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The conformations of the cis and trans isomers of 4,6-diphenyl-, 4,5-diphenyl-and 5,6-&phenyltetrahydrro-1,3-oxazin-2-one and 4,5-diphenylhexahydropyrimidin-2-one, and of some of their N-substituted derivatives, have been studied by 'HNMR. Conformers with 4a, 6e-, 4a,5e-and Sa,se-phenyl groups are preferred in the respective isomers of the N-H oxazinones, confirming a half-chair conformation of the ring. Auylic strain caused by N-substituents shifts strongly the a,e =$ e,a equilibria in trans-4,6-diphenyl-and cis-4,s-diphenyloxazinones, but only moderately the e,e*a,a equilibria in the compounds with truns-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.An important feature of six-membered rings containing double bonds is the allylic strain2 between substituents at an endocyclic or exocyclic double bond and an equatorial ring substituent, resulting in a preference for the axial conformation. The effect of N-substituents in saturated heterocycles with an endocyclic amide bond has received less attention, although cases of preferred axial conformation have been r e p~r t e d .~ A striking example4 was encountered in the case of the r-4, c-5, t-6-and r-4, t-5, t-6-isomers of 4,5,6-triphenyltetrahydro-1,3-oxazin-2-one. An N-substituent strongly shifted the e , e , a e a,a,e or e,a,a* a,e,e equilibria towards the conformer with an axial neighbouring phenyl group, irrespective of whether this involved two equatorial or two axial phenyl groups. It was of interest to assess the effect of allylic strain in similar systems where the axial conformation of the phenyl group can be attained with a varying degree of steric strain. We now report a 'HNMR study of the conformations of the compounds shown in Scheme 1.Thus, in the trans-4,6-diphenyl-and cis-4,5-diphenyltetrahydro-1,3-oxazin-2-ones (trans-1 and cis-2 respectively), allylic strain will affect a , e Z e,a equilibria, while in the trans-4,5-diphenyloxazinones (trans -2) and -hexahydropyrimidin-2-ones (truns-4), it will act against the strain of two axial phenyl groups. RESULTS AND DISCUSSIONThe coupling constant data are summarized in Table 1.Because of the biased diequatorial conformation of cis-4,6-diphenyloxazinone (cis-la, formula A, Scheme 2), the observed coupling constants J(4a5a) and * Author to whom correspondence should be addressed. J(5a6a) can be used as references in the estimates of conformational populations. The limitations of such estimates are well known, and one particular complication is discussed below. The following rounded off 'neat' values were assumed: J(4a5a) = 11.5 Hz; J(5a6a) = 12.0 Hz and J(4e5e) = J(5e6e) = 2 Hz. The J(4e5e) value is taken from data for strongly biased N-substituted triphenyloxa~inones;~ for J(5e6e) the choice is arbitrary.The form of the oxazinone ring approaches a halfchair,4 because of the greater double bond character of the N-CO bond than that of the 0-CO bond, and this e...

The rate data for the acid-catalysed ring closure of eight methyl-substituted 3-ureidopropionic acids constitute an isokinetic series according to Exner's statistical criteria. Linear relationships were obtained for the free energies in the reaction series for rates and equilibria exhibiting the gem-dimethyl effect with steric strain energies estimated from enthalpies of formation of homomorphic hydrocarbons. These estimates were obtained by means of lstomin and Palm's procedure and are of t w o kinds, the P values measuring the total strain energy and A P values measuring the strain between t w o fragments in the model hydrocarbon. Some reaction series correlated with A Pvatues when one of the functional groups was taken as an invariable fragment and the rest of the molecule as a variable substituent. Other reaction series correlated with P values. The approach has the advantage that it takes into account directly the non-additivity of gauche-interactions and permits evaluation of the data for substituents at various positions of the chain in a single correlation. For cyclization t o five-and six-membered rings, the data form a single reaction series when the rates for the parent compounds are close in value. Only certain hydrocarbon models and manners of fragmentation lead to satisfactory correlations. These, and the correlation with P or with ri P values, or whether a correlation can be obtained at all, indicate the steric requirements of the reaction and thus its mechanism. Linear relationships with strain energies were obtained for reaction series where the rate accelerations have been attributed to entropy effects.