Chemoenzymatic Total Synthesis of Deoxy-, epi-, and Podophyllotoxin and a Biocatalytic Kinetic Resolution of Dibenzylbutyrolactones

Fe(II) and 2-oxoglutarate-dependent dioxygenases for natural product synthesis: molecular insights into reaction diversity

Covering: up to 2024Fe(II) and 2-oxoglutarate-dependent dioxygenases (Fe/2OG DOs) are a superfamily of enzymes that play important roles in a variety of catalytic reactions, including hydroxylation, ring formation, ring reconstruction, desaturation, and demethylation. Each member of this family has similarities in their overall structure, but they have varying specific differences, making Fe/2OG DOs attractive for catalytic diversity. With the advancement of current research, more Fe/2OG DOs have been discovered, and their catalytic scope has been further broadened; however, apart from hydroxylation, many reaction mechanisms have not been accurately demonstrated, and there is a lack of a systematic understanding of their molecular basis. Recently, an increasing number of X-ray structures of Fe/2OG DOs have provided new insights into the structural basis of their function and substrate-binding properties. This structural information is essential for understanding catalytic mechanisms and mining potential catalytic reactions. In this review, we summarize most of the Fe/2OG DOs whose structures have been resolved in recent years, focus on their structural features, and explore the relationships between various structural elements and unique catalytic mechanisms and their associated reaction type classification.

Concise and Modular Chemoenzymatic Total Synthesis of Bisbenzylisoquinoline Alkaloids

The bisbenzylisoquinoline alkaloids (bisBIAs) have attracted tremendous attention from the synthetic community due to their diverse and intriguing biological activities. Herein, we report the convergent and modular chemoenzymatic syntheses of eight bisBIAs bearing various substitutes and linkages in 5-7 steps. The gram-scale synthesis of various well-designed enantiopure benzylisoquinoline monomers was accomplished through an enzymatic stereoselective Pictet-Spengler reaction, followed by regioselective enzymatic methylation or chemical functionalization in a sequential one-pot process. A modified intermolecular copper-mediated Ullmann coupling enabled the concise and efficient total synthesis of five different linear bisBIAs with either head-to-tail or tail-to-tail linkage. A biomimetic oxidative phenol dimerization selectively formed the sterically hindered, electron-rich diaryl ether bond, and subsequent intramolecular Suzuki-Miyaura domino reaction or Ullmann coupling facilitated the first enantioselective total synthesis of three macrocyclic bisBIAs, including ent-isogranjine, tetrandrine and O-methylrepandine. This study highlights the great potential of chemoenzymatic strategies in the total synthesis of diverse bisBIAs and paves the way to further explore the biological functions of these natural products.

Genomic characterization and identification of candidate genes for putative podophyllotoxin biosynthesis pathway in Penicillium herquei HGN12.1C

Previous studies have reported the functional role, biochemical features and synthesis pathway of podophyllotoxin (PTOX) in plants. In this study, we employed combined morphological and molecular techniques to identify an endophytic fungus and extract PTOX derivatives. Based on the analysis of ITS sequences and the phylogenetic tree, the isolate was classified as Penicillium herquei HGN12.1C, with a sequence identity of 98.58%. Morphologically, the HGN12.1C strain exhibits white colonies, short-branched mycelia and densely packed hyphae. Using PacBio sequencing at an average read depth of 195×, we obtained a high-quality genome for the HGN12.1C strain, which is 34.9 Mb in size, containing eight chromosomes, one mitochondrial genome and a GC content of 46.5%. Genome analysis revealed 10 genes potentially involved in PTOX biosynthesis. These genes include VdtD, Pinoresinollariciresinol reductase (PLR), Secoisolariciresinol dehydrogenase (SDH), CYP719A23, CYP71BE54, O-methyltransferase 1 (OMT1), O-methyltransferase 3 (OMT3), 2-ODD, CYP71CU and CYP82D61. Notably, the VdtD gene in fungi shares functional similarities with the DIR gene found in plants. Additionally, we identified peltatin, a PTOX derivative, in the HGN12.1C extract. Docking analysis suggests a potential role for the 2-ODD enzyme in converting yatein to deoxypodophyllotoxin. These findings offer invaluable insights into the synthesis mechanism of PTOX in fungi, shedding light on the relationship between host plants and endophytes.

