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Structure of preussomerin A: an unusual new antifungal metabolite from the coprophilous fungus Preussia isomera
... The preussomerins are a family of about 20 natural compounds identified for the first time in 1990 from the coprophilous fungus Preussia isomera [108] and subsequently from other fungal species, including Sporormiella vexans, and Edenia gomezpompae. Preussomerins possess two naphthalene units linked by three oxygen atoms, generating a bis-spiroacetal system. ...
... Cloropreussomerins A and B (101,102) and preussomerins A, C, F, G and H (103,104,(106)(107)(108) showed cytotoxicity against five human cell lines [15]. ...
... Moreover, the synergistic effect of 119 and 112 has been assessed against the cervical cancer cell line, HeLa [115]. The preussomerins are a family of about 20 natural compounds identified for the first time in 1990 from the coprophilous fungus Preussia isomera [108] and subsequently from other fungal species, including Sporormiella vexans, and Edenia gomezpompae. Preussomerins possess two naphthalene units linked by three oxygen atoms, generating a bis-spiroacetal system. ...
Lasiodiplodia theobromae is a plant pathogenic fungus from the family Botryosphaeriaceae that is commonly found in tropical and subtropical regions. It has been associated with many hosts, causing diverse diseases and being responsible for serious damages on economically important crops. A diverse array of bioactive low molecular weight compounds has been described as being produced by L. theobromae cultures. In this review, the existing literature on secondary metabolites of L. theobromae, their bioactivity, and the implications of their occurrence are compiled. Moreover, the effects of abiotic factors (e.g., temperature, nutrient availability) on secondary metabolites production are highlighted, and possible avenues for future research are presented. Currently, a total of 134 chemically defined compounds belonging to the classes of secondary metabolites and fatty acids have been reported from over 30 L. theobromae isolates. Compounds reported include cyclohexenes and cyclohexenones, indoles, jasmonates, lactones, melleins, phenols, and others. Most of the existing bioactivity studies of L. theobromae metabolites have assessed their potential phytotoxic, cytotoxic, and antimicrobial activities. In fact, its host adaptability and its ability to cause diseases in plants as well as in humans may be related to the capacity to produce bioactive compounds directly involved in host-fungus interactions.
... Type II SBNs are mainly preussomerins, with the first member, preussomerin A, being isolated in 1990. [5] The antifungal preussomerin A, [5][6] antibacterial, antiplasmodial, and cytotoxic preussomerin E, [6][7] are the representatives of this type ( Figure 1B). Type III SBNs only include spiroxins, which were isolated in 1999. ...
... Type II SBNs are mainly preussomerins, with the first member, preussomerin A, being isolated in 1990. [5] The antifungal preussomerin A, [5][6] antibacterial, antiplasmodial, and cytotoxic preussomerin E, [6][7] are the representatives of this type ( Figure 1B). Type III SBNs only include spiroxins, which were isolated in 1999. ...
Spirobisnaphthalenes (SBNs) are a class of highly oxygenated, fungal bisnaphthalenes containing a unique spiroketal bridge, that displayed diverse bioactivities. Among the reported SBNs, palmarumycins are the major type, which are precursors for the other type of SBNs structurally. However, the biosynthesis of SBNs is unclear. In this study, we elucidated the biosynthesis of palmarumycins, using gene disruption, heterologous expression, and substrate feeding experiments. The biosynthetic gene cluster for palmarumycins was identified to be distant from the polyketide synthase gene cluster, and included two cytochrome P450s (PalA and PalB), and one short chain dehydrogenase/reductase (PalC) encoding genes as key structural genes. PalA is an unusual, multifunctional P450 that catalyzes the oxidative dimerization of 1,8‐dihydroxynaphthalene to generate the spiroketal linkage and 2,3‐epoxy group. Chemical synthesis of key intermediate and in vitro biochemical assays proved that the oxidative dimerization proceeded via a binaphthyl ether. PalB installs the C‐5 hydroxy group, widely found in SBNs. PalC catalyzes 1‐keto reduction, the reverse 1‐dehydrogenation, and 2,3‐epoxide reduction. Moreover, an FAD‐dependent oxidoreductase, encoded by palD, which locates outside the cluster, functions as a 1‐dehydrogenase. These results provided the first genetic and biochemical evidence for the biosynthesis of palmarumycin SBNs.
... 6,8,9 Both Preussia and Sporormiella are known to be prolific producers of bioactive polyketides, including depsidones. 10−12 In particular, Preussia similis (Khan & Cain) Arenal 13,14 was found to be a rich source of antifungal compounds, such as similins A and B 15 and preussomerin A. 16 From our collection of endophytic fungi isolated from the medicinal plant Globularia alypum Linn. (Plantaginaceae) collected in Batna, Algeria, P. similis DSM 104666 was selected for further study based on a phylogenetic preselection as outlined previously 17,18 and screened in order to search for novel biologically active secondary metabolites. ...
... This was supported by an NOE between H-14 and H-18. Furthermore, the 13 C NMR shift for H 3 -25 (δ C 20.8), which is downfield compared to H 3 -25 in all preussilides confirms the Z-configuration at Δ 16,17 . Consequently, the geometry of the double bonds in 4 was assigned as 14E, 16Z, 18E. ...
