Chemical racemization

The hydrolysis of various para-substituted a-methylstyrene oxides was studied using 10 EHs [184]. The hydrolysis of the isobutyl compound with the enzyme from A. niger WHS the key step in the synthesis of (S)-ibuprofen (Figure 6.65). The (R)-diol was recycled via chemical racemization. 

At present, the purification by chromatographic processes is the most powerful high-resolution bioseparation technique for many different products from the laboratory to the industrial scale. In this context, continuous simulated moving bed (SMB) systems are of increasing interest for the purification of pharmaceuticals or specialty chemicals (racemic mixtures, proteins, organic acids, etc.).This is particularly due to the typical advantages of SMB-systems, such as reduction of solvent consumption, increase in productivity and purity obtained as well as in investment costs in comparison to conventional batch elution chromatography [1]. 

The enzymatic synthesis approaches are discussed in more detail in Chapter 19. A protease can be used to catalyze the synthesis of a peptide bond (Scheme 31.21). When the stoichiometry of the reactions is such that two moles of phenylalanine methyl ester are used with one mole of Z-Asp, the Z-APM PM product precipitates and shifts the equilibrium to >95% conversion.232 This is the basis of the commercial TOSOH process operated by Holland Sweetener that uses thermolysin.233 One significant variation has been the use of racemic PM instead of the L-isomer. Because the enzyme will only recognize the l-PM isomer to form the peptide bond, the unreacted d-PM isomer forms a salt and then, after acidification, the d-PM can be chemically racemized and recycled. 

Chemical racemization of the 5-monosubstituted hydantoins proceeds via keto-enol tautomerism under alkaline conditions, as shown in Figure 12.1 [6]. The racemization velocity is highly dependent on the buUdness and electronic factors of the substituent in 5-position (see Table 12.1) and is usually a very slow process... 

High velocities of chemical racemization have only been observed for d,l-5-phenyl- and D,L-5-p-hydroxy-phenylhydantoin, because of the resonance stabilization by the 5-subshtuent, while all other hydantoins take many hours to racemize... 

Although on the whole the hydantoin racemases have shown high thermal stability, with optimal activity at 55 °C (Table 12.2), the optimal temperatures for the ones from Pseudomonas and Sinorhizobium decrease to 45 and 40 °C, respectively. However, the optimal pH is higher than 8, except for both hydantoin racemases from Agrobacterium. This low alkaline pH avoids chemical racemization. Consequently, the racemization of the d- or L-5-monosubstituted hydantoins in an industrial process will only occur enzymatically. 

Figure 12.9 Enzymatic racemization by hydantoin racemase from S. meiiioti CECT 4114 of the D-isomer (O) and L-isomer ( ) of 5-methylthioethylhydantoin. Chemical racemization of the D-isomer (V) and L-isomer (T) of each substrate was also measured at the same intervals.

The kinetic resolution of racemic secondary alcohols by enzymatic acylation is a well-established method for obtaining optically pure alcohols or their esters in near-50% yield [293]. Coupling the enzymatic method with a catalytic redox ability of a Ru complex makes the process a dynamic kinetic resolution, increasing the theoretical yield from 50 to 100% [294]. Thus, a reaction system consisting of an achiral Ru catalyst for the chemical racemization of an alcoholic substrate, a suitable enzyme,... 

Immobilized enzymes have been used for years in the preparation of fine chemicals. Racemic amines were resolved by enzyme-mediated acylation (Figure 13.13) [24]. Solutions of the racemic amine 13 and the ester 14 were combined just before being pumped onto the top of a column packed with the enzyme immobilized on glass beads. The nonenzymatic acylation was slowed down by decreasing the contact time of 13 and 14, and by conducting the reaction in 3-methyl-3-pentanol. The eluant was drained directly into concentrated HC1 in order to convert the unreacted amine into the hydrochloride salt 15 and prevent any further nonenzymatic acylation. 

During enzymatic hydrolysis of 5-monosubstituted hydantoin derivatives in some cases the remaining, non-hydrolyzed enantiomer is racemizing chemically under alkaline reaction conditions. The velocity of this chemical racemization is strongly dependent on electronic factors of the substituent in the 5-position (see Table 12.4-6). High velocities of racemization are observed particularly for 5-phenyl- and 5-p-OH-phenylhydantoin. 

The (Z)-l-HETCA-CoA isomers would chemically racemize at C-4 and thus reduction of only the erythro isomer would produce HTCA or its CoA derivative with the correct stereochemistry at C-4 and C-5. Retaining the CoA portion of the molecule as indicated in Figure 4 would produce an activated carboxylic acid that could be used for the incorporation of HTCA into the final methanofuran. The biosynthetic intermediates, erythro-HPTCA, threo-WVTCh, erythro-VTCh, and threo-VTCh were each identified in cell extracts of Methanococcus vannidii, M. thermophilia strain TM-1, and M. thermoautotrophicus, indicating the widespread occurrence of this pathway in the methanogens. 

The two main sources of stereoselectivity in drug disposition are the circulatory proteins and enzymes in both the gastrointestinal tract and the liver. Both binding of drugs to proteins and metabolism by various isozymes are, therefore, often stereoselective. Many examples of stereoselective systemic clearance and presystemic metabolism exist (see Chapters 6 and 7). For a few classes of drugs, metabolism may include chiral inversion (see Chapter 8). This, if unidirectional, adds to the overall stereoselectivity in disposition of drugs. Bidirectional bioinversion (see Chapter 8), on the other hand, similar to chemical racemization (thalidomide, see Chapter 5), may diminish stereoselectivity. 

It should also be kept in mind that ketorolac is an example of a chiral NSAID that demonstrates complete spontaneous chemical racemization during chromatographic procedures. The racemization process is dependent on an alkaline pH and ionic strength [121,122]. Further studies are required to clarify the mechanisms of ketorolac bioinversion. 

Time-resolved CPL from racemic mixtures chemical racemization... 

In practice, however, deracemization via repeated resolution is often plagued by low overall yields due to the harsh reaction conditions required for (chemical) racemization [71]. In view of the mild reaction conditions displayed by enzymes, there is a great potential for biocatalytic racemization based on the use of racemases of EC-class 5 [72, 73]. 

J.M., Mingorance-Cazorla, L., and Rodriguez-Vico, F. (2002) Complete conversion of D,L-5-monosubstituted hydantoins with a low velocity of chemical racemization into D-amino acids using whole cells of recombinant Escherichia coli. Biotechnol. Prog., 18... 

Deoxygenation and Desulfurization. Hexachlorodisilane has been shown to effect the stereospecific reduction of phosphine oxides to phosphines with inversion of configuration (eq 1 ), although short reaction times are required to prevent chemical racemization of the phosphine products under the reaction conditions. This protocol complements trichlorosilane which, under appropriate conditions, reduces phosphine oxides with retention of configuration. Reductions of bridged bicyclic phosphine oxides and cyclic halophospholene oxides by hexachlorodisilane have also been reported. 

As an alternative approach to classical OYE-like ene reductases, also cell-free extracts of anaerobic bacteria such as Clostridium sporogenes were tested for the bioreduction of a library of nitroalkenes [106]. The iron-sulfur cluster dependent enoate reductases responsible for the C=C bioreduction in Clostridia showed the same stereopreference of OYEl-3 on ( )-81 and related nitroolefins. As with ene reductases, nonaromatic substrates were converted more slowly and a,P-disubstituted nitroalkenes yielded almost racemic products. The relatively low deuterium exchange rate at the a-position (employing a biphasic system) confirmed also in this case that the low optical purity of the products is mostly due to the bioreduction itself, rather than to chemical racemization. 

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