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Optically active heterodimeric 5,5'-linked bis-isochromans, containing a stereogenic ortho-trisubstituted biaryl axis and up to four chirality centers, were synthesized stereoselectively by using a Suzuki-Miyaura biaryl coupling reaction of optically active isochroman and 1-arylpropan-2-ol derivatives, providing the first access to synthetic biaryl-type isochroman dimers. Enantiomeric pairs and stereoisomers up to seven derivatives were prepared with four different substitution patterns, which enabled us to test how OR, ECD, and VCD measurements and DFT calculations can be used to determine parallel central and axial chirality elements in three isolated blocks of chirality. In contrast to natural penicisteckins A-D and related biaryls, the ECD spectra and OR data of (aS) and (aR) atropodiastereomers did not reflect the opposite axial chirality, but they were characteristic of the central chirality. The atropodiastereomers showed consistently near-mirror-image VCD curves, allowing the determination of axial chirality with the aid of DFT calculation or by comparison of characteristic VCD transitions.

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A chemical investigation of Laburnicola nematophila, isolated from cysts of the plant parasitic nematode Heterodera filipjevi, affored three dactylfungin derivatives (1-3) and three tetralone congeners (4-6). Dactylfungin C (1), laburnicolin (4), and laburnicolenone (5) are previously undescribed natural products. Chemical structures of the isolated compounds were determined based on 1D and 2D NMR spectroscopic analyses together with HR-ESI-MS spectrometry and comparison with data reported in the literature. The relative configurations of compounds 1, 2, and 4-6 were determined based on their ROESY data and analysis of their coupling constants (J values). The absolute configurations of 4-6 were determined through the comparison of their measured and calculated TDDFT-ECD spectra. Compounds 1-3 were active against azole-resistant Aspergillus fumigatus.

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A simplified molecular-dynamics-based electronic circular dichroism (ECD) approach was tested on three condensed derivatives with limited conformational flexibility and an isochroman-2H-chromene hybrid, the ECD spectra of which could not be precisely reproduced by the conventional ECD calculation protocol. Application of explicit solvent molecules at the molecular mechanics (MD) level in the dynamics simulations and subsequent TDDFT-ECD calculation for the unoptimized MD structures was able to improve the agreements between experimental and computed spectra. Since enhancements were achieved even for molecules with limited conformational flexibility, deformations caused by the solvent molecules and multitudes of conformers produced with unoptimized geometries seem to be key factors for better agreement. The MD approach could confirm that aggregation of the phenanthrene natural product luzulin A had a significant contribution to a specific wavelength range of the experimental ECD. The MD approach has proved that dimer formation occurred in solution and this was responsible for the anomalous ECD spectrum. The scope and limitations of the method have also been discussed.

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Chemical exploration for two isolates of the recently described ascomycete species Polyphilus sieberi, derived from the eggs of the plant parasitic nematode Heterodera filipjevi, afforded the identification of many compounds that belong to various metabolite families: two previously undescribed chlorinated cyclotetrapeptides, omnipolyphilins A (1) and B (2), one new pyranonaphthoquinone, ventiloquinone P (3), a 6,6'-binaphto-α-pyranone dimer, talaroderxine D (4) in addition to nine known metabolites (5-13) were isolated from this biocontrol candidate. All isolated compounds were characterized by comprehensive 1D, 2D NMR, and HR-ESI-MS analyses. The absolute configurations of the cyclotetrapeptides were determined by a combination of advanced Marfey's method, ROE correlation aided by conformational analysis, and TDDFT-ECD calculations, while ECD calculations, Mosher's method, and experimental ECD spectra were used for ventiloquinone P (3) and talaroderxine D (4). Among the isolated compounds, talaroderxine D (4) showed potent antimicrobial activities against Bacillus subtilis and Staphylococcus aureus with MIC values of 2.1 and 8.3 µg mL-1, respectively. Additionally, promising inhibitory effects on talaroderxine D (4) against the formation of S. aureus biofilms were observed up to a concentration of 0.25 µg mL-1. Moreover, ophiocordylongiiside A (10) showed activity against the free-living nematode Caenorhabditis elegans.

