Biosynthesis of α-pyrones

Facile construction of 2-pyrones under carbene catalysis

2-Pyrones are valuable structural motifs in organic chemistry, found in numerous natural products and pharmaceuticals. The synthesis of these heterocycles has been significantly advanced by the application of N-Heterocyclic Carbene (NHC) catalysis. This review examines the recent advancements in NHC-catalyzed synthesis of 2-pyrones, highlighting key methodologies, mechanisms, and synthetic applications. NHC catalysis has revolutionized the synthesis of 2-pyrones, providing efficient, selective, and versatile methods for constructing these valuable heterocycles.

Natural resorcylic lactones derived from alternariol

In this overview, naturally occurring resorcylic lactones biosynthetically derived from alternariol and almost exclusively produced by fungi, are discussed with view on their isolation, structure, biological activities, biosynthesis, and total syntheses. This class of compounds consists until now of 127 naturally occurring compounds, with very divers structural motifs. Although only a handful of these toxins (i.e., alternariol and its 9-O-methyl ether, altenusin, dehydroaltenusin, altertenuol, and altenuene) were frequently found and isolated as fungal contaminants in food and feed and have been investigated in significant detail, further metabolites, which were much more rarely found as natural products, similarly show interesting biological activities.

Human AKR1C3 binds agonists of GPR84 and participates in an expanded polyamine pathway

Altered human aldo-keto reductase family 1 member C3 (AKR1C3) expression has been associated with poor prognosis in diverse cancers, ferroptosis resistance, and metabolic diseases. Despite its clinical significance, the endogenous biochemical roles of AKR1C3 remain incompletely defined. Using untargeted metabolomics, we identified a major transformation mediated by AKR1C3, in which a spermine oxidation product "sperminal" is reduced to "sperminol." Sperminal causes DNA damage and activates the DNA double-strand break response, whereas sperminol induces autophagy in vitro. AKR1C3 also pulls down acyl-pyrones and pyrone-211 inhibits AKR1C3 activity. Through G protein-coupled receptor ligand screening, we determined that pyrone-211 is also a potent agonist of the semi-orphan receptor GPR84. Strikingly, mammalian fatty acid synthase produces acyl-pyrones in vitro, and this production is modulated by NADPH. Taken together, our studies support a regulatory role of AKR1C3 in an expanded polyamine pathway and a model linking fatty acid synthesis and NADPH levels to GPR84 signaling.

α-Pyrone mediates quorum sensing through the conservon system in Nocardiopsis sp

Actinobacteria produce a plethora of bioactive secondary metabolites that are often regulated by quorum-sensing signaling molecules via specific binding to their cognate TetR-type receptors. Here, we identified monocyclic α-pyrone as a new class of actinobacterial signaling molecules influencing quorum sensing process in Nocardiopsis sp. LDBS0036, primarily evidenced by a significant reduction in the production of phenazines in the pyrone-null mutant compared to the wild-type strain. Exogenous addition of the α-pyrone can partially restore the expression of some pathways to the wild strain level. Moreover, a unique multicomponent system referred to as a conservon, which is widespread in actinobacteria and generally contains four or five functionally conserved proteins, may play an important role in detecting and transmitting α-pyrone signals in LDBS0036. We found the biosynthetic gene clusters of α-pyrone and their associated conservon genes are highly conserved in Nocardiopsis, indicating the widespread prevalence and significant function of this regulate mechanism within Nocardiopsis genus. Furthermore, homologous α-pyrones from different actinobacterial species were also found to mediate interspecies communication. Our results thus provide insights into a novel quorum-sensing signaling system and imply that various modes of bacterial communication remain undiscovered.

Triplinones A-H: Anti-Inflammatory Arylalkenyl α,β-Unsaturated-δ-Lactones Isolated from the Leaves of Australian Rainforest Plant Cryptocarya triplinervis (Lauraceae)

Our ongoing exploration of Australian rainforest plants for the biodiscovery of anti-inflammatory agents led to the isolation and structural elucidation of eight new arylalkenyl α,β-unsaturated-δ-lactones, triplinones A-H (1-8), from the leaves of the Australian rainforest plant Cryptocarya triplinervis B. Hyland (Lauraceae). The chemical structures of these compounds were established by NMR spectroscopic data analysis, while their relative and absolute configurations were established using a combination of Mosher ester analysis utilizing both Riguera's and Kishi's methods, ECD experiments, and X-ray crystallography analysis. Compounds 1-8 exhibited good inhibitory activities toward nitric oxide (NO) production in lipopolysaccharide (LPS) and interferon (IFN)-γ induced RAW 264.7 macrophages, in particular compounds 1-3 and 5, with IC50 values of 7.3 ± 0.5, 6.0 ± 0.3, 5.6 ± 0.3, and 5.4 ± 2.5 μM, respectively.

