Pentacyclic Ring System Research Articles

Identification of antibacterial constituents from Rhododendron simsii Planch with an activity-guided method.

Bacterial infections and antibiotic resistance pose significant public health challenges globally. Natural products serve as valuable sources for discovering antimicrobial agents. Rhododendron simsii Planch, a folk medicine, is traditionally used to treat various inflammatory diseases. In this study, we investigated the antibacterial metabolites derived from R. simsii Planch. Rhodosimsiin A (1), bearing a 1,5-seco-1,6 and 3,6-epoxy grayanane diterpene skeleton, representing a novel 5/6/7/6/5 pentacyclic ring system, and 3β,16α-dihydroxy-6β-ethoxy-14β-acetoxy-grayan-1(5)-ene-10-one (4), which represents the first example of the degradation of C-20 and carbonylation in C-10 diterpenoid, together with two new grayanane diterpenes (2-3), three new triterpenes (13-15), and known analogs (5-12, 16-30), were isolated from the leaves of R. simsii Planch by using the bioassay-guided method. Their structures were elucidated by comprehensive spectroscopic analyses, and absolute configurations were established by single-crystal X-ray diffraction and calculated ECD spectra. Compounds 14, 15, 18, 20, 27, 28, and 30 exhibited potent antibacterial activity with an MIC50 of 1.4-24.3μg/mL against Staphylococcus aureus. The findings of this research indicate that secondary metabolites derived from R. simsii Planch are promising natural antimicrobial candidates.

Hyperkouytones A—O, New Polyprenylated Acylphloroglucinols from Hypericum kouytchense with Multidrug Resistance Reversal Activity

Comprehensive SummaryGiven the lack of systematic polyprenylated acylphloroglucinols (PPAPs) research on traditional Chinese medicine of Hypericum kouytchense, this plant was applied for the phytochemical study, which led to fifteen new PPAPs (1—15, PPAPs), along with 36 known PPAP derivatives. Their structures and absolute configurations were established by comprehensive spectral analysis and theoretical ECD and NMR calculations. Structurally, compound 1 possesses a rare fused 6/6/6/5/5 pentacyclic ring system. Eleven compounds exhibited good multidrug resistance reversal activity (RF ranging from 5 to 53) in HepG2/ADR cells. Importantly, compound 34, the most potential MDR modulator, showed better reversal effect (RF: 53) than positive control, verapamil. The primary mechanistic study of compound 34, implied that this compound could prohibit the function of P‐gp transport rather than its expression. The possible recognition mechanism between compound 34 and P‐gp was predicted by molecular docking.

Lycopodium Alkaloids from Huperzia goebelii.

One new fawcettimine-type Lycopodium alkaloid, hupertimine F (1), together with five known (2-6) Lycopodium alkaloids were isolated from Huperzia goebelii. The structure of 1 was elucidated by 1D and 2D NMR spectra, HRESIMS, and X-ray diffraction. Structurally, 1 represents the fourth example of Lycopodium alkaloids characterized by a 5/5/5/5/6 pentacyclic ring system with a 1-aza-7-oxabicyclo[2.2.1]heptane moiety. These known compounds 2, 3, 5, and 6 were isolated from H. goebelii for the first time. Compounds 1-6 were evaluated for acetylcholinesterase, butyrylcholinesterase and monoamine oxidase B inhibitory activities in vitro.

Enantioselective Total Synthesis of (-)-Hunterine A Enabled by a Desymmetrization/Rearrangement Strategy.

The first enantioselective total synthesis of (-)-hunterine A is disclosed. Our strategy employs a catalytic asymmetric desymmetrization of a symmetrical diketone and subsequent Beckmann rearrangement to construct a 5,6-α-aminoketone. A convergent 1,2-addition joins a vinyl dianion nucleophile and the enantioenriched ketone. The endgame of the synthesis features an aza-Cope/Mannich reaction and azide-olefin dipolar cycloaddition to complete the pentacyclic ring system. The synthesis is completed through a regioselective aziridine ring opening.

