Carbonyl compounds prochiral

Reaction of optically active a-sulphinyl acetate 298a with prochiral carbonyl compounds proceeds with a high asymmetric induction - , the degree of which depends on the nature of substituents at the carbonyl group (equation 252 Table 22) . The jS-hydroxy sulphoxides 422 formed may be transformed to optically active p-hydroxycarboxylic esters 423 (equation 253) and optically active long-chain lactones 424 99 (equation 254). Corey and coworkers have used this method to introduce a chiral centre at C-3 in their synthesis of maytansin °°, and Papageorgiou and Benezra for the synthesis of chiral a-hydroxyalkyl acrylates 425 ° (equation 255). 

One approach to enantioselective reduction of prochiral carbonyl compounds is to utilize chiral ligand-modified metal hydride reagents. In these reagents, the number of reactive hydride species is minimized in order to get high chemo-selectivity. Enantiofacial differentiation is due to the introduced chiral ligand. 

Chiral 4-chloro-3-hydroxybutanoate esters are important chiral C4-building blocks [43-53]. For example, (i )- and (S)-isomers can be converted to L-car-nitine and the hydroxymethyl glutaryl-CoA reductase inhibitor. Since these compounds are used as pharmaceuticals, a high optical purity is required. A practical enzymatic method for the production of chiral 4-chloro-3-hydroxy-butanoate esters from prochiral carbonyl compounds, i.e.,4-chloroacetoacetate esters, or racemic 4-chloro-3-hydroxybutanoate esters is described. 

The reaction shown in Scheme 39 was also performed starting from a chiral carbamoyl chloride (91, Y = O) derived from (f )-iV-methyl-iV-(l-phenylethyl)amine, in order to study the possible asymmetric induction using prochiral carbonyl compounds. Thus, with pivalaldehyde or benzaldehyde the mixture of diastereomers obtained was ca 1 1. This behavior was also observed with other chiral functionalized organolithium compounds ". ... 

Electroenzymatic reactions are not only important in the development of ampero-metric biosensors. They can also be very valuable for organic synthesis. The enantio- and diasteroselectivity of the redox enzymes can be used effectively for the synthesis of enantiomerically pure compounds, as, for example, in the enantioselective reduction of prochiral carbonyl compounds, or in the enantio-selective, distereoselective, or enantiomer differentiating oxidation of chiral, achiral, or mes< -polyols. The introduction of hydroxy groups into aliphatic and aromatic compounds can be just as interesting. In addition, the regioselectivity of the oxidation of a certain hydroxy function in a polyol by an enzymatic oxidation can be extremely valuable, thus avoiding a sometimes complicated protection-deprotection strategy. 

Asymmetric hydrosilylation of prochiral carbonyl compounds, imines, alkenes and 1,3-dienes has been extensively studied and continues to be one of the most important subjects in the hydrosilylation reactions. This topic has been reviewed at each stage of its development as a useful synthetic method based on asymmetric catalytic processes1,3,187-189. In the last decade, however, substantial progress has been made in the efficiency of this reaction. Accordingly, this section summarizes the recent advances in this reaction. 

A nucleophilic addition to a prochiral carbonyl compound R -CO-R gives a racemic mixture of alcohols. If the substituent R is chiral, the two faces of the carbonyl become... 

Asymmetric reduction of prochiral carbonyl compounds for the production of chiral alcohols D-pantoyl lactone production by... 

Bottlenecks for bioreduction of prochiral carbonyl compounds by microbial cells for industrial use production... 

ASYMMETRIC REDUCTION OF PROCHIRAL CARBONYL COMPOUNDS FOR THE PRODUCTION OF CHIRAL ALCOHOLS D-PANTOYL LACTONE PRODUCTION BY ASYMMETRIC REDUCTION... 

Chiral alcohols are some of the most important chiral building blocks for the production of pharmaceuticals. The creation of chiral alcohols through the asymmetric reduction of prochiral carbonyl compounds using biocatalysts, such as microbial cells and commercially available oxidoreductases, has been... 

