SYNTHESIS OF COTTON BOLL WEEVIL PHEROMONES: GRANDLURE II, III AND IV

Abstract

Cotton is the fourth most important crop in Brazilian agriculture, after soybeans, sugar cane and corn. In the world ranking, Brazil occupies the fifth position of largest cotton producer. Pest insects are a limiting factor for cotton cultivation and, among them, Anthonomus grandis (Coleoptera: Curculionidae) is the main cotton pest in South America, causing losses of up to 70% of production. The cotton boll weevil, as it is known, in its larval phase, develops inside the flower buds and, as adults, feed mainly on pollen, thus causing a reduction in productivity due to the attack on the flower buds, significantly decreasing the number of bolls at harvest. The main management method used for boll weevil control is the use of insecticides. However, in addition to the environmental impact caused by these, there are already reports of the emergence of resistant insects. In this context, semiochemicals appear as an efficient and sustainable control method. Four compounds are described in the literature as pheromones of the boll weevil: (+)-cis-2-isopropenyl-1-methyl-cyclobutane-ethanol, (Z) -3,3-dimethylcycloexylidene-ethanol, (E) -3,3-dimethylcycloexylidene acetaldehyde and (Z)-3,3-dimethylcycloexylidene acetaldehyde known as grandilure I, II, III and IV, respectively. Grandilure I is a widely studied molecule and there is a range of synthetic methodologies described. The present work aims to propose an efficient and fast synthetic route for grandillures II, III and IV. For the synthesis of the three grandilures, the intermediate (E,Z)-α, β-unsaturated ester [(E,Z)-ethyl 2-(3,3-dimethylcyclohexylidene) acetate], containing first an exocyclic double bond, was formed through the Horner-Wadsworth-Emmons reaction. Grandilure II was then quantitatively prepared by reducing this ester with LiAlH4. Grandilures III and IV were obtained by oxidation of grandilure II alcohol using PCC and DCM. By synthesizing these pheromones, it will be possible to develop traps for the control of A. grandis and thus increase the productivity of the cotton crop.