Gold-Oxazoline Complex-Catalyzed Cross-Dehydrogenative Coupling of Glycine Derivatives and Alkenes

Oxidative Tandem Cyclization of Glycine Esters with Propargyl Alcohols

A facile and efficient aerobic oxidative (4 + 2)-cyclization/aromatization/lactonization tandem reaction of N-aryl glycine esters with propargyl alcohols to access quinoline-fused lactones is reported. The reaction can be extended to homopropargylic alcohols too. The transformation is straightforward to perform under mild conditions and scalable, and both reaction components are readily available.

Photocatalytic Azetidine Synthesis by Aerobic Dehydrogenative [2 + 2] Cycloadditions of Amines with Alkenes

Photocatalytic dehydrogenative [2 + 2] cycloadditions between amines and alkenes were developed that allow for the stereoselective and high-yielding synthesis of functionalized azetidines. The oxidative formal Aza Paternò-Büchi reactions are induced by photoredox-catalyzed aerobic oxidation of dihydroquinoxalinones 1 as the amines, and in the presence of structurally diverse alkenes 3 intermolecular [2 + 2] cyclization to dihydro-1H-azeto[1,2-a]quinoxalin-3(4H)-ones 4 occurs. The utility of the method is illustrated by the selective conversion of amino acid derived dihydroquinoxalinones 1, including oxidation-prone lysine and tryptophan derivatives.

The Povarov Reaction: A Versatile Method to Synthesize Tetrahydroquinolines, Quinolines and Julolidines

The multicomponent Povarov reaction represents a powerful approach for the construction of substances containing N-heterocyclic frameworks. By using the Povarov reaction, in addition to accessing tetrahydroquinolines, quinolines and julolidines in a single step, it is possible to form the following new bonds: two Csp 3–Csp 3 and one Csp 3–Nsp 3, two Csp 2–Csp 2 and one Csp 2–Nsp 2, and four Csp 3–Csp 3 and two Csp 3–Nsp 1, respectively. This short review discusses the main features of the Povarov reaction, including its mechanism, the reaction scope by employing different catalysts and substrates, as well as stereoselective versions.

1 Introduction

2 Mechanism of the Povarov Reaction

3 Tetrahydroquinolines

4 Quinolines

5 Julolidines

6 Concluding Remarks

Employing Arylacetylene as a Diene Precursor and Dienophile: Synthesis of Quinoline via the Povarov Reaction

A novel I₂-mediated Povarov reaction of arylacetylenes and anilines for the synthesis of 2,4-substituted quinolines has been developed, in which arylacetylene first acts as both a diene precursor and dienophile. This work further develops the Povarov reaction to expand the types of diene precursors. Preliminary mechanistic studies indicate that the I₂/DMSO system realized the oxidative carbonylation of C(sp)-H of arylacetylene and then undergoes a [4 + 2] cycloaddition reaction.

I2-Catalyzed oxidative dehydrogenative tandem cyclization of 2-methylquinolines, arylamines and 1,4-dioxane

A novel I2-catalyst oxidative dehydrogenative tandem cyclization of 2-methylquinolines, arylamines and 1,4-dioxane for the construction of 2-([2,2′-biquinolin]-3-yloxy)ethan-1-ol derivatives has been developed under metal-free conditions.

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.

Visible-Light-Mediated Aerobic Tandem Dehydrogenative Povarov/Aromatization Reaction: Synthesis of Isocryptolepines

A metal‐free, photoinduced aerobic tandem amine dehydrogenation/Povarov cyclization/aromatization reaction between N‐aryl glycine esters and indoles leads to tetracyclic 11H‐indolo[3,2‐c]quinolines under mild conditions and with high yields. The reaction can be performed by using molecular iodine along with visible light, or by combining an organic photoredox catalyst with a halide anion. Mechanistic studies reveal that product formation occurs through a combination of radical‐mediated oxidation steps with an iminium ion or N‐haloiminium ion [4+2]‐cycloaddition, and the N‐heterocyclic products constitute new analogues of the antiplasmodial natural alkaloid isocryptolepine.

Visible-light-enabled aerobic oxidative Csp3-H functionalization of glycine derivatives using an organic photocatalyst: access to substituted quinoline-2-carboxylates

A practical visible-light-induced aerobic oxidative dehydrogenative coupling reaction of glycine derivatives with olefins has been developed to efficiently synthesize quinoline-2-carboxylates. This metal-free process proceeds smoothly under mild conditions and exhibits good tolerance of functional groups. Given the low cost of the catalyst and feedstock materials, the mild reaction conditions and the absence of hazardous byproducts, this protocol should find broad applications in the synthesis of quinoline-2-carboxylate derivatives.

