Decalin-Containing Tetramic Acids and 4-Hydroxy-2-pyridones with Antimicrobial and Cytotoxic Activity from the Fungus Coniochaeta cephalothecoides Collected in Tibetan Plateau (Medog)

Biosynthesis, biological activities, and structure-activity relationships of decalin-containing tetramic acid derivatives isolated from fungi

Covering: up to December 2023Decalin-containing tetramic acid derivatives, especially 3-decalinoyltetramic acids (3-DTAs), are commonly found as fungal secondary metabolites. Numerous biological activities of this class of compounds, such as antibiotic, antiviral, antifungal, antiplasmodial, and antiprotozoal properties, have been the subject of ongoing research. For this reason, these molecules have attracted a lot of interest from the scientific community and various efforts including semi-synthesis, co-culturing with bacteria and biosynthetic gene sequencing have been made to obtain more derivatives. In this review, 3-DTAs are classified into four major groups based on the absolute configuration of the bicyclic decalin ring. Their biosynthetic pathways, various biological activities, and structure-activity relationship are then introduced.

Metabolism characterization and toxicity of N-hydap, a marine candidate drug for lung cancer therapy by LC-MS method

N-Hydroxyapiosporamide (N-hydap), a marine product derived from a sponge-associated fungus, has shown promising inhibitory effects on small cell lung cancer (SCLC). However, there is limited understanding of its metabolic pathways and characteristics. This study explored the in vitro metabolic profiles of N-hydap in human recombinant cytochrome P450s (CYPs) and UDP-glucuronosyltransferases (UGTs), as well as human/rat/mice microsomes, and also the pharmacokinetic properties by HPLC-MS/MS. Additionally, the cocktail probe method was used to investigate the potential to create drug-drug interactions (DDIs). N-Hydap was metabolically unstable in various microsomes after 1 h, with about 50% and 70% of it being eliminated by CYPs and UGTs, respectively. UGT1A3 was the main enzyme involved in glucuronidation (over 80%), making glucuronide the primary metabolite. Despite low bioavailability (0.024%), N-hydap exhibited a higher distribution in the lungs (26.26%), accounting for its efficacy against SCLC. Administering N-hydap to mice at normal doses via gavage did not result in significant toxicity. Furthermore, N-hydap was found to affect the catalytic activity of drug metabolic enzymes (DMEs), particularly increasing the activity of UGT1A3, suggesting potential for DDIs. Understanding the metabolic pathways and properties of N-hydap should improve our knowledge of its drug efficacy, toxicity, and potential for DDIs.

Description of Two Fungal Endophytes Isolated from Fragaria chiloensis subsp. chiloensis f. patagonica: Coniochaeta fragariicola sp. nov. and a New Record of Coniochaeta hansenii

Prospection of the endosphere of the native plant Fragaria chiloensis subsp. chiloensis f. patagonica from the foothills of the Chilean Andes led to the isolation of two strains of the genus Coniochaeta. We addressed the taxonomic placement of these strains based on DNA sequencing data using the ITS and LSU genetic markers, morphological features, and biochemical traits. One of these strains was identified as Coniochaeta hansenii, for which the anamorph and teleomorph states were described. The second strain did not seem to match any of the currently described species of this genus; therefore, we propose the name Coniochaeta fragariicola sp. nov.

Two New 4-Hydroxy-2-pyridone Alkaloids with Antimicrobial and Cytotoxic Activities from Arthrinium sp. GZWMJZ-606 Endophytic with Houttuynia cordata Thunb

Two new 4-hydroxy-2-pyridone alkaloids furanpydone A and B (1 and 2), along with two known compounds N-hydroxyapiosporamide (3) and apiosporamide (4) were isolated from the endophytic fungus Arthrinium sp. GZWMJZ-606 in Houttuynia cordata Thunb. Furanpydone A and B had unusual 5-(7-oxabicyclo[2.2.1]heptane)-4-hydroxy-2-pyridone skeleton. Their structures including absolute configurations were determined on the basis of spectroscopic analysis, as well as the X-ray diffraction experiment. Compound 1 showed inhibitory activity against ten cancer cell lines (MKN-45, HCT116, K562, A549, DU145, SF126, A-375, 786O, 5637, and PATU8988T) with IC₅₀ values from 4.35 to 9.72 µM. Compounds 1, 3 and 4 showed moderate inhibitory effects against four Gram-positive strains (Staphylococcus aureus, methicillin-resistant S. aureus, Bacillus Subtilis, Clostridium perfringens) and one Gram-negative strain (Ralstonia solanacarum) with MIC values from 1.56 to 25 µM. However, compounds 1-4 showed no obvious inhibitory activity against two Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) and two pathogenic fungi (Candida albicans and Candida glabrata) at 50 µM. These results show that compounds 1-4 are expected to be developed as lead compounds for antibacterial or anti-tumor drugs.

