(+)- and (-)-Alternarilactone A: Enantiomers with a Diepoxy-Cage-like Scaffold from an Endophytic Alternaria sp

Identification of antibacterial constituents from Rhododendron simsii Planch with an activity-guided method

Bacterial infections and antibiotic resistance pose significant public health challenges globally. Natural products serve as valuable sources for discovering antimicrobial agents. Rhododendron simsii Planch, a folk medicine, is traditionally used to treat various inflammatory diseases. In this study, we investigated the antibacterial metabolites derived from R. simsii Planch. Rhodosimsiin A (1), bearing a 1,5-seco-1,6 and 3,6-epoxy grayanane diterpene skeleton, representing a novel 5/6/7/6/5 pentacyclic ring system, and 3β,16α-dihydroxy-6β-ethoxy-14β-acetoxy-grayan-1(5)-ene-10-one (4), which represents the first example of the degradation of C-20 and carbonylation in C-10 diterpenoid, together with two new grayanane diterpenes (2-3), three new triterpenes (13-15), and known analogs (5-12, 16-30), were isolated from the leaves of R. simsii Planch by using the bioassay-guided method. Their structures were elucidated by comprehensive spectroscopic analyses, and absolute configurations were established by single-crystal X-ray diffraction and calculated ECD spectra. Compounds 14, 15, 18, 20, 27, 28, and 30 exhibited potent antibacterial activity with an MIC50 of 1.4-24.3 μg/mL against Staphylococcus aureus. The findings of this research indicate that secondary metabolites derived from R. simsii Planch are promising natural antimicrobial candidates.

Natural resorcylic lactones derived from alternariol

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

Wheldone Revisited: Structure Revision Via DFT-GIAO Chemical Shift Calculations, 1,1-HD-ADEQUATE NMR Spectroscopy, and X-ray Crystallography Studies

Wheldone is a fungal metabolite isolated from the coculture of Aspergillus fischeri and Xylaria flabelliformis, displaying cytotoxic activity against breast, melanoma, and ovarian cancer cell lines. Initially, its structure was characterized as an unusual 5-methyl-bicyclo[5.4.0]undeca-3,5-diene scaffold with a 2-hydroxy-1-propanone side chain and a 3-(2-(1-hydroxyethyl)-2-methyl-2,5-dihydrofuran-3-yl)acrylic acid moiety. Upon further examination, minor inconsistencies in the data suggested the need for the structure to be revisited. Thus, the structure of wheldone has been revised using an orthogonal experimental-computational approach, which combines 1,1-HD-ADEQUATE NMR experiments, DFT-GIAO chemical shift calculations, and single-crystal X-ray diffraction (SCXRD) analysis of a semisynthetic p-bromobenzylamide derivative, formed via a Steglich-type reaction. The summation of these data now permits the unequivocal assignment of both the structure and absolute configuration of the natural product.

Discovery of neo-Clerodane Diterpenoids from Ajuga campylantha as Neuroprotective Agents against Ferroptosis and Neuroinflammation

Twelve new neo-clerodane diterpenoids, eight undescribed methoxy/ethoxy acetal analogues, and one new nor-iridane monoterpenoid were isolated from Ajuga campylantha. Their structures were elucidated using a combination of spectroscopic data, quantum chemical calculations, and X-ray crystallography. This research reveals the distinctive structural features of A. campylantha diterpenes, including distinct C rings and 4,18-double bonds, distinguishing them from diterpenes of other plants in the Ajuga genus. Compound 2 represents the first example of a 19(5→6)-abeo-clerodane formed through a Wagner-Meerwein rearrangement. The isolated compounds were assessed for their neuroprotective effects against RSL3-induced ferroptosis in HT22 cells and LPS-induced neuroinflammation in BV-2 cells. Notably, compound 7 inhibits ferroptosis (EC50 = 10 μM) with a potentially new mechanism of action. The preliminary structure-activity relationship studies revealed that the furan-clerodane diterpenoids possess potential ferroptosis inhibitory activity, while the lactone-clerodanes do not. This study represents the first report of furan-containing clerodanes within the Ajuga genus, providing fresh insights into the phytochemistry and pharmacological potential of A. campylantha.

