Preparation of insecticidal optically active isovalerate esters

The method is for preparing an isomer of an alpha -cyano-3-phenoxybenzyl-2-(4-chlorphenyl)isovalerate or of the derivative having 4-fluoro on the phenoxy nucleus; these compounds have two asymmetric carbon atoms. … The starting material is a solution which consists of or is rich in a racemic mixture of the isomers of the compound having respectively an (S) configuration on the acid moieties and respectively an (S) or (R) configuration on the alcohol moiety; in the presence of seed crystal of substantially pure (S)-(S) isomer, the (S)-(S) isomer is crystallized from the solution. The solution may be heated to at least 40 DEG C. The seed crystal can be one obtained by recrystallization. The solvent is preferably an alcohol, aliphatic hydrocarbon and/or alicyclic hydrocarbon. A basic catalyst, e.g. ammonia or triethylamine, can be present. Preferably the solution is at a temperature and concentration so as to be supersaturated in racemic mixture. … The product isomer has high insecticidal and/or acaricidal efficacy.