Acid-Catalyzed Multicomponent Tandem Cyclizations: Access to Polyfunctional Dihydroindolizino[8,7-b]indoles

POCl3/Sulfoxide-Promoted Synthesis of Indolizino[8,7-b]indoles

A mild chlorocyclization of pyrrole-tethered indoles has been realized using POCl3 as the chlorine source and tetramethylene sulfoxide as the promoter. A variety of chlorinated indolizino[8,7-b]indole derivatives have been constructed efficiently under this reaction system in moderate to good yields (19 examples, up to 93% yield).

Synthesis and Biological Evaluation of 12-Aryl-11-hydroxy-5,6-dihydropyrrolo[2″,1″:3',4']pyrazino[1',2':1,5]pyrrolo[2,3-d]pyridazine-8(9H)-one Derivatives as Potential Cytotoxic Agents

In the present paper, a facile and efficient synthetic procedure has been applied to obtain dihydrodipyrrolo[1,2-a:2',1'-c]pyrazine-2,3-dicarboxylates (5a-s), which have subsequently gone through the cyclization in the presence of hydrazine hydrate to afford 12-aryl-11-hydroxy-5,6-dihydropyrrolo[2″,1″:3',4']pyrazino[1',2':1,5]pyrrolo[2,3-d]pyridazine-8(9H)-ones (7a-q). The molecular structures of these novel compounds were extensively examined through the analysis of spectroscopic data in combination with X-ray crystallography techniques. Following that, the in vitro cytotoxic activities of all derivatives against three human cancer cell lines (Panc-1, PC3, and MDA-MB-231) were comprehensively evaluated alongside the assessment on normal human dermal fibroblast (HDF) cells using the MTT assay. Among the compounds, the 3-nitrophenyl derivative (7m) from the second series showed the best antiproliferative activity against all tested cell lines, particularly against Panc-1 cell line, (IC50 = 12.54 μM), being nearly twice as potent as the standard drug etoposide. The induction of apoptosis and sub-G1 cell cycle arrest in Panc-1 cancer cells by compound 7m was confirmed through further assessment. Moreover, the inhibition of kinases and the induction of cellular apoptosis by compound 7m in Panc-1 cancer cells were validated using the Western blotting assay.

Iodine-Catalyzed Synthesis of Benzo-β-carbolines through Desulfurative Cyclization of 2-(1H-Indol-3-ylsulfanyl)-phenylamines with Aryl Methyl Ketones

A novel transition metal-free strategy for the synthesis of benzene-fused β-carboline scaffolds has been developed. This protocol offers a rapid and direct pathway to access the benzene fused β-carboline from 2-(1H-indol-3-ylsulfanyl)-phenylamines and aryl methyl ketones using an efficient catalytic system of I2/DMSO. The present mild protocol proceeds through the sequential reactions of Kornblum oxidation, Pictet-Spengler cyclization, and desulfurization to afford the desired products in excellent yields up to 99%. Moreover, this method has a wide range of substrate tolerance and is operationally simple and applicable in gram-scale synthesis.

Brønsted acid and Lewis acid co-promoted cascade cyclization reaction and application to the total synthesis of Erysotramidine

A strategy for the syntheses of pyrrolo [2,1-a] isoquinolines and indolizinoindolones via a Brønsted acid and Lewis acid co-promoted cascade reaction has been developed. The strategy was exemplified in the total synthesis of Erysotramidine.

Synthesis of Tetrahydroindolizino[8,7-b]indole Derivatives in the Presence of Fe(OTf)3 or CF3SO3H through Intramolecular Dearomatization of Indole

We have developed a convenient synthesis of tetrahydroindolizino[8,7-b]indole derivatives via intramolecular dearomatization of indole. Highly functionalized tetrahydroindolizinoindoles can be prepared in the presence of trifluoromethanesulfonic acid in good to excellent yields (up to >99% yield) with novel designed pyrrole-tethered indoles. The same products can also be synthesized through a mild Fe(OTf)₃-catalyzed process in acceptable to good yields (up to 75% yield).

Acid-Catalyzed Multicomponent Rearrangements via 2-((Quinoxalin-3(4H)-on-2-yl)(aryl)methylene)malononitriles, Generated In Situ, for Divergent Synthesis of Pyrroles with Different Substitution Patterns

New three-component domino reactions, providing divergent approaches to multifunctionalized pyrroles with different substitution patterns, have been established (47 examples). In this work, a new rearrangement of quinoxalinones with the participation of the in situ-generated 2-en-1-imine moiety of the substituent at C3 makes it possible to construct two new heterocyclic systems, namely, a benzimidazolone and a pyrrole, simultaneously under one-pot reaction conditions. The reaction is easy to perform simply by mixing three common reactants of acetic acid with heating. Secondary amines or primary alcohols as the third component of the reaction, along with quinoxalin-3(4H)-ones and malononitrile, not only initiate the rearrangement but also are responsible for the nature of substituents at position 5 of the pyrrole ring in the newly formed new biheterocyclic system. The reaction proceeds smoothly and can be finished within 7 h, which makes workup convenient to give up to 97% chemical yields.

