The Domino Chromone Annulation and a Transient Halogenation-Mediated C–H Alkenylation toward 3-Vinyl Chromones

Eco-Friendly and Efficient Synthesis of 2-Hydroxy-3-Hydrazono-Chromones Through α,β-C(sp2)-H Bond Difunctionalization/Chromone Annulation Reaction of o-Hydroxyaryl Enaminones, Water, and Aryldiazonium Salts

A novel, eco-friendly, and efficient method for constructing 2,3-disubstituted chromone skeletons from readily available water, o-hydroxyaryl enaminones (o-HPEs), and aryldiazonium salts has been developed under mild reaction conditions. This α,β-C(sp2)-H bond difunctionalization/chromone annulation reaction strategy is achieved by building two C(sp3)-O bonds and a C(sp2)-N bond, which provides a practical pathway for the preparation of 2-hydroxy-3-hydrazono-chromones in moderate to excellent yields, enabling broad substrate scope and good functional group tolerance, as well as gram-scale synthesis. This protocol offers a valuable tool for synthesizing diverse functionalized chromones with potential applications in drug discovery and industrial synthesis.

Recent advances in site-selective transformations of β-enaminones via transition-metal-catalyzed C-H functionalization/annulation

β-Enaminone transformation strategies are widely employed in the synthesis of numerous biologically active drugs and natural products, highlighting their significance in medicinal chemistry. In recent years, various strategies have been developed for synthesizing several five- and six-membered heterocycles, as well as substituted polyaromatic scaffolds, which serve as crucial synthons in drug development, from β-enaminones. Among these approaches, site-selective transformations of β-enaminones via C-H activation and annulation have been particularly well explored. This review summarizes the most recent literature (over the past eight years) on β-enaminone transformations for developing bioactive scaffolds through site-selective C-H bond functionalization and annulation.

Visible-light-induced cascade chromone cyclization/chalcogenation to access 3-chalcogenyl-chromones using elemental sulfur/selenium

A mild and efficient visible-light-induced cascade undergoing a chromone cyclization/chalcogenation at room temperature has been developed. This three-component reaction employs user-friendly elemental S8 and Se as the chalcogenide source, providing an attractive route for the convenient synthesis of 3-chalcogenyl-chromones with a wide substrate scope and good functional group tolerance.

Electrocatalytic Cascade Selenylation/Cyclization/Deamination of 2-Hydroxyaryl Enaminones: Synthesis of 3-Selenylated Chromones under Mild Conditions

Herein, we disclosed a highly efficient pathway toward 3-selenylated chromone derivatives via electrocatalytic cascade selenylation/cyclization/deamination of 2-hydroxyaryl enaminones with diselenides. This method showed mild conditions, easy operation, wide substrate scope, and good functional group tolerance. Furthermore, this electrosynthesis strategy was amendable to scale-up the reaction. Additionally, the preliminary experiments revealed that this reaction probably proceeded via a cation pathway instead of a radical pathway.

Divergent Synthesis of 2-Chromonyl-3-hydrazono-chromones and 2-Alkoxy-3-hydrazono-chromones through Switchable Annulation Reactions of o-Hydroxyphenylenaminones with Aryldiazonium Salts

An unprecedented selective chromone annulation reaction controlled by solvent for the divergent synthesis of two types of 2,3-disubstituted chromone skeletons has been developed. A variety of 2-chromonyl-3-hydrazono-chromones and 2-alkoxy-3-hydrazono-chromones were constructed efficiently from readily available o-hydroxyphenylenaminones (o-HPEs) and aryldiazonium salts at room temperature. This strategy is highly chemoselective and features mild reaction conditions, broad substrate scope, broad functional group tolerance, easy gram-scale preparation, and simple filtration to obtain the pure products without tedious column chromatography.

