Chiral ketone-catalyzed asymmetric epoxidation

Chiral ketone-catalyzed asymmetric epoxidation has received intensive interest since the first reported by Curci et al. in 1984. The reaction is performed with oxone (potassium peroxomonosulfate) as the primary oxidant which generates the chiral dioxirane catalytic species in situ, which in turn, transfers the oxygen... 

Frohn, M. Shi, Y. Chiral Ketone-Catalyzed Asymmetric Epoxidation of Olefins, Synthesis 2000, 1979-2000. 

Chiral ketone-catalyzed asymmetric epoxidation of olefins 00S1979. 

The asymmetric epoxidation of alkenes constitutes a powerful approach to enantiomerically enriched epoxides, a class of highly versatile intermediates in organic synthesis [1]. Various effective epoxidation systems have been developed, including epoxidation of allylic [2, 3] and homoallylic [4] alcohols, metalunfunctionalized alkenes [5-7], and the nucleophilic epoxidation of electron-deficient alkenes [8]. During the past 10-15 years, much effort has been devoted to chiral ketone-catalyzed asymmetric epoxidation (Scheme 3.1). The subject has been described in great detail in the first edition [9] and other reviews [10]. This chapter provides an update on progress in this area since the first edition [9]. 

Warren JD, Shi Y. Chiral ketone-catalyzed asymmetric epoxidation of 2, 2-disubstituted vinylsilanes. J. Org. Chem. 1999 64(20) 7675-7677. 

For recent reviews on chiral ketone catalyzed asymmetric epoxidation see ... 

Zhang Z, Tang J, Wang X, Shi H. Chiral ketone- or chiral amine-catalyzed asymmetric epoxidation of cis-l-propenyl-phosphonic acid using hydrogen peroxide as oxidant. J. Mol. Catal. A Chem. 2008 285 68-71. 

Abstract Organo-catalyzed asymmetric epoxidation has received much attention in the past 30 years and significant progress has been made for various types of olefins. This review will cover the advancement made in the field of chiral ketone and chiral iminium salt-catalyzed epoxidations. 

Since the pioneering report of Curci in 1984 that a chiral ketone can catalyze asymmetric epoxidation reactions (methylcyclohexene oxide produced with up to 12% ee using (- -)-isopinocamphone/KHSOs) <1984CC155>, this area has attracted considerable interest with particular advances being achieved in the past decade by numerous research groups. 

Shi asymmetric epoxidation Chiral-ketone catalyzed epoxidation of unfunctionalized olefins. 410... 

Ketone-catalyzed asymmetric and diastereoselective epoxidation of olefins by di-oxiranes generated in situ from chiral ketones and oxone (2KHS05 KH-S04 K2S04) 04ACR497. 

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

Asymmetric epoxidation of olefins is an effective approach for the synthesis of enan-tiomerically enriched epoxides. A variety of efficient methods have been developed [1, 2], including Sharpless epoxidation of allylic alcohols [3, 4], metal-catalyzed epoxidation of unfunctionalized olefins [5-10], and nucleophilic epoxidation of electron-deficient olefins [11-14], Dioxiranes and oxazirdinium salts have been proven to be effective oxidation reagents [15-21], Chiral dioxiranes [22-28] and oxaziridinium salts [19] generated in situ with Oxone from ketones and iminium salts, respectively, have been extensively investigated in numerous laboratories and have been shown to be useful toward the asymmetric epoxidation of alkenes. In these epoxidation reactions, only a catalytic amount of ketone or iminium salt is required since they are regenerated upon epoxidation of alkenes (Scheme 1). 

Sharpless asymmetric epoxidation of allylic alcohols, asymmetric epoxidation of conjugated ketones, asymmetric sulfoxidations catalyzed, or mediated, by chiral titanium complexes, and allylic oxidations are the main classes of oxidation where asymmetric amplification effects have been discovered. The various references are listed in Table 4 with the maximum amplification index observed. 

Scheme 12.2 Asymmetric epoxidation catalyzed by C2-symmetrical chiral ketones 4. (For experimental details see Chapter 14.13.4).

Chen, R., Qian, C. and de Vries, J. G. Asymmetric epoxidation of a,/ -unsaturated ketones catalyzed by chiral ytterbium complexes, Tetrahedron Lett., 2001, 42, 6919-6921. 

NAP-MgO acts as a bifunctional heterogeneous catalyst for the Claisen-Schmidt condensation (CSC) of benzaldehydes with acetophenones to yield chalcones, followed by asymmetric epoxidation (AE) to afford chiral epoxy ketones in moderate to good yields and impressive enantioselectivities (ee s). NAP-MgO, in combination with the chiral auxiliary (11 ,21 )-(- -)-1,2-diphenyl-1,2-ethylenediamine (DPED), catalyzed the asymmetric Michael addition of malonates to cyclic and acyclic enones. 

Organocatalytic Oxidation. Asymmetric Epoxidation of Olefins Catalyzed by Chiral Ketones and Iminium Salts Wong, O.A. Shi, Y. Chem. Rev. 2008,108,3958. 

Previous reviews have dealt with metal-catalyzed [93] and stoichiometric [94] oxidation of amines in a broad sense. This section will be limited to the selective oxidation of tertiary amines to N-oxides. Amine N-oxides are synthetically useful compounds [95, 96] and are frequently used as stoichiometric oxidants in osmium-[97-99] manganese- [100] and ruthenium-catalyzed [101,102] oxidations, as well as in other organic transformations [103-105]. Aliphatic tert-amine N-oxides are usefid surfactants [96] and are essential components in hair conditioners, shampoos, toothpaste, cosmetics, and so on [106]. Chiral N-oxides have been used in asymmetric catalysis involving metal-free catalytic transformations [107] as well as metal-catalyzed reactions where the N-oxide serves as a ligand [107, 108]. Chiral tertiary amine N-oxides were recently used as reagents in asymmetric epoxidation of a,(3-unsaturated ketones [109]. 

It was found that 4-substitnted a,a-diarylprolinol 162 catalyzed the asymmetric epoxidation of a, 3-enones to give the corresponding chiral epoxides in good yields and high enantioselectivities (np to 96%) nnder mild reaction conditions (Scheme 1.68) [109]. The extension of epoxidation to cyclic ot,P-unsaturated ketones with chiral primary salts was reported by Wang et al. in good yields and excellent enantioselectivities (up to 99%) [110]. 

Wong OA, Shi Y. Organocatalytic oxidation. Asymmetric epoxidation of olefins catalyzed by chiral ketones and imi-nium salts. Chem. Rev. 2008 108 3958-3987. 

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