Naturally Occurring Cleistanthane Diterpenoids and Their Biological Activities

Cleistanthane diterpenoids, an important class of active natural products, are characterized by a 6/6/6 tricycle system and contain a variety of functional groups. Studies have shown that cleistanthane diterpenoids are widespread in various higher plants and endophytic fungi and exhibit a wide range of biological activities. This review provides extensive coverage of naturally occurring cleistanthane diterpenoids discovered from 1973 to 2024 and sheds light on the sources, structures, biological effects, and primary molecular mechanisms of them and provides a useful reference for the further study and development of these compounds.

Identification of promising dipeptidyl peptidase-4 and protein tyrosine phosphatase 1B inhibitors from selected terpenoids through molecular modeling

Motivation

Investigating novel drug-target interactions is crucial for expanding the chemical space of emerging therapeutic targets in human diseases. Herein, we explored the interactions of dipeptidyl peptidase-4 and protein tyrosine phosphatase 1B with selected terpenoids from African antidiabetic plants.

Results

Using molecular docking, molecular dynamics simulations, molecular mechanics with generalized Born and surface area solvation-free energy, and density functional theory analyses, the study revealed dipeptidyl peptidase-4 as a promising target. Cucurbitacin B, 6-oxoisoiguesterin, and 20-epi-isoiguesterinol were identified as potential dipeptidyl peptidase-4 inhibitors with strong binding affinities. These triterpenoids interacted with key catalytic and hydrophobic pockets of dipeptidyl peptidase-4, demonstrating structural stability and flexibility under dynamic conditions, as indicated by dynamics simulation parameters. The free energy analysis further supported the binding affinities in dynamic environments. Quantum mechanical calculations revealed favorable highest occupied molecular orbital and lowest unoccupied molecular orbital energy profiles, indicating the suitability of the hits as proton donors and acceptors, which likely enhance their molecular interactions with the targets. Moreover, the terpenoids showed desirable drug-like properties, suggesting their potential as safe and effective dipeptidyl peptidase-4 inhibitors. These findings may pave the way for the development of novel antidiabetic agents and nutraceuticals based on these promising in silico hits.

Availability and implementation

Not applicable.

Discovery of ent-kaurane diterpenoid glucosides as potent analgesics from the leaves of Pieris formosa

To search for structurally novel analgesics from Ericaceae plants, the leaves of Pieris formosa collected at Yichang, Hubei, China, were phytochemically investigated for the first time. A total of fifteen ent-kaurane diterpene glucosides (1-15) including twelve new ones, named forminosides A-L (1-12), were isolated. Their structures were elucidated by comprehensive spectroscopic data analyses, quantum chemical calculations (13C NMR and ECD calculations and DP4+ analysis), and chemical methods. The absolute configures of 1-3, 5-8, 11, and 13 were further determined by single-crystal X-ray diffraction analysis. Forminoside A (1) represents the first 3α-(β-d-glucopyranosyloxy)-11,16-epoxy-ent-kaurane diterpenoid bearing a unique 12-oxa-pentacyclo[9.3.3.01,10.04,9.013,16]heptadecane core. Forminoside J (10) is the first 17-nor-ent-kaurane type diterpenoid from Ericaceae family, while forminoside L (12) represents the first example of 4,5-seco-ent-kaurane diterpenoid glycoside bearing an unusual α-hydroxyl-α,β-unsaturated ketone block. Notably, the structure of mollisside A was revised to 3β-(β-d-glucopyranosyloxy)-16β,17-dihydroxy-ent-kaurane based on the NMR and single-crystal X-ray diffraction data analysis of forminoside C (3). All the isolates 1-15 showed potent analgesic activity in the HOAc-induced writhing test in mice. Among them, compounds 1-3, 5-12, and 15 exhibited significant analgesic effects at a dose of 5.0 mg/kg with the inhibition rates over 50%. Compounds 1, 5, 7, and 9-12 still displayed significant analgesic effects with the inhibition rates exceeding 50% at a lower dose of 1.0 mg/kg. Forminosides J (10) and L (12) still showed significant analgesic potency even at a lower dose of 0.2 mg/kg, comparable to that of the positive control, morphine. This is first report of the analgesic activity of 11,16-epoxy-ent-kaurane diterpenoid. A preliminary structure-activity relationship was explored, providing new clues to design novel analgesics based on the ent-kaurane and related diterpenoids.

