Aureochaeglobosins A–C, Three [4 + 2] Adducts of Chaetoglobosin and Aureonitol Derivatives from Chaetomium globosum

Antimicrobial sesterterpenoids with a unique 5/8/6/5 tetracyclic carbon-ring-system and diepoxide polyketides from a deep sea-sediment-sourced fungus Chaetomium globosum SD-347

Two new sesterterpenoids, sesterchaetins A and B (1 and 2), and two new diepoxide polyketides, chaetoketoics A and B (3 and 4), were characterized from the culture extract of Chaetomium globosum SD-347, a fungal strain derived from deep sea-sediment. Their structures and absolute configurations were unambiguously determined by detailed NMR, mass spectra, and X-ray crystallographic analysis. Compounds 1 and 2 contained a distinctive 5/8/6/5 tetracyclic carbon-ring-system, which represented a rarely occurring natural product framework. The new isolates 1-4 exhibited selective antimicrobial activities against human and aquatic pathogenic bacteria and plant-pathogenic fungi.

Chaepseubakerins A-J, ten undescribed cytotoxic [11]-chaetoglobosins from an endophytic fungus Pseudeurotium bakeri

Ten previously unreported [11]-chaetoglobosins, chaepseubakerins A-J (1-10), were characterized from the solid rice-based culture of Pseudeurotium bakeri P1-1-1, an endophyte harbored in the roots of Macrocoma tenue subsp. sullivantii Vitt. (Orthotrichaceae). Their structures were determined by spectroscopic analysis, single-crystal X-ray diffraction (Cu Kα radiation), and chemical methods. Chaepseubakerin A (1) exhibited significant cytotoxic effects against seven human cancer cell lines, A549, A427, HCT116, HT-29, HeLa, HepG2, and MCF-7, with IC50 values of 2.9, 3.0, 4.0, 4.4, 7.1, 6.7, and 8.9 μM, respectively. Mechanistically, 1 induced G2/M cell cycle arrest and apoptosis in A549, Hela, and HCT116 cells in a dose dependent manner.

Chemical constituents from the medicinal herb-derived fungus Chaetomium globosum Km1226

BACKGROUND

Endophytic fungi have proven to be a rich source of novel natural products with a wide-array of biological activities and higher levels of structural diversity.

RESULTS

Chemical investigation on the liquid- and solid-state fermented products of Chaetomium globosum Km1226 isolated from the littoral medicinal herb Atriplex maximowicziana Makino resulted in the isolation of compounds 1-14. Their structures were determined by spectroscopic analysis as three previously undescribed C13-polyketides, namely aureonitol C (1), mollipilins G (2), and H (3), along with eleven known compounds 4-14. Among these, mollipilin A (5) exhibited significant nitric oxide production inhibitory activity in LPS-induced BV-2 microglial cells with an IC50 value of 0.7 ± 0.1 µM, and chaetoglobosin D (10) displayed potent anti-angiogenesis property in human endothelial progenitor cells (EPCs) with an IC50 value of 0.8 ± 0.3 µM.

CONCLUSIONS

Three previously unreported compounds 1-3 were isolated and identified. Mollipilin A (5) and chaetoglobosin D (10) could possibly be developed as anti-inflammatory and anti-angiogenic lead drugs, respectively.

Caryophyllene Sesquiterpenes from a Chaetomium globosum Endophyte of the Canadian Medicinal Plant Empetrum nigrum

Punctaporonins T (1) and U (2), new caryophyllene sesquiterpenes, were isolated with three known punctaporonins, A (3), B (4), and C (5), from the endophytic fungus Chaetomium globosum (TC2-041). The structures and relative configurations of punctaporonins T and U were elucidated based on a combination of HRESIMS, 1D/2D NMR spectroscopic analysis, and X-ray diffraction analysis, while their absolute configuration is presumed to be consistent with the co-isolated 3-5 on biogenetic arguments. Compound 1 showed weak inhibitory activity against both Mycobacterium tuberculosis and Staphylococcus aureus.

