The copper‐catalysed oxidation of unsaturated carbonyl compounds: II. Base‐catalysed isomerization of isomesityl oxide and Δ5‐cholestenone

Abstract The mechanism of the base‐catalysed isomerization of the βγ‐unsaturated ketones isomesityl oxide and Δ5‐cholestenone to their αβ‐unsaturated isomers was studied. The results are interpreted on the basis of a reaction scheme in which a dienolate anion, formed by deprotonation of the starting material, is an intermediate. The factors determining the rate of deprotonation of unsaturated ketones are discussed. Methoxide ions were found to deprotonate both ketones studied, whereas triethylamine, which deprotonates isomesityl oxide directly, was found to react with Δ5‐cholestenone only via the intermediate formation of methoxide ions from the solvent methanol.