Enantioselective Synthesis of 1-Aryl-Substituted Tetrahydroisoquinolines Employing Imine Reductase

Engineered Imine Reductase Catalyzed Enantiodivergent Synthesis of Alkylated Amphetamines

Less steric ketones exhibited low stereoselectivity toward M5 due to their difficulty in restricting the free rotation of the imine intermediate. An engineered enantio-complementary imine reductase from M5 was obtained with catalytic activity. We identified four key residues that play essential roles in controlling stereoselectivity. Two mutants, I149Y-W234L (up to 99%S ee) and L200M-F260M (up to 99%R ee), were achieved, showing excellent stereoselectivity toward the tested substrates, offering valuable biocatalysts for synthesizing alkylated amphetamines.

Biocatalytic Synthesis of Ruxolitinib Intermediate via Engineered Imine Reductase

An enzyme catalyzed strategy for the synthesis of a chiral hydrazine from 3-cyclopentyl-3-oxopropanenitrile 5 and hydrazine hydrate 2 is presented. An imine reductase (IRED) from Streptosporangium roseum was identified to catalyze the reaction between 3-cyclopentyl-3-oxopropanenitrile 5 and hydrazine hydrate 2 to produce trace amounts of (R)-3-cyclopentyl-3-hydrazineylpropanenitrile 4. We employed a 2-fold approach to optimize the catalytic performance of this enzyme. First, a transition state analogue (TSA) model was constructed to illuminate the enzyme-substrate interactions. Subsequently, the Enzyme_design and Funclib methods were utilized to predict mutants for experimental evaluation. Through three rounds of site-directed mutagenesis, site saturation mutagenesis, and combinatorial mutagenesis, we obtained mutant M6 with a yield of 98% and an enantiomeric excess (ee) of 99%. This study presents an effective method for constructing a hydrazine derivative via IRED-catalyzed reductive amination of ketone and hydrazine. Furthermore, it provides a general approach for constructing suitable enzymes, starting from nonreactive enzymes and gradually enhancing their catalytic activity through active site modifications.

Engineered Imine Reductase for Asymmetric Synthesis of Dextromethorphan Key Intermediate

(S)-1-(4-Methoxybenzyl)-1,2,3,4,5,6,7,8-octahydroisoquinoline ((S)-1-(4-methoxybenzyl)-OHIQ) is the key intermediate of the nonopioid antitussive dextromethorphan. In this study, (S)-IR61-V69Y/P123A/W179G/F182I/L212V (M4) was identified with a 766-fold improvement in catalytic efficiency compared with wide-type IR61 through enzyme engineering. M4 could completely convert 200 mM of 1-(4-methoxybenzyl)-3,4,5,6,7,8-hexahydroisoquinoline into (S)-1-(4-methoxybenzyl)-OHIQ in 77% isolated yield, with >99% enantiomeric excess and a high space-time yield of 542 g L-1 day-1, demonstrating a great potential for the synthesis of dextromethorphan intermediate in industrial applications.

An imine reductase that captures reactive intermediates in the biosynthesis of the indolocarbazole reductasporine

Imine reductases (IREDs) and reductive aminases have been used in the synthesis of chiral amine products for drug manufacturing; however, little is known about their biological contexts. Here we employ structural studies and site-directed mutagenesis to interrogate the mechanism of the IRED RedE from the biosynthetic pathway to the indolocarbazole natural product reductasporine. Cocrystal structures with the substrate-mimic arcyriaflavin A reveal an extended active site cleft capable of binding two indolocarbazole molecules. Site-directed mutagenesis of a conserved aspartate in the primary binding site reveals a new role for this residue in anchoring the substrate above the NADPH cofactor. Variants targeting the secondary binding site greatly reduce catalytic efficiency, while accumulating oxidized side-products. As indolocarbazole biosynthetic intermediates are susceptible to spontaneous oxidation, we propose the secondary site acts to protect against autooxidation, and the primary site drives catalysis through precise substrate orientation and desolvation effects. The structure of RedE with its extended active site can be the starting point as a new scaffold for engineering IREDs and reductive aminases to intercept large substrates relevant to industrial applications.

