An experimental and theoretical study on stereocontrolled glycosylations by a “one-pot” procedure

Abstract

A generalized strategy for stereocontrolled glycosylation to generate both isomers from a single donor is highly demanded to assemble the desired oligosaccharides. Herein we describe a “one-pot” strategy to install the stereoselectivity of both α- and β-glycosides by changing reaction conditions. Specifically, β-glycosides were selectively obtained when the perbenzylated trichloroacetimidate was activated by a catalytic amount of SnCl₄ at a low temperature of −40 °C. On the other hand, using 3.0 equivalents of SnCl₄ and/or TiCl₄ provided α-glycosides as the major products at room temperature via anomerization. The control experiments combined with DFT calculations revealed the mechanistic details of the glycosyl reaction, which elucidated that β-glycosylation occurred first and anomerization occurred subsequently (glycosidation–anomerization).