A 13C‐NMR. Study of cis‐trans Isomeric Vitamins A, Carotenoids and Related Compounds

The 1H‐decoupled 13C‐NMR. spectra of 35 all‐trans, 17 mono‐cis vitamin A compounds (acetates, alcohols, aldehydes, acids and esters) and of one 11, 13‐di‐cis compound (11, 13‐di‐cis retinol) are reported. Included in this investigation are desmethyl‐, desmethylethyl, and aryl‐vitamin A analogues and others as well as 30 reference compounds of smaller molecular weight. Furthermore, the 13C‐NMR. spectra of 23 β‐apo‐ and other carotenoids were studied. A complete assignment of the signals of all 106 compounds to the specific carbon atoms was achieved by extensive application of lanthanide shift reagents, mainly Yb(dpm)3, by CW‐offset and selective 1H‐decoupling experiments, by comparison of the shifts of related compounds, and in three cases by utilization of specifically deuteriated compounds (11, 12‐D2‐retinol and retinyl acetate, 15, 15′‐D2‐β‐carotene). The chemical shift differences between the cis‐ and trans‐vitamin A compounds and the applicability of the shift reagents for the assignment of the 13C‐NMR. spectra are discussed.