R-Hexitronic acid, a new tetronic acid derivative isolated from a soil fungus FG9RK.

A new tetronic acid derivative (1) together with terrestric acid (2), a known metabolite of Penicillium species, was isolated from the soil fungus, FG9RK following fermentation on solid rice medium. The structure of 1 was elucidated by one- and two-dimensional NMR and MS measurements. The absolute configuration of the oxygenated carbon in the side chain of 1 was identified as S by converting the compound into its Mosher ester whereas the absolute configuration of the lactone ring was deduced based on biogenetic considerations and comparison with 2.

Dithiodiketopiperazine derivatives from endophytic fungi Trichoderma harzianum and Epicoccum nigrum.

A new epidithiodiketopiperazine (ETP), pretrichodermamide G (1), along with three known (epi)dithiodiketopiparazines (2-4) were isolated from cultures of Trichoderma harzianum and Epicoccum nigrum, endophytic fungi associated with medicinal plants Zingiber officinale and Salix sp., respectively. The structure of the new compound (1) was established on the basis of spectroscopic data, including 1D/2D NMR and HRESIMS. The isolated compounds were investigated for their antifungal, antibacterial and cytotoxic potential against a panel of microorganisms and cell lines. Pretrichodermamide A (2) displayed antimicrobial activity towards the plant pathogenic fungus Ustilago maydis and the human pathogenic bacterium Mycobacterium tuberculosis with MIC values of 1 mg/mL (2 mM) and 25 µg/mL (50 µM), respectively. Meanwhile, epicorazine A (3) exhibited strong to moderate cytotoxicity against L5178Y, Ramos, and Jurkat J16 cell lines with IC50 values ranging from 1.3 to 28 µM. Further mechanistic studies indicated that 3 induces apoptotic cell death.

Azacoccones F-H, new flavipin-derived alkaloids from an endophytic fungus Epicoccum nigrum MK214079.

Three new flavipin-derived alkaloids, azacoccones F-H (1-3), along with six known compounds (4-9) were isolated from the endophytic fungus Epicoccum nigrum MK214079 associated with leaves of Salix sp. The structures of the new compounds were established by analysis of their 1D/2D nuclear magnetic resonance (NMR) and high-resolution electrospray ionization mass spectroscopy (HRESIMS) data. The absolute configuration of azacoccones F-H (1-3) was determined by comparison of experimental electronic circular dichroism (ECD) data with reported ones and biogenetic considerations. Epicocconigrone A (4), epipyrone A (5), and epicoccolide B (6) exhibited moderate antibacterial activity against Staphylococcus aureus ATCC 29213 with minimal inhibitory concentration (MIC) values ranging from 25 to 50 µM. Furthermore, epipyrone A (5) and epicoccamide A (7) displayed mild antifungal activity against Ustilago maydis AB33 with MIC values of 1.6 and 1.8 mM, respectively. Epicorazine A (8) showed pronounced cytotoxicity against the L5178Y mouse lymphoma cell line with an IC50 value of 1.3 µM.

Didymellanosine, a new decahydrofluorene analogue, and ascolactone C from Didymella sp. IEA-3B.1, an endophyte of Terminalia catappa.

Didymellanosine (1), the first analogue of the decahydrofluorene-class of natural products bearing a 13-membered macrocyclic alkaloid conjugated with adenosine, and a new benzolactone derivative, ascolactone C (4) along with eight known compounds (2, 3, 5-10), were isolated from a solid rice fermentation of the endophytic fungus Didymella sp. IEA-3B.1 derived from the host plant Terminalia catappa. In addition, ascochitamine (11) was obtained when (NH4)2SO4 was added to rice medium and is reported here for the first time as a natural product. Didymellanosine (1) displayed strong activity against the murine lymphoma cell line L5178Y, Burkitt's lymphoma B cells (Ramos) and adult lymphoblastic leukemia T cells (Jurkat J16), with IC50 values of 2.0, 3.3 and 4.4 µM, respectively. When subjected to a NFκB inhibition assay, didymellanosine (1) moderately blocked NFκB activation in the triple-negative breast cancer cell line MDA-MB 231. In an antimicrobial assay, ascomylactam C (3) was the most active compound when tested against a panel of Gram-positive bacteria including drug-resistant strains with MICs of 3.1-6.3 µM, while 1 revealed weaker activity. Interestingly, both compounds were also found active against Gram-negative Acinetobacter baumannii with MICs of 3.1 µM, in the presence of a sublethal concentration (0.1 µM) of colistin.

Bioactive Secondary Metabolites from Endophytic Fungi.

