Cu-Catalyzed Three-Component Cascade Annulation Reaction: An Entry to Functionalized Pyridines

Substituent-Oriented Synthesis of Substituted Pyridines/Pyrido[3,2-C]coumarins via Sequential Reactions of α-H and Me/Aryl Substituted Oxime Acetates and 3-Formylchromones

A facile one pot sequential and highly chemoselective synthesis of substituted pyridines/pyrido[3,2-c]coumarins has been developed from oxime acetates and 3-formylchromones in the presence of FeCl2. This reaction was oriented by different substituents on the α-position (H, methyl/aryl) of oxime acetates. Mechanistic investigations suggested that substituted pyridines were formed via intramolecular aza-Michael addition followed by ring opening, while pyrido[3,2-c]coumarins were formed via intermolecular Michael addition. Besides, anti-TB activity was screened for some of the synthesized pyridines. Among the tested compounds, 3ga, 3ha, 3ja, 3ma, 3ac, 4pa, 4pb and 4sb were found to have good activity with an MIC of 12.5 μg/mL.

Synthesis of nicotinimidamides via a tandem CuAAC/ring-cleavage /cyclization/oxidation four-component reaction and their cytotoxicity

Nicotinamide and its derivatives, recognized as crucial drug intermediates, have been a focal point of extensive chemical modifications and rigorous pharmacological studies. Herein, a series of novel nicotinamide derivatives, nicotinimidamides, were synthesized via a tandem CuAAC/ring-cleavage/cyclization/oxidation four-component reaction procedure from O-acetyl oximes, terminal ynones, sulfonyl azides, and NH4OAc. This strategy is significantly more efficient than previously reported, and the cytotoxicity of the nicotinimidamides is also tested. This project not only exhibits a sustainable and eco-friendly domino methodology for the creation of nicotinimidamides but also presents a promising candidate for liver cancer treatment.

Transition metal-free [3 + 3] annulation of cyclopropanols with β-enamine esters to assemble nicotinate derivatives

A metal-free and efficient approach for the synthesis of structurally important nicotinates through 4-HO-TEMPO-mediated [3 + 3] annulation of cyclopropanols with β-enamine esters is presented. This protocol features high atom efficiency, green waste, simple operation and broad substrate scope. Moreover, the experiments of gram-scale synthesis and recovery of oxidants make this strategy more sustainable and practical.

Copper-catalyzed three-component annulation toward pyrroles via the cleavage of two C-C bonds in 1,3-dicarbonyls

The first copper-catalyzed three-component annulation of α,β-unsaturated ketoximes, 1,3-dicarbonyls and paraformaldehyde has been documented. This novel strategy achieved the two C-C bond cleavage of 1,3-dicarbonyl compounds directly as a single-carbon synthon and provided a new and highly efficient method for the synthesis of 2,3-disubstituted pyrroles in moderate to good yields with broad functional group compatibility.

Base-Catalyzed One-Pot Synthesis of 2,3,6-Substituted Pyridines

A three-component reaction catalyzed by base was established, which mainly consisted of ynals, isocyanates, amines and alcohols. This strategy provides a wide range of substrates and represents a simple process for the preparation of different pyridine derivatives in good yields with high regioselectivities.

Oxime Esters: Flexible Building Blocks for Heterocycle Formation

Oxime esters as the applicable building blocks, internal oxidizing agents, and directing groups in the synthesis of -, S-, and O-containing heterocycle scaffolds have gained great attention in the last decade. This review provides an overview of recent advances in the cyclization of oxime esters with various functional group reagents under transition metal and transition metal-free catalyzed conditions. Moreover, the mechanistic aspects of these protocols are explained in detail.

