Synthesis and isolation of trans‐7, cis‐9 octadecadienoic acid and other CLA isomers by base conjugation of partially hydrogenated γ‐linolenic acid

• Published in: Lipids Vol. 38; no. 5; pp. 579 - 583
• Main Authors: Delmonte, Pierluigi, Roach, John A. G., Mossoba, Magdi M., Morehouse, K. M., Lehmann, Lutz, Yurawecz, Martin P.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer‐Verlag 01-05-2003
• Subjects: Chemistry, Organic - methods; Chromatography, Gas; Chromatography, High Pressure Liquid; Fatty Acids, Unsaturated - chemical synthesis; gamma-Linolenic Acid - chemistry; Gas Chromatography-Mass Spectrometry; Hydrogenation; Isomerism; Linoleic Acids, Conjugated - chemical synthesis
• ISSN: 0024-4201; 1558-9307
• DOI: 10.1007/s11745-003-1499-5

CLA is of considerable interest because of reported potentially beneficial effects in animal studies. CLA, while not yet unambiguously defined, is a mixture of octadecadienoic acids with conjugated double bonds. The major isomer in natural products is generally considered to be cis‐9,trans‐11‐octadecadienoic acid (c9, t11), which represents >75% of the total CLA in most cases. Other isomers are drawing increased attention. The t7,c9 isomer, which is often the second‐most prevalent CLA in natural products, has been reported to represent as much as 40% of total CLA in milk from cows fed a high‐fat diet. The need for a reference material became apparent in a recent study directed specifically at measuring t7,c9‐CLA in milk, plasma, and rumen. A suitable standard mixture was produced by stirring 0.5 g of γ‐linolenic acid (all cis‐6,9, 12‐C18∶3) with 100 mL of 10% hydrazine hydrate in methanol for 2.5 h at 45°C. The solution was diluted with H2O and acidified with HCl. The resulting partially hydrogenated FA were extracted with ether/petroleum ether, dried with Na2SO4, and conjugated by adding of 6.6% KOH in ethlylene glycol and heating for 1.5 h at 150–160°C. Approximately 20 mg each of cis‐6, trans‐8; trans‐7, cis‐9; cis‐9, trans‐11; and rans‐10, cis‐12 were obtained along with other FA. Methyl esters (FAME) of these four cis/trans isomers were resolved by Ag+HPLC (UV 233) and partially resolved by GC/(MS or FID) (CP‐Sil 88). Treatment of these FAME with I2 yielded all possible cis/trans (geometric) isomers for the four positions 6,8; 7,9; 9,11; and 10,12.