SULFUR ATOM PARTICIPATION IN THIOOLIGOSACCHARIDE CHEMISTRY - SYNTHESIS OF 1'-THIOTREHALULOSE AND 1'-EPI-THIOTREHALULOSE AND COMPARATIVE REACTIVITY WITH THE O-LINKED DISACCHARIDE ANALOG, TREHALULOSE

1'-Thiotrehalulose (1-S-alpha-D-glucopyranosyl-1-thio-D-fructose, 6) a nd 1'-epi-thiotrehalulose (1-S-beta-D-glucopyranosyl-1-thio-D-fructose , 12) have been prepared, and their reactivity toward base and acid ca talysts has been compared with that of their alpha-O-linked natural co unterpart trehalulose (14). Under conventional, pyridine-catalyzed ace tylation conditions, 6 and 12 afforded exclusively the C-2 E open-chai n enol acetates at the D-fructose moiety 7 and 13, when 14 led to the expected peracetylated disaccharides. Upon protonic activation, 6, 12, and 14 underwent intramolecular glycosidation reactions, yielding D-f ructosyl D-glucosyl 1,1':2,2' mixed thioacetals and acetals, respectiv ely, whose structures are under control of stereoelectronic factors. A much higher intramolecular glycosylation reaction rate was observed f or the S-linked thioanalogues as compared to trehalulose, supporting i nvolvement of episulfonium intermediates at the anomeric position acti vation step. Nevertheless, the close transformation patterns with eith er S- or O-linked disaccharides favors in both cases oxocarbenium enti ties as main intermediates in glycosylation reactions involving proton ic activation. An unexpected tautomerization reaction at the D-glucose moiety, resulting in a fructofuranosyl glucofuranosyl spiro-oxathiane derivative presumably via a sulfonium intermediate, was also observed for derivatives of 6 and 12 under strong protonating conditions. Thes e results stress on the importance of sulfur atom participation reacti ons in thiooligosaccharide chemistry.