Emeriones A-C: Three Highly Methylated Polyketides with Bicyclo[4.2.0]octene and 3,6-Dioxabicyclo[3.1.0]hexane Functionalities from Emericella nidulans

Chemical Diversity, Pharmacology, Synthesis and Detection of Naturally Occurring Peroxides

Natural occurring peroxides are interesting bioprospecting targets due to their molecular structural diversity and the wide range of pharmacological activities. In this systematic review, a total of 123 peroxide compounds were analysed from 99 published papers with the compounds distributed in 31 plants, 18 animals and 41 microorganisms living in land and water ecosystems. The peroxide moiety exists as both cyclic and acyclic entities and can include 1,2-dioxolanes, 1,2-dioxane rings and common secondary metabolites with a peroxo group. These peroxides possessed diverse bioactivities including anticancer, antimalarial, antimicrobial, anti-inflammatory, neuroprotective, adipogenic suppressor, antituberculosis, anti-melanogenic and anti-coagulant agents. Biosynthetic pathways and mechanisms of most endoperoxides have not been well established. Method development in peroxide detection has been a challenging task requiring multidisciplinary investigation and exploration on peroxy-containing secondary metabolites are necessary.

Induction of apoptosis by emestrin from the plant endophytic fungus Emericella nidulans ATCC 38163 in Huh-7 human hepatocellular carcinoma cells

This research aimed to investigate the anticancer properties of emestrin, a major constituent of Emericella nidulans ATCC 38163 through the induction of apoptosis in Huh-7 human hepatocellular carcinoma cells. In this study, this fungus was isolated from the fresh leaves of Ruprechtia salicifolia (Cham. & Schltdl.) C.A. Mey, and identified by morphology and 18S rDNA followed by large-scale fermentation in liquid biomalt broth medium. Epidithiodioxopiperazine derivative emestrin along with ten known metabolites were isolated and identified from the fungal extract. The cytotoxic assay revealed that emestrin had the strongest cytotoxicity against Huh-7 and A-549 cells with IC50 values of 4.89 and 6.3 μM, respectively. Using annexin V-FITC assay, treatment of Huh-7 cells with 4.89 µM for 24 h resulted in a significant increase in the percentage of early and late apoptosis (3.16% and 22.84%, respectively) compared to untreated cells. Additionally, Bax and bcl-2 protein levels were regulated, which induced apoptosis in treated cells. These results indicate that emestrin induces mitochondrial pathway to stimulate apoptosis and inhibits cell proliferation in hepatocellular carcinoma.

Aquilariperoxide A, a Sesquiterpene Dimer from Agarwood of Aquilaria sinensis with Dual Antitumor and Antimalarial Effects

Aquilariperoxide A (1), an unprecedented sesquiterpene dimer characterized by a dioxepane ring connecting two sesquiterpene units via a C-C bond, was isolated from agarwood of Aquilaria sinensis-containing resins. The structure was elucidated by spectroscopic and computational methods. A bioassay revealed that 1 significantly inhibits cell proliferation and migration in human cancer cells. The mechanism of 1 against cancer cells was briefly discussed by analysis of RNA sequence data and epithelial-mesenchymal transition. Besides, the antimalarial activity of 1 was also evaluated.

Quadristerols A-G: Seven undescribed ergosterols from Aspergillus quadrilineata

Quadristerols A-G, seven undescribed ergosterols, were obtained from Aspergillus quadrilineata. Their structures and absolute configurations were determined based on HRESIMS, NMR, quantum-chemical calculations, and single-crystal X-ray diffraction analyses. Quadristerols A-G featured ergosterol skeletons with different attachments; quadristerols A-C were three diastereoisomers possessing a 2-hydroxy-propionyloxy group at C-6, and quadristerols D-G were two pairs of epimers with a 2,3-butanediol group at C-6. All of these compounds were evaluated for their immunosuppressive activities in vitro. Quadristerols B and C showed excellent inhibitory effects against concanavalin A-induced T lymphocyte proliferation with IC₅₀ values of 7.43 and 3.95 μM, respectively, and quadristerols D and E strongly inhibited lipopolysaccharide-induced B lymphocyte proliferation with IC₅₀ values of 10.96 and 7.47 μM, respectively.

