Paracaseolide A, First α-Alkylbutenolide Dimer with an Unusual Tetraquinane Oxa-Cage Bislactone Skeleton from Chinese Mangrove Sonneratia paracaseolaris

Two New Protolimonoids from the Chinese Mangrove Xylocarpus granatum Koenig

Two new compounds including one apotirucallane protolimonoid, xylogranatriterpin A (1), and one glabretal protolimonoid, xylocarpusin A (2), along with three known related compounds were isolated from the twigs and leaves of the Chinese mangrove Xylocarpus granatum. The apotirucallane xylogranatriterpin A (1) bears an unprecedented 24-ketal carbon connecting ring E with an epoxide ring. The structures of new compounds were elucidated by extensive spectroscopic analysis and by comparison of the spectroscopic data with those reported in the literatures. Plausible biosynthetic pathway to xylogranatriterpin A (1) was also proposed. None of them showed cytotoxic, neuroprotective, or protein tyrosine phosphatase 1B (PTP1B) inhibitory activity.

Nonenzymatic Reactions in Natural Product Formation

Biosynthetic mechanisms of natural products primarily depend on systems of protein catalysts. However, within the field of biosynthesis, there are cases in which the inherent chemical reactivity of metabolic intermediates and substrates evades the involvement of enzymes. These reactions are difficult to characterize based on their reactivity and occlusion within the milieu of the cellular environment. As we continue to build a strong foundation for how microbes and higher organisms produce natural products, therein lies a need for understanding how protein independent or nonenzymatic biosynthetic steps can occur. We have classified such reactions into four categories: intramolecular, multicomponent, tailoring, and light-induced reactions. Intramolecular reactions is one of the most well studied in the context of biomimetic synthesis, consisting of cyclizations and cycloadditions due to the innate reactivity of the intermediates. There are two subclasses that make up multicomponent reactions, one being homologous multicomponent reactions which results in dimeric and pseudodimeric natural products, and the other being heterologous multicomponent reactions, where two or more precursors from independent biosynthetic pathways undergo a variety of reactions to produce the mature natural product. The third type of reaction discussed are tailoring reactions, where postmodifications occur on the natural products after the biosynthetic machinery is completed. The last category consists of light-induced reactions involving ecologically relevant UV light rather than high intensity UV irradiation that is traditionally used in synthetic chemistry. This review will cover recent nonenzymatic biosynthetic mechanisms and include sources for those reviewed previously.

Weizhouochrones: Gorgonian-Derived Symmetric Dimers and Their Structure Elucidation Using Anisotropic NMR Combined with DP4+ Probability and CASE-3D

Natural product dimers have intriguing structural features and often have remarkable pharmacological activities. We report here two uncommon marine gorgonian-derived symmetric dimers, weizhouochrones A (1) and B (2), with indenone-derived monomers, that were isolated from the coral Anthogorgia ochracea collected from the South China Sea. These dimers are difficult targets for structure elucidation that solely relies upon conventional NMR data such as NOEs and J-couplings. Here, to explore the application of emerging methods on the structure elucidation of challenging molecules, we explored a number of different anisotropic and computational NMR approaches. The measurements of anisotropic NMR parameters of weizhouochrone A, including residual dipolar couplings (RDCs) and residual chemical shift anisotropy (RCSA), allowed us to successfully determine the planar structure and its relative configuration. This result was corroborated by a computational NMR analysis based on DP4+ probability and computer-assisted 3D structure elucidation (CASE-3D).

Lagerindicine, a New Pyrrole Alkaloid Isolated from the Flowers of Lagerstroemia indica Linnaeus

A phytochemical investigation of the EtOH extract of the flowers of Lagerstroemia indica L. led to the isolation and characterization of a new pyrrole alkaloid, named lagerindicine (1), along with four known compounds (2-5). Their structures were elucidated by the detailed spectroscopic analysis and comparison with literature data, whereas the structure, in particularly, the absolute configuration (AC) of 1, was firmly determined by total synthesis. All the isolates were evaluated for their cytotoxic effects against human colon cancer cell (HCT-116), and compound 3 exhibited weak cytotoxicity with IC₅₀ value of 28.4 μM.

Catalytic Hydroxycyclopropanol Ring-Opening Carbonylative Lactonization to Fused Bicyclic Lactones

A novel palladium-catalyzed ring opening carbonylative lactonization of readily available hydroxycyclopropanols was developed to efficiently synthesize tetrahydrofuran (THF) or tetrahydropyran (THP)-fused bicyclic γ-lactones, two privileged scaffolds often found in natural products. The reaction features mild reaction conditions, good functional group tolerability, and scalability. Its application was demonstrated in a short total synthesis of (±)-paeonilide. The fused bicyclic γ-lactone products can be easily diversified to other medicinally important scaffolds, which further broadens the application of this new carbonylation method.

