Total synthesis and biological evaluation of 22-hydroxyacuminatine

Recent advances in the radical cascade reaction for constructing nitrogen heterocycles using azides as radical acceptors

Due to the high conversion properties, azide compounds are widely utilized in organic synthesis. For instance, azide compounds readily release nitrogen to form a new N-C bond when they function as radical acceptors for the active intermediates in the reaction. Over the past decade, strategies employing azides as radical acceptors to construct nitrogen heterocycles have been extensively developed. This approach has emerged as a crucial method for synthesizing nitrogen heterocycles. Therefore, this paper provides a review of the research advancements in tandem cyclization reactions using azides as radical acceptors, summarizing the process of reaction design, exploration, reasoning of the mechanism, and prospects for further research of these reactions.

Biological properties of pyrroloquinoline and pyrroloisoquinoline derivatives

In this review, we summarize pyrroloquinoline and pyrroloisoquinoline derivatives (PQs and PIQs) that act on a broad spectrum of biological targets and are used as bacteriostatic, antiviral, plasmodial, anticancer, antidiabetic and anticoagulant agents. Many of these compounds play important roles in the study of DNA and its interactions, the regulation of the cell cycle and programmed cell death. This review involves twenty-five types of skeletally analogical compounds bearing pyrrole and (iso)quinoline scaffolds with different mutual annelations.

Novel Approach to the Construction of Fused Indolizine Scaffolds: Synthesis of Rosettacin and the Aromathecin Family of Compounds

Camptothecin-like compounds are actively employed as anticancer drugs in clinical treatments. The aromathecin family of compounds, which contains the same indazolidine core structure as the camptothecin family of compounds, is also expected to display promising anticancer activity. Therefore, the development of a suitable and scalable synthetic method of aromathecin synthesis is of great research interest. In this study, we report the development of a new synthetic approach for constructing the pentacyclic scaffold of the aromathecin family by forming the indolizidine moiety after synthesizing the isoquinolone moiety. Thermal cyclization of 2-alkynylbenzaldehyde oxime to the isoquinoline N-oxide, followed by a Reissert-Henze-type reaction, forms the key strategy in this isoquinolone synthesis. Under the optimum reaction conditions for the Reissert-Henze-type reaction step, microwave irradiation-assisted heating of the purified N-oxide in acetic anhydride at 50 °C reduced the formation of the 4-acetoxyisoquinoline byproduct to deliver the desired isoquinolone at a 73% yield after just 3.5 h. The eight-step sequence employed afforded rosettacin (simplest member of the aromathecin family) at a 23.8% overall yield. The synthesis of rosettacin analogs was achieved by applying the developed strategy and may be generally applicable to the production of other fused indolizidine compounds.

Mn-mediated oxidative radical cyclization of 2-(azidomethyl)phenyl isocyanides with carbazate: access to quinazoline-2-carboxylates

Mn-TBHP mediated oxidative radical cyclization of 2-(azidomethyl)phenyl isocyanides using methyl carbazate has been described.

Synthesis of C-Ring-Substituted Vasicinones and Luotonins via Regioselective Aza-Nazarov Cyclization of Quinazolinonyl Enones

A facile synthesis of C-ring substituted luotonins and vasicinones has been realized via a super-acid-mediated aza-Nazarov cyclization of quinazolinonyl enones. The regioselectivity of the cyclization is highly dependent on proton availability in the reaction medium.

Antiparasitic effect of synthetic aromathecins on Leishmania infantum

Background

Canine leishmaniasis is a zoonotic disease caused by Leishmania infantum, being the dogs one of the major reservoirs of human visceral leishmaniasis. DNA topology is a consolidated target for drug discovery. In this regard, topoisomerase IB – one of the enzymes controlling DNA topology – has been poisoned by hundreds of compounds that increase DNA fragility and cell death. Aromathecins are novel molecules with a multiheterocyclic ring scaffold that have higher stability than camptothecins.

Results

Aromathecins showed strong activity against both forms of L. infantum parasites, free-living promastigotes and intra-macrophagic amastigotes harbored in ex vivo splenic explant cultures obtained from infected BALB/c mice. However, they prevented the relaxation activity of leishmanial topoisomerase IB weakly, which suggests that the inhibition of topoisomerase IB partially explains the antileishmanial effect of these compounds. The effect of aromathecins was also studied against a strain resistant to camptothecin, and results suggested that the trafficking of these compounds is not through the ABCG6 transporter.

