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 1 H 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.