Secondary metabolites of Alternaria: A comprehensive review of chemical diversity and pharmacological properties

Fungi are considered to be one of the wealthiest sources of bio-metabolites that can be employed for yielding novel biomedical agents. Alternaria, including parasitic, saprophytic, and endophytic species, is a kind of dark fungi that can produce a broad array of secondary metabolites (SMs) widely distributed in many ecosystems. These are categorized into polyketides, nitrogen-containing compounds, quinones, terpenes, and others based on the unique structural features of the metabolites. New natural products derived from Alternaria exhibit excellent bioactivities characterized by antibacterial, antitumor, antioxidative, phytotoxic, and enzyme inhibitory properties. Thus, the bio-metabolites of Alternaria species are significantly meaningful for pharmaceutical, industrial, biotechnological, and medicinal applications. To update the catalog of secondary metabolites synthesized by Alternaria fungi, 216 newly described metabolites isolated from Alternaria fungi were summarized with their diverse chemical structures, pharmacological activity, and possible biosynthetic pathway. In addition, possible insights, avenues, and challenges for future research and development of Alternaria are discussed.

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.

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.

A critical review on the use of DP4+ in the structural elucidation of natural products: the good, the bad and the ugly. A practical guide

Covering: 2015 up to the end of 2020Even in the golden age of NMR, the number of natural products being incorrectly assigned is becoming larger every day. The use of quantum NMR calculations coupled with sophisticated data analysis provides ideal complementary tools to facilitate the elucidation process in challenging cases. Among the current computational methodologies to perform this task, the DP4+ probability is a popular and widely used method. This updated version of Goodman's DP4 synergistically combines NMR calculations at higher levels of theory with the Bayesian analysis of both scaled and unscaled data. Since its publication in late 2015, the use of DP4+ to solve controversial natural products has substantially grown, with several predictions being confirmed by total synthesis. To date, the structures of more than 200 natural products were determined with the aid of DP4+. However, all that glitters is not gold. Besides its intrinsic limitations, on many occasions it has been improperly used with potentially important consequences on the quality of the assignment. Herein we present a critical revision on how the scientific community has been using DP4+, exploring the strengths of the method and how to obtain optimal results from it. We also analyze the weaknesses of DP4+, and the paths to by-pass them to maximize the confidence in the structural elucidation.

Enantiomeric Dibenzo-α-Pyrone Derivatives from Alternaria alternata ZHJG5 and Their Potential as Agrochemicals

Three pairs of enantiomeric dibenzo-α-pyrone derivatives (1-3) including two pairs of new racemates (±)-alternaone A (1) and (±)-alternaone B (2) and one new enantiomer (-)-alternatiol (3), together with five known compounds (4-8) were isolated from the fungus Alternaria alternata ZHJG5. Their structures were confirmed by spectroscopic data and single-crystal X-ray diffraction analysis. All enantiomers were separated via chiral high-performance liquid chromatography, with their configurations determined by electronic circular dichroism calculation. Biogenetically, a key epoxy-rearrangement step was proposed for the formation of skeletons in 1-3; (+) 1, (-)-1, and 5 presented moderate antibacterial inhibition on phytopathogenic bacteria Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola. In the antifungal test, compounds 7 and 8 showed a moderate protective effect against Botrytis cinerea in vivo.

Total synthesis and complete configurational assignment of amphirionin-2

Amphirionin-2 is a linear polyketide metabolite that exhibits potent and selective cytotoxic activity against certain human cancer cell lines. We disclose herein the first total synthesis of amphirionin-2 and determination of its absolute configuration. Our synthesis featured an extensive use of cobalt-catalyzed Mukaiyama-type cyclization of γ-hydroxy olefins for stereoselective formation of all the tetrahydrofuran rings found in the natural product, and a late-stage Stille-type coupling for convergent assembly of the entire carbon backbone. Four candidate diastereomers of amphirionin-2 were synthesized in a unified, convergent manner, and their spectroscopic/chromatographic properties were compared with those of the authentic material. The present study culminated in the reassignment of the C5/C7 relative configuration, assignment of the C12/C18 relative configuration, and determination of the absolute configuration of amphirionin-2.