In silico prediction of Antifungal compounds from Natural sources towards Lanosterol 14-alpha demethylase (CYP51) using Molecular docking and Molecular dynamic simulation

An increase in the occurrence of fungal infections throughout the world, as well as the rise of novel fungal strains and antifungal resistance to commercially available drugs, suggests that new therapeutic choices for fungal infections are needed. The purpose of this research was to find new antifungal candidates or leads of secondary metabolites derived from natural sources that could effectively inhibit the enzymatic activity of Candida albicans lanosterol 14-alpha demethylase (CYP51) while also having good pharmacokinetics. In silico prediction of the drug-likeness, chemo-informatics and enzyme inhibition indicate that the 46 compounds derived from fungi, sponges, plants, bacteria and algae sources have a high novelty to meet all five requirements of Lipinski's rules and impede enzymatic function. Among the 15 candidate molecules with strong binding affinity to CYP51 investigated by molecular docking simulation, didymellamide A-E compounds demonstrated the strongest binding energy against the target protein at -11.14, -11.46, -11.98, -11.98, and -11.50 kcal/mol, respectively. Didymellamide molecules bind to comparable active pocket sites of antifungal ketoconazole and itraconazole medicines by hydrogen bonds forming to Tyr132, Ser378, Met508, His377 and Ser507, and hydrophobic interactions with HEM601 molecule. The stability of the CYP51-ligand complexes was further investigated using molecular dynamics simulations that took into account different geometric features and computed binding free energy. Using the pkCSM ADMET descriptors tool, several pharmacokinetic characteristics and the toxicity of candidate compounds were assessed. The findings of this study revealed that didymellamides could be a promising inhibitor against these CYP51 protein. However, there is still a need for further in vivo and in vitro studies to support these findings.

Discovery of Anti-MRSA Secondary Metabolites from a Marine-Derived Fungus Aspergillus fumigatus

Methicillin-resistant Staphylococcus aureus (MRSA), a WHO high-priority pathogen that can cause great harm to living beings, is a primary cause of death from antibiotic-resistant infections. In the present study, six new compounds, including fumindoline A–C (1–3), 12β, 13β-hydroxy-asperfumigatin (4), 2-epi-tryptoquivaline F (17) and penibenzophenone E (37), and thirty-nine known ones were isolated from the marine-derived fungus Aspergillus fumigatus H22. The structures and the absolute configurations of the new compounds were unambiguously assigned by spectroscopic data, mass spectrometry (MS), electronic circular dichroism (ECD) spectroscopic analyses, quantum NMR and ECD calculations, and chemical derivatizations. Bioactivity screening indicated that nearly half of the compounds exhibit antibacterial activity, especially compounds 8 and 11, and 33–38 showed excellent antimicrobial activities against MRSA, with minimum inhibitory concentration (MIC) values ranging from 1.25 to 2.5 μM. In addition, compound 8 showed moderate inhibitory activity against Mycobacterium bovis (MIC: 25 μM), compound 10 showed moderate inhibitory activity against Candida albicans (MIC: 50 μM), and compound 13 showed strong inhibitory activity against the hatching of a Caenorhabditis elegans egg (IC₅₀: 2.5 μM).

Structural Analysis of the Michael-Michael Ring Closure (MIMIRC) Reaction Products

A representative number of decalin and hydrindane derivatives 2a–l were prepared in 11–91% yield by means of a cascade reaction of cyclohexanone/cyclopentanone enolates and methyl acrylate through a Michael–Michael ring closure (MIMIRC) process. The relative stereochemistry of the four stereogenic centers formed in all products was determined by analyzing the vicinal coupling constants from the ¹H NMR and X-ray crystallography. Such a stereochemical outcome was corroborated by conformational analysis supported by DFT calculations and simulating the ¹H NMR spectra of representative products. All products showed the same relative stereochemistry at C-1 and C-8a, while at C-3 and bridgehead carbon C-4a, configurational changes were observed. The present results provide some insights about the scope and limitations of the triple cascade reaction between cycloalkanone enolates with methyl acrylate. This synthetic protocol is still a simple and very practical alternative to generate decalin and hydrindane derivatives with great structural diversity.