Enzymatic Oxy- and Amino-Functionalization in Biocatalytic Cascade Synthesis: Recent Advances and Future Perspectives

Biocatalytic cascades are a powerful tool for building complex molecules containing oxygen and nitrogen functionalities. Moreover, the combination of multiple enzymes in one pot offers the possibility to minimize downstream processing and waste production. In this review, we illustrate various recent efforts in the development of multi-step syntheses involving C-O and C-N bond-forming enzymes to produce high value-added compounds, such as pharmaceuticals and polymer precursors. Both in vitro and in vivo examples are discussed, revealing the respective advantages and drawbacks. The use of engineered enzymes to boost the cascades outcome is also addressed and current co-substrate and cofactor recycling strategies are presented, highlighting the importance of atom economy. Finally, tools to overcome current challenges for multi-enzymatic oxy- and amino-functionalization reactions are discussed, including flow systems with immobilized biocatalysts and cascades in confined nanomaterials.

Chirality Sensing of N-Heterocycles via 19F NMR

Methods to rapidly detect and differentiate chiral N-heterocyclic compounds become increasingly important owing to the widespread application of N-heterocycles in drug discovery and materials science. We herein report a ¹⁹F NMR-based chemosensing approach for the prompt enantioanalysis of various N-heterocycles, where the dynamic binding between the analytes and a chiral ¹⁹F-labeled palladium probe create characteristic ¹⁹F NMR signals assignable to each enantiomer. The open binding site of the probe allows the effective recognition of bulky analytes that are otherwise difficult to detect. The chirality center distal to the binding site is found sufficient for the probe to discriminate the stereoconfiguration of the analyte. The utility of the method in the screening of reaction conditions for the asymmetric synthesis of lansoprazole is demonstrated.

Introduction of chirality at C1 position of 1-substituted-3,4-dihydroisoquinoline by its enantioselective reduction: synthesis of chiral 1-substituted-1,2,3,4-tetrahydroisoquinoline - a review

There is a wide range of biological activities associated with C1 chiral carbon containing 1-substituted-1,2,3,4-tetrahydroisoquinolines (1-substituted-THIQs) which constitute the isoquinoline alkaloids, a large group of natural products. This work summarizes several novel catalytic stereoselective approaches to enantioselectively reduce the 1-substituted-3,4-dihydroisoquinolines (1-substituted-DHIQs) to produce the desired 1-substituted-THIQs. The 1-substituted-DHIQs were prepared by using the Bischler-Napieralski reaction. The enantioselective reduction of 1-substituted-DHIQs was accomplished by using chiral hydride reducing agents, by hydrogenation with a chiral catalyst, by enantioselective reduction of DHIQs possessing a chiral auxiliary at the imine nitrogen by achiral metallic hydride reducing agents, or by enzymatic catalysis. Among these methods, much more work was carried out on the hydrogenation of 1-substituted-DHIQs in the presence of a chiral catalyst. This review summarizes articles and advancements on this topic from 1972 to 2023.

Engineering a norcoclaurine synthase for one-step synthesis of (S)-1-aryl-tetrahydroisoquinolines

Tetrahydroisoquinoline alkaloids (THIQAs) are ubiquitous compounds with important pharmaceutical and biological activity. Their key N-heterocyclic structural motifs are synthesised via Pictet-Spengler (P-S) reaction by norcoclaurine synthases (NCS) in plants. The synthesis of 1-aryl-tetrahydroisoquinoline alkaloids has attracted increasing attention due to their antitumor and antivirus activities. Herein, the L68T/M97V mutant of NCS from Thalictrum flavum with improved activity was developed by semi-rational design. This mutant not only showed higher catalytic performance (> 96% conversion) toward benzaldehyde and dopamine over the wild-type enzyme, but also catalysed the P-S reaction of the bulky substrate 4-biphenylaldehyde and dopamine with high conversion (> 99%) for the effective synthesis of 1-aryl-THIQA. In terms of stereoselectivity, all products synthesised by the L68T/M97V mutant showed high optical purity (92-99% enantiomeric excess).

The Discovery of Imine Reductases and their Utilisation for the Synthesis of Tetrahydroisoquinolines

Imine reductases (IREDs) are NADPH-dependent enzymes with significant biocatalytic potential for the synthesis of primary, secondary, and tertiary chiral amines. Their applications include the reduction of cyclic imines and the reductive amination of prochiral ketones. In this study, twenty-nine novel IREDs were revealed through genome mining. Imine reductase activities were screened at pH 7 and 9 and in presence of either NADPH or NADH; some IREDs showed good activities at both pHs and were able to accept both cofactors. IREDs with Asn and Glu at the key 187 residue showed preference for NADH. IREDs were also screened against a series of dihydroisoquinolines to synthesise tetrahydroisoquinolines (THIQs), bioactive alkaloids with a wide range of therapeutic properties. Selected IREDs showed high stereoselectivity, as well high THIQ yields (>90 %) when coupled to a glucose-6-phosphate dehydrogenase for NADPH cofactor recycling.

