Unexpected Orthogonality of S-Benzoxazolyl and S-Thiazolinyl Glycosides: Application to Expeditious Oligosaccharide Assembly

Thorough mechanistic studies of the alkylation pathway for the activation of glycosyl thioimidates have led to the development of the thioimidate-only orthogonal strategy. Discrimination among S-thiazolinyl (STaz) and S-benzoxazolyl (SBox) anomeric leaving groups was achieved by fine-tuning of the activation conditions. Preferential glycosidation of a certain thioimidate is not simply determined by the strength of activating reagents; instead, the type of activation-direct vs indirect-comes to the fore and plays the key role.