Spectasterols, Aromatic Ergosterols with 6/6/6/5/5, 6/6/6/6, and 6/6/6/5 Ring Systems from Aspergillus spectabilis

Spectasterols A-E (1-5), aromatic ergosterols with unique ring systems, were isolated from Aspergillus spectabilis. Compounds 1 and 2 possess a 6/6/6/5/5 ring system with an additional cyclopentene, while 3 and 4 have an uncommon 6/6/6/6 ring system generated by the D-ring expansion via 1,2-alkyl shifts. Compound 3 exhibited cytotoxic activity (IC50 6.9 μM) and induced cell cycle arrest and apoptosis in HL60 cells. Compound 3 was anti-inflammatory; it decreased COX-2 levels at the transcription and protein levels and inhibited the nuclear translocation of NF-κB p65.

Aspergillus nidulans—Natural Metabolites Powerhouse: Structures, Biosynthesis, Bioactivities, and Biotechnological Potential

Nowadays, finding out new natural scaffolds of microbial origin increases at a higher rate than in the past decades and represents an auspicious route for reinvigorating the pool of compounds entering pharmaceutical industries. Fungi serve as a depository of fascinating, structurally unique metabolites with considerable therapeutic significance. Aspergillus genus represents one of the most prolific genera of filamentous fungi. Aspergillus nidulans Winter G. is a well-known and plentiful source of bioactive metabolites with abundant structural diversity, including terpenoids, benzophenones, sterols, alkaloids, xanthones, and polyketides, many of which display various bioactivities, such as cytotoxicity, antioxidant, anti-inflammatory, antiviral, and antimicrobial activities. The current work is targeted to survey the reported literature on A. nidulans, particularly its metabolites, biosynthesis, and bioactivities, in addition to recent reports on its biotechnological potential. From 1953 till November 2022, relying on the stated data, 206 metabolites were listed, with more than 100 references.

Synthesis and hydrolysis of monocarbamate from allylic 1,4-dicarbamate: Bis-homodichloroinositol

The synthesis of novel bis-homodichloroinositol with a configuration similar to that of conduritol-D is reported for the first time. The photooxygenation of cis-dichloro-diene obtained using cyclooctatetraene as the starting molecule afforted the tricyclic endoperoxide. The reduction of the endoperoxide with thiourea gave the corresponding allylic cis-diol. Formation of the bis-carbamate groups with p-TsNCO of allylic cis-diol followed by the [(dba)₃Pd₂CHCl₃] in the presence of trimethylsilyl azide, gave a new monocarbamate as well as oxazolidinone derivative. Oxidation of the double bond in the monocarbamate with osmium tetraoxide followed by acetylation furnished the desired monocarbamate triacetate. Eventually, the desired halogenated bicyclo[4.2.0] inositol (bis-homodichloroinositol) were obtained in high yield by hydrolysis of the acetate groups and monocarbanate group by potassium carbonate in methanol. Characterization of all the synthesized compounds were performed by FT-IR, ¹H NMR, ¹³C NMR, COSY (2D-NMR), HRMS, and Elemental Analysis techniques.

Bioinspired Asymmetric Total Synthesis of Emeriones A–C**

We report asymmetric bioinspired total syntheses of the fungal metabolites emeriones A–C via stereoselective oxidations of two bicyclo[4.2.0]octadiene diastereomers. The central bicyclic scaffolds are prepared in an 8π/6π electrocyclization cascade of a stereodefined pentaene, which contains the fully assembled side chains of the emeriones. The anti‐aldol side chain is made using a Paterson‐aldol addition, and the epoxide of the dioxabicyclo[3.1.0]hexane side chain via ring‐closure onto an oxidized acetal. Our work has enabled the structural revision of emerione C, and resulted in the synthesis of a “missing” family member, which we call emerione D. DFT calculations identified two methyl groups that govern torquoselectivity in the 8π/6π cascade.

Stereoselective synthesis of novel bis-homoinositols with bicyclo[4.2.0]octane motifs

Starting from cyclooctatetraene, bis-homoconduritols with cis-inositol and allo-inositol (or bicyclo[4.2.0]octane motif) structures were synthesized. Photooxygenation of trans-7,8-dibromo-bicyclo[4.2.0]octa-2,4-diene allowed the preparation of tricyclic endoperoxide. The compound diacetate was obtained by reduction of endoperoxide with thiourea followed by acetylation reaction. Removal of halides with zinc dust in acetic acid yielded the dien-diacetate, a key compound of the designed molecules. OsO₄ oxidation of diendiacetate followed by acetylation gave the corresponding hexaacetates. Finally, the novel desired bis-homoinositols were obtained in high yield by the ammonolysis of acetate groups. The structures of all synthesized compounds were characterized by spectroscopic methods.

