Mechanisms of bromination of uracil derivatives. 4. Formation of adducts in acidic aqueous solutions and their dehydration to 5-bromouracils

Abstract

The reactions of bromine with uracil, 1-methyluracil, 3-methyluracil, and with 1,3-dimethyluracil in 0.125–2.0 M aqueous sulfuric acid have been examined. Under these conditions rapid attack by bromine upon the uracil leads to the formation of an observable intermediate, a 5-bromo-5,6-dihydro-6-hydroxyuracil, which subsequently undergoes slow acid-catalyzed dehydration to the appropriate 5-bromouracil. The intermediates derived from uracil and 3-methyluracil also show acid independent dehydration. The dehydration of the adduct derived from 6-methyluracil proceeds much more rapidly and so precludes observation of that adduct. The dehydrations show significant primary isotope effects for the rupture of C(5)—H bonds of the intermediates. The mechanistic implications of these results are discussed.