Terpenoids of plants from Chloranthaceae family: chemistry, bioactivity, and synthesis

Covering: 1976 to December 2023Chloranthaceae is comprised of four extant genera (Chloranthus, Sarcandra, Hedyosmum, and Ascarina), totaling about 80 species, many of which have been widely used as herbal medicines for diverse medical purposes. Chloranthaceae plants represent a rich source of structurally interesting and diverse secondary metabolites, with sesquiterpenoids and diterpenoids being the predominant structural types. Lindenane sesquiterpenoids and their oligomers, chemotaxonomical markers of the family Chloranthaceae, have shown a wide spectrum of bioactivities, attracting significant attention from organic chemists and pharmacologists. Recent achievements also demonstrated the research value of two unique structural types in this plant family, sesquiterpenoid-monoterpenoid heterodimers and meroterpenoids. This review systematically summarizes 682 structurally characterized terpenoids from 22 Chloranthaceae plants and their key biological activities as well as the chemical synthesis of selected terpenoids.

Sarcanoids A and B, two new lindenane-type sesquiterpenoid dimers from the aerial parts of Sarcandra glabra

Sarcanoids A and B (1 and 2), two new lindenane-type sesquiterpenoid dimers with a γ-hydroxysenecioate moiety at C-15', were isolated from the ethyl acetate extract of Sarcandra glabra. The structures were elucidated by extensive analysis of spectroscopic data, and their absolute configurations were determined by single-crystal X-ray crystallography. Compounds 1 and 2 showed moderate inhibitory activities on the nitric oxide (NO) production induced by lipopolysaccharide (LPS) in RAW264.7 macrophages.

Discovery of sesquiterpenoids from the roots of Chloranthus henryi Hemsl. var. hupehensis (Pamp.) K. F. Wu and their anti-inflammatory activity by IKBα/NF-κB p65 signaling pathway suppression

Phytochemical analysis of Chloranthus henryi var. hupehensis roots led to the identification of a new eudesmane sesquiterpenoid dimer, 18 new sesquiterpenoids, and three known sesquiterpenoids. Among the isolates, 1 was a rare sesquiterpenoid dimer that is assembled by a unique oxygen bridge (C11-O-C8') of two highly rearranged eudesmane-type sesquiterpenes with the undescribed C16 carbon framework. (+)-2 and (-)-2 were a pair of new skeleton dinorsesquiterpenoids with a remarkable 6/6/5 tricyclic ring framework including one γ-lactone ring and the bicyclo[3.3.1]nonane core. Their structures were elucidated using spectroscopic data, single-crystal X-ray diffraction analysis, and quantum chemical computations. In the LPS-induced BV-2 microglial cell model, 17 suppressed IL-1β and TNF-α expression with EC50 values of 6.81 and 2.76 µM, respectively, indicating its excellent efficacy in inhibiting inflammatory factors production in a dose dependent manner and without cytotoxicity. In subsequent mechanism studies, compounds 3, 16, and 17 could reduce IL-1β and TNF-α production by inhibiting IKBα/p65 pathway activation.

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.

Structurally diverse amides from Chloranthus henryi var. hupehensis and their anti-inflammatory activities by blocking Akt phosphorylation

Eleven new amides, four racemic pairs of (±)-chlorahupetamides A, B, D, E (1, 2, 4, 5) and chlorahupetamides C, F, G (3, 6, 7), have been isolated from Chloranthus henryi var. hupehensis. Compounds 1-3 are the first naturally occurring dimers via an unprecedented [2 + 2] cycloaddition derived from two dissimilar cinnamic acid amides, while compounds 4 and 5 represent the first examples of lignanamides in Chloranthus; together with two new hydroxycinnamic acid amide monomers (6-7), these compounds were obtained. Their structures were characterized by nuclear magnetic resonance (NMR), electronic circular dichroism (ECD), and X-ray diffraction analysis. Meanwhile, an LPS-induced BV-2 cell inflammatory model was used to determine the potential anti-inflammatory activity of all the isolated compounds. Intriguingly, compound -1 treatment showed a much greater inhibition of TNF-α expression with an EC50 value of 1.80 µM, while compound + 1 had more advantages in reducing IL-1β expression with an EC50 value of 19.93 µM. Moreover, compounds + 1 and -1 could significantly suppress inflammation and inhibit the Akt signaling pathway by decreasing the phosphorylated protein levels of Akt.

