Preparation of sterically hindered geometric isomers of 7-cis-β-ionyl and β-ionylidene derivatives in the vitamin A series

The preparation of the relatively unknown 7-cis isomers of β-ionyl and β-ionylidene derivatives via one way sensitized geometric isomerization is described. These highly sterically crowded isomers, once prepared do not readily thermally revert back to the trans. But the trienes, at temperatures ≥ 100° undergo irreversible cyclization to cyclohexadienes. 7-cis Isomers of β-ionylideneacetone and acetaldehyde as well as higher tetraenes and pentaenes in this series could not be prepared by sensitized isomerization. Partial reduction of 7-cis-β-ionylideneacetonitrile isomers led to the preparation of 7-cis- and 7,9-dicis-β-ionylideneacetaldehyde; and methyl Grignard reaction with the same nitrile gave the triene-methyl ketones.