Cu(TFA)2-Catalyzed Oxidative Tandem Cyclization/1,2-Alkyl Migration of Enamino Amides for Synthesis of Pyrrolin-4-ones

Rhodium(III)-Catalyzed Coupling of Quinolin-8-carboxaldehydes with CF3-Imidoyl Sulfoxonium Ylides by Chelation-Assisted C(sp2)-H Bond Activation for the Synthesis of Trifluoromethyl-Substituted Enaminones

A rhodium(III)-catalyzed aldehydic C(sp2)-H imidoylmethylation of quinolin-8-carboxaldehydes with CF3-imidoyl sulfoxonium ylides (TFISYs) has been developed for the generation of α-imino ketones, which could be readily tautomerized to enaminones in moderate to excellent yields. In the transformation, TFISYs act as a kind of masked alkenylating reagents for the aldehyde moiety, and the obtained CF3-enaminone products have been successfully converted into other useful trifluoromethyl-substituted heterocycles.

Photooxidative Reaction of β-Oxoamides with Amines for the Synthesis of Pyrrolin-4-ones under External Photocatalyst-Free Conditions

The incorporation of oxygen atoms from air under aerobic conditions plays an important role in organic synthesis. Herein, Brønsted acids are found to be a two-in-one strategic catalyst to transform enamines from β-oxoamides and amines to pyrrolin-4-ones without an external photocatalyst under visible-light conditions. The Brønsted acid can inhibit the C-C bond fragmentation of the [2 + 2] adduct from enamine and 1O2, but most importantly, it can form photosensitizers with enamine and pyrrolin-4-one product by acidochromism to promote the 1O2 generation.

Multicomponent Cyclizative 1,2-Rearrangement Enabled Enantioselective Construction of 2,2-Disubstituted Pyrrolinones

The 1,2-rearrangement reaction is one of the most important approaches to construct carbon-carbon bonds in organic synthesis. However, the development of catalytic asymmetric 1,2-rearrangements is still far from mature and often suffers from problems such as complex substrates, single product structure, and lack of synthetic application. Multicomponent reaction has been recognized as a robust tool for the synthesis of diverse and tunable products from readily available starting material. Conceptionally and practically, the development of multicomponent asymmetric 1,2-rearrangements is highly desirable. In this regard, we report herein a three-component benzilic acid-type rearrangement of 2,3-diketoesters, aromatic amines and aldehydes for the asymmetric construction of synthetically challenging pyrrolinones bearing aza-quaternary stereocenters. To the best of our knowledge, this reaction represents the first example of organocatalyzed multicomponent asymmetric 1,2-rearrangements.

Theoretical Insight into the Mechanism of Cu(I)-Catalyzed [2 + 2 + 1] Cycloaddition to β-Pyrrolinones: Azaheterocycle Formation and Assisted Dehydrogenation with Solvent MeNO2

The construction of multisubstituted β-pyrrolinones from simple starting materials remains a great challenge. Recently, a novel Cu(I)-catalyzed [2 + 2 + 1] cycloaddition reaction was developed for rapid access to fully substituted β-pyrrolinones, which are difficult to synthesize through traditional methods as this approach may involve unusual C-nucleophilic addition of enamines and umpolung of imines. Elucidating the reaction mechanism may inspire the development of new methodologies via the unusual C-nucleophilic addition of enamines and imines. However, the reaction mechanism is still unclear because none of the intermediates was observed during the reaction process. In this work, we employed theoretical and computational chemistry to investigate the possible pathway. Finally, the calculated results indicate that ketene formed by the Wolff rearrangement of α-diazo-β-ketoester reacts with enamine formed by the addition of alkynes and amine, affording the five-membered azaheterocycle, and this process involves the formation of a six-membered ring intermediate and sequential isomerization, and the further dehydrogenation needs to be assisted with solvent MeNO₂.

