Highly Site-Selective Metal-Free C–H Acyloxylation of Stable Enamines

Platform of Oxidative Transformation of α-Methyl Secondary Enaminones toward Tetrahydropyridines

In this paper, the application of α-methyl secondary enaminones in the synthesis of tetrahydrofuropyridines is described. The key step of the methodology is the in situ generation of 1-azadiene from oxidation of α-methyl secondary enaminone, followed by a subsequent inverse-electron-demand hetero-Diels-Alder reaction proceeded to give the desired product. Mechanistic studies and density functional theory (DFT) calculations revealed the detailed reaction pathway. Gram-scale preparation experiments and further transformation of the product demonstrate the potential applicability of this method. In addition, the amide derivatives could be obtained by employing β-methyl secondary enaminones as substrates under similar oxidative conditions. The present work opens a new window to the application of rarely reported α-methyl secondary enaminones.

Preparation and Transformations of Acetophenone-Derived Enamino Ketones, BF2-β-Ketoiminates, and BF2-β-Diketonates

A series of differently substituted β-enaminones 2a,b, 4a-i, 8a-d, and 9-13, their BF2-β-ketoiminate complexes 5a-d, and BF2-β-diketonate complexes 6a-d were prepared as model substrates for photochemical transformations. The attempted photochemical transformations of enaminones 2, 4, 8 and BF2-β-ketoiminate complexes 5 failed. On the other hand, irradiation of mixtures of BF2-β-diketonate complexes 6a-d and cycloalkanes with UV-A light (365 nm) gave the corresponding De Mayo reaction products 7a-f in 9-30% yields. The photochemical ring-expansion of acetyl tetralone-derived BF2-complex 6d gave novel diannulated cyclooctane derivatives 7e and 7f, which would be difficult to obtain using conventional cyclization methods.

Direct synthesis of enone-hydrazones under solvent free and additive free conditions

A series of enone-hydrazones were synthesized through the reaction of enaminones with hydrazones under solvent-free, additive-free, and metal-catalyst-free conditions. The target products were obtained in high yields, with outstanding group tolerance, stereoselectivity, and chemical selectivity. In this work, a series of novel enaminone skeleton compounds were synthesized, and deuteration experiments showed that enone-hydrazones have different properties from traditional electron-deficient enketones.

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.

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.

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.

I2-DMSO-Mediated Transannulation of Benzo[d]isoxazol-3-amine: Direct Access to 2,4,5-Substituted Pyrimidine Derivatives

An I2-DMSO-mediated multicomponent [3+1+2] cascade annulation reaction using aryl methyl ketones, enaminones, and benzo[d]isoxazol-3-amine as substrates has been developed. This metal-free reaction involved the transannulation of benzo[d]isoxazol-3-amines with the formation of two C-N bonds and a C-C bond in one pot. Notably, a pyrimidine ring with a 1,4-dicarbonyl scaffold could efficiently transform into a pyrimido[4,5-d]pyridazine skeleton. The phenolic hydroxyl group of the target product could undergo further modification with pharmaceuticals, demonstrating the utility of this method.

One Stone, Three Birds: One-Pot Synthesis of Pyrido[3,2-a]phenoxazin-5-one Derivatives from o-Aminophenols with Triple Roles, Paraformaldehyde, and Enaminones via the Povarov Reaction

A novel multicomponent cascade cyclization reaction in one pot for the preparation of pyrido[3,2-a]phenoxazin-5-ones from simple o-aminophenols, paraformaldehyde, and enaminones has been established. It is noteworthy that o-aminophenol plays multiple roles serving as both a bis-nucleophile and an iminoquinone precursor, which can in situ generate aminophenoxazinones to undergo the Povarov reaction for the first time to yield pyrido[3,2-a]phenoxazin-5-ones with a high efficiency. Moreover, the photoluminescence of pyrido[3,2-a]phenoxazin-5-ones has polarity sensitivity and features aggregation-induced emission (AIE) characteristics, which is promising for bioimaging and theranostic applications.

Targeting Transcriptional CDKs 7, 8, and 9 with Anilinopyrimidine Derivatives as Anticancer Agents: Design, Synthesis, Biological Evaluation and In Silico Studies

Cyclin-dependent kinases (CDKs) are promising targets in chemotherapy. In this study, we report a series of 2-anilinopyrimidine derivatives with CDK inhibitory activity. Twenty-one compounds were synthesized and their CDK inhibitory and cytotoxic activities were evaluated. The representative compounds demonstrate potent antiproliferative activities toward different solid cancer cell lines and provide a promising strategy for the treatment of malignant tumors. Compound 5f was the most potent CDK7 inhibitor (IC₅₀ = 0.479 µM), compound 5d was the most potent CDK8 inhibitor (IC₅₀ = 0.716 µM), and compound 5b was the most potent CDK9 inhibitor (IC₅₀ = 0.059 µM). All the compounds satisfied the Lipinski's rule of five (molecular weight < 500 Da, number of hydrogen bond acceptors <10, and octanol-water partition coefficient and hydrogen bond donor values below 5). Compound 5j is a good candidate for lead optimization because it has a non-hydrogen atom (N) of 23, an acceptable ligand efficiency value of 0.38673, and an acceptable ligand lipophilic efficiency value of 5.5526. The synthesized anilinopyrimidine derivatives have potential as anticancer agents.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Expanding the Promiscuity of a Copper-Dependent Oxidase for Enantioselective Cross-Coupling of Indoles