Secondary metabolites with fungicide potentials from the deep-sea seamount-derived fungus Talaromyces scorteus AS-242

Marine natural products play an important role in biopesticides. Seven new secondary metabolites with different structural classes, including two cycloheptapeptides, scortide A (1) and scortide B (2), two 19-nor-diterpenoids, talascortene H (3) and talascortene I (4), two diterpenoid acids, talascortene J (5) and talascortene K (6), and one triterpenoid, talascortene L (7) were isolated and identified from the sea-anemone-derived endozoic fungus Talaromyces scorteus AS-242. Their structures were comprehensively assigned by spectroscopic data analysis, single-crystal X-ray diffraction, tandem mass spectrometry, and electronic circular dichroism (ECD) calculations. The result of the antimicrobial assay demonstrated that compounds 1 - 6 have inhibitory activity against several human, aquatic, and plant pathogens with minimum inhibitory concentration (MIC) values ranging from 1 to 64 μg/mL. Specially, compounds 2 and 4 showed significant activities against the pathogenic fungus Curvularia spicifera with the MIC value of 1 μg/mL, providing an experimental basis of 2 and 4 with the potential as lead compounds to be developed into biopesticides.

Structurally diverse (9β-H)-pimarane derivatives with six frameworks from the leaves of Icacina oliviformis and their cytotoxic activities

Thirteen previously undescribed (9β-H)-pimarane derivatives, icacinolides A-G (1-7) and oliviformislactones C-H (8-13), together with four known analogs (14-17), were isolated from the leaves of Icacina oliviformis. Their structures were constructed by extensive spectroscopic analysis, 13C NMR-DP4+ analysis, ECD calculation, single-crystal X-ray diffraction, and chemical methods. These structurally diverse isolates were classified into six framework types: rearranged 3-epi-17-nor-(9β-H)-pimarane, rearranged 17-nor-(9β-H)-pimarane, 16-nor-(9β-H)-pimarane, 17-nor-(9β-H)-pimarane, 17,19-di-nor-(9β-H)-pimarane, and (9β-H)-pimarane. Among them, compounds 1, 5, and 7 were the first examples of three rearranged 3-epi-17-nor-(9β-H)-pimaranes featuring a unique (11S)-carboxyl-9-oxatricyclo[5.3.1.02,7]dodecane motif with contiguous stereogenic centers, whereas their C-3 epimers, compounds 2-4 and 6 were the second examples of four rearranged 17-nor-(9β-H)-pimaranes. Additionally, compounds 8 and 12/13 represented the second examples of a 16-nor-(9β-H)-pimarane and two 17,19-di-nor-(9β-H)-pimaranes, respectively. In cytotoxic bioassay, compound 2 exhibited significant cytotoxic against HT-29 with IC50 values of 7.88 μM, even stronger than 5-fluorouracil, and 15 showed broad-spectrum cytotoxic activities against HepG2, HT-29, and MIA PaCa-2 with IC50 values of 11.62, 9.77, and 4.91 μM, respectively. Meanwhile, a preliminary structure-activity relationship suggested that 3,20-epoxy, 6,19-lactone, 2-OH, 7-OH, and 8-OH in (9β-H)-pimarane derivatives might be active groups, whereas ring C aromatization may decrease the cytotoxic activities.

Status of research on natural protein tyrosine phosphatase 1B inhibitors as potential antidiabetic agents: Update

Protein tyrosine phosphatase 1B (PTP1B) is a crucial therapeutic target for multiple human diseases comprising type 2 diabetes (T2DM) and obesity because it is a seminal part of a negative regulator in both insulin and leptin signaling pathways. PTP1B inhibitors increase insulin receptor sensitivity and have the ability to cure insulin resistance-related diseases. However, the few PTP1B inhibitors that entered the clinic (Ertiprotafib, ISIS-113715, Trodusquemine, and JTT-551) were discontinued due to side effects or low selectivity. Molecules with broad chemical diversity extracted from natural products have been reported to be potent PTP1B inhibitors with few side effects. This article summarizes the recent PTP1B inhibitors extracted from natural products, clarifying the current research progress, and providing new options for designing new and effective PTP1B inhibitors.

Callnudoids A-H: Highly modified labdane diterpenoids with anti-inflammation from the leaves of Callicarpa nudiflora

Eight undescribed 3,4-seco-norlabdane diterpenoids, callnudoids A-H, as well as two known analogues were isolated from the leaves of Callicarpa nudiflora. The structures were elucidated using spectroscopic methods and were compared with published NMR spectroscopic data. The absolute configurations of callnudoids D and E were defined based on ECD data or single-crystal X-ray diffraction. Callnudoids A-C are the highly modified labdane diterpenoids featuring rearranged 3,4-seco-ring and the formation of an undescribed cyclohexene moiety via C2-C18 cyclization. They only contain 15 carbon atoms on the carbon skeleton. Callnudoid D represents the unusual 3,4-seco-15,16-norlabdane diterpenoid with C13-C17 cyclization, and a putative biosynthesis pathway for callnudoids A, B, D, and E was proposed. All compounds were evaluated for their anti-inflammatory activities by inhibiting the lipopolysaccharide (LPS)-induced nitric oxide (NO) released in RAW264.7 cells; callnudoids A-E and H, and methylcallicarpate obviously inhibited pro-inflammatory cytokines TNF-α and IL-1β in a dose-dependent manner.

