Enaminone, Enaminoesters, and Related Compounds in the Metal-Free Synthesis of Pyridines and Fused Pyridines

The most Recent Compilation of Reactions of Enaminone Derivatives with various Amine Derivatives to Generate Biologically Active Compounds

Heterocyclic derivatives serve as the fundamental components of both natural and synthetic drugs. Enaminones play a crucial role as foundational units in the synthesis of numerous bioactive heterocyclic compounds, including pyrazoles, pyridines, oxazoles, isoxazoles, as well as fused heterocyclic structures like indoles, carbazoles, quinolines, acridines, and phenanthridines. These diverse heterocyclic rings are well-known for their various therapeutic activities, encompassing anticancer, anti-inflammatory, antimicrobial, antidepressant, and antiviral properties. By reacting with nitrogenbased nucleophiles, enaminones can generate bioactive azoles, azines, and their fused systems. This study focuses on the recent advancements in enaminone reactions with (a) nitrogen-based nucleophiles, such as aliphatic amines, derivatives of aniline, heterocyclic amines, hydroxylamine, hydrazine derivatives, guanidine derivatives, urea, and thiourea derivatives, and (b) nitrogen-based electrophiles, such as diazonium salts. These reactions have led to the synthesis of a wide range of bioactive fused heterocyclic compounds from 2010 to the end of 2022.

Synthesis, biological activities, and future perspectives of steroidal monocyclic pyridines

Steroidal pyridines are a class of compounds that have been the subject of extensive research in recent years due to their potential biological activities. The introduction of a pyridine ring into the steroid skeleton can significantly alter the chemical and biological properties of the compound, making it more potent and/or selective for a particular target. Different synthetic methods have been developed for the preparation of steroidal pyridines. This review provides an overview of the synthesis, biological activities, and future perspectives of steroidal monocyclic dihydropyridines, tetrahydropyridines, and pyridines from 2005 to the present. The different synthetic methods that have been developed for the preparation of these steroids are discussed, as well as the proposed mechanisms and the biological activities that have been reported. Finally, the potential of steroidal monocyclic pyridines for the development of new drugs is discussed. This review is intended to provide a comprehensive overview of the field of steroidal monocyclic pyridines for researchers and scientists who are interested in this area of research. It is also hoped that this review will stimulate further research into the synthesis and biological activities of steroidal pyridines to develop new and improved drugs for the treatment of diseases.

Enaminone Formation Drives the Coupling of Biosynthetic Pathways to Generate Cyclic Lipopeptides

A family of novel cyclic lipopeptides named tasikamides A-H (Tsk A-H) were discovered recently in Streptomyces tasikensis P46. Aside from the unique cyclic pentapeptide scaffold shared by the tasikamides, Tsk A-C contain a hydrazone bridge that connects the cyclic pentapeptide to the lipophilic alkyl 5-hydroxylanthranilate (AHA) moiety. Here we report the production of tasikamides I-K (Tsk I-K) by a mutant strain of S. tasikensis P46 that overexpresses two pathway-specific transcription regulators. Unlike Tsk A-C, Tsk I-K feature a rare enaminone-bridge that links the cyclic peptide scaffold to the AHA moiety. Our experimental data suggest that Tsk I-K are generated by the coupling of two biosynthetic pathways via a nonenzymatic condensation reaction between an arylamine and a β-keto aldehyde-containing precursor. The results underscore the nucleophilic and electrophilic reactivity of the β-keto aldehyde moiety and its ability to promote fragment coupling reactions in live microbial cells.

Thiazole-5-carbaldehyde Synthesis by Cascade Annulation of Enaminones and KSCN with Dess-Martin Periodinane Reagent

The Dess-Martin periodinane (DMP) reagent-mediated reactions of tertiary enaminones with potassium thiocyanate for the synthesis of thiazole-5-carbaldehydes are developed. The product formation involves cascade hydroxyl thiocyanation of the C═C double bond, intramolecular hydroamination of the C≡N bond, and thiazole annulation by condensation on the ketone carbonyl site, representing novel reaction pathways in the reactions between enaminones and thiocyanate salt. DMP plays dual roles in mediating the free radical thiocyanation and inducing the unconventional selective thiazole-5-carbaldehyde formation by masking the in situ generated formyl group during the reaction process.

