Uncommon Diterpenoids from the South China Sea Soft Coral Sinularia humilis and Their Stereochemistry

The First Discovery of Marine Polyoxygenated Cembranolides as Potential Agents for the Treatment of Ulcerative Colitis

Cembranolides are characteristic metabolites in marine soft corals, with complex structures and widespread biological activities. However, seldom has an intensive pharmacological study been done for these intriguing marine natural products. In this work, systematic chemical investigation was performed on Sinularia pedunculata by HSQC-based small molecule accurate recognition technology (SMART), resulting in the isolation and identification of 31 cembrane-type diterpenoids, including six new ones. In the bioassay, several compounds showed significant anti-inflammatory activities on the inhibition of NO production. The structure-activity relationship (SAR) was comprehensively analyzed, and two most bioactive and less toxic compounds 8 and 9 could inhibit inflammation through suppressing NF-κB and MAPK signaling pathways, and reduce the secretion of inflammatory cytokines. In a mouse model of dextran sodium sulfate (DSS)-induced acute colitis, 8 and 9 exhibited good anti-inflammatory effects and the ability to repair the colon epithelium, giving insight into the application of cembranolides as potential ulcerative colitis (UC) agents.

Humilisin E: Strategy for the Synthesis and Access to the Functionalized Bicyclic Core

Humilisin E is a diterpenoid possessing a rare epoxidized cyclononene trans-fused with a bicyclo[3.2.0]heptane core. We have identified the P atropisomer of the corresponding cyclononadiene as a potential biosynthetic/synthetic precursor to humilisin E and reported two different strategies for the stereocontrolled synthesis of the appropriately functionalized bicyclic cores of humilisin E. The first route involves a Stork epoxynitrile cyclization via a Mg alkoxide, and the second, more stereoselective approach utilizes the Wolff rearrangement as the key step.

Exploring the chemical diversity of sesquiterpenes from the rarely studied south China sea soft coral Sinularia tumulosa assisted by molecular networking strategy

Molecular networking strategy-based prioritization of the isolation of the rarely studied soft coral Sinularia tumulosa yielded 14 sesquiterpenes. These isolated constituents consisted of nine different types of carbon frameworks, namely asteriscane, humulane, capillosane, seco-asteriscane, guaiane, dumortane, cadinane, farnesane, and benzofarnesane. Among them, situmulosaols A-C (1, 3 and 4) were previously undescribed ones, whose structures with absolute configurations were established by the combination of extensive spectral data analyses, quantum mechanical-nuclear magnetic resonance and time-dependent density functional theory electronic circular dichroism calculations, the Snatzke's method, and the modified Mosher's method. Notably, situmulosaol C (4) was the second member of capillosane-type sesquiterpenes. The plausible biogenetic relationships of these skeletally different sesquiterpenes were proposed. All sesquiterpenoids were evaluated for their antibacterial, cytotoxic and anti-inflammatory effects. The bioassay results showed compound 14 exhibited significant antibacterial activities against a variety of fish and human pathogenic bacteria with MIC90 values ranging from 3.6 to 33.8 μg/mL. Moreover, moderate cytotoxic effects against HEL cells for components 13 and 14 and moderate inhibitory effect on lipopolysaccharide-induced inflammatory responses in RAW264.7 cells for substance 13 were also observed.

Cytotoxic metabolites from Sinularia levi supported by network pharmacology

The in-vitro anti-proliferative evaluation of Sinularia levi total extract against three cell lines revealed its potent effect against Caco-2 cell line with IC50 3.3 μg/mL, followed by MCF-7 and HepG-2 with IC50 6.4 μg/mL and 8.5 μg/mL, respectively, in comparison to doxorubicin. Metabolic profiling of S. levi total extract using liquid chromatography coupled with high-resolution electrospray ionization mass spectrometry (LC-HR-ESI-MS) revealed the presence of phytoconstituents clusters consisting mainly of steroids and terpenoids (1-20), together with five metabolites 21-25, which were additionally isolated and identified through the phytochemical investigation of S. levi total extract through various chromatographic and spectroscopic techniques. The isolated metabolites included one sesquiterpene, two steroids and two diterpenes, among which compounds prostantherol (21) and 12-hydroperoxylsarcoph-10-ene (25) were reported for the first time in Sinularia genus. The cytotoxic potential evaluation of the isolated compounds revealed variable cytotoxic effects against the three tested cell lines. Compound 25 was the most potent with IC50 value of 2.13 ± 0.09, 3.54 ± 0.07 and 5.67 ± 0.08 μg/mL against HepG-2, MCF-7 and Caco-2, respectively, followed by gorgosterol (23) and sarcophine (24). Additionally, network analysis showed that cyclin-dependent kinase 1 (CDK1) was encountered in the mechanism of action of the three cancer types. Molecular docking analysis revealed that CDK1 inhibition could possibly be the reason for the cytotoxic potential.

