SYNTHESIS OF PYRETHROIC ACIDS FROM NATURAL-PRODUCTS AND FROM ISOMERICCOMPOUNDS - STRATEGY AND PRACTICE

Pyrethroic acids are essential constituents of the pyrethroids, which are now some of the most powerful commercially available insecticides. A number of syntheses of pyrethroic acids have been published since t he elucidation of the structure of chrysanthemic acid, one of the natu rally occurring acids present as esters in Chrysanthemum cinerarefoliu m Vis. This paper reports some results of the stereoselective synthesi s of the most biologically active enantiomers of pyrethroic acids obta ined over the last five years in our laboratory. Two different approac hes, which have been patented by two leading European companies in the agrochemical field, have been carried out simultaneously. The first u ses the cheap chiral natural products mannitol and tartaric acid as st arting materials and allows the rapid construction of the cyclopropane framework. The second takes advantage of the isomeric relationship be tween dimethyl dimedone, a readily available, non-chiral compound and chrysanthemic acid. Enantioselective isomerisation of dimethyl dimedon e to (1R)-cis- and (1R)-trans-chrysanthemic acid has been achieved suc cessfully using a lipase. These two approaches to the synthesis of chr ysanthemic acid are compared with related approaches in the literature , which use other natural products or isomers of chrysanthemic acid as starting materials.