Occurrence, Biogenesis, and Synthesis of Biologically Active Carbazole Alkaloids

A NRAS mRNA G-quadruplex structure-targeting small-molecule ligand reactivating DNA damage response in human cancer cells for combination therapy with clinical PI3K inhibitors

The Neuroblastoma RAS (NRAS) oncogene homologue plays crucial roles in diverse cellular processes such as cell proliferation, survival, and differentiation. Several strategies have been developed to inhibit NRAS or its downstream effectors; however, there is no effective drug available to treat NRAS-driven cancers and thus new approaches are needed to be established. The mRNA sequence expressing NRAS containing several guanine(G)-rich regions may form quadruplex structures (G4s) and regulate NRAS translation. Therefore, targeting NRAS mRNA G4s to repress NRAS expression at translational level with ligands may be a feasible strategy against NRAS-driven cancers but it is underexplored. We reported herein a NRAS mRNA G4-targeting ligand, B3C, specifically localized in cytoplasm in HeLa cells. It effectively downregulates NRAS proteins, reactivates the DNA damage response (DDR), causes cell cycle arrest in G2/M phase, and induces apoptosis and senescence. Moreover, combination therapy with NARS mRNA G4-targeting ligands and clinical PI3K inhibitors for cancer cells inhibition treatment is unexplored, and we demonstrated that B3C combining with PI3Ki (pictilisib (GDC-0941)) showed potent antiproliferation activity against HeLa cells (IC50 = 1.03 μM (combined with 10 μM PI3Ki) and 0.42 μM (combined with 20 μM PI3Ki)) and exhibited strong synergistic effects in inhibiting cell proliferation. This study provides new insights into drug discovery against RAS-driven cancers using this conceptually new combination therapy strategy.

Synthesis of di/tri-substituted carbazoles involving Pd-mediated Sonogashira coupling of indolyltriflates with aryl acetylenes

In this study, we present our preliminary findings on the synthesis of carbazole derivatives involving the Sonogashira coupling reaction of 2-(trimethylamino)methylindolyltriflates with aryl acetylenes followed by isomerization, thermal electrocyclization and 1,3-H shift, furnishing the respective di- and tri-substituted carbazoles.

A rapid approach towards a specific carbazolelactam system related to calothrixin B

Reported here is a rapid and versatile synthetic approach towards the specific pentacyclic carbazolelactams related to calothrixin B. The crucial transformation is an acid-promoted cascade involving a dehydration/electrocyclization/C-N bond cleavage/lactamization sequence.

Gram-Scale Total Synthesis of Carbazomycins A-D through Late-Stage Regioselective Demethylation of Aryl Methyl Ethers

Gram-scale total synthesis of carbazomycins A-D is described. The total synthesis of carbazomycin A was achieved in 44% overall yield over six steps from symmetrical 5-chloro-1,2,3-trimethoxybenzene. The key aryne-mediated carbazole formation and methylation steps provided the multiply substituted carbazole. The regioselective demethylation of the trimethoxycarbazole was performed using boron trichloride. Thereafter, the phenolic hydroxy group was converted into the methyl group to provide carbazomycin A. Subsequent installation of the methoxy group realized the total synthesis of carbazomycin D. Regioselective demethylation was performed using 1-dodecanethiol, effecting the conversions of carbazomycins A and D into carbazomycins B and C, respectively.

Integrated synthesis of 3,4-carbazoquinone alkaloids N-Me-carbazoquinocin A, B and D-F

Carbazole alkaloids carbazoquinocin A-F possessing a 1-alkyl-2-methyl-3,4-ortho-carabazoquinone framework were isolated from the microorganism Streptomyces violaceus 2448-SVT2 in 1995. Furthermore, they were found to exhibit strong inhibitory activity against lipid peroxidation. Herein, we report the integrated synthesis of N-Me-analogues of 5 members of the carbazoquinocin family of natural products, namely, N-Me-carbazoquinocin A, B and D-F. We employed an acid-catalyzed, intramolecular benzannulation of indole-appended Z-enoate propargylic alcohols, which was developed earlier in our laboratory, for the construction of the required carbazole framework. All five natural products were obtained in an overall yield of 50-60%, starting from a commercially available indole.

Novel convenient 2-step synthesis of pyrido[1,2-a]indoles from pyrylium salts and o-bromoanilines

A novel convenient 2-step synthesis of substituted pyrido[1,2-a]indoles is developed starting from easily available pyrylium tetrafluoroborates and ortho-bromoanilines. A conversion of the pyrylium tetrafluoroborates to pyridinium ones followed by their palladium catalyzed intramolecular cyclization allows the formation of 24 examples of N-fused heterocycles. A one-pot two-stage cyclization procedure was developed. The utility of the methodology was demonstrated with the synthesis of new pyrido[1,2-a]indoles bearing different alkyl, aryl, chlorine, fluorine and methoxy substituents.

