Photocatalytic Pyridine Synthesis with Enaminones and TMEDA under Metal-Free Conditions

Reported herein is a new photocatalytic annulation for the synthesis of 2,3,4,6-tetrasubstituted pyridines with enaminones and N,N,N',N'-tetramethyl ethylenediamine (TMEDA). The photocatalytic reactions take place without requiring a transition metal reagent and provide products with broad scope. The methyl in TMEDA acts as the carbon source in pyridine ring construction, and BrCF2CO2Et plays the role of the terminal oxidant for free radical quenching.

Green Synthesis of Aromatic Nitrogen-Containing Heterocycles by Catalytic and Non-Traditional Activation Methods

Recent advances in the environmentally benign synthesis of aromatic N-heterocycles are reviewed, focusing primarily on the application of catalytic methods and non-traditional activation. This account features two main parts: the preparation of single ring N-heterocycles, and their condensed analogs. Both groups include compounds with one, two and more N-atoms. Due to the large number of protocols, this account focuses on providing representative examples to feature the available methods.

Rongalite as C1 Synthon in the Synthesis of Divergent Pyridines and Quinolines

Rongalite has been used as a cheap and efficient carbon synthon for the synthesis of divergent N-heteroaromatics, including different pyridines and quinolines. The selective synthesis of different products can be achieved by employing enaminones or enaminones/anilines as reaction partners. In addition, compared with the reaction using conventional aldehyde synthons, rongalite displays an evident advantage in providing products with considerably higher product yields under milder conditions. The GC-MS analysis of the reaction process has been performed to probe the possible reaction mechanism.

Thiazole-5-carbaldehyde Synthesis by Cascade Annulation of Enaminones and KSCN with Dess-Martin Periodinane Reagent

The Dess-Martin periodinane (DMP) reagent-mediated reactions of tertiary enaminones with potassium thiocyanate for the synthesis of thiazole-5-carbaldehydes are developed. The product formation involves cascade hydroxyl thiocyanation of the C═C double bond, intramolecular hydroamination of the C≡N bond, and thiazole annulation by condensation on the ketone carbonyl site, representing novel reaction pathways in the reactions between enaminones and thiocyanate salt. DMP plays dual roles in mediating the free radical thiocyanation and inducing the unconventional selective thiazole-5-carbaldehyde formation by masking the in situ generated formyl group during the reaction process.

Lewis Acid-Relayed Singlet Oxygen Reaction with Enamines: Selective Dimerization of Enamines to Pyrrolin-4-ones

Singlet oxygen (¹O₂)-mediated oxidation represents an attractive strategy for incorporation of oxygen atoms from air under mild and environmentally benign conditions. However, the ¹O₂ reaction with enamine suffers from fragmentation, leading to very unsuccessful transformation. Here, Lewis acid is introduced to intercept [2 + 2] or "ene" reaction intermediates of the ¹O₂ reaction and enables oxidative dimerization of enamines to produce pyrrolin-4-ones in good to excellent yields. Mechanistic studies reveal the formation of the imino ketone intermediate from the interaction of ¹O₂ and enamine, which is able to interact with Lewis acid, relaying the ¹O₂ reaction in enamine chemistry. For the first time, selective cross-dimerization of two different enamines is achieved. Due to the advantages of mild conditions, high chemoselectivity, and up to 99% yield, a promising strategy has been developed for synthesizing aza-heterocycles under ambient conditions, which can be further applied for the synthesis of imidazolone, quinoxaline, and highly functionalized imine.

Synthesis of Diastereoenriched α-Aminomethyl Enaminones via a Brønsted Acid-Catalyzed Asymmetric aza-Baylis-Hillman Reaction of Chiral N-Phosphonyl Imines

An effective chiral GAP methodology for preparing α-aminomethyl enaminones through a (R)-CSA-catalyzed asymmetric aza-Baylis-Hillman reaction is reported. Excellent yields and high diastereoselectivity could be obtained under mild conditions and convenient GAP techniques. The confirmations of the absolute configuration of N-phosphonyl imine and chiral enaminone by X-ray diffraction provides an explicit explanation of the chirality mechanism for GAP chemistry.

Merging catalyst-free synthesis and iodine catalysis: one-pot synthesis of dihydrofuropyrimidines and spirodihydrofuropyrimidine pyrazolones

A new and efficient one-pot strategy combining catalyst-free synthesis and iodine catalysis has been developed for the synthesis of dihydrofuropyrimidines and spirodihydrofuropyrimidine pyrazolones. This approach affords products in moderate to high yields (up to 96%) with excellent diastereoselectivities (up to >25 : 1 dr). The reaction is simple to carry out and is metal-free.

A new and efficient one-pot strategy combining catalyst-free synthesis and iodine catalysis has been developed for the synthesis of dihydrofuropyrimidines and spirodihydrofuropyrimidine pyrazolones.

Controllable synthesis of 3-iodo-2H-quinolizin-2-ones and 1,3-diiodo-2H-quinolizin-2-onesviaelectrophilic cyclization of azacyclic ynones

An efficient approach was developed to divergently synthesize 3-ipdo-2H-quinolizin-2-ones and 1,3-diiodo-2H-quinolizin-2-ones from azacyclic ynones with high regioselectivity under metal-free, room temperature conditions in air.