Asymmetric Total Synthesis of Griseofamine B and Its Three Stereoisomers

The first total synthesis of griseofamine B is described starting from l-4-bromo tryptophan methyl ester hydrochloride via five steps and in 18% overall yield. Its three stereoisomers were also synthesized following the same procedure with the yields of 5%, 19%, and 5%, respectively. In vitro antibacterial activities were also evaluated. All four compounds exhibited less potent activity than griseofamine A.

Natural Enantiomers: Occurrence, Biogenesis and Biological Properties

The knowledge that natural products (NPs) are potent and selective modulators of important biomacromolecules (e.g., DNA and proteins) has inspired some of the world’s most successful pharmaceuticals and agrochemicals. Notwithstanding these successes and despite a growing number of reports on naturally occurring pairs of enantiomers, this area of NP science still remains largely unexplored, consistent with the adage “If you don’t seek, you don’t find”. Statistically, a rapidly growing number of enantiomeric NPs have been reported in the last several years. The current review provides a comprehensive overview of recent records on natural enantiomers, with the aim of advancing awareness and providing a better understanding of the chemical diversity and biogenetic context, as well as the biological properties and therapeutic (drug discovery) potential, of enantiomeric NPs.

Exploring Verrucosidin Derivatives with Glucose-Uptake-Stimulatory Activity from Penicillium cellarum Using MS/MS-Based Molecular Networking

Under the guidance of LC-MS/MS-based molecular networking, seven new verrucosidin derivatives, penicicellarusins A-G (3–9), were isolated together with three known analogues from the fungus Penicillium cellarum. The structures of the new compounds were determined by a combination of NMR, mass and electronic circular dichroism spectral data analysis. The absolute configuration of penicyrone A (10) was corrected based on X-ray diffraction analyses. Bioactivity screening indicated that compounds 1, 2, and 4 showed much stronger promising hypoglycemic activity than the positive drug (rosiglitazone) in the range of 25–100 μM, which represents a potential new class of hypoglycemic agents. Preliminary structure-activity relationship analysis indicates that the formation of epoxy ring on C₆-C₇ in the structures is important for the glucose uptake-stimulating activity. The gene cluster for the biosynthesis of 1–12 is identified by sequencing the genome of P. cellarum and similarity analysis with the gene cluster of verrucosidins in P. polonicum.

Iterative synthesis of nitrogen-containing polyketide via oxime intermediates

Typical polyketides consist of C, H, and O atoms, whereas several types of N-containing polyketides are known to show intriguing properties. Because conventional synthetic approaches for such compounds focus on only specific structures, a more general method is desirable. Here, we have developed an iterative synthesis of nitrogen-containing polyketide. Chain elongation of carboxylic acid via decarboxylative Claisen condensation with malonic acid half thioester was iteratively performed to construct carbon frameworks. β-Keto groups formed by the chain elongation were appropriately converted to O-methyl oximes for incorporation of nitrogen atoms. Cyclization of the resulting oxime intermediates followed by reductive N–O cleavage afforded structurally diverse nitrogen-containing polyketides such as 2-pyridone, 4-aminopyrone, and 4-aminosalicylate. This method was finally applied to the synthesis of (R)-6-aminomellein, which is a nitrogen-substituted derivative of bioactive compound, (R)-6-methoxymellein. The versatility of the present method would enable the synthesis of diverse polyketides with nitrogen functional groups, which can be potentially utilized for the development of novel bioactive compounds.

An iterative synthesis of nitrogen-containing polyketides has been developed. Structurally diverse polyketides with nitrogen functional groups were obtained by the uniform protocol.

A Cell Factory of a Fungicolous Fungus Calcarisporium arbuscula for Efficient Production of Natural Products

Fungal natural products are rich sources of clinical drugs. Particularly, the fungicolous fungi have a large number of biosynthetic gene clusters (BGCs) to produce numerous bioactive natural products, but most BGCs are silent in the laboratory. We have shown that a fungicolous fungus Calcarisporium arbuscula NRRL 3705 predominantly produces the highly reduced polyketide-type mycotoxins aurovertins. Here after evaluation of the aurovertin-null mutant ΔaurA as an efficient host, we further screened two strong promoters aurBp and A07068p based on RNA-Seq, and successfully activated an endogenous gene cluster from C. arbuscula as well as three additional exogenous BGCs from other fungi to produce polyketide-type natural products. Thus, we showed an efficient expression system from the fungicolous fungus C. arbuscula, which will be highly beneficial and complementary to the conventional Aspergillus and Penicillium fungal cell factories, and provides a useful toolkit for genome-wide mining of bioactive natural products from fungicolous fungi.

