NMR spectroscopic study of configurations and conformations of 5-pyridylmethylenehydantoins

Nine 5-pyridylmethylenehydantoins were prepared. Each of the 1-methyl-substitute compounds was obtained in two stereoisomeric forms. Only one form of each of the 3-methyl-substituted and N-unsubstituted compounds was obtained directly from synthesis but could be partially converted into the other stereoisomer photochemically. The Z/E configurations and the conformational relationship between the pyridine and hydantoin rings were studied by 1H and 13C NMR spectroscopy, including variable-temperature 1H NMR. The existence of NH…N or CH…N interactions and the possibility of tautomerism are suggested for some of the compounds. The Z-isomers of compounds with 2- or 3-pyridyl rings prefer an s-cis conformation whereas the E-isomers prefer an s-trans conformation.