Efficient and Practical One-Pot Conversions of N-Tosyltetrahydroisoquinolines into Isoquinolines and of N-Tosyltetrahydro-β-carbolines into β-Carbolines through Tandem β-Elimination and Aromatization

Cyclometalated ruthenium (II) complexes induced HeLa cell apoptosis through intracellular reductive injury

The main challenge of cancer chemotherapy is the resistance of tumor cells to oxidative damage. Herein, we proposed a novel antitumor strategy: cyclic metal‑ruthenium (Ru) complexes mediate reductive damage to kill tumor cells. We designed and synthesized Ru(II) complexes with β-carboline as ligands: [Ru (phen)2(NO2-Ph-βC)](PF6) (RuβC-7) and [Ru(phen)2(1-Ph-βC)](PF6) (RuβC-8). In vitro experimental results showed that RuβC-7 and RuβC-8 can inhibit cell proliferation, promote mitochondrial abnormalities, and induce DNA damage. Interestingly, RuβC-7 with SOD activity could reduce intracellular reactive oxygen species (ROS) levels, while RuβC-8 has the opposite effect. Accordingly, this study identified the reductive damage mechanism of tumor apoptosis, and may provide a new ideas for the design of novel metal complexes.

Syntheses of 3-Aryl Tetrahydroisoquinolines via an Intermolecular [4 + 2] Cycloaddition of Sultines with Imines

The development of an intermolecular aza-Diels-Alder (DA) cycloaddition of sultines and imines is reported. By exploiting sultines as o-quinodimethane precursors and aryl imines as dienophiles in the presence of Cu(OTf)₂, an aza-DA reaction proceeds to provide a wide variety of 3-aryl tetrahydroisoquionlines in moderate to excellent yield (up to 89%). The synthetic utility of these products was demonstrated in the preparation of tetracyclic N-heterocycles, including a tetrahydroprotoberberine skeleton.

Cyclometalated Ru(II)-isoquinoline complexes overcome cisplatin resistance of A549/DDP cells by downregulation of Nrf2 via Akt/GSK-3β/Fyn pathway

Both ruthenium (Ru) and isoquinoline (IQ) compounds are regarded as potential anticancer drug candidates. Here, we report the synthesis and characterization of three novel cyclometalated Ru(II)-isoquinoline complexes: RuIQ-3, RuIQ-4, and RuIQ-5, and evaluation of their in vitro cytotoxicities against a panel of cell lines including A549/DDP, a cisplatin-resistant human lung cancer cell line. A549/DDP 3D multicellular tumor spheroids (MCTSs) were also used to detect the drug resistance reversal effect of Ru(II)-IQ complexes. Our results indicated that the cytotoxic activities against cancer cells of Ru(II)-IQ complexes, especially RuIQ-5, were superior compared with cisplatin. In addition, RuIQ-5 exhibited low toxicity towards both normal HBE cells in vitro and zebrafish embryos in vivo. Further investigation on cellular mechanism of action indicated that after absorption by A549/DDP cells, RuIQ-5 was mainly distributed in the nucleus, which is different from cisplatin. Besides, RuIQ-5 could induce apoptosis through mitochondrial dysfunction, reactive oxygen species (ROS) accumulation, ROS-mediated DNA damage, and cycle arrest at both S and G2/M phases. Moreover, RuIQ-5 could inhibit the overexpression of Nrf2 through regulation of Akt/GSK-3β/Fyn signaling pathway and hindering the nuclear translocation of Nrf2. Based on these findings, we firmly believe that the studied Ru(II)-IQ complexes hold great promise as anticancer therapeutics with high effectiveness and low toxicity.