Asymmetric Synthesis of Fused-Ring Tetrahydroisoquinolines and Tetrahydro-β-carbolines from 2-Arylethylamines via a Chemoenzymatic Approach

While chiral fused-ring tetrahydroisoquinoline (THIQ) and tetrahydro-β-carboline (THβC) scaffolds have attracted considerable interest due to their wide spectrum of biological activities, the synthesis of optically pure chiral fused-ring THIQs and THβCs remains a challenging task. Herein, a group of active imine reductases were identified to convert the imine precursors into the corresponding enantiocomplementary fused-ring THIQs and THβCs with high enantioselectivity and conversion, establishing an efficient and green chemoenzymatic approach to fused-ring alkaloids from 2-arylethylamines.

Multistep enzyme cascades as a route towards green and sustainable pharmaceutical syntheses

Enzyme cascades are a powerful technology to develop environmentally friendly and cost-effective synthetic processes to manufacture drugs, as they couple different biotransformations in sequential reactions to synthesize the product. These biocatalytic tools can address two key parameters for the pharmaceutical industry: an improved selectivity of synthetic reactions and a reduction of potential hazards by using biocompatible catalysts, which can be produced from sustainable sources, which are biodegradable and, generally, non-toxic. Here we outline a broad variety of enzyme cascades used either in vivo (whole cells) or in vitro (purified enzymes) to specifically target pharmaceutically relevant molecules, from simple building blocks to complex drugs. We also discuss the advantages and requirements of multistep enzyme cascades and their combination with chemical catalysts through a series of reported examples. Finally, we examine the efficiency of enzyme cascades and how they can be further improved by enzyme engineering, process intensification in flow reactors and/or enzyme immobilization to meet all the industrial requirements.

Computational Study of Mechanism and Enantioselectivity of Imine Reductase from Amycolatopsis orientalis

Imine reductases (IREDs) are NADPH-dependent enzymes (NADPH=nicotinamide adenine dinucleotide phosphate) that catalyze the reduction of imines to amines. They exhibit high enantioselectivity for a broad range of substrates, making them of interest for biocatalytic applications. In this work, we have employed density functional theory (DFT) calculations to elucidate the reaction mechanism and the origins of enantioselectivity of IRED from Amycolatopsis orientalis. Two substrates are considered, namely 1-methyl-3,4-dihydroisoquinoline and 2-propyl-piperideine. A model of the active site is built on the basis of the available crystal structure. For both substrates, different binding modes are first evaluated, followed by calculation of the hydride transfer transition states from each complex. We have also investigated the effect of mutations of certain important active site residues (Tyr179Ala and Asn241Ala) on the enantioselectivity. The calculated energies are consistent with the experimental observations and the analysis of transition states geometries provides insights into the origins of enantioselectivity of this enzyme.

Inverting the Stereoselectivity of an NADH-Dependent Imine-Reductase Variant

Imine reductases (IREDs) offer biocatalytic routes to chiral amines and have a natural preference for the NADPH cofactor. In previous work, we reported enzyme engineering of the (R)-selective IRED from Myxococcus stipitatus (NADH-IRED-Ms) yielding a NADH-dependent variant with high catalytic efficiency. However, no IRED with NADH specificity and (S)-selectivity in asymmetric reductions has yet been reported. Herein, we applied semi-rational enzyme engineering to switch the selectivity of NADH-IRED-Ms. The quintuple variant A241V/H242Y/N243D/V244Y/A245L showed reverse stereopreference in the reduction of the cyclic imine 2-methylpyrroline compared to the wild-type and afforded the (S)-amine product with >99 % conversion and 91 % enantiomeric excess. We also report the crystal-structures of the NADPH-dependent (R)-IRED-Ms wild-type enzyme and the NADH-dependent NADH-IRED-Ms variant and molecular dynamics (MD) simulations to rationalize the inverted stereoselectivity of the quintuple variant.

