Palladium nanoparticles for the synthesis of phenanthridinones and benzo[c]chromenes via C-H activation reaction

In the present work, derivatives of phenanthridine-6(5H)-ones and benzo[c]chromenes were efficiently prepared through an intramolecular C-H bond functionalization reaction catalyzed by photochemically synthesized Pd-PVP nanoparticles. The heterocycles were obtained via intramolecular arylation of the corresponding N-methyl-N-aryl-2-halobenzamide or aryl-(2-halo)benzyl ethers using K2CO3 as base in a mixture of H2O : DMA as solvent without additives or ligands. High yields of the heterocyclic compounds were achieved (up to 95%) using a moderately low catalyst loading (1-5 mol%) under an air atmosphere at 100 °C. The reaction exhibited very good tolerance to diverse functional groups (OMe, Me, t Bu, Ph, OCF3, CF3, F, Cl, -CN, Naph), and both bromine and iodine substrates showed great reactivity. Finally, the in vitro antiproliferative activity of phenanthridine-6(5H)-ones and benzo[c]chromenes was evaluated against six human solid tumor cell lines. The more active compounds exhibit activity in the low micromolar range. 1-Isopropyl-4-methyl-6H-benzo[c]chromene was identified as the best compound with promising values of activity (GI50 range 3.9-8.6 μM). Thus, the benzochromene core was highlighted as a novel organic building block to prepare potential antitumor agents.

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.

Aryl-aryl cross-coupling reactions without reagents or catalysts: photocyclization of ortho-iodoaryl ethers and related compounds via triplet aryl cation intermediates

Cyclisations of benzyl ortho-iodoaryl ethers to benzo[c]chromenes can be effected without reagents or catalysts by irradiation with UVC under flow. Reactions proceed via triplet aryl cation generation, 5-exo and 3-exo-cyclisations, and rearomatisation. They have wide scope, are easy to effect and extend to a myriad of related ring systems.

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.

The Stereoselective Total Synthesis of Axially Chiral Naphthylisoquinoline Alkaloids

The naphthylisoquinoline (NIQ) alkaloids are a thrilling class of natural biaryls─structurally, biosynthetically, and pharmacologically. A common feature of these metabolites is the biaryl bond between their naphthalene and isoquinoline moieties, which in most cases is rotationally hindered, leading to the phenomenon of axial chirality. Depending on their individual structures, including the respective axial configurations, NIQs show promising bioactivities. Their total synthesis is a challenging but rewarding goal, with the stereocontrolled construction of the biaryl linkage as the key step.The position of the biaryl axis and its configuration determine the overall molecular shape and thus the choice of the best possible method for efficient asymmetric aryl-aryl bond formation. The axis in NIQs can cover a broad range of steric hindrance, from freely rotating to configurationally stable. For dioncophylline B (1) and dioncophylline F (2a/b), with only two ortho-substituents next to the axis, the synthesis is easy to accomplish by direct coupling of the intact naphthalene moiety with the isoquinoline unit, and no atropo-selectivity is required.Naphthylisoquinolines with a configurationally stable biaryl axis are the focus of the present Account. They are more difficult to synthesize because, in addition to the problem of decreased chemical yields with increasing steric hindrance at the axis, the synthesis needs to proceed stereoselectively. Within this class of NIQs, 5,8'-coupled representatives, such as korupensamine A (3a), have received considerable synthetic attention because the rotational barrier is high enough for the existence of atropisomerism without being too excessive, and they show potent bioactivities. Their synthesis, as systematically presented herein, thus occupies a central role in this report. For their aryl-aryl bond formation, both intra- and intermolecular approaches can be successfully applied. Axial stereoinformation is introduced by internal asymmetric induction from stereogenic elements already present in the isoquinoline or its precursors, from chiral auxiliary elements artificially introduced, or by external asymmetric induction using chiral catalysts.To overcome even higher steric hindrance, as in ancistrocladine (4a), innovative approaches were developed. A most successful strategy is the "lactone concept" developed by the Bringmann group, which allows the directed synthesis of any desired atropisomer in high chemical and optical yields, thus permitting the atropo-divergent preparation of the two isomers from a single joint precursor. In this approach, the two formal tasks of stereoselective biaryl synthesis, which are usually done simultaneously─the C-C linkage and the asymmetric induction─are achieved consecutively. The coupling step is performed intramolecularly after prefixation of the coupling partners by an ester bridge. The resulting biaryl lactone already possesses the biaryl axis but is still configurationally unstable; it can then, with internal or external asymmetric induction, be cleaved atropo-divergently with high stereoselectivities. Besides its unique concept, the procedure excels by its broad applicability; among all presented methods, it has been used for the synthesis of the largest number of NIQs, more than 20 representatives, including those with the highest steric hindrance.This Account gives comprehensive insight into the plethora of conceptual approaches for the efficient formation of the hindered biaryl bond of NIQs.

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.

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.