| Enzymes have specific substrate recognition in metabolism, which are optimized through evolution of a specific chemical transformation. Despite this, more and more enzymes have been found to have the ability to catalyze reactions, or act on substrates, other than those for which they evolved; this phenomenon is called enzyme promiscuity. Enzymes, as green and efficient biocatalysts, have huge superiority in many important respects such as mild reaction conditions, high yields, good stereoselectivity, easy processing, economically and ecologically advantageous, etc. Since Klibanov et al.(re)discovered that enzymes can maintain their activities in organic solvents, enzymatic reactions have attracted extensive attention. Enzymes not only can work in organic media, but also acquire remarkable properties such as enhanced stability, altered substrate specificities, and the ability to catalyze unusual reactions which are impossible in aqueous media. Many examples of enzyme catalytic promiscuity have been reported, such as the aldol reactions, Markovnikov additions, Michael additions, Mannich reactions, the asymmetric synthesis of α-aminonitrile amides, multi-component cascade or domino reactions, etc. In this thesis, we reported anti-selective direct asymmetric Mannich reaction using protease for the synthesis of chiral anti-β-amino-α-hydroxycarbonyl compounds, asymmetric domino thia-Michael/aldol condensation using pepsin for the synthesis of chiral dihydrothiophenes and aminoalkylation reaction via a relay catalysis strategy in one pot with combining enzyme and photoredox catalysis in divided sections.Firstly, we described enzyme-catalyzed anti-selective direct asymmetric Mannich reaction in organic media. The asymmetric Mannich reaction is one of the most important C-C bond forming reactions for the preparation of valuable optically active β-amino carbonyl compounds and their derivatives that contain two adjacent stereocenters. These enantiomerically enriched nitrogenous molecules are common building blocks for numerous biologically active and medicinal agents with high pharmaceutical value, as well as complex α-amino acid derivatives. Because of the versatility of these optically active β-amino carbonyl units obtained from Mannich reactions, it is highly desirable to obtain either syn- or anti-products with high enantioselectivity. Whereas syn-selective Mannich reactions now represent a rather well-solved problem, enantioselective anti-selective Mannich reactions are considerably more challenging. Development of the direct asymmetric three-component Mannich reactions that use unmodified carbonyl compounds as nucleophile sources has been of interest, since these reactions are more atom economical than those that use preactivated carbonyl compounds, such as silyl enol ethers or preformed enamines. Considering the above advantages of enzyme catalysis and direct asymmetric Mannich reactions, and the challenging of preparing enantioselective anti-selective Mannich reactions, it is important for us to develop an enzyme-catalyzed, direct asymmetric, anti-selective Mannich reaction as a more sustainable complement to chemical catalysis. The direct asymmetric Mannich reaction of 4-nitrobenzaldehyde, 4-anisidine and O-benzyl hydroxyacetone catalyzed by protease type XIV from Streptomyces griseus(SGP) in MeCN in the presence of water was chosen as the model reaction. After condition optimization, through substrate expansion, a series of corresponding products with the excellent diastereoselectivities of up to > 99 : 1(anti/syn) and good enantioselectivities of up to 90% ee were achieved.Secondly, we studied enzyme-catalyzed asymmetric domino thia-Michael/aldol condensation. The novel catalytic promiscuity of pepsin from porcine gastric mucosa for the asymmetric catalysis of domino thia-Michael/aldol condensation reaction in MeCN/buffer was discovered for the first time. The influence of reaction conditions including solvents, pH, molar ratio of substrates, phosphate buffer content, solvent volume, enzyme loading, temperature and reaction time were investigated. Broad substrate specificity was tested and 13 examples of corresponding products were obtained with enantioselectivities of up to 84% ee and yields of 35-53%. This specific catalysis was demonstrated by using recombinant pepsin and the control experiments with denatured and inhibited pepsin. The reaction was also proved to occur in the active site by site-directed mutagenesis(the Asp32 Ala mutant of pepsin) and a possible mechanism was proposed. The performance of pepsin in the present work as well as its other promiscuous activities reported previously demonstrated that pepsin has the considerable potential on broad catalytic promiscuity, which may enable organic chemists to rapidly develop new synthetic applications of pepsin to expand the repertoire of synthetic organic methodologies.Although the enzyme catalytic promiscuity can be realized by many biocatalytic organic reactions, most enzyme catalytic activity is not high. In order to solve this problem, we attempted a novel strategy of combining enzyme and photoredox catalysis for mild aminoalkylation. This method features a relay catalysis protocol consisting of enzyme–catalyzed hydrolysis and visible-light excited decarboxylation. The resulting α-amino radical can be further oxidized to the iminium ion and then reacted with nucleophile indole and acetone, and the α-amino radical can also directly react with electrophilic reagent diazo-compound and maleimide. After condition optimization, through substrate expansion, 34 examples of the α-amino-alkylation products were obtained in good to excellent yields(up to 99%). This work demonstrates the potential benefits of combining enzyme and photocatalyst in a relay catalysis. |
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