Exploring the Impact of Elements on the Reactivity of a Straightforward Procedure for Generating Vinyl-Carbazole Derivatives via a Frustrated Lewis Pair Mechanism

The effect of chemical element on the reactivity for carbazolation reaction of phenylacetylene utilizing G13(C6F5)3 (Lewis acid) and G15-carbazole (Lewis base) was theoretically investigated using density functional theory (M06-2X-D3/def2-TZVP), where G13 represents Group 13 elements and G15 represents Group 15 elements. Through activation strain model (ASM) analysis, it is apparent that the reactivity of the entire carbazolation reaction is chiefly governed by the structural strain energy of the alkyne fragment. In other words, if G13(C6F5)3 or G15-carbazole features an atomic radius that is either too small (e.g., B atom) or too large (e.g., Tl or Bi atom), it results in inadequate orbital overlap between the reactants due to the impact of steric effects. This, in turn, results in an elevation of the activation energy for such reactions, thereby impeding the alkyne from undergoing the carbazole catalytic reaction. In light of the above analyses, our theoretical findings suggest that, except for Tl(C6F5)3, the other four Lewis acid catalysts (B(C6F5)3, Al(C6F5)3, Ga((C6F5)3, and In((C6F5)3) demonstrate effectiveness in catalyzing the carbazolation reaction of alkyne alongside with N-carbazole. Additionally, it is anticipated that, among the five categories of G15-carbazole molecules studied, only N-carbazole can participate in the carbazolation reaction with alkyne catalyzed by B(C6F5)3, considering both kinetic and thermodynamic factors at room temperature. Our theoretical investigations, as outlined in this study, indicate that the carbazolation reaction of the alkyne, catalyzed by G13(C6F5)3 and G15-carbazole, follows Hammond's postulate. To put it more plainly, when the transition state of the chemical reaction occurs earlier, it results in a decrease in activation energy.

Au(I)/Sc(III)-co-catalyzed tandem spiroannulation/cycloisomerization of 3-(2-ethynylaryl)-N-tosylaziridines with indoles to access 5H-benzo[b]carbazoles

An unusual spiroannulation/cycloisomerization cascade of 3-(2-ethynylaryl)-N-tosylaziridines with indoles enabled by cooperative gold/scandium catalysis is presented, which facilitates the synthesis of 5H-benzo[b]carbazoles in moderate to excellent yields. This protocol features a broad substrate scope and good functional-group compatibility. Additionally, the resulting 5H-benzo[b]carbazoles exhibit good fluorescence properties, demonstrating the synthetic practicality of this method. Moreover, control experiments were performed to illustrate the reaction mechanism.

Palladium-catalyzed amidation of carbazole derivatives via hydroamination of isocyanates

The first amidation of carbazoles at the N9 position via palladium-catalyzed hydroamination of isocyanates is demonstrated. This simple, general and efficient method could deliver a wide range of carbazole-N-carboxamides in up to 99% yield. The salient features of this transformation include simple conditions with no need for a strong base, high chemo- and regio-selectivities and good functional group tolerance. In particular, this work-up-free and chromatography-free protocol is time-saving, cost-effective and user-friendly.

Practical Access to Fused Carbazoles via Oxidative Benzannulation and their Photophysical Properties

An aryne annulation strategy for the synthesis of fused carbazoles is developed using indolyl β-ketonitrile in a cascade manner. The reaction sequence involves aryne-mediated [2 + 2] cycloaddition cleavage and intramolecular Michael addition, followed by oxidation under transition-metal-free reaction conditions. Subsequently, conversion of benzo[b]carbazole-6-carbonitrile to carbazole quinone is observed upon prolongation of the reaction time. Furthermore, these materials exhibit high quantum efficiency, which promotes the light-emitting diode applications.

Investigating the potential of monocyclic B9N9 and C18 rings for the electrochemical sensing, and adsorption of carbazole-based anti-cancer drug derivatives: DFT-based first-principle study

CONTEXT

For the first time, the use of monocyclic rings C18 and B9N9 as sensors for the sensing of carbazole-based anti-cancer drugs, such as tetrahydrocarbazole (THC), mukonal (MKN), murrayanine (MRY), and ellipticine (EPT), is described using DFT simulations and computational characterization. The geometries, electronic properties, stability studies, sensitivity, and adsorption capabilities of C18 and B9N9 counterparts towards the selected compounds confirm that the analytes interact through active cavities of the C18 and B9N9 rings of the complexes.

METHODS

Based on the interaction energies, the sensitivity of surfaces towards EPT, MKN, MRY, and THC analytes is observed. The interaction energy of EPT@B9N9, MKN@B9N9, MRY@B9N9, and THC@B9N9 complexes are observed - 20.40, - 19.49, - 20.07, and - 18.27 kcal/mol respectively which is more exothermic than EPT@C18, MKN@C18, MRY@C18, and THC@C18 complexes are - 16.37, - 13.97, - 13.96, and - 11.39 kcal/mol respectively. According to findings from the quantum theory of atoms in molecules (QTAIM) and the reduced density gradient (RDG), dispersion forces play a significant role in maintaining the stability of these complexes. The electronic properties including FMOs, density of states (DOS), natural bond orbitals (NBO), charge transfer, and absorption studies are carried out. In comparison of B9N9 and C18, the analyte recovery time for C18 is much shorter (9.91 × 10-11 for THC@C18) than that for B9N9 shorter recovery time value of 3.75 × 10-9 for EPT@B9N9. These results suggest that our reported sensors B9N9 and C18 make it faster to detect adsorbed molecules at room temperature. The sensor response is more prominent in B9N9 due to its fine energy gap and high adsorption energy. Consequently, it is possible to think of these monocyclic systems as a potential material for sensor applications.

Synthesis of Functionalized 4-Hydroxy Carbazoles and Carbazole Alkaloids via Ring Expansion of Indole Cyclopentanone

The exploration of a ring expansion reaction from indole cyclopentanone to generate a range of diversely functionalized 4-hydroxyl carbazole frameworks, representing the core structure of numerous carbazole alkaloids, has been conducted under mild reaction conditions. This approach exhibits broad functional group tolerance and moderate to good yields. The practical applicability of this strategy has been demonstrated through the concise syntheses of carbazomycins A, D, and G.

