Pyrethroid insecticides chemical properties

Even if the phytoalexins so far isolated have little commercial utility, it is still possible that useful substances may yet appear. But the more exciting possibility is that consideration of the chemical structures of natural phytoalexins and of their modes of action against fungal infections may provide clues for the development of synthetic pesticides. The complexity of the chemical structures of the natural phytoalexins may make them uneconomical to manufacture but a comparison should be made with the synthetic pyrethroid insecticides. The natural pyrethrums have complex chemical structures but simpler compounds, economical to manufacture, have been developed on the basis of the structures of natural pyrethrums and many of these have much more desirable properties for use in agriculture than the natural substances. There would seem to be no reason why simpler compounds based on the structures of natural phytoalexins should not provide synthetic fungicides as important and useful as the synthetic pyrethroids. This is a future challenge for the synthetic organic chemists in this area. 

Natural pyrethrins are a neurotoxin and repel, knock down, and kill by contact with insects at a low concentration. On the other hand, they have ideal features for household insecticides because of their quite low dermal and oral toxicides to warm-blooded animals. Neither plants other than pyrethrum nor synthetic insecticides have been reported to have such properties. Numerous synthetic pyrethroids have been developed by chemists since the complicated chemical structure of natural pyrethrins was elucidated in the middle of the twentieth century. Allethrin was the first synthetic pyrethroid put into practical use. 

Thus, the form of insecticides should be selected in consideration of chemical and physical properties as well as the safety of pyrethroids, and their excessive use should be restricted in rooms. 

Organophosphorus pesticides have been the insecticides most commonly used by professional pest control bodies and homeowners for the past three decades (Jeannot and Dagnac 2006). Nevertheless, the decision of the US Environmental Protection Agency (EPA) to phase out certain uses of the organophosphate insecticides because of their potentially toxic effects to humans has led to their gradual replacement by other pesticides. Among them, synthetic pyrethroids have been manufactured since 1950 s based upon the structure of natural pyrethrins, which are chemicals with active insecticidal properties in the pyrethrum extract (a mixture of chemicals found in certain chrysanthemum flowers). Pyrethrins are very unstable in the environment, due to oxidation and UV-radiation. 

---