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The OSMAC (one strain many compounds) concept is a cultivation-based approach to increase the diversity of secondary metabolites in microorganisms. In this study, we applied the OSMAC-approach to the endophytic fungus Trichocladium sp. by supplementation of the cultivation medium with 2.5% phenylalanine. This experiment yielded five new compounds, trichocladiol (1), trichocladic acid (2), colletodiolic acid (3), colletolactone (4) and colletolic acid (5), together with five previously described ones (6-10). The structures were elucidated via comprehensive spectroscopic measurements, and the absolute configurations of compound 1 was elucidated by using TDDFT-ECD calculations. For formation of compounds 3-5, a pathway based on colletodiol biosynthesis is proposed. Compound 6 exhibited strong antibacterial activity against methicillin-resistant Staphylococcus aureus with a minimal inhibitory concentration (MIC) of 0.78 µM as well as a strong cytotoxic effect against the human monocytic cell line THP1 with an IC50 of 0.7 µM. Compound 8 showed moderate antibacterial activity against Mycobacterium tuberculosis with a MIC of 25 µM and a weak cytotoxic effect against THP1 cells with an IC50 of 42 µM.

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Abundisporin A (1), together with seven previously undescribed drimane sesquiterpenes named abundisporins B-H (2-8), were isolated from a polypore, Abundisporus violaceus MUCL 56355 (Polyporaceae), collected in Kenya. Chemical structures of the isolated compounds were elucidated based on exhaustive 1D and 2D NMR spectroscopic measurements and supported by HRESIMS data. The absolute configurations of the isolated compounds were determined by using Mosher's method for 1-4 and TDDFT-ECD calculations for 4 and 5-8. None of the isolated compounds exhibited significant activities in either antimicrobial or cytotoxicity assays. Notably, all of the tested compounds demonstrated neurotrophic effects, with 1 and 6 significantly increasing outgrowth of neurites when treated with 5 ng/mL NGF.

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In this study, we report the isolation of two new meroterpenoids, miniolutelide D (1) and miniolutelide E (13-epi-miniolutelide C) (2), along with two meroterpenoidal analogues (3 and 4) and two phenolic compounds (5 and 6) from the endophytic fungus Talaromyces purpureogenus derived from Punica granatum fruits. Their structures were elucidated using extensive MS, 1D, and 2D NMR spectroscopic analyses as well as by comparing with data in the literature. The absolute configurations of 1 and 2 were determined using TDDFT-ECD calculations. Antimicrobial activity was evaluated. Compound 5 displayed significant activity against methicillin-resistant Staphylococcus aureus strain ATCC 700699 and moderate activity against S. aureus strain ATCC 29213.

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Even with the aid of the available methods, the configurational assignment of natural products can be a challenging task that is prone to errors, and it sometimes needs to be corrected after total synthesis or single-crystal X-ray diffraction (XRD) analysis. Herein, the absolute configuration of amidochelocardin is revised using a combination of XRD, NMR spectroscopy, experimental ECD spectra, and time-dependent density-functional theory (TDDFT)-ECD calculations. As amidochelocardin was obtained via biosynthetic engineering of chelocardin, we propose the same absolute configuration for chelocardin based on the similar biosynthetic origins of the two compounds and result of TDDFT-ECD calculations. The evaluation of spectral data of two closely related analogues, 6-desmethyl-chelocardin and its semisynthetic derivative 1, also supports this conclusion.

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Clonostachys rosea is a fungus widely distributed on Earth and has a high capacity to adapt to complex environments in soil, plants, or sea. It is an endophyte that can be used as a potential biocontrol agent to protect plants from pathogenic fungi, nematodes, and insects. However, the spectrum of secondary metabolites produced by C. rosea has only scarcely been studied. In the present study, eight new phenalenones, asperphenalenones F-M (1-8), together with two known derivatives, asperphenalenones E and B (9 and 10), were isolated from the axenic rice culture of this fungus. The structures of the new compounds were elucidated by nuclear magnetic resonance, high-resolution electrospray ionization mass spectrometry, electronic circular dichroism, and gas chromatography-mass spectrometry analyses. Asperphenalenones J-M (5-8) are unusual phenalenone adducts that are conjugated to diterpenoid glycosides. Asperphenalenones F and H showed moderate antibacterial activity against methicillin-resistant Staphylococcus aureus, with minimal inhibitory concentrations of 12.5 and 25 µM, respectively. Asperphenalenone B exhibited low antiviral activity against the human immunodeficiency virus replication. Furthermore, asperphenalenones F and H exhibited low cytotoxicity against Jurkat cells, while all other compounds were devoid of cytotoxicity.

