Biocatalytic method

Biotechnology has attracted enormous interest and high expectations over the past decade. However, the implementation of new technologies into industrial processes has been slower than initially predicted. Although biocatalytic methods hold great industrial potential, there are relatively few commercial applications of biocatalysts in organic chemical synthesis. The main factors that limit the application of biocatalysts are ... 

An efficient biocatalytic method for the production of amides in multigrara scale has been developed for the synthesis of a pyrrole-amide, which is an intermediate for the synthesis of the dipeptidyl peptidase IV that regulates plasma levels of the insulinotropic proglucagon. CALB catalyzes the ammonolysis of the ester with ammonium carbamate as source of ammonia (Scheme 7.8) [22]. The use of ascarite and calcium chloride as adsorbents for carbon dioxide and ethanol by-products. 

Stereoinversion Stereoinversion can be achieved either using a chemoenzymatic approach or a purely biocatalytic method. As an example of the former case, deracemization of secondary alcohols via enzymatic hydrolysis of their acetates may be mentioned. Thus, after the first step, kinetic resolution of a racemate, the enantiomeric alcohol resulting from hydrolysis of the fast reacting enantiomer of the substrate is chemically transformed into an activated ester, for example, by mesylation. The mixture of both esters is then subjected to basic hydrolysis. Each hydrolysis proceeds with different stereochemistry - the acetate is hydrolyzed with retention of configuration due to the attack of the hydroxy anion on the carbonyl carbon, and the mesylate - with inversion as a result of the attack of the hydroxy anion on the stereogenic carbon atom. As a result, a single enantiomer of the secondary alcohol is obtained (Scheme 5.12) [8, 50a]. 

The aim of this short review was to signal the new possibilities offered by the fast-expanding repertoire of biocatalytic methods and approaches. These recent achievements make it possible to optimize enzymatic processes on levels reserved previously only for synthetic catalysts, and also to reach far beyond, employing the special features and origin of proteins. 

Most catalysts originally developed for C=C bonds show a rather poor performance for the hydrogenation of many ketones. However, this situation changed dramatically when it was found that selected Ru-binap and later Ru-binap-dia-mine complexes achieve excellent enantioselectivities, as well as very high TONs and TOFs, for a variety of ketones [92]. Since then, it has been demonstrated that many a- and yS-functionalized, as well as aromatic ketones, are suitable substrates for hydrogenation with industrially viable catalytic results. For the reduction of various ketones biocatalytic methods are an industrially viable alternative to chemocatalysts [15]. 

No attempt has been made to cover all drug classes or enzyme classes instead, a flavour of the potential benefits that can be achieved by the adoption of biocatalytic methods as a compliment to chemical approaches is given. Biocatalytic methods of accessing chiral building blocks will only occasionally be discussed here and the reader is referred to a number of comprehensive reviews that have been published elsewhere. 

Semi-synthetic penicillins are accessed from 6-aminopenicillanic acid, (6-APA), derived from fermented penicillin G. Starting materials for semi-synthetic cephalosporins are either 7-aminodesacetoxycephalosporanic acid (7-ADCA), which is also derived from penicillin G or 7-aminocephalosporanic acid (7-ACA), derived from fermented cephalosporin C (Scheme 1.10). These three key building blocks are produced in thousands of tonnes annually worldwide. The relatively labile nature of these molecules has encouraged the development of mild biocatalytic methods for selective hydrolysis and attachment of side chains. 

This novel biocatalytic method for the production of (5)-bis(trifluoromethyl)phenyletha-nol was easily and reproducibly demonstrated up to pilot plant scale in reactions generating 25 kg of >99% ee material. Substrate concentrations were increased up to 580 mM,... 

With the aim of developing a biocatalytic method which might reduce the tendency for non-enzymic addition of HCN and be broadly applicable, the group of Kyler et al. first introduced transhydrocyanation [85] using the PaHnl. 

This biocatalytic method is therefore quite promising for the synthesis of complex molecules. Very recently, it was reported 1121] that HRP catalyzes the oxidation of 2-naphthols to l,l -binaphthyl-2,2 -diols with moderate enantiomeric excess (ee 38-64%) (Eq. 5). However, in view of the analytical techniques used, these data have to be questioned [167]. As shown recently, atrop-selective biaryl coupling can only be achieved by means of dirigent protein as chiral auxiliary [122]. 

On the other hand, a number of biocatalytic methods provide a useful arsenal of methods as valuable alternatives to the above-mentioned techniques [9-14]. One is where prochiral or racemic synthetic precursors of epoxides, such as halohydrins, can be asymmetrized or resolved using hydrolytic enzym-... 

Often, it appears that the possible role of heterogeneous catalysis in this scenario is not receiving sufficient attention in comparison with that of the biocatalytic methods. Therefore, in the present chapter we will highlight some of the existing possibilities for converting bio-resources, primary... 

Enzyme catalysis of reactions (biocatalysis) is a branch of biotechnology (Hauer 1999 Crameri 1999). The superiority of biocatalytic methods of synthesis, particularly if carried out in a continuum (Orsat 1999), is often manifestly clear, only limited by the cost of replacing the old chemical plants (Pachlatko 1999 Schmid 2001). Illustrative examples of biocatalytic plants are illustrated in Chart 14.2. 