Zn(II)-Catalyzed Multicomponent Sustainable Synthesis of Pyridines in Air

Herein, we report a Zn(II)-catalyzed solvent-free sustainable synthesis of tri- and tetra-substituted pyridines using alcohols as the primary feedstock and NH₄OAc as the nitrogen source. Using a well-defined air-stable Zn(II)-catalyst, 1a, featuring a redox-active tridentate azo-aromatic pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (L^a), a wide variety of unsymmetrical 2,4,6-substituted pyridines were prepared by three-component coupling of primary and secondary alcohols with NH₄OAc. Catalyst 1a is equally compatible with the four-component coupling. Unsymmetrical 2,4,6-substituted pyridines were also prepared via a four-component coupling of a primary alcohol with two different secondary alcohols and NH₄OAc. A series of tetra-substituted pyridines were prepared up to 67% yield by coupling primary and secondary alcohols with 1-phenylpropan-1-one or 1,2-diphenylethan-1-one and NH₄OAc. The 1a-catalyzed reactions also proceeded efficiently upon replacing the secondary alcohols with the corresponding ketones, producing the desired tri- and tetra-substituted pyridines in higher yields in a shorter reaction time. A few control experiments were performed to unveil the mechanistic aspects, which indicates that the active participation of the aryl-azo ligand during catalysis enables the Zn(II)-complex to act as an efficient catalyst for the present multicomponent reactions. Aerial oxygen acts as an oxidant during the Zn(II)-catalyzed dehydrogenation of alcohols, producing H₂O and H₂O₂ as byproducts.

Copper-Catalyzed Annulation of O-Acyl Oximes with Cyclic 1,3-Diones for the Synthesis of 7,8-Dihydroindolizin-5(6H)-ones and Cyclohexanone-Fused Furans

A copper-catalyzed annulation of O-acyl oximes with cyclic 1,3-diones has been developed for the concise synthesis of 7,8-dihydroindolizin-5(6H)-ones and cyclohexanone-fused furans through the substituent-controlled selective radical coupling process. 2-Alkyl cyclic 1,3-diones undergo C-C radical coupling, while 2-unsubstituted cyclic 1,3-diones undergo C-O radical coupling.

Iron-Catalysed [3+3] Annulation of Oxime Acetates and Enaminones towards the Synthesis of Multi-Substituted Pyridines

A direct access to unsymmetrical and symmetrical multi-substituted pyridines has been accomplished via iron-catalysed [3+3] annulation of oxime acetates with enaminones. This protocol is featured by easily available starting materials, no requirement of expensive additives and ligands, operational simplicity, and good tolerance with diverse functional groups.

Regioselective access to di- and trisubstituted pyridines via a metal-oxidant-solvent-free domino reaction involving 3-chloropropiophenones

A remarkable metal-oxidant-solvent- and base-free domino route for regioselective access to a wide range of 2,4-di- and 2,3,4/6-trisubstituted pyridines including carbo- and heterocyclic fused pyridines is reported. This [3C + 2C + 1N] cyclization reaction occurs between 3-chloropropiophenones (3C units), enolizable acyclic/cyclic ketones (2C sources) and NH₄OAc as a robust N source under neat conditions under an open atmosphere, producing new C=C and C=N-C bonds in highly chemo- and regioselective manners. Interestingly, this eco-friendly method has many positive features: excellent functional group tolerance, broad substrate scope, good to excellent regioselectivities, promising yields, no-unwanted products, neutral reaction conditions and appropriateness for large-scale synthesis. Mechanism studies reveal that the in situ generated β-amino ketone from 3-chloropropiophenone and an ammonium salt undergoes C=N bond formation with a ketone followed by an intramolecular cyclization process (C=C bond), which are the decisive steps for pyridine synthesis.

Synthesis of Substituted Pyridines via Formal (3+3) Cycloaddition of Enamines with Unsaturated Aldehydes and Ketones

An organocatalyzed, formal (3+3) cycloaddition reaction is described for the practical synthesis of substituted pyridines. Starting from readily available enamines and enal/ynal/enone substrates, the protocol affords tri- or tetrasubstituted pyridine scaffolds bearing various functional groups. This method was demonstrated on a 50 g scale, enabling the synthesis of 2-isopropyl-4-methylpyridin-3-amine, a raw material used for the manufacture of sotorasib. Mechanistic analysis using two-dimensional nuclear magnetic resonance (NMR) spectrometry revealed the transformation proceeds through the reversible formation of a stable reaction off-cycle species that precedes pyridine formation. In situ reaction progress kinetic analysis and control NMR studies were employed to better understand the role of FeCl₃ and pyrrolidine hydrochloride in promoting the reaction.

