Diterpenoids from Cephalotaxus fortunei var. alpina and their cytotoxic activity

Bioinspired Total Synthesis of Cephalotaxus Diterpenoids and Their Structural Analogues

Herein, we present a unified chemical synthesis of three subgroups of cephalotaxus diterpenoids. Key to the success lies in adopting a synthetic strategy that is inspired by biosynthesis but is opposite in nature. By employing selective one-carbon introduction and ring expansion operations, we have successfully converted cephalotane-type C18 dinorditerpenoids (using cephanolide B as a starting material) into troponoid-type C19 norditerpenoids and intact cephalotane-type C20 diterpenoids. This synthetic approach has enabled us to synthesize cephinoid H, 13-oxo-cephinoid H, 7-oxo-cephinoid H, fortalpinoid C, 7-epi-fortalpinoid C, cephanolide E, and 13-epi-cephanolide E. Furthermore, through the development of an intermolecular asymmetric Michael reaction between β-oxo esters and β-substituted enones, we have achieved the enantioselective synthesis of advanced intermediates within our synthetic sequence, thus formally realizing the asymmetric total synthesis of the cephalotaxus diterpenoids family.

Highly modified cephalotane-type diterpenoids from Cephalotaxus fortunei var. alpina and C. sinensis

Cephalotanes are a rare class of diterpenoids occurring exclusively in Cephalotaxus plants. The intriguing structures and promising biological activities for this unique compound class prompt us to investigate C. fortunei var. alpina and C. sinensis, leading to the isolation of six undescribed cephalotane-type diterpenoids and/or norditerpenoids, ceforloids A-F (1-6). Their structures were elucidated by comprehensive analysis of spectroscopic data, including ECD and single-crystal X-ray diffraction studies, as well as quantum chemical calculations. Compound 1 possesses an unprecedented norditerpenoid skeleton featuring an unusual acetophenone moiety, and originated putatively from a disparate biogenetic pathway. Compounds 4 and 5 incorporate a unique 12,13-p-hydroxybenzylidene acetal motif. Compound 6 is a rare cephalotane-type diterpenoid glycoside. Immunosuppressive assays showed that compounds 2 and 6 exhibited mild suppressive activity against the activated T and B lymphocytes proliferation. These findings not only expanded the structural diversity of this small group of diterpenoids, but also explored their potential as novel structures for the development of immunosuppressive agents.

Bioassay-guided isolation of anti-leukemic steroids from Aglaia abbreviata by inducing apoptosis

The strategy of bioactivity-guided isolation is widely used to obtain active compounds as quickly as possible. Thus, the inhibitory effects on human erythroleukemia cells (HEL) were applied to guide the isolation of the anti-leukemic compounds from Aglaia abbreviata. As a result, 19 compounds (16 steroids, two phenol derivatives, and a rare C12 chain nor-sesquiterpenoid), including 13 new compounds, were isolated and identified based on spectroscopic data analysis, single-crystal X-ray diffraction data, and electronic circular dichroism (ECD) calculations. Among them, 9 steroids exhibited good selective anti-leukemic activity against HEL and K562 (human chronic myeloid leukemia cells) cells with IC50 values between 2.29 ± 0.18 μM and 19.58 ± 0.13 μM. Notably, all the active compounds had relatively lower toxicity on the normal human liver cell line (HL-7702). Furthermore, five compounds (1, 4, 8, 10, and 19) displayed good anti-inflammatory effects, with IC50 values between 7.15 ± 0.16 and 27.1 ± 0.37 μM. An α,β-unsaturated ketone or a 5,6Δ double bond was crucial for improving anti-leukemic effect from the structure-activity relationship analysis. The compound with the most potential, 14 was selected for the preliminary mechanistic study. Compound 14 can induce apoptosis and cause cell cycle arrest. The expression of the marker proteins, such as PARP and caspase 3, were notably effected by this compound, thus inducing apoptosis. In conclusion, our investigation implied that compound 14 may serve as a potential anti-leukemia agent.

