Transition-Metal-Free Approach for the Synthesis of 4-Aryl-quinolines from Alkynes and Anilines

Utilization of DMSO as a solvent-cum-reactant: synthesis of fused 2-aryl-4-methylquinolines

Herein, we disclose the synthesis of a variety of disubstituted fused quinoline, such as 4-methyl-2-arylbenzo[h]quinolines (3a-j), 1-methyl-3-arylbenzo[f]quinolines (3k-r), 1-methyl-3-arylnaphtho[2,3-f]quinolines (3s-x), and 2-aryl-5,7-dimethoxy-4-methylquinolines (4a-c), derivatives from the reaction of an aryl aldehyde with 1-naphthylamine (1a), 2-naphthylamine (1b), 2-aminoanthracene (1c) and 3,5-dimethoxyaniline (1d), respectively, at 80 °C in the presence of 30 mol% (±)-10-camphorsulfonic acid ((±)-CSA) using DMSO as the solvent-cum-reactant. DMSO molecules regioselectively incorporate three carbon atoms into the target molecule in this distinct reaction. The other advantages of the present protocol are that it can be performed under mild reaction conditions and does not require metal catalysts, additional additives or oxidants.

Recent advancement in the synthesis of quinoline derivatives via multicomponent reactions

The synthesis of quinoline derivatives through multicomponent reactions (MCRs) has emerged as an efficient and versatile strategy in organic synthesis. MCRs offer the advantage of constructing complex molecular architectures in a single step, utilising multiple starting materials in a convergent manner. This review provides an overview of recent advancements in the field of quinoline synthesis via MCRs. Various MCRs, such as the Povarov reaction, the Gewald reaction, and the Ugi reaction have been successfully employed for the synthesis of diverse quinoline scaffolds. These methodologies not only showcase high atom economy but also allow the incorporation of structural diversity into the final products. The versatility of MCRs enables the introduction of functional groups and substitution patterns tailored to specific applications. This review highlights the significance of quinoline derivatives in medicinal chemistry, materials science, and other interdisciplinary areas. The continuous innovation and development of novel MCR-based approaches for quinoline synthesis hold great promise for the rapid and efficient generation of valuable compounds with a wide range of biological and physicochemical properties.

p-TSA·H2O catalyzed metal-free and environmentally benign synthesis of 4-aryl quinolines from arylamine, arylacetylene, and dimethyl sulfoxide

An environmentally benign and metal-free synthesis of 4-aryl quinolines is reported by employing readily available arylamine, arylacetylene, and DMSO in the presence of 20 mol% p-TSA·H2O. In the present protocol, the solvent DMSO serves as a reactant cum solvent for providing the C2 carbon atom of the quinoline skeleton. Notably, the reaction proceeds effectively and efficiently without the involvement of any metal catalyst, ligand, and co-catalyst as additives and inert atmospheric reaction conditions. This method provides high atom economy and good yields, and two C-C and one C-N bonds are formed in a single step through a three-component reaction.

H2O2-Mediated Synthesis of a Quinazolin-4(3H)-one Scaffold: A Sustainable Approach

A quinazolin-4(3H)-one ring system is a privileged heterocyclic moiety with distinctive biological properties. From this perspective, the development of an efficient strategy for the synthesis of quinazolin-4(3H)-one has always been in demand for the synthetic chemistry community. In this report, we envisaged an efficient protocol for the synthesis of quinazolin-4(3H)-one using substituted 2-amino benzamide with dimethyl sulfoxide (DMSO) as a carbon source and H2O2 as an effective oxidant. Mechanistically, the reaction proceeds through the radical approach with DMSO as one carbon source. To further substantiate the synthetic claim, the synthetic protocol has been extended to the synthesis of the anti-endotoxic active compound 3-(2-carboxyphenyl)-4-(3H)-quinazolinone.

Facile access to 3-sulfonylquinolines via Knoevenagel condensation/aza-Wittig reaction cascade involving ortho-azidobenzaldehydes and β-ketosulfonamides and sulfones

Quinoline-based sulfonyl derivatives, and especially sulfonamides, are relevant and promising structures for drug design. We have developed a new convenient protocol for the synthesis of 3-sulfonyl-substituted quinolines (sulfonamides and sulfones). The approach is based on a Knoevenagel condensation/aza-Wittig reaction cascade involving o-azidobenzaldehydes and ketosulfonamides or ketosulfones as key building blocks. The protocol is appropriate for both ketosulfonyl reagents and α-sulfonyl-substituted alkyl acetates providing the target quinoline derivatives in good to excellent yields.