Unraveling the Mechanism of the Oxidative C-C Bond Coupling Reaction Catalyzed by Deoxypodophyllotoxin Synthase

Deoxypodophyllotoxin synthase (DPS), a nonheme Fe(II)/2-oxoglutarate (2OG)-dependent oxygenase, is a key enzyme that is involved in the construction of the fused-ring system in (-)-podophyllotoxin biosynthesis by catalyzing the C-C coupling reaction. However, the mechanistic details of DPS-catalyzed ring formation remain unclear. Herein, our quantum mechanics/molecular mechanics (QM/MM) calculations reveal a novel mechanism that involves the recycling of CO2 (a product of decarboxylation of 2OG) to prevent the formation of hydroxylated byproducts. Our results show that CO2 can react with the FeIII-OH species to generate an unusual FeIII-bicarbonate species. In this way, hydroxylation is avoided by consuming the OH group. Then, the C-C coupling followed by desaturation yields the final product, deoxypodophyllotoxin. This work highlights the crucial role of the CO2 molecule, generated in the crevice between the iron active site and the substrate, in controlling the reaction selectivity.

Functions and mechanisms of enzymes assembling lignans and norlignans

Lignans and norlignans are distributed throughout the plant kingdom and exhibit diverse chemical structures and biological properties that offer potential for therapeutic use. Originating from the phenylpropanoid biosynthesis pathway, their characteristic carbon architectures are formed through unique enzyme catalysis, featuring regio- and stereoselective C-C bond forming processes. Despite extensive research on these plant natural products, their biosynthetic pathways, and enzyme mechanisms remain enigmatic. This review highlights recent advancements in elucidating the functions and mechanisms of the biosynthetic enzymes responsible for constructing the distinct carbon frameworks of lignans and norlignans.

Computational Study Into the Oxidative Ring-Closure Mechanism During the Biosynthesis of Deoxypodophyllotoxin

The nonheme iron dioxygenase deoxypodophyllotoxin synthase performs an oxidative ring-closure reaction as part of natural product synthesis in plants. How the enzyme enables the oxidative ring-closure reaction of (-)-yatein and avoids substrate hydroxylation remains unknown. To gain insight into the reaction mechanism and understand the details of the pathways leading to products and by-products we performed a comprehensive computational study. The work shows that substrate is bound tightly into the substrate binding pocket with the C7'-H bond closest to the iron(IV)-oxo species. The reaction proceeds through a radical mechanism starting with hydrogen atom abstraction from the C7'-H position followed by ring-closure and a final hydrogen transfer to form iron(II)-water and deoxypodophyllotoxin. Alternative mechanisms including substrate hydroxylation and an electron transfer pathway were explored but found to be higher in energy. The mechanism is guided by electrostatic perturbations of charged residues in the second-coordination sphere that prevent alternative pathways.

Diastereoselective allylation-based asymmetric total synthesis of 1,10-seco-guaianolides

A Cr(II)-mediated Nozaki-Hiyama allylation of aldehydes with functionalized chiral allylbromolactone paved the way to easily access β-hydroxy-aryl/alkyl-α-methylene-γ-butyrolactones in good yields with high diastereoselectivities. A subsequent undemanding translactonization was orchestrated in the efficient first asymmetric total synthesis of two 1,10-seco-guaianolides as a valuable extension of the strategy developed.

Chemistry and Biology of Podophyllotoxins: An Update

Podophyllotoxin is an aryltetralin lignan lactone derived from different plants of Podophyllum. It consists of five rings with four chiral centers, one trans-lactone and one aryl tetrahydronaphthalene skeleton with multiple modification sites. Moreover, podophyllotoxin and its derivatives showed lots of bioactivities, including anticancer, anti-inflammatory, antiviral, and insecticidal properties. The demand for podophyllotoxin and its derivatives is rising as a result of their high efficacy. As a continuation of our previous review (Chem. Eur. J., 2017, 23, 4467-4526), herein, total synthesis, biotransformation, structural modifications, bioactivities, and structure-activity relationships of podophyllotoxin and its derivatives from 2017 to 2022 are summarized. Meanwhile, a piece of update information on the origin of new podophyllotoxin analogues from plants from 2014 to 2022 was compiled. We hope that this review will provide a reference for future high value-added applications of podophyllotoxin and its analogues in the pharmaceutical and agricultural fields.

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.