Electrochemical-induced solvent-tuned selective C(sp3)-H bond activation towards the synthesis of C3-functionalized chromone derivatives

An unprecedented solvent-tuned electrochemical method for selective C(sp3)-H bond activation towards the synthesis of C3 functionalized chromone derivatives has been developed. This electrosynthesis protocol provides an efficient and green way to access various C3-functionalized chromones by avoiding traditionally employed transition metals and high temperatures. The swappable chemoselectivity was controlled mainly by altering the solvent and the current. A plausible reaction mechanism has been proposed with the help of radical capture and cyclic voltammetry experiments.

Photoredox-Catalyzed Tandem Cyclization of Enaminones with N-Sulfonylaminopyridinium Salts toward the Synthesis of 3-Sulfonaminated Chromones

A photoredox-catalyzed intermolecular tandem sulfonamination/cyclization of enaminones was realized by using N-aminopyridinium salts as the sulfonaminated reagents without transition-metal catalysts or bases. The reaction exhibits a broad scope and good functional group tolerance, good yields, and regioselectivity. Preliminary mechanistic studies support the radical property of the reaction and the involvement of N-centered radical intermediates.

Visible-light-enabled cascade cross-dehydrogenative-coupling/cyclization to construct α-chromone substituted α-amino acid derivatives

Organophotocatalytic cascade cross-dehydrogenative-coupling/cyclization reaction of o-hydroxyarylenaminones with α-amino acid derivatives for the construction of α-chromone substituted α-amino acid derivatives was developed. Various N-arylglycine esters, amides and dipeptides underwent the cascade cyclization reaction well with o-hydroxyarylenaminones to afford the corresponding 3-aminoalkyl chromones in good to excellent yields. This approach consists of visible-light-promoted oxidation of α-amino acid derivatives, the Mannich reaction, and intramolecular nucleophilic cyclization under acidic conditions, and features a wide reaction scope, a simple operation and mild reaction conditions, which may have the potential to be used for the synthesis of bioactive molecules.

Metal-free synthesis of difluoro/trifluoromethyl carbinol-containing chromones via tandem cyclization of o-hydroxyaryl enaminones

We herein propose a HFIP-promoted tandem cyclization reaction for the synthesis of difluoro/trifluoromethyl carbinol-containing chromones from o-hydroxyphenyl enaminones at room temperature. This protocol provides a facile and efficient approach to access diverse difluoro/trifluoromethylated carbinols in good to excellent yields. In addition, gram-scale and synthetic derivatization experiments have also been performed.

Visible-Light-Induced Synthesis of 3-Alkyl Chromones under Catalyst- and Additive-Free Conditions

Herein, we reported an efficient and facile visible-light-induced 3-alkyl chromone synthesis from easily accessible o-hydroxyaryl enaminones and α-diazo esters. In this protocol, excellent yields were obtained with a broad substrate scope at room temperature, tolerating various functional groups. Of note is that this eco-friendly methodology features catalyst- and additive-free, mild reaction conditions, simple operation procedure, and easy scale-up, which affords a convenient pathway for the preparation of 3-alkyl chromones. Experimental results and density functional theory (DFT) computation analyses confirm the participation of carbene species and active cyclopropane intermediate.

Selectfluor-mediated tandem cyclization of enaminones with diselenides toward the synthesis of 3-selenylated chromones

A practical and metal-free approach for the regioselective selenation of chromones employing Selectfluor reagent under mild conditions is described. The developed method is suitable for a wide substrate scope and affords 3-selenylated chromones in good to excellent yield with high selectivity. An ionic mechanism is proposed for this transformation. Furthermore, the application of potassium thiocyanate with enaminones for the synthesis of thiocyano chromones in this transformation is also successful.

Palladium-Catalyzed Direct Esterification via C-H Bond Activation of Aldehydes

A concise and highly efficient synthesis method of direct esterification of aldehydes via Pd-catalyzed C-H bond activation of aldehyde group has been developed. The strategy avoids the preoxidation step of aldehyde or use of condensing agents in ester synthesis, which is not only applicable to various alcohols but also suitable for the esterification of phenolics which are usually difficult to be esterified. The methodology has the significant advantages of broad substrate scope, mild reaction conditions, and nonrequirement of additional oxidants.

