Enantiomer by enzymes

A suitable enzyme catalyses enantioselective hydrolysis of ester 39. This gives an alcohol of one configuration and leaves behind the ester of the other. Differentiation between enantiomers by enzymes is almost total. 

Racemic amino acids may be resolved into their (S)- and (7 )-enantiomers by enzymic or chemical procedures11 l4. An interesting method has been developed for the conversion of racemic amino acids to the (S)-enantiomer in a yield of >95% and with >98% ee. This method has been applied to the conversion of (/ /. (-methionine. (7 /5)-alanine and (R/S)-leucine into their (S)-enantiomers 15 Thus, Lhe (/J)-amino acid in the racemate is fully converted to the (S )-amino acid... 

Lactams, 803 Lactase, 993 Lactic acid, 737, 1015 biological oxidation of, 602 (5 ) enantiomer by enzymic reduction of pyruvic acid, 681-682,1015 Lactones, 758-759, 788 formation of... 

Optically inactive starting materials can give optically active products only if they are treated with an optically active reagent or if the reaction is catalyzed by an optically active substance The best examples are found m biochemical processes Most bio chemical reactions are catalyzed by enzymes Enzymes are chiral and enantiomerically homogeneous they provide an asymmetric environment m which chemical reaction can take place Ordinarily enzyme catalyzed reactions occur with such a high level of stereo selectivity that one enantiomer of a substance is formed exclusively even when the sub strate is achiral The enzyme fumarase for example catalyzes hydration of the double bond of fumaric acid to malic acid m apples and other fruits Only the S enantiomer of malic acid is formed m this reaction... 

In all the reported examples, the enzyme selectivity was affected by the solvent used, but the stereochemical preference remained the same. However, in some specific cases it was found that it was also possible to invert the hydrolases enantioselectivity. The first report was again from iQibanov s group, which described the transesterification of the model compound (13) with n-propanol. As shown in Table 1.6, the enantiopreference of an Aspergillus oryzae protease shifted from the (l)- to the (D)-enantiomer by moving from acetonitrile to CCI4 [30]. Similar observations on the inversion of enantioselectivity by switching from one solvent to another were later reported by other authors [31]. 

An interesting example of the above difference is l-DOPA 4, which is used in the treatment of Parkinson s disease. The active drug is the achiral compound dopamine formed from 4 via in vivo decarboxylation. As dopamine cannot cross the blood-brain barrier to reach the required site of action, the prodrug 4 is administered. Enzyme-catalyzed in vivo decarboxylation releases the drug in its active form (dopamine). The enzyme l-DOPA decarboxylase, however, discriminates the stereoisomers of DOPA specifically and only decarboxylates the L-enantiomer of 4. It is therefore essential to administer DOPA in its pure L-form. Otherwise, the accumulation of d-DOPA, which cannot be metabolized by enzymes in the human body, may be dangerous. Currently l-DOPA is prepared on an industrial scale via asymmetric catalytic hydrogenation. 

The E-value is related to the difference in free energy of activation for reaction with the two enantiomers by A A G =-RT InE. A reaction between the enzyme and one enantiomer passes through a transition state of different energy from the transition state resulting from reaction with the other (Figure 2.7) and E is constant throughout the reaction. As mentioned above there are two products in a resolution process the enantiomeric excesses of which depend on the degree of conversion. This feature is a major difference from asymmetric synthesis. In asymmetric synthesis there is only one product and the ee is independent of conversion. 

Quartey, E.G.K., Hustad, J.A., Faber, K. and Anthonsen, T. (1996) Selectivity enhancement of PPL-catalysed resolution by enzyme-fractionation and mediumengineering Synthesis of both enantiomers of tetrahydropyran-2-methanol. Enzyme. Microb. Technol., 19, 361-366. 

A very important process is the kinetic resolution of acetylated amino acids, mediated by enzymes obtained from Aspergillus strains, which leads in a predictable manner to the L-amino acids 32 the o-enantiomers react very slowly250. 