Improving the Enantioselectivity of CHMOBrevi1 for Asymmetric Synthesis of Podophyllotoxin Precursor

(R)-β-piperonyl-γ-butyrolactones are key building blocks for the synthesis of podophyllotoxin, which have demonstrated remarkable potential in cancer treatment. Baeyer-Villiger monooxygenases (BVMOs)-mediated asymmetric oxidation is a green approach to produce chiral lactones. While several BVMOs were able to oxidize the corresponding cyclobutanone, most BVMOs gave the (S) enantiomer while Cyclohexanone monooxygenase (CHMO) from Brevibacterium sp. HCU1 gave (R) enantiomer, but with a low enantioselectivity (75 % ee). In this study, we use a strategy called "focused rational iterative site-specific mutagenesis" (FRISM) at residues ranging from 6 Å from substrate. The mutations by using a restricted set of rationally chosen amino acids allow the formation of a small mutant library. By generating and screening less than 60 variants, we achieved a high ee of 96.8 %. Coupled with the cofactor regeneration system, 9.3 mM substrate was converted completely in a 100-mL scale reaction. Therefore, our work reveals a promising synthetic method for (R)-β-piperonyl-γ-butyrolactone with the highest enantioselectivity, and provides a new opportunity for the chem-enzymatic synthesis of podophyllotoxin.

Industrial total synthesis of natural medicines

Total synthesis offers a key approach to the production of natural medicines if sufficient quantities cannot be obtained due to low natural abundance or lack of efficient fermentation or semi-synthesis methods. This Viewpoint outlines the previous and current states of research as they apply to the total synthesis of natural medicines on an industrial scale and provides a holistic view on the potential for future developments in the field.

Organocatalytic aldol approach for the protecting group-free asymmetric synthesis of (7R')-parabenzlactone, (-)-hinokinin, (-)-yatein, (-)-bursehernin, (-)-pluviatolide, (+)-isostegane and allied lignans

A short and efficient catalytic asymmetric protection-free synthesis of dibenzylbutyrolactone lignans, such as (-)-hinokinin, (-)-yatein, (-)-bursehernin, (-)-pluviatolide, and their 7'-hydroxylignans - (7'R)-parabenzlactone, (7'R)-hydroxyyatein, (7'R)-hydroxybursehernin, and (7'R)-hydroxy pluviatolide, respectively, is described. The syntheses of (+)-isostegane and the formal synthesis of (-)-podophyllotoxin and bicubebins are also described. Organocatalytic aldol-reduction-lactonization and Pd/C-catalyzed hydrogenative debromination are two-pot sequential reactions for the enantioselective synthesis of hydroxybutyrolactone 13b with excellent diastereo- and enantioselectivity (dr 33 : 1 and >99% ee). The protecting group-free chemoselective α-alkylation of 13b directly led to 7'-hydroxydibenzylbutyrolactone lignans, followed by hydrogenative dehydroxylation, which led to their (deoxy) dibenzylbutyrolactone lignans, and the syntheses were completed in three to five steps from 6-bromopiperonal.

Chemoenzymatic Synthesis of 13-Oxoverruculogen

Verruculogens are rare fumitremorgin alkaloids that contain a highly unusual eight-membered endoperoxide. In this paper, we report a concise chemoenzymatic synthesis of 13-oxoverruculogen using enzymatic C-H peroxidation and rhodium-catalyzed C-C bond activation reactions to install the eight-membered endoperoxide and the pentacyclic core of the natural product, respectively. Our strategy involves the use of 13-epi-fumitremorgin B as a substrate analog for endoperoxidation by verruculogen synthase, FtmOx1. The resulting product, 13-epi-verruculogen, is the first unnatural endoperoxide generated by FtmOx1 and is used in the first synthesis of 13-oxoverruculogen. This strategy enables a 10-step synthesis of this natural product from commercially available starting materials and illustrates a hybrid approach utilizing biocatalytic and transition-metal-catalyzed reactions to access challenging alkaloid architectures. Moreover, this work demonstrates the use of native enzyme promiscuity as a viable strategy for the chemoenzymatic synthesis of natural products.

Non-Native Site-Selective Enzyme Catalysis

The ability to site-selectively modify equivalent functional groups in a molecule has the potential to streamline syntheses and increase product yields by lowering step counts. Enzymes catalyze site-selective transformations throughout primary and secondary metabolism, but leveraging this capability for non-native substrates and reactions requires a detailed understanding of the potential and limitations of enzyme catalysis and how these bounds can be extended by protein engineering. In this review, we discuss representative examples of site-selective enzyme catalysis involving functional group manipulation and C-H bond functionalization. We include illustrative examples of native catalysis, but our focus is on cases involving non-native substrates and reactions often using engineered enzymes. We then discuss the use of these enzymes for chemoenzymatic transformations and target-oriented synthesis and conclude with a survey of tools and techniques that could expand the scope of non-native site-selective enzyme catalysis.

