Biocatalytic transformation

Another example of a biocatalytic transformation ousting a chemical one, in a rather simple reaction, is provided by the Lonza nitotinamide process (Fig. 2.34) (Heveling, 1996). In the final step a nitrile hydratase, produced by whole cells of Rh. rhodoccrous, catalyses the hydrolysis of 3-cyano-pyridine to give nitotinamide in very high purity. In contrast, the conventional chemical hydrolysis afforded a product contaminated with nicotinic acid. 

Some companies are successfully integrating chemo- and biocatalytic transformations in multi-step syntheses. An elegant example is the Lonza nicotinamide process mentioned earlier (.see Fig. 2.34). The raw material, 2-methylpentane-1,5-diamine, is produced by hydrogenation of 2-methylglutaronitrile, a byproduct of the manufacture of nylon-6,6 intermediates by hydrocyanation of butadiene. The process involves a zeolite-catalysed cyciization in the vapour phase, followed by palladium-catalysed dehydrogenation, vapour-pha.se ammoxidation with NH3/O2 over an oxide catalyst, and, finally, enzymatic hydrolysis of a nitrile to an amide. 

Recently, ///)H.YL has been found to catalyze the stereoselective addition of nitroalkanes to aldehydes in an. S -selective fashion, which is in agreement with the known stereopreference of this enzyme. This is the first example for a substitution of HCN by another carbon nucleophile, expanding the synthetic scope of this biocatalytic transformation. The addition of nitromethane to different aldehydes with moderate to good yields and enantioselectivity has been demonstrated (Figure 5.9) [58]. However, large amounts of enzyme are required to... 

Enantiometrically pure alcohols are important and valuable intermediates in the synthesis of pharmaceuticals and other fine chemicals. A variety of synthetic methods have been developed to obtain optically pure alcohols. Among these methods, a straightforward approach is the reduction of prochiral ketones to chiral alcohols. In this context, varieties of chiral metal complexes have been developed as catalysts in asymmetric ketone reductions [ 1-3]. However, in many cases, difficulties remain in the process operation, and in obtaining sufficient enantiomeric purity and productivity [2,3]. In addition, residual metal in the products originating from the metal catalyst presents another challenge because of the ever more stringent regulatory restrictions on the level of metals allowed in pharmaceutical products [4]. An alternative to the chemical asymmetric reduction processes is biocatalytic transformation, which offers... 

Garcia-Arellano, H. Buenrostro-Gonzalez, E. and Vazquez-Duhalt, R., Biocatalytic transformation of petroporphyrins by chemical modified cytochrome, C. Biotechnology and Bioengineering, 2004. 85(7) pp. 790-798. 

Scheme 26.5 Biocatalytic transformation of glucose to vanillin (adapted from [35])...

The viscosity of ionic liquids is high compared with molecular solvents and increases with the chain length. Consequently, diffusion is bound to be slow in ionic liquids. The effects on biocatalytic transformations seem to be insignificant, however, except in extreme cases, presumably because the reaction times are measured in hours rather than minutes. 

Besides the broad applications of electrically contacted enzyme electrodes as amperometric biosensors, substantial recent research efforts are directed to the integration of these functional electrodes as biofuel cell devices. The biofuel cell consists of an electrically contacted enzyme electrode acting as anode, where the oxidation of the fuel occurs, and an electrically wired cathode, where the biocatalyzed reduction of the oxidizer proceeds (Fig. 12.4a). The biocatalytic transformations occurring at the anode and the cathode lead to the oxidation of the fuel substrate and the reduction of the oxidizer, with the concomitant generation of a current through the external circuit. Such biofuel cells can, in principle, transform chemical energy stored in biomass into electrical energy. Also, the use... 

The supramolecular bis-aniline cross-linked metallic NPs/enzyme composite does not only act as a conducting matrix that electrically contacts the redox center with the electrode, but the NPs may also provide catalytic sites that enhance the biocatalytic transformations at the enzyme active site. This has been demonstrated by the effective... 

Bacterial De-Novo Synthesis. The basic idea behind this variant is to use the synthetic potential of bacteria to produce the indole precursor (Scheme 3). Although indole (23) does not occur as an intermediate in bacterial metabolism, it appears as an enzyme-linked intermediate in the biocatalytic transformation ofD-glucose (17) to L-tryptophan (22). The crucial biosynthetic step is the conversion of indole-3-glycerine phosphate (21) to L-tryptophan (22) by the enzyme tryptophan synthase. 

The biocatalytic transformation generates a product, and hence the light-triggered formation or inhibition of the respective product may be tailored. 

The biocatalytic transformation and the enzyme turnover provides a means to amplify the light-triggered process. 

In order to increase the efficiency of biocatalytic transformations conducted under continuous flow conditions, Honda et al. (2006, 2007) reported an integrated microfluidic system, consisting of an immobilized enzymatic microreactor and an in-line liquid-liquid extraction device, capable of achieving the optical resolution of racemic amino acids under continuous flow whilst enabling efficient recycle of the enzyme. As Scheme 42 illustrates, the first step of the optical resolution was an enzyme-catalyzed enantioselective hydrolysis of a racemic mixture of acetyl-D,L-phenylalanine to afford L-phenylalanine 157 (99.2-99.9% ee) and unreacted acetyl-D-phenylalanine 158. Acidification of the reaction products, prior to the addition of EtOAc, enabled efficient continuous extraction of L-phenylalanine 157 into the aqueous stream, whilst acetyl-D-phenylalanine 158 remained in the organic fraction (84—92% efficiency). Employing the optimal reaction conditions of 0.5 gl min 1 for the enzymatic reaction and 2.0 gl min-1 for the liquid-liquid extraction, the authors were able to resolve 240 nmol h-1 of the racemate. 

Another route to 2 uses both chemical and biocatalytic transformations and starts from glucose (3) (Scheme 4.2).28 29... 

Figure 3.3 Biocatalytic transformation of the natural terpenol, Lavandulol, in supercritical carbon...

The chloroperoxidase of C. fumago was able to transform 17 of 20 PAHs assayed [100]. In this case, only halogenated products were observed, and no oxygenated products could be detected. This biocatalytic transformation should be carefully considered because the toxicity and environmental impact of aromatic compounds may be increased. 

Supercritical fluids (SCFs) and in particular supercritical carbon dioxide (scC02) have also been shown to provide a unique medium in which to perform biocatalytic transformations. The ability to perform biocatalysis in a supercritical fluid was first demonstrated in 1985 by several groups. There are several advantages to the use of SCFs, as well as some drawbacks that need to be addressed if large-scale processes are to be developed. 

Kragl U. et al. Biocatalytic transformations in ionic liquids, Ionic Liquids in Synthesis, Wiley-VCH, Verlag GmbH Co. KGaA, Weinheim, 2003. 

Another example in which a biocatalytic transformation has replaced a chemo-catalytic one, in a very simple reaction, is the Mitsubishi Rayon process for the production of acrylamide by hydration of acrylonitrile (Fig. 1.42). Whole cells of Rhodococcus rhodocrous, containing a nitrile hydratase, produced acrylamide in >99.9% purity at >99.9% conversion, and in high volumetric and space time yields [121]. The process (Fig. 1.42) currently accounts for more than 100000 tons annual production of acrylamide and replaced an existing process which employed a copper catalyst. A major advantage of the biocatalytic process is the high product purity, which is important for the main application of acrylamide as a specialty monomer. 

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