Himachalene hydrocarbons

Six male specific compounds were isolated from the crucifer flea beetle, Phyllotreta cruciferae, and the same compounds plus two additional compounds were isolated from males of Aphthona flava, A. czwalinae, and A. cyparissiae. Three of the compounds were identified as (-F)-i r-himachalene, (-F)- ra w-a-himachalene (-F)-y-cadinene. Two other compounds were identified as new enantiomers of himachalene hydrocarbons that were previously identified from the fir trees, Abies alba and Abies nordmanniana. Finally, there were two himachalene alcohols and one nonsesquiterpene ketone that is a himachalene analog that were identified. The chemical and electrophysiological patterns are consistent with, but do not prove, a pheromonal function. 

Collection of volatiles from males feeding on pieces of cabbage revealed a blend of six male-specific sesquiterpenes, including hydrocarbons 24-28 and ketone 29, shown in Figure 19.5 (Bartelt et al., 2001). Compound 24 was the most abundant. One or more of these were thought likely to account for the pheromonal activity reported earlier. Hydrocarbon 28 [(+)-v-cadincnc is known from citronella oil (Herout and Sykora, 1958), but structures 24-27 and 29 were new. The opposite enantiomers of hydrocarbons 24 and 25 were previously isolated from the trees Nordmann fir (Abies nordmanniana) and silver fir (Abies alba) (Bartelt et al., 2001 Khan and Pentegova, 1988 Khan et al., 1989), and the opposite enantiomer of 27, known as ar-himachalene, is found in Himalayan cedar (Pandey and Dev, 1968). Neither compound 26 nor its enantiomer were known from nature, but... 

Why was our stereochemical conclusion opposite to that of Bartelt s To answer this question, I synthesized (RH/r-himachalene (84) by employing Evans asymmetric alkylation as the key step (Figure 4.25).49 The steric course of the Evans asymmetric process (A->-B) is well established and unambiguous. When the specific rotation of my synthetic (RH/r-himachalene (84) was measured, I immediately found the reason why Bartelt made a mistake. The hydrocarbon 84 was levorotatory in chloroform, but dextrorotatory in hexane. Although Sukh Dev used chloroform in his rotation measurement, Bartelt did not care about the solvent and used hexane, without knowing the fact that a different solvent may change the sign of rotation. 

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