Regioselective Annulation of 6-Carboxy-Substituted Pyrones as a Two-Carbon Unit in Formal [4 + 2] Cycloaddition Reactions

Heterocycles serve as a critical motif in chemistry, but despite being present in more than 85% of pharmaceuticals, there are limited methods for their construction. Here, we describe the incorporation of intact pyrone (2H-pyran-2-one) into larger ring systems via annulation. In a formal [4 + 2] cycloaddition, the pyrone regioselectively accepts a benzylic anion as a nucleophile in a conjugate addition fashion, with the subsequent pyrone-derived enolate attaching to a pendant ester on the initial nucleophile. Subsequent base-driven enolate formation and elimination establish aromaticity of the newly formed ring. After optimization of this process using an NMR-based assessment to overcome solubility and separation challenges, the reaction was successfully applied to a library of 6-ester and -amide-substituted pyrones and using a phenyl ester and other substituted sulfoxides. This technology enables the incorporation of intact pyrone rings into more complex systems, such as for the total synthesis of the natural product thermorubin.

Demonstrating the Applicability of Proton Transfer Reaction Mass Spectrometry to Quantify Volatiles Emitted by the Mycoparasitic Fungus Trichoderma atroviride in Real Time: Monitoring of Trichoderma-Based Biopesticides

The present study aims to explore the potential application of proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) for real-time monitoring of microbial volatile organic compounds (MVOCs). This investigation can be broadly divided into two parts. First, a selection of 14 MVOCs was made based on previous research that characterized the MVOC emissions of Trichoderma atroviride, which is a filamentous fungus widely used as a biocontrol agent. The analysis of gas-phase standards using PTR-ToF-MS allowed for the categorization of these 14 MVOCs into two groups: the first group primarily undergoes nondissociative proton transfer, resulting in the formation of protonated parent ions, while the second group mainly undergoes dissociative proton transfer, leading to the formation of fragment ions. In the second part of this investigation, the emission of MVOCs from samples of T. atroviride was continuously monitored over a period of five days using PTR-ToF-MS. This also included the first quantitative online analysis of 6-amyl-α-pyrone (6-PP), a key MVOC emitted by T. atroviride. The 6-PP emissions of T. atroviride cultures were characterized by a gradual increase over the first two days of cultivation, reaching a plateau-like maximum with volume mixing ratios exceeding 600 ppbv on days three and four. This was followed by a marked decrease, where the 6-PP volume mixing ratios plummeted to below 50 ppbv on day five. This observed sudden decrease in 6-PP emissions coincided with the start of sporulation of the T. atroviride cultures as well as increasing intensities of product ions associated with 1-octen-3-ol and 3-octanone, whereas both these MVOCs were previously associated with sporulation in T. atroviride. The study also presents the observations and discussion of further MVOC emissions from the T. atroviride samples and concludes with a critical assessment of the possible applications and limitations of PTR-ToF-MS for the online monitoring of MVOCs from biological samples in real time.

Comprehensive updates on the biological features and metabolic potential of the versatile extremophilic actinomycete Nocardiopsis dassonvillei

Nocardiopsis dassonvillei prevails under harsh environmental conditions and the purpose of this review is to highlight its biological features and recent biotechnological applications. The organism prevails in salt-rich soils/marine systems and some strains endure extreme temperatures and pH. A few isolates are associated with marine organisms and others cause human diseases. Comparative genomic analysis indicates its versatility in producing biotechnologically relevant metabolites. Antimicrobial, cytotoxic, anticancer and growth promoting biomolecules are obtained from this organism. It also synthesizes biotechnologically important enzymes. Bioactive compounds and enzymes obtained from this actinomycete provide evidence regarding its metabolic competence and its potential economic value.

