A crystalline insecticidal material, i.e. alphamesrin is prepd. by reacting alpha-cyano-3-phenoxy benzyl- cis-3-(2,2-dichlorovinyl)-2,2-dimethyl cyclopropane carboxylate isomers with C1-5 alkyl alcohol and strong base at room temp. The alphamesrin has each crystal consisting of equimolar amounts of the enantiomer pair alpha (S)-cyano phenoxy benzyl (1R,cis)-3-(2,2-dichlorovinyl)-2,2-dimethyl cyclopropane carboxylate and the alpha (S)-cyano-3-phenoxy benzyl (1S,cis) (2,2-dichlorovinyl)-2,2-dimethyl cyclopropane carboxylate. The obtd. alphamesrin has a high level of insecticidal activity.
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