Thermoinduced Free-Radical C–H Acyloxylation of Tertiary Enaminones: Catalyst-Free Synthesis of Acyloxyl Chromones and Enaminones

Transition-Metal-Free C-H Trifluoromethylthiolation of N,N-Disubstituted Enaminones To Access CF3S-Functionalized Enaminones and Their Application in the Synthesis of CF3S-Heteroaryls

The α-C-H trifluoromethylthiolation of N,N-disubstituted enaminones has been achieved with simple and cheap CF3SO2Na as the CF3S source. The reactions were run at mild temperature (0 °C to rt) using POCl3 as the only reducing reagent. The work represents the first example on the synthesis of α-trifluoromethylthio enaminones via direct C-H functionalization. In addition, the resulting CF3S-functionalized enaminones have been proven as useful building blocks in the synthesis of various CF3S-functionalized heteroaromatic compounds by simple annulation reactions.

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.

Benzopyrone, a privileged scaffold in drug discovery: An overview of FDA-approved drugs and clinical candidates

Natural products have always served as an important source of drugs for treating various diseases. Among various privileged natural product scaffolds, the benzopyrone class of compounds has a substantial presence among biologically active compounds. One of the pioneering anticoagulant drugs, warfarin approved in 1954 bears a benzo-α-pyrone (coumarin) nucleus. The widely investigated psoriasis drugs, methoxsalen, and trioxsalen, also contain a benzo-α-pyrone nucleus. Benzo-γ-pyrone (chromone) containing drugs, cromoglic acid, and pranlukast were approved as treatments for asthma in 1982 and 2007, respectively. Numerous other small molecules with a benzopyrone core are under clinical investigation. The present review discusses the discovery, absorption, distribution, metabolism, excretion properties, and synthetic approaches for the Food and Drug Administration-approved and clinical-stage benzopyrone class of compounds. The role of the pyrone core in biological activity has also been discussed. The present review unravels the potential of benzopyrone core in medicinal chemistry and drug development.

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.

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.

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.

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.

Access to Sulfur-Containing Bisheterocycles through Base-Promoted Consecutive Tandem Cyclization/Sulfenylation with Elemental Sulfur

A convenient and efficient tandem cyclization/sulfenylation of o-alkynyl-phenols/-anilines/enaminones for the synthesis of diverse sulfur-containing bisheterocycles has been developed using stable, odorless, and easy-to-handle elemental S₈ as a building block under green chemistry conditions. Notably, a one-step simple base-mediated organic transformation affords a benzofuran (indole or chromone) ring and two C-S bonds. Attractive features of this methodology include the absence of a metal catalyst, mild conditions, good functional group tolerance, and valuable product structures.

Transition-Metal-Free Synthesis of 3-Acyl Chromones by the Tandem Reaction of Ynones and Methyl Salicylates

A facile and effective tandem reaction of ynones and methyl salicylates was developed to obtain a broad range of 3-acyl chromones in moderate-to-excellent yields. This protocol underwent a Michael addition and cyclization process, which exhibited easily accessible substrates, broad substrate scope, and high regioselectivity under mild and transition-metal-free conditions. Moreover, gram-scale reaction and further chemical transformation of the products were also further studied.

The polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs

The novel polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs has been developed to prepare 2-pyrrolidinone derivatives with polychloromethyl units with the use of an inexpensive copper salt under mild conditions. This strategy includes two dual-role designs, not only improving atomic utilization but also allowing a cleaner process. The wide substrate scope and simple reaction conditions demonstrate the practicability of this protocol.

A Straightforward Approach to Fluorinated Pyrimido[1,2-b]indazole Derivatives via Metal/Additive-Free Annulation with Enaminones, 3-Aminoindazoles, and Selectfluor

A novel and efficient three-component reaction with two C-N bonds and one C-F bond formation has been reported, which provides a straightforward route to a variety of fluorinated pyrimido[1,2-b]indazole derivatives. This transformation has the advantage of excellent functional group compatibility, including aliphatic and aromatic substituents enaminones. Moreover, metal and additives are not necessary for this reaction, which is of great significance for the synthesis and application of fluorinated heterocycles.

Lewis-Acid-Catalyzed Regioselective Construction of Diversely Functionalized Polycyclic Fused Furans

A novel, facile, and efficient Lewis-acid-catalyzed [4 + 1] annulation protocol for the construction of functionalized polycyclic-fused furans is developed. This methodology is free of transition metals and ligands and provides a rapid synthetic route to divergently orientated polycyclic furans in good yields. In addition, this protocol can also be used to synthesize multisubstituted furans.

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.

Selective Thiocyanation and Aromatic Amination To Achieve Organized Annulation of Enaminone with Thiocyanate

A tandem insertion of thiocyanate to enamine was performed for the regioselective synthesis of multisubstituted benzoimidazo[2,1-b]thiazoles. This method was shown to be effective in addressing the issue of isomerization encountered in common strategies. With a change made to the leading group on the aniline fragment of enamine, the reaction achieved different transformations, thus enabling multisubstituted benzo[4,5]imidazo[2,1-b]thiazoles and thiazoles in satisfactory yields.

