Nanomolar Antimalarial Agents against Chloroquine-Resistant Plasmodium falciparum from Medicinal Plants and Their Structure-Activity Relationships

Anti-inflammatory lindenane sesquiterpenoid dimers from the roots of Chloranthus holostegius var. trichoneurus

Two new lindenane-type sesquiterpenoid dimers, chlotrichenes C and D (1 and 2) together with five known lindenane-type sesquiterpenoid dimers (3-7) were isolated from the roots of Chloranthus holostegius var. trichoneurus, a famous natural medicine named as "Sikuaiwa" for subduing swellings and relieving pain. The structures including absolute configuration were elucidated by their 1D and 2D NMR, HRESIMS, and ECD data. Compounds 1 and 2 were classical [4 + 2] lindenane-type sesquiterpenoid dimers that differed from known analogs in oxidation profile, side chain profile, and double bond position. The new isolates and compound 3 exhibited significant inhibitory activity on IL-1β production (IC50: 1-15 μM) in LPS-induced THP-1 cells and other compounds exhibited inhibitory activity on NO production in LPS-induced RAW 264.7 cells (IC50: 24-33 μM).

Anti-Inflammatory Peroxidized Chlorahololide-Type Dimers Are Artifacts of Shizukaol-Type Dimers: From Phenomena Discovery and Confirmation to Potential Underlying Mechanism

In our research on naturally occurring sesquiterpenes, eight shizukaol-type dimers, one chlorahololide-type dimer, and one sarcanolide-type dimer were isolated from the roots of Chloranthus fortunei. As the project was implemented, we accidentally discovered that shizukaol-type dimers can be converted into peroxidized chlorahololide-type dimers. This potential change was discovered after simulations of the changes in corresponding shizukaols showed that three peroxide products were generated (1-3), indicating that peroxidation reactions occurred. HPLC-HR-MS analysis results obtained for the shizukaol derivatives further demonstrate that the reaction occurred, and the type of substituent of small organic ester moieties at positions C-15' and C-13' of unit B were not decisively related to the reaction. Quantum chemical calculations of the mode dimer further demonstrated this phenomenon. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy of the precursor and production revealed the advantageous yield of 4β-hydroperoxyl production. Additionally, the potential reaction mechanism was speculated and validated using the free energy in the reaction which successfully explained the feasibility of the reaction. Finally, the anti-inflammatory activity of the precursors and products was evaluated, and the products of peroxidation showed better anti-inflammatory activity.

Chemistry and bioactivity of lindenane sesquiterpenoids and their oligomers

Covering: 1925 to July 2023Among the sesquiterpenoids with rich structural diversity and potential bioactivities, lindenane sesquiterpenoids (LSs) possess a characteristic cis, trans-3,5,6-carbocyclic skeleton and mainly exist as monomers and diverse oligomers in plants from the Lindera genus and Chloranthaceae family. Since the first identification of lindeneol from Lindera strychnifolia in 1925, 354 natural LSs and their oligomers with anti-inflammatory, antitumor, and anti-infective activities have been discovered. Structurally, two-thirds of LSs exist as oligomers with interesting skeletons through diverse polymeric patterns, especially Diels-Alder [4 + 2] cycloaddition. Fascinated by their diverse bioactivities and intriguing polycyclic architectures, synthetic chemists have engaged in the total synthesis of natural LSs in recent decades. In this review, the research achievements related to LSs from 1925 to July of 2023 are systematically and comprehensively summarized, focusing on the classification of their structures, chemical synthesis, and bioactivities, which will be helpful for further research on LSs and their oligomers.

