Mbandakamines A and B, Unsymmetrically Coupled Dimeric Naphthylisoquinoline Alkaloids, from a Congolese Ancistrocladus Species

Structural variety and pharmacological potential of naphthylisoquinoline alkaloids

Naphthylisoquinoline alkaloids are a fascinating class of natural biaryl compounds. They show characteristic mono- and dimeric scaffolds, with chiral axes and stereogenic centers. Since the appearance of the last comprehensive overview on these secondary plant metabolites in this series in 1995, the number of discovered representatives has tremendously increased to more than 280 examples known today. Many novel-type compounds have meanwhile been discovered, among them naphthylisoquinoline-related follow-up products like e.g., the first seco-type (i.e., ring-opened) and ring-contracted analogues. As highlighted in this review, the knowledge on the broad structural chemodiversity of naphthylisoquinoline alkaloids has been decisively driven forward by extensive phytochemical studies on the metabolite pattern of Ancistrocladus abbreviatus from Coastal West Africa, which is a particularly "creative" plant. These investigations furnished a considerable number of more than 80-mostly new-natural products from this single species, with promising antiplasmodial activities and with pronounced cytotoxic effects against human leukemia, pancreatic, cervical, and breast cancer cells. Another unique feature of naphthylisoquinoline alkaloids is their unprecedented biosynthetic origin from polyketidic precursors and not, as usual for isoquinoline alkaloids, from aromatic amino acids-a striking example of biosynthetic convergence in nature. Furthermore, remarkable botanical results are presented on the natural producers of naphthylisoquinoline alkaloids, the paleotropical Dioncophyllaceae and Ancistrocladaceae lianas, including first investigations on the chemoecological role of these plant metabolites and their storage and accumulation in particular plant organs.

Naphthylisoquinoline alkaloids: novel agents against the causative pathogens of eumycetoma and actinomycetoma-en route to broad-spectrum antimycetomal drugs

Mycetoma is a devastating neglected tropical infection of the subcutaneous tissues. It is caused by fungal and bacterial pathogens recognized as eumycetoma and actinomycetoma, respectively. Mycetoma treatment involves diagnosing the causative microorganism as a prerequisite to prescribing a proper medication. Current therapy of fungal eumycetoma causative agents, such as Madurella mycetomatis, consists of long-term antifungal medication with itraconazole followed by surgery, yet with usually unsatisfactory clinical outcomes. Actinomycetoma, on the contrary, usually responds to treatment with co-trimoxazole and amikacin. Therefore, there is a pressing need to discover novel broad-spectrum antimicrobial agents to circumvent the time-consuming and costly diagnosis. Using the resazurin assay, a series of 23 naphthylisoquinoline (NIQ) alkaloids and related naphthoquinones were subjected to in vitro screening against two fungal strains of M. mycetomatis and three bacterial strains of Actinomadura madurae and A. syzygii. Seven NIQs, mostly dimers, showed promising in vitro activities against at least one strain of the mycetoma-causative pathogens, while the naphthoquinones did not show any activity. A synthetic NIQ dimer, 8,8'''-O,O-dimethylmichellamine A (18), inhibited all tested fungal and bacterial strains (IC50 = 2.81-12.07 µg/mL). One of the dimeric NIQs, michellamine B (14), inhibited a strain of M. mycetomatis and significantly enhanced the survival rate of Galleria mellonella larvae infected with M. mycetomatis at concentrations of 1 and 4 µg/mL, without being toxic to the uninfected larvae. As a result, broad-spectrum dimeric NIQs like 14 and 18 with antimicrobial activity are considered hit compounds that could be worth further optimization to develop novel lead antimycetomal agents.

Synthesis and pharmacodynamic evaluation of naphthalene derivatives against influenza A virus in vitro and in vivo

Influenza A virus is a highly mutable pathogenic pathogen that could cause a global pandemic. It is necessary to find new anti-influenza drugs to resist influenza epidemics due to the seasonal popularity of a certain area every year. Naphthalene derivatives had potential antiviral activity. A series of naphthalene derivatives were synthesized via the metal-free intramolecular hydroarylation reactions of alkynes. Evaluation of their biological efficacy showed that compound 2-aminonaphthalene 4d had better antiviral activity in vitro than ribavirin. By studying the mechanism of action of 2-aminonaphthalene 4din vivo and in vitro, we found that 4d had antiviral activity to three different subtype influenza viruses of A/Weiss/43 (H1N1), A/Virginia/ATCC2/2009 (H1N1) and A/California/2/2014 (H3N2). Compound 4d had the best effect after viral adsorption, and mainly played in the early stage of virus replication. 2-Aminonaphthalene 4d could reduce the replication of virus by inhibiting the NP and M proteins of virus. Compound 4d cut down ROS accumulation, autophagy and apoptosis induced by influenza virus. Inflammatory response mediated by RIG-1 pathway were suppressed in the cell and mice. In addition, the pathological changes of lung tissue and virus titer in mice were reduced by the administration of 4d. Therefore, naphthalene derivative 4d is a potential drug for the treatment of influenza A virus infection.

