Increased understanding of the important roles that oligosaccharides and glycoconjugates play in biological processes has led to a demand for significant amounts of these materials for biological, medicinal, and pharmacological studies. Therefore, tremendous effort has been made to develop new procedures for the synthesis of glycosides, whereby the main focus is often the formation of the glycosidic bonds. Accordingly, quite a few review articles have been published over the past few years on glycoside synthesis; however, most are confined to either a specific type of glycoside or a specific strategy for glycoside synthesis. In this Review, new principles for the formation of glycoside bonds are discussed. Developments, mainly in the last ten years, that have led to significant advances in oligosaccharide and glycoconjugate synthesis have been compiled and are evaluated.

New principles for glycoside-bond formation.

The main focus of this perspective lies in the discussion of the recent mechanistic theories and supporting experimental evidences that have been put forth in an attempt to advance our understanding of the factors affecting chemical glycosylation.

Mechanism of chemical O-glycosylation: from early studies to recent discoveries.

Carbohydrates have been shown to play important roles in biological processes. The pace of development in carbohydrate research is, however, relatively slow due to the problems associated with the complexity of carbohydrate structures and the lack of general synthetic methods and tools available for the study of this class of biomolecules. Recent advances in synthesis have demonstrated that many of these problems can be circumvented. In this Review, we describe the methods developed to tackle the problems of carbohydrate-mediated biological processes, with particular focus on the issue related to the development of the automated synthesis of oligosaccharides. Further applications of carbohydrate microarrays and vaccines to human diseases are also highlighted.

Toward automated oligosaccharide synthesis.

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 ...

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

Catalytic glycosylation has been a central reaction in carbohydrate chemistry since its introduction by Fischer 125 years ago, but it is only in the past three to four decades that catalytic methods for synthesizing oligosaccharides have appeared. Despite the development of numerous elegant and ingenious catalytic glycosylation methods, only a few are in general use. This review covers all methods of catalytic glycosylation with the focus on the development and application in oligosaccharide synthesis and provide an overview of the scope and limitations of these. The review also includes relevant mechanistic studies of catalytic glycosylations. The future of catalytic glycosylation chemistry is discussed, including specific, upcoming methods and possible directions for the field of research in general.

Catalytic Glycosylations in Oligosaccharide Synthesis

Highly beta-selective O-glucosidation due to the restricted twist-boat conformation.

A completely β-selective glycosylation that does not rely on neighboring group participation is described. The novelty of this work is the design of the glycosyl donor locked into the axial-rich form by the o-xylylene bridge between the 3-O and 6-O of d-glucopyranose. The synthesized 2,4-di-O-benzyl-3,6-O-(o-xylyene)glucopyranosyl fluoride could efficiently react with various alcohols in a SnCl(2)-AgB(C(6)F(5))(4) catalytic system. The mechanism composed of the glycosylation and isomerization cycles was revealed through comparative experiments using acidic and basic molecular sieves. The achieved perfect stereocontrol is attributed to the synergy of the axial-rich conformation and convergent isomerization caused by HB(C(6)F(5))(4) generated in situ.

Completely β-Selective Glycosylation Using 3,6-O-(o-Xylylene)-Bridged Axial-Rich Glucosyl Fluoride

Highly beta-selective and direct formation of 2-O-glycosylated glucosides by ring restriction into twist-boat.

A new O-benzylation reagent, benzyl N-phenyl-2,2,2-trifluoroacetimidate, has been developed. It even reacts with sterically hindered alcohols and base-sensitive hydroxy esters to afford the corresp...

Yasunori Okada

Papers

Highly beta-selective O-glucosidation due to the restricted twist-boat conformation.

Completely β-Selective Glycosylation Using 3,6-O-(o-Xylylene)-Bridged Axial-Rich Glucosyl Fluoride

Highly beta-selective and direct formation of 2-O-glycosylated glucosides by ring restriction into twist-boat.

Benzyl N-Phenyl-2,2,2-trifluoroacetimidate: A New and Stable Reagent for O-Benzylation

Facile and regioselective preparation of partly O-benzylated d-glucopyranose acetates via acid-mediated simultaneous debenzylation–acetolysis