Enantioselective Reduction of Ethyl 3-Oxo-5-phenylpentanoate with Whole-Cell Biocatalysts

Microbial alcohol dehydrogenases: recent developments and applications in asymmetric synthesis

Alcohol dehydrogenases are a well-known group of enzymes in the class of oxidoreductases that use electron transfer cofactors such as NAD(P)+/NAD(P)H for oxidation or reduction reactions of alcohols or carbonyl compounds respectively. These enzymes are utilized mainly as purified enzymes and offer some advantages in terms of green chemistry. They are environmentally friendly and a sustainable alternative to traditional chemical synthesis of bulk and fine chemicals. Industry has implemented several whole-cell biocatalytic processes to synthesize pharmaceutically active ingredients by exploring the high selectivity of enzymes. Unlike the whole cell system where cofactor regeneration is well conserved within the cellular environment, purified enzymes require additional cofactors or a cofactor recycling system in the reaction, even though cleaner reactions can be carried out with fewer downstream work-up problems. The challenge of producing purified enzymes in large quantities has been solved in large part by the use of recombinant enzymes. Most importantly, recombinant enzymes find applications in many cascade biotransformations to produce several important chiral precursors. Inevitably, several dehydrogenases were engineered as mere recombinant enzymes could not meet the industrial requirements for substrate and stereoselectivity. In recent years, a significant number of engineered alcohol dehydrogenases have been employed in asymmetric synthesis in industry. In a parallel development, several enzymatic and non-enzymatic methods have been established for regenerating expensive cofactors (NAD+/NADP+) to make the overall enzymatic process more efficient and economically viable. In this review article, recent developments and applications of microbial alcohol dehydrogenases are summarized by emphasizing notable examples.

Stereoselective Bioreduction of α-diazo-β-keto Esters

Diazo compounds are versatile reagents in chemical synthesis and biology due to the tunable reactivity of the diazo functionality and its compatibility with living systems. Much effort has been made in recent years to explore their accessibility and synthetic potential; however, their preparation through stereoselective enzymatic asymmetric synthesis has been scarcely reported in the literature. Alcohol dehydrogenases (ADHs, also called ketoreductases, KREDs) are powerful redox enzymes able to reduce carbonyl compounds in a highly stereoselective manner. Herein, we have developed the synthesis and subsequent bioreduction of nine α-diazo-β-keto esters to give optically active α-diazo-β-hydroxy esters with potential applications as chiral building blocks in chemical synthesis. Therefore, the syntheses of prochiral α-diazo-β-keto esters bearing different substitution patterns at the adjacent position of the ketone group (N₃CH₂, ClCH₂, BrCH₂, CH₃OCH₂, NCSCH₂, CH₃, and Ph) and in the alkoxy portion of the ester functionality (Me, Et, and Bn), were carried out through the diazo transfer reaction to the corresponding β-keto esters in good to excellent yields (81–96%). After performing the chemical reduction of α-diazo-β-keto esters with sodium borohydride and developing robust analytical conditions to monitor the biotransformations, their bioreductions were exhaustively studied using in-house made Escherichia coli overexpressed and commercially available KREDs. Remarkably, the corresponding α-diazo-β-hydroxy esters were obtained in moderate to excellent conversions (60 to >99%) and high selectivities (85 to >99% ee) after 24 h at 30 °C. The best biotransformations in terms of conversion and enantiomeric excess were successfully scaled up to give the expected chiral alcohols with almost the same activity and selectivity values observed in the enzyme screening experiments.

Green synthesis of enantiopure quinoxaline alcohols using Daucus carota

Green chemistry comprises a new approach in the synthesis of biologically active compounds using biocatalysts, thus diminishing the hazards for human health and environmental pollution. Asymmetric bioreduction is one of the most widely employed strategies in chemoenzymatic synthesis to produce enantiomerically pure chiral alcohols. The present study highlights the use biocatalyst Daucus carota for selective bioreduction of quinoxaline ketones 1a-6a to their corresponding optically pure alcohols 1b-6b in high yields (up to 84%) and good enantioselectivity (up to 98%). The absolute configuration of the chiral product (R)-1-(3-methyl 7-nitroquinoxalin-2-yl) ethan-1-ol 2b was confirmed by X-ray crystallography studies. The chiral R-configuration of the products obtained was confirmed by absolute configuration studies and was assigned following anti-Prelogs rule. Quinoxaline pharmacophores form a part of well-known potent drug molecules; hence, the chiral products were studied for determination of their molecular properties using SwissADME property analyser. All the chiral products show no Lipinski rule violations and are expected to have good oral bioavailability. As per the molecular properties prediction studies, the compound 6b (R)-1-(6,7-dichloro-3- methylquinoxalin-2-yl) ethanol is observed to show the best physicochemical properties to be a good lead molecule. Thus, the sustainable methodology was developed, and it confirms the synthesis of novel quinoxaline chiral alcohols in a simple, inexpensive, and eco-friendly condition using D carota.

Salt mine microorganisms used for the biotransformation of chlorolactones

The aim of the project was to find new catalysts capable of chlorolactone biotransformation. Three bicyclic chlorolactones with structures possessing one or two methyl groups in their cyclohexane ring were subjected to screening biotransformation using seven bacterial strains and one fungal strain from a salt mine. Three strains of bacteria (Micrococcus luteus Pb10, Micrococcus luteus WSP45, Gordonia alkanivorans Pd25) and one fungal strain (Aspergillus sydowii KGJ10) were able to catalyse hydrolytic dehalogenation of one substrate. The classification of the strains that were effective biocatalysts was confirmed by 16S rDNA analysis. The best result (76%) was obtained using Aspergillus sydowii KGJ10. All strains catalysed hydrolytic dehalogenation without changing the conformation. The equatorial position of the chlorine atom in the substrate turned out to be warrant of the positive result of the biotransformation process.

Enantioselective synthesis of 1,2,3,4-tetrahydroquinoline-4-ols and 2,3-dihydroquinolin-4(1H)-ones via a sequential asymmetric hydroxylation/diastereoselective oxidation process using Rhodococcus equi ZMU-LK19

A cascade biocatalysis system involving asymmetric hydroxylation and diastereoselective oxidation was developed using Rhodococcus equi ZMU-LK19, which gave chiral 2-substituted-1,2,3,4-tetrahydroquinoline-4-ols (2) (up to 57% isolated yield, 99 : 1 dr, and >99% ee) and chiral 2-substituted-2,3-dihydroquinolin-4(1H)-ones (3) (up to 25% isolated yield, and >99% ee) from (±)-2-substituted-tetrahydroquinolines (1). In addition, a possible mechanism for this cascade biocatalysis was tentatively proposed.