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.

Pogocablenes A-O, fifteen undescribed sesquiterpenoids with structural diversity from Pogostemon cablin

Fifteen previously undescribed sesquiterpenoids (pogocablenes A-O), three first discovered natural patchoulol-type ones, coupled with fourteen known ones, were isolated from the aerial parts of Pogostemon cablin. Among them, pogocablenes A and B, a pair of C2 epimers, possessed an unusual carbon skeleton with bicyclo[4.3.1]decane core. Pogocablene C, originated from eudesmane-type sesquiterpenoid, had an unprecedented bicyclo[5.4.0]undecane scaffold with a peroxy hemiactetal moiety. Pogocablene D possessed a rare tricyclo[5.2.2.01,5]undecane carbon skeleton derived from guaiane-type sesquiterpenoid. Pogocablene E was a 4,5-seco-guaiane derivative owning a peroxy hemiactetal unit and a spirocyclic skeleton. Pogocablene M was a nor-patchoulol-type sesquiterpenoid with α,β-unsaturated ketone moiety. Their structures with absolute configuration were determined by extensive spectroscopic analysis, in combination with quantum chemical calculation. In addition, the plausible biogenetic pathways of pogocablenes A-E were proposed. Furthermore, all isolates were evaluated for anti-influenza virus and anti-inflammatory effects.

Anti-neuroinflammatory sesquiterpenoids from Chloranthus henryi

Hupelactones A (1) and B (2), two new eudesmanolide-type enantiomers of the corresponding compounds, along with four mono- (3-6) and nine dimeric- (7-15) known sesquiterpenoids were isolated from the whole plant of Chloranthus henryi var. hupehensis (syn. C. henryi). The new structures including the absolute configurations were determined by comparison with previously reported enantiomers, extensive spectroscopic methods in combination with electronic circular dichroism (ECD) calculations. All the isolates were evaluated for their inhibitory activities against the lipopolysaccharide (LPS)-induced nitric oxide (NO) production in murine BV-2 microglial cells. Among them, the dimeric lindenane sesquiterpenoids shizukaols F (8) and G (11) exhibited the most potent activities, with IC₅₀ values of 2.65 and 4.60 μM, respectively.

A DFT study of the endo-selectivity mechanism of the Diels-Alder reaction in lindenane dimeric sesquiterpene synthesis promoted by pyridines

The lindenane dimeric sesquiterpenoids with versatile biological activities are accessible via biometric synthesis, in which the endo-selective Diels-Alder reaction plays an important role. To explore the endo-selectivity of the Diels-Alder reaction between lindenane sesquiterpenes promoted by pyridines, density functional theory (DFT) calculations were performed to explore the reaction mechanism between pyridines and D-A monomers. The calculations performed on the reaction pathways explain why pyridines can promote endo-selectivity via hydrogen bonding, and the hydrogen bond strength is a key factor driving the Diels-Alder reaction in major biochemical systems. These DFT-level insights will pave the way for designing better promoters for Diels-Alder reactions in biometric synthesis applications.

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.

[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.

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.

Automatic MS/MS Data Mining Strategy for Discovering Target Natural Products: A Case of Lindenane Sesquiterpenoids

Untargeted liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a widely used method for discovering natural products (NPs); however, automatic MS/MS data mining for the discovery of NPs remains a challenge. In this work, LindenaneExtractor, a program based on characteristic MS/MS ions of lindenane sesquiterpenoids (LSs) was developed to automatically extract the LSs features for target LS discovery in plant extracts. To build this program, fragmentation mechanisms of characteristic ions of LSs were elucidated and confirmed by quantum chemical calculation and deuterium-labeled compounds. Subsequently, the information of characteristic ions was integrated and coded to develop LindenaneExtractor, which was further examined by standards and several public databases. Finally, the target LS features in Sarcandra hainanensis extract were automatically extracted by LindenaneExtractor and visualized by feature-based molecular networking and two-dimensional (2D) retention time-m/z plot, leading to the discovery of 96 target LSs in total, 37 of these compounds were potentially new NPs and one was confirmed by further isolation. This work proposed a new strategy for target NP analysis and discovery based on automatic MS/MS data mining, which could significantly improve the efficiency and accuracy of NP discovery.

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.

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A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as chlorahupetone A from Chloranthus henryi var. hupehensis.