Lindenane sesquiterpenoid monomers and oligomers: Chemistry and pharmacological activities

Lindenane sesquiterpenoid monomers and oligomers, characterized by a sterically congested cyclopentane and an unusual trans-5/6 ring junction, are mainly found in Chloranthaceae species and the genus Lindera Thunb (Lauraceae). Numerous studies have shown that lindenane sesquiterpenoid monomers and oligomers exhibit a broad range of biological activities, such as cytotoxicity, anti-inflammation, neuroprotection, antifungal, and anti-malarial activities. This review covers publications from the first identification of lindeneol in 1925-2023 and classifies the lindenane sesquiterpenoid derivatives into sesquiterpenoid monomers, sesquiterpenoid-monoterpene conjugates, sesquiterpenoid homodimers, sesquiterpenoid heterodimers, and trimeric sesquiterpenoids. In addition, their biological activities are summarized. This review will establish a scientific basis and provide guidance for utilizing this unique class of natural products as potential lead compounds to develop their application in treating diseases corresponding to inflammation, cancer, and plasmodium.

Lindenane sesquiterpenoid dimers from Chloranthus holostegius with anti-neuroinflammatory activities in BV-2 microglia

Fifteen undescribed lindenane-type sesquiterpenoid dimers, designated chloranholides F-T (1-15), together with twenty-five known analogs (16-40), were isolated from the whole plants of Chloranthus holostegius. The isolate structures were elucidated by analysis of spectroscopic data and chemical methods, and their absolute configurations were determined by X-ray crystallography and electronic circular dichroism spectra. In anti-neuroinflammatory assays, all isolates were evaluated by examination of their inhibitory effect on nitric oxide (NO) in LPS-stimulated BV-2 cells, and the results showed that 21-24, 26, 30, 32 and 36 significantly inhibited the production of the inflammatory mediator NO, with IC50 values ranging from 3.18 to 11.46 μM, which was better than that of quercetin. Structure-activity relationship analysis revealed that two essential functional groups played an indispensable role in the anti-inflammatory effects. Moreover, 22 and 24 inhibited the LPS-induced upregulation of iNOS and COX-2 enzymes in BV-2 microglia at the protein level.

Chlorahololide D, a Lindenane-Type Sesquiterpenoid Dimer from Chloranthus holostegius Suppressing Breast Cancer Progression

Aimed at discovering small molecules as anticancer drugs or lead compounds from plants, a lindenane-type sesquiterpene dimer, chlorahololide D, was isolated from Chloranthus holostegius. The literature review showed that there were few reports on the antitumor effects and mechanisms of chlorahololide D. Our biological assay suggested that chlorahololide D blocked the growth and triggered apoptosis of MCF-7 cells by stimulating the reactive oxygen species (ROS) levels and arresting the cell cycle at the G2 stage. Further mechanism exploration suggested that chlorahololide D regulated apoptosis-related proteins Bcl-2 and Bax. Moreover, chlorahololide D inhibited cell migration by regulating the FAK signaling pathway. In the zebrafish xenograft model, chlorahololide D was observed to suppress tumor proliferation and migration significantly. Considering the crucial function of angiogenesis in tumor development, the anti-angiogenesis of chlorahololide D was also investigated. All of the research preliminarily revealed that chlorahololide D could become an anti-breast cancer drug.

Prioritised identification of structural classes of natural products from higher plants in the expedition of antimalarial drug discovery

