SYNTHESIS OF METHYL (ETHYL 2-O-ACYL-3,4-DI-O-BENZYL-1-THIO-BETA-D-GLUCOPYRANOSID)URONATES AND EVALUATION OF THEIR USE AS REACTIVE BETA-SELECTIVE GLUCURONIC-ACID DONORS

The synthesis of derivatives of methyl (ethyl 2-0-acyl-3,4-di-O-benzyl-1-thio-beta-D-glucopyranosid)-uronate with different ester groups (acetyl, benzoyl, pivaloyl, and anisoyl) at O-2 is described. The synthesis proceeds via a two-step oxidation (DMSO/DCC followed by PDC/MeOH) of C-6 on a suitably protected glucose derivative to give directly the methyl glucuronic ester without affecting the thioglycoside. The thioglucuronides were tested as donors in coupling reactions with unreactive carbohydrate alcohols using dimethyl(methylthio)sulfonium triflate (DMTST) as promoter, Due to the activating benzyl protecting groups at O-3 and -4, the sluggishness of fully acylated glucuronic acid donors could be overcome and glucuronide disaccharides were produced in fair to good yields. The glucuronides were obtained with the desired P-configuration because of the participating group at O-2. Among the different ester groups tried, the benzoyl group was found to give the highest yield in the couplings.