Cyclometalated Ru(II) β-carboline complexes induce cell cycle arrest and apoptosis in human HeLa cervical cancer cells via suppressing ERK and Akt signaling

Two new cyclometalated Ru(II)-β-carboline complexes, [Ru(dmb)₂(Cl-Ph-βC)](PF₆) (dmb = 4,4'-dimethyl-2,2'-bipyridine; Cl-Ph-βC = Cl-phenyl-9H-pyrido[3,4-b]indole; RuβC-3) and [Ru(bpy)₂(Cl-Ph-βC)](PF₆) (bpy = 2,2'-bipyridine; RuβC-4) were synthesized and characterized. The Ru(II) complexes display high cytotoxicity against HeLa cells, the stabilized human cervical cancer cell, with IC₅₀ values of 3.2 ± 0.4 μM (RuβC-3) and 4.1 ± 0.6 μM (RuβC-4), which were considerably lower than that of non-cyclometalated Ru(II)-β-carboline complex [Ru(bpy)₂(1-Py-βC)] (PF₆)₂ (61.2 ± 3.9 μM) by 19- and 15-folds, respectively. The mechanism studies indicated that both Ru(II) complexes could significantly inhibit HeLa cell migration and invasion, and effectively induce G0/G1 cell cycle arrest. The new Ru(II) complexes could also trigger apoptosis through activating caspase-3 and poly (ADP-ribose) polymerase (PARP), increasing the Bax/Bcl-2 ratio, enhancing reactive oxygen species (ROS) generation, decreasing mitochondrial membrane potential (MMP), and inducing cytochrome c release from mitochondria. Further research revealed that RuβC-3 could deactivate the ERK/Akt signaling pathway thus inhibiting HeLa cell invasion and migration, and inducing apoptosis. In addition, RuβC-3-induced apoptosis in HeLa cells was closely associated with the increase of intracellular ROS levels, which may act as upstream factors to regulate ERK and Akt pathways. More importantly, RuβC-3 exhibited low toxicity on both normal BEAS-2B cells in vitro and zebrafish embryos in vivo. Consequently, the developed Ru(II) complexes have great potential on developing novel low-toxic anticancer drugs.

Discovery of β-Carboline Derivatives as a Highly Potent Cardioprotectant against Myocardial Ischemia-Reperfusion Injury

Timely myocardial reperfusion salvages ischemic myocardium from infarction, whereas reperfusion itself induces cardiomyocyte death, which is called myocardial ischemia/reperfusion (MI/R) injury. Herein, β-carboline derivative 17c was designed and synthesized with obvious myocardial protective activity for the first time. Pretreatment of 17c effectively protected the cardiomyocyte H9c2 cells from H₂O₂-induced lactate dehydrogenase leakage and restored the endogenous antioxidants, superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px). Besides, 17c effectively protected the mitochondria through decreasing the reactive oxygen species overproduction and enhancing the mitochondrial membrane potential. As a result, 17c significantly reduced the necrosis of cardiomyocytes in H₂O₂-induced oxidative stress, which was more potent than polydatin. In MI/R injury rats, 17c pretreatment obviously increased the levels of SOD and GSH-Px and inhibited the apoptosis of cardiomyocytes. Through this way, the size of myocardial infarction was significantly reduced after MI/R injury in vivo, better than that of polydatin, suggesting that 17c is a promising cardioprotectant for the prevention of MI/R injury.

Palladium-Catalyzed Synthesis of 1-Vinyltetrahydro-β-carbolines and Aza-spiroindolenines: Access to the Syntheses of 1-Vinyl-β-carbolines and Eudistomins Y1 and Y2

A general synthesis of 1-vinyltetrahydro-β-carbolines (THBCs) has been achieved via palladium(0)-catalyzed cyclocondensation between allenyltryptamines and aryl iodides. Aza-spiroindolenines could also be accessed from the N-unsubstituted indole substrates by simply tweaking the reaction conditions. DDQ-mediated oxidation of THBCs easily afforded β-carbolines, which could be synthetically transformed into 1-aroyl-β-carbolines of pharmacological interest. Formal total syntheses of eudistomins Y1 and Y2 have also been achieved.

Diprotonative stabilization of ring-opened carbocationic intermediates: conversion of tetrahydroisoquinoline to triarylmethanes

Superacid-promoted conversion of tetrahydroisoquinolines to triarylmethanes via tandem reactions of C-N bond scission, Friedel-Crafts alkylation, C-O bond scission, and electrophilic aromatic amidation was developed. Dication formation was important for stabilizing the ring-opened carbocationic intermediate, which is a new role for diprotonation in reaction mechanisms.