Formation of an amide bond (peptide bond) will take place if an amine and not an alcohol attacks the acyl enzyme. If an amino acid (acid protected) is used, reactions can be continued to form oligo peptides. If an ester is used the process will be a kinetically controlled aminolysis. If an amino acid (amino protected) is used it will be reversed hydrolysis and if it is a protected amide or peptide it will be transpeptidation. Both of the latter methods are thermodynamically controlled. However, synthesis of peptides using biocatalytic methods (esterase, lipase or protease) is only of limited importance for two reasons. Synthesis by either of the above mentioned biocatalytic methods will take place in low water media and low solubility of peptides with more than 2-3 amino acids limits their value. Secondly, there are well developed non-biocatalytic methods for peptide synthesis. For small quantities the automated Merrifield method works well. 

Aspartame is a high intensity dipeptide sweetener, ca. 200 times as sweet as sncrose. It was originally developed by G.D.Searle Co. prior to their acqnisition by Monsanto. Chemically synthesised aspartame has rapidly acqnired a major share of the world high intensity sweetener market, particnlarly in soft drinks. Until recently it has all been snpplied by a monopoly snpplier, the Nntrasweet Corp (a Monsanto-AJinomoto joint ventnre) protected by prodnct patents. Recently biocatalytic methods... 

Other biocatalytic methods of producing D-p-hydroxyphenylglycine have not proved competitive, for instance transaminase based processes require glutamate to be supplied. Others include the hydrolysis of N-acyl derivatives by acylase and amides by aminopeptidase (DSM), the use of L-specrfic hydantoinases and immobilised subtilisin for the resolution of D,L-2-acetamido-/>-hydroxyphenylacetic acid methyl ester (Bayer). 

C-3 synthons are important in the manufacture of a number of products. These intermediates include (R) and f -glycidyl derivatives. Small scale chemical synthesis from D or L-serines and other raw materials has been carried out, but all these routes have problems, such as the use of expensive substrates or production of polluting wastes. Therefore various biocatalytic methods for producing enantiopure glycidyl butanoate have been attempted (EUerink et al, 1991). 

CJ Sih, Q-M Gu, X Holdgrun, K Harris. Optically active compounds via biocatalytic methods. Chirality 4 91-97, 1992. 

Here, we describe several successful examples of the development of chiral technologies using biocatalytic methods. These developments were based on extensive screening of microorganisms, and the consequent discovery of novel enzymes and their functions. 

A variety of monooxygenases (see above) can perform epoxidations. Some biocatalytic methods come into sight, which will become attractive for industrial use. In these cases chiral epoxides are the targeted products, and therefore this subject will be dealt with in Section 4.6. 

When comparing chemical and biocatalytic methods, one could say that, especially for asymmetric oxidations, enzymatic methods enter the scene. This is most evident in the area of asymmetric Baeyer-Villiger oxidation, where biocatalysts take the lead and homogeneous chiral catalysts lag far behind in terms of ee values. Significant progress can be expected in the area of biocatalysis due to the advancement in enzyme production technologies and the possibility of tailor-made enzymes. 

This chapter focuses on biocatalytic methods that have been demonstrated in the aldol and related reactions. Additionally strategies for enzyme engineering and future challenges faced in this field are addressed. 

Even for simple, nonstereoselective conversions of functional groups, biocatalytic methods can be highly competitive (and superior) compared with chemical standard processes. 

There is a strong need for new biocatalytic methods producing enantiomerically pure compounds which are the basis for synthesis and production of biologically active compounds for agrochemicals and pharmaceuticals. 

Several other studies dedicated to the comparison of biocatalytic and nonenzymatic transformations have been published [56-58]. The major aspect can be summarized as follows if high substrate and product concentration can be reached using low-priced substrates, this will result in an advantage for biocatalytic methods relative to nonenzymatic processes, at least if highly enantioenriched products are needed. 

On the other hand, virtually all target molecules can be synthesized by chemists, using nonenzymatic and/or biocatalytic methods. However, the scale up is still not an easy task, especially in the case of nonenzymatic methods. Discodermolide, for instance, a potent inhibitor of tumor cell growth, has been synthesized on a 60 g scale. However, the 39-step synthesis is the result of the work of 43 industrial chemists and was build on the preparatory work of two leading academic groups (Scheme 4.21) [83]. 

As a conclusion, examination of the present literature clearly indicates that, depending on the circumstances, any of the methods described in this review may be the best for the preparation of a given enantiopure epoxide. In particular, the recent progress achieved by using metal-catalyzed chemical processes obviously has to be taken into account. As far as biocatalytic methods are concerned, one can anticipate that, in the near future, lipases or, better, epoxide hydrolases, will prove to be the best choice, particularly as far as industrial applications are concerned. Research is ongoing in diverse laboratories to explore the scope and limitations of these very promising enzymes. 

Give an overview of the different biocatalytic methods that can be used to prepare enantiomerically pure chiral amines. 

Fig. 3 Overview of asymmetric biocatalytic methods for the production of L-amino acids.

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