Synthesis and nuclear magnetic resonance properties of all geometrical isomers of conjugated linoleic acids.

Pure geometric isomers of conjugated linoleic acid were prepared from castor oil as the primary starting material. Methyl octadeca-9Z,11E-dienoate (2) and methyl octadeca-9Z,11Z-dienoate (4) were obtained by zinc reduction of methyl santalbate (1, methyl octadec-11E-en-9-ynoate) and methyl octadec-11Z-en-9-ynoate (3), respectively, as the key intermediates. Methyl octadeca-9E,11E-dienoate (8) and methyl octadeca-9E,11Z-dienoate (9) were prepared by demesylation of the mesyloxy derivative of methyl ricinelaidate (6, methyl 12-hydroxy-octadec-9E-enoate). A study of the nuclear magnetic resonance spectral properties was carried-out, and the shifts of the olefinic carbon atoms of 18:2(9Z,11E) (2) and 18:2(9E,11Z) (9) were readily identified by a combination of incredible natural abundance double quantum transfer experiment, heteronuclear multiple bond correlation, and 1H-13C correlation spectroscopy correlation techniques. Doubts remain in the absolute identification of the individual olefinic carbon atoms of the 18:2(9Z,11Z) (4) and 18:2(9E,11E) (8), except the fact that the shifts of the "inner" (C-10 and C-11) and "outer" (C-9 and C-12) positioned olefinic carbon atoms of the conjugated diene system are distinguishable.