Synthetic oligosaccharides and glycoconjugates are increasingly used as probes for biological research and as lead compounds for drug and vaccine discovery. These endeavors are, however, complicated by a lack of general methods for the routine preparation of this important class of compounds. Recent development such as one-pot multi-step protecting group manipulations, the use of unified monosaccharide building blocks, the introduction of stereoselective glycosylation protocols, and convergent strategies for oligosaccharide assembly, are beginning to address these problems. Furthermore, oligosaccharide synthesis can be facilitated by chemo-enzymatic methods, which employ a range of glycosyl transferases to modify a synthetic oligosaccharide precursor. Glycosynthases, which are mutant glycosidases, that can readily form glycosidic linkages are addressing a lack of a wide range glycosyltransferases. The power of carbohydrate chemistry is highlighted by an ability to synthesize glycoproteins.

/pdf/opportunities-and-challenges-in-synthetic-oligosaccharide-4802bp03rp.pdf

Opportunities and challenges in synthetic oligosaccharide and glycoconjugate research

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

/pdf/exploration-of-the-sialic-acid-world-37d4w0kjhu.pdf

Exploration of the Sialic Acid World.

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor–acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with ...

/pdf/the-experimental-evidence-in-support-of-glycosylation-qm4cq0in86.pdf

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1–SN2 Interface

Saccharides are polyhydroxy compounds, and their synthesis requires complex protecting group manipulations. Protecting groups are usually used to temporarily mask a functional group which may interfere with a certain reaction, but protecting groups in carbohydrate chemistry do more than protecting groups usually do. Particularly, protecting groups can participate in reactions directly or indirectly, thus affecting the stereochemical outcomes, which is important for synthesis of oligosaccharides. Herein we present an overview of recent advances in protecting groups influencing stereoselectivity in glycosylation reactions, including participating protecting groups, and conformation-constraining protecting groups in general.

https://www.mdpi.com/1420-3049/15/10/7235/pdf?y=1

Protecting Groups in Carbohydrate Chemistry: Influence on Stereoselectivity of Glycosylations

This review is the counterpart of a 2018 Chemical Reviews article (Adero, P. O.; Amarasekara, H.; Wen, P.; Bohe, L.; Crich, D. Chem. Rev. 2018, 118, 8242–8284) that examined the mechanisms of chemi...

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

C-5 modifications in N-acetyl-neuraminic acid: scope and limitations.

Solvent Effect in the Synthesis of Sialosyl α(2–6) Galactosides: Is Acetonitrile the only Choice?

Glycoconjugates containing sialic acid are involved in a large variety of biological phenomena, including cell–cell adhesion, recognition by viruses and bacteria, and oncogenesis. Therefore, they are important synthetic targets for the design of drugs and vaccines. In the last decades, different methodologies that improve yield and stereoselectivity in sialylation reactions have been investigated. This review summarizes the latest developments in the synthesis of C-5 modified sialic acid glycosyl donors and glycosyl acceptors and their application in the synthesis of α-sialosides.

C‐5 Modifications in N‐Acetyl‐neuraminic Acid — Scope and Limitations

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C-5 modifications in N-acetyl-neuraminic acid: scope and limitations.

Solvent Effect in the Synthesis of Sialosyl α(2–6) Galactosides: Is Acetonitrile the only Choice?

C‐5 Modifications in N‐Acetyl‐neuraminic Acid — Scope and Limitations