PIDA-mediated intramolecular oxidative C–N bond formation for the direct synthesis of quinoxalines from enaminones

A intramolecular oxidative C(sp²)–N bond formation mediated by hypervalent iodine(iii) to obtain quinoxalines from readily available N-(2-acetaminophenyl)enaminones was developed. A tandem process involving PIDA-mediated intramolecular condensation cyclization and a subsequent elimination was postulated, which was highly efficient and metal-free under mild conditions. Moreover, flexible structural modifications of quinoxalines bearing carbonyl groups are of interest for further transformations as building blocks in organic synthesis.

An expedient hypervalent iodine(iii)-mediated approach to obtain substituted quinoxalines from readily available enaminones has been developed under mild conditions.

I2-Catalyzed Oxidative Cross-Coupling Reaction of Methyl Ketones and 2-(2-Aminophenyl) Benzimidazole: Facile Access to Benzimidazo[1,2-c]quinazoline

A general and efficient iodine-catalyzed metal-free oxidative cross-coupling reaction of methyl ketones with 2-(1H-benzo[d]imidazol-2-yl)aniline has been established. This is a new synthetic strategy for the synthesis of benzimidazo[1,2-c]quinazoline derivatives involving C(sp³)-H oxidation, condensation, and cyclization processes.

FeCl3-Catalyzed synthesis of pyrrolo[1,2-a]quinoxaline derivatives from 1-(2-aminophenyl)pyrroles through annulation and cleavage of cyclic ethers

A straightforward Fe-catalyzed method for the synthesis of pyrrolo[1,2-a]quinoxalines from 1-(2-aminophenyl)pyrroles and cyclic ethers, which includes functionalization of C(sp³)-H bonds and the construction of C-C and C-N bonds, has been developed. The features of this reaction are Fe catalysis, low-cost and readily accessible starting materials. Moreover, this procedure exhibits good functional group tolerance and a series of pyrrolo[1,2-a]quinoxaline derivatives are obtained in moderate to good yields.

One-pot asymmetric synthesis of a spiro[dihydrofurocoumarin/pyrazolone] scaffold by a Michael addition/I2-mediated cyclization sequence

An asymmetric formal one-pot reaction of 4-hydroxycoumarins with unsaturated pyrazolones has been developed by merging a chiral bifunctional organocatalyst with molecular iodine, which furnished a series of optically active spiro[dihydrofurocoumarin/pyrazolone] heterocycles with spiro quaternary stereogenic centers in moderate to excellent yields (up to 99%) with excellent diastereoselectivities (up to >99 : 1 dr) and good to excellent enantioselectivities (up to 99% ee). The application in the gram-scale synthesis of chiral spiro[dihydrofurocoumarin/pyrazolone] compounds was also successfully realized.

tert-Butyl nitrite (TBN) as the N atom source for the synthesis of substituted cinnolines with 2-vinylanilines and a relevant mechanism was studied

A green method to synthesize cinnolines by 6π electrocyclic reaction with alkenyl amines and TBN has been developed.

Regioselective three-component synthesis of 2,3-disubstituted quinolines via the enaminone modified Povarov reaction

The enaminone modified three-component Povarov reactions enable the regioselective synthesis of 2,3-disubstituted quinolines.

Solvent-free iodine-promoted synthesis of 3,2′-pyrrolinyl spirooxindoles from alkylidene oxindoles and enamino esters under ball-milling conditions

A solvent-free mechanochemical reaction of alkylidene oxindoles with enamino esters via an iodine-promoted tandem Michael/cyclization sequence has been developed to provide a variety of spirocyclic oxindoles in moderate to good yields with excellent diastereoselectivities.

Base-catalyzed cascade synthesis of 2,3-dihydrofuro[2,3-b]pyridines and 2,3-dihydro-1H-pyrrolo[2,3-b]pyridines from N-propargylic β-enaminones

A novel protocol to prepare dihydrofuropyridine and dihydropyrrolopyridine derivatives from KOH-catalyzed reactions between N-propargylic β-enaminones and arylaldehydes or N-sulfonyl imines has been developed.

Metal-Free Synthesis of Fully Substituted Pyridines via Ring Construction Based on the Domino Reactions of Enaminones and Aldehydes

An unprecedented domino reaction involving primary enaminones/enaminoesters and aldehydes has been developed for the synthesis of fully substituted pyridines. The construction of the products has been accomplished via the cascade generation of two C-C and one C-N bond by simply using TfOH as a promoter.

Metal-free synthesis of 3-methylthiofurans from homopropargylic alcohols and DMSO via a tandem sulfenylation/cyclization reaction in a one-pot manner

A tandem sulfenylation/cyclization reaction for the synthesis of 3-methylthiofurans from homopropargylic alcohols and DMSO in a one-pot manner has been developed.

Metal free synthesis of 2,4-diarylquinoline derivatives with enamides and imines

A simple and efficient way to construct quinoline derivatives using enamides and imines in metal free condition.