Stereoselective Total Synthesis of Formosanol, Tsugacetal, and Methyl β-Conidendral

The first enantioselective total synthesis of aryltetralin lignan acetals, (-)-formosanol, (+)-tsugacetal, (+)-methyl β-conidendral, and their enantiomers have been accomplished on the basis of the Pd-catalyzed asymmetric allylic cycloaddition as a key step. Six stereoisomers of the lignan acetals have been synthesized via a 7-8 step sequence in up to 14% overall yield. The in vitro cytotoxicity against several cancer cells has preliminarily been examined for the obtained six stereoisomers of lignan acetals.

Sinensiols H–J, three new lignan derivatives from Selaginella sinensis (Desv.) Spring

One new lignan sinensiol H (1) and two new bisnorlignans, sinensiols I and J (2 and 3), along with three known compounds were isolated from the whole plants of Selaginella sinensis. Their structures were elucidated on the basis of 1D and 2D NMR spectroscopy as well as high-resolution mass spectrometry. The absolute configuration of 1 was established by ECD calculation. Compounds 2 and 3 represent rare examples of naturally occurring 9,9'-bisnorlignans. All the isolated compounds were assayed for their inhibitory effects on LPS-induced nitric oxide production in RAW 264.7 macrophages.

Arylboronic Acid-Catalyzed Racemization of Secondary and Tertiary Alcohols

The use of 2-carboxyphenylboronic acid (5 mol %) and oxalic acid (10 mol %) with 2-butanone as a solvent for the racemization of a range of enantiomerically pure secondary and tertiary alcohols is demonstrated. The process is postulated to proceed via reversible Brønsted acid-catalyzed C–O bond cleavage through an achiral carbocation intermediate.

Enzyme- and Chemo-enzyme-Catalyzed Stereodivergent Synthesis

Multiple stereoisomers can be found when a substance contains chiral carbons in its chemical structure. To obtain the desired stereoisomers, asymmetric synthesis was proposed in the 1970s and developed rapidly at the beginning of this century. Stereodivergent synthesis, an extension of asymmetric synthesis in organic synthesis with the hope to produce all stereoisomers of chiral substances in high conversion and selectivity, enriches the variety of available products and serves as a reference suggestion for the synthesis of their derivatives and other compounds. Since biocatalysis has outstanding advantages of economy, environmental friendliness, high efficiency, and reaction at mild conditions, the biocatalytic reaction is regarded as an efficient strategy to perform stereodivergent synthesis. Thus, in this review, we summarize the stereodivergent synthesis catalyzed by enzymes or chemo-enzymes in cases where a compound contains two or three chiral carbons, i.e., at most four or eight stereoisomers are present. The types of reactions, including reduction of substituent ketones, cyclization reactions, olefin addition, and nonredox transesterification reactions, are also discussed for the understanding of the progress and application of biocatalysis in stereodivergent synthesis.

Biosynthesis, total synthesis, and pharmacological activities of aryltetralin-type lignan podophyllotoxin and its derivatives

Covering: up to 2022Podophyllotoxin (PTOX, 1), a kind of aryltetralin-type lignan, was first discovered in the plant Podophyllum peltatum and its structure was clarified by W. Borsche and J. Niemann in 1932. Due to its potent anti-cancer and anti-viral activities, it is considered one of the molecules most likely to be developed into modern drugs. With the increasing market demand and insufficient storage of natural resources, it is crucial to expand the sources of PTOXs. The original extraction method from plants has gradually failed to meet the requirements, and the biosynthesis and total synthesis have become the forward-looking alternatives. As key enzymes in the biosynthetic pathway of PTOXs and their catalytic mechanisms being constantly revealed, it is possible to realize the heterogeneous biosynthesis of PTOXs in the future. Chemical and chemoenzymatic synthesis also provide schemes for strictly controlling the asymmetric configuration of the tetracyclic core. Currently, the pharmacological activities of some PTOX derivatives have been extensively studied, laying the foundation for clinical candidate drugs. This review focuses primarily on the latest research progress in the biosynthesis, total synthesis, and pharmacological activities of PTOX and its derivatives, providing a more comprehensive understanding of these widely used compounds and supporting the future search for clinical applications.

