Effect of allylic strain on the conformations of diphenyl-substituted tetrahydro-1,3-oxazin-2-ones and hexahydropyrimidin-2-ones

The conformations of the cis and trans isomers of 4,6-diphenyl-, 4,5-diphenyl- and 5,6-diphenyltetrahydro-1,3-oxazin-2-one and 4,5-diphenylhexahydropyrimidin-2-one, and of some of their N-substituted derivatives, have been studied by ¹H NMR. Conformers with 4a, 6e-, 4a, 5e- and 5a, 6e-phenyl groups are preferred in the respective isomers of the N-H oxazinones, confirming a half-chair conformation of the ring. Allylic strain caused by N-substituents shifts strongly the a,e?e, a equilibria in trans-4,6-diphenyl- and cis-4,5-diphenyl-oxazinones, but only moderately the e,e?a,a equilibria in the compounds with trans-vicinal phenyl groups. In the latter, the diaxial conformation is preferred only in the case of bulky N-substituents. The diaxial conformation is more favoured in the trans-4,5-diphenylpyrimidones.