Temporary (P[double bond, length as m-dash]O) directing group enabled carbazole ortho arylation via palladium catalysis

A palladium-catalyzed, temporary P(O) directing group assisted C-H bond arylation of carbazoles was achieved. The release of the directing group occurs spontaneously in the reaction and the mechanistic studies indicate that acid is essential for N-P bond cleavage.

Recent advances in the synthesis of carbazoles from indoles

Synthesis of carbazoles using indoles as precursors through CH activation/annulation.

Imidazolium and benzimidazolium-containing compounds: from simple toxic salts to highly bioactive drugs

The toxicity of simple imidazolium and benzimidazolium salts started to be more and more investigated in the last few years and was taken in consideration in the context of microorganisms, plants and more evolved organisms' exposure. However, the toxicity of these salts can be exploited in the development of different biological applications by incorporating them in the structure of compounds that specifically target microorganisms and cancer cells. We highlight in this minireview the way researchers became aware of the inherent problem of the stability and bioaccumulation of imidazolium and benzimidazolium salts and how they found inspiration to exploit their toxicity by incorporating them into new highly potent drugs.

Oxidative C-H/C-H Coupling Reactions between Two (Hetero)arenes

Transition metal-mediated C-H bond activation and functionalization represent one of the most straightforward and powerful tools in modern organic synthetic chemistry. Bi(hetero)aryls are privileged π-conjugated structural cores in biologically active molecules, organic functional materials, ligands, and organic synthetic intermediates. The oxidative C-H/C-H coupling reactions between two (hetero)arenes through 2-fold C-H activation offer a valuable opportunity for rapid assembly of diverse bi(hetero)aryls and further exploitation of their applications in pharmaceutical and material sciences. This review provides a comprehensive overview of the fundamentals and applications of transition metal-mediated/catalyzed oxidative C-H/C-H coupling reactions between two (hetero)arenes. The substrate scope, limitation, reaction mechanism, regioselectivity, and chemoselectivity, as well as related control strategies of these reactions are discussed. Additionally, the applications of these established methods in the synthesis of natural products and exploitation of new organic functional materials are exemplified. In the last section, a short introduction on oxidant- or Lewis acid-mediated oxidative Ar-H/Ar-H coupling reactions is presented, considering that it is a very powerful method for the construction of biaryl units and polycylic arenes.

Anti-tuberculosis activity and structure-activity relationships of oxygenated tricyclic carbazole alkaloids and synthetic derivatives

A series of 49 oxygenated tricyclic carbazole derivatives has been tested for inhibition of the growth of Mycobacterium tuberculosis and a mammalian cell line (vero cells). From this series, twelve carbazoles showed a significant anti-TB activity. The four most active compounds were the naturally occurring carbazole alkaloids clauszoline-M (45), murrayaline-C (41), carbalexin-C (27), and the synthetic carbazole derivative 22 with MIC₉₀ values ranging from 1.5 to 3.7μM. The active compounds were virtually nontoxic for the mammalian cell line in the concentration range up to 50μM.

Synthesis of Stable Diarylpalladium(II) Complexes: Detailed Study of the Aryl-Aryl Bond-Forming Reductive Elimination

The synthesis of diarylpalladium(II) complexes by twofold aryl C-H bond activation was developed. These intermediates of oxidative cyclization reactions are stabilized by chelation with acetyl groups while still maintaining sufficient reactivity to study their reductive elimination. Four distinct triggers were found for the reductive elimination of these complexes to dibenzofurans and carbazoles. Thermal elimination occurs at very high temperatures, whereas ligand-promoted and oxidatively induced reductive eliminations proceed readily at room temperature. Under these conditions, no isomerization occurs. In contrast, weak Brønsted acids, such as acetic acid, lead to a sequence of proto-demetalation, isomerization to a κ(3) -diarylpalladium(II) complex, and reductive elimination to non-symmetrical cyclization products.

One-pot synthesis of carbazoles via tandem C–C cross-coupling and reductive amination

We have developed a highly efficient synthetic route to carbazoles that employs sequential C–C/C–N bond formation via Suzuki cross-coupling and Cadogan cyclization. The developed method is compatible with electron neutral, rich or deficient substrates. The synthetic utility of this method was demonstrated by the concise syntheses of four natural products.

Silver catalyzed domino aza-annulation/Diels–Alder cyclization of 2-ene-yne anilines: a facile one-pot access to carbazole, dihydrocarbazole and tetrahydrocarbazole frameworks

AgSbF₆-catalyzed domino aza-annulation/Diels–Alder/aromatization cascade for the construction of carbazoles, dihydrocarbazoles, tetrahydrocarbazoles and 3-indolyl-substituted tetrahydrocarbazoles was achieved.