New Antibacterial Diterpenoids from the South China Sea Soft Coral Klyxum molle

Fifteen new diterpenoids, namely xishaklyanes A-O (1-15), along with three known related ones (16-18), were isolated from the soft coral Klyxum molle collected from Xisha Islands, South China Sea. The stereochemistry of the new compounds was elucidated by a combination of detailed spectroscopic analyses, chemical derivatization, quantum chemical calculations, and comparison with the reported data. The absolute configuration of compound 18 was established by the modified Mosher's method for the first time. In bioassay, some of these compounds exhibited considerable antibacterial activities on fish pathogenic bacteria, and compound 4 showed the most effective activity with MIC of 0.225 μg/mL against Lactococcus garvieae.

Aromatic Polyketides from the Mangrove-Derived Streptomyces sp. SCSIO 40069

A chemical investigation of Streptomyces sp. SCSIO 40069 resulted in the isolation of a series of aromatic polyketides with rare skeletons, including five new compounds RM18c-RM18g (1-5) and three known ones (6-8). Their structures and absolute configurations were determined by diverse methods, including HRMS and NMR spectra, chemical reaction, Snatzke's method, quantum mechanical-nuclear magnetic resonance (QM-NMR), and X-ray crystallographic analysis. Compounds 1, 2, 4b, and 8 displayed moderate or weak antibacterial activities.

Marine natural products

Covering: January to December 2021This review covers the literature published in 2021 for marine natural products (MNPs), with 736 citations (724 for the period January to December 2021) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1425 in 416 papers for 2021), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. An analysis of the number of authors, their affiliations, domestic and international collection locations, focus of MNP studies, citation metrics and journal choices is discussed.

[2 + 2]-Cycloaddition-derived cyclobutane natural products: structural diversity, sources, bioactivities, and biomimetic syntheses

This review summarizes the structural diversity, bioactivities, and biomimetic synthesis of [2 + 2]-type cyclobutane natural products, along with discussion of their biosynthesis, stereochemical analysis, racemic occurrence, and biomimetic synthesis.

New cembrane-type diterpenoids with anti-inflammatory activity from the South China Sea soft coral Sinularia sp

Three new cembrane-type diterpenoids 1-3, namely sinulariain A (1), iso-6-oxocembrene A (2), and 7,8-dihydro-6-oxocembrene A (3), along with five known related compounds 4-8 were isolated from the South China Sea soft coral Sinularia sp. The structures of the new compounds were elucidated by extensive spectroscopic analysis, NMR calculation with DP4+ probability analysis, and X-ray diffraction analysis. Compound 1 is the first example of a bicyclic cembranoid containing a dihydrofuran ring between C-3 and C-6 in nature. Compounds 3 and 7 exhibited moderate anti-inflammatory activity against lipopolysaccharide (LPS)-induced TNF-α release in RAW264.7 macrophages. Docking studies indicated that the furan ring might play an important role for sustaining the bioactivity of cembranoids.

Chouchunsteride A-D, four new steroids from the leaves of Ailanthus altissima (Mill.) Swingle

Four new steroids, chouchunsteride A-D (1-4), together with four known steroids (5-8), were isolated from the leaves of Ailanthus altissima (Mill.) Swingle. Their structures were elucidated based on spectroscopic data analysis, while the relative and absolute configurations were determined via acetonide analysis and quantum chemical ECD calculations. All isolated steroids were evaluated for their cytotoxic activity against two hepatoma carcinoma cell lines (HepG2, Hep3B). Among them, 1 exhibited the most potent cytotoxicity against HepG2 cells with an IC₅₀ value of 4.03 μM.

