Synthesis of Permethric Acid via Cyclizing 1,3-Dehydrohalogenation

In the case of the dimethylpentenoic dimethylamide 775, intended to circumvent the patents, the addition of CBr4 takes another course (Reaction scheme 66). The carbonyl oxygen is involved, leading to the formation of the immoniolactone 7 74 [184]. 

The stereochemical control in the formation of the final product (Reaction scheme 65) is complex. The product can be obtained by different routes starting with the CCl4-adduct 112. Bases, solvents and solvent mixtures are involved in shifting the cis/trans ratio at will between predominant trans- and high cis-content up to 90%. 

Na-t-butylate in hexane plus some hexamethyl phosphoric triamide yields the high cis-product quite independent of temperature [185], in contrast to the usually cis/trans 40/60 mixture, which is the trade-product for permethrin, cypermethrin and cyfluthrin. 

Patents for similar sequences were filed soon after by Sumitomo [186] and ICI [187], and later worked on and modified by other companies [188,189,190] as shown in the Reaction scheme 67, in which the relatively expensive acetic orthoester 115 is substituted by the cheaper acetacetic ester derivative 116  

1 Synthesis of 2,2-Dimethylpentenoic Acid Derivatives as Key Intermediates for 1,3-Cycloeliminations 

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