A Synthetic View on Momilactones and Related 9β-H Pimarane Skeleton Diterpenoids

Allelochemicals are secondary metabolites produced from plants and used to prevent and control the invasion of other plants and microorganisms, with broad application prospects in crop protection. Structurally, momilactones belong to 9β-H pimarane diterpenoids, one of rice’s significant allelochemicals with anti-weeds and antibacterial activity. Rare studies have been reported with the synthesis challenges of the unique 9β-H pimarane skeleton. Hence, synthetic strategies of momilactones and related 9β-H pimarane skeleton are reviewed from 1984 to 2021.

Potential of Diterpenes as Antidiabetic Agents: Evidence from Clinical and Pre-Clinical Studies

Diterpenes are a diverse group of structurally complex natural products with a wide spectrum of biological activities, including antidiabetic potential. In the last 25 years, numerous diterpenes have been investigated for antidiabetic activity, with some of them reaching the stage of clinical trials. However, these studies have not been comprehensively reviewed in any previous publication. Herein, we critically discussed the literature on the potential of diterpenes as antidiabetic agents, published from 1995 to September, 2021. In the period under review, 427 diterpenes were reported to have varying degrees of antidiabetic activity. Steviol glycosides, stevioside (1) and rebaudioside A (2), were the most investigated diterpenes with promising antidiabetic property using in vitro and in vivo models, as well as human subjects. All the tested pimaranes consistently showed good activity in preclinical evaluations against diabetes. Inhibitions of α-glucosidase and protein tyrosine phosphatase 1B (PTP 1B) activities and peroxisome proliferator-activated receptors gamma (PPAR-γ) agonistic property, were the most frequently used assays for studying the antidiabetic activity of diterpenes. The molecular mechanisms of action of the diterpenes include increased GLUT4 translocation, and activation of phosphoinositide 3-kinase (PI3K) and AMP-activated protein kinase (AMPK)-dependent signaling pathways. Our data revealed that diterpenes hold promising antidiabetic potential. Stevioside (1) and rebaudioside A (2) are the only diterpenes that were advanced to the clinical trial stage of the drug discovery pipeline. Diterpenes belonging to the abietane, labdane, pimarane and kaurane class have shown promising activity in in vitro and in vivo models of diabetes and should be further investigated.

Isolation, absolute configurations and bioactivities of pestaphilones A-I: Undescribed methylated side chain containing-azaphilones from Pestalotiopsis oxyanthi

Nine undescribed side chain containing azaphilones, pestaphilones A-I, were isolated from the Anoectochilus roxburghii endophytic fungus Pestalotiopsis oxyanthi. The structures of these isolates were identified by spectroscopic data, electronic circular dichroism (ECD) calculations and comparisons, quantum-chemical ¹³C NMR calculations with DP4+ probability analysis, Rh₂(OCOCF₃)₄-induced ECD, acetonide formation, selective oxidation reaction and X-ray crystallographic data. Structurally, pestaphilones A-I were the first azaphilones characteristically formed via a methyl group at C-9 in the C₇ side chain. More importantly, a selective oxidation reaction was firstly set up to resolve the absolute configuration of flexible side chain containing azaphilones, and an acetonide formation based Rh₂(OCOCF₃)₄-induced ECD experiment was performed to identify the configurations of the oxygenated pyranoquinone core in the azaphilones. In bioassay, pestaphilones A-F displayed potential immunosuppressive activity in concanavalin A (Con A)-induced T lymphocyte proliferation, with IC₅₀ values ranging from (9.36 ± 1.14) μM to (35.21 ± 3.25) μM.