Total Synthesis of Carbazomycins E and F

Total synthesis of carbazomycins E and F was achieved by double functionalization of an aryne intermediate generated from a 2-aminobiphenyl derivative. The tethered amino group underwent nucleophilic addition to the aryne intermediate to construct the carbazole skeleton. The resulting carbanion was formylated to give the multiply substituted carbazole. This formyl group caused several problems. For example, it was difficult to perform regioselective demethylation of the methoxy group proximal to the formyl group without protecting the carbazole nitrogen. In addition, the formyl group was unexpectedly reduced to give a methoxymethyl group under heating conditions with copper iodide and sodium methoxide. Oxidation of this compound in the presence of water was effective for obtaining the formylated carbazole, leading to the first total synthesis of carbazomycin F.

Perfluoroalkylation of Triarylamines by EDA Complexes and Ulterior Sensitized [6π]-Electrocyclization to Perfluoroalkylated Endo-Carbazoles. Mechanistic and Photophysical Studies

Blue LEDs-irradiation of a mixture of N,N,N',N'-tetramethylethylenediamine (TMEDA) and perfluoroalkyl iodides (RF-I) - Electron Donor Acceptor (EDA)-complex - in the presence of triphenylamines (TPAs) in an aqueous solvent mixture afforded mono-perfluoroalkylated triphenylamines (RF-TPA) in good yields. These RF-TPA were further subjected to acetone-sensitized [6π]-electrocyclization at 315 nm-irradiation affording exclusively perfluoroalkylated endo-carbazole derivatives (RF-CBz) in quantitative yields. Mechanistic studies and photophysical properties of products are studied.

HNTf2-Catalyzed Formal [3 + 2] Cycloaddition of Vinyldiazoacetates to Indole-Substituted Diazo Compounds and Their Conversion to Carbazoles

Triflimide catalysis of the [3 + 2]-cycloaddition of 3-indolymethanols with vinyldiazoacetates provides general access to β-tetrahydrocyclopenta[b]indol-substituted α-diazoesters. Initiated by addition of the in situ generated vinylogous iminium electrophile from 3-indolymethanol to the vinylogous position of the vinyldiazo compound and completed by intramolecular cyclization from the vinyldiazonium ion intermediate, this transformation occurs in good yields and excellent diastereoselectivity with a broad substrate scope under mild conditions. The resulting α-diazoesters undergo Rh2(OAc)4-catalyzed substrate-dependent 1,2-migration to form multisubstituted carbazoles in high yields.

Photocatalytic Thio/Selenosulfonylation-Bicyclization of Indole-Tethered 1,6-Enynes Leading to Substituted Benzo[c]pyrrolo[1,2,3-lm]carbazoles

The photocatalyzed radical-triggered thio/selenosulfonylation-bicyclization of indole-tethered 1,6-enynes has been established for the first time, enabling the synthesis of various previously unreported thio/selenosulfonylated benzo[c]pyrrolo[1,2,3-lm]carbazoles with moderate to good yields under mild conditions. The reaction pathway was proposed, consisting of energy transfer, homolytic cleavage, radical addition, 5-exo-dig, radical coupling, and a Mallory reaction cascade. This approach exhibits a wide substrate compatibility and excellent tolerability toward various functional groups and is characterized by its remarkable efficiency in both bond formation and annulation.

Cu(OTf)2/HFIP catalyzed regioselective cycloisomerization of indole-C3-functionalized alkynols to carbazoles

We report here a simple and atom economic cycloisomerization reaction of indole-tethered alkynols for constructing diverse carbazoles using Cu(OTf)2/HFIP as the excellent promoter system. The reaction proceeds through a one-pot, domino process of spiro cyclization and 1,2-migration followed by aromatization to deliver carbazoles.

Derivatization of 2,1,3-Benzothiadiazole via Regioselective C-H Functionalization and Aryne Reactivity

Despite growing interest in 2,1,3-benzothiadiazole (BTD) as an integral component of many functional molecules, methods for the functionalization of its benzenoid ring have remained limited, and many even simply decorated BTDs have required de novo synthesis. We show that regioselective Ir-catalyzed C-H borylation allows access to versatile 5-boryl or 4,6-diboryl BTD building blocks, which undergo functionalization at the C4, C5, C6, and C7 positions. The optimization and regioselectivity of C-H borylation are discussed. A broad reaction scope is presented, encompassing ipso substitution at the C-B bond, the first examples of ortho-directed C-H functionalization of BTD, ring closing reactions to generate fused ring systems, as well as the generation and capture reactions of novel BTD-based heteroarynes. The regioselectivity of the latter is discussed with reference to the Aryne Distortion Model.

Dehydrogenative synthesis of N-functionalized 2-aminophenols from cyclohexanones and amines: Molecular complexities via one-shot assembly

Polyfunctionalized arenes are privileged structural motifs in both academic and industrial chemistry. Conventional methods for accessing this class of chemicals usually involve stepwise modification of phenyl rings, often necessitating expensive noble metal catalysts and suffering from low reactivity and selectivity when introducing multiple functionalities. We herein report dehydrogenative synthesis of N-functionalized 2-aminophenols from cyclohexanones and amines. The developed reaction system enables incorporating amino and hydroxyl groups into aromatic rings in a one-shot fashion, which simplifies polyfunctionalized 2-aminophenol synthesis by circumventing issues associated with traditional arene modifications. The wide substrate scope and excellent functional group tolerance are exemplified by late-stage modification of complex natural products and pharmaceuticals that are unattainable by existing methods. This dehydrogenative protocol benefits from using 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) as oxidant that offers interesting chemo- and regio-selective oxidation processes. More notably, the essential role of in situ generated water is disclosed, which protects aliphatic amine moieties from overoxidation via hydrogen bond-enabled interaction.

