The role of biocatalysis in the asymmetric synthesis of alkaloids

Multigram-scale chemoenzymatic synthesis of diverse aminopolycarboxylic acids as potential metallo-β-lactamase inhibitors

Toxin A, a precursor to naturally occurring aspergillomarasmine A, aspergillomarasmine B, lycomarasmine and related aminopolycarboxylic acids, was synthesized as the desired (2S,2'S)-diastereomer on a multigram-scale (>99% conversion, 82% isolated yield, dr > 95 : 5) from commercially available starting materials using the enzyme ethylenediamine-N,N'-disuccinic acid lyase. A single-step protection route of this chiral synthon was developed to aid N-sulfonylation/-alkylation and reductive amination at the terminal primary amine for easy derivatization, followed by global deprotection to give the corresponding toxin A derivatives, including lycomarasmine, in moderate to good yields (23-66%) and with high stereopurity (dr > 95 : 5). Furthermore, a chemoenzymatic route was developed to introduce a click handle on toxin A (yield 72%, dr > 95 : 5) and its cyclized congener for further analogue design. Finally, a chemoenzymatic route towards the synthesis of photocaged aspergillomarasmine B (yield 8%, dr > 95 : 5) was established, prompting further steps into smart prodrug design and precision delivery. These new synthetic methodologies have the prospective of facilitating research into the finding of more selective and potent metallo-β-lactamase (MBL) inhibitors, which are urgently needed to combat MBL-based infections.

Concise synthesis of (R)-reticuline and (+)-salutaridine by combining early-stage organic synthesis and late-stage biocatalysis

Efficient access to the morphinan scaffold remains a major challenge in both synthetic chemistry and biotechnology. Here, a biomimetic chemo-enzymatic strategy to synthesize the natural promorphinan intermediate (+)-salutaridine is demonstrated. By combining early-stage organic synthesis with enzymatic asymmetric key step transformations, the prochiral natural intermediate 1,2-dehydroreticuline was prepared and subsequently stereoselectively reduced by the enzyme 1,2-dehydroreticuline reductase obtaining (R)-reticuline in high ee and yield (>99% ee, up to quant. conversion, 92% isol. yield). In the final step, membrane-bound salutaridine synthase was used to perform the selective ortho-para phenol coupling to give (+)-salutaridine. The synthetic route shows the potential of combining early-stage advanced organic chemistry to minimize protecting group techniques with late-stage multi-step biocatalysis to provide an unprecedented access to the medicinally important compound class of promorphinans.

Uncovering the Mechanism of Azepino-Indole Skeleton Formation via Pictet-Spengler Reaction by Strictosidine Synthase: A Quantum Chemical Investigation

Strictosidine synthase (STR) catalyzes the Pictet-Spengler (PS) reaction of tryptamine and secologanin to produce strictosidine. Recent studies demonstrated that the enzyme can also catalyze the reaction of non-natural substrates to form new alkaloid skeletons. For example, the PS condensation of 1H-indole-4-ethanamine with secologanin could be promoted by the STR from Rauvolfia serpentina (RsSTR) to generate a rare class of skeletons with a seven-membered ring, namely azepino-[3,4,5-cd]-indoles, which are precursors for the synthesis of new compounds displaying antimalarial activity. In the present study, the detailed reaction mechanism of RsSTR-catalyzed formation of the rare seven-membered azepino-indole skeleton through the PS reaction was revealed at the atomic level by quantum chemical calculations. The structures of the transition states and intermediates involved in the reaction pathway were optimized, and the energetics of the complete reaction were analyzed. Based on our calculation results, the most likely pathway of the enzyme-catalyzed reaction was determined, and the rate-determining step of the reaction was clarified. The mechanistic details obtained in the present study are important in understanding the promiscuous activity of RsSTR in the formation of the rare azepino-indole skeleton molecule and are also helpful in designing STR enzymes for the synthesis of other new alkaloid skeleton molecules.