Domino synthesis of functionalized pyridine carboxylates under gallium catalysis: Unravelling the reaction pathway and the role of the nitrogen source counter anion

The catalytic potential of various metal Lewis acid catalysts have been assessed to derive a high-yielding, multi-component domino synthesis of functionalized pyridines from (E)-3-(dimethylamino)-1-aryl/heteroaryl-prop-2-en-1-ones, 1,3-dicarbonyl compounds, and an ammonium salt (as the nitrogen precursor). Amongst the various metal halides, tetrafluoroborates, perchlorates, and triflates used as the catalyst GaI3 proved to be the most effective. The mechanistic course of the most plausible pathway has been outlined as the intermediate formation of imine/enamine by the reaction of the 1,3-dicarbonyl compound with ammonia (liberated in situ from the ammonium salt used as the nitrogen source), which participates in the domino nucleophilic Michael reaction to the (2E)-3-(dimethylamino)-1-aryl/hetroarylprop-2-en-1-one by its active methylene carbon through its enamine form followed by intramolecular cyclization and aromatization. The effect of different ammonium salts as the nitrogen source has been investigated and NH4OAc was found to be best. The influence of the acetate counter anion of NH4OAc on the progress of the reaction was studied and its specific role in the cyclization and subsequent aromatization has been revealed. This work offers distinct advantages compared to the literature reported methodologies on the count of several green index parameters.

Facile synthesis of fully substituted 1H-imidazoles from oxime esters via dual photoredox/copper catalyzed multicomponent reactions

A novel and efficient photoredox/copper cocatalyzed domino cyclization of oxime esters, aldehydes, and amines has been achieved, affording a broad range of fully substituted 1H-imidazoles in good yields.

Divergent copper-catalyzed syntheses of 3-carboxylpyrroles and 3-cyanofurans from O-acetyl oximes and β-ketoesters/nitriles

The reaction between O-acetyl oximes and β-ketoesters/nitriles catalyzed by copper generated polysubstituted pyrroles and furans, respectively, under redox–neutral reaction conditions. Using this protocol, pyrroles or furans could be obtained simply by choosing an appropriate active methylene compound. Although both transformations occur essentially under the same reaction conditions, control experiments indicated that they follow different mechanistic pathways.

A copper-catalyzed heteroannulation of O-acetyl oximes and β-ketoesters/nitriles to afford polysubstituted pyrroles and furans was successfully achieved.

Chemoselective [3 + 2] annulation of oxime acetate with 2-aryl-3-ethoxycarbonyl-pyrroline-4,5-dione: an entry to pyrrolo[2,3-b]pyrrole derivatives

A novel chemoselective [3 + 2] annulation reaction of easily accessible ketoxime acetate with 2-aryl-3-ethoxycarbonyl pyrroline-4,5-dione has been developed for the synthesis of unknown pyrrolo[2,3-b]pyrrole frameworks. This method involves copper-mediated N-O bond cleavage followed by the formation of carbon-carbon and carbon-nitrogen bonds. This operationally simple protocol provides broader functional group compatibility and good yields.

Copper catalyzed five-component domino strategy for the synthesis of nicotinimidamides

A library of medicinally and synthetically important nicotinimidamides was synthesized by a copper-catalyzed multicomponent domino reaction of oxime esters, terminal ynones, sulfonyl azides, aryl aldehydes and acetic ammonium. Its synthetic pathway involves the formation of a highly reactive N-sulfonyl acetylketenimine, characterized by high selectivity, combinations of potential nucleophiles and electrophiles, mild reaction conditions and a wide substrate scope, and is a rare five-component example of a CuAAC/ring-opening reaction.

Mild and efficient copper-catalyzed oxidative cyclization of oximes with 2-aminobenzyl alcohols at room temperature: synthesis of polysubstituted quinolines

A simple and efficient ligand-free Cu-catalyzed protocol for the synthesis of polysubstituted quinolines via oxidative cyclization of oxime acetates with 2-aminobenzyl alcohols at room temperature has been developed. The presented approach provides a new synthetic pathway leading to polysubstituted quinolines with good functional group tolerance under mild conditions. Moreover, this transformation can be applied for the preparation of quinolines on a gram scale. Oxime acetates serve as the internal oxidants in the reactions, thus making this method very attractive.

Copper-catalyzed aerobic oxidative domino cyclization of methyl azaarenes with 6-amino-pyrimidine-2,4-diones and pyrazol-5-amines: access to dipyrimidine/dipyrazolo-fused pyridines

An aerobic copper-catalyzed oxidative domino cyclization of methyl azaarenes with 6-amino-pyrimidine-2,4-diones and pyrazol-5-amines for the synthesis of dipyrimidine/dipyrazolo-fused pyridines was reported.

