Epoxides achiral

Rearrangement of an achiral epoxide to give an optically active allyl alcohol, e.g., 1, induced by enantioselective deprotonation with a homochiral base (see p 436 for the determination of absolute configuration)55. 

Epoxidation of (Z)-2-butene gives the meso (achiral) epoxide. Oxygen transfer from the peroxy acid can occur at either face of the double bond, but the product formed is the same because the two mirror-image forms of the epoxide are superposable. 

A review of the metal-catalysed ring opening of achiral epoxides by achiral carbon-, sulfur-, nitrogen- and halogen-containing nucleophiles and kinetic resolution of racemic epoxides has been published.24 The review also discusses the reactions of chiral bases with epoxides that give allylic alcohols. 

A. Solid Achiral Epoxidation Catalysts Based on Organometallic Ti Sources... 

Enantioselective Ring Opening of Epoxides. In two steps, (l/ ,25)-N-pyrrolidinylnorephedrine can be converted into the analogous diamine 2. Treatment with butyllithium affords an amide salt that can be used as a chiral base. In a limited study, the opening of achiral epoxides to chiral allylic alcohols proceeds in high yield and with good enantioselectivity (eq 4). ... 

Examples of the use of vitamin B12 as a catalyst for enan-tioselective processes have been reported. The rearrangement of aziridines can proceed catalytically with ee s of up to 95% (eq 9). Analogous rearrangements on achiral epoxides (typically 60% ee) and achiral peroxides (low ee s) have been reported. 

Epoxidation of cyclohexene adds an O atom from either above or below the plane of the double bond to form a single achiral epoxide, so only one representation is shown. Opening of the epoxide ring then occurs with backside attack at either C—O bond. Because the epoxide is drawn above the plane of the six-membered ring, nucleophilic attack occurs from below the plane. This reaction is a specific example of the opening of epoxide rings with strong nucleophiles, first presented in Section 9.15. 

The selectivity is determined by the substrate since an achiral epoxidizing agent is used. To obtain the other diastereomer, one must exert reagent control to override the substrate s preference. 

Another privileged structure is the salen complex we have already seen as the catalyst 199 for Jacobsen epoxidations. Here the equivalent cobalt complex 235 is used to catalyse the opening of an achiral epoxide.53... 

As discussed in Hodgson et al, in this volume, transformations of achiral epoxides based on enantioselective a-deprotonation have been extensively studied by Hodgson and coworkers since 1996 [49, 50] ligands used were (-)-sparteine 2 [49], (-)-a-isosparteine 28 and (usually less effectively) bisoxazolines (e.g., 30), with the bisoxazoHne study [51] being the first report of such Hg-ands in an enantioselective deprotonation. 

Despite the tremendous success of hydroxyl-directed asymmetric epoxidation of allylic alcohols and homoallylic alcohols, the development of efficient asymmetric epoxidation methods for unfunctionalized olefins is still of great importance. In the mid-1980s, Kochi and co-workers studied achiral epoxidation of unfunctionalized olefins using achiral Cr(III)-salen and Mn(lll)-salen comlexes as catalysts, and they proposed that a high valent metal oxo (such as 0=Cr(V)-salen and 0=Mn(V)-salen species) as the reactive intermediate was responsible for the epoxidation of olefins. Such a reactive intermediate was formed in a catalytic cycle by oxidation of the catalyst (such as Cr(III)-salen or Mn(lll)-salen complex) with the stoichiometric oxidant (such as PhlO or NaOCl). ... 

Enantiopure organic compounds, which can easily be obtained from natural sources, e.g., D- and L-tartaric acid, D-mannitol, L-lactic acid, and L-amino acids, have served as precursors for chiral oxiranes through multistep syntheses. New chemical methods allowing for the desymmetrization of achiral epoxides have been reviewed recently [15]. 

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