The emergence and spread of drug-recalcitrant Plasmodium falciparum parasites threaten to reverse the gains made in the fight against malaria. Urgent measures need to be taken to curb this impending challenge. The higher plant-derived sesquiterpene, quinoline alkaloids, and naphthoquinone natural product classes of compounds have previously served as phenomenal chemical scaffolds from which integral antimalarial drugs were developed. Historical successes serve as an inspiration for the continued investigation of plant-derived natural products compounds in search of novel molecular templates from which new antimalarial drugs could be developed. The aim of this study was to identify potential chemical scaffolds for malaria drug discovery following analysis of historical data on phytochemicals screened in vitro against P. falciparum. To identify these novel scaffolds, we queried an in-house manually curated database of plant-derived natural product compounds and their in vitro biological data. Natural products were assigned to different structural classes using NPClassifier. To identify the most promising chemical scaffolds, we then correlated natural compound class with bioactivity and other data, namely (i) potency, (ii) resistance index, (iii) selectivity index and (iv) physicochemical properties. We used an unbiased scoring system to rank the different natural product classes based on the assessment of their bioactivity data. From this analysis we identified the top-ranked natural product pathway as the alkaloids. The top three ranked super classes identified were (i) pseudoalkaloids, (ii) naphthalenes and (iii) tyrosine alkaloids and the top five ranked classes (i) quassinoids (of super class triterpenoids), (ii) steroidal alkaloids (of super class pseudoalkaloids) (iii) cycloeudesmane sesquiterpenoids (of super class triterpenoids) (iv) isoquinoline alkaloids (of super class tyrosine alkaloids) and (v) naphthoquinones (of super class naphthalenes). Launched chemical space of these identified classes of compounds was, by and large, distinct from that of 'legacy' antimalarial drugs. Our study was able to identify chemical scaffolds with acceptable biological properties that are structurally different from current and previously used antimalarial drugs. These molecules have the potential to be developed into new antimalarial drugs.

Anti-Inflammatory Lindenane Sesquiterpenoid Dimers from the Roots of Sarcandra glabra

Sarglaroids A-H (1-8), eight new lindenane dimers, and a monomer sarglaroid I (9), along with fourteen known analogues (10-23), were isolated from the roots of Sarcandra glabra. The planar structures and the absolute configurations were elucidated by HR-MS, NMR, ECD calculations, and X-ray diffraction crystallography. Sarglaroid A (1) was identified as a rare 8,9-seco lindenane dimer with a unique 5/5/5 tricyclic system. The biological evaluation showed that compounds 1 and 13 potently inhibited NO production with IC50 values at 19.8 ± 1.06 and 10.7 ± 0.25 μM, respectively, and had no cytotoxicity to RAW264.7 cells. Compound 6 significantly inhibited the LPS-/ATP-induced IL-1β release by inactivating the NLRP3 inflammasome through inhibiting the initiation and assembly by affecting the K+ efflux. Compounds 2 and 3 inhibited the proliferation of MCF-7 and MDA-MB-231 with IC50 values ranging from 5.4 to 10.2 μM.

Plant Extracts as a Source of Natural Products with Potential Antimalarial Effects: An Update from 2018 to 2022

Malaria kills more than 500,000 people yearly, mainly affecting Africa and Southeast Asia. The disease is caused by the protozoan parasite from the genus Plasmodium, with Plasmodium vivax and Plasmodium falciparum being the main species that cause the disease in humans. Although substantial progress has been observed in malaria research in the last years, the threat of the spread of Plasmodium parasites persists. Artemisinin-resistant strains of this parasite have been reported mainly in Southeast Asia, highlighting the urgent need to develop more effective and safe antimalarial drugs. In this context, natural sources, mainly from flora, remain underexplored antimalarial spaces. The present mini-review explores this space focusing on plant extracts and some of their isolated natural products with at least in vitro antiplasmodial effects reported in the literature comprising the last five years (2018-2022).

Constituents from Chloranthus multistachys and their cytotoxic activities against various human cancer cell lines

Two new furanoeremophilane sesquiterpenoids, namely, 6,9-dioxo-1α,4α-dihydroxy-furanoeremophilane (1) and 4α,5α-epoxy-6,9-dioxo-1α-hydroxyl-furanoeremophilane (2), and 10 known compounds were isolated from the whole plant of Chloranthus multistachys, and compound 3 was converted to derivative 3a. Their structures were determined based on extensive spectroscopic analysis. All compounds were evaluated by using five cancer cell lines: PC3, LNcap, A549, K562, and HEL. The derivative 3a exhibited excellent cytotoxic activities, with the IC₅₀ against HEL cells being the lowest at 1.322 ± 0.08 μM, which was comparable to that of the positive control (doxorubicin). Mechanism studies showed that the anticancer activity of 3a may be associated with cell cycle regulation.