Rh(III)-Catalyzed C8-Spiroannulation of 1-Aminonaphthalenes with Maleimides

The rhodium(III)-catalyzed C8-spiroannulation of 1-aminonaphthalenes with maleimides is described herein. Initially formed C8-alkenylated 1-aminonaphthalenes can intercept nucleophilic 1-amino groups through the intramolecular aza-Michael reaction, resulting in the formation of spirofused tetracyclic frameworks. This protocol displayed a wide substrate scope and a broad functional group compatibility. The synthetic utility of this process is demonstrated by the gram-scale synthesis, late-stage modification, and synthetic transformations.

Asian Ancistrocladus Lianas as Creative Producers of Naphthylisoquinoline Alkaloids

This book describes a unique class of secondary metabolites, the mono- and dimeric naphthylisoquinoline alkaloids. They occur in lianas of the paleotropical Ancistrocladaceae and Dioncophyllaceae families, exclusively. Their unprecedented structures include stereogenic centers and rotationally hindered, and thus likewise stereogenic, axes. Extended recent investigations on six Ancistrocladus species from Asia, as reported in this review, shed light on their fascinating phytochemical productivity, with over 100 such intriguing natural products. This high chemodiversity arises from a likewise unique biosynthesis from acetate-malonate units, following a novel polyketidic pathway to plant-derived isoquinoline alkaloids. Some of the compounds show most promising antiparasitic activities. Likewise presented are strategies for the regio- and stereoselective total synthesis of the alkaloids, including the directed construction of the chiral axis.

A nonneglectable stereochemical factor in drug development: Atropisomerism

Chirality is one of the key factors affecting the medicinal efficacy of compounds. In addition to central chirality, sterically hindered chiral axes commonly appear in drugs and the resulting chirality is known as atropisomerism. With developments in medicinal chemistry, atropisomerism has attracted increasing attention. This review discusses the classification, biological activity, pharmacokinetics, toxicity and side effects of atropisomers, and can serve as a reference in the research and development of potential chiral drugs.

In Vitro Antiviral Activity of Potential Medicinal Plant Extracts Against Dengue and Chikungunya Viruses

Dengue and chikungunya are two important mosquito-borne infections which are known to occur extensively in tropical and subtropical areas. Presently, there is no treatment for these viral diseases. In vitro antiviral screening of 25 extracts prepared from the plants of Vitex negundo, Plumeria alba, Ancistrocladus heyneanus, Bacopa monnieri, Anacardium occidentale, Cucurbita maxima, Simarouba glauca, and Embelia ribes using different solvents and four purified compounds (anacardic acid, chloroquinone, glaucarubinone, and methyl gallate) were carried out for their anti-dengue virus (DENV) and anti-chikungunya virus (CHIKV) activities. Maximum nontoxic concentrations of the chloroform, methanol, ethyl acetate, petroleum ether, dichloromethane, and hydroalcoholic extracts of eight plants were used. The antiviral activity was assessed by focus-forming unit assay, quantitative real-time RT-PCR, and immunofluorescence assays. Extracts from Plumeria alba, Ancistrocladus heyneanus, Bacopa monnieri, and Cucurbita maxima showed both anti-DENV and CHIKV activity while extract from Vitex negundo showed only anti-DENV activity. Among the purified compounds, anacardic acid, chloroquinone and methyl gallate showed anti-dengue activity while only methyl gallate had anti-chikungunya activity. The present study had identified the plant extracts with anti-dengue and anti-chikungunya activities, and these extracts can be further characterized for finding effective phytopharmaceutical drugs against dengue and chikungunya.

Antimalarial Natural Products

Natural products have made a crucial and unique contribution to human health, and this is especially true in the case of malaria, where the natural products quinine and artemisinin and their derivatives and analogues, have saved millions of lives. The need for new drugs to treat malaria is still urgent, since the most dangerous malaria parasite, Plasmodium falciparum, has become resistant to quinine and most of its derivatives and is becoming resistant to artemisinin and its derivatives. This volume begins with a short history of malaria and follows this with a summary of its biology. It then traces the fascinating history of the discovery of quinine for malaria treatment and then describes quinine's biosynthesis, its mechanism of action, and its clinical use, concluding with a discussion of synthetic antimalarial agents based on quinine's structure. The volume then covers the discovery of artemisinin and its development as the source of the most effective current antimalarial drug, including summaries of its synthesis and biosynthesis, its mechanism of action, and its clinical use and resistance. A short discussion of other clinically used antimalarial natural products leads to a detailed treatment of other natural products with significant antiplasmodial activity, classified by compound type. Although the search for new antimalarial natural products from Nature's combinatorial library is challenging, it is very likely to yield new antimalarial drugs. The chapter thus ends by identifying over ten natural products with development potential as clinical antimalarial agents.

Tandem Synthesis of 1,3-Disubstituted Naphthalenes via TfOH-Promoted Directed-Aldol and Friedel-Crafts Reactions

A TfOH-promoted tandem synthesis of 1,3-disubstituted naphthalenes is developed via a directed-aldol reaction and a Friedel-Crafts reaction. Two new C-C bonds and one new benzene ring are created efficiently in one pot due to the discovery of a TfOH-promoted highly chemoselective directed-aldol reaction between two different ketones with α-hydrogens.

