Comparing n-pentenyl orthoesters and n-pentenyl glycosides as alternative glycosyl donors

As is well known, cyclic 1,2-glycosyl orthoesters undergo ready acid catalyzed rearrangement to 2-O-acyl glycosides in which the alkoxy group is transferred from the orthoester to the anomeric center in a highly stereocontrolled process. The related n-pentenyl derivatives are unique in that either the orthoester (NPOE) or its rearrangement product (NPGAC) can function as a glycosyl donor, and mechanistic considerations indicate that both should (or could!) lead to the same product(s) arising from trans-orthoesterification, glycosidation, glycosyl esterification, etc. Experiments are described which show that the product obtained from a given reaction can be optimized by careful choice of the donor, NPOE or related NPGAC, and careful attention to reaction conditions, electrophilic promoter, ‘size’ of the glycosyl acceptor, and experimental protocol.