Enantioselective-biocatalysis

Biocatalysis has emerged as an important tool for the enantioselective synthesis of chiral pharmaceutical intermediates and several review articles have been published in recent years [133-137]. For example, quinuclidinol is a common pharmacophore of neuromodulators acting on muscarinic receptors (Figure 6.50). (JJ)-Quinudidin-3-ol was prepared via Aspergillus melleus protease-mediated enantioselective hydrolysis of the racemic butyrate [54,138]. Calcium hydroxide served as a scavenger of butyric acid to prevent enzyme inhibition and the unwanted (R) enantiomer was racemized over Raney Co under hydrogen for recycling. 

Since stereoselectivities of biocatalytic reductions are not always satisfactory, modification of biocatalysis are necessary for practical use. This section explains how to find, prepare, and modify the suitable biocatalysts, how to recycle the coenzyme, and how to improve productivity and enantioselectivity of the reactions. 

However, the reactions were not enantioselective ones, though the most important aspect of the biocatalysis reaction should be in the enantioselective reaction. We and KragF independently reported the first enantioselective lipase-catalyzed reaction in February-March 2001. Since lipase was anchored by the IL solvent and remained in it after the extraction work-up of the product, we succeeded in demonstrating that recyclable use of the lipase in the [bmim][PFg] solvent system was possible (Fig. 2). ... 

Jaeger, K.E. and Eggert, T. (2004) Enantioselective biocatalysis optimized by directed evolution. Current... 

Bergeron S, Chaplin D, Edwards JH, Ellis BS, Hill CL, Holt-Tiffin K, Knight JR, Mahoney T, Osborne AP, Ruecroft G (2006) Nitrilase-catalyzed desym-metrization of 3-hydroxyglutaronitrile preparation of a statin side-chain intermediate. Org Proc Res Dev 10 661-665 Burns M, Weaver J, Wong J (2005) Stereoselective enzymic bioconversion of aliphatic dinitriles into cyano carboxylic acids. WO 2005100580 DeSantis G, Zhu Z, Greenberg W, Wong K, Chaplin J, Hanson SR, Farwell B, Nicholson LW, Rand CL, Weiner DP, Robertson D, Burk MJ (2002) An enzyme library approach to biocatalysis development of nitrilases for enantioselective production of carboxylic acid derivatives. J Am Chem Soc 124 9024-9025... 

There is huge potential in the combination of biocatalysis and electrochemistry through reaction engineering as the linker. An example is a continuous electrochemical enzyme membrane reactor that showed a total turnover number of 260 000 for the enantioselective peroxidase catalyzed oxidation of a thioether into its sulfone by in situ cathodic generated hydrogen peroxide - much higher than achieved by conventional methods [52],... 

One of the most actively investigated aspects of the biohydrolysis of carboxylic acid esters is enantioselectivity (for a definition of the various stereochemical terms used here, see [7], particularly its Sect. 1.5) for two reasons, one practical (preparation of pure enantiomers for various applications) and one fundamental (investigations on the structure and function of hydrolases). The synthetic and preparative aspects of enantioselective biocatalysis by hydrolases have been extensively investigated for biotechnology applications but are of only secondary interest in our context (e.g., [16-18], see Sect. 7.3.5). In contrast, the fundamental aspects of enantioselectivity in particular and of structure-metabolism relationships in general are central to our approach and are illustrated here with a number of selected examples. 

Microbial reduction has been recognized for decades as a laboratory method of preparing alcohols from ketones with exquisite enantioselectivity. The baker s yeast system represents one of the better known examples of biocatalysis, taught on many undergraduate chemistry courses. Numerous other microorganisms also produce the ADH enzymes (KREDs) responsible for asymmetric ketone reduction, and so suitable biocatalysts have traditionally been identified by extensive microbial screening. Homann et have... 

A strain of Pseudomonas aeruginosa has been recently described, which shows the opposite enantioselectivity, converting racemic arylaminonitriles efficientiy into the D-amino acids. Again, whole-cell biocatalysis worked well, the cells being entrapped in alginate beads. It is unclear whether this biotransformation involves an amide intermediate. 

Thus far only three reports regarding the directed evolution of enantioselective EHs have appeared 95,96,143), notwithstanding the fact that these enzymes, even as wild types, constitute important catalysts in synthetic organic chemistry 7-12,144,145). Indeed, since two EHs became commercially available recently, this type of biocatalysis offers exciting prospects for the practicing organic chemist. 

Promising developments of ionic liquids for biocatalysis reflect their enhanced thermal and operational stabilities, sometimes combined with high regio- or enantioselectivities. Ionic liquids are particularly attractive media for certain biotransformations of highly polar substrates, which cannot be performed in water owing to equilibrium limitations 297). 

Rakels, J.L.L., Straathof, A.J.J. and Heijnen,J.J. (1993) A Simple Method to Determine the Enantiomeric Ratio in Enantioselective Biocatalysis. Enzyme Microb. Technol.,... 

In Chapter 2, the two methods of enantioselective biocatalysis have been introduced Asymmetric synthesis and kinetic resolution. 

An early success story, which has been pivotal for the advancement of enantioselective biocatalysis among organic synthetic chemists, touches on the work of... 

It thus appears that the enantioselectivity of CALB for this reaction in the solid/ gas bioreactor is similar to that in an organic liquid medium. Solid/gas biocatalysis therefore offers important potential for production of enantiomerically pure compounds, provided that these transformations involve components having a degree of volatility. Furthermore, as the addition of solvents is avoided in this system, separation and purification during downstream processing are simplified, and side reactions are suppressed. 

Antibody Catalysis. Recent advances in biocatalysis have led to the generation of catalytic antibodies exhibiting aldolase activity by Lemer and Barbas. The antibody-catalyzed aldol addition reactions display remarkable enantioselectivity and substrate scope [18]. The requisite antibodies were produced through the process of reactive immunization wherein antibodies were raised against a [Tdiketone hapten. During the selection process, the presence of a suitably oriented lysine leads to the condensation of the -amine with the hapten. The formation of enaminone at the active site results in a molecular imprint that leads to the production of antibodies that function as aldol catalysts via a lysine-dependent class I aldolase mechanism (Eq. 8B2.12). 

The notion of selectivity needs to be specified further point selectivity is the incremental selectivity, usually towards product, at a specific degree of conversion, whereas integral selectivity is the overall average selectivity at the same specific degree of conversion. Owing to the importance of enantiomeric purity of target product molecules in life science applications and the pre-eminent position biocatalysts enjoy with respect to the achievement of that goal, enantioselectivity is the most important kind of selectivity in the context of biocatalysis. 

C. S. Chen and C. J. Sih, Enantioselective biocatalysis in organic solvents. Lipase catalyzed reactions, Angew. Chem. 1989,... 

Hudlicky T, Reed JW (2009) Celebrating 20 years of SYNLETT - special account on the merits of biocatalysis and the impact of arene cis-dihydrodiols on enantioselective synthesis. Synlett 2009 685-703... 

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