β-Ureido acids and dihydrouracils—VII (original) (raw)

Ahatraet-The NMR spectra of the cis and trunr-isomers of 5.6tctramethylenedihydrouracil(2,4dioxodecahydroquinaxoline, TMDHU), 5,Wimethyldihydrouracif (DMeDHU), 5,6diphenyldihydrouracil (DPLDHU), 5-methyldihydroorotic acid (McDHO), as well as those of dihydroorotic acid (DHO) and its methyl ester, and 6-phenyldihydrouracil (WDHU) are reported The J,, and I,, coupling constants of these and other dihydrouracils are used in the conformational analysis Contrary to earlier suggestions3** of widely varying distortion of the dihydrouracil ring, the variation of cis J,s and Jr, is best explained by electronegativity and equilibrium effects. Carboxy and methoxycarbonyl groups at Cd prefer the axial orientation. A phenyl group at C-6, and the two phenyls in trans-DPhDHU, are predominantly axial in dimethyl sulphoxide, but equatorial in trilluoroacetic acid. The coupling constants of the cis and tram isomers of 2-ureidocyclohexane atrboxylic (UCHA) and of the eryrho and three isomers of 2-methyL3ureidobutyric acid (MeUBA) and of2,34iphenyl-3-ureidopropionic acid (DPhUPA) indicate that with both isomers of McURA conformers with gauche hydrogens are preferred, while in the case of DPhUPA trans hydrogens predominate.