Marsupellins A-F, ent-longipinane-type sesquiterpenoids from the Chinese liverwort Marsupella alpine with acetylcholinesterase inhibitory activity

Total Syntheses of (±)-Acetoxymarsupellone and (±)-Marsupellins A, B, and D Enabled by a Reductive Cyclization and Semipinacol Rearrangement Strategy

We report herein the total syntheses of three marsupellin-family sesquiterpenoids, (±)-acetoxymarsupellone and (±)-marsupellins B and D, in 14-19 steps from our known precursor, making marsupellin A also accessible from marsupellin B through a known procedure. The critical tricyclic framework bearing the challenging C7 bridgehead all-carbon quaternary center is strategically constructed through a Ti-mediated reductive cyclization and semipinacol rearrangement sequence. This study provides a general approach to the syntheses of (ent-)longipinane-type molecules.

Prenylated bibenzyls from the Chinese liverwort Radula apiculata

Three new prenylated bibenzyls (1-3) and seven known congeners were purified from the Chinese liverwort Radula apiculata. Their structures were identified by the analysis of spectroscopic data and comparison of reported NMR data. All isolated compounds were tested for several human cancer cell lines with adriamycin served as a positive control.

Correlations between the ECD spectra and absolute configuration of bridged-ring lactones: revisiting Beecham's rule

Bridged lactones frequently appear as structural fragments in natural products. To elucidate their stereochemistry using electronic circular dichroism (ECD) spectra, Beecham correlated the sign of the Cotton effect (CE) from the n → π* transition of lactones at approximately 220 nm with the skeleton of bridged lactones. By combining experimental and theoretical ECD analyses of various bridged lactones using time-dependent density functional theory calculations and a methodology for extracting core structures, Beecham's rule was revisited and revised to define the scope of application. Both the position of the β-C atom in the larger lactone system and the additive contribution of groups at β-C exerted effects on the sign of the CE. The revised rule provides an alternative way to interpret experimental ECD data in addition to quantum-chemical calculation for various bridged lactones.

Acetylcholinesterase Inhibitory Potential of Various Sesquiterpene Analogues for Alzheimer's Disease Therapy

Alzheimer's disease (AD) is a gradually growing irreversible illness of the brain that almost affects every fifth person (aged > 80 years) in the world. World Health Organization (WHO) also revealed that the prevalence of this disease will enhance (upto double) significantly upto 2030. The poor cholinergic transmission at the synapse is considered to be one of the main reasons behind the progression and occurrence of this disorder. Natural inhibitors of acetylcholine (ACh) such as galanthamine and rivastigmine are used commercially in the treatmentof AD. The biomolecules such assesquiterpenes, possess a great structural diversity and are responsible for a plethora of pharmacological properties. The potential of various sesquiterpenes as anticholinesterase has been reviewed in this article. For this purpose, the various databases, mainly PubMed, Scopus, and Web of Science were investigatedwith different keywords such as "sesquiterpenes+acetylcholinesterase" and "sesquiterpenes+cholinesterase+inhibitors" in the surveyed time frame (2010-2020). A vast literature was evident in the last decade, which affirms the potential of various sesquiterpenes in the improvement of cholinergic transmission by inhibiting the AChE. After data analysis, it was found that 12 compounds out of a total of 58 sesquiterpenes were reported to possess IC50 < 9μM and can be considered as potential candidates for the improvement of learning and memory. Sesquiterpene is an important category of terpenoids, found to possess a large spectrum of biological activities. The outcome of the review clearly states that sesquiterpenes (such as amberboin, lipidiol,etc) from herbs could offer fresh, functional compounds for possible prevention and treatment of AD.

Artemyrianolides A-S, Cytotoxic Sesquiterpenoids from Artemisia myriantha

Random screening suggested that the EtOH extract of Artemisia myriantha (Asteraceae) and its EtOAc fraction had cytotoxicity against HepG2 cells with inhibitory ratios of 30.6% and 53.5% at 50.0 μg/mL. Bioassay-guided isolation of the most active fractions (Fr. C and Fr. D) afforded 19 new sesquiterpenolides, artemyrianolides A-S (1-19), involving 13 germacranolides (1-13), four guaianolides (14-17), and two eudesmanolides (18 and 19), together with 16 known sesquiterpenoids (20-35). The new compounds were characterized by physical data analyses (HRESIMS, IR, 1D and 2D NMR, ECD), and the absolute configurations of compounds 1, 2, and 11 were determined by X-ray crystallography. Structurally, compounds 2 and 11-13 maintain an uncommon cis-fused 10/5 bicyclic system and compound 12 possesses an unusual (7S) configuration. Twenty of the compounds exhibited cytotoxicity against HepG2, Huh7, and SMMC-7721 cell lines. Compound 9 showed cytotoxic activity on both HepG2 and Huh7 cells with IC₅₀ values of 8.6 and 8.8 μM, and compounds 8 and 33 showed cytotoxicity to the three human hepatoma cell lines with IC₅₀ values of 4.9 and 7.4 μM (HepG2), 4.3 and 7.8 μM (Huh7), and 3.1 and 9.8 μM (SMMC-7721), respectively.