N,C-Coupled naphthylisoquinoline alkaloids: a versatile new class of axially chiral natural products

Covering: up to April 2021During the past decades, a plethora of natural products with restricted rotation about a biaryl axis have been discovered, among them the naphthylisoquinoline (NIQ) alkaloids, mostly C,C-coupled and having remarkable bioactivities. Within this fascinating class of naturally occurring biaryl compounds, NIQ alkaloids bearing an N,C-heterobiaryl axis have attracted particular attention. They are structurally and biosynthetically unprecedented, with interesting stereochemical implications and biological activities. In contrast to existing articles and reviews about axially chiral - yet C,C-coupled - natural products, this is the first, comprehensive review on the new subclass of N,C-coupled NIQs, their isolation and structural elucidation, their N,C-axial chirality, their biosynthetic origin, their promising antiparasitic and antileukemic activities, and their total synthesis.

Spirombandakamine A3 and Cyclombandakamines A8 and A9, Polycyclic Naphthylisoquinoline Dimers, with Antiprotozoal Activity, from a Congolese Ancistrocladus Plant

Spirombandakamine A₃ (7) is only the third known naphthylisoquinoline dimer with a spiro-fused novel molecular framework and the first such representative to possess a relative trans-configuration at the two chiral centers in both tetrahydroisoquinoline subunits. It was found in the leaves of a botanically as yet unidentified Congolese Ancistrocladus plant, which is morphologically closely related to the Central African taxon Ancistrocladus ealaensis. Likewise isolated were the new cyclombandakamines A₈ (8) and A₉ (9), which belong to another most recently discovered type of unusual oxygen-bridged naphthylisoquinoline dimers and two previously described "open-chain" analogues, mbandakamines C (10) and D (11). The full absolute stereostructures of these compounds were assigned by combining spectroscopic, chemical, and chiroptical methods. Preliminary biomimetic investigations indicated that both spirombandakamine- and cyclombandakamine-type dimers result from the oxidation of their open-chain mbandakamine-type congeners. The new dimeric alkaloids 7-9 displayed potent growth-inhibitory activity against Plasmodium falciparum, the protozoal pathogen causing malaria, and moderate effects on Trypanosoma brucei rhodesiense, the parasite responsible for African sleeping sickness.

Computational studies of biologically active alkaloids of plant origin: an overview

Computational studies nowadays constitute a crucial source of information for drug development, because they provide information on many molecular properties and also enable predictions of the properties of not-yet-synthesized compounds. Alkaloids are a vast group of natural products exhibiting a variety of biological activities, many of which are interesting for drug development. On the other hand, computational studies of biologically active alkaloids have so far mostly focused on few particularly relevant or “popular” molecules, such as quinine, caffeine, or cocaine, with only few works on the other molecules. The present work offers an overview of existing computational studies on alkaloid molecules, from the earliest ones to the most recent, and considering all the theoretical approaches with which studies have been performed (both quantum mechanics and molecular dynamics). The considered studies are grouped according to their objectives and outcomes, such as conformational analysis of alkaloid molecules, effects of selected solvents on their properties, docking studies aimed at better understanding of the interactions between alkaloid molecules and biological targets, studies focusing on structure activity relationships, and computational studies performed to confirm experimental results. It is concluded that it would be important that computational studies on many other alkaloid molecules are performed and their results made available, covering their different classes as well as the variety of their biological activities, to attain better understanding of the properties not only of individual molecules, but also of groups of related molecules and of the overall alkaloids family.

Biologically active isoquinoline alkaloids covering 2014-2018

Isoquinoline alkaloids, an important class of N-based heterocyclic compounds, have attracted considerable attention from researchers worldwide since the early 19th century. Over the past 200 years, many compounds from this class were isolated, and most of them and their analogs possess various bioactivities. In this review, we survey the updated literature on bioactive alkaloids and highlight research achievements of this alkaloid class during the period of 2014-2018. We reviewed over 400 molecules with a broad range of bioactivities, including antitumor, antidiabetic and its complications, antibacterial, antifungal, antiviral, antiparasitic, insecticidal, anti-inflammatory, antioxidant, neuroprotective, and other activities. This review should provide new indications or directions for the discovery of new and better drugs from the original naturally occurring isoquinoline alkaloids.

Induction of apoptosis in breast cancer cells by naphthylisoquinoline alkaloids

Breast cancer is one of the most common types of cancer in the world and a major cause of mortality. Present therapeutic strategies against breast cancer have severe drawbacks such as allergies, damage to healthy tissues, reoccurrence of cancer, and emergence of drug resistance. Naphthylisoquinoline alkaloids are a group of structurally unique natural products produced by tropical lianas belonging to the plant families Dioncophyllaceae and Ancistrocladaceae indigenous to Asia and Africa. These secondary metabolites have been reported to show anti-infective activity, but they also act against leukemic and pancreatic cancer cells. In the present study we have tested the potential of eleven mono- and dimeric naphthylisoquinoline compounds against two breast cancer cell lines, MCF-7 and MDA-MB-231. Three out of the compounds (agents 1, 4, and 11) showed significant activities against both tested cancer cell lines. Further mechanistic investigations revealed that all of the three substances induce apoptotic cell death via its intrinsic pathway by causing deformation of the nuclear membrane, disruption of the mitochondrial membrane potential (MMP), and elevated reactive oxygen species (ROS) production in both cell lines. Flow cytometric analysis using Annexin V - FITC/PI double staining showed an increased number of apoptotic cells in both, the early and the late phases.

