Visible-Light Photocatalysis for Sustainable Chromene Synthesis and Functionalization

Chromenes represent a biologically significant family of heterocycles, widely present in pharmaceutical compounds and secondary metabolites. Their importance in synthesis and medicinal chemistry has driven continuous efforts to develop advanced and efficient synthetic methodologies. Over the past decade, photoredox catalysis has revolutionized Organic Synthesis, offering innovative and sustainable strategies for the construction and functionalization of complex molecules. The synthesis of chromene derivatives has greatly benefited from the integration of visible-light-promoted methodologies, which utilize light as an abundant and non-toxic energy source, enabling greener chemical transformations. In this review, the latest progress in visible-light-mediated approaches for the synthesis of chromene derivatives, along with the photochemical reactivity of these relevant frameworks, are summarized.

Water-Controlled Geminal Hydroxyphosphinoylation and Diphosphinoylation of Enaminones with H-Phosphine Oxides

A water-controlled geminal phosphinoylation of enaminones with H-phosphine oxides has been established through AlCl3-mediated C-N bond cleavage in this work, which provides a novel strategy for accessing various hydroxy and diphosphinoyl products 3a and 4a in high yields. The transformation features excellent functional group tolerance, operational simplicity, and high atom economy, and is amenable for phosphinoylation of complex molecule skeletons. Preliminary mechanism studies suggest the conversion from 3a to 4a involve the elimination of hydroxyl group, and water and temperature plays a critical role in influencing the reaction pathway and product selectivity. This research provides significant value to the geminal functionalization of enaminones.

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.

Cascade Annulation for Synthesizing Chromenopyrrolones from o-Hydroxyphenyl Enaminones and 2-Halo-N-alkyloxyacetamides

A cascade cyclization reaction comprising two halogenation reactions and a Michael addition was developed for the synthesis of chromeno[2,3-c]pyrrole-3-ones 4. Additionally, another cascade cyclization reaction, which involves a halogenation reaction followed by two intramolecular Michael additions, was established for the synthesis of chromeno[2,3-b]pyrrole-2-ones 5. Both types of compounds were synthesized from o-hydroxyphenyl enaminones and 2-halo-N-alkyloxyacetamides through a process that facilitated the intramolecular formation of C-C, C-O, and C-N bonds to effectively establish two fused rings in a single operation. This novel protocol is efficient, uses readily accessible starting materials, and operates under mild conditions, demonstrating tolerance for various functional groups while achieving good yields.

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.

Catalyst-free reactions of anilines with β-chloroenones: synthesis of α-chloroenaminones and 1,4-benzodiazepines

The Michael addition of anilines to β-chloroenones gives enaminones by the elimination of hydrochloric acid (HCl). These enaminones are transformed into α-chloroenaminones via in situ sp2 C-H functionalization. Anilines that are attached to an electron-donating group react more readily with β-chloroenone to give the corresponding products in excellent yields. A highly atom-economical method has been developed using dimethyl sulfoxide (DMSO) as a green oxidant and solvent. The desired α-functionalized enaminones are formed in good yields with excellent Z-selectivity. We have established the generality of this reaction with many substrates, and scaled-up reactions have been performed to showcase the practical applications. A catalyst-free double annulation of β-chloroenones with o-phenylenediamine has also been demonstrated for the synthesis of 1,4-benzodiazepine derivatives in moderate yields under mild reaction conditions.

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.

Iodine-Promoted Thioylation and Dicarbonylation of Enaminone α-C Sites: Synthesis of Fully Substituted Thiazoles via C═C Bond Cleavage

A novel iodine-promoted difunctionalization of α-C sites in enaminones was demonstrated as a means of synthesizing a variety of fully substituted thiazoles by constructing C-C(CO), C-S, and C-N bonds. This transformation allows the realization of enaminones as unusual aryl C2 synthons and simultaneously allows the thioylation and dicarbonylation of α-C sites. A preliminary mechanistic study was performed and indicated that the cleavage of C═C bonds in enaminones involves a bicyclization/ring-opening and oxidative coupling sequence.

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.

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.

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.

Cascade in Situ Iodination, Chromone Annulation, and Cyanation for Site-Selective Synthesis of 2-Cyanochromones

A facile cascade reaction for the site selective synthesis of 2-cyanochromones is described. By using simple o-hydroxyphenyl enaminones and potassium ferrocyanide trihydrate (K₄[Fe(CN)₆]₃·3H₂O) as starting materials and I₂/AlCl₃ as promoters, the products are furnished via tandem chromone ring formation and C-H cyanation. The in situ formation of 3-iodochromone and a formal 1,2-hydrogen atom transfer (HAT) process account for the unconventional site selectivity. In addition, the synthesis of 2-cyanoquinolin-4-one has been realized by employing corresponding 2-aminophenyl enaminone as substrate.

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.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

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.

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...

Controllable methylenation with ethylene glycol as the methylene source: bridging enaminones and synthesis of tetrahydropyrimidines

Controllable methylenation using renewable ethylene glycol as the methylene source has been developed for the introduction of one or two methylene building blocks.