TRAIL-sensitizing Cytochalasins from the Endophytic Fungus Phoma multirostrata

Seven undescribed cytochalasins, multirostratins K - Q (2: -8: ), together with one known analogue, cytochalasin Z3 (1: ), were isolated from the culture of Phoma multirostrata XJ-2-1, an endophytic fungus obtained from the root of Parasenecio albus. Their structures with absolute configurations were determined by 1D and 2D NMR, high-resolution electrospray ionization mass spectrometry (HRESIMS), electronic circular dichroism (ECD), single-crystal X-ray crystallography, and chemical methods. The structure of ascochalasin was revised from Δ ¹³ to Δ ²¹ by detailed analysis of the NMR data and by comparison with the data for 7: . In a TRAIL (tumor necrosis factor related apoptosis inducing ligand)-resistance-overcoming experiment, co-treatment of 2: or 6: with TRAIL reduced the cell viability of A549 cells by 30.3% and 27.5% at 10 µM, respectively.

[11]-chaetoglobosins with cytotoxic activities from Pseudeurotium bakeri

Fourteen new [11]-chaetoglobosins (1-14), along with two known congeners, cytochalasins X and Y (15 and 16), were isolated from the cultures of an endophytic fungus Pseudeurotium bakeri P1-1-1. Their structures incorporating absolute configurations were elucidated based on the comprehensive analyses of one- and two-dimensional NMR data, HRESIMS spectrometry, chemical methods, and single-crystal X-ray diffraction analysis (Cu Kα). All isolates were evaluated for their cytotoxic activities and chaetopseudeurin M (1) displayed significant cytotoxic effects against seven human cancer cell lines, with IC₅₀ values ranging from 5.1 ± 0.9 to 10.8 ± 0.1 μM. Western blot experiments exhibited that compound 1 exerted its cytotoxic effect in MCF-7 cells by inducing G2/M cell cycle arrest and apoptosis via downregulating the expression of cyclin B1 and Cdk1, and activating Bcl-2/caspase-3/PARP pathway, respectively.

Stereocontrolled Synthesis of Enantiopure cis-Fused Octahydroisoindolones via Chiral Oxazoloisoindolone Lactams

Kinetically controlled cyclocondensation of stereoisomeric and ring-chain tautomeric mixture of (±)-hydroxylactone 1 and 0.5 equiv of (R)-phenylglycinol provided tricyclic oxazoloisoindolone lactam (3R,5aS,9aR,9bS)-2a, a versatile intermediate for further stereocontrolled transformations to access enantiopure cis-fused octahydroisoindolones. An extension of this methodology was successfully applied to the synthesis of the 5,6-dihydroxy derivative (3aR,5R,6S,7aS)-17.

Cytochalasans and azaphilones: suitable chemotaxonomic markers for the Chaetomium species

The accurate taxonomic concept of the fungal Chaetomium species has been a hard work due to morphological similarity. Chemotaxonomy based on secondary metabolites is a powerful tool for taxonomical purposes, which could be used as an auxiliary reference to solve the problems encountered in the classification of Chaetomium. Among secondary metabolites produced by Chaetomium, cytochalasans and azaphilones exhibited a pattern of distribution and frequency of occurrence that establish them as chemotaxonomic markers for the Chaetomium species. This review attempted to elucidate the composition of the Chaetomium species and its relationship with classical taxonomy by summarizing the pattern of cytochalasans and azaphilones distribution and biosynthesis in the Chaetomium species. KEY POINTS: • Secondary metabolites from the genus Chaetomium are summarized. • Cytochalasans and azaphilones could be characteristic metabolites of the Chaetomium species. • Cytochalasans and azaphilones could be used to analyze for taxonomical purposes.

Endophytic fungi: A treasure trove of novel anticancer compounds

Cancer is a multifactorial disease with a convoluted genesis and progression. The emergence of multidrug resistance to presently be offered drug and relapse is by far, the most critical concern to tackle this deteriorating disease. Henceforth, there is undeniably an inflated necessity for safe, promising, and less harmful new anticancer drugs. Natural compounds from various sources like plants, animals, and microorganisms have occupied a center stage in drug discovery due to their tremendous chemical diversity and potential as therapeutic agents. Endophytic microbes are symbiotically associated with plants and have been proven to produce novel or analogues of host bioactive metabolites exhibiting a variety of biological activities including anticancer activity. This review emphasizes on structurally diverse unprecedented anticancer natural compounds that have been reported exclusively from endophytic fungi from 2016 to 2020. It covers chemical nature of metabolites, its fungal source associated with terrestrial, as well as marine plants and anticancer activity based on their cytotoxicity profile against various cancer cell lines. Many of these fungal metabolites with promising anticancer activity can be used as lead molecules for in silico experiments and deserve special attention from scientists for further in vitro and clinical research.