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.

Stereoelectronic power of oxygen in control of chemical reactivity: the anomeric effect is not alone

Although carbon is the central element of organic chemistry, oxygen is the central element of stereoelectronic control in organic chemistry. Generally, a molecule with a C-O bond has both a strong donor (a lone pair) and a strong acceptor (e.g., a σ*_C-O orbital), a combination that provides opportunities to influence chemical transformations at both ends of the electron demand spectrum. Oxygen is a stereoelectronic chameleon that adapts to the varying situations in radical, cationic, anionic, and metal-mediated transformations. Arguably, the most historically important stereoelectronic effect is the anomeric effect (AE), i.e., the axial preference of acceptor groups at the anomeric position of sugars. Although AE is generally attributed to hyperconjugative interactions of σ-acceptors with a lone pair at oxygen (negative hyperconjugation), recent literature reports suggested alternative explanations. In this context, it is timely to evaluate the fundamental connections between the AE and a broad variety of O-functional groups. Such connections illustrate the general role of hyperconjugation with oxygen lone pairs in reactivity. Lessons from the AE can be used as the conceptual framework for organizing disjointed observations into a logical body of knowledge. In contrast, neglect of hyperconjugation can be deeply misleading as it removes the stereoelectronic cornerstone on which, as we show in this review, the chemistry of organic oxygen functionalities is largely based. As negative hyperconjugation releases the "underutilized" stereoelectronic power of unshared electrons (the lone pairs) for the stabilization of a developing positive charge, the role of orbital interactions increases when the electronic demand is high and molecules distort from their equilibrium geometries. From this perspective, hyperconjugative anomeric interactions play a unique role in guiding reaction design. In this manuscript, we discuss the reactivity of organic O-functionalities, outline variations in the possible hyperconjugative patterns, and showcase the vast implications of AE for the structure and reactivity. On our journey through a variety of O-containing organic functional groups, from textbook to exotic, we will illustrate how this knowledge can predict chemical reactivity and unlock new useful synthetic transformations.

Diacyl peroxides: practical reagents as aryl and alkyl radical sources

Diacyl peroxides, which can be easily synthesized from corresponding carboxylic acids, are commonly utilized as radical initiators and one electron oxidants. Under thermal, transition-metal catalysis or irradiation conditions the cleavage of relatively weak O-O bonds would occur followed by CO₂ extrusion to generate the corresponding aryl or alkyl radicals. Thus, diacyl peroxides can be employed as ideal arylating and alkylating reagents in organic synthesis, including C-H/N-H arylation/alkylation, aryl/alkyl radical addition to unsaturated bonds, hetero arylation/alkylation, eliminative arylation/alkylation, perfluoroalkylation etc. Moreover, these arylation/alkylation protocols have been successfully utilized in the synthesis and late-stage functionalization of natural products as well as bioactive molecules. In this review, recent advances on arylation and alkylation using diacyl peroxides as aryl and alkyl radical sources are summarized and discussed.

Visible Light Promoted, Catalyst-Free Radical Carbohydroxylation and Carboetherification under Mild Biomimetic Conditions

Metal and catalyst-free carbohydroxylations and carboetherifications at room temperature have been achieved by a combination of beneficial factors including high aryl diazonium concentration and visible light irradiation. The acceleration of the reaction by visible light irradiation is particularly remarkable against the background that neither the aryldiazonium salt nor the alkene show absorptions in the respective range of wavelength. These observations point to weak charge transfer interactions between diazonium salt and alkene, which are nevertheless able to considerably influence the reaction course. As highly promising perspective, many more aryldiazonium-based radical arylations might benefit from simple light irradiation without requiring a photocatalyst or particular additive.

Recent advances in reactions using diacyl peroxides as sources of O- and C-functional groups

This review summarizes recent advances in reactions utilizing diacyl peroxides as O- and C-sources, with examples illustrating how the reactivity of diacyl peroxides in organic reactions can be controlled.

Silver-catalyzed decarboxylative radical relay difluoroalkylation–carbocyclization: convenient access to CF2-containing quinolinones

A practical Ag-catalyzed formal decarboxylation and radical difluoroalkylation–carbocyclization–hydrolysis route is established to construct a series of structurally diverse CF2-containing N-heterocycles.

Trisulfur-Radical-Anion-Triggered C(sp2)-H Amination of Electron-Deficient Alkenes

A trisulfur-radical-anion (S₃̇^-)-triggered C(sp²)-H amination of α,β-unsaturated carbonyl derivatives with simple amines has been demonstrated. This protocol provides convenient access to a variety of synthetically valuable N-unprotected and secondary β-enaminones with absolute Z selectivity and tertiary β-enaminones with E selectivity. Mechanistic probe and electronic structure theory calculations suggest that S₃̇^- initiates the nucleophilic attacks via a thiirane intermediate.

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

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.