Natural sesquiterpenoid oligomers: A chemical perspective

Artemselenoids A-H, eight guaiane-type sesquiterpenoid dimers from Artemisia selengensis and their antihepatoma activities

Eight undescribed guaiane-type sesquiterpenoid dimers (GSDs), artemselenoids A-H (1-8), together with nine known GSDs (9-17), were isolated from Artemisia selengensis. Their structures and absolute configurations were determined through comprehensive spectral analyses, theoretical ECD calculations, and NMR computations. Chemically, compound 1 represented the first example of two guaianolide lactone units dimerizing through unprecedented C-3-C-11' and C-4-C-13' bonds via a [2 + 2] cycloaddition reaction and producing a structurally unique 5,4 spirocyclic system; compounds 2-8 were biogenetically formed through a [4 + 2] cycloaddition reaction. Specifically, compounds 2-7 were connected by C-1-C-13' and C-4-C-11' bonds, while compound 8 was linked by C-1-C-13' and C-4-C-11' bonds. Antihepatoma assays indicated that the most active compounds 3 and 8 demonstrated significant inhibitory effects on three hepatoma cell lines, with IC50 values of 7.4 and 5.1 μM (HepG2), 5.9 and 8.6 μM (Huh7), and 20.9 and 9.6 μM (SK-Hep-1), which were equal to those of the positive control sorafenib.

Highly Fused Lindenane Sesquiterpenoid Dimers with Apoptosis-Inducing Properties from Chloranthus holostegius var. shimianensis

Six undescribed lindenane sesquiterpenoid dimers (LSDs), shimianolides A-F (1-6), were isolated from the whole plants of Chloranthus holostegius var. shimianensis. Shimianolides A-C (1-3) represent an unprecedented class of LSDs, distinguished by a highly fused 3/5/6/6/6/5/6/6/5/3 decacyclic scaffold. All isolates were evaluated for their inhibitory effects on LPS-induced NO production in RAW264.7 macrophages and their cytotoxicity against five human cancer cell lines. Compound 4 exhibited moderate cytotoxicity against the HL-60 cell line with an IC50 value of 15.6 ± 1.11 μM, comparable to that of the positive control cisplatin. Further investigation demonstrated that compound 4 induced apoptosis in HL-60 cells, with apoptosis rates of 36.9% at 15.0 μM and 47.5% at 30.0 μM. These results highlight the potential of LSDs as lead compounds for leukemia therapy.

Heterocyclic pseudoguaianolide oligomers and seco-pseudoguaianolide derivatives from the inflorescence of Ambrosia artemisiifolia

Two heterocyclic sesquiterpenoid oligomers (1, 2) and four previously undescribed seco-pseudoguaianolide derivatives (3-6) were isolated from the inflorescence of Ambrosia artemisiifolia. Ambrosiadimer A (1) is an unprecedented dimer featuring a hexahydropyrrolizine core scaffold and two pseudoguaianolide units. Ambrosiatrimer A (2) is a trimer formed from three pseudoguaianolide units via a pyrrolidine ring. The structures and absolute configurations of these compounds were determined through NMR, MS, and Cu Kα X-ray crystallographic analysis. A plausible cycloaddition reaction was proposed for 1 and 2. Moreover, compounds 2 and 7 exhibited moderate cytotoxicities in human cancer cell lines, with IC50 values ranging from 7.22 to 27.45 μM and 9.32-33.45 μM, respectively.

Chamilactones A and B, Sesquiterpenes from the Endophytic Fungus Chaetomium nigricolor F5

A bioassay-guided chemical investigation of the endophytic fungus Chaetomium nigricolor F5 resulted in the discovery of two novel sesquiterpenes, chamilactones A and B (1 and 2), with a new 9,10-seco-15-nor-isoilludalane carbon skeleton, together with several biosynthetically related precursors (3-8). Their structures and absolute configurations were elucidated by the analysis of MS, NMR, calculated 13C chemical shifts, ECD calculations, and single-crystal X-ray diffraction data. It was proposed that an unprecedented carbon-carbon bond cleavage between C-9 and C-10 in 3-8 was the key step in the biosynthetic pathway of 1 and 2. Compound 3 displayed potent neuroprotective effects by reducing the phosphorylation level of p65 and inhibiting its nuclear translocation in the TLR4-mediated NF-κB signaling pathway in LPS-induced BV2 microglial cells.

