Efficient Cu(OTf) 2 -catalyzed and microwave-assisted rapid synthesis of 3,4-fused chromenopyridinones under neat conditions

Synthesis of 2H-Chromeno[2,3-b]tetrahydropyridine Frameworks through Cs2CO3-Promoted Annulation of 2-Amino-4H-chromen-4-ones and 4-Benzylideneoxazol-5(4H)-ones

This innovative reaction involved the [3+3] annulation of 2-amino-4H-chromen-4-ones and 4-benzylideneoxazol-5(4H)-ones. The process provided quick and easy access to a broad range of structurally diverse and highly functionalized 1,3,4,5-tetrahydro-2H-chromeno[2,3-b]pyridines in moderate to good yields, which possess trans-form C3 and C4 substituents.

ZnCl2-Promoted Three-Component Reaction of 2-Aminochromenones, Aromatic Aldehydes, and Quinone Monoketals: Access to 5,6-Dihydro-12H-chromeno[2,3-c]isoquinolin-12-one Derivatives

A three-component reaction of 2-amino-4H-chromen-4-ones, aromatic aldehydes, and 4,4-dialkoxycyclohexa-2,5-dien-1-ones for the concise synthesis of chromeno[2,3-c]dihydroisoquinoline derivatives has been investigated. This reaction involved consecutive ZnCl2-promoted Micheal addition and intramolecular Friedel-Crafts alkylation. This synthetic protocol offered several advantages, including the readily accessible starting materials, good functional group tolerance, and simplicity of operation. Additionally, the structures of products obtained were determined based on X-ray diffraction studies.

One-pot domino synthesis of five- and six-membered fused dihydropyridines promoted by PPh3-NBS in aqueous medium

A facile one-pot synthesis of five- and six-membered fused dihydropyridines such as chromenodihydropyridines, pyrazolodihydropyridines and isoxazolopyridines was accomplished for the first time by employing PPh₃-NBS via a formal [3 + 2 + 1] cycloaddition of 1,3-bisnucleophiles (i.e., 2-aminochromone, 4-aminochromone, 5-aminopyrazole and 5-aminoisoxazole), β-enaminones and aldehydes in aqueous medium. The present approach involves a Michael type addition followed by intramolecular cyclization leading to the formation of two new C-C bonds and one C-N bond. High compatibility and excellent yields are the advantages of this protocol.

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.

DBU-Mediated Domino Annulation Reaction of 2-Amino-4H-chromen-4-ones and Aromatic Aldehydes for Synthesis of Polysubstituted 3-Hydroxy-5H-chromeno[2,3-b]pyridinones

A pseudo-three-component annulation reaction of substituted 2-amino-4H-chromen-4-ones with aromatic aldehydes promoted by DBU was investigated to access polysubstituted 5H-chromeno[2,3-b]pyridines. This reaction included a sequential intermolecular nucleophilic addition/Michael cyclization/intramolecular epoxidation/ring opening/aromatization sequence, which possessed excellent step and atom economy in a single operation for generating 3-hydroxy-5H-chromeno[2,3-b]pyridines from readily available substrates.

DABCO-Promoted Cyclization of 2-Amino-4H-chromen-4-ones with 2,6-Dibenzylidenecyclohexan-1-ones for the Synthesis of Chromeno[2,3-b]tetrahydroquinoline

DABCO-promoted cyclization reaction of substituted 2-amino-4H-chromen-4-ones with substituted 2,6-dibenzylidenecyclohexan-1-ones was investigated under mild conditions. This reaction provided a novel and efficient access to the 7,8,9,10-tetrahydro-12H-chromeno[2,3-b]quinolin-12-ones in good yields, the exocyclic double bond of which is predominantly E-selective.

Recent Advances in Organic Synthesis of 3-Amino- or 4-Aminocoumarins

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Coumarin is a privileged scaffold that contains the unique 2H-chromen-2-one motif, and its derivatives are widely distributed in nature, especially in plants. In recent years, due to their diverse pharmacological activities and remarkable photochemical properties, they have attracted significant attention from scientists, which has also prompted the research on the synthesis approaches and the availability of substrates for these compounds. This article is a brief description of the methods for the synthesis of various coumarin derivatives via two- or multi-component reactions involving 3-amino or 4-aminocoumarin reported during 2015-2021. This review may help expand the development of various analogues with coumarin as the basic unit.

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.

