The first synthesis of a ribo-hexos-5-ulose: The L-enantiomer

The title compound, previously unreported in either enantioform, and its 2, 6-di-O-benzyl derivative have been synthesized through a stereocontrolled e pimerization at C-2 of 6-O-protected methyl 3,4-O-isopropylidene-5-C-methox y-beta-D-galactopyranosides The epimerization, performed through a high yie lding sequence of oxidation-reduction owing to the cooperative role of the equatorial C-l aglycon and the steric hindrance of the isopropylidene group , turned out to be completely diastereoselective. Whereas the unprotected L -ribo-hexos-5-ulose exists, as proved by NMR in D2O, in five main tautomeri c forms in a ratio of about 4:2:2:1:1, only two anomeric 1,4-furanosic form s are present at equilibrium in its 2,6-di-O-benzyl derivative, in ratios r anging from 10:1 to 7:3, depending on the prevalence of D2O or CD3CN in the solvent mixture.