[11]-Chaetoglobosins from Pseudeurotium bakeri Induce G2/M Cell Cycle Arrest and Apoptosis in Human Cancer Cells

Twelve new [11]-chaetoglobosins, chaetopseudeurins A-L (1-12), were identified from the fermentation of the endophytic fungus Pseudeurotium bakeri P1-1-1. Their structures with absolute configurations were elucidated by detailed interpretation of NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. Compounds 2 and 5-7 exhibited significant cytotoxicities against seven human cancer cell lines, with IC₅₀ values ranging from 4.7 ± 1.0 to 12.2 ± 0.7 μM, and induced G2/M cell cycle arrest and apoptosis in MCF-7 and A427 cells dose dependently. Western blot analysis showed that compound 2 induced apoptosis of MCF-7 cells via the Bcl-2/caspase-3/PARP pathway.

Z, Cubilla-Rios L. Structurally Uncommon Secondary Metabolites Derived from Endophytic Fungi

Among microorganisms, endophytic fungi are the least studied, but they have attracted attention due to their high biological diversity and ability to produce novel and bioactive secondary metabolites to protect their host plant against biotic and abiotic stress. These compounds belong to different structural classes, such as alkaloids, peptides, terpenoids, polyketides, and steroids, which could present significant biological activities that are useful for pharmacological or medical applications. Recent reviews on endophytic fungi have mainly focused on the production of novel bioactive compounds. Here, we focus on compounds produced by endophytic fungi, reported with uncommon bioactive structures, establishing the neighbor net and diversity of endophytic fungi. The review includes compounds published from January 2015 to December 2020 that were catalogued as unprecedented, rare, uncommon, or possessing novel structural skeletons from more than 39 different genera, with Aspergillus and Penicillium being the most mentioned. They were reported as displaying cytotoxic, antitumor, antimicrobial, antiviral, or anti-inflammatory activity. The solid culture, using rice as a carbon source, was the most common medium utilized in the fermentation process when this type of compound was isolated.

Acautalides A-C, Neuroprotective Diels-Alder Adducts from Solid-State Cultivated Acaulium sp. H-JQSF

The solid-state cultivation of Acaulium sp. H-JQSF isolated from Armadillidium vulgare produces acautalides A-C (1-3) as skeletally unprecedented Diels-Alder adducts of a 14-membered macrodiolide to an octadeca-9,11,13-trienoic acid. The acautalide structures, along with the intramolecular transesterifications of 1-acylglycerols, were elucidated by mass spectrometry, nuclear magnetic resonance, chemical transformation, and single-crystal X-ray diffraction. Compounds 1-3 were found to be neuroprotective with antiparkinsonic potential in the 1-methyl-4-phenylpyridinium-challenged nematode model, with the magnitude impacted by the glycerol esterification.

Bioinspired Network Analysis Enabled Divergent Syntheses and Structure Revision of Pentacyclic Cytochalasans

We accomplished the divergent total syntheses of ten pentacyclic cytochalasans (aspergillin PZ, trichodermone, trichoderones, flavipesines, and flavichalasines) from a common precursor aspochalasin D and revised the structures of trichoderone B, spicochalasin A, flavichalasine C, aspergilluchalasin based on structure network analysis of the cytochalasans biosynthetic pathways and DFT calculations. The key steps of the syntheses include transannular alkene/epoxyalkene and carbonyl-ene cyclizations to establish the C/D ring of pentacyclic aspochalasans. Our bioinspired approach to these pentacyclic cytochalasans validate the proposed biosynthetic speculation from a chemical view and provide a platform for the synthesis of more than 400 valuable cytochalasans bearing different macrocycles and amino-acid residues.

Visible-Light-Induced Formation of Thiavinyl 1,3-Dipoles: A Metal-Free [3+2] Oxidative Cyclization with Alkynes as Easy Access to Thiophenes

A visible-light-induced [3+2] oxidative cyclization of various alkynes with easily available ketene dithioacetals as the previously unknown thiavinyl 1,3-dipoles in the presence of an acridine photosensitizer is reported. A series of multisubstituted thiophenes were achieved regioselectively in ≤98% yields under very mild metal-free conditions without other additives. This reaction could tolerate a wide range of substrates and achieve good efficiency in large-scale syntheses. The reaction mechanism and their applications are described in detail to reveal the reactivity of the new 1,3-dipoles and the selectivity of the reactions.