Cobalt Homeostatic Catalysis for Coupling of Enaminones and Oxadiazolones to Quinazolinones

Transition metal catalysis has revolutionized modern synthetic chemistry for its diverse modes of coordination reactivity. However, this versatility in reactivity is also the predominant cause of catalyst deactivation, a persisting issue that can significantly compromise its synthetic value. Homeostatic catalysis, a catalytic process that can sustain its productive catalytic cycle even when chemically disturbed, is proposed herein as an effective tactic to address the challenge. In particular, a cobalt homeostatic catalysis process has been developed for the water-tolerant coupling of enaminones and oxadiazolones to quinazolinones. Dynamic covalent bonding serves as a mechanistic handle for the preferred buffering of water onto enaminone and reverse exchange by a released secondary amine, thus securing reversible entry into cobalt's dormant and active states for productive catalysis. Through this homeostatic catalysis mode, a broad structural scope has been achieved for quinazolinones, enabling further elaboration into distinct pharmaceutically active agents.

Rh(III)-Catalyzed Oxidative C-H Activation/Annulation of Salicylaldehydes with Masked Enynes for the Synthesis of Chromones

A rhodium(III)-catalyzed oxidative C-H activation/annulation of salicylaldehydes with propargylic acetates has been developed for the regioselective synthesis of 3-vinyl chromones in good yields with broad functional group tolerance. 3-Vinyl chromones were converted into biologically active benzo[c]xanthone by I₂-mediated oxidative electrocyclization.

Combining Enzyme and Photoredox Catalysis for the Construction of 3-Aminoalkyl Chromones

Herein, we reported a practical and efficient strategy combining photoredox and enzyme catalysis for the construction of 3-aminoalkyl chromones from o-hydroxyaryl enaminones and N-arylglycine esters. A variety of 3-aminoalkyl chromones were synthesized with good yields under mild conditions in one pot. This synthetic protocol consists of sequential enzymatic hydrolysis and photoredox decarboxylation of N-arylglycine esters, oxidation of aminoalkyl radicals, Mannich reaction, and intramolecular nucleophilic cyclization, which affords a convenient pathway for the preparation of various 3-substituted chromones.

Transition-metal-free approach to quinolines via direct oxidative cyclocondensation reaction of N,N-dimethyl enaminones with o-aminobenzyl alcohols

A transition-metal-free method for the construction of 3-substituted or 3,4-disubstituted quinolines from readily available N,N-dimethyl enaminones and o-aminobenzyl alcohols is reported. The direct oxidative cyclocondensation reaction tolerates broad functional groups, allowing the efficient synthesis of various quinolines in moderate to excellent yields. The reaction involves a C (sp3)-O bond cleavage and a C=N bind and a C=C bond formation during the oxidative cyclization process, and the mechanism was proposed.

Copper-Promoted N-Alkylation and Bromination of Arylamines/Indazoles Using Alkyl Bromides as Reagents for Difunctionalization

Practical copper-promoted N-alkylation and bromination of arylamines/indazoles with alkyl bromides are described; the N-alkylation-C-4-bromination and N-dialkylation-C-4-bromination of arylamines, and N-alkylation-C-3-bromination of indazoles, with alkyl bromides have been analyzed. The full use of alkyl bromides as alkylating and brominating building blocks without atom wastage, indicating excellent atom and step economy, has been highlighted. Eco-friendly oxygen and water are the reaction oxidant and byproduct, respectively.

