Selective Anomeric Acetylation of Unprotected Sugars with Acetic Anhydride in Water

Abstract

Unprotected sugars are selectively acetylated simply by stirring in aqueous solution in the presence of acetic anhydride and a weak base such as sodium carbonate. The reaction is selective for acetylation of the anomeric hydroxyl group of mannose, 2-acetamido, and 2-deoxy sugars and can be performed on a large scale. Competitive intramolecular migration of the 1-O-acetate to the 2-hydroxyl group when these two substituents are cis causes over-reaction and the formation of product mixtures.

Glycosyl benzoates as novel substrates for glycosynthases

“…While the galactosyl benzoate did form under our reaction conditions, we were unable to separate galactosyl benzoate 6 from formed by-products (Table S5, ESI, entry 1 † ). The rationale for the stereoselective formation of esters for sugars bearing a 2-OH was recently elucidated by Qiu et al 14 whereby any formed 1,2- cis -glycosyl acetates undergo migration of the ester from the anomeric position to 2-OH and, accordingly, we also isolated migration products by RP-HPLC. Moreover, the low yields observed in our DMC-mediated esterification, compared to that of the earlier work is likely due to the insolubility of thiobenzoic acid under our reaction conditions.…”