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

Oxidative Free-Radical C(sp2)-H Bond Chlorination of Enaminones with LiCl: Access to Highly Functionalized α-Chlorinated Enaminones

An oxidative free-radical C(sp2)-H bond chlorination strategy of enaminones has been developed by using LiCl as a chlorinating reagent and K2S2O8 as an oxidant. This transformation provides a new and straightforward synthetic methodology to afford highly functionalized α-chlorinated enaminones with a Z-configuration in good to excellent yields.

Copper-Catalyzed Oxidative [3 + 2] Cycloaddition of Enamines and Pyridotriazoles toward Indolizines

An efficient copper catalytic system has been established for the synthesis of highly functional indolizines through oxidative [3 + 2] cycloaddition of enamines and pyridotriazoles. This modular platform is compatible with a broad range of functional groups, including natural and complex skeletons, allowing for late-stage modifications. It features a step-economical, highly regioselective, and easy-handling procedure and has been applied in constructing small molecules of potent activity toward inhibiting the VEGF-NRP1 interaction through a one-pot reaction of pyridotriazoles, amines, and aldehydes.

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.

Tf2O-Mediated Tandem Reaction of Enaminones for the Synthesis of Functionalized Conjugated-Enals/β-Naphthalaldehydes

A highly efficient and regioselective method for constructing functionalized conjugated enals via the Tf2O-mediated tandem reaction of enaminones with thiophenols has been described. Chain products with excellent stereoselectivity could be obtained through substrate regulation. Additionally, a feasible method for synthesizing β-naphthalaldehydes through PhSO2Na/DABCO promoting hydrogen atom transfer process has also been reported here. Mechanism studies have shown that 2-formyl vinyl triflate 8 and sulfonylated enal 9 were the key intermediates in this process.

Synthesis of functionalized tetrahydrodibenzo[b,g][1,8]naphthyridin-1(2H)-ones through base-promoted annulation of quinoline-derived dipolarophiles and cyclic enaminones

An eco-friendly and metal-free method for the synthesis of tetrahydrodibenzo[b,g][1,8]naphthyridin-1(2H)-ones was established. Quinoline-derived dipolarophiles and cyclic enaminones as starting materials undergo a 1,4-Michael addition/SNAr tandem annulation reaction affording the target products. This approach features transition metal-free conditions, good functional group tolerance and operational simplicity.

Mn(OAc)2-promoted [3+2] cyclization of enaminone with isocyanoacetate: Rapid access to pyrrole-2-carboxylic ester derivatives with potent anticancer activity

The practical and facile Mn(OAc)2-promoted [3+2] cycloaddition reaction of enaminones with isocyanoacetate was developed, that delivered a diversity of 3-aroyl pyrrole-2-carboxylic esters with broad substrates scope. The most of the newly synthesized compounds exhibit moderate antiproliferative activity against four cancer cells. Notably, compound 2n demonstrate the most potent activity with average IC50 values of 5.61 μM against four distinct cancer cell lines. Moreover, 2n exhibit favorable anti-migration activity and drug-like properties. The further investigation suggests that compound 2n possesses the ability to inhibit ERK5 activity and exhibits effective binding with the ERK5 protein, making it a promising candidate as a lead compound for a new class of ERK5 inhibitors discovery.

Synthesis of Polysubstituted Furans via Rh(II)-Catalyzed [2 + 3] Annulation of N-Sulfonyl-1,2,3-triazoles with Enaminones

An unprecedented [2 + 3] annulation of N-sulfonyl-1,2,3-triazoles with enaminones is reported for the access of polysubstituted furans. The key to the success of the transformations lies in the use of Rh(II)-Brønsted acid as cooperative catalysts. Unlike the conventional annulations of N-sulfony-l-1,2,3-triazoles, the Rh(II)-azavinyl carbenes species play dual functions in this work, enabled by the cleavage of the C(sp2)-N bond. The mechanism studies suggested that an intermolecular rearrangement of the TsNH- group is crucial to the annulation process.

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.

Rh-Catalyzed C-H Activation/Annulation of Enaminones and Cyclic 1,3-Dicarbonyl Compounds: An Access to Isocoumarins

A facile access to isocoumarins has been established via rhodium(III)-catalyzed C-H bond activation and intramolecular C-C cascade annulation of enaminones and cyclic 1,3-dicarbonyl compounds. The synthetic protocol features a wide range of substrates with high functional group tolerance, mild reaction conditions, and the selective cleavage of the enaminone C-C bond. Notably, the cyclic 1,3-dicarbonyl compounds can in situ-generate iodonium ylide as a carbene precursor to prepare polycyclic scaffolds by reacting with PhI(OAc)₂. The application of this method to prepare useful synthetic precursors and bioactive skeletons is also exemplified.

