The dihydro-β-agarofuran sesquiterpenoids

Kandenols A-E, eudesmenes from an endophytic Streptomyces sp. of the mangrove tree Kandelia candel

Five novel eudesmene-type sesquiterpenes, kandenols A-E (1-5), have been isolated from Streptomyces sp. HKI0595 derived from the mangrove plant Kandelia candel. Their structures were established through NMR and mass spectrometry, and absolute configurations were established by the Mosher method and comparison of CD spectra with α-rotunol and β-rotunol. The kandenols are reminiscent of various plant-derived eudesmenes, yet kandenols B and C are unusual because of their hydroperoxide moieties. Kandenol E is the first bacterial agarofuran, which belongs to an important group of antibiotics. Whereas the kandenols display no cytotoxicity against 12 human cell lines, weak to moderate antimicrobial activities were detected against Bacillus subtilis ATCC 6633 and Mycobacterium vaccae IMET 10670.

The triterpenoid pristimerin induces U87 glioma cell apoptosis through reactive oxygen species-mediated mitochondrial dysfunction

It has become evident that some of the natural or synthetic triterpenoids are natural proteasome inhibitors that have great potential for use in cancer prevention and treatment. However, the mechanisms for the antitumor activity of triterpenoids remain to be elucidated. In the present study, we investigated the anticancer activities of a natural triterpenoid, pristimerin, and the signaling pathways affected. Pristimerin was found to possess potent cytotoxic effects, inducing apoptosis and inhibiting proliferation in U87 human glioma cells. Hoechst 33258 staining and Annexin V/PI double staining exhibited the typical nuclear features of apoptosis and increased the proportion of apoptotic Annexin V-positive cells in a dose-dependent manner, respectively. Moreover, western blotting assay revealed that this apoptotic induction was associated with activated caspase-9, caspase-3, PARP cleavage and downregulation of Bcl-xl/Bax in a concentration-dependent manner. Pristimerin also increased the generation of reactive oxygen species and induced the subsequent release of cytochrome c from the mitochondria into the cytosol. Additionally, pristimerin downregulated EGFR protein expression and inhibited downstream signaling pathways in U87 cells. Our results suggest that pristimerin may have potential as a new targeting therapeutic strategy in the treatment of EGFR-overexpressing gliomas.

Antiprotozoal sesquiterpene pyridine alkaloids from Maytenus ilicifolia

As part of a bioprospecting program aimed at the discovery of antiprotozoal agents from the Brazilian flora, two new sesquiterpene pyridine alkaloids, ilicifoliunines A (1) and B (2), along with the known alkaloids aquifoliunine E-I (3) and mayteine (4), were isolated from the root bark of Maytenus ilicifolia. The structures of 1 and 2 were established on the basis of spectroscopic data interpretation. Alkaloid 3 displayed potent in vitro antiprotozoal activity against Leishmania chagasi and Trypanosoma cruzi, with IC(50) values of 1.4 and 41.9 μM, respectively, as well as low cytotoxicity against murine peritoneal macrophages (IC(50) of 1.8 mM).

Simultaneous identification of multiple celangulins from the root bark of Celastrus angulatus using high-performance liquid chromatography-diode array detector-electrospray ionisation-tandem mass spectrometry

INTRODUCTION

Celangulins are a small family of β-dihydroagarofuran sesquiterpenoids endowed with diverse polyoxygenated polyol esters and various biological properties. Since our research focuses on celangulins, the development of rapid and sensitive online analytical methods to analyse and characterise them is of great significance.

OBJECTIVE

To develop an HPLC-DAD-ESI-MS/MS method capable of simple and rapid analysis of celangulins in crude extract of root bark of C. angulatus extracts.

METHODOLOGY

High-performance liquid chromatography coupled with a diode array detector and electrospray ionisation tandem mass spectrometry was established for the efficient and rapid identification of the celangulins. Chromatographic separations of celangulins were performed on a Hypersil Gold C(18) reverse-phase column by gradient elution with acetonitrile-water as mobile phase at a flow-rate 0.2 mL/min.

RESULTS

ESI/MS/MS analysis of sodium adduct ion ([M + Na](+) ) of each celangulin shows that all the celangulins produced very similar fragmentation profiles, and that the characteristic fragments at m/z 245, m/z 229 and m/z 231 were defined as the diagnostic ions for celangulins. Simultaneously, 46 components in the extracts of this plant were separated, and 36 of them were characterised as celangulins by online ESI/MS/MS and by comparing their retention times, UV and MS spectra with those of authentic compounds.