Conclusions

Aromathecins are promising novel compounds against canine leishmaniasis that can circumvent potential resistances based on drug efflux pumps.

Topoisomerase 1B poisons: Over a half-century of drug leads, clinical candidates, and serendipitous discoveries

Topoisomerases are DNA processing enzymes that relieve supercoiling (torsional strain) in DNA, are necessary for normal cellular division, and act by nicking (and then religating) DNA strands. Type 1B topoisomerase (Top1) is overexpressed in certain tumors, and the enzyme has been extensively investigated as a target for cancer chemotherapy. Various chemical agents can act as "poisons" of the enzyme's religation step, leading to Top1-DNA lesions, DNA breakage, and eventual cellular death. In this review, agents that poison Top1 (and have thus been investigated for their anticancer properties) are surveyed, including natural products (such as camptothecins and indolocarbazoles), semisynthetic camptothecin and luotonin derivatives, and synthetic compounds (such as benzonaphthyridines, aromathecins, and indenoisoquinolines), as well as targeted therapies and conjugates. Top1 has also been investigated as a therapeutic target in certain viral and parasitic infections, as well as autoimmune, inflammatory, and neurological disorders, and a summary of literature describing alternative indications is also provided. This review should provide both a reference for the medicinal chemist and potentially offer clues to aid in the development of new Top1 poisons.

Use of Bromine and Bromo-Organic Compounds in Organic Synthesis

Bromination is one of the most important transformations in organic synthesis and can be carried out using bromine and many other bromo compounds. Use of molecular bromine in organic synthesis is well-known. However, due to the hazardous nature of bromine, enormous growth has been witnessed in the past several decades for the development of solid bromine carriers. This review outlines the use of bromine and different bromo-organic compounds in organic synthesis. The applications of bromine, a total of 107 bromo-organic compounds, 11 other brominating agents, and a few natural bromine sources were incorporated. The scope of these reagents for various organic transformations such as bromination, cohalogenation, oxidation, cyclization, ring-opening reactions, substitution, rearrangement, hydrolysis, catalysis, etc. has been described briefly to highlight important aspects of the bromo-organic compounds in organic synthesis.

A review on anticancer potential of bioactive heterocycle quinoline

The advent of Camptothecin added a new dimension in the field anticancer drug development containing quinoline motif. Quinoline scaffold plays an important role in anticancer drug development as their derivatives have shown excellent results through different mechanism of action such as growth inhibitors by cell cycle arrest, apoptosis, inhibition of angiogenesis, disruption of cell migration, and modulation of nuclear receptor responsiveness. The anti-cancer potential of several of these derivatives have been demonstrated on various cancer cell lines. In this review we have compiled and discussed specifically the anticancer potential of quinoline derivatives, which could provide a low-height flying bird's eye view of the quinoline derived compounds to a medicinal chemist for a comprehensive and target oriented information for development of clinically viable anticancer drugs.

Structure-activity relationships for a novel series of citalopram (1-(3-(dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile) analogues at monoamine transporters

(+/-)-Citalopram (1, 1-(3-(dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile), and its eutomer, escitalopram (S-(+)-1) are selective serotonin reuptake inhibitors (SSRIs) that are used clinically to treat anxiety and depression. To further explore structure-activity relationships at the serotonin transporter (SERT), a series of (+/-)-4- and 5-substituted citalopram analogues were designed, synthesized, and evaluated for binding at the SERT, dopamine transporter (DAT) and norepinephrine transporter (NET) in native rodent tissue. Many of these analogues showed high SERT binding affinities (Ki=1-40 nM) and selectivities over both NET and DAT. Selected enantiomeric pairs of analogues were synthesized and both retained enantioselectivity as with S- and R-1, wherein S>R at the SERT. In addition, the enantiomeric pairs of 1 and 5 were tested for binding at the homologous bacterial leucine transporter (LeuT), wherein low affinities and the absence of enantioselectivity suggested distinctive binding sites for these compounds at SERT as compared to LeuT. These novel ligands will provide molecular tools to elucidate drug-protein interactions at the SERT and to relate those to behavioral actions in vivo.

