Elucidation of the concise biosynthetic pathway of the communesin indole alkaloids

A Cytochrome P450 Catalyzes Oxidative Coupling Formation of Insecticidal Dimeric Indole Piperazine Alkaloids

Cytochrome P450 (CYP450)-catalyzed oxidative coupling is an efficient strategy for using simple building blocks to construct complex structural scaffolds of natural products. Among them, heterodimeric coupling between two different monomers is relatively scarce, and the corresponding CYP450s are largely undiscovered. In this study, we discovered a fungal CYP450 (CpsD) and its associated cps cluster from 37208 CYP450s of Pfam PF00067 family member database and subsequently identified a group of new skeleton indole piperazine alkaloids (campesines A-G) by combination of genome mining and heterologous synthesis. Importantly, CYP450 CpsD mainly catalyzes intermolecular oxidative heterocoupling of two different indole piperazine monomers to generate an unexpected 6/5/6/6/6/6/5/6 eight-ring scaffold through the formation of one C-C bond and two C-N bonds, illuminating its first dimerase role in this family of natural products. The proposed catalytic mechanism of CpsD was deeply investigated by diversified substrate derivatization. Moreover, dimeric campesine G shows good insecticidal activity against the global honeybee pest Galleria mellonella. Our study shows a representative example of discovering new skeleton monomeric and dimeric indole piperazine alkaloids from microbial resources, expands our knowledge of bond formation by CYP450s and supports further development of the newly discovered and engineered campesine family compounds as potential biopesticides.

One-step Bio-guided Isolation of Secondary Metabolites from the Endophytic Fungus Penicillium crustosum Using High-resolution Semi-preparative HPLC

BACKGROUND

An endophytic fungal strain Penicillium crustosum was isolated from the seagrass Posidonia oceanica and investigated to identify its antimicrobial constituents and characterize its metabolome composition. The ethyl acetate extract of this fungus exhibited antimicrobial activity against methicillin-resistant Staphylococcus aureus (MRSA) as well as an anti-quorum sensing effect against Pseudomonas aeruginosa.

METHODS

The crude extract was profiled by UHPLC-HRMS/MS, and the dereplication was assisted by feature-based molecular networking. As a result, more than twenty compounds were annotated in this fungus. To rapidly identify the active compounds, the enriched extract was fractionated by semipreparative HPLC-UV applying a chromatographic gradient transfer and dry load sample introduction to maximise resolution. The collected fractions were profiled by 1H-NMR and UHPLC-HRMS.

RESULTS

The use of molecular networking-assisted UHPLC-HRMS/MS dereplication allowed preliminary identification of over 20 compounds present in the ethyl acetate extract of P. crustosum. The chromatographic approach significantly accelerated the isolation of the majority of compounds present in the active extract. The one-step fractionation allowed the isolation and identification of eight compounds (1-8).

CONCLUSION

This study led to the unambiguous identification of eight known secondary metabolites as well as the determination of their antibacterial properties.

Methodological Development and Applications of Tryptamine-Ynamide Cyclizations in Synthesizing Core Skeletons of Indole Alkaloids

Over the past two decades, synthetic strategies for synthesizing the skeletons of various indole alkaloids based on tryptamine-ynamide have been continuously developed and applied to the total syntheses or formal total syntheses of related molecules. In this synopsis, we summarized the cyclization pathways of tryptamine-ynamide under different catalytic conditions, emphasizing the reaction mechanism and applications in the syntheses of indole alkaloids.

Finding activity through rigidity: syntheses of natural products containing tricyclic bridgehead carbon centers

Covering: up to 2022Tricyclic bridgehead carbon centers (TBCCs) are a synthetically challenging substructure found in many complex natural products. Here we review the syntheses of ten representative families of TBCC-containing isolates, with the goal of outlining the strategies and tactics used to install these centers, including a discussion of the evolution of the successful synthetic design. We provide a summary of common strategies to inform future synthetic endeavors.

Purpurascenines A-C, Azepino-Indole Alkaloids from Cortinarius purpurascens: Isolation, Biosynthesis, and Activity Studies on the 5-HT2A Receptor