Six novel bioactive bicyclic polyketides (1−6) were isolated from cultures of an endophytic fungus of the
medicinal plant Globularia alypum collected in Batna, Algeria. The producer organism was identified as Preussia similis using morphological and molecular phylogenetic methods. The structures of metabolites 1−6, for which the trivial names preussilides A−F are proposed, were elucidated using a combination of spectral methods, including extensive 2D NMR spectroscopy, high- resolution mass spectrometry, and CD spectroscopy. Preussilides were tested for antimicrobial and antiproliferative effects, and, in particular, compounds 1 and 3 showed selective activities against eukaryotes. Subsequent studies on the influence of 1 and 3 on the morphology of human osteosarcoma cells (U2OS) suggest that these two polyketides might target an enzyme involved in coordination of the cell division cycle. Hence, they might, for instance, affect timing or spindle assembly mechanisms, leading to defects in chromosome segregation and/or spindle geometry
... Frequent previous reports indicate that Preussia species are rich sources of bioactive natural products (Bergstrom et al., 1995;Chen et al., 2020;Chen et al., 2009;Du et al., 2012;Du et al., 2014;Gonzalez-Menendez et al., 2017;Noumeur et al., 2017;Poch & Gloer, 1991;Rangel-Grimaldo et al., 2017;Talontsi et al., 2014;Weber et al., 1990;Weber & Gloer, 1988;Weber & Gloer, 1991;Xu et al., 2019;Zhang et al., 2012). However, to the best of our knowledge, no studies have yet associated secondary metabolites produced by any Preussia strain with its respective biosynthetic gene clusters (BGCs). ...
Cryptococcus neoformans is a serious human pathogen with limited options for treatment. We have interrogated extracts from fungal fermentations to find Cryptococcus-inhibiting natural products using assays for growth inhibition and differential thermosensitivity. Extracts from fermentations of four fungal strains from wild and domestic animal dung from Arkansas and West Virginia, USA were identified as Preussia typharum. The extracts exhibited two antifungal regions. Purification of one region yielded new 24-carbon macrolides incorporating both a phosphoethanolamine unit and a bridging tetrahydrofuran ring. The structures of these metabolites were established mainly by analysis of high-resolution mass spectrometry and 2D NMR data. Relative configurations were assigned using NOESY data, and the structure assignments were supported by NMR comparison with similar compounds. These new metabolites are designated preussolides A and B. The second active region was caused by the cytotoxin, leptosin C. Genome sequencing of the four strains revealed biosynthetic gene clusters consistent with those known to encode phosphoethanolamine-bearing polyketide macrolides and the biosynthesis of dimeric epipolythiodioxopiperazines. All three compounds showed moderate to potent and selective antifungal activity towards the pathogenic yeast C. neoformans.
... The preussomerins and deoxypreussomerins are phenolic fungal metabolites extracted from the coprophilous fungus Preussia isomera and the endophytic fungus Harmonema dematioides with FTase and GGTase inhibitory properties [172,[184][185][186]. Low-toxicity synthetic esters derived from these compounds required reductive activation, specifically at the cancer cells, resulting from hypoxia and the overexpression of reductases. ...
RAS genes encode signaling proteins, which, in mammalian cells, act as molecular switches regulating critical cellular processes as proliferation, growth, differentiation, survival, motility, and metabolism in response to specific stimuli. Deregulation of Ras functions has a high impact on human health: gain-of-function point mutations in RAS genes are found in some developmental disorders and thirty percent of all human cancers, including the deadliest. For this reason, the pathogenic Ras variants represent important clinical targets against which to develop novel, effective, and possibly selective pharmacological inhibitors. Natural products represent a virtually unlimited resource of structurally different compounds from which one could draw on for this purpose, given the improvements in isolation and screening of active molecules from complex sources. After a summary of Ras proteins molecular and regulatory features and Ras-dependent pathways relevant for drug development, we point out the most promising inhibitory approaches, the known druggable sites of wild-type and oncogenic Ras mutants, and describe the known natural compounds capable of attenuating Ras signaling. Finally, we highlight critical issues and perspectives for the future selection of potential Ras inhibitors from natural sources.
... Spirobisnaphthalenes consist of two naphthalene-derived C10 units joined together through aspiroketal linkage,and belong to an ovel family of natural products isolated from fungi. [4,5] They possess av ariety of biological properties, [6] including antimicrobial, [7,8] antiplasmodial, [9] antileishmanial, [10] nematicidal, [11] and antitumor activities. [12] In 2014, Pudhom and coworkers identified af amily of novel spirobisnaphthalene compounds from endophytic fungi Rhytidhysteron rufulum AS21B,namely rhytidenones A-F (Scheme 1). ...
The rhytidenone family comprises spirobisnaphthalene natural products isolated from the mangrove endophytic fungus Rhytidhysteron rufulum AS21B. The biomimetic synthesis of rhytidenone A was achieved by a Michael reaction/aldol/lactonization cascade in a single step from the proposed biosynthetic precursor rhytidenone F. Moreover, the mode of action of the highly cytotoxic rhytidenone F was investigated. The pulldown assay coupled with mass spectrometry analysis revealed the target protein PA28γ is covalently attached to rhytidenone F at the Cys92 residue. The interactions of rhytidenone F with PA28γ lead to the accumulation of p53, which is an essential tumor suppressor in humans. Consequently, the Fas‐dependent signaling pathway is activated to initiate cellular apoptosis. These studies have identified the first small‐molecule inhibitor targeting PA28γ, suggesting rhytidenone F may serve as a promising natural product lead for future anticancer drug development.
