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 NH…︁N or CH…︁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.