THE STRUCTURE OF HYDANTOINS IN SOLUTION AND IN THE SOLID-STATE

The results of NMR spectroscopic and X-ray crystallographic studies ar e critically discussed with respect to the structure of hydantoins, th eir tautomerism, and their acidity. The imide NH proton of the preferr ed, nearly planar 2,4-imidazolidine-dione tautomer proved to be more a cidic than the corresponding amide NH proton. Phenyl substituents at t he ring nitrogen atoms and at C-5 are twisted from the plane of the hy dantoin ring; in case of ortho substituents restricted rotation about the N-aryl bond was found and the barrier to rotation determined by dy namic NMR spectroscopy. For 5-benzyl substituents, a folded conformati on of the two rings, due to intramolecular interactions, was found and for 5-exo-methylene substituted hydantoins the relevant E/Z isomerism at the exo-cyclic C, C double bond was studied. In addition, the H-1 and C-13 chemical shifts of the hydantoins proved to excellently indic ate the electronic distribution along the hydantoin ring moiety. Final ly, the mass spectrometric fragmentation of the hydantoins is critical ly discussed.