5 - Synthesis and Activation of Carbohydrate Donors: Acetimidates, n-Pentenyl and Vinyl Glycosides

Abstract

This chapter provides an account of three glycosylation methodologies that have found particular utility in oligosaccharide synthesis. Each method relies upon the preparation of a donor carbohydrate with specific anomeric functionality that is displaced selectively during the glycosylation reaction by the acceptor alcohol. The three methods—anomeric acetimidates, anomeric n-pentenyl groups and vinyl glycosides—all involve the use of a donor sugar that has at its anomeric position a function, which contains an unsaturated component. The trichloroacetimidate method is undoubtedly the best and most often employed technique for carrying out glycosylation chemistry. However, its one limitation is that it relies on selective deprotection of the anomeric position and successful installation of the imidate when required. n-Pentenyl glycosides are useful glycosyl donors because the n-pentenyl group may be carried through many different synthetic manipulations to be activated on demand. In combination with the reactivity tuning and latent/active variants, it is often possible to construct large oligosaccharide targets from a relatively small number of initial monosaccharides. Finally, while the latent/active strategy employing vinyl glycosides cannot really compete with the imidate or n-pentenyl methods for the construction of large specific oligosaccharide targets, it finds great applicability in the synthesis of oligosaccharide libraries from small numbers of building blocks.