Pyrethroid insecticide synthesis, alcohol

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). 

Besides the use of stereoselective nitrile-converting enzymes as described above, useful chiral building blocks have also been obtained by stereoselective nitrile-forming enzymes. The main product class of nitrile-forming enzymes are cyanohydrins (a-hydroxynitriles, 1-cyanoalkanols), which are versatile synthons in organic synthesis that are readily convertible to a-hydroxy acids [90], a-hydroxy aldehydes [91], ethanolamines [92], amino alcohols, pyrethroid insecticides [93], imidazoles, and heterocycles [94]. Examples of valuable bioactive products derived from chiral cyanohydrins are (i )-adrenaline, L-ephedrin, and (5)-amphetamines [95]. For the synthesis of chiral cyanohydrins, stereoselective enzymes from both plant and bacterial sources have been used. 

Asymmetric synthesis by means of a cyandiydrin is an imprvtant process in organic synthesis, because the cyanohydrin can be easily converted into a variety of valuable synthetic intermediates, such as a-hy-droxy ketones, a-hydroxy acids, y-diketones, p-amino alcohols, 4-oxocarboxylic esters, 4 xonitriles, a-amino acids and acyl cyanides. More specifically, the (S)-cyanohydrin of m-phenoxybenzaldehyde is a building block for the synthesis of the insecticide deltamethrin, or (IR)-cis-pyrethroids. ... 

In view of the diversity in the synthesis of pyrethroid ester components it is small wonder, that the final step of formation of the actual insecticidal ester may be accomphshed in many ways. Often, novel methods were needed to make it possible on a larger technical scale. Classical methods like azeotropic esterification of free acid [767] or reaction of acid chloride with simple alcohols, in many cases preferably in the absence of any auxiliary additional reagent or solvent [768], mostly proceeds in a textbook-like manner. 

CF3CCl2-substituted alcohols in good yields. In this case, when insitu-geaeiaied zinc alkoxides are subsequently treated with AC2O and zinc dust at 50°C, trifluoromethylated alkenes can be obtained in good yields (Scheme 26.33). This practical one-pot synthesis can be applied for the preparation of highly potent insecticides, so-called synthetic pyrethroids, such as bifenthrin and others. ... 

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