Three previously undescribed azepino-indole alkaloids, named purpurascenines A-C (1-3), together with the new-to-nature 7-hydroxytryptophan (4) as well as two known compounds, adenosine (5) and riboflavin (6), were isolated from fruiting bodies of Cortinarius purpurascens Fr. (Cortinariaceae). The structures of 1-3 were elucidated based on spectroscopic analyses and ECD calculations. Furthermore, the biosynthesis of purpurascenine A (1) was investigated by in vivo experiments using 13C-labeled sodium pyruvate, alanine, and sodium acetate incubated with fruiting bodies of C. purpurascens. The incorporation of 13C into 1 was analyzed using 1D NMR and HRESIMS methods. With [3-13C]-pyruvate, a dramatic enrichment of 13C was observed, and hence a biosynthetic route via a direct Pictet-Spengler reaction between α-keto acids and 7-hydroxytryptophan (4) is suggested for the biosynthesis of purpurascenines A-C (1-3). Compound 1 exhibits no antiproliferative or cytotoxic effects against human prostate (PC-3), colorectal (HCT-116), and breast (MCF-7) cancer cells. An in silico docking study confirmed the hypothesis that purpurascenine A (1) could bind to the 5-HT2A serotonin receptor's active site. A new functional 5-HT2A receptor activation assay showed no functional agonistic but some antagonistic effects of 1 against the 5-HT-dependent 5-HT2A activation and likely antagonistic effects on putative constitutive activity of the 5-HT2A receptor.

Biomimetic Total Synthesis of Clavicipitic Acid: A DDQ-Mediated Intramolecular Cross-Dehydrogenative Coupling Approach

Clavicipitic acid, a communesin alkaloid precursor, has attracted significant attention due to its unique azepino[5,4,3-cd]indole framework. Herein, we report a novel biomimetic synthesis of clavicipitic acid diastereomers by utilizing a DDQ-mediated cross-dehydrogenative coupling (CDC) reaction. The synthesis involves Suzuki coupling for the prenylation of a 4-bromotryptophan derivative, followed by an intramolecular CDC reaction to construct the azepinoindole core. The trans isomer was obtained as the major product, and the two diastereomers were separable. The CDC reaction conditions, including temperature, solvent, and protecting groups, were investigated, and a plausible mechanism for the observed diastereoselectivity was proposed.

Derivation of the multiply-branched ergot alkaloid pathway of fungi

Ergot alkaloids are a large family of fungal specialized metabolites that are important as toxins in agriculture and as the foundation of powerful pharmaceuticals. Fungi from several lineages and diverse ecological niches produce ergot alkaloids from at least one of several branches of the ergot alkaloid pathway. The biochemical and genetic bases for the different branches have been established and are summarized briefly herein. Several pathway branches overlap among fungal lineages and ecological niches, indicating activities of ergot alkaloids benefit fungi in different environments and conditions. Understanding the functions of the multiple genes in each branch of the pathway allows researchers to parse the abundant genomic sequence data available in public databases in order to assess the ergot alkaloid biosynthesis capacity of previously unexplored fungi. Moreover, the characterization of the genes involved in the various branches provides opportunities and resources for the biotechnological manipulation of ergot alkaloids for experimentation and pharmaceutical development.

Evolutionary relationships of adenylation domains in fungi

Non-ribosomal peptide synthetases (NRPSs) and NRPS-like enzymes are abundant in microbes as they are involved in the production of primary and secondary metabolites. In contrast to the well-studied NRPSs, known to produce non-ribosomal peptides, NRPS-like enzymes exhibit more diverse activities and their evolutionary relationships are unclear. Here, we present the first in-depth phylogenetic analysis of fungal NRPS-like A domains from functionally characterized pathways, and their relationships to characterized A domains found in fungal NRPSs. This study clearly differentiated amino acid reductases, including NRPSs, from CoA/AMP ligases, which could be divided into 10 distinct phylogenetic clades that reflect their conserved domain organization, substrate specificity and enzymatic activity. In particular, evolutionary relationships of adenylate forming reductases could be refined and explained the substrate specificity difference. Consistent with their phylogeny, the deduced amino acid code of A domains differentiated amino acid reductases from other enzymes. However, a diagnostic code was found for α-keto acid reductases and clade 7 CoA/AMP ligases only. Comparative genomics of loci containing these enzymes revealed that they can be independently recruited as tailoring genes in diverse secondary metabolite pathways. Based on these results, we propose a refined and clear phylogeny-based classification of A domain-containing enzymes, which will provide a robust framework for future functional analyses and engineering of these enzymes to produce new bioactive molecules.

Design, Synthesis, and Bioactivity Study of Novel Tryptophan Derivatives Containing Azepine and Acylhydrazone Moieties

Based on the scaffolds widely used in drug design, a series of novel tryptophan derivatives containing azepine and acylhydrazone moieties have been designed, synthesized, characterized, and evaluated for their biological activities. The bioassay results showed that the target compounds possessed moderate to good antiviral activities against the tobacco mosaic virus (TMV), among which compounds 5c, 6a, 6h, 6t, 6v, and 6y exhibited higher inactivation, curative, and protection activities in vivo than that of ribavirin (40 ± 1, 37 ± 1, 39 ± 2% at 500 mg/L). Especially, 6y showed comparable activities to that of ningnanmycin (57 ± 2, 55 ± 3, 58 ± 1% at 500 mg/L). Meanwhile, we were pleased to find that almost all these derivatives showed good larvicidal activities against Plutella xylostella. Meanwhile, these derivatives also showed a broad spectrum of fungicidal activities.