Enantioselective Total Synthesis of (-)-Cephalotanin B

Cephalotaxus diterpenoids are attractive natural products with intriguing molecular frameworks and promising biological features. As a structurally unusual member, (-)-cephalotanin B possesses an extraordinarily congested heptacyclic skeleton, three lactone units, and nine consecutive stereocenters. Herein, we report an enantioselective total synthesis of (-)-cephalotanin B based on a divergent asymmetric Michael addition reaction, a novel Pauson-Khand/deacyloxylation process discovered in the development of a second-generation stereoselective Pauson-Khand reaction protocol, and an epoxide-opening/elimination/dual-lactonization cascade to construct the challenging propeller-shaped A-B-C ring system as key transformations.

Asymmetric Total Synthesis of Cephalotaxus Diterpenoids: Cephinoid P, Cephafortoid A, 14-epi-Cephafortoid A and Fortalpinoids M-N, P

The asymmetric total syntheses of cephalotaxus C19 diterpenoids, bearing a unique cycloheptene A ring with a chiral methyl group at C-12, were disclosed based on a universal strategy. Six members, including cephinoid P, cephafortoid A, 14-epi-cephafortoid A and fortalpinoids M-N, P, were accomplished for the first time. The concise approach relies on two crucial steps: (1) a Nicholas/Hosomi-Sakurai cascade reaction was developed to efficiently generate the cycloheptene ring bearing a chiral methyl group; (2) an intramolecular Pauson-Khand reaction was followed to facilitate the construction of the complete skeleton of target molecules. Our studies provide a new strategy for the synthetic analysis of cephalotaxus diterpenoids and structurally related polycyclic natural products.

Total Synthesis of (-)-Ceforalide B and (-)-Cephanolides B-D

(-)-Ceforalide B (1) and (-)-cephanolides B-D (2-4) are benzenoid cephanolide diterpenoids possessing the same pentacyclic skeleton, which contains three C13-C15 substituent patterns and different benzylic oxidation states. An olefination/6π-electrocyclization/oxidative aromatization cascade has been verified as divergent access to three C13-C15 patterns. The benzylic aerobic oxidations enabled by the Co(OAc)2·4H2O/bromide salt/O2/PPh3/N-hydroxyphthalimide system have been developed to deliver expected site-selectivity and different oxidation states. Through the divergent strategy, total synthesis of (-)-ceforalide B and (-)-cephanolides B-D is accomplished.

Hainanolide inhibits the progression of colon cancer via inducing the cell cycle arrest, cell apoptosis and activation of the MAPK signaling pathway

Hainanolide (HN) is a norditerpenoid metabolite extract from Cephalotaxus fortunei Hook. f. C. fortunei Hook. f. is renowned for the active alkaloids, such as harringtonine (HT) and homoharringtonin (HTT), which have been clinically used to treat chronic myeloid leukemia. Nowadays, diterpenoids, another important metabolite, attracted the attention of chemists. Among them, Hainanolide (HN), a cephalotane-type diterpenoid, has been proven to possess potent antitumor activities. However, the underlying therapeutic mechanisms of HN in anti-tumor have not been investigated yet. Our present study demonstrated that HN inhibited HCT-116 and HCT-15 cell proliferation in a dose- and time-dependent manner. Further studies demonstrated that HN can induce G2/M phase arrest and alter the Cdc25C/Cdc2/CyclinB1 proteins. Western blot indicated that HN promoted apoptosis by up-regulating Bax and down-regulated Bcl-2. And the caspase-3 and caspase-9 activities of HCT-116 and HCT-15 cells were increased. Transcriptome analysis is used to reveal the possible mechanism. Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses suggested the genes were mainly enriched in the MAPK signaling pathway. Certainly, HN activates MAPK signaling pathway. In vivo, HN prevented the AOM/DSS-induced tumorigenesis of colon cancer in C57BL/6 mice. Our study indicated that HN inhibits the progression of colon cancer cells by blocking the cell cycle, inducing apoptosis, and activating the MAPK pathway. This study provides a theoretical and experimental scientific basis for future investigations of the antitumor effects of HN against colon cancer.