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During the course of our search for biologically active secondary metabolites from fungal cultures, a new oligocyclic diterpenoidal derivative, panapophenanthrin (1), was isolated from Panus strigellus. In addition, two known metabolites, panepophenanthrin (2) and dihydrohypnophilin (3), were also obtained. The chemical structures of the isolated compounds were elucidated based on extensive 1D and 2D NMR spectral analyses together with high-resolution electrospray ionization mass spectrometry (HR-ESI-MS). The absolute configuration was determined through TDDFT-ECD calculations. All of the compounds were assessed for their antimicrobial and cytotoxic activities. Compounds 1 and 3 showed moderate to weak activities in the performed antimicrobial assays, while compound 1 exhibited potent cytotoxic activity against the mammalian cell lines mouse fibroblast (L929) and human endocervical adenocarcinoma (KB3.1).

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Verrucosidins, a methylated α-pyrone class of polyketides rarely reported upon, have been implicated in one or more neurological diseases. Despite the significance of verrucosidins as neurotoxins, the absolute configurations of most of the derivatives have not been accurately characterized yet. In this study, three pairs of C-9 epimeric verrucosidin derivatives, including the known compounds penicyrones A and B (1a/1b) and 9-O-methylpenicyrones A and B (2a/2b), the new compounds 9-O-ethylpenicyrones A and B (3a/3b), together with the related known derivative verrucosidin (4), were isolated and identified from the culture extract of Penicillium cyclopium SD-413, which was obtained from the marine sediment collected from the East China sea. Their structures were established based on an in-depth analysis of nuclear magnetic resonances (NMR) and mass spectroscopic data. Determination of the absolute configurations of these compounds was accomplished by Mosher's method and time-dependent density functional theory (TDDFT) calculations of electronic circular dichroism (ECD) and optical rotation (OR). The configurational assignment of penicyrone A demonstrated that the previously reported C-6 absolute configuration of verrucosidin derivatives needs to be revised from (6S) to (6R). The 9R/9S epimers of compounds 1-3 were found to exhibit growth inhibition against some pathogenic bacteria, indicating that they have potential as lead compounds for the creation of antimicrobial agents. Supplementary Information: The online version contains supplementary material available at 10.1007/s42995-023-00173-2.

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A new flexible germacranolide (1, lobatolide H) was isolated from the aerial parts of Neurolaena lobata. The structure elucidation was performed by classical NMR experiments and DFT NMR calculations. Altogether, 80 theoretical level combinations with existing 13C NMR scaling factors were tested, and the best performing ones were applied on 1. 1H and 13C NMR scaling factors were also developed for two combinations utilizing known exomethylene containing derivatives, and the results were complemented by homonuclear coupling constant (JHH) and TDDFT-ECD calculations to elucidate the stereochemistry of 1. Lobatolide H possessed remarkable antiproliferative activity against human cervical tumor cell lines with different HPV status (SiHa and C33A), induced cell cycle disturbance and exhibited a substantial antimigratory effect in SiHa cells.

