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

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 AgClO4·H2O) was highly effective in activating the 5-N,7-O-carbonyl donor, providing moderate α-selectivity (α/β = ∼1.8/1).

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