oxygen atmosphere have been used to epoxidize some classes of enones (see equation (8) below). The evolution of ethane gas and uptake of oxygen are evidence for ligand exchange followed by oxidation of the intermediate zinc alkoxide species. A catalytic version of this transformation has been achieved using chiral zinc alkylperoxides. (4) Lithium, magnesium, and calcium alkylperoxides have also been employed as asymmetric nucleophilic epoxidation reagents. Simple tartrate and pseudoephedrine ligands are effective in combination with these metals; however, little detailed information about the precise mechanisms of these systems is known. In combination with BINOL ligands and cumene hydroperoxide, lanthanide alkoxides can be used to epoxidize both trans and cis enones with high enantioselectivity. Studies of non-linear effects with these catalyst systems suggest that the active catalyst is oligomeric. (5) Homopolymers of amino acids (polypeptides) can also be used to effect enantioselective epoxidations in the presence of an enone and a peroxide. Structure-reactivity relationships have not emerged, but enantioselectivities in these reactions are often high, and polypeptides can often be used when other methods fail. Phase-transfer catalysis of nucleophilic epoxidation is also possible using cinchona-based alkaloid catalysts. Phase-transfer methods allow some variability in the oxidant used: hydroperoxides, hydrogen peroxide, and hypochlorites have all been used with some success. Scope and Limitations Optimal conditions for enantio-selective nucleophilic epoxidation depend on the substrate employed. Although a variety of substrates may be epoxidized using nucleophilic methods, each particular method tends to have limited substrate scope. This section describes asymmetric nucleophilic epoxidation methods, organizing them according to the constitution and configuration of the unsaturated substrate. Enones Dialkyl (E)-enones have been most commonly epoxidized using either lanthanide/BINOL systems or a magnesium tartrate catalyst. (6) For alkyl aryl (E)-enones, both polypeptides and lanthanide/BINOL catalysts give good yields and enantioselectivities. The most common polypeptide employed is poly-L-leucine. (7) Aryl alkyl (E)-enones have been epoxidized with high enantioselectivity using stoichiometric zinc peroxide systems. Polyleucine may be used with these substrates as well; when an existing stereocenter in the substrate biases the sense of selectivity of the epoxidation, polyleucine is able to overcome this bias. (8) Phase-transfer catalysis has been applied successfully to epoxidations of diaryl (E)-enones (chalcones). Lanthanide/BINOL is effective for this class of substrates as well. (9) (Z)-Enones are difficult to epoxidize without intermediate bond rotation to afford trans-epoxides. Lanthanide catalysts do effectively prevent bond rotation, however, and provide access to cis epoxide products. (10) With the