Cephalotaxine-type and homoerythrina-type alkaloids with antiproliferative effects from Cephalotaxus fortunei

Twenty-two cephalotaxine-type and ten homoerythrina-type alkaloids, including seven previously undescribed ones, were isolated from the twigs and leaves and the seed kernels of Cephalotaxus fortunei. Their structures were established by spectroscopic analysis, single crystal X-ray diffraction, and ECD calculation methods. Cephalofortunine A β-N-oxide (1) is the first nitrogen-oxidized homoerythrina-type alkaloid. The isolated compounds were evaluated for their in vitro antiproliferative effects against two human leukemia cell lines (THP-1 and K562). All compounds showed different levels of antiproliferation in THP-1 and K562 cells with GI₅₀ values of 0.24-29.55 μM. Hainanensine (31) was the most active against two cancer cell lines with GI₅₀ values of 0.24 ± 0.07, and 0.29 ± 0.01 μM, respectively.

Asymmetric Total Syntheses of Cephalotane-Type Diterpenoids Cephanolides A-D

Cephanolides A-D are cephalotane-type diterpenoids featuring a novel 6/6/6/5 tetracyclic core embedded with a bridged δ-lactone. The asymmetric and divergent total syntheses of cephanolides A-D have been accomplished, proceeding in 11-14 steps from a known alcohol. The salient features of the present work include (i) a substrate-controlled diastereoselective intermolecular Diels-Alder reaction to form the 6-6 cis-fused rings, (ii) a palladium-catalyzed formal bimolecular [2 + 2 + 2] cycloaddition reaction via a partially intermolecular cascade reaction sequence involving multiple carbometalations to rapidly install the key tetracyclic skeleton, and (iii) lactonization and late-stage oxidative diversification to complete total syntheses of the four benzenoid cephanolides.

Photoaffinity Probe Reveals the Potential Target of Harringtonolide for Cancer Cell Migration Inhibition

Harringtonolide (HO, 1) is a bioactive diterpenoid tropone isolated from Cephalotaxus harringtonia with antiproliferation activity. Until now there have been no reports to elucidate its anticancer mechanism. Herein we report the synthesis of HO-derived probes (10, 11, and 12) to identify the possible target of HO. As a result, the application of a novel photoaffinity alkyne-tagged probe from HO (compound 12) showed direct engagement between HO and receptor for activated C kinase 1 (RACK1). Furthermore, HO could suppress the epithelial-mesenchymal transition (EMT) process and inhibit activation of the FAK/Src/STAT3 signaling pathway in A375 cells. This study provides a groundwork for HO as an effective antitumor agent that targets RACK1 to suppress cancer cell migration.

A nickel(ii)-catalyzed enantioselective all-carbon-based inverse-electron-demand Diels–Alder reaction of 2-pyrones with indenes

An asymmetric IEDDA reaction of 2-pyrones with indenes catalyzed by a chiral N,N′-dioxide/Ni(OTf)2 complex has been disclosed to construct highly functionalized hexahydrofluorenyl bridged-lactone scaffolds in high yields and ee.

Progress in structure, synthesis and biological activity of natural cephalotane diterpenoids

The Cephalotaxus genus is well-known owing to the numerous complex, biologically relevant natural products that can be obtained from its constituent species. The successful identification of various Cephalotaxus alkaloids and natural, structurally diverse cephalotane diterpenoids that exhibit antitumor activities and excellent pharmacological properties has encouraged the discovery of previously undescribed compounds from this genus. The present review summarizes the different strategies for the total synthesis of cephalotane diterpenoids as well as their diverse chemical structures, antitumor activities, structure-activity relationships (SARs), and biosynthetic pathways.

Catalytic Asymmetric Inverse-Electron-Demand Diels-Alder Reactions of 2-Pyrones with Indenes: Total Syntheses of Cephanolides A and B

An inverse-electron-demand Diels-Alder (IEDDA) reaction could complement the conventional normal-electron-demand Diels-Alder reaction in the synthesis of six-membered carbocycles. However, catalytic asymmetric all-carbon-based IEDDA reactions are underdeveloped. Herein, we disclosed a copper-catalyzed asymmetric IEDDA reaction using electron-deficient 3-carboalkoxyl-2-pyrones and electronically unbiased indenes as reactants. This method enables the rapid and enantioselective construction of a wide range of hexahydrofluorenyl bridged-lactone scaffolds. Using this method, asymmetric total syntheses of cephanolides A and B were accomplished.

