Enzyme-Based Organic Synthesis. Cheanyeh Cheng

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2 Organic Synthesis with Oxidoreductases

      2.1 Oxidation Reactions

      Enzymatic and microbial oxidations can be dated back to 2000 BCE with vinegar production that is based on the oxidation of ethanol by acetic acid bacteria. Enzymes involved in the biocatalyzed oxidations are dehydrogenases and oxidases. The more common coenzymes associated with dehydrogenases are nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide hydrogen (NAD⁺/NADH), nicotinamide adenine dinucleotide phosphate/nicotinamide adenine dinucleotide phosphate hydrogen (NADP⁺/NADPH), and flavin adenine dinucleotide/flavin adenine dinucleotide hydrogen (FAD/FADH₂), whereas oxidases are usually assisted by flavoproteins for transferring electrons to molecular oxygen. Dehydrogenases can be found in aerobic and anaerobic organisms or microorganisms; however, oxidases are not present in strictly anaerobic species. Nowadays, dehydrogenases and oxidases have been extensively used for selective oxidations and as an alternative synthetic strategy for conventional oxidations with the advantages of being environmentally friendly and highly chemo‐, regio‐, and stereoselective. Nevertheless, due to poor stability of the two enzymes at high substrate/product/organic solvent concentration and temperature, large‐scale bio‐oxidation processes were few. The functional groups involved in those bio‐oxidations include hydroxyls of primary and secondary alcohols, carbonyls of aldehydes, saturated C–C bonds, C–N bond of amino acids, amines, nitroalkanes, and thiols [1].

      2.1.1 Oxidation of Alcohols and Aldehydes

Since primary alcohols can be oxidized to aldehydes and further to carboxylic acids, which are versatile building blocks in organic synthesis, the selective oxidation of primary alcohols using enzymes to produce corresponding aldehydes is important for both fundamental and industrial research. Benzaldehyde, which is important and widely applied in cosmetic, flavor, and pharmaceutical industries [2, 3], is generally prepared by oxidation of toluene or hydrolysis of benzyl chloride. Due to environmental concern, scientific reports have shown that Gluconobacter oxydans can be used for the selective oxidation of benzyl alcohol to obtain benzaldehyde in an organic/aqueous biphasic system to substitute for these environmentally unfavorable processes [4, 5]. Optimization of the immobilization parameters for G. oxydans not only improves the bioconversion of the selective oxidative process but also increases the stability of immobilized cells so that the immobilized cells can be used repeatedly for 10 cycles and a 53.2% of the oxidative activity of the immobilized cells compared with that of free cells was retained [6].

Fourteen commercial alcohol oxidases, 33 selected commercial alcohol dehydrogenases (ADHs), and 218 microorganisms were tested for the oxidation of benzyl alcohol for the production of benzaldehyde via different types of hydrogen transfer [7]. If alcohol oxidases were used for the oxidation, the reaction just required molecular oxygen as oxidant and hydrogen peroxide was produced as a side product, which subsequently is decomposed disproportionately by a catalase to form water and molecular oxygen (Scheme 2.1). If isolated ADHs or lyophilized microorganisms were employed for the oxidation of benzyl alcohol, the hydrogen acceptor of the hydrogen transfer reaction could be acetaldehyde (Scheme 2.2), chloroacetone, or acetone.

Direct oxidation of simple primary alcohols such as methanol and ethanol to corresponding aldehydes using either free whole cell or immobilized whole cell was also reported in literature. For methanol oxidation to formaldehyde, methanol oxidase (MOX) in the yeast Hansenula polymorpha was involved for the biotransformation [8]; for ethanol oxidation to acetaldehyde, ADH in Saccharomyces cerevisiae was employed for the biotransformation. In these studies, the immobilization of microbial cells was found to be of great help for both the cell activity and stability [9].

Scheme 2.1 Oxidation of benzyl alcohol using an oxidase and a catalase.

Scheme 2.2 Oxidation of benzyl alcohol via alcohol dehydrogenases or microbial cells.

      The stereoselective oxidation of secondary alcohols to produce ketones is of greatest interest in organic synthesis for its applications in pharmaceutical industries. Simple sec‐alcohol, 2‐butanol, has been oxidized to butanone by the immobilized yeast S. cerevisiae with a 45% yield [9]. Three screened yeast, Williopsis californica, Williopsis saturnus, and Pachysolen tannophilus, have been used for the oxidation of six cycloalkanols with different ring size including cyclobutanol, cyclopentanol, cyclohexanol, cycloheptanol, cyclooctanol, and cyclododecanol. The results show that W. californica and P. tannophilus are active against all six cycloalkanols and can be thought as nonselective, while W. saturnus is active against cycloalkanols of four, five, and six carbon atoms and is selective for small cyclohexanols [10]. These three selected strains have also been employed for exploring the stereoselectivity of several sec‐alcohols such as (1R)‐(2‐furyl)‐ethanol, (1S)‐(2‐furyl)‐ethanol, (1R)‐phenyl‐ethanol, (1S)‐phenolethanol, (1R)‐tetrahydronapthol, (1S)‐tetrahydronapthol, (−)‐neo‐menthol((1R,2R,5S)‐2‐isopropyl‐5‐methyl‐cyclohexanol),(+)‐menthol ((1S,2R,5S)‐2‐isopropyl‐5‐methyl‐cyclohexanol), and iso‐menthol ((1S,2R,5R)‐2‐isopropyl‐5‐methyl‐cyclohexanol). The results indicate that all the strains are stereoselective toward the S‐enantiomer [10].

The application of primary alcohol oxidation is proved to be valuable for the preparation of enantiopure 1,2‐diols [11]. СКАЧАТЬ