Mechanism of Chemical Glycosylation: Focus on the Mode of Activation and Departure of Anomeric Leaving Groups

Many glycosyl donors used in chemical glycosylation are too stable to undergo spontaneous glycosylation. Hence, the first key step of practically every glycosylation reaction is the activation of the leaving group that usually takes place via its interaction with a promoter/activator or, more rarely, a catalyst. The focus of this review is the first key step of glycosylation with the emphasis on the modes by which the leaving groups are activated and departed. Studies wherein the exact nature of the activation process has been reliably elucidated are still scarce. Therefore, this review particularly details the mechanistic studies wherein the mode of activation was proven either by characterizing the activated intermediates or by isolation of the departed leaving group–activator adducts.