Research advances in palladium-catalysed intermolecular C-H annulation of aryl halides with various aromatic ring precursors

Transition-metal-catalysed C-H functionalization has emerged as a powerful approach for the transformation of organic molecules due to its high atom and step economy. Palladium-catalysed intermolecular C-H annulation of aryl halides, especially those involving annulation of a five-membered C,C-palladacycle with coupling reagents, have attracted considerable attention in the past decades. This review summarizes the progress on palladium-catalysed intermolecular C-H annulation of aryl halides with various aromatic ring precursors. Mechanistically, five-membered C,C-palladacycles as intermediates are involved in the majority of reactions.

Copper-Mediated Cyclization of o-Hydroxyaryl Enaminones with 3-Indoleacetic Acids toward the Synthesis of 3-Indolmethyl-Chromones

Here, we describe a copper-mediated tandem decarboxylative coupling/annulation protocol of o-hydroxyaryl enaminones with 3-indoleacetic acids. A series of 3-indolmethyl-chromones were afforded in up to 97% yield. A one-pot method for 3-indolmethyl-chromones from o-hydroxy acetophenones, N, N-dimethylformamide dimethyl acetal, and 3-indoleacetic acids was also developed. Derivatization of the products was conducted to provide various indolmethyl-substituted pyrimidines. Moreover, a biological evaluation revealed that some compounds had anti-influenza viral activities.

Transition-Metal-Free Radical-Triggered Hydrosulfonylation and Disulfonylation Reaction of Substituted Maleimides with Sulfonyl Hydrazides

A convenient and practical hydrosulfonylation and disulfonylation of substituted maleimides was realized using sulfonyl hydrazides as the sulfur reagent and tert-butyl hydroperoxide as the oxidant. The advantages of the reactions include mild and transition-metal-free reaction conditions, good functional group tolerance, and readily available starting materials. The radical species-induced pathway is also demonstrated by mechanistic studies.

Copper-catalyzed C-H/N-H annulation of enaminones and alkynyl esters for densely substituted pyrrole synthesis

Herein, the copper-catalyzed annulation of enaminones with alkynyl esters for the facile synthesis of different pyrroles with a 2,3,4,5-tetrasubstituted structure has been developed. With Cu(OAc)₂ as the only catalyst, the tunable synthesis of 2-vinyl and 2,3-dicarboxyl-functionalized pyrroles has been achieved by using terminal and internal alkynyl esters, respectively. The synthesis of 2-vinyl pyrroles represents the first example accessing 2-vinyl substituted pyrroles via direct cascade reactions involving vinylation and pyrrole ring formation.

Regioselective synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds via transition metal-free C-C and C-N bond formation

Few methods are known for the synthesis of nitroindole derivatives. A simple and practical Cs₂CO₃-promoted method for the synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds has been developed. Two new C-C and C-N bonds were formed in a highly regioselective manner under transition metal-free conditions.

Advances in the site-selective C-5, C-3 and C-2 functionalization of chromones via sp2 C-H activation

In this work, site-selective C-H activation at C-5, C-3 and C-2 positions of chromones for the introduction of structural diversity to the chromone scaffold was studied. The keto group of the chromone moiety acts as the directing group for the selective functionalization of chromones at the C-5 position. Furthermore, the C-H functionalization at the electron-rich C-3 position of the chromone can be achieved using electrophilic coupling partners. The C-H functionalization at the C-2 position can be possible using nucleophilic coupling partners. The direct functionalization methods provide a better pathway for the generation of C-5, C-3 and C-2-substituted chromones with good atom economy than that of classical pre-functionalized reaction protocols.

Photoredox-Catalyzed Cascade of o-Hydroxyarylenaminones to Access 3-Aminated Chromones

Reported herein is a photoredox-catalyzed amination of o-hydroxyarylenaminones with tert-butyl ((perfluoropyridin-4-yl)oxy)carbamate, a versatile amidyl-radical precursor developed in our laboratory. This work establishes a new cascade pathway for the assembly of a range of 3-aminochromones under mild conditions. Downstream transformations of the obtained 3-aminochromones to construct diverse amino pyrimidines greatly broaden the applications of this photocatalyzed protocol.