Medicinal chemistry insights into novel CDC25 inhibitors

Cell division cycle 25 (CDC25) phosphatases, a kind of cell cycle regulators, have become an attractive target for drug discovery, as they have been found to be over-expressed in various human cancer cells. Several CDC25 inhibitors have achieved significant attention in clinical trials with possible mechanistic actions. Prompted by the significance of CDC25 inhibitors with medicinal chemistry prospect, it is an apt time to review the various drug discovery methods involved in CDC25 drug discovery including high throughput screening (HTS), virtual screening (VS), fragment-based drug design, substitution decorating approach, structural simplification approach and scaffold hopping method to seek trends and identify promising new avenues of CDC25 drug discovery.

Compounds from the fruits of mangrove Sonneratia apetala: Isolation, molecular docking and antiaging effects using a Caenorhabditis elegans model

The chemical investigation of the fruits of a mangrove Sonneratia apetala collected from the Beibu Gulf led to the isolation of four new compounds, sonneradons A-D (1-4), as well as 18 known compounds (5-22). The structures of the new compounds were elucidated based on extensive spectroscopic analysis, and the structures of the known compounds were established by comparison of their spectroscopic data with those of related metabolites. The antiaging activities of all isolates were evaluated using the nematode Caenorhabditis elegans as a model organism. The results showed that 10 compounds could protect C. elegans by extending its lifespan. Compound 1 possessed the most potent effect in the anti-heat stress assay and significantly attenuated aging-related decreases in the pumping and bending of the nematodes in the healthspan assay. Molecular docking studies suggested that compound 1 was bound to the DNA binding domain of HSF-1 and promoted the conformation of HSF-1, thus strengthening the interaction between the HSF-1 and related DNA. GLN49, ASN-74, and LYS-80 of the binding region might be the key amino residues during the interaction.

A Biomimetic Synthesis Elucidates the Origin of Preuisolactone A

A short, biomimetic synthesis of the fungal metabolite preuisolactone A is described. Its key steps are a purpurogallin-type (5 + 2)-cycloaddition, followed by fragmentation, vinylogous aldol addition, oxidative lactonization, and a final benzilic acid rearrangement. Our work explains why preuisolactone A has been isolated as a racemate and suggests that the natural product is not a sesquiterpenoid but a phenolic polyketide.

Cytotoxic and Antiviral Triterpenoids from the Mangrove Plant Sonneratia paracaseolaris

A chemical investigation was conducted on the aerial parts of the mangrove plant Sonneratia paracaseolaris, yielding five new triterpenoid paracaseolins A-E (1-4, and 11) together with twelve known analogues (5-10, 12-17). Their structures were established by extensive spectroscopic methods and comparisons their spectroscopic data with those of the known related compounds. The cytotoxicities against P388, HeLa, A549, and K562 tumor cell lines and anti-H1N1 (Influenza A virus) activities for the isolates were evaluated. Compound 4 showed potent cytotoxicity against the A549 cell line with an IC50 value of 1.89 µM, and compound 1 exhibited significant anti-H1N1 virus activity with an IC50 value of 28.4 µg/mL. A preliminary structure activity relationship was discussed.

Synthesis and Structural Modification of Marine Natural Products

In the last decades, marine natural products (MNPs), have attracted extensive interest from both chemists and pharmacologists due to their chemical and bioactive diversities. This special issue, collecting total synthesis and structural modification of six different type of bioactive MNPs, is expected to inspire and attract more research effects invested into MNP research.

Appreciation of symmetry in natural product synthesis

This review defines symmetric molecules from a synthetic perspective and shows various strategies that take advantage of molecular symmetry to construct them.

Novel and Neuroprotective Tetranortriterpenoids from Chinese Mangrove Xylocarpus granatum Koenig

Eight new tetranortriterpenoids (1–8) were isolated from the twigs and leaves of the Chinese mangrove plant Xylocarpus granatum, together with four related known ones (9–12). The structures of new compounds were elucidated by detailed spectroscopic analysis. The absolute configuration of 9-epixylogranatin A (1) was determined by time-dependent density functional theory-electronic circular dichroism (TDDFT-ECD) calculations of the solution conformers. Xylogranatumin A (2) represents the first example of the 9, 10-seco limonoid with an unprecedented oxygen-bridged B ring (2,7-dioxabicyclo[2.2.1]-heptane). All the isolates were evaluated for the in vitro neuroprotective activity, both compounds 11 and 12 displayed moderate effects against H₂O₂-induced neurotoxicity in PC12 cells at the concentration of 10 μM, with an increase in cell viability of 12.0% and 11.6%, respectively.

