An Update on Cyclohepta[ b ]indoles

Efficient Synthesis of Cyclohepta[b]indoles and Cyclohepta[b]indole-Indoline Conjugates via RCM, Hydrogenation, and Acid-Catalyzed Ring Expansion: A Biomimetic Approach

Cyclohepta[b]indoles, prevalent in natural products and pharmaceuticals, are conventionally accessed via metal or Lewis acid-mediated cycloadditions with prefunctionalized substrates. Our study introduces an innovative sequential catalytic assembly for synthesizing cyclohepta[b]indoles from readily available isatin derivatives. The process involves three catalytic sequences: ring-closing metathesis, catalytic hydrogenation, and acid-catalyzed ring expansion. The RCM of 2,2-dialkene-3-oxindoles, formed by butenyl Grignard addition to 3-allyl-3-hydroxy-2-oxindoles, yields versatile spirocyclohexene-3-oxindole derivatives. These derivatives undergo further transformations, including dibromination, dihydroxylation, epoxidation, Wacker oxidation at the double bond. Hydrogenation of spirocyclohexene-3-oxindole yields spirocyclohexane-3-oxindoles. Their subsequent acid-catalyzed ring expansion/aromatization, dependent on the acid catalyst, results in either cyclohepta[b]indoles or cyclohepta[b]indole-indoline conjugates, adding a unique synthetic dimension. The utility of this methodology is exemplified through the synthesis of an A-FABP inhibitor, showcasing its potential in pharmaceutical applications.

Synthesis of cyclohepta[b]indoles via gold mediated energy transfer photocatalysis

Photocatalysis involving energy transfer (EnT) has become a valuable technique for building intricate organic frameworks mostly through [2+2]-cycloaddition reactions. Herein, we report a synthetic method leading to functionalized cyclohepta[b]indoles, an important structural motif in natural products and pharmaceuticals, using gold-mediated energy transfer photocatalysis. The scope of this operationally simple and atom-economical strategy is presented. Density functional theory studies were employed in order to gain insights into the mechanism of formation of the cyclohepta[b]indole core.

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.

Molecular-iodine catalyzed selective construction of cyclopenta[b]indoles from indoles and acetone: a green gateway to indole-fused cycles

Molecular-iodine catalyzed access to an important class of bio-relevant indole derivatives, cyclopenta[b]indoles, has been achieved via a cascade addition/intramolecular cyclization reaction of indoles and acetone. Explorations of diverse substitution patterns revealed an essential substrate-control in the reaction. The high-density electronic core of indole is pivotal in favouring the formation of indolyl-cyclopenta[b]indole derivatives; in contrast, the electron deficiency of the core hindered the cyclization process, directing the formation of bis(indolyl)propanes. Investigations on the mechanistic pathway revealed that bis(indolyl)alkanes were the intermediates for the addition-cyclization process. This simple experimental method provides sustainable synthetic access to cyclopentannulated indoles.

Intramolecular Dearomative Inverse-Electron-Demand Diels Alder Strategy for the Total Synthesis of (+)-Alstonlarsine A

An evolution of a synthetic route leading to a successful enantioselective total synthesis of monoterpenoid indole alkaloid (+)-alstonlarsine A is represented. The unique 9-azatricyclo[4.3.1.03,8]decane core was assembled through an efficient domino sequence comprising enamine formation in situ, followed by intramolecular dearomative inverse-electron-demand Diels Alder reaction. The preparation of the tricyclic dihydrocyclohepta[b]indole key intermediate via the intramolecular Horner-Wadsworth-Emmons reaction required a development of a new general method for the introduction of the phosphonoacetate moiety into the indole C-2 position, through copper-carbenoid insertion. The modular nature of the represented synthetic approach makes it suitable for the synthesis of analogues with different substituents' patterns.

Rh(III)-Catalyzed Double C-H Functionalization of Indoles with Cyclopropenones via Sequential C-H/C-C/C-H Bond Activation

An unprecedented Rh(III)-catalyzed double C-H functionalization of indoles with cyclopropenones via sequential C-H/C-C/C-H bond activation has been developed. This procedure represents the first example for assembling of cyclopenta[b]indoles utilizing cyclopropenones as 3C synthons. This powerful approach shows excellent chemo- and regioselectivity, wide functional group tolerance, and good reaction yields.

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.

Construction of a Pentacyclic Framework Enabled by Nickel Catalysis

We present a novel nickel-catalyzed reaction of indole-tethered 2-alkynylphenol esters with various (hetero)aryl boronic acids, resulting in the synthesis of diversely functionalized pentacyclic benzofurocyclohepta[b]indole derivatives. This unprecedented cascade reaction involves the arylative cyclization of alkynes, nucleophilic attack of the indole moiety on the oxonium ion intermediate, 1,2-alkyl group migration, and aromatization. The synthesized molecules exhibit exceptional cytotoxicity against multiple cancer cell lines while maintaining biocompatibility toward healthy cells.

DDQ/Fe(NO3)3-Catalyzed Aerobic Synthesis of 3-Acyl Indoles and an In Silico Study for the Binding Affinity of N-Tosyl-3-acyl Indoles toward RdRp against SARS-CoV-2

In the present study, we herein report a DDQ-catalyzed new protocol for the synthesis of substituted 3-acylindoles. Being a potential system for virtual hydrogen storage, introduction of catalytic DDQ in combination with Fe(NO₃)₃·9H₂O and molecular oxygen as co-catalysts offers a regioselective oxo-functionalization of C-3 alkyl-/aryllidine indolines even with scale-up investigations. Intermediate isolation, their spectroscopic characterization, and the density functional theory calculations indicate that the method involves dehydrogenative allylic hydroxylation and 1,3-functional group isomerization/aromatization followed by terminal oxidation to afford 3-acylindoles quantitatively with very high regioselectivity. This method is very general for a large number of substrates with varieties of functional groups tolerance emerging high-yield outcome. Moreover, molecular docking studies were performed for some selected ligands with an RNA-dependent RNA polymerase complex (RdRp complex) of SARS-CoV-2 to illustrate the binding potential of those ligands. The docking results revealed that few of the ligands possess the potential to inhibit the RdRp of SARS-Cov-2 with binding energies (-6.7 to -8.19 kcal/mol), which are comparably higher with respect to the reported binding energies of the conventional re-purposed drugs such as Remdesivir, Ribavirin, and so forth (-4 to -7 kcal/mol).