Stability, chemical pyrethroids

In 1958, Barthel et al. [25] reported dimethrin (15), which was the first substituted benzyl alcohol ester of chrysanthemic acid. This compound was not put into practical use due to its low insecticidal activities. Phenothrin (16), one of the m-phenoxybenzyl alcohol esters developed by Fujimoto et al. [26], was found to have superior chemical stability as well as safety, and has been the sole pyrethroid used as a lice control agent for humans. Further improvement was made by Matsuo et al. [27] who introduced a cyano function at the a position of the benzyl part of phenothrin, leading to a-cyano-m-phenoxybenzyl alcohol esters (17). Thereafter, this alcohol moiety has been used as a component for a number of photostable pyrethroids for agricultural purposes however, the development of cross-resistance can be seen in some pests. 

Pyrethrum and pyrethrin products are used mainly for indoor pest control. They are not sufficiently stable in light and heat to remain as active residues on crops. The synthetic insecticides known as pyrethroids (chemically similar to pyrethrins) do have the stability needed for agricultural application. 

These modem synthetic insecticides are chemically similar to natural pyre-thrins, but modified to increase stability in the natural enviromnent. They ate now widely used in agriculture, in homes and gardens, and for treatment of ectoparasitic disease. Figure 5.6 shows several different products containing various synthetic pyrethroids. 

Another method to classify pesticides is related to their main chemical structural elements or their mode of action. Examples are organochlo-rines, organophosphates, carbamates, pyrethroids, and so on (Table 11.2). In commercial products, the active ingredients are formulated (mixed) with other compounds, such as solvents, surfactants, stabilizers, and so on, that make the pesticides ready for use on farms and for private pest control. The formulations for crop protection are usually concentrates that are diluted with water before being applied on a field. 

The most remarkable compound listed is probably permethrin, a rebuilt chemical with much higher stability and insecticidal activity than the natural pyrethroid. Not much later the difference in activity between the various stereoisomers was taken into account. Permethrin is a racemic mixture, but in the products called bio-, as in bioallethrin and bioresmethrin, as well as in deltamethrin and several other newer pyrethroids, the inactive stereoisomers have been removed. Deltamethrin has a cyano group, making mirror-image isomerism possible. The one shown is the most potent. Substances without the cyclopropane moiety were also found. Fenvalerate was developed by Sumitomo Chemical Co. Ltd. and described in 1974, whereas its most active isomer was found and described in 1979. 

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. 

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