Competition between the beta-hydroxylation of a primary and a tertiary carbon atom in rats.

In order to study the effect of steric hindrance on competition between two kinds of beta-hydroxylation, a compound bearing on a pyrimidinetrione nucleus both a branched side chain with a tertiary carbon atom in position beta (isobutyl group) and a linear side chain (ethyl group), was selected and administered to rats. Urine and faeces were collected and extracted. Hydroxymetabolites and their derivatives were isolated and then identified. The beta-hydroxylation of the linear chain was more important than the beta-hydroxylation of the branched chain. Steric hindrance plays a decisive role in this regioselectivity.