Interplay between σ Holes, Anion···H-C, and Cation-π Interactions in Dibromo[2,2]paracyclophane Complexes

Theoretical calculations were performed to investigate the interplay between σ-hole, anion-HC and cation-π interactions in the complexes of dibromo[2,2]paracyclophane (DBr[2,2]PCP) with alkali (Li⁺, Na⁺, K⁺), alkaline earth metal cations (Be²⁺, Mg²⁺, and Ca²⁺), and halogen anions (F^-, Cl^-, and Br^-) using the wave function (MP2) and density functional theory (M06-2X and B3LYP) methods with the 6-311++G(d,p) basis set. The study reveals that DBr[2,2]PCP behaves as amphoteric molecule with a predominance of basic character. It prefers to interact with hard cations and hard anions such as Be²⁺ and F^- through cation-π and anion···HC interactions, respectively. Substitution of Br by F and Cl atoms in DBr[2,2]PCP decreases slightly the interaction energies of DX[2,2]PCP-halogen complexes (X = F, Cl, and Br) by 2.0 and 0.3 kcal/mol (M06-2X), respectively. The anion-HC interactions in DBr[2,2]PCP complexes are ∼10 kcal/mol stronger (B3LYP; ∼15 kcal/mol at M06-2X and 7 kcal/mol at MP2) than the σ-hole interactions.

Alkaloids from Plants with Antimalarial Activity: A Review of Recent Studies

Malaria is one of the major health problems in developing countries. The disease kills a large number of people every year and also affects financial status of many countries. Resistance of the plasmodium parasite, the causative agent, to the existing drugs, including chloroquine, mefloquine, and artemisinin based combination therapy (ACT), is a serious global issue in malaria treatment and control. This warrants an urgent quest for novel compounds, particularly from natural sources such as medicinal plants. Alkaloids have over the years been recognized as important phytoconstituents with interesting biological properties. In fact, the first successful antimalarial drug was quinine, an alkaloid, which was extracted from Cinchona tree. In the present review work, the alkaloids isolated and reported recently (2013 till 2019) to possess antimalarial activity are presented. Several classes of alkaloids, including terpenoidal, indole, bisindole, quinolone, and isoquinoline alkaloids, were identified with a promising antimalarial activity. It is hoped that the reports of the review work will spur further research into the structural modification and/or development of the interesting compounds as novel antimalarial drugs.

Antiplasmodial natural products: an update

Background

Malaria remains a significant public health challenge in regions of the world where it is endemic. An unprecedented decline in malaria incidences was recorded during the last decade due to the availability of effective control interventions, such as the deployment of artemisinin-based combination therapy and insecticide-treated nets. However, according to the World Health Organization, malaria is staging a comeback, in part due to the development of drug resistance. Therefore, there is an urgent need to discover new anti-malarial drugs. This article reviews the literature on natural products with antiplasmodial activity that was reported between 2010 and 2017.

Methods

Relevant literature was sourced by searching the major scientific databases, including Web of Science, ScienceDirect, Scopus, SciFinder, Pubmed, and Google Scholar, using appropriate keyword combinations.

Results and Discussion

A total of 1524 compounds from 397 relevant references, assayed against at least one strain of Plasmodium, were reported in the period under review. Out of these, 39% were described as new natural products, and 29% of the compounds had IC₅₀ ≤ 3.0 µM against at least one strain of Plasmodium. Several of these compounds have the potential to be developed into viable anti-malarial drugs. Also, some of these compounds could play a role in malaria eradication by targeting gametocytes. However, the research into natural products with potential for blocking the transmission of malaria is still in its infancy stage and needs to be vigorously pursued.

A Near-Complete Series of Four Atropisomeric Jozimine A2-Type Naphthylisoquinoline Dimers with Antiplasmodial and Cytotoxic Activities and Related Alkaloids from Ancistrocladus abbreviatus

Three new naphthylisoquinoline dimers, jozibrevines A-C (1a-c), were isolated from the West African shrub Ancistrocladus abbreviatus, along with the known dimer jozimine A₂ (1d). The two molecular moieties of 1a-d are coupled via the sterically constrained 3',3″-positions of their two naphthalene units, so that the central biaryl linkage is rotationally hindered. With the two outer axes also being chiral, 1a-d possess three consecutive stereogenic axes. The four isolated dimers all have the same constitutions and identical absolute configurations at the four stereogenic centers, but differ by their axial chirality. They belong to the extremely small class of Dioncophyllaceae-type naphthylisoquinoline dimers, i.e., being devoid of oxygen functions at C-6 and bearing the R-configuration at C-3 in their isoquinoline portions. Besides these dimers, the plant produces predominantly typical Ancistrocladaceae-type monomeric compounds, i.e., with the S-configuration at C-3 and an oxygen function at C-6, such as the new ancistrobrevines K (5) and L (6). Furthermore, a new hybrid-type (i.e., mixed Ancistrocladaceae/Dioncophyllaceae-type) alkaloid was identified, named ancistrobrevine M (7), which is 3R-configured and 6-oxygenated. Remarkable was the discovery of its "inverse hybrid-type" counterpart, dioncoline A (8). It is the as yet only known 3S-configured naphthylisoquinoline lacking an O-functionality at C-6. The new jozibrevines A-C (1a-c) exhibited pronounced antiplasmodial activities in the submicromolar range, with 1a being the most potent compound (IC₅₀, 0.012 μM). Furthermore, jozimine A₂ (1d) showed cytotoxicity against human colon carcinoma (HT-29), fibrosarcoma (HT1080), and multiple myeloma (MM.1S) cancer cells, displaying IC₅₀ values of 12.0, 9.0, and 5.0 μM, respectively, whereas jozibrevines A (1a) and B (1b) were nontoxic in this concentration range.

