Solvent-Free Glycosylation from per-O-Acylated Donors Catalyzed by Methanesulfonic Acid

The huge importance of carbohydrates and their derivatives in biomedical and industrial applications call for the development of streamlined and sustainable procedures for their synthetic elaboration. Here reported a novel glycosylation method based on direct activation of readily available per-O-acylated (acetylated or benzoylated) donors, promoted under air by methanesulfonic acid as a cheap and green catalyst in the absence of any solvent. Besides the beneficial avoidance of toxic and polluting organic solvents, these conditions were found critical for activating such poorly reactive donors with a very small catalyst loading (only 5 mol %), instead of stoichiometric Lewis acid promoters typically employed. Desired glycosides were quickly obtained, in most cases with high 1,2-trans stereoselectivity. Other main advantages over reported glycosylations with similar donors are the limited stoichiometric excess of the acceptor (or the donor), the easy applicability and low cost of the procedure and the wide target scope, also covering the synthesis of disaccharides and other non-trivial glycosides with applicable potential.