THE CONVERSION OF 1,5-ANHYDRO-D-GALACTITOL INTO THE ENDOCYCLIC GLYCALOF L-SORBOPYRANOSE AND INTO L-TAGATOSE

Several fully protected 3,4-O-isopropylidene derivatives of 1,5-anhydr o-D-galactitol have been converted into the corresponding 2,6-anhydro- 3-deoxy-L-threo-hex-2-enitols through an elimination of acetone promot ed by t-BuOK in DMSO, and into the parent compound (L-2-sorbal) by fur ther hydrolytic removal of methoxyisopropyl (MIP) protecting groups. T his method provides an easy access to endocyclic glycals of ketopyrano ses, a hitherto completely neglected class of unsaturated sugars that can be useful synthetic intermediates. Their reaction with MCPBA in Me OH, gave, in a regio- and diastereospecific manner, derivatives of met hyl alpha-L-tagatopyranoside, from which L-tagatose was obtained by de benzylation and hydrolysis. Structures and conformations of new compou nds were confirmed through NMR and computational methods.