A new longipinane ketone from Achillea abrotanoides (Vis.) Vis.: chemical transformation of the essential oil enables the identification of a minor constituent

INTRODUCTION

Minor plant constituents are difficult to identify due to the challenging isolation and acquiring of reliable spectral data. Essential oils abound in such minor constituents that might be of high importance for their (e.g. olfactory) properties. The presence of new minor constituents is usually inferred from gas chromatography mass spectrometry (GC-MS) analyses that provide only a mass spectrum and retention data, which are insufficient to allow a positive identification.

OBJECTIVE

To identify a minor unknown constituent of the essential oil of Achillea abrotanoides (Vis.) Vis. (Asteraceae).

METHODOLOGY

The application of chemical transformations (oxidation and reduction) performed directly on crude essential-oil samples, followed by preparative chromatography and detailed spectral analysis, to identify a new longipinane ketone from the mentioned sample.

RESULTS

GC-MS analyses of the essential oil revealed, among other constituents, the presence of a known rare longipinane alcohol (α-longipinen-7β-ol) representing 2.5% of the total GC-peak areas, and a related unidentified oxygenated sesquiterpene (3.8%). Interpretation of their mass spectra led to an assumption that the unidentified one could represent α-longipinen-7-one. Oxidation of the entire essential-oil sample by pyridinium chlorochromate confirmed the assumed relationship among the compounds and gave a simplified enriched mixture containing the ketone (ca. 16%). A straightforward chromatographic separation of the ketone was followed by corroboration of its structure by nuclear magnetic resonance (NMR) (one- and two-dimensional), infrared (IR) and MS.

CONCLUSIONS

The complementing use of chemical transformations of crude essential oils, chromatographic separations, and detailed spectral analysis could have a more general application in the identification of new natural products.

Total Synthesis of (±)-Marsupellins A and B via Acetoxymarsupellone Using an Intramolecular Reductive Cyclization of Epoxycyanohydrin Derivative with Cp2TiI

The first total synthesis of C3- and C9-oxidized ent-longipinane-type sesquiterpenoids containing acetoxymarsupellone, marsupellins A and B, has been accomplished. This unique core common to C3- and C9-oxidized ent-longipinane-type sesquiterpenoids was constructed via a new intramolecular reductive cyclization reaction of an epoxycyanohydrin derivative using Cp₂TiI.

Prenylated Bibenzyls from the Chinese Liverwort Radula constricta and Their Mitochondria-Derived Paraptotic Cytotoxic Activities

Nine new prenylated bibenzyls, radstrictins A-I (1-9), and 11 known congeners were obtained from the Chinese liverwort Radula constricta. Their structures were identified by analysis of HRMS, NMR, and electronic circular dichroism data. Radstrictins A-F (1-6) were isolated as a racemate or scalemic mixtures. All the isolated compounds were subjected to cytotoxicity assessment. Methyl 2,4-dihydroxy-3-(3-methyl-2-butenyl)-6-phenethylbenzoate (10) exhibited significant activity against human lung cancer cell lines A549 and NCI-H1299 with IC₅₀ values of 6.0 and 5.1 μM, respectively. Further research revealed that cell death triggered by 10 occurred via mitochondria-derived paraptosis.

Sacculatane diterpenoids from the Chinese liverwort Pellia epiphylla with protection against H2O2-induced apoptosis of PC12 cells

Eight previously undescribed sacculatane diterpenoids, epiphyllins A-H, and one unknown bibenzyl-based isopentene along with seven known compounds were isolated from the Chinese liverwort Pellia epiphylla (L.) Corda. Their structures were established unequivocally on the basis of spectroscopic data and CD measurement. The quinine reductase-inducing activity evaluation demonstrated that epiphyllins A-D, 1β-hydroxysacculatanolide and pellianolactone B displayed moderate antioxidant effect. Further investigation of pellianolactone B revealed its protective effects on H₂O₂-induced oxidative insults and apoptosis in PC12 cells.