Copper-catalyzed [3+3] annulation of ketones with oxime acetates for the synthesis of pyridines

A novel and efficient copper-catalyzed strategy for the synthesis of diverse pyridines through the [3+3] annulation of ketones with oxime acetates has been reported. It is very convenient to obtain various unsymmetrical 2,6-diarylpyridines by changing different substrates. The rare copper-catalyzed direct difunctionalization of saturated ketones for the synthesis of nitrogen heterocycles is developed. This protocol has excellent functional group tolerance, readily available raw materials, high chemoselectivity and broad substrate scope.

Synthesis, Structural Determination, and Antioxidant Activities of Acyclic and Substituted Heterocyclic Phosphonates Linearly Linked 4-hydroxy-2(1H)-quinolinone

The chemical reactivity of 3-[(E)-3-(dimethylamino)-2-propenoyl]-4-hydroxy-1-methy-2(1H)-quinolinone (1) towards some phosphorus reagents was studied. The enaminone 1 was cyclized into pyranoquinolinylphosphonate 2 via treatment with diethyl phosphite in basic medium. However, its reaction with triethoxy phosphonoacetate gave the substituted oxopyranylphosphonate 3. Using the same reaction conditions, both thioxopyridinylphosphonate 4 and oxopyranylphosphonate 5 were produced via a reaction of enaminone 1 with both diethyl 2-amino-2-thioxoethylphosphonate and diethyl vinylphosphonate, respectively, in low yields. In addition, the two novel oxopyridinylphosphonates 6 and 7 were obtained by treatment of enaminone 1 with a diethyl cyanomethylphosphonate reagent. Two oaxathiaphosphininyl derivatives, 8 and 9, were obtained by treatment of the enaminone 1 with O, O-diethyl dithiophosphoric acid under different reaction conditions. Diazaphosphininyl 11 and oxazaphosphininyl 12 derivatives were obtained in excellent yields using a P-phenylphosphonic diamide reagent under different reaction conditions. The treatment of the enaminone 1 with phosphorus pentasulfide produced the non-phosphorylated product thioxothiopyranoquinolinone 13. Finally, the enaminone was turned into oxathiaphosphininyl 14 using Lawesson’s reagent. The possible reaction mechanisms of the formation of these products were discussed. The structures of newly isolated products were established by elemental analysis and spectral tools. The compounds were evaluated for their antioxidant activities.

Recent Advances in Synthesis of Multiply Arylated/Alkylated Pyridines

Multiply aryl/alkyl-substituted pyridines are some of the untapped synthetic targets because of the challenge in regioselectively introducing less polar aryl/alkyl groups at the desired positions in the pyridine framework. Interestingly, the importance of this family of compounds has increased annually, particularly in biological and materials engineering applications. The syntheses of such pyridines have been extensively reported, but there is a lack of comprehensive review articles. Hence, this review discusses recent advances by grouping reaction patterns that generally deliver tri-, tetra-, and penta-aryl/alkyl pyridines.

Anticorrosive Effect of Halogenated Aniline Enaminoesters on Carbon Steel in HCl

Four enaminoesters derived from halogenated aniline, with potential anticorrosion activity, were synthesized and tested against carbon steel AISI 1020 in acid medium using 1.0 mol L-1 HCl. The synthesis was demonstrated through the reaction of ethyl acetoacetate with four different amines, in the presence of glacial acetic acid and molecular sieve, using ethanol as solvent for 24 h. The evaluation of the anticorrosive activity was performed using the gravimetric technique and electrochemical methods, such as electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), and potentiodynamic polarization (PP). Results indicated that the F-EN (Ethyl (2Z)-3-[(4-fluoro-phenyl)-amino]-but-2-enoate) inhibitor had higher corrosion inhibition efficiency, of 98% by mass loss, and 85% by electrochemical techniques. Adsorption obeyed the Langmuir isotherm, thus suggesting that the inhibitors form a monolayer film in metal surface. These results also contributed to the calculations of the physicochemical parameters of $E_{\text{a}}$ , $△{H^{\ne }}_{\text{ads}}$ , and $△{S^{\ne }}_{\text{ads}}$ , which confirmed the corrosion inhibition when compared to the absence of the inhibitors.

Synthesis of Highly Functionalized Pyridines: A Metal-Free Cascade Process

Herein, we report a new process for the synthesis of highly functionalized pyridines based on a tandem Pummerer-type rearrangement, aza-Prins cyclization, and elimination-induced aromatization. This formal [5+1] cyclization provides pyridines in good yields with easily accessible starting materials. The synthetic potential of our new method is further demonstrated in the modification of the frameworks of BINOL and some natural products.

