Chemistry of glucal halohydrins(II): An unusual protecting group effect in the competitive formation of formyl furanosides and methyl glycosides

A remarkable protecting group influence was observed in the base-induced reaction of protected halohydrins derived from d-glycals. Tri-O-methyl and tri-O-benzyl halohydrins react with cesium carbonate in methanol at room temperature to give methyl glycosides as the major product and unsaturated formyl furanosides as the minor product. Whereas, the tri-O-tert-butyldimethylsilyl (t-BuMe2Si)-protected halohydrins reacted with cesium carbonate in methanol at room temperature to give a mixture of epimeric formyl furanosides, and at reflux to give an unsaturated formyl furanoside, as the only products. The tri-O-methyl and tri-O-benzyl halohydrins react slowly at elevated temperature to give predominantly furans. In comparison, the tri-O-t-BuMe2Si halohydrins reacted completely after five minutes to give a mixture of epimeric formyl furanosides. The tri-O-t-BuMe2Si iodohydrins were oxidized to the corresponding iodolactones, which also underwent a based-induced ring contraction in methanol to give the furanose 1-methylcarboxylate esters.