Ealamines A-H, a Series of Naphthylisoquinolines with the Rare 7,8'-Coupling Site, from the Congolese Liana Ancistrocladus ealaensis, Targeting Pancreatic Cancer Cells

From the twigs and leaves of the Central African liana Ancistrocladus ealaensis (Ancistrocladaceae), a series of ten 7,8'-coupled naphthylisoquinoline alkaloids were isolated, comprising eight new compounds, named ealamines A-H (4a, 4b, 5-10), and two known ones, 6-O-demethylancistrobrevine A (11) and yaoundamine A (12), which had previously been found in related African Ancistrocladus species. Only one of the new compounds within this series, ealamine H (10), is a typical Ancistrocladaceae-type alkaloid, with 3S-configuration at C-3 and an oxygen function at C-6, whereas seven of the new alkaloids are the first 7,8'-linked "hybrid-type" naphthylisoquinoline alkaloids, i.e., 3R-configured and 6-oxygenated in the tetrahydroisoquinoline part. The discovery of such a broad series of 7,8'-coupled naphthyltetrahydroisoquinolines is unprecedented, because representatives of this subclass of alkaloids are normally found in Nature quite rarely. The stereostructures of the new ealamines were assigned by HRESIMS, 1D and 2D NMR, oxidative degradation, and experimental and quantum-chemical ECD investigations, and-in the case of ealamine A (4a)-also confirmed by X-ray diffraction analysis. Ealamines A-D exhibited distinct-and specific-antiplasmodial activities, and they displayed pronounced preferential cytotoxic effects toward PANC-1 human pancreatic cancer cells in nutrient-deprived medium, without causing toxicity under normal, nutrient-rich conditions, with ealamine C (5) as the most potent agent.

An Unusually Broad Series of Seven Cyclombandakamines, Bridged Dimeric Naphthylisoquinoline Alkaloids from the Congolese Liana Ancistrocladus ealaensis

A series of seven unusual dimeric naphthylisoquinoline alkaloids was isolated from the leaves of the tropical liana Ancistrocladus ealaensis J. Léonard, named cyclombandakamine A (1), 1-epi-cyclombandakamine A (2), and cyclombandakamines A_3–7 (3–7). These alkaloids have a chemically thrilling structural array consisting of a twisted dihydrofuran-cyclohexenone-isochromene system. The 1′″-epimer of 4, cyclombandakamine A₁ (8), had previously been discovered in an unidentified Ancistrocladus species related to A. ealaensis. Both lianas produce the potential parent precursor, mbandakamine A (9), but only A. ealaensis synthesizes the corresponding cyclized form, along with a broad series of slightly modified analogs. The challenging isolation required, besides multi-dimensional chromatography, the use of a pentafluorophenyl stationary phase. Featuring up to six stereocenters and two types of chiral axes, their structures were elucidated by means of 1D and 2D NMR, HRESIMS, in combination with oxidative chemical degradation experiments as well as chiroptical (electronic circular dichroism spectroscopy) and quantum chemical calculations. Compared to the ‘open-chain’ parent compound 9, these dimers displayed rather moderate antiplasmodial activities.

Antiprotozoal dimeric naphthylisoquinolines, mbandakamines B3 and B4, and related 5,8'-coupled monomeric alkaloids, ikelacongolines A-D, from a Congolese Ancistrocladus liana

From the leaves of a botanically and phytochemically as yet unexplored Ancistrocladus liana discovered in the rainforests of the Central region of the Democratic Republic of the Congo in the vicinity of the town of Ikela, six new naphthylisoquinoline alkaloids were isolated, viz., two constitutionally unsymmetric dimers, the mbandakamines B3 (3) and B4 (4), and four related 5,8'-linked monomeric alkaloids, named ikelacongolines A-D (5a, 5b, 6, and 7). The dimers 3 and 4 are structurally unusual quateraryls comprising two 5,8'-coupled monomers linked via a sterically strongly constrained 6',1''-connection between their naphthalene units. These compounds contain seven elements of chirality, four stereogenic centers and three consecutive chiral axes. They were identified along with two known related compounds, the mbandakamines A (1) and B2 (2), which had so far only been detected in two Ancistrocladus species indigenous to the Northwestern Congo Basin. In addition, five known monomeric alkaloids, previously found in related Central African Ancistrocladus species, were isolated from the here investigated Congolese liana, three of them belonging to the subclass of 5,8'-coupled naphthylisoquinoline alkaloids, whereas two compounds exhibited a less frequently occurring 7,8'-biaryl linkage. The stereostructures of the new alkaloids were established by spectroscopic (in particular HRESIMS, 1D and 2D NMR), chemical (oxidative degradation), and chiroptical (electronic circular dichroism) methods. The mbandakamines B3 (3) and B4 (4) displayed pronounced activities in vitro against the malaria parasite Plasmodium falciparum and the pathogen of African sleeping sickness, Trypanosoma brucei rhodesiense.

Dimeric naphthylisoquinoline alkaloids: polyketide-derived axially chiral bioactive quateraryls

This is the first review on dimeric naphthylisoquinolines, a group of structurally intriguing, biosynthetically unique, and pharmacologically promising alkaloids.

