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-1H-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 ₂O, leading to the formation of a 1,2-oxazoline (=4,5-dihydrooxazole) moiety in good yield. Since the reaction proceeds in H ₂O 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.