Status of research on natural protein tyrosine phosphatase 1B inhibitors as potential antidiabetic agents: Update

Protein tyrosine phosphatase 1B (PTP1B) is a crucial therapeutic target for multiple human diseases comprising type 2 diabetes (T2DM) and obesity because it is a seminal part of a negative regulator in both insulin and leptin signaling pathways. PTP1B inhibitors increase insulin receptor sensitivity and have the ability to cure insulin resistance-related diseases. However, the few PTP1B inhibitors that entered the clinic (Ertiprotafib, ISIS-113715, Trodusquemine, and JTT-551) were discontinued due to side effects or low selectivity. Molecules with broad chemical diversity extracted from natural products have been reported to be potent PTP1B inhibitors with few side effects. This article summarizes the recent PTP1B inhibitors extracted from natural products, clarifying the current research progress, and providing new options for designing new and effective PTP1B inhibitors.

One-Pot Biocatalytic Synthesis of Primary, Secondary, and Tertiary Amines with Two Stereocenters from α,β-Unsaturated Ketones Using Alkyl-Ammonium Formate

The efficient asymmetric catalytic synthesis of amines containing more than one stereogenic center is a current challenge. Here, we present a biocatalytic cascade that combines ene-reductases (EReds) with imine reductases/reductive aminases (IReds/RedAms) to enable the conversion of α,β-unsaturated ketones into primary, secondary, and tertiary amines containing two stereogenic centers in very high chemical purity (up to >99%), a diastereomeric ratio, and an enantiomeric ratio (up to >99.8:<0.2). Compared with previously reported strategies, our strategy could synthesize two, three, or even all four of the possible stereoisomers of the amine products while precluding the formation of side-products. Furthermore, ammonium or alkylammonium formate buffer could be used as the only additional reagent since it acted both as an amine donor and as a source of reducing equivalents. This was achieved through the implementation of an NADP-dependent formate dehydrogenase (FDH) for the in situ recycling of the NADPH coenzyme, thus leading to increased atom economy for this biocatalytic transformation. Finally, this dual-enzyme ERed/IRed cascade also exhibits a complementarity with the recently reported EneIRED enzymes for the synthesis of cyclic six-membered ring amines. The ERed/IRed method yielded trans-1,2 and cis-1,3 substituted cyclohexylamines in high optical purities, whereas the EneIRED method was reported to yield one cis-1,2 and one trans-1,3 enantiomer. As a proof of concept, when 3-methylcyclohex-2-en-1-one was converted into secondary and tertiary chiral amines with different amine donors, we could obtain all the four possible stereoisomer products. This result exemplifies the versatility of this method and its potential for future wider utilization in asymmetric synthesis by expanding the toolbox of currently available dehydrogenases via enzyme engineering and discovery.

Recent Advances in the Synthesis of Marine-Derived Alkaloids via Enzymatic Reactions

Alkaloids are a large and structurally diverse group of marine-derived natural products. Most marine-derived alkaloids are biologically active and show promising applications in modern (agro)chemical, pharmaceutical, and fine chemical industries. Different approaches have been established to access these marine-derived alkaloids. Among these employed methods, biotechnological approaches, namely, (chemo)enzymatic synthesis, have significant potential for playing a central role in alkaloid production on an industrial scale. In this review, we discuss research progress on marine-derived alkaloid synthesis via enzymatic reactions and note the advantages and disadvantages of their applications for industrial production, as well as green chemistry for marine natural product research.

