Nuclear magnetic resonance spectra, configuration and conformation of diastereomers—II : Ethyl esters of 3-substituted 2,3-diphenylpropanoic acids: Magnetic nonequivalence induced by two asymmetric centres

Abstract

The NMR spectra of the erythro and threo forms of the esters PhCH aXCH b(Ph)COOC 2H 5, where X = OH, NH 2, NHPh, and NHCONH 2 have been investigated in CDCl 3, DMSO-d 6, benzene and pyridine solutions. Study of the conformational distribution based on the values of the vicinal coupling constants J ab showed that the conformer with antiperiplanar methine protons is the favored one for both diastereomers. Some chemical-shift criteria permitting the assignment of relative configuration of diastereomeric compounds of such type were established. It was shown by use of suitable model compounds that the magnetic nonequivalence of the methylene protons observed in some solvents might be attributed not to either of the asymmetric centres taken alone, but rather to the dissymmetry of the molecule as a whole.