Taming the radical cation intermediate enabled one-step access to structurally diverse lignans

Lignans, in spite of their structural diversity, are all biosynthetically derived from coniferyl alcohol. We report herein a divergent synthesis of lignans from biomass-derived monolignols in a short synthetic sequence. Blue LED irradiation of a dichloromethane solution of dicinnamyl ether derivatives in the presence of Cu(TFA)₂, an alcohol (2.0 equiv) and a catalytic amount of Fukuzumi’s salt affords the C7-alkoxylated aryltetralin cyclic ethers. Increasing the amount of alcohol under otherwise identical conditions diverts the reaction course to furnish the C7,C7’-dialkoxylated dibenzyltetrahydrofurans, while replacing Cu(TFA)₂ with diphenyl disulfide (PhSSPh) provides selectively the C7-monoalkoxylated dibenzyltetrahydrofurans. Aza-, thia- and carba-analogues of lignans are equally accessible by simply changing the tethering atom of the allylic alcohols. Concise total syntheses of aglacins A, E, F, brassilignan, and dehydrodimethylconidendrin are documented featuring these transformations.

Lignans, in spite of their structural diversity, are all biosynthetically derived from coniferyl alcohol. Here, the authors describe a divergent and stereoselective chemical synthesis of three types of lignans from biomass-derived monolignols, exploiting the different reaction manifolds of highly reactive radical cation intermediate.

Chemoenzymatic approaches to plant natural product inspired compounds

Covering: 2003 up to the end of 2021Complex molecules produced by plants have provided us with a range of medicines, flavour and fragrance compounds and pesticides. However, there are challenges associated with accessing these in an economically viable manner, including low natural abundance and the requirement for complex multi-step synthetic strategies. Chemoenzymatic approaches provide a valuable alternative strategy by combining traditional synthetic methods with biocatalysis. This review highlights recent chemoenzymatic syntheses towards plant natural products and analogues, focusing on the advantages of incorporating biocatalysts into a synthetic strategy.

Recent strategies and tactics for the enantioselective total syntheses of cyclolignan natural products

Covering: 2000 to 2021Lignan natural products are found in many different plant species and possess numerous useful biological properties, such as anti-inflammatory, antiviral, antioxidant, antibacterial, and antitumor activities. Their utility in both traditional and conventional medicine, coupled with their structural diversity has made them popular synthetic targets over many decades. This review specifically addresses the cyclolignan subclass of the family, which possess both a C8-C8' and a C2-C7' linkage between two different phenylpropene units. We present a comprehensive overview of the diverse strategies employed by chemists to achieve enantioselective total syntheses of cyclolignans covering: 2000 to 2021.

Recent advances in the total synthesis of 2,7'-cyclolignans

The synthetic progress of bioactive 2,7'-cyclolignans is reviewed. After a short introduction to biosynthesis and chemoenzymatic synthesis, the chemical synthesis of various aryltetralin, dihydronaphthalene and 7'-arylnaphthalene-types of these lignans is demonstrated. Notably, newly developed methods, such as Pd-catalyzed C-H arylation, organocatalysis and photocatalysis under visible-light, are discussed during the construction of their skeleton. These efforts will stimulate further development of novel synthetic strategies for this kind of natural product with important biological activities.

Mechanistic analysis of carbon-carbon bond formation by deoxypodophyllotoxin synthase

The completion of the tetracyclic core of etoposide, classified by the World Health Organization as an essential medicine, by the Fe/2OG oxygenase deoxypodophyllotoxin synthase follows a hybrid radical-polar pathway not previously seen in other members of this enzyme class. The implication of a substrate-based benzylic carbocation in this mechanism will inform ongoing efforts to create analogs of this important drug with improved or emergent properties and represents a new route for resolution of the initial substrate radical that is common to members of the class. This study adds to our understanding on a growing number of biochemical transformations in which carbocation intermediates are likely to be crucial.

Deoxypodophyllotoxin contains a core of four fused rings (A to D) with three consecutive chiral centers, the last being created by the attachment of a peripheral trimethoxyphenyl ring (E) to ring C. Previous studies have suggested that the iron(II)- and 2-oxoglutarate–dependent (Fe/2OG) oxygenase, deoxypodophyllotoxin synthase (DPS), catalyzes the oxidative coupling of ring B and ring E to form ring C and complete the tetracyclic core. Despite recent efforts to deploy DPS in the preparation of deoxypodophyllotoxin analogs, the mechanism underlying the regio- and stereoselectivity of this cyclization event has not been elucidated. Herein, we report 1) two structures of DPS in complex with 2OG and (±)-yatein, 2) in vitro analysis of enzymatic reactivity with substrate analogs, and 3) model reactions addressing DPS’s catalytic mechanism. The results disfavor a prior proposal of on-pathway benzylic hydroxylation. Rather, the DPS-catalyzed cyclization likely proceeds by hydrogen atom abstraction from C7', oxidation of the benzylic radical to a carbocation, Friedel–Crafts-like ring closure, and rearomatization of ring B by C6 deprotonation. This mechanism adds to the known pathways for transformation of the carbon-centered radical in Fe/2OG enzymes and suggests what types of substrate modification are likely tolerable in DPS-catalyzed production of deoxypodophyllotoxin analogs.

