Bioactive Resorcylic Acid Lactones with Different Ring Systems from Desert Plant Chaetosphaeronema hispidulum. [corrected]

Natural ten-membered lactones: sources, structural diversity, biological activity, and intriguing future

Covering: 2012 to 2022Ten-membered lactones (TMLs) are an interesting and diverse group of natural polyketides that are abundant in fungi and, to a lesser extent, in bacteria, marine organisms, and insects. TMLs are known for their ability to exhibit a wide spectrum of biological activity, including phytotoxic, cytotoxic, antifungal, antibacterial, and others. However, the random discovery of these compounds by scientific groups with various interests worldwide has resulted in patchy information about their distribution among different organisms and their biological activity. Therefore, despite more than 60 years of research history, there is still no common understanding of the natural sources of TMLs, their structural type classification, and most characteristic biological activities. The controversial nomenclature, incorrect or erroneous structure elucidation, poor identification of producing organisms, and scattered information on the biological activity of compounds - all these factors have led to the problems with dereplication and the directed search for TMLs. This review consists of two parts: the first part (Section 2) covers 104 natural TMLs, published between 2012 and 2022 (after the publishing of the previous review), and the second part (Section 3) summarizes information about 214 TMLs described during 1964-2022 and as a result highlights the main problems and trends in the study of these intriguing natural products.

Bioactive secondary metabolites from endophytic strains of Neocamarosporium betae collected from desert plants

Endophytic fungi from desert plants belong to a unique microbial community that has been scarcely investigated chemically and could be a new resource for bioactive natural products. In this study, 13 secondary metabolites (1-13) with diverse carbon skeletons, including a novel polyketide (1) with a unique 5,6-dihydro-4H,7H-2,6-methanopyrano[4,3-d][1,3]dioxocin-7-one ring system and three undescribed polyketides (2, 7, and 11), were obtained from the endophytic fungus Neocamarosporium betae isolated from two desert plant species. Different approaches, including HR-ESI-MS, UV spectroscopy, IR spectroscopy, NMR, and CD, were used to determine the planar and absolute configurations of the compounds. The possible biosynthetic pathways were proposed based on the structural characteristics of compounds 1-13. Compounds 1, 3, 4, and 9 exhibited strong cytotoxicity toward HepG2 cells compared with the positive control. Several metabolites (2, 4-5, 7-9, and 11-13) were phytotoxic to foxtail leaves. The results support the hypothesis that endophytic fungi from special environments, such as desert areas, produce novel bioactive secondary metabolites.

Dereplication of Fungal Metabolites by NMR-Based Compound Networking Using MADByTE

Strategies for natural product dereplication are continually evolving, essentially in lock step with advances in MS and NMR techniques. MADByTE is a new platform designed to identify common structural features between samples in complex extract libraries using two-dimensional NMR spectra. This study evaluated the performance of MADByTE for compound dereplication by examining two classes of fungal metabolites, the resorcylic acid lactones (RALs) and spirobisnaphthalenes. First, a pure compound database was created using the HSQC and TOCSY data from 19 RALs and 10 spirobisnaphthalenes. Second, this database was used to assess the accuracy of compound class clustering through the generation of a spin system feature network. Seven fungal extracts were dereplicated using this approach, leading to the correct prediction of members of both families from the extract set. Finally, NMR-guided isolation led to the discovery of three new palmarumycins (20-22). Together these results demonstrate that MADByTE is effective for the detection of specific compound classes in complex mixtures and that this detection is possible for both known and new natural products.

Phaeosphspirone (1/1'), a pair of unique polyketide enantiomers with an unusual 6/5/5/6 tetracyclic ring from the desert plant endophytic fungus Phaeosphaeriaceae sp

Phaeosphspirone, an undescribed polyketide with a unique 6/5/5/6-fused tetracyclic system, and two known analogues, herbarin and O-methylherbarin, were purified from the endophytic fungus Phaeosphaeriaceae sp. isolated from the desert plant Bassia dasyphylla. The connectivity and relative configuration of phaeosphspirone was elucidated by comprehensive HR-ESI-MS and NMR analysis together with a computer-assisted structure elucidation (CASE) method. A pair of enantiomers existing in phaeosphspirone were separated by HPLC chromatography after reacting with chiral reagents, from which the absolute configuration of phaeosphspirone was simultaneously determined based on Mosher's rule. This tandem strategy provides a useful approach for the separation and stereochemical determination of enantiomers possessing secondary hydroxyl groups. The structural feature of phaeosphspirone, herbarin and O-methylherbarin together with gene cluster analysis suggested their polyketide biosynthetic origin. Herbarin and O-methylherbarin exhibited moderate cytotoxicity against three cancer cell lines.