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Toxoplasma gondii is an apicomplexan pathogen able to infect a wide range of warm-blooded animals, including humans, leading to toxoplasmosis. Current treatments for toxoplasmosis are associated with severe side-effects and a lack efficacy to eradicate chronic infection. Thus, there is an urgent need for developing novel, highly efficient agents against toxoplasmosis with low toxicity. For decades, natural products have been a useful source of novel bioactive compounds for the treatment of infectious pathogens. In the present study, we isolated eight natural products from the crude extract of the endophytic fungus Paraboeremia selaginellae obtained from the leaves of the plant Philodendron monstera. The natural products were tested for inhibiting Toxoplasma gondii proliferation, and their cytotoxicity was evaluated in different human cell lines. Six natural products showed antitoxoplasma activity with low or no cytotoxicity in human cell lines. Together, these findings indicate that biphenyl ethers, bioxanthracenes, and 5S,6S-phomalactone from P. selaginellae are potential candidates for novel anti-toxoplasma drugs.

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Epoxide ring opening reactions are common and important in both biological processes and synthetic applications and can be catalyzed in a non-redox manner by epoxide hydrolases or reductively by oxidoreductases. Here we report that fluostatins (FSTs), a family of atypical angucyclines with a benzofluorene core, can undergo nonenzyme-catalyzed epoxide ring opening reactions in the presence of flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide (NADH). The 2,3-epoxide ring in FST C is shown to open reductively via a putative enol intermediate, or oxidatively via a peroxylated intermediate with molecular oxygen as the oxidant. These reactions lead to multiple products with different redox states that possess a single hydroxyl group at C-2, a 2,3-vicinal diol, a contracted five-membered A-ring, or an expanded seven-membered A-ring. Similar reactions also take place in both natural products and other organic compounds harboring an epoxide adjacent to a carbonyl group that is conjugated to an aromatic moiety. Our findings extend the repertoire of known flavin chemistry that may provide new and useful tools for organic synthesis.

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VCD analysis of 16 diastereomeric pairs of NHC precursors containing two isolated chirality centers and different substitution patterns identified VCD transitions characteristic of the chirality center in the imidazolium ring or in the side chain, which, in contrast to ECD and OR, could be utilized to assign the two chirality centers separately by simple comparison, regardless of the type and position of achiral aromatic substituents. While the ECD and OR data showed great dependence on the position of an achiral substituent such as a methoxy group, characteristic experimental VCD transitions remained consistent and they could be used to determine the absolute configuration of all the regio- and stereoisomers and substituted analogues. VCD, ECD and OR approaches were evaluated, and several carbene precursors were found, for which only the VCD method could distinguish the four stereoisomers. With t-butyl, phenyl or 2-naphthyl substituents at the C-1' chirality center, the ECD spectra of the C-1' epimers were near-identical, and hence it was only the VCD approach that showed distinct differences suitable for the configurational assignment. The chiroptical characterization of our diastereomeric pairs of NHC precursors enables the future application of related derivatives having different substitution patterns in stereoselective transformations.

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Chemical investigation of the fungal endophyte Pseudopestalotiopsis theae isolated from leaves of Caloncoba welwitschii, collected in Cameroon, resulted in two previously undescribed sulfur-containing xanthone derivatives sydoxanthones D and E, in addition to three previously undescribed monomeric diisoprenyl-cyclohexene-type meroterpenoids biscognienynes D-F and five known natural products. The structures of the undescribed compounds were unambiguously identified by their mass spectra and by extensive 1D and 2D NMR spectroscopic analysis. Mosher's reaction was performed to determine the absolute configuration of sydoxanthones D and E while TDDFT-ECD calculations were used to assign the configuration of biscognienyne D. Biscognienynes B and D showed significant cytotoxicity against the mouse lymphoma cell line L5178Y with IC50 values of 7.7 and 6.7 µM and against the human leukemic cell lines HL60, and Hal-01 with IC50 values ranging from 4.3 to 12.1 µM.