Ancistrobreveines A–D and related dehydrogenated naphthylisoquinoline alkaloids with antiproliferative activities against leukemia cells, from the West African lianaAncistrocladus abbreviatus

A unique series of six biaryl natural products displaying four different coupling types (5,1′, 7,1′, 7,8′, and 5,8′) were isolated from the roots of the West African liana Ancistrocladus abbreviatus (Ancistrocladaceae). Although at first sight structurally diverse, these secondary metabolites all have in common that they belong to the rare group of naphthylisoquinoline alkaloids with a fully dehydrogenated isoquinoline portion. Among the African Ancistrocladus species, A. abbreviatus is so far only the second one that was found to produce compounds with such a molecular entity. Here, we report on four new representatives, named ancistrobreveines A–D (12–14, and 6). They were identified along with the two known alkaloids 6-O-methylhamateine (4) and ent-dioncophylleine A (10). The two latter naphthylisoquinolines had so far only been detected in Ancistrocladus species from Southeast Asia. All of these fully dehydrogenated alkaloids have in common being optically active despite the absence of stereogenic centers, due to the presence of the rotationally hindered biaryl axis as the only element of chirality. Except for ent-dioncophylleine A (10), which lacks an oxygen function at C-6, the ancistrobreveines A–D (12–14, and 6) and 6-O-methylhamateine (4) are 6-oxygenated alkaloids, and are, thus, typical ‘Ancistrocladaceae-type’ compounds. Ancistrobreveine C (14), is the first – and so far only – example of a 7,8′-linked fully dehydrogenated naphthylisoquinoline discovered in nature that is configurationally stable at the biaryl axis. The stereostructures of the new alkaloids were established by spectroscopic (in particular HRESIMS, 1D and 2D NMR) and chiroptical (electronic circular dichroism) methods. Ancistrobreveine C (14) and 6-O-methylhamateine (4) exhibited strong antiproliferative activities against drug-sensitive acute lymphoblastic CCRF-CEM leukemia cells and their multidrug-resistant subline, CEM/ADR5000.

Ancistrobreveines A–D belong to the rare group of naphthylisoquinoline alkaloids with a non-hydrogenated isoquinoline portion, some of them, like ancistrobreveine C, occurring in the plants only in a scalemic, yet nearly enantiopure form.

Mbandakamine-Type Naphthylisoquinoline Dimers and Related Alkaloids from the Central African Liana Ancistrocladus ealaensis with Antiparasitic and Antileukemic Activities

Four new dimeric naphthylisoquinoline alkaloids, michellamine A₅ (2) and mbandakamines C-E (4-6), were isolated from the Congolese plant Ancistrocladus ealaensis, along with the known dimer mbandakamine A (3). They represent constitutionally unsymmetric dimers, each consisting of two 5,8'-coupled naphthylisoquinoline monomers. While the molecular halves of michellamine A₅ (2) are linked via C-6' of both of the naphthalene moieties, i.e., via the least-hindered positions, so that the central biaryl axis is configurationally unstable and not an additional element of chirality, the mbandakamines 3-6 possess three consecutive stereogenic axes. Their monomeric units are linked through an unprecedented 6',1″-coupling in the binaphthalene core, leading to a high steric load, since the central axis is located in one of the peri-positions, neighboring one of the outer axes. In addition, four new 5,8'-coupled monomeric naphthylisoquinolines, viz., ancistroealaines C-F (7-10), were identified, along with four "naphthalene-devoid" tetra- and dihydroisoquinolines, named ealaines A-D (11-14). The new mbandakamines C (4) and D (5) showed pronounced activities against the malaria parasite Plasmodium falciparum, and they were likewise found to display strong cytotoxic activities against human leukemia (CCRF-CEM) and multi-drug-resistant tumor cells (CEM/ADR5000).

Michellamines A6 and A7, and further mono- and dimeric naphthylisoquinoline alkaloids from a Congolese Ancistrocladus liana and their antiausterity activities against pancreatic cancer cells

Michellamines A6 (1) and A7 (2) are the first dimers of 5,8'-coupled naphthylisoquinoline alkaloids with cis-configured stereocenters in both tetrahydroisoquinoline subunits. They were isolated from the leaves of a recently discovered, yet unidentified Congolese Ancistrocladus liana that shares some morphological characteristics with Ancistrocladus likoko. Two further new dimeric analogs, michellamines B4 (3) and B5 (4), were obtained, along with two previously likewise unknown monomers, ancistrobonsolines A1 (5) and A2 (6), which, besides one single known other example, are the only naphthyldihydroisoquinolines with an M-configured biaryl axis and R-configuration at C-3. Moreover, five compounds earlier reported from other Ancistrocladus species were identified, ancistroealaine C (7), korupensamines A (8a) and B (8b), and michellamines A2 (9) and E (10). Their complete structural elucidation succeeded due to the fruitful interplay of spectroscopic, chemical, and chiroptical methods. Chemotaxonomically, the stereostructures of the metabolites clearly delineate this Congolese Ancistrocladus liana from all known related species, showing that it might be a new taxon. Ancistrobonsolines A1 (5) and A2 (6) exhibited strong preferential cytotoxicities against human PANC-1 pancreatic cancer cells under nutrient-deprived conditions, without displaying toxicity in normal, nutrient-rich medium. Against cervical HeLa cancer cells, the dimeric alkaloids michellamines A6 (1) and E (10) displayed the highest cytotoxic activities, comparable to that of the standard agent, 5-fluorouracil. Furthermore, ancistrobonsolines A1 (5) and A2 (6) showed weak-to-moderate antiprotozoal activities.