PW06 suppresses cancer cell metastasis in human pancreatic carcinoma MIA PaCa-2 cells via the inhibitions of p-Akt/mTOR/NF-κB and MMP2/MMP9 signaling pathways in vitro

PW06 [(E)-3-(9-ethyl-9H-carbazol-3-yl)-1-(2,5-dimethoxyphenyl) prop-2-en-1-one], a kind of the carbazole derivative containing chalcone moiety, induced cell apoptosis in human pancreatic carcinoma in vitro. There is no investigation to show that PW06 inhibits cancer cell metastasis in human pancreatic carcinoma in vitro. Herein, PW06 (0.1-0.8 μM) significantly exists in the antimetastatic activities of human pancreatic carcinoma MIA PaCa-2 cells in vitro. Wound healing assay shows PW06 at 0.2 μM suppressed cell mobility by 7.45 and 16.55% at 6 and 24 hours of treatments. PW06 at 0.1 and 0.2 μM reduced cell mobility by 14.72 and 21.8% for 48 hours of treatment. Transwell chamber assay indicated PW06 (0.1-0.2 μM) suppressed the cell migration (decreased 26.67-35.42%) and invasion (decreased 48.51-68.66%). Atomic force microscopy assay shows PW06 (0.2 μM) significantly changed the shape of cell morphology. The gelatin zymography assay indicates PW06 decreased MMP2's and MMP9's activities at 48 hours of treatment. Western blotting assay further confirms PW06 reduced levels of MMP2 and MMP9 and increased protein expressions of EGFR, SOS1, and Ras. PW06 also increased the p-JNK, p-ERK, and p-p38. PW06 increased the expression of PI3K, PTEN, Akt, GSK3α/β, and E-cadherin. Nevertheless, results also show PW06 decreased p-Akt, mTOR, NF-κB, p-GSK3β, β-catenin, Snail, N-cadherin, and vimentin in MIA PaCa-2 cells. The confocal laser microscopy examination shows PW06 increased E-cadherin but decreased vimentin in MIA PaCa-2 cells. Together, our findings strongly suggest that PW06 inhibited the p-Akt/mTOR/NF-κB/MMPs pathways, increased E-cadherin, and decreased N-cadherin/vimentin, suppressing the migration and invasion in MIA PaCa-2 cells in vitro.

A biomimetic approach to lycogarubin C, lynamicin D and related analogues

An efficient and biomimetic synthetic approach to 3,4-diindolylpyrrole-2,5-dicarboxylate derivatives, including lycogarubin C, lynamicin D and related analogues, was discovered. The crucial transformation included the one-pot formation of two C-N bonds and one C-C bond to construct characteristic pyrrole rings.

Preparation of Carbazoles Involving 6π-Electrocyclization, Photoredox-, Electrochemical-, and Thermal Cyclization Reactions: Mechanistic Insights

Carbazole is a heterocyclic motif that can be found in a diverse array of natural and unnatural products displaying a wide range of biological and physiological properties. Furthermore, this heterocycle is part of electronic materials like photoconducting polymers and organic optoelectronic materials owing to its excellent photophysical characteristics. Consequently, the development of synthetic strategies for carbazole scaffolds holds potential significance in biological and material fields. In this regard, a variety of preparation methods has been developed to exploit their efficient and distinct formation of new C-C and C-heteroatom bonds under mild conditions and enabling broad substrate diversity and functional group tolerance. Therefore, this review focuses on the synthesis of a set of carbazole derivatives describing a variety of methodologies that involve direct irradiation, photosensitization, photoredox, electrochemical and thermal cyclization reactions.

Computational molecular perspectives on novel carbazole derivative as an anti-cancer molecule against CDK1 of breast and colorectal cancers via gene expression studies, novel two-way docking strategies, molecular mechanics and dynamics

With increase in cancer incidences, alternative strategies for disease management are of utmost importance. Carbazole, is a compound that is being studied extensively as an anti-cancer compound. In this work, we aimed to investigate a carbazole derivative against specific cancer types such as breast and colorectal, based on the off-target analyses of carbazole derivative. The present work shortlisted 6 proteins that have an association in both cancer types, and then employed two different molecular docking strategies to examine the binding stability of carbazole derivative: a blind-docking state, where the pockets were undefined and mutation-docking state, where possible mutations were induced within the proteins. The results showed that CDK1 bound best in both states to carbazole derivative, and performed better than an array of positive controls. Molecular dynamic simulations at 100 ns further proved its stability, with carbazole derivative-CDK1-blind and mutated complex having RMSD values between 3.2 and 3.6 Å, and 2.8-3.2 Å respectively. Molecular-mechanics generalized born and surface area solvation disclosed free energy of binding for the complexes as -28.79 ± 3.97 kcal/mol and -31.86 ± 5.09 kcal/mol respectively, with carbazole derivative bound stably within the binding pocket at every 10 ns of the 100 ns trajectory. Radial distribution functions showed that the bell curve was well within 6 Å, thus showing that carbazole derivative and its atoms do not deviate away from the pocket, suggesting its ability to be used as a good anti-cancer compound against breast and colorectal.

