Total Synthesis of Himachalene Sesquiterpenes ofAphthonaandPhyllotretaFlea Beetles

Abstract
A total synthesis is presented for himachalene-related norsesquiterpene ketone 1 (racemic), based on the Robinson annulation and other standard reactions. The route involved five steps from cycloheptanone. A favorable result was that the desired dia­stereomer of 1 was obtained preferentially (97:3) over the unwanted one. Molecular modeling (ab initio calculations) aided in confirming the assignment of relative stereochemistry of 1 and in rationalizing the observed ratio of diastereomers. Three related sesquiterpene hydrocarbons and two alcohols were subsequently produced from 1. The six compounds occur naturally in several flea beetle species and likely serve a pheromonal function.