The role of biocatalysis in the asymmetric synthesis of alkaloids - an update

Alkaloids are a group of natural products with interesting pharmacological properties and a long history of medicinal application. Their complex molecular structures have fascinated chemists for decades, and their total synthesis still poses a considerable challenge. In a previous review, we have illustrated how biocatalysis can make valuable contributions to the asymmetric synthesis of alkaloids. The chemo-enzymatic strategies discussed therein have been further explored and improved in recent years, and advances in amine biocatalysis have vastly expanded the opportunities for incorporating enzymes into synthetic routes towards these important natural products. The present review summarises modern developments in chemo-enzymatic alkaloid synthesis since 2013, in which the biocatalytic transformations continue to take an increasingly 'central' role.

Recent trends in biocatalysis

Biocatalysis has undergone revolutionary progress in the past century. Benefited by the integration of multidisciplinary technologies, natural enzymatic reactions are constantly being explored. Protein engineering gives birth to robust biocatalysts that are widely used in industrial production. These research achievements have gradually constructed a network containing natural enzymatic synthesis pathways and artificially designed enzymatic cascades. Nowadays, the development of artificial intelligence, automation, and ultra-high-throughput technology provides infinite possibilities for the discovery of novel enzymes, enzymatic mechanisms and enzymatic cascades, and gradually complements the lack of remaining key steps in the pathway design of enzymatic total synthesis. Therefore, the research of biocatalysis is gradually moving towards the era of novel technology integration, intelligent manufacturing and enzymatic total synthesis.

Screening and characterization of a diverse panel of metagenomic imine reductases for biocatalytic reductive amination

Finding faster and simpler ways to screen protein sequence space to enable the identification of new biocatalysts for asymmetric synthesis remains both a challenge and a rate-limiting step in enzyme discovery. Biocatalytic strategies for the synthesis of chiral amines are increasingly attractive and include enzymatic asymmetric reductive amination, which offers an efficient route to many of these high-value compounds. Here we report the discovery of over 300 new imine reductases and the production of a large (384 enzymes) and sequence-diverse panel of imine reductases available for screening. We also report the development of a facile high-throughput screen to interrogate their activity. Through this approach we identified imine reductase biocatalysts capable of accepting structurally demanding ketones and amines, which include the preparative synthesis of N-substituted β-amino ester derivatives via a dynamic kinetic resolution process, with excellent yields and stereochemical purities.

Single step syntheses of (1S)-aryl-tetrahydroisoquinolines by norcoclaurine synthases

The 1-aryl-tetrahydroisoquinoline (1-aryl-THIQ) moiety is found in many biologically active molecules. Single enantiomer chemical syntheses are challenging and although some biocatalytic routes have been reported, the substrate scope is limited to certain structural motifs. The enzyme norcoclaurine synthase (NCS), involved in plant alkaloid biosynthesis, has been shown to perform stereoselective Pictet-Spengler reactions between dopamine and several carbonyl substrates. Here, benzaldehydes are explored as substrates and found to be accepted by both wild-type and mutant constructs of NCS. In particular, the variant M97V gives a range of (1 S)-aryl-THIQs in high yields (48-99%) and e.e.s (79-95%). A co-crystallised structure of the M97V variant with an active site reaction intermediate analogue is also obtained with the ligand in a pre-cyclisation conformation, consistent with (1 S)-THIQs formation. Selected THIQs are then used with catechol O-methyltransferases with exceptional regioselectivity. This work demonstrates valuable biocatalytic approaches to a range of (1 S)-THIQs.

Hydrolysis of dihydroisoquinoline derivatives activated by sulfonyl or acyl chloride

A new, efficient, and mild strategy for hydrolysis of 3,4-dihydroisoquinoline imines activated by sulfonyl chloride or acyl chloride has been developed, by which method ketosulfonamides and ketoamides have been synthesized. This process tolerates broad scope with respect to both the sulfonyl chloride and acyl chloride with moderate to excellent yields. This protocol features a broad substrate scope for various kinds of 3,4-dihydroisoquinoline and mild reaction conditions without using strong acidic or basic conditions. These features show that this user-friendly and simple system could be applied in the future to the synthesis of a broader range of amino benzophenones.