Novel cyclometalated Ru(II) complexes containing isoquinoline ligands: Synthesis, characterization, cellular uptake and in vitro cytotoxicity

Two novel cyclometalated Ru(II) complexes containing isoquinoline ligand, [Ru(bpy)₂(1-Ph-IQ)](PF₆), (bpy = 2,2'-bipyridine; 1-Ph-IQ = 1-phenylisoquinoline; RuIQ-1) and [Ru(phen)₂(1-Ph-IQ)](PF₆) (phen = 1,10-phenanthroline; RuIQ-2) were found to show high cytotoxic activity against NCI-H460, A549, HeLa and MCF-7 cell lines. Notably, both of them exhibited IC₅₀ values that were an order of magnitude lower than those of clinical cisplatin and two structurally similar Ru(II)-isoquinoline complexes [Ru(bpy)₂(1-Py-IQ)](PF₆)₂ (Ru3) and [Ru(phen)₂(1-Py-IQ)](PF₆)₂ (Ru4) (1-Py-IQ = 1-pyridine-2-yl). The cellular uptake and intracellular localization displayed that the two cyclometalated Ru(II) complexes entered NCI-H460 cancer cells dominantly via endocytosis pathway, and preferentially distributed in the nucleus. Further investigations on the apoptosis-inducing mechanisms of RuIQ-1 and RuIQ-2 revealed that the two complexes could cause S, G2/M double-cycle arrest by regulating cell cycle related proteins. The two complexes also could reduce the mitochondrial membrane potential (MMP), promote the generation of intracellular ROS and trigger DNA damage, and then lead to apoptosis-mediated cell death. More importantly, RuIQ-2 exhibits low toxicity both towards normal HBE cells in vitro and zebrafish embryos in vivo. Accordingly, the developed complexes hold great potential to be developed as novel therapeutics for effective and low-toxic cancer treatment.

A facile metal-free one-pot synthesis of 3-aminoisoquinolines by intramolecular transannulation of 1-sulfonyl-4-(2-aminomethylphenyl)-1,2,3-triazoles

A metal-free one-pot intramolecular transannulation of 1-sulfonyl-4-(2-aminomethylphenyl)-1,2,3-triazoles has been developed, which enables the facile synthesis of the various 3-aminoisoquinolines as well as relevant scaffolds from readily available starting materials.

An efficient method for the construction of 3-aminoisoquinolines through a metal-free one-pot intramolecular transannulation of 1-sulfonyl-4-(2-aminomethylphenyl)-1,2,3-triazoles has been developed.

CuBr2-Catalyzed Mild Oxidation of 3,4-Dihydro-β-Carbolines and Application in Total Synthesis of 6-Hydroxymetatacarboline D

A green chemical method for the conversion of 3,4-dihydro-β-carbolines to β-carbolines has been developed using air as the oxidant. With 15 mol % CuBr₂ as the catalyst, 3,4-dihydro-β-carbolines could be efficiently oxidized to β-carbolines in dimethyl sulfoxide at room temperature in the presence of 1,8-diazabicyclo[5,4,0]undec-7-ene (or Et₃N). By applying this method, the first total synthesis of 6-hydroxymetatacarboline D was performed through 12 steps in 22% overall yield starting from l-5-hydroxy-tryptophan.

