Nucleosides. Part 5. Isolation and characterization of the stable cyclic adducts, (5R,6S)- and (5S,6S)-bromo-O6,5′-cyclo-5,6-dihydrouridines in the bromination of 2′,3′-O-isopropylideneuridine with N-bromosuccinimide

Abstract

Bromination of 2′,3′-O-isopropylideneuridine (1) with N-bromosuccinimide in chloroform containing acetic acid gave two diastereoisomeric cyclic adducts, (5R,6S)- and (5S,6S)-bromo-O⁶,5′-cyclo-5,6-dihydro-2′,3′-O-isopropylideneuridines (4a) and (4b), whose structures were determined on the basis of their chemical reactivities and ¹H n.m.r. spectral results. The cyclic adducts (4a) and (4b) formed an equilibrium mixture under acidic conditions [(4a)/(4b)= 9:11], while under neutral and basic conditions both adducts were converted into 5-bromo-2′,3′-O-isopropylideneuridine (2).