Bottlenecks for Bioreduction of Prochiral Carbonyl Compounds by Microbial Cells for Industrial Use Production of (fl)-4-Chloro-3-Hydroxybutanoate Ethyl Ester... 

In 1951 Bothner-By first attempted asymmetric reductions based on the conversion of lithium aluminum hydride (LAH) into a chiral alkoxy derivative by reaction with (+)-camphor. Since this pioneering work, the use of chirally modified LAH reagents has been the focus of much attention. In 1979, the first virtually complete enantiofacial recognition of prochiral carbonyl compounds was accomplished by using LAH modified with optically pure 2,2 -dihydroxy-1,1 -binaphthyl and a simple alcohol (BINAL-H). Asymmetric reduction with chiral 2,5-dimethylborolane also gave alcohols in high optical yields." Recently, excellent results have been obtained using a chirally modified sodium borohydride... 

A number of reports involving the addition of chiral nucleophiles to prochiral carbonyl compounds have appeared in Ae literature. While many of these examples involve the use of stabili carbanions... 

Enantioselective hydrogenation of prochiral carbonyl compounds with Wilkinson-type catalysts is less successful than the hydrogenation of prochiral olefins. Both rates and enantioselectivities are greatly diminished in the hydrogenation of ketones, compared with olefins. Enantioselectivities only occasionally reach 80% ee, e. g., in the hydrogenation of acetophenone with the in-situ catalyst [Rh(nbd)Cl]2/DIOP, where nbd = norbomadiene [71]. The Ru-based BINAP catalysts improved this situation, by allowing the hydrogenation of a variety of functionalized ketones in enantioselectivities close to 100% ee [72]. 

Asymmetric hydrosilylation of prochiral carbonyl compounds, alkenes, 1,3-dienes, and imines has been extensively studied and remains one of the most important subjects in the field. This reaction is strongly affected by the nature of the catalyst (metal, type of chiral ligand) and the substrate as well as the reaction conditions (solvent, temperature, etc.). In recent years, many papers have been published on asymmetric hydrosilylation, describing new catalytic systems (mainly new optically active ligands) and new synthetic applications of the reaction [4, 24]. 

Asymmetric induction from chiral ligands on the metal center can be used to produce enantiomerically enriched products from simple prochiral carbonyl compounds. More often, this aldol control element is employed to reinforce or overturn the inherent stereochemical bias of a chiral ketone or aldehyde. 

Using optically active aldehydes, ketones or chiral complexes of prochiral carbonyl compounds can induce the diastereoselective addition of hydrogen cyanide to the carbonyl carbon. In this case the substrate range is also very narrow and only in some cases are the obtainable optical yields satisfactory [101-106]. 

Scheme 2.131 Enantioselective nucleophilic trifluoromethylation of prochiral carbonyl compounds with chiral fluoride sources [78].

That is, in order for the phenomenon to be observed, both reactants must show inherent diastereoface selectivity in their reactions with achiral partners. If one of the reactants shows no inherent diastereoface selectivity in its reactions with achiral reactants, then mutual kinetic resolution will not be observed regardless of the stereoselectivity of the other reactant. For an example, consider the case of an enzyme which mediates some reaction, say reduction of the carbonyl group. We can let the enzyme be A and assume that, because of its uniquely-evolved molecular structure, it shows very high inherent diastereoface selectivity (thus, it will reduce prochiral carbonyl compounds to chiral alcohols with very high enantiomeric excess). For B, let us take a chiral aldehyde that shows no inherent diastereoface selectivity in its... 

Cross-coupling of o-trimethylsilylbenzylmagnesium bromide with alkenyl bromides catalyzed by the PPFA-palladium complex gave optically active allylsilanes 11 (eq. 6). Allylsilane 11a reacted enantioselectively with the prochiral carbonyl compounds 12 in the presence of TiCl4 to produce alcohols 13 of over 90% enantiomeric purity (eq. 7). ... 

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