Preparation of optically active alpha-hydroxynitriles

A process for the preparation of an optically-active cyanomethyl pyrethroid ester or mixture enriched therein which comprises treating an alpha-chiral (optically-active) pyrethroid carboxylic acid halide or reactive derivative thereof or a mixture enriched therein with an optically-active alpha-hydroxynitrile of formula I ##STR11## wherein m is 0 or 1;

• Y is 0, CH.sub.2, or C(O);
• A, D and E each independently is a hydrogen atom, a halogen atom having an atomic number of from 9 to 35, inclusive, or an alkyl or alkoxy group containing 1 to 6 carbon atoms, each optionally substituted by one or more halogen atoms having an atomic number of from 9 to 35, inclusive, or a mixture enriched therein, which is prepared by treating the corresponding aldehyde with a source of hydrogen cyanide in the presence of a substantially water-immiscible, aprotic solvent and a cyclo(D-phenylalanyl-D-histidine) as a catalyst.