excess, which is labelled as "ee") variant of the Johnson–Corey–Chaykovsky reaction remains an active area of academic research. The use of chiral 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 organosulfide reagents are not prohibitively expensive and the racemic reactions can be carried out with equimolar amounts of ylide without raising costs significantly. Chiral sulfides, on the other hand, are more costly to prepare, spurring the advancement of catalytic enantioselective methods. Stoichiometric reagents The most successful reagents employed in a stoichiometric fashion are shown below. The first is a bicyclic oxathiane that has been employed in the synthesis of the β-adrenergic compound dichloroisoproterenol (DCI) but is limited by the availability of only one enantiomer of the reagent. The synthesis of the axial diastereomer is rationalized via the 1,3-anomeric effect which reduces the nucleophilicity of the equatorial lone pair. The conformation of the ylide is limited by transannular strain and approach of the aldehyde is limited to one face of the ylide by steric interactions with the methyl substituents. The other major reagent is a camphor-derived reagent developed by Varinder Aggarwal of the University of Bristol. Both enantiomers are easily synthesized, although the yields are lower than for the oxathiane reagent. The ylide conformation is determined by interaction with the bridgehead hydrogens and approach of the aldehyde is blocked by the camphor moiety. The reaction employs a phosphazene base to promote formation of the ylide. Catalytic reagents Catalytic reagents have been less successful, with most variations suffering from poor yield, poor enantioselectivity, or both. There are also issues with substrate scope, most having limitations with methylene transfer and aliphatic aldehydes. The trouble stems from the need for a nucleophilic sulfide that efficiently generates the ylide which can also act as a good leaving group to form the epoxide. Since the factors underlying these desiderata are at odds, tuning of the catalyst properties has proven difficult. Shown below are several of the most successful catalysts along with the yields and enantiomeric excess for their use in synthesis of (E)-stilbene oxide. Aggarwal has developed an alternative method employing the same sulfide as above and a novel alkylation involving a rhodium carbenoid formed in situ. The method too has limited substrate scope, failing for any electrophiles