Serendipity at work: Unexpected ring transformations of 4-aminopyrazolidin-3-ones into N-aminohydantoins

Abstract

Catalytic hydrogenation of (4R*,5R*)-4-benzyloxycarbonylamino- 5-phenylpyrazolidin-3-one 2 in the presence of Pd-C furnished an unexpected 'ring switching' transformation product, 3-amino-5-benzylimidazolidine-2,4-dione 4. Furthermore, heating of azomethine imines 3a,b (derived from 2 and aromatic aldehydes) afforded the corresponding (Z)-5-benzylidene-3-[(E)-benzylideneamino] imidazolidine-2,4-diones 8a,b as ring transformation products. Both reactions are explainable by cleavage of the C(5)-N(1) single bond in substrates 2 and 3 followed by cyclocondensation of the amide nitrogen to the carbamate carbonyl group. The structure of hydantoin 8a was confirmed by X-ray diffraction. © ARKAT-USA, Inc.