General Papers ARKIVOC 2005 (xiii) 8-20 ISSN 1424-6376 Page 8 © ARKAT USA, Inc Tetrahydropyrimidin-2(1H)-ones with three neighbouring phenyl groups. Synthesis and allylic strain effects Vanya B. Kurteva* and Maria J. Lyapova Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl.9, 1113 Sofia, Bulgaria E-mail: vkurteva@orgchm.bas.bg (received 26 Jan 05; accepted 17 Feb 05; published on the web 27 Feb 05) Abstract Favoured conformations of differently N-substituted cis,trans-4,5,6-triphenyl- tetrahydropyrimidin-2(1H)-ones and the allylic strain caused by different groups were studied. It was found that the diequatorial conformer is strongly preferred in equally N,N’-disubstituted products, where the effects of the two substituents negate each other. It was shown that a benzyl group gives rise to much stronger allylic strain than a methyl group. Additionally, unusual azetidine formation was observed upon acid hydrolysis of benzylideneamino azide via S N 2 mechanism. It was suggested that N-substitution does not significantly influence the ring geometry angle, while quaternisation affords serious increase of puckering. Keywords: Tetrahydropyrimidin-2(1H)-ones, 1,2,3-triphenylpropane skeleton, favoured conformations, allylic strain, azetidines, ring geometry Introduction Compounds with a 1,2,3-triphenylpropane skeleton are useful models for conformational relationship studies due to their symmetry and the limited number of allowed conformations. Heterocyclic systems of this type are of special interest due to the tendency of phenyl groups to adopt axial vs equatorial position in the conformations of the various isomers. The conformational behaviour of tetrahydrooxazinones 1 and tetrahydrothiazinethiones 2 with three neighbouring phenyl groups (Figure 1) has been investigated. It was found that an N-substituent shifts the conformational equilibrium towards the conformer with an axial substituent at the carbon next to the nitrogen, due to the partial double character of the N-CO bond. The latter brings the N-substituent close to the equatorial one at the adjacent carbon, thus giving rise to allylic strain, A (1,2) . 3 It has been observed that the effect is most clearly revealed in the case of trans,cis diastereoisomers, 4 while with the rest of the isomers either the same conformer is favoured for unsubstituted compounds or an equilibrium between both allowed forms exists.