Achiral 2-Hydroxy Protecting Group for the Stereocontrolled Synthesis of 1,2-cis-α-Glycosides by Six-Ring Neighboring Group Participation

Glycosylation of a fully armed donor bearing a 2-O-(trimethoxybenzenethiol) ethyl ether protecting group is completely a-selective with a range of carbohydrate alcohol acceptors. Low-temperature NMR studies confirm the intermediacy of cyclic sulfonium ion intermediates arising from six-membered,beta-sulfonium ring neighboring group participation. Selective protecting group removal is achieved in high yield in a single operation by S-methylation and base-induced beta-elimination.