Visible-light-promoted trifluoromethylselenolation of ortho-hydroxyarylenaminones

Development of an efficient process that employs easy to handle and shelf-stable reagents for the synthesis of trifluoromethylselenylated heterocyclics remains a daunting challenge in organic synthesis. Herein, we report a...

Recent advances in transition metal-free annulation toward heterocycle diversity based on the C-N bond cleavage of enaminone platform

The enaminones and analogous stable enamines are well known as platform building blocks in organic synthesis for construction of heterocyclic compounds, especially N-heterocycles. To date, especially enaminones have been successfully...

Visible light-mediated radical-cascade addition/cyclization of arylacrylamides with aldehydes to form quaternary oxindoles at room temperature

The visible light-induced oxidative radical cascade coupling of N-arylacrylamides with aldehydes using bromide as the hydrogen atom transfer agent to synthesize functional oxindoles is described.

Iodine-Mediated Domino Cyclization for One-Pot Synthesis of Indolizine-Fused Chromones via Metal-Free sp3 C-H Functionalization

An efficient method for the synthesis of new indolizine-fused chromones has been accomplished from ethyl (E)-3-(2-acetylphenoxy)acrylates and pyridines in a "one-pot" manner. Facile operation in open-air, metal-free, and mild conditions renders this protocol particularly practical and attractive. Moreover, this method can simultaneously construct two molecular fragments of chromone and indolizine. Scale-up experiment and the construction of natural products further prove the practicability of this strategy.

Synthesis of 3,3-Dihalogenated 2-Aminochromanones via Tandem Dihalogenation and Cyclization of o-Hydroxyarylenaminones with NXS (X = Cl or Br)

An unprecedented method for the synthesis of dichlorinated and dibrominated 2-amino-substituted chromanones is developed by employing enaminones and NCS/NBS as starting materials under microwave irradiation. The reactions proceed quickly to deliver products without using any catalyst or additive, thus providing practical access to 3,3-dihalogenated 2-aminochromanones.

One-Pot Synthesis of Chromone-Fused Pyrrolo[2,1-a]isoquinolines and Indolizino[8,7-b]indoles: Iodine-Promoted Oxidative [2 + 2 + 1] Annulation of O-Acetylphenoxyacrylates with Tetrahydroisoquinolines and Noreleagnines

An iodine-promoted one-pot cascade oxidative annulation reaction has been developed for the synthesis of chromone-fused-pyrrolo[2,1-a]isoquinolines and indolizino[8,7-b]indoles from o-acetylphenoxyacrylates, tetrahydroisoquinolines, and noreleagnines. This process underwent a logical approach to both chromone-fused-pyrrolo[2,1-a]isoquinolines and chromone-fused-indolizino[8,7-b]indoles isolamellarin derivatives. Manipulations of l-menthol and dl-α-tocopherol demonstrate the applications of this strategy.

Access to Chiral Chromenones through Organocatalyzed Mannich/Annulation Sequence

Herein we report an efficient and practical method to access chiral chromenones bearing one α-amino stereogenic center in the β position of the carbonyl group. The quinine-derived squaramide could efficiently promote Mannich/cycloketalization/dehydration tandem reactions between 1-(2-hydroxyaryl)-1,3-diketones and functionalized imines generated in situ, providing a wide range of chiral chromenones with propargylamine or α-amino ester moieties with good results (54 examples, up to 98% ee).

Solvent-controlled two-step one-pot syntheses of α-X (X = Br or Cl) enamino ketones/esters and 3-(2,5-dioxopyrrolidin-1-yl)acrylate by using terminal carbonyl alkynes

A new two-step one-pot aminobromination/chlorination of carbonyl alkynes has been achieved via a Michael addition of aliphatic secondary amines and subsequent β-bromination/chlorination of the obtained enamines to afford various α-X (X = Br or Cl) enamino ketones/esters in moderate to good yields. A solvent-controllable protocol has been developed to produce versatile 3-(2,5-dioxopyrrolidin-1-yl)acrylates in moderate yields by using toluene as the solvent and chain alkyl propiolates as alkynyl substrates.

