Determination of erythro and threo configurations by nuclear magnetic resonance spectroscopy

An examination of the nuclear magnetic resonance spectra of ten pairs of racemic erythro and threo isomers of 1,2-disubstituted-1-arylpropanes shows that the signals of the methyl protons of the erythro isomer always appear at lower field than the threo isomer. The vicinal coupling constant (Jab) of the erythro isomer is found to be smaller than that of the threo isomer in six of the isomeric pairs indicating that the magnitude of Jab is a poor criterion of configuration of the 1,2-disubstituted-1-arylpropanes.