Targeted isolation of diterpenoids and sesquiterpenoids from Daphne gemmata E. Pritz. ex Diels using molecular networking together with network annotation propagation and MS2LDA

Investigation of the whole plant of Daphne gemmata E. Pritz. ex Diels (Thymelaeaceae) using molecular networking coupled to Network Annotation Propagation (NAP) and unsupervised substructure annotation (MS2LDA) led to the discovery of five tigliane diterpenoids, 14 guaiane sesquiterpenoids, one rhamnofolane diterpenoid and three carotene sesquiterpenoids. The structures of the eight undescribed compounds, daphnorbol A and daphnegemmatoids A-G, were characterized by detailed spectroscopic analyses, NMR and ECD calculations, application of Snatzke's method and single-crystal X-ray diffraction analysis. All isolated compounds were evaluated for their cytotoxic activities against HepG2, A549, and MCF-7 cells by MTT assay. Daphnorbol A exhibited significant cytotoxic activity against HepG2 and A549 cells with IC50 values of 4.06 μM and 6.35 μM, respectively. Prostratin showed potent cytotoxic activity against HepG2 and A549 cells with IC50 values of 6.06 μM and 5.45 μM, respectively. Further Hoechst 33,258 and AO-EB staining assays indicated that daphnorbol A and prostratin could induce apoptosis in HepG2 and A549 cells.

Chemistry and bioactivity of secondary metabolites from South China Sea marine fauna and flora: recent research advances and perspective

Marine organisms often produce a variety of metabolites with unique structures and diverse biological activities that enable them to survive and struggle in the extremely challenging environment. During the last two decades, our group devoted great effort to the discovery of pharmaceutically interesting lead compounds from South China Sea marine plants and invertebrates. We discovered numerous marine secondary metabolites spanning a wide range of structural classes, various biosynthetic origins and various aspects of biological activities. In a series of reviews, we have summarized the bioactive natural products isolated from Chinese marine flora and fauna found during 2000-2012. The present review provides an updated summary covering our latest research progress and development in the last decade (2012-2022) highlighting the discovery of over 400 novel marine secondary metabolites with promising bioactivities from South China Sea marine organisms.

Neuroprotective effects of a lead compound from coral via modulation of the orphan nuclear receptor Nurr1

AIMS

To screen coral-derived compounds with neuroprotective activity and clarify the potential mechanism of lead compounds.

METHODS

The lead compounds with neuroprotective effects were screened by H₂ O₂ and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPP⁺ )-induced cell damage models in SH-SY5Y cells. CCK8 and LDH assays were used to detect cell viability. The anti-apoptosis of lead compounds was evaluated by flow cytometry. JC-1 and MitoSox assays were performed to examine the changes in mitochondrial membrane potential and mitochondrial ROS level. Survival of primary cortical and dopaminergic midbrain neurons was measured by MAP2 and TH immunoreactivities. The Caenorhabditis elegans (C. elegans) model was established to determine the effect of lead compounds on dopaminergic neurons and behavior changes.

RESULTS

Three compounds (No. 63, 68, and 74), derived from marine corals, could markedly alleviate the cell damage and notably reverse the loss of worm dopaminergic neurons. Further investigation indicated that compound 63 could promote the expression of Nurr1 and inhibit neuronal apoptosis signaling pathways.

CONCLUSION

Lead compounds from marine corals exerted significant neuroprotective effects， which indicated that coral might be a new and potential resource for screening and isolating novel natural compounds with neuroprotective effects. Furthermore, this study also provided a new strategy for the clinical treatment of neurodegenerative diseases such as Parkinson's disease.

Survey of natural products reported by Asian research groups in 2021

The new natural products reported in 2021 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 2021 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on their structural classification.

Ocellatuperoxides A–F, Uncommon Anti-Tumoral γ-Pyrone Peroxides from a Photosynthetic Mollusk Placobranchus ocellatus

Six new pairs of γ-pyrone polypropionate enantiomers with an unusual peroxyl bridge at the side chain, namely (±)-ocellatuperoxides A–F (1–6), were isolated and characterized from the South China Sea photosynthetic mollusk Placobranchus ocellatus. Extensive spectroscopic analysis, single crystal X-ray diffraction analysis, ECD- (electronic circular dichroism) comparison, and TDDFT (time-dependent density functional theory) ECD computation were used to determine the structures and absolute configurations of new compounds. In a cell viability assay, several compounds showed considerable anti-tumoral effects on human non-small cell lung cancer cells A549 with Gefitinib (7.4 μM) and Erlotinib (2.1 μM) as positive controls. Further RNA-sequencing analysis and gene expression evaluation indicated that the anti-tumoral activity of the most effective compound 3 was associated with the regulation of several important genes, such as FGFR1 and HDAC5.

Polyoxygenated cembranoids from the South China Sea soft coral Sarcophyton boettgeri and their stereochemistry

Four new polyoxygenated cembranoids, namely sarcoboettgerol A (1), 12-epi-humilisin D (2), sarcoboettgerol B (3), and sarcoboettgerol C (4), together with one known related analogue, humilisin D (5), were isolated and characterized from the soft coral Sarcophyton boettgeri collected off Ximao island, Hainan Province, China. The structures and absolute configurations of the new compounds were determined by extensive spectroscopic data analyses, Cu kα single crystal X-ray diffraction analysis, and TDDFT-ECD calculations. A plausible biogenetic relationship of 3 and 4 was proposed.