BACKGROUND: Endophytes represent a complex community of microorganisms colonizing asymptomatically internal tissues of higher plants. Several reports have shown that endophytes enhance the fitness of their host plants by direct production of bioactive secondary metabolites, which are involved in protecting the host against herbivores and pathogenic microbes. In addition, it is increasingly apparent that endophytes are able to biosynthesize medicinally important "phytochemicals", originally believed to be produced only by their host plants. OBJECTIVE: The present review provides an overview of secondary metabolites from endophytic fungi with pronounced biological activities covering the literature between 2010 and 2017. Special focus is given on studies aiming at exploration of the mode of action of these metabolites towards the discovery of leads from endophytic fungi. Moreover, this review critically evaluates the potential of endophytic fungi as alternative sources of bioactive "plant metabolites". RESULTS: Over the past few years, several promising lead structures from endophytic fungi have been described in the literature. In this review, 65 metabolites are outlined with pronounced biological activities, primarily as antimicrobial and cytotoxic agents. Some of these metabolites have shown to be highly selective or to possess novel mechanisms of action, which hold great promises as potential drug candidates. CONCLUSION: Endophytes represent an inexhaustible reservoir of pharmacologically important compounds. Moreover, endophytic fungi could be exploited for the sustainable production of bioactive "plant metabolites" in the future. Towards this aim, further insights into the dynamic endophyte - host plant interactions and origin of endophytic fungal genes would be of utmost importance.

Polyketides and a Dihydroquinolone Alkaloid from a Marine-Derived Strain of the Fungus Metarhizium marquandii.

Three new natural products (1-3), including two butenolide derivatives (1 and 2) and one dihydroquinolone derivative (3), together with nine known natural products were isolated from a marine-derived strain of the fungus Metarhizium marquandii. The structures of the new compounds were unambiguously deduced by spectroscopic means including HRESIMS and 1D/2D NMR spectroscopy, ECD, VCD, OR measurements, and calculations. The absolute configuration of marqualide (1) was determined by a combination of modified Mosher's method with TDDFT-ECD calculations at different levels, which revealed the importance of intramolecular hydrogen bonding in determining the ECD features. The (3R,4R) absolute configuration of aflaquinolone I (3), determined by OR, ECD, and VCD calculations, was found to be opposite of the (3S,4S) absolute configuration of the related aflaquinolones A-G, suggesting that the fungus M. marquandii produces aflaquinolone I with a different configuration (chiral switching). The absolute configuration of the known natural product terrestric acid hydrate (4) was likewise determined for the first time in this study. TDDFT-ECD calculations allowed determination of the absolute configuration of its chirality center remote from the stereogenic unsaturated γ-lactone chromophore. ECD calculations aided by solvent models revealed the importance of intramolecular hydrogen bond networks in stabilizing conformers and determining relationships between ECD transitions and absolute configurations.

Indole Diterpenoids from an Endophytic Penicillium sp.

A chemical investigation of the endophyte Penicillium sp. (strain ZO-R1-1), isolated from roots of the medicinal plant Zingiber officinale, yielded nine new indole diterpenoids (1-9), together with 13 known congeners (10-22). The structures of the new compounds were elucidated by 1D and 2D NMR analysis in combination with HRESIMS data. The absolute configuration of the new natural products 1, 3, and 7 was determined using the TDDFT-ECD approach and confirmed for 1 by single-crystal X-ray determination through anomalous dispersion. The isolated compounds were tested for cytotoxicity against L5178Y, A2780, J82, and HEK-293 cell lines. Compound 1 was the most active metabolite toward L5178Y cells, with an IC50 value of 3.6 µM, and an IC50 against A2780 cells of 8.7 µM. Interestingly, 1 features a new type of indole diterpenoid scaffold with a rare 6/5/6/6/6/6/5 heterocyclic system bearing an aromatic ring C, which is suggested to be important for the cytotoxic activity of this natural product against L5278Y and A2780 cells.

Expanding the chemical diversity of an endophytic fungus Bulgaria inquinans, an ascomycete associated with mistletoe, through an OSMAC approach.

An endophytic fungus Bulgaria inquinans (isolate MSp3-1), isolated from mistletoe (Viscum album), was subjected to fermentation on solid Czapek medium. Chromatographic workup of the crude EtOAc extract yielded five new natural products (1-5). Subsequent application of the "One Strain, MAny Compounds" (OSMAC) strategy on this strain by the addition of a mixture of salts (MgSO4, NaNO3 and NaCl) to solid Czapek medium induced the accumulation of nine additional new secondary metabolites (6-13, 16), with most of them (8, 10-12) not detectable in cultures lacking the salt mixture. The structures of the new compounds were established on the basis of the 1D/2D NMR and HRESIMS data. The TDDFT-ECD method was applied to determine the absolute configurations of the new compounds 1, 4 and 6 as well as of the previously reported bulgarialactone B (14), for which the absolute configuration was unknown so far. The modified Mosher's method was performed to assign the absolute configurations of 12 and 13. TDDFT-ECD analysis also allowed determining the absolute configuration of (+)-epicocconone, which had an enantiomeric absolute configuration in the tricyclic moiety compared to that of bulgarialactone B (14). All the isolated metabolites were evaluated for their cytotoxic activity. Compound 2 was found to possess strong cytotoxic activity against the murine lymphoma cell line L5178Y with an IC50 value of 1.8 µM, while the remaining metabolites were shown to be inactive.