Chemoenzymatic and Protecting-Group-Free Synthesis of 1,4-Substituted 1,2,3-Triazole-α-d-glucosides with Potent Inhibitory Activity toward Lysosomal α-Glucosidase

α-Glucosyl triazoles have rarely been tested as α-glucosidase inhibitors, partly due to inefficient synthesis of their precursor α-d-glucosylazide (αGA1). Glycosynthase enzymes, made by nucleophile mutations of retaining β-glucosidases, produce αGA1 in chemical rescue experiments. Thermoanaerobacterium xylanolyticus glucosyl hydrolase 116 β-glucosidase (TxGH116) E441G nucleophile mutant catalyzed synthesis of αGA1 from sodium azide and pNP-β-d-glucoside (pNPGlc) or cellobiose in aqueous medium at 45 °C. The pNPGlc and azide reaction product was purified by Sephadex LH-20 column chromatography to yield 280 mg of pure αGA1 (68% yield). αGA1 was successfully conjugated with alkynes attached to different functional groups, including aryl, ether, amine, amide, ester, alcohol, and flavone via copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry reactions. These reactions afforded the 1,4-substituted 1,2,3-triazole-α-d-glucoside derivatives AGT2-14 without protection and deprotection. Several of these glucosyl triazoles exhibited strong inhibition of human lysosomal α-glucosidase, with IC50 values for AGT4 and AGT14 more than 60-fold lower than that of the commercial α-glucosidase inhibitor acarbose.

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.

Marine Alkaloid Pityriacitrin and Its Analogues: Discovery, Structures, Synthetic Methods and Biological Properties

Pityriacitrin is a natural marine alkaloid with a typical β-carboline scaffold, and which has been demonstrated to exhibit diverse biological functions. The special structural features for pityriacitrin lead to the increasing research interest and the emergence of versatile derivatives, and many pityriacitrin analogues have been isolated or synthesized over the past decades. The structural diversity and evolved biological activity of these natural alkaloids can offer opportunities for the development of highly potential novel drugs with a new mechanism of action, and therefore, the aim of this brief review is to describe the discovery, synthesis, and biological properties of natural pityriacitrin and its derivatives, as well as the isolation source.

High Regio- and Stereoselective Multi-enzymatic Synthesis of All Phenylpropanolamine Stereoisomers from β-Methylstyrene

We present a one-pot cascade for the synthesis of phenylpropanolamines (PPAs) in high optical purities (er and dr up to >99.5 %) and analytical yields (up to 95 %) by using 1-phenylpropane-1,2-diols as key intermediates. This bioamination entails the combination of an alcohol dehydrogenase (ADH), an ω-transaminase (ωTA) and an alanine dehydrogenase to create a redox-neutral network, which harnesses the exquisite and complementary regio- and stereo-selectivities of the selected ADHs and ωTAs. The requisite 1-phenylpropane-1,2-diol intermediates were obtained from trans- or cis-β-methylstyrene by combining a styrene monooxygenase with epoxide hydrolases. Furthermore, in selected cases, the envisioned cascade enabled to obtain the structural isomer (1S,2R)-1-amino-1-phenylpropan-2-ol in high optical purity (er and dr >99.5 %). This is the first report on an enzymatic method that enables to obtain all of the four possible PPA stereoisomers in great enantio- and diastereo-selectivity.

Enzymatic Kinetic Resolution by Addition of Oxygen

Kinetic resolution using biocatalysis has proven to be an excellent complementary technique to traditional asymmetric catalysis for the production of enantioenriched compounds. Resolution using oxidative enzymes produces valuable oxygenated structures for use in synthetic route development. This Minireview focuses on enzymes which catalyse the insertion of an oxygen atom into the substrate and, in so doing, can achieve oxidative kinetic resolution. The Baeyer-Villiger rearrangement, epoxidation, and hydroxylation are included, and biological advancements in enzyme development, and applications of these key enantioenriched intermediates in natural product synthesis are discussed.

