Epoxidation organocatalytic, asymmetric

Wu in Comprehensive Asymmetric Catalysis I-III (Eds. Jacobsen, E. N. Pfaltz, A. Yamamoto H.), Springer, Berlin, 1999, p. 649f. (c) For organocatalytic asymmetric epoxidations, see chapter 10. 

Shi Y (2004) Organocatalytic asymmetric epoxidation of olefins by chiral ketones. Acc Chem Res 32 488... 

Organocatalytic asymmetric epoxidation of olefins by chiral ketones 04ACR488. 

Organocatalytic Asymmetric Epoxidation of Enones via Iminium Catalysis... 

W. Zhuang, M. Marigo, K. A. Jorgensen, Organocatalytic asymmetric epoxidation reactions in water-alcohol solutions, Org. Biomol. Chem. 3 (2005) 3883. 

Organocatalytic Asymmetric Epoxidation of Olefins by Chiral Ketones ... 

Likewise, [bis(acyloxy)iodo]arenes can be used as the oxidants in organocatalytic, asymmetric epoxidation of a,p-unsaturated aldehydes using chiral imidazolidinone catalyst 207 [266]. In a specific example, the... 

The straightforward reaction of carbonyls with an oxidant is a very well-known route towards the widely present a-hydro>y-carbonyl structural motive and several types of organometallic or organocatalytic asymmetric protocols have been developed. The reaction is generally accepted as a variant of Rubottom oxidation (oxidation with peracids), with a mechanism proceeding via an epoxide intermediate obtained from an enol. The first catal5Aic asymmetric reaction was developed in 1988 using a quaternised Cinchona alkaloid. (See Chapter 16). 

Scheme 30 Organocatalytic asymmetric epoxidation of a,p-unsaturated aldehydes 77...

Epoxides can also be accessed asymmetrically using hypervalent iodine reagents in combination with imidazolidinone catalysts 78 (Scheme 30). The methodology developed by MacMillan et al. includes participation of hypervalent iodine reagent in a 1,4-heteroconjugate addition reaction for the organocatalytic, asymmetric epoxidation of a,p-unsaturated aldehydes 77. This organocatalytic reaction allows for the enantioselective formation of epoxides 78 from a wide array of electronically and sterically diverse a,p-unsaturated aldehydes [92]. 

Scheme 40.52 Organocatalytic asymmetric epoxidation/semipinacol rearrangement.

Iminium-Activated Epoxidations Similar to ylide 153 and bromomalo-nates 157, it was proposed that hydrogen peroxide could also be used as an amphiphilic reactant for [2-1-1] reactions of a,P-unsaturated aldehydes to furnish epoxidation products. Inspired by this hypothesis, Jprgensen s group developed an organocatalytic asymmetric epoxidation system of a,P-unsaturated aldehydes with HjOj as the oxidant (Scheme 1.63) [ 106]. The reactions take place under mild conditions in good to high yields and enantio- and diastereoselectivities. 

The catalytic asymmetric epoxidation of a,p-unsaturated aldehydes has also been an important challenge in iminium catalysis and for chemical synthesis in general. More recently, Jprgensen and coworkers have developed an asymmetric organocatalytic approach to ot, (3-epoxy aldehydes using pyrrolidine catalyst 20 and H2O2 as the stoichiometric oxidant. The reaction appears to be extremely general and will likely receive wide attention from the chemical synthesis community (Scheme 11.6b). 

Nucleophilic addition of sulfur ylides to C=0 double bonds is an important means of synthesis of epoxides [198], Because optically active epoxides are widely applied as versatile intermediates in the preparation of, e.g., pharmaceuticals, the asymmetric design of this sulfur ylide-based reaction has attracted much interest [199, 200, 212, 213], One aspect of this asymmetric organocatalytic process which has been realized by several groups is shown in Scheme 6.87A. In the first step a chiral sulfur ylide of type 204 is formed in a nucleophilic substitution reaction starting from a halogenated alkane, a base, and a chiral sulfide of type 203 as organocata-... 

Asymmetric sulfur-ylide-type epoxidation is an excellent tool for enantioselective and diastereoselective synthesis of epoxides. By use of Aggarwal-type methodology a broad range of aromatic, enolizable, and base-sensitive aldehydes can be converted into the desired epoxides. In addition to an excellent diastereomeric ratio, the optimized organocatalytic systems of this sulfur-ylide-type epoxidation also... 

The vast majority of organocatalytic reactions proceeds via covalent formation of the catalyst-substrate adduct to form an activated complex. Amine-based reactions are typical examples, in which amino acids, peptides, alkaloids and synthetic nitrogen-containing molecules are used as chiral catalysts. The main body of reactions includes reactions of the so-called generalized enamine cycle and charge accelerated reactions via the formation of iminium intermediates (see Chapters 2 and 3). Also, Morita-Baylis-Hillman reactions (see Chapter 5), carbene-mediated reactions (see Chapter 9), as well as asymmetric ylide reactions including epoxidation, cyclopropanation, and aziridination (see Chapter 10), and oxidation with the in situ generation of chiral dioxirane or oxaziridine catalysts (see Chapter 12), are typical examples. 

Oxidation reactions - notably alkene epoxidations - were some of the first asymmetric organocatalytic processes to develop into generally useful synthetic methods applicable to a range of substrates [1], This chapter surveys these reactions, with emphasis placed on the most practical and general. Some recent, very useful oxidation reactions involving a-oxidation of carbonyl compounds are covered elsewhere (see Chapter 2). 

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

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