Process, continuous resolution, enzymic

DL-Amino acid resolution processes for various amino acids had already been successfully pioneered by the Tanabe Seiyabu Co. using immobilised, oryzae aminoacylase acting on DL-N-acetyl acids. This step was first carried out as a continuous immobilized enzyme process in 1969. 

Several L-amino acids are produced on a large scale by enzymatic resolution of N-acetyl-D,L-amino adds (Figure A8.4). Acylase immobilised on DEAE-Sephadex is for example employed in a continuous process while Degussa uses the free acylase retained in a membrane reactor. In the latter process the advantage of reuse of the enzyme and homogeneous catalysis are combined. 

A novel continuous-flow SCCO2 process for the kinetic resolution of 1-phenyethanol enantiomers (Figure 30) using Novozym 435 immobilized enzyme from Candida antarctica was described by Matsuda et al. [51], The lipase enzyme, selectively acetylated the R)-alcohol component. A mixture of starting material and vinyl acetate was passed through the enzyme with supercritical carbon-dioxide (Figure 31). The reaction zone was pressurized and heated, so the reaction could be performed imder supercritical conditions, synthesizing the desired (i )-acetate with 99.7% ee. and 47% yield. 

It is worth noting here that with two enzymes displaying opposite enantioselec-tivity it is possible to produce both enantiomers of the ester products. If the remaining alcohols can be continuously and rapidly racemized during the much slower acylation reaction, either the R- or S-esters can be obtained in high yields (>>5096) from reactions catalyzed by two hydrolases that display opposite enantio-preference. The combined process of racemization and simultaneous resolution, dynamic kinetic resolution (DKR), is described in Chapter 6. 

Researchers at Degussa AG focused on an alternative means towards commercial application of the Julia-Colonna epoxidation [41]. Successful development was based on design of a continuous process in a chemzyme membrane reactor (CMR reactor). In this the epoxide and unconverted chalcone and oxidation reagent pass through the membrane whereas the polymer-enlarged organocatalyst is retained in the reactor by means of a nanofiltration membrane. The equipment used for this type of continuous epoxidation reaction is shown in Scheme 14.5 [41]. The chemzyme membrane reactor is based on the same continuous process concept as the efficient enzyme membrane reactor, which is already used for enzymatic a-amino acid resolution on an industrial scale at a production level of hundreds of tons per year [42]. 

The only difference is that in conventional kinetic resolution the enantiomer (5)-substrate is left behind as unreacted starting material while in case of dynamic kinetic resolution the substrate is continuously isomerised during the resolution process, thus (R) and ( )-substrates are in equilibrium, which allows for the possibility of converting all starting materials of (A)-substrate into (A)-product. Several conditions should be applied and are reviewed in literature.21 For instance, Backvall et al20 used a combination of enzyme and transition metal complex (Ru-catalyst) to perform the DKR of a set of secondary alcohols. Depending on the substrate, the chemical yield was ranging from 60 to 88 % with more... 

The chemzyme membrane reactor is based on the same continuous process concept as the efficient enzyme membrane reactor, which has already been applied for enzymatic a-amino acid resolution on industrial scale at a production level of hundreds of tons per year (Drauz and Waldmann 2002 Wandrey and I iaschcl 1979 Wandrey et al. 1981 Groger and Drauz 2004). 

Deracemization by DKR is in principle a kinetic resolution process in which the non-transformed enantiomer is racemized in situ. The conditions are that a chiral catalyst promotes the transformation of one enantiomer (Rj) into the product (Rp) while the other enantiomer is racemized at a comparable rate and the racemic mixture (R -i- S ) is restored. The product (Rp) is not racemized under the same conditions. While a simple kinetic resolution yields a maximum of 50% of the product, with this technique a 100% conversion can be reached. Although the majority of chiral molecules of industrial interest are stiU prepared by kinetic resolution, the continuous development of industrial enzymes and racemizing processes fosters new chemo- or biocatalytic systems for DKR to appear. A great impulse for deracemization methods based on the one-pot/two-steps resolution racemization process was brought about by BackvaU et al. over a 10-year period... 

In addition to the above described procedures implying either direct oxidation of an olefinic double bond or stereoselective reduction of a ketone precursor, which, as discussed above, do not really provide very efficient ways for the large scale synthesis of enantiopure epoxides, some indirect strategies have also been explored. These are essentially based on the resolution of epoxide-ring bearing substrates as exemplified below. As will be seen, these approaches imply the use of cofactor-independent enzymes, which are in practice much easier to work with, and lead to very interesting results. As a matter fact, some of these processes are already used on an industrial scale, and it can be predicted that future industrial applications will continue to be essentially based on the use of these very promising easy-to-use biocatalysts. 

So far SECM applications have been considered where enzymes immobilized at a surface catalyze redox reactions of low molecular weight compounds. The reaction products are detected at the ultramicroelectrode tip under diffusion-controlled conditions. This approach requires that the biochemically active layer continuously generate or consume redox active (for amperometric detection) or charged (for potentiometric detection) species. Since the tip signal depends on the diffusion coefficients and/or convective effects as well as the local concentration, it is possible to image localized mass transport phenomena instead of localized chemical fluxes (Chapter 9). In a general sense it is a process that is recorded with lateral resolution. 

The success of an enzyme-catalyzed kinetic resolution is limited by the maximum chemical yield of 50% for each enantiomer. However, this drawback can be overcome by a process called dynamic kinetic resolution. The key idea of this principle is to racemize the slow reacting enantiomer continuously reproducing the faster one. In an ideal case at the end of the conversion one enantiomer is formed in 100% yield with 100% of enantiomeric excess[13S 1371. The kinetic requirements for a dynamic kinetic resolution are shown in Scheme 11.1-16[8bl. 

The separation of the products from the IL catalytic mixture can be performed in various cases by simple decanting and phase separation or by product distillation. In this respect, a continuous-flow process using toluene as extractant has been appHed for the selective Pd-catalyzed dimerization of methyl acrylate in ILs [136]. However, in cases where the products are retained in the IL phase, extraction with supercritical carbon dioxide can be used instead of classical liquid-liquid extractions that necessitate the use of organic solvents, which may result in cross-contamination of products. This process was successfully used in catalyst recycling and product separation for the hydroformylation of olefins employing a continuous-flow process in supercritical carbon dioxide-IL mixtures [137]. Similarly, free and immobilized Candida antarctica lipase B dispersed in ILs were used as catalyst for the continuous kinetic resolution of rac-l-phenylethanol in supercritical carbon dioxide at 120°C and 150°C and 10 Mpa with excellent catalytic activity, enzyme stability and enantioselectivity levels (Fig. 3.5-11). 

The kinetic resolution of stereoisomer mixtures is a process based on the stereoselectivity of the enzyme, which speeds up the reaction selectivity that produces just one enantiomer the reaction equilibrium always prescribes the racemic solution formation, but continuously removing the isomers (distributed with a large excess of the enantiomer of interest) pushes back the reaction towards the formation of products of the speeded reaction. Thus, the removed solution is enriched by the enantiomer of interest if the reaction rate differences are great, the purity of the permeate solution is high with respect to the enantiomeric form of interest. 

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