Z-E ISOMERISM OF 5-CYCLOHEXYLMETHYLENEHYDANTOINS

5-Cyclohexylmethylenehydantoin and its 1- and 3-methyl derivatives were studied as the aliphatic analogues of the 5-benzylidenehydantoins to examine the effects of replacing a benzene by a cyclohexane ring on Z-E isomerism. The relative stabilities of the Z- and E-isomers were compared by estimating the free energy differences from thermal equilibration experiments and the heats of formation by AM1 calculation. In the Z-isomers, the possible existence of weakly attractive interaction between the C(4) = 0 and C(6)-H groups, which form parts of a stereochemically favourable, conjugated alpha,beta-unsaturated system, is suggested as a contributory factor to the observed configurational preference. Assignment of configurations was based mainly on NMR and is supported by IR and UV spectra. NMR spectra also revealed the conformation of the cyclohexane ring and the cyclohexyl proton signals were assigned by double resonance and by high-field NMR in two solvents where solubility allows.