A Practical One‐Step Synthesis of 1,2‐Oxazoline Derivatives from Unprotected Sugars and Its Application to Chemoenzymatic β ‐ N ‐Acetylglucosaminidation of Disialo‐oligosaccharide

A facile and practical method for synthesis of sugar oxazolines (=dihydrooxazoles) from the corresponding N ‐acetyl‐2‐amino sugars has been developed by using 2‐chloro‐1,3‐dimethyl‐1 H ‐benzimidazol‐3‐ium chloride (CDMBI) as a dehydrative condensing agent. The intramolecular dehydrative reaction between the 2‐acetamido group and the anomeric OH group of unprotected N ‐acetyl‐2‐amino sugars took place smoothly in H 2 O, leading to the formation of a 1,2‐oxazoline (=4,5‐dihydrooxazole) moiety in good yield. Since the reaction proceeds in H 2 O without using any protecting groups, the resulting oxazolines can be utilized as effective glycosyl donors for the subsequent enzymatic glycosylation. We have successfully demonstrated a highly efficient chemoenzymatic transglycosylation of a disialo‐oligosaccharide moiety to p ‐nitrophenyl N ‐acetylglucosaminide catalyzed by a mutant endo‐ N ‐acetylglucosaminidase without isolating disialo‐oligosaccharide oxazoline as synthetic intermediate.