Improved synthesis of the chrysomelid pheromone (6R,7 S)-(+)-himachala-9, 11-diene via spontaneous bromination and didehydrobromination of 2,6,6,9-tetramethyl-bicyclo[5.4.0]undec-8-ene

A convenient synthesis of (6R,7S)-(+)-himachala-9,11-diene, the pheromone of the chrysomelid beetle Phyllotreta striolata is described. The diene is obtained in a single operation by a spontaneous "bromination/ dehydrobromination" of 2,6,6,9-tetramethylbicyclo[5.4.0]undec-8-ene. The halogenation/dehalogenation sequence proceeds spontaneously in CCl₄, and is less uniform in CH₂Cl₂ and CHCl₃. ¹H NMR experiments carried out in presence of the radical scavenger di-tert-butyl-4-methylphenol suggest an ionic mechanism for this reaction. Theoretical calculations demonstrate that the spontaneous reaction profits from the strongly exergonic addition of Br₂ to the double bond and an almost neutral energy difference between the starting olefin and the diene pheromone.