1
Introduction

Chemical processes are vital for the manufacturing of goods that meet the human's growing needs. Especially since early last century, organic chemical reactions have greatly contributed to the production of fine chemicals as materials, pharmaceuticals, food additives, cosmetics, etc., which are related to our daily lives. In the meantime, fine chemical industry has contributed to increasing air pollution and environmental contamination, which have adverse effects on the earth, our health, and the quality of our daily lives. The E (environmental) factor (mass of waste/mass of product, kgs/kg), which is often used to assess the environmental impact of a manufacturing process, for the production of fine chemicals is usually 5-50, or even higher for pharmaceuticals (25?=?100) [1]. Therefore, it is desirable to develop green organic chemical processes for the manufacturing of the desired chemical products, thus enabling sustainable development of the fine chemical industry [2].

On the other hand, Nature has created and evolved a diversity of enzymes that catalyze numerous kinds of reactions in live organisms and show advantages over traditional chemical reactions, such as high chemo-, regio- and stereoselectivities, and mild reaction conditions. Enzymes can catalyze various reactions that are difficult to be achieved by traditional chemical reactions. When being incorporated into organic synthesis, enzyme catalysis can reduce the number of reaction steps by eliminating the protection and deprotection steps or redesigning the synthetic route with enzymatic reactions to achieve greater efficiency or atom economy [3]. For example, 19-Nor-steroids are key intermediates for the production of contraceptives, such as norethindrone, mifepristone, and tibolone. By employing the biocatalytic hydroxylation at C-19 of steroids, the synthesis of 19-nor-steroids can be achieved in three chemoenzymatic steps [4]. However, the chemical demethylation of steroids usually requires many more steps as shown in Scheme 1.1 [5].

Retrosynthetic analysis involving both chemical and enzyme catalysis enables the design of novel synthetic sequences for the preparation of complex organic molecules such as active pharmaceutical ingredients [6]. Chemoenzymatic cascade reactions thus play an important role in developing green chemical processes by reducing the waste, energy consumption, and production cost. Great advances in this field have been achieved in the last two decades, and industries are paying increasing attention to enzyme application in the green production of chemicals used for pharmaceuticals, food additives, cosmetics, and so on [7].

Scheme 1.1 Chemoenzymatic and chemical demethylation of steroids.

Source: Based on Wang et al. [5].

In the first chapter of this book, the unique features of enzyme catalysis compared to traditional chemical reactions will be discussed. Next, different modes of chemoenzymatic transformations will be covered with some examples of the operating processes. In the remaining chapters, recent advances in this field, organized according to the modes of chemoenzymatic transformations, will be presented in more detail.

1.1 Advantages of Enzyme Catalysis

1.1.1 Chemoselectivity

Enzyme catalysis is usually highly chemoselective, and the specific transformation of one functional group can be achieved without affecting the other active functional groups in the same molecule, a feature that otherwise cannot be realized by traditional chemical reactions. For example, chemical hydrolysis of nitrile group to carboxylic acid requires strong basic or acidic conditions at elevated temperature, under which the ester group is also hydrolyzed. Thus, it is impossible to chemo-selectively hydrolyze the nitrile group indiscriminately in the presence of ester group in the same molecule. On the other hand, this challenge can be addressed by using nitrilases, which can catalyze the chemo-specific hydrolysis of nitriles under neutral conditions to give the corresponding carboxylic acids in the presence of other acid- or base-sensitive functional groups [8]. For example, ethyl (R)-4-cyano-3-hydroxybutyate was hydrolyzed by a recombinant nitrilase from Aarabidopsis thaliana (AtNIT2) to give ethyl (R)-3-hydroxyglutarate. The ester group in the molecule remained intact (Scheme 1.2) [9]. This key building block for the synthesis of the cholesterol-lowering drug, rosuvastatin, was produced with excellent biocatalyst productivity (55.6?g/g wet cells weight) and space-time productivity (625.5?g/l?d).

Scheme 1.2 Nitrilase-catalyzed chemospecific hydrolysis of ethyl (R)-4-cyano-3-hydroxybutyate.

Source: From Yao et al. [9]. © 2014, John Wiley & Sons.