Hauser-Kraus Annulation Initiated Multi-Cascade Reactions for Facile Access to Functionalized and Fused Oxazepines, Carbazoles and Phenanthridinediones

The Hauser-Kraus (H-K) annulation of N-unsubstituted 3-olefinic oxindoles with 3-nucleophilic phthalides triggers a cascade of ring expansion and ring contraction reactions through several regioselective steps in one pot. While oxazepines were isolated in the presence of stoichiometric amounts of base at room temperature, carbazoles and phenanthridinediones were the products in the presence of excess base and microwave irradiation. Mechanistic studies guided by stepwise reactions and control experiments revealed that the isolable oxazepine intermediate, formed via ring expansion of the H-K adduct, is the key precursor to carbazole and phenanthridinedione via decarboxylative regioselective cyclizations.

Cascade radical cyclization on 3-propargyl-2-alkenyl indole gives stereoselective access to cyclohepta[b]indole over carbazole

A protecting group-dependent diastereoselective synthesis of cyclohepta[b]indole over carbazole derivatives is developed. This strategy involves a regioselective 6-exo-trig radical cyclization-cyclopropanation-ring expansion cascade of 3-propargyl-2-alkenyl indole. The cascade radical cyclization was also performed on indole derivatives possessing alkyne, acrylate and vinylogous carbamate moieties, which delivered pyridocarbazole giving credence to the mechanistic hypothesis. Furthermore, cyclohepta[b]indole could be selectively converted to sulfone and sulfoxide as well as benzoazulenoindole via intramolecular Friedel-Crafts acylation.

Site-selective synthesis and pharmacological elucidation of novel semi-synthetic analogues of koenimbine as a potential anti-inflammatory agent

Koenimbine (1), a carbazole alkaloid isolated from Murraya koenigii, belongs to the Rutaceae family. Various pharmacological effects such as anti-diabetic, melanogenesis inhibition, anti-diarrheal, anti-cancer, and anti-inflammatory properties of koenimbine have already been reported. In the current study, we investigated the anti-inflammatory role of koenimbine (1) and its novel semi-synthetic derivative 8-methoxy-3,3,5-trimethylpyrano[3,2-a] carbazole-11(3H)-yl) (3-(trifluoromethyl) phenyl) methanone (1G) in both in vitro and in vivo biological systems. Our results demonstrated that the anti-inflammatory activity of 1G significantly lowered the production of NO, pro-inflammatory cytokines (IL-6, TNF-α & IL-1β), LTB4 following LPS stimulation in RAW 264.7 macrophages. Furthermore, 1G significantly attenuated the expression levels of nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in a dose dependent manner and also decreased the production of reactive oxygen species (ROS) in LPS-activated RAW 264.7 cells. In addition, the oral administration of 1G reduced the inflammatory response in carrageenan-induced paw edema in BALB/C mice. Moreover, it effectively reduced NO, IL-6, IL-1β & TNF-α levels, liver markers (AST, ALT), and kidney markers (BUN, CRE, and Urea). Also, 1G reverted the infiltration of inflammatory cells and tissue damage in lungs, liver and kidney enhanced the survival rate in LPS-challenged mice. 1G blocks NF-κB p65 from translocating into the nucleus and activating inflammatory gene transcription. These results illustrated that 1G suppresses the inflammatory effects both in-vitro and in-vivo studies via downregulating the nuclear factor kappa-B (NF-κB) signaling pathway. In conclusion, our results demonstrate that semi-synthetic derivative 1G can effectively attenuate the inflammatory response via NF-κB and MAPK signaling pathways; suggesting 1G is a potential novel anti-inflammatory drug candidate in treating inflammatory disorders.

Cascade hydroarylation/Diels-Alder cycloaddition of alkynylindoles with electron-deficient alkynes and alkenes

Herein, a novel cascade gold(I)-catalyzed hydroarylation of alkynylindoles and subsequent Diels-Alder cycloaddition with electron-deficient alkynes and alkenes is described. A variety of azepino-fused hydrocarbazoles and carbazoles were obtained in moderate to excellent yields. Key features of this methodology are low catalyst loadings, high regioselectivity, broad functional group tolerances, access to important heterocycles, and 100% atom economy.

Direct Access to Quinone-Fused 5-Substituted-1,4-Benzodiazepine Scaffolds from Azidoquinones with/without [1,2]-Azide-Nitrogen Migration: Mechanistic Insights

Seven-membered nitrogen heterocycles have a strong influence in drug discovery due to their inherent 3D character, which allows the ability to explore a vast conformational space with a biological target. Notably, the privileged 1,4-benzodiazepine scaffold is dominant in treating the central nervous system due to its binding affinity with the GABAA receptor. Herein, we report a protocol for the transformation of azidoquinones to p-quinone fused 5-substituted-1,4-benzodiazepines (p-QBZDs) from InCl3-catalyzed intermolecular tandem cycloannulation of azidoquinones with amines and aldehydes. Detailed mechanistic studies reveal that the EDA complex between azidoquinones and InCl3 is crucial in determining the reaction pathway. In the absence of EDA complex formation, the reaction proceeds via the intermediacy of 2,3-bridged-2H-azirine followed by regiospecific addition of an amine to C═N/ring opening/cyclization to deliver p-QBZD with 1,2-azide-nitrogen migration. In the case of EDA complex formation, the reaction proceeds through regioselective aza-Michael addition/nitrene insertion with aldehyde and subsequent cyclization to deliver p-QBZD and p-quinone fused imidazole as a secondary product without 1,2-azide-nitrogen migration. This protocol provides straightforward access to redox-active quinone embedded 5-substituted-1,4-benzodiazepines from azidoquinones with diverse substrate scopes that would find potential applications in medicinal chemistry and drug discovery.

