Synthetic studies on farnesene isomers and homologues from the Dufour glands of ants

Abstract

(Z,E)-q-farnesene and its homologues homo-, bishomo- and trishomo-farnesenes had been identified as constituents, of the Dufour gland secretion of several species of Myrmica ant. Although farnesene isomers had been found previously in plant and insect sources, the homo-, bishomo- and trishomo-farnesenes were a new group of substances.
A general preparative route, that would permit the synthesis of this homologous series, was sought to confirm identification and for biological testing.
A partially stereospecific synthesis of (Z,E)- and (Z,Z)-a- -farnesenes, involving a Wittig reaction between 6-methyl-5-hepten-2- -one and the ylid from (Z)-4-methyl-3,5-hexadienyltriphenylphosphonium iodide, is described. Although the Wittig reaction was performed under conditions reported to give trans-selective olefin formation, it gave a mixture in which (Z,E)-a-farnesene was the minor component. The ' isomers were separated on a silver nitrate loaded liquid chromatography column. The identity of the natural and synthetic isomers was confirmed.
Two ketones, 7-methyl-6-octen-3-one and 7-methyl-6-nonen-3-one were successfully prepared by a multistage route from 4-chlorobutyryl chloride, the former was used in condensations with the ylid described above. Attempts were made to find optimum conditions for the condensation but yields of (Z,E)- and (Z,Z) -homo fame senes were poor.
The mass spectrum of the (Z,E) isomer, obtained by GC-MS, was identical to that of ant homofarnesene.
Another route to the series, a variation of the Wittig reaction using a sulphoxide reagent, was tried in an attempt to improve the yield of farnesene homologues at the final stage. Trial condensations of the prepared t-butylsjulphoxide reagent with 6-methyl-5-hepten-2- -one gaye the desired *-hydrqxysulphoxide but the elimination step to giye farnesene could not he successfully achieved.