Biocatalysis in Drug Development—Highlights of the Recent Patent Literature

Redox Out of the Box: Catalytic Versatility Across NAD(P)H-Dependent Oxidoreductases

The asymmetric reduction of double bonds using NAD(P)H-dependent oxidoreductases has proven to be an efficient tool for the synthesis of important chiral molecules in research and on industrial scale. These enzymes are commercially available in screening kits for the reduction of C=O (ketones), C=C (activated alkenes), or C=N bonds (imines). Recent reports, however, indicate that the ability to accommodate multiple reductase activities on distinct C=X bonds occurs in different enzyme classes, either natively or after mutagenesis. This challenges the common perception of highly selective oxidoreductases for one type of electrophilic substrate. Consideration of this underexplored potential in enzyme screenings and protein engineering campaigns may contribute to the identification of complementary biocatalytic processes for the synthesis of chiral compounds. This review will contribute to a global understanding of the promiscuous behavior of NAD(P)H-dependent oxidoreductases on C=X bond reduction and inspire future discoveries with respect to unconventional biocatalytic routes in asymmetric synthesis.

Effective engineering of a ketoreductase for the biocatalytic synthesis of an ipatasertib precursor

Semi-rational enzyme engineering is a powerful method to develop industrial biocatalysts. Profiting from advances in molecular biology and bioinformatics, semi-rational approaches can effectively accelerate enzyme engineering campaigns. Here, we present the optimization of a ketoreductase from Sporidiobolus salmonicolor for the chemo-enzymatic synthesis of ipatasertib, a potent protein kinase B inhibitor. Harnessing the power of mutational scanning and structure-guided rational design, we created a 10-amino acid substituted variant exhibiting a 64-fold higher apparent kcat and improved robustness under process conditions compared to the wild-type enzyme. In addition, the benefit of algorithm-aided enzyme engineering was studied to derive correlations in protein sequence-function data, and it was found that the applied Gaussian processes allowed us to reduce enzyme library size. The final scalable and high performing biocatalytic process yielded the alcohol intermediate with ≥ 98% conversion and a diastereomeric excess of 99.7% (R,R-trans) from 100 g L-1 ketone after 30 h. Modelling and kinetic studies shed light on the mechanistic factors governing the improved reaction outcome, with mutations T134V, A238K, M242W and Q245S exerting the most beneficial effect on reduction activity towards the target ketone.

Recent Developments and Applications of Biocatalytic and Chemoenzymatic Synthesis for the Generation of Diverse Classes of Drugs

Biocatalytic and chemoenzymatic biosynthesis are powerful methods of organic chemistry that use enzymes to execute selective reactions and allow the efficient production of organic compounds. The advantages of these approaches include high selectivity, mild reaction conditions, and the ability to work with complex substrates. The utilization of chemoenzymatic techniques for the synthesis of complicated compounds has lately increased dramatically in the area of organic chemistry. Biocatalytic technologies and modern synthetic methods are utilized synergistically in a multi-step approach to a target molecule under this paradigm. Chemoenzymatic techniques are promising for simplifying access to essential bioactive compounds because of the remarkable regio- and stereoselectivity of enzymatic transformations and the reaction diversity of modern organic chemistry. Enzyme kits may include ready-to-use, reproducible biocatalysts. Its use opens up new avenues for the synthesis of active therapeutic compounds and aids in drug development by synthesizing active components to construct scaffolds in a targeted and preparative manner. This study summarizes current breakthroughs as well as notable instances of biocatalytic and chemoenzymatic synthesis. To assist organic chemists in the use of enzymes for synthetic applications, it also provides some basic guidelines for selecting the most appropriate enzyme for a targeted reaction while keeping aspects like cofactor requirement, solvent tolerance, use of whole cell or isolated enzymes, and commercial availability in mind.