Novel oligomeric neolignans with PTP1B inhibitory activity from the bark of Magnolia officinalis var. biloba

The barks of Magnolia officinalis var. biloba, Magnoliae cortex, have been used as traditional Chinese medicines for several centuries. In this study, phytochemical investigation of M. officinalis var. biloba bark extract afforded five pairs of novel enantiomeric oligomeric neolignans, (±)-mooligomers A-E (1-5). (±)-1 and (±)-2 were two diastereomeric pairs of enantiomers with six C6-C3 subunits, and (±)-4 was a pair of previously unreported tetrameric neolignans bearing eight C6-C3 subunits. (±)-5 is the first example of a naturally occurring trilignan featuring an eight-membered ring with a magnolol moiety. The absolute configurations of (±)-1-(±)-5 were elucidated on the basis of HRESIMS, 1D and 2D NMR spectroscopy and electronic circular dichroism (ECD) calculations. Among the compounds tested for their PTP1B inhibitory activities, (±)-2, (±)-4 and (±)-5 displayed significant PTP1B inhibitory activities with IC₅₀ values of 0.14-2.10 μM. Furthermore, a Molecular docking simulation of PTP1B and active compounds [(±)-2, (±)-4 and (±)-5] exhibited that these active compounds possess low binding affinities ranging from - 5.9 to - 7.7 kcal/mol.

Reisolation and Configurational Reinvestigation of Cottoquinazolines E-G from an Arthropod-Derived Strain of the Fungus Neosartorya fischeri

The reported fumiquinazoline-related alkaloids cottoquinazolines E-G (1-3) were reisolated from solid cultures of the fungus Neosartorya fischeri, which was isolated from the medicinal arthropod Cryptotympana atrata. The unresolved issues regarding the absolute configurations (for cottoquinazolines E and F) prompted a reinvestigation of the configurations for all three compounds, as enabled by extensive spectroscopic methods, comparisons of experimental electronic circular dichroism data, and X-ray crystallography. In addition, cottoquinazoline F (2) showed significant antibacterial activity against ESBL-producing Escherichia coli, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterococcus faecalis with MIC values of 8, 32, 32, and 16 μg/mL, respectively.

Rhizovagine A, an unusual dibenzo-α-pyrone alkaloid from the endophytic fungus Rhizopycnis vagum Nitaf22

Rhizovagine A (1), a novel dibenzo-α-pyrone alkaloid with an unprecedented 5/5/6/6/6 fused pentacyclic skeleton, was isolated from the endophytic fungus Rhizopycnis vagum Nitaf22. The structure was elucidated by comprehensive spectroscopic analysis, in combination with quantum chemical ¹³C NMR and electronic circular dichroism (ECD) calculations for configurational assignment. A plausible biosynthetic pathway for 1 was proposed. Compound 1 displayed acetylcholinesterase inhibitory activity.

Rhizovagine A (1), a dibenzo-α-pyrone alkaloid with a 5/5/6/6/6 fused pentacyclic skeleton and acetylcholinesterase inhibitory activity, was isolated from the endophytic fungus Rhizopycnis vagum Nitaf22.

Nidulaxanthone A, a xanthone dimer with a heptacyclic 6/6/6/6/6/6/6 ring system from Aspergillus sp.-F029

Nidulaxanthone A (1), a xanthone dimer bearing an unprecedented heptacyclic 6/6/6/6/6/6/6 system, together with a new monomeric nidulalin D (2) and four known analogues (3, 4, 5 and 6), were isolated from Aspergillus sp. F029.

Cytotoxic and neuroprotective activities of constituents from Alternaria alternate, a fungal endophyte of Psidium littorale

Chemical investigation of the EtOAc extract of the plant-associated fungus Alternaria alternate in rice culture led to the isolation of a novel liphatic polyketone, alternin A (1), a new indole alkaloid (8), and a new sesquiterpene (11), together with 12 known compounds. Their structures were elucidated by the interpretation of extensive spectroscopic data, and the absolute configurations of 1-3 were established using calculations of ECD spectra, NMR data, and optical rotation values. Compound 1 possesses an unprecedented C25 liphatic polyketone skeleton. Compounds 5 and 10 exhibited potential cytotoxic activities against MCF-7 and HepG cells, and compounds 2, 7, and 9 exhibited potential neuroprotective activities in glutamate induced-PC12 injured cells.