Hyperhenrones: Prenylated α-pyrones with anti-inflammatory activity from Hypericum henryi

Fourteen previously undescribed α-pyrone derivatives (1-14) together with four known analogs (15-18) were isolated from a traditional Chinese medicinal plant Hypericum henryi. Compounds (+)/(-)-1, 2, and 3 share a rare 6/6/4/6/6 polycyclic skeleton. Compound 14 was the first example of a 7,7-dimethyl-pyran-4-one moiety. Their structures were elucidated using comprehensive spectroscopic analyses and electronic circular dichroism calculations. The anti-inflammatory activities of 1-18 were screened in lipopolysaccharide-induced RAW264.7 cells. Among them, compounds 14, (+)-18, and (-)-18 exhibited inhibitory effects against nitric oxide production in LPS-induced RAW264.7 cells. Additionally, compound 14 suppressed the expression of cyclooxygenase-2 and inducible nitric oxide synthase in LPS-induced RAW264.7 cells. Furthermore, preliminary mechanism studies indicated that compound 14 suppressed the phosphorylation and degradation of the inhibitor of NF-κB, and this led to the inhibition of NF-κB activation.

Unravelling the Secrets of α-Pyrones from Aspergillus Fungi: A Comprehensive Review of Their Natural Sources, Biosynthesis, and Biological Activities

Aspergillus, one of the most product-rich and genetically robust genera, contains a diverse range of species with potential economic and ecological implications. Chemically, Aspergillus is one of the essential sources of polyketides, alkaloids, diphenyl ethers, diketopiperazines, and other miscellaneous compounds, displaying a variety of pharmacological activities. The α-pyrones are unsaturated six-membered lactones. Although α-pyrone has a small structure, it is responsible for the structural diversity of several natural and synthetic compounds and multiple biological activities. In this review, we have summarized approximately 178 α-pyrone containing metabolites derivatives identified/reported from terrestrial, marine, endophytic, and filamentous Aspergillus species, including their sources, biological properties, and biosynthetic pathways until mid-2023, for the first time. This review is the first to compile and analyze the available data on α-pyrone metabolites from Aspergillus, which could facilitate further research and innovation in this field. Additionally, it offers a valuable source of scaffolds for future bioactive drug development, as some of these metabolites have shown potent antimicrobial, anti-inflammatory, and anticancer effects. Therefore, this review has significant implications for the advancement of natural product chemistry, pharmacology, biotechnology, and medicine.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Secondary Metabolites and Biosynthetic Gene Clusters Analysis of Deep-Sea Hydrothermal Vent-Derived Streptomyces sp. SCSIO ZS0520

Streptomyces sp. SCSIO ZS0520 is a deep-sea hydrothermal vent-derived actinomycete. Our previous metabolism investigation showed that Streptomyces sp. SCSIO ZS0520 is a producer of cytotoxic actinopyrones. Here, another four types of secondary metabolites were identified, including six salinomycin isomers (2–7), the macrolide elaiophylin (8), the triterpene N-acetyl-aminobacteriohopanetriol (9), and the pyrone minipyrone (10). Among them, compounds 2–6 and 10 are new compounds. To understand the biosynthetic pathway of these compounds, a bioinformatic analysis of the whole genome was carried out, which identified 34 secondary metabolite biosynthetic gene clusters. Next, the biosynthetic pathways responsive to four types of products were deduced on the basis of gene function predictions and structure information. Taken together, these findings prove the metabolite potential of ZS0520 and lay the foundations to solve the remaining biosynthetic issues in four types of marine natural products.

Discovery of prescopranone, a key intermediate in scopranone biosynthesis

A key intermediate in scopranone biosynthesis, prescopranone, accumulated in the mycelium of Streptomyces avermitilis SUKA carrying the biosynthetic gene cluster for scopranone lacking the sprT encoding the monooxygenase. The structure of prescopranone was elucidated by NMR and other spectral data. Prescopranone consists of a 2-pyranone ring with two atypical scoop-like moieties (1-ethyl-1-propenyl and 2-ethylbutyl groups), which was deduced as a product of the modular polyketide syntheses encoded by sprA, sprB, and sprC. Prescopranone inhibited bone morphogenetic protein (BMP)-induced alkaline phosphatase activity in a BMP receptor mutant cell line.

