Biosynthesis of epilachnene, a macrocyclic defensive alkaloid of the Mexican bean beetle

The carbon skeleton of the azamacrolide epilachnene, the principal defensive alkaloid of Epilachna varivestis pupae, can be derived from oleic acid and serine. Analytical evidence from three experiments in which insects were fed (²H₃₅)octadecanoic acid, (Z)-9- [11,11,12,12,13,13,14,14,15,15,16,16,17,17,18,18,18-²H₁₇]octadecenoic acid and (Z)-9-[11,11,12,12,13,13,14,14,16,16,17,17,18,18,18-²H₁₅]octadecenoic acid, indicates that only the C-15 methylene group of oleic acid is involved in the mechanism leading to carbon-nitrogen bond formation in epilachnene. Support for this scheme is provided by the observation of an unusual ion at m/z 170 in the electron-ionization mass spectrum of epilachnene, rationalized as CH₂=CH-COO-CH₂-CH₂-NH⁺=CH-CH₂-CH₂-CH₃.