Polyketides as Secondary Metabolites from the Genus Aspergillus

Asperinsuterpenes A-C from the Fungus Aspergillus insuetus BTBU20220155

Three new meroterpenoids, asperinsuterpenes A-C (1-3), and eight previously reported natural products, namely asnovolin I (4), (2'E,4'E,6'E)-6-(1'-carboxyocta-2',4',6'-triene)-9-hydroxydrim-7-ene-11,12-olide (5), (2'E,4'E,6'E)-6-(1'-carboxyocta-2',4',6'-triene)-11,12-epoxy-9,11-dihydroxydrim-7-ene (6), cinereain (7), carnequinazolines A and B (8 and 9), carnemycin B (10), and stromemycin (11) were isolated from the fungus Aspergillus insuetus, strain BTBU20220155. The structures of the compounds were determined based on spectroscopic techniques, including 1D and 2D NMR, HRESIMS, and ECD experiments. The in vitro antimicrobial evaluation revealed that compounds 5 and 11 exhibited inhibitory activity against Candida albicans, with minimum inhibitory concentration (MIC) values of 12.5 and 25 μg/mL, respectively. These findings suggest that A. insuetus is a promising source of bioactive natural products with potential applications in antifungal therapy.

Comparative metabolomics study on the secondary metabolites of the red alga, Corallina officinalis and its associated endosymbiotic fungi

Marine endosymbionts have gained remarkable interest in the last three decades in terms of natural products (NPs) isolated thereof, emphasizing the chemical correlations with those isolated from the host marine organism. The current study aimed to conduct comparative metabolic profiling of the marine red algae Corallina officinalis, and three fungal endosymbionts isolated from its inner tissues namely, Aspergillus nidulans, A. flavipes and A. flavus. The ethyl acetate (EtOAc) extracts of the host organism as well as the isolated endosymbionts were analyzed using ultra-high performance liquid chromatography coupled to high resolution tandem mass spectrometry (UHPLC-MS/MS)in both positive and negative ion modes, applying both full scan (FS) and all ion fragmentation (AIF) modes. Extensive interpretation of the LC-MS/MS spectra had led to the identification of 76 metabolites belonging to different phytochemical classes including alkaloids, polyketides, sesquiterpenes, butyrolactones, peptides, fatty acids, isocoumarins, quinones, among others. Metabolites were tentatively identified by comparing the accurate mass and fragmentation pattern with metabolites previously reported in the literature, as well as bioinformatics analysis using GNPS. A relationship between the host C. officinalis and its endophytes (A. flavus, A. nidulans, and A. flavipes) was discovered. C. officinalis shares common metabolites with at least one of the three endosymbiotic fungi. Some metabolites have been identified in endophytes and do not exist in their host. Multivariate analysis (MVA) revealed discrimination of A. flavipes from Corallina officinalis and other associated endophytic Aspergillus fungi (A. flavus and A. nidulans).

Purification of andibenin and a chromanone analogue from rhizospheric Aspergillus flavus and soil-borne Penicillium notatum exhibiting cytotoxic and antibacterial properties

The discovery of bioactive compounds from fungal natural sources holds immense potential for the development of novel therapeutics. The present study investigates the extracts of soil-borne Penicillium notatum and rhizosphere-inhabiting Aspergillus flavus for their antibacterial, antifungal, and cytotoxic potential. Additionally, two compounds were purified using chromatographic and spectroscopic techniques. The results demonstrated that the ethyl acetate fraction of A. flavus exhibited prominent cytotoxic activity against Artemia salina, whereas the ethyl acetate fraction of P. notatum displayed promising antibacterial potential. At dose concentrations of 10, 100, and 1000 µg mL-1, the ethyl acetate fraction of A. flavus showed mortality percentages of 7.6%, 66.4%, and 90%, respectively. The ethyl acetate fraction of P. notatum extract exhibited significant antibacterial activity, forming inhibition zones measuring 41, 38, 34, 34, and 30 mm against B. subtilis, S. flexneri, E. coli, K. pneumoniae, and S. aureus, respectively, at 1000 µg mL-1. At this concentration, inhibition zones of 28, 27, and 15 mm were recorded for P. vulgaris, S. typhi, and X. oryzae. Using bioassay-guided approach, one compound each was purified from the fungal extracts. The initial purification involved mass spectroscopic analysis, followed by structural elucidation using 500 MHz nuclear magnetic resonance (NMR) spectroscopy. Compound 1, derived from A. flavus, was identified as ethyl 2-hydroxy-5,6-dimethyl-4-oxocyclohex-2-ene-1-carboxylate, with a mass of 212, whereas compound 2, isolated from P. notatum, was identified as 3-amino-2-(cyclopenta-2,4-dien-1-ylamino)-8-methoxy-4H-chromen-4-one, with an exact mass of 270. Based on bioassay results, compound 1 was subjected to brine shrimp lethality assay and compound 2 was tested for its antibacterial potential. Compound 1 exhibited 30% lethality against brine shrimp larvae at a concentration of 100 µg mL-1, whereas at 1000 µg mL-1 the mortality increased to 70%. Compound 2 displayed notable antibacterial potential, forming inhibition zones of 30, 24, 19, and 12 mm against S. aureus, E. coli, B. subtilis, and S. flexneri, respectively. In comparison, the standard antibiotic tetracycline produced inhibition zones of 18, 18, 15, and 10 mm against the respective bacterial strains at the same concentration.

Immunosuppressive steroids quadrilisteroids A-C and derivatives from the terrestrial fungus Aspergillus quadrilineatus

Seven undescribed compounds (1-7) along with six known compounds (8-13) were isolated from Eurotiaceae Aspergillus quadrilineatus. Their structures and absolute configurations were elucidated by NMR, HRESIMS, and ECD calculations. Quadrilisteroids A (1) and B (2) possessed an unprecedented 6/5/6/6/6/5 hexacyclic ring system in conjugation with a highly fused benzene ring, while quadrilisteroid C (3) featured a surprising 6/6/6/5/5-fused carbocyclic skeleton. Quadrilisteroid C (3) exhibited potent inhibitory activity against LPS-induced proliferation of B lymphocyte cells with an IC50 value of 1.03 μM. Compound 4, demonstrated inhibitory activity against Con A-induced proliferation of T lymphocyte cells with IC50 values of 6.42 μM.

Bioinspired synthesis and biological evaluation of ent-protulactones A and B

The bioinspired and stereoselective synthesis of the furo[3,2-b] furan lactone (-)-protulactone A and the dioxabicyclo[3.3.1]nonane lactone (+)-protulactone B has been achieved based on the chiron approach. The synthesis features the utilization of a number of one-pot, sequential transformations, including a cascade reaction of reductive elimination and nucleophilic addition in a one-pot process and a one-pot sequence via cross-metathesis/acetonide deprotection/O-Michael addition/lactonization to streamline the synthesis route and avoid the tedious work of product purification. Synthetic protulactones and their analogues were evaluated for their in vitro antiproliferative activity against selected tumor cell lines (MCF-7 and Capan 2) and showed minor cytotoxicity.