Biosynthesis and biological function of secondary metabolites of the rice blast fungus Pyricularia oryzae

Filamentous fungi have many secondary metabolism genes and produce a wide variety of secondary metabolites with complex and unique structures. However, the role of most secondary metabolites remains unclear. Moreover, most fungal secondary metabolism genes are silent or poorly expressed under laboratory conditions and are difficult to utilize. Pyricularia oryzae, the causal pathogen of rice blast disease, is a well-characterized plant pathogenic fungus. P. oryzae also has a large number of secondary metabolism genes and appears to be a suitable organism for analyzing secondary metabolites. However, in case of this fungus, biosynthetic genes for only four groups of secondary metabolites have been well characterized. Among two of the four groups of secondary metabolites, biosynthetic genes were identified by activating secondary metabolism. These secondary metabolites include melanin, a polyketide compound required for rice infection; tenuazonic acid, a well-known mycotoxin produced by various plant pathogenic fungi and biosynthesized by a unique nonribosomal peptide synthetase-polyketide synthase hybrid enzyme; nectriapyrones, antibacterial polyketide compounds produced mainly by symbiotic fungi, including plant pathogens and endophytes, and pyriculols, phytotoxic polyketide compounds. This review mainly focuses on the biosynthesis and biological functions of the four groups of P. oryzae secondary metabolites.

Secondary Metabolites of Purpureocilliumlilacinum

Fungi can synthesize a wealth of secondary metabolites, which are widely used in the exploration of lead compounds of pharmaceutical or agricultural importance. Beauveria, Metarhizium, and Cordyceps are the most extensively studied fungi in which a large number of biologically active metabolites have been identified. However, relatively little attention has been paid to Purpureocillium lilacinum. P. lilacinum are soil-habituated fungi that are widely distributed in nature and are very important biocontrol fungi in agriculture, providing good biological control of plant parasitic nematodes and having a significant effect on Aphidoidea, Tetranychus cinnbarinus, and Aleyrodidae. At the same time, it produces secondary metabolites with various biological activities such as anticancer, antimicrobial, and insecticidal. This review attempts to provide a comprehensive overview of the secondary metabolites of P. lilacinum, with emphasis on the chemical diversity and biological activity of these secondary metabolites and the biosynthetic pathways, and gives new insight into the secondary metabolites of medical and entomogenous fungi, which is expected to provide a reference for the development of medicine and agrochemicals in the future.

Defueling the cancer: ATP synthase as an emerging target in cancer therapy

Reprogramming of cellular metabolism is a hallmark of cancer. Mitochondrial ATP synthase (MAS) produces most of the ATP that drives the cell. High expression of the MAS-composing proteins is found during cancer and is linked to a poor prognosis in glioblastoma, ovarian cancer, prostate cancer, breast cancer, and clear cell renal cell carcinoma. Cell surface-expressed ATP synthase, translocated from mitochondrion to cell membrane, involves the angiogenesis, tumorigenesis, and metastasis of cancer. ATP synthase has therefore been considered a therapeutic target. We review recent various ATP synthase inhibitors that suppress tumor growth and are being tested for the clinic.

Emergence of 2-Pyrone and Its Derivatives, from Synthesis to Biological Perspective: An Overview and Current Status

Pyrone moieties are present in natural products and can be synthesized by a diverse range of synthetic methods, resulting in the formation of various derivatives through chemical modifications. Many pyrone-based derivatives are commercially available and are biocompatible. They are building blocks of various intermediates in organic synthesis. They possess remarkable biological properties including antimicrobial, antiviral, cytotoxic, and antitumor activity. These characteristics have made them valuable for the development of drugs. We have summarized recent developments in the synthesis of 2-pyrone and its derivatives and their potential applications. With regard to synthetic approaches, the focus has been on metal-free and transition metal-catalyzed reactions.

Precursor Supply Increases the Accumulation of 4-Hydroxy-6-(4-hydroxyphenyl)-α-pyrone after NRPS-PKS Gene Expression

Expression of a nonribosomal peptide synthetase-nonreducing polyketide synthase hybrid gene pcr10109 from Penicillium crustosum PRB-2 in Aspergillus nidulans led to the accumulation of 4-hydroxy-6-(4-hydroxyphenyl)-α-pyrone (1). Adding para-hydroxybenzoic acid into the medium in which the overexpressing mutant is growing increased the product yield up to 5-fold. This strategy could be helpful for heterologous gene expression experiments requiring special substrates for product formation.

