Imidazole-promoted 1,4-migration of the tert-butyldiphenylsilyl group: influence on the selectivity control of the silylation reactions of carbohydrate OH groups

Abstract

The regioselective protection of secondary hydroxy groups of gluco-, galacto-, manno-, rhamno- and fucopyranosides using TBDPSCl with imidazole in DMF has been studied. It was found that the relative spatial arrangement of the OH groups modulates the silylation selectivity which arises from the combination of kinetic factors and the intramolecular migrations of the secondary TBDPS groups. The rearrangement of the TBDPS groups has a much larger effect on the α-D-manno- and α-L-rhamnopyranosides, allowing the protection of the OH groups at positions 2, 3 or 2 and 4, in synthetically useful yields, by changing the reaction conditions. The relative reactivity of the secondary OH groups seems particularly likely to be governed by steric factors. This trend provides a valuable approach to the synthesis of 3-O-tert-butyldiphenylsilyl-1-thio-β-L-fucopyranoside.