Dissecting the Essential Role of Anomeric β-Triflates in Glycosylation Reactions

Glycosylations promoted by triflate-generating reagents are widespread synthetic methods for the construction of glycosidic scaffolds and glycoconjugates of biological and chemical interest. These processes are thought to proceed with the participation of a plethora of activated high energy intermediates such as the alpha- and beta-glycosyl triflates, or even increasingly unstable glycosyl oxocarbenium-like species, among which only alpha-glycosyl triflates have been well characterized under representative reaction conditions. Interestingly, the remaining less accessible intermediates, yet to be experimentally described, seem to be particularly relevant in a-selective processes, involving weak acceptors. Herein, we report a detailed analysis of several paradigmatic and illustrative examples of such reactions, employing a combination of chemical, NMR, kinetic and theoretical approaches, culminating in the unprecedented detection and quantification of the true beta-glycosyl triflate intermediates within activated donor mixtures. This achievement was further employed as a stepping-stone for the characterization of the triflate anomerization dynamics, which along with the acceptor substitutions, govern the stereochemical outcome of the reaction. The obtained data conclusively show that, even for highly dissociative reactions involving beta-close ion pair (beta-CIP) species, the formation of the alpha-glycoside is necessarily preceded by a bimolecular a. beta triflate interconversion, which under certain circumstances becomes the rate-limiting step. Overall, our results rule out the prevalence of the Curtin-Hammett fast-exchange assumption for most glycosylations and highlight the distinct reactivity properties of alpha- and beta-glycosyl triflates against neutral and anionic acceptors.