The Reaction of O-Benzylidene Sugars with N-Bromosuccinimide.: VII. Stereochemical and Mechanistic Aspects in the Pyrimidine Nucleoside Series

Abstract

The reaction of 2′,3′-O-benzylideneuridine with N-bromosuccinimide (NBS) leads to 3'-O-benzoyl-2′,5-dibromouridine. The stereoselectivity of bromination in the sugar ring can be explained by invoking the sequential formation of benzoxonium ion and 2,2′-anhydronucleoside intermediates. Protonation of the appropriate 2,2′-anhydronucleoside, followed by treatment with NBS, recreates the conditions which prevail in the original reaction of the benzylidene acetal with NBS.