Semiochemicals of the Scarabaeinae VIII. Identification of active constituents of the abdominal sex-attracting secretion of the male dung beetle, Kheper bonellii, using gas chromatography with flame ionization and electroantennographic detection in parallel

Abstract

Using gas chromatography with flame ionization detection and electroantennographic detection in parallel (GC-FID/EAD), the active constituents of the sex attractant of male dung beetles of Kheper bonellii were located in the gas chromatogram of an extract of the secretion. These constituents were identified as propanoic acid, butanoic acid, indole, 3-methylindole (skatole) and methyl cis-cascarillate (methyl cis-2-2'-hexylcyclopropylacetate) by, inter alia, GC-MS, (1)H and (13)C NMR analysis, and synthesis. These compounds elicited EAD responses in male as well as female antennae. Racemic methyl cis-cascarillate was synthesized for comparison with the natural methyl ester. Enantioselective GC-FID/EAD using a capillary column coated with OV-1701-OH containing 10% heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin showed that the natural compound co-eluted with the first-eluting enantiomer of the racemic methyl cis-cascarillate, which was the only enantiomer that elicited EAD responses in the antennae of male and female K. bonellii. The absolute configuration of this enantiomer was established by a stereoselective synthesis, which gave methyl (R,R)-cascarillate [methyl (1'R,2'R)-2-2'-hexylcyclopropylacetate] in an enantiomeric excess of 69%.