Catalytic Acceptorless Dehydrogenation of Amino Alcohols and 2-Hydroxybenzyl Alcohols for Annulation Reaction under Neutral Conditions

A base-free and acceptorless Ru-catalyzed dehydrogenative approach has been developed for the synthesis of N-heterocycles by using 1,3-dicarbonyls and amino alcohols through a domino sequential enamine formation and intramolecular oxidative cyclization strategy. This unified approach is also applicable for the synthesis of O-heterocycles involving 2-hydroxybenzyl alcohol as a coupling reactant via consecutive C-alkylation and intramolecular cyclization steps. The present protocol is general for the synthesis of varieties of biologically important scaffolds, such as tetrahydro-4H-indol-4-one, 3,4-dihydroacridin-1(2H)-one, and tetrahydro-1H-xanthen-1-ones derivatives using a single catalytic system, viz. RuH₂CO(PPh₃)₃. Environmentally benign H₂O and H₂ are the only byproducts in this domino process. Moreover, RuH₂CO(PPh₃)₃-catalyzed C3-alkylation of tetrahydro-4H-indol-4-one using alcohol as a alkylating partner is also described in this report. For the first time, a solvent-free gram-scale reaction for the acceptorless dehydrogenative annulation has been demonstrated. A plausible mechanism for the Ru-catalyzed base-free and acceptorless dehydrogenative annulation of amino alcohols or 2-hydroxybenzyl alcohols has been provided with several experimental investigations and spectroscopic evidence.

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.

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.

Generation of (E)-β-sulfonyl enamines from sulfur dioxide via a radical process

An iron(ii)-catalyzed three-component reaction of O-acyl oximes, sulfur dioxide, and N-vinylacetamides is accomplished. Diverse (E)-β-sulfonyl enamines are obtained in moderate to good yields by using this protocol with excellent stereoselectivity and regioselectivity.

Pleiotropic Properties of Amphiphilic Dihydropyridines, Dihydropyridones, and Aminovinylcarbonyl Compounds

Three groups of synthetic lipids are chosen for studies: (1) 1,4-dihydropyridines (1,4-DHPs) containing two cationic moieties and their analogues; (2) 3,4-dihydro-2(1H)-pyridones containing a cationic moiety; and (3) acyclic, open-chain analogues, i.e., 2-amino-3-alkoxycarbonylalkylammonium derivatives. 1,4-DHPs possessing dodecyl alkyl chains in the ester groups in positions 3 and 5 and cationic nitrogen-containing groups in positions 2 and 6 have high cytotoxicity in cancer cells HT-1080 (human lung fibrosarcoma) and MH-22A (mouse hepatoma), but low cytotoxicity in the noncancerous NIH3T3 cells (mouse embryonic fibroblast). On the contrary, similar compounds having short (methyl, ethyl, or propoxyethyl) chains in the ester groups in positions 3 and 5 lack cytotoxicity in the cancer cells HT-1080 and MH-22A even at high doses. Inclusion of fluorine atoms in the alkyl chains in positions 3 and 5 of the DHP cycle decreases the cytotoxicity of the mentioned compounds. Structurally related dihydropyridones with a polar head group are substantially more toxic to normal and cancerous cells than the DHP analogues. Open-chain analogues of DHP lipids comprise the same conjugated aminovinylcarbonyl moiety and possess anticancer activity, but they also have high basal cytotoxicity. Electrochemical oxidation data demonstrate that oxidation potentials of selected compounds are in the range of 1.6-1.7 V for cationic 1,4-DHP, 2.0-2.4 V for cationic 3,4-dihydropyridones, and 1.2-1.5 V for 2-amino-3-alkoxycarbonylalkylammonium derivatives. Furthermore, the tested cationic 1,4-DHP amphiphiles possess antiradical activity. Molecular topological polar surface area values for the tested compounds were defined in accordance with the main fragments of compound structures. The determined logP values were highest for dodecyl ester groups in positions 3 and 5 of the 1,4-DHP and lowest for short alkyl chain-containing amphiphiles.

Aza-Henry and aza-Knoevenagel reactions of nitriles for the synthesis of pyrido[1,2-a]indoles

N-(Propargyl)indole-2-carbonitriles undergo DBU-catalyzed addition of CH-acids to nitriles, followed by cyclization to give 9-aminopyrido[1,2-a]indoles.

A copper-catalyzed insertion of sulfur dioxide via radical coupling

A copper-catalyzed reaction of O-acyl oximes, DABCO·(SO₂)₂, and 2H-azirines is developed under mild conditions, leading to diverse tetrasubstituted β-sulfonyl N-unprotected enamines with excellent stereoselectivity and regioselectivity.