Marker-free CRISPR-Cas9 based genetic engineering of the phytopathogenic fungus, Penicillium expansum

Filamentous fungi are prolific producers of secondary metabolites (SecMets), including compounds with antibiotic properties, like penicillin, that allows the producing fungus to combat competitors in a shared niche. However, the biological function of the majority of these small complex metabolites for the producing fungi remains unclear (Macheleidt et al., 2016). In an effort to address this lack of knowledge, we have chosen to study the microbial community of moldy apples in the hope of shedding more light on the role of SecMets for the dynamics of the microbial community. Penicillium expansum is one of the prevalent fungal species in this system, and in co-culture experiments with other apple fungal pathogens, we have observed up- and downregulation of several SecMets when compared to monocultures. However, molecular genetic dissection of the observed changes is challenging, and new methodologies for targeted genetic engineering in P. expansum are needed. In the current study, we have established a CRISPR-Cas9 dependent genetic engineering toolbox for the targeted genetic manipulation of P. expansum to allow for single-step construction of marker-free strains. The method and effect of different combinations of a Cas9-sgRNA expressing plasmids and repair template substrates in the NHEJ-proficient WT strain is tested by targeted deletion of melA, encoding a PKS responsible for pigment formation, which upon deletion resulted in white mutants. Co-transformation with a linear double-stranded DNA fragment consisting of two 2 kb homology arms flanking the PKS gene proved to be the most efficient strategy with 100% confirmed deletions by diagnostic PCR. Shorter homology arms (500-1000bp) resulted in 20-30% deletion efficiency. Furthermore, we demonstrate the application of the CRISPR-Cas9 method for targeted deletion of biosynthetic genes without a visible phenotype, insertion of a visual reporter-encoding gene (mRFP), and overexpression of biosynthetic genes. Combined, these tools will advance in enabling the deciphering of SecMet biosynthetic pathways, provide in situ insight into when and where SecMets are produced, and provide an avenue to study the role of P. expansum SecMets in shaping the microbial community development on moldy apples via marker-free targeted genetic engineering of P. expansum.

Branching and converging pathways in fungal natural product biosynthesis

In nature, organic molecules with great structural diversity and complexity are synthesized by utilizing a relatively small number of starting materials. A synthetic strategy adopted by nature is pathway branching, in which a common biosynthetic intermediate is transformed into different end products. A natural product can also be synthesized by the fusion of two or more precursors generated from separate metabolic pathways. This review article summarizes several representative branching and converging pathways in fungal natural product biosynthesis to illuminate how fungi are capable of synthesizing a diverse array of natural products.

Ag(I)/PPh3-catalyzed diastereoselective syntheses of spiro[indole-3,4'-piperidine] derivatives via cycloisomerizations of tryptamine-ynamides

A Ag(I)/PPh₃-catalyzed chelation-controlled cycloisomerization of tryptamine-ynamide was developed to access the spiro[indole-3,4'-piperidine] scaffold in a racemic and diastereoselective manner. The diastereoselective products were achieved by a chiron approach. Density functional theory (DFT) calculations indicated that strong non-covalent effects between the substrate and catalyst/ligand complex stabilized the spiroindoleninium intermediate via cation-π-π interactions.

One-pot synthesis of natural-product inspired spiroindolines with anti-cancer activities

A post-Ugi/diastereoselective cascade reaction was developed to construct the spiroindoline scaffold through a 5-exo-dig indole cyclization and the intramolecular trapping of the spiro intermediate forming a quaternary carbon center.

Antibiotic Biosynthesis Pathways from Endophytic Streptomyces SUK 48 through Metabolomics and Genomics Approaches

