The Stereospecific Synthesis of Abscisic Acid

A stereospecific synthesis of 3-methyl-5-(1-hydroxy-4-oxo-2,6,6-trimethyl-2-cyclohexen-1-yl)-cis, trans-2,4-pentadienoic acid (abscisic acid) from ${\alpha}-ionone$ has been investigated. Ethyl 5-(2,6,6-trimetyl-2-cyclohexen-1-yl)-trans-4-penten-2-ynoate $({\alpha},{\beta}-acetylenic\;ester)$, which was synthesized from alpha-ionone in two steps, was stereospecifically converted in good yield into ethyl 3-methyl-5-(2,6,6-trimethyl-2-cyclohexen-1-yl)-cis, trans-2, 4-pentadienoate $({\alpha}-ionylideneacetate)$ by the conjugate addition of lithium dimethylcuprate at $-78^{\circ}C$. Basic hydrolysis of the ethyl ${\alpha}-ionylideneacetate$ gave an abscisic acid precursor, 3-methyl-5-(2,6,6-trimethyl-2-cyclohexen-1-yl)-cis, trans-2,4-pentadienoic acid, which can be oxidized to yield abscisic acid.