Abstract

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. H-1 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 Br-2 to the double bond and an almost neutral energy difference between the starting olefin and the diene pheromone.