Streptomyces sp. has been known to be a major antibiotic producer since the 1940s. As the number of cases related to resistance pathogens infection increases yearly, discovering the biosynthesis pathways of antibiotic has become important. In this study, we present the streamline of a project report summary; the genome data and metabolome data of newly isolated Streptomyces SUK 48 strain are also analyzed. The antibacterial activity of its crude extract is also determined. To obtain genome data, the genomic DNA of SUK 48 was extracted using a commercial kit (Promega) and sent for sequencing (Pac Biosciences technology platform, Menlo Park, CA, USA). The raw data were assembled and polished using Hierarchical Genome Assembly Process 4.0 (HGAP 4.0). The assembled data were structurally predicted using tRNAscan-SE and rnammer. Then, the data were analyzed using Kyoto Encyclopedia of Genes and Genomes (KEGG) database and antiSMASH analysis. Meanwhile, the metabolite profile of SUK 48 was determined using liquid chromatography-mass spectrophotometry (LC-MS) for both negative and positive modes. The results showed that the presence of kanamycin and gentamicin, as well as the other 11 antibiotics. Nevertheless, the biosynthesis pathways of aurantioclavine were also found. The cytotoxicity activity showed IC50 value was at 0.35 ± 1.35 mg/mL on the cell viability of HEK 293. In conclusion, Streptomyces sp. SUK 48 has proven to be a non-toxic antibiotic producer such as auranticlavine and gentamicin.

Molecular insights into the endoperoxide formation by Fe(II)/α-KG-dependent oxygenase NvfI

Endoperoxide-containing natural products are a group of compounds with structurally unique cyclized peroxide moieties. Although numerous endoperoxide-containing compounds have been isolated, the biosynthesis of the endoperoxides remains unclear. NvfI from Aspergillus novofumigatus IBT 16806 is an endoperoxidase that catalyzes the formation of fumigatonoid A in the biosynthesis of novofumigatonin. Here, we describe our structural and functional analyses of NvfI. The structural elucidation and mutagenesis studies indicate that NvfI does not utilize a tyrosyl radical in the reaction, in contrast to other characterized endoperoxidases. Further, the crystallographic analysis reveals significant conformational changes of two loops upon substrate binding, which suggests a dynamic movement of active site during the catalytic cycle. As a result, NvfI installs three oxygen atoms onto a substrate in a single enzyme turnover. Based on these results, we propose a mechanism for the NvfI-catalyzed, unique endoperoxide formation reaction to produce fumigatonoid A.

Biosynthesis and synthetic biology of psychoactive natural products

Psychoactive natural products play an integral role in the modern world. The tremendous structural complexity displayed by such molecules confers diverse biological activities of significant medicinal value and sociocultural impact. Accordingly, in the last two centuries, immense effort has been devoted towards establishing how plants, animals, and fungi synthesize complex natural products from simple metabolic precursors. The recent explosion of genomics data and molecular biology tools has enabled the identification of genes encoding proteins that catalyze individual biosynthetic steps. Once fully elucidated, the "biosynthetic pathways" are often comparable to organic syntheses in elegance and yield. Additionally, the discovery of biosynthetic enzymes provides powerful catalysts which may be repurposed for synthetic biology applications, or implemented with chemoenzymatic synthetic approaches. In this review, we discuss the progress that has been made toward biosynthetic pathway elucidation amongst four classes of psychoactive natural products: hallucinogens, stimulants, cannabinoids, and opioids. Compounds of diverse biosynthetic origin - terpene, amino acid, polyketide - are identified, and notable mechanisms of key scaffold transforming steps are highlighted. We also provide a description of subsequent applications of the biosynthetic machinery, with an emphasis placed on the synthetic biology and metabolic engineering strategies enabling heterologous production.

A Biological Route to Conjugated Alkenes: Microbial Production of Hepta-1,3,5-triene

Conjugated alkenes such as dienes and polyenes have a range of applications as pharmaceutical agents and valuable building blocks in the polymer industry. Development of a renewable route to these compounds provides an alternative to fossil fuel derived production. The enzyme family of the UbiD decarboxylases offers substantial scope for alkene production, readily converting poly unsaturated acids. However, biochemical pathways producing the required substrates are poorly characterized, and UbiD-application has hitherto been limited to biological styrene production. Herein, we present a proof-of-principle study for microbial production of polyenes using a bioinspired strategy employing a polyketide synthase (PKS) in combination with a UbiD-enzyme. Deconstructing a bacterial iterative type II PKS enabled repurposing the broad-spectrum antibiotic andrimid biosynthesis pathway to access the metabolic intermediate 2,4,6-octatrienoic acid, a valuable chemical for material and pharmaceutical industry. Combination with the fungal ferulic acid decarboxylase (Fdc1) led to a biocatalytic cascade-type reaction for the production of hepta-1,3,5-triene in vivo. Our approach provides a novel route to generate unsaturated hydrocarbons and related chemicals and provides a blue-print for future development and application.

Regulation of Secondary Metabolism in the Penicillium Genus

Penicillium, one of the most common fungi occurring in a diverse range of habitats, has a worldwide distribution and a large economic impact on human health. Hundreds of the species belonging to this genus cause disastrous decay in food crops and are able to produce a varied range of secondary metabolites, from which we can distinguish harmful mycotoxins. Some Penicillium species are considered to be important producers of patulin and ochratoxin A, two well-known mycotoxins. The production of these mycotoxins and other secondary metabolites is controlled and regulated by different mechanisms. The aim of this review is to highlight the different levels of regulation of secondary metabolites in the Penicillium genus.

