Microbial reduction catalyzed

Patel and coworkers reported that microbial reduction of ethyl l-benzyl-3-oxopiperidine-4-carboxylate by Candida parapsilosis SC16 347 gave ethyl cis-(3R,4R)-1 -benzyl-3AMiydro-xypiperidine-4/ -carboxylate as the major product in 97.4% diastereomeric excess (de) and 99.8% ee (Figure 7.2), while 99.5% de and 98.2% ee were achieved in the reduction catalyzed by Pichia methanolica SC16 415 [14]. 

Several catalysts are used in the field of microbial reductions. The common features of these catalysts are the high selectivity and their use only on a laboratorial scale. They are applied, for example, in the stereoselective synthesis of pharmaceutical intermediates. The reductions are exclusively selective either in the hydrogenation of the C=C double bond or in that of other reducible groups. One of the most widely used catalysts is baker s yeast. In the following hydrogenations, which are catalyzed by Saccharomyces cerevisiae, high enantioselectivities were achieved (equations 35-38)105-108. 

Enantioselective enzymatic transesterifications have been used as a complementary method to enantioselective enzymatic ester hydrolyses. The first example of this particular type of biotransformation is the synthesis of the optically active 2-acetoxy-l-silacyclohexane (5 )-78 (Scheme 19). This compound was obtained by an enantioselective transesterification of the racemic l-silacyclohexan-2-ol rac-43 with triacetin (acetate source) in isooctane, catalyzed by a crude lipase preparation from Candida cylindracea (CCL, E.C. 3.1.1.3)62. After terminating the reaction at 52% conversion (relative to total amount of substrate rac-43), the product (S)-78 was separated from the nonreacted substrate by column chromatography on silica gel and isolated in 92% yield (relative to total amount of converted rac-43) with an enantiomeric purity of 95% ee. The remaining l-silacyclohexan-2-ol (/ )-43 was obtained in 76% yield (relative to total amount of nonconverted rac-43) with an enantiomeric purity of 96% ee. Repeated recrystallization of (R)-43 led to an improvement of enantiomeric purity by up to >98% ee. Compound (R)-43 has already earlier been prepared by an enantioselective microbial reduction of the l-silacyclohexan-2-one 42 (see Scheme 8)53. The l-silacyclohexan-2-ol (R)-43 is the antipode of compound (.S j-43 which was obtained by a kinetic enzymatic resolution of the racemic 2-acetoxy-l-silacyclohexane rac-78 (see Scheme 15)62. For further enantioselective enzymatic transesterifications of racemic organosilicon substrates, with a carbon atom as the center of chirality, see References 64 and 70-72. 

The microbial reduction of 4-benzyloxy-3-methanesulfonylamino-2 -bromoace-tophenone (6) to the corresponding (A)-alcohol (7) was demonstrated by S. paucimobilis SC 16113 (Fig. 6). Among cultures evaluated, Hansenula anamola SC 13833, H. anamola SC 16142, Rhodococcus rhodochrous ATCC 14347, and S. paucimobilis SC 16113, gave desired alcohol (7) in >96% e.e. and >15% reaction yield. S. paucimobilis SC 16113, in the initial screening, catalyzed the efficient conversion of ketone (6) to the desired chiral alcohol (7) in 58% reaction yield and >99.5% e.e. 

Several alternative methods with high ee s for various types of ketones are known reductions catalyzed by enzymes or baker s yeast [30] and microbial reagents [31], homogeneous hydrogenation (cf. Chapter 6.1), and stoichiometric reductions with chiral metal hydrides [32]. 

The opposite enantiomer selectivity towards these cage-shaped C2 ketones was demonstrated in oxidation-reduction mediated by horse liver alcohol dehydrogenase (HLADH) (170). Incubation of racemic C2 ketones with HLADH in a phosphate buffer (pH 7.0) containing coenzyme NADH afforded a mixture of the alcohols corresponding to the M C2 ketone and the recovered P C2 ketones, both with much higher optical purities than that found in the microbial reduction. In the oxidative direction (with NAD coenzyme), HLADH was found to preferentially catalyze oxidation of the alcohols corresponding to the M C2 ketone with excellent selectivity. 

In an alternate approach, the enantioselective microbial reduction of methyl-4-(2 -acetyl-5 -fluorophenyl) butanoates 80 (Figure 16.19B) was demonstrated using strains of Candida and Pichia. Reaction yields of 40%-53% and EEs of 90%-99% were obtained for the corresponding (5)-hydroxy esters 77. The reductase that catalyzed the enantioselective reduction of ketoesters was purified to homogeneity from cell extracts of Pichia methanolica SC 13825. It was cloned and expressed in E. coli, and recombinant cultures were used for the enantioselective reduction of the keto-methyl ester 80 to the corresponding (5)-hydroxy methyl ester 77. On a preparative scale, a reaction yield of 98% with an EE of 99% was obtained [99]. 