Lipase-Catalyzed Kinetic Resolution of Alcohols as Intermediates for the Synthesis of Heart Rate Reducing Agent Ivabradine

Ivabradine (Corlanor®), is a chiral benzocycloalkane currently employed and commercialized for the treatment of chronic stable angina pectoris and for the reduction in sinus tachycardia. The eutomer (S)-ivabradine is usually produced via chiral resolution of intermediates, by employing enantiopure auxiliary molecules or through preparative chiral HPLC separations. Recently, more sustainable biocatalytic approaches have been reported in literature for the preparation of the chiral amine precursor. In this work, we report on a novel biocatalyzed pathway, via a resolution study of a key alcohol intermediate used as a precursor of the chiral amine. After screening several enzymatic reaction conditions, employing different lipases and esterases both for the esterification and hydrolysis reactions, the best result was achieved with Pseudomonas cepacia Lipase and the final product was obtained in up to 96:4 enantiomeric ratio (e.r.) of an ivabradine alcohol precursor. This enantiomer was then efficiently converted into the desired amine in a facile three step synthetic sequence.

Retracted Article: The synthesis and biological activity of marine alkaloid derivatives and analogues

The ocean is the origin of life, with a unique ecological environment, which has given birth to a wealth of marine organisms. The ocean is an important source of biological resources and tens of thousands of monomeric compounds have been separated from marine organisms using modern separation technology. Most of these monomeric compounds have some kind of biological activity that has attracted extensive attention from researchers. Marine alkaloids are a kind of compound that can be separated from marine organisms. They have complex and special chemical structures, but at the same time, they can show diversity in biological activities. The biological activities of marine alkaloids mainly manifest in the form of anti-tumor, anti-fungus, anti-viral, anti-malaria, and anti-osteoporosis properties. Many marine alkaloids have good medicinal prospects and can possibly be used as anti-tumor, anti-viral, and anti-fungal clinical drugs or as lead compounds. The limited amounts of marine alkaloids that can be obtained by separation, coupled with the high cytotoxicity and low selectivity of these lead compounds, has restricted the clinical research and industrial development of marine alkaloids. Marine alkaloid derivatives and analogues have been obtained via rational drug design and chemical synthesis, to make up for the shortcomings of marine alkaloids; this has become an urgent subject for research and development. This work systematically reviews the recent developments relating to marine alkaloid derivatives and analogues in the field of medical chemistry over the last 10 years (2010-2019). We divide marine alkaloid derivatives and analogues into five types from the point-of-view of biological activity and elaborated on these activities. We also briefly discuss the optimization process, chemical synthesis, biological activity evaluation, and structure-activity relationship (SAR) of each of these compounds. The abundant SAR data provides reasonable approaches for the design and development of new biologically active marine alkaloid derivatives and analogues.

The Pictet-Spengler Reaction Updates Its Habits

The Pictet-Spengler reaction (P-S) is one of the most direct, efficient, and variable synthetic method for the construction of privileged pharmacophores such as tetrahydro-isoquinolines (THIQs), tetrahydro-β-carbolines (THBCs), and polyheterocyclic frameworks. In the lustro (five-year period) following its centenary birthday, the P-S reaction did not exit the stage but it came up again on limelight with new features. This review focuses on the interesting results achieved in this period (2011-2015), analyzing the versatility of this reaction. Classic P-S was reported in the total synthesis of complex alkaloids, in combination with chiral catalysts as well as for the generation of libraries of compounds in medicinal chemistry. The P-S has been used also in tandem reactions, with the sequences including ring closing metathesis, isomerization, Michael addition, and Gold- or Brønsted acid-catalyzed N-acyliminium cyclization. Moreover, the combination of P-S reaction with Ugi multicomponent reaction has been exploited for the construction of highly complex polycyclic architectures in few steps and high yields. The P-S reaction has also been successfully employed in solid-phase synthesis, affording products with different structures, including peptidomimetics, synthetic heterocycles, and natural compounds. Finally, the enzymatic version of P-S has been reported for biosynthesis, biotransformations, and bioconjugations.

Integrating biocatalysis and multicomponent reactions

While often multicomponent reactions (MCR) are used for the diversity-oriented synthesis of racemic (or achiral) molecular entities, this short review describes two alternative approaches for accessing enantiopure products exploiting the power of biocatalysis. Enzymes or microorganisms may be used for preparing enantiopure MCR inputs or for resolving racemic (or achiral) MCR adducts.