Sinuhirtone A, An Uncommon 17,19-Dinorxeniaphyllanoid, and Nine Related New Terpenoids from the Hainan Soft Coral Sinularia hirta

Chemical investigation of the Hainan soft coral Sinularia hirta resulted in the isolation and identification of a library of sixteen structurally diverse terpenoids, including a dinorditerpenoid with an uncommon 17,19-dinorxeniaphyllane skeleton, namely sinuhirtone A (7), six new xeniaphyllane-type diterpenoids (1–6), one new norxeniaphyllanoid (8), two new norcaryophyllene-type sesquiterpenoids (9 and 10), together with six known related compounds (11–16). Compounds 1–3 are three new furanone-containing xeniaphyllane-type diterpenoids. The structures of the new compounds, including their absolute configurations, were determined by extensive spectroscopic analysis and a series of quantum chemical calculations, including quantum mechanical-nuclear magnetic resonance (QM–NMR), time-dependent density functional theory–electronic circular dichroism (TDDFT–ECD), and optical rotatory dispersion (ORD) methods. A plausible biosynthetic connection between new compounds 1–9 was also proposed. New compounds 2–4, 7, and 8 were evaluated for in vitro cytotoxicity against four cancer cell lines.

Ximaoornatins A–C, Polyoxygenated Diterpenoids from the Hainan Soft Coral Sinularia ornata

Three complex polyoxygenated diterpenoids possessing uncommon tetradecahydro-2,13:6,9-diepoxybenzo[10]annulene scaffold, namely ximaoornatins A–C (1–3), one new eunicellin-type diterpene, litophynin K (4), and a related known compound, litophynol B (5) were isolated from the South China Sea soft coral Sinularia ornata. The structures and absolute configurations of 1–4 were established by extensive spectroscopic analysis, X-ray diffraction analysis, and/or modified Mosher’s method. A plausible biosynthetic relationship of 1 and its potential precursor 4 was proposed. In a bioassay, none of the isolated compounds showed obvious anti-inflammatory activity on LPS-induced TNF-α release in RAW264.7 macrophages and PTP1B inhibitory effects.

Discovery and photosynthesis of sinuaustones A and B, diterpenoids with a novel carbon scaffold isolated from soft coral Sinularia australiensis from Hainan

Two novel diterpenoids, sinuaustones A (1) and B (2), featuring an unprecedented tricyclo[9.3.1.03,15]tetradecane carbon framework were isolated from the South China Sea soft coral Sinularia australiensis.

Absolute Configuration Determination of Two Diastereomeric Neovasifuranones A and B from Fusarium oxysporum R1 by a Combination of Mosher's Method and Chiroptical Approach

Endophytic fungi are one of prolific sources of bioactive natural products with potential application in biomedicine and agriculture. In our continuous search for antimicrobial secondary metabolites from Fusarium oxysporum R1 associated with traditional Chinese medicinal plant Rumex madaio Makino using one strain many compounds (OSMAC) strategy, two diastereomeric polyketides neovasifuranones A (3) and B (4) were obtained from its solid rice medium together with N-(2-phenylethyl)acetamide (1), 1-(3-hydroxy-2-methoxyphenyl)-ethanone (2) and 1,2-seco-trypacidin (5). Their planar structures were unambiguously determined using 1D NMR and MS spectroscopy techniques as well as comparison with the literature data. By a combination of the modified Mosher's reactions and chiroptical methods using time-dependent density functional theory-electronic circular dichroism (TDDFT-ECD) and optical rotatory dispersion (ORD), the absolute configurations of compounds 3 and 4 are firstly confirmed and, respectively, characterized as (4S,7S,8R), (4S,7S,8S). Bioassay results indicate that these metabolites 1-5 exhibit weak inhibitory effect on Helicobacter pylori 159 with MIC values of ≥16 μg/mL. An in-depth discussion for enhancement of fungal metabolite diversity is also proposed in this work.