From nature to industry: Harnessing enzymes for biocatalysis

Biocatalysis harnesses enzymes to make valuable products. This green technology is used in countless applications from bench scale to industrial production and allows practitioners to access complex organic molecules, often with fewer synthetic steps and reduced waste. The last decade has seen an explosion in the development of experimental and computational tools to tailor enzymatic properties, equipping enzyme engineers with the ability to create biocatalysts that perform reactions not present in nature. By using (chemo)-enzymatic synthesis routes or orchestrating intricate enzyme cascades, scientists can synthesize elaborate targets ranging from DNA and complex pharmaceuticals to starch made in vitro from CO2-derived methanol. In addition, new chemistries have emerged through the combination of biocatalysis with transition metal catalysis, photocatalysis, and electrocatalysis. This review highlights recent key developments, identifies current limitations, and provides a future prospect for this rapidly developing technology.

Stairway to Stereoisomers: Engineering Short- and Medium-Chain Ketoreductases To Produce Chiral Alcohols

The short- and medium-chain dehydrogenase/reductase superfamilies are responsible for most chiral alcohol production in laboratories and industries. In nature, they participate in diverse roles such as detoxification, housekeeping, secondary metabolite production, and catalysis of several chemicals with commercial and environmental significance. As a result, they are used in industries to create biopolymers, active pharmaceutical intermediates (APIs), and are also used as components of modular enzymes like polyketide synthases for fabricating bioactive molecules. Consequently, random, semi-rational and rational engineering have helped transform these enzymes into product-oriented efficient catalysts. The rise of newer synthetic chemicals and their enantiopure counterparts has proved challenging, and engineering them has been the subject of numerous studies. However, they are frequently limited to the synthesis of a single chiral alcohol. The study attempts to defragment and describe hotspots of engineering short- and medium-chain dehydrogenases/reductases for the production of chiral synthons.

Evolving New Chemistry: Biocatalysis for the Synthesis of Amine-Containing Pharmaceuticals

Biocatalysis has become an attractive tool in modern synthetic chemistry both in academic and industrial settings, offering access to enantiopure molecules. In industry, biocatalysis found use in small molecule pharmaceutical development. For several amine-containing drugs, biotransformations were applied in the process routes, improving the original syntheses employing classical chemical methods. This review illustrates how and why biocatalysis has been applied to create safer, more efficient and less costly processes for the manufacture of chiral amine-containing pharmaceuticals and alkaloids. Several enzyme classes have been applied to syntheses of natural products, pharmaceutical products and their intermediates, including transaminases, imine reductases, monoamine oxidases and Pictet-Spenglerases. The routes with and without application of biocatalysis are compared, and the potential of these enzyme classes in redesigned synthetic routes to natural products, alkaloids and high-value chemicals is evaluated, using syntheses of sitagliptin, suvorexant, PF-04449913, MK-7246, vernakalant, GSK-2879552, boceprevir and (−)-strictosidine as examples. Application of biocatalysis in the synthesis of amine-containing pharmaceuticals constitutes a greener alternative to transition metal-catalysed routes, facilitates installation of chiral amine functionalities at a late stage of the synthesis and provides exquisite stereocontrol. Opportunities and challenges of biocatalysis for the synthesis of chiral amines are reviewed with respect to use in drug discovery and development.

Metal–Organic Cages: Applications in Organic Reactions

Supramolecular metal–organic cages, a class of molecular containers formed via coordination-driven self-assembly, have attracted sustained attention for their applications in catalysis, due to their structural aesthetics and unique properties. Their inherent confined cavity is considered to be analogous to the binding pocket of enzymes, and the facile tunability of building blocks offers a diverse platform for enzyme mimics to promote organic reactions. This minireview covers the recent progress of supramolecular metal–organic coordination cages for boosting organic reactions as reaction vessels or catalysts. The developments in the utilizations of the metal–organic cages for accelerating the organic reactions, improving the selectivity of the reactions are summarized. In addition, recent developments and successes in tandem or cascade reactions promoted by supramolecular metal–organic cages are discussed.