Natural products fragment-based design and synthesis of a novel pentacyclic ring system as potential MAPK inhibitor

The synthesis of compounds based on fragments derived from natural products (NPs) serves as a source of inspiration for the design of pseudo-natural products (PNPs), to identify bioactive molecules that exhibit similar characteristics to NPs. These novel molecular scaffolds exhibit previously unexplored biological activities as well. This study reports the development and synthesis of a novel pentacyclic ring system, the indole-pyrimidine-quinoline (IPQ) scaffold. The design of this scaffold was based on the structural characteristics of four natural products, namely tryptanthrin, luotonin A, rutaecarpine, and camptothecin. Several successive steps accomplished the effective synthesis of the IPQ scaffold. The constituent components of the pentacycle, containing the indole, quinazolinone, pyrimidone, and quinoline units, possess significant biological significance. Compound 1a demonstrated noteworthy anti-tumor activity efficacy against A549 cell lines among the tested compounds. The compound 1a was observed to elicit cell cycle arrest in both the G2/M and S phases, as well as trigger apoptosis in A549 cells. These effects were attributed to its ability to modulate the activation of mitochondrial-related mitogen-activated protein kinase (MAPK) signaling pathways.

Boerelasins A-D, Four Unprecedented Cytochalasins from the Endophytic Fungus Boeremia Exigua.

Four undescribed cytochalasins (1-4) were isolated from the endophytic fungus Boeremia exigua. Structurally, boerelasin A (1) represents the first example of a cytochalasin with a rare 5/5 bicyclic carbon core. Boerelasin B (2) possesses an unprecedented 5/6/5/6/8 pentacyclic ring system. Boerelasin C (3), a derivative from the common biosynthetic intermediate to 1, is a macrocyclic ring-opening cytochalasin, and boerelasin D (4) contains an uncommon six-carbon alkyl acid side chain. The structures were elucidated based on spectroscopic methods, electronic circular dichroism, spin-spin coupling constants, and calculated nuclear magnetic resonance with DP4+ analysis. These compounds exhibited significant cytotoxicity against the tumor cells.

Astellolides R-W, Drimane-Type Sesquiterpenoids from an Aspergillus parasiticus Strain Associated with an Isopod.

Sesquiterpenoids with a cage-like multiring frame are rarely found in nature. Mining of the isopod-derived fungus Aspergillus parasiticus SDU001 by the one strain-many compounds (OSMAC) strategy unexpectedly led to the discovery of fungal drimane-type sesquiterpenoids astellolide R (1), featuring an unusual cage-like 6/6/5/6/5 pentacyclic ring system, astellolide S (2), possessing a rare nicotinic acid building block, and astellolides T-W (3-6). Their structures were comprehensively assigned by spectroscopic data analysis, single-crystal X-ray diffraction, and electronic circular dichroism calculations. Furthermore, compounds 3 and 5 exhibited anti-inflammatory activity by inhibiting the lipopolyssacharide-induced NO production in RAW264.7 macrophages with IC50 values of 6.1 ± 0.8 and 6.8 ± 0.8 μM, respectively. A putative biosynthetic pathway for 1 is proposed. Our results enlarge the chemical space of the drimane-type sesquiterpenoids generated from endophytic fungi.

Revision of the Peniroquesine Biosynthetic Pathway by Retro-Biosynthetic Theoretical Analysis: Ring Strain Controls the Unique Carbocation Rearrangement Cascade.

Peniroquesine, a sesterterpenoid featuring a unique 5/6/5/6/5 fused pentacyclic ring system, has been known for a long time, but its biosynthetic pathway/mechanism remains elusive. Based on isotopic labeling experiments, a plausible biosynthetic pathway to peniroquesines A-C and their derivatives was recently proposed, in which the characteristic peniroquesine-type 5/6/5/6/5 pentacyclic skeleton is synthesized from geranyl-farnesyl pyrophosphate (GFPP) via a complex concerted A/B/C-ring formation, repeated reverse-Wagner-Meerwein alkyl shifts, three successive secondary (2°) carbocation intermediates, and a highly distorted trans-fused bicyclo[4.2.1]nonane intermediate. However, our density functional theory calculations do not support this mechanism. By applying a retro-biosynthetic theoretical analysis strategy, we were able to find a preferred pathway for peniroquesine biosynthesis, involving a multistep carbocation cascade including triple skeletal rearrangements, trans-cis isomerization, and 1,3-H shift. This pathway/mechanism is in good agreement with all of the reported isotope-labeling results.

