Alkyltriflate-Triggered Annulation of Arylisothiocyanates and Alkynes Leading to Multiply Substituted Quinolines through Domino Electrophilic Activation

I2-mediated Csp2–P bond formation via tandem cyclization of o-alkynylphenyl isothiocyanates with organophosphorus esters

A highly efficient molecular-iodine-catalyzed cascade cyclization reaction has been developed, creating a series of 4H-benzo[d][1,3]thiazin-2-yl phosphonates in moderate to excellent yields. This approach benefits from metal-free catalysts and available raw materials.

Catalyst-free aerobic radical cascade reactions of o-vinylphenylisocyanides with thiols to access 2-thio-substituted quinolines

We herein report an efficient and green aerobic radical cascade reaction of o-vinylphenylisocyanides with thiols to access a broad range of 2-thio-substituted quinolines without the need for additional catalysts or oxidants.

An environmentally benign regioselective synthesis of 2-benzyl-4-arylquinoline derivatives using aryl amines, styrene oxides and aryl acetylenes

A novel and an expedient metal- and solvent-free synthesis of a wide variety of 2-benzyl-4-arylquinoline derivatives is described from readily available aryl amines, styrene oxides and aryl acetylenes in the presence of 10 mol% molecular iodine. This domino reaction occurs under metal- and solvent-free conditions at 120 °C, which avoids the usage of metal catalyst and as a consequence generation of metal waste. The salient features of this methodology are the use of simple starting materials, ease of handling, high regioselectivity, shorter reaction time, atom-economical, step-economical, the formation of one C-N and two C-C bonds and a wide range of functional groups tolerance.

MeOTf/KI-catalyzed efficient synthesis of 2-arylnaphthalenes via cyclodimerization of styrene oxides

The MeOTf/KI-catalyzed synthesis of 2-arylnaphthalene derivatives from aryl ethylene oxides in alcohol under ambient conditions is described. The present protocol has a higher atom efficiency and wider substrate applicability with excellent yields. The reaction proceeded using the aryl ethylene oxides to give 2-arylnaphthalenes either in homo-coupling or in cross-coupling. The reaction could also be carried out at the gram scale in minutes.

Metal-free synthesis of quinoline-2,4-dicarboxylate derivatives using aryl amines and acetylenedicarboxylates through a pseudo three-component reaction

An efficient, useful and one-pot protocol for the synthesis of quinoline-2,4-dicarboxylate scaffolds is accomplished from aryl amines and dimethyl/diethyl acetylenedicarboxylates using 20 mol% molecular iodine as a catalyst in acetonitrile at 80 °C. In addition, the mechanistic explanation for the formation of the desired products is disclosed. The pivotal role of molecular iodine in the formation of the major products, diester quinoline derivatives, and the minor product, triesters, in two cases is described in the mechanism. The notable advantages of this method are non-involvement of a metal catalyst, avoiding of metal contamination in the final product as well as waste generation, use of a low cost and eco-friendly catalyst, ease of handling, high regioselectivity, shorter reaction time, the formation of one C-N and two C-C bonds and a broad substrate scope with good yields.

Copper(ii) triflate catalyzed three-component reaction for the synthesis of 2,3-diarylquinoline derivatives using aryl amines, aryl aldehydes and styrene oxides

An efficient and expedient synthetic protocol is reported for the synthesis of 2,3-diarylquinoline derivatives from readily available aryl amines, aryl aldehydes and styrene oxides using 10 mol% copper(ii) triflate by employing three-component reaction.

Recent Progress in the Synthesis of Quinolines

BACKGROUND

Quinoline-containing compounds present in both natural and synthetic products are an important class of heterocyclic compounds. Many of the substituted quinolines have been used in various areas including medicine as drugs. Compounds with quinoline skeleton possess a wide range of bioactivities such as antimalarial, anti-bacterial, anthelmintic, anticonvulsant, antiviral, anti-inflammatory, and analgesic activity. Due to such a wide range of applicability, the synthesis of quinoline derivatives has attracted a lot of attention of chemists to develop effective methods. Many known methods have been expanded and improved. Furthermore, various new methods for quinoline synthesis have been established. This review will focus on considerable studies on the synthesis of quinolines date which back to 2014.