Progress in the Chemistry of Cytochalasans

Cytochalasans are a group of fungal-derived natural products characterized by a perhydro-isoindolone core fused with a macrocyclic ring, and they exhibit a high structural diversity and a broad spectrum of bioactivities. Cytochalasans have attracted significant attention from the chemical and pharmacological communities and have been reviewed previously from various perspectives in recent years. However, continued interest in the cytochalasans and the number of laboratory investigations on these compounds are both growing rapidly. This contribution provides a general overview of the isolation, structural determination, biological activities, biosynthesis, and total synthesis of cytochalasans. In total, 477 cytochalasans are covered, including "merocytochalasans" that arise by the dimerization or polymerization of one or more cytochalasan molecules with one or more other natural product units. This contribution provides a comprehensive treatment of the cytochalasans, and it is hoped that it may stimulate further work on these interesting natural products.

Large-scale culture as a complementary and practical method for discovering natural products with novel skeletons

Covering: up to July 2020Fungal metabolites with diverse and novel scaffolds can be assembled from well-known biosynthetic precursors through various mechanisms. Recent examples of novel alkaloids (e.g., cytochalasans and diketopiperazine derivatives), terpenes (e.g., sesterterpenes and diterpenes) and polyketides produced by fungi are presented through case studies. We show that large-scale culture is a complementary and practical method for genome mining and OSMAC approaches to discover natural products of unprecedented skeletal classes from fungi. We also summarize the discovery strategies and challenges for characterizing these compounds.

Pseudeurglobosins A–F, six rearranged [11]-chaetoglobosins with immunosuppressive activities from Pseudeurotium bakeri P1-1-1

Six novel rearranged [11]-chaetoglobosins (1–6) were isolated from an endophytic fungus Pseudeurotium bakeri P1-1-1.

Pchaeglobolactone A, Spiropchaeglobosin A, and Pchaeglobosals A and B: Four Rearranged Cytochalasans from Chaetomium globosum P2-2-2

Four novel rearranged cytochalasans (1-4) were isolated from an endophytic fungus Chaetomium globosum P2-2-2. Pchaeglobolactone A (1) possessed an unprecedented 13-aza-21-oxa-tetracyclo-[10.6.1.2^17,19.0^15,19]henicosane core. Spiropchaeglobosin A (2) was the first example of cytochalasans featuring a novel spiro[5.10]hexadecane unit. Pchaeglobosals A (3) and B (4) featured a unique 5/5/13 fused tricyclic ring system. Compounds 1-4 were tested for their antiproliferative, apoptosis, cell cycle arrest, and TRAIL-resistance-overcoming activities on cancer cell lines.

Progress in the Chemistry of Macrocyclic Meroterpenoids

In the last decade, the chemistry of meroterpenoids—conjugated molecules formed from isoprenyl fragments through biosynthetic pathways—has developed rapidly. The class includes some natural metabolites and fully synthetic fragments formed through nonbiological synthesis. In the field of synthetic receptors, a range of structures can be achieved by combining fragments of different classes of organic compounds into one hybrid macrocyclic platform which retains the properties of these fragments. This review discusses the successes in the synthesis and practical application of both natural and synthetic macrocycles. Among the natural macrocyclic meroterpenoids, special attention is paid to isoprenylated flavonoids and phenols, isoprenoid lipids, prenylated amino acids and alkaloids, and isoprenylpolyketides. Among the synthetic macrocyclic meroterpenoids obtained by combining the “classical” macrocyclic platforms, those based on cyclodextrins, together with meta- and paracyclophanes incorporating terpenoid fragments, and meroterpenoids obtained by macrocyclization of several terpene derivatives are considered. In addition, issues related to biomedical activity, processes of self-association and aggregation, and the formation of host–guest complexes of various classes of macrocyclic merotenoids are discussed in detail.

