Preparation of L-lyxo-hexos-5-ulose through C-3 epimerization of bis-glycopyranosides of L-arabino-hexos-5-ulose

The unreported title compound and its 2,6-di-O-benzyl derivative have been prepared from methyl beta-D-galactopyranoside through a sequence involving the bis-glycoside methyl 2,6-di-O-benzyl-5-C-methoxy-beta-D-galactopyranoside 8, the precursor of L-arabino-hexos-5-ulose, that was converted to the L-lyxo series by inversion at C-3. The inversion was achieved in acceptable yields by selective triflation, followed by displacement with benzoate, and by an oxidation/reduction sequence. Whereas 2,5-di-O-benzyl-L-lyxo-hexos-5-ulose exists entirely as a mixture of the two anomeric 1,4-furanosic forms, the unprotected hexos-5-ulose involves at equilibrium in CD3CN/D2O at least eight tautomers, one of which is predominant.