NMR study of the conformational distribution in methyl esters and N,N-dimethylamides of diastereoisomeric 4-cyano-3,4-diphenyl- and 4-cyano-3-phenyl-4-(1-naphthyl)-butyric acids. Relation to the configurations

Abstract

The conformational behaviour of Ar 2CH(CN)CH(Ar 1)CH 2COR (R  OCH 3; N(CH 3) 2; Ar 1  Ph; Ar 2  Ph or 1-naphthyl) has been investigated by 1H and 13C NMR spectroscopy. The vicinal proton-proton coupling constants were used together with those of carbon-proton couplings to provide information on the conformational preference in the compounds studied. The conformational distribution with respect to the C(2)C(3) bond is shifted to the conformation having trans Ph and COR groups. Coupling constants involving the cyano carbon atom and the methine proton are J g CN,H = 2.0 Hz and J t CN,H = 14.1 Hz. The conformation with the cyano group and the methine proton trans to each other, with gauche-oriented protons around the C(3)C(4) bond is the preferred one for both isomers in the compounds investigated. The results obtained support the assumption that electrostatic (non-bonded attractive) interactions rather than steric demands operate as dominant factors in controlling the conformational equilibrium.