Attila Mándi

Structure, Absolute Configuration, and Conformational Study of 12-Membered Macrolides from the Fungus Dendrodochium sp. Associated with the Sea Cucumber Holothuria nobilis Selenka

Dendrodolides A-M (1-13), 13 new 12-membered macrolides, were isolated from Dendrodochium sp., a fungus associated with the sea cucumber Holothuria nobilis Selenka, which was collected from the South China Sea. The structures of the dendrodolides were elucidated by means of detailed spectroscopic analysis and X-ray single-crystal diffraction. The absolute configurations were assigned using the modified Mosher method, exciton-coupled circular dichroism (ECCD), electronic solution and solid-state circular dichroism (ECD) supported by time-dependent density functional theory (TDDFT) ECD calculations, and X-ray analysis. A detailed conformational analysis of the 13 derivatives indicated that the conformation of the flexible macrolide ring plays a decisive role in their chiroptical properties. Thus, it is highly recommended to apply advanced levels of theory and to avoid simple comparison of ECD spectra to determine the absolute configurations of these derivatives. In an in vitro bioassay, compounds 1-5, 7-9, 11, and 12 exhibited different levels of growth inhibitory activity against SMMC-7721 and HCT116 cells. This is the first report of 12-membered macrolides from the fungus of the genus Dendrodochium . The coisolation of four pairs of epimers is extremely interesting and indicates the complexity of β-ketoreductase stereospecificity in the biosynthesis of enigmatic iterative fungal polyketides.

An enzymatic [4+2] cyclization cascade creates the pentacyclic core of pyrroindomycins

The [4+2] cycloaddition remains one of the most intriguing transformations in synthetic and natural products chemistry. In nature, however, there are remarkably few enzymes known to have this activity. We herein report an unprecedented enzymatic [4+2] cyclization cascade that has a central role in the biosynthesis of pyrroindomycins, which are pentacyclic spirotetramate natural products. Beginning with a linear intermediate that contains two pairs of 1,3-diene and alkene groups, the dedicated cyclases PyrE3 and PyrI4 act in tandem to catalyze the formation of two cyclohexene rings in the dialkyldecalin system and the tetramate spiro-conjugate of the molecules. The two cyclizations are completely enzyme dependent and proceed in a regio- and stereoselective manner to establish the enantiomerically pure pentacyclic core. Analysis of a related spirotetronate pathway confirms that homologs are functionally exchangeable, establishing the generality of these findings and explaining how nature creates diverse active molecules with similar rigid scaffolds.

Bioactive nonanolide derivatives isolated from the endophytic fungus Cytospora sp.

Cytospolides F-Q (6-17) and decytospolides A and B (18 and 19), 14 unusual nonanolide derivatives, were isolated from Cytospora sp., an endophytic fungus from Ilex canariensis. The structures were elucidated by means of detailed spectroscopic analysis, chemical interconversion, and X-ray single crystal diffraction. The solution- and solid-state conformers were compared by the combination of experimental methods (X-ray, NMR) supported by DFT calculations of the conformers. Absolute configurations were assigned using the modified Moshers method and solution- and solid-state TDDFT ECD calculations. In an in vitro cytotoxicity assay toward the tumor cell lines of A549, HCT116, QGY, A375, and U973, the γ-lactone 17 demonstrated a potent growth inhibitory activity toward the cell line A-549, while nonanolide 16 with (2S) configuration showed the strongest activity against cell lines A-549, QGY, and U973. A cell cycle analysis indicated that compound 16 can significantly mediate G1 arrest in A549 tumor cells, confirming the important role of the C-2 methyl in the growth inhibition toward the tumor line. The discovery of an array of new nonanolides demonstrates the productivity of the fungus, and it is an example of chemical diversity, extending the nonanolide family by derivatives formed by ring cleavage, oxidation, esterification, and Michael addition.

Varioxepine A, a 3H-oxepine-containing alkaloid with a new oxa-cage from the marine algal-derived endophytic fungus Paecilomyces variotii.

A new 3H-oxepine-containing alkaloid, varioxepine A (1), characterized by a structurally unprecedented condensed 3,6,8-trioxabicyclo[3.2.1]octane motif, was isolated from the marine algal-derived endophytic fungus Paecilomyces variotii. Due to the low proton/carbon ratio, the unambiguous assignment of the planar structure and relative configuration was precluded by NMR experiments and solved by single crystal X-ray analysis. The absolute configuration was established by DFT conformational analysis and TDDFT-ECD calculations. Compound 1 inhibited plant pathogenic fungus Fusarium graminearum.

Penibruguieramine A, a Novel Pyrrolizidine Alkaloid from the Endophytic Fungus Penicillium sp. GD6 Associated with Chinese Mangrove Bruguiera gymnorrhiza

A novel pyrrolizidine alkaloid, penibruguieramine A (1), characterized by an unprecedented 1-alkenyl-2-methyl-8-hydroxymethylpyrrolizidin-3-one skeleton, was isolated from the endophytic fungus Penicillium sp. GD6, associated with the Chinese mangrove Bruguiera gymnorrhiza. The absolute configuration of penibruguieramine A (1) was established by TDDFT ECD calculations of the vacuum and solution conformers, exploiting the transitions of the lactam chromophore. A plausible pathway for its biosynthesis has been proposed.

