Stereoselective synthesis of nucleoside analogues of chiral imidazolidines from sugar isothiocyanates

The syntheses of the disaccharide 5 and the trisaccharide 6 glycosyl isothiocyanates starting from di- or monosaccharide N-dialkoxycarbonylvinylglycosylamines 1 and 2 are described. Compounds 5, 6, alpha-(D-ribopyranosyl) and beta-(D-glucopyranosyl, D-ribopyranosyl) isothiocyanates and 1,3,4,6-tetra-O-acetyl-2-deoxy-2-isothiocyanato-beta-D-glucopyranose are; used in the diastereoselective synthesis of the nucleoside analogues of (5R)- and (5S)-5-hydroxyimidazolidine-2-thiones 7, 11, 14, 16, 17, 18a, and 18b. The 5R:5S ratio-depends on the anomeric configuration (configuration of C-2' for 18a, 18b) of the sugar isothiocyanate, beta-Glycosyl isothiocyanates give mostly the (5R)-epimer, whereas when a alpha-glycosyl isothiocyanate is used, there is practically no stereoselectivity.