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, obtaine d by oxidation of 3,4,6-protected D-galactopyranosides, can be alkylat ed in their anionic 2,6-pyranose forms to produce bis-glycosides conta ining the 2,5-dioxabicyclo[2.2.2]octane ring system. The 1-benzyl-2-me thyl bis-glycoside 4b, when subjected to catalytic hydrogenolysis, pro duces the methyl D-lyxo-hexopyranos-2-uloside 10, existing as an 8:2 m ixture of 1,5-pyranose anomers 9. Computational and NMR evidence is pr esented in favour of the hypothesis that the major anomer has the alph a configuration. Reduction of 9/10 with NaBH4 gives methyl 3,4-O-isopr opylidene-beta-D-tagatopyranoside, that can be hydrolyzed to D-tagatos e. A simple synthesis of 1,5-anhydro-D-tagatose, starting from 1,5-anh ydro-D-galactitol, is also presented. All new compounds were fully cha racterized by elemental analysis and by H-1 and C-13 NMR spectroscopy.