Herein, we disclose the highly enantioselective oxidative cross-coupling of 3-hydroxyindole esters with various nucleophilic partners as catalyzed by copper efflux oxidase. The biocatalytic transformation delivers functionalized 2,2-disubstituted indolin-3-ones with excellent optical purity (90-99 % ee), which exhibited anticancer activity against MCF-7 cell lines, as shown by preliminary biological evaluation. Mechanistic studies and molecular docking results suggest the formation of a phenoxyl radical and enantiocontrol facilitated by a suited enzyme chiral pocket. This study is significant with regard to expanding the catalytic repertoire of natural multicopper oxidases as well as enlarging the synthetic toolbox for sustainable asymmetric oxidative 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.

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.

Nickel-Catalyzed Reductive Borylation of Enaminones via C(sp2)-N Bond Cleavage

The cleavage and transformation of alkenyl C(sp²)-N bonds is a significant synthetic challenge. Herein we described an unprecedented nickel-catalyzed reductive borylation of enaminones to synthesize β-ketone boronic esters. Notably, B₂pin₂ played the dual role in this process, and water served as a hydrogen source, which was transferred to target products. The air-stable nickel catalyst was applied to the cleavage of alkenyl C(sp²)-N bonds, concomitant with the reductive process of the alkenyl boronic ester intermediates, on the basis of the mechanism study.

Ultrasound-Promoted Synthesis of α-Thiocyanoketones via Enaminone C═C Bond Cleavage and Tunable One-Pot Access to 4-Aryl-2-aminothiazoles

Ultrasound has been successfully employed to promote the thiocyanation of the C═C bond in enaminones for the synthesis of α-thiocyanoketones and 2-aminothiazoles. The reactions of enaminones with ammonium thiocyanate provide α-thiocyanoketones with ultrasound irradiation at room temperature. More interestingly, simply further heating the vessel after ultrasonic irradiation leads to the selective synthesis of 2-aminothiazoles with an unconventional 4-aryl substructure.

Selective Mono- and Diamination of Ketones in a Combined Copper-Organocatalyst System

Herein, we report a simple and mild protocol for the chemoselective mono- and diamination of ketone using pyrazole as the amine source in a combined copper-organocatalyst system. Various substrates are compatible, providing the corresponding products in moderate to good yields. This strategy gives an efficient and convenient solution for the synthesis of α-pyrazole and α,α-dipyrazole ketone derivatives. The control experiment demonstrates that in situ generated hydrazone is a key intermediate in the transformation.

Metal-free synthesis of gem-difluorinated heterocycles from enaminones and difluorocarbene precursors

A cascade strategy to synthesise gem-difluorinated 2H-furans from reactions of BrCF₂CO₂Et with enaminones has been described. The reactions tolerate a wide variety of functional groups under metal-free conditions. An active aminocyclopropane is proposed to be a key intermediate through the cyclopropanation of difluorocarbene with enaminones, which further triggers a regioselective C-C bond cleavage in situ to afford the corresponding gem-difluorinated 2H-furans.

Metal-free synthesis of β-aminoketones by the reductive hydroamination of ynones

This study describes a cascade method for the synthesis of β-aminoketones through the reductive hydroamination of alkynes under very mild metal-free conditions. It allows for the rapid conversion of ynones and amines into corresponding β-aminoketones with a broad substrate scope and diverse functionalities. This straightforward and easy-to-handle reaction process can be successfully applied for the synthesis of Proroxan and Propipocaine, offering a potential option for the synthesis of drug molecules with the β-aminoketone skeleton.

A General Method for α-Oxyacylation of Vinyl Ketones Using Koser's Reagent

A direct general method for the preparation of α-oxyacylated vinyl ketones using Koser's hypervalent iodine reagent is reported. A variety of acyloxy groups from long-chain aliphatic, aromatic, α,β-unsaturated carboxylic acids have been installed efficiently for the first time. The oxyacylated adducts were used for the preparation of densely functionalized chiral δ-lactones and cyclopentenes using carbene organocatalysis.

One-Pot Synthesis of Symmetrical and Asymmetrical 3-Amino Diynes via Cu(I)-Catalyzed Reaction of Enaminones with Terminal Alkynes

An economical and efficient protocol for the direct construction of amino skipped diynes through the Cu(I)-catalyzed reaction of enaminones and terminal alkynes has been described. Different kinds of symmetrical and asymmetrical 3-amino diynes could be obtained in up to 83% yield through a one-pot reaction under mild conditions.

