Enone Epoxidation, enantioselective

Berkessel A, Gasch N, Glaubitz K, Koch C (2001) Highly enantioselective enone epoxidation catalyzed by short solid phase-bound peptides. Org Lett 3 3839-3842... 

High enantioselectivity and good yields have been obtained in asymmetric epoxidation of enones. Roberts modification of the Julia epoxidation using an immobilized polyleucine catalyst now represents a simple, practical method for enone epoxidation. Of the metal-based systems, the most economical and practical method is probably Enders protocol, despite the fact that it uses stoichiometric amounts of metal and hgand, as all the reagents are commercially available and cheap. It is difficult to compare the polypeptide-based catalysts with the metal based catalysts in terms of overall efficiency. 

An exploration of structural modifications on the activity of prolinol catalysts has been published <06T12264>. More electron-rich aromatic rings on the prolinol scaffold improve the activity in the epoxidation of a, 3-enones. The reaction of 10 with an enone and f-BuOOH provides the epoxy-ketones with moderate levels of enantioselectivity. 

Excellent enantioselectivities have not been attained in the asymmetric phase transfer catalyzed epoxidation of (Z)-enones in contrast to that of (E)-enones. However, a few promising results168,691 have been reported on the epoxidation of 2-substituted... 

E. J. Corey, F.-Y. Zhang, Mechanism and Conditions for Highly Enantioselective Epoxidation of a,p-Enones Using Charge-Accelerated Catalysis by a Rigid Quaternary Ammonium Salt , Org. Lett. 1999,1,1287-1290. 

The use of vinyl epoxides as substrates in enantioselective copper-catalyzed reactions, on the other hand, has met with more success. An interesting chiral ligand effect on Cu(OTf)2-catalyzed reactions between cyclic vinyloxiranes and dialkylzinc reagents was noted by Feringa et al. [51]. The 2,2 -binaphthyl phosphorus amidite ligands 32 and 43 (Fig. 8.5), which have been successfully used in copper-catalyzed enantioselective conjugate additions to enones [37], allowed kinetic resolution of racemic cyclic vinyloxiranes (Scheme 8.26). 

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

In 1996, Enders and coworkers reported the asymmetric epoxidation of ( )-enones 91 in the presence of stoichiometric amounts of diethylzinc and (lR,2R)-A-methylpseudo-ephedrine (120) under an oxygen atmosphere to give fraw -epoxides 92 with excellent yields (94-99%), almost complete diastereoselectivity (>98% de) and with very good enantioselectivities (61-92%) (Scheme 54) . For the same reaction Pu and coworkers utilized achiral polybinaphthyl 121 as ligand (in excess) instead of the chiral aminoalcohol. For each substrate, only one diastereomer was formed, but in most cases yields were lower than observed with the Enders system. Enders catalyst shows high asymmetric induction for alkyl-substituted enones (ee 82-92%), but for substrates bearing only aromatic substituents only modest enantioselectivity was obtained (R = R = Ph ... 

Ye, J., Wang, Y, Liu, R., Zhang, G., Zhang, Q., Chen, J. and Liang, X. A Highly Enantioselective Phase-transfer Catalyzed Epoxidation of Enones with a mild Oxidant, Trichloroisocyanuric acid. Chem. Commun. 2003, 2714-2715. 

Catalytic enantioselective epoxidation of 0, -ENONES WITH A BiNOL-ziNC-coMPLEx. 249... 

CATALYTIC ENANTIOSELECTIVE EPOXIDATION OF a, -ENONES WITH A BINOL-ZINC-COMPLEX... 

Asymmetric epoxidation of a,jS-unsaturated ketones represents an efficient method for the preparation of optically active a,jS-epoxy ketonesJ Recently, a new and very efficient catalytic system for enantioselective epoxidation of ( )-a,jS-enones to the corresponding trans-epoxy ketones has been developed based on a BlNOL-zinc complexJ Very high yields and excellent diastereo- and enantioselectivities are achieved at room temperature using cumene hydroperoxide (CMHP) as the terminal oxidant and performing the reaction in diethyl ether. A combination of enantio-merically pure BINOL and diethylzinc readily affords the active catalyst in situ (Figure 6.13). ... 

Table 6.12 Enantioselective catalytic epoxidation of aryl- and alkyl-substituted a,/l-enones using a (i )-BINOL-zinc catalyst (see Figure 6.13).

The remaining chapters deal with a variety of catalysts for effecting oxidation reactions. Chapter 5 describes three simple protocols for the controlled oxidation of primary or secondary alcohols. The importance of stereocontrolled epoxidation and hydroxylation reactions is reflected by the fact that Chapter 6, directed at this field, is one of the most extensive sections of the book. An interesting example of an enantioselective Baeyer-Villiger reaction is featured in Chapter 7, together with an industrially important ketone to enone conversion. Oxidative carbon-carbon... 

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