Rearranged Lindenane Sesquiterpenoid Trimers from Chloranthus fortunei: Target Discovery and Biomimetic Conversion

Seven lindenane-type sesquiterpenoid trimers, including four new ones (1-4) and three known analogues (5-7), were isolated from Chloranthus fortunei guided by high-performance liquid chromatography with photodiode array detection with characteristic absorption at 210 and 350 nm. Their structures, including absolute configurations, were achieved by high-resolution mass spectrometry, nuclear magnetic resonance, electronic circular dichroism, and quantum chemical calculations. Compound 1 was the first example of two lindenane units connected by a C-15-C-15' bond. The 5/7/5-fused ring system in 2 was presumably formed biogenetically by key keto-enol tautomerism and Cope rearrangement from 5. The 5/3/6 carbon skeleton in 3-5 and epi-cyclopropane in 3 and 6 might have originated from trishizukaol A (7) with a normal 3/5/6-fused ring system through vinylcyclopropane rearrangement. The biomimetic conversion from 7 to 3-6 was successfully achieved by adding a 365 nm ultraviolet lamp and a free radical initiator, and 2 was also spontaneously converted to 5 in methanol and CDCl₃, which proved the correctness of the structural identification and the speculation described above. Compounds 1-7 exhibited anti-inflammatory activity with IC₅₀ values in the range of 2.90-22.80 μmol/L.

Natural products are the treasure pool for antimalarial agents

Despite the success in malaria control, it remains a life-threatening infectious disease due mainly to the persistent emergence of drug resistance. Sharpened insight into the historical achievements and current trends in antimalarial drug discovery provides more hopes and advantages on natural products for the development of the next antimalarial treatment.

Chlorfortunones A and B, Two Sesquiterpenoid Dimers, Possessing Dispiro[4,2,5,2]pentadecane-6,10,14-tren Moiety from Chloranthus fortunei

Chlorfortunones A (1) and B (2), two novel sesquiterpenoid dimers, were isolated from the roots of Chloranthus fortunei. Their structures were elucidated by spectroscopic analysis and X-ray diffraction analysis. Compounds 1 and 2 represent a new type of sesquiterpenoid dimer possessing an unprecedented 3/5/6/6/6/5 hexacyclic system with a unique dispiro[4,2,5,2]pentadecane-6,10,14-trien moiety. A plausible biosynthetic pathway of 1 and 2 was proposed. Compound 1 showed transforming growth factor (TGF)-β inhibitory activity in MDA-MB-231 cells.

Spicatulides A-G, Phenolic-Monoterpenoid Hybrids from Chloranthus spicatus

Spicatulides A-G (1-7), seven new phenolic-monoterpenoid hybrid molecules, along with two known compounds, 8 and 9, were isolated and identified from Chloranthus spicatus. Compound 1 represents an unprecedented skeleton featuring an aryl-fused 2-oxabicyclo[4.3.1]decane moiety, and compound 2 is the first example of a denudaquinol-normonoterpenoid adduct. Their structures with absolute configurations were elucidated on the basis of spectroscopic data analyses and TDDFT-ECD calculations. Compounds 3, 5, 6, and 9 exhibited the activity of reducing lipogenesis in HepG2 cells in a dose-dependent manner.

Ursane and 24-Noroleanane-Type Triterpenoids with Anti-HIV Activity from the Twigs and Leaves of Antirhea chinensis

Investigations on the twigs and leaves of Antirhea chinensis have led to the discovery of two undescribed pentacyclic triterpenoids ( 1 and 2 ) and nine known analogues. Compounds 1 and 2 , each assigned as the ursane and 24-noroleanane-type triterpenoids, featuring similar oxidation pattern of 3 β ,6 β ,19 α -trihydroxy-28-carboxyl. Their structures were elucidated via comprehensive analyses of spectroscopic data. Compound 1 displayed potent anti-HIV activity (EC 50 = 1.24 μ M) and high selectivity index (SI > 32.3).