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.

An expedient synthesis of highly functionalized 1,3-dienes by employing cyclopropenes as C4 units

An efficient method has been described to synthesize dicarbonyl functionalized 1,3-dienes by cleaving the C[double bond, length as m-dash]C bond of enaminones with cyclopropenes in the presence of a rhodium catalyst. The acetate-substituted cyclopropenes are judiciously chosen as standard C4 units of 1,3-diene precursors. The reactions are believed to undergo a unique cutting and insertion process, involving a C[double bond, length as m-dash]C bond cleavage of the enaminone and insertion of a new C(sp2) source with the formation of two C-C single bonds. A broad range of substrates can be used to synthesize the corresponding 1,3-dienes under very mild reaction conditions, including low catalyst-loading, ambient temperature, and a neutral reaction solvent.

Pd-Catalyzed Triple-Fold C(sp2)-H Activation with Enaminones and Alkenes for Pyrrole Synthesis via Hydrogen Evolution

The synthesis of NH-free pyrroles via Pd-catalyzed annulation of enaminones and alkenes is reported. With the catalysis of Pd(II), the activation of triple C(sp²)-H bonds, including one internal C(sp²)-H bond in enaminone, has been activated to provide various pyrroles. The interesting evolution of hydrogen gas from the reactions has been observed by a hydrogen detector.

Arylation of ortho-Hydroxyarylenaminones by Sulfonium Salts and Arenesulfonyl Chlorides: An Access to Isoflavones

Herein we disclose three new methods for the straightforward and efficient synthesis of 3-arylchromones following the arylation of ortho-hydroxyarylenaminones by vast diversities of bench-stable and easy-to-use sulfonium salts and arenesulfonyl chlorides. Both developed methods, namely the light-mediated photoredox and electrophilic arylation, showed good efficiency, and are feasible for the preparation of 3-arylchromones in good-to-excellent yields. This work showcases the first described attempt where the sulfonium salts and arenesulfonyl chlorides were successfully utilized for the construction of the chromone heterocycle system.

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.

Azido-Enolonium Species in C-C and C-N Bond-Forming Coupling Reactions

Vinyl azides react with boron trifluoride activated Koser's hypervalent iodine reagent to afford azido-enolonium species. These previously unknown azido-enolonium species react efficiently with aromatic compounds, allyltrimethylsilane, and azoles under mild conditions, with no need for a transition-metal catalyst, forming C-C and C-N bonds to give a variety of α-functionalized ketones. The intermediacy of the proposed azido-enolonium species is supported by spectroscopic studies.

Toward C2-nitrogenated chromones by copper-catalyzed β-C(sp2)–H N-heteroarylation of enaminones

The synthesis of C2-nitrogenated chromones has been performed via reactions of enaminones and nitrogen nucleophiles based on an unconventional β-C–H bond functionalization and a featured chromone annulation of enaminones.

Stereoselective synthesis of trifluoromethyl-substituted 2H-furan-amines from enaminones

A straightforward strategy for synthesis of highly functionalized trifluoromethyl 2H-furans is described.

Synthesis of 2-Amino-5-acylthiazoles by a Tertiary Amine-Promoted One-Pot Three-Component Cascade Cyclization Using Elemental Sulfur as a Sulfur Source

A novel one-pot three-component cascade cyclization strategy for the synthesis of 2-amino-5-acylthiazoles using enaminones, cyanamide, and elemental sulfur has been developed. The reported methods have demonstrated good tolerance of various functional groups. Up to 28 2-amino-5-acylthiazole compounds bearing diverse structural differences were successfully synthesized from easily obtained starting materials with moderate to excellent yields. Our method provides an effective way for the access of valuable and potentially bioactive 2-amino-5-acylthiazole derivatives.

Transition metal-free functionalized hydration of alkynes: one-pot synthesis of fluorinated β-keto-imidates using Selectfluor

A transition metal-free, four-component one-pot synthesis of polyfunctionalized fluorinated β-keto-imidates via the functionalized hydration of alkynes has been described. The intermediate 1,3-ketoamino moiety was obtained from easily accessible arylpropioladehyde and arlyhydroxylamine and was treated with Selectfluor delivering fluorinated β-keto-imidates. A wide variety of functional groups are tolerated under mild reaction conditions and the product applicability is highlighted.

The synthesis of densely functionalised α-acyloxy enaminals and enaminones via a novel homogeneous silver(i) catalysed rearrangement

A synthesis of densely functionalised α-acyloxy enaminals and enaminones via a novel homogeneous silver(i) catalyzed rearrangement of 1-acyloxy-3-azido ketones is reported.

Metal-free, green and efficient oxidative α halogenation of enaminones by halo acid and DMSO

Metal free oxidative halogenation of N-aryl enaminones has been demonstrated using a DMSO–halo acid combination under mild reaction conditions. The present method is a straightforward approach and is also applied for the synthesis of chromenone derivatives in excellent yields.