Chemical Synthesis and Gas Chromatographic Behaviour of γ-Stearidonic (18:4n-6) Acid

gamma-Stearidonic acid, 18:4n-6, a potential product of beta-oxidation of arachidonic acid (20:4n-6), was only recently positively identified in a living organism-a thermophilic cyanobacterium Tolypothrix sp., albeit at low levels, whilst some indirect evidence suggests its wider presence, e.g. in a unicellular marine alga. We have prepared 18:4n-6 using an iodolactonisation chain-shortening approach from 22:5n-6 and obtained its H-1-, C-13-, COSY- and HSQC NMR spectra, with 18:5n-3 spectra also recorded for a comparison. The GC and GC-MS behaviour of its methyl ester was also studied. Like another Delta 3 polyunsaturated acid, octadecapentaenoic (18:5n-3), 18:4n-6 rapidly yields 2-trans isomer upon formation of dimethyloxazoline derivative. On a polar ionic liquid phase (SLB-IL100, 200 A degrees C) the methyl ester could be mistaken for 18:3n-3, while on methylsilicone phase (BP1, 210 A degrees C) it eluted ahead of 18:3n-6 and 18:4n-3, suggesting that when present it may be easily misidentified during GC analysis of fatty acids.