Highly Stereoselective α-Glycosylation by Acid-Base Catalysis with the Help of Two Additives: The Counterion of the Acid and a Heterocyclic Base.

The stereoselective synthesis of 1,2-cis glycosides remains a longstanding challenge in carbohydrate chemistry. Herein, we report a cooperative acid-base catalysis strategy based on N-iodosuccinimide/triethylsilane and 8-nitroquinoline for highly α-selective glycosylation with O-glycosyl trichloroacetimidate donors. This catalytic system enables the synthesis of over 50 glycosides in good to excellent yields under mild conditions. The synthetic utility was further demonstrated through gram-scale reactions and the convergent synthesis of an α-(1→6)-linked hexasaccharide.