CRISPR-Cas9-Based Discovery of the Verrucosidin Biosynthesis Gene Cluster in Penicillium polonicum

Penicillium polonicum, commonly found on food matrices, is a mycotoxigenic species able to produce a neurotoxin called verrucosidin. This methylated α-pyrone polyketide inhibits oxidative phosphorylation in mitochondria and thereby causes neurological diseases. Despite the importance of verrucosidin as a toxin, its biosynthetic genes have not been characterized yet. By similarity analysis with the polyketide synthase (PKS) genes for the α-pyrones aurovertin (AurA) and citreoviridin (CtvA), 16 PKS genes for putative α-pyrones were identified in the P. polonicum genome. A single PKS gene, verA, was found to be transcribed under verrucosidin-producing growth conditions. The annotated functions of the genes neighboring verA correspond to those required for verrucosidin biosynthesis. To prove the involvement of verA in verrucosidin biosynthesis, the clustered regularly interspaced short palindrome repeats (CRISPR) technology was applied to P. polonicum. In vitro reconstituted CRISPR-Cas9 was used to induce targeted gene deletions in P. polonicum. This approach allowed identifying and characterizing the verrucosidin biosynthetic gene cluster. VerA deletion mutants were no longer able to produce verrucosidin, whereas they were displaying morphological characteristics comparable with the wild-type strain. The available CRISPR-Cas9 technology allows characterizing the biosynthetic potential of P. polonicum as a valuable source of novel compounds.

Bifunctional Urea/Hg(OAc)2-Mediated Synthesis of 4-Aryl-6-oxycarbonyl-2-pyrones and 2-Pyridones from Dithiomalonate and β,γ-Unsaturated α-Keto Esters

Disubstituted 2-pyrones and 2-pyridones were obtained by bifunctional urea-catalyzed Michael addition/lactonization or lactamization followed by a Hg(OAc)₂- or Hg(OAc)₂/DBU-mediated hydrolysis/decarboxylation/dehydrogenation process. This one-pot two-stage protocol enabled the rapid synthesis of 4,6-disubstituted 2-pyrones and 2-pyridones from dithiomalonate and β,γ-unsaturated α-keto esters in practical yields under mild reaction conditions. Additionally, the obtained 2-pyridones were facilely transformed to 2,4,6-trisubstituted pyridines in excellent yields.

A formal [3 + 3] cycloaddition of allenyl imide and activated ketones for the synthesis of tetrasubstituted 2-pyrones

CsOH·H₂O-catalyzed formal [3 + 3] cycloadditions of allenyl imide with β-ketoesters, 1,3-diketones or β-ketonitriles for the synthesis of tetrasubstituted 2-pyrone derivatives have been demonstrated. The allenyl imide was utilized as a C3-synthon, and a ketenyl intermediate was proposed via the process of 1,4-addition of carbon anion to allene followed by elimination of the 2-oxazolidinyl group.

CsOH·H₂O-catalyzed formal [3 + 3] cycloadditions of allenyl imide with β-ketoesters, 1,3-diketones or β-ketonitriles for the synthesis of tetrasubstituted 2-pyrone derivatives were reported.

Divergent rhodium-catalyzed electrochemical vinylic C-H annulation of acrylamides with alkynes

α-Pyridones and α-pyrones are ubiquitous structural motifs found in natural products and biologically active small molecules. Here, we report an Rh-catalyzed electrochemical vinylic C-H annulation of acrylamides with alkynes, affording cyclic products in good to excellent yield. Divergent syntheses of α-pyridones and cyclic imidates are accomplished by employing N-phenyl acrylamides and N-tosyl acrylamides as substrates, respectively. Additionally, excellent regioselectivities are achieved when using unsymmetrical alkynes. This electrochemical process is environmentally benign compared to traditional transition metal-catalyzed C-H annulations because it avoids the use of stoichiometric metal oxidants. DFT calculations elucidated the reaction mechanism and origins of substituent-controlled chemoselectivity. The sequential C-H activation and alkyne insertion under rhodium catalysis leads to the seven-membered ring vinyl-rhodium intermediate. This intermediate undergoes either the classic neutral concerted reductive elimination to produce α-pyridones, or the ionic stepwise pathway to produce cyclic imidates.

