Reconsidering glycosylations at high temperature: precise microwave heating.
Current methods for glycosylation of complex alcohols, e.g. with glycosyl trichloroacetimidates, generally occur in the presence of a strong Lewis acid 'promoter', and at sub-ambient temperatures. However, the older literature reports high-temperature glycosylations, especially of phenols. We have described an efficient method for glycosylation of alcohols under neutral conditions, using as anomeric leaving group methyl 3,5-dinitrosalicylate (DISAL). Only a very few reports have described the use of microwaves to promote glycosylations, mainly of simple alcohols. Here we describe fast, high-temperature glycosylations using precise microwave heating in the synthesis of oligosaccharides, with both DISAL and widely used trichloroacetimidate glycosyl donors in the absence of strong Lewis acids. Also, we have applied microwave heating as a general protocol for evaluating new, potential glycosyl donors.