Visible-Light Photoredox-Catalyzed Formal [5 + 1] Cycloaddition of N-Tosyl Vinylaziridines with Difluoroalkyl Halides

A visible-light photoredox-catalyzed formal [5 + 1] cycloaddition of N-tosyl vinylaziridines with difluoroalkyl halides as unique C1 synthons was developed. The procedure provides an efficient and practical method to synthesize diverse pyridines in moderate to good yields. The reaction underwent a radical-initiated kinetically controlled ring-opening of vinylaziridines and involved a key α,β-unsaturated imine intermediate, followed by an E2 elimination, a 6π electrocyclization, and defluorinated aromatization.

Cu(I)-Catalyzed Ligand-Free Tandem One-Pot or Sequential Annulation via Knoevenagel Intermediates: An Entry into Multifunctional Naphthalenes, Phenanthrenes, Quinolines, and Benzo[b]carbazoles

A simple but efficient one-pot or sequential copper-catalyzed protocol using 2-bromoaldehydes and active methylene group containing substrates that affords multifunctional naphthalenes, phenanthrenes, quinolines, and benzo[b]carbazoles via Knoevenagel condensation, C-arylation, and decarboxylation, followed by aromatization, is developed. The reaction utilizes the potential of Knoevenagel intermediates and does not require any ancillary ligand. The phenanthrene products thus obtained show moderate fluorescence activity. Structural elaboration of the products to obtain dihydrobenzoquinazolines is also highlighted.

NH4OAc-Promoted Domino Route to Hydroxyarylated Unsymmetrical Pyridines under Neat Conditions

A new metal-, oxidant-, and solvent-free ecofriendly domino method has been established for modular synthesis of a diverse range of medicinally promising hydroxyarylated unsymmetrical pyridines in good to high chemical yields with an excellent regioselectivity. This domino process involves a range of N-sulfonyl ketimines as C,N-binucleophiles, enolizable ketones, and aromatic/heteroaromatic aldehydes using ammonium acetate as an ideal promoter under neat conditions, which creates two new C-C bonds and one C-N bond. Notably, the neutral reaction conditions are mild enough to tolerate a range of functionalities and cover a variety of substrates, thus bestowing a powerful avenue to access tri- and tetrasubstituted pyridines including carbo- and heterocyclic fused ones. Interestingly, a practical, scalable, and high-yielding synthesis of pyridylphenol derivatives was successfully accomplished by our unique method.

An unorthodox metal-free synthesis of dihydro-6H-quinoline-5-ones in ethanol/water using a non-nucleophilic base and their cytotoxic studies on human cancer cell line

One-pot metal-free catalysed green methodology for dihydro-6H-quinoline-5-ones and their biological activities in HeLa cells.

Access to pyrrolo[2,1-a]isoindolediones from oxime acetates and ninhydrin via Cu(i)-mediated domino annulations

A copper-mediated domino condensation reaction of easily accessible oxime acetates with ninhydrin to give pyrrolo[2,1-a]isoindolediones is reported.

Direct access to bis-S-heterocyclesviacopper-catalyzed three component tandem cyclization using S8as a sulfur source

An elemental sulfur atom donor strategy for constructing a thiophene-fused thiazole bis-S-heterocyclic skeletonviaCu-catalyzed three-component tandem cyclization has been developed.

Three-component bis-heterocycliation for synthesis of 2-aminobenzo[4,5]thieno[3,2-d]thiazoles

A cooperative base system has been developed for the novel three-component synthesis of 2-aminobenzo[4,5]thieno[3,2-d]thiazoles via bis-heterocyclization of methylketoxime acetates.

DABCO- and DBU-promoted one-pot reaction of N-sulfonyl ketimines with Morita-Baylis-Hillman carbonates: a sequential approach to (2-hydroxyaryl)nicotinate derivatives

An intriguing DABCO-catalyzed and DBU-promoted one-pot synthesis of an important class of (2-hydroxyaryl)pyridine derivatives bearing a carboxylate or a nitrile group suitably placed at C3 position of the aza-ring has been achieved in acceptable chemical yields with a broad functional group tolerance. This sequential C-C/C-N bond making process proceeds through a regioselective allylic alkylation/aza-Michael reaction between MBH carbonates derived from an acrylate/acrylonitrile and N-sulfonyl ketimines as C,N-binucleophiles catalyzed by DABCO, followed by elimination of SO2 under the influence of base and subsequent aromatization in an open atmosphere.