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 alk ali-isomerized linoleic acid contained mainly the methyl cis-9,trans-11- an d trans-10,cis-12-octadecadienoates (44 and 47%, respectively). These two i somers were then separated and purified by a series of low-temperature crys tallizations from acetone. The isomeric purity obtained for the cis-9, tran s-11-octadecadienoate isomer was >90% and that of the trans-10,cis-12-octad ecadienoate isomer was 89 to 97%. The isolated yield of the two isomers cor responded to 18 and 25.7%, respectively, of the starting material. The stru cture of the two isomers was confirmed using partial hydrazine reduction, s ilver nitrate-thin-layer chromatography of the resulting monoenes and gas c hromatography coupled with mass spectrometry of the 4,4-dimethyloxazoline d erivatives. Fourier transform infrared spectroscopy of the monoenes gave th e confirmation of the geometry of each double bond.