Emerione A, a novel fungal metabolite as an inhibitor of New Delhi metallo-β-lactamase-1, restores carbapenem susceptibility in carbapenem-resistant isolates

OBJECTIVES

Bacterial strains that produce New Delhi metal-β-lactamase 1 (NDM-1) are worldwide threats. It is still a challenging task to find a potent NDM-1 inhibitor for clinical practice.

METHODS

Molecular docking and virtual screening of an in-house fungal natural product database for NDM-1 inhibitors were performed. Based on the screening results, the affinity and inhibition analysis of potential NDM-1 inhibitors was determined using purified NDM-1. The efficacy of compounds in combination with four β-lactam antibiotics (meropenem, imipenem, ceftriaxone and ampicillin) was evaluated. The morphological transforms of K. pneumoniae ATCC BAA2146 after treatment with the compounds were visualized by transmission electron microscopy.

RESULTS

In silico screening led to the identification of four fungal products as potential NDM-1 inhibitors. Emerione A (1), a methylated polyketide with bicyclo[4.2.0]octene and 3,6-dioxabicyclo[3.1.0]hexane, has significant activity in cells (K_d = 11.8 ± 0.6 μM; IC₅₀ = 12.1 ± 0.9 μM) and potentiates the activity of meropenem against two kinds of NDM-1-producing Enterobacteriaceae. To the best of our knowledge, emerione A (1) is the second fungal metabolite reported to exhibit NMD-1 inhibitory activity. According to the structural novelty of our database, we also found a structural new compound, asperfunolone A (2), with potential NMD-1 inhibitory activity.

CONCLUSION

Considering the low toxicity characteristic of emerione A (1), it may be processed as a potential lead compound for anti-NDM-1 drug development.

Plakortinic acids C and D: a pair of peroxide-polyketides possessing a rare 7,8-dioxatricyclo[4.2.2.02,5]dec-9-ene core from a two-sponge association of Plakortis symbiotica-Xestospongia deweerdtae

Plakortinic acids C (3) and D (4), two unprecedented peroxide-polyketides with 7,8-dioxatricyclo[4.2.2.0^2,5]dec-9-ene scaffold, as well as known biogenetically related congeners, plakortinic acids A (1) and B (2), were isolated from a two-sponge association of Plakortis symbiotica-Xestospongia deweerdtae. Upon chemical derivatization, the structures and relative configurations of 3 and 4 were characterized by analysis of HRESIMS and NMR spectroscopic data, molecular modeling studies, and chiroptical comparisons with known natural products and published values of [α]_D of related synthetic analogs. A mixture of methyl ester derivatives 5 and 6 displayed negligible cytotoxicity against a panel of 60 cell lines of various human cancers at a concentration of 10 μM.

Survey of natural products reported by Asian research groups in 2019

The new natural products reported in 2019 in peer-reviewed articles in journals with good reputations were reviewed and analyzed. The advances made by Asian research groups in the field of natural products chemistry in 2019 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on their structural classification.

Nidulantes of Aspergillus (Formerly Emericella): A Treasure Trove of Chemical Diversity and Biological Activities

The genus Emericella (Ascomycota) includes more than thirty species with worldwide distribution across many ecosystems. It is considered a rich source of diverse metabolites. The published classes of natural compounds that are discussed here are organized according to the following biosynthetic pathways: polyketides (azaphilones, cyclopentenone pigments, dicyanides, furan derivatives, phenolic ethers, and xanthones and anthraquinones); shikimate derivatives (bicoumarins); mevalonate derivatives (meroterpenes, sesquiterpenes, sesterterpenes and steroids) and amino acids derivatives (alkaloids (indole-derivatives, isoindolones, and piperazine) and peptides (depsipeptides)). These metabolites produce the wide array of biological effects associated with Emericella, including antioxidant, antiproliferative, antimalarial, antiviral, antibacterial, antioxidant, antihypertensive, anti-inflammatory, antifungal and kinase inhibitors. Careful and extensive study of the diversity and distribution of metabolites produced by the genus Emericella (either marine or terrestrial) revealed that, no matter the source of the fungus, the composition of the culture medium effectively controls the metabolites produced. The topic of this review is the diversity of metabolites that have been identified from Emericella, along with the contextual information on either their biological or geographic sources. This review presents 236 natural compounds, which were reported from marine and terrestrial Emericella. Amongst the reported compounds, only 70.2% were biologically assayed for their effects, including antimicrobial or cytotoxicity. This implies the need for substantial investigation of alternative activities. This review includes a full discussion of compound structures and disease management, based on materials published from 1982 through December 2019.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as peyssonnoside A from a Peyssonnelia species.