Green Chemistry in the Synthesis of Pharmaceuticals

The principles of green chemistry (GC) can be comprehensively implemented in green synthesis of pharmaceuticals by choosing no solvents or green solvents (preferably water), alternative reaction media, and consideration of one-pot synthesis, multicomponent reactions (MCRs), continuous processing, and process intensification approaches for atom economy and final waste reduction. The GC's execution in green synthesis can be performed using a holistic design of the active pharmaceutical ingredient's (API) life cycle, minimizing hazards and pollution, and capitalizing the resource efficiency in the synthesis technique. Thus, the presented review accounts for the comprehensive exploration of GC's principles and metrics, an appropriate implication of those ideas in each step of the reaction schemes, from raw material to an intermediate to the final product's synthesis, and the final execution of the synthesis into scalable industry-based production. For real-life examples, we have discussed the synthesis of a series of established generic pharmaceuticals, starting with the raw materials, and the intermediates of the corresponding pharmaceuticals. Researchers and industries have thoughtfully instigated a green synthesis process to control the atom economy and waste reduction to protect the environment. We have extensively discussed significant reactions relevant for green synthesis, one-pot cascade synthesis, MCRs, continuous processing, and process intensification, which may contribute to the future of green and sustainable synthesis of APIs.

Shortening Synthetic Routes to Small Molecule Active Pharmaceutical Ingredients Employing Biocatalytic Methods

Biocatalysis, using enzymes for organic synthesis, has emerged as powerful tool for the synthesis of active pharmaceutical ingredients (APIs). The first industrial biocatalytic processes launched in the first half of the last century exploited whole-cell microorganisms where the specific enzyme at work was not known. In the meantime, novel molecular biology methods, such as efficient gene sequencing and synthesis, triggered breakthroughs in directed evolution for the rapid development of process-stable enzymes with broad substrate scope and good selectivities tailored for specific substrates. To date, enzymes are employed to enable shorter, more efficient, and more sustainable alternative routes toward (established) small molecule APIs, and are additionally used to perform standard reactions in API synthesis more efficiently. Herein, large-scale synthetic routes containing biocatalytic key steps toward >130 APIs of approved drugs and drug candidates are compared with the corresponding chemical protocols (if available) regarding the steps, reaction conditions, and scale. The review is structured according to the functional group formed in the reaction.

A Tailor-Made Deazaflavin-Mediated Recycling System for Artificial Nicotinamide Cofactor Biomimetics

Nicotinamide adenine dinucleotide (NAD) and its 2'-phosphorylated form NADP are crucial cofactors for a large array of biocatalytically important redox enzymes. Their high cost and relatively poor stability, however, make them less attractive electron mediators for industrial processes. Nicotinamide cofactor biomimetics (NCBs) are easily synthesized, are inexpensive, and are also generally more stable than their natural counterparts. A bottleneck for the application of these artificial hydride carriers is the lack of efficient cofactor recycling methods. Therefore, we engineered the thermostable F420:NADPH oxidoreductase from Thermobifida fusca (Tfu-FNO), by structure-inspired site-directed mutagenesis, to accommodate the unnatural N1 substituents of eight NCBs. The extraordinarily low redox potential of the natural cofactor F420H2 was then exploited to reduce these NCBs. Wild-type enzyme had detectable activity toward all selected NCBs, with K m values in the millimolar range and k cat values ranging from 0.09 to 1.4 min-1. Saturation mutagenesis at positions Gly-29 and Pro-89 resulted in mutants with up to 139 times higher catalytic efficiencies. Mutant G29W showed a k cat value of 4.2 s-1 toward 1-benzyl-3-acetylpyridine (BAP+), which is similar to the k cat value for the natural substrate NADP+. The best Tfu-FNO variants for a specific NCB were then used for the recycling of catalytic amounts of these nicotinamides in conversion experiments with the thermostable ene-reductase from Thermus scotoductus (TsOYE). We were able to fully convert 10 mM ketoisophorone with BAP+ within 16 h, using F420 or its artificial biomimetic FOP (FO-2'-phosphate) as an efficient electron mediator and glucose-6-phosphate as an electron donor. The generated toolbox of thermostable and NCB-dependent Tfu-FNO variants offers powerful cofactor regeneration biocatalysts for the reduction of several artificial nicotinamide biomimetics at both ambient and high temperatures. In fact, to our knowledge, this enzymatic method seems to be the best-performing NCB-recycling system for BNAH and BAPH thus far.