Marine natural products

This review covers the literature published in 2014 for marine natural products (MNPs), with 1116 citations (753 for the period January to December 2014) 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 (1378 in 456 papers for 2014), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Regenerative γ-Lactone Annulations: A Modular, Iterative Approach to Oligo-tetrahydrofuran Molecular Stairs and Related Frameworks

A unified, stereocontrolled, regenerative γ-butyrolactone annulation approach has been conceptualized and validated through syntheses of a range of oligo-THFs. The new protocol is short (four steps), simple (table-top reagents), and efficient (50-61% overall yields). Although the scope of this approach is unlimited, it has been demonstrated up to five iterations on commercial γ-butyrolactone to assemble six fused tetrahydrofuran moieties in a staircase-like architecture. A selection of exploratory transformations is presented to exemplify the potential applications of this protocol.

Diels-Alderase-free, bis-pericyclic, [4+2] dimerization in the biosynthesis of (±)-paracaseolide A

The natural product paracaseolide A is a tetracyclic dilactone containing six adjacent stereocenters. It has an unprecedented skeleton and occupies unique structural space among the >200,000 characterized secondary metabolites. Six different research groups have reported a chemical synthesis of this compound, five of which used a thermal, net Diels–Alder [4+2] cycloaddition and dehydration at 110 °C to access the target by dimerization of a simple butenolide precursor. Here we report that this dimerization proceeds under much milder conditions and with a different stereochemical outcome than previously recognized. This can be rationalized by invoking a bis-pericyclic transition state. Furthermore, we find that spontaneous epimerization, necessary to correct the configuration at one key stereocenter, is viable and that natural paracaseolide A is racemic. Together these facts point to the absence of enzymatic catalysis (i.e., Diels–Alderase activity) in the cycloaddition and strongly suggest that a non-enzyme-mediated dimerization is the actual event by which paracaseolide A is produced in Nature.

Apotirucallane protolimonoids from the Chinese mangrove Xylocarpus granatum Koenig

A series of previously unreported compounds including seven new apotirucallane protolimonoids, xylogranatumines A-G (1-7), were isolated together with three known analogues (8-10) from the twigs and leaves of the Chinese mangrove, Xylocarpus granatum. The structures of these new compounds were elucidated by extensive spectroscopic analysis including 1D and 2D NMR and high-resolution electrospray ionization mass spectrometry (HRESIMS) and by comparison with those of related known compounds in the literature. Xylogranatumine F (6) exhibited cytotoxic activity against A549 tumor cell in vitro.

Marine natural products

This review covers the literature published in 2012 for marine natural products, with 1035 citations (673 for the period January to December 2012) 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 (1241 for 2012), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Cipatrijugin G, a new trijugin-type limonoid bearing an uncommon γ-hydroxybutenolide unit from the aerial parts of Cipadessa cinerascens

A new trijugin-type limonoid, cipatrijugin G (1), together with the related known cipatrijugin A (2), were isolated from the aerial parts of Cipadessa cinerascens. The structure of the new compound was determined by extensive analysis of its spectroscopic data and by comparison of its NMR data with those reported in the literature. Compound 1 showed cytotoxicity against tumor cell line A549 with an IC₅₀ value of 9.78 μM. This is the first report of a trijugin-type limonoid bearing a γ-hydroxybutenolide unit, in comparison to the common furan unit as ring E.

Natural products chemistry research: progress in China in 2011

This article reviews the progress made by Chinese scientists in the field of natural products chemistry in 2011. Selected compounds with unique structural features and/or promising bioactivities are described herein on the basis of structural types.

Bioactive natural products from Chinese marine flora and fauna

In recent decades, the pharmaceutical application potential of marine natural products has attracted much interest from both natural product chemists and pharmacologists. Our group has long been engaged in the search for bioactive natural products from Chinese marine flora (such as mangroves and algae) and fauna (including sponges, soft corals, and mollusks), resulting in the isolation and characterization of numerous novel secondary metabolites spanning a wide range of structural classes and various biosynthetic origins. Of particular interest is the fact that many of these compounds show promising biological activities, including cytotoxic, antibacterial, and enzyme inhibitory effects. By describing representative studies, this review presents a comprehensive summary regarding the achievements and progress made by our group in the past decade. Several interesting examples are discussed in detail.