Pyrethroid insecticide alcohols, structures

Enantioselection can be controlled much more effectively with the appropriate chiral copper, rhodium, and cobalt catalyst.The first major breakthrough in this area was achieved by copper complexes with chiral salicylaldimine ligands that were obtained from salicylaldehyde and amino alcohols derived from a-amino acids (Aratani catalysts ). With bulky diazo esters, both the diastereoselectivity (transicis ratio) and the enantioselectivity can be increased. These facts have been used, inter alia, for the diastereo- and enantioselective synthesis of chrysan-themic and permethrinic acids which are components of pyrethroid insecticides (Table 10). 0-Trimethylsilyl enols can also be cyclopropanated enantioselectively with alkyl diazoacetates in the presence of Aratani catalysts. In detailed studies,the influence of various parameters, such as metal ligands in the catalyst, catalyst concentration, solvent, and alkene structure, on the enantioselectivity has been recorded. Enantiomeric excesses of up to 88% were obtained with catalyst 7 (R = Bz = 2-MeOCgH4). 

Pyrethroid insecticides are generally classified into one of two large groups on the basis of the central neurotoxic syndrome that they produce [5, 6]. Type I pyrethroids are esters of chrysanthemic acid and an alcohol, having a furan ring and terminal side chain moieties, and absence of a cyano moiety. Allethrin was the first pyrethroid identified in 1949. Allethrin and other pyrethroids such as phenothrin and permethrin with the basic cyclopropane carboxylic ester structure are type I pyrethroids. The insecticidal activity of these synthetic pyrethroids was enhanced further by the addition of a cyano group to give ot-cyano type II pyrethroids such as deltamethrin, fenvalerate, cyfluthrin, cyhalothrin, and lambda-cyhalothrin (Fig. 137.2). 

Figure 7 shows the course of development of various synthetic pyrethroids developed by retaining chrysanthemic acid as the acid moiety and modifying the alcohol moiety. Numerous useful compounds with favorable characteristics have been derived from the structural modification of natural cinerin I (7). These underlined compounds have been put into practical use as active ingredients, mainly for household insecticides. 

Pyrethroid Esters of Benzene Acetate. These insecticides have more extensive structural optimization in both acid and alcohol moieties. Fenvalerate [51630-58-1]y 0t-cyano-(3-phenoxyphenyl)methyl (+)-(2R,T)-a-isopropyl-4-chlorophenylacetate (24) (d 1.17, vp 1.4 J,Pa at 25°C), a mixture of four isomers, is soluble in water to 0.3 mg/L The rat oral LD ( is 450 mg/kg. Esfenvalerate [66230-04-4] is the (+)-2-(3, 3)-isomer (mp 59°C). The rat LD5Qs are 75, 458 (oral), and the rabbit dermal LD50 is 2000 mg/kg. These pyrethroids are widely used general-purpose insecticides for field, vegetable, and fruit crops. 

Synthetic pyrethroids are a group of ester compounds having excellent insecticidal activities. After the discovery of allethrin (1), a variety of useful synthetic pyrethroids have been produced mainly by structural modification of an alcohol having an asymmetric center. The insecticidal activities greatly depend upon the stereoisomers. Therefore, much effort has been expended to develop technologies for obtaining optically active isomers. However, contrary to the case of chrysanthemic acid, chemical methods of optical resolution were not very effective for these alcohols. 

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