Continuous flow asymmetric synthesis of chiral active pharmaceutical ingredients and their advanced intermediates

Catalytic enantioselective transformations provide well-established and direct access to stereogenic synthons that are broadly distributed among active pharmaceutical ingredients (APIs). These reactions have been demonstrated to benefit considerably from the merits of continuous processing and microreactor technology. Over the past few years, continuous flow enantioselective catalysis has grown into a mature field and has found diverse applications in asymmetric synthesis of pharmaceutically active substances. The present review therefore surveys flow chemistry-based approaches for the synthesis of chiral APIs and their advanced stereogenic intermediates, covering the utilization of biocatalysis, organometallic catalysis and metal-free organocatalysis to introduce asymmetry in continuously operated systems. Single-step processes, interrupted multistep flow syntheses, combined batch/flow processes and uninterrupted one-flow syntheses are discussed herein.

Enzymatic strategies for asymmetric synthesis

Enzymes, at the turn of the 21st century, are gaining a momentum. Especially in the field of synthetic organic chemistry, a broad variety of biocatalysts are being applied in an increasing number of processes running at up to industrial scale. In addition to the advantages of employing enzymes under environmentally friendly reaction conditions, synthetic chemists are recognizing the value of enzymes connected to the exquisite selectivity of these natural (or engineered) catalysts. The use of hydrolases in enantioselective protocols paved the way to the application of enzymes in asymmetric synthesis, in particular in the context of biocatalytic (dynamic) kinetic resolutions. After two decades of impressive development, the field is now mature to propose a panel of catalytically diverse enzymes for (i) stereoselective reactions with prochiral compounds, such as double bond reduction and bond forming reactions, (ii) formal enantioselective replacement of one of two enantiotopic groups of prochiral substrates, as well as (iii) atroposelective reactions with noncentrally chiral compounds. In this review, the major enzymatic strategies broadly applicable in the asymmetric synthesis of optically pure chiral compounds are presented, with a focus on the reactions developed within the past decade.

Efficient synthesis of bepotastine and cloperastine intermediates using engineered alcohol dehydrogenase with a hydrophobic pocket

(S)-4-Chlorophenylpyridylmethanol and (R)-4-chlorobenzhydrol are key pharmaceutical intermediates for the synthesis of bepotastine and cloperastine, respectively. However, the biocatalytic approach to prepare these bulky diaryl ketones remains challenging because of the low activity of naturally occurring alcohol dehydrogenases (ADH). In the present study, ADH seq5, which has an adequate binding pocket volume and accepts bulky diaryl ketones, was further engineered with a binding pocket of increased hydrophobicity. Based on molecular simulation and binding free energy analyses, a small mutation library was constructed, and mutant seq5-D150I with a threefold increase in k_cat and a low K_m was obtained successfully. The comparison of kinetic parameters, binding free energy, docking conformation, and critical catalytic distances calculated by molecular dynamic simulations revealed the source of increased activity. To develop a practical approach with seq5-D150I, reaction conditions including pH, temperature, buffer, and metal ions were optimised and applied to synthesise (S)-4-chlorophenylpyridylmethanol and (R)-4-chlorobenzhydrol with high enantiomeric excess. The space-time yields for (S)-4-chlorophenylpyridylmethanol and (R)-4-chlorobenzhydrol increased dramatically to as high as 263.4 g∙L^-1 day^-1 and 150 g∙L^-1 day^-1, respectively, which, to our knowledge, is the highest reported yield to date. These results show that the biocatalytic approach with seq5-D150I may be practical for future industrial applications.Key points An alcohol dehydrogenase was engineered based on binding free energy analysis. The mutant seq5-D150I obtained a threefold increase in k_cat and a low K_m. Two important pharmaceutical intermediates were obtained with high space-time yield.

