A 1H and 13C NMR study of methoxyretinoids. Spectral assignment and determination of configuration

A number of modified retinals and retinoic esters carrying one or two methoxy groups or one methoxy and one methyl group on the polyene chain were investigated by 1H and 13C NMR spectroscopy. Spectral assignments were made from homo- and selective 13C{1H} hetero-decoupling experiments and from chemical shift comparisons. The configurations of the polyene double bonds were derived from vicinal H,H coupling constants, from 1H and 13C chemical shifts and by measuring nuclear Overhauser enhancements. It is found that all double bonds with no additional substituents occur in the trans (E) and all methoxy-substituted double bonds in the cis (also E) configuration. Double bonds carrying methyl groups give rise to both cis (Z) and trans (E) isomers.