SYNTHESES WITH PARTIALLY BENZYLATED SUGARS .11. STUDIES ON SYNTHESIS OF ANOMERIC 5,6-DIMETHYL-1-D-RIBOFURANOSYLBENZIMIDAZOLES (RIBAZOLES . COMPARISON OF CONDENSATION OF 2,3,5-TRI-O-BENZOYL-D-RIBOFURANOSYL BROMIDE AND 2,3,5-TRI-O-BENZYL-D-RIBOFURANOSYL CHLORIDE WITH 5,6-DIMETHYLBENZIMIDAZOLE

The amorphous 2,3,5-tri-O-benzyl-D-ribofuranosyl chloride (3) which is readily preparable from 2,3,5-tri-Qbenzyl-1-O-p-nitrobenzoyl- (or p-phenylazobenzoyl-) β-D-ribofuranose, is a mixture of anomers in which the β form predominates. On condensation with an excess of 5,6-dimethylbenzimidazole, this halide (3) gives the cis-glycoside derivative 1-(2,3,5-tri-O-benzyl-α-D-ribofuranosyl)-5,6-dimethylbenzimidazoie (7) in 66% yield; catalytic debenzylation of 7 affords 5,6-dimethyl-1-α-D-ribofuranosylbenzimidazole (5, -ribazole). As usually obtained, syrupy 2,3,5-tri-O-benzoyl-D-ribofuranosyl bromide (9) is a mixture of anomers with the a form in slight excess. On condensation with an excess of 5,6-dimethylbenzimidazole, 9 gives 1-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)- 5,6-dimethylbenzimidazole (6, isolated as its picrate) in 51% yield and the corresponding α anomer (8, also isolated as its picrate) in 28% yield. The substantial quantity of 8 formed from 9 emphasizes the fact that the presence of a participating acyl group at C-2 in a glycosyl halide cannot be relied upon to ensure, even under mild conditions, the exclusive formation of glycosides which are trans at C-1-C-2. © 1968, American Chemical Society. All rights reserved.