Planar chirality is a valuable feature of the ferrocene chemistry (Chart 2B,C) (205). This unnatural chirality type has attracted attention of several groups. Sadeghi and co-workers have demonstrated that the planar chiral ferrocenes are recognized by cytochrome c peroxidase (206). The rate constants for the oxidation of R and S enantiomers by the wild type enzyme equal 2.9 x 106 and 1.6 x 106 M 1 s respectively. Interestingly, the enantioselectivity inverts for the aspartate 34 for lysine mutant and the rate constants become equal to 5.9 x 106 and 14.8 x 106 M-1 s-1, respectively. The discrimination of planar chiral ferrocenes is the case, but the stereoselectivity factors are lower than 3. 

Yeast-mediated reductions predominantly form a single enantiomer and it is often difficult to find conditions which produce the opposite stereoisomer selectively. It has, however, been possible to obtain both enantiomers in 50% yield in 100% via enzymatic optical resolution. Chiral fluorinated secondary alcohols possessing the mono-, di- and/or trifluoromethyl group have been prepared by enzyme-catalyzed kinetic resolutions [27]. 

A kinetic resolution is a chemical reaction in which one enantiomer of a racemate reacts faster than the other. Most kinetic resolutions of pharmaceutical compounds are catalyzed processes. Catalysts used in a kinetic resolution must be chiral. Binding of a chiral catalyst with a racemic material can form two different diastereomeric complexes. Since the complexes are diastereomers, they have different properties different rates of formation, stabilities, and rates of reaction. The products form from the diastereomeric substrate-catalyst complexes at different rates. Therefore, a chiral catalyst is theoretically able to separate enantiomers by reacting with one enantiomer faster than the other. The catalysts used in kinetic resolutions are often enzymes. Enzymes are constructed from chiral amino acids and often differentiate between enantiomeric substrates. 

The proper stereochemistry was achieved by enzyme catalyzed desymmetrization of the prochiral 1,3-diol 30. Candida antarctica lipase (CAL)-catalyzed transesterification yielded the monoacetate 31, which gave rise to the methyl with the proper stereochemistry 32. The generation of the desired chiral epoxide 35 was achieved by asymmetric dihydroxylation employing AD-mix-a,42 followed by epoxide formation. Base-catalyzed etherification yielded the mixture of the enantiopure (+)-heliannuol A and (-)-heliannuol D. Unfortunately these compounds correspond to the opposite d/l series and correspond to the enantiomers of the natural products (-)-heliannuol A and (+)-heliannuol D (Fig. 5.6.A). 

Differences in enantiomers become apparent in their interactions with other chiral molecules, such as enzymes. Still, we need a simple method to distinguish between enantiomers and measure their purity in the laboratory. Polarimetry is a common method used to distinguish between enantiomers, based on their ability to rotate the plane of polarized light in opposite directions. For example, the two enantiomers of thyroid hormone are shown below. The (5) enantiomer has a powerful effect on the metabolic rate of all the cells in the body. The (R) enantiomer is useless. In the laboratory, we distinguish between the enantiomers by observing that the active one rotates the plane of polarized light to the left. 

Synthetic statins are important lipid regulating drugs for the treatment of atherosclerosis and other diseases related to hyperlipidaemia, especially coronary heart disease. As the pharmacophore, all synthetic statins contain a saturated or partially unsaturated syn-3,5-dihydroxy C7-carboxylate. An important building block for the synthesis of the side chain is represented by 4-chloro-3-hydroxybutanoate esters (CHBE). Both enantiomers can be obtained by enzyme-catalyzed reduction of the (1-keto ester. The group of Kataoka and Shimizu found that an aldehyde reductase of Sporobolomyces salmonicolor [161] and a carbonyl reductase of Candida... 

The resolution of racemic compounds mediated by enzymes has become a valuable tool for the synthesis of chiral intermediates. In most cases, however, only one enantiomer of the intermediate is required for the next step in the synthesis thus, the unwanted isomer must be either discarded or racemized for reuse in the enzymatic resolution process. Dynamic kinetic resolution is one way of avoiding this problem the unwanted enantiomer is racemized during the selective enzymatic process and serves as fresh starting material in the resolution. 

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