... 334 Preussomerin F was extracted from the coprophilous fungus Preussia isomera by Gloer and co-workers in 1990. 335 Preussomerin K was also extracted independently by Isaka et al. from a lichenicolous fungus Microsphaeropsis sp. BCC 3050. ...
Over the years, Friedel-Crafts (FC) reactions have been acknowledged as the most useful and powerful synthetic tools for the construction of a special kind of carbon-carbon bond involving an aromatic moiety. Its stoichiometric and, more recently, its catalytic procedures have extensively been studied. This reaction in recent years has frequently been used as a key step (steps) in the total synthesis of natural products and targeted complex bioactive molecules. In this review, we try to underscore the applications of intermolecular and intramolecular FC reactions in the total syntheses of natural products and complex molecules, exhibiting diverse biological properties.
Spirobisnaphthalenes (SBNs) are a class of highly oxygenated, fungal bisnaphthalenes containing an unique spiroketal bridge, that displayed diverse bioactivities. Among the reported SBNs, palmarumycins are the major type, which are precursors for the other type of SBNs structurally. However, the biosynthesis of SBNs is unclear. In this study, we elucidated the biosynthesis of palmarumycins, using gene disruption, heterologous expression, and substrate feeding experiments. The biosynthetic gene cluster for palmarumycins was identified to be distant from the polyketide synthase gene cluster, and included two cytochrome P450s (PalA and PalB), and one short chain dehydrogenase/reductase (PalC) encoding genes as key structural genes. PalA is an unusual, multifunctional P450 that catalyzes the oxidative dimerization of 1,8‐dihydroxynaphthalene to generate the spiroketal linkage and 2,3‐epoxy group. Chemical synthesis of key intermediate and in vitro biochemical assays proved that the oxidative dimerization proceeded via a binaphthyl ether. PalB installs the C‐5 hydroxyl group, widely found in SBNs. PalC catalyzes 1‐keto reduction, the reverse 1‐dehydrogenation, and 2,3‐epoxide reduction. Moreover, an FAD‐dependent oxidoreductase, encoded by palD, which locates outside the cluster, functions as 1‐dehydrogenase. These results provided the first genetic and biochemical evidence for the biosynthesis of palmarumycin SBNs.
Dung-inhabiting fungi are specialized to grow on dung. The coprophilous habit required these fungi to have physiological adaptations to survive on dung, so they developed a variety of metabolites to exploit the resource and tolerate intra and interspecific competition. These fungi are important cyclers of matter/energy in terrestrial ecosystems, but they are poorly studied. We analyzed the global scientific production involving these fungi, using a systematic review and scientometric methods. We found 661 publications, with temporal distribution showing a marked increase in publications from the 1970s onwards with the widespread of the moist chamber technique facilitating the studying these fungi. Twelve journals account 37% of publications, with Fungal Biology with most publications (33). A total of 15 countries from a total of 54 concentrate more than half (65%) of the publications, with the USA (79), United Kingdom (66), Brazil (36), and India (33) having the greatest production (≥ 30), but there are lots of unexplored geographic areas regarding this biodiversity, to which attention should be given. Most studies are divided between paleobiological or taxonomic/ecological approach, with the latter group being trend among studies produced in developing countries. Fifty-eight species were involved in studies on secondary metabolites or another biological processes, with Podospora anserina (syn. Triangularia anserina and Thermochaetoides thermophila (syn. Chaetomium thermophilum) being the most studied. We hope our data reinforce the need for new research/funding aimed this fungal group with the purpose of learning its potential and guaranteeing conservation and knowledge of their biodiversity through the valuation of the goods and services they can generate.
Lasiodiplodiapyrones A and B (1 and 2), two new preussomerin derivatives, possessing an unexpected 6-methyl-4H-furo[3,2-c]pyran-4-one moiety and a highly functionalized conjoint and complicated polycyclic ring system, along with two known congeners (3 and 4), were isolated from the fungus Lasiodiplodia pseudotheobromae. Their structures including absolute configurations were determined by spectroscopic analyses, Mosher's method, and ECD calculations. A biosynthetic pathway was proposed to explain the origin of lasiodiplodiapyrones A and B as well as their relationship with preussomerins. Compounds 1-4 showed suppressive effects on the production of NO with IC50 values of 4.8 ± 0.3, 8.5 ± 1.1, 5.9 ± 0.8, and 12.8 ± 1.3 μM, respectively.
Photochemical reactions of naphthoquinones triggered by 1,6‐hydrogen atom transfer have been developed. This reaction proceeds in stereospecific manner without loss of the stereochemical information of the substrate, enabling the enantioselective total synthesis of preussomerin EGs. The constructed spiroacetal center is the sole stereogenic center in the product, which had been previously impossible to set by exploiting conventional methods. image
We report the enantioselective total syntheses of preussomerins EG1, EG2, and EG3. The key transformation is a stereospecific photochemical reaction involving 1,6‐hydrogen atom transfer to achieve retentive replacement of a C−H with a C−O bond, enabling otherwise‐difficult control of the spiroacetal stereogenic center. Photochemical reactions of naphthoquinones triggered by 1,6‐hydrogen atom transfer have been developed. This reaction proceeds in stereospecific manner without loss of the stereochemical information of the substrate, enabling the enantioselective total synthesis of preussomerin EGs. The constructed spiroacetal center is the sole stereogenic center in the product, which had been previously impossible to set by exploiting conventional methods.