A Pummerer Reaction-Enabled Modular Synthesis of Alkyl Quinoline-3-carboxylates and 3-Arylquinolines from Amino Acids

Concise synthesis of functionalized quinolines has received continuous research attention owing to the biological importance and synthetic potential of bicyclic N-heterocycles. However, synthetic routes to the 2,4-unsubstituted alkyl quinoline-3-carboxylate scaffold, which is an important motif in drug design, remain surprisingly limited, with modular protocols that proceed from readily available materials being even more so. We herein report an acidic I₂-DMSO system that converts readily available aspartates and anilines into alkyl quinoline-3-carboxylate. This method can be extended to a straightforward synthesis of 3-arylquinolines by simply replacing the aspartates with phenylalanines. Mechanistic studies revealed that DMSO was activated by HI via a Pummerer reaction to provide the C1 synthon, while the amino acid catabolized to the C2 synthon through I₂-mediated Strecker degradation. A formal [3 + 2 + 1] annulation of these two concurrently generated synthons with aniline was responsible for the selective formation of the quinoline core. The synthetic utility of this protocol was illustrated by the efficient synthesis of human 5-HT4 receptor ligand. Moreover, an unprecedented chemoselective synthesis of 2-deuterated, 3-substituted quinoline, featuring this reaction, has been established.

Synthesis, characterization, anticancer, and antioxidant activities of chitosan Schiff bases bearing quinolinone or pyranoquinolinone and their silver nanoparticles derivatives

In this work, new chitosan-based Schiff bases were synthesized by the reaction of chitosan with quinolinone and pyranoquinolinone giving CSQ, and CSP, respectively. The novel Chitosan Schiff bases were grafted on silver nanoparticles providing CSQ/Ag, CSP/Ag structures. Characterization of isolated compounds was carried out by FT-IR, TGA, XRD, SEM, and TEM. The target compounds CSQ, CSP, CSQ/Ag, and CSP/Ag were evaluated as antitumor agents against three cancer cell lines, liver (HepG-2), colon (HCT-116), and breast (MCF-7). Compound CSQ/Ag disclosed potent cytotoxic effect with IC50 values in the range of 41.9–55.1 μg/ml in comparison with 5-fluorouracil against different cancer cell lines. Besides, the antioxidant activity of chitosan and its quinolinone and pyranoquinolinone analogues was assessed as radical scavengers versus 1,1-diphenyl-2-picrylhydrazyl radicals (DPPH%). The compound CSQ/Ag emerged as the most active member in scavenging the DPPH radicals. The obtained findings proved that the new Schiff bases/silver nanoparticles of chitosan showed higher antiproliferative and antioxidant activities than the blank CS and would be highly applicable in biomedical fields.

Graphical abstract

Synthesis of 3-substituted quinolines by ruthenium-catalyzed aza-Michael addition and intramolecular annulation of enaminones with anthranils

A method for the synthesis of 3-substituted quinolines by ruthenium-catalyzed aza-Michael addition and intramolecular annulation of enaminones with anthranils has been developed.

Ammonium iodide-catalyzed radical-mediated tandem cyclization of aromatic aldehydes, arylamines and 1,4-dioxane

We have developed a novel approach for the construction of 2-((2-arylquinolin-4-yl)oxy)ethan-1-ol derivatives involving a radical-mediated tandem cyclization reaction.

Methylene-Tethered Arylsulfonation and Benzotriazolation of Aryl/Heteroaryl C-H Bonds with DMSO as a One-Carbon Surrogate

The Selectfluor-mediated approach toward the synthesis of methylene-tethered arylsulfonation and benzotriazolation of imidazopyridines has been described. The reaction involves imidazopyridine, aryl sulfinate, or benzotriazole and dimethyl sulfoxide (DMSO) in the presence of Selectfluor, where DMSO acts as a one-carbon synthon. The protocol has been extended to the methylene-tethered arylsulfonation and benzotriazolation of β-naphthols. The mechanistic insights show that the intermediate 3-((methylthio)methyl)-2-phenylimidazo[1,2-a]pyridine is generated from imidazopyridine, DMSO, and Selectfluor. The nucleophilic displacement by the aryl sulfinate salt or benzotriazole on the intermediate afforded the product.

DMSO as a C1 Source for [2 + 2 + 1] Pyrazole Ring Construction via Metal-Free Annulation with Enaminones and Hydrazines

A cascade reaction between enaminones, hydrazines, and dimethyl sulfoxide (DMSO) for the synthesis of 1,4-disubstituted pyrazoles catalyzed by molecular iodine in the presence of Selectfluor has been realized. DMSO plays a dual role as the C₁ source and the reaction medium. In addition, the synthesis of 1,3,4-trisubstituted pyrazoles using aldehydes as alternative C₁ building blocks has also been achieved.