Interaction of Norsecurinine-Type Oligomeric Alkaloids with α-Tubulin: A Molecular Docking Study

The medicinal plant Securinega virosa (Roxb ex. Willd) Baill., also known as Flueggea virosa (Roxb. ex Willd.) Royle, is commonly used in traditional medicine in Africa and Asia for the management of diverse pathologies, such as parasite infections, diabetes, and gastrointestinal diseases. Numerous alkaloids have been isolated from the twigs and leaves of the plant, notably a variety of oligomeric indolizidine alkaloids derived from the monomers securinine and norsecurinine which both display anticancer properties. The recent discovery that securinine can bind to tubulin and inhibit microtubule assembly prompted us to investigate the potential binding of two series of alkaloids, fluevirosines A-H and fluevirosinine A-J, with the tubulin dimer by means of molecular modeling. These natural products are rare high-order alkaloids with tri-, tetra-, and pentameric norsecurinine motifs. Despite their large size (up to 2500 Å3), these alkaloids can bind easily to the large drug-binding cavity (about 4800 Å3) on α-tubulin facing the β-tubulin unit. The molecular docking analysis suggests that these hydrophobic macro-alkaloids can form stable complexes with α/β-tubulin. The tubulin-binding capacity varies depending on the alkaloid size and structure. Structure-binding relationships are discussed. The docking analysis identifies the trimer fluevirosine D, tetramer fluevirosinine D, and pentamer fluevirosinine H as the most interesting tubulin ligands in the series. This study is the first to propose a molecular target for these atypical oligomeric Securinega alkaloids.

Inubritantrimers A-D: Trimerized Sesquiterpenoid [4 + 2] Adducts Featuring a Distinctive Spiro-Polycyclic Scaffold from Inula britannica

Inubritantrimer A (1), a trace trimerized sesquiterpenoid [4 + 2] adduct featuring an unusual exo-exo type spiro-polycyclic scaffold, together with three new endo-exo [4 + 2] adducts, inubritantrimers B-D (2-4), were discovered from the flowers of Inula britannica. Their structures were elucidated using 1D/2D NMR, X-ray diffraction, and ECD approaches. 1 is characterized as a novel exo-exo trimer, synthesized biogenetically from three sesquiterpenoid monomers, featuring a unique linkage of C-11/C-1', C-13/C-3' and C-13'/C-3″, C-11'/C-1″ through a two-step exo [4 + 2] cycloaddition process. Compounds 1-4 exhibited modest cytotoxicity against breast cancer cells with IC50 values in the range of 5.84-12.01 μM.

Glaucatotones A-I: Guaiane-type sesquiterpenoids from the roots of Lindera glauca with anti-inflammatory activity

Glaucatotones A - I, nine new guaiane-type sesquiterpenoids, along with two reported compounds, namely (1β,5β)-1-hydroxyguaia-4(15),11(13)-dieno-12,5-lactone (10) and pseudoguaianelactone C (11), were isolated from the roots of Lindera glauca. The structures and absolute configurations of these compounds were elucidated by extensive spectroscopic analyses, single-crystal X-ray diffraction, and comparison of experimental and calculated electronic circular dichroism (ECD) data. Structurally, glaucatotone A (1) is characterized as a dihomosesquiterpenoid with an unprecedented 5/5/7/6 ring system. A pair of enantiomers, (±)-glaucatotone B (2a/2b), represent the first rearranged norsesquiterpenoid with a (cyclopentylmethyl)cyclohexane skeleton. 3 is defined as a dinorsesquiterpenoid possessing a 5/7/5 ring system. 4-6 are three guaiane-type norsesquiterpenoids. In vitro bioactivity, 2a selectively inhibited Bcap-37 with IC50 value of 5.60 μM, and 9 selectively inhibited Du-145 with IC50 value of 5.52 μM. The anti-inflammatory activity of 1-9 were tested, and of these compounds, 1, 2a, 2b and 7 exhibited potent inhibitory effects.