Chlorahupetenes A-D, Four Eudesmane-Type Sesquiterpenoid Dimer Enantiomers with Two Unusual Carbon Skeletons from Chloranthus henryi Var. hupehensis

(+)- and (-)-Chlorahupetenes A (1a and 1b), B (2a and 2b), C (3a and 3b), and D (4a and 4b), four unique enantiomeric pairs of eudesmane-type sesquiterpenoid dimers with two new carbon skeletons, were isolated from the aerial parts of Chloranthus henryi var. hupehensis. Compounds 1 and 2 possess an unprecedented 6/6/5/6/6 pentacyclic carbon skeleton with a new dimerization pattern of two eudesmane-type sesquiterpenoids. Compounds 3 and 4, which are fused with two eudesmane-type sesquiterpenoids via an unprecedented five-membered O-heterocyclic ring, represent a new 6/6/5/5/6/6/5 heptacyclic ring system. The structures of the compounds were determined through spectroscopic data and X-ray crystallography. Compounds 1a-3b significantly inhibited NO production with IC₅₀ values ranging from 9.62 to 12.91 μM. Moreover, compounds 1b and 3a suppressed the production of a proinflammatory mediator (TNF-α) and enzyme expression (iNOS) at the mRNA level.

Phorneroids A-M, diverse types of diterpenoids from Euphorbia neriifolia

A chemical investigation on the aerial parts of Euphorbia neriifolia led to the identification of thirteen undescribed diterpenoids, phorneroids A-M, including ent-abietane (A-D), ent-kaurane (E-G), ent-atisane (H-K), and ent-isopimarane (L and M) types, together with three known compounds. Phorneroid A represents the first example of 8-spiro-fused 9,10-seco-ent-abietane diterpenoid lactone featuring a unique 6/5/6/5 spirocyclic framework. Biological assays showed that some of the compounds displayed moderate cytotoxicity against two human tumor cell lines, A549 and HL-60.

Sarglaromatics A-E: A Class of Naphthalene-Like Architecture Fused Norlindenane Sesquiterpene Dimers from Sarcandra glabra

Sarglaromatics A-E (1-5, respectively), five unprecedented naphthalene-like architecture-fused norlindenane sesquiterpene dimers, were discovered from the roots of Sarcandra glabra. The unique naphthalene core skeleton was obtained from classical lindenane [4 + 2] dimers via a free-radical-mediated C11-C11' bond formation reaction and 12'-decarboxylation. The highly fused octonary ring skeleton was elucidated by HRMS, NMR, ECD, quantum chemical calculations, and biogenetic inspiration. Compounds 1 and 2 showed significant anti-nonalcoholic steatohepatitis (NASH) activity by inhibiting lipid deposition in L02 cells.

Antimalarial Natural Products

Natural products have made a crucial and unique contribution to human health, and this is especially true in the case of malaria, where the natural products quinine and artemisinin and their derivatives and analogues, have saved millions of lives. The need for new drugs to treat malaria is still urgent, since the most dangerous malaria parasite, Plasmodium falciparum, has become resistant to quinine and most of its derivatives and is becoming resistant to artemisinin and its derivatives. This volume begins with a short history of malaria and follows this with a summary of its biology. It then traces the fascinating history of the discovery of quinine for malaria treatment and then describes quinine's biosynthesis, its mechanism of action, and its clinical use, concluding with a discussion of synthetic antimalarial agents based on quinine's structure. The volume then covers the discovery of artemisinin and its development as the source of the most effective current antimalarial drug, including summaries of its synthesis and biosynthesis, its mechanism of action, and its clinical use and resistance. A short discussion of other clinically used antimalarial natural products leads to a detailed treatment of other natural products with significant antiplasmodial activity, classified by compound type. Although the search for new antimalarial natural products from Nature's combinatorial library is challenging, it is very likely to yield new antimalarial drugs. The chapter thus ends by identifying over ten natural products with development potential as clinical antimalarial agents.

