Biotransformations enantioselective hydrolysis reaction

Pantenoic acid is used as a vitamine B2 complex, d- and L-pantolactone are used as chiral intermediates in chemical synthesis. The enantioselective hydrolysis is carried out in the aqueous phase with a substrate concentration of 2.69 M = 350 g L 1 (Fig. 19-17). For the synthesis whole cells are immobilized in calcium alginate beads and used in a fixed bed reactor. The immobilized cells retain more than 90 % of their initial activity after 180 days of continuous use. At the end of the reaction l-pantolactone is extracted and reracemized to d,L-pantolactone, which is recycled to the reactor. The D-pantenoic acid is chemically lactonized to D-pantolactone and extracted. By applying cells from Brevibacterium protophormia the L-lactone is available. The biotransformation eliminates several steps that are necessary in the chemical resolution process (Fig. 19-18). 

Organic aqueous two-phase solvent systems are also widely applied for other biotransformations, for example, in hydrolase-catalyzed processes [37,49]. A typical example is the enantioselective hydrolysis of a racemic, water-immisdble ester as a substrate. The reaction yields a water-soluble carboxylic acid (in depro-tonated form as a salt). Ionic liquids have also been used in biphasic systems, a topic that is discussed in detail in Chapter 3 in this book [50]. 

In some cases enzymes can increase the rate of reaction by up to lO times. Carnell and Roberts (1997) have briefly discussed the scope of biotransformations that are used to make pharmaceuticals like penicillins, cephalosporines, erythromycin, lovastatin, cyclosporin, etc., and for food additives like citric acid, L-glutamate, and L-lysine. A very successful transformation by Zeneca has been that of benzene reduction, with Pseudomonase Putida, to dihydrocatechol and catechol the dihydro derivative is used to produce (+/-) pinitol. Fluorobenzene has been converted to fluorodihydrocatechol, an intermediate for pharmaceuticals. The highly stereo selective Bayer-Villeger reaction has been carried out with genetically engineered S-cerevisvae. Hydrolases have allowed enantioselective, and in some cases regioselective, hydrolysis of racemic esters. 

One of the reactions catalyzed by esterases and lipases is the reversible hydrolysis of esters (Figure 19.1, Reaction 2). These enzymes also catalyze transesterilications and the desymmetrization of mew-substrates (vide infra). Many esterases and lipases are commercially available, making them easy to use for screening desired biotransformations without the need for culture collections and/or fermentation capabilities.160 In addition, they have enhanced stability in organic solvents, require no co-factors, and have a broad substrate specificity, which make them some of the most ideal industrial biocatalysts. Alteration of reaction conditions with additives has enabled enhancement and control of enantioselectivity and reactivity with a wide variety of substrate structures.159161164... 

Nifrilases catalyze the conversion of organonitriles directly to the corresponding carboxylic acids. Synthetic hydrolysis of nitriles into the corresponding amides and carboxylic acids requires severe reaction conditions. A typical synthetic approach would require the use of 70% H2SO4 and heat (13). Such a reaction condition is not compatible when selectivity and the conservation of other hydrolysable functional groups in a substrate are desired. Biotransformation of nitrites can be accomplished under mild conditions, in an aqueous environment (13). Additionally, enantioselectivity of the biocatalytic conversion of nitriles to chiral acids has been demonstrated (14-16). Therefore, nifrilases provide an alternative route for synthetic processes that require conversion of nitriles to corresponding acids. 

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