State-of-the-Art Biocatalysis

The use of enzyme-mediated reactions has transcended ancient food production to the laboratory synthesis of complex molecules. This evolution has been accelerated by developments in sequencing and DNA synthesis technology, bioinformatic and protein engineering tools, and the increasingly interdisciplinary nature of scientific research. Biocatalysis has become an indispensable tool applied in academic and industrial spheres, enabling synthetic strategies that leverage the exquisite selectivity of enzymes to access target molecules. In this Outlook, we outline the technological advances that have led to the field's current state. Integration of biocatalysis into mainstream synthetic chemistry hinges on increased access to well-characterized enzymes and the permeation of biocatalysis into retrosynthetic logic. Ultimately, we anticipate that biocatalysis is poised to enable the synthesis of increasingly complex molecules at new levels of efficiency and throughput.

Chemoselective and Site-Selective Reductions Catalyzed by a Supramolecular Host and a Pyridine-Borane Cofactor

Supramolecular catalysts emulate the mechanism of enzymes to achieve large rate accelerations and precise selectivity under mild and aqueous conditions. While significant strides have been made in the supramolecular host-promoted synthesis of small molecules, applications of this reactivity to chemoselective and site-selective modification of complex biomolecules remain virtually unexplored. We report here a supramolecular system where coencapsulation of pyridine-borane with a variety of molecules including enones, ketones, aldehydes, oximes, hydrazones, and imines effects efficient reductions under basic aqueous conditions. Upon subjecting unprotected lysine to the host-mediated reductive amination conditions, we observed excellent ε-selectivity, indicating that differential guest binding within the same molecule is possible without sacrificing reactivity. Inspired by the post-translational modification of complex biomolecules by enzymatic systems, we then applied this supramolecular reaction to the site-selective labeling of a single lysine residue in an 11-amino acid peptide chain and human insulin.

Biocatalysis: Enzymatic Synthesis for Industrial Applications

Biocatalysis has found numerous applications in various fields as an alternative to chemical catalysis. The use of enzymes in organic synthesis, especially to make chiral compounds for pharmaceuticals as well for the flavors and fragrance industry, are the most prominent examples. In addition, biocatalysts are used on a large scale to make specialty and even bulk chemicals. This review intends to give illustrative examples in this field with a special focus on scalable chemical production using enzymes. It also discusses the opportunities and limitations of enzymatic syntheses using distinct examples and provides an outlook on emerging enzyme classes.

Biocatalytic routes to anti-viral agents and their synthetic intermediates

An assessment of biocatalytic strategies for the synthesis of anti-viral agents, offering guidelines for the development of sustainable production methods for a future COVID-19 remedy.

Stereo-selective synthesis of non-canonical γ-hydroxy-α-amino acids by enzymatic carbon–carbon bond formation

A feasible and sustainable approach for stereo-selective synthesis of non-canonical γ-hydroxy-α-amino acids from l-aspartic acid and different aldehydes has been developed.

Biocatalytic ketone reductions using Biobeads for miniaturized high throughput experimentation

Miniaturized reactions conducted in parallel can lead to increased productivity in laboratories without depleting high value reagents.

Synthesis and Applications of Cinchona Squaramide-Modified Poly(Glycidyl Methacrylate) Microspheres as Recyclable Polymer-Grafted Enantioselective Organocatalysts

This work presents the immobilization of cinchona squaramide organocatalysts on poly(glycidyl methacrylate) solid supports. Preparation of the well-defined monodisperse polymer microspheres was facilitated by comprehensive parameter optimization. By exploiting the reactive epoxy groups of the polymer support, three amino-functionalized cinchona derivatives were immobilized on this carrier. To explore the effect of the amino linker, these structurally varied precatalysts were synthesized by modifying the cinchona skeleton at different positions. The catalytic activities of the immobilized organocatalysts were tested in the Michael addition of pentane-2,4-dione and trans-β-nitrostyrene with excellent yields (up to 98 %) and enantioselectivities (up to 96 % ee). Finally, the catalysts were easily recovered five times by centrifugation without loss of activity.

Treating a Global Health Crisis with a Dose of Synthetic Chemistry

The SARS-CoV-2 pandemic has prompted scientists from many disciplines to work collaboratively toward an effective response. As academic synthetic chemists, we examine how best to contribute to this ongoing effort.