Stereoselective synthesis of (Z)-1,3-bis(α,β-unsaturated carbonyl)-isoindolines from aldehydes and phenacyl azides under metal free conditions

Here in the present manuscript, we report our observation of an unprecedented stereoselective synthesis of 2H-isoindolin-1,3-ylidenes from 2-(formylphenyl)acrylates and phenacylazide in the presence of piperidine. Unlike in our previous findings, in which we accessed 3-keto-isoquinolines from the same starting materials under slightly modified reaction conditions, this unexpected one-pot tandem reaction allows an efficient and simple method to access a variety of highly functionalized ethyl (Z)-2-((Z)-3-(2-oxo-2-arylethylidene)-2,3-dihydro-1H-benzo[e]isoindol-1-ylidene)-acetates in very good to excellent yields (up to 91%). The present methodology is compatible with a wide variety of functional groups.

Recent advances in directed sp2 C-H functionalization towards the synthesis of N-heterocycles and O-heterocycles

Heterocyclic compounds are widely present in the core structures of several natural products, pharmaceuticals and agrochemicals, and thus great efforts have been devoted to their synthesis in a mild and simpler way. In the past decade, remarkable progress has been made in the field of heterocycle synthesis by employing C-H functionalization as an emerging synthetic strategy. As a complement to previous protocols, transition metal catalyzed C-H functionalization of arenes using various directing groups has recently emerged as a powerful tool to create different classes of heterocycles. This review is mainly focussed on the recent key progress made in the field of the synthesis of N,O-heterocycles from olefins and allenes by using nitrogen based and oxidizing directing groups.

Synthetic and medicinal perspective of quinolines as antiviral agents

In current scenario, various heterocycles have come up exhibiting crucial role in various medicinal agents which are valuable for mankind. Out of diverse range of heterocycle, quinoline scaffold have been proved to play an important role in broad range of biological activities. Several drug molecules bearing a quinoline molecule with useful anticancer, antibacterial activities etc have been marketed such as chloroquine, saquinavir etc. Owing to their broad spectrum biological role, various synthetic strategies such as Skraup reaction, Combes reaction etc. has been developed by the researchers all over the world. But still the synthetic methods are associated with various limitations as formation of side products, use of expensive metal catalysts. Thus, several efforts to develop an efficient and cost effective synthetic protocol are still carried out till date. Moreover, quinoline scaffold displays remarkable antiviral activity. Therefore, in this review we have made an attempt to describe recent synthetic protocols developed by various research groups along with giving a complete explanation about the role of quinoline derivatives as antiviral agent. Quinoline derivatives were found potent against various strains of viruses like zika virus, enterovirus, herpes virus, human immunodeficiency virus, ebola virus, hepatitis C virus, SARS virus and MERS virus etc.

Sequential condensation/biannulation reactions of β-(2-aminophenyl)-α,β-ynones with 1,3-dicarbonyls

A divergent domino condensation/biannulation reaction of β-(2-aminophenyl) α,β-ynones with 1,3-dicarbonyls to construct a polycyclic 4H-pyrano[3,4-c]quinoline core has been developed. The p-TsOH·H2O catalyzed reaction of β-(2-aminophenyl) α,β-ynones with β-ketoesters in ethanol proceeds with good to excellent yields to provide a simple and effective method for the synthesis of functionalized 4H-pyrano[3,4-c]quinolinones. Further elaboration of these latter derivatives with an excess of 20% NH4OH in EtOH at 50 °C helps achieve the synthesis of the perlodinine analogues benzo[c][2,7]naphthyridin-4(3H)-one derivatives in high yields. Moreover, the p-TsOH·H2O mediated reaction of β-(2-aminophenyl) α,β-ynones with β-di-ketones leads to the formation of a variety of structurally diverse 4H-pyrano[3,4-c]quinoline polycyclic ketals by the incorporation of an alcohol solvent molecule in a cascade fashion.

Direct oxidation of N-ynylsulfonamides into N-sulfonyloxoacetamides with DMSO as a nucleophilic oxidant

N-Arylethynylsulfonamides are oxidized into N-sulfonyl-2-aryloxoacetamides directly and efficiently with dimethyl sulfoxide (DMSO) as both an oxidant and solvent with microwave assistance. DFT calculations indicate that DMSO nucleophilically attacks the ethylic triple bond and transfers its oxygen atom to the triple bond to form zwitterionic anionic N-sulfonyliminiums to trigger the reaction. Then it nucleophilically attacks the generated iminium intermediates to accomplish the oxidation via the second oxygen atom transfer. The current method provides a straightforward and efficient strategy to transform various N-arylethynylsulfonamides into N-sulfonyl-2-aryloxoacetamides, sulfonyl oxoacetimides, without any other electrophilic activators or oxidants.

Microwave-assisted direct oxidation of N-arylethynylsulfonamides with DMSO as a nucleophilic oxidant and solvent affords N-sulfonyl-2-aryloxoacetamides without any other additional electrophilic activators or oxidants.

