Directed sequential synthesis of conjugated linoleic acid isomers from Δ7,9 to Δ12,14

Position and configuration isomers of conjugated linoleic acid (CLA), from 7,9- through 12,14-C18:2, were synthesized by directed sequential isomerizations of a mixture of rumenic (cis-9,trans-11 C18:2) and trans-10,cis-12 C18:2 acids. Indeed, the synthesized conjugated fatty acids cover the range of unsaturated systems as found in milk fat CLA. The two-step sequence consisted in initial sigmatropic rearrangement of cis/trans CLA isomers at 200degreesC for 13 h under inert atmosphere (Helium, He), followed by selenium-catalyzed geometrical isomerization of double bonds at 120degreesC for 20 h under He. Product analysis was achieved by gas-liquid chromatography using a 120 m polar capillary, column coated with 70% cyanoalkylpolysiloxane equivalent polymer. Migration of conjugated systems was geometrically controlled as follows: the cis-C-n,trans-Cn+2 double bond system was rearranged through a pericyclic [1,5] sigmatropic mechanism into a trans-Cn-1,cis-Cn+1 unsaturated system, while the trans-C-n,cis-Cn+2 double bond system was rearranged through a similar pericyclic mechanism into a cis-Cn+l,trans-Cn+3 unsaturated system. Selenium-catalyzed geometrical isomerization under mild conditions then allowed cis/trans double bond configuration transitions, resulting in the formation of all cis, all trans, cis-trans and trans-cis isomers. A sequential combination of the two reactions resulted in a facile controlled synthesis of CLA isomers, useful for the chromatographic identification of milk fat CLA, as well as for the preparation of CLA standard mixture.