CONCLUSION

HPLC-DAD-ESI-MS/MS was demonstrated to be a powerful tool for the characterisation of minor celangulins in complex samples.

Synthesis and insecticidal activities of new ester-derivatives of Celangulin-V

In order to develop new biorational pesticides, ten new 6-substituted ester derivatives of Celangulin-V were designed and synthesized. The structures of the new derivatives were confirmed by IR, (1)H-NMR, (13)C-NMR and ESI-MS spectral analysis. Insecticidal activities of these compounds were tested against the third-instar larvae of Mythimna separata. Two derivatives (1.1, 1.2) showed higher insecticidal activities than Celangulin-V, with mortality of 75.0% and 83.3%, respectively. While four compounds (1.3, 1.4, 1.7, 1.8) denoted lower insecticidal activities, the others (1.5, 1.6, 1.9, 1.10) revealed no activities at a concentration of 10 mg.mL(-1). The results suggest that C-6 substitutions of Celangulin-V are very important in determining the insecticidal activities of its ester-derivatives. That the acetyl (1.1) and propionyl (1.2) derivatives possessed much higher insecticidal activities than Celangulin-V itself supported the view that Celangulin-V has the potential to be a lead structure of semi-synthetic green insecticides.

4α,6α-Dihy-droxy-1β-methyl-sulfonyl-8α,9α-ep-oxy-2β,12-epoxymethano-β-dihydro-agarofuran

The title mol­ecule, C₁₆H₂₄O₈S, is a dihydro­agrofuran derivative and has a heteropolycyclic structure. One cyclohexane ring exhibits a chair conformation and the other a non-chair conformation. In the crystal structure there is an inter­molecular C—H⋯O hydrogen-bonding inter­action to stabilize the packing.

Ultrastructural effects of Celangulin V on midgut cells of the oriental armyworm, Mythimna separata walker (Lepidoptera: Noctuidae)

The ultrastructural effects of ingested Celangulin V (CA-V), an insecticidal component extracted from the root bark of Chinese bittersweet (Celastrus angulatus Maxim), on the midgut epithelial cells of the oriental armyworm larva (Mythimna separata Walker) were studied. Transmission electron microscopy showed that CA-V could induce a severe, time-dependent cytotoxicity of the midgut epithelial cells, which had large cytoplasmic spaces, disrupted microvilli and swollen mitochondria. The cisternae of the rough endoplasmic reticulum were excessively dilated, vesiculated and then fragmented. The nuclei were pycnotic and contained one or two divided nucleoli and agglomerated chromatin. Numerous lysosome-like vacuoles and secretion granules were observed. Finally, the CA-V resulted in cell death by necrosis with plasma membrane lysis and the cytoplasm content leakage into the migdut lumen.

Dihydro-β-agarofuran sesquiterpenes isolated from Celastrus vulcanicola as potential anti-Mycobacterium tuberculosis multidrug-resistant agents

In the present study, we report four new dihydro-β-agarofuran sesquiterpenes (1-4), which were isolated from the leaves of Celastrus vulcanicola, in addition to five derivatives (5-9). Their stereostructures were elucidated on the basis of spectroscopic analysis, including 1D and 2D NMR techniques, X-ray studies, chemical correlations and biogenetic means. Compounds 1-9 and the previously reported sesquiterpenes 10-25 have been tested as potential antimycobacterial agents against sensitive and resistant Mycobacterium tuberculosis strains. 1α-Acetoxy-6β,9β-dibenzoyloxy-dihydro-β-agarofuran (20) exhibited antituberculosis activity against the MDR TB strain with a MIC value of 6.2 μg/mL, comparable to or better than isoniazid or rifampin, two of the best first-line drugs commonly used in the treatment of TB. The structure-activity relationship is discussed.

Cytotoxic triterpenoids from Maytenus retusa

Seven new triterpenoids (1-7) and 36 known compounds were isolated from the root bark of Maytenus retusa. Their structures were determined by 1D and 2D spectroscopic studies. Several compounds were evaluated for their cytotoxicity against the human tumor cell lines HL-60 and MCF-7. Some of them were cytotoxic, with IC(50) values ranging between 0.2 and 4.7 μM.