Regio- and stereoselective intermolecular phenol coupling enzymes in secondary metabolite biosynthesis

Covering: 2005 to 2020Phenol coupling is a key reaction in the biosynthesis of important biopolymers such as lignin and melanin and of a plethora of biarylic secondary metabolites. The reaction usually leads to several different regioisomeric products due to the delocalization of a radical in the reaction intermediates. If axial chirality is involved, stereoisomeric products are obtained provided no external factor influences the selectivity. Hence, in non-enzymatic organic synthesis it is notoriously difficult to control the selectivity of the reaction, in particular if the coupling is intermolecular. From biosynthesis, it is known that especially fungi, plants, and bacteria produce biarylic compounds regio- and stereoselectively. Nonetheless, the involved enzymes long evaded discovery. First progress was made in the late 1990s; however, the breakthrough came only with the genomic era and, in particular, in the last few years the number of relevant publications has dramatically increased. The discoveries reviewed in this article reveal a remarkable diversity of enzymes that catalyze oxidative intermolecular phenol coupling, including various classes of laccases, cytochrome P450 enzymes, and heme peroxidases. Particularly in the case of laccases, the catalytic systems are often complex and additional proteins, substrates, or reaction conditions have a strong influence on activity and regio- and atroposelectivity. Although the field of (selective) enzymatic phenol coupling is still in its infancy, the diversity of enzymes identified recently could make it easier to select suitable candidates for biotechnological development and to approach this challenging reaction through biocatalysis.

The brlA Gene Deletion Reveals That Patulin Biosynthesis Is Not Related to Conidiation in Penicillium expansum

Dissemination and survival of ascomycetes is through asexual spores. The brlA gene encodes a C2H2-type zinc-finger transcription factor, which is essential for asexual development. Penicillium expansum causes blue mold disease and is the main source of patulin, a mycotoxin that contaminates apple-based food. A P. expansum PeΔbrlA deficient strain was generated by homologous recombination. In vivo, suppression of brlA completely blocked the development of conidiophores that takes place after the formation of coremia/synnemata, a required step for the perforation of the apple epicarp. Metabolome analysis displayed that patulin production was enhanced by brlA suppression, explaining a higher in vivo aggressiveness compared to the wild type (WT) strain. No patulin was detected in the synnemata, suggesting that patulin biosynthesis stopped when the fungus exited the apple. In vitro transcriptome analysis of PeΔbrlA unveiled an up-regulated biosynthetic gene cluster (PEXP_073960-PEXP_074060) that shares high similarity with the chaetoglobosin gene cluster of Chaetomium globosum. Metabolome analysis of PeΔbrlA confirmed these observations by unveiling a greater diversity of chaetoglobosin derivatives. We observed that chaetoglobosins A and C were found only in the synnemata, located outside of the apple, whereas other chaetoglobosins were detected in apple flesh, suggesting a spatial-temporal organization of the chaetoglobosin biosynthesis pathway.

Enzyme-Catalyzed Azepinoindole Formation in Clavine Alkaloid Biosynthesis

(-)-Aurantioclavine (1), which contains a characteristic seven-membered ring fused to an indole ring, belongs to the azepinoindole class of fungal clavine alkaloids. Here we show that starting from a 4-dimethylallyl-l-tryptophan precursor, a flavin adenine dinucleotide (FAD)-binding oxidase and a catalase-like heme-containing protein are involved in the biosynthesis of 1. The function of these two enzymes was characterized by heterologous expression, in vitro characterization, and deuterium labeling experiments.

Biosynthesis of a New Benzazepine Alkaloid Nanangelenin A from Aspergillus nanangensis Involves an Unusual l-Kynurenine-Incorporating NRPS Catalyzing Regioselective Lactamization