Secondary metabolites of Phlebopus species from Northern Thailand

Submerged cultures of the edible mushrooms Phlebopus portentosus and Phlebopus spongiosus were screened for their secondary metabolites by HPLC-UV/Vis and HR-LC-ESI-MS. Two new compounds, 9′-hydroxyphenyl pulvinone (1), containing an unusual pulvinone structure, and phlebopyron (2), together with the seven known pigments, atromentic acid (3), xerocomic acid (4), variegatic acid (5), methyl atromentate (6), methyl isoxerocomate (7), methyl variegatate (8), and variegatorubin (9) were isolated from the cultures. Their structures were assigned on the basis of extensive 1D/2D NMR spectroscopic analyses, as well as HR-ESI-MS, and HR-ESI-MS/MS measurements. Furthermore, the isolated compounds were evaluated for their antimicrobial and cytotoxic properties. 9′-hydroxyphenyl pulvinone (1), xerocomic acid (4), and methyl variegatate (8) exhibited weak to moderate cytotoxic activities against several tumor cell lines. The present paper provides a comprehensive characterization of pigments from the class of pulvinic acids that are present in the basidiomes of many edible bolete species.

Secondary Metabolites of the Rice Blast Fungus Pyricularia oryzae: Biosynthesis and Biological Function

Plant pathogenic fungi produce a wide variety of secondary metabolites with unique and complex structures. However, most fungal secondary metabolism genes are poorly expressed under laboratory conditions. Moreover, the relationship between pathogenicity and secondary metabolites remains unclear. To activate silent gene clusters in fungi, successful approaches such as epigenetic control, promoter exchange, and heterologous expression have been reported. Pyricularia oryzae, a well-characterized plant pathogenic fungus, is the causal pathogen of rice blast disease. P. oryzae is also rich in secondary metabolism genes. However, biosynthetic genes for only four groups of secondary metabolites have been well characterized in this fungus. Biosynthetic genes for two of the four groups of secondary metabolites have been identified by activating secondary metabolism. This review focuses on the biosynthesis and roles of the four groups of secondary metabolites produced by P. oryzae. These secondary metabolites include melanin, a polyketide compound required for rice infection; pyriculols, phytotoxic polyketide compounds; nectriapyrones, antibacterial polyketide compounds produced mainly by symbiotic fungi including endophytes and plant pathogens; and tenuazonic acid, a well-known mycotoxin produced by various plant pathogenic fungi and biosynthesized by a unique NRPS-PKS enzyme.

Xylariaopyrones E-I, five new α-pyrone derivatives from the endophytic fungus Xylariales sp. (HM-1)

Five new α-pyrones, xylariaopyrones E-I (1-5), along with three known analogues (6-8) were isolated from the cultivation broth of the endophytic fungus Xylariales sp. (HM-1). The structures of the new compounds including their absolute configurations were elucidated by comprehensive spectroscopic methods and quantum ECD calculations. Xylariaopyrone E (1) is the first example of α-pyrone derivative with a novel [3, 2, 0] bridge ring system via a ketal function group in the side chain. In bioactivity assays, xylariaopyrones E-G (1-3) showed moderate inhibiting activities against Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa with MIC values from 25.4 to 64.5 μg/mL, whereras xylariaopyrone G (3) exhibited significant inhibition of monoamine oxidase B with an IC₅₀ value of 15.6 μmol/L. Xylariaopyrone H (4) and the known compound 7 showed moderate toxicity against brine shrimp larvae with inhibition rates of 42.8% and 44.5%, respectively.

The Lipoxygenase Lox1 Is Involved in Light- and Injury-Response, Conidiation, and Volatile Organic Compound Biosynthesis in the Mycoparasitic Fungus Trichoderma atroviride

The necrotrophic mycoparasite Trichoderma atroviride is a biological pest control agent frequently applied in agriculture for the protection of plants against fungal phytopathogens. One of the main secondary metabolites produced by this fungus is 6-pentyl-α-pyrone (6-PP). 6-PP is an organic compound with antifungal and plant growth-promoting activities, whose biosynthesis was previously proposed to involve a lipoxygenase (Lox). In this study, we investigated the role of the single lipoxygenase-encoding gene lox1 encoded in the T. atroviride genome by targeted gene deletion. We found that light inhibits 6-PP biosynthesis but lox1 is dispensable for 6-PP production as well as for the ability of T. atroviride to parasitize and antagonize host fungi. However, we found Lox1 to be involved in T. atroviride conidiation in darkness, in injury-response, in the production of several metabolites, including oxylipins and volatile organic compounds, as well as in the induction of systemic resistance against the plant-pathogenic fungus Botrytis cinerea in Arabidopsis thaliana plants. Our findings give novel insights into the roles of a fungal Ile-group lipoxygenase and expand the understanding of a light-dependent role of these enzymes.

