Mechanistic studies in the chemistry of urea. Part 4. Reactions of urea, 1-methylurea, and 1,3-dimethylurea with benzil in acid solution

In acid solution benzil reacts with urea and with 1-methylurea to give the bicyclic products (2) and (3b) and with 1,3-dimethylurea to give 1,3-dimethyl-5,5-diphenylhydantoin (5). The initial step in the reaction is attack by urea on protonated benzil to give the diol (6). Further reaction of (6) is governed by its tendency to eliminate water.