An In-Depth Exploration of Six Decades of the Kröhnke Pyridine Synthesis.

The Kröhnke Pyridine Synthesis has been discovered about six decades ago (1961), by Fritz Kröhnke and Wilfried Zecher at the University of Giessen. The original method involved the reaction of α-pyridinium methyl ketone salts with α,ß-unsaturated carbonyl compounds in the presence of a nitrogen source, frequently ammonium acetate. Since its discovery, the Kröhnke methodology has been demonstrated to be suitable for the preparation of mono-, di-, tri- and tetra-pyridines, with important applications in several research fields. Over the years, a number of modifications to the original approach have been developed and reported, enabling for the broad applicability of these methods even in modern days, also for the synthesis of non-pyridine compounds. In this critical and tutorial review, we will thoroughly explore and discuss the potential of the original method, the refinements that have been made over the years, as well as some applications arising from each type of pyridine and/or non-pyridine compounds produced by Kröhnke's approach.

The proof of concept of 2-{3-[N-(1-benzylpiperidin-4-yl)propyl]amino}-6-[N-methyl-N-(prop-2-yn-1-yl)amino]-4-phenylpyridine-3,5-dicarbonitrile for the therapy of neuropathic pain.

In the search for new small molecules for the therapy of neuropathic pain, we found that 2-{3-[N-(1-benzylpiperidin-4-yl)propyl]amino}-6-[N-methyl-N-(prop-2-yn-1-yl)amino]-4-phenylpyridine-3,5-dicarbonitrile (12) induced a robust antiallodynic effect in capsaicin-induced mechanical allodynia, a behavioural model of central sensitization, through σ1R antagonism. Furthermore, administration of compound 12 to neuropathic animals, fully reversed mechanical allodynia, increasing its mechanical threshold to levels that were not significantly different from those found in paclitaxel-vehicle treated mice or from basal levels before neuropathy was induced. Ligand 12 is thus a promising hit-compound for the therapy of neuropathic pain.

Discovery of indoleninyl-pyrazolo[3,4-b]pyridines as potent chemotherapeutic agents against colorectal cancer cells.

A new series of indolenines decorated with pyrazolo[3,4-b]pyridines were designed and synthesized in up to 96% yield from the acid-catalyzed cyclocondensation of 1,3-dialdehydes with 3-aminopyrazoles. X-ray crystallography on a representative derivative, 5n, revealed two close to planar conformations whereby the N-atom of the pyridyl residue was syn or anti to the pyrrole-N atom in the two independent molecules of the asymmetric unit. The computational and DNA binding data suggest that 5n is a strong DNA intercalator with the results in agreement with its potent cytotoxicity against two colorectal cancer cell lines (HCT 116 and HT-29). In contrast to doxorubicin, compounds 5k-o have higher druggability (compliance to more criteria stated in Lipinski's rule of five and Veber's rule), higher bioavailability, and better medicinal chemistry properties, indicative of their potential application as chemotherapeutical agents.

Identification of indole-grafted pyrazolopyrimidine and pyrazolopyridine derivatives as new anti-cancer agents: Synthesis, biological assessments, and molecular modeling insights.

In the current medical era, developing new PIM-1 inhibitors stands as a significant approach to cancer management due to the pivotal role of PIM-1 kinase in promoting cell survival, proliferation, and drug resistance in various cancers. This study involved designing and synthesizing new derivatives of pyrazolo[1,5-a]pyrimidines (6a-i) and pyrazolo[3,4-b]pyridines (10a-i) as potential anti-cancer agents targeting PIM-1 kinase. The cytotoxicity was screened on three cancer cell lines: A-549 (lung), PANC-1 (pancreatic), and A-431 (skin), alongside MRC5 normal lung cells to assess selectivity. Several pyrazolo[1,5-a]pyrimidines (6b, 6c, 6g, 6h, and 6i) and pyrazolo[3,4-b]pyridine (10f) demonstrated notable anticancer properties, particularly against A-549 lung cancer cells (IC50 range: 1.28-3.52 µM), also they exhibited significantly lower toxicity towards MRC5 normal cells. Thereafter, the compounds were evaluated for their inhibitory activity against PIM-1 kinase. Notably, 10f, bearing a 4-methoxyphenyl moiety, demonstrated good inhibition of PIM-1 with an IC50 of 0.18 µM. Additionally, 10f induced apoptosis and arrested cell cycle progression in A-549 cells. Molecular docking and dynamics simulations provided insights into the binding interactions and compounds' stability with PIM-1 kinase. The results highlight these compounds, especially 10f, as promising selective anticancer agents targeting PIM-1 kinase.

