Cyclic fatty esters: Synthesis and characterization of methyl ω‐(6‐alkyl‐3‐cyclohexenyl) alkenoates

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Abstract
Diunsaturated C18 cyclic fatty acid methyl esters of known structure and configuration were synthesized as model derivatives of cyclic fatty acids formed in heat‐abused vegetable oils for characterization and further biological evaluation. The Wittig reaction was used to prepare 5 pure methyl esters: (a) 12‐(3‐cyclohexenyl)‐11‐dodecenoate, (b) 11‐(6‐methyl‐3‐cyclohexenyl)‐10‐undecenoate, (c) 10‐(6‐ethyl‐3‐cyclohexenyl)‐9‐decenoate, (d) 9‐(6‐propyl‐3‐cyclohexenyl)‐8‐nonenoate and (e) 8‐(6‐butyl‐3‐cyclohexenyl)‐7‐octenoate. Diels‐Alder cycloaddition reactions between 1,3‐butadiene and appropriate (E)‐2‐alkenals produced 3‐cyclohexenal intermediates. The appropriate methyl ω‐bromoesters and their triphenylphosphonium bromides were made and converted to their respective ylids with NaOCH3 in DMF. The appropriate 3‐cyclohexenals and phospho‐ylids were reacted, and the desired cyclic ester products were isolated in crude yields of 30–83% as liquids and fractionally distilled. The crude cyclic esters were purified either by preparative TLC or by saponification‐esterification. Double bonds in purified cyclic esters weretrans‐isomerized and hydrogenated. Each derivative was characterized by IR,1H‐NMR,13C‐NMR, capillary GLC and GC‐MS. On the basis of these analyses, no positional isomers were detected, Z‐unsaturated isomers were produced in better than 90% purity, and the alkyl and ester ring substituents were predominantlytrans to each other.