Substrate-Dependent Denitrogenative Rearrangements of Rhodium Azavinyl Carbenes Involving 1,2-Aryl Migration

Herein, we disclose substrate-dependent rearrangements of 4-substituted N-sulfonyl-1,2,3-triazoles under Rh(II)-catalysis via denitrogenation. The reaction pathways included key 1,2-aryl migration via the formation of intermediatory phenonium ion, which is elusive so far with Rh-azavinyl carbenes. Intriguingly, the transformations were completely dependent on the substituent present leading to different scaffolds like enaminones, pyrrol-3-ones, and azadienes. Hammett studies provided essential insights into the carbocationic intermediate formation. The developed methodology featured simple reaction conditions, excellent functional group compatibility, and broad substrate scope.

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.

Recent developments in the synthesis of nitrogen-containing heterocycles from β-aminovinyl esters/ketones as CC-N donors

Nitrogen-containing heterocycles are ubiquitous fragments of numerous natural products, pharmaceuticals, designed bioactive drug candidates and agrochemicals. During the past few decades, these compounds have received considerable attention from the synthetic chemistry community, and great efforts have been focused on the development of concise and efficient methods for the synthesis of these heterocyclic skeletons. In this review, we summarize a diverse range of synthetic methods employing β-aminovinyl esters(ketones) as key CC-N-synthons to furnish useful bioactive heterocyclic frameworks, such as quinolines, pyridines, pyrazines, pyrroles, indoles, oxazoles, imidazoles, thiazoles, isothiazoles, pyrazoles, triazoles, and azepines, thus offering new opportunities and expanding the toolbox of synthetic chemistry reactions.

Iodine-Promoted Formal [3+2] Cycloaddition of Enaminone: Access to 2-Hydroxy-1,2-dihydro-pyrrol-3-ones with Quaternary Carbon Center

A novel iodine promoted cyclization of enaminone with aryl methyl ketones has been developed as a straightforward method for constructing 2-hydroxy-pyrrol-3(2H)-ones. This strategy affords structurally diverse 2-hydroxy-pyrrol-3(2H)-ones rings in high yields. Moreover, a quarternary alcohol has been constructed efficiently in the reaction. Product purification required only washing with CH₂Cl₂ solvent, thereby avoiding traditional chromatography and recrystallization, making this an example of group-assisted purification chemistry.

Cu(I)-Catalyzed [2 + 2 + 1] Cycloaddition of Amines, Alkynes, and Ketenes: An Umpolung and Regioselective Approach to Full-Substituted β-Pyrrolinones

Described here is a Cu-catalyzed [2 + 2 + 1] modular synthesis of full-substituted β-pyrrolinones from simple amines, alkynes, and α-diazo-β-ketoesters. This approach involving the regioselective C-nucleophilic attack of enamines, uncommon C-nucleophilic addition to ketenes, and umpolung of imines enables the direct synthesis of full-substituted β-pyrrolinones, which were hardly constructed by traditional synthetic strategies.

Selectfluor-triggered fluorination/cyclization of o-hydroxyarylenaminones: A facile access to 3-fluoro-chromones

A fast and efficient Selectfluor-triggered fluorination/cyclization reaction of o-hydroxyarylenaminones has been successfully developed. The reaction successfully provides an expedient method for the synthesis of 3-fluoro-chromones promoted by potassium carbonate, which shows readily available starting materials and is easy to operate. In addition, a plausible mechanism of this tandem cyclization reaction was proposed where 4 H-chromen-4-one, 2-(dimethylamino)-3,3-difluorochroman-4-one, and 3,3-difluoro-2-hydroxychroman-4-one were not found to be the reactive intermediates. Moreover, these novel compounds have been obtained in moderate to good yields, and their structures have been confirmed by 1H NMR, 13C NMR, and high-resolution mass spectrometry.

Synthesis of 1 H-Pyrrol-3(2 H)-ones via Three-Component Reactions of 2,3-Diketo Esters, Amines, and Ketones

An efficient one-pot, three-component reaction of 2,3-diketo esters with amines and ketones has been developed for the synthesis of 1 H-pyrrol-3(2 H)-ones. By using trifluoroacetic acid (TFA) as the additive and acetonitrile (MeCN) as the solvent, this convenient method provides a library of 1 H-pyrrol-3(2 H)-ones in moderate to good yields. The simple protocol features readily available starting materials, a straightforward process, good functional group tolerance, and broad substrate scope.

