Selective 3-O- and/or 6-O-Glycosidation of Unprotected <i>O</i>- and <i>S</i>-Glycosides Promoted by an Intramolecularly Coordinated Arylboronic Compound

Abstract

Abstract Arylboronic compound 1 that has intramolecular coordination promotes highly regioselective 3-O- and/or 6-O-glucosidation and -galactosidation of methyl fucopyanoside, methyl galactopyranoside, methyl mannopyranoside, p-methylphenyl 1-thiogalactopyranoside, and p-methylphenyl 1-thiomannopyranoside with peracetylated glucopyranosyl bromide or galactopyranosyl bromide as a glycosyl donor in the presence of Ag2CO3, Et4NI, and molecular sieves 4A in THF. Glycosyl acceptors and promoter 1 form cyclic boronates at cis-vicinal diol (3,4-diol or 2,3-diol) and/or 4,6-diol moieties with concomitant activation of the less hindered 3-O (equatorial) and/or 6-O (primary) nucleophiles via intramolecular O → B coordination. This reaction provides a novel method of simultaneous one-pot 3,6-O-double glycosidation (glucosidation and galactosidation) to give trisaccharides. However, attempted mannosidation with peracetylated mannopyranosyl bromide only gives rise to orthoesters.