Diterpenoids from the Chinese liverwort Frullania hamatiloba and their Nrf2 inducing activities

Six previously undescribed labdane diterpenoids, frullanians A-F, along with five known diterpenoids, were isolated from the Chinese liverwort Frullania hamatiloba Stephani. Their structures were determined using NMR data, electronic circular dichroism (ECD) calculations as well as the single crystal X-ray diffraction measurement. NAD(P)H: QR (quinone reductase) assay demonstrated that frullanian D and four known compounds displayed antioxidant effect mediated via Nrf2 (Nuclear factor-erythroid 2-related factor 2) induction. Further investigation of the most bioactive frullanian D in MOVAS cells revealed that it ameliorated H₂O₂-induced oxidative insults without toxicity by increasing cell viability, attenuating morphological changes, and reducing intracellular ROS production. In addition, frullanian D promoted the nuclear translocation of Nrf2 and upregulated the expressions of antioxidant proteins NQO1 (NAD(P)H quinone oxidoreductase 1) and γ-GCS (γ-glutamyl cysteine synthetase). Docking analysis using MOE software further supported the activation of the Nrf2 pathway by frullanian D.

A concise approach to the tricyclic framework of longipinane- and ent-longipinane-type sesquiterpenoids

A general strategy for the synthesis of the common 6,6-dimethyltricyclic[5.4.0.0^2,8]undecane framework of longipinane- and ent-longipinane-type sesquiterpenoids has been successfully developed.

Cyperane and eudesmane-type sesquiterpenoids from Chinese liverwort and their anti-diabetic nephropathy potential

Six new sesquiterpenoids, including cyperane (1 and 2) and eudesmane (3-6) types, and one known eudesmane derivative (7), were isolated from the Chinese liverwort Chiloscyphus polyanthus var. rivularis (Schrad.) Nees. The structures of 1-7 were determined based on a combination of their spectroscopic data (NMR, HRESIMS and IR), single-crystal X-ray diffraction, and electronic circular dichroism (ECD) calculations. Preliminary anti-diabetic nephropathy activity testing showed that the cyperane-type sesquiterpenoids could inhibit cell proliferation and extracellular matrix accumulation in high glucose cultured mesangial cells in a dose-dependent manner.

Terpenoids with vasorelaxant effects from the Chinese liverwort Scapania carinthiaca

Four new diterpenoids scapanacins A-D (1-4) including one kaurane and three clerodane derivatives, along with eleven known compounds (9-15), were isolated from the Chinese liverwort Scapania carinthiaca J.B. Jack ex Lindb. Their structures were determined based on extensive spectroscopic analyses, and electronic circular dichroism (ECD) calculations. Vasorelaxant activity assays of the clerodane-type diterpenoids 2, and 4-8 revealed that they relaxed 3^rd-order rat mesenteric arterioles pre-contracted with norepinephrine (NE). Further assays with scapanacin D (4) confirmed that the vasodilatation was mediated through inhibition of Ca²⁺ influx via voltage-dependent Ca²⁺ channels (VDCs), and this Ca²⁺ channel blocking effect was also confirmed by inhibiting the extracellular Ca²⁺ influx in MOVAS cells. Besides, very little decrease of the relaxant activity caused by 4 on endothelium-denuded mesenteric arterioles with NE also suggested the vasodilatation was mainly produced by inhibiting Ca²⁺-induced contraction of smooth muscle. In addition, cytotoxicity testing showed that compounds 1 and 9 with α,β-unsaturated ketone exhibited inhibitory activities against a small panel of human cancer cell lines.

Bibenzyl-Based Meroterpenoid Enantiomers from the Chinese Liverwort Radula sumatrana

Six new pairs of bibenzyl-based meroterpenoid enantiomers, (±)-rasumatranin A-D (1-4) and (±)-radulanin M and N (5 and 6), and six known compounds were isolated from the adnascent Chinese liverwort, Radula sumatrana. Their structures were elucidated based on spectroscopic data and chiral phase HPLC-ECD analyses. The structures of 1 and 7 were also confirmed by single-crystal X-ray diffraction analysis. Cytotoxicity tests of the isolated compounds showed that 6-hydroxy-3-methyl-8-phenylethylbenzo[b]oxepin-5-one (8) showed activity against the human cancer cell lines MCF-7, PC-3, and SMMC-7721, with IC₅₀ values of 3.86, 6.60, and 3.58 μM, respectively, and induced MCF-7 cell death through a mitochondria-mediated apoptosis pathway.

Palladium-Promoted Neutral 1,4-Brook Rearrangement/Intramolecular Allylic Cyclization Cascade Reaction: A Strategy for the Construction of Vinyl Cyclobutanols

A cascade reaction to build vinyl cyclobutanol rings through activation of vinyl epoxides by palladium, followed by 1,4-Brook rearrangement and intramolecular cyclization with a palladium complex of the resulting carbon anion, is described. Through this cascade reaction, several highly substituted cyclobutanol substrates were achieved in good yields with high stereoselectivities.