Sarcanolides C–E: Three new lindenane sesquiterpenoid dimers with anti-inflammatory activities from Sarcandra glabra

Three new lindenane-type sesquiterpenoid dimers, sarcanolides C–E (1–3), were isolated from the roots of Sarcandra glabra. Sarcanolide C (1) possesses a unique decacyclic scaffold with an unusual orthoformate unit. The...

Chlomultiols A-L, sesquiterpenoids from Chloranthus multistachys and their anti-inflammatory activities

Twelve undescribed sesquiterpenoids, named chlomultiols A-L, involving three lindenane sesquiterpenoid dimers, three eudesmane sesquiterpenoids, three guaiane sesquiterpenoids, and three cadinane sesquiterpenoids, along with four known compounds, were obtained from the whole plant of Chloranthus multistachys. Their structures were determined through spectroscopic techniques (HRESIMS, 1D and 2D NMR). In addition, the absolute and relative configurations of the undescribed compounds were established by using single crystal X-ray crystallography, NOESY and CD spectroscopy. The inhibitory effects of chlomultiols A-M on the production of nitric oxide in RAW 264.7 cells induced by lipopolysaccharide were evaluated. Chlomultiols A-C, and chlomultiols K-L showed moderate anti-inflammatory activities with IC₅₀ values of 3.34 ± 0.73, 15.06 ± 1.08, 13.13 ± 3.99, 6.63 ± 1.11, and 16.16 ± 1.88 μM, respectively.

Cephalotane-type C20 diterpenoids from Cephalotaxus fortunei var. alpina

Seventeen new cephalotane-type diterpenoids were isolated from Cephalotaxus fortunei var. alpina. Compounds 14 and 15 contain an unusual 7-oxabicyclo[4.1.1]octane moiety.

Sublyratins A-O, Labdane-Type Diterpenoids from Croton sublyratus

Fifteen new labdane-type diterpenoids, sublyratins A-O (1-15), along with four known analogues (16-19) were isolated from the aerial parts of Croton sublyratus. Their structural assignments were challenging due to the stereoisomeric features evident and were achieved by analyzing comprehensively the spectroscopic data and electronic circular dichroism spectra and using X-ray crystallographic analysis. Compounds 9 and 16-18 displayed cytotoxic activity against the HL-60 cell line with IC₅₀ values of 1.5-2.8 μM.

Discovery of four modified classes of triterpenoids delineated a metabolic cascade: compound characterization and biomimetic synthesis

Chemical studies on Dichapetalum gelonioides have afforded 18 highly modified complex triterpenoids belonging to four compound classes as defined by the newly adapted functional motifs associated with the A ring of the molecules. Their structures were determined by solid data acquired by diverse methods. The biosynthetic pathway for the four compound classes was rationalized via cascade modifications involving diverse chemical events. The subsequent biomimetic syntheses afforded all the desired products, including compounds 16 and 19 that were not obtained in our purification, which validated the proposed biosynthetic pathway. Besides, some compounds exhibited strong cytotoxic activities, especially 2 and 4 showed nanomolar potency against the NAMALWA tumor cell line, and a gross structure–activity relationship (SAR) of these compounds against the tested tumor cell lines was delineated.

Characterization of four classes of highly modified triterpenoids from Dichapetalum gelonioides sheds light on an unprecedented biosynthetic cascade, which was validated by the subsequent biomimetic syntheses. Moreover, some isolates exhibited nanomolar cytotoxic activities.