Total Synthesis and Anti-Tobacco Mosaic Virus Activity of the Furofuran Lignan (±)-Phrymarolin II and Its Analogues

Phrymarolin II, a furofuran lignan isolated from Phryma leptostachya L., features a 3,7-dioxabicyclo[3.3.0]octane skeleton. Herein, we report an alternative total synthesis of (±)-phrymarolin II (2), which was performed in 9 steps from commercially available sesamol. The key steps of the synthesis included a zinc-mediated Barbier-type allylation and a copper-catalyzed anomeric O-arylation. Our total synthesis allowed the synthesis of analogues of (±)-phrymarolin II. Most derivatives displayed good to excellent in vivo activity against tobacco mosaic virus (TMV). (±)-Phrymarolin II (2) and compounds (±)-31d and (±)-31g exhibited similar or higher activity than commercial ningnanmycin, which indicated that phrymarolin lignans are a promising new class of plant virus inhibitors.

Synthesis of α-exo-Methylene-γ-butyrolactones: Recent Developments and Applications in Natural Product Synthesis

The α-exo-methylene-γ-butyrolactone moiety is present in a vast array of structurally diverse natural products and is often central to their biological activity. In this short review, we summarize new approaches to α-exo-methylene-γ-butyrolactones developed over the past decade as well as their applications in total synthesis.

1 Introduction

2 Approaches to α-exo-Methylene-γ-butyrolactones

2.1 Enantioselective Synthesis via Lactonization Approaches

2.2 Enantioselective Halolactonizations

2.3 Enantioselective Barbier-Type Allylation

2.4 C–H Insertion/Olefination Sequences

2.5 Alkene Cyclization

2.6 Strain-Driven Dyotropic Rearrangement

3 β-(Hydroxymethylalkyl)-α-exo-methylene-γ-butyrolactones

4 Applications in Total Synthesis

4.1 Sesquiterpene Lactones

4.2 Lignans

4.3 Other Monocyclic Natural Products

4.4 Choice of Methodology in Recent Total Syntheses

5 Summary and Outlook

Synthesis of (-)-deoxypodophyllotoxin and (-)-epipodophyllotoxin via a multi-enzyme cascade in E. coli

Background

The aryltetralin lignan (−)−podophyllotoxin is a potent antiviral and anti-neoplastic compound that is mainly found in Podophyllum plant species. Over the years, the commercial demand for this compound rose notably because of the high clinical importance of its semi-synthetic chemotherapeutic derivatives etoposide and teniposide. To satisfy this demand, (−)−podophyllotoxin is conventionally isolated from the roots and rhizomes of Sinopodophyllum hexandrum, which can only grow in few regions and is now endangered by overexploitation and environmental damage. For these reasons, targeting the biosynthesis of (−)−podophyllotoxin precursors or analogues is fundamental for the development of novel, more sustainable supply routes.

Results

We recently established a four-step multi-enzyme cascade to convert (+)−pinoresinol into (−)−matairesinol in E. coli. Herein, a five-step multi-enzyme biotransformation of (−)−matairesinol to (−)−deoxypodophyllotoxin was proven effective with 98 % yield at a concentration of 78 mg/L. Furthermore, the extension of this cascade to a sixth step leading to (−)−epipodophyllotoxin was evaluated. To this end, seven enzymes were combined in the reconstituted pathway involving inter alia three plant cytochrome P450 monooxygenases, with two of them being functionally expressed in E. coli for the first time.

Conclusions

Both, (−)−deoxypodophyllotoxin and (−)−epipodophyllotoxin, are direct precursors to etoposide and teniposide. Thus, the reconstitution of biosynthetic reactions of Sinopodophyllum hexandrum as an effective multi-enzyme cascade in E. coli represents a solid step forward towards a more sustainable production of these essential pharmaceuticals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01673-5.

