Synthesis of Permethric Acid

From the technical point of view an interesting alternative for PCI5 chlorination is the Zn-catalyzed chlorination of a carbonyl group with the cheap a-dichloromethyl ether 90, forming the useful byproduct formic ester [126], thereby avoiding waste water which contains phosphoric add. 

The breakthrough for pyrethroids as agricultural insecticides became possible when the high activity of its pyrethroid ester in connection with the photostabihty of 3-dichlorovinyl-2,2-dimethyl cyclopropane carboxyhc acid 91 was found. This acid, synthesized first by Farkas, Sorm and Kurim in 1957 [127], was rediscovered during a straight forward search for higher stabihty by Elhott and Janes in 1969 and tested as the photostable ester permethrin. 

Synthesis of permethric acid (also called DV-acid) 91 became one of the central items in the years of industrial pyrethroid research in the 1970 s. Many original proposals have been made. Only a very small number have appeared in the scientific hterature. But the wealth of findings as pubhshed in patent apphcations show, in a fascinating way, that modem synthetic organic chemistry can turn many diverse and simple chemicals into permethric acid for less and less money as shown in the following chapter, in which the vast majority of the pubhshed discoveries has been taken into account. 

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For the synthesis of permethric acid esters 16 from l,l-dichloro-4-methyl-l,3-pentadiene and of chrysanthemic acid esters from 2,5-dimethyl-2,4-hexadienes, it seems that the yields are less sensitive to the choice of the catalyst 72 77). It is evident, however, that Rh2(OOCCF3)4 is again less efficient than other rhodium acetates. The influence of the alkyl group of the diazoacetate on the yields is only marginal for the chrysanthemic acid esters, but the yield of permethric acid esters 16 varies in a catalyst-dependent non-predictable way when methyl, ethyl, n-butyl or f-butyl diazoacetate are used77). 

An analogous reaction applied to l,l-dichloro-4-methyl-l,3-pentadiene would lead to a selective synthesis of permethric acid, of which the cis isomer was more desirable. It is interesting that this seemingly simple structural change provided... 

As already mentioned, the selective reduction of the add stage down to the aldehyde can be accomplished by reduction of an acylphosphonic ester 189. Based on this fact a synthesis of permethric acid is proposed, which comprises several typical synthetic advantages of Homer-Hoflfmann phosphonic acid reagents of ready accessibility (Reaction scheme 118). In fact, the trans-caronaldehyde adduct of diethylphosphite... 

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