A “Traceless” Directing Group Enables Catalytic S_<i>N</i>2 Glycosylation toward 1,2-<i>cis</i>-Glycopyranosides

Abstract

Generally applicable and stereoselective formation of 1,2-<i>cis</i>-glycopyranosidic linkage remains a long sought after yet unmet goal in carbohydrate chemistry. This work advances a strategy to this challenge via stereoinversion at the anomeric position of 1,2-<i>trans</i> glycosyl ester donors. This S_<i>N</i>2 glycosylation is enabled under gold catalysis by an oxazole-based directing group optimally tethered to a leaving group and achieved under mild catalytic conditions, in mostly excellent yields, and with good to outstanding selectivities. The strategy is also applied to the synthesis of oligosaccharides.