Non-asymmetric epoxidation

3 Asymmetric epoxidation using chiral modified diethylzinc. 61 

4 Asymmetric epoxidation of ( -benzylideneacetophenone using THE La-(R)-BINOL-Ph3PO/CUMENE hydroperoxide system 

Hydrogen peroxide can cause burns wear suitable protective clothing, including eye and face protection. Store in a cool place. 

In a 50 mL dry round-bottomed flask was dissolved the a, (3-unsaturated ketone (2 mmol) in anhydrous methanol (10 mL) hydrogen peroxide (300 mg) was added. 

The reaction mixture was stirred at room temperature and the reaction monitored by TLC. After completion, the reaction was carefully quenched with water (10 mL). A white precipitate appeared. 

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Although comparison studies were carried out with a variety of other amide bases including diethylamide (which had proved superior to LDA in the non-asymmetric epoxide rearrangement) and the more sterically hindered dicyclohexylamide, LDA proved to give the highest yield and ee by combining the proper amounts of steric hindrance and amide reactivity. ... 

Spino and Frechette reported the synthesis of non-racemic allenic alcohol 168 by a combination of Shi s asymmetric epoxidation of 166 and its organocopper-mediat-ed ring-opening reaction (Scheme 4.43) [74]. Reduction of the ethynyl epoxide 169 with DIBAL-H stereoselectively gave the allenic alcohol 170, which was converted to mimulaxanthin 171 (Scheme 4.44) [75] (cf. Section 18.2.2). The DIBAL-H reduction was also applied in the conversion of 173 to the allene 174, which was a synthetic intermediate for peridinine 175 (Scheme 4.45) [76], The SN2 reduction of ethynyl epoxide 176 with DIBAL-H gave 177 (Scheme 4.46) [77]. 

The assumed boundaries between organic and carbohydrate chemistry were first breached in Sydney where D. H. R. Barton, a non-carbohydrate chemist, was invited to talk on new synthetic methods applicable to sugars. This trend was continued in Vancouver where K. B. Sharpless, another non-carbohydrate chemist, spoke on asymmetric epoxidation, a reaction important for carbohydrate syntheses. Then the boundary disappeared altogether. In Stockholm (1988) S. J. Danishefsky spoke on synthetic methods and in Yokohama (1990) K. C. Nicolaou also discussed new methods and synthetic strategies applicable... 

Preparation and activation of silica-supported poly-L-leucine[150] has been studied under a variety of reaction conditions leading to an efficient procedure for the preparation of material suitable for use in the Julia-Colonna asymmetric epoxidation reaction. Poly-L-leucine, can be added to the list of natural11511 and non-natural[152] oxidation catalysts that benefit from being supported on commercially available silica gel. 

The applicability of the Sharpless asymmetric epoxidation is however limited to functionalized alcohols, i.e. allylic alcohols (see Table 4.11). The best method for non-functionalized olefins is the Jacobsen-Kaksuki method. Only a few years after the key publication of Kochi and coworkers on salen-manganese complexes as catalysts for epoxidations, Jacobsen and Kaksuki independently described, in 1990, the use of chiral salen manganese (111) catalysts for the synthesis of optically active epoxides [276, 277] (Fig. 4.99). Epoxidations can be carried out using commercial bleach (NaOCl) or iodosylbenzene as terminal oxidants and as little as 0.5 mol% of catalyst. The active oxidant is an oxomanganese(V) species. 

Asymmetric ylide reactions such as epoxidation, cyclopropanation, aziridination, [2,3]-sigmatropic rearrangement and alkenation can be carried out with chiral ylide (reagent-controlled asymmetric induction) or a chiral C=X compound (substrate-controlled asymmetric epoxidations). Non-racemic epoxides are significant intermediates in the synthesis of, for instance, pharmaceuticals and agrochemicals. 

Asymmetric epoxidation The catalytic asymmetric epoxidation of alkenes has been the focus of many research efforts over the past two decades. The non-racemic epoxides are prepared either by enantioselective oxidation of a prochiral carbon-carbon double bond or by enantioselective alkylidenation of a prochiral C=0 bond (e.g. via a ylide, carbene or the Darzen reaction). The Sharpless asymmetric epoxidation (SAE) requires allylic alcohols. The Jacobsen epoxidation (using manganese-salen complex and NaOCl) works well with ds-alkenes and dioxirane method is good for some trans-alkenes (see Chapter 1, section 1.5.3). 

Katsuki has studied asymmetric epoxidation of non-functionalized olefins catalyzed by chiral Mn(salen) complex. Recently they proposed that the ligands of Mn(salen) complexes take non-planar stepped conformation and the direction of the folding ligands is strongly related to the sense of chirality in the asymmetric epoxidation (Eq. (7.26)) [71]. On the basis of this proposal, conformational con-... 

E. Da Palma Carreiro, G. Young-En, A. J. Burke, Approaches towards catalytic asymmetric epoxidations with methyltrioxorhenium(VII) (MTO) Synthesis and evaluation of chiral non-racemic 2-substituted pyridines, J. Mol. Cat. A.. Chem. 235, 285-292 (2005). 

P.C.B. Page, B.R. Buckley, D. Barros, H. Heaney, A.J. Blacker, B.A. Marples, Non-aqueous iminium salt mediated catalytic asymmetric epoxidation, Tetrahedran 62 (2006) 6607. 

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