Stannylene acetal-mediated regioselective open glycosylation of methyl β-d-galactopyranoside and methyl α-l-rhamnopyranoside

Open glycosylation of stannylated methyl β-d-galactopyranoside with per-O-benzoyl or -pivaloyl-glucopyranosyl bromide and -glucuronyl bromide, promoted by Ag-silica alumina, afforded regio- and stereoselectively glycosyl-β(1→6)-galactose and its orthoester derivative in good yield. Similar glycosylation of unprotected methyl α-l-rhamnopyranoside with per-O-pivaloylglucuronyl bromide provided glucuronyl-β(1→3)-rhamnose in moderate yield.