Weitz-Scheffer epoxidation enones

Compared to metal-catalyzed asymmetric epoxidation reactions, asymmetric versions of this reaction without the need of a catalyst (apart from a base) are rarely known. In 2000 Adam and coworkers reported a method for the asymmetric Weitz-Scheffer epoxidation of substituted enones 91 by the secondary, optically active hydroperoxide (5 )-(l-phenyl)ethyl hydroperoxide (equation 27, Table 10). ... 

TABLE 10. Enantioselective Weitz-Scheffer epoxidation of substituted enones p-XC6H4C(0)CH= CR with (5)-(l-phenyl)ethyl hydroperoxide... 

As discussed in Section 10.1, asymmetric epoxidation of C=C double bonds usually requires electrophilic oxygen donors such as dioxiranes or oxaziridinium ions. The oxidants typically used for enone epoxidation are, on the other hand, nucleophilic in nature. A prominent example is the well-known Weitz-Scheffer epoxidation using alkaline hydrogen peroxide or hydroperoxides in the presence of base. Asymmetric epoxidation of enones and enoates has been achieved both with metal-containing catalysts and with metal-free systems [52-55]. In the (metal-based) approaches of Enders [56, 57], Jackson [58, 59], and Shibasaki [60, 61] enantiomeric excesses > 90% have been achieved for a variety of substrate classes. In this field, however, the same is also true for metal-free catalysts. Chiral dioxiranes will be discussed in Section 10.2.1, peptide catalysts in Section 10.2.2, and phase-transfer catalysts in Section 10.2.3. 

In the early 1980s, Julia and Colonna reported that the Weitz-Scheffer epoxidation of chalcone (45a) can be catalyzed by poly-amino acids such as poly-L-alanine, and that the resulting epoxide is formed with enantiomeric excesses > 90% (Scheme 10.8) [66]. In the original three-phase procedure the enone is dissolved in an... 

Finally, it should be mentioned that poly-amino acid catalysts have been used either to enhance or override substrate-induced diastereoselectivity in the Weitz-Scheffer epoxidation of chiral enones (Scheme 10.12) [90]. 

In the 1980s, Julia and Colonna discovered that the Weitz-Scheffer epoxidation of enones such as chalcone (4, Scheme 2) by alkaline hydrogen peroxide is catalyzed in a highly enantioselective fashion by poly-amino acids such as poly-alanine or poly-leucine (Julia et al. 1980, 1982). The poly-amino acids used for the Julia-Colonna epoxidation are statistical mixtures, the maximum length distribution being around 20-25 mers (Roberts et al. 1997). The most fundamental question to be addressed refers to the minimal structural element (i.e. the minimal peptide length) required for catalytic activity and enantioselectiv-ity. To tackle this question, we have synthesized the whole series of L-leucine oligomers from 1- to 20-mer on a solid support (Berkessel... 

The asymmetric epoxidation of electron deficient alkenes like a,p-unsaturated esters, ketones and nitriles often is not efficient with the reagents suitable for electron rich systems. Prominent examples for the successful epoxidation of a,P-enones are the well-known Weitz-Scheffer epoxidation using alkaUne hydrogen peroxide or... 

The unusual nucleophilic epoxidation of /i-hydroxyenones under Sharpless conditions (see Section 4.5.1.3.2.1.) is also applicable to compounds 1 with endocyclic double bonds. The. sj H-epoxides are produced with complete selectivity. The stereochemical outcome of the reaction under Weitz-Scheffer conditions significantly differs from that observed for acyclic compounds. While the acyclic enones afforded preferentially the moderate ratio, cyclic ones gave predominantly s>7i-epoxides32. 

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