Carbasugar analogues of galactofuranosides: α-O-linked derivatives

Using an indirect method, we have synthesised α-linked carbasugar analogues of galactofuranosides for the first time. Ring opening of a β-talo configured carbasugar 1,2-epoxide by alcohol nucleophiles under Lewis acidic conditions proceeded with very good regioselectivity to give α-talo configured C1-substituted ethers with a free OH-group at the C2 position. Inversion of configuration at C2 by an oxidation-reduction sequence gave the α-galacto configured carbahexofuranose C1 ethers. A carbadisaccharide corresponding to the Galf(α1→3)Manp substructure from Apodus deciduus galactomannan was synthesised to exemplify the method.