3 - Synthesis and Activation of Carbohydrate Donors: Acetates, Halides, Phenyl selenides and Glycals

Abstract

This chapter deals with a mixed bag of glycosyl donor systems whose glycosylating reactivities rely on a range of differing mechanisms that exploit familiar leaving group chemistries on one hand, and less familiar leaving groups or electrophiles on the other. Of these various donors, peracetylated derivatives of glycosyl bromides continue to be the most popular by virtue of their long history of use, their commercial availability, and relative ease of preparation. This chapter presents certain advantages offered by less frequently used donors in terms of convenience and strategy. Anomeric acetates, typically peracetylated carbohydrates, are frequently encountered as intermediates in the preparation of other glycosyl donors, including glycosyl halides. Although they are less reactive than many other donors, they can, in fact, be used as effective glycosyl donors in their own right and may offer certain strategic advantages. It is a combination of these two approaches—variation of the activating agent and reactivity tuning by variation of the protecting group regime—that has led to the increasing use of phenyl selenoglycosides in various elegant oligosaccharide syntheses.