Structurally Diverse Diterpenoids from the Roots of Salvia deserta Based on Nine Different Skeletal Types

Twenty-four diterpenoids (1-24), classified into nine diverse carbon skeletal types, 8-nor-7(8→14),9(8→7)-di-abeo-abietane (1, 2, and 13), 7(8→14),9(8→7)-di-abeo-abietane (3 and 4), 6-nor-6,7-seco-abietane (5 and 6), 6,7-seco-abietane (7 and 11), 9,10-seco-abietane (8), abietane (9, 10, and 14-21), 11(9→8),20(10→11)-di-abeo-abietane (12), 15(13→12)-abeo-abietane (22 and 23), and 4,5-seco-20(10→5)-abeo-abietane (24), respectively, were isolated from the roots of Salvia deserta. The structures of 10 new diterpenoids, named salviadesertins A-J (1-10), were elucidated by spectroscopic data interpretation, quantum-chemical calculations including calculated ¹³C NMR-DP4+ analysis and electronic circular dichroism as well as X-ray crystallography analysis. The absolute configurations of compounds 1-3, 7, 14, and 22 were defined by single-crystal X-ray diffraction analysis. All the isolated diterpenoids 1-24 were evaluated for their cytotoxicity against five cancer cell lines, and 6-hydroxysalvinolone (14) showed micromolar potencies against MCF-7, A-549, SMMC-7721, and HL-60 cells, whereas the other diterpenoids were inactive (half-maximal inhibitory concentration greater than 10.0 μM).

Gelstriamine A, a Triamino Monoterpene Indole Alkaloid with a Caged 6/5/7/6/6/5 Scaffold and Analgesic Alkaloids from Gelsemium elegans Stems

A novel triamino monoterpene indole alkaloid with an unprecedented skeleton, gelstriamine A (1), four new monoterpene indole alkaloids (2-5), and 12 known analogues (6-17) were isolated from Gelsemium elegans. The structures of 1-5 were established using extensive spectroscopic techniques, NMR calculations with iJ/dJ-DP4 and 2D C-H COSY ANNs analysis, ECD calculations, chemical methods, and single crystal X-ray diffraction analysis. Gelstriamine A (1) possesses an unprecedented 6/5/7/6/6/5 heterohexacyclic scaffold bearing a unique hexahydrooxazolo[4,5-b]pyridin-2(3H)-one motif, and a plausible biosynthetic pathway was proposed. All the isolated alkaloids 1-17 showed discernible analgesic activities in an acetic acid-induced writhing test in mice, and N-desmethoxyhumantenine N₄-oxide (3) exhibited more potent analgesic activities than those of morphine at doses of 0.04 and 0.2 mg/kg.

Pimarane-derived diterpenoids with anti-Helicobacter pylori activity from the tuber of Icacina trichantha

Two novel diterpenoids and ten known analogs were obtained from the tuber of Icacina trichantha. All compounds exhibited antibacterial activity against Helicobacter pylori strains with MIC values ranging from 8 to 64 μg mL⁻¹.

Novel Protein Tyrosine Phosphatase 1B Inhibitor-Geranylated Flavonoid from Mulberry Leaves Ameliorates Insulin Resistance

Mulberry leaf is a common vegetable with a variety of beneficial effects, such as hypoglycemic activity. However, the underlying mechanism of its hypoglycemic effect have not been fully revealed. In this study, two flavonoid derivatives were isolated from mulberry leaves, a new geranylated flavonoid compound (1) and its structural analogue (2). The structures of compounds 1 and 2 were elucidated using spectroscopic analysis. To study the potential hypoglycemic properties of these compounds, their regulatory effects on protein tyrosine phosphatase 1B (PTP1B) were investigated. In comparison to oleanolic acid, compounds 1 and 2 showed significant inhibitory activities (IC₅₀ = 4.53 ± 0.31 and 10.53 ± 1.76 μM) against PTP1B, the positive control (IC₅₀ = 7.94 ± 0.76 μM). Molecular docking predicted the binding sites of compound 1 to PTP1B. In insulin-resistance HepG2 cell, compound 1 promoted glucose consumption in a dose-dependent manner. Furthermore, western blot and polymerase chain reaction analyses indicated that compound 1 might regulate glucose consumption through the PTP1B/IRS/PI3K/AKT pathway. In conclusion, geranylated flavonoids in mulberry leaves inhibite PTP1B and increase the glucose consumption in insulin-resistant cells. These findings provide an important basis for the use of mulberry leaf flavonoids as a dietary supplement to regulate glucose metabolism.

Spirodesertols A and B, two highly modified spirocyclic diterpenoids with an unprecedented 6-isopropyl-3H-spiro[benzofuran-2,1′-cyclohexane] motif from Salvia deserta

Two highly modified spirocyclic diterpenoids with an unprecedented 6-isopropyl-3H-spiro[benzofuran-2,1′-cyclohexane] motif and four new icetexane diterpenoids were isolated from Salvia deserta. 1 showed more potent cytotoxicity than cis-platin.