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Chemical investigation of an alcoholic extract from the stem of Daphne papyracea ("Xuehuagou") led to the isolation of the tetracyclic sesquiterpenoid daphnepapytone A (1), containing a unique caged skeleton with a cyclobutane ring having three tetrasubstituted chirality centers. Also isolated were new guaiane sesquiterpenoids, namely, daphnepapytones B-H (2-8), and one 1,5-diphenylpentanone 2-hydroxy-5-oxo-daphneone (9), together with 26 known compounds. The cyclic metabolites share a 5-isoprenyl-hexahydroazulene-2(1H)-one skeleton with different substitution patterns and a bridged cyclobutane, oxetane, or tetrahydrofuran ring. The planar structures and relative configuration of the new compounds were elucidated on the basis of spectroscopic analysis aided by DFT 13C NMR calculations. The absolute configurations of 1-7 were determined by X-ray single-crystal diffraction or TDDFT-ECD calculations. Daphnepapytones A and C (1 and 3), 2-hydroxy-5-oxodaphneone (9), daphnenone (10), daphneone (11), and 3-methyldaphneolone (12) showed α-glycosidase inhibitory activity, with IC50 values of 159.0, 102.3, 139.3, 43.3, 145.0, and 126.1 µM, respectively.

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Seven new germacranolides (1-3, 5-8), among them a heterodimer (7), and known germacranolide (4), eudesmane (9) and isodaucane (10) sesquiterpenes were isolated from the aerial parts of Neurolaena lobata. Their structures were determined by using a combination of different spectroscopic methods, including HR-ESIMS and 1D and 2D NMR techniques supported by DFT-NMR calculations. The enantiomeric purity of the new compounds was investigated by chiral HPLC analysis, while their absolute configurations were determined by TDDFT-ECD and OR calculations. Due to the conformationally flexible macrocycles and difficulties in assigning the relative configuration, 13C and 1H NMR chemical shift and ECD and OR calculations were performed on several stereoisomers of two derivatives. The isolated compounds (1-10) were shown to have noteworthy antiproliferative activities against three human cervical tumor cell line with different HPV status (HeLa, SiHa and C33A). Additionally, lobatolide C (6) exhibited substantial antiproliferative properties, antimigratory effect, and it induced cell cycle disturbance in SiHa cells.

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Penicisteckins A-D (1-4), two pairs of atropodiastereomeric biaryl-type hetero- and homodimeric bis-isochromans with 7,5'- and 7,7'-linkages and a pair of atropodiastereomeric 2-(isochroman-5-yl)-1,4-benzoquinone derivatives [penicisteckins E (5) and F (6)], were isolated from the Penicillium steckii HNNU-5B18. Their structures including the absolute configuration were determined by extensive spectroscopic and single-crystal X-ray diffraction analysis and TDDFT-ECD calculations. Both the bis-isochromans and the isochroman/1,4-benzoquinone conjugates represent novel biaryl scaffolds containing both central and axial chirality elements. The monomer anserinone B (8) exhibited potent antibacterial activities against Staphylococcus aureus ATCC 29213 and methicillin-resistant Staphylococcus aureus with minimal inhibition concentration values ranging from 2 to 8 µg mL-1. Plausible biosynthetic pathways of 1-6 are proposed, which suggest how the absolute configurations of the isolates were established during the biosynthetic scheme.

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Super-carbon-chain compounds (SCCCs) are marine organic molecules featuring long polyol carbon chains with numerous stereocenters. Polyol-polyene compounds (PPCs) and ladder-frame polyethers (LFPs) are two major families. It is highly challenging to establish the absolute configurations of SCCCs. In this century, few new SCCC families have been reported. Benthol A, an aberrant SCCC, was obtained from a South China Sea benthic dinoflagellate that should belong to a new taxon. Its planar structure and absolute configuration, containing thirty-five carbon stereocenters, were unambiguously established by a combination of extensive NMR spectroscopic investigations, periodate degradation of the 1,2-diol groups, ozonolysis of the carbon-carbon double bonds, J-based configurational analysis, NOE interactions, modified Mosher's MTPA ester method, and DFT-NMR 13C chemical-shift calculations aided by DP4+ statistical analysis. Benthol A displayed potent antimalarial activity against Plasmodium falciparum 3D7 parasites. This new molecule combines extraordinary structural features, particularly eight scattered ether rings on a C72 backbone chain, which places it within a new SCCC family between PPCs and LFPs, herein termed polyol-polyether compounds. This suggestion was strongly supported by principal component analysis. The discovery of benthol A does not only provide new insights into the untapped biosynthetic potential of marine dinoflagellates, but also opens up a new window for skeletal diversity of SCCCs.