A Sm(II)-Mediated Cascade Approach to Dibenzoindolo[3,2-b]carbazoles: Synthesis and Evaluation

Synthesis of N-Alkyl and N-H-Carbazoles through SN Ar-Based Aminations of Dibenzothiophene Dioxides

Alkyl amines have become available for the synthesis of diverse N-alkyl carbazoles through twofold S_N Ar aminations of dibenzothiophene dioxides by using alkali metal bases. Of particular importance is the choice of counter cations on alkali metal bases, that is, i) the use of Li base for the efficient intermolecular reaction and ii) the sequential addition of heavier alkali metal bases (Na, K, or Cs) to promote intramolecular cyclization in a one-pot manner. This protocol also enables the cascade synthesis of N-H-carbazoles by using 2-phenylethylamine by removal of the 2-phenethyl group from N-(2-phenethyl) carbazoles in a single operation.

Crystal structure of 5,15-dihexyl-5,15-di-hydro-benzo[2,1-b:4,3-c']dicarbazole hexane 0.375-solvate

The title compound, C38H40N2·0.375C6H14, crystallizes in the monoclinic space group P21/c and has a host-guest structure with the helicene molecules forming a porous structure and mol-ecules of hexane inserted into the holes. The dihedral angles between the two carbazole sections of the right- and left-handed helicenes are 27.44 (3) and 25.63 (3)°, respectively. There are no classical π-π inter-actions or hydrogen-bonding inter-actions present between adjacent mol-ecules in the crystal structure. The hexane solvent mol-ecule shows positional disorder.

Chemistry and Properties of Indolocarbazoles

The indolocarbazoles are an important class of nitrogen heterocycles which has evolved significantly in recent years, with numerous studies focusing on their diverse biological effects, or targeting new materials with potential applications in organic electronics. This review aims at providing a broad survey of the chemistry and properties of indolocarbazoles from an interdisciplinary point of view, with particular emphasis on practical synthetic aspects, as well as certain topics which have not been previously accounted for in detail, such as the occurrence, formation, biological activities, and metabolism of indolo[3,2- b]carbazoles. The literature of the past decade forms the basis of the text, which is further supplemented with older key references.

Nitration of 5,11-dihydroindolo[3,2-b]carbazoles and synthetic applications of their nitro-substituted derivatives

A new general approach to double nitration of 6,12-di(hetero)aryl-substituted and 6,12-unsubstituted 5,11-dialkyl-5,11-dihydroindolo[3,2-b]carbazoles by acetyl nitrate has been developed to obtain their 2,8-dinitro and 6,12-dinitro derivatives, respectively. A formation of mono-nitro derivatives (at C-2 or C-6) from the same indolo[3,2-b]carbazoles has also been observed in several cases. Reduction of 2-nitro and 2,8-dinitro derivatives with zinc powder and hydrochloric acid has afforded 2-amino- and 2,8-diamino-substituted indolo[3,2-b]carbazoles, while reduction of 6,12-dinitro derivatives under similar reaction conditions has been accompanied by denitrohydrogenation of the latter compounds into 6,12-unsubstituted indolo[3,2-b]carbazoles. Formylation of 6,12-dinitro derivatives has proved to occur only at C-2, while bromination of these compounds has taken place at both C-2 and C-8 of indolo[3,2-b]carbazole scaffold. Moreover, 6,12-dinitro-substituted indolo[3,2-b]carbazoles have been modified by the reactions with S- and N-nucleophiles. Notably, the treatment of 6,12-dinitro compounds with potassium thiolates has resulted in the displacement of both nitro groups, unlike potassium salts of indole or carbazole, which have caused substitution of only one nitro group.

Selective construction of quaternary stereocentres in radical cyclisation cascades triggered by electron-transfer reduction of amide-type carbonyls

Radical cyclisation cascades triggered by electron-transfer to amide-type carbonyls using SmI₂–H₂O–LiBr, result in the selective construction of quaternary carbon stereocentres.

Synthesis of Linearly Fused Benzodipyrrole Based Organic Materials

The objective of this review is to give an overview of the synthetic methods to prepare different indolo[3,2-b]carbazoles and similar systems with a potential use in electro-optical devices such as OLEDs (organic light emitting diode), OPVs (organic photovoltaic) and OFETs (organic field effect transistor). Some further modifications to the core units and their implications for specific applications are also discussed.

