Revisiting Indolo[3,2-b]carbazole: Synthesis,Structures, Properties,and Applications

Indolo[3,2-b]carbazole presents a π-skeleton with a remarkable electronic structure and interesting potential applications. It is, however, also associated with ambiguity and controversy. Herein, new derivatives of indolo[3,2-b]carbazole are reported and they have enabled a comprehensive study on the electronic structure of indolo[3,2-b]carbazole and the development of a new n-type organic semiconductor. Experimental and computational studies show that indolo[3,2-b]carbazole has a largely localized p-benzoquinonediimine moiety and significant antiaromaticity. When substituted with (4-silylethynyl)phenyl groups, the indolo[3,2-b]carbazole exhibits one-dimensional π–π stacking and functions as an n-type organic semiconductor in solution-processed field effect transistors.     

Synthesis,photophysical and electrochemical properties of novel 6,12-di(thiophen-2-yl) substituted indolo[3,2-b]carbazoles

Novel 5,11-dialkyl-6,12-di(thiophen-2-yl) substituted 5,11-dihydroindolo[3,2-b]carbazoles have been obtained and plausible ways for their further modifications via the Friedel–Crafts reaction are presented. The formylation of these indolo[3,2-b]carbazoles with dichloromethyl alkyl esters catalysed by Lewis acids leads to the formation of the corresponding 2,8-diformyl derivatives. Applicability of this formylation method for modification of indolo[3,2-b]carbazoles bearing electron-rich aromatic substituents at C-6 and C-12 has also been demonstrated. The Knoevenagel condensation of 2,8-dialdehydes with active methylene nitriles has been studied. The measurements of optical and redox properties for a number of new indolo[3,2-b]carbazoles have been performed.     

Synthesis, photophysical and electrochemical properties and theoretical studies on three novel indolo[3,2-b]carbazole derivatives containing benzothiazole units

Three novel indolo[3,2-b]carbazoles derivatives were successfully synthesized by condensation reaction and structurally characterized by elemental analysis, mass spectrometry and proton nuclear magnetic resonance spectroscopy methods, which belong to donor-π-acceptor systems comprising an indolo[3,2-b]carbazole group as an electron donor and two benzothiazole rings as electron acceptors. The thermal, electrochemical and photophysical properties of the compounds were characterized by thermogravimetric analysis combined with electrochemistry, UV-vis absorption spectroscopy and fluorescence spectroscopy. On the other hand, the geometries, molecular orbitals, charge-injection and transport properties were determined by quantum chemical calculations. The results show that the compounds synthesized exhibit good thermal stability and high fluorescence quantum yields, indicating the potential application as optoelectronic materials.     

Palladium-catalyzed amination and cyclization to heteroannellated indoles and carbazoles

New ortho-bromodiarylamines in the benzo[b]thiophene series were prepared by palladium-catalyzed amination, either in the benzene or in the thiophene ring. These were submitted to palladium-catalyzed cyclization, under different required conditions, to give several differently substituted thieno[3,2-c] or [2,3-b]carbazoles and indolo[3,2-b]benzo[b]thiophenes. This constitutes a novel synthetic route to both tetracyclic systems.     

Structure elucidation of two tryptophan-derived, high affinity Ah receptor ligands

Background: Environmental contaminants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other structurally related ‘environmental hormones’, exert their harmful biological effects through the Ah receptor signaling pathway. Several naturally occurring substances also bind to this receptor, but its natural role is still obscure. Tryptophan derivatives of the indolo[3,2-b]carbazole type, earlier suggested by us to be endogenous ligands for the receptor, should be a powerful tool in understanding receptor function. We therefore: set out to determine their identity.Results: The two tryptophan-derived Ah receptor ligands have been chemically analyzed and characterized by means of mass spectrometry, ¹H NMR and ¹³C NMR. UV, infra-red and fluorescence spectra were also recorded. All data are in accordance with the two compounds being closely related indolo[3,2-b]carbazole derivatives. Evidence is presented that compound A (MW = 312) is the symmetrical 6,12-diformylindolo[3,2-b]carbazole, and compound B (MW = 284) is the monosubstituted 6-formylindolo[3,2-b]carbazole.Conclusions: The elucidation of the structures of the two high affinity Ah receptor ligands 6,12-diformylindolo[3,2-b]carbazole and 6-formylindolo[3,2-b]carbazole provides the necessary basis for further mechanistic studies of this important group of compounds, and will help in determining the natural role of the Ah receptor.     

