7-Cis isomers of retinal via 7-cis- and 7,9-dicis-β-C₁₈-tetraene ketones. New geometric isomers of vitamin A and carotenoids-I

Based catalyzed condensation of 7-cis- or 7,9-dicis-β-ionylideneacetaldehyde with acetone gave either 7-cis or 7,9-dicis-β-C₁₈-tetraene ketone. Reaction of the tetraene ketone mixture with diethyl cyanomethylphosphonate gave a mixture of 4 isomers of retinonitrile, all believed to have the 7-cis geometry. Partial redution of the retinonitrile mixture with di-isobutylaluminum hydride gave a mixture of retinal isomers. Repeated column chromatography resulted in partial separation of the retinal isomers. Based on NMR data the geometry of the isomers prepared are believed to be 7-cis, 7,13-dicis, and 7,9,13-tricis.