1-Benzazepine is a pharmaceutically important scaffold but is rare among natural products. Nanangelenin A (1), containing an unprecedented 3,4-dihydro-1-benzazepine-2,5-dione-N-prenyl-N-acetoxy-anthranilamide scaffold, was isolated from a novel species of Australian fungus, Aspergillus nanangensis. Genomic and retrobiosynthetic analyses identified a putative nonribosomal peptide synthetase (NRPS) gene cluster (nan). The detailed biosynthetic pathway to 1 was established by heterologous pathway reconstitution in A. nidulans, which led to biosynthesis of intermediates nanagelenin B-F (2-5 and 7). We demonstrated that the NRPS NanA incorporates anthranilic acid (Ant) and l-kynurenine (l-Kyn), which is supplied by a dedicated indoleamine-2,3-dioxygenase NanC encoded in the gene cluster. Using heterologous in vivo assays and mutagenesis, we demonstrated that the C-terminal condensation (C_T) and thiolation (T₃) domains of NanA are responsible for the regioselective cyclization of the tethered Ant-l-Kyn dipeptide to form the unusual benzazepine scaffold in 1. We also showed that NanA-C_T catalyzes the regioselective cyclization of a surrogate synthetic substrate, Ant-l-Kyn-N-acetylcysteamine, to give the benzazepine scaffold, while spontaneous cyclization of the dipeptide yielded the alternative kinetically favored benzodiazepine scaffold. The discovery of 1 and the characterization of NanA have expanded the chemical and functional diversities of fungal NRPSs.

Brocaeloid D, a novel compound isolated from a wheat pathogenic fungus, Microdochium majus 99049

Microbes serve as the most important resource for drug discovery. During our screening for bioactive compounds from our natural products library, a pathogenic fungus, Microdochium majus strain 99049, from wheat was selected for further investigation. A new alkaloid named brocaeloid D (1), together with six previously characterized compounds (2–7) were identified. Compound 1 belongs to 4-oxoquinoline with C-2 reversed prenylation and a succinimide substructure. All the structures of these newly isolated compounds were determined by different means in spectroscopic experiments. The absolute configurations of 1 was further deduced from comparison of its CD spectrum with that of known compound 2. The bioactivities of these identified compounds were evaluated against several pathogenic microorganisms and cancer cell lines. Compounds 1–5 showed activity against HUH-7 human hepatoma cells with IC₅₀ values of 80 μg/mL. Compound 6 showed mild activity against HeLa cells (IC₅₀ = 51.9 μg/mL), weak anti-MTB activity (MIC = 80  μg/mL), and moderate anti-MRSA activity (MIC = 25 μg/mL), and compound 7 showed weak anti-MRSA activity (MIC = 100 μg/mL).

Total Synthesis and Anti-Cancer Activity of All Known Communesin Alkaloids and Related Derivatives

A unified enantioselective total synthesis and anticancer evaluation of all known epoxide-containing communesin alkaloids and related derivatives is described. Our synthesis is predicated on the convergent and modular diazene-directed assembly of two complex fragments to secure the critical C3a-C3a' linkage followed by a guided biomimetic aminal reorganization to deliver the heptacyclic core of these alkaloids. Concise enantioselective syntheses of the fragments were devised, with highlights including the application of a rationally designed sulfinamide chiral auxiliary, an efficient calcium trifluoromethanesulfonate promoted intramolecular amination, and a diastereoselective epoxidation that simultaneously converts the new chiral auxiliary to a versatile amine protective group. The modularity of our convergent approach enabled the rapid synthesis of all epoxide-containing members of the communesin family from a single heterodimeric intermediate, including the first total synthesis of communesins C-E, and G-I, and facilitated our stereochemical revision of (-)-communesin I, the most recently isolated communesin alkaloid. Furthermore, the generality of our biogenetically inspired heterodimer rearrangement was demonstrated in a guided synthesis of a communesin derivative with an unnatural topology. Finally, we report the first comparative analysis of the anticancer activities of all naturally occurring communesin alkaloids A-I and eight complex derivatives against five human cancer cell lines. From these data, we have identified (-)-communesin B as the most potent natural communesin and discovered that derivatives with N8'-sulfonamide substitution exhibit up to a 10-fold increase in potency over the natural alkaloids.

Total Synthesis as a Vehicle for Collaboration

"Collaboration" is not the first word most would associate with the field of total synthesis. In fact, the spirit of total synthesis is all-too-often reputed as being more competitive, rather than collaborative, sometimes even within individual laboratories. However, recent studies in total synthesis have inspired a number of collaborative efforts that strategically blend synthetic methodology, biocatalysis, biosynthesis, computational chemistry, and drug discovery with complex molecule synthesis. This Perspective highlights select recent advances in these areas, including collaborative syntheses of chlorolissoclimide, nigelladine A, artemisinin, ingenol, hippolachnin A, communesin A, and citrinalin B. The legendary Woodward-Eschenmoser collaboration that led to the total synthesis of vitamin B₁₂ is also discussed.