Three pairs of sesquiterpene enantiomers from Chloranthus multistachys pei

Phytochemical investigation on the whole plant of Chloranthus multistachys pei (Chloranthaceae) afforded three pairs of new sesquiterpene enantiomers (+)/(-)-chlorantene M [(+)/(-)-1], (+)/(-)-chlorantene M1 [(+)/(-)-2] and (+)/(-)-chlorantene N [(+)/(-)-3]. The structures of new compounds were determined through spectroscopic techniques (HR-ESI-MS, 1 D and 2 D NMR), besides, their absolute and relative configurations were established by using Single-crystal X-ray diffraction analysis and CD spectrum. The anti-inflammatory potential of all compounds was evaluated by applying LPS induced RAW 264.7 macrophage inflammatory model, and the results were that none of these compounds showed activity (IC₅₀ > 100 μM).

Horienoids A and B, Two Heterocoupled Sesquiterpenoid Dimers from Hedyosmum orientale

Two eudesmane-guaiane/lindenane heterocoupled sesquiterpenoid dimers, horienoids A (1) and B (2) with new carbon skeletons, from Hedyosmum orientale were characterized by a combined method. Compound 1 featured a unique 2,10-dioxabicyclo[6.2.1]undecane core moiety with an anti-Bredt bridgehead double bond. Their biogenetic pathways were proposed to involve Diels-Alder and cascade rearrangement reactions as the key steps. Compound 2 exhibited a potent anti-inflammatory effect on LPS-induced BV-2 microglial cells.

Biomimetic Approaches to the Synthesis of Natural Disesquiterpenoids: An Update

Natural disesquiterpenoids represent a small group of secondary metabolites characterized by complex molecular scaffolds and interesting pharmacological profiles. In the last decade, more than 400 new disesquiterpenoids have been discovered and fully characterized, pointing out once more the "magic touch" of nature in the design of new compounds. The perfect blend of complex and unique architectures and biological activity has made sesquiterpene dimers an attractive and challenging synthetic target, inspiring organic chemists to find new and biomimetic approaches to replicate the efficiency and the selectivity of natural processes under laboratory conditions. In this work, we present a review covering the literature from 2010 to 2020 reporting all the efforts made in the total synthesis of complex natural disesquiterpenoids.

Trichloranoids A–D, antimalarial sesquiterpenoid trimers from Chloranthus spicatus

Four new anti-malarial sesquiterpenoid trimers including two rearranged skeletons were isolated from Chloranthus spicatus. The structurally different but biogenetically related compounds 1, 2 and 5 rationalized the biradical pathway, and vice versa.

Resistance to Some But Not Other Dimeric Lindenane Sesquiterpenoid Esters Is Mediated by Mutations in a Plasmodium falciparum Esterase

Unique lindenane sesquiterpenoid dimers from Chloranthecae spp. were recently identified with promising in vitro antiplasmodial activity and potentially novel mechanisms of action. To gain mechanistic insights to this new class of natural products, in vitro selection of Plasmodium falciparum resistance to the most active antiplasmodial compound, chlorajaponilide C, was explored. In all selected resistant clones, the half-maximal effective concentration (EC50) of chlorajaponilide C increased >250-fold, and whole genome sequencing revealed mutations in the recently discovered P. falciparum prodrug activation and resistance esterase (PfPARE). Chlorajaponilide C was highly potent (mean EC50 = 1.6 nM, n = 34) against fresh Ugandan P. falciparum isolates. The analysis of the structure-resistance relationships revealed that in vitro potency of a subset of lindenane sesquiterpenoid dimers was not mediated by PfPARE mutations. Thus, chlorajaponilide C, but not some related compounds, required parasite esterase activity for in vitro potency, and those compounds serve as the foundation for development of potent and selective antimalarials.

Naturally occurring [4 + 2] type terpenoid dimers: sources, bioactivities and total syntheses

This review article highlights recent progress on their sources, bioactivities, biosynthetic hypotheses and total chemical syntheses of naturally occurring [4 + 2] type terpenoid dimers.

Natural disesquiterpenoids: an update

This review highlights the progress on the isolation, bioactivity, biogenesis and total synthesis of dimeric sesquiterpenoids since 2010.