Synthesis of novel (benzimidazolyl)isoquinolinols and evaluation as adenosine A1 receptor tools

G protein-coupled receptors (GPCRs) constitute the largest family of transmembrane receptors in eukaryotes. The adenosine A₁ receptor (A₁AR) is a class A GPCR that is of interest as a therapeutic target particularly in the treatment of cardiovascular disease and neuropathic pain. Increased knowledge of the role A₁AR plays in mediating these pathophysiological processes will help realise the therapeutic potential of this receptor. There is a lack of enabling tools such as selective fluorescent probes to study A₁AR, therefore we designed a series of (benzimidazolyl)isoquinolinols conjugated to a fluorescent dye (31–35, 42–43). An improved procedure for the synthesis of isoquinolinols from tetrahydroisoquinolinols via oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and atmospheric oxygen is reported. This synthetic method offers advantages over previous metal-based methods for the preparation of isoquinolinols and isoquinolines, which are important scaffolds found in many biologically active compounds and natural products. We report the first synthesis of the (benzimidazolyl)isoquinolinol compound class, however the fluorescent conjugates were not successful as A₁AR fluorescent ligands.

Mild, metal free aromatization of tetrahydroisoquinolinols. Synthesis of (benzimidazolyl)isoquinolinols.

Synthesis, characterization, cellular uptake and apoptosis-inducing properties of two highly cytotoxic cyclometalated ruthenium(II) β-carboline complexes

Two new cyclometalated Ru(II) complexes of the general formula [Ru(N-N)₂(1-Ph-βC)](PF₆), where N-N = 4,4'-dimethyl-2,2'-bipyridine (dmb, Ru1), 2,2'-bipyridine (bpy, Ru2), and 1-Ph-βC (1-phenyl-9H-pyrido[3,4-b]indole) is a β-carboline alkaloids derivatives, have been synthesized and characterized. The in vitro cytotoxicities, cellular uptake and localization, cell cycle arrest and apoptosis-inducing mechanisms of these complexes have been extensively explored by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, inductively coupled plasma mass spectrometry (ICP-MS), flow cytometry, comet assay, inverted fluorescence microscope as well as western blotting experimental techniques. Notably, Ru1 and Ru2 exhibit potent antiproliferative activities against selected human cancer cell lines with IC₅₀ values lower than those of cisplatin and other non-cyclometalated Ru(II) β-carboline complexes. The cellular uptake and localization exhibit that these complexes can accumulate in the cell nuclei. Further antitumor mechanism studies show that Ru1 and Ru2 can cause cell cycle arrest in the G0/G1 phase by regulating cell cycle relative proteins and induce apoptosis through mitochondrial dysfunction, reactive oxygen species (ROS) accumulation and ROS-mediated DNA damage.

Rhodium(ii)-catalyzed intramolecular annulation of 1-sulfonyl-1,2,3-triazoles with indoles: facile synthesis of functionalized tetrahydro-β-carbolines

A novel rhodium(ii)-catalyzed intramolecular annulation of 1-sulfonyl-1,2,3-triazoles with indoles has been developed, which enables the facile synthesis of various 4-functionalized tetrahydro-β-carbolines and relevant scaffolds from readily available starting materials.

Preparation and pH-switching of fluorescent borylated arylisoquinolines for multilevel molecular logic

The preparation of pH-switchable fluorescent borylated arylisoquinoline dyes via a flexible iridium-catalyzed route is reported. The obtained dyes feature aromatic amino substitution and lateral aliphatic amino groups as electron donors. The photophysical properties of the internal charge transfer dyes were studied, which was complemented by density functional theory calculations. Appreciable fluorescence quantum yields (Φf up to ca. 0.4) and characteristic spectral fingerprints in the green to red emission range were observed. The fluorescence modulation upon multiple and orthogonal protonation with triflic acid was studied and led to the interpretation of multilevel switching including off-on-off, ternary, and quaternary responses.

A New Route to Azafluoranthene Natural Products via Direct Arylation

Microwave-assisted direct arylation was successfully employed in the synthesis of azafluoranthene alkaloids for the first time. Direct arylation reactions on a diverse set of phenyltetrahydroisoquinolines produces the indeno[1,2,3-ij]isoquinoline nucleus en route to a high yielding azafluoranthene synthesis. The method was used as a key step in the efficient preparation of the natural products rufescine and triclisine. As demonstrated herein, this synthetic approach should be generally applicable to the preparation of natural and un-natural azafluoranthene alkaloids as well as "azafluoranthene-like" isoquinoline alkaloids.