Allantopyrone E, a rare α-pyrone metabolite from the mangrove derived fungus Aspergillus versicolor

A new α-pyrone derivative, with a rare substitution pattern, was isolated from the solid rice media of the fungal endophyte Aspergillus versicolor associated with the fruit of the mangrove Avicennia marina. It was named allantopyrone E, after allantopyrones C and D. The aforementioned three are the only diaryl-substituted α-pyrones to be reported from nature. The unique structure was elucidated based on one- and two dimensional NMR data as well as high resolution mass spectroscopy. A biosynthetic scheme was proposed for the new compound. Furthermore, allantopyrone E showed cytotoxic effect on HeLa cells with IC₅₀= 50.97 μM.

Cyclocondensation of coumarin-3-thioformates with 3-hydroxyoxindoles and 3-aminooxindoles for the synthesis of spiro-fused pentaheterocyclic compounds

A range of spiro-fused pentaheterocyclic compounds including spiro-butyrolactoneoxindole[3,4-c]coumarins and spiro-butyrolactamoxindole[3,4-c]coumarins were smoothly obtained via tandem Michael addition-lactonization/lactamization process.

Electrochemistry-Enabled Ir-Catalyzed Vinylic C-H Functionalization

Synergistic use of electrochemistry and organometallic catalysis has emerged as a powerful tool for site-selective C-H functionalization, yet this type of transformation has thus far mainly been limited to arene C-H functionalization. Herein, we report the development of electrochemical vinylic C-H functionalization of acrylic acids with alkynes. In this reaction an iridium catalyst enables C-H/O-H functionalization for alkyne annulation, affording α-pyrones with good to excellent yields in an undivided cell. Preliminary mechanistic studies show that anodic oxidation is crucial for releasing the product and regeneration of an Ir(III) intermediate from a diene-Ir(I) complex, which is a coordinatively saturated, 18-electron complex. Importantly, common chemical oxidants such as Ag(I) or Cu(II) did not give significant amounts of the desired product in the absence of electrical current under otherwise identical conditions.

Production of New Antibacterial 4-Hydroxy-α-Pyrones by a Marine Fungus Aspergillus niger Cultivated in Solid Medium

Four 4-hydroxy-α-pyrones including three new ones named nipyrones A-C (1-3) together with one known analogue germicidin C (4) were discovered from a marine sponge-derived fungus Aspergillus niger cultivated in a solid rice culture. Their structures and absolute configurations were elucidated through a combination of spectroscopic data and electronic circular dichroism (ECD) calculations as well as comparison with literature data. Compounds 1-4 were evaluated for their antibacterial activities against five pathogenic bacteria (Staphylococcus aureus, Escherichia coli, Bacillus subtilis, methicillin-resistant Staphylococcus aureus, and Mycobacterium tuberculosis). Compound 3 showed promising activity against S. aureus and B. subtilis, with minimum inhibitory concentration (MIC) values of 8 μg/mL and 16 μg/mL, respectively, and displayed weak antitubercular activities against M. tuberculosis, with MIC value of 64 μg/mL, while compounds 1 and 2 exhibited moderate antibacterial efficacy against four pathogenic bacteria with MIC values of 32-64 μg/mL.

New α-pyrones from an endophytic fungus, Hypoxylon investiens J2

Four new α-pyrones, hypotiens A–D (1–4), were isolated from a fungal endophyte, Hypoxylon investiens J2, harbored in the medicinal plant Blumea balsamifera. Their structures were determined through detailed HRMS and NMR spectroscopic data. Compounds 1–4 are new α-pyrone derivatives containing an unusual dimethyl substitution in the highly unsaturated side chain. Their plausible biosynthetic pathway was discussed. Biological assay indicated that compounds 1–4 showed no antimicrobial, quorum sensing inhibitory, and cytotoxic activities. The specific side chain in α-pyrone derivatives 1–4 might be responsible for the weak pharmacological activities.

