Configuration prochiral

Reductive alkylation with chiral substrates may afford new chiral centers. The reaction has been of interest for the preparation of optically active amino acids where the chirality of the amine function is induced in the prochiral carbonyl moiety 34,35). The degree of induced asymmetry is influenced by substrate, solvent, and temperature 26,27,28,29,48,51,65). Asymmetry also has been obtained by reduction of prochiral imines, using a chiral catalyst 44). Prediction of the major configurational isomer arising from a reductive alkylation can be made usually by the assumption that amine formation comes via an imine, not the hydroxyamino addition compound, and that the catalyst approaches the least hindered side (57). 

CHMO is known to catalyze a number of enantioselective BV reactions, including the kinetic resolution of certain racemic ketones and desymmetrization of prochiral substrates [84—87]. An example is the desymmetrization of 4-methylcyclohexanone, which affords the (S)-configurated seven-membered lactone with 98% ee [84,87]. Of course, many ketones fail to react with acceptable levels of enantioselectivity, or are not even accepted by the enzyme. 

Finally, non-racemic phosphorothioic and phosphonothioic acids 98 were obtained via a PTE-catalysed stereoselective hydrolysis of the prochiral substrates 97 (Equation 48). ° The absolute configurations of the thioacids 98 depended on whether native PTE or its mutants were used. 

Asymmetric conjugate addition of dialkyl or diaryl zincs for the formation of all carbon quaternary chiral centres was demonstrated by the combination of the chiral 123 and Cu(OTf)2-C H (2.5 mol% each component). Yields of 94-98% and ee of up to 93% were observed in some cases. Interestingly, the reactions with dialkyl zincs proceed in the opposite enantioselective sense to the ones with diaryl zincs, which has been rationalised by coordination of the opposite enantiofaces of the prochiral enone in the alkyl- and aryl-cuprate intermediates, which precedes the C-C bond formation, and determines the configuration of the product. The copper enolate intermediates can also be trapped by TMS triflate or triflic anhydride giving directly the versatile chiral enolsilanes or enoltriflates that can be used in further transformations (Scheme 2.30) [110],... 

Prochiral organic acids were hydrogenated on clay-supported Rh-chiral phosphine complexes.205,206 Hectorite-supported chiral Rh(I)-phosphine complexes were used for the asymmetric hydrogenation of a,P-unsaturated carboxylic acids.207 It was found that the interaction between the a-ester group of itaconates and phenyl groups of phosphine can play an important role in the determination of the configuration of products. 

Ethyl 2-phenylcyclopropanecarboxylate, obtained in the presence of 207a, has S configuration at C-l in both the cis- and trans-isomer. As that carbon has been furnished by the diazo ester, this result indicates enantiofacial selection at the carbenoid. In contrast, hardly any discrimination between the enantiofaces of the prochiral olefin occurs. Only when the ester substitutents become bulkier, does this additional stereochemical feature gain importance, and the S configuration at C-2 of the cyclopropane is favored. 

The enantioface and also the configuration (s-trans, s-cis) of the prochiral butadienes involved in the several elementary steps are of crucial importance for the stereocontrol of the cyclo-oligomer formation. Oxidative coupling, for example, can occur between two cA-butadienes, two /rum-butadienes or between cis- and /nmv-butadiene with either the same or the opposite enantioface of the two butadienes involved. The several stereoisomers are exemplified for the [Ni°(butadiene)2L] active catalysts for cyclodimer formation, that are schematically depicted in Fig. 1, together with the related stereoisomers of the ry ri fC1) and bis(r 3) octadienediyl-Ni11 species 2a and 4a, respectively. For each of the individual elementary steps there are several stereochemical pathways, which are exemplified in Fig. 1 for the... 

In recent years, extensive attention has been focused on finding cultured plant cells that can be used as catalysts for organic functional group transformations. A number of transformations employing freely suspended or immobilized plant cell cultures have been reported.24 For example, Akakabe et al.25 report that immobilized cells of Daucus carota from carrot can be used to reduce prochiral carbonyl substrates such as keto esters, aromatic ketones, and heterocyclic ketones to the corresponding secondary alcohols in ( -configuration with enantiomeric excess of 52-99% and chemical yields of 30 63%). 

Moreover, Soai et al.53c found that the enantioselective addition of Reformatsky reagents to prochiral ketones proceeds well when N,N-dialkylnorephedine 59 is used as the chiral ligand. When (15, 2R)-59a is used, the //-hydroxyl ester is obtained in 74% ee and 65% yield with ( -configuration predominant. When (lR,25,)-59a is used, the product is obtained in 74% ee and at 47% yield with (R)-configuration prevailing. 

Our final example is that of cyclic anhydrides, namely prochiral 3-sub-stituted glutaric anhydrides (7.101, R = Me, Et, or Pr). When incubated with lipase in an inert solvent in the presence of an alcohol (methanol, butan-l-ol, etc.), these compounds underwent nucleophilic ring opening with formation of a hemiester (7.102) of (/ -configuration (60-90% ee) [180]. This product enantioselectivity and, of course, the lack of reactivity in the absence of lipase show the enzymatic nature of the reaction. 

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