Monoterpenoid 5-methylcoumarins from Centrapalus pauciflorus with antiproliferative activity

Thirteen undescribed monoterpene-fused 5-methylcoumarins, named centrapalus coumarins A–M, were isolated from the aerial parts of the Centrapalus pauciflorus together with seven known compounds. The structures were established by extensive spectroscopic analyses, including 1D NMR, 2D NMR, and HR-ESI-MS experiments. The compounds represent a wide range of chemical diversity depending on the connection of the head-to-tail coupled diisoprene unit. Centrapalus coumarins A–H and I–L are based on 6–6–6- and 6–6-7-membered tricyclic ring systems, respectively. Centrapalus coumarins D and E exhibit cyclic hemiketal structures, while centrapalus coumarins F is unique because its monoterpene part forms an additional lactone ring. Centrapalus coumarin L is the only compound containing a modified trinor-monoterpene part. Centrapalus coumarin M is unprecedented as it contains a pentacyclic heterocyclic ring system. Sixteen isolated compounds were investigated for antiproliferative activity on the human breast (MCF-7 and MDA-MB-231), cervical (HeLa and SiHa), and ovarian (A2780) cancer-cell lines by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, and a few of them exhibited substantial activity. Centrapalus coumarin F demonstrated the highest potency against MCF-7, HeLa, and A2780 cells with IC50 values of 6.59, 2.28, and 15.41 μM, respectively. The cytotoxic activity of centrapalus coumarin F showed moderate cancer selectivity, as determined using intact fibroblast cells (NIH-3 T3). The antiproliferative activity of these 5-methylcoumarin derivatives provides evidence for the establishment of structure–activity relationships.

Investigation of the Biosynthetic Mechanism of Bipentaromycin Featuring an Unprecedented Cyclic Head-to-Tail Dimeric Scaffold.

Bipentaromycins are heterodimeric aromatic polyketides featuring two distinctive 5/6/6/6/5 pentacyclic ring systems and exhibit antibacterial activities. However, their overall biosynthetic mechanism, particularly the mechanism for early-stage modifications, such as hydrogenation and methylation, and late-stage dimerization, remains unknown. Herein, by integrating heterologous expression, isotope labeling, gene knockout and complementation, and computational modeling, we determined the biosynthetic origin of the skeleton, identified the enzymes involved in stereo-/regioselective hydrogenation and methylation, and provided new mechanistic insights into the dimerization. This work not only deciphers the biosynthetic mechanism of bipentaromycins but also provides new strategies for creating biologically active dimeric pharmacophores for drug discovery and development.

New prenylated indole diketopiperazine alkaloids and polyketides from the mangrove-derived fungus Penicillium sp

Two new prenylated indole diketopiperazine alkaloids (PIDAs) penicamides A and B (1 and 2) and three new polyketides penicinones A−C (6−8), along with four known compounds deoxybrevianamide E (3), brevianamide V (4), 12,13-dehydroprolyl-2-(1,1-dimethylallyltryptophyl)diketopiperazine (5), and 4-hydroxyphenethyl 2-(4-hydroxyphenyl)acetate (9), were isolated and identified from the culture extract of the mangrove-derived fungus Penicillium sp. Their structures were fully elucidated by analyzing spectroscopic data. The absolute configurations of these compounds were determined by the comparison of experimental and calculated electronic circular dichroism (ECD) data and Mo2(OAc)4-induced and Rh2(OCOCF3)4-induced ECD experiments. Structurally, compound 1 is the first example of PIDAs featuring a 6/5/8/6/5 pentacyclic ring system with an α-hydroxy group at C-11, while compound 2 is a new analogue of PIDAs possessing the unique 3-methyleneindolin-2-ol moiety. In addition, compound 6 is a new lactone with the furo[3,4-b]pyran-5-one moiety. Compound 6 displayed potent cytotoxicity against murine melanoma (B16) cells, human breast adenocarcinoma (MCF-7) cells, and human hepatocellular carcinoma (HepG2) cells at 50.0 µM with inhibitory ratios of 82.7%, 75.1% and 95.9%, respectively. In addition, compound 6 exhibited significantly cytotoxic activity against the HepG2 cells, with an IC50 value of 3.87 ± 0.74 µM.

Two Terpenoid Derivatives with Different 3/5/6/6/5‐FusedPentacyclic Skeletons fromHedyosmum orientale

Comprehensive SummaryChemical investigation ofHedyosmum orientaleled to the identification of two terpenoid derivatives, dyosmunoids A and B (1and2) featuring two different 3/5/6/6/5‐fused pentacyclic ring systems, respectively. Compound1is the second natural product of this C25terpenoid compound class, and compound2is a rare dinor‐C25terpenoid with a peroxide bridge. Their structural elucidation was achieved by the comprehensive analysis of spectroscopic data, single‐crystal X‐ray diffraction, and ECD calculation. Putative biosynthetic pathways for compounds1and2are proposed. Compound2exhibited strong antimalarial activity with an IC50value of 0.42 μmol·L–1.