Enzymatic Cascade Reactions in Biosynthesis

Enzyme-mediated cascade reactions are widespread in biosynthesis. To facilitate comparison with the mechanistic categorizations of cascade reactions by synthetic chemists and delineate the common underlying chemistry, we discuss four types of enzymatic cascade reactions: those involving nucleophilic, electrophilic, pericyclic, and radical reactions. Two subtypes of enzymes that generate radical cascades exist at opposite ends of the oxygen abundance spectrum. Iron-based enzymes use O2 to generate high valent iron-oxo species to homolyze unactivated C-H bonds in substrates to initiate skeletal rearrangements. At anaerobic end, enzymes reversibly cleave S-adenosylmethionine (SAM) to generate the 5'-deoxyadenosyl radical as a powerful oxidant to initiate C-H bond homolysis in bound substrates. The latter enzymes are termed radical SAM enzymes. We categorize the former as "thwarted oxygenases".

Detecting and prioritizing biosynthetic gene clusters for bioactive compounds in bacteria and fungi

Secondary metabolites (SM) produced by fungi and bacteria have long been of exceptional interest owing to their unique biomedical ramifications. The traditional discovery of new natural products that was mainly driven by bioactivity screening has now experienced a fresh new approach in the form of genome mining. Several bioinformatics tools have been continuously developed to detect potential biosynthetic gene clusters (BGCs) that are responsible for the production of SM. Although the principles underlying the computation of these tools have been discussed, the biological background is left underrated and ambiguous. In this review, we emphasize the biological hypotheses in BGC formation driven from the observations across genomes in bacteria and fungi, and provide a comprehensive list of updated algorithms/tools exclusively for BGC detection. Our review points to a direction that the biological hypotheses should be systematically incorporated into the BGC prediction and assist the prioritization of candidate BGC.

Synthesis of the ABCDG ring skeleton of communesin F based on carboborylation of 1,3-diene and Bi(OTf)3-catalyzed cyclizations

Communesins, isolated from the mycelium of a strain of Penicillium sp., are cytotoxic heptacyclic indole alkaloids bearing a bis-aminal structure and two contiguous quaternary carbon centers. Toward a total synthesis of communesin F, we synthesized a pentacyclic ABCDG ring skeleton via carboborylation of 1,3-diene and a Friedel-Crafts-type cyclization, resulting in the formation of an azepine ring through a Bi(OTf)₃-catalyzed S_N2' reaction.

4-Aminoindoles as 1,4-bisnucleophiles for diversity-oriented synthesis of tricyclic indoles bearing 3,4-fused seven-membered rings

A straightforward access to tricyclic indoles bearing 3,4-fused seven-membered rings was established by using 4-aminoindoles as 1,4-bisnucleophiles in three-component reactions.

Non-monoterpenoid azepinoindole alkaloids

This review discusses various biological and chemical aspects of the non-monoterpenoid azepinoindole class of alkaloids, including their isolation, biosynthesis and total synthesis.

Heterologous expression reveals the biosynthesis of the antibiotic pleuromutilin and generates bioactive semi-synthetic derivatives

The rise in antibiotic resistance is a major threat for human health. Basidiomycete fungi represent an untapped source of underexploited antimicrobials, with pleuromutilin-a diterpene produced by Clitopilus passeckerianus-being the only antibiotic from these fungi leading to commercial derivatives. Here we report genetic characterisation of the steps involved in pleuromutilin biosynthesis, through rational heterologous expression in Aspergillus oryzae coupled with isolation and detailed structural elucidation of the pathway intermediates by spectroscopic methods and comparison with synthetic standards. A. oryzae was further established as a platform for bio-conversion of chemically modified analogues of pleuromutilin intermediates, and was employed to generate a semi-synthetic pleuromutilin derivative with enhanced antibiotic activity. These studies pave the way for future characterisation of biosynthetic pathways of other basidiomycete natural products in ascomycete heterologous hosts, and open up new possibilities of further chemical modification for the growing class of potent pleuromutilin antibiotics.

Organocatalytic and Late-Stage CH-Functionalization Enabled Asymmetric Synthesis of Communesin F and Putative Communesins

Herein we report the total syntheses of communesin F and putative members of the communesin family of polycyclic bis-aminal alkaloids. The successful strategy featured a novel organocatalytic reaction between two oxindole subunits to cast, after extensive optimization, the all-carbon vicinal quaternary stereocenters of the target molecule with high enantiocontrol. The resulting bis-oxindole intermediate further underwent a Ti(O ⁱPr)₄-mediated dehydrative skeletal rearrangement to furnish the communesin core structure. Consider the ready availability and low-cost of unsubstituted isatin, and the inferior organocatalytic reaction employing a bromo-substituted substrate, a Pd(OAc)₂-catalyzed and oxalamide-directed aryl CH-alkenylation reaction was implemented to assemble the complete skeletal backbone of the target molecule. Collectively, the synthetic technologies disclosed herein constitute the first asymmetric organocatalytic approach to the communesins, together with a highly effective late-stage CH-functionalization in stark contrast to the bromoarene substrates employed in all of the past synthetic work.

