Asymmetric Synthesis and Biological Activity of Contact Pheromone of Western Flower Thrips, Frankliniella occidentalis

Abstract

Western flower thrips, Frankliniella occidentalis, is a serious worldwide pest of agriculture and horticulture, and its contact pheromone is 7-methyltricosane. Two enantiomers of 7-methyltricosane were synthesized for the first time. The centra of our strategy were chiral auxiliaries to introduce stereocenter, and Wittig coupling to connect two blocks. The overall yields of our synthesis were 29-30% with seven steps. The electroantennogram (EAG) and the contact behavioral responses revealed that (R)-, (S)- and racemic 7-methyltricosane were separately bioactive, and the racemate was the most bioactive in the male arrestant activity and the female EAG test. This result provides valuable insights, showing that the racemate could be used for the support of the control of western flower thrips, which could be more easily prepared relative to more expensive enantiopure pheromone.

Keywords: Frankliniella occidentalis; asymmetric synthesis; biological activity; contact pheromone.

Figure 1

Two enantiomers of the contact pheromone of F. occidentalis.

Figure 2

Synthesis of chiral intermediates (R)- and (S)-6.

Figure 3

Synthesis of the contact pheromones (R)- and (S)-1.

Figure 4

Electrophysiological responses to different concentration of synthetic pheromones from the antennas of the adult F. occidentalis males and females. Data in line charts are mean (±SEM, n = 5). EAG responses were obtained by subtracting EAG value of the solvent control from the results of antenna stimulation. (A) is the EAG responses to (R)-7-methyltricosane. (B) is the EAG responses to (S)-7-methyltricosane. (C) is the EAG responses to racemic 7-methyltricosane.

Figure 5

Mean EAG responses (±SEM, n = 5) of F. occidentalis females and males to (R)-, (S)- and racemic 7-methyltricosane. EAG responses were obtained by subtracting EAG value of the solvent control from the results of antenna stimulation. Bars with different letters indicate significant differences from each other (Tukey’s pairwise comparisons, p < 0.05). Capital letters (A–C) represent significant differences in three compounds of (R)-, (S)- and racemic 7-methyltricosane, while lowercase letters (a–c) represent significant differences in three concentrations of 0.5, 1.0 and 2.0 μg/μL.