Vinylogous Elimination/C-H Functionalization/Allylation Cascade Reaction of Allenoate Adducts: Synthesis of Ring-Fused Dihydropyridinones

A palladium-catalyzed cascade reaction of β'-allenoate adducts with aryl/heteroaryl carboxamides through a vinylogous elimination/C-H functionalization/intramolecular allylation reaction sequence has been developed with high Z stereoselectivity. Various ring-fused dihydropyridinones bearing an α,β-unsaturated ester substituent are obtained. It is the first example of application of the allenoate adducts to C-H functionalization annulations as practical precursors of hard-to-get functionalized electron-deficient 1,3-butadienes. Using air as the terminal oxidant also shows a great advantage in environmental friendliness.

Cytochalasins from an endophytic fungus Phoma multirostrata XJ-2-1 with cell cycle arrest and TRAIL-resistance-overcoming activities

Nine new (1-9) and four known (10-13) [13]cytochalasins, along with three known 24-oxa[14]cytochalasins (14-16), were isolated from the culture of Phoma multirostrata XJ-2-1, an endophytic fungus obtained from the fibrous root of Parasenecio albus. Their structures were elucidated by interpretation of the nuclear magnetic resonance (NMR) and high-resolution electrospray ionization mass spectroscopy (HRESIMS). The absolute configurations were assigned by single-crystal X-ray crystallography, modified Mosher's method, and by analysis of their experimental electronic circular dichroism (ECD) spectra. Compound 6 could induce cell cycle arrest at G2-phase in CT26 and A549 cells, and displayed moderate cytotoxicity against CT26 and A549 cell lines with IC₅₀ values of 6.03 and 5.04 μM, respectively. Co-treatment of 7-9, 13 and 16 with tumor necrosis factor related apoptosis inducing ligand (TRAIL) could significantly decrease the cell viability of A549, which revealed that cytochalasins could possibly be a new group of TRAIL sensitizers in lung cancer therapy.

Bioactivities and Future Perspectives of Chaetoglobosins

Chaetoglobosins belonging to cytochalasan alkaloids represent a large class of fungal secondary metabolites. To date, around 100 chaetoglobosins and their analogues have been isolated and identified over the years from a variety of fungi, mainly from the fungus Chaetomium globosum. Studies have found that chaetoglobosins possess a broad range of biological activities, including antitumor, antifungal, phytotoxic, fibrinolytic, antibacterial, nematicidal, anti-inflammatory, and anti-HIV activities. This review will comprehensively summarize the biological activities and mechanisms of action of nature-derived chaetoglobosins.

HPLC-DAD-Guided Isolation of Diversified Chaetoglobosins from the Coral-Associated Fungus Chaetomium globosum C2F17

Cytochalasans have continuously aroused considerable attention among the chemistry and pharmacology communities due to their structural complexities and pharmacological significances. Sixteen structurally diverse chaetoglobosins, 10-(indol-3-yl)-[13]cytochalasans, including a new one, 6-O-methyl-chaetoglobosin Q (1), were isolated from the coral-associated fungus Chaetomium globosum C2F17. Their structures were accomplished by extensive spectroscopic analysis combined with single-crystal X-ray crystallography and ECD calculations. Meanwhile, the structures and absolute configurations of the previously reported compounds 6, 12, and 13 were confirmed by single-crystal X-ray analysis for the first time. Chaetoglobosins E (6) and Fex (11) showed significant cytotoxicity against a panel of cancer cell lines, K562, A549, Huh7, H1975, MCF-7, U937, BGC823, HL60, Hela, and MOLT-4, with the IC₅₀ values ranging from 1.4 μM to 9.2 μM.

Photoinduced C(sp2)-H/C(sp2)-H Cross-Coupling of Alkenes: Direct Synthesis of 1,3-Dienes

A highly concise route to substituted 1,3-dienes from vinylarenes and ketene dithioacetals under photoinduced cross-coupling reaction is described. The reaction proceeded in a highly regio- and stereoselective manner and showed broad functional group tolerance. More than 35 substituted 1,3-dienes were synthesized with good to excellent yields through the construction of the Csp²-Csp² bond without using noble metal and external oxidants, and natural sunlight could also induce the reaction to afford gram-scale synthesis under ambient conditions.

Ergocytochalasin A, a polycyclic merocytochalasan from an endophytic fungus Phoma multirostrata XJ-2-1

Ergocytochalasin A (1), an unprecedented merocytochalasan with a 5/6/14/6/5/6/6/6 fused octacyclic ring system, was isolated from an endophytic fungus Phoma multirostrata XJ-2-1.