Dihydro-beta-agarofuran sesquiterpenes and pentacyclic triterpenoids from the root bark of Osyris lanceolata

Three new dihydro-beta-agarofuran polyesters, 1alpha,9beta-difuranoyloxy-2-oxodihydro-beta-agarofuran (1), 1alpha,9beta-difuranoyloxy-2-oxo-3-enedihydro-beta-agarofuran (2), and 1alpha,9beta-difuranoyloxydihydro-beta-agarofuran (3), have been isolated from the CHCl(3) extract of the root bark of Osyris lanceolata, together with two known pentacylic triterpenoids, 4 and 5. Compounds 1-5 did not scavenge the DPPH radical within 30 min of reaction time. All five compounds displayed antifungal activity against Candida albicans. Compounds 1, 3, 4, and 5 showed antibacterial activity against the Gram-positive Bacillus subtilis and Staphylococcus aureus and Gram-negative Escherichia coli and Pseudomonas aeruginosa, with 4 and 5 being the most active. Compound 2 displayed weak antibacterial activity only against Escherichia coli.

Isolation, biological evaluation and 3D-QSAR studies of insecticidal/narcotic sesquiterpene polyol esters

For the first time, a set of (43) natural sesquiterpene polyol esters isolated from the root bark of Celastrus angulatus Maxim and Euonymus japonicus Thunb were subjected to 3D-QSAR comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) studies, with the aim of proposing novel sesquiterpene-based compounds with optimal narcotic or insecticidal activities. The established 3D-QSAR models exhibit reasonable statistical quality and prediction capabilities, with internal cross-validated Q (2) values of ∼ 0.5 and external predicted R (2) values of >0.9, respectively. The relative contributions of the steric/electrostatic fields of the 3D-QSAR models show that the electronic effect governs the narcotic activities of the molecules, but the hybrid effect of the electrostatic and hydrophobic interactions is more influential in the insecticidal activities of the compounds. These findings may have valuable implications for the development of novel natural insecticides.

Synthesis and Insecticidal Activities of New Ether-Derivatives of Celangulin-V

In order to find new biorational pesticides, eight new 6-substituted ether derivatives of Celangulin-V were synthesized. The structures were confirmed by IR, 1H NMR and 13C NMR spectroscopic and HRMS analyses. The insecticidal activities of these compounds were tested against third-instar larvae of Mythimna separata. Four derivatives (b, c, f, g) showed higher insecticidal activities than Celangulin-V, with KD50 values of 135.9, 101.33, 169.0 and 219.2 μg.g-1, respectively. However, two compounds (a, d) showed lower insecticidal activities and two (e, h) no activity. The results suggest that C-6 substitutions in these compounds are very important for their insecticidal activities. The insecticidal activities of the derivatives with two or three carbon substitutions at C-6 are higher than that of Celangulin-V showing that this compound has the potential to be a lead structure for semi-synthetic, green insecticides.

Three New Insecticidal Sesquiterpene Polyol Esters from Celastrus angulatus

Three new insecticidal sesquiterpene polyol esters with dihydro-β-agarofuran skeletons, Kupiteng esters A (1), B (2) and C (3), were isolated from the methanol extracts of the root bark of Celastrus angulatus and their structures elucidated by analyses of MS and NMR spectral data. The insecticidal activities of these compounds against the 4th instar larvae of Mythimna separata were evaluated. The KD50 values of compounds 1-3, were 260.2, 445.5 and 1260.0 μg-g−1, respectively.

Sesquiterpenes from Maytenus jelskii as potential cancer chemopreventive agents

Seven new (1-4 and 7-9) sesquiterpenes with a dihydro-beta-agarofuran skeleton, along with four known compounds (5, 6, 10, and 11), have been isolated from the leaves of Maytenus jelskii. The structures of the new compounds were elucidated by means of spectroscopic data analysis, including 1D and 2D NMR techniques, and their absolute configurations were determined by circular dichroism and chemical correlations. The compounds have been tested for their inhibitory effects on Epstein-Barr virus early antigen (EBV-EA) activation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA). Compound 10 was found to be an effective antitumor-promoting agent and also showed a potent chemopreventive effect in an in vivo two-stage carcinogenesis model.

Facile synthesis of hydroxy wilfordic acid, a esterifying unit of anti-HIV sesquiterpene pyridine alkaloids

Sesquiterpene pyridine alkaloids were isolated mainly from plants of the genus Tripterygium (Celastraceae) which have been used traditionally in Chinese medicine. These compounds have polyhydroxy dihydro-beta-agarofuran core and esterifying substituent with dilactone bridges, and recently demonstrated promising anti-HIV activity. We have achieved the synthesis of hydroxy wilfordic acid and its ester via asymmetric cyanosilylation. With a NMR study of (S)- and (R)-PGME (phenylglycine methyl ester) amide, the tertiary alcohol stereochemistry of synthetic hydroxyl wilfordic acids was determined. Our synthetic approach will provide a contribution to the synthesis of sesquiterpene pyridine alkaloids and the development of their analogs for anti-HIV activity.