Spirobisnaphthalenes are a family of natural products containing two naphthalenes connected with spiroketal or dinaphthalene spiroether. This family of natural compounds has a wide range of biological activities and complex structures, and has attracted great attention from synthetic chemists in recent years. Hundreds of novel spirobisnaphthalenes have been identified from the first separation in 1989. Despite the slow pace of research in recent years, since the latest review in 2017, 30 new natural products have been isolated from the endolichenic and endophytic fungus, including Type A, B, D and E, a new special skeleton with a spiro quaternary carbon center, as well as significant breakthroughs are made in the total synthesis. Here, the last research advances in the isolation of new spirobisnaphthalenes, their activities, mode of action and the total synthesis of spirobisnaphthalenes in the last five years were afforded based on our research in this field in this review.
Spiro compounds are biologically active organic compounds with unique structures, found in a wide variety of natural products and drugs. They do not readily lead to drug resistance due to their unique mechanisms of action and have, therefore, attracted considerable attention regarding pesticide development. Analyzing structure-activity relationships (SARs) and summarizing the characteristics of spiro compounds with high activity are crucial steps in the design and development of new pesticides. This review mainly summarizes spiro compounds with insecticidal, bactericidal, fungicidal, herbicidal, antiviral, and plant growth regulating functions to provide insight for the creation of new spiro compound pesticides.
Molecular photoswitches that offer simultaneous precise control over geometrical and electronic changes are rare yet highly sought tools for the development of responsive nanosystems. Here we present such an advantageous combination of property control within a novel multiphotoswitch architecture. Hemithioindigo-based trioxobicyclononadiene (HTI-TOND) offers a rigid three-dimensional molecular structure that undergoes different exotic rearrangement reactions upon photochemical and thermal signaling. Three to four different states with distinct geometric and electronic properties can be accessed reversibly in high yields within this molecular framework. Thus, a highly promising and unique switching tool has become available to instill the next level of addressability at the smallest scales.
Sparticols A (1) and B (2), two catechol-bearing naphthalenedioxy derivatives, were isolated from the submerged culture of the Spanish broom inhabiting Dothideomycetes fungus, Sparticola junci. The structures of 1 and 2 were established by NMR spectroscopic analysis and high-resolution mass spectrometry. The 8S absolute configuration of their β-hydroxy functionalities was determined by ECD-TDDFT. Both compounds exhibited inhibitory activity against Staphylococcus aureus with an MIC value of 66.6 μg/ mL. Polyketides 1 and/or 2 may be associated with pathways cascading to seco-spirodioxynapthalene derivatives.
This paper reviews the research data available in the literature on the chemistry of 4-chloro-3-formylcoumarins reported over the last two to three decades. It covers the synthetic applicability of 4-chloro-3-formylcoumarins in the development of 3,4-substituted coumarins as well as 5-, 6- and 7-membered ring fused coumarin derivatives via classical reaction protocols, microwave-mediated reactions, organo-catalyzed reactions, transition metal-catalyzed reactions, and green reaction protocols. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
Light-mediated [1,3] sigmatropic shift of chromeno-5-methyl-2,6,9-trioxabicyclo[3.3.1]nonadienes to yield chromeno-3-methyl-2,6,9-trioxabicyclo[3.3.1]nonadienes was reported. The migration was observed to proceed without the requirement of UV light if a strong electron-withdrawing group on the...
As one of the world’s five terminally ills, tumours can cause important genetic dysfunction. However, some current medicines for tumours usually have strong toxic side effects and are prone to drug resistance. Studies have found that farnesyltransferase inhibitors (FTIs) extracted from natural materials have a good inhibiting ability on tumours with fewer side effects. This article describes several FTIs extracted from natural materials and clarifies the current research progress, which provides a new choice for the treatment of tumours.
Executive summary
Farnesyltransferase (FTase)
When Ras gene is activated, it becomes an oncogene with oncogenic activity. Ras protein plays a crucial role in cancer cells. FTase is the first step to activate Ras protein.
FTIs not only have anti-tumour effects, but also makes great contribution to the treatment of plasmodium falciparum, parasitic diseases and progeria.
There are still some problems with FTIs, but natural products FTIs are worthy of further research as a new class of low-toxic, safe and effective anticancer drugs. Summarised natural product-derived FTase inhibitors
This review summarised several FTIs extracted from natural materials and clarifies their anti-tumour activity (IC50 value) and structure, providing a reference for further research on tumour therapy.
The endophytes from medicinal plants are important resources for discovery of natural products and bioactive compounds having the potential biotechnological applications in agriculture, industry, medicine, and allied sectors. Endophytes have been sorted out from all the parts of medicinal plant such as roots, stem, leaves, fruits, flowers, bark, scales, resin canals, and even from meristem. The medicinal plants which are known to be used since centuries as a substitute of medicine are a precious source for bioprospecting endophytes. The endophytic fungi such as Acremonium, Alternaria, Apiospora, Aspergillus, Aureobasidium, Bartalinia, Cephalosporium, Chaetomium, Chloridium, Choanephora, Colletotrichum, Cryptosporiopsis, Emericella, Eupenicillium, Eutypella, Eutypella, Fusarium, Gliocladium, Hypoxylon, Paecilomyces, Penicillium, Pestalotiopsis, Pseudomassari, Quercina, Talaromyces, and Trichoderma have been reported from different medicinal plants. These endophytic fungi have ability to produce the natural and potential bioactive compounds for different processes. The present chapter reveals the importance of endophytic fungi from medicinal plants as a source of bioactive and chemically novel compounds as well as its use in the pharmaceutical and agrochemical industries.