Structure and Biocatalytic Scope of Coclaurine N-Methyltransferase

Benzylisoquinoline alkaloids (BIAs) are a structurally diverse family of plant secondary metabolites, which have been exploited to develop analgesics, antibiotics, antitumor agents, and other therapeutic agents. Biosynthesis of BIAs proceeds via a common pathway from tyrosine to (S)-reticulene at which point the pathway diverges. Coclaurine N-methyltransferase (CNMT) is a key enzyme in the pathway to (S)-reticulene, installing the N-methyl substituent that is essential for the bioactivity of many BIAs. In this paper, we describe the first crystal structure of CNMT which, along with mutagenesis studies, defines the enzymes active site architecture. The specificity of CNMT was also explored with a range of natural and synthetic substrates as well as co-factor analogues. Knowledge from this study could be used to generate improved CNMT variants required to produce BIAs or synthetic derivatives.

Chemoenzymatic Approaches to the Synthesis of the Calcimimetic Agent Cinacalcet Employing Transaminases and Ketoreductases

Several chemoenzymatic routes have been explored for the preparation of cinacalcet, a calcimimetic agent. Transaminases (TAs) and ketoreductases (KREDs) turned out to be useful biocatalysts for the preparation of key optically active precursors. Thus, the asymmetric amination of 1-acetonaphthone yielded an enantiopure (R)-amine, which can be alkylated in one step to yield cinacalcet. Alternatively, the bioreduction of the same ketone resulted in an enantiopure (S)-alcohol, which was easily converted into the previous (R)-amine. In addition, the reduction was efficiently performed with the KRED and its cofactor co-immobilized on the same porous surface. This self-sufficient heterogeneous biocatalyst presented an accumulated total turnover number (TTN) for the cofactor of 675 after 5 consecutive operational cycles. Finally, in a preparative scale synthesis the TA-based approach was performed in aqueous medium and led to enantiopure cinacalcet in two steps and 50% overall yield.

One-pot chemoenzymatic synthesis of trolline and tetrahydroisoquinoline analogues

Chemoenzymatic reaction cascades can provide access to chiral compounds from low-cost starting materials in one pot. Here we describe one-pot asymmetric routes to tetrahydroisoquinoline alkaloids (THIAs) using the Pictet-Spenglerase norcoclaurine synthase (NCS) followed by a cyclisation, to give alkaloids with two new heterocyclic rings. These reactions operated with a high atom economy to generate THIAs in high yields.

Protein tyrosine phosphatase 1B inhibitors from natural sources

Since PTP1B enzyme was discovered in 1988, it has captured the research community's attention. This landmark discovery has stimulated numerous research studies on a variety of human diseases, including cancer, inflammation, and diabetes. Tremendous progress has been made in finding PTP1B inhibitors and exploring PTP1B regulatory mechanisms. This review investigates for the natural PTP1B inhibitors, and focuses on the common characteristics of the discovered structures and structure-activity relationships. To facilitate understanding, all the natural compounds are here divided into five different classes (fatty acids, phenolics, terpenoids, steroids, and alkaloids), according to their skeletons. These PTP1B inhibitors of scaffold structures could serve as a theoretical basis for new concept drug discovery and design.

Stereoselective strategies for the construction of polysubstituted piperidinic compounds and their applications in natural products’ synthesis

An abridged and far-reaching review communication on the construction of the polysubstituted piperidinic core using diverse methodologies for the benefit of organic chemists interested in the total synthesis of biologically active compounds.

Deciphering Piperidine Formation in Polyketide-Derived Indolizidines Reveals a Thioester Reduction, Transamination, and Unusual Imine Reduction Process

Piperidine and indolizidine are two basic units of alkaloids that are frequently observed in natural and synthetic compounds. Their biosynthesis in natural products is highly conserved and mostly derived from the incorporation of lysine cyclization products. Through in vitro reconstitution, we herein identified a novel pathway involving a group of polyketide-derived indolizidines, which comprises the processes of tandem two-electron thioester reduction, transamination, and imine reduction to convert acyl carrier protein (ACP)-tethered polyketide chains into the piperidine moieties of their indolizidine scaffolds. The enzymes that catalyze the imine reduction are distinct from previous known imine reductases, which have a fold of acyl-CoA dehydrogenase but do not require flavin for reduction. Our results not only provide a new way for the biosynthesis of the basic units of alkaloids but also show a novel class of imine reductases that may benefit the fields of biocatalysis and biomanufacturing.

