Enzyme-Based Organic Synthesis. Cheanyeh Cheng

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Scheme 2.35 The asymmetric synthesis of 3,5‐bis(trifluoromethyl) acetophenone to (1R)‐[3,5‐bis(trifluoromethyl)phenyl] ethanol.

The family of imidazole derivatives, including miconazole, econazole, isoconazole, ketoconazole, sertaconazole, and sulconazole, is well known for antifungal imidazolium compounds. Miconazole and econazole are usually employed in the treatment of vaginal diseases and several fungal infections in the skin of both human and animals by interfering with ergosterol biosynthesis of fungal organisms. Since the therapeutic efficacies of their enantiomers were usually different, the demand for synthesis of optically pure compounds rather than their racemates is required. However, the synthesis of miconazole and econazole single enantiomers with asymmetric chemical transformations [151–154] was rarely reported. Recently, a simple and novel bioreduction of prochiral ketones using ADHs has been applied for the synthesis of the precursors of miconazole and econazole single enantiomers. Then the target fungicides miconazole and econazole were produced from corresponding enantiomeric pure precursors by a series of chemical modifications. The best results were the synthesis of enantiopure precursor, (R)‐2‐chloro‐1‐(2,4‐dichlorophenyl)ethanol, from 2‐chloro‐1‐(2,4‐dichlorophenyl)ethanone using screened ADHs under very mild conditions (Scheme 2.36) [155].

The use of S. cerevisiae (baker’s yeast) for the reduction of ketones or aldehydes may result in mixtures of products due to the existence of a variety of reductases possessing overlapping substrate specificity and mixed stereoselectivity [156]. Therefore, S. cerevisiae and the gram‐negative bacterium E. coli were overexpressed with gene encoding the NADPH‐dependent aldo‐keto reductase YPR1 and compared for producing optically active alcohols through whole‐cell bioreduction. The NADPH‐dependent carbonyl reduction of bicycle[2.2.2]octane‐2,6‐dione to produce optically pure (−)‐(1R,4S,6S)‐6‐hydroxy‐bicyclo[2.2.2]octane‐2‐one was used as a model reaction (Scheme 2.37). High purity of the (−)‐keto alcohol (>99% e.e., 97–98% de) was obtained for both engineered microorganisms. However, E. coli had higher initial rate but S. cerevisiae continued the reaction longer to give a higher substrate conversion (95%). S. cerevisiae also demonstrated higher viability during reaction than E. coli [157].

Scheme 2.36 Bioreduction of α‐haloketones in aqueous medium using different alcohol dehydrogenase followed by a series of chemical modifications to optically pure miconazole or econazole.

Scheme 2.37 E. coli or S. cerevisiae catalyzed reduction of bicycle[2.2.2]octane‐2,6‐dione to (−)‐(1R,4S,6S)‐6‐hydroxy‐bicyclo[2.2.2]octane‐2‐one and (+)‐(1S,4R,6S)‐6‐hydroxy‐bicyclo[2.2.2]octane‐2‐one.

The asymmetric reduction of β‐ketoesters mediated by microorganisms has become a standard method for the synthesis of chiral β‐hydroxyesters [158, 159]. Recently, the immobilization of ADH from permeabilized brewer’s yeast on derived attapulgite nanofibers through glutaraldehyde covalent binding for the bioreduction of ethyl 3‐oxobutyrate (EOB) to ethyl (S)‐3‐hydroxybutyrate ((S)‐EHB) was investigated. The effect of immobilization on ADH activity for the bioreduction shows that the immobilized ADH retained higher activity over wider ranges of pH and temperature than those of the free enzyme. The optimum temperature and pH of the immobilized ADH were 7.5 and 35 °C, respectively. Under the optimum conditions, the immobilized enzyme retained 58% of the original activity after 32 hours of incubation. The conversion of substrate (EOB) and the enantiomeric excess value of (S)‐product reached 88 and 99.2%, respectively, within two hours. The immobilized ADH retained about 42% of the initial activity after eight cycles [160]. Also, although the yields and the enantioselectivity are low to moderate, the enantioselective bioreduction of ethyl benzoylacetate and their p‐nitro and p‐methoxy substituted derivatives to form corresponding chiral ethyl 3‐hydroxy‐3‐phenylpropionate and substituted derivatives (Scheme 2.38) has long been of interest in pharmaceutical industry for synthesizing the key chiral building blocks of many compounds such as fluoxetine [161], chloramphenicol [162], and diltiazem [163]. However, the coupling of simple screening procedures and reaction engineering strategy can increase the (S)‐enantioselectivity to 99% e.e. and shows a significant improvement in the yields to around 85%. In this way, yeasts Pichia kluyveri, Pichia stipites, and Candida utilis were screened and better yields and e.e.’s for ethyl benzoylacetate, p‐nitrobenzoylacetate, and p‐methoxybenzoylacetate can be achieved by the addition of glucose, α‐chloroacetophenone as inhibitor, and immobilization of the yeast in alginate beads, respectively. These processes can also be implemented on a preparative scale and still maintain the same yield and e.e. [164].

Scheme 2.38 Yeast mediated enantioselective reduction of ethyl benzoylacetate and substituted derivatives.

Although water is the first choice as solvent for biocatalysis, the low solubility of organic compounds in water, difficult product separation, and potential side reactions caused by other enzymes in the cell have led to alternative solvents being sought. Since optically pure (S)‐1‐(4′‐methoxyphenyl) ethanol ((S)‐MOPE) is a potential synthon for the preparation of cycloalkyl [b] indoles in clinical treatment of general allergic response, whole microbial cells have been used to synthesize for enantiopure (S)‐MOPE from 4′‐methoxyacetophenone (MOAP) in aqueous systems [165]. Recently, the bioreduction of MOAP to (S)‐MOPE has been successfully performed in a hydrophilic ionic liquid (IL) containing cosolvent system using immobilized Rhodotorula sp. AS2.2241 cells. The novel IL 1‐(2′‐hydroxy)ethyl‐3‐methylimidazolium nitrate (C₂OHMIM•NO₃) gave СКАЧАТЬ