lone exception of methylidene tetralone substrates, no general methods are available for the asymmetric nucleophilic epoxidation of trisubstituted double bonds. Other Electron-poor Alkenes Unsaturated esters may be epoxidized using either electrophilic or nucleophilic methods. Lanthanide-mediated epoxidation has been successfully applied to cinnamates and β-heteroaryl unsaturated esters. Amides are also epoxidized under lanthanide-mediated conditions. (11) Epoxidations of other electron-deficient double bonds (substituted by electron-withdrawing groups other than carbonyls) are limited in scope, although a few examples have been reported. The ability of the carbonyl group to coordinate Lewis acidic functionality is critical for most existing methods. Comparison with Other Methods The asymmetric Darzens reaction between aldehydes and (alpha)-haloesters is an effective method for the synthesis of glycidic esters. Chiral auxiliaries, chiral boron enolates, and asymmetric phase transfer catalysis have been used successfully to effect asymmetric induction in the Darzens reaction. (12) Diastereoselective epoxidations of chiral, non-racemic alkenes suffer from the limitation that removal of the auxiliary without disturbing the epoxide is often difficult. Nonetheless, diastereoselectivity is high in some cases. (13) Oxidation of epoxy alcohols generated via Sharpless epoxidation is a third method for the enantioselective synthesis of chiral α,β-epoxy carbonyl compounds. Swern and Parikh-Doering conditions are most commonly applied to accomplish these oxidations. (14) Typical experimental conditions Generally, nucleophilic epoxidations are carried out under inert atmosphere in anhydrous conditions. For zinc-mediated epoxidations, diethylzinc and ligand are first mixed and oxidized, then the enone is introduced. Lanthanide-mediated epoxidations typically require an additive to stabilize the catalyst; this is most commonly triphenylphosphine oxide or triphenylarsine oxide. Phase-transfer catalyzed epoxidations may be carried out using one of three possible sets of reaction conditions: (1) sodium hypochlorite at room temperature, (2) freshly prepared 8 M potassium hypochlorite, or (3) trichloroisocyanuric acid in aqueous or non-aqueous conditions. Among polypeptide-based methods, employing a phase transfer catalyst and triphasic media permits lower catalyst loadings. Biphasic conditions using an organic base in conjunction with urea/H2O2 may also be used. References Organic reactions