Abstract
Treatment of methyl tri- O-acetyl-β- D-arabinopyranoside ( 1a) with hydrogen bromide in benzene or in acetic acid gave, in addition to the pyranosyl bromide ( 2a), a considerable proportion of tri- O-acetyl- D-arabinofuranosyl bromide ( 5). Similar treatment of methyl tri- O-benzoyl-β- D-arabinopyranoside ( 1b) gave a good yield of the pyranosyl bromide ( 2b); no furanoid derivative was formed. Ring contraction also took place when methyl 4- O-acetyl-2,3-di- O-benzoyl-β- D-arabinopyranoside ( 7) was treated with hydrogen bromide, whereas the isomeric 3- O-acetyl-2,4-di- O-benzoyl compound ( 12) gave the pyranosyl bromide 13 in high yield. Thus, methyl pyranosides with an O-acetyl group at C-4 undergo ring contraction on treatment with hydrogen bromide. The corresponding compounds with O-benzoyl groups at C-4 gave pyranosyl bromides only.
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