Die biomimetische Synthese von Rhytidenon A wurde durch eine Kaskade aus Michael-Reaktion, Aldolreaktion und Lactonisierung in einem einzigen Schritt aus der postulierten Biosynthesevorstufe Rhytidenon F erreicht. Das hoch zytotoxische Rhytidenon F greift PA28γ an und führt zur Anreicherung des Tumorsuppressors p53. Es ist daher ein vielversprechender Naturstoff für die zukünftige Entwicklung von Krebsmedikamenten.
Abstract
The rhytidenone family comprises spirobisnaphthalene natural products isolated from the mangrove endophytic fungus Rhytidhysteron rufulum AS21B. The biomimetic synthesis of rhytidenone A was achieved by a Michael reaction/aldol/lactonization cascade in a single step from the proposed biosynthetic precursor rhytidenone F. Moreover, the mode of action of the highly cytotoxic rhytidenone F was investigated. The pulldown assay coupled with mass spectrometry analysis revealed the target protein PA28γ is covalently attached to rhytidenone F at the Cys92 residue. The interactions of rhytidenone F with PA28γ lead to the accumulation of p53, which is an essential tumor suppressor in humans. Consequently, the Fas-dependent signaling pathway is activated to initiate cellular apoptosis. These studies have identified the first small-molecule inhibitor targeting PA28γ, suggesting rhytidenone F may serve as a promising natural product lead for future anticancer drug development.
The first representative of a new type of spirobisnaphthalene with a previously unknown skeletal system, Spiroaxillarone A (1), was isolated from Cyanotis axillaris. The planar structure was determined through analysis of spectroscopic evidence, while electronic circular dichroism (ECD) calculations were used to determine its absolute configuration. The structure of 1 was further confirmed through X-ray diffraction studies of its tetrabromobenzoate derivative (1a). Spiroaxillarone A exhibited significant antimalarial activity against resistant Plasmodium falciparum (IC50 = 2.32 μM).
In the course of our studies of coprophilous fungi as sources of antifungal agents, a strain of an undescribed species in the genus Niesslia (TTI-0426) was isolated from horse dung collected in Texas. An extract from fermentation cultures of this strain afforded two new antifungal wortmannin derivatives, wortmannins C and D (1 and 2), as well as four additional new related compounds, wortmannines B1-B4 (3-6), containing an unusual ring system. The structures of these metabolites were established mainly by analysis of HRESIMS and 2D NMR data. Relative configurations were assigned using NOESY data, and the structure assignments were supported by NMR comparison with similar compounds. Wortmannins C and D showed activity against Cryptococcus neoformans and Candida albicans in disk assays, but low MIC potency observed for 1 was suggested to be due in part to efflux processes on the basis of assay results for a Schizosaccharomyces pombe efflux mutant in comparison to wild-type.
A pair of enantiomeric norsesquiterpenoids, (+)- and (-)-preuisolactone A (1) [(+)-1 and (-)-1)] featuring an unprecedented tricyclo[4.4.0 1,6 .0 2,8 ]decane carbon scaffold were isolated from Preussia isomera. Their structures were determined by spectroscopic and computed methods and X-ray crystallography. Compounds (+)-1 and (-)-1 are two rare naturally occurring sesquiterpenoidal enantiomers. A plausible biosynthetic pathway for 1 is proposed. Additionally, (±)-1 exhibited remarkable antibacterial activity against Micrococcus luteus with an MIC value of 10.2 μM.
This work documents 32 new Preussia isolates from the Iberian Peninsula, including endophytic and saprobic strains. The morphological study of the teleomorphs and anamorphs was combined with a molecular phylogenetic analysis based on sequences of the ribosomal rDNA gene cluster and chemotaxonomic studies based on liquid chromatography coupled to electrospray mass spectrometry. Sixteen natural compounds were identified. On the basis of combined analyses, 11 chemotypes are inferred. © 2017 German Mycological Society and Springer-Verlag Berlin Heidelberg
Studies of the genome-sequenced, flutimide-producing coprophilous fungus Delitschia confertaspora (ATCC 74209), originally obtained from a sample of rock hyrax (Procavia capensis) dung collected in Namibia, led to the discovery of three new highly aromatic natural products named delicoferones A–B (1–2) and fimetarone B (3). The new benzophenone derivatives 1 and 2 have a somewhat unusual skeleton that incorporates three aromatic rings linked via two ketone carbonyl groups, while 3 contains a spiro[chroman-3,7′-isochromene]-4,6′(8′H) skeleton reported only once previously. The structures of these compounds were assigned mainly by analysis of 2D NMR and HRESITOFMS data.
Two new chlorinated preussomerins, chloropreussomerins A and B (1 and 2), together with nine known preussomerin analogues, 3-11, were obtained from the endophytic fungus Lasiodiplodia theobromae ZJ-HQ1. Their structures were elucidated by a combination of spectroscopic analyses. The absolute configurations of 1 and 2 were both determined by single-crystal X-ray diffraction using Cu Kα radiation. Chloropreussomerins A and B (1 and 2) are the first chlorinated compounds in the preussomerin family, and preussomerin M (3) is reported for the first time as a natural product. Compounds 1 and 2 showed potent in vitro cytotoxicity against A549 and MCF-7 human cancer cell lines, with IC50 values ranging from 5.9 to 8.9 μM, and compounds 4-7 exhibited significant bioactivity against A549, HepG2, and MCF-7 human cancer cell lines, with IC50 values of 2.5-9.4 μM. In the antibacterial assay, compounds 1, 2, 5-7, and 11 exhibited significant activities against Staphylococcus aureus, with MIC values between 1.6 and 13 μg/mL.
