Process for preparing cyclopropane carboxylic acid ester derivatives

Abstract

This invention relates to the preparation of cyclopropane carboxylic acid ester derivatives, which are useful as pesticides. There is provided a process for preparing a 1:1 mixture of 1R cis S- and 1S cis R- isomers substantially free of 1S cis S- and 1R cis R- isomers of a compound of formula: ##STR1## wherein R.sup.1 and R.sup.2 are each independently selected from chlorine, bromine and methyl, which process comprises dissolving a mixture of 1S cis S- and 1R cis R- isomers of the compound of formula I, alone or in the presence of 1R cis S- and 1S cis R-isomers, in organic amine base containing from 5 to 7 carbon atoms and being a secondary amine containing two branched alkyl groups or a tertiary amine, and crystallizing out from the resulting solution of cis - isomers of formula I in the organic amine base a 1:1 mixture of the 1R cis S- and 1S cis R- isomers substantially free of 1S cis S- and 1R cis R- isomers. The process of the invention yields a product which contains substantially twice as much of the most active isomer of the compound of formula I as a racemic mixture of all four cis - isomers, and is a readily effected process which does not involve any asymmetric synthesis or optical resolution steps.