6-(Aryldiazenyl)pyrazolo[1,5-a]pyrimidines as Strategic Intermediates for the Synthesis of Pyrazolo[5,1-b]purines

p-TSA catalyzed 6-endo-trig/dig cyclization of 5-aminopyrazoles and 3°/2°-propargylic alcohols: access to pyrazolo[1,5-a]dihydropyrimidines and pyrazolo[3,4-b]pyridines

A facile, straightforward synthesis of fused pyrazolo[1,5-a]dihydropyrimidines and pyrazolo[3,4-b]pyridines is accomplished by using 5-aminopyrazoles, 3°/2°-propargylic alcohols and ynones in the presence of p-TSA. The reaction proceeds through allenylation (N-alkylation)/propargylation (C-alkylation) of 5-aminopyrazoles, followed by intramolecular 6-endo-trig/dig cyclization leading to the title products with the formation of new C-N and C-C bonds. Operationally simple reaction conditions, inexpensive reagents, better yields, and gram-scale synthesis are the advantages of this protocol.

Synthesis of triazole bridged N-glycosides of pyrazolo[1,5-a]pyrimidinones as anticancer agents and their in silico docking studies

In the pursuit of novel therapeutic agents, we present a comprehensive study on the design, synthesis, and evaluation of a diverse library of triazole bridged N-glycosides of pyrazolo[1,5-a]pyrimidinones, employing a microwave-assisted synthetic approach via 'click chemistry'. This methodology offers efficient and accelerated access to the glycohybrids, showcasing improved reaction conditions that yield high-quality products. In this research endeavor, we have successfully synthesized a series of twenty-seven triazole bridged N-glycosides of pyrazolo[1,5-a]pyrimidinones. Our investigation extends beyond synthetic endeavors to explore the potential therapeutic relevance of these compounds. We subjected them to rigorous in vitro screening against prominent breast cancer cell lines MCF-7, MDA-MB231, and MDA-MB453. Among the library of compounds synthesized, (2S,3S,4R,5S,6S)-2-(acetoxymethyl)-6-(4-((5-(4-methoxyphenyl)-7-oxopyrazolo[1,5-a]pyrimidin-1(7H)-yl)methyl)-1H-1,2,3-triazol-1-yl)tetrahydro-2H-pyran-3,4,5-triyl triacetate emerged as a potent compound, exhibiting remarkable anti-cancer activity with an IC50 value of 27.66 μM against the MDA-MB231 cell line. Additionally, (2S,3R,4R,5S,6S)-2-(acetoxymethyl)-6-(4-((7-oxo-5-(4-(trifluoromethyl)phenyl)pyrazolo[1,5-a]pyrimidin-1(7H)-yl)methyl)-1H-1,2,3-triazol-1-yl)tetrahydro-2H-pyran-3,4,5-triyl triacetate displayed notable inhibitory potential against the MCF-7 cell line, with an IC50 value of 4.93 μM. Furthermore, in silico docking analysis was performed to validate our experimental findings. These findings underscore the promise of our triazole bridged N-glycosides of pyrazolo[1,5-a]pyrimidinones as potential anti-cancer agents. This research not only enriches the field of glycohybrid synthesis but also contributes valuable insights into the development of novel anti-cancer therapeutics.

Recent developments on microwave-assisted organic synthesis of nitrogen- and oxygen-containing preferred heterocyclic scaffolds

In recent decades, the utilization of microwave energy has experienced an extraordinary surge, leading to the introduction of innovative and revolutionary applications across various fields of chemistry such as medicinal chemistry, materials science, organic synthesis and heterocyclic chemistry. Herein, we provide a comprehensive literature review on the microwave-assisted organic synthesis of selected heterocycles. We highlight the use of microwave irradiation as an effective method for constructing a diverse range of molecules with high yield and selectivity. We also emphasize the impact of microwave irradiation on the efficient synthesis of N- and O-containing heterocycles that possess bioactive properties, such as anti-cancer, anti-proliferative, and anti-tumor activities. Specific attention is given to the efficient synthesis of pyrazolopyrimidines-, coumarin-, quinoline-, and isatin-based scaffolds, which have been extensively studied for their potential in drug discovery. The article provides valuable insights into the recent synthetic protocols and trends for the development of new drugs using heterocyclic molecules.