Jozilebomines A and B, Naphthylisoquinoline Dimers from the Congolese Liana Ancistrocladus ileboensis, with Antiausterity Activities against the PANC-1 Human Pancreatic Cancer Cell Line

Two new naphthylisoquinoline dimers, jozilebomines A (1a) and B (1b), were isolated from the roots of the Congolese plant Ancistrocladus ileboensis, along with the known dimer jozimine A₂ (2). These compounds are Dioncophyllaceae-type metabolites, i.e., lacking oxygen functions at C-6 and with an R-configuration at C-3 in their tetrahydroisoquinoline moieties. The dimers 1a and 1b consist of two 7,1'-coupled naphthylisoquinoline monomers linked through an unprecedented 3',6″-coupling in the binaphthalene core and not, as in 2, via the C-3-positions of the two naphthalene units. Thus, different from the C₂-symmetric jozimine A₂ (2), the new jozilebomines are constitutionally unsymmetric. The central biaryl axis of each of the three dimers is rotationally hindered, so that 1a, 1b, and 2 possess three consecutive chiral axes. The two jozilebomines have identical constitutions and the same absolute configurations at all four stereogenic centers, but differ from each other in their axial chirality. Their structural elucidation was achieved by HRESIMS, 1D and 2D NMR, oxidative degradation, and experimental and calculated ECD data. They exhibited distinct and specific antiplasmodial activities. All dimers showed potent cytotoxicity against HeLa human cervical cancer cells and preferential cytotoxicity against PANC-1 human pancreatic cancer cells under nutrition-deprived conditions. Furthermore, these dimers significantly inhibited the colony formation of PANC-1 cells, even when exposed to noncytotoxic concentration for a short time. Jozilebomines A (1a) and B (1b) and jozimine A₂ (2) represent novel potential candidates for future drug development against pancreatic cancer.

Antiplasmodial Ealapasamines A-C,'Mixed' Naphthylisoquinoline Dimers from the Central African Liana Ancistrocladus ealaensis

Three unusual heterodimeric naphthylisoquinoline alkaloids, named ealapasamines A-C (1-3), were isolated from the leaves of the tropical plant Ancistrocladus ealaensis J. Léonard. These 'mixed', constitutionally unsymmetric dimers are the first stereochemically fully assigned cross-coupling products of a 5,8'- and a 7,8'-coupled naphthylisoquinoline linked via C-6' in both naphthalene portions. So far, only two other West and Central Ancistrocladus species were known to produce dimers with a central 6,6″-axis, yet, in contrast to the ealapasamines, usually consisting of two 5,8'-coupled monomers, like e.g., in michellamine B. The new dimers 1-3 contain six elements of chirality, four stereogenic centers and the two outer axes, while the central biaryl axis is configurationally unstable. The elucidation of the complete stereostructures of the ealapasamines was achieved by the interplay of spectroscopic methods including HRESIMS, 1D and 2D NMR (in particular ROESY measurements), in combination with chemical (oxidative degradation) and chiroptical (electronic circular dichroism) investigations. The ealapasamines A-C display high antiplasmodial activities with excellent half-maximum inhibition concentration values in the low nanomolar range.

Cimicifoetones A and B, Dimeric Prenylindole Alkaloids as Black Pigments of Cimicifuga foetida

Two dimeric prenylindole alkaloids with a unique indole-benzoindolequinone skeleton, cimicifoetones A (1) and B (2), were isolated as black pigments from the rhizomes of Cimicifuga foetida. The structures were elucidated by spectroscopic methods including HRMS and 2D NMR, as well as single-crystal X-ray diffraction and computational chemical modeling techniques. Cimicifoetones A and B represented the first examples of dimeric indole alkaloids generated through [4+2] Diels-Alder cycloaddition between the prenyl side chain in one 3-prenylindole and the aromatic ring in another. The two compounds showed promising anti-proliferative activity on seven tumor cell lines with IC₅₀ values in the range of 1.36-21.09 μm. Flow cytometric and western blot analysis revealed that compound 2 induced cell apoptosis via death receptor-mediated extrinsic and mitochondrial-mediated intrinsic pathways.

Dioncophyllines C2, D2, and F and Related Naphthylisoquinoline Alkaloids from the Congolese Liana Ancistrocladus ileboensis with Potent Activities against Plasmodium falciparum and against Multiple Myeloma and Leukemia Cell Lines