Dimerized sesquiterpenoid [4 + 2] adducts with ferroptosis-promoting activity from Inula britannica Linn

Inubritanolides C and D (1 and 2), two exo sesquiterpenoid [4 + 2] adducts with unprecedented interconverting conformations of twist-chair and chair, together with two previously undescribed endo [4 + 2] dimers (3 and 4) were discovered from Inula britannica flowers. Dimers 1 and 2 have an undescribed carbon skeleton comprising of eudesmanolide and guaianolide units with the linkage mode of C-11/C-1' and C-13/C-3' via a Diels-Alder cycloaddition reaction. Their structures were elucidated using 1D/2D NMR, X-ray diffraction, ECD, and variable-temperature NMR experiments. Dimer 2 displayed a strong inhibitory effect on breast cancer cells by promoting lipid ROS production, showing its potential as ferroptosis inducer.

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.

A comprehensive review on the chemical constituents, sesquiterpenoid biosynthesis and biological activities of Sarcandra glabra

Sarcandra glabra (Thunb.) Nakai is a perennial evergreen herb categorised within the Sarcandra Gardner genus under the Chloranthaceae family. Indigenous to tropical and subtropical regions of East Asia and India, this species is extensively distributed across China, particularly in the southern regions (Sichuan, Yunnan, and Jiangxi). In addition to its high ornamental value, S. glabra has a rich history of use in traditional Chinese medicine, evident through its empirical prescriptions for various ailments like pneumonia, dysentery, fractures, bruises, numbness, amenorrhea, rheumatism, and other diseases. Besides, modern pharmacological studies have revealed various biological activities, such as antitumour, anti-bacterial, anti-viral anti-inflammatory and immunomodulatory effects. The diverse chemical constituents of S. glabra have fascinated natural product researchers since the 1900s. To date, over 400 compounds including terpenoids, coumarins, lignans, flavonoids, sterols, anthraquinones, organic acids, and organic esters have been isolated and characterised, some featuring unprecedented structures. This review comprehensively examines the current understanding of S. glabra's phytochemistry and pharmacology, with emphasis on the chemistry and biosynthesis of its unique chemotaxonomic marker, the lindenane-type sesquiterpenoids.

Structurally Diverse Sesquiterpenoids from the Genus of Ainsliaea

The genus of Ainsliaea embraces approximately 70 recognized species, many of which have been used to treat various diseases in folklore medicines. As the main metabolites of Ainsliaea plants, Ainsliaea sesquiterpenoids have drawn considerable attention in related scientific communities due to their intriguing structures and a variety of bioactivities. In this review, we intend to provide a full-aspect coverage of sesquiterpenoids reported from the genus of Ainsliaea, including 145 monomeric sesquiterpenoids and 30 oligomeric ones. Multiple aspects will be summarized, including their classification, distributions, structures, bioactivities, and biomimetic syntheses. In addition, their possible biosynthetic pathway will be discussed in detail.