Pyrido[1,2-e]purine: Design and Synthesis of Appropriate Inhibitory Candidates against the Main Protease of COVID-19

A series of tricyclic and polycyclic pyrido[1,2-e]purine derivatives were designed and synthesized via a two-step, one-pot reaction of 2,4-dichloro-5-amino-6-methylpyrimidine with pyridine under reflux conditions. Various derivatives of pyrido[1,2-e]purine were also synthesized by substituting the chlorine atom with secondary amines. After careful physiochemical and pharmacokinetic predictions, the inhibitory effects of the synthesized compounds against the main protease of SARS-CoV-2 have been evaluated by molecular docking and molecular dynamics approaches. The in silico results revealed that among all of the studied compounds, the morpholine/piperidine-substituted pyrido[1,2-e]purine derivatives are the best candidates as effective inhibitors of SARS-CoV-2.

Pyrimidine-fused Dinitrogenous Penta-heterocycles as a Privileged Scaffold for Anti-Cancer Drug Discovery

Pyrimidine-fused derivatives that are the inextricable part of DNA and RNA play a key role in the normal life cycle of cells. Pyrimidine-fused dinitrogenous penta-heterocycles including pyrazolopyrimidines and imidazopyrimidines is a special class of pyrimidine-fused compounds contributing to an important portion in anti-cancer drug discovery, which have been discovered as core structure for promising anti-cancer agents used in clinic or clinical evaluations. Pyrimidine-fused dinitrogenous penta-heterocycles have become one privileged scaffold for anti-cancer drug discovery. This review consists of the recent progress of pyrimidine-fused dinitrogenous penta-heterocycles as anti-cancer agents and their synthetic strategies. In addition, this review also summarizes some key structure-activity relationships (SARs) of pyrimidine-fused dinitrogenous penta-heterocycle derivatives as anti-cancer agents.

Bicyclic 5-6 Systems: Comprehensive Synthetic Strategies for the Annulations of Pyrazolo[ 1,5-a]pyrimidines

Pyrazolopyrimidines are a privileged class of 5-6 bicyclic systems with three or four nitrogen atoms, including four possible isomeric structures. The significance of this class of compounds is that they can be applied in medical and pharmaceutical fields due to their unlimited biological aptitude, hence it is the basic skeleton of several synthetic drugs. The current review aimed to highlight all the synthetic routes that have been applied to construct the pyrazolo[1,5-a]pyrimidine ring systems up to date. The sections in this study included the synthesis of pyrazolo[1,5- a]pyrimidines by condensation reactions of 5-aminopyrazoles with each of β-diketones, 1,5-diketones, β- ketoaldehydes, α-cyanoaldehydes, β-enaminones, enamines, enaminonitriles, ethers, with unsaturated ketones, unsaturated thiones, unsaturated esters, unsaturated dienones "1,2-allenic", unsaturated aldehydes, unsaturated imines, and unsaturated nitriles. The routes adopted to synthesize this class of heterocyclic compounds were extended for ring construction from acyclic reagents and multicomponent reactions under catalytic or catalyst-free conditions.

Functional Pyrazolo[1,5-a]pyrimidines: Current Approaches in Synthetic Transformations and Uses As an Antitumor Scaffold

Pyrazolo[1,5-a]pyrimidine (PP) derivatives are an enormous family of N-heterocyclic compounds that possess a high impact in medicinal chemistry and have attracted a great deal of attention in material science recently due to their significant photophysical properties. Consequently, various researchers have developed different synthesis pathways for the preparation and post-functionalization of this functional scaffold. These transformations improve the structural diversity and allow a synergic effect between new synthetic routes and the possible applications of these compounds. This contribution focuses on an overview of the current advances (2015-2021) in the synthesis and functionalization of diverse pyrazolo[1,5-a]pyrimidines. Moreover, the discussion highlights their anticancer potential and enzymatic inhibitory activity, which hopefully could lead to new rational and efficient designs of drugs bearing the pyrazolo[1,5-a]pyrimidine core.

Vinylation of α-Aminoazoles with Triethylamine: A General Strategy to Construct Azolo[1,5-a]pyrimidines with a Nonsubstituted Ethylidene Fragment

A new general synthesis of pharmaceutically important azolo[1,5-a]pyrimidines starting from widely available 3(5)-aminoazoles, aldehydes, and triethylamine is developed. The key is to enable the vinylation reaction that allows the in situ generation of elusive acyclic enamines and the subsequent annulation reaction to occur. This direct and practical strategy is capable of constructing a range of 5,6-unsubstituted pyrazolo[1,5-a]pyrimidines and [1,2,4]triazolo[1,5-a]pyrimidines. More importantly, this protocol provides a concise synthetic route to prepare the clinically used zaleplon.

