BETA-UREIDO ACIDS AND DIHYDROURACILS .7. APPLICATIONS OF PROTON RESONANCE SPECTROSCOPY .32. NMR SPECTRA AND CONFORMATION OF DIHYDROURACILS AND RELATED COMPOUNDS

The NMR spectia of the cis and trans-isomers of 5,6-tetramethylenedihydrouracil (2,4-dioxo-decahydroquinazoline, TMDHU), 5,6-dimethyldihydrouracil (DMeDHU), 5,6-diphenyldihydrouracil (DPhDHU), 5-methyldihydroorotic acid (MeDHO), as well as those of dihydroorotic acid (DHO) and its methyl ester, and 6-phenyldihydrouracil (PhDHU) are reported. The Jtrans and J16 coupling constants of these and other dihydrouracils are used in the conformational analysis. Contrary to earlier suggestions3,4 of widely varying distortion of the dihydrouracil ring, the variation of cis J56 and J16 is best explained by electronegativity and equilibrium effects. Carboxy and methoxycarbonyl groups at C-6 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 trifluoroacetic acid. The coupling constants of the cis and trans isomers of 2-ureidocyclohexane carboxylic (UCHA) and of the erytho and threo isomers of 2-methyl-3-ureidobutyric acid (MeUBA) and of 2,3-diphenyl-3-ureidopropionic acid (DPhUPA) indicate that with both isomers of MeUBA conformers with gauche hydrogens are preferred, while in the case of DPhUPA trans hydrogens predominate. © 1969.