Dibenzo-Fused Heterocycles: A Decade Update on the Syntheses of Carbazole, Dibenzofuran, and Dibenzothiophene

Polycyclic heterocycles are the most common and critical structural motifs found in a variety of natural products, medicines, fertilizers, and advanced materials. Because of their widespread use in biologically active compounds and material chemistry, functionalised dibenzo heterocyclic compounds, especially dibenzofuran, dibenzothiophene, and carbazole derivatives, garnered much attention over time. Scientists are especially interested in elucidating more efficient techniques for developing these industrially essential compounds. Dibenzo-fused heterocycles can rapidly be synthesised using highly efficient transition metal-catalysed strategies as well as by economic metal-free reaction conditions. This review includes a detailed overview of the most recent significant synthetic techniques, both metal-catalysed and metal-free, to produce these industrially significant and medicinally important dibenzo-fused heterocycles.

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.

Acid-Promoted Domino Access to Substituted Benzo[b]carbazoles

Herein, a straightforward Bro̷nsted acids-promoted domino pathway to build substituted benzo[b]carbazoles has been described from easily accessible ortho-formyl (or ortho-acyl) cinnamate esters and indoles. Noticeably, the protocol was amenable to protecting group-free indoles. Notably, this methodology is based on a single-pot regioselective construction of two new C-C bonds and aromatization sequences under mild and metal-free reaction conditions. The mechanistic studies suggested the initial formation of bis-indole substituted intermediate via a dual aromatic substitution with two indole molecules at the carbonyl carbon of ortho-formyl (or ortho-acyl) cinnamate ester followed by intramolecular cyclization and aromatization with exclusion of a second indole molecule. Besides, the efficacy of this approach was also illustrated by scale-up and derivatization reactions, including the photophysical properties study.

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.

Benzannulation strategies for the synthesis of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines

This review presents a critical and authoritative analysis of several exciting benzannulation approaches developed in the past decade for the construction of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines.

Pd-Catalyzed Domino Reactions Involving Alkenes To Access Substituted Indole Derivatives

Palladium-catalyzed domino reactions are advanced tools in achieving various nitrogen-containing heterocycles in an efficient and economical manner due to the reduced number of steps in the process. This review highlights recent advances in domino processes aimed at the synthesis of indole derivatives and polycyclic systems containing the indole nucleus in intra/intra- or intra/intermolecular reactions. In particular, we consider domino processes that involve a double bond in a step of the sequence, which allow the issue of regioselectivity in the cyclization to be faced and overcome. The different sections in this review focus on the synthesis of the indole nucleus and functionalization of the scaffold starting from different substrates that have been identified as activated starting materials, which involve a halogenated moiety or unactivated unsaturated systems. In the former case, the reaction is under Pd(0) catalysis, and in the second case a Pd(II) catalytic species is required and then an oxidant is necessary to reconvert the Pd(0) into the active Pd(II) species. On the other hand, the second method has the advantage that it uses easy available and inexpensive substrates.

1 Introduction

2 Indole Scaffold Synthesis

2.1 Activated Substrates

2.2 Unactivated Substrates

3 Functionalization of Indole Scaffold

3.1 Activated Substrates

3.2 Unactivated Substrates

4 Conclusions

Cu(I)-Catalyzed Ligand-Free Tandem One-Pot or Sequential Annulation via Knoevenagel Intermediates: An Entry into Multifunctional Naphthalenes, Phenanthrenes, Quinolines, and Benzo[b]carbazoles

A simple but efficient one-pot or sequential copper-catalyzed protocol using 2-bromoaldehydes and active methylene group containing substrates that affords multifunctional naphthalenes, phenanthrenes, quinolines, and benzo[b]carbazoles via Knoevenagel condensation, C-arylation, and decarboxylation, followed by aromatization, is developed. The reaction utilizes the potential of Knoevenagel intermediates and does not require any ancillary ligand. The phenanthrene products thus obtained show moderate fluorescence activity. Structural elaboration of the products to obtain dihydrobenzoquinazolines is also highlighted.

One-pot access to tetrahydrobenzo[c]carbazoles from simple ketones by using O2 as an oxidant

An effective and operationally simple one-pot Brønsted acid catalyzed cascade method is demonstrated for the synthesis of diversely functionalized carbazole frameworks starting from protecting group free 2-alkenyl indoles.

A catalyst-free aqueous mediated multicomponent reaction of isocyanide: expeditious synthesis of polyfunctionalized cyclo[b]fused mono-, di- and tricarbazoles

A new multi-component reaction is developed for the synthesis of cyclo[b]fused mono-, di- and tricarbazoles from o-alkenyl arylisocyanides, aldehydes, and cyclic diketones in ethanol/water (1 : 1) as the solvent system.