Pyrazolo[1,5-a]pyrimidines-based fluorophores: a comprehensive theoretical-experimental study

Fluorescent molecules are crucial tools for studying the dynamics of intracellular processes, chemosensors, and the progress of organic materials. In this study, a family of pyrazolo[1,5-a]pyrimidines (PPs) 4a-g has been identified as strategic compounds for optical applications due to several key characteristics such as their simpler and greener synthetic methodology (RME: 40-53%) as compared to those of BODIPYS (RME: 1.31-17.9%), and their tunable photophysical properties (going from ε = 3320 M-1 cm-1 and ϕ F = 0.01 to ε = 20 593 M-1 cm-1 and ϕ F = 0.97), in which electron-donating groups (EDGs) at position 7 on the fused ring improve both the absorption and emission behaviors. The PPs bearing simple aryl groups such as 4a (4-Py), 4b (2,4-Cl2Ph), 4d (Ph) and 4e (4-MeOPh), allow good solid-state emission intensities (QYSS = 0.18 to 0.63) in these compounds and thus, solid-state emitters can be designed by proper structural selection. The properties and stability found in 4a-g are comparable to commercial probes such as coumarin-153, prodan and rhodamine 6G. Ultimately, the electronic structure analysis based on DFT and TD-DFT calculations revealed that EDGs at position 7 on the fused ring favor large absorption/emission intensities as a result of the ICT to/from this ring; however, these intensities remain low with electron-withdrawing groups (EWGs), which is in line with the experimental data and allows us to understand the optical properties of this fluorophore family.

Applications of alkyl orthoesters as valuable substrates in organic transformations, focusing on reaction media

In this review we focus on applications of alkyl orthoesters as valuable and efficient substrates to perform various classes of two-component and multi-component organic reactions. The article has classified them according to two aspects, which are: (i) a focus on the reaction medium (solvent-free conditions, aqueous media, and organic solvents); and (ii) an examination of product structures. Reaction accomplishment under solvent-free conditions is an eco-friendly process with the absence of volatile toxic solvents, which puts it in line with green chemistry goals. Water is an interesting choice in organic transformations due to its inexpensiveness and safety. The authors hope their assessment will help chemists to attain new approaches for utilizing alkyl orthoesters in various organic synthetic methods. The review covers the corresponding literature up to the beginning of 2020.

A New Family of Fused Azolo[1,5-a]pteridines and Azolo[5,1-b]purines

The nitration of azolo[1,5-a]pyrimidin-7-amines with several nitration agents (such as acetic nitric anhydride, nitronium tetrafluoroborate, and a mixture of concentrated nitric acid and sulfuric acid) has been investigated. It has been shown that, depending on the conditions, the nitration of pyrazolopyrimidin-7-amines bearing electron-withdrawing groups in the pyrazole ring leads to nitration products in the pyrimidine and/or pyrazole ring. The nitration of triazolo[1,5-a]pyrimidin-7-amines with "nitrating mixture" has been optimized, thus allowing us to obtain a series of 6-nitro[1,2,4]triazolo[1,5-a]pyrimidin-7-amines, followed by their reduction into the corresponding [1,2,4]triazolo[1,5-a]pyrimidin-6,7-diamines (yields 86-89%). The latter have been subjected to heterocyclization by a variety of electrophilic compounds (such as CS₂, glyoxal, triethyl orthoformate) with the formation of five- or six-membered annulated cycles.

One-Pot Acid-Promoted Synthesis of 6-Aminopyrazolopyrimidines from 1H-Pyrazol-5-yl-N,N-dimethylformamidines or 5-Amino-1H-pyrazole-4-carbaldehydes with Cyanamide

A convenient and efficient one-pot acid-promoted synthesis of 6-aminopyrazolo[3,4-d]pyrimidine has been developed by treatment of 1H-pyrazol-5-yl-N,N-dimethylformamidines or 5-amino-1H-pyrazole-4-carbaldehydes with cyanamide (NH₂C≡N) in an acid-mediated solution. This synthetic route involves four steps of deprotection, imination, the key acid-promoted heterocyclization, and aromatization. On the basis of optimized studies, methanesulfonylchloride is considered to be the best solvent. Furthermore, the microwave-assisted synthetic technique was also carried out to improve the major product 6-aminopyrazolo[3,4-d]pyrimidines in this method. Moreover, our proposed mechanism was confirmed in this study, which demonstrates that N-[(5-amino-1,3-diaryl-1H-pyrazol-4-yl)methylene]cyanamide is the intermediate.

Synthesis of Fluorescent 1,7-Dipyridyl-bis-pyrazolo[3,4-b:4',3'-e]pyridines: Design of Reversible Chemosensors for Nanomolar Detection of Cu2

An efficient access toward novel tridentate ligands based on 1,7-dipyridinyl-substituted bis-pyrazolo[3,4-b:4',3'-e]pyridines (BPs) and their usefulness as fluorescent probes for cation detection is reported. The synthesis proceeds by a three-step sequence starting from 2-chloropyridine (1