Sugar episulfides as precursor of thiooligosaccharides

UHRIG M. L.; REPETTO E.; MANZANO V.; VARELA O.

Cap Hornu, Bahía del Somme, Picardie, Francia

Jornada; XXIIIèmes Journées du GFG (Groupe Français des Glucides) 2010; 2010

Groupe Français des Glucides

As part of our ongoing project on the synthesis of thiodisaccharides, we explored the ring-opening reaction of sugar thiiranes as the key step for the construction of the thioglycosidic bond of both hexopyranose and hexofuranose-containing thiooligosaccharides. Thus, sugar thiiranes (1 or 5) have been stereoselectively prepared by treatment with thiourea or KSCN, from the epoxides readily obtained in our laboratory. We conducted the ring-opening of the thiirane ring under varied conditions. This reaction was successfully performed using an excess of the sodium salt of 1-thioglucose or 1-thiogalactose in the presence of 18-crown-6. 5,6-Episulfides of furanoses underwent the attack of the 1-thiosugar to the less hindered C-6, to give thiodisaccharide 6 as major product. The ring opening of 3,4-thiiranes of pyranoses was a more complex reaction, affording a mixture of products with gluco configuration for the reducing end. This result suggests the approach of the nucleophile to C-4 of 1. We consistently observed the formation of disulfides, characterized as 3 and 7, respectively, probably due to the oxidative coupling of sulfides intermediates 2 and 6 with the 1-thiosugar. Treatment of 3 with LiAlH4, followed by acetylation afforded 4 in 70% overall yield. The removal of the S-acetyl group of 4 was selectively achieved by treatment with cysteamine. The products obtained, bearing a free thiol group, are immediate precursors of 1,2-branched thiotrisaccharides