One-electron oxidative dehydrogenative annulation and cyclization reactions

This review focuses on the recent advances in one-electron oxidation involved oxidative dehydrogenative annulations and cyclizations for the intermolecular and intramolecular construction of valuable ring structures.

Photoinduced iodine-mediated tandem dehydrogenative Povarov cyclisation/C–H oxygenation reactions

Visible light and iodine mediate dehydrogenative imine [4+2]-cycloaddition/C–H oxygenation reactions to furnish highly functionalised 3-quinolones under metal-free conditions.

Oxidative [4+2] Cycloaddition of α-(N-Arylamino) Carbonyls with Aryl Alkenes by Multiple C-H Functionalizations and [1,2]-Aryl Shifts

A new, general copper-catalyzed oxidative tandem [4+2] cycloaddition of α-(N-arylamino) carbonyl compounds with aryl alkenes to produce highly substituted quinolines has been developed, which allows the formation of three new C-C bonds through a sequence of multiple C-H functionalizations, annulation, and [1,2]-aryl shifts.

TBHP promoted demethylation of α-amino carbonyl compounds: a concise approach to substituted γ-lactams

A novel tert-butyl hydroperoxide (TBHP) promoted CH₂-extrusion reaction of α-amino carbonyl compounds has been developed, which is driven by a demethylenation process to give various ring contraction products γ-lactams under radical conditions.

Oxidative cross-dehydrogenative [2 + 3] annulation of α-amino ketones with α-keto esters: concise synthesis of clausenamide analogues

A one-pot oxidative cross-dehydrogenative [2 + 3] annulation of α-amino ketones with α-keto esters at room temperature is reported.

A Computational Mechanistic Investigation into Reduction of Gold(III) Complexes by Amino Acid Glycine: A New Variant for Amine Oxidation

Density functional theory (DFT) was utilized to explore the reduction of gold(III) complexes by the amino acid glycine (Gly). Interestingly, when the nitrogen atom of Gly coordinates to the gold(III) center, its C^α -hydrogen atom becomes so acidic that it can be easily deprotonated by a mild base like water. The deprotonation converts the amino acid into a potent reductant by which gold(III) is reduced to gold(I) with a moderate activation energy. To our knowledge, this is the first contribution suggesting that primary amines are oxidized to imines via direct α-carbon deprotonation. This finding may provide new insights into the mechanistic interpretation of amine oxidations catalyzed/mediated by a center with high cathodic reduction potential. This work also provides a rationalization behind why gold(III) complexes with amine-based polydentate ligands are reluctant to undergo a redox process. Gold(III) reduction occurs most efficiently if the C^α proton leaves in the plane of the C^α , N and Au atoms. Chelation prevents this alignment, resulting in the gold(III) complex being unreactive toward reduction. It has been experimentally found that gold(III) is capable of oxidizing Gly to glyoxylic acid (GA) as the initial product. The latter, in the presence of another gold(III) complex, has been reported to undergo oxidative decarboxylation to afford CO₂ and HCOOH. This process is found to be mediated by formation of a geminal diol intermediate produced by reaction of water with the aldehyde functional group of the coordinated GA.

Functionalization of N-arylglycine esters: electrocatalytic access to C-C bonds mediated by n-Bu4NI

An efficient electrocatalytic functionalization of N-arylglycine esters is reported. The protocol proceeds in an undivided cell under constant current conditions employing the simple, cheap and readily available n-Bu₄NI as the mediator. In addition, it is demonstrated that the mediated process is superior to the direct electrochemical functionalization.

Acid-promoted iron-catalysed dehydrogenative [4 + 2] cycloaddition for the synthesis of quinolines under air

An acid-promoted iron-catalysed dehydrogenative [4 + 2] cycloaddition reaction was developed for the synthesis of quinolines using air as a terminal oxidant. Various quinoline derivatives were obtained, and no other byproducts besides water.

Auto-oxidation promoted sp3 C–H arylation of glycine derivatives

An auto-oxidation promoted sp³ C–H arylation reaction between N-aryl glycine derivatives and electron-rich arenes, leading to the formation of N-aryl α-aryl α-amino acid derivatives, is described. This atom-economical and environmentally benign reaction proceeds smoothly under mild reaction conditions and requires only Brønsted acid and oxygen (balloon). A plausible radical involved mechanism is proposed.

Palladium-catalyzed synthesis of quinolines from allyl alcohols and anilines

A process for quinoline synthesis through palladium-catalyzed oxidative cyclization of aryl allyl alcohols and anilines is described.