Coprophilous fungi are a large group of saprotrophic fungi, mostly found in herbivore dung. The number of these fungi are investigated is continuously increasing and new species and genera continue to be described.
Dung-inhabiting fungi play an important ecological role in decomposing and recycling nutrients from animal dung. They produce a large array of bioactive secondary metabolites and have a potent enzymatic arsenal able to utilize even complex molecules. Bioactive secondary metabolites are actively involved in the interaction with and defence against other organisms whose growth can be inhibited, resulting in an enhanced ecological fitness of producer strains.
Currently these antibiotics and bioactive secondary metabolites are of interest especially in medicine, while very little information is available concerning their potential use in agriculture.
This review introduces the ecology of dung-inhabiting fungi with particular emphasis on the production of antibiotic compounds as a mean to compete with other microorganisms. Due to the fast pace of technological progress, new approaches are proposed in order to predict the biosynthesis of bioactive metabolites.
Coprophilous fungi should be considered as elite candidate organisms for the discovery of novel antifungal compounds, above all in view of their exploitation for crop protection.
Mechanisms of fungal antagonism and defense often include the production of biologically active metabolites by one species that exert effects on potential competitors and (or) predators. Studies carried out in our laboratory and others clearly indicate that such ecological phenomena can serve as valuable leads to the discovery of novel and potentially useful bioactive fungal metabolites. There is evidence that some of these compounds may render advantages to the producing organism, although careful and definitive ecological studies are required to determine this. Nevertheless, the results summarized here demonstrate the broad array of possible benefits that can arise from interdisciplinary studies in this area. This paper focuses primarily on our own investigations of the chemistry involved in fungal antagonism and defense using coprophilous and sclerotial fungi as model systems. These results have potential implications in many areas of study, including fungal ecology, secondary metabolism, chemotaxonomy, organic chemistry, structure determination, antifungal chemotherapy, and insect control.
This chapter describes some of the advances that have been made since 1985 in the chemistry of aromatic natural products.1 The coverage, which is not meant to be comprehensive, parallels that in the corresponding chapter in the first edition of this book.2 While the selection of topics is inevitably subjective and, because of space limitations, highly selective, an effort has been made to address many of the areas in which significant developments have taken place. Discussion is focussed on novel structures, and on aspects of the biosynthesis, synthesis and chemistry of naturally occurring aromatic compounds. Only representative, rather than exhaustive, literature references are cited.
Not only medicinal plants, but also the endophytic fungi that colonize them are excellent sources of antifungal metabolites. In some cases, the substances detected in plants have been shown to be secondary metabolites synthesized by their fungal inhabitants. As we have shown, most endophytic fungi produce biologically active metabolites, many of which are antifungal. We speculate that in situ these antifungal metabolites protect the endophytic fungi from competitors within the plant host. The metabolites belong to diverse structural groups and include many unprecedented carbon skeletons, but also derivatives of known structural groups: the benzofuranes, biaryl ethers, botryanes, coniothyriomycins, dinemasones, epoxydon derivatives, fusidilactones, isocoumarins, massarilactones, palmarumycins, pestalotheols, pyranocines, pyrenocines, naphthalenes, oblongolides and xanthones. The chemical diversity of the fungal metabolites by functional group transformation and the quality of diversity by ring closure of open-chain compounds and dimerization is demonstrated with examples from our research. We conclude that endophytic fungi are creative masters of chemical synthesis.
Studies of the coprophilous fungus Hypocopra rostrata (TTI-0009, NRRL 66178) isolated from a sample of horse dung collected in Texas led to the isolation of three new sesquiterpenoids that we named hypocoprins A-C (1-3), together with the known fungal metabolite helvolic acid. The new metabolites have a distinctive ring system consisting of fused cyclopropane and cyclodecene units not previously reported from a fungal source. Compounds 1 and 3 moderately inhibited growth of Staphylococcus aureus. The structures of these metabolites were assigned mainly by analysis of 2D NMR and HRESITOFMS data. Relative and absolute configurations were assigned by interpretation of NMR J-values and NOESY data and by application of Mosher's method. These results represent the first report of chemistry from any strain of the genus Hypocopra.
One new and three known spiro-dioxynaphthalene compounds, related to palmarumycins, were isolated from the extracts of fungus Lasiodiplodia pseudotheobromae XSZ-3. Their structures were established by spectroscopic methods (1D- and 2D-NMR, HR-ESI-MS, etc.). The absolute configuration of 1 was determined by comparison of quantum-chemical TDDFT (time-dependent density-functional theory) calculated and experimental ECD (electronic circular dichroism) spectra. The cytotoxic activities of 1–4 against HL-60 cell line were evaluated by trypan blue-staining assay.
Oxidative cyclization of bis-hydroxynaphthyl ethers allows concise total syntheses of palmarumycin CP1 and deoxypreussomerin A in 8-9 steps and 15–35% overall yield from 5-hydroxy-8-methoxy-1-tetralone (8). Polymer-bound triphenyl phosphine was found to be a superior reagent for the rapid preparation of a small library of palmarumycin analogs. Preliminary biological evaluation of naphthoquinone spiroketals against MCF-7 and MDA-MB-231 human breast cancer cells revealed several low-micromolar growth inhibitors.