A Glimpse into Green Chemistry Practices in the Pharmaceutical Industry

In this Minireview, the importance and implementation of green chemistry practices in the pharmaceutical industry are illustrated. With notable examples, some of the most important industrial organic transformations are discussed along with their applications in the synthesis of drug molecules. A brief comparison between traditional unsustainable methods and modern green methods is made to shed light on the economic and environmental benefits of greener methods. Finally, green chemistry practices in the pharmaceutical industries of India and China are also discussed.

One Pot Use of Combilipases for Full Modification of Oils and Fats: Multifunctional and Heterogeneous Substrates

Lipases are among the most utilized enzymes in biocatalysis. In many instances, the main reason for their use is their high specificity or selectivity. However, when full modification of a multifunctional and heterogeneous substrate is pursued, enzyme selectivity and specificity become a problem. This is the case of hydrolysis of oils and fats to produce free fatty acids or their alcoholysis to produce biodiesel, which can be considered cascade reactions. In these cases, to the original heterogeneity of the substrate, the presence of intermediate products, such as diglycerides or monoglycerides, can be an additional drawback. Using these heterogeneous substrates, enzyme specificity can promote that some substrates (initial substrates or intermediate products) may not be recognized as such (in the worst case scenario they may be acting as inhibitors) by the enzyme, causing yields and reaction rates to drop. To solve this situation, a mixture of lipases with different specificity, selectivity and differently affected by the reaction conditions can offer much better results than the use of a single lipase exhibiting a very high initial activity or even the best global reaction course. This mixture of lipases from different sources has been called “combilipases” and is becoming increasingly popular. They include the use of liquid lipase formulations or immobilized lipases. In some instances, the lipases have been coimmobilized. Some discussion is offered regarding the problems that this coimmobilization may give rise to, and some strategies to solve some of these problems are proposed. The use of combilipases in the future may be extended to other processes and enzymes.

The Hitchhiker's guide to biocatalysis: recent advances in the use of enzymes in organic synthesis

Enzymes are excellent catalysts that are increasingly being used in industry and academia. This perspective is primarily aimed at synthetic organic chemists with limited experience using enzymes and provides a general and practical guide to enzymes and their synthetic potential, with particular focus on recent applications.

Combined Photoredox/Enzymatic C-H Benzylic Hydroxylations

Chemical transformations that install heteroatoms into C-H bonds are of significant interest because they streamline the construction of value-added small molecules. Direct C-H oxyfunctionalization, or the one step conversion of a C-H bond to a C-O bond, could be a highly enabling transformation due to the prevalence of the resulting enantioenriched alcohols in pharmaceuticals and natural products,. Here we report a single-flask photoredox/enzymatic process for direct C-H hydroxylation that proceeds with broad reactivity, chemoselectivity and enantioselectivity. This unified strategy advances general photoredox and enzymatic catalysis synergy and enables chemoenzymatic processes for powerful and selective oxidative transformations.

Biocatalysis in Medicinal Chemistry: Challenges to Access and Drivers for Adoption

The use of biocatalysis in the manufacture of small molecule active pharmaceutical ingredients has seen a marked increase over the past decade. Driven by academic and industrial interest in the application of enzymes as catalysts for transforming chemical routes, the biocatalytic toolbox available to a chemist has continued to expand. Despite this, the application of biocatalysis in early discovery chemistry has trailed in comparison to its use in manufacturing routes. The authors offer their perspective on the adoption of biocatalysis in the early discovery space: highlighting challenges including enzyme supply and the biocatalysis business model, as well as recent trends that could spur more collaboration and access to enzymes for early discovery R&D activities.

Biocatalysis as Useful Tool in Asymmetric Synthesis: An Assessment of Recently Granted Patents (2014–2019)

The broad interdisciplinary nature of biocatalysis fosters innovation, as different technical fields are interconnected and synergized. A way to depict that innovation is by conducting a survey on patent activities. This paper analyses the intellectual property activities of the last five years (2014–2019) with a specific focus on biocatalysis applied to asymmetric synthesis. Furthermore, to reflect the inventive and innovative steps, only patents that were granted during that period are considered. Patent searches using several keywords (e.g., enzyme names) have been conducted by using several patent engine servers (e.g., Espacenet, SciFinder, Google Patents), with focus on granted patents during the period 2014–2019. Around 200 granted patents have been identified, covering all enzyme types. The inventive pattern focuses on the protection of novel protein sequences, as well as on new substrates. In some other cases, combined processes, multi-step enzymatic reactions, as well as process conditions are the innovative basis. Both industries and academic groups are active in patenting. As a conclusion of this survey, we can assert that biocatalysis is increasingly recognized as a useful tool for asymmetric synthesis and being considered as an innovative option to build IP and protect synthetic routes.

