Tetrahedron: Asymmetry Report Number 108Recent progress in biocatalysis for asymmetric oxidation and reduction
Graphical abstract
Recent developments in biocatalytic approach to reduction, oxidation, dynamic kinetic resolution, and deracemization are reviewed.
Introduction
Asymmetric synthesis is increasingly getting important for variety of purposes such as drug synthesis as more than half of drug candidate molecules have more than one chiral centers.1 To synthesize them in environmentally friendly methods, catalysts play an important role, and both chemical and biological catalysts need to be developed because they are complementary to each other. Now, about 10% of the total drug synthesis depend on the biocatalysts. Biocatalytic processes used in the synthesis of chiral intermediates for pharmaceuticals have been reviewed.2
The kind of biocatalytic reaction in majority of reports has been hydrolysis due to the high stability and easiness in handling of hydrolases. Oxidation and reduction have accounted for the second largest portion of the studies of biocatalysis. With recent remarkable progress, the number of commercially available and easy-to-handle oxidoreductases is increasing. For example, 38 of various types of reducing enzymes (ChiralScreenTM OH) are available from Daicel.3 Codexis also prepares a large number of various types of screening kit for reduction of ketones, diketones, and keto esters as well as for reduction of olefin.4(a), 4(b) With the increased number of available enzymes, suitable biocatalysts having high enantioselectivities for a variety of substrates are easier to find.
Here, latest advances for asymmetric synthesis through reduction and oxidation including deracemization by biocatalysts are reviewed. Most of the literature reviewed in this report are published after 2003, and other reviews5(a), 5(b) and books6(a), 6(b) cover the studies with high originality conducted before.
Section snippets
Reaction mechanism
Dehydrogenase, reductase, oxidase, and oxygenase require a coenzyme such as NADH (nicotinamide adenine dinucleotide), NADPH (nicotinamide adenine dinucleotide phosphate), and flavin. Enzyme and coenzyme work together to catalyze reaction. For example, the reduction with NADH proceeds as follows:
- (1)
Coenzyme and substrate bind to an enzyme.
- (2)
The substrate is reduced, while the coenzyme is oxidized.
- (3)
The coenzyme and product dissociate from the enzyme.
Preparation of biocatalysts and investigation of reaction conditions
Methods to find, mutate, and immobilize enzyme as well as to examine reaction conditions are described. Screening sources from which to search new biocatalysts became diverse with the recent progress in molecular biology. The following sources are available.
- (1)
Enzymes expressed in microorganisms were found in environment and cultivated in laboratory.
- (2)
Enzymes expressed using DNA were extracted from environment (metagenome).
- (3)
Enzymes expressed using information from databank.
Examples for asymmetric reductions
Various examples for asymmetric reduction are described in this section. Reduction of carbonyl compounds as well as reduction of olefines and nitro group are described.
Oxidations
Enantioselective oxidation of racemic alcohols, oxidation of C–H bonds, Baeyer–Villiger oxidation and oxidative polymerization are described in this section.
Dynamic kinetic resolution
Dynamic kinetic resolution of racemic substrate proceeds through asymmetric reduction or oxidation when the substrate does racemize and the product does not under the applied experimental conditions. Recent development in dynamic kinetic resolution has been reviewed.128
Deracemization
Deracemization reaction, transformation of racemic compounds into chiral forms in one pot without changing their chemical structures, has been performed using microorganism containing several enzymes with different stereochemistries. Instead, use of two isolated enzymes or use of one enzyme for enantioselective conversion and a chemical reagent is also possible to perform deracemization. This section shows some examples for deracemization.
Conclusions
Asymmetric reduction and oxidations are covered in this review. Asymmetric reductions have been reported continuously for decades, and an increased number of reports about oxidation have been published recently. Now, enzyme source becomes diverse; enzymes expressed and unexpressed in natural environment can be used as long as the gene for the enzymes can be obtained and the enzymes can be expressed in laboratory. An increase in available gene in data bank has also promoted the development of
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