Sequential Dy(OTf) 3 ‐Catalyzed Solvent‐Free Per‐ O ‐Acetylation and Regioselective Anomeric De‐ O ‐Acetylation of Carbohydrates

Dysprosium(III) trifluoromethanesulfonate‐catalyzed per‐ O ‐acetylation and regioselective anomeric de‐ O ‐acetylation of carbohydrates can be tuned by adjusting the reaction medium. In this study, the per‐ O ‐acetylation of unprotected sugars by using a near‐stoichiometric amount of acetic anhydride under solvent‐free conditions resulted in the exclusive formation of acetylated saccharides as anomeric mixtures, whereas anomeric de‐ O ‐acetylation in methanol resulted in a moderate‐to‐excellent yield. Reactions with various unprotected monosaccharides or disaccharides followed by a semi‐one‐pot sequential conversion into the corresponding acetylated glycosyl hemiacetal also resulted in high yields. Furthermore, the obtained hemiacetals could be successfully transformed into trichloroimidates after Dy(OTf) 3 ‐catalyzed glycosylation.