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Reduction of aromatic nitro compounds to anilines is of great interest to the chemical industry. Biocatalytic reduction of nitroarenes has made it possible to effectively produce anilines by applying nitroreductase enzymes (NR) in combination with vanadium pentoxide. Herein, the NR-catalyzed reduction of 2-methyl-5-nitro-pyridine (2) to give the desired aniline (1) was studied as a model reaction. It demonstrates the importance of process development and enzyme engineering as key approaches to overcome scale-up issues and improve yield and productivity. Moving to fed-batch allowed controlling the feeding rate of 2to prevent the accumulation of intermediates and formation of undesired side products. Starting with a substrate (2) concentration of 200 mM (28 g/L) and enzyme loading of 5 mg/mL (18% w/w), it was possible to achieve complete conversion and 1in 95% yield by high-performance liquid chromatography (89.1% isolated yield) over 18 h, whereas, with 500 mM (69 g/L) 2and an enzyme loading of 10 mg/mL (14.5% w/w), the same conversion and yield were achieved in 26 h. A rational engineering of NR-4 yielded faster variants, including NR-5, in only one round. The improved rate of the new variants allowed increasing the feeding rate of 2to shorten the reaction time to less than a day as well as decreasing the enzyme loading to 3.6%.

The biocatalytic stereocontrolled synthesis of various acyclic anti- or syn-α-alkyl-β-amino amides through a dynamic kinetic resolution strategy is demonstrated. A series of commercially available and in-house transaminases (TAs) were employed to perform the transamination of a series of chemically synthesized racemic α-alkyl-β-keto amides. Among them, commercial (R)-selective TAs showed the best activities and selectivities, giving access preferentially to the anti-diastereoisomers with low to high diastereomeric ratios (up to 96%) and excellent enantiomeric excess (>99%). The stereoselective biotransamination experiments were successfully demonstrated at a semi-preparative scale (25 mM, 100 mg substrate), leading to the corresponding optically active α-alkyl-β-amino amides in 45–90% isolated yields after a simple liquid–liquid extraction protocol.