A simple method of preparation of methyl trans‐10,cis‐12‐ and cis‐9,trans‐11‐octadecadienoates from methyl linoleate

Pure conjugated isomers of linoleic acid were prepared on a large scale by alkali‐isomerization of purified methyl linoleate. The methyl esters of alkali‐isomerized linoleic acid contained mainly the methyl cis ‐9, trans ‐11‐ and trans ‐10, cis ‐12‐octadecadienoates (44 and 47%, respectively). These two isomers were then separated and purified by a series of low‐temperature crystallizations from acetone. The isomeric purity obtained for the cis ‐9, trans ‐11‐octadecadienoate isomer was >90% and that of the trans ‐10, cis ‐12‐octadecadienoate isomer was 89 to 97%. The isolated yield of the two isomers corresponded to 18 and 25.7%, respectively, of the starting material. The structure of the two isomers was confirmed using partial hydrazine reduction, silver nitrate‐thin‐layer chromatography of the resulting monoenes and gas chromatography coupled with mass spectrometry of the 4,4‐dimethyloxazoline derivatives. Fourier transform infrared spectroscopy of the monoenes gave the confirmation of the geometry of each double bond.