Ancistrolikokines A-C: new 5,8'-coupled naphthylisoquinoline alkaloids from Ancistrocladus likoko

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.

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.

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.

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.

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.

The discovery of the two unsymmetrically coupled dimeric naphthylisoquinoline alkaloids, the mbandakamines B₃ and B₄, showing a high steric hindrance at the central biaryl axis and displaying pronounced antiplasmodial activities, is described.

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.

The first dimers of 5,8′-coupled naphthylisoquinolines with two 1,3-cis-configured tetrahydroisoquinoline portions and their cytotoxicities against cancer cells are described.

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.

Synthesis of 6,6'-binaphthopyran-2-one natural products: pigmentosin A, talaroderxines A and B

Efficient and stereoselective syntheses of pigmentosin A, talaroderxine A, and its diastereomer talaroderxine B are reported. The binaphthyl ring system is assembled by vanadium-catalyzed phenolic coupling of tricyclic precursors. These key intermediates were prepared by Michael-Dieckmann annulation of a protected orsellinate ester, with the requisite pyranones accessed by a new variant of Ghosez's sulfone-epoxide annulation. Preliminary biological experiments are reported for pigmentosin.

Total Synthesis of the 7,3‘-Linked Naphthylisoquinoline Alkaloid Ancistrocladidine

The first total synthesis of ancistrocladidine, a rare 7,3'-linked naphthylisoquinoline alkaloid, has been completed, with the key feature of the synthesis being the formation of the extremely hindered biaryl linkage by ortho-arylation of a naphthol with an aryllead triacetate. Initial efforts were focused on the generation of a heteroaryl lead species, which would have allowed a convergent synthesis to be developed. However, it was not possible to generate such a lead species. A simpler aryl lead triacetate was prepared and reacted. The resulting biaryl aldehyde was elaborated in 10 steps to form a 1:1 mixture of ancistrocladidine and its atropisomer. Recrystallization of the mixture afforded ancistrocladidine, which was identical in all respects to the reported data.

Ancistrolikokine D, a 5,8'-coupled naphthylisoquinoline alkaloid, and related natural products from Ancistrocladus likoko

A new naphthylisoquinoline alkaloid, ancistrolikokine D, and the likewise 5,8'-coupled alkaloid ancistroealaine A, as well as two further, biosynthetically related, but nitrogen-free natural products, ancistronaphthoic acid B and cis-isoshinanolone, have been isolated from Ancistrocladus likoko J. LEACUTE;ONARD (Ancistrocladaceae). The 5,8'-coupling of the new alkaloids and of the alkaloids isolated earlier hints at a close phylogenetic relationship of A. likoko to other Central African Ancistrocladus species. The compounds show moderate activities against Leishmania donovani, Trypanosoma cruzi, and Trypanosoma brucei rhodesiense.