Total Synthesis of (+)-3-Deoxyfortalpinoid F, (+)-Fortalpinoid A, and (+)-Cephinoid H

3-Deoxyfortalpinoid F, fortalpinoid A, and cephinoid H are members of the Cephalotaxus diterpenoids class of natural products, which feature diverse chemical structures and valuable biological activities. We report herein the development of a diastereoselective Pauson-Khand reaction as an effective pathway to access the core tetracyclic skeleton, which is found widely in Cephalotaxus diterpenoids. Furthermore, we enabled the construction of the tropone moiety through a ring-closing metathesis/elimination protocol. Based on the developed strategy, asymmetric synthesis of the title compounds has been achieved for the first time.

Glaucocalyxin A induces cell cycle arrest and apoptosis via inhibiting NF-κB/p65 signaling pathway in melanoma cells

AIMS

Melanoma is a malignant tumor of the skin with a high metastasis rate and poor prognosis. Glaucocalyxin A (GLA), isolated from Rabdosia japonica, is a diterpenoid compound with anticancer properties. Here, we investigated the anticancer properties and explored the mechanisms underlying GLA activity in melanoma cells in vitro and in vivo.

MAIN METHODS

Cell Counting Kit-8 and colony formation assays were used to assess the effects of GLA on cell proliferation. Flow cytometry was used to evaluate the cell cycle, apoptosis, mitochondrial membrane potential (MMP), and reactive oxygen species (ROS), and western blot analysis and immunofluorescence staining were used to examine protein expression. Immunohistochemical analysis was performed to examine animal tissues and tumors in mice.

KEY FINDINGS

GLA could effectively inhibit cell proliferation and induce cell apoptosis. GLA induced an overproduction of cellular ROS, decreased MMP, and upregulated the Bax/Bcl-2 ratio, which is an indicator of apoptosis. Phosphorylation of nuclear factor κB (NF-κB)/p65 and NF-κB/p65 nuclear expression decreased after GLA treatment in vitro and in vivo, suggesting that the anticancer effects of GLA are mediated through the NF-κB/p65 pathway. Moreover, we observed that GLA was effective in inhibiting tumor growth without obvious toxicity to major organs in mice.

SIGNIFICANCE

This is the first study to show that GLA inhibits cell proliferation, arrests the cell cycle in the G2/M phase, and induces mitochondrial apoptosis via the NF-κB/p65 pathway in melanoma cells. Overall, our results demonstrate that GLA may be a potential anticancer agent for the treatment of melanoma.

Cephalodiones A-D: Compound Characterization and Semisynthesis by [6+6] Cycloaddition

Cephalodiones A-D (1-4), the first example of C₁₉ -norditerpenoid dimers, were isolated and fully characterized from a Cephalotaxus plant. These new skeletal natural products shared a unique tricyclo[6.4.1.1^2,7 ]tetradeca-3,5,9,11-tetraene-13,14-dione core that was capped in both ends with rigid multicyclic ring systems either C₂ -symmetrically or asymmetrically. Compounds 1-4 were proposed to be biosynthetically produced by the [6+6]-cycloaddition of two identical C₁₉ -norditerpenoid troponoids, which was validated by the semisyntheses of dimers 2-4. Moreover, some compounds showed significant inhibition on Th17 cell differentiation.

Semi-Synthesis of Harringtonolide Derivatives and Their Antiproliferative Activity

Harringtonolide (HO), a natural product isolated from Cephalotaxus harringtonia, exhibits potent antiproliferative activity. However, little information has been reported on the systematic structure-activity relationship (SAR) of HO derivatives. Modifications on tropone, lactone, and allyl positions of HO (1) were carried out to provide 17 derivatives (2-13, 11a-11f). The in vitro antiproliferative activity against four cancer cell lines (HCT-116, A375, A549, and Huh-7) and one normal cell line (L-02) was tested. Amongst these novel derivatives, compound 6 exhibited comparable cell growth inhibitory activity to HO and displayed better selectivity index (SI = 56.5) between Huh-7 and L-02 cells. The SAR results revealed that the tropone and lactone moieties are essential for the cytotoxic activities, which provided useful suggestions for further structural optimization of HO.