Biocatalysis and Pharmaceuticals: A Smart Tool for Sustainable Development

Biocatalysis is the term used to describe the application of any type of biocatalyst (enzymes, as isolated preparations of wild-type or genetically modified variants, or whole cells, either as native cells or as recombinant expressed proteins inside host cells) in a given synthetic schedule [...]

Comparison of Cinchona Catalysts Containing Ethyl or Vinyl or Ethynyl Group at Their Quinuclidine Ring

Numerous cinchona organocatalysts with different substituents at their quinuclidine unit have been described and tested, but the effect of those saturation has not been examined before. This work presents the synthesis of four widely used cinchona-based organocatalyst classes (hydroxy, amino, squaramide, and thiourea) with different saturation on the quinuclidine unit (ethyl, vinyl, ethynyl) started from quinine, the most easily available cinchona derivative. Big differences were found in basicity of the quinuclidine unit by measuring the pKa values of twelve catalysts in six solvents. The effect of differences was examined by testing the catalysts in Michael addition reaction of pentane-2,4-dione to trans-β-nitrostyrene. The 1.6-1.7 pKa deviation in basicity of the quinuclidine unit did not result in significant differences in yields and enantiomeric excesses. Quantum chemical calculations confirmed that the ethyl, ethynyl, and vinyl substituents affect the acid-base properties of the cinchona-thiourea catalysts only slightly, and the most active neutral thione forms are the most stable tautomers in all cases. Due to the fact that cinchonas with differently saturated quinuclidine substituents have similar catalytic activity in asymmetric Michael addition application of quinine-based catalysts is recommended. Its vinyl group allows further modifications, for instance, recycling the catalyst by immobilization.

Nitroalkanes as Versatile Nucleophiles for Enzymatic Synthesis of Noncanonical Amino Acids

C-C bond-forming reactions often require nucleophilic carbon species rarely compatible with aqueous reaction media, thus restricting their appearance in biocatalysis. Here we report the use of nitroalkanes as a structurally versatile class of nucleophilic substrates for C-C bond formation catalyzed by variants of the β-subunit of tryptophan synthase (TrpB). The enzymes accept a wide range of nitroalkanes to form noncanonical amino acids, here the nitro group can serve as a handle for further modification. Using nitroalkane nucleophiles greatly expands the scope of compounds made by TrpB variants and establishes nitroalkanes as a valuable substrate class for biocatalytic C-C bond formation.

Structure-guided engineering of ChKRED20 from Chryseobacterium sp. CA49 for asymmetric reduction of aryl ketoesters

ChKRED20 is a robust NADH-dependent ketoreductase identified from the genome of Chryseobacterium sp. CA49 that can use 2-propanol as the ultimate reducing agent. The wild-type can reduce over 100 g/l ketones for some pharmaceutical relevant substrates, exhibiting a remarkable potential for industrial application. In this work, to overcome the limitation of ChKRED20 to aryl ketoesters, we first refined the X-ray crystal structure of ChKRED20/NAD⁺ complex at a resolution of 1.6 Å, and then performed three rounds of iterative saturation mutagenesis at critical amino acid sites to reshape the active cavity of the enzyme. For methyl 2-oxo-2-phenylacetate and ethyl 3-oxo-3-phenylpropanoate, several gain-of-activity mutants were achieved, and for ethyl 2-oxo-4-phenylbutanoate, improved mutants were achieved with k_cat/K_m increasing to 196-fold of the wild-type. All three substrates were completely reduced at 100 g/l loading catalyzed with selected ChKRED20 mutants, and deliver the corresponding chiral alcohols with >90% isolated yield and 97 - >99%ee.