Pro-angiogenic New Chloro-Azaphilone Derivatives From the Hadal Trench-Derived Fungus Chaetomium globosum YP-106.

Five new chloro-azaphilones, chaetofanixins A–E (1–5), and five known analogs (6–10) were isolated and identified from the hadal trench-derived fungus Chaetomium globosum YP-106. The structure of chaetofanixin E (5) is unique and interesting, bearing a highly rigid 6/6/5/3/5 penta-cyclic ring system, which is first encountered in natural products. The structures of these compounds, including absolute configurations, were determined based on the spectroscopic analysis, electronic circular dichroism (ECD) calculations, and analysis of biogenetic origins. Compounds 1–7 significantly promoted angiogenesis in a dose-dependent manner, and thus, these compounds might be used as promising molecules for the development of natural cardiovascular disease agents.

Anti-infective Potential of Manzamine Alkaloids - A Review.

From time immemorial, natural products have been used for the treatment of various diseases. Various natural products, their semisynthetic derivatives, and synthetic analogs have been explored for their anti-infective properties. One such group of natural compounds that has been widely explored is manzamine alkaloids. Manzamine alkaloids are complex natural compounds consisting of a β-carboline nucleus attached to a pentacyclic ring system; they were first isolated from a marine sponge during the 1980s. This review aims to provide a critical overview on the anti-infective potential of manzamine alkaloids. A comprehensive and exhaustive review of the literature on manzamine alkaloids, and their isolation, anti-infective properties, and mechanism of action, is presented. Various manzamine alkaloids have been isolated and have been found to exhibit potent antiinfective activities like antibacterial, antimalarial, antiviral, antifungal, antileishmanial, among others. These manzamine alkaloids exhibit their anti-infective activity by inhibiting targets like GSK-3β, MtSK. This present review along with structure-activity relationship study of manzamine alkaloids for their anti-infective activity will be useful for further development of semisynthetic manzamine analogs as potent anti-infective agents with better therapeutic potential and reduced toxicity.

Hupertimines A-E, Fawcettimine-Type Lycopodium Alkaloids from Huperzia serrata.

Five new fawcettimine-type Lycopodium alkaloids, hupertimines A-E (1-5), were discovered from the whole plant of Huperzia serrata, along with two known alkaloids, 8α-hydroxyphlegmariurine B (6) and 8β-hydroxyphlegmariurine B (7). The structures of 1-7 were identified through HR-MS, IR, 1 H, 13 C, and 2D NMR, and single-crystal X-ray diffraction analysis. Structurally, compound 1 was the fourth example of Lycopodium alkaloid with an ether linkage between C-5 and C-13 and 2 was the third example of Lycopodium alkaloid with a 5/5/5/5/6 pentacyclic ring system and featuring a 1-aza-7-oxabicyclo[2.2.1]heptane unit. Compounds 1-7 were tested for their BACE1 inhibitory activity. In addition, the correct 1 H- and 13 C-NMR data for 7 were reported in current study.

Norprzewalsone A, a Rearranged Polycyclic Polyprenylated Acylphloroglucinol with a Spiro[cyclopentane-1,3'-tricyclo[7.4.0.01,6]tridecane] Core from Hypericum przewalskii.

Norprzewalsone A (1), a rearranged polyprenylated polycyclic acylphloroglucinol (PPAP) with a new carbon skeleton, along with a new congener, norprzewalsone B (2), were isolated from Hypericum przewalskii. Compound 1 possessed a new 5/6/5/6/6 pentacyclic ring system based on a spiro[cyclopentane-1,3'-tricyclo[7.4.0.01,6]tridecane] core, which might be derived from the common [3.3.1]-type bicyclic polyprenylated acylphloroglucinol (BPAP) via the key retro-Claisen, intramolecular cyclization, and Diels-Alder cyclization reactions. Their structures and absolute configurations were confirmed by spectroscopic data, calculated 1D NMR data with DP4+ probability analyses, and electronic circular dichroism calculations and comparison. More significantly, compound 1 exhibited a moderate inhibitory effect on NO production in lipopolysaccharide-stimulated RAW264.7 cells.

Cytochalasan Alkaloids as TRAIL Sensitizers from an Endophytic Fungus Chaetomium sp.