Four new α-pyrones, hypotiens A–D (1–4), were isolated from a fungal endophyte, Hypoxylon investiens J2, harbored in the medicinal plant Blumea balsamifera.

Chemotaxonomy of Mycotoxigenic Small-Spored Alternaria Fungi - Do Multitoxin Mixtures Act as an Indicator for Species Differentiation?

Necrotrophic as well as saprophytic small-spored Alternaria (A.) species are annually responsible for major losses of agricultural products, such as cereal crops, associated with the contamination of food and feedstuff with potential health-endangering Alternaria toxins. Knowledge of the metabolic capabilities of different species-groups to form mycotoxins is of importance for a reliable risk assessment. 93 Alternaria strains belonging to the four species groups Alternaria tenuissima, A. arborescens, A. alternata, and A. infectoria were isolated from winter wheat kernels harvested from fields in Germany and Russia and incubated under equal conditions. Chemical analysis by means of an HPLC-MS/MS multi-Alternaria-toxin-method showed that 95% of all strains were able to form at least one of the targeted 17 non-host specific Alternaria toxins. Simultaneous production of up to 15 (modified) Alternaria toxins by members of the A. tenuissima, A. arborescens, A. alternata species-groups and up to seven toxins by A. infectoria strains was demonstrated. Overall tenuazonic acid was the most extensively formed mycotoxin followed by alternariol and alternariol mono methylether, whereas altertoxin I was the most frequently detected toxin. Sulfoconjugated modifications of alternariol, alternariol mono methylether, altenuisol and altenuene were frequently determined. Unknown perylene quinone derivatives were additionally detected. Strains of the species-group A. infectoria could be segregated from strains of the other three species-groups due to significantly lower toxin levels and the specific production of infectopyrone. Apart from infectopyrone, alterperylenol was also frequently produced by 95% of the A. infectoria strains. Neither by the concentration nor by the composition of the targeted Alternaria toxins a differentiation between the species-groups A. alternata, A. tenuissima and A. arborescens was possible.

Fungal endophytes from arid areas of Andalusia: high potential sources for antifungal and antitumoral agents

Native plant communities from arid areas present distinctive characteristics to survive in extreme conditions. The large number of poorly studied endemic plants represents a unique potential source for the discovery of novel fungal symbionts as well as host-specific endophytes not yet described. The addition of adsorptive polymeric resins in fungal fermentations has been seen to promote the production of new secondary metabolites and is a tool used consistently to generate new compounds with potential biological activities. A total of 349 fungal strains isolated from 63 selected plant species from arid ecosystems located in the southeast of the Iberian Peninsula, were characterized morphologically as well as based on their ITS/28S ribosomal gene sequences. The fungal community isolated was distributed among 19 orders including Basidiomycetes and Ascomycetes, being Pleosporales the most abundant order. In total, 107 different genera were identified being Neocamarosporium the genus most frequently isolated from these plants, followed by Preussia and Alternaria. Strains were grown in four different media in presence and absence of selected resins to promote chemical diversity generation of new secondary metabolites. Fermentation extracts were evaluated, looking for new antifungal activities against plant and human fungal pathogens, as well as, cytotoxic activities against the human liver cancer cell line HepG2. From the 349 isolates tested, 126 (36%) exhibited significant bioactivities including 58 strains with exclusive antifungal properties and 33 strains with exclusive activity against the HepG2 hepatocellular carcinoma cell line. After LCMS analysis, 68 known bioactive secondary metabolites could be identified as produced by 96 strains, and 12 likely unknown compounds were found in a subset of 14 fungal endophytes. The chemical profiles of the differential expression of induced activities were compared. As proof of concept, ten active secondary metabolites only produced in the presence of resins were purified and identified. The structures of three of these compounds were new and herein are elucidated.

Total Synthesis of Pyrophen and Campyrones A-C

The first total syntheses of the natural products pyrophen and campyrones A-C, isolated from the fungus Aspergillus niger, have been achieved in six steps starting from commercially available N-Boc amino acids. Key steps in this sequence include a vinylogous Claisen condensation to achieve fragment coupling and a dioxinone thermolysis/cyclization cascade to form the α-pyrone ring. The route described herein afforded the natural products in 15-25% overall yield, furnishing sufficient material for testing in biological assays.