Revisiting Indolo[3,2‐b]carbazole: Synthesis,Structures, Properties,and Applications

Indolo[3,2‐b]carbazole presents a π‐skeleton with a remarkable electronic structure and interesting potential applications. It is, however, also associated with ambiguity and controversy. Herein, new derivatives of indolo[3,2‐b]carbazole are reported and they have enabled a comprehensive study on the electronic structure of indolo[3,2‐b]carbazole and the development of a new n‐type organic semiconductor. Experimental and computational studies show that indolo[3,2‐b]carbazole has a largely localized p‐benzoquinonediimine moiety and significant antiaromaticity. When substituted with (4‐silylethynyl)phenyl groups, the indolo[3,2‐b]carbazole exhibits one‐dimensional π–π stacking and functions as an n‐type organic semiconductor in solution‐processed field effect transistors.     

A biomimetic synthesis of the indolo[3,2-j]phenanthridine alkaloid, calothrixin B

A biomimetic approach to calothrixin B via a hypothetical metabolite, 6-formylindolo[2,3-a]carbazole, is described. The construction of a suitable indolo[2,3-a]carbazole ring system was carried out using an allene-mediated electrocyclic reaction involving two [b]-bonds of indoles as the key step.     

Synthesis of 2,10,11-Trisubstituted Indolo[3,2-b]quinolines

A new method has been developed for the synthesis of derivatives of indolo[3,2-b]quinolines-11 based on N-oxidization of 2-nitro-10-substituted indolo[3,2-b]quinolines with subsequent conversion of the mixtures obtained into 2-nitro-11-substituted indolo[3,2-b]quinolinones-11. A series of 2-nitro-11-substituted indolo[3,2-b]quinolines was prepared.     

Synthesis of the calothrixins, pentacyclic indolo[3,2-j]phenanthridine alkaloids, using a biomimetic approach

Oxidation of the indolo[2,3-a]carbazole 16, readily obtained in six steps from indigo, followed by deprotection results in formation of the indolo[3,2-j]phenanthridine quinone alkaloid calothrixin B 2, demonstrating the viability of the proposed biosynthetic route to this unique ring system.     

A biomimetic synthesis of calothrixin B

Oxidation of the indolo[2,3-a]carbazole 15, readily obtained in six steps from indigo, followed by deprotection results in formation of the indolo[3,2-j]phenanthridine quinone alkaloid calothrixin B 2, demonstrating the viability of the proposed biosynthetic route to this unique ring system.     

Studies of the reactions between indole-2,3-diones (isatins) and 2-aminobenzylamine

Reflux of equimolecular amounts 2-aminobenzylamine and isatins in acetic acid produced indolo[3,2-c]quinolin-6-ones in good yields. A proposed mechanism involving initial formation of a spiro compound is given. This isolable intermediate subsequently rearranges via a sequential isocyanate ring opening and a cyclisation process to a urea derivative which finally cyclized to the indolo[3,2-c]quinolin-6-ones. The urea derivative could be prepared separately and cyclized selectively to indolo[3,2-c]quinolin-6-one. Reaction of N-acetylisatin with 2-aminobenzylamine at room temperature yielded the 1,4-benzodiazepinone 3-(2-acetamidophenyl)-1,5-dihydro-1,4-benzodiazepin-2-one whereas its isomer 2(2-acetamidophenyl)-4,5-dihydro-1,4-benzodiazepin-3-one was obtained from 2-(2-acetylaminophenyl)-N-(2-aminobenzyl)-2-oxoacetamide in acetic acid at room temperature.The previously unknown linear isomer of indolo[3,2-c]quinolin-6-one, i.e. indolo[2,3-b]quinolin-11-one, has been prepared by thermal (260°C) cyclization of methyl 2-phenylamino indole-3-carboxylate, which in turn was prepared in two steps from methyl indole-3-carboxylate.     

