54833-61-3

From 1-13C-labelled 4-monosubstituted benzils 1a-c and the ureas 3a-d the 5-aryl-5-phenylhydantoines 6a-c and the 3-substituted 5-aryl-5-phenylhydantoines 9a-i were obtaincd as mixtures of isotopomeres. The distribution of 13C-labelling of C-4 and C-5 was determined by mass- and 13C-nmr-spectroscopy. As in the case of benzilic acid rearrangement the a-methylphenyl- and 4-methoxyphenyl-increment, respectively, migrate less easily than does the unsubstituted phenyl group, which in turn migrates less easily than the 4-chlorophenylrest. In the latter case the differences are small and in one example cven slightly reversed. The characteristics of the rearrangement is explained by thermodynamic control of the reaction by preceding equilibria (Schemes 2 and 3).

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