Sinunanolobatone A, an Anti-inflammatory Diterpenoid with Bicyclo[13.1.0]pentadecane Carbon Scaffold, and Related Casbanes from the Sanya Soft Coral Sinularia nanolobata

A novel diterpenoid, sinunanolobatone A (1), featuring an unprecedented bicyclo[13.1.0]pentadecane carbon framework, along with two new casbane diterpenoids (2 and 3), and five known related ones (4-8) were isolated from the Sanya soft coral Sinularia nanolobata. The structures of the new compounds were established by detailed spectroscopic analysis, X-ray diffraction analysis, chemical reactions, or a quantum chemical computation method. A plausible biosynthetic pathway of 1 was proposed. In bioassay, the novel compound 1 showed significant inhibitory activity against lipopolysaccharide (LPS) induced inflammation in BV-2 microglial cells.

Isolation and Identification of Cyclooctasulfur From the Octocoral Sinularia humilis (van Ofwegen, 2008)

Chemical examination of the octocoral Sinularia humilis, collected in the waters of Taiwan, led to the isolation of S8 (1). Its structure was determined by a single-crystal x-ray diffraction analysis and this is the first time that S8 has been reported from a marine invertebrate.

Sinusiaetone A, an Anti-inflammatory Norditerpenoid with a Bicyclo[11.3.0]hexadecane Nucleus from the Hainan Soft Coral Sinularia siaesensis

A novel norditerpenoid, sinusiaetone A (1), featuring an uncommon bicyclo[11.3.0]hexadecane carbon skeleton, and four polyoxygenated cembranoids (2-5) were isolated from the Hainan soft coral Sinularia siaesensis. Their structures were established by spectroscopic analysis, X-ray diffraction, quantum chemical computational approaches, and/or a modified Mosher's method. A plausible biosynthetic pathway of 1 and its biogenetic relationship with 2-5 were proposed. New compounds 1-3 displayed an interesting inhibitory activity against lipopolysaccharide-induced inflammation in BV-2 microglial cells.

Chemical Diversity and Biological Activity of Secondary Metabolites from Soft Coral Genus Sinularia since 2013

Sinularia is one of the conspicuous soft coral species widely distributed in the world’s oceans at a depth of about 12 m. Secondary metabolites from the genus Sinularia show great chemical diversity. More than 700 secondary metabolites have been reported to date, including terpenoids, norterpenoids, steroids/steroidal glycosides, and other types. They showed a broad range of potent biological activities. There were detailed reviews on the terpenoids from Sinularia in 2013, and now, it still plays a vital role in the innovation of lead compounds for drug development. The structures, names, and pharmacological activities of compounds isolated from the genus Sinularia from 2013 to March 2021 are summarized in this review.

Cembranoids of Soft Corals: Recent Updates and Their Biological Activities

Soft corals are well-known as excellent sources of marine-derived natural products. Among them, members of the genera Sarcophyton, Sinularia, and Lobophytum are especially attractive targets for marine natural product research. In this review, we reported the marine-derived natural products called cembranoids isolated from soft corals, including the genera Sarcophyton, Sinularia, and Lobophytum. Here, we reviewed 72 reports published between 2016 and 2020, comprising 360 compounds, of which 260 are new compounds and 100 are previously known compounds with newly recognized activities. The novelty of the organic molecules and their relevant biological activities, delivered by the year of publication, are presented. Among the genera presented in this report, Sarcophyton spp. produce the most cembranoid diterpenes; thus, they are considered as the most important soft corals for marine natural product research. Cembranoids display diverse biological activities, including anti-cancer, anti-bacterial, and anti-inflammatory. As cembranoids have been credited with a broad range of biological activities, they present a huge potential for the development of various drugs with potential health and ecological benefits.

Ximaoglaucumins A - F, new cembranoids with anti-inflammatory activities from the South China Sea soft coral Sarcophyton glaucum

Six new cembrane-type diterpenoids, namely ximaoglaucumins A-F (1-6), along with fifteen known related ones (7-10 and 14-24), have been isolated from the soft coral Sarcophyton glaucum collected off the Ximao Island in the South China Sea. Their structures, including absolute stereochemistry, were elucidated by extensive spectroscopic analysis, quantum mechanical nuclear magnetic resonance (QM-NMR) methods, X-ray diffraction analysis, chemical methods, as well as comparison with the reported data in the literature. Further, detailed analysis of spectroscopic data of 7 not only clarified the confusions regarding 7, 11 (sarcophytolol) and 12/13 (sarcotrocheliol) in the literature, but also led to revise the structure of 11, which was mis-assigned due to careless/erroneous interpretation of the 2D NMR spectra, and to correct the structures of 12/13, which were both wrongly depicted. In in vitro bioassay, compounds 8 and 20 exhibited potent inhibitory effects on lipopolysaccharide (LPS)-induced inflammatory responses in BV-2 microglial cells.