Evolution of Glucose Dehydrogenase for Cofactor Regeneration in Bioredox Processes with Denaturing Agents

Glucose dehydrogenase (GDH) is a general tool for driving nicotinamide (NAD(P)H) regeneration in synthetic biochemistry. An increasing number of synthetic bioreactions are carried out in media containing high amounts of organic cosolvents or hydrophobic substrates/products, which often denature native enzymes, including those for cofactor regeneration. In this work, we attempted to improve the chemical stability of Bacillus megaterium GDH (BmGDH_M0 ) in the presence of large amounts of 1-phenylethanol by directed evolution. Among the resulting mutants, BmGDH_M6 (Q252L/E170K/S100P/K166R/V72I/K137R) exhibited a 9.2-fold increase in tolerance against 10 % (v/v) 1-phenylethanol. Moreover, BmGDH_M6 was also more stable than BmGDH_M0 when exposed to hydrophobic and enzyme-inactivating compounds such as acetophenone, ethyl 2-oxo-4-phenylbutyrate, and ethyl (R)-2-hydroxy-4-phenylbutyrate. Coupled with a Candida glabrata carbonyl reductase, BmGDH_M6 was successfully used for the asymmetric reduction of deactivating ethyl 2-oxo-4-phenylbutyrate with total turnover number of 1800 for the nicotinamide cofactor, thus making it attractive for commercial application. Overall, the evolution of chemically robust GDH facilitates its wider use as a general tool for NAD(P)H regeneration in biocatalysis.

Biosynthesis of plant tetrahydroisoquinoline alkaloids through an imine reductase route

Herein, we report a biocatalytic approach to synthesize plant tetrahydroisoquinoline alkaloids (THIQAs) from dihydroisoquinoline (DHIQ) precursors using imine reductases and N-methyltransferase (NMT). The imine reductase IR45 was engineered to significantly expand its substrate specificity, enabling efficient and stereoselective conversion of 1-phenyl and 1-benzyl 6,7-dimethoxy-DHIQs into the corresponding (S)-tetrahydroisoquinolines (S-THIQs). Coclaurine N-methyltransferase (CNMT) was able to further efficiently convert these (S)-THIQ intermediates into (S)-THIQAs. By assembling IRED, CNMT, and glucose dehydrogenase (GDH) in one reaction, we effectively constituted two artificial biosynthetic pathways in Escherichia coli and successfully applied them to the production of five (S)-THIQAs. This highly efficient (100% yield from DHIQs) and easily tailorable (adding other genes) biosynthetic approach will be useful for producing a variety of plant THIQAs.

A Method for Bischler-Napieralski-Type Synthesis of 3,4-Dihydroisoquinolines

A new method for the Bischler-Napieralski-type synthesis of 3,4-dihydroisoquinolines was developed by a Tf₂O-promoted tandem annulation from phenylethanols and nitriles. Its success was mainly due to the fact that a phenonium ion was formed in the process and practically functioned as a stable and reactive primary phenylethyl carbocation.

Sequence-Based In-silico Discovery, Characterisation, and Biocatalytic Application of a Set of Imine Reductases

Imine reductases (IREDs) have recently become a primary focus of research in biocatalysis, complementing other classes of amine-forming enzymes such as transaminases and amine dehydrogenases. Following in the footsteps of other research groups, we have established a set of IRED biocatalysts by sequence-based in silico enzyme discovery. In this study, we present basic characterisation data for these novel IREDs and explore their activity and stereoselectivity using a panel of structurally diverse cyclic imines as substrates. Specific activities of >1 U/mg and excellent stereoselectivities (ee>99 %) were observed in many cases, and the enzymes proved surprisingly tolerant towards elevated substrate loadings. Co-expression of the IREDs with an alcohol dehydrogenase for cofactor regeneration led to whole-cell biocatalysts capable of efficiently reducing imines at 100 mM initial concentration with no need for the addition of extracellular nicotinamide cofactor. Preparative biotransformations on gram scale using these 'designer cells' afforded chiral amines in good yield and excellent optical purity.

Approach to Chiral 1-Substituted Isoquinolone and 3-Substituted Isoindolin-1-one by Addition-Cyclization Process

An approach to access 1-substituted isoquinolones has been developed through the addition-cyclization of imines with Grignard reagents in the presence of 2,2'-dipyridyl. A number of substituted aromatic magnesium reagents were amenable to this process, and the desired products were obtained with excellent yields and outstanding diastereoselectivities ( dr > 99:1). The utility of this convenient approach is demonstrated by the formal synthesis of ( S)-cryptostyline II. Moreover, N-methylmorpholine (NMM) was found to be an effective additive for the formation of 3-substituted isoindolin-1-ones using one-pot addition-cyclization-deprotection of imine with Grignard reagents.

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A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as tryptorubin A isolated from a Streptomyces species.