Construction of highly functionalized carbazoles via condensation of an enolate to a nitro group

This paper describes a novel synthesis of highly functionalized and diverse carbazoles via transition-metal-free and mild base-promoted condensations of readily available 2-nitrocinnamaldehyde or 2-nitrochalcones with various β-ketoesters or 1,3-diaryl-2-propanones. The method selectively forms four bonds by the intramolecular conjugate addition of an enolate to the enal or chalcone bearing an o-nitro group. This group then undergoes in situ N-O bond cleavage under non-reductive conditions in a one-pot procedure. This protocol allows for the introduction of various functional groups at all positions of the newly formed aromatic ring of the carbazole moiety. The utility of this methodology is further illustrated by the concise synthesis of naturally occurring hyellazole and chlorohyellazole.

Synthesis and antitumor activity of novel N-substituted carbazole imidazolium salt derivatives

A series of novel N-substituted carbazole imidazolium salt derivatives has been prepared and investigated for their cytotoxic activity against five human tumor cell lines by MTS assay. The results indicated that the existence of 5,6-dimethyl-benzimidazole ring, substitution of the imidazolyl-3-position with a 2-bromobenzyl or naphthylacyl group, as well as alkyl chain length between carbazole and imidazole ring were important for the antitumor activity. Compound 61, bearing a 2-bromobenzyl substituent at position-3 of the 5,6-dimethyl-benzimidazole, showed powerful inhibitory activities and was more selective to HL-60, SMMC-7721, MCF-7 and SW480 cell lines with IC50 values 0.51-2.48 μM. Mechanism of action studies revealed that this new compound could remarkably induce cell cycle arrest and apoptosis in SMMC-7721 cells. This work provides alternative novel way for future drug development based on carbazole and imidazolium salt scaffolds.

Palladium(ii)-catalysed total synthesis of naturally occurring pyrano[3,2-a]carbazole and pyrano[2,3-b]carbazole alkaloids

Seven naturally occurring pyranocarbazole alkaloids (pyrayafoline A–E,O-methylmurrayamine A andO-methylmahanine) have been synthesised using a palladium(ii)-catalysed cyclisation of a diarylamine to an orthogonally diprotected 2,7-dihydroxycarbazole as key step.

Brønsted acid-catalyzed synthesis of carbazoles from 2-substituted indoles

A simple and efficient approach for the synthesis of disubstituted carbazoles has been developed from o-haloanilines and terminal alkynes using a two-step strategy, namely, Sonogashira coupling and intramolecular cyclization.

First total syntheses of chrestifoline-B and (±)-chrestifoline-C, and improved synthetic routes to bismurrayafoline-A, bismurrayafolinol and chrestifoline-D

We describe an efficient synthesis of the methylene-bridged biscarbazole alkaloids bismurrayafoline-A, bismurrayafolinol and chrestifoline B–D using an Ullmann-type coupling at the benzylic position.

Total synthesis of the cyclic monoterpenoid pyrano[3,2-a]carbazole alkaloids derived from 2-hydroxy-6-methylcarbazole

The synthesis of seven pyrano[3,2-a]carbazole alkaloids has been achieved using their putative biogenetic precursor 2-hydroxy-6-methylcarbazole as key intermediate.

An efficient Au-catalyzed synthesis of isomukonidine, clausine L, mukonidine, glycosinine, mukonal, and clausine V from propadienyl methyl ether

Naturally occurring carbazole alkaloids such as isomukonidine, clausine L, mukonidine, glycosinine, mukonal, and clausine V have been synthesized through a gold-catalyzed cyclization reaction of 1-(indol-2-yl)-2-methoxy-2,3-allenols, which are readily available from indolecarbaldehydes and methoxypropadiene. Compared to the reported procedures, our approach is general, atom economic, highly selective, and the starting materials are easily available.

Palladium-catalyzed synthesis of natural and unnatural 2-, 5-, and 7-oxygenated carbazole alkaloids from N-arylcyclohexane enaminones

A palladium-catalyzed synthesis of the carbazole framework is described, including the preparation of 2-, 5-, and 7-oxygenated natural and unnatural carbazole alkaloids. A series of N-arylcyclohexane enaminones, generated by condensation of cyclohexane-1,3-dione with diverse anilines, were aromatized by a Pd(0)-catalyzed thermal treatment to afford the corresponding diarylamines. The latter were submitted to a Pd(II)-catalyzed cyclization and methylation processes to provide the desired carbazoles, including clausine V. Following an inverse strategy, a new and short total synthesis of glycoborine is also reported.