Based on critical reviews, Lovley (1991, 2004) concluded that there are potential mechanisms for the abiotic reduction of Fe(III) and Mn(IV), but the significance of this process is minimal as compared to biotic reduction catalyzed by microbial activities. Typically, the end products of Fe(II) and Mn(II) are measured as indicators of the biotic and abiotic reduction of Fe(III) and Mn(IV) in anaerobic environments. The reduction of Fe(III) and Mn(IV) as a function of Eh is shown in Figures 10.10 and 10.11. Sodium acetate extractable iron and manganese in anaerobic soils represents Fe(II) and Mn(II), end products of reduction. As expected, extractable Mn(II) and Fe(II) concentrations are low nnder oxidized conditions and increase with a decrease in the Eh of soil. The accumulation of Mn(II) occurs at higher Eh values than the accumulation of Ee(II), suggesting Mn(IV) reduction precedes Fe(III) reduction. Because the reduction of Ee(III) and Mn(IV) occurs... 

Both biological and chemical (abiotic) pathways exist for the reduction of chromate and uranyl however, the reaction kinetics for the two broad classes differ appreciably. Reduction of chromate will probably occur through chemical means, albeit that the chemical reactant may result from microbial processes, in anaerobic environments ferrous Fe will dominate the reduction of chromate at mildly acidic to alkaline pH values while sulfide will dominate at lower pH values given equal availability. The microbial reduction of Fe (hydr)oxides promotes reduction of Cr(Vl) to Cr(Ill)— the result of a coupled, two-step, biotic-abiotic reaction pathway in which Fe(ll) produced during Fe respiration catalyzes the reduction of Cr(Vl). Thus, attenuation of chromate in saturated soil environments may be in large part attributable to dissimilatory Fe reduction. Enhancing Fe reduction may therefore promote the reductive stabilization of Cr. 

Synthesis of BMY-14802 (228) commenced from pyrimidine derivative 243 which reacted with piperazine 244 to give derivative 245 (Scheme 58) [215, 216]. Reduction of the compound 245 followed by deprotection gave amine 246, which was alkylated with chloride 247 and then subjected to acidic hydrolysis to form ketone 248. Reduction of 248 allowed BMY-14802 (228) to be obtained. Pure enantiomers of 228 were also obtained. To achieve this, the following methods were used resolution of 228 with using reaction with a-phenylethyl isocyanate [217] or lipase-catalyzed acetylation or hydrolysis [218], alkylation of 245 with enantiopure alcohols 249 [219] and microbial reduction [305] or Ru-catalyzed enantioselective hydrogenation [220] of 248. 

Reductive reactions typically occur in anaerobic environments where there is an abundant supply of electron donors. Electron donors are typically of microbial origin, eg, porphyrins or cysteine, which sometimes leads to confusion regarding the nature, ie, chemical vs enzymatic, of the reductive reaction. By definition, all reductive reactions which are not enzymatically catalyzed are chemical. The most significant chemical reductive reaction is reductive dechlorination. 

Based on the previous publications, azo dye can be reduced by azoreductase-catalyzed reduction under anaerobic conditions. But still there is a speculation whether bacterial flavin reductases are responsible for the azo reductase activity observed with bacterial cell extracts. In a published report, it is reported that flavin reductases are indeed able to act as azo reductases [24]. Bacteria produce extracellular oxidative enzymes, which are relatively nonspecific enzymes catalyzing the oxidation of a variety of dyes. It was reported that so many diverse groups of bacteria play a role in decolorization. It has been also reported that mixed microbial community could reduce various azo dyes, and members of the y-proteabacteria and sulfate reducing bacteria (SRB) were found to be prominent members of mixed bacterial population by using molecular methods to determine the microbial population dynamics [1],... 

We take two cases in which mineral surfaces catalyze oxidation or reduction, and one in which a consortium of microbes, modeled as if it were a simple enzyme, promotes a redox reaction. In Chapter 33, we treat the question of modeling the interaction of microbial populations with geochemical systems in a more general way. 

Smith and Rosazza have suggested that microbial transformation experiments could best be carried out by using a series of perhaps 10 metabolitically prodigious microorganisms as microbial models. Microorganisms for such work may be selected on the basis of considerable literature precedence for their abilities to catalyze the desired biotransformation reaction (i.e., O-dealkylation, N-dealkylation, aromatic hydroxylation, and reductions). The alkaloid substrate... 

TCE is the other major contaminant at the site and is a common groundwater contaminant in aquifers throughout the United States [425]. Since TCE is a suspected carcinogen, the fate and transport of TCE in the environment and its microbial degradation have been extensively studied [25,63, 95,268,426,427]. Reductive dechlorination under anaerobic conditions and aerobic co-metabolic processes are the predominant pathways for TCE transformation. In aerobic co-metabolic processes, oxidation of TCE is catalyzed by the enzymes induced and expressed for the initial oxidation of the growth substrates [25, 63, 268, 426]. Several growth substrates such as methane, propane, butane, phenol, and toluene have been shown to induce oxygenase enzymes which co-metabolize TCE [428]. 

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