Reversing agents for ATP-binding cassette drug transporters

The multidrug resistance (MDR) phenotype exhibited by cancer cells is believed to be the major barriers to successful chemotherapy in cancer patients. The major form of MDR phenotype is contributed by a group of ATP-binding cassette (ABC) drug transporters which include P-glycoprotein, multidrug resistance-associated protein 1, and breast cancer resistance protein. There has been intense search for compounds which can act to reverse MDR phenotype in cultured cells, in animal models, and ultimately in patients. The ongoing search for MDR modulators, compounds that act directly on the ABC transporter proteins to block their activity, has led to three generations of drugs. Some of the third-generation MDR modulators have demonstrated encouraging results compared to earlier generation MDR modulators in clinical trials. These modulators are less toxic and they do not affect the pharmacokinetics of anti-cancer drugs. Significant numbers of natural products have also been identified for their effectiveness in reversing MDR in a manner similar to the MDR modulators. Other MDR reversing strategies that have been studied quite extensively are also reviewed and discussed in this chapter. These include strategies aimed at destroying mRNAs for ABC drug transporters, approaches in inhibiting transcription of ABC transporter genes, and blocking of ABC transporter activity using antibodies. This review summarizes the development of reversing agents for ABC drug transporters up to the end of 2008, and provides an optimistic view of what we have achieved and where we could go from here.

Reactive oxygen species-dependent activation of Bax and poly(ADP-ribose) polymerase-1 is required for mitochondrial cell death induced by triterpenoid pristimerin in human cervical cancer cells

Naturally occurring triterpenoid compounds have long been used as anti-inflammatory, antimalarial, and insecticidal agents. It has become evident that some of the natural or synthetic triterpenoids have promising clinical potential as both a therapeutic and chemopreventive agent for cancer. However, the molecular basis for the antitumor activity of triterpenoid has yet to be defined. In this study, we show that pristimerin, a natural triterpenoid, induces mitochondrial cell death in human cervical cancer cells and that reactive oxygen species (ROS)-dependent activation of both Bax and poly(ADP-ribose) polymerase-1 (PARP-1) is critically required for the mitochondrial dysfunction. We also showed that c-Jun N-terminal kinase (JNK) is involved in ROS-dependent Bax activation. Treatment of pristimerin induced an increase in intracellular ROS, JNK activation, conformational change, and mitochondrial redistribution of Bax, mitochondrial membrane potential loss, and cell death. The PARP-1 was also found to be activated by pristimerin treatment. An antioxidant, N-acetyl-l-cysteine (NAC), inhibited pristimerin-induced JNK activation, Bax relocalization, and PARP-1 activation, as well as mitochondrial cell death. Moreover, inhibition of JNK clearly suppressed conformational change and mitochondrial translocation of Bax and subsequent mitochondrial cell death but did not affect PARP-1 activation. Inhibition of PARP-1 with 1,5-dihydroxyisoquinoline (DIQ) or with small interfering RNA of PARP-1 significantly attenuated pristimerin-induced mitochondrial membrane potential loss and cell death but did not affect JNK activation and Bax relocalization. These results indicate that the natural triterpenoid pristimerin induces mitochondrial cell death through ROS-dependent activation of both Bax and PARP-1 in human cervical cancer cells and that JNK is involved in ROS-dependent Bax activation.

Ribosomal peptide natural products: bridging the ribosomal and nonribosomal worlds

Ribosomally synthesized bacterial natural products rival the nonribosomal peptides in their structural and functional diversity. The last decade has seen substantial progress in the identification and characterization of biosynthetic pathways leading to ribosomal peptide natural products with new and unusual structural motifs. In some of these cases, the motifs are similar to those found in nonribosomal peptides, and many are constructed by convergent or even paralogous enzymes. Here, we summarize the major structural and biosynthetic categories of ribosomally synthesized bacterial natural products and, where applicable, compare them to their homologs from nonribosomal biosynthesis.

A new insecticidal sesquiterpene ester from Celastrus angulatus

A new sesquiterpene polyol ester with a β-dihydroagarofuran skeleton, NW37 (1), and three known compounds NW13 (2), NW16 (3) and NW35 (4) were isolated by bioassay-guided fractionation from the highly polar MeOH extracts of the root bark of Celastrus angulatus. Their chemical structures were elucidated mainly by analyses of MS and NMR spectral data. The insecticidal activity of compound 1 against 4^th instar Mythimna separata larvae with a KD₅₀ value of 252.3 μg·g^-1 was demonstrated.

Natural sesquiterpenoids

This review covers the isolation, structural determination, synthesis and chemical and microbiological transformations of natural sesquiterpenoids. 423 references are cited.