Isolation by HPLC. and Identification by NMR. Spectroscopy of 11 mono‐, di‐ and tri‐cis isomers of an aromatic analogue of retinoic acid, ethyl all‐trans‐9‐(4‐methoxy‐2,3,6‐trimethylphenyl)‐3,7‐dimethyl‐nona‐2,4,6, 8‐tetraenoate

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Abstract
Photoisomerization of an aromatic analogue of retinoic acid, ethyl all‐trans‐9‐(4‐methoxy‐2,3,6‐trimethylphenyl)‐3,7‐dimethyl‐nona‐2,4,6, 8‐tetraenoate 1 in dilute solutions of hexane, benzene, and ethanol yielded multi‐component mixtures of cis isomers which were separated by HPLC. FT‐1H‐NMR. at 270 MHz and, in some cases, homonuclear decoupling and Overhauser experiments as well as 13C‐NMR. were applied to establish the structures of 4 mono‐cis, 4 (of 6 possible) di‐cis, and 3 (of 4 possible) tri‐cis isomers. The structures of 3 isomeric esters, namely (2Z, 4E, 6E, 8E) 6, (2Z, 4Z, 6E, 8E) 9, and (2Z, 4Z, 6Z, 8E) 7 were independently confirmed by direct syntheses. The 1H‐NMR. data of all these compounds and the 13C‐NMR. data of the all‐trans and of 6 cis isomers available in sufficiently large quantities are discussed.