Copper-Catalyzed Double Additions and Radical Cyclization Cascades in the Re-Engineering of the Antibacterial Pleuromutilin

A general synthetic sequence involving simply prepared starting materials provides rapid access to diverse, novel tricyclic architectures inspired by pleuromutilin. Sm(II) -mediated radical cyclization cascades of dialdehydes, prepared using a new, one-pot, copper-catalyzed double organomagnesium addition to β-chlorocyclohexenone, proceed with complete sequence selectivity and typically with high diastereocontrol to give analogues of the target core. Our expedient approach (ca. 7 steps) allows non-traditional, de novo synthetic access to analogues of the important antibacterial that can't be prepared from the natural product by semisynthesis.

A new synthetic approach to fused nine-ring systems of the indolo[3,2-b]carbazole family through double Pd-catalyzed intramolecular C–H arylation

A series of new fused 5,11-dihydroindolo[3,2-b]carbazole derivatives, having nine-ring ladder-type structure, has been synthesized by regioselective Pd-catalyzed cyclization.

Extension of Pyrrolopyrrole π-System: Approach to Constructing Hexacyclic Nitrogen-Containing Aromatic Systems

A facile three-step approach to synthesizing quinoline-fused pyrrolopyrroles is reported. The crucial step in this synthesis is the condensation of 2-aminophenyl substituted pyrrolopyrroles with aromatic aldehydes. The resulting hexacyclic ladder-type dyes strongly absorb UV radiation and exhibit fluorescence at 450-510 nm. The presence of pyridine-type and pyrrole-type nitrogen atoms is important for the electronic properties of this almost planar heterocycle. These heteroatoms, along with the addition of moderate electron-withdrawing and electron-accepting substituents, provide a means for fine-tuning of the emission characteristics of the polycyclic conjugates.

Sm(II)-Mediated Electron Transfer to Carboxylic Acid Derivatives: Development of Complexity-Generating Cascades

Reductive electron transfer (ET) to organic compounds is a powerful method for the activation of substrates via the formation of radicals, radical anions, anions, and dianions that can be exploited in bond-cleaving and bond-forming processes. Since its introduction to the synthetic community in 1977 by Kagan, SmI2 has become one of the most important reducing agents available in the laboratory. Despite its widespread application in aldehyde and ketone reduction, it was widely accepted that carboxylic acid derivatives could not be reduced by SmI2; only recently has our work led to this dogma being overturned, and the reduction of carboxylic acid derivatives using SmI2 can now take its place alongside aldehyde/ketone reduction as a powerful activation mode for synthesis. In this Account, we set out our studies of the reduction of carboxylic acid derivatives using SmI2, SmI2-H2O, and SmI2-H2O-NR3 and the exploitation of the unusual radical anions that are now accessible in unprecedented carbon-carbon bond-forming processes. The Account begins with our serendipitous discovery that SmI2 mixed with H2O is able to reduce six-membered lactones to diols, a transformation previously thought to be impossible. After the successful development of selective monoreductions of Meldrum's acid and barbituric acid heterocyclic feedstocks, we then identified the SmI2-H2O-NR3 reagent system for the efficient reduction of a range of acyclic carboxylic acid derivatives that typically present a significant challenge for ET reductants. Mechanistic studies have led us to propose a common mechanism for the reduction of carboxylic acid derivatives using Sm(II), with only subtle changes observed as the carboxylic acid derivative and Sm(II) reagent system are varied. At the center of our postulated mechanism is the proposed reversibility of the first ET to the carbonyl of carboxylic acid derivatives, and this led us to devise several strategies that allow the radical anion intermediates to be exploited productively in efficient new processes. First, we have used internal directing groups in substrates to "switch on" productive ET to esters and amides and have exploited such an approach in tag-removal cyclization processes that deliver molecular scaffolds of significance in biology and materials science. Second, we have exploited external ligands to facilitate ET to carboxylic acid derivatives and have applied the strategy in telescoped reaction sequences. Finally, we have employed follow-up cyclizations with alkenes, alkynes, and allenes to intercept radical anion intermediates formed along the reaction path and have employed this strategy in complexity-generating cascade approaches to biologically significant molecular architectures. From our studies, it is now clear that Sm(II)-mediated ET to carboxylic acid derivatives constitutes a general strategy for inverting the polarity of the carbonyl, allowing nucleophilic carbon-centered radicals to be formed and exploited in novel chemical processes.