Three-Component Fusion to Pyrazolo[5,1-a]isoquinolines via Rh-Catalyzed Multiple Order Transformation of Enaminones

A facile and practical method for the synthesis of fused tricyclic pyrazolo[5,1-a]isoquinolines has been realized via the reactions of enaminones, hydrazine hydrochloride, and internal alkynes. By means of Rh catalysis, the extraordinary high-order bond functionalization, including the transformation of aryl C-H, ketone C═O, and alkenyl C-N bonds in the enaminones, marks the major feature of the cascade reactions. The results disclose the individual advantage of enaminones in the design of novel and efficient synthetic methods.

Synthesis of Indolizines via Tf2O-Mediated Cascade Reaction of Pyridyl-enaminones with Thiophenols/Thioalcohols

A cost-effective, highly regioselective and metal-free version for the synthesis of indolizine derivatives by means of Tf₂O-mediated cascade reaction of pyridyl-enaminones and thiophenols/thioalcohols under mild reaction conditions has been reported. Diverse electron-rich indolizine derivatives could be obtained in up to 94% yield via the selective 1,4-addition of vinyl iminium triflate tandem cyclization/aromatization, which allowed the simultaneous construction of C-N and C-S/and one example of C-Se bonds.

B2pin2-Mediated Cascade Cyclization/Aromatization Reaction: Facial Access to Functionalized Indolizines

Herein, a B₂pin₂-mediated radical cascade cyclization/aromatization reaction of enaminone with pyridine is described. This strategy provides a practical way for the construction of valuable functionalized indolizines under metal-, external oxidant-, and base-free conditions, which could be compatible with various kinds of functional groups, such as halogen, π-system, heterocycle, ferrocenyl, etc. A preliminary mechanism investigation indicated that the pyridine-boryl radical formed in situ triggered the reaction to occur.

Electrochemical Synthesis of Polysubstituted Sulfonated Pyrazoles via Cascade Intermolecular Condensation, Radical-Radical Cross Coupling Sulfonylation, and Pyrazole Annulation

Electrochemical synthesis of polysubstituted sulfonated pyrazoles from enaminones and sulfonyl hydrazides was established under metal-free, exogenous-oxidant-free, and mild conditions. By judicious choice of different electrochemical reaction conditions, NH₂-functionalized enaminones or N,N-disubstituted enaminones can react with aryl/alkyl sulfonyl hydrazides to afford tetra- or trisubstituted sulfonated pyrazoles in moderate to good yields, respectively. The gram-scale electrochemical transformation demonstrated the efficiency and practicability of this synthetic strategy. In addition, the sulfonated NH-pyrazole can be obtained via the dissociation of the N-tosyl group. Mechanistic studies reveal that the electrochemical cascade reaction synthesis of polysubstituted sulfonated pyrazoles proceeded via the sequence of intermolecular condensation, radical-radical cross coupling sulfonylation, and pyrazole annulation.

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.

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.

Cascade Reaction of Tertiary Enaminones, KSCN, and Anilines: Temperature-Controlled Synthesis of 2-Aminothiazoles and 2-Iminothiazoline

A one-pot cascade strategy for the synthesis of 2-aminothiazole derivatives by tertiary enaminones, KSCN, and anilines was developed. By changing the reaction temperature, the three-component reaction could be transformed in different ways to obtain moderate to good yields of polysubstituted 2-aminothiazoles and 2-iminothiazolines. This protocol provides an efficient and concise approach to accessing 2-aminothiazole derivatives with potential bioactivity from readily accessible building blocks and reagents.

Regiodivergent Synthesis of 3,4- and 4,5-Disubstituted N-Methylpyrazoles from 4-Acyl-1H-pyrrole-2,3-dione and Methylhydrazine

A simple, efficient and highly regioselective method for the preparation of 3,4- and 4,5-disubstituted N-methylpyrazoles in a one-pot procedure is reported. The methodology developed was based on the regiochemical control of the reaction of 4-acyl-1H-pyrrole-2,3-diones and methylhydrazine with an influence of the addition or absence of acid and the substrate structure.

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.

Design, synthesis, and fungicidal activity of novel N-substituted piperazine-containing phenylpyridines against cucumber downy mildew

BACKGROUND

Cucumber downy mildew (CDM) is a severe plant disease and affects the yield of cucumber production worldwide. As the resistance toward conventional fungicides is emerging as a ubiquitous issue, it is urgent to discover efficient fungicides with unique structures.

RESULTS

In this study, a series of novel phenylpyridine derivatives were designed and synthesized. Bioassays revealed that most of these compounds possessed excellent fungicidal activities against CDM. Among the phenylpyridine compounds, 2-(4-(4-(tert-butyl)benzyl)piperazin-1-yl)-6-phenylnicotinonitrile (C8) [half-maximal effective concentration (EC₅₀ ) = 4.40 mg L^-1 ] displayed the highest activity, which was better than those of the commercial fungicides, such as azoxystrobin (EC₅₀  = 42.77 mg L^-1 ) and flumorph (EC₅₀  = 41.94 mg L^-1 ). Furthermore, the molecular electrostatic potential of high-activity compound C8 indicated that nitrogen atom of the cyano group on the pyridine ring was in the negative region and may easily form hydrogen bonds and allow for electrostatic interactions with potential receptors.

