Non-Ester Pyrethroids

We have earlier discussed the pyrethroid area of insecticides. A number of ester and non-ester pyrethroid insecticides have incorporated the difluoromethoxy group as a means of widening their biological activity to the control of mites [115], Flucythrinate (Cybolt , Cythrin , Pay-Off ) [116] provides control of a variety of sucking insects, beetles, and lepidoptera in cotton and pome fruits. Later, a close analog, flubrocythrinate, was commercialized [117]. 

In addition to the classical ester pyrethroids previously discussed in Sections 2.1.1.2, 2.2.3.2, and 2.2.4.1, two non-ester pyrethroid molecules were reported by the FMC Corporation in the late 1980s and early 1990s protrifenbute [161] and eflusilanate [162]. Structure-activity investigations of eflusilanate found that removal of the fluorine atom resulted in a 10-fold loss of biological activity [163]. 

The non-ester pyrethroid flufenprox is a broad spectrum insecticide with residual activity against hemiptera, lepidoptera, and coleopteran insects in rice. It is reported that flufenprox is safe to beneficial insects such as spiders and predaceous mites [175]. 

The chemistry of the non-ester pyrethroids is still under investigation in some industrial laboratories. Forthcoming improvements are steadily published in patents. 

In the time after completion of this manuscript a number of structurally and commercially interesting non-ester pyrethroids have been published. A few of them are now under development for commercial use. These new structures have certain similarities to etofenprox, but are characterized by the occurrence of an a-Cp3 group or a dimethylsilyl partial structure in the bridge between the two aromatic moieties. The technical-synthetic problems connected with the realization of these insecticides with slightly differennt biological properties are certainly another worthy but concluding climax of pyrethroid chemistry. 

Synthetic pyrethroids block the nerve signal by prolonging the opening of the VgSCh. Beside natural pyrethrins, 37 ester (Type A and B) and two non-ester type pyrethroids (Type C) are currently registered worldwide (Scheme 35.2) [62]. 

The achiral etofenprox (25 X = O, A = carbon R = OEt) containing no halogen atom shows a non-ester exerting pyrethroid-like efficacy and is highly advantageous regarding the rice insecticide market (Table 35.5). 

For example, cycloaddition of carbethoxycarbene (from ethyl diazoacetate as precursor) on XXXIV (R=methyl, Cl, Br) in the presence of various rhodium carboxylates yields cyclopropanecarboxylic acid esters (pyrethroids) that are useful for their insecticidal activity [56] (slightly volatile, biodegradable, non toxic for mammalians). 

Wether the latest aftermath in pyrethroid research, the non-ester variations of the ether, hydrocarbon or the silane types are sufficiently different or more advantageous to capture market shares has to be shown in the future. 

The synthesis of chrysanthemic acid 1 as mixture of stereoisomers, as racemate of pure stereoisomers or as single optically active isomer was important before the advent of synthetic photostable pyrethroids. Because of the approved and favourable properties of pyrethrin I as a non-toxic and fast-acting contact insecticide, not only pyrethrum, but also other esters of chrysanthemic acids with similar properties are of commercial interest. Therefore cheap methods for the synthesis of this add or even total synthesis of the natural compound deserve the attention of chemists involved in synthesis. Many interesting reactions, involving rearrangements, eliminations and additions were apphed in the synthesis of this archetypical pyrethroid cydopropane carboxyhc acid. 

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