Heterocyclic Pyrethroid Alcohols

This alcohol 312 was the first one conveying very high insecticidal activity to its pyrethroid esters, and must be regarded as the direct forerunner of modem pyrethroid alcohols. The technical process for resmethrin 315 (Reaction scheme 219) builds up a furane ring in a classical manner [676, 677]. 

A variation of this process utilizes the same starting material, but applies other protecting groups and means of introducing the additional carbonyl-function [678] (Reaction scheme 220). 

Retrodiene reaction [679] of the Diels-Alder adduct 313 of 2-benzyl-4-methyloxazole and propynol, by splitting off acetonitril, also affords the desired alcohol (Reaction scheme 221). 

A recent proposal looks very interesting, and consists of the Lewis acid-catalyzed addition of phenylacetyl chloride on 2 chloromethyl-l-chloropopen-2 (Reaction scheme 222). The resulting product 314 after two dehydro chlorinations under the influence of collidine yields the right component for esterifications with the chrysan-themate anion to give resmethrin 315 [680,681]. 

A similar precursor is subjected to a 1,3-dipolar cycloaddition as outlined in Reaction scheme 223. From the bicyclic intermediate 315 a the supernumerary heteroatoms are removed hydrogenolytically to yield 312 [682]. 

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R = (Cl, F, Nalk2, aryl)alkyl heterocycl. aryl = pyrethroid alcohol... 

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. 

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