Biotransformations enantioselective transformations

Hiihnerfuss et al. [ 14,23] reported the enantioselective biotransformation of PCBs in blue mussels Mytillus edulis). Subsequently, Blanch et al. [24] also reported the enantioselective transformation of PCBs in shark (Centroscymnus coelolepis B and C) liver. Similarly, various reports have been published on the enantioselective biotransformation of PCBs in different animals [23, 24, 25-34], Faller etal. [12] reported that the different rates of PCB biotransformation were due to their different reaction rates with the biological enzymes. In another study, Hiilmerfuss etal. [35-37] reported the enantiomeric biotransformation of PCBs in the liver of humans and rats. Wong etal. [38] reported the different... 

Enantioselective transformations catalyzed by nitrilases often suffer from poor chiral recognition. Exceptions from this trend are benzaldehyde and phenylac-etaldehyde cyanohydrins. As an additional advantage, these substrates racemize readily at near-neutral pH via reversible loss of hydrogen cyanide representing good starting materials for dynamic kinetic resolution processes. This was demonstrated using 22 substituted phenyl and heteroaryl derivates 25 with two recombinant nitrilases a preparative biotransformation yielded (S)-phenyllactic add 26 in 84% yield and 96% ee on 1 g scale (Scheme 9.7) [31]. 

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. 

Rhodococcus sp. AJ270 was applied to the transformation of a number of racemic cis- and traray-3-aryl-2-methyloxiranecarbonitriles (Figure 8.7). In all cases, the NHase activity proceeded very rapidly and with poor enantioselectivity. In contrast, the amidase activity was strongly dependent upon substrate structure. In general, the biocatalyst displays a strong preference for the unsubstituted phenyl side chain or /wa-substituted phenyl side chain compared with ortho- or meta-, and this is manifest both with respect to observed conversion and rate and also observed enantioselectivity. In contrast, the biotransformations of... 

Biooxidation of chiral sulfides was initially investigated in the 1960s, especially through the pioneering work of Henbest et al. [101]. Since then, many developments have been reported and are summarized in reviews [102,103], It would be helpful to reveal some structural or mechanistic details of enzymes involved in theoxidation processes. Biotransformations are also of great current interest for the preparation of chiral sulfoxides, which are useful as synthetic intermediates and chiral auxiliaries. Because extensive review of these transformations is beyond the scope of this chapter, only highlights are discussed in comparison with the abiotic enantioselective oxidations described earlier. Biooxidations by microorganisms and by isolated enzymes are discussed in Sections 6C.12.1. and 6C.12.2. 

Biotransformation pathways have also been used to establish this chiral center (Scheme 7). The bromoacetophenone 27 was mixed with sodium laurel sulfate and added to a microbial culture of Rhodotorula rubra to produce enantiomencally pure alcohol 28 with 95% ee that was eventually converted to (R)-salmeterol (2).17 In a similar transformation, the azidoketone 29 was enantioselectively reduced, using the microorganism Pichia angusta, to the alcohol 30 with > 98% ee that was eventually converted to (5 )-salmeterol (2).18... 

The rate constants calculated by EF profiles (Equation (4.6)) are necessarily crude as several assumptions must hold the initial enantiomer composition is known, only a single stereoselective reaction is active, and the amount of time over which transformation takes place is known. These assumptions may not necessarily hold. For example, for reductive dechlorination of PCBs in sediments, it is possible for degradation to take place upstream followed by resuspension and redeposition elsewhere [156, 194]. The calculated k is an aggregate of all reactions, enantioselective or otherwise, involving the chemical in question. This includes degradation and formation reactions, so more than one reaction will confound results. Biotransformation may not follow first-order kinetics (e.g. no lag phase is modeled). The time period may be difficult to estimate for example, in the Lake Superior chiral PCB study, the organism s lifespan was used [198]. Likewise, in the Lake Hartwell sediment core PCB dechlorination study, it is likely that microbial activity stopped before the time periods selected [156]. However, it should be noted that currently all methods to estimate biotransformation rate constants in field studies are equally crude [156]. 