Iodine-Catalyzed Regioselective Synthesis of Diphenyl-Substituted Carbazoles via [4 + 2] Annulation of β-Formyl Ketones with Indoles

The [4 + 2] annulation of β-formyl ketones with an indole has been developed for the regioselective synthesis of diphenyl-substituted carbazoles in the presence of a catalytic amount of iodine. The 1,4-dicarbonyl compound containing a phenyl group at the α-position of an aldehyde group reacts more readily with indoles to form carbazole derivatives. Using this method, a variety of carbazole derivatives can be readily accessed under mild reaction conditions.

π-Extension of Indoles Using Acrolein Linker: Synthesis of Indolo[3,2-a]carbazole-6-carbaldehydes and Racemosin B

A simple method for the synthesis of indolo[3,2-a]carbazole-6-carbaldehydes by the π-extension of indoles with acrolein is reported. The scope of the method is demonstrated with 33 examples. The products exhibit high activities toward advanced synthesis and are proved to be able to produce valuable chemicals, such as natural products, dyes, and organic fluorescent materials. In addition, the alkaloid racemosin B can be prepared by this method in two steps in ∼50% overall yield.

Trends in carbazole synthesis - an update (2013-2023)

The interest of scientists in the carbazole core has risen steadily over the last 30 years, particularly over the last decade given its presence in several active pharmaceutical ingredients, functional materials and a wide range of biologically active natural products. The continuous development of more efficient, more (regio-)selective and "greener" methodologies to access the carbazole core is thus imperative. This review compares and evaluates synthetic strategies towards the carbazole core that have been reported since 2013, with a focus on their applicability towards the total synthesis of carbazole-containing natural products.

Catalyst- and Substrate-Controlled Regiodivergent Synthesis of Carbazoles through Gold-Catalyzed Cyclizations of Indole-Functionalized Alkynols

A wide variety of regioselectively substituted carbazole derivatives can be synthesized by the gold-catalyzed cyclization of alkynols bearing an indol-3-yl and an additional group at the homopropargylic positions. The regioselectivity of the process can be controlled by both the oxidation state of the gold catalyst and the electronic nature of the substituents of the alkynol moiety. The 1,2-alkyl migration in the spiroindoleninium intermediate, generated after indole attack to the activated alkyne, is favored with gold(I) complexes and for electron-rich aromatic substituents at the homopropargylic position, whereas the 1,2-alkenyl shift is preferred when using gold(III) salts and for alkyl or non-electron-rich aromatic groups.

Recent synthetic strategies for the construction of functionalized carbazoles and their heterocyclic motifs enabled by Lewis acids

This article demonstrates recent innovative cascade annulation methods for preparing functionalized carbazoles and their related polyaromatic heterocyclic compounds enabled by Lewis acid catalysts. Highly substituted carbazole scaffolds were synthesized via Lewis acid mediated Friedel-Crafts arylation, electrocyclization, intramolecular cyclization, cycloaddition, C-N bond-formations, aromatization and cascade domino reactions, metal-catalyzed, iodine catalyzed reactions and multi-component reactions. This review article mainly focuses on Lewis acid-mediated recent synthetic methods to access a variety of electron-rich and electron-poor functional groups substituted carbazole frameworks in one-pot reactions. Polyaromatic carbazole and their related nitrogen-based heterocyclic compounds were found in several synthetic applications in pharma industries, energy devices, and materials sciences. Moreover, the review paper briefly summarised new synthetic strategies of carbazole preparation approaches will assist academic and pharma industries in identifying innovative protocols for producing poly-functionalized carbazoles and related highly complex heterocyclic compounds and discovering active pharmaceutical drugs or carbazole-based alkaloids and natural products.

Towards bioresource-based aggregation-induced emission luminogens from lignin β-O-4 motifs as renewable resources

One-pot synthesis of heterocyclic aromatics with good optical properties from phenolic β-O-4 lignin segments is of high importance to meet high value added biorefinery demands. However, executing this process remains a huge challenge due to the incompatible reaction conditions of the depolymerization of lignin β-O-4 segments containing γ-OH functionalities and bioresource-based aggregation-induced emission luminogens (BioAIEgens) formation with the desired properties. In this work, benzannulation reactions starting from lignin β-O-4 moieties with 3-alkenylated indoles catalyzed by vanadium-based complexes have been successfully developed, affording a wide range of functionalized carbazoles with up to 92% yield. Experiments and density functional theory calculations suggest that the reaction pathway involves the selective cleavage of double C-O bonds/Diels-Alder cycloaddition/dehydrogenative aromatization. Photophysical investigations show that these carbazole products represent a class of BioAIEgens with twisted intramolecular charge transfer. Distinctions of emission behavior were revealed based on unique acceptor-donor-acceptor-type molecular conformations as well as molecular packings. This work features lignin β-O-4 motifs with γ-OH functionalities as renewable substrates, without the need to apply external oxidant/reductant systems. Here, we show a concise and sustainable route to functional carbazoles with AIE properties, building a bridge between lignin and BioAIE materials.