Chaetolines A and B, Pyrano[3,2- f]isoquinoline Alkaloids from Cultivation of Chaetomium sp. in the Presence of Autoclaved Pseudomonas aeruginosa.

The first members of a new alkaloid class, chaetolines A (1) and B (2), which feature a pyrano[3,2- f]isoquinoline core structure, were obtained from a crude extract of the fungal endophyte Chaetomium sp. after cultivation in the presence of autoclaved Pseudomonas aeruginosa. The structures of the new compounds, including the absolute configuration of the major stereoisomer, were determined through detailed analysis of HRESIMS, 1D/2D NMR, and calculation of ECD data. The possible biosynthetic origin of the unprecedented scaffold of 1 and 2 is proposed. The current study provides further evidence for mixed fermentation as a powerful tool to induce the accumulation of cryptic fungal natural products even in the absence of viable bacterial cells.

Lead Compounds from Mangrove-Associated Microorganisms.

The mangrove ecosystem is considered as an attractive biodiversity hotspot that is intensively studied in the hope of discovering new useful chemical scaffolds, including those with potential medicinal application. In the past two decades, mangrove-derived microorganisms, along with mangrove plants, proved to be rich sources of bioactive secondary metabolites as exemplified by the constant rise in the number of publications, which suggests the great potential of this important ecological niche. The present review summarizes selected examples of bioactive compounds either from mangrove endophytes or from soil-derived mangrove fungi and bacteria, covering the literature from 2014 to March 2018. Accordingly, 163 natural products are described in this review, possessing a wide range of potent bioactivities, such as cytotoxic, antibacterial, antifungal, α-glucosidase inhibitory, protein tyrosine phosphatase B inhibitory, and antiviral activities, among others.

Natural Products from Deep-Sea-Derived Fungi ̶ A New Source of Novel Bioactive Compounds?

BACKGROUND: Over the last two decades, deep-sea-derived fungi are considered to be a new source of pharmacologically active secondary metabolites for drug discovery mainly based on the underlying assumption that the uniqueness of the deep sea will give rise to equally unprecedented natural products. Indeed, up to now over 200 new metabolites have been identified from deep-sea fungi, which is in support of the statement made above. RESULTS: This review summarizes the new and/or bioactive compounds reported from deepsea- derived fungi in the last six years (2010 - October 2016) and critically evaluates whether the data published so far really support the notion that these fungi are a promising source of new bioactive chemical entities.

Structure Elucidation of Antibiotics by Nmr Spectroscopy.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for the structure elucidation of antibiotics in solution. Over the past 30 years there have been numerous publications describing the use of NMR to characterize naturally derived or synthetic antibiotics. A large number of one-dimensional (1D) and two-dimensional (2D) NMR methods are available today and the list continues to expand. In this chapter, we will consider the key NMR experiments that provide useful information for compound structure elucidation.

Antimycobacterial Metabolites from Marine Invertebrates.

Marine organisms play an important role in natural product-based drug research due to accumulation of structurally unique and bioactive metabolites. The exploration of marine-derived compounds may significantly extend the scientific knowledge of potential scaffolds for antibiotic drug discovery. Development of novel antitubercular agents is especially significant as the emergence of drug-resistant Mycobacterium tuberculosis strains remains threateningly high. Marine invertebrates (i.e., sponges, corals, gorgonians) as a source of new chemical entities are the center of research for several scientific groups, and the wide spectrum of biological activities of marine-derived compounds encourages scientists to carry out investigations in the field of antibiotic research, including tuberculosis treatment. The present review covers published data on antitubercular natural products from marine invertebrates grouped according to their biogenetic origin. Studies on the structure-activity relationships of these important leads are highlighted as well.

Flavonoids from Stellaria nemorum and Stellaria holostea.

Stellaria nemorum L. and S. holostea L. (Caryophyllaceae) were investigated for their flavonoids. The new flavonoid 6-C-[(α-arabinopyranosyl)-( 1-->2)-O-ß- xylopyranosyl]apigenin (1) and the four known C-glycosides, 6-C-[(α-arabinopyranosyl)-(1-->2)-O-ß-glucopyranosyl]apigenin (2), apigenin 6-C-ß- galactopyranoside-8-C-ß-glucopyranoside (3), apigenin 6-C-ß-glucopyranoside-8-C-α-arabinopyranoside (4), and apigenin 6-C-ß-glucopyranoside-8-C-ß- xylopyranoside (5) were isolated from the aerial parts of S. nemorum for the first time. Furthemore, five known flavonoids, 3,5,7-trihydroxy-3',5'- dimethoxyflavone (9), diosmetin 6-C-ß-glucopyranoside (8), schaftoside (4), isoorientin (6) and orientin (7) were obtained from the aerial parts of S. holostea. Compounds 4, 8 and 9 are reported for the first time from this species. The structures of all isolated compounds were unambiguously elucidated by one- and two- dimensional NMR and mass spectral analysis, by acid hydrolysis, as well as by comparison with literature data. The crude extracts of the investigated species exhibited antimicrobial activity against Staphylococcus aureus, while none of the isolated compounds was found to be active.