Enhanced Aryltetralin Lignans Production in Linum Adventi-Tious Root Cultures

Lignans are the main secondary metabolites synthetized by Linum species as plant defense molecules. They are also valuable for human health, in particular, for their potent antiviral and antineoplastic properties. In this study, the adventitious root cultures of three Linum species (L. flavum, L. mucronatum and L. dolomiticum) were developed to produce aryltetralin lignans. The effect of two elicitors, methyl jasmonate and coronatine, on aryltetralin lignans production was also evaluated. The adventitious root cultures from L. dolomiticum were obtained and analyzed for the first time and resulted as the best producer for all the aryltetralins highlighted in this system: Podophyllotoxin, 6-methoxypodophyllotoxin and 6-methoxypodophyllotoxin-7-O-β-glucoside, the last showing a productivity of 92.6 mg/g DW. The two elicitors differently affected the production of the 6-methoxypodophyllotoxin and 6-methoxypodophyllotoxin-7-O-β-glucoside.

Practical Enzymatic Production of Carbocycles

The Pd-catalyzed carbon-carbon bond formation pioneered by Heck in 1969 has dominated medicinal chemistry development for the ensuing fifty years. As the demand for more complex three-dimensional active pharmaceuticals continues to increase, preparative enzyme-mediated assembly, by virtue of its exquisite selectivity and sustainable nature, is poised to provide a practical and affordable alternative for accessing such compounds. In this minireview, we summarize recent state-of-the-art developments in practical enzyme-mediated assembly of carbocycles. When appropriate, background information on the enzymatic transformation is provided and challenges and/or limitations are also highlighted.

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.

Asymmetric Total Synthesis of Aglacins A, B, and E

An asymmetric photoenolization/Diels-Alder (PEDA) reaction between electron-rich 2-methylbenzaldehydes and unsaturated γ-lactones was developed to directly construct the basic tricyclic core of aryltetralin lactone lignans. This methodology enabled the first asymmetric total synthesis of aglacins A, B, and E and revision of the absolute configuration of these natural lignans. The strategy was also used to prepare the naturally occurring aryldihydronaphthalene-type lignans (-)-7,8-dihydroisojusticidin B and (+)-linoxepin in four and six steps, as well as 27 natural-product-like molecules containing a C8' quaternary center. We believe that the synthetic aglacins and small-molecule library provide new opportunities to carry out the SAR studies of the podophyllotoxin family of natural products.

A unified strategy to prostaglandins: chemoenzymatic total synthesis of cloprostenol, bimatoprost, PGF2α, fluprostenol, and travoprost guided by biocatalytic retrosynthesis

Development of efficient and stereoselective synthesis of prostaglandins (PGs) is of utmost importance, owing to their valuable medicinal applications and unique chemical structures. We report here a unified synthesis of PGs cloprostenol, bimatoprost, PGF_2α, fluprostenol, and travoprost from the readily available dichloro-containing bicyclic ketone 6a guided by biocatalytic retrosynthesis, in 11-12 steps with 3.8-8.4% overall yields. An unprecedented Baeyer-Villiger monooxygenase (BVMO)-catalyzed stereoselective oxidation of 6a (99% ee), and a ketoreductase (KRED)-catalyzed diastereoselective reduction of enones 12 (87 : 13 to 99 : 1 dr) were utilized in combination for the first time to set the critical stereochemical configurations under mild conditions. Another key transformation was the copper(ii)-catalyzed regioselective p-phenylbenzoylation of the secondary alcohol of diol 10 (9.3 : 1 rr). This study not only provides an alternative route to the highly stereoselective synthesis of PGs, but also showcases the usefulness and great potential of biocatalysis in construction of complex molecules.

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.

Total Syntheses of Paraconic Acids and 1,10-seco-Guaianolides via a Barbier Allylation/Translactonization Cascade of 3-(Bromomethyl)-2(5H)-furanone

A palladium-catalyzed Barbier allylation/translactonization cascade reaction was established for the rapid construction of β,γ-disubstituted α-exo-methylene-γ-butyrolactone, an important motif in sesquiterpenes. Dimethyl zinc played significant roles in both steps for the umpolung of π-allylpalladium as a nucleophile and promoting a Lewis acid-mediated translactonization. This sequence showed a broad substrate scope and was further harnessed for the synthesis of two paraconic acids as well as the first protecting-group-free total synthesis of two 1,10-seco-guaianolides.