Visible-Light Photoredox-Catalyzed Three-Component Reaction of Carbazoles with Alkenes and S-(Difluoromethyl)sulfonium Salt: A Practical Approach to Access Difluoroalkyl-Functionalized Carbazoles

A mild and elegant multicomponent protocol for construction of CF₂H-containing carbazoles was accomplished by visible-light photoredox catalysis with formation of two new C-C bonds in a single step to deliver a wide variety of structurally diverse difluoroalkylated carbazoles in moderate to good yields, featuring mild reaction conditions, synthetic simplicity, broad substrates, and good functional group tolerance.

Rh(III)-Catalyzed Direct C-H Cyclization of N-Nitrosoanilines with 1,3-Dicarbonyl Compounds: A Route to Tetrahydrocarbazol-4-ones

A novel and efficient Rh(III)-catalyzed direct C-H bond tandem annulation of N-nitrosoanilines with 1,3-dicarbonyl compounds through two C-H bond cleavage was developed. This protocol provides a rapid access to a series of valuable tetrahydrocarbazol-4-ones with the feature of readily available starting materials, broad functional group tolerance, and in situ generation of carbene precursors. Importantly, some reaction products exhibited promising antiproliferative activity in cancer cell lines.

Anodic Dehydrogenative Aromatization of Tetrahydrocarbazoles Leading to Carbazoles

Anodic oxidation-promoted aromatization of 1,2,3,4-tetrahydrocarbazoles was achieved. Nitrogen-protected tetrahydrocarbazoles could be converted to the corresponding carbazoles with the use of bromide as a mediator. LiBr, an inexpensive bromide source, allowed for efficient transformation in the presence of AcOH.

Role of Polarization Interactions in the Formation of Dipole-Bound States

Even though there is a critical dipole moment required to support a dipole-bound state (DBS), how molecular polarizability may influence the formation of DBSs is not well understood. Pyrrolide, indolide, and carbazolide provide an ideal set of anions to systematically examine the role of polarization interactions in the formation of DBSs. Here, we report an investigation of carbazolide using cryogenic photodetachment spectroscopy and high-resolution photoelectron spectroscopy (PES). A polarization-assisted DBS is observed at 20 cm^-1 below the detachment threshold for carbazolide, even though the carbazolyl neutral core has a dipole moment (2.2 D) smaller than the empirical critical value (2.5 D) to support a dipole-bound state. Photodetachment spectroscopy reveals nine vibrational Feshbach resonances of the DBS, as well as three intense and broad shape resonances. The electron affinity of carbazolyl is measured accurately to be 2.5653 ± 0.0004 eV (20,691 ± 3 cm^-1). The combination of photodetachment spectroscopy and resonant PES allows fundamental frequencies for 14 vibrational modes of carbazolyl to be measured. The three shape resonances are due to above-threshold excitation to the three low-lying electronic states (S₁-S₃) of carbazolide. Resonant PES of the shape resonances is dominated by autodetachment processes. Ultrafast relaxation from the S₂ and S₃ states to S₁ is observed, resulting in constant kinetic energy features in the resonant PES. The current study provides decisive information about the role that polarization plays in the formation of DBSs, as well as rich spectroscopic information about the carbazolide anion and the carbazolyl radical.

Alashanines A-C, Three Quinone-Terpenoid Alkaloids from Syringa pinnatifolia with Cytotoxic Potential by Activation of ERK

Three quinone-terpenoid alkaloids, alashanines A-C (1-3), possessing an unprecedented 6/6/6 tricyclic conjugated backbone and quinone-quinoline-fused characteristic, were isolated from the peeled stems of Syringa pinnatifolia. Their structures were elucidated by analysis of extensive spectroscopic data and quantum chemical calculations. A hypothesis of biosynthesis pathways for 1-3 was proposed on the basis of the potential precursor iridoid and benzoquinone. Compound 1 exhibited antibacterial activities against Bacillus subtilis and cytotoxicity against HepG2 and MCF-7 human cancer cell lines. The results of the cytotoxic mechanism revealed that compound 1 induced apoptosis of HepG2 cells through activation of ERK.

Site-Selective C-H Functionalization of Carbazoles

Carbazole alkaloids hold great potential in pharmaceutical and material sciences. However, the current approaches for C1 functionalization of carbazoles rely on the use of a pre-installed directing group, severely limiting their applicability and hindering their overall efficiency. Herein, we report for the first time the development of direct Pd-catalyzed C-H alkylation and acylation of carbazoles assisted by norbornene (NBE) as a transient directing mediator. Notably, the involvement of a six-membered palladacycle intermediate was suggested in this case, representing the first example of such intermediacy within the extensively studied Pd/norbornene reactions realm.

Synthesis of Cyclohepta[b]indoles and Furo[3,4-b]carbazoles from Indoles, Tertiary Propargylic Alcohols, and Activated Alkynes

A robust metal-free and environmentally friendly approach to cyclohepta[b]indole and furo[3,4-b]carbazole frameworks via a three-component reaction from indoles, tertiary propargylic alcohols, and activated alkynes is described. A probable mechanism was proposed on the basis of the isolation and characterization of a key intermediate of the reaction. A gram-scale reaction and product derivatizations were also performed to demonstrate the practicality of the developed methodology.

Deprotonative Generation and Trapping of Haloaryllithium in a Batch Reactor

A method for the regioselective functionalization of haloarenes through deprotonative lithiation is disclosed. The generated haloaryllithiums were trapped in a batch reactor with a zinc chloride diamine complex to provide organozinc species without aryne formation, which reacted with electrophiles to afford the corresponding products in 38-98% yields. This method was applied to the five-step total synthesis of carbazomycin A on a gram scale in 33% overall yield.