Taxonomic Re-Examination of Nine Rosellinia Types (Ascomycota, Xylariales) Stored in the Saccardo Mycological Collection

In a recent monograph on the genus Rosellinia, type specimens worldwide were revised and re-classified using a morphological approach. Among them, some came from Pier Andrea Saccardo's fungarium stored in the Herbarium of the Padova Botanical Garden. In this work, we taxonomically re-examine via a morphological and molecular approach nine different Roselliniasensu Saccardo types. ITS1 and/or ITS2 sequences were successfully obtained applying Illumina MiSeq technology and phylogenetic analyses were carried out in order to elucidate their current taxonomic position. Only the ITS1 sequence was recovered for Rosellinia areolata, while for R. geophila, only the ITS2 sequence was recovered. We proposed here new combinations for Rosellinia chordicola, R. geophila and R. horridula, while for R. ambigua, R. areolata, R. australis, R. romana and R. somala, we did not suggest taxonomic changes compared to the current ones. The name Rosellinia subsimilis Sacc. is invalid, as it is a later homonym of R. subsimilis P. Karst. & Starbäck. Therefore, we introduced Coniochaeta dakotensis as a nomen novum for R. subsimilis Sacc. This is the first time that these types have been subjected to a molecular study. Our results demonstrate that old types are an important source of DNA sequence data for taxonomic re-examinations.

Arthpyrone L, a New Pyridone Alkaloid from a Deep-Sea Arthrinium sp., Inhibits Proliferation of MG63 Osteosarcoma Cells by Inducing G0/G1 Arrest and Apoptosis

Fractionation of the ethanol extract of a marine fungus, Arthrinium sp., afforded a new pyridone alkaloid (arthpyrone L (1)), the structure with absolute configuration of which was established by comprehensive spectroscopic analyses. In vitro cell viability assays revealed that compound 1 showed antiproliferative effects toward human A549 (lung), MG63, U2OS (bone), MCF-7 and MDA-MB-231 (breast) cancer cells. MG63 cell lines were chosen for further biological evaluations and presented apoptosis and cell cycle arrest (G0/G1 phase) upon treatment of 1. Subsequent mechanism studies demonstrated that the growth inhibition of 1 against MG63 cells was via activation of caspase-modulated apoptotic pathway and inhibition of PI3K/Akt pathway.

An efficient method for the synthesis of 2-pyridones via C-H bond functionalization

A simple and practical method to access N-substituted 2-pyridones via a formal [3+3] annulation of enaminones with acrylates based on RhIII-catalyzed C-H functionalization was developed. Control and deuterated experiments led to a plausible mechanism involving C-H bond cross-coupling and aminolysis cyclization. This strategy provides a short synthesis of structural motifs of N-substituted 2-pyridones.

Immunosuppressive Nor-isopimarane Diterpenes from Cultures of the Fungicolous Fungus Xylaria longipes HFG1018

Eighteen new nor-isopimarane diterpenes, xylarinorditerpenes A-R (1-18), along with two previously reported compounds, 14α,16-epoxy-18-norisopimar-7-en-4α-ol (19) and the labdane-type diterpene agatadiol (20), were isolated from cultures of the fungicolous fungus Xylaria longipes HFG1018 isolated from the wood-rotting basidiomycete Fomitopsis betulinus. The structure elucidation and relative configuration assignments of 1-18 were accomplished by interpretation of spectroscopic data and through computational methods. The absolute configurations of 1, 4, and 16 were determined by single-crystal X-ray diffraction. Compounds 1-16 possess an 18- or 19-nor-isopimarane skeleton, and compounds 17 and 18 possess an 18,19-dinor-isopimarane skeleton. Compounds 2-5, 9, 14, 19, and 20 showed immunosuppressive activity but were devoid of cytotoxicity against the cell proliferation by concanavalin A-induced T lymphocytes and lipopolysaccharide-induced B lymphocytes, with IC₅₀ values varying from 1.0 to 27.2 μM and from 16.1 to 51.8 μM, respectively.

