Synthese von cis‐konfigurierten Bis(trifluormethyl)pyrethroiden

Synthesis of cis-Configurated Bis(trifluoromethyl)pyrethroids trans-2,2-Bis(trifluoromethyl)cyclopropanecarboxylates 2a, b are converted into the cis-diastereomers by base-catalyzed epimerization via the bicyclic lactones 3a, b. Dehydration of cis-2a and cleavage of the ester function leads to the fluorinated chrysanthemic acid 4b, which is converted into the pyrethroids 4d, e. cis-2b is oxidized by pyridinium chlorochromate to give methyl cis-3-formyl-2-2-bis(trifluoromethyl)-cyclopropanecarboxylate (hexafluorocaronaldehydic acid methyl ester) (5). This compound, which is very sensitive to isomerization, is converted into the pyrethroid esters 6a, b predominately retaining the cis configuration; however, by introducing the 2,2-dichlorovinyl side chain the trans diastereomer is obtained. The starting materials trans-2a, b are prepared by the reaction of aldehydes 9a, b with 2,2-dichlorohexafluoropropane and triphenylphosphine followed by cyclopropanization of the resulting 1,1-bis(trifluoromethyl)olefins 10a, b with (ethoxycarbonylmethylene)dimethylsulfurane (14).