The structure-activity relationships of A-ring-substituted aromathecin topoisomerase I inhibitors strongly support a camptothecin-like binding mode

Aromathecins are inhibitors of human topoisomerase I (Top1). These compounds are composites of several heteroaromatic systems, namely the camptothecins and indenoisoquinolines, and they possess notable Top1 inhibition and cytotoxicity when substituted at position 14. The SAR of these compounds overlaps with indenoisoquinolines, suggesting that they may intercalate into the Top1-DNA complex similarly. Nonetheless, the proposed binding mode for aromathecins is purely hypothetical, as an X-ray structure is unavailable. In the present communication, we have synthesized eight novel series of A-ring-substituted (positions 1-3) aromathecins, through a simple, modular route, as part of a comprehensive SAR study. Certain groups (such as 2,3-ethylenedioxy) moderately improve Top1 inhibition, and, often, antiproliferative activity, whereas other groups (2,3-dimethoxy and 3-substituents) attenuate bioactivity. Strikingly, these trends are very similar to those previously observed for the A-ring of camptothecins, and this considerable SAR overlap lends further support (in the absence of crystallographic data) to the hypothesis that aromathecins bind in the Top1 cleavage complex as interfacial inhibitors in a 'camptothecin-like' pose.

3-Benzyl-9-phenyl-2-tosyl-2,3,3a,4,9,9a-hexa-hydro-1H-pyrrolo[3,4-b]quinoline

In the title compound, C₃₁H₃₀N₂O₂S, the pyrrolidine ring adopts a twist conformation while the tetra­hydro­pyridine ring is in a half-chair conformation. The two rings are trans-fused. The pyridine-bound phenyl ring forms dihedral angles of 17.7 (1) and 48.1 (1)°, respectively, with the tosyl and benzyl phenyl rings. The mol­ecular structure is stabilized by an N—H⋯π inter­action involving the benzyl phenyl ring. In the crystal structure, mol­ecules translated by one unit along the a axis are linked into chains by C—H⋯π inter­actions involving the benzene ring of the tosyl group.

Synthesis and biological evaluation of 14-(aminoalkyl-aminomethyl)aromathecins as topoisomerase I inhibitors: investigating the hypothesis of shared structure-activity relationships

The aromathecin topoisomerase I (top1) inhibitors offer promising scaffolds for the development of novel cancer chemotherapeutics. They are 'composites' of the camptothecin and indenoisoquinoline top1 inhibitors. Interestingly, some structure-activity relationship (SAR) overlap between the aromathecins and the indenoisoquinolines has been observed. For both classes, placement of certain polar groups in similar regions of the heteroaromatic system improves top1 inhibitory and antiproliferative activities. A series of water-soluble aromathecins substituted at position 14 with diaminoalkanes of various lengths has been prepared. These compounds all possess similar antiproliferative potency, but a general trend is observed: aromathecins with longer diaminoalkane substituents (>6 carbons) possess lower anti-top1 activity than their smaller counterparts (2-4 carbons), presumably as a result of unfavorable hydrophobic interactions. This trend is also noted with the indenoisoquinolines, revealing additional SAR overlap that supports the hypothesis that there is a 'universal' top1 inhibitor SAR.

The indenoisoquinoline noncamptothecin topoisomerase I inhibitors: update and perspectives

Because camptothecins are effective against previously resistant tumors and are the only class of topoisomerase I (Top1) inhibitors approved for cancer treatment, we developed the indenoisoquinolines. Like camptothecins, the indenoisoquinolines selectively trap Top1-DNA cleavage complexes and have been cocrystallized with the Top1-DNA cleavage complexes. Indenoisoquinolines show antitumor activity in animal models. They have several advantages over the camptothecins: (a) They are synthetic and chemically stable. (b) The Top1 cleavage sites trapped by the indenoisoquinolines have different genomic locations, implying differential targeting of cancer cell genomes. (c) The Top1 cleavage complexes trapped by indenoisoquinolines are more stable, indicative of prolonged drug action. (d) They are seldom or not used as substrates for the multidrug resistance efflux pumps (ABCG2 and MDR-1). Among the >400 indenoisoquinolines synthesized and evaluated, three have been retained as leads for clinical development by the National Cancer Institute: NSC 706744, NSC 725776 (Indimitecan), and NSC 724998 (Indotecan). The trapping of Top1 cleavage complexes by indenoisoquinolines in cells results in the rapid and sustained phosphorylation of histone H2AX (γ-H2AX). We discuss the use of γ-H2AX as a pharmacodynamic biomarker for the clinical development of the indenoisoquinolines.