Cytochalasins from endophytic Diaporthe sp. GDG-118

The plant Sophora tonkinensis, possessed a range of active compounds, was traditionally used in the medicine of Chinese minorities. Endophytic fungi were isolated from this plant, of which the fungus Diaporthe sp. GDG-118 was fermented and extracted with methanol. The extract was screened by antifungal and antibacterial assays leading to the discovery of two new 21-acetoxycytochalasins (1-2) and five known cytochalasins (3-7). These two new compounds were elucidated by spectroscopic analyses, and further their absolute configurations were determined by the X-ray of compound 3 and comparing their experimental CD spectra. The antibacterial and antifungal effects of these compounds were evaluated. Compound 2 showed significant inhibitory activity against Bacillus anthraci and Escherichia coli with MIC value of 12.5 μg/mL, and 7 showed strong antifungal activity against Alternaria oleracea, Pestalotiopsis theae and Colletotrichum capsici with MIC values of 3.125, 1.56 and 1.56 μg/mL, respectively.

Survey of natural products reported by Asian research groups in 2018

The new natural products reported in 2018 in peer-reviewed articles in journals with good reputations were reviewed and analyzed. The advances made by Asian research groups in the field of natural products chemistry in 2018 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on their structural classification.

Antimicrobial potential of endophytic fungi from Astragalus chinensis

The purpose of the present study was to discover antimicrobial endophytic fungi from Astragalus chinensis. Three fungal endophytes with antibacterial activity were isolated and determined as Chaetomium sp. HQ-1, Fusarium sp. HQ-7 and Fusarium sp. HQ-9 based on the neighbor-joining phylogenetic tree. Chaetomium sp. HQ-1 showed the best antibiotic potential and was thus selected for large-scale fermentation. Bioactivity-directed separation of ME fermentation of strain HQ-1 led to the discovery of three compounds, which were identified as differanisole A (1), 2,6-dichloro-4-propylphenol (2) and 4,5-dimethylresorcinol (3), from the HR-ESI-MS and NMR data analysis. All three compounds exhibited moderate antibacterial activity against Listeria monocytogenes, Staphylococcus aureus, and methicillin-resistant S. aureus, with MIC values ranging from 16 to 128 μg/mL. Compounds 1 and 3 also displayed promising antifungal activity against Selerotium rolfsii with IC₅₀ values of less than 16 and 32 μg/mL, respectively, which were comparable to that of actidione (8 μg/mL). The findings of the present study suggest that the endophytic fungi from A. chinensis have the potential to be used as bactericides and fungicides.

Investigating the Function of Cryptic Cytochalasan Cytochrome P450 Monooxygenases Using Combinatorial Biosynthesis

Tailoring enzymes in cytochalasan biosynthesis are relatively promiscuous. Exploiting this property, we deduced the function of four cryptic cytochrome P450 monooxygenases via heterologous expression of six cytochrome P450-encoding genes, originating from Hypoxylon fragiforme and Pyricularia oryzae, in pyrichalasin H ΔP450 strains. Three cryptic cytochrome P450 enzymes (HffD, HffG, and CYP1) restored pyrichalasin H production in mutant strains, while CYP3 catalyzed a site-selective epoxidation leading to the isolation of three novel cytochalasans.

Pyonitrins A-D: Chimeric Natural Products Produced by Pseudomonas protegens

Bacterial symbionts frequently provide chemical defenses for their hosts, and such systems can provide discovery pathways to new antifungals and structurally intriguing metabolites. This report describes a small family of naturally occurring small molecules with chimeric structures and a mixed biosynthesis that features an unexpected but key nonenzymatic step. An insect-associated Pseudomonas protegens strain's activity in an in vivo murine candidiasis assay led to the discovery of a family of highly hydrogen-deficient metabolites. Bioactivity- and mass-guided fractionation led to the pyonitrins, highly complex aromatic metabolites in which 10 of the 20 carbons are quaternary, and 7 of them are contiguous. The P. protegens genome revealed that the production of the pyonitrins is the result of a spontaneous reaction between biosynthetic intermediates of two well-studied Pseudomonas metabolites, pyochelin and pyrrolnitrin. The combined discovery of the pyonitrins and identification of the responsible biosynthetic gene clusters revealed an unexpected biosynthetic route that would have prevented the discovery of these metabolites by bioinformatic analysis alone.