Calamene-Type Sesqui-, Mero-, and Bis-sesquiterpenoids from Cultures of Heimiomyces sp., a Basidiomycete Collected in Africa

New meroterpenoids bis-heimiomycins A-D (1-4) and heimiomycins D and E (5 and 6) were isolated from solid rice cultures of Heimiomyces sp., while new calamene-type sesquiterpenoids heimiocalamene A (7) and B (8) were isolated from shake cultures, respectively. Structures of the metabolites were elucidated by 1D and 2D NMR in addition to HRESIMS data. While relative configurations were assigned by ROESY data, absolute configurations were derived from the structurally related, previously described calamenes, which we herein name heimiocalamenes C-E (9-11). A plausible biosynthetic pathway was proposed for 1-6, with a radical reaction connecting their central para-benzoquinone building block to calamene-sesquiterpenoids. Based on the assumption of a common biosynthesis, we reviewed the structure of the known nitrogen-containing derivative 11, calling the validity of the originally proposed structure into question. Subsequently, the structure of 11 was revised by analysis of HMBC and ROESY NMR data. Only heimiomycin D (5) displayed cytotoxic effects against cell line KB3.1.

Sesquiterpenoids and sesquiterpenoid dimers from the stem bark of Dysoxylum parasiticum (osbeck) kosterm

Two undescribed sesquiterpenoids, namely dysoticans A and B, and three undescribed sesquiterpenoid dimers, namely dysoticans C-E, together with six analogs, were isolated from the stem bark of Dysoxylum parasiticum (Osbeck) Kosterm. (Meliaceae), growing in West Java, Indonesia. Their structures were elucidated based on extensive spectroscopic analysis and theoretical simulations of ECD spectra and ¹³C NMR shifts. Dysoticans A and B possessed undescribed cadinanes with minor modifications, while C and D featured unprecedented pseudo-sesquiterpenoid dimers through O-ether linkages of cadinanes and guaianes, respectively. Dysotican E was also characterized as the true-sesquiterpenoid dimer featuring eudesmane-germacrene hybrid framework from the Meliaceae family. Furthermore, A-C and E showed moderate activities against the human breast cancer MCF-7 and cervical cancer HeLa cell lines with IC₅₀ values ranging from 22.15 to 45.14 μM. D selectively exhibited significant cytotoxicity against the HeLa cell line with an IC₅₀ value of 13.00 ± 0.13 μM.

Biogenetic and biomimetic synthesis of natural bisditerpenoids: hypothesis and practices

Covering: up to March 2022Bisditerpenoids, or diterpenoid dimers, are a group of natural products with high structural variance, deriving from homo- or hetero-dimeric coupling of two diterpenoid units. They usually possess complex architectures resulting from the diversity of monomeric diterpenoids as building blocks and the dimerization processes. These compounds have attracted the attention of synthetic and biological scientists owing to the rarity of their natural origin and their significant biological activities. Herein, we provide a review highlighting some of the interesting bisditerpenoids reported since 1961 and showcase the chemical diversity in both their structures and biosynthesis, as well as their biological functions. This review focuses on the biosynthetic dimerization pathways of interesting molecules and their biomimetic synthesis, which may act as useful inspiration for the discovery and synthesis of more bisditerpenoids and further pharmacological investigations.

Asymmetric Total Synthesis of Shizukaol J, Trichloranoid C and Trishizukaol A

The asymmetric total synthesis of three lindenane sesquiterpenoid oligomers, shizukaol J, trichloranoid C and trishizukaol A, has been accomplished concisely in 15, 16 and 18 longest linear steps, respectively. The expeditious construction of molecular architectures was facilitated by Nelson's catalytic asymmetric ketene-aldehyde cycloaddition, a sequence of allylic alkylation/reduction/acidic cyclization to forge a lactone, and a double aldol condensation cascade to construct the 5/6 bicyclic system. Diastereoselective nucleophilic substitution promoted by a phase transfer catalyst constructed the C₁₁ quaternary stereogenic center, thus prompting synthetic efficacy toward shizukaol J. The synthesis of trichloranoid C and trishizukaol A was achieved after a cascade involving furanyl diene formation and a Diels-Alder reaction, as well as a one-pot sequence involving furan oxidation and global deprotection. Furthermore, our biological evaluation revealed that two compounds exhibited unexpected toxicity against tumor cell lines.