CONCLUSIONS

This study demonstrated that the novel N-substituted piperazine-containing phenylpyridine derivatives could be further developed as a candidate compound to control CDM.

Base-Promoted Annulative Difluoromethylenation of Enaminones with BrCF2CO2Et toward 2,2-Difluorinated 2,3-Dihydrofurans

A practical method for the synthesis of 2,2-difluorinated 2,3-dihydrofurans has been established via the [4 + 1] annulation of enaminones and BrCF₂CO₂Et with Na₂CO₃ promotion. This new protocol does not employ any transition metal reagent and enables the annulative difluoromethylation by the partial cleavage of the C═C double bond. In addition, the further treatment with hydrochloric acid in one pot leads to β-keto enoic acids (4-oxo-2-butenoic acids) via a formal enaminone C-N carboxylation.

PhIO-Mediated Oxidative C═C Bond Cleavage and Reassembly toward Highly Functionalized Oxazolones

An efficient PhIO-mediated oxidative C═C bond cleavage and reassembly of enaminone toward oxazolone with high regioselectivity has been reported. DFT calculations revealed that the reaction proceeded through an oxygen atom transfer, C═C bond cleavage, alkylthio migration, and reassembly cascade. This strategy is highlighted by high atom and step economy with formation of five bonds in one pot and generation of a high-valued oxazolone skeleton under mild conditions.

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.

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.

Ultrasound-assisted synthesis of pyrimidines and their fused derivatives: A review

The pyrimidine scaffold is present in many bioactive drugs; therefore, efficient synthetic routes that provide shorter reaction times, higher yields, and site-selective reactions are constantly being sought. Ultrasound (US) irradiation has emerged as an alternative energy source in the synthesis of these heterocyclic scaffolds, and over the last ten years there has been a significant increase in the number of publications mentioning US in either the construction or derivatization of the pyrimidine core. This review presents a detailed summary (with 140 references) of the effects of US (synergic or not) on the construction and derivatization of the pyrimidine core through classical reactions (e.g., multicomponent, cyclocondensation, cycloaddition, and alkylation reactions). The main points that were taken into consideration are as follows: chemo- and regioselectivity issues, and the results of conventional heating methods compared to US and mechanistic insights that are also presented and discussed for key reactions.

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).

Rh(III)-Catalyzed [5 + 1] Annulation of Indole-enaminones with Diazo Compounds To Form Highly Functionalized Carbazoles

A novel Rh(III)-catalyzed C-H activation/annulation cascade of indole-enaminones with diazo compounds was reported to construct diversely functionalized carbazole frameworks. The most notable characteristic is that this transformation could smoothly furnish a novel [5 + 1] cyclization product with good to excellent yields (up to 95%), accompanied by the thorough removal of acetyl and N,N-dimethyl groups of two substrates from the target products, rather than the normally expected [4 + 2] cyclization products.

Highly Regioselective and Chemoselective [3 + 3] Annulation of Enaminones with ortho-Fluoronitrobenzenenes: Divergent Synthesis of Aposafranones and Their N-Oxides

A base-promoted unprecedented strategy for the regioselective and chemoselective divergent synthesis of highly functionalized aposafranones and their N-oxides has been developed from the [3 + 3] annulation of enaminones with o-fluoronitrobenzenenes. This novel synthetic strategy offers an alternative method for the construction of aposafranones and their N-oxides are meaningful in the fields of both biology and organic synthesis. The established protocol explores the annulation scope of enaminones, and it expands the application of nitro-based cyclization.

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.

Synthesis of 6,7-Dihydro-1H,5H-pyrazolo[1,2-a]pyrazoles by Azomethine Imine-Alkyne Cycloadditions Using Immobilized Cu(II)-Catalysts

A series of 12 silica gel-bound enaminones and their Cu(II) complexes were prepared and tested for their suitability as heterogeneous catalysts in azomethine imine-alkyne cycloadditions (CuAIAC). Immobilized Cu(II)-enaminone complexes showed promising catalytic activity in the CuAIAC reaction, but these new catalysts suffered from poor reusability. This was not due to the decoordination of copper ions, as the use of enaminone ligands with additional complexation sites resulted in negligible improvement. On the other hand, reusability was improved by the use of 4-aminobenzoic acid linker, attached to 3-aminopropyl silica gel via an amide bond to the enaminone over the more hydrolytically stable N-arylenamine C-N bond. The study showed that silica gel-bound Cu(II)-enaminone complexes are readily available and suitable heterogeneous catalysts for the synthesis of 6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazoles.