hydroxyl group when these two substituents are cis causes over-reaction and the formation of product mixtures.

Pathogen-sugar interactions revealed by universal saturation transfer analysis

Many pathogens exploit host cell-surface glycans. However, precise analyses of glycan ligands binding with heavily modified pathogen proteins can be confounded by overlapping sugar signals and/or compounded with known experimental constraints. Universal saturation transfer analysis (uSTA) builds on existing nuclear magnetic resonance spectroscopy to provide an automated workflow for quantitating protein-ligand interactions. uSTA reveals that early-pandemic, B-origin-lineage severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike trimer binds sialoside sugars in an "end-on" manner. uSTA-guided modeling and a high-resolution cryo-electron microscopy structure implicate the spike N-terminal domain (NTD) and confirm end-on binding. This finding rationalizes the effect of NTD mutations that abolish sugar binding in SARS-CoV-2 variants of concern. Together with genetic variance analyses in early pandemic patient cohorts, this binding implicates a sialylated polylactosamine motif found on tetraantennary N-linked glycoproteins deep in the human lung as potentially relevant to virulence and/or zoonosis.

Protecting group free glycosylation: one-pot stereocontrolled access to 1,2-trans glycosides and (1→6)-linked disaccharides of 2-acetamido sugars

Unprotected 2-acetamido sugars may be directly converted into their oxazolines using 2-chloro-1,3-dimethylimidazolinium chloride (DMC), and a suitable base, in aqueous solution. Freeze drying and acid catalysed reaction with an alcohol as solvent produces the corresponding 1,2-trans-glycosides in good yield. Alternatively, dissolution in an aprotic solvent system and acidic activation in the presence of an excess of an unprotected glycoside as a glycosyl acceptor, results in the stereoselective formation of the corresponding 1,2-trans linked disaccharides without any protecting group manipulations. Reactions using aryl glycosides as acceptors are completely regioselective, producing only the (1→6)-linked disaccharides.

Streamlined Iterative Assembly of Thio-Oligosaccharides by Aqueous S-Glycosylation of Diverse Deoxythio Sugars

A streamlined iterative assembly of thio-oligosaccharides was developed by aqueous glycosylation. Facile syntheses of various deoxythio sugars with the sulfur on different positions from commercially available starting materials were described. These syntheses featured efficient chemical methods including our recently reported BTM-catalyzed site-selective acylation. The resulting deoxythio sugars could then be used for the Ca(OH)₂ -promoted protecting group-free S-glycosylation in water at room temperature. The aqueous glycosylation reaction proceeded smoothly to afford the corresponding 1,2-trans S-glycosides in good yields with high chemo- and stereoselectivity. An appropriate choice of protecting groups for the thiol in the glycosyl donor was necessary for the development of iterative synthesis of thio-oligosaccharides. The aqueous glycosylation was then applied to the synthesis of a trimannoside moiety of N-linked glycans core region.

Chemical (neo)glycosylation of biological drugs

Biological drugs, specifically proteins and peptides, are a privileged class of medicinal agents and are characterized with high specificity and high potency of therapeutic activity. However, biologics are fragile and require special care during storage, and are often modified to optimize their pharmacokinetics in terms of proteolytic stability and blood residence half-life. In this review, we showcase glycosylation as a method to optimize biologics for storage and application. Specifically, we focus on chemical glycosylation as an approach to modify biological drugs. We present case studies that illustrate the success of this methodology and specifically address the highly important question: does connectivity within the glycoconjugate have to be native or not? We then present the innovative methods of chemical glycosylation of biologics and specifically highlight the emerging and established protecting group-free methodologies of glycosylation. We discuss thermodynamic origins of protein stabilization via glycosylation, and analyze in detail stabilization in terms of proteolytic stability, aggregation upon storage and/or heat treatment. Finally, we present a case study of protein modification using sialic acid-containing glycans to avoid hepatic clearance of biological drugs. This review aims to spur interest in chemical glycosylation as a facile, powerful tool to optimize proteins and peptides as medicinal agents.

Applications of Shoda's reagent (DMC) and analogues for activation of the anomeric centre of unprotected carbohydrates

2-Chloro-1,3-dimethylimidazolinium chloride (DMC, herein also referred to as Shoda's reagent) and its derivatives are useful for numerous synthetic transformations in which the anomeric centre of unprotected reducing sugars is selectively activated in aqueous solution. As such unprotected sugars can undergo anomeric substitution with a range of added nucleophiles, providing highly efficient routes to a range of glycosides and glycoconjugates without the need for traditional protecting group manipulations. This mini-review summarizes the development of DMC and some of its derivatives/analogues, and highlights recent applications for protecting group-free synthesis.

Scope of the DMC mediated glycosylation of unprotected sugars with phenols in aqueous solution

Activation of reducing sugars in aqueous solution using DMC and triethylamine in the presence of phenols allows direct stereoselective conversion to the corresponding 1,2-trans aryl glycosides without the need for any protecting groups.

Photochemistry of XCH2CN (X = -Cl, -SH) in Argon Matrices

We report infrared spectra and photochemical behavior of the potentially astrochemically significant species, mercaptoacetonitrile (HS-CH₂C≡N) and, for comparison purposes, chloroacetonitrile (Cl-CH₂C≡N), both suspended in an argon matrix at 6 K. Photolytic formation of the isocyano products HS-CH₂-NC and Cl-CH₂-NC were observed as well as CH₃NSC and CH₃SCN (in HS-CH₂CN photolysis). While no dissociation products were observed for Cl-CH₂-CN, photolysis of HS-CH₂-CN produced compounds necessitating the loss of the CN group to form CH₂═S, the SH group to form H₂C-CN and HC-CN, or both CN and SH to form CH₃ and CH₄. Observation of emission spectra upon annealing indicates the presence of free sulfur atom in matrices of photolyzed HS-CH₂-CN.

Meet the Board of ChemistryOpen: Antony J. Fairbanks

Antony J. Fairbanks is a Professor in the Department of Chemistry at the University of Canterbury in New Zealand. The research of his group focuses on the broad areas of organic synthesis, particularly applied to carbohydrates. He currently serves as an active Editorial Board member for ChemistryOpen.

A double-click approach to the protecting group free synthesis of glycoconjugates

The use of a bi-functional linker, containing an alkyne and an alkene, allows the protecting group free conjugation of reducing sugars to thiols via a double click process.

Selective anomeric acetylation of unprotected sugars in water

High yielding selective acetylation of only the anomeric hydroxyl of unprotected sugars is possible in aqueous solution using 2-chloro-1,3-dimethylimidazolinium chloride (DMC), thioacetic acid, and a suitable base. The reaction, which may be performed on a multi-gram scale, is stereoselective for sugars that possess a hydroxyl group at position-2, exclusively yielding the 1,2-trans products. The use of an iterative reagent addition procedure allows the use of sodium carbonate as the base, avoiding the formation of triethylammonium salts, which may hamper product purification. The glycosyl acetate products may be used as donor substrates for glycosidase-catalysed synthesis. The crude aqueous acetylation reaction mixture may also be used for this purpose.

Protecting group free synthesis of glycosyl thiols from reducing sugars in water; application to the production of N-glycan glycoconjugates

Un-protected 2-acetamido terminated reducing sugars may be converted into the corresponding glycosyl thiols in water, and conjugated to peptides using the thiol–ene click reaction without recourse to any protecting groups.