Structural elucidation of the main cis beta-carotenes.

Different isomerization experiments on all trans-beta-carotene (I) yielded multi-component mixtures of cis isomers. Separation and isolation of these isomers were achieved by classic adsorption chromatography on a lime column. Investigation of their 200 MHz 1H- and 50.3 MHz 13C-NMR spectra led to the structural elucidation of the main cis-beta-carotenes, namely the 9-monocis structures (II) for neo beta-carotene U, and the 13-monocis structure (III) for neo beta-carotene B. Formation of other hindered monocis (7-cis and 11-cis) and 15-monocis isomers should be carefully examined by a suitable method such as high-performance liquid chromatography.