Two new cytochalasans with a rare 6/6/5/5/7 pentacyclic ring system, named chaetoconvosins C-D (1: -2: ), together with two known congeners (3: -4: ), were isolated from the fermentation of an endophytic fungus, Chaetomium sp. SG-01, harbored in the fibrous roots of Schisandra glaucescens Diels. Their structures including the absolute configuration were elucidated by extensive spectroscopic (HRESIMS, NMR, and ECD) and X-ray crystallographic analyses. The TRAIL-resistance-overcoming activity of 1: -4: in a TRAIL-resistant HT29 colorectal cancer cell line was evaluated, which revealed that co-treatment of 1: -4: at 50 µM with TRAIL (150 ng/mL) reduced the HT29 cell viability by 19.0%, 24.1%, 17.9%, and 15.5%, respectively, compared to treatment with 1: -4: alone.

Towards Tianeptine Analogues: Synthesis of New Ring Systems Containing a Dibenzo[c,f][1,2]thiazepine S,S-Dioxide Core

AbstractIn the course of the synthesis of fused-ring analogues of the antidepressant drug tianeptine, representatives of three new heterocyclic ring systems, indolo[1,7-bc][1,2]benzothiazepines (and their 4,5-dihydro analogues), 5,6-dihydroquino[1,8-bc][1,2]benzothiazepines, and [1,2]benzothiazepino[4,3,2-jk]carbazoles, as well as their intermediates, have been prepared. The tetracyclic and pentacyclic ring systems containing either an oxo or a hydroxy functional group are suitable for introducing various side chains, including potential pharmacophores. For this latter transformation, examples are demonstrated by conversion of the hydroxy group into a chloro moiety and subsequent reaction with amines or with primary alcohols bearing a tertiary amino side chain. Two fused-ring derivatives exhibiting the side-chain characteristics of tianeptine have also been synthesized. Altogether 40 compounds are described in the present manuscript, eight of them are also characterized by single-crystal X-ray diffraction.

Interaction of new tri-, tetra-, and pentacyclic azaphenothiazine derivatives with calf thymus DNA: Spectroscopic and molecular docking studies

Azaphenothiazines (AZA), modified phenothiazine derivatives, have been reported to exhibit a wide spectrum of biological activities, including anticancer activities, but the mechanisms of their interactions with biomolecules are not fully recognized. In this work, the mode of interaction of selected AZA with calf thymus DNA was investigated using UV–Vis absorption, fluorescence spectroscopy (competition experiment with ethidium bromide, quenching of fluorescence) and molecular docking. The investigated AZA represent dipyrido[3,4-b;3′4′-e][1,4]thiazine, quino[3,2-b]benzo[1,4]thiazine and diquino[3,2-b;2′,3′-e][1,4]thiazine possessing tricyclic, tetracyclic and pentacyclic ring system with the additional N,N-dimethylaminopropyl group at the nitrogen atom in the 1,4 thiazine ring. The results obtained from spectroscopic studies showed that AZA bind to DNA by insertion of a fragment of the fused rings system between the base pair stack in the double helix of DNA. In addition, the number of rings in the AZA structures seemed to be related to the strength of the interaction, because pentacyclic AZA (binding constant Kb = 6.31 × 106 L/mol) demonstrated 10-fold higher affinity towards DNA than the tetracyclic AZA and about 100-fold higher affinity than that of tricyclic AZA. The molecular docking results showed that the binding mode of AZA to DNA helix was an intercalation mode with the partial insertion of one planar part of the AZA structure (the pyridine or quinoline ring) into the neighboring bases of one of the DNA chains with additional hydrogen bonding with the minor groove through the positively charged N,N-dimethylaminopropyl group. Chemical potential (μ), chemical hardness (ƞ), electronegativity (χ) and the value of electrons transferred from one system to another (ΔN) calculated from the HOMO and LUMO energies by the density functional theory method indicated that AZA acted as the electron acceptors to the DNA bases.

Ascandinines A-D, Indole Diterpenoids, from the Sponge-Derived Fungus Aspergillus candidus HDN15-152.

Four new indole diterpenoids, ascandinines A-D (1-4), were isolated from an Antarctic sponge-derived fungus Aspergillus candidus HDN15-152. Their structures, including absolute configurations, were established based on NMR data, computational calculations, and biosynthetic considerations. Ascandinine A (1) possesses an unprecedented 2-oxabicyclo[2.2.2]octan-3-ol motif embedded in a pentacyclic ring system, while compounds 2-4 represent a rare type of indole diterpenoid featuring the 6/5/5/6/6/6/6-fused ring system. Compound 3 displayed anti-influenza virus A (H1N1) activity with an IC50 value of 26 μM, while compound 4 showed cytotoxicity against HL-60 cells with an IC50 value of 7.8 μM.