OBJECTIVE

In this review, we discussed recent achievements on the synthesis of quinoline compounds. Some classical methods have been modified and improved, while other new methods have been developed. A vast variety of catalysts were used for these transformations. In some studies, quinoline synthesis reaction mechanisms were also displayed.

CONCLUSION

Many methods for the synthesis of substituted quinoline rings have been developed recently. Over the past five years, the majority of those reported have been based on cycloisomerization and cyclization processes. Undoubtedly, more imaginative approaches to quinoline synthesis will appear in the literature in the near future. The application of known methods to natural product synthesis is probably the next challenge in the field.

Organocatalyzed Synthesis of Functionalized Quinolines

As one of the most widely investigated compound skeleton, quinolines possess important medicinal and biological activities. As such, a great number of literatures, including reviews, have reported various methodologies to construct quinolines. Recently, organocatalyzed reactions have attracted the attention of organic chemists due to its being "green" because the reactions avoid the use of toxic metals. In this review, various distinctive contributions are surveyed with specific emphasis on organocatalyzed reactions for quinoline core constructions.

Concise and Efficient Synthesis of Indole-Indolone Scaffolds through MeOTf-Induced Annulation of N-(2-Cyanoaryl)indoles

MeOTf-triggered annulation of N-(2-cyanoaryl)indoles to provide indolo[1,2-a]indol-10-imines and the corresponding indolo[1,2-a]indol-10-ones have been described under metal-free conditions. A variety of functional groups are tolerated in their scaffolds with good to excellent yields. The reaction could be carried to gram scale.

Preparation of 2-arylquinolines from β-arylpropionitriles with aryllithiums and NIS through iminyl radical-mediated cyclization

Treatment of β-arylpropionitriles with aryllithiums, followed by the reaction with water and then with NIS under irradiation with a tungsten lamp gave 2-arylquinolines in good to moderate yields.

Rh(iii)-catalyzed alkynylation: synthesis of functionalized quinolines from aminohydrazones

Rhodium-catalyzed, chemo- and regioselective synthesis of functionalized quinolines using 2-aminohydrazones and terminal alkynes has been described.

ROTf-induced annulation of heteroatom reagents and unsaturated substrates leading to cyclic compounds

The development of metal-free organic reactions is one of the hotspots in the synthesis of cyclic compounds. ROTf (alkyl trifluoromethanesulfonates), due to their good electrophilicity, are powerful alkylating reagents at heteroatoms such as nitrogen, oxygen, sulfur and phosphorus to induce an electrophilic centre for carbon-carbon or carbon-heteroatom bond formation. Inspired by this chemistry, a variety of research concentrating on heterocycles synthesis has been carried out. In this review, we mainly summarize the ROTf-induced annulation of heteroatom reagents such as nitriles, carbodiimides, azobenzenes, isothiocyanates, aldehydes, isocyanates and phosphaalkene with themselves or alkynes to afford cyclic compounds.

Base-catalyzed thio-lactamization of 2-(1-arylvinyl)anilines with CS2 for the synthesis of quinoline-2-thiones

Quinoline-2-thiones were prepared through a base-catalyzed thio-lactamization of 2-(1-arylvinyl)anilines with CS₂.

Base-controlled chemoselectivity reaction of vinylanilines with isothiocyanates for synthesis of quinolino-2-thione and 2-aminoquinoline derivatives

Quinolino-2-thione and 2-aminoquinoline derivatives were obtained by a base-controlled chemo-selective reaction of vinylanilines with alkyl/aryl isothiocyanates.

Multicomponent spiropolymerization of diisocyanides, alkynes and carbon dioxide for constructing 1,6-dioxospiro[4,4]nonane-3,8-diene as structural units under one-pot catalyst-free conditions

A novel multicomponent spiropolymerization was developed by using diisocyanide, alkyne and CO₂, and 1,6-dioxospiro[4,4]nonane-3,8-diene was instantly formed.