LaeA regulation of secondary metabolism modulates virulence in Penicillium expansum and is mediated by sucrose

Penicillium expansum, the causal agent of blue mould rot, is a critical health concern because of the production of the mycotoxin patulin in colonized apple fruit tissue. Although patulin is produced by many Penicillium species, the factor(s) activating its biosynthesis are not clear. Sucrose, a key sugar component of apple fruit, was found to modulate patulin accumulation in a dose-responsive pattern. An increase in sucrose culture amendment from 15 to 175 mm decreased both patulin accumulation and expression of the global regulator laeA by 175- and five-fold, respectively, whilst increasing expression of the carbon catabolite repressor creA. LaeA was found to regulate several secondary metabolite genes, including the patulin gene cluster and concomitant patulin synthesis in vitro. Virulence studies of ΔlaeA mutants of two geographically distant P. expansum isolates (Pe-21 from Israel and Pe-T01 from China) showed differential reduction in disease severity in freshly harvested fruit, ranging from no reduction for Ch-Pe-T01 strains to 15%-25% reduction for both strains in mature fruit, with the ΔlaeA strains of Is-Pe-21 always showing a greater loss in virulence. The results suggest the importance of abiotic factors in LaeA regulation of patulin and other secondary metabolites that contribute to pathogenicity.

The family of berberine bridge enzyme-like enzymes: A treasure-trove of oxidative reactions

Biological oxidations form the basis of life on earth by utilizing organic compounds as electron donors to drive the generation of metabolic energy carriers, such as ATP. Oxidative reactions are also important for the biosynthesis of complex compounds, i.e. natural products such as alkaloids that provide vital benefits for organisms in all kingdoms of life. The vitamin B₂-derived cofactors flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) enable an astonishingly diverse array of oxidative reactions that is based on the versatility of the redox-active isoalloxazine ring. The family of FAD-linked oxidases can be divided into subgroups depending on specific sequence features in an otherwise very similar structural context. The sub-family of berberine bridge enzyme (BBE)-like enzymes has recently attracted a lot of attention due to the challenging chemistry catalyzed by its members and the unique and unusual bi-covalent attachment of the FAD cofactor. This family is the focus of the present review highlighting recent advancements into the structural and functional aspects of members from bacteria, fungi and plants. In view of the unprecedented reaction catalyzed by the family's namesake, BBE from the California poppy, recent studies have provided further insights into nature's treasure chest of oxidative reactions.

Ergot alkaloids: synthetic approaches to lysergic acid and clavine alkaloids

Covering: 2000 to 2017Ergot alkaloids are among the most important pharmaceuticals and natural toxins. Significant progress has been achieved in recent years on the research of ergot alkaloids. In this review, we re-introduced the history of ergot alkaloids. Meanwhile, we summarized all the natural products and semi-synthetic derivatives of ergot alkaloids. We also briefly described the biosynthesis and semi-synthesis of ergot alkaloid drugs from raw materials obtained by fermentation. Moreover, we reviewed the advances that have been made in the total synthesis of ergot alkaloids since 2000.

Marine natural products

Covering: 2015. Previous review: Nat. Prod. Rep., 2016, 33, 382-431This review covers the literature published in 2015 for marine natural products (MNPs), with 1220 citations (792 for the period January to December 2015) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1340 in 429 papers for 2015), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Oxidative Cyclization in Natural Product Biosynthesis

Oxidative cyclizations are important transformations that occur widely during natural product biosynthesis. The transformations from acyclic precursors to cyclized products can afford morphed scaffolds, structural rigidity, and biological activities. Some of the most dramatic structural alterations in natural product biosynthesis occur through oxidative cyclization. In this Review, we examine the different strategies used by nature to create new intra(inter)molecular bonds via redox chemistry. This Review will cover both oxidation- and reduction-enabled cyclization mechanisms, with an emphasis on the former. Radical cyclizations catalyzed by P450, nonheme iron, α-KG-dependent oxygenases, and radical SAM enzymes are discussed to illustrate the use of molecular oxygen and S-adenosylmethionine to forge new bonds at unactivated sites via one-electron manifolds. Nonradical cyclizations catalyzed by flavin-dependent monooxygenases and NAD(P)H-dependent reductases are covered to show the use of two-electron manifolds in initiating cyclization reactions. The oxidative installations of epoxides and halogens into acyclic scaffolds to drive subsequent cyclizations are separately discussed as examples of "disappearing" reactive handles. Last, oxidative rearrangement of rings systems, including contractions and expansions, will be covered.