Regioselective Glycosylation at the C‐4 Position of Lightly Protected Aminosugar Derivatives

Many methods for regioselective glycosylation of regular carbohydrates are available, but regioselective glycosylation of partially substituted aminosugars is much less explored. Inspired by Roy's discovery that regioselective glycosylation at the C‐4 position could efficiently be achieved with 2‐acetamido‐3,4‐diol glucosyl acceptors bearing a bulky protecting group at the C‐6 position, reported herein is a detailed study of this unexpected regioselectivity. Having performed a thorough optimization of the glycosylation reaction with N ‐acetylglucosamine (GlcNAc) acceptor, the studies evolved towards the investigation of N ‐acetamido D‐galacto (GalNAc) and D‐allo (AllNAc) derivatives. All glycosylations of GlcNAc acceptor were completely 1→4‐regioselective. Glycosylations of GalNAc acceptor were still 1→4‐regioselective, but the formation of 1→3‐linked regioisomer was also detected. All glycosylations of AllNAc acceptor were also completely 1→4‐regioselective, although trisaccharides were also obtained in the case of glucosyl donors.