The synthetic pyrethroid isomers II. Biological activity

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Abstract
Pure optical isomers of phenothrin, permethin and cypermethrin were synthesised and tested on a wide spectrum of insects (Blattella germanica, Leptinotarsa decemlineata, Tribolium confusum, Oncopeltus fasciatus, Musca domestica, Aedes aegypti). Our results suggest that in the structurally related series phenothrin, permethrin and cypermethrin the chemical modifications had only a small influence on intrinsic activity of the compounds at the target site. The activity of piperonylbutoxide synergised 1Rtrans phenothrin and 1Rcis permethrin (LD50=1.2 ng/fly in both cases) is in a similar range to the most active 1RcisS cypermethrin isomer alone or with PB (LD50=0.9 and 0.4 ng/fly, respectively). Some cypermethrin isomers considered previously to be totally inactive show significant activity depending on the tested insect species. The 1ReisR and 1RtransR isomers proved to be as active as the most potent isomers on mosquito. A similarly unexpected result was that in the case of mosquito all four trans isomers were significantly more active than the corresponding eis isomers. The superiority of the (S)‐α configuration over (R)‐α was found. It does not result from a higher intrinsic activity at the target site, but rather from the enhanced metabolic resistance of (S)‐α‐esters to ester cleavage as confirmed by Synergist studies. The activity of the two most potent stereoisomers, 1ReisS and 1RtransS, could not be elevated by quinalphos, while the inactive 1ScisR and 1StransR isomers showed significant activity with the esterase inhibitor quinalphos administered sublethally. In contrast with the antagonism between (S)‐ and (R)‐α‐epimers of cypermethrin reported earlier a considerable synergism was observed between 1RtransS and 1StransR enantiomer pair on flour beetle and housefly in tarsal contact tests. The results with stereoisomers and their mixtures were utilised in the development of two new products, Chinmix (beta‐cypermetrhin) and Transmix (theta‐cypermetrhin).