Copper (II) complex supported on magnetic nanoparticles as a novel nanocatalyst for the synthesis of imidazo[1,2-a]pyridines.

Research on the synthesis of imidazo[1,2-a]pyridines has gained great importance among synthetic chemists because there have been numerous reports of their biological and medicinal activities. In this respect, we fabricated CuCl2 immobilized on Fe3O4 nanoparticles modified with 1,10-phenanthroline-5,6-diol [Fe3O4@Diol/Phen-CuCl2] and investigated its catalytic activity for the preparation of imidazo[1,2-a]pyridine derivatives through one-pot three-component reaction of 2-aminopyridines, aldehydes and terminal alkynes under ecofriendly conditions. FT-IR spectroscopy, EDX, SEM, TEM, XRD, TGA, VSM and ICP-OES techniques employed in order to identify the structure of the as-constructed Fe3O4@Diol/Phen-CuCl2 nanocatalyst. This catalytic system has a series of advantages such as the synthesis of imidazo[1,2-a]pyridine products with high yields in suitable time, performing the reactions in an environmentally friendly solvent (PEG), easy preparation of the catalyst with a simple method, and the recyclability of the Fe3O4@Diol/Phen-CuCl2 nanocatalyst.

Synthesis, spectroscopic characterization, DFT calculations, in silico-ADMET and molecular docking analysis of novel quinoline-substituted 5H-chromeno [2,3-b] pyridine derivatives as antibacterial agents.

A convenient, straightforward, and effective one-step reaction for the synthesis of a three-component compound of biologically relevant novel 2,4-diamino-5-(8-hydroxyquinolin-7-yl)-5H-chromeno[2,3-b] pyridine-3-carbonitrile derivatives was designed and synthesized. The synthesis was developed by the reaction between salicylaldehyde 1, 8-hydroxyquinoline 2, 2-aminopropene-1,1,3-tricarbonitrile 3, and the catalytic amount of triethylamine in ethanol at 78 °C. This methodology has many beneficial features, including the use of inexpensive and non-hazardous starting materials, single-flask reactions, optimized reaction conditions, the termination of intermediate isolation, easy workup, reducing organic waste products, being chromatography-free, and decreasing the reaction time along with quantitative yields with high functional group tolerance. A proposed mechanism with supporting experimental data is presented, including 1H NMR, 13C NMR, 2D NMR (HMBC, COSY, HSQC), mass, and IR spectroscopy, which are used to characterize the complete derivatives. All synthesized compounds were evaluated in vitro for their antibacterial activities against Bacillus subtilis, Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa bacterial strains via the agar-well diffusion method compared with the reference drug gentamicin. The data indicated that compounds 4A, 4F, 4G, 4 J, and 4K consistently demonstrated strong antimicrobial activity against Gram-positive and Gram-negative bacteria. Furthermore, a molecular docking investigation was carried out to gain insight into the binding mode of the most promising compounds via the crystal structure of the S. aureus DNA gyrase complex with ciprofloxacin (PDB ID: 2XCT). Density functional theory (DFT) calculations were performed to determine the various molecular properties of the synthesized novel 2,4-diamino-5-(8-hydroxyquinolin-7-yl)-5H-chromeno [2,3-b] pyridine-3-carbonitrile derivatives (4A-4 M). On the basis of the reactive sites explored by the molecular electrostatic potential maps, the antibacterial activities of the compounds were screened.

Synthesis, Evaluation and Docking Studies of Disubstituted N-Heterocyclic Derivatives as Anticancer Agents.

Cancer is a chronic disease reported with alarming rates of mortalities every year.  Herein, we reported the synthesis of nitrogen based novel heterocyclic disubstituted derivatives and evaluated them against L929 and A549 cell lines using MTT assay. Among all, 6a2 and 6c1 were significantly active against L929 with IC50 value of 2.61±9.58 and 2.64±8.97 µg/mL respectively. Compounds 6a2 and 6c1 were also active against A549 with IC50 value of 2.36±9.20 and 2.43±6.28 µg/mL respectively and were found to be more potent than the standard drug Doxorubicin. A molecular docking study of the active compounds was also done against EGFR, conferring good binding affinity and binding interactions. Further biological investigations may provide valuable insights towards exploring the therapeutic potential of the active compounds in future.

Conformation-Associated C···dz2-PtII Tetrel Bonding: The Case of Cyclometallated Platinum(II) Complex with 4-Cyanopyridyl Urea Ligand.