Preparation of 3-Iodoquinolines from N-Tosyl-2-propynylamines with Diaryliodonium Triflate and N-Iodosuccinimide

4-Aryl and 4-alkyl substituted 3-iodoquinolines could be smoothly obtained in one pot by treating N-tosyl-2-propynylamines with diaryliodonium triflate in the presence of K₃PO₄ and a catalytic amount of CuCl at room temperature, followed by treatment with N-iodosuccinimide and BF₃·OEt₂ at 0 °C, and then NaOH in methanol solution. The product, 3-iodo-4-phenylquinoline was smoothly transformed into 4-phenylquinoline with zinc; 4-phenyl-3-toluenesulfenylquinoline with toluenethiol, K₂CO₃, and CuI; 4-phenyl-3-phenylethynylquinoline with the Sonogashira coupling reaction; 4-phenyl-3-styrylquinoline with the Heck coupling reaction; 3,4-diphenylquinoline with the Suzuki-Miyaura coupling reaction; 2-cyclohexyl-3-iodo-4-phenylquinoline with cyclohexanecarboxylic acid, Ag₂CO₃, and K₂S₂O₈; and 3-iodo-2-(2',5'-dioxan-1'-yl)-4-phenylquinoline with benzoyl peroxide in dioxane.

Synthesis of 4-substituted 3-(2-hydroxyphenyl)-quinolines through an unexpected iron(iii) chloride promoted reaction of cyclic imine dibenzo[b,f][1,4]oxazepines with alkynes

An unexpected FeCl₃-promoted reaction of cyclic imine dibenzo[b,f][1,4]oxazepines with alkynes was studied to produce novel 4-substituted 3-(2-hydroxyphenyl)-quinolines with 34–88% yield.

A copper-catalyzed arylation/nucleophilic addition/fragmentation/C–S bond formation cascade: synthesis of bis(arylthio)imines

A new arylation/nucleophilic addition/fragmentation/C–S bond formation cascade was developed using isothiocyanates and diaryliodonium salts.

Recent Advances in Metal-Free Quinoline Synthesis

The quinoline ring system is one of the most ubiquitous heterocycles in the fields of medicinal and industrial chemistry, forming the scaffold for compounds of great significance. These include anti-inflammatory and antitumor agents, the antimalarial drugs quinine and chloroquine, and organic light-emitting diodes. Quinolines were first synthesized in 1879, and since then a multitude of synthetic routes have been developed. Many of these methods, such as the Skraup, Doebner–Von Miller, and Friedlander quinoline syntheses, are well-known but suffer from inefficiency, harsh reaction conditions, and toxic reagents. This review focuses on recent transition metal-free processes toward these important heterocycles, including both novel routes and modifications to established methods. For example, variations on the Skraup method include microwave irradiation, ionic liquid media, and novel annulation partners, all of which have shown increased reaction efficiency and improved yield of the heteroring-unsubstituted quinoline products. Similarly, modifications to other synthetic routes have been implemented, with the quinoline products displaying a wide variety of substitution patterns.

Metal- and Protection-Free [4 + 2] Cycloadditions of Alkynes with Azadienes: Assembly of Functionalized Quinolines

A base promoted, protection-free, and regioselective synthesis of highly functionalized quinolines via [4 + 2] cycloaddition of azadienes (generated in situ from o-aminobenzyl alcohol) with internal alkynes has been discovered. The reaction tolerates a wide variety of functional groups which has been successfully extended with diynes, (2-aminopyridin-3-yl)methanol, and 1,4-bis(phenylethynyl)benzene to afford (Z)-phenyl-2-styrylquinolines, phenylnaphthyridine, and alkyne-substituted quinolines, respectively. The proposed mechanism and significant role of the solvent were well supported by isolating the azadiene intermediate and deuterium-labeling studies.