Submitted to: American Chemical Society Symposium Series
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2001
Publication Date: N/A
Interpretive Summary: Due to customer concerns about the use of broad-spectrum pesticides and their effect on the environment, we are developing pest control strategies requiring the use of less pesticide. Sap beetles of the genus Carpophilus are pests of corn that can introduce and transmit damaging fungi from ear to ear. Male sap beetles produce a chemical substance, termed an aggregation pheromone, that selectively attracts male and female members of its own species. We can use pheromone-baited traps in fields to monitor for the presence and numbers of specific insect pests and thus precisely time the use of pesticide sprays. There is strong interest by growers in the use of sap beetle pheromones as part of an effective pest management strategy, but there is no commercial supplier because of the difficulty and cost of synthesis. We have improved critical steps in the synthetic pathway leading to the compounds. The potential use of these pheromones in agriculture is more likely as a result of our research efforts.
Technical Abstract: Sap beetles, belonging to the genus Carpophilus, produce aggregation pheromone blends that are composed of conjugated tetriene and tetraene hydrocarbons having all E-double-bond configuration. The preparation of (2E,4E,6E,8E)-3,5,7-trimethyl-2,4,6,8-decatetraene (1) and (3E,5E,7E,9E)- 6,8-diethyl-4-methyl-3,5,7,9-dodecatetraene (2) was improved in terms of overall yield: 46% vs 40% for compound 1 and 20% vs 8% for compound 2. A mild Horner-Wadsworth-Emmons olefination of a starting aldehyde was used to build up the carbon chain, two carbons at a time with either methyl or ethyl branches. The disubstituted double bond containing the terminal methyl group was added with a Wittig reaction in the case of compound 1, but a new E-double-bond selective alkylidenation sequence that allows much better stereochemical control than the Wittig reaction was used in the construction of compound 2. The alkylidenation sequence consists of four steps. Horner-Wadsworth-Emmons olefination of an aldehyde with triethylphosphonoacetate yields an alpha,beta- unsaturated ester with >99% E-double-bond configuration. Reduction of the unsaturated ester with lithium aluminum hydride affords an allylic alcohol. The allylic alcohol is acetylated under mild conditions with acetic anhydride, triethylamine, and 4-dimethylaminopyridine (DMAP) as the acylation catalyst. Displacement of the acetate function with a methyl group is accomplished with a methylmagnesium chloride Grignard reagent in the presence of lithium tetrachlorocuprate as the carbon-carbon-cross coupling catalyst. The overall yield for the alkylidenation sequence was 41%, but >99% E-double bond configuration was obtained.