Enantioselective reduction alkene epoxidation

Although the chiral ketoiminatomanganese(lll) complexes were reported to catalyze the asymmetric aerobic alkene epoxidations, an aldehyde such as pivalaldehyde is required as a sacrihcial reducing agent. Groves reported that the dioxo(porphyrinato)ruthenium complexes 31, prepared with m-chloroperoxyben-zoic acid, catalyzed the aerobic epoxidation without any reductant. " On the basis of these reports, Che synthesized the optically active D4-porphyrin 35 and applied it to the truly aerobic enantioselective epoxidation of alkenes catalyzed by the chiral frani-dioxo (D4-porphyrinato)ruthenium(Vl) complex. The dioxoruthenium complex catalyzed the enantioselective aerobic epoxidation of alkenes with moderate to good enantiomeric excess without any reductant. In the toluene solvent, the turnovers for the epoxidation of T-(3-methylstyrene reached 20 and the ee of the epoxide was increased to 73% ee. 

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],... 

Control of enantioselectivity will be discussed in the corresponding sections on carbonyl reduction (Chapter 4) alkene hydrogenation, epoxidation, and dihydroxylation (Chapter 5) aldol condensation (Chapter 6) allylation and crotylation (Chapter 7) Claisen rearrangement (Chapter 8) and the Diels-Alder reaction (Chapter 9). 

As for the diols, the symmetric compounds have found most uses for nonsymmetric diols, a versatile synthesis via silyl ketones using the SAMP/RAMP methodology has been developedl5. Both enantiomers of the simplest symmetric diol, 2,3-butanediol (11), are often used in asymmetric synthesis, mostly for the formation of acetals and ketals with carbonyl compounds and subsequent reactions with acidic catalysts (Section D. 1.1.2.2.), Grignard reagents (Section D. 1.3.1.4.) and other carbanions (Sections D. 1.5.1., D. 1.5.2.4.), and diastereoselective reductions (Section D.2.3.3.). Precursors of chiral alkenes for cycloprotonations (Section D.1.6.1.5.) and for chiral allenes (Section B.I.), and chiral haloboronic acids (Section D. 1.1.2.1.) are other applications. The free diol has been employed as a chiral ligand in molybdenum peroxo complexes used for enantioselective epoxidation of alkenes (Section D.4.5.2.2.). 

For the enantioselective preparations of chiral synthons, the most interesting oxidations are the hydroxylations of unactivated saturated carbons or carbon-carbon double bonds in alkene and arene systems, together with the oxidative transformations of various chemical functions. Of special interest is the enzymatic generation of enantiopure epoxides. This can be achieved by epoxidation of double bonds with cytochrome P450 mono-oxygenases, w-hydroxylases, or biotransformation with whole micro-organisms. Alternative approaches include the microbial reduction of a-haloketones, or the use of haloperoxi-dases and halohydrine epoxidases [128]. The enantioselective hydrolysis of several types of epoxides can be achieved with epoxide hydrolases (a relatively new class of enzymes). These enzymes give access to enantiopure epoxides and chiral diols by enantioselective hydrolysis of racemic epoxides or by stereoselective hydrolysis of meso-epoxides [128,129]. 

The reduction of carbon-carbon double bonds to single bonds has been studied with various substrates. For example, Ohta et al. demonstrated that the reduction of a number of 1-nitro-l-alkenes by fermenting baker s yeast was enantioselective, resulting in the formation of optically active 1-nitroalkanes as shown in Fig. 10.13(a).On the other hand, Fuganti et al. reduced a,/3-unsaturated y-lactones to produce enantiomerically pure (-1-)-(J )-goniothalamin (Fig. 10.13(b)), which shows central nervous system activity. They also performed the kinetic resolution of the corresponding embryotoxic epoxide with yeast. 8... 

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