Asymmetric Synthesis of Isoquinoline Alkaloids: 2004-2015

In the past decade, the asymmetric synthesis of chiral nonracemic isoquinoline alkaloids, a family of natural products showing a wide range of structural diversity and biological and pharmaceutical activity, has been based either on continuation or improvement of known traditional methods or on new, recently developed, strategies. Both diastereoselective and enantioselective catalytic methods have been applied. This review describes the stereochemically modified traditional syntheses (the Pictet-Spengler, the Bischler-Napieralski, and the Pomeranz-Fritsch-Bobbitt) along with strategies based on closing of the nitrogen-containing ring B of the isoquinoline core by the formation of bonds between C₁-N₂, N₂-C₃, C₁-N₂/N₂-C₃, and C₁-N₂/C₄-C_4a atoms. Methods involving introduction of substituents at the C1 carbon of isoquinoline core along with syntheses applying various biocatalytic techniques have also been reviewed.

High-performance enzymatic membrane bioreactor based on a radial gradient of pores in a PSF membrane via facile enzyme immobilization

Hollow fiber polysulfone membranes with perfect radial gradient distributed pores were selected as substrates, and subsequently the enzyme-immobilization process was achieved by filtration and crosslinking, to finally construct an enhanced EMBR.

Dicarboxylic esters: Useful tools for the biocatalyzed synthesis of hybrid compounds and polymers

Dicarboxylic acids and their derivatives (esters and anhydrides) have been used as acylating agents in lipase-catalyzed reactions in organic solvents. The synthetic outcomes have been dimeric or hybrid derivatives of bioactive natural compounds as well as functionalized polyesters.

Reactivation of lipases by the unfolding and refolding of covalently immobilized biocatalysts

Lipases immobilized on octyl–glyoxyl agarose may be partially reactivated by unfolding/refolding strategies.

A new concept for production of (3S,4R)-6-[(benzyloxycarbonyl)amino]-5,6-dideoxyhex-2-ulose, a precursor of d-fagomine

A novel cascade reaction combining three enzymes in one pot for the production of aldol adduct (3S,4R)-6-[(benzyloxycarbonyl)amino]-5,6-dideoxyhex-2-ulose was studied and 79% yield on aldol adduct was achieved in the batch reactor.

Unraveling the coptisine–ctDNA binding mechanism by multispectroscopic, electrochemical and molecular docking methods

This study provides evidences of coptisine–DNA intercalation, which may help to develop new efficient, safe probes for the fluorometric detection of DNA instead of traditional toxic and carcinogenic probes.

Deracemization by simultaneous bio-oxidative kinetic resolution and stereoinversion

Deracemization, that is, the transformation of a racemate into a single product enantiomer with theoretically 100% conversion and 100% ee, is an appealing but also challenging option for asymmetric synthesis. Herein a novel chemo-enzymatic deracemization concept by a cascade is described: the pathway involves two enantioselective oxidation steps and one non-stereoselective reduction step, enabling stereoinversion and a simultaneous kinetic resolution. The concept was exemplified for the transformation of rac-benzylisoquinolines to optically pure (S)-berbines. The racemic substrates were transformed to optically pure products (ee>97%) with up to 98% conversion and up to 88% yield of isolated product.

Deracemisation of benzylisoquinoline alkaloids employing monoamine oxidase variants

Deracemisation of benzylisoquinoline alkaloids was performed employing a recently developed variant of monoamine oxidase from Aspergillus niger (MAO-N variant D11).