Synthesis of Oligosaccharides by Preactivation-Based Chemoselective Glycosylation of Thioglycosyl Donors

Increasing recognition of important biological functions of carbohydrates has stimulated the development of many innovative methodologies in carbohydrate synthesis during the past two decades.1-4 In most glycosylation reactions, a promoter is added to a mixture of a glycosyl donor and an acceptor. The glycosyl donor is activated by the promoter, which undergoes an in situ nucleophilic addition or displacement reaction with the acceptor leading to the glycoside product. Alternatively, the glycosyl donor can be activated in the absence of an acceptor (preactivation).5-12 Upon complete donor activation, the acceptor is added to the reaction mixture initiating glycoside formation. Using the preactivation strategy, unique stereochemical outcomes5, 9, 11, 13 and chemoselectivities6-8, 10, 14 have been observed.