Reduction of aldehyde and ketone to give alcohols is an important transformation in organic synthesis. Traditional chemical carbonyl reduction methods often show low chemo-selectivity toward either aldehyde or ketone. To achieve selective reduction of aldehyde in the presence of keto group, and vice versa, careful selection of the reducing agent and control of the reaction conditions are usually required. On the other hand, Nature has evolved many aldehyde or ketone reductases, which can catalyze the chemo-specific reduction of aldehyde functional group in the presence of keto group, or vice versa, to give the corresponding alcohols. For example, an NADPH-dependent aldehyde reductase from Oceanospirillum sp. MED92 (OsAR) catalyzes the selective reduction of the aldehyde group in 4-acetylbenzaldehyde without reducing the keto group, affording 4-acetylbenzyl alcohol as the sole product (Scheme 1.3) [10].

Carboxylic acid functional group is difficult to be reduced and usually requires strong reducing agents, which in turn can often reduce C=O, C=N, and other functional groups. Thus, it is quite challenging to achieve chemo-specific reduction of carboxylic acid group in the presence of other reducible functional groups such as C=O and C=N groups in the same molecule. Furthermore, the reduction of carboxylic acid is difficult to stop at the aldehyde intermediate, since the latter can be further reduced to the corresponding alcohol by the reducing agents [11, 12]. However, by using carboxylic acid reductases (CAR, E.C.1.2.1.30) as biocatalyst these problems can be solved, because they catalyze the selective reduction of carboxylic acids into the corresponding aldehydes under mild conditions [13, 14]. This enzyme does not catalyze the reduction of other functional groups such as keto groups and C=N double bonds. Scheme 1.4 shows the enzymatic reduction of a representative keto acid (4-methyl-5-oxo-5-phenylpentanoic acid) to the corresponding keto aldehyde, leaving the keto group intact using a recombinant CAR from Mycobacterium marinum (MmCAR) [15].

Scheme 1.3 The reduction of 4-acetylbenzaldehyde catalyzed by OsAR.

Source: Based on Li et al. [10].

Scheme 1.4 CAR-catalyzed reduction of keto acid to keto aldehyde.

Ene reductase [16], carbonyl reductase [17], and imine reductase [18] catalyze the specific reductions of C=C, C=O, or C=N functional group, respectively. Ene reductase catalyzes the reduction of C=C bond without affecting the C=O or C=N functional group in the molecule (Scheme 1.5) [19]. This is difficult to be realized by the metal-catalyzed hydrogenation reaction since the C=O or C=N functional group may also be hydrogenated during the reduction of the C=C bond.

Scheme 1.5 Reduction of (R)-carvone by an ene reductase (LacER) from Lactobacillus casei.

Source: Based on Chen et al. [19].

1.1.2 Regioselectivity

Enzymes can differentiate the same functional group at different positions in one molecule; thus, enzymatic reactions are usually highly regioselective. Incorporation of regiospecific enzymatic reaction into organic synthesis often simplifies the synthetic route of a target compound by eliminating the protecting and de-protecting steps. The regiospecific reduction of either the trifluoromethyl or the methyl keto group in the methyl/trifluoromethyl diketones was achieved by using some commercially available ketoreductases (KREDs) to give either the R or the S enantiomer with >98% enantiomeric excess (ee), as shown in Scheme 1.6 [20]. Among the three keto groups in the molecule of cortisone, the keto group at 11-position was highly regio- and stereospecifically reduced to give 11ß-hydrocortisone by a mutant 11ß-hydroxysteroid dehydrogenase (11ß-HSDH) from guinea pig (Scheme 1.7) [21].

Scheme 1.6 Enzymatic regiospecific reduction of the methyl/trifluoromethyl diketone.

Source: Based on Grau et al. [20].

Scheme 1.7 Enzymatic regio- and stereospecific reduction of cortisone.

Source: From Zhang et al. [21]. © 2014, Springer Nature.

Like alcohol dehydrogenase, other oxidoreductases also show excellent regioselectivity. For example, meso-diaminopimelate dehydrogenase (meso-DAPDH, EC 1.4.1.16) acts on the...