Bioactive metabolites from the desert plant-associated endophytic fungus Chaetomium globosum (Chaetomiaceae)

Globosumin, an undescribed chromene-4,7(4aH)-dione-tetramic acid PKS-PKS-NRPS hybrid, and globosumone, an undescribed azaphilone, together with ten known metabolites, were isolated from the desert plant-associated endophytic fungus Chaetomium globosum (Chaetomiaceae). The planar structures and relative configurations of globosumin and globosumone were determined by high-resolution ESI-MS and NMR data, and the absolute configurations of these two metabolites were determined by electronic circular dichroism (ECD) and circular dichroism (CD) combined with time-dependent density functional theory (TDDFT)-based quantum-chemical calculations. Chaetoglobosin A displayed biological effects against the seedling growth of Arabidopsis thaliana (Brassicaceae) in a dose-dependent manner, and this compound also exhibited biological activity against two cancer cell lines, A549 and HepG2, with IC₅₀ values of 6.82 ± 2.34 and 38.62 ± 7.44 μM, respectively.

Bioactive seco-Sativene Sesquiterpenoids from an Artemisia desertorum Endophytic Fungus, Cochliobolus sativus

A series of seco-sativene sesquiterpenoids (1-11) including two new natural products (2 and 3), four new analogues (4-7), and six known analogues, helminthosporic acid (1), drechslerine A (8), drechslerine B (9), helminthosporol (10), helminthosporal acid (11), and isosativenediol (12), were purified from the endophytic fungus Cochliobolus sativus isolated from a desert plant, Artemisia desertorum. The stereochemistry of helminthosporic acid (1) was established for the first time by X-ray diffraction, and the structures including relative and absolute configurations of these new compounds were determined by NMR and CD spectra together with biosynthetic considerations. Compounds 5-7 are the first seco-sativene sesquiterpenoids possessing a glucose group on C-15, C-15, and C-14, respectively. Compounds 1, 7, 9, and 11 displayed strong phytotoxic effects on corn leaves by producing visible lesions, and helminthosporic acid (1) was shown to promote division of leaves and roots of Arabidopsis thaliana with a dose-dependent relationship.

Stereochemical determination of four 10-membered ring resorcylic acid lactones from the desert plant endophytic fungus Chaetosphaeronema hispidulum

Four 10-membered ring resorcylic acid lactones (RALs) including a new compound hispidulactone F (1) and three known analogs hispidulactone B (2), 2 R, 4R-sonnerlactone (3), and 2 R, 4S-sonnerlactone (4) were isolated from the special bioenvironmental desert plant endophytic fungus Chaetosphaeronema hispidulum. The structure of the new compound hispidulactone F (1) was determined by extensive spectra analysis including HR-ESI-MS, NMR (¹H, ¹³C, ¹H-¹H COSY, HSQC, and HMBC). Hispidulactone F (1) and hispidulactone B (2) were a pair of stereoisomers at C-3, whereas 2 R, 4R-sonnerlactone (3) and 2 R, 4S-sonnerlactone (4) were another pair of stereoisomers at C-4. The stereochemistries of the hydroxyl groups at C-3 in 1 and 2, and at C-4 in 3 and 4 were first determined by modified Mosher's reactions. Thus, the absolute configuration C-3 in hispidulactone B (2) was not right in our previous report, and was rectified to be R. Compounds 1 and 4 were evaluated for their cytotoxic effects on the proliferation of HepG2. The possible biosynthetic pathway of compounds 1-4 was also presented.

Herbicidal Polyketides and Diketopiperazine Derivatives from Penicillium viridicatum

Herbicidal activity-guided isolation from the fermentation extract of Penicillium viridicatum had obtained two herbicidal series of polyketides (1-7) and diketopiperazine derivatives (8-11), especially including three novel polyketides (1-3). The structures and absolute configurations of new polyketides 1-3 were elucidated by extensive spectroscopic analyses, as well as comparisons between measured and calculated ECD spectra. Novel polyketides 1-3 and known 4, all bearing the heptaketide skeleton with a trans-fused decalin ring of 8-CH₃ substitution, could significantly inhibit the radicle growth of Echinochloa crusgalli seedlings with a dose-dependent relationship. Especially at the concentration of 10 μg/mL, 1-4 exhibited the inhibition rates with 81.5% ± 2.0, 76.4% ± 0.8, 79.6% ± 1.1, and 80.0 ± 1.8%, respectively, even better than the commonly used synthetic herbicide of acetochlor with 76.1 ± 1.4%. Further greenhouse bioassay revealed that 4 showed pre-emergence herbicidal activity against E. crusgalli with the fresh-weight inhibition rate of 74.1% at a dosage of 400 g ai/ha, also better than acetochlor, while the other isolated metabolites (5-11) exhibited moderate herbicidal activities. The structure-activity differences of isolated polyketides indicated that the heptaketide skeleton, characterized by a trans-fused decalin ring with 8-CH₃ substitution, should be the key factor of their herbicidal activities, which could give new insights for the bioherbicide developments.

