Thiol-catalyzed cis-trans isomerization of oleic acid

Various thiols were found to catalyze the geometrical isomerization of oleic acid to trans-Δ⁹-octadecenoic acid. The reaction proceeds in neutral aqueous solution at mild temperatures and at relatively low thiol concentration, 5–20 meq/liter. Hydrogen from the medium was not incorporated into the product, and no trace of Δ⁸ or Δ¹⁰ octadecenoic acid could be detected among the products. The reaction is proposed to involve the formation of a mixed micelle of fatty acid and thiol, nucleophilic attack of the double bond by the thiol, rotation about the former double bond, and elimination of the thiol to produce the thermodynamically more stable trans isomer. The cationic reagent, 2-mercaptoethylamine, was the most efficient catalyst tested. This system should prove to be useful for the preparation of labeled trans unsaturated fatty acids.