Atropisomeric Dihydroanthracenones as Inhibitors of Multiresistant Staphylococcus aureus

Two bisdihydroanthracenone atropodiastereomeric pairs, including homodimeric flavomannin A (1) and the previously unreported flavomannin B (2), two new unsymmetrical dimers (3 and 4), and two new mixed dihydroanthracenone/anthraquinone dimers (5 and 6) were isolated from Talaromyces wortmannii , an endophyte of Aloe vera . The structures of 2-6 were elucidated by extensive NMR and mass spectrometric analyses. The axial chirality of the biaryls was determined using TDDFT ECD and VCD calculations, the combination of which however did not allow the assignment of the central chirality elements of 1. The compounds exhibited antibacterial activity against Staphylococcus aureus , including (multi)drug-resistant clinical isolates. Reporter gene analyses indicated induction of the SOS response for some of the derivatives, suggesting interference with DNA structure or metabolism. Fluorescence microscopy demonstrated defective segregation of the bacterial chromosome and DNA degradation. Notably, the compounds showed no cytotoxic activity, encouraging their further evaluation as potential starting points for antibacterial drug development.

Pro-apoptotic and immunostimulatory tetrahydroxanthone dimers from the endophytic fungus Phomopsis longicolla.

Four tetrahydroxanthone dimers (1-4) and four biogenetically related monomers (5-8), including the new derivatives 4-6, were isolated from the endophyte Phomopsis longicolla. The absolute configurations of 2-4 were established for the first time by TDDFT electronic circular dichroism calculations, and that of phomoxanthone A (1) was revised by X-ray crystallography. Phomoxanthone A (1) showed the strongest pro-apoptotic activity when tested against a panel of human cancer cell lines, including cisplatin-resistant cells, whereas it was up to 100-fold less active against healthy blood cells. It was also the most potent activator of murine T lymphocytes, NK cells, and macrophages, suggesting an activation of the immune system in parallel to its pro-apoptotic activity. This dual effect in combating cancer cells could help in fighting resistance during chemotherapy. Preliminary structure-activity studies of isolated compounds and derivatives obtained by semisynthesis (9a-11) hinted at the location of the biaryl axis and the presence of acetyl groups as important structural elements for the biological activity of the studied tetrahydroxanthones.

Bicunningines A and B, Two New Dimeric Diterpenes from Cunninghamia lanceolata

Two unprecedented dimeric diterpenoids, with a 2,3-dihydrofuran ring fusing an abietane and a 4,5-seco-abietane diterpene, were isolated from Cunninghamia lanceolata. Their structures were elucidated by spectroscopic measurements, and their absolute configurations were determined by quantum chemical TDDFT ECD calculations, chemical transformations, and Moshers method. The Mosher method carried out with MPA and MTPA esters of the sterically hindered sec-hydroxyl group gave contradictory results, while MPA afforded the correct absolute configuration.

Hybrid isoprenoids from a reeds rhizosphere soil derived actinomycete Streptomyces sp. CHQ-64.

Indotertine A (1), a hybrid isoprenoid with a condensed pentacyclic skeleton, together with two related hybrid isoprenoids, drimentines F (2) and G (3), were isolated from a reeds rhizosphere soil derived actinomycete Streptomyces sp. CHQ-64. Their structures including absolute configurations were elucidated by spectroscopic methods, X-ray single crystal diffraction analysis, and TDDFT ECD calculations. Drimentines G (3) showed strong cytotoxicity against human cancer cells lines with IC(50)s down to 1.01 μM, while 1 and 2 showed no significant activity.

Cytotoxic 14-Membered Macrolides from a Mangrove-Derived Endophytic Fungus, Pestalotiopsis microspora

Seven new 14-membered macrolides, pestalotioprolides C (2), D-H (4-8), and 7-O-methylnigrosporolide (3), together with four known analogues, pestalotioprolide B (1), seiricuprolide (9), nigrosporolide (10), and 4,7-dihydroxy-13-tetradeca-2,5,8-trienolide (11), were isolated from the mangrove-derived endophytic fungus Pestalotiopsis microspora. Their structures were elucidated by analysis of NMR and MS data and by comparison with literature data. Single-crystal X-ray diffraction analysis was used to confirm the absolute configurations of 1, 2, and 10, while Moshers method and the TDDFT-ECD approach were applied to determine the absolute configurations of 5 and 6. Compounds 3-6 showed significant cytotoxicity against the murine lymphoma cell line L5178Y with IC50 values of 0.7, 5.6, 3.4, and 3.9 μM, respectively, while compound 5 showed potent activity against the human ovarian cancer cell line A2780 with an IC50 value of 1.2 μM. Structure-activity relationships are discussed. Coculture of P. microspora with Streptomyces lividans caused a roughly 10-fold enhanced accumulation of compounds 5 and 6 compared to axenic fungal control.