Hydantoins ring transformations

When 5-bromo-6-methyluracils possessing a phenyl group at the N-l position are treated with alkylamines and hydrazine, ring contraction occurs to hydantoins and 3-pyrazolones, respectively. The latter conversion into the pyrazolone is a double-ring transformation via a hydantoin intermediate. The N-l-phenyl group is essential for of the ring contraction to... 

Hydantoins have been prepared from cyclic derivatives of urea by mechanisms that are not very reliable in most cases.1 Some new ring transformations have clarified some mechanistic aspects, however. 

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