Stereoelectronic Effects Determine Oxa-carbenium vs β-Sulfonium Ion Mediated Glycosylations

摘要

Activation of a glycosyl donor protected with a 2-O-(S)-(phenylthiomethyl)benzyl ether chiral auxiliary results in the formation of an anomeric β-sulfonium ion, which can be displaced with sugar alcohols to give corresponding α-glycosides. Sufficient deactivation of such glycosyl donors by electron withdrawing protecting groups is, however, critical to avoid glycosylation of an oxa-carbenium ion intermediate. The latter type of glycosylation pathway can also be suppressed by installing additional substituents in the chiral auxiliary.