Bioactive Terpenoids from Salvia plebeia: Structures, NO Inhibitory Activities, and Interactions with iNOS

A phytochemical investigation to obtain new NO inhibitors resulted in the identification of six new (1-6) and four known (7-10) terpenoids from Salvia plebeia. Compounds 1 and 2 are new diterpenoids, 3-5 are new meroditerpenoids, 6-9 are sesquiterpenoids, and 10 is a known meroditerpenoid. The structures of these isolates were determined by routine NMR experiments and X-ray diffraction, as well as the electronic circular dichroism spectra. Compounds 1-4 are diterpenoids carrying an oxygen bridge, and 6 is a rare copane-type sesquiterpenoid with a bridged tricyclic framework. The isolates inhibited NO generation induced by lipopolysaccharide in BV-2 cells. The possible mechanism of NO inhibition of some bioactive compounds was also investigated using molecular docking, which revealed interactions of bioactive compounds with the iNOS protein.

Acetylcholinesterase inhibitory activity and molecular docking study of steroidal alkaloids from Holarrhena pubescens barks

An alkaloidal extract of the bark of Holarrhena pubescens showed several inhibition zones of acetylcholinesterase (AChE) inhibitor, using a bioautographic assay. Activity-guided fractionation afforded three new steroidal alkaloids, mokluangins A-C (1-3), together with three known compounds, antidysentericine (4), holaphyllamine (5), methylholaphyllamine (6). All structures were elucidated by analysis of NMR and MS spectroscopic data. Compound 2 showed moderate antibacterial activity against Bacillus subtilis and Escherichia coli with the MIC value of 16 μg/mL, while compound 3 exhibited moderate selective activity against E. coli with the MIC value of 16 μg/mL. In addition, compounds 1-4 also showed strong AChE inhibiting activity with IC50 values ranging from 1.44 to 23.22 μM. Molecular docking calculations were also performed and the results demonstrated that all compounds can bind at the aromatic gorge of AChE with estimated binding free energies correlated well with the in vitro inhibitory profiles. Hydrophobic and hydrogen bonding interactions contribute mainly to the binding of the alkaloids where the substituents at C-3 serving as key functional groups for the AChE inhibition. Our results will allow the development of new AChE-inhibitors based on steroidal alkaloid skeleton bearing the cyclic amide moiety.

Bioactive Xanthones from Cratoxylum cochinchinense

Chemical investigation of Cratoxylum cochinchinense stem bark has led to the isolation and identification of a new xanthone, cochinchinone M (1), together with 12 known compounds. Their structures were elucidated on the basis of spectroscopic methods, including UV, IR, NMR and MS. Some compounds were evaluated for their antibacterial and acetylcholinesterase inhibitory activities.

New apoptosis-inducing sesquiterpenoids from the mycelial culture of Chinese edible fungus Pleurotus cystidiosus

Two new bisabolane-type sesquiterpenoids, pleuroton A (1) and pleuroton B (2), and three clitocybulol derivatives, clitocybulol D (3), clitocybulol E (4), and clitocybulol F (5), were obtained from the mycelial culture of edible fungus Pleurotus cystidiosus O. K. Mill by repeated column chromatography over RP-18, Sephadex LH-20, and silica gel. Their structures were determined according to nuclear magnetic resonance data, high-resolution electron impact mass spectrometry, and circular dichroism spectra. These new sesquiterpenoids exhibited significant cytotoxicity against two human prostate cancer DU-145 and C42B cells in the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The median inhibitory concentration (IC50) of compounds 1, 2, 3, 4, and 5 was 174, 28, 233, 162, and 179 nM, respectively, against the DU-145 cell and was 104, 52, 163, 120, and 119 nM, respectively, against the C42B cell. Especially, pleuroton B (2) exhibited the strongest cytotoxity among these sesquiterpenoids, which was confirmed by the colony formation assay. Furthermore, pleuroton B (2) could trigger the apoptosis of DU-145 cells through the detection of apoptosis cells using annexin V-FITC staining by flow cytometry, the observation of condensed nuclei in the apoptosis cells, and the western blot analysis for the expression of apoptosis-related proteins Bcl-2, Bak, and Bax. Analysis of structure-activity relationships of these sesquiterpenoids revealed that the unusual functional moiety of pleuroton B should contribute to its significant bioactivity. These results display the pharmacological potential of P. cystidiosus.