Leporin A (5), an antiinsectan N-methoxy-2-pyridone, was isolated from the sclerotia of Aspergillus leporis. The structure was determined through analysis of various 1D- and 2D-NMR experiments. Two previously reported aflavinine derivatives (3 and 4) were also isolated from this source.
Similins A and B (2 and 3), two new metabolites from the coprophilous fungus Sporormiella similis, have been isolated and identified. Similin A was found to be responsible for the antifungal activity exhibited by S. similis. The relative stereochemistry of similin B was established by x-ray crystallography.
Diepoxins (1-4), unique, spiroketal-linked bisepoxides have been isolated from a filamentous fungus. Structures were assigned to these compounds primarily on the basis of NMR data.
The tertiary naphthoquinol is a key structural component of the antitumor natural products spiroxins A–E. Herein we report the first catalytic asymmetric approach to the tertiary naphthoquinol C4′ stereogenic center present in the spiroxin framework, via tandem oxidation/ring-opening of a cyclic 3,4-epoxyalcohol. This new route allows a facile entry into relatively inaccessible tertiary naphthoquinols with high enantioselectivity and without the need of chiral auxiliaries.
Bipendensin (1), an unusual phenolic acetal, has been isolated from the sapwood of Afzelia bipendensis.
Chemical analysis of a terrestrial-derived Streptomyces sp. Lv20-195 cultivated from the root zone of Olea europea yielded oleaceran, 1, possessing a novel spiro[isobenzofuran-1,2'-naptho[1,8-b,c]furan] carbon skeleton. The structure of 1 was determined by detailed spectroscopic analysis.
The exciton coupled CD method has been applied to determine the absolute configuration of the diepoxins, spiroketal-linked naphthodiepoxydecalinones of fungal origin. The CD spectra of the bis-dimethylaminobenzoate derivatives of the diepoxins η, ι and κ, reveal a positive chiral twist between the two substituted hydroxyl groups and thus infer theS configuration at both of these stereogenic centers. The absolute configuration of the remaining chiral centers is deduced from their relative configurations as established by X-ray diffraction of diepoxin κ. The twist boat conformation of the epoxycyclohexanone ring and the continued axial orientation of the substituents at C-4 and C-5 after dimethylaminobenzoate derivatization was corroborated by1H-NMR coupling constants.
Three known preussomerins, G (1), H (2) and I (3), and three new representatives, J (4), K (5) and L (6), were isolated from an endophytic fungus, a Mycelia sterila, from Atropa belladonna. Their absolute configuration was determined by comparison of calculated and experimental CD spectra.
Interactions between Sporormiellasimilis Khan & Cain and the blue-stain fungus Ophiostomapiliferum (Fr.) H. & P. Sydow, both isolated from Populustremuloides Michx. wood, were investigated. Sporormiellasimilis significantly reduced the growth of O. piliferum in vitro when grown in dual culture, in addition to inhibiting the growth of O. piliferum on agar media and in liquid culture when treated with a culture filtrate of S. similis. Ophiostomapiliferum failed to colonize P. tremuloides wood chips when they were preinoculated with S. similis. Ten known compounds were isolated and identified from the culture filtrate of S. similis. These compounds showed varied fungitoxic effect against O. piliferum at concentrations of 1 to 1000 μg/mL. The potential for S. similis as a biological control agent against O. piliferum on P. tremuloides is discussed.
Mechanisms of fungal antagonism and defense often include the production of biologically active metabolites by one species that exert effects on potential competitors and (or) predators. Studies carried out in our laboratory and others clearly indicate that such ecological phenomena can serve as valuable leads to the discovery of novel and potentially useful bioactive fungal metabolites. There is evidence that some of these compounds may render advantages to the producing organism, although careful and definitive ecological studies are required to determine this. Nevertheless, the results summarized here demonstrate the broad array of possible benefits that can arise from interdisciplinary studies in this area. This paper focuses primarily on our own investigations of the chemistry involved in fungal antagonism and defense using coprophilous and sclerotial fungi as model systems. These results have potential implications in many areas of study, including fungal ecology, secondary metabolism, chemotaxonomy, organic chemistry, structure determination, antifungal chemotherapy, and insect control. Key words: fungi, antifungal, insecticide, antagonism, chemical defense, secondary metabolites.
An efficient synthesis of novel chromeno-2,6,9-trioxabicyclo-[3.3.1]nonadienes (CTOBN) scaffolds was
achieved by mild base mediated reaction of 4-chloro-3-formylcoumarin and o-hydroxyacetophenones.
The structure of the product was confirmed by X-ray analysis and the proposed mechanism was validated computationally at B3LYP/6-31G(d) level.
Tubingensin B (4), a cytotoxic carbazole alkaloid with a novel ring system, was isolated from the sclerotia of the fungus . The structure was assigned primarily on the basis of selective INEPT, homonuclear decoupling, and COSY experiments.