Anti-inflammatory sesquiterpenoid dimers from Artemisia atrovirens

Eight new sesquiterpenoid dimers, artatrovirenolides A-H (1-8), along with three known analogues (9-11), were isolated from Artemisia atrovirens by using the LC-MS guided isolation. Compound 1 was a compound dimerized from a guaianolide and a 1,10-seco-guaianolide unit while others were from two guaianolide units. Their structures were established by comprehensive analysis of spectroscopic data, and their absolute configurations were determined by the aid of time-dependent density functional theory electronic circular dichroism (TDDFT ECD) calculation. Compound 8 showed anti-inflammatory effect in LPS-stimulated BV-2 microglial cells at 1 μM, while compounds 1, 2, 5, and 6 inhibited microglial inflammation at 10 μM.

Unprecedented diterpenoid dimers with soluble epoxide hydrolase inhibitory effect from Euphorbia fischeriana

Biseuphoids A (1) and B (2), two unprecedented ent-abietane-type diterpenoid dimers linked by monomeric blocks through C-17-C-12' and C-17-C-11', respectively, were isolated from Euphorbia fischeriana, along with their biogenesis related diterpenoid monomers, 17-hydroxyjolkinolide B (3), caudicifolin (4), and fischeriabietane C (5). Their structures were elucidated by extensive spectroscopy assisted by quantum chemical NMR and ECD calculations. The unusual dimeric skeletons are possibly derived from the adduct of diterpenoid monomers through Michael-like reactions. The novel dimers 1 and 2 exhibited inhibitory activities on soluble epoxide hydrolase (sEH) with IC₅₀ values of 8.17 and 5.61 μM, respectively. Molecular dynamics studies illustrated that both 1 and 2 can occupy the catalytic pocket of sEH by forming stable hydrogen bonds with the key amino acid residues including Gln384, Asn378, Pro361, Ala365, Asn366, and Asn472.

Bisabolane-type sesquiterpenoids: Structural diversity and biological activity

Bisabolane-type sesquiterpenoids, a class of monocyclic sesquiterpenoids, are widely distributed in nature and have a variety of biological activities. To provide a reference for the further research and development of these compounds, the phytochemical and biological properties of natural bisabolane-type sesquiterpenoids (356 compounds in total) isolated between 1985 and 2020 from 24 families, primarily Compositae, Zingiberaceae, Aspergillaceae, Halichondriidae, and Aplysiidae were reviewed. In vitro and in vivo studies have indicated that antibacterial, anti-inflammatory, and cytotoxic effects are the most commonly reported pharmacological properties of bisabolane-type sesquiterpenoids. Owing to their extensive significant effects, a lot of traditional medicines containing this type of compounds have been used for a long history. Thus, bisabolane-type sesquiterpenoids are a rich source of important natural products, which show great potential for the development of new drugs.

Chemical characterization and multifunctional neuroprotective effects of sesquiterpenoid-enriched Inula britannica flowers extract

Dried flowers of Inula britannica commercially serve as pharmaceutical/nutraceutical herbs in the manufacture of medicinal products and functional tea that has been reported to possess extensive biological property. However, the neuroprotective constituents in I. britannica flowers are not known. In the current study, phytochemicals of sesquiterpenoid-enriched I. britannica flowers extract and their potential multifunctional neuroprotective effects were investigated. Nineteen structurally diverse sesquiterpenoids, including two new sesquiterpenoid dimers, namely, inubritanolides A and B (1, 2), and four new sesquiterpenoid monomers (3-6), namely, 1-O-acetyl-6-O-chloracetylbritannilactone (3), 6-methoxybritannilactone (4), 1-hydroxy-10β-methoxy-4αH-1,10-secoeudesma-5(6),11(13)-dien-12,8β-olide (5) and 1-hydroxy-4αH-1,10-secoeudesma-5(6),10(14),11(13)-trien-12,8β-olide (6), as well as 13 known congeners (7-19) were isolated from this source. The structures of compounds 1-6 were elucidated by 1D- and 2D- NMR and HR-ESI-MS data, and their absolute configurations were discerned by electronic circular dichroism (ECD) data analysis and single crystal X-ray diffraction. Interestingly, inubritannolide A (1) is a new type [4 + 2] Diels-Alder dimer featuring a hepta-membered cycloether skeleton. Most of the compounds showed potential multifunctional neuroprotective effects, including antioxidative, anti-neuroinflammatory, and microglial polarization properties. Specifically, 1 and 6 displayed slight strong neuroprotective potency against different types of neuronal cells mediated by various inducers including H₂O₂, 6-hydroxydopamine (6-OHDA), and lipopolysaccharide (LPS). Overall, this is the first report on multifunctional neuroprotective effects of sesquiterpenoid-enriched I. britannica flowers extract, which supports its potential pharmaceutical/nutraceutical application in neurodegenerative diseases.