Hole-transporting glass-forming indolo[3,2-b]carbazole-based diepoxy monomer and polymers

Hole-transporting indolo[3,2-b]carbazole-based diepoxy monomer and polymers are reported. The polymers were prepared by polyaddition reaction of indolo[3,2-b]carbazole diepoxide with aromatic dithiols in the presence of triethylamine. Their number average molecular weights range from 11500 to 14310 and polydispersity indices are in the range of 2.96–3.08. Thermal, optical, photophysical, electrochemical and photoelectrical properties of the title compounds were studied. Both the monomer, 5,11-di-2,3-epoxypropyl-6-pentyl-5,11-dihydroindolo[3,2-b]carbazole and the polymers were found to form glasses with the glass transition temperatures ranging from 37 °C for the monomer to 99 °C for one of the polymers. Time-of-flight hole drift mobilities observed in the solid amorphous films of the monomer exceeded 10^?4 cm² V^?1 s^?1 at an electric field of 10⁶ V cm^?1.     

Heterocycles [h]-fused onto 4-oxoquinoline-3-carboxylic acid, Part VI [1]. Synthesis and X-ray structure of model indolo[3,2-b]- and [2,3-b]pyrido[2,3-f]quinoxaline-3-carboxylic esters

Cyclocondensation reaction of ethyl 7,8-diamino-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylate with 1-methylisatin produced a separable mixture of the corresponding indolo[3,2-b]- and [2,3-b]pyrido[2,3-f]quinoxaline-3-carboxylates, of which the latter isomer predominates. On the other hand, interaction with 1H-isatin or 5-chloroisatin gave the respective indolo[2,3-b]pyrido[2,3-f]quinoxaline-3-carboxylates as the sole regiospecific products. The structures of these new pentacyclic derivatives are based on microanalytical, spectral (IR, MS, and NMR) and X-ray crystal structure data.     

Intramolecular 4+2 cycloaddition of thieno[2,3-e][1,2,4]triazines: routes towards condensed thieno[2,3-b]pyridines

Synthetic approaches towards new condensed thienopyridine ring systems including furo[2,3-b]thieno[3,2-e]pyridines, bisthieno[2,3-b:3′,2′-e]pyridines, 5H-chromeno[2,3-b]thieno[3,2-e]pyridines, 5H-benzo(f)chromeno[2,3-b]thieno[3,2-e]pyridines have been achieved by application of intramolecular 4+2 cycloaddition reactions of suitably designed thieno[2,3-e][1,2,4]triazines tethered with alkene or alkyne terminals.     

Convenient synthesis of 10H-indolo[3,2-b]quinolines and 6H-indolo[2,3-b]quinolines by sequential chemoselective Suzuki reaction followed by double C-N coupling

A convenient and regioselective synthesis of two series of indolo[2,3-b]quinolines, namely 10H-indolo[3,2-b]quinolines and 6H-indolo[2,3-b]quinolines, has been developed. The synthesis, proceeds in moderate to high yields, involving chemoselective palladium catalyzed Suzuki reaction of 2,3-dihaloquinolines with 2-bromophenylboronic acid, followed by a double Buchwald-Hartwig C-N coupling.     

