Improved synthesis of three methyl-branched pheromone components produced by the female lichen moth

Female moths of Lyclene dharma dharma (Arctiidae, Lithosiinae) produce a novel sex pheromone composed of three methyl-branched ketones: 6-methyl-2-octadecanone (I), 14-methyl-2-octadecanone (II), and 6,14-dimethyl-2-octadecanone (III). Their structures were confirmed by syntheses accomplished by a different route for each component. In order to obtain a sufficient amount of the synthetic pheromone, we developed new routes via methyl-branched 1-alkenes: 6-methyl-1-octadecene (1), 14-methyl-1-octadecene (2), and 6,14-dimethyl-1-octadecene (3). Compound 1 was synthesized by coupling between a C(10)-chain bromide and a 3-methyl-branched C(8) unit (A) prepared from 3-methyl-1,5-pentanediol, 2, by coupling between a C(11)-chain bromide and a 3-methyl-branched C(7) unit (B) prepared from 2-hexanone, and 3, by connecting A and B, using propargyl alcohol as a C(3) linchpin. The use of 3-chloro-1-propanol and tert-butyl acetoacetate as the linchpin was also examined to connect the two synthetic blocks in the synthesis of 3. Components I-III were obtained by Wacker oxidation of the corresponding 1-alkenes 1-3 in good yields.