χ-Shaped bis(areno)-1,4-dihydropyrrolo[3,2-b]pyrroles generated by oxidative aromatic coupling

A synthesis of dihydropyrrolo[3,2-b]pyrroles fused with two peripheral arenes or heterocyclic units has been realized through the concise route. These nearly planar compounds were prepared starting from assembling the central core via condensation of 2-aryl or 2-heteroarylbenzaldehydes with aromatic amines and diacetyl, followed by double intramolecular oxidative aromatic coupling. This two-step procedure afforded the desired products in overall yields of 5-36%, and it tolerates structural diversity of starting materials. All the final dyes exhibit strong blue fluorescence in solution.

Polycyclic imidazo[1,2-a]pyridine analogs – synthesis via oxidative intramolecular C–H amination and optical properties

Oxidative C–H amination has proved to be an efficient strategy to construct pyrido[2′,1′:2,3]imidazo[4,5-b]indoles and their π-expanded analogs.

Why vertically π-expanded imidazo[1,2-a]pyridines are weak fluorescence emitters: experimental and computational studies

Two novel π-expanded analogs of imidazo[1,2-a]pyridine are studied by spectroscopic, kinetic and DFT methods. Low fluorescence quantum yield has been attributed to intersystem crossing.

SmI2-H2O-mediated 5-exo/6-exo lactone radical cyclisation cascades

The SmI2-H2O reagent system mediates challenging 5-exo/6-exo lactone radical cascade cyclisations that deliver carbo[5.4.0]bicyclic motifs in a diastereoselective, one-pot process that establish two new carbocyclic rings and four stereocentres.

Diversity oriented convergent access for collective total synthesis of bioactive multifunctional carbazole alkaloids: synthesis of carbazomycin A, carbazomycin B, hyellazole, chlorohyellazole, and clausenaline D

Facile syntheses of imperative carbazole alkaloids carbazomycin A, carbazomycin B, hyellazole, chlorohyellazole, and clausenaline D have been demonstrated starting from readily available Boc-protected 3-formylindole and dimethyl maleate. The suitably substituted aromatic rings have been designed comprising three/four significant C-C bond forming reactions. The competent Wittig reaction, selective monoalkylations, one-pot regioselective Weinreb amide formation and Boc-deprotection, well designed Grignard reactions, dehydrative intramolecular cyclizations, and Baeyer-Villiger rearrangement of aromatic aldehydes were the main features.

Diindolo[2,3-b:2',3'-f]pyrrolo[3,2-b]pyrroles as electron-rich, ladder-type fluorophores: synthesis and optical properties

Previously unknown ladder-type heteroacenes were synthesized in just two steps from aromatic amines, 2-nitrobenzaldehydes, and biacetyl. A recently discovered multicomponent process affording 1,4-dihydropyrrolo[3,2-b]pyrroles was followed by the Cadogan reaction, which resulted in simultaneous closure of two additional pyrrole rings. Rod-like compounds synthesized in this way were stable in the absence of light, and they were transformed into bis(tert-butoxycarbonyl) derivatives. The rigid herringbone-like structure resulted in a strong absorption in the λ=350-410 nm region and very strong violet and blue emission, which was bathochromically shifted relative to the emission of the parent 1,4-dihydropyrrolo[3,2-b]pyrroles. The optical properties of these compounds strongly suggest that the conjugation spanned the entire π system. The HOMO energy level was located at -4.6 to -5.1 eV.

Facile synthesis of 5H-benzo[b]carbazol-6-yl ketones via sequential reaction of Cu-catalyzed Friedel–Crafts alkylation, iodine-promoted cyclization, nucleophilic substitution and aromatization

A convenient method to access 5H-benzo[b]carbazol-6-yl ketones from indoles and 2-(2-(alkynyl)benzylidene)malonates has been developed.