The nucleophilic addition of 3-(4-cyanopyridin-2-yl)-1,1-dimethylurea (1) to cis-[Pt(CNXyl)2Cl2] (2) gave a new cyclometallated compound 3. It was characterized by NMR spectroscopy (1H, 13C, 195Pt) and high-resolution mass spectrometry, as well as crystallized to obtain two crystalline forms (3 and 3·2MeCN), whose structures were determined by X-ray diffraction. In the crystalline structure of 3, two conformers (3A and 3B) were identified, while the structure 3·2MeCN had only one conformer 3A. The conformers differed by orientation of the N,N-dimethylcarbamoyl moiety relative to the metallacycle plane. In both crystals 3 and 3·2MeCN, the molecules of the Pt(II) complex are associated into supramolecular dimers, either {3A}2 or {3B}2, via stacking interactions between the planes of two metal centers, which are additionally supported by hydrogen bonding. The theoretical consideration, utilizing a number of computational approaches, demonstrates that the C···dz2(Pt) interaction makes a significant contribution in the total stacking forces in the geometrically optimized dimer [3A]2 and reveals the dz2(Pt)→π*(PyCN) charge transfer (CT). The presence of such CT process allowed for marking the C···Pt contact as a new example of a rare studied phenomenon, namely, tetrel bonding, in which the metal site acts as a Lewis base (an acceptor of noncovalent interaction).

6-Amino-4-aryl-7-phenyl-3-(phenylimino)-4,7-dihydro-3H-[1,2]dithiolo[3,4-b]pyridine-5-carboxamides: Synthesis, Biological Activity, Quantum Chemical Studies and In Silico Docking Studies.

New [1,2]dithiolo[3,4-b]pyridine-5-carboxamides were synthesized through the reaction of dithiomalondianilide (N,N'-diphenyldithiomalondiamide) with 3-aryl-2-cyanoacrylamides or via a three-component reaction involving aromatic aldehydes, cyanoacetamide and dithiomalondianilide in the presence of morpholine. The structure of 6-amino-4-(2,4-dichloro- phenyl)-7-phenyl-3-(phenylimino)-4,7-dihydro-3H-[1,2]dithiolo[3,4-b]pyridine-5-carboxamide was confirmed using X-ray crystallography. To understand the reaction mechanism in detail, density functional theory (DFT) calculations were performed with a Grimme B97-3c composite computational scheme. The results revealed that the rate-limiting step is a cyclization process leading to the closure of the 1,4-dihydropyridine ring, with an activation barrier of 28.8 kcal/mol. Some of the dithiolo[3,4-b]pyridines exhibited moderate herbicide safening effects against 2,4-D. Additionally, ADMET (Absorption, Distribution, Metabolism, Excretion, Toxicity) parameters were calculated and molecular docking studies were performed to identify potential protein targets.

Linear and Angular Heteroannulated Pyridines Tethered 6-Hydroxy-4,7-Dimethoxybenzofuran: Synthesis and Antimicrobial Activity.

2-Chloropyridine-3-carbonitrile derivative 1 was utilized as a key precursor to build a series of linear and angular annulated pyridines linked to a 6-hydroxy-4,7-dimethoxybenzofuran moiety. Reaction of substrate 1 with various hydrazines afforded pyrazolo[3,4-b]pyridines. Treatment of substrate 1 with 1,3-N,N-binucleophiles including 3-amino-1,2,4-triazole, 5-amino-1H-tetrazole, 3-amino-6-methyl-1,2,4-triazin-5(4H)-one and 2-aminobenzimidazole produced the novel angular pyrido[3,2-e][1,2,4]triazolo[4,3-a]pyrimidine, pyrido[3,2-e][1,2,4]tetrazolo[1,5-a]pyrimidine, pyrido[3',2':5,6] pyrimido[2,1-c][1,2,4]triazine and benzo[4,5]imidazo[1,2-a]pyrido[3,2-e]pyrimidine, respectively. Reaction of substrate 1 with 1,3-C,N-binucleophiles including cyanoacetamides and 1H-benzimidazol-2-ylacetonitrile furnished 1,8-naphthyridines and benzoimidazonaphthyridine. Moreover, reacting substrate 1 with 5-aminopyrazoles gave pyrazolo[3,4-b][1,8]naphthyridines. Finally, reaction of compound 1 with 6-aminouracils as cyclic enamines yielded pyrimido[4,5-b][1,8]naphthyridines. Some of the synthesized products showed noteworthy antimicrobial efficiency against all types of microbial strains. Structures of the produced compounds were established using analytical and spectroscopic tools.

Serendipitous Identification of Azine Anticancer Agents Using a Privileged Scaffold Morphing Strategy.