Enantiomer-based methods exploit the fact that some pesticide compounds are applied in known ratios of enantiomers, most commonly as racemic mixtures, i.e., 1 1 ratios (Buser et al., 2000 Monkiedje et al., 2003). Although most abiotic transformation and partitioning processes are not affected by the structural differences between enantiomers (Bidleman, 1999), the biotransformation of some pesticide compounds has been found to be an enantioselective process, i.e., one that exhibits a preference for one enantiomer over the other (e.g., Harner et al., 1999 Monkiedje et al., 2003). The measurement of enantiomer concentration ratios for a pesticide compound that is applied as a racemic mixture but may undergo enantioselective biotransformation in the environment can thus provide an indication of whether or not the compound has undergone biotransformation since it was applied—and thus a rough... 

Analogous enantioselective biotransformations have been achieved with the silicon compounds 217 and 219, which were transformed into the respective optically active products (S)-218 (yield 90%, enantiomeric purity 84% ee) and (S)-220 (yield 80%, enantiomeric purity 76% ee) using growing cells of Kloeckera corticis (ATCC 20109)278,279. Particularly remarkable is the conversion of the hydridosilane 219 which could be performed without a noticeable degree of hydrolytic cleavage of the Si-H bond (incubation conditions pH 5.5, 27 °C). 

By analogy to the biotransformation of rac-221, the structurally related acetyldisilane rac-229 and the acetylsilane rac-231 were also reduced enantioselectively using resting free cells of Trigonopsis variabilis (DSM 70714)282,287. The conversion of rac-229 leads to a mixture of the optically active disilanes (R,R)-230 and (S,R)-230 (yield 75%, enantiomeric purity of both diastereomers >98% ee) and the transformation of rac-231 leads to a mixture of the optically active silanes (R,R)-232 and (S,R)-232 (yield 72%, enantiomeric purity of both diastereomers 94% ee). These reactions were carried out at 37 (rac-229) and 44 °C (rac-231), respectively the substrate concentrations used were 0.53 g/1. Especially remarkable is the biotransformation of the disilane rac-229 which could be realized without a noticeable degree of hydrolytic cleavage of the Si-Si bond. 

For the enantioselective preparations of chiral synthons, the most interesting oxidations are the hydroxylations of unactivated saturated carbons or carbon-carbon double bonds in alkene and arene systems, together with the oxidative transformations of various chemical functions. Of special interest is the enzymatic generation of enantiopure epoxides. This can be achieved by epoxidation of double bonds with cytochrome P450 mono-oxygenases, w-hydroxylases, or biotransformation with whole micro-organisms. Alternative approaches include the microbial reduction of a-haloketones, or the use of haloperoxi-dases and halohydrine epoxidases [128]. The enantioselective hydrolysis of several types of epoxides can be achieved with epoxide hydrolases (a relatively new class of enzymes). These enzymes give access to enantiopure epoxides and chiral diols by enantioselective hydrolysis of racemic epoxides or by stereoselective hydrolysis of meso-epoxides [128,129]. 

Stereoselective biotransformation with growing cells, resting free or immobilized cells, or isolated enzymes has been demonstrated to be a useful preparative method for the synthesis of centrochiral optically active organosilicon compounds [1-3]. In continuation of our own studies in this field, we have investigated stereoselective microbial transformations of rac-1-(4-fluorophenyl)-l-methyl-l-sila-2-cyclohexanone (rac-1) and rac-(Si5,CR/SiR,C5)-2-acetoxy-l-(4-fluorophenyl)-l-methyl-1-silacyclohexane [rac-(Si5,C/ /Si/f,C5)-3a]. We report here on (i) the synthesis of rac-1 and rac- SiS,CR/SiR,CS)-3a, (ii) the diastereoselective microbial reduction of rac-1 [— (Si5,C/ )-2a, (SiR,C5)-2a], and (iii) the enantioselective microbial hydrolysis of rac-(SiS,CR/SiR,CS)-3a [- (SiR,C5)-2a],... 

Scheme 32 outlines a high yielding approach to the enantiopure appetite suppressant drug 77. The reduction of the unsaturated aldehyde 74 was reported to occur with modest enantioselectivity in normal fermenting conditions with baker s yeast [155]. When the biotransformation is performed at very low substrate concentration, the e.e. can be raised to more than 90%, suggesting that incomplete enantioselectivity is due to the action of enzymes operating on the same substrate with opposite stereochemical preference [115]. However, an efficient transformation can be performed if the substrate concentration is controlled with the addition of absorbing hydrophobic resins. At 5 g/L 97% recovery and 98% e.e. was obtained. The halohydrin 75 obtained was transformed into the epoxide 76 and finally into the enantiopure 2-/ -benzylmorpholine 77, the more active enantiomer with appetite suppressant activity [26]. 

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