Biocatalysis has become an important aspect of modern organic synthesis, both in academia and across the chemical and pharmaceutical industries. Its success has been largely due to a rapid expansion of the range of chemical reactions accessible, made possible by advanced tools for enzyme discovery coupled with high-throughput laboratory evolution techniques for biocatalyst optimization. A wide range of tailor-made enzymes with high efficiencies and selectivities can now be produced quickly and on a gram to kilogram scale, with dedicated databases and search tools aimed at making these biocatalysts accessible to a broader scientific community. This Primer discusses the current state-of-the-art methodology in the field, including route design, enzyme discovery, protein engineering and the implementation of biocatalysis in industry. We highlight recent advances, such as de novo design and directed evolution, and discuss parameters that make a good reproducible biocatalytic process for industry. The general concepts will be illustrated by recent examples of applications in academia and industry, including the development of multistep enzyme cascades.

Reductive amination of carbonyl compounds constitutes one of the most efficient ways to rapidly construct chiral and achiral amine frameworks. Imine reductase (IRED) biocatalysts represent a versatile family of enzymes for amine synthesis through NADPH‐mediated imine reduction. The reductive aminases (RedAms) are a subfamily of IREDs that were recently shown to catalyze imine formation as well as imine reduction. Herein, a diverse library of novel enzymes were expressed and screened as cell‐free lysates for their ability to facilitate reductive amination to expand the known suite of biocatalysts for this transformation and to identify more enzymes with potential industrial applications. A range of ketones and amines were examined, and enzymes were identified that were capable of accepting benzylamine, pyrrolidine, ammonia, and aniline. Amine equivalents as low as 2.5 were employed to afford up to >99 % conversion, and for chiral products, up to >98 % ee could be achieved. Preparative‐scale reactions were conducted with low amine equivalents (1.5 or 2.0) of methylamine, allylamine, and pyrrolidine, achieving up to >99 % conversion and 76 % yield.

A multi‐enzymatic cascade process involving transaminases (TAs) and reductive aminases (RedAms) to produce enantiomerically pure 2,5‐disubstituted pyrrolidine alkaloids from their respective 1,4‐diketones is reported. Several TAs were screened and the best results for diketone monoamination were obtained with an R‐selective TA from Mycobacterium chlorophenicum and with an S‐selective TA from Bacillus megaterium. Pyrroline reduction was best performed by a reductive aminase from Ajellomyces dermatitidis (AdRedAm). Finally, a biocatalytic one‐pot cascade was implemented using the aforementioned enzymes and a variety of 2‐methyl‐5‐alkylpyrrolidines were produced with high (>99 %) conversion, diastereomeric and enantiomeric excess values.

Biocatalysis is a highly valued enabling technology for pharmaceutical research and development as it can unlock synthetic routes to complex chiral motifs with unparalleled selectivity and efficiency. This perspective aims to review recent advances in the pharmaceutical implementation of biocatalysis across early and late-stage development with a focus on the implementation of processes for preparative-scale syntheses.

New drugs introduced to the market are privileged structures that have affinities for biological targets implicated in human diseases and conditions. These new chemical entities (NCEs), particularly small molecules and antibody–drug conjugates (ADCs), provide insight into molecular recognition and simultaneously function as leads for the design of future medicines. This Review is part of a continuing series presenting the most likely process-scale synthetic approaches to 44 new chemical entities approved for the first time anywhere in the world during 2020.

Imine reductases (IREDs) catalyze the asymmetric reduction of cyclic imines, but also in some cases the coupling of ketones and amines to form secondary amine products in an enzymecatalyzed reductive amination (RedAm) reaction. Enzymatic RedAm reactions have typically used small hydrophobic amines, but many interesting pharmaceutical targets require that larger amines be used in these coupling reactions. Following the identification of IR77 from Ensifer adhaerens as a promising biocatalyst for the reductive amination of cyclohexanone with pyrrolidine, we have characterized the ability of this enzyme to catalyze couplings with larger bicyclic amines such as isoindoline and octahydrocyclopenta(c)pyrrole. By comparing the activity of IR77 with reductions using sodium cyanoborohydride in water, it was shown that, while the coupling of cyclohexanone and pyrrolidine involved at least some element of reductive amination, the amination with the larger amines likely occurred ex situ, with the imine recruited from solution for enzyme reduction. The structure of IR77 was determined, and using this as a basis, structureguided mutagenesis, coupled with point mutations selecting improving amino acid sites suggested by other groups, permitted the identification of a mutant A208N with improved activity for amine product formation. Improvements in conversion were attributed to greater enzyme stability as revealed by X-ray crystallography and nano differential scanning fluorimetry. The mutant IR77-A208N was applied to the preparative scale amination of cyclohexanone at 50 mM concentration, with 1.2 equiv of three larger amines, in isolated yields of up to 93%.

Biocatalytic retrosynthetic analysis of dibenz-[c,e]azepines has highlighted the use of imine reductase (IRED) and w-transaminase (w-TA) biocatalysts to establish the key stereocentres of these molecules.S everal enantiocomplementary IREDs were identified for the synthesis of (R)-and (S)-5-methyl-6,7-dihydro-5H-dibenz[c,e]azepine with excellent enantioselectivity,b yr eduction of the parent imines. Crystallographic evidence suggests that IREDs may be able to bind one conformer of the imine substrate such that, upon reduction, the major product conformer is generated directly. w-TAb iocatalysts were also successfully employed for the production of enantiopure 1-(2-bromophenyl)ethan-1-amine, thus enabling an orthogonal route for the installation of chirality into dibenz[c,e]azepine framework.

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