New syntheses of d-tagatose and of 1,5-anhydro-d-tagatose from d-galactose derivatives

3,4-O-Isopropylidene-d-lyxo-hexopyranosid-2-ulose derivatives, obtained by oxidation of 3,4,6-protected d-galactopyranosides, can be alkylated in their anionic 2,6-pyranose forms to produce bis-glycosides containing the 2,5-dioxabicyclo[2.2.2]octane ring system. The 1-benzyl-2-methyl bis-glycoside 4b, when subjected to catalytic hydrogenolysis, produces the methyl d-lyxo-hexopyranos-2-uloside 10, existing as an 8:2 mixture of 1,5-pyranose anomers 9. Computational and NMR evidence is presented in favour of the hypothesis that the major anomer has the α configuration. Reduction of 9/10 with NaBH4 gives methyl 3,4-O-isopropylidene-β-d-tagatopyranoside, that can be hydrolyzed to d-tagatose. A simple synthesis of 1,5-anhydro-d-tagatose, starting from 1,5-anhydro-d-galactitol, is also presented. All new compounds were fully characterized by elemental analysis and by 1H and 13C NMR spectroscopy.