A “Traceless” Directing Group Enables Catalytic S N 2 Glycosylation toward 1,2- cis -Glycopyranosides

Generally applicable and stereoselective formation of 1,2- cis -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- trans glycosyl ester donors. This S N 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.