Two-dimensional heteronuclear chemical-shift correlation (1-4) has proven to be a very effective way for identifying which proton is bonded to which 13C nucleus. Unfortunately, the experiment is much less effective in determining long-range con-nectivity (5), i.e., correlating shifts from protons and 13C nuclei that are separated by two or more bonds. A possible way for determining two-bond connectivity is the two-dimensional RELAY experiment (6-9). Although this experiment is considerably less sensitive than conventional heteronuclear chemical-shift correlation, it can provide very useful information. In practice, there is a great interest in determining three-bond 'H-13C connectivity, as this would allow "bridging" of nonprotonated carbons, and nuclei other than carbon, for example, oxygen and nitrogen. In most practical cases, none of the tech-niques mentioned above are very effective at doing this. It will be demonstrated here, that a modified version of the refocused INEPT experiment (10-12) is a very sensitive and simple way for determining two-and three-bond connectivity. It has been dem-onstrated elsewhere that this modified INEPT experiment is also very useful for enhancing sensitivity of nonprotonated "N nuclei (13). For a description of how the regular INEPT experiment functions, the reader is referred to the original literature (10-12). Here the effect of replacing all nonselective proton pulses in the INEPT experiment by "soft" pulses (YH, = 25 Hz) will be discussed. The pulse scheme is sketched in Fig. 1. As will be shown below, the effect of this sequence can easily be understood. The first soft 90: proton pulse rotates the magnetization of one preselected proton into a position parallel to the y axis of the rotating frame (Fig. 2a). Consider the case where there is a 13C nucleus in the molecule that has a long-range coupling, irJcH, with the proton considered. The proton mag-netization will then evolve during the first half of the delay A,, under the influence of chemical shift, static magnetic field inhomogeneity, homonuclear J coupling, and coupling with the i3C nucleus (Fig. 2b). For clarity, the 'rJcH interaction is assumed to be larger than the homonuclear proton-proton coupling in the diagram. The selective 180; proton pulse then rotates the proton magnetization vectors 180" about the y axis (Fig. 2c) and all the effects of 'H-'H and 'H-13C J coupling, static field inhomogeneity, and chemical shift would refocus at the end of the interval, A1.
High-resolution, 25.16 MHz, proton coupled 13C Fourier transform NMR spectra of the natural abundant 13C-toluenes have been obtained and completely analyzed. Signs and accurate magnitudes of all possible 13C1H coupling constants in toluene as well as 1H chemical shift differences, including long-range 13C isotope shifts, could be determined from the analysis of these complex spectra. The unperturbed relative 1H chemical shifts of toluene containing only 12C carbons and the long-range 13C isotope effects on these shifts have been calculated from the latter data. Finally, the signs and magnitudes of the long-range 13C1H coupling constants are discussed with respect to the 1H1H couplings in toluene and other model compounds. Special attention has been paid to the δ-π exchange polarization mechanism for π-electron couplings.
The proton-coupled 13C NMR spectrum of salicylaldehyde was analysed. The second-order character of the seven independent sub-spectra allowed the signs of most of the couplings to be determined. The input values for the analyses were obtained from the known J(CH) values of benzene, phenol and benzaldehyde using the additivity principle. In most instances, very good agreement between the calculated and observed values was obtained. Significant couplings of the ring carbons to both the aldehyde and hydroxyl protons were also observed. The magnitude of these couplings was very dependent on the stereochemical relationships between the coupled nuclei.
A new α-tetralone derivative designated (−)-regiolone has been isolated with juglone, betulinic acid and sitosterol from the stem-bark of Juglans regia. (−)-Regiolone has been shown to be 4,8-dihydroxy-1-tetralone on the basis of its spectral data and chemical transformation to juglone. Its conformation has been deduced from the 1H NMR spectral data. The absolute stereochemistry of its only chiral centre has been shown to be S by the application of the dibenzoate chirality rule.
Podosporin A (1), a new tetracyclic sesquiterpene quinone exhibiting antifungal activity, antibacterial activity, and brine shrimp toxicity, has been isolated from the coprophilous fungus Podospora decipiens (NRRL 6461), a colonist of cattle dung. Podosporin A was obtained from ethyl acetate extracts of liquid cultures of P. decipiens by silica gel chromatography and reversed-phase HPLC, and its structure was assigned by single-crystal X-ray diffraction analysis. The isolation process was guided by in vitro bioassays for antifungal antagonism toward other coprophilous fungi.
A new synthetic route to pentaprismane (1) has been developed. The key intermediate, endo-carbomethoxypentacyclo[4.4.0.0 2,5.0 3,9.0 4,8]decan-7-one (5a), was prepared in six steps from 4,4-dimethoxycyclohexa-2,5-dien-1-one (10b) in an overall yield of 23%. Photochemical closure of the Diels-Alder adduct 8b, obtained from cyclobutadiene and 10b, gave 10,10-dimethoxypentacyclo[4.4.0.0 2,5.0 3,9.0 4,8]decan-7-one (7b). Methylenation of 7b followed by hydroboration and oxidation afforded 9-hydroxy-10-oxahexacyclo[6.4.0.0 2,7.0 3,6.0 4,12.0 5,9]dodecan-11-one (18a), which was esterified to give 5a.
Compound 1 (2,3′-dihydroxy-4-methoxy-5′,6-dimethyldiphenyl ether) has been established as the major causative agent of interference competition between Preussia fleischhakii, a fungal colonist of rabbit dung, and two early successional dung colonists, Ascobolus furfuraceus and Sordaria fimicola. Compound 1 was isolated from EtOAc extracts of liquid cultures of P. fleischhakii by Si gel chromatography and reversed-phase hplc and was identified by analysis of 1H-nmr, 13C-nmr, and ms data. The isolation process was guided by in vitro bioassays for antifungal activity against A. furfuraceus and S. fimicola. These results suggest that studies of interference competition observed in relatively unexplored natural fungal communities may be a valuable, ecologically rational approach to the discovery of antifungal agents.