Dioncophylline F (1), the first 5,8'-coupled dioncophyllaceous alkaloid (i.e., lacking an oxygen function at C-6 and possessing an R-configuration at C-3), was isolated from the recently described Congolese liana Ancistrocladus ileboensis. Two further, likewise Dioncophyllaceae-type, alkaloids, the dioncophyllines C₂ (2) and D₂ (3), were identified, along with the Ancistrocladaceae-type compound ancistrocladisine B (4), which is oxygenated at C-6 and S-configured at C-3. The structures of the new compounds were determined by spectroscopic, chemical, and chiroptical methods. The stereostructure of 1 was further confirmed by total synthesis. As a consequence of the lack of a methyl group ortho to their biaryl axes, both dioncophylline F (1) and the 7,8'-coupled dioncophylline D₂ (3) occur as pairs of configurationally semistable and, thus, slowly interconverting atropo-diastereomers, whereas dioncophylline C₂ (2), with its 5,1'-linkage, is configurationally stable at the axis. Eight further known naphthylisoquinolines were isolated from A. ileboensis, among them dioncophylline A (P-10), its 4'-O-demethyl analogue P-11, and 5'-O-methyldioncophylline D (7), which were found to display strong cytotoxic activities against multiple myeloma INA-6 cells (P-10 even stronger than the standard drug melphalan) and against drug-sensitive acute lymphoblastic CCRF-CEM leukemia cells and their multidrug-resistant subline, CEM/ADR5000. Moreover, the dioncophyllines 1, 3, and 7 showed high-and specific-activities against the malaria parasite Plasmodium falciparum.

Ancistrolikokines E–H and related 5,8′-coupled naphthylisoquinoline alkaloids from the Congolese lianaAncistrocladus likokowith antiausterity activities against PANC-1 human pancreatic cancer cells

A striking feature of the metabolite profile ofAncistrocladus likoko(Ancistrocladaceae) is the exclusive production of 5,8′-linked naphthylisoquinoline alkaloids with strong activities against pancreatic cancer cells.

HIV-inhibitory michellamine-type dimeric naphthylisoquinoline alkaloids from the Central African liana Ancistrocladus congolensis

Five michellamine-type dimeric naphthylisoquinoline alkaloids (NIQs), named michellamines A2, A3, A4, B2, and B3, have been isolated from the root bark of the Central African liana Ancistrocladus congolensisJ. Léonard (Ancistrocladaceae), along with their two known parent compounds, the michellamines A and B, which had so far only been detected in the Cameroonian species Ancistrocladus korupensis. Five monomeric representatives, viz., korupensamine D, ancistrobrevine B, hamatine, 5'-O-demethylhamatine, and 6-O-methylhamatine, already known from related Ancistrocladus species, have likewise been identified. The structure elucidation was achieved by spectroscopic analysis including HRESIMS, 1D and 2D NMR, and by chemical and chiroptical methods. The michellamines A2, A3, B3, and A4 were evaluated for their cytotoxic and anti-HIV activities at a concentration range of 0-100 μM against the HIV reference strain IIIB/LAI in A3.01 T lymphoblast cell cultures, and their effects were compared to the ones displayed by the known michellamines A and B. Inhibitory activities for HIV replication were monitored for the michellamines A2 (IC50 = 29.6 μM), A3 (IC50 = 15.2 μM), A4 (IC50 = 35.9 μM), and B (IC50 = 20.4 μM). The michellamines A and B3, by contrast, did not inhibit HIV replication. No cytotoxicity was observed. Furthermore, the chemotaxonomic significance of the previously undescribed michellamines is discussed.

Anstifolines A and B, two dimeric furoquinoline alkaloids from the root bark of Dictamnus angustifolius

Anstifolines A and B, two dimeric furoquinoline alkaloids with unique coupling patterns were isolated from the root bark of Dictamnus angustifolius.

Highly antiplasmodial non-natural oxidative products of dioncophylline A: synthesis, absolute configuration, and conformational stability

Four new compounds, the monomeric dioncotetralones A (6 a) and B (6 b) and the dimeric compounds jozimine A3 (7) and jozimine A4 (9), were semi-synthesized from the natural product dioncophylline A (4) and its 5'-O-demethylated derivative (5), respectively, under phenol oxidative reaction conditions. Dioncotetralones A (6 a) and B (6 b) possess an unprecedented Z-configured double bond, in contrast to the classic biaryl axis that is present in the precursor dioncophylline A (4), and an additional stereogenic center at the C2' atom was generated due to the dearomatization. The resulting steric repulsion forced the expected planar double bond into a helical distorted conformation. The homocoupling of 5 yielded compounds 7 and 9, the latter of which is the first sp(3) -sp(2) coupled product of a monomeric naphthylisoquinoline with a reduced one and, thus, contains a newly generated stereogenic center. The full stereostructures of 6 a, 6 b, 7, and 9 were successfully elucidated by the interplay of spectroscopic methods (1D/2D NMR and electronic circular-dichroism spectroscopy) in combination with quantum-chemical calculations. In addition, compounds 6 a and 7 exhibited high antiplasmodial activities with excellent half-maximal inhibitory concentration values.

Fistulains A and B, New Bischromones from the Bark of Cassia fistula, and Their Activities

Fistulains A and B (1 and 2), two novel bischromones with unique coupling patterns, alone with their biosynthetic related compound 3, were isolated from the bark of Cassia fistula. Fistulain A represents a new type of dimeric chromone alkaloid biogenetically derived from a chromone and a tricyclic alkaloid through an unusual C-14-N linkage. Fistulain B has a new carbon skeleton with a C-14-C-5' linkage formed between two different chromone units. Fistulain A displayed anti-TMV activity, and both 1 and 2 showed weak cytotoxicities.

A twist of nature – the significance of atropisomers in biological systems

This review encompasses the synthesis and identification of recently detected natural atropisomers with potential therapeutic activity.