Chirality, enzyme introduction

Enzyme-mediated oxidation reactions offer highly diverse options for the modification of existing functional groups as well as for the introduction of novel function in chiral catalysis. Biooxidations often enable us to obtain complementary solutions to metal-assisted transformations and organocatalysis and are considered one of the important strategies of green chemistry . 

One 7i-bond of an aromatic ring can be converted to a cyclohexadiene 1,2-diol by reaction with enzymes associated with P. putida A variety of substituted aromatic compounds can be oxidized, including bromobenzene, chlorobenzene, " and toluene. In these latter cases, introduction of the hydroxyl groups generates a chiral molecule that can be used as a template for asymmetric syntheses. " ... 

Chem. Soc., 126, 14411-14418 Skander, M., Malan, C., Ivanova, A. and Ward, TR. (2005) Chemical optimization of artificial metaUoenzymes based on the biotin-avidin technology (S)-selective and solvent-tolerant hydrogenation catalysts via the introduction of chiral amino acid spacers. Chem. Commun., 4815-4817 Ward, TR. (2005) Artificial metallo-enzymes for enantioselective catalysis based on the noncovalent incorporation of organometallic moieties in a host protein. Chem.-Eur. J., 11, 3798-3804 Letondor, C. and Ward, TR. (2006) Artificial metaUoenzymes for enantioselective catalysis Recent advances. Chem. Bio. Chem., 7, 1845-1852. 

Electroenzymatic reactions are not only important in the development of ampero-metric biosensors. They can also be very valuable for organic synthesis. The enantio- and diasteroselectivity of the redox enzymes can be used effectively for the synthesis of enantiomerically pure compounds, as, for example, in the enantioselective reduction of prochiral carbonyl compounds, or in the enantio-selective, distereoselective, or enantiomer differentiating oxidation of chiral, achiral, or mes< -polyols. The introduction of hydroxy groups into aliphatic and aromatic compounds can be just as interesting. In addition, the regioselectivity of the oxidation of a certain hydroxy function in a polyol by an enzymatic oxidation can be extremely valuable, thus avoiding a sometimes complicated protection-deprotection strategy. 

Manufacture of optically pure (5)-(+)-ibuprofen (13), an NSAID similar to naproxen, is another example demonstrating the role of resolution in production of chiral fine chemicals, although from a somewhat different angle. Unlike naproxen, ibuprofen (14) was introduced to the market as a racemate almost 30 years ago.30 31 At the time of the introduction, it was thought that both R- and 5-isomers of ibuprofen had the same in vivo activity.32 It has been demonstrated that the R-isomer is converted to the 5-isomer in vivo29 by a unique enzyme system called invertase.34 Based on these data, ibuprofen has since been marketed as a racemate and has achieved sales of more than a billion... 

Enzymes often prove to be the catalyst of choice for numerous transformations, and their prowess is particularly noteworthy for the synthesis of chiral molecules. The ability of biocatalysts to impart chirality through conversion of prochiral molecules or by transformation of only one stereoisomer of a racemic mixture stems from the inherent chirality of enzymes. As noted in the introduction to this book (Chapter 1), the chiral drug market is increasing, partly as a result of the need to produce single enantiomers as advocated by the U.S. Food and Drag Administration.1 The ability to extend the patent life of a drug through a racemic switch also plays a role in this increase. An example of a racemic switch is Astra Zeneca s Esomeprazole, a proton pump inhibitor (see Chapter 31).2... 

The number of enzymes for industrial synthetic applications is growing fast. Enzymatic synthesis can be performed under mild reaction conditions so that many problems of chemical synthesis like isomerization orracemization can be prevented. Furthermore, enzymes are highly specific and selective, especially for enantio- or regio-selective introduction of functional groups. For the preparation of chiral enantiopure compounds, the resolution of racemic mixtures by hydrolases is a well-established route, which has the advantage to be able to use enzymes free of coenzymes. Otherwise, only a maximum yield of 50% can be reached by the primary reaction and further steps of reracemization must follow to avoid loss of the undesired enantiomer. 

The percentage distribution of the number of chiral centres in three classes of compounds (drugs, natural products and combinatorial) was markedly different.18 The mean numbers of chiral centres were 6.2 for natural products, 2.3 for drugs and 0.4 for combinatorial molecules.18 Since natural products are synthesised by enzymes the introduction of a chiral centre appears to be effortless, whereas it requires special attention to synthesise chiral compounds in the laboratory. 

Surprisingly, the introduction of the pyridine ring not only influences the velocity of the enzymatic transformations, but also induces promising stereochemical effects (Table 1). For instance, at 40% conversion (R)-phenylethanol is obtained from the pyridyl acetate 25 with 73 % ee, whereas the value for the corresponding phenylacetate is only 28%. Also, the secondary alcohol liberated from the ester 26 displays 98% ee at 40% conversion, whereas the respective phenylacetate leads to 1-phenylpropanol with 94% ee but at a conversion rate of 12% only [19,20]. These results demonstrate that the stereoselecting properties of penicillin acylase may be enhanced by appropriate engineering of the substrate. This is of particular interest since this enzyme has already been used for the kinetic resolution of various chiral alcohols [21-24], e.g. furyl alkyl carbinols [24], which are valuable precursors for the de novo synthesis, with moderate to high ee values, of carbohydrates. 

The chapter on resolutions has a number of examples as illustrations showing that this methodology is still important to obtain chiral compounds. Although, ultimately, it may not be the most cost-effective method, it can provide material in a rapid manner, and can usually be scaled up. The introduction of large-scale chromatographic techniques, as well as the availability of a large number of enzymes that can be used to perform reactions on only one enantiomer, will ensure that this approach remains a useful tool in the future. 

Recent employment of optically active fluorinated compounds for biologically active substances (7-2) or ferroelectric liquid crystals (3-5) has emphasized the versatility of these chiral molecules, while few methods have been reported for the preparation of such materials in a highly diastereo- as well as enantioselective manner. On the other hand, recent investigations in this field have opened the possibility for the introduction of chirality via asymmetric reduction or optical resolution by employing biocatalysts such as baker s yeast (6-75) or hydrolytic enzymes (16-20), respectively (27-23), along with the conventional chemical methodology (24-27). Chiral materials thus obtained may also be utilized in diastereoselective reactions which create new chiral centers (77). In this paper, the authors would like to discuss our recent progress in the preparation of optically active fluorinated compoounds and the effect of fluorine atom(s) on the reactivity and selectivity. 

The polyketides are a family of natural products containing many important pharmaceutical agents that are synthesized through the multienzyme complex, polyketide synthase, which can display substantial molecular diversity with respect to chain length, monomer incorporated, reduction of keto groups, and stereochemistry at chiral centers (189). This variability, together with the existence of several discrete forms of polyketide synthase, allows the generation of diverse structures like erythromycin, avermectin, and rapamycin. This biochemical diversity has been considerably expanded by the introduction of new sub strate species that were used by the enzymes to produce new or unnatural polyketides (190). 

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