Antiplasmodial natural products: an update

Background

Malaria remains a significant public health challenge in regions of the world where it is endemic. An unprecedented decline in malaria incidences was recorded during the last decade due to the availability of effective control interventions, such as the deployment of artemisinin-based combination therapy and insecticide-treated nets. However, according to the World Health Organization, malaria is staging a comeback, in part due to the development of drug resistance. Therefore, there is an urgent need to discover new anti-malarial drugs. This article reviews the literature on natural products with antiplasmodial activity that was reported between 2010 and 2017.

Methods

Relevant literature was sourced by searching the major scientific databases, including Web of Science, ScienceDirect, Scopus, SciFinder, Pubmed, and Google Scholar, using appropriate keyword combinations.

Results and Discussion

A total of 1524 compounds from 397 relevant references, assayed against at least one strain of Plasmodium, were reported in the period under review. Out of these, 39% were described as new natural products, and 29% of the compounds had IC₅₀ ≤ 3.0 µM against at least one strain of Plasmodium. Several of these compounds have the potential to be developed into viable anti-malarial drugs. Also, some of these compounds could play a role in malaria eradication by targeting gametocytes. However, the research into natural products with potential for blocking the transmission of malaria is still in its infancy stage and needs to be vigorously pursued.

Total synthesis, structural revision and biological evaluation of γ-elemene-type sesquiterpenes

Total synthesis and absolute configuration confirmation of γ-elemene-type sesquiterpenes, which possess vast potential for biological activities, was investigated based on a convergent synthetic strategy. A key intermediate with all functional groups of this family of natural products was accessed by an intermolecular aldol reaction and then an acetylation of a known ketone (12) derived from commercially available verbenone. The versatile intermediate can be easily transformed into structurally different γ-elemene-type sesquiterpenes based on control of base-promoted cyclization manipulation in different solvents. The utility of this robust approach is illustrated by the first syntheses of elema-1,3,7(11),8-tetraen-8,12-lactam (4') and 8β-methoxy-isogermafurenolide (6a), as well as the syntheses of elem-1,3,7,8-tetraen-8,12-olide (3) and hydroxyisogermafurenolide (5) in only 6 or 7 steps. In addition, the structure of the reported 5βH-elem-1,3,7,8-tetraen-8,12-olide (1) was revised as elem-1,3,7,8-tetraen-8,12-olide (3) by comparison of their identified datum, and the absolute configuration of elema-1,3,7(11),8-tetraen-8,12-lactam was confirmed as 4'. Furthermore, the inhibitory effect of all synthesized natural compounds and their natural analogues on cancer cell proliferation was evaluated. Among them compounds 3, 4 and 4' were found to possess potent inhibitory activity against Kasumi-1 and Pfeiffer. Meanwhile, preliminary structure-activity relationships for these compounds are discussed.

17- nor-Cephalotane-Type Diterpenoids from Cephalotaxus fortunei

Seventeen new 17- nor-cephalotane-type diterpenoids, fortalpinoids A-Q (1-17), were isolated from the seeds of Cephalotaxus fortunei var. alpine. Compound 12 represents the first 17- nor-cephalotane-type diterpenoid featuring an 8-oxabicyclo[3.2.1]oct-2-ene moiety. The absolute configuration of fortunolide A (18) was determined for the first time, and the structure of cephinoid Q was revised to 14- epi-cephafortoid A (24) by X-ray crystallographic data analysis. Some of the compounds showed significant cytotoxicity against A549 and HL-60 cells, and the structure-activity relationship of this compound class is discussed.

Cytotoxic Tigliane Diterpenoids from Croton damayeshu

Chemical investigation of an EtOH extract of the twigs and leaves of Croton damayeshu afforded 10 new tigliane diterpenoids, crodamoids A-J (1-10), along with five known compounds. Their structures were elucidated by physical data analysis. Compounds 8, 9, and 15 displayed cytotoxic effects against two human tumor cell lines, A549 and HL-60 (IC₅₀: 0.9-2.4 μM).