Pyrethroids

Pyrethrins described originally are natural products obtained from chrysanthemum flowers. Pyrethrins are chiral and composed of several optically active isomers. They have been extracted and purified to be used as insectcontrolling agents. Later, many synthetic compounds of simpler structures based on modifications of the natural pyrethrins have been examined for their insecticidal activities, and numerous “synthetic pyrethroids” have been developed and marketed nowadays. Many of them are optically active and enantiopure because, since the initiation of such efforts, strategy of the synthetic chemists for constructing active pyrethroid structures involved extensive utilization of naturally occurring enantiopure pyrethrin molecules. For example, chrysanthemic acid (acid portion of the natural pyrethrin I) was esterified with a simpler alcohol of a structure similar to natural pyrethrolone, etc. (S)-bioallethrin (6) (2), the ester of (1R)- trans-chrysanthemic acid with (S)-allethrolone, is one of the earliest examples. In this study, the large difference in biological activity between stereoisomers was clearly indicated.

The pyrethroids formed chronologically the fourth major group of insecticides after the organochlorines, the organophosphates and the carbamates. They are synthetic analogues of the pyrethrins, the naturally occurring insecticidal constituents of the flowers of Tanaceturn cinerariaefdiurn. The pyrethroids were first commercialised in 1978 (fenvalerate) and the class now consists of 42 active ingredients, some of which differ only in relative stereoisomer content. This figure (PM) includes natural pyrethrum and four non-ester pyrethroids. The members of the class control a wide range of insects, especially Lepidoptera (and also Coleoptera, Diptera, Hemiptera and others) in a wide range of crops, ornamentals and trees. They are also important in public health and veterinary medicine where they are used as ectoparasiticides. The more volatile compounds and those prone to photodecomposition have uses in domestic insect control.
The pyrethroids are non-systemic (in plants), and are contact and stomach poisons; they also have an anti-feeding action. They possess good residual activity on treated plants. The compounds act on the central and peripheral nervous systems of target insects at very low doses. Their action occurs on sodium channels within the lipophilic environment of the membranes at, or close to, the Na⁺ gate proteins and they act by modulating the opening and closing of the channels, leading to synaptic discharge, repetitive discharge, depolarisation and ultimately death.
The major components of the natural insecticide mixture are: the pyrethrins (I and II), the cinerins (I and II) and the jasmolins (I and II) in the percentage ratio 71 : 21 : 7. The designation I refers to the chrysanthemate esters and II refers to the ’pyrethrate’ esters. Their structures are shown in the pyrethrum entry. Pyrethrin I (1) is the most important component for insecticidal action but pyrethrin II is very effective at rapid ‘knock-down’. This is not synonymous with death but it is very effective in the rapid removal of a nuisance factor (important in domestic use). Pyrethrin I is an ester of 2,2-dimethyl-3-dimethylvinylcyclopropanecarboxylica cid and 2-pentadienyl-3-methylcyclopentenolone. The potential for complex stereochemistry is immediately apparent.

Pyrethroids are neurotoxie synthetic compounds used as insecticides. Cypermethrin and fenvalerate have been reported as causing positive allergie patch tests, but only fenvalerate was relevant in an agricultural worker.