Metal-free enaminone C-N bond cyanation for the stereoselective synthesis of (E)- and (Z)-β-cyano enones

A highly practical method for C-CN bond formation by C-N bond cleavage on enaminones leading to the efficient synthesis of β-cyano enones is developed. The reactions take place efficiently to provide (E)-β-cyano enones with only a molecular iodine catalyst. In addition, the additional employment of oxalic acid enables the selective synthesis of (Z)-β-cyano enones.

A Flexible Synthetic Approach to Fluorescent Chromeno[4,3-b]pyridines and Pyrano[3,2-c]chromenes from Electron-Deficient 3-Vinylchromones

We report a flexible approach to the synthesis of phenanthrene-like heterocycles through organocatalytic ANRORC (Addition of the Nucleophile, Ring Opening, and Ring Closure) reaction of electron-deficient 3-vinylchromones with cyanoacetamide. Addition of highly basic DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) or tetramethylguanidine (TMG) at 80 °C leads to chromeno[4,3-b]pyridines in good yields, whereas Et3 N at 20 °C made it possible to obtain the less accessible pyrano[3,2-c]chromenes and their 2-imines. The synthesis proceeds in mild conditions (EtOH, 20-80 °C), is versatile and applicable for a wide scope of reactants. The obtained compounds show bright fluorescence in the range 460-595 nm with high quantum yields (up to 0.84) in various solvents (MeCN, DMSO, EtOH, H2 O).

Electrochemical C-H Halogenations of Enaminones and Electron-Rich Arenes with Sodium Halide (NaX) as Halogen Source for the Synthesis of 3-Halochromones and Haloarenes

Without employing an external oxidant, the simple synthesis of 3-halochromones and various halogenated electron-rich arenes has been realized with electrode oxidation by employing the simplest sodium halide (NaX, X = Cl, Br, I) as halogen source. This electrochemical method is advantageous for the simple and mild room temperature operation, environmental friendliness as well as broad substrate scope in both C-H bond donor and halogen source components.

Copper-Catalyzed Enaminone C(sp2)-N Bond Phosphonation for Stereoselective Synthesis of Alkenylphosphonates

A direct enaminone C-N bond coupling functionalization that generates a new C-P bond using dialkyl phosphonate for the efficient and stereoselective synthesis of (E)-alkenylphosphonates is reported. The reactions toward target products proceed well with a broad scope, disclosing a valuable new synthetic application of enaminones by the interesting C(sp²)-N bond elaboration.

Tunable Trifunctionalization of Tertiary Enaminones for the Regioselective and Metal-Free Synthesis of Discrete and Proximal Phosphoryl Nitriles

This paper reports an unprecedented trifunctionalization of tertiary enaminones for the synthesis phosphoryl nitriles by the reactions of enaminones with diarylphosphine oxides and trimethylsilyl cyanide (TMSCN) without the use of any metal reagent. Employing tetrabutyl ammonium hydroxide (TBAH) as the catalyst (0.2 equiv) enables discrete cyanophosphonation. On the other hand, selective proximal cyanophosphonation has been realized in the presence of acetic acid only (AcOH).

Metal-Free Photosynthesis of Alkylated Benzimidazo[2,1-a]isoquinoline-6(5H)-ones and Indolo[2,1-a]isoquinolin-6(5H)-ones in PEG-200

A visible-light-induced decarboxylation reaction was developed for the synthesis of alkylated benzimidazo[2,1-a]isoquinoline-6(5H)-ones and indolo[2,1-a]isoquinolin-6(5H)-ones under metal-free conditions. Impressively, metal catalysts and traditionally volatile organic solvents could be effectively avoided.