Sialylation reactions with 5- N,7- O-carbonyl-protected sialyl donors: unusual stereoselectivity with nitrile solvent assistance

Abstract

A 5- N,7- O-carbonyl-protected sialyl donor was synthesized, and, unexpectedly, this donor showed β-selectivity (α/β = 1/2.4–1/20) on coupling with sugar acceptors in acetonitrile upon treatment with various promoter systems. For the coupling reaction in dichloromethane, a modified Ellervik’s method (IBr and AgClO 4·H 2O) was highly effective in activating the 5- N,7- O-carbonyl donor, providing moderate α-selectivity (α/β = ∼1.8/1).