Ketones enantioselectivity

Keywords aldehydes, ketones, enantioselective hetera-Diels-Alder reactions... 

The introduction of various metal-catalyzed reactions, however, remarkably expanded the scope of the epoxidation of Q,.3-unsaturatcd ketones. Enders et al. have reported that a combination of diethylzinc and A-methyl-pseudoephedrine epoxidizes various o,. j-unsaturatcd ketones, under an oxygen atmosphere, with good to high enantioselectivity (Scheme 23).126 In this reaction, diethylzinc first reacts with the chiral alcohol, and the resulting ethylzinc alkoxide is converted by oxygen to an ethylperoxo-zinc species that epoxidizes the a,/3-unsaturated ketones enantioselectively. Although a stoichiometric chiral auxiliary is needed for this reaction, it can be recovered in almost quantitative yield. 

CATALYTIC ASYMMETRIC ADDITIONS OF DIALKYLZINC TO KETONES ENANTIOSELECTIVE FORMATION OF TERTIARY ALCOHOLS... 

The results demonstrate clearly the capacity of the catalytic system to reduce aliphatic ketones enantioselectively which has not been accomplished so far with other ruthenium catalysts under hydrogen transfer conditions (46). 

Benzamido-cinnamic acid, 20, 38, 353 Benzofuran polymerization, 181 Benzoin condensation, 326 Benzomorphans, 37 Benzycinchoninium bromide, 334 Benzycinchoninium chloride, 334, 338 Bifiinctional catalysts, 328 Bifiinctional ketones, enantioselectivity, 66 BINAP allylation, 194 allylic alcohols, 46 axial chirality, 18 complex catalysts, 47 cyclic substrates, 115, 117 double hydrogenation, 72 Heck reaction, 191 hydrogen incorporation, 51 hydrogen shift, 100 hydrogenation, 18, 28, 57, 309 hydrosilylation, 126 inclusion complexes, oxides, 97 ligands, 19, 105 molecular structure, 50, 115 mono- and bis-complexes, 106 NMR spectra, 105 olefin isomerization, 96... 

Keywords ketone, enantioselective reduction, BH3-ethylenediamin complex, inclusion complex, alcohol... 

The boron atom dominates the reactivity of the boracyclic compounds because of its inherent Lewis acidity. Consequently, there have been very few reports on the reactivity of substituents attached to the ring carbon atoms in the five-membered boronated cyclic systems. Singaram and co-workers developed a novel catalyst 31 based on dicarboxylic acid derivative of 1,3,2-dioxaborolane for the asymmetric reduction of prochiral ketones 32. This catalyst reduces a wide variety of ketones enantioselectively in the presence of a co-reductant such as LiBH4. The mechanism involves the coordination of ketone 32 with the chiral boronate 31 and the conjugation of borohydride with carboxylic acid to furnish the chiral borohydride complex 34. Subsequent transfer of hydride from the least hindered face of the ketone yields the corresponding alcohol 35 in high ee (Scheme 3) <20060PD949>. 

Knochel and coworkers have reported the use of lithiated /V,/V-dialkylurcas (such as 47), which have proved to be useful for enantioselective deprotonation and alkylation of ketones. Enantioselectivities up to 88% were achieved across a range of 4-substituted cyclohexanones in the absence of HMPA. On addition of HMPA, both yield and enantioselectivity were lowered (Scheme 31)71,72. 

Several novel catalysts in which borohydride is complexed with a difiinctional chiral ligand have been developed and used for the enantioselective reduction of prochiral ketones to chiral alcohols. Corey-Bakshi-Shibatareduction (CBS reduction) is an organic reaction which reduces ketones enantioselectively into alcohols by using chiral oxazaborolidines and BHs-THF or catecholborane as stoichiometric reductants (CBS reagent, 1.64) (also see Chapter 6, section 6.4.2). 

Chiral Ligand of LiAlH4 for the Enantioselective Reduction of a,p-Unsaturated Ketones. Enantioselective reductions of a,p-unsaturated ketones afford optically active ally lie alcohols which are useful intermediates in natural product synthesis. Enantioselective reduction of a,p-unsaturated ketones with LiAlH4 modified with chiral amino alcohol (1) affords optically active (S)-allylic alcohols with high ee s. When 2-cyclohexen-l-one is employed, (5)-2-cyclohexen-l-ol with 100% ee is obtained in 95% yield (eq 2). This is comparable with the results obtained using LiAlH4-chiral binaphthol and chiral 1,3,2-oxazaborolidine. ... 

Microorganisms like baker s yeast are able to reduce some ferrocenyl ketones enantioselectively (for a review on microorganisms and enzymes for the reduction... 

With NiCl2-CrCl2 to mediate the reaction of alkenyl halides to ketones, enantioselectivity is induced by 40. ... 

Catecholborane reduces ketones enantioselectively in the presence of the titanium complexes such as 86. 

J -Sulfonyloxazaborohdine also acts as catalyst for the borane reduction of ketones [30]. This catalyst may form a borane 0-adduct which reduces ketones enantioselectively. 

Apart from the asymmetric metal catalysis, enantioselective Baeyer-Villiger oxidations mediated by enzymes have been known for some time [32,33,34]. Both whole-cell cultures and isolated enzymes, usually flavin-dependent monooxygenases, can be used to oxidize ketones enantioselectively. For future improvements in the asymmetric Baeyer-VilHger oxidation the use of chiral Lewis acids in combination with an appropriate oxidant seems worthy of intensive investigation. 

A tetracyclic oxazaborolidine derived from tryptophol can be used to reduce ketones enantioselectively. It is not effective for imines. 

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