Fungi: outstanding source of novel chemical scaffolds

A large number of remarkable studies on the secondary metabolites of fungi have been conducted in recent years. This review gives an overview of one hundred and sixty-seven molecules with novel skeletons and their bioactivities that have been reported in seventy-nine articles published from 2013 to 2017. Our statistical data showed that endophytic fungi and marine-derived fungi are the major sources of novel bioactive secondary metabolites.

New Tetramic Acids Comprising of Decalin and Pyridones From Chaetomium olivaceum SD-80A With Antimicrobial Activity

Cycloaddition reactions such as intramolecular Diels–Alder (IMDA) are extremely important in constructing multicyclic scaffolds with diverse bioactivities. Using MycB as a biomarker, three new polyketides – Chaetolivacines A (1), B (3), and C (4) – with one known compound Myceliothermophin E (2) comprising of decalin and 4-hydroxy-2-pyridones were obtained from the culture of Chaetomium olivaceum SD-80A under the guidance of gene mining. The structures of these compounds were established using detailed 1D, 2D NMR, and high-resolution electron spray ionization mass spectroscopy (HRESIMS) analysis. The relative and absolute configurations of the compounds 1, 3, and 4 were elucidated by NOESY and ECD. The biosynthesis pathways of these compounds were proposed, which involves in three key genes ChaA [polyketide synthase-non-ribosomal peptide synthetases (PKS-NRPS)], ChaB, and ChaC. Compounds 1–4 were tested for their antimicrobial activities, and compounds 2 and 3 showed moderate bioactivity against Staphylococcus aureus (SA) and methicillin-resistant S. aureus (MRSA) with MIC values of 15.8 and 27.1 μM. The results showed that configuration of C-21 in 3 and 4 is important for anti-SA and anti-MRSA activities. This study reveals the significant potential of the genus Chaetomium in producing new PKS-NRPS, therefore increasing the speed in the mining for new sources of antimicrobial agents.

A Decade of Antifungal Leads from Natural Products: 2010-2019

In this review, we discuss novel natural products discovered within the last decade that are reported to have antifungal activity against pathogenic species. Nearly a hundred natural products were identified that originate from bacteria, algae, fungi, sponges, and plants. Fungi were the most prolific source of antifungal compounds discovered during the period of review. The structural diversity of these antifungal leads encompasses all the major classes of natural products including polyketides, shikimate metabolites, terpenoids, alkaloids, and peptides.

Total Synthesis and Antibacterial Activity Evaluation of Griseofamine A and 16- epi-Griseofamine A

The first total synthesis of griseofamine A and its diastereomer, 16- epi-griseofamine A, is described over seven steps with yields of 23% and 7%, respectively. Their antibacterial activities are also disclosed for the first time. Griseofamine A exhibited in vitro activities against a panel of drug-resistant Gram-positive bacteria with minimum inhibitory concentration (MIC) values of 8-16 μg/mL. Notably, 16- epi-griseofamine A was 2-3 times more potent than griseofamine A with MIC values of 2-8 μg/mL.

Ag NPs decorated on a COF in the presence of DBU as an efficient catalytic system for the synthesis of tetramic acids via CO2 fixation into propargylic amines at atmospheric pressure

Ag NPs are decorated at the surface of a COF material TpPa-1 and the resulting Ag@TpPa-1 catalyzes efficiently for the synthesis of tetramic acids from a variety of propargylic amines using CO₂ as reagent.

Bioactive Pyridone Alkaloids from a Deep-Sea-Derived Fungus Arthrinium sp. UJNMF0008

Eight new 4-hydroxy-2-pyridone alkaloids arthpyrones D⁻K (1⁻8), along with two known analogues apiosporamide (9) and arthpyrone B (10), were isolated from a deep-sea-derived fungus Arthrinium sp. UJNMF0008. The structures of the isolated compounds were elucidated on the basis of spectroscopic methods with that of 1 being established by chemical transformation and X-ray diffraction analysis. Compounds 1 and 2 bore an ester functionality linking the pyridone and decalin moieties first reported in this class of metabolites, while 3 and 4 incorporated a rare natural hexa- or tetrahydrobenzofuro[3,2-c]pyridin-3(2H)-one motif. Compounds 3⁻6 and 9 exhibited moderate to significant antibacterial activity against Mycobacterium smegmatis and Staphylococcus aureus with IC₅₀ values ranging from 1.66⁻42.8 μM, while 9 displayed cytotoxicity against two human osteosarcoma cell lines (U2OS and MG63) with IC₅₀ values of 19.3 and 11.7 μM, respectively.