Epoxides mediated

The aerobic degradation of chloroethene (vinyl chloride) by Mycobacterium aurum strain LI proceeded by initial formation of an epoxide mediated by an alkene monooxygenase (Hartmans and de Bout 1992). This reaction has also been demonstrated to occur with Methylosinus trichosporium, even though subsequent reactions were purely chemical (Castro et al. 1992b). 

Mechanistic studies103 revealed that chiral ketone-mediated asymmetric epoxidation of hydroxyl alkenes is highly pH dependent. Lower enantioselectivity is obtained at lower pH values at high pH, epoxidation mediated by chiral ketone out-competes the racemic epoxidation, leading to higher enantioselectivity. (For another mechanistic study on ketone-mediated epoxidation of C=C bonds, see Miaskiewicz and Smith.104)... 

Scheme 23. Reaction for Thiolysis of enantiomerically pure epoxide mediated by 51,...

Diaryl-2-hydroxypropiophenones (607) are obtained from 4-hydroxychalcone and a reactive phenol on treatment with alkaline hydrogen peroxide in an epoxide-mediated coupling reaction. The ketones undergo a base-catalyzed a-ketol rearrangement to the isomeric l-hydroxypropan-2-ones (608) and acid-catalyzed ring closure provides a route to 4-arylflavan-3-ones (Scheme 231) (80JCS(Pl)1025). 

Chiral 1,3-diols. Coupling of diaryl ketones with chiral epoxides mediated by Yb (15, 366) provides chiral 1,3-diols in high optical yields. Generally two diols are formed by coupling at both carbon atoms, but coupling with styrene oxide occurs mainly at the more-substituted carbon. 

The synthesis of spiro y-lactones has been extended to the synthesis of spiro -lactones. This one-pot transformation of 1,3-dienes reacted with epoxides, mediated by Rieke magnesium to spiro -lactones in good isolated yields, was recently reported [15]. Much work has been done to elucidate novel synthetic routes for these types of molecules [16]. In particular, -substituted 5-lactones have recently attracted considerable attention, mainly because molecules of this class include many natural products that exhibit substantial biological activity [17]. However, most methods for the synthesis of these molecules require... 

This convenient, one-pot process for the synthesis of spiro -lactones from 1,3-dienes and epoxides, mediated by Rieke magnesium, provides a proficient route to this class of complex organic molecules in good isolated yields. 

In 2009, the Magnus group disclosed the synthesis of racemic codeine [50]. In their synthesis (1) an intramolecular phenol alkylation generating an A-C bicyclic ring system with a quaternary carbon, (2) a stereo-controlled, one-pot formation of a B-ring by way of the combination of Henry reaction and Michael addition of nitromethane with a cyclohexadienone-aldehyde, and (3) an introduction of the requisite functionalities in the C-ring by the epoxide-mediated selenylation followed by oxidation, were employed as the key transformations. 

The germfree experiment showed that minor amounts of triCB-SQ2CH3 are formed by a nonmicrofloral mediated pathway. This could be mediated by tissue C-S lyase activity as shown in Figure 5(C), or by the pathway shown in Equation 13. This epoxide mediated pathway... 

P. C. B. Page, D. Barros, B. R. Buckley, A. Ardakani, B. A. Marples, Organocatalysis of asymmetric epoxidation mediated by iminium salts under nonaqueous conditions, /. Org. Chem. 69 (2004) 3595. 

Catalysts based on a binaphthalene structure 2.6. Catalytic asymmetric anhydrous epoxidation mediated by tetraphenyIphosphonium monoperoxysulphate and 196... 

SCHEME 5.4 Catalytic cycle for epoxidations mediated by oxaziridinium salts. 

Armstrong has also reported an in situ epoxidation, mediated by an intramolecular oxaziridinium salt, which gave good regioselectivity (Scheme 5.9) [16]. 

TABLE 5.3 Catalytic asymmetric epoxidation mediated by the new dibenzo[c,c]azepinium salts (30 and 31) a comparison to the corresponding dihydroisoquinolinium salts (17 and 24) ... 

SCHEME 5.21 The standard conditions applied for catalytic asymmetric epoxidation mediated by Oxone and iminium salts. 

V.K. Aggarwal, M.F. Wang, Catalytic asymmetric synthesis of epoxides mediated by chiral iminium salts, Chem. Commun. (1996) 191. 

P-18 - A mechanistic exploration of alkene epoxidation mediated by H2O2 within porous titanosiiicate catalysts... 

Asami, M., Ishizuka, T. and Inoue, S. (1994) Catal3ftic enantioselective deprotonation of mejo-epoxides by the use of chiral lithium amide. Tetrahedron.Asymmetry, 5, 793-796 Seki, A. and Asami, M. (2002) Catalytic enantioselective rearrangement of mejo-epoxides mediated by chiral lithium amides in the presence of excess cross-linked polymer-bound hthium amides. Tetrahedron, 58, 4655 663. 

Experimental findings and a computational study supported a non-covalent general acid/base activation of the reagents in the epoxidation mediated by diaiyl prolinols, where the oxa-Michael addition of the nucleophilic peroxide is the rate- and stereoselectivity determining-step, followed by a fast ring-closure step to the epoxide. The most stable transition... 

With conputational results in hand that anticipated the success of the reactions shown in Scheme 10.20. two substrates (E)-92 with eiAer anti or syn configuration were investigated under Payne rearrangement conditions. The requisite substrates (E)-92 fScheme 10.211 were synthesized from the a,p-unsaturated ketones (E)-94 by nucleophilic epoxidation mediated by... 

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