Acyloxy neighboring-group participation in the acid-catalyzed cleavage of methyl 2,3-anhydro-β- d-ribofuranoside

Abstract

The reaction of methyl 2,3-anhydro-β- d-ribofuranoside with hydrogen bromide in an acetic acid—acetic anhydride solution leads to the formation of methyl 2,3-di- O-acetyl-5-bromo -5-deoxy-α,gb- d-xylofuranoside. Similar treatment of methyl 2,3-anhydro-5- O-benzoyl-β- d-ribofuranoside provided methyl 2- O-acetyl-3- O-benzoyl-5-bromo-5-deoxy-α,β- d-xylofuranosides. The position of halogen substitution was ascertained by hydrogenolysis to the resultant 5-deoxy sugars, which were characterized by their n.m.r. spectra. Confirmation of the structural assignment for methyl 2- O-acetyl-3- O-benzoyl-5-deoxy-α,β- d-xylofuranoside was obtained by synthesis from 1,2- O-isopropylidene-α- d-xylofuranose. The formation of the 5-bromo derivatives under the reported conditions probably occurred through the intermediacy of the 3,5-acyloxonium ions. Similar conversions were observed when the starting compound was treated with hydrogen chloride, acetyl bromide, or acetyl chloride in acetic acid—acetic anhydride solutions.