Power of Biocatalysis for Organic Synthesis

Biocatalysis, using defined enzymes for organic transformations, has become a common tool in organic synthesis, which is also frequently applied in industry. The generally high activity and outstanding stereo-, regio-, and chemoselectivity observed in many biotransformations are the result of a precise control of the reaction in the active site of the biocatalyst. This control is achieved by exact positioning of the reagents relative to each other in a fine-tuned 3D environment, by specific activating interactions between reagents and the protein, and by subtle movements of the catalyst. Enzyme engineering enables one to adapt the catalyst to the desired reaction and process. A well-filled biocatalytic toolbox is ready to be used for various reactions. Providing nonnatural reagents and conditions and evolving biocatalysts enables one to play with the myriad of options for creating novel transformations and thereby opening new, short pathways to desired target molecules. Combining several biocatalysts in one pot to perform several reactions concurrently increases the efficiency of biocatalysis even further.

Exploration of Iron- and a-Ketoglutarate-Dependent Dioxygenases as Practical Biocatalysts in Natural Product Synthesis

Catalytic C─H oxidation is a powerful transformation with enormous promise to streamline access to complex molecules. In recent years, biocatalytic C─H oxidation strategies have received tremendous attention due to their potential to address unmet regio- and stereoselectivity challenges that are often encountered with the use of small-molecule-based catalysts. This Account provides an overview of recent contributions from our laboratory in this area, specifically in the use of iron- and α-ketoglutarate-dependent dioxygenases in the chemoenzymatic synthesis of complex natural products.

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.

Assembly of Plant Enzymes in E. coli for the Production of the Valuable (-)-Podophyllotoxin Precursor (-)-Pluviatolide

Lignans are plant secondary metabolites with a wide range of reported health-promoting bioactivities. Traditional routes toward these natural products involve, among others, the extraction from plant sources and chemical synthesis. However, the availability of the sources and the complex chemical structures of lignans often limit the feasibility of these approaches. In this work, we introduce a newly assembled biosynthetic route in E. coli for the efficient conversion of the common higher-lignan precursor (+)-pinoresinol to the noncommercially available (-)-pluviatolide via three intermediates. (-)-Pluviatolide is considered a crossroad compound in lignan biosynthesis, because the methylenedioxy bridge in its structure, resulting from the oxidation of (-)-matairesinol, channels the biosynthetic pathway toward the microtubule depolymerizer (-)-podophyllotoxin. This oxidation reaction is catalyzed with high regio- and enantioselectivity by a cytochrome P450 monooxygenase from Sinopodophyllum hexandrum (CYP719A23), which was expressed and optimized regarding redox partners in E. coli. Pinoresinol-lariciresinol reductase from Forsythia intermedia (FiPLR), secoisolariciresinol dehydrogenase from Podophyllum pleianthum (PpSDH), and CYP719A23 were coexpressed together with a suitable NADPH-dependent reductase to ensure P450 activity, allowing for four sequential biotransformations without intermediate isolation. By using an E. coli strain coexpressing the enzymes originating from four plants, (+)-pinoresinol was efficiently converted, allowing the isolation of enantiopure (-)-pluviatolide at a concentration of 137 mg/L (ee ≥99% with 76% isolated yield).

Radical-Cation Cascade to Aryltetralin Cyclic Ether Lignans Under Visible-Light Photoredox Catalysis

The development of concise, sustainable, and cost-effective synthesis of aryltetralin lignans, bearing either a fused lactone or cyclic ether, is of significant medicinal importance. Reported is that in the presence of Fukuzumi's acridinium salt under blue LED irradiation, functionalized dicinnamyl ether derivatives are converted into aryltetralin cyclic ether lignans with concurrent generation of three stereocenters in good to high yields with up to 20:1 diastereoselectivity. Oxidation of an alkene to the radical cation is key to the success of this formal Diels-Alder reaction of electronically mismatched diene and dienophile. Applying this methodology, six natural products, aglacin B, aglacin C, sulabiroin A, sulabiroin B, gaultherin C, and isoshonanin, are synthesized in only two to three steps from readily available biomass-derived monolignols. A revised structure is proposed for gaultherin C.

Photoredox Generated Carbonyl Ylides Enable a Modular Approach to Aryltetralin, Dihydronaphthalene, and Arylnaphthalene Lignans

A one-pot synthesis of dihydronaphthalenes and arylnaphthalenes from epoxides and common dipolarophiles is described. The reaction proceeds through photoredox activation of epoxides to carbonyl ylides, which undergo concerted [3 + 2] dipolar cycloaddition with dipolarophiles to provide tetrahydrofurans or 2,5-dihydrofurans. In the same flask, acid promoted rearrangement affords densely functionalized dihydronaphthalenes and arylnaphthalenes, respectively, in an overall redox-neutral sequence of transformations. Succinct total synthesis (4-6 steps) of pycnanthulignene B and C and justicidin E are reported.