 The development of an enantioselective ( i.e. yielding an enantiomeric excess , which is labelled as " ee " ) variant of the Johnson – Corey – Chaykovsky reaction remains an active area of academic research . The use of <unk> sulfides in a stoichiometric fashion has proved more successful than the corresponding catalytic variants , but the substrate scope is still limited in all cases . The catalytic variants have been developed almost exclusively for enantioselective purposes ; typical <unk> reagents are not <unk> expensive and the <unk> reactions can be carried out with <unk> amounts of ylide without raising costs significantly . <unk> sulfides , on the other hand , are more costly to prepare , spurring the advancement of catalytic enantioselective methods . 
