Potent Pyrethroid Insecticides from Modified Cyclopropane Acids

Abstract

THE natural pyrethrins^1–3, of which the most important constituent is pyrethrin I (1), are very active against a wide range of insect species, do not leave persistent residues, and have a long record of safe use; despite surprisingly high intravenous toxicity, they have low oral toxicity to mammals⁴. The related synthetic compound, bioresmethrin (2)^5,6, has greater insecticidal activity (for example, about fifty times that of pyrethrin I, by topical application⁵, to a normal strain of houseflies), yet is even less toxic to mammals, both orally and intravenously^4,7. Pyrethrin I and bioresmethrin are esters of (+)-trans [1R,3R]-chrysan-themic acid. We have now found that a simple modification of the acid side chain in bioresmethrin by which the four carbon atoms in the isobutenyl group are redisposed to a but-1-enyl substituent (as in 7) produces still greater insecticidal activity (about 100 times that of pyrethrin I to houseflies) whilst retaining low mammalian toxicity. Other changes of the acid side chain in bioresmethrin increase insecticidal activity further; for example the compound (10) with a cis-(Z)-butadienyl group and the dichlorovinyl compound (12) are both 2–3 times as potent as bioresmethrin.