Bisfischoids A and B, dimeric ent-abietane-type diterpenoids with anti-inflammatory potential from Euphorbia fischeriana Steud

Two undescribed ent-abietane-type diterpenoid dimers with nonacyclic backbone formed by intermolecular [4 + 2] cycloaddition into a spirocyclic skeleton, bisfischoids A (1) and B (2), along with a known one fischdiabietane A (3), were identified from Euphorbia fischeriana Steud. Their structures were elucidated by extensive spectroscopic analysis, ECD and NMR calculation combined with DP4+ probability analysis, as well as X-ray diffraction. The anti-inflammatory potential of dimers 1-3 were examined using their inhibitory effects on soluble epoxide hydrolase (sEH), which revealed that 1 and 2 exhibited promising activities with inhibition constant (Ki) of 3.20 and 1.95 μM, respectively. Further studies of molecular docking and molecular dynamics indicated that amino acid residue Tyr343 in the catalytic cavity of sEH was the key site for their inhibitory function.

Cytotoxic Guaianolide Sesquiterpenoids from Ainsliaea fragrans

Twelve guaianolide-type sesquiterpene oligomers with diverse structures were isolated from the whole plants of Ainsliaea fragrans, including a novel trimer (1) and two new dimers (2, 3). The chemical structures of the new compounds were elucidated through spectroscopic data interpretation and computational calculations. Ainsfragolide (1) is an unusual guaianolide sesquiterpene trimer generated with a novel C-C linkage at C_2'-C_15″, which may be biosynthesized prospectively through a further Michael addition. Cytotoxicity results showed that ainsfragolide (1) was the most potent compound against five cancer cell lines with IC₅₀ values in the range of 0.4-8.3 μM.

Medicinal Inula Species: Phytochemistry, Biosynthesis, and Bioactivities

As a genus of the Asteraceae, Inula is widely distributed all over the world, and several of them are being used in traditional medicines. A number of metabolites were isolated from Inula species, and some of these have shown to possess ranges of pharmacological activities. The genus Inula contains abundant sesquiterpenoids, such as eudesmanes, xanthanes, and sesquiterpenoid dimers and trimers. In addition, other types of terpenoids, flavonoids, and lignins also exist in the genus Inula. Since 2010, more than 300 new secondary metabolites, including several known natural products that were isolated for the first time from the genus Inula. Most of them exhibited potential bioactivities in various diseases. The review aimed to summarize the advance of recent researches (2010-2020) on phytochemical constituents, biosynthesis, and pharmacological properties of the genus Inula for providing a scientific basis and supporting its application and exploitation for new drug development.

Trimeric and dimeric sesquiterpenoids from Artemisia atrovirens and their cytotoxicities

Two sesquiterpenoid trimers and two sesquiterpenoid dimers possessing unprecedented scaffolds with a methylene-bridged connection were identified from Artemisia atrovirens. Compound 3 showed obvious cytotoxicity against HepG2 and Huh7 cell lines.