Synthesis of 5H-chromeno[3,4-b]pyridines via DABCO-catalyzed [3 + 3] annulation of 3-nitro-2H-chromenes and allenoates

The DABCO-catalyzed [3 + 3] annulation between 3-nitro-2H-chromenes and benzyl 2,3-butadienoate has been developed as a route to 5H-chromeno[3,4-b]pyridine derivatives. Under optimal reaction conditions, 5H-chromeno[3,4-b]pyridines incorporating two allenoate units were obtained in moderate to good yields (30-76%). The same type of transformation could be carried out using butynoates as allene surrogates. Mechanistic studies by mass spectrometry allowed the identification of the key intermediates involved in the reaction mechanism. The reported synthetic methodology represents an entirely new approach for the synthesis of the 5H-chromeno[3,4-b]pyridine core structure based on allene chemistry.

The synthesis, biological evaluation and fluorescence study of chromeno[4,3-b]pyridin/quinolin-one derivatives, the backbone of natural product polyneomarline C scaffolds: a brief review

This review presents the synthesis, biological and fluorescence study of chromeno[4,3-b]pyridin/quinolin-ones via classical reactions including metal-catalyzed and green reaction protocols.

Recent Advances in Microwave-Assisted Copper-Catalyzed Cross-Coupling Reactions

Cross-coupling reactions furnishing carbon–carbon (C–C) and carbon–heteroatom (C–X) bond is one of the most challenging tasks in organic syntheses. The early developed reaction protocols by Ullmann, Ullman–Goldberg, Cadiot–Chodkiewicz, Castro–Stephens, and Corey–House, utilizing elemental copper or its salts as catalyst have, for decades, attracted and inspired scientists. However, these reactions were suffering from the range of functional groups tolerated as well as severely restricted by the harsh reaction conditions often required high temperatures (150–200 °C) for extended reaction time. Enormous efforts have been paid to develop and achieve more sustainable reaction conditions by applying the microwave irradiation. The use of controlled microwave heating dramatically reduces the time required and therefore resulting in increase in the yield as well as the efficiency of the reaction. This review is mainly focuses on the recent advances and applications of copper catalyzed cross-coupling generation of carbon–carbon and carbon–heteroatom bond under microwave technology.

A derivatization-directed three-component synthesis of fluorescent spiro [dihydropyridine-4,3ʹ-indoline]s

The synthesis of spiro[chromeno[4,3- b]pyrazolo[4,3- e]pyridine-7,3ʹ-indoline]s is achieved via three-component reactions of 5-amino-3-methylpyrazole, 4-aminocoumarin, and isatin derivatives. This protocol provides expedient synthesis of 10-unsubstituted derivatives of the parent heterocyclic spiro framework and does not lead to coumarin ring opening. The synthesis is highly convergent as no by-products are present in the reaction mixtures. The spiro products show violet fluorescence emissions depending on the nature of their substituents.

Selective construction of functionalized chromeno[3,4-b]pyrroles and benzo[c]chromenesviaa K3PO4promoted three-component reaction

A K₃PO₄promoted three-component reaction of pivaloylacetonitrile (benzoylacetonitrile), dialkyl but-2-ynedioates and 2-aryl-3-nitrochromenes afforded functionalized chromeno[3,4-b]pyrroles.

Organic Synthesis Using Environmentally Benign Acid Catalysis

Recent advances in the application of environmentally benign acid catalysts in organic synthesis are reviewed. The work includes three main parts; (i) description of environmentally benign acid catalysts, (ii) synthesis with heterogeneous and (iii) homogeneous catalysts. The first part provides a brief overview of acid catalysts, both solid acids (metal oxides, zeolites, clays, ion-exchange resins, metal-organic framework based catalysts) and those that are soluble in green solvents (water, alcohols) and at the same time could be regenerated after reactions (metal triflates, heteropoly acids, acidic organocatalysts etc.). The synthesis sections review a broad array of the most common and practical reactions such as Friedel-Crafts and related reactions (acylation, alkylations, hydroxyalkylations, halogenations, nitrations etc.), multicomponent reactions, rearrangements and ring transformations (cyclizations, ring opening). Both the heterogeneous and homogeneous catalytic synthesis parts include an overview of asymmetric acid catalysis with chiral Lewis and Brønsted acids. Although a broad array of catalytic processes are discussed, emphasis is placed on applications with commercially available catalysts as well as those of sustainable nature; thus individual examples are critically reviewed regarding their contribution to sustainable synthesis.

Molecular diversity of the domino annulation reaction of 2-aryl-3-nitrochromenes with pivaloylacetonitriles

In the presence of triethylamine, the domino annulation reaction of two molecules of pivaloylacetonitrile with one molecule of 2-aryl-3-nitrochromene in tetrahydrofuran resulted in the unprecedented imino-substituted dihydrofuro[2,3-c]chromene derivatives in high yields.