Cysteine Residue Containing Merocytochalasans and 17,18-seco-Aspochalasins from Aspergillus micronesiensis

Two cysteine residue containing merocytochalasans (cyschalasins A and B, 1 and 2) and two 17,18-seco-aspochalasins (secochalasins A and B, 3 and 4) were isolated from the endophytic fungus Aspergillus micronesiensis. Cyschalasins A and B represent a new type of merocytochalasan featuring the fusion of an aspochalasin with a modified cysteine residue. Secochalasins A and B are the first 17,18-seco-aspochalasins to be reported and represent a previously undescribed carbon skeleton. Plausible biosynthetic pathways of 1-4 were proposed. Compounds 1 and 2 were cytotoxic and active against Gram-positive bacteria.

Enzymatic Intermolecular Hetero-Diels-Alder Reaction in the Biosynthesis of Tropolonic Sesquiterpenes

Diels-Alder reactions are among the most powerful synthetic transformations to construct complex natural products. Despite that increasing of enzymatic intramolecular Diels-Alder reactions have been discovered, natural intermolecular Diels-Alderases are rarely described. Here, we report an intermolecular hetero-Diels-Alder reaction in the biosynthesis of tropolonic sesquiterpenes and functionally characterize EupfF as the first fungal intermolecular hetero-Diels-Alderase. We demonstrate that EupfF catalyzed the dehydration of a hydroxymethyl-containing tropolone (5) to generate a reactive tropolone o-quinone methide (6) and might further stereoselectively control the subsequent intermolecular hetero-Diels-Alder reaction with (1E,4E,8Z)-humulenol (8) to produce enantiomerically pure neosetophomone B (1). Our results reveal the biosynthetic pathway of 1 and expand the repertoire of activities of Diels-Alder cyclases.

Diagnostic fragmentation filtering for the discovery of new chaetoglobosins and cytochalasins

RATIONALE

Microbial natural products are often biosynthesized as classes of structurally related compounds that have similar tandem mass spectrometry (MS/MS) fragmentation patterns. Mining MS/MS datasets for precursor ions that share diagnostic or common features enables entire chemical classes to be identified, including novel derivatives that have previously been unreported. Analytical data analysis tools that can facilitate a class-targeted approach to rapidly dereplicate known compounds and identify structural variants within complex matrices would be useful for the discovery of new natural products.

METHODS

A diagnostic fragmentation filtering (DFF) module was developed for MZmine to enable the efficient screening of MS/MS datasets for class-specific product ions(s) and/or neutral loss(es). This approach was applied to series of the structurally related chaetoglobosin and cytochalasin classes of compounds. These were identified from the culture filtrates of three fungal genera: Chaetomium globosum, a putative new species of Penicillium (called here P. cf. discolor: closely related to P. discolor), and Xylaria sp. Extracts were subjected to LC/MS/MS analysis under positive electrospray ionization and operating in a data-dependent acquisition mode, performed using a Thermo Q-Exactive mass spectrometer. All MS/MS datasets were processed using the DFF module and screened for diagnostic product ions at m/z 130.0648 and 185.0704 for chaetoglobosins, and m/z 120.0808 and 146.0598 for cytochalasins.

RESULTS

Extracts of C. globosum and P. cf. discolor strains revealed different mixtures of chaetoglobosins, whereas the Xylaria sp. produced only cytochalasins; none of the strains studied produced both classes of compounds. The dominant chaetoglobosins produced by both C. globosum and P. cf. discolor were chaetoglobosins A, C, and F. Tetrahydrochaetoglobosin A was identified from P. cf. discolor extracts and is reported here for the first time as a natural product. The major cytochalasins produced by the Xylaria sp. were cytochalasin D and epoxy cytochalasin D. A larger unknown "cytochalasin-like" molecule with the molecular formula C₃₈ H₄₇ NO₁₀ was detected from Xylaria sp. culture filtrate extracts and is a current target for isolation and structural characterization.

CONCLUSIONS

DFF is an effective LC/MS data analysis approach for rapidly identifying entire classes of compounds from complex mixtures. DFF has proved useful in the identification of new natural products and allowing for their partial characterization without the need for isolation.