Asymmetric synthesis using biocatalysts

Chiral epoxides and their corresponding vicinal diols are very important intermediates in asymmetric synthesis [163]. Chiral nonracemic epoxides can be obtained through asymmetric epoxidation using either chemical catalysts [164] or enzymes [165-167]. Biocatalytic epoxidations require sophisticated techniques and have thus far found limited application. An alternative approach is the asymmetric hydrolysis of racemic or meso-epoxides using transition-metal catalysts [168] or biocatalysts [169-174]. Epoxide hydrolases (EHs) (EC 3.3.2.3) catalyze the conversion of epoxides to their corresponding vicinal diols. EHs are cofactor-independent enzymes that are almost ubiquitous in nature. They are usually employed as whole cells or crude... 

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. 

Although the use of an epoxide hydrolase was already claimed for the industrial synthesis of L- and meso-tartaric acid in 1969 [51], it was only recently that applications to asymmetric synthesis appeared in the hterature. This fact can be attributed to the limited availabihty of these biocatalysts from sources such as mammals or plants. Since the production of large amounts of crude enzyme is now feasible, preparative-scale applications are within reach of the synthetic chemist. For instance, fermentation of Nocardia EHl on a 701-scale afforded > 700 g of lyophilized cells [100]). 

Oxidoreductases are, after lipases, the second most-used kinds of biocatalysts in organic synthesis. Two main processes have been reported using this type of enzymes-bioreduction of carbonyl groups [39] and biohydroxylation of non-activated substrates [40]. However, in recent few years other processes such as deracemization of amines or alcohols [41] and enzymatic Baeyer-Villiger reactions of ketones and aldehydes [42] are being used with great utility in asymmetric synthesis. 

Kinetic resolutions in general are regularly applied in organic synthesis. Since enzymes are highly attractive for asymmetric synthesis, various types of biocatalysts have been used in enzymatic (dynamic) kinetic resolutions, but the focus will remain on lipase- and esterase-mediated resolutions as the most common tools in early steps of natural product syntheses. 

Naturally occurring redox enzymes have been successfully exploited for asymmetric synthesis for some years.1 Although impressive chemo-, regio-, and enantioselectivities have been achieved in some cases, these biocatalysts have prescribed selectivity and often require expensive cofactors that must be recycled for preparative work. Catalytic antibodies offer an attractive alternative, since they are not limited a priori by Nature s choices. Thus the need for cofactor recycling can be circumvented through the use of inexpensive oxidants and reductants, and, as we have seen above, selectivity can be tailored through appropriate hapten design. 

An alternative approach may also consist of the de novo synthesis of the two carbohydrate substructures that would have to be grown from the termini of much simpler precursors. For classical chemical syntheses, tremendous difficulties are immediately obvious by the simultaneous needs of high relative stereocontrol and terminus differentiation for each individual step. A somewhat better perspective may be seen for the use of stereoselective biocatalysts for asymmetric C-C bond coupling reactions. Indeed, more than 30 aldolases are known from which recently a larger number has been studied extensively for their utility in asymmetric synthesis [30,37,38,41,42]. 

Hydrolases catalyze the addition of water to a substrate by means of a nucleophilic substitution reaction. Hydrolases (hydrolytic enzymes) are the biocatalysts most commonly used in organic synthesis. They have been used to produce intermediates for pharmaceuticals and pesticides, and chiral synthons for asymmetric synthesis. Of particular interest among hydrolases are amidases, proteases, esterases, and lipases. These enzymes catalyze the hydrolysis and formation of ester and amide bonds. 

Biotransformations for the synthesis of asymmetric compounds can be divided into two types of reactions those where an achiral precursor is converted into a chiral product (true asymmetric synthesis) and those involving the resolution of a racemic mixture. Both types of reaction are used at Lonza, which is a leading producer of intermediates for the life science industry. Lonza also uses biocatalysis for the synthesis of achiral molecules, for example, an immobilized whole-cell biocatalyst is used for the nitrile hydratase-catalyzed synthesis of thousands of tons per year of nicotinamide from 3-cyanopyridine. 

Over the past few years, an impressive array of epoxide hydrolases has been identified from microbial sources. Due to the fact that they can be easily employed as whole-cell preparations or crude cell-free extracts in sufficient amounts by fermentation, they are just being recognized as highly versatile biocatalysts for the preparation of enantiopure epoxides and vicinal diols. The future will certainly bring an increasing number of useful applications of these systems to the asymmetric synthesis of chiral bioactive compounds. As for all enzymes, the enantioselectivity of... 

One of the rare examples for the use of immobilized oxynitrilases has been published by Degussa [146]. The company investigated the asymmetric synthesis of (i )-cyanohydrins and used (i )-oxynitrilase, which had been cross-linked and subsequently polyvinyl alcohol-entrapped. The obtained immobilized lens-shaped biocatalysts were much more satisfying in terms of long-term stability and activity compared to the free enzyme and also showed less catalyst leaching than other enzyme supports. Moreover, the immobilization method is cheap, efficient, feasible on an industrial scale, and gives particles of defined size. The utility of these entrapped enzymes could be shown, as indicated in Scheme 57, in the synthesis of (i )-mandelonitrile (R)-175) from aldehyde 174. No catalyst deactivation was observed even after 20 cycles of reuse and yields as well as optical purities of (R)-175 remained constant within normal limits. 

It involves the attack by the deprotonated a-carbon atom of an aldehyde or ketone on the carbonyl atom of another aldehyde or ketone, resulting in a (3-hydroxyaldehyde or a (3-hydroxyketone. While the donor compound for aldolases is usually invariable, the acceptor may vary, allowing the use of these biocatalysts in asymmetric synthesis (Takayama et al. 1997 Samland and Sprenger 2006). 

Examples involving four common synthetic steps, oxidation, reduction, addition, and epoxidation, are given in Figure 20.8 to illustrate the more general role of biocatalysts (microbial and enzymatic) in asymmetric synthesis. Table 20.11 contains several other examples of bioasymmetric synthesis. The classical application of this route is the well-known total synthesis of penicillins starting from various building blocks using a variety of Penicillium strains. 

Biocatalysts. In biocatalysis, enzymes and antibodies are used in homogeneous or heterogeneous systems. Reactions that use biocatalysts often proceed with exceptionally high selectivity and as a result, these catalysts are becoming increasingly more important in the asymmetric synthesis of organic compounds. 

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