A three-component, general and practical route for diastereoselective synthesis of aza-spirocyclic pyrazolones via a decarboxylative annulation process

An efficient, general, and practical route for highly diastereoselective synthesis of aza-spirocyclic pyrazolones from easily available α-amino acids, aldehydes, and alkylidene pyrazolones by means of a decarboxylative annulation process is reported.

Recent synthetic efforts in the preparation of 2-(3,4)-alkenyl (aryl) quinoline molecules towards anti-kinetoplastid agents

Leishmaniasis, Chagas disease and African sleeping sickness have been considered some of the most important tropical protozoan afflictions. As the number of drugs currently available to treat these human illnesses is severely limited and the majority has poor safety profiles and complicated administration schedules, actually there is an urgent need to develop new effective, safe and cost-effective drugs. Because quinoline alkaloids with antiprotozoal activity (quinine, chimanine, cryptolepine or huperzine groups) were historically and are still essential models for drug research to combat these parasitic infections, synthetic or semi-synthetic quinoline-based molecules are important for anti-kinetoplastid drug design approaches and synthetic methods of their preparation become a key task that is the central subject of this review. Its goal is to highlight the advances in the conventional and current syntheses of new 2-(3,4)-alkenyl (aryl) quinoline derivatives, which kill the most important kinetoplastid protozoa, – Leishmania and Trypanosoma and could be useful models for antileishmanial and antitrypanosomal research. An attempt has been made to present and discuss the more recent contributions in this field over the period 2015–2019, paying special attention to molecular design, synthetic efforts to new green reaction conditions for classical methods such as Skraup synthesis, Friedländer synthesis, Conrad–Limpach, Doebner–Miller, as well as contemporary methods like Gould–Jacobs, Meth–Cohn and Povarov reactions. This review includes brief general information on these neglected tropical diseases, their current chemotherapies, and primary natural models (quinoline alkaloids), suitable for development of anti-kinetoplastid quinoline-based agents. The main part of the review comprises critical discussion on the synthesis and chemistry of new quinolines diversely substituted by alkyl (alkenyl, aryl) fragments on the pyridine part of the quinoline skeleton, which could be considered interesting analogues of chimanine alkaloids. The methods described in this review were developed with the aim of overcoming the drawbacks of the traditional protocols using revolutionary precursors and strategies.

Leishmaniasis, Chagas disease and African sleeping sickness have been considered some of the most important tropical protozoan afflictions.

The recent advances in K2S2O8-mediated cyclization/coupling reactions via an oxidative transformation

Recently the K₂S₂O₈ mediated cyclization/coupling reactions to construct carbon–carbon/carbon–heteroatom bond via oxidative transformation is became much interesting in organic synthesis.

Transition-Metal-Free One-Pot Tandem Synthesis of 3-Ketoisoquinolines from Aldehydes and Phenacyl Azides

An efficient and transition-metal-free strategy for the synthesis of 3-keto-isoquinolines in one pot has been developed from the easily accessible 2-(formylphenyl)acrylates and phenacyl azides. Various substituted aldehydes and phenacyl azides were successfully employed in this transformation to furnish a variety 3-keto-isoquinolines in very good yields. A number of controlled experiments were conducted to postulate the reaction mechanism. Secondary functionalizations of 2-keto-isoquinolins were also performed to showcase the synthetic utility.

Potassium tert-Butoxide-Mediated Condensation Cascade Reaction: Transition Metal-Free Synthesis of Multisubstituted Aryl Indoles and Benzofurans

An efficient and facile method to synthesize valuable disubstituted 2-aryl indoles and benzofurans in good yields has been demonstrated, based on a tert-butoxide-mediated condensation reaction involving a vinyl sulfoxide intermediate. Products are obtained from N- or O-benzyl benzaldehydes using dimethyl sulfoxide as a carbon source. The methodology features a wide functional group tolerance and transition metal-free environment. Preliminary mechanistic studies suggest that the reaction involves a tandem aldol reaction/Michael addition/dehydrosulfenylation/isomerization sequence through an ionic protocol.

An efficient 3-acylquinoline synthesis from acetophenones and anthranilviaC(sp3)–H bond activation mediated by Selectfluor

An efficient method for the synthesis of 3-functionalized quinolines from commercially available acetophenones and anthranil has been described. Selectfluor propels the C(sp³)–H bond activation of the acetophenones and aza-Michael addition of anthranil resulting in annulated 3-acylquinolines in moderate to high yields. DMSO acts not only as a solvent but also as a one carbon donor in the reaction.

An efficient method for the synthesis of 3-functionalized quinolines from acetophenones and anthranil mediated by Selectfluor was achieved.