Sex pheromone and analogs of the citrus mealybug, Planococcus citri: synthesis and biological activity

The citrus mealybug, is a cosmopolitan pest and affects many crops. The female sex pheromone has been identified by Bierl-Leonhardt et al., in 1981 as (+)-(1R)-cis-2,2-dimethyl-3isopropenylcyclobutanemethanol acetate 1. Several groups including our team have synthesized the pheromone. A number of analogs have also been prepared in order to study the structure-activity relationship. We present here a modified synthetic route for the pheromone, preparation of some analogs and the biological activity of these compounds. The starting materials for the pheromone and analogs are cis-pinonic acid or cis-pinonic aldehyde which can be easily obtained from cheap commercial (+)-α-pinene by cleavage with permanganate or ozonolysis. Conversion of the pinonic derivatives to the pinononic compounds was achieved either by a modified Hundsdiecker reaction (Wolk et al., 1986) or by ozonolysis of the enol acetate of cis-pinonic aldehyde (Barton and Fontana, 1996). In the present study, we report our results using the second method, which avoids the use of the unstable pinononyl halides. The key element of the synthesis of 1 is the use of pinononyl aldehyde 2 and its selective reduction to pinononyl alcohol 3. The latter was submitted to a Wittig reaction and then acetylated; alternatively the sequence was reversed and the Wittig reaction was performed after the acetylation to form the pheromone 1. The stereochemistry was preserved in all steps and no racemization was observed. Field tests indicated that a number of analogs display considerable biological activity. One of them, a homolog containing an elongated acetate side chain by one carbon 10 has a relatively high activity. This observation has practical importance because the synthesis of the homolog is shorter and more convenient than that of the pheromone. The field tests indicated that the acetate group and the double bond in the pheromone molecule are essential for biological activity.