Hispidulones A and B, two new phenalenone analogs from desert plant endophytic fungus Chaetosphaeronema hispidulum

Two new phenalenone analogs hispidulones A (1) and B (2) were isolated from the specially bioenvironmental desert plant endophytic fungus Chaetosphaeronema hispidulum. The structure of these two compounds were elucidated by extensive spectra analysis including HR-ESI-MS, NMR (¹H, ¹³C, ¹H-¹H COSY, HSQC, and HMBC), CD, and electronic circular dichroism (ECD) combined with quantum-chemical calculations adopting time-dependent density functional theory (TDDFT) approaches. The W long-ranged ¹H-¹H COSY and HMBC correlations are very important in the structural elucidation of these two compounds. Hispidulone A (1) possesses a cyclohexa-2,5-dien-1-one moiety, whereas hispidulone B (2) contains a hemiacetal OCH₃ group, which are very rare in the structures of phenalenone analogs. According to structural features of these two compounds together considering the literature, the possible biosynthetic pathway of 1 and 2 was postulated. Hispidulone B (2) displayed cytotoxic activities against three cancer cell lines A549, Huh7, and HeLa with IC₅₀ values of 2.71 ± 0.08, 22.93 ± 1.61, and 23.94 ± 0.33 μM.

Chaetomadrasins A and B, Two New Cytotoxic Cytochalasans from Desert Soil-Derived Fungus Chaetomium madrasense 375

Two new cytochalasans, Chaetomadrasins A (1) and B (2), along with six known analogues (3-8), were isolated from the solid-state fermented culture of desert soil-derived Chaetomium madrasense 375. Their structures were clarified by comprehensive spectroscopic analyses, and the absolute configurations of Compounds 1 and 2 were confirmed by electronic circular dichroism (ECD) and calculated ECD. For the first time, Chaetomadrasins A (1), which belongs to the chaetoglobosin family, is characterized by the presence of all oxygen atoms in the form of Carbonyl. Chaetomadrasin B (2) represents the first example of chaetoglobosin type cytochalasan characterized by a hydroxy unit and carbonyl group fused to the indole ring. Compounds 1 and 2 displayed moderate cytotoxicity against HepG2 human hepatocellular carcinoma cells.

Sesquiterpenoids and mycotoxin swainsonine from the locoweed endophytic fungus Alternaria oxytropis

Oxytropiols A-J, ten undescribed guaiane-type sesquiterpenoids, and the mycotoxin swainsonine (SW) were isolated from the locoweed endophytic fungus Alternaria oxytropis. The chemical structures of these sesquiterpenoids were elucidated on the basis of HR-ESI-MS and NMR data including ¹H, ¹³C, HSQC, ¹H-¹H COSY, HMBC, and NOESY spectra, and the absolute configurations of these compounds were determined using a modified Mosher's method and X-ray diffraction spectroscopy. A possible biosynthetic pathway of these guaiane-type sesquiterpenoids is discussed, and proposed that post-modification oxidative enzymes might form these highly polyhydroxylated structures. Compound 1 displayed biological effects on the root growth of Arabidopsis thaliana, and SW displayed cytotoxicity against A549 and HeLa cancer cell lines.

Rhinoclactones A-E, Resorcylic Acid Analogs from Desert Plant Endophytic Fungus Rhinocladiella similis

Seven resorcylic acid lactones (RALs) including five new analog rhinoclactones, A–E (1, 2, 4–6), were isolated from an endophytic fungus Rhinocladiella similis in the plant Agriophyllum squarrosum collected from the Tengger Desert of the Ningxia Province, China. The structures of these new compounds were determined by HR-ESI-MS (High Resolution Electrospray Ionization Mass Spectrometry), NMR data, modified Mosher’s method, and X-ray diffraction experiments. All compounds isolated from this fungus possessed the 16-OMe/14-OH, not the common 16-OH/14-OH or 16-OH/14-OMe groups on the aromatic ring, which are rarely found in nature. Compound 7 displayed cytotoxic activities against HCT116 and HeLa cancer cell lines. The possible biosynthesis of 1–7 is suggested, and the potential ecological roles of these fungal secondary metabolites is discussed.

Antifungal Prenylated Diphenyl Ethers from Arthrinium arundinis, an Endophytic Fungus Isolated from the Leaves of Tobacco (Nicotiana tabacum L.)

An endophytic fungus Arthrinium arundinis TE-3 was isolated and purified from the fresh leaves of cultivated tobacco (Nicotiana tabacum L.). Chemical investigation on this fungal strain afforded three new prenylated diphenyl ethers (1-3) as well as three known analogues (4-6). Structure elucidation of the isolated compounds was carried out by analysis of 1D and 2D nuclear magnetic resonance (NMR) and high-resolution electrospray ionization mass spectroscopy (HRESIMS) spectra, as well as by comparison of those data with literature data. The absolute configuration of the stereogenic center at C-8 in 1 was assigned by comparison of the experimental and calculated ECD spectra. Compounds 1 and 2 showed selective antifungal activity against Mucor hiemalis with minimum inhibitory concentration (MIC) values of 8 and 4 μg/mL, respectively. Compounds 5 and 6 exhibited inhibitory activity against Alteraria alternata with an MIC value of 8 μg/mL. In the cytotoxic assay, 2, 5, and 6 displayed moderate in vitro cytotoxicity against the human monocytic cell line (THP-1 cell line), with IC50 values of 40.2, 28.3, and 25.9 μM, respectively. This study indicated that endophytic fungi possess great potential for exploring new bioactive secondary metabolites.