Aza-Annulation of 1,2,3,4-Tetrahydro-β-carboline Derived Enaminones and Nitroenamines: Synthesis of Functionalized Indolizino[8,7-b]indoles, Pyrido[1,2-a:3,4-b']diindoles, Indolo[2,3-a]quinolizidine-4-ones and Other Tetrahydro-β-carboline Fused Heterocycles

Aza-annulation of novel 1,2,3,4-tetrahydro-β-carboline derived enaminones and nitroenamines with various 1,2- and 1,3-bis electrophiles, such as oxalyl chloride, maleic anhydride, 1,4-benzoquinone, 3-bromopropionyl chloride, itaconic anhydride, and imines (from formaldehyde and primary amines), has been investigated. These methodologies provide simple one-step pathways for efficient construction of highly functionalized tetrahydro-β-carboline 1,2-fused, five- and six-membered heterocyclic frameworks, such as indolizino[8,7-b]indoles, pyrido[1,2-a:3,4-b']diindoles, indolo[2,3-a]quinolizidines, and pyrimido[1',6':1,2]pyrido[3,4-b]indoles, which are core structures of many naturally occurring indole alkaloids with diverse bioactivity.

I2-Promoted Multicomponent Dicyclization and Ring-Opening Sequences: Direct Synthesis of Benzo[e][1,4]diazepin-3-ones via Dual C-O Bond Cleavage

A novel and efficient formal [4 + 2+1] annulation of aryl methyl ketones and 2-aminobenzyl alcohols for the synthesis of benzo[e][1,4]diazepin-3-ones is reported. This reaction successfully affords diverse seven-membered ring lactams via dual C-O bond cleavage. A preliminary mechanistic study showed that a multicomponent dicyclization and ring-opening sequence might occur, with the introduction of methyl sulfide proposed as the last step. This efficient strategy with mild reaction conditions and a broad substrate scope has potential applications in chemistry and medicine.

Convenient construction of tetrahydrochromeno[4',3':2,3]indolizino[8,7-b]indoles and tetrahydroindolizino[8,7-b]indoles via one-pot domino reaction

The functionalized tetrahydrochromeno[4′,3′:2,3]indolizino[8,7-b]indoles were conveniently synthesized in high yields by one-pot domino reaction of tryptamines, alkyl propiolates and 2-aryl-3-nitro-2H-chromenes. Under similar conditions, the one-pot reaction of tryptamines, alkyl propiolates and β-nitroalkenes resulted in functionalized tetrahydroindolizino[8,7-b]indoles. The reaction mechanism involved sequential generation of β-enamino ester, Michael addition, Pictet–Spengler reaction and annulation process. The reaction showed high atomic economy and met the goals of sustainable chemistry.

The functionalized tetrahydrochromeno[4′,3′:2,3]indolizino[8,7-b]indoles were conveniently synthesized in high yields by one-pot domino reaction of tryptamines, alkyl propiolates and 2-aryl-3-nitro-2H-chromenes.

Recent advances in the application of indoles in multicomponent reactions

Indoles are some of the most versatile and common nitrogen-based heterocyclic scaffolds and are frequently used in the synthesis of various organic compounds. Indole based compounds are very important among heterocyclic structures due to their biological and pharmaceutical activities. The last decade, in particular, has witnessed considerable activity towards the synthesis of indole derivatives due to the possibilities for the design of polycyclic structures by the incorporation of multiple fused heterocyclic scaffolds in an attempt to achieve promising new heterocycles with chemical and biomedical relevance. In this study, we provide an overview on recent applications of indole in the multicomponent reactions for the synthesis of various heterocyclic compounds during the period of 2012 to 2017.

Synthesis of α-functionalized α-indol-3-yl carbonyls through direct SN reactions of indol-3-yl α-acyloins

A new and efficient synthesis of α-functionalized α-indol-3-yl ketones from easily available indolyl α-acyloins is reported. This process, catalyzed by Brønsted or Lewis acids, involves an uncommon direct nucleophilic substitution reaction of a secondary α-carbonyl-substituted hydroxyl group. The described methodology allows the introduction of a variety of nucleophiles such as (hetero)arenes, thiophenols, nitroanilines and 1,3-dicarbonyl derivatives. The synthesized α-indol-3-yl carbonyl compounds are important synthetic targets also useful for accessing functionalized tryptophols and furan-3-yl indoles.

Acid-Catalyzed Multicomponent Tandem Double Cyclization: A One-pot, Metal-free Route to Synthesize Polyfunctional 4,9-Dihydropyrrolo[2,1-b]quinazolines

An acid-catalyzed multicomponent tandem double cyclization protocol has been developed for the synthesis of polyfunctional 4,9-dihydropyrrolo[2,1-b]quinazolines from simple and readily available arylglyoxal monohydrates, 2-aminobenzylamine, and trans-β-nitrostyrenes. This practical and metal-free reaction proceeds through an imine formation/cyclization/Michael addition/Henry cyclization protocol, resulting in the construction of four new bonds and two ring moieties directly in one pot.