Chromone a Privileged Scaffold in Drug Discovery: Developments on the Synthesis and Bioactivity

Chromones are the class of secondary metabolites broadly occurred in the plant kingdom in a noticeable quantity. This rigid bicyclic system has been categorized "as privileged scaffolds in compounds" in medicinal chemistry. The wide biological responses made them an important moiety in a drug discovery program. This review provides updates on the various methods of synthesis of chromones and biological applications in medicinal chemistry. Various synthetic strategies for the construction of chromones include readily available phenols, salicylic acid and its derivatives, ynones, chalcones, enaminones, chalcones and 2-hydroxyarylalkylketones as starting materials. Synthesis of chromones by using metal, metal free, nanomaterials and different catalysts are included. Details of diverse biological activities such as anti-cancer agents, antimicrobial agents, anti-viral property, anti-inflammatory agents, antioxidants, Monoamine Oxidase-B (MAO-B) Inhibitors, anti-Alzheimer's agents, anti-diabetic agent, antihistaminic potential, antiplatelet agents of chromone derivatives are diecussed.

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.

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.

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.

Base-Promoted Stereoselective Hydrogenation of Ynamides with Sulfonyl Hydrazide to Give Z-Enamides

A base-mediated semihydrogenation of ynamides using p-toluenesulfonyl hydrazide as an inexpensive and easy-to-handle hydrogen donor is reported. This transition-metal-free protocol avoids overhydrogenation and reduction of other functional groups, generating the thermodynamically unfavorable Z-enamides exclusively.

The cascade coupling/iodoaminocyclization reaction of trifluoroacetimidoyl chlorides and allylamines: metal-free access to 2-trifluoromethyl-imidazolines

A metal-free cascade coupling/iodoaminocyclization reaction for the rapid assembly of 2-trifluoromethyl-imidazolines has been disclosed. The transformation applies readily accessible trifluoroacetimidoyl chlorides, allylamines and N-iodosuccinimides as the starting substrates, achieving an efficient and straightforward pathway to construct diverse imidazoline derivatives. Excellent efficiency of the reaction is observed (higher than 90% isolated yield for half of the examples), and the obtained imidazoline products bearing a pendent iodomethyl group could be easily transformed into other synthetically valuable compounds.

Recent Advances in Organic Synthesis Based on N,N-Dimethyl Enaminones

Enaminones are gaining increasing interest because of their unique properties and their importance in organic synthesis as versatile building blocks. N,N-Dimethyl enaminones offer a better leaving group (a dimethylamine group) than other enaminones, and allow further elaboration via a range of facile chemical transformations. Over the past five years, there have been an increasing number of reports describing the synthetic applications of N,N-dimethyl enaminones. This review provides a comprehensive overview on the synthetic applications of N,N-dimethyl enaminones that have been reported since 2016.

1 Introduction

2 Direct C(sp2)–H α-Functionalization

2.1 Synthesis of α-Sulfenylated N,N-Dimethyl Enaminones

2.2 Synthesis of α-Thiocyanated N,N-Dimethyl Enaminones

2.3 Synthesis of α-Acyloxylated N,N-Dimethyl Enaminones

3 Functionalization Reactions via C=C Double Bond Cleavage

3.1 Synthesis of Functionalized Methyl Ketones

3.2 Synthesis of α-Ketoamides, α-Ketoesters and 1,2-Diketones

3.3 Synthesis of N-Sulfonyl Amidines

4 Construction of All-Carbon Aromatic Scaffolds

4.1 Synthesis of Benzaldehydes

4.2 Synthesis of the Naphthalenes

5 Construction of Heterocyclic Scaffolds

5.1 Synthesis of Five-Membered Heterocycles

5.2 Synthesis of Six-Membered Heterocycles

5.3 Synthesis of Quinolines

5.4 Synthesis of Functionalized Chromones

5.5 Synthesis of Other Fused Polycyclic Heterocycles

6 Conclusions and Perspectives

C4-Alkylamination of C4-Halo-1H-1-tritylpyrazoles Using Pd(dba)2 or CuI

Alkylamino coupling reactions at the C4 positions of 4-halo-1H-1-tritylpyrazoles were investigated using palladium or copper catalysts. The Pd(dba)₂ catalyzed C-N coupling reaction of aryl- or alkylamines, lacking a β-hydrogen atom, proceeded smoothly using ^tBuDavePhos as a ligand. As a substrate, 4-Bromo-1-tritylpyrazole was more effective than 4-iodo or chloro-1-tritylpyrazoles. Meanwhile, the CuI mediated C-N coupling reactions of 4-iodo-1H-1-tritylpyrazole were effective for alkylamines possessing a β-hydrogen atom.

Transition metal-free synthesis of 3-trifluoromethyl chromones via tandem C–H trifluoromethylation and chromone annulation of enaminones

The synthesis of 3-trifluoromethyl chromones has been realized via transition metal-free reactions of o-hydroxyphenyl enaminones and the Langlois reagent via cascade C–H trifluoromethylation and chromone annulation.