The binding orientation of a norindenoisoquinoline in the topoisomerase I-DNA cleavage complex is primarily governed by pi-pi stacking interactions

High level ab initio quantum chemical studies have shown that the binding orientations of topoisomerase I (top1) inhibitors such as camptothecins and indenoisoquinolines are primarily governed by pi-pi stacking. However, a recently discovered norindenoisoquinoline antitumor compound was observed by X-ray crystallography to adopt a "flipped" orientation (relative to indenoisoquinolines), which facilitates the formation of a characteristic hydrogen bond with the Arg364 of top1 in its binding with the top1-DNA complex. This observation raises the possibility that hydrogen bonding between the norindenoisoquinoline nitrogen and the Arg364 side chain of top1 might be responsible for the "flip". It also brings into question whether pi-pi stacking, as opposed to hydrogen bonding, is primarily responsible for the binding orientations of indenoisoquinolines and norindenoisoquinolines. In this study, the forces responsible for the binding orientation of a norindenoisoquinoline in the DNA cleavage site were systematically investigated using MP2 methods. The theoretical calculation of the preferred binding orientation based solely on pi-pi stacking was completely consistent with the actual orientation observed by X-ray crystallography, indicating that the binding of the norindenoisoquinoline in the top1-DNA complex is mainly governed by pi-pi stacking forces and that the "flip" can occur independently from hydrogen bonding.

Design, synthesis, and biological evaluation of 14-substituted aromathecins as topoisomerase I inhibitors

The aromathecin or "rosettacin" class of topoisomerase I (top1) inhibitors is effectively a "composite" of the natural products camptothecin and luotonin A and the synthetic indenoisoquinolines. The aromathecins have aroused considerable interest following the isolation and total synthesis of 22-hydroxyacuminatine, a rare cytotoxic natural product containing the 12 H-5,11a-diazadibenzo[ b, h]fluoren-11-one system. We have developed two novel syntheses of this system and prepared a series of 14-substituted aromathecins as novel antiproliferative topoisomerase I poisons. These inhibitors are proposed to act via an intercalation and "poisoning" mechanism identical to camptothecin and the indenoisoquinolines. Many of these compounds possess greater antiproliferative activity and anti-top1 activity than the parent unsubstituted compound (rosettacin) and previously synthesized aromathecins, as well as greater top1 inhibitory activity than 22-hydroxyacuminatine. In addition to potentially aiding solubility and localization to the DNA-enzyme complex, nitrogenous substituents located at the 14-position of the aromathecin system have been proposed to project into the major groove of the top1-DNA complex and hydrogen-bond to major-groove amino acids, thereby stabilizing the ternary complex.

Synthesis and bioevaluation of 22-hydroxyacuminatine analogs

A series of 22-hydroxyacuminatine analogs was prepared by using different Friedländer condensations. Several of the new compounds were tested for antiproliferative activity on cancer cell lines and for topoisomerase I inhibitory activity.

3-Ethyl-9-phenyl-2-tosyl-2,3,3a,4,9,9a-hexahydro-1H-pyrrolo[3,4-b]quinoline

In the mol­ecule of the title compound, C₂₆H₂₈N₂O₂S, the pyrrolidine ring adopts an envelope conformation and the tetra­hydro­pyridine ring is in a half-chair conformation; these two rings are trans-fused. The dihedral angle between the pyridine- and sulfonyl-bound benzene rings is 36.15 (5)°. In the crystalline state, the mol­ecules are linked into a two-dimensional network parallel to the ab plane by C—H⋯O and C—H⋯π inter­actions.

Short and efficient total synthesis of luotonin A and 22-hydroxyacuminatine using a common cascade strategy

Total syntheses of 22-hydroxyacuminatine and luotonin A were achieved in direct fashion and high overall yields (16% in eight steps and 47% in five steps, respectively). A mild cascade methodology was successfully applied in both syntheses as a common strategy. The new approach presents the advantages of short routes, high overall yields, use of stable intermediates, and ease of operations.

Synthesis and topoisomerase poisoning activity of A-ring and E-ring substituted luotonin A derivatives

A series of A-ring and E-ring analogues of the natural product luotonin A, a known topoisomerase I poison, was evaluated for growth inhibition in human carcinoma and leukemia cell lines. Rational design of structures was based on analogues of the related alkaloid camptothecin, which has been demonstrated to exert cytotoxic effects by the same mechanism of action. When compared to luotonin A, several compounds exhibited an improved topoisomerase I-dependent growth inhibition of a human leukemia cell line.