Enantioselective synthesis of the sex pheromone of the grey pineapple mealybug, Dysmicoccus neobrevipes (Hemiptera: Pseudococcidae), for determination of the absolute configuration
The grey pineapple mealybug, Dysmicoccus neobrevipes Beardsley, originally found on Hawaii and on Australasian islands, was recently discovered on a southwestern island (Ishigaki Island) of Japan. Because D. neobrevipes is known to attack many fruits and other crops, it is essential to establish a strategy to prevent the spread of this potential pest. Detection and monitoring by use of pheromone traps would provide important information about its distribution. The sex pheromone of D. neobrevipes has been isolated and identified as (E)-2-isopropyl-5-methylhexa-3,5-dien-1-yl acetate, although its absolute configuration was unknown. In this study, we achieved enantioselective synthesis of this compound by use of porcine pancreas lipase (PPL)-catalyzed acylation. Acetyl transfer from vinyl acetate to prochiral 2-isopropyl-1,3-propanediol in the presence of PPL in an organic solvent predominantly produced the (R) enantiomer of the monohydroxy acetate (86 % ee). In contrast, PPL-catalyzed hydrolysis of 2-isopropyl-1,3-diacetoxypropane in phosphate buffer yielded the (S) enantiomer of the monohydroxy acetate (75 % ee). Each enantiomer of the monohydroxy acetate was oxidized to a formyl acetate then coupled with a Wittig reagent, to produce the (R)-(−) and (S)-(+) pheromones. Analysis by gas chromatography with a chiral column and by polarimetry revealed the natural pheromone to be the (S)-(+) enantiomer. In a field trap experiment, the attractiveness of the pheromone produced by this route was equivalent to that of the pure (+) enantiomer (>99 % ee).