Efficient regioselective heterocyclisation leading to fluorescent fused pyrazine derivatives

The regioselective syntheses of substituted pyrrolo[2,3-b]quinoxaline, pyrido[2,3-b]pyrrolo[2,3-e]pyrazine, pyrido[2,3-b]pyrrolo[3,2-e]pyrazine and pyrido[3,4-b]pyrrolo[3,2-e]pyrazine are reported. Differential reactivity between two amino groups in ortho-diaminopyridine can be exploited to obtain new regio-defined unsymmetrical pyridopyrrolopyrazine derivatives. Weak electron-donating methyl or moderately electron-withdrawing carboxylic groups attached to the aromatic ortho-diamines reduce the regioselectivity of obtaining unsymmetrical substituted pyrrolo[2,3-b]quinoxaline. The fluorescence properties of the resultant 1-alkyl pyridopyrrolopyrazine and substituted pyrrolo[2,3-b]quinoxaline derivatives are presented.     

2-Benzyliden-2H-thieto[3,2-b]quinoline: a new heterocycle and its rearrangement to 2-phenylthieno[3,2-b]quinoline

Synthesis of the strained 2H-thieto[3,2-b]quinoline ring system is reported for the first time. Treatment of (Z)-2-benzyliden-2H-thieto[3,2-b]quinoline derivatives of this heterocycle with base, at reflux in ethanol, causes a novel rearrangement to 2-phenylthieno[3,2-b]quinolines. Indeed, the one-pot reaction of 2-aminobenzaldehydes and (Z)-2-benzylidenethietan-3-one in refluxing basic ethanol leads directly to 2-phenylthieno[3,2-b]quinolines in good yields.     

Synthesis of azecino[5,4-b]indoles and indolo[3,2-e][2]benzazonines via tandem transformation of hydrogenated indoloquinolizines and indolizines

The reactions of partially hydrogenated indole-fused quinolizines and indolizines with activated alkynes in methanol, acetonitrile, and dichloromethane were studied. The reactions were shown to be accompanied by the cleavage of the bridging C-N bond. Azecino[5,4-b]-indole and indolo[3,2-e][2]benzazonine derivatives were synthesized.     

Palladium-catalyzed reductive N-heterocyclization of alkenyl-substituted nitroarenes as a viable method for the preparation of bicyclic pyrrolo-fused heteroaromatic compounds

Palladium-catalyzed, carbon monoxide-mediated reductive N-heterocyclization of nitro-heteroaromatic compounds having an alkene adjacent to the nitro-group affords bicyclic pyrrolo-fused heteroaromatic molecules. This type of reaction was used to prepare the fused bicyclo[3.3.0] ring-system: thieno[3,2-b]pyrrole, thieno[2,3-b]pyrrole, furo[2,3-b]pyrrole, pyrrolo[3,2-d]thiazole, and pyrrolo[2,3-d]imidazole and the bicyclo[4.3.0] ring-systems: pyrrolo[3,2-b]pyridine, pyrrolo[2,3-b]pyridine, pyrrolo[3,2-c]pyridine, pyrrolo[2,3-c]pyridine, pyrrolo[3,2-c]pyridazine, and pyrrolo[3,2-d]pyrimidine in 32-94% yield.     

Regio- and diastereoselective synthesis of new functionalized indolo[2,3-b]indole-pyrimidine based on C–N bond formation via a four-component reaction

A facile and diastereoselective synthetic procedure has been designed for the preparation of new substituted indolo[2,3-b]indole (IDID) moiety at position-5 of the pyrimidine ring by a one-pot four-component reaction of dimedone, aniline and substituted derivatives, barbituric acid/thiobarbituric acid and isatin under mild conditions. This method proceeds rely on a CsbndN bond formation under catalyst-free conditions affording a range of skeletally diversity 5-indolo[3,2-b]indole-pyrimidine-based heterocycles. 5-Indolo[2,3-b]indol-pyrimidine-2,4,6-trione and 5-indolo[2,3-b]indol-2-thioxo-pyrimidine-4,6-dione were obtained in EtOH solvent in high yield, short reaction time and diastereoselectivity. The structural diversities of the synthesized compounds have been confirmed spectroscopically, by IR ¹H- and ¹³C NMR, EI-MS spectra and elemental analyses which agree with the proposed structures.