Synthesis and Stereochemistry of the Four Himachalene‐Type Sesquiterpenes Isolated from the Flea Beetle (Aphthona flava) as Pheromone Candidates

Both the (1S,2R) and (1R,2S) isomers of 2,6,6‐trimethylbicyclo[5.4.0]undec‐7‐en‐9‐one (1) were synthesized by starting from (S)‐ and (R)‐citronellal (5), respectively. The stereochemistry of (1R,2S)‐(−)‐1 was established by an X‐ray analysis; it exhibits a CD spectrum similar to that of cholest‐4‐en‐3‐one, in agreement with its absolute configuration. The ketones (1S,2R)‐ and (1R,2S)‐1 were converted into the enantiomers of three other himachalene hydrocarbons, 2,2,6‐trimethyl‐10‐methylenebicyclo[5.4.0]undec‐1(11)‐ene (2), 1,1,5,8‐tetramethyl‐1,2,3,4,5,6,5a‐heptahydrobenzo[1,2‐a][7]annulene (3), and 1,1,5,8‐tetramethyl‐1,2,3,4,5‐pentahydrobenzo[a][7]annulene (ar‐himachalene, 4). The stereochemistry of these himachalene‐type sesquiterpenes isolated from flea beetles such as Aphthona flava and Phyllotreta cruciferae as their male‐pheromone components was revised to (1S,2R)‐1, (6R,7S)‐2, (5R,5aS)‐3, and (R)‐4. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)