Pyrethrins, development from natural products

The change in the alcohol moiety to allethrone led to the development of the first synthetic pyrethroid, allethrin. It also led to improved chemical stability of the natural pyrethrins and to reduced cost, because the original pyrethrins had been sourced from natural products (LaForge and Soloway 1947). The stability of alle-thrin made it superior to the natural pyrethrins in both kill and knock-down effects against mosquitoes. Allethrin s successful discovery was followed by the development of other successful pyrethroids, to wit, tetramethrin (Kato et al. 1965) (1965, patent appl date) from tetrahydrophtalimide, resmethrin (Elliott et al. 1967) (1967, patent appl date) from 5-benzyl-3-furylmethyl alcohol and phenothrin (Fujimoto et al. 1973) (1968, patent appl date) by changing a-benzylfuran to phenoxyphenyl. 

Currently pest control by natural plant extracts is practiced primarily by subsistence farmers in those less developed part of the world where it is still an economic necessity.(ref. 3). Of the approximately 2000 plant species with known insecticidal properties (ref. 4), few have been developed commercially. These include the pyrethrins, rotenones and some of the alkaloids. Pyrethrins were the most important natural plant extracts in the early commercial insecticide formulations and were already in use in Persia and Yugoslavia during the early 1800s. By 1939 pyrethrum imports to the United States were 13-5 million lbs, declining from this peak as the synthetic analogs (e.g., the allethrins) appeared on the market. The addition of stabilizers (antioxidants) and synergists to the original pyrethrum formulations saved the natural product from commercial extinction. Currently the demand for pyrethrum flowers is still over 25,000 tons per annum—met by hand-harvested crops from Ecuador, Kenya and Tanzania (ref. 5) ... 

Pyrethrin is a natural insecticide obtained from the powdered flower heads of several species of Chrysanthemum. The active substances in pyrethrum, principally pyrethrins I and II, are contact poisons for insects and cold-blooded vertebrates. Although powders made from Chrysanthemum extracts have found widespread use, the active substances in them are destroyed rapidly in the environment. In an effort to develop synthetic compounds as effective as the natural insecticides but with greater biostability, chemists prepared a series of esters related in structure. Among the synthetic pyrethrenoids now in common use in household and agricultural products are permethrin and bifenthrin. 

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