Copper-Promoted Regioselective Synthesis of Polysubstituted Pyrroles from Aldehydes, Amines, and Nitroalkenes via 1,2-Phenyl/Alkyl Migration

The facile copper-catalyzed synthesis of polysubstituted pyrroles from aldehydes, amines, and β-nitroalkenes is reported. Remarkably, the use of α-methyl-substituted aldehydes provides efficient access to a series of tetra- and pentasubstituted pyrroles via an overwhelming 1,2-phenyl/alkyl migration. The present methodology is also accessible to non α-substituted aldehydes, yielding the corresponding trisubstituted pyrroles. On the contrary, the use of ketones, in place of aldehydes, does not promote the organic transformation, signifying the necessity of α-substituted aldehydes. The reaction proceeds under mild catalytic conditions with low catalyst loading (0.3-1 mol %), a broad scope, very good functional-group tolerance, and high yields and can be easily scaled up to more than 3 mmol of product, thus highlighting a useful synthetic application of the present catalytic protocol. Based on formal kinetic studies, a possible radical pathway is proposed that involves the formation of an allylic nitrogen radical intermediate, which in turn reacts with the nitroalkene to yield the desired pyrrole framework via a radical 1,2-phenyl or alkyl migration.

Asymmetric synthesis of 1H-pyrrol-3(2H)-ones from 2,3-diketoesters by combination of aldol condensation with benzilic acid rearrangement

An efficient two-step protocol for the asymmetric synthesis of 1H-pyrrol-3(2H)-one derivatives in 99% ee from conveniently accessed 2,3-diketoesters has been developed.

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.

Synthesis of furans and pyrroles via migratory and double migratory cycloisomerization reactions of homopropargylic aldehydes and imines

A novel gold-catalyzed divergent sysnthesis of furans and pyrroles employing readily available homopropargylic aldehydes and imines have been developed. The regiochemical outcome of this reaction is dependent on the substituent on the terminal alkyne of substrate. Thus, substrates possessing alkyl and aryl substituent at the alkyne moiety produce 2,3,5-substituted furans and pyrroles via a migratory cycloisomerizaton reaction. Whereas, their silicon analogues are capable to undergo a double migratory process leading to 2,3,4-substituted heterocycles.

Copper-mediated intramolecular oxidative C-H/N-H cross-coupling of α-alkenoyl ketene N,S-acetals to synthesize pyrrolone derivatives

CuCl2 and CuBr2-mediated intramolecular oxidative C-H/N-H cross-coupling/halogenation of β-thioalkyl-substituted α-alkenoyl ketene N,S-acetals occurred efficiently, affording 4-halo-5-thioalkyl-3-pyrrolones. Tunable C-S and C-halo bond transformations of the resultant pyrrolone derivatives led to highly functionalized N-heterocyclic compounds.

A novel one-pot method for the synthesis of substituted furopyridines: iodine-mediated oxidation of enaminones by tandem metal-free cyclization

A novel iodine-mediated oxidative tandem cyclization reaction of simple enaminones has been developed for the synthesis of substituted furopyridines through C–C/C–N/C–O bond formation in a one-pot procedure.

Catalytic asymmetric α-Iminol rearrangement: new chiral platforms

A series of 19 different asymmetric catalysts were screened in an effort to identify the first chiral catalyst for the rearrangement of α-hydroxy imines to α-amino ketones involving a 1,2-carbon shift. Although aluminate complexes of VAPOL, VANOL, and 7,7'-(t)Bu2VANOL were quite effective catalysts giving up to 88% ee, the ne plus ultra catalyst for this reaction was found to be a zirconium complex of VANOL which gives 97 to >99% ee for the majority of the substrates examined. An X-ray diffraction study of the catalyst reveals that the zirconium exists as a homoleptic complex with three VANOL ligands and two protonated N-methyl imidazoles.