SummaryPhotoisomerization 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-comppnent mixtures of cis isomers which were separated by HPLC. FT-'H-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, ( 2 2 , 4 Z , 6E, 8E) 9, and (2Z, 4 Z , 6 Z , 8E) 7 were independently confirmed by direct syntheses. The 'H-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.Introduction. -In recent years considerable experimental and theoretical efforts have been focussed on the study of the primary photochemical step and the subsequent chemical events following the absorption of light by the visual protein rhodopsin. For better understanding of the physical and chemical factors influencing the photochemically initiated isomerization process several groups have studied extensively the photoisomerization of vitamin A and related polyenes in solution. In the course of these studies, several new cis isomeric photo-products have been isolated and identified by spectroscopic methods ([ 11 and ref. therein).In the following we report on the photoisomerization of an aromatic analogue of retinoic acid, ethyl all-trans-9-(4-methoxy-2,3,6-trimethylphenyl)-3,7-dimethylnona-2,4,6,8-tetraenoate 1 (Scheme I ) . As expected, irradiation of dilute solutions of 1