Enantioselectivity, enzyme-catalysed reaction

Enzymes provide catalytic pathways which are often superior to non-enzymic routes. The advantage of enzymes can overcome their disadvantages as relatively unstable species which workbest in aqueous solution. In synthetic processes, the selectivity of the enzyme-catalysed process is not approached by chemical catalysis particularly regarding the enantioselective synthesis of chiral molecules. Furthermore, protective group chemistry is often not required in enzyme-catalysed reactions. 

Enzyme Catalysed Reactions, Enantioselectivity And Stability Under High Hydrostatic Pressure... 

Considering that the enantioselectivity (E) is dependent on high hydrostatic pressure the KM and v value should also be dependent on pressure. In view of the enzyme catalysed reactions volume changes due to conformational changes of the enzymes should expect larger sensitivity of the reaction rates compared to the uncatalysed ones. 

Keywords Cyanogenesis, Enantioselective syntheses. Cyanohydrins, Enzyme catalysed reactions. 

In contrast to Mori s synthesis, Pawar and Chattapadhyay used enzymatically controlled enantiomeric separation as the final step [300]. Butanone H was converted into 3-methylpent-l-en-3-ol I. Reaction with trimethyl orthoacetate and subsequent Claisen-orthoester rearrangement yielded ethyl (E)-5-methyl-hept-4-enoate K. Transformation of K into the aldehyde L, followed by reaction with ethylmagnesium bromide furnished racemic ( )-7-methylnon-6-ene-3-ol M. Its enzyme-catalysed enantioselective transesterification using vinylacetate and lipase from Penicillium or Pseudomonas directly afforded 157, while its enantiomer was obtained from the separated alcohol by standard acetylation. 

Starting from achiral materials, two stereoisomeric phosphonylated dihydroxy pyrrolidines (275) and (276), containing four stereogenic centers, have been synthesized enantioselectively, employing a combination of enzymatic and transition-metal-mediated methods. Both compounds contain features of the transition state of the enzyme-catalysed fucosyl transfer reaction and represent building blocks of potential inhibitors against this class of enzymes. The synthesis of new sugar-derived phosphonic acids e.g. (277) from protected... 

Exploiting the ability of an enzyme to catalyse a reaction, usually either alkaline hydrolysis of an ester, or an esterification, of one enantiomer exclusively. If the enantiomers of the racemic starting material can be made to equilibrate while the enzyme-catalysed enantioselective reaction is taking place, yields of one enantiomer in excess of 50% can be obtained. 

From a synthetic point of view, aldolases offer a number of advantages as catalysts for C—C bond formation. For example, they operate best on unprotected substrates, thus avoiding the problem of complex protection/ deprotection schemes for polyfunctional molecules (e.g. carbohydrates). They also catalyse C—C bond synthesis with high diastereoselectivity and enantioselectivity. Such simultaneous control is often difficult to achieve using non-enzymic aldol reactions. 

By studying the ring opening of (rac)-2-phenyl-4-benzyl-5(4H)-oxazolone with butanol catalysed by CALB in organic media, it has been possible to correlate the protonation state of the enzyme with the enantioselectivity of the reaction [36]. The protonation state was controlled by the use of either organo-soluble bases or solid-state buffers of known pfC. Both triethylamine and the buffer pair CAPSO/CAPSO.Na [CAPSO = 3-(cyclohexylamino)-2-hydroxy-l-propanesulfonic acid] were found to increase the enantioselectivity of reactions catalysed by CALB and also the lipase from Mucor miehei. The effect of solvent, water activity and temperature on the enantioselectivity of reactions catalysed by lipases and hydroxynitrile lyases (enzymes that catalyse the addition of cyanide to aldehydes) has been reported [37]. 

In 2010, Janssen and co-workers reported that the kinetic resolution of p-phenylalanine catalysed by a tandem biocatalytic system composed of phenylalanine aminomutase (PAM) and phenylalanine ammonia lyase (PAL) yielded the corresponding enantiopure (5)-p-phenylalanine in good yield (48%) and excellent enantiomeric excess of >99% ee (Scheme 4.13). The process was based upon the PAM-catalysed, reversible, enantioselective transformation of (I )-p-phenylalanine to (S)-a-phenylalanine. The latter one was transformed in a PAL-catalysed regioselective process into ( )-cinnamic acid, with liberation of ammonia. This constituted an example of a tandem biocatalytic, kinetic resolution in which one enzyme catalysed the equilibration between the substrate and reaction intermediate, while the other shifted this equilibrium between the substrate towards the final product... 

The solvent present in biphasic reactions can still have an effect on the enzyme even though the enzyme functions primarily in an aqueous microenvironment. A particularly dramatic example is the lipase AH (lipase from Burkholderia cepac/fl)-catalysed desym-metrization of prochiral 1,4-dihydropyridine dicarboxylic esters, where either enantiomer can be accessed in high enantioselectivity by using either water-saturated cyclohexane or diisopropyl ether (DIPE) respectively (Scheme 1.60). The acyl group used in acylation and deacylation can also have a dramatic effect on enantioselectivity. " ... 

The results actually showed a deracemization of the racemic hydroxyester 10 as opposed to enantioselective hydrolysis with formation of optically pure (R)-hydroxyester 10 and only 20 % loss in mass balance. Small quantities of ethyl 3-oxobutanoate 9 (<5%) were also detected throughout the reaction, leading the authors to suggest a multiple oxidation-reduction system with one dehydrogenase enzyme (DH-2) catalysing the irreversible reduction to the (R)-hydroxy-ester (Scheme 5). 

The haem peroxidases are a superfamily of enzymes which oxidise a broad range of structurally diverse substrates by using hydroperoxides as oxidants. For example, chloroperoxidase catalyses the regioselective and stereoselective haloge-nation of glycals, the enantioselective epoxidation of distributed alkenes and the stereoselective sulfoxidation of prochiral thioethers by racemic arylethyl hydroperoxides [62]. The latter reaction ends in (i )-sulfoxides, (S)-hydroperoxides and the corresponding (R)-alcohol, all In optically active forms. 

Recently, the first asymmetric cell-free application of styrene monooxygenase (StyAB) from Pseudomonas sp. VLB 120 was reported [294]. StyAB catalyses the enantiospecific epoxidation of styrene-type substrates and requires the presence of flavin and NADH as cofactor. This two-component system enzyme consists of the actual oxygenase subunit (StyA) and a reductase (StyB). In this case, the reaction could be made catalytic with respect to NADH when formate together with oxygen were used as the actual oxidant and sacrificial reductant respectively. The whole sequence is shown in Fig. 4.106. The total turnover number on StyA enzyme was around 2000, whereas the turnover number relative to NADH ranged from 66 to 87. Results for individual substrates are also given in Fig. 4.106. Excellent enantioselectivities are obtained for a- and -styrene derivatives. 

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