The use of privileged scaffolds as a starting point for the construction of libraries of bioactive compounds is a widely used strategy in drug discovery and development. Scaffold decoration, morphing and hopping are additional techniques that enable the modification of the chosen privileged framework and better explore the chemical space around it. In this study, two series of highly functionalized pyrimidine and pyridine derivatives were synthesized using a scaffold morphing approach consisting of triazine compounds obtained previously as antiviral agents. Newly synthesized azines were evaluated against lymphoma, hepatocarcinoma, and colon epithelial carcinoma cells, showing in five cases acceptable to good anticancer activity associated with low cytotoxicity on healthy fibroblasts. Finally, ADME in vitro studies were conducted on the best derivatives of the two series showing good passive permeability and resistance to metabolic degradation.

Imidazopyridine Family: Versatile and Promising Heterocyclic Skeletons for Different Applications.

In recent years, there has been increasing attention focused on various products belonging to the imidazopyridine family; this class of heterocyclic compounds shows unique chemical structure, versatile optical properties, and diverse biological attributes. The broad family of imidazopyridines encompasses different heterocycles, each with its own specific properties and distinct characteristics, making all of them promising for various application fields. In general, this useful category of aromatic heterocycles holds significant promise across various research domains, spanning from material science to pharmaceuticals. The various cores belonging to the imidazopyridine family exhibit unique properties, such as serving as emitters in imaging, ligands for transition metals, showing reversible electrochemical properties, and demonstrating biological activity. Recently, numerous noteworthy advancements have emerged in different technological fields, including optoelectronic devices, sensors, energy conversion, medical applications, and shining emitters for imaging and microscopy. This review intends to provide a state-of-the-art overview of this framework from 1955 to the present day, unveiling different aspects of various applications. This extensive literature survey may guide chemists and researchers in the quest for novel imidazopyridine compounds with enhanced properties and efficiency in different uses.

C3-Alkylation of Imidazo[1,2-a]pyridines via Three-Component Aza-Friedel-Crafts Reaction Catalyzed by Y(OTf)3.

As an important class of nitrogen-containing fused heterocyclic compounds, imidazo[1,2-a]pyridines often exhibit significant biological activities, such as analgesic, anticancer, antiosteoporosis, anxiolytic, etc. Using Y(OTf)3 as a Lewis acid catalyst, a simple and efficient method has been developed for the synthesis of C3-alkylated imidazo[1,2-a]pyridines through the three-component aza-Friedel-Crafts reaction of imidazo[1,2-a]pyridines, aldehydes, and amines in the normal air atmosphere without the protection of inert gas and special requirements for anhydrous and anaerobic conditions. A series of imidazo[1,2-a]pyridine derivatives were obtained with moderate to good yields, and their structures were confirmed by 1H NMR, 13C NMR, and HRMS. Furthermore, this conversion has the advantages of simple operation, excellent functional group tolerance, high atomic economy, broad substrate scope, and can achieve gram-level reactions. Notably, this methodology may be conveniently applied to the further design and rapid synthesis of potential biologically active imidazo[1,2-a]pyridines with multifunctional groups.

Meta-Selective Copper-Catalyzed C-H Arylation of Pyridines and Isoquinolines through Dearomatized Intermediates.

C(sp2)-H functionalization offers an efficient strategy for the synthesis of various elaborated N-containing heteroarenes. Along these lines, oxazino pyridines that can be readily prepared from pyridines, have been introduced as powerful substrates in radical- and ionic-mediated meta-C-H functionalization. However, the regioselective meta-C-H arylation of pyridines remains a great challenge. Herein, a copper-catalyzed meta-selective C-H arylation of pyridines and isoquinolines through bench-stable dearomatized intermediates is reported. Electrophilic aryl-Cu(III) species, generated from readily accessible aryl I(III) reagents, enable the efficient meta-arylation of a broad range of pyridines and isoquinolines. The method also allows the meta-selective alkenylation of these heteroarenes using the corresponding alkenyl I(III)-reagents. Late-stage arylation of drug-derived pyridines and larger-scale experiments demonstrate the potential of this synthetic methodology.

Reactivity and Steric Parameters from 2D to 3D Bulky Pyridines: Increasing Steric Demand at Nitrogen with Chiral Azatriptycenes.

