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 NaBH 4 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 1 H and 13 C NMR spectroscopy.