Development and Characterization of Benzoselenazole Derivatives as Potent and Selective c-MYC Transcription Inhibitors

Developing c-MYC transcription inhibitors that target the G-quadruplex has generated significant interest; however, few compounds have demonstrated specificity for c-MYC G-quadruplex and cancer cells. In this study, we designed and synthesized a series of benzoazole derivatives as potential G-quadruplex ligand-based c-MYC transcription inhibitors. Surprisingly, benzoselenazole derivatives, which are rarely reported as G-quadruplex ligands, demonstrated greater c-MYC G-quadruplex selectivity and cancer cell specificity compared to their benzothiazole and benzoxazole analogues. The most promising compound, benzoselenazole m-Se3, selectively inhibited c-MYC transcription by specifically stabilizing the c-MYC G-quadruplex. This led to selective inhibition of hepatoma cell growth and proliferation by affecting the MYC target gene network, as well as effective tumor growth inhibition in hepatoma xenografts. Collectively, our study demonstrates that m-Se3 holds significant promise as a potent and selective inhibitor of c-MYC transcription for cancer treatment. Furthermore, our findings inspire the development of novel selenium-containing heterocyclic compounds as c-MYC G-quadruplex-specific ligands and transcription inhibitors.

Palladium-Catalyzed Oxidative Annulation Leading to Substituted Pyrrolo[3,2,1-jk]carbazoles by Sequential C-N and C-C Bond Formation

A novel regioselective annulation of propargylic alcohols with simple carbazoles for the construction of [3,2,1-jk]carbazole scaffolds is described to be the first example of intermolecular synthesis of [3,2,1-jk]carbazoles from simple carbazoles. In situ synthesis of propargyl alcohols from simple, cheap, and easily accessible ketones has also been developed during the one-pot synthesis of [3,2,1-jk]carbazoles.

Construction of heterocycle-fused tetrahydrocarbazoles through a formal [3 + 3]-annulation of 2-indolylmethanols with para-quinone methides

A metal-free approach for the synthesis of tetrahydroindolo[2,3-b]carbazoles has been developed through an acid-mediated one-pot [3 + 3]-annulation of 2-indolylmethanols and 3-indolyl-substituted para-quinone methides. This operationally simple protocol allowed us to prepare many unsymmetrical tetrahydroindolo[2,3-b]carbazoles in good to excellent yields with a broad substrate scope. This concept was also elaborated to the synthesis of tetrahydrothieno[2,3-b]carbazoles and tetrahydrothieno[3,2-b]carbazoles.

Iodine-Catalyzed Annulation Reaction of Ortho-Formylarylketones with Indoles: A General Strategy for the Synthesis of Indolylbenzo[b]carbazoles

The iodine-catalyzed cascade reaction of ortho-formylarylketones with indoles for the synthesis of indolylbenzo[b]carbazoles is reported. The reaction is initiated in the presence of iodine by two successive nucleophilic additions of indoles with an aldehyde group of ortho-formylarylketones, and the ketone does not undergo a nucleophilic addition and only involves in the Friedel-Crafts-type cyclization. A variety of substrates are tested, and the efficiency of this reaction is demonstrated with gram-scale reactions.

B(C6F5)3-Catalyzed [4 + 2] Cyclization Strategy: Synthesis and Photophysical Properties of 5H-Naphtho[2,3-c]carbazole-8,13-dione Derivatives

In this paper, a series of novel carbazolequinones were efficiently obtained by a B(C₆F₅)₃-catalyzed [4 + 2] cyclization reaction. This protocol not only had a simple operation, broad substrate range, and high atomic economy, but also had a low catalyst loading and avoided using metal catalysts. In addition, we constructed diverse new carbazole-fused compounds under different reduction conditions. The results of photophysical characterization showed that the structure of carbazole-fused derivatives had a significant impact on the fluorescence properties.

Brønsted Acid-Catalyzed Reaction of N-arylnaphthalen-2-amines with Quinone Esters for the Construction of Carbazole and C-N Axially Chiral Carbazole Derivatives

We demonstrated here an efficient synthetic method of carbazole derivatives from readily available N-arylnaphthalen-2-amines and quinone esters catalyzed by Brønsted acid. With this strategy, a series of carbazole derivatives were obtained in good to excellent yields (76 to >99) under mild conditions. Large scale reaction illustrated the synthetic utility of this protocol. Meanwhile, a series of C-N axially chiral carbazole derivatives were also constructed in moderate to good yields (36-89% yield) with moderate to excellent atroposelectivities (44-94% ee) by using chiral phosphoric acid as a catalyst, which provides a novel strategy for the atroposelective construction of C-N axially chiral compounds and a new member of the C-N atropisomers.

Skeletal photoinduced rearrangement of diarylethenes: ethene bridge effects

A skeletal photorearrangement involving UV-induced 6π-electrocyclization of diarylethenes with various ethene bridges has been studied. It has been found that deprotonation is the predominant step among the three possible alternative reaction pathways (radical abstraction, deprotonation, or sigmatropic shift) following 6π-electrocyclization, and incorporation of an electronegative carbonyl group into the geminal position to the phenyl residue results in a reduction in the reaction time and an increase in the yield of the desired product. The significant increase in the reaction time in less polar solvents (toluene, TCM) also indicates a large contribution of the deprotonation step to the skeletal photorearrangement of diarylethenes. Performing the reaction in toluene in the presence of tertiary amines leads to a reduction in the reaction time and an increase in the yield of the desired product. The best results were achieved when the reaction was carried out in toluene in the presence of DIPEA. The experimental results are in good agreement with the DFT calculations.