Sterically hindered pyridines embedded in a three-dimensional triptycene framework have been synthesized, and their resolution by chiral HPLC enabled access to unprecedented enantiopure pyridines exceeding the known steric limits. The design principles for new axially chiral pyridine derivatives are then described. To rationalize their associations with Lewis acids and transition metals, a comprehensive determination of the steric and electronic parameters for this new class of pyridines was performed. This led to the general parameterization of the steric parameters (percent buried volume %VBur, Tolman cone angle θ, and He8_steric descriptor) for a large set of two- and three-dimensional pyridine derivatives. These parameters are shown to describe quantitatively their interactions with carbon- and boron-centered Lewis acids and were used to predict the ΔG° of association with the prototypical B(C6F5)3 Lewis acid widely used in frustrated Lewis pair catalysis. This first parameterization of pyridine sterics is a fundamental basis for the future development of predictive reactivity models and for guiding new applications of bulky and chiral pyridines in organocatalysis, frustrated Lewis pairs, and transition-metal catalysis.

Accessing Pyridines via a Nitrene Internalization Process.

Pyridines are valuable pharmacophores, and their access via direct and selective transmutation of carbon atom with desired nitrogen could become crucial in drug discovery processes. However, only scarce examples can be found when it comes C-to-N-transmutation reactions of aromatics that could lead to the facile synthesis of pyridines or other azaarenes. In this context, Levin and co-workers recently disclosed a process leading to pyridines from the corresponding aryl azides via the regioselective nitrene internalization process. Notably, the transformation did not lead to any further modification of the rest of the aromatic skeleton. This innovative work enabled selectively accessing various pyridine derivatives through direct nitrogen scan operations on benzene derivatives, which were otherwise not feasible.

Copper-Catalyzed C4-selective Carboxylation of Pyridines with CO2 via Pyridylphosphonium Salts.

Pyridine motifs are widespread pharmacophores in many drugs. Installing various substituents through pyridine C-H bond functionalization is significant for new drug design and discovery. Developments of late-stage functionalization reactions enrich the strategies for selective functionalization of pyridines. However, late-stage C-H carboxylation of pyridines is a long-standing challenge, especially selectively carboxylation with CO2 on pyridine motifs. Herein, we describe a practical method for C4-H carboxylation of pyridines via one-pot C-H phosphination and copper-catalyzed carboxylation of the resulted phosphonium salts with CO2 . The reaction is conducted under mild conditions and compatible with multiple active groups and several pyridine drugs, providing diverse valuable isonicotinic acid compounds, demonstrating the application potential of this strategy.

The Groebke-Blackburn-Bienaymé reaction in its maturity: innovation and improvements since its 21st birthday (2019-2023).

The Groebke-Blackburn-Bienaymé (GBB) three-component reaction, discovered in 1998, is a very efficient strategy to assemble imidazo[1,2-a]-heterocycles starting from amidines, aldehydes and isocyanides. This review aims to exhaustively describe innovative aspects of this reaction achieved during the last five years, and classifies them into five categories: synthetic methods, building blocks, scaffolds, biological activities and physical properties.

Mild and efficient synthesis and base-promoted rearrangement of novel isoxazolo[4,5-b]pyridines.

An efficient method for the synthesis of isoxazolo[4,5-b]pyridines has been developed on the basis of readily available 2-chloro-3-nitropyridines via the intramolecular nucleophilic substitution of the nitro group as a key step. The previously unknown base-promoted Boulton-Katritzky rearrangement of isoxazolo[4,5-b]pyridine-3-carbaldehyde arylhydrazones into 3-hydroxy-2-(2-aryl[1,2,3]triazol-4-yl)pyridines was observed.

Synthesis, Molecular Docking, and Dynamic Simulation Targeting Main Protease (Mpro) of New, Thiazole Clubbed Pyridine Scaffolds as Potential COVID-19 Inhibitors.

Many biological activities of pyridine and thiazole derivatives have been reported, including antiviral activity and, more recently, as COVID-19 inhibitors. Thus, in this paper, we designed, synthesized, and characterized a novel series of N-aminothiazole-hydrazineethyl-pyridines, beginning with a N'-(1-(pyridine-3-yl)ethylidene)hydrazinecarbothiohydrazide derivative and various hydrazonoyl chlorides and phenacyl bromides. Their Schiff bases were prepared from the condensation of N-aminothiazole derivatives with 4-methoxybenzaldehyde. FTIR, MS, NMR, and elemental studies were used to identify new products. The binding energy for non-bonding interactions between the ligand (studied compounds) and receptor was determined using molecular docking against the SARS-CoV-2 main protease (PDB code: 6LU7). Finally, the best docked pose with highest binding energy (8a = -8.6 kcal/mol) was selected for further molecular dynamics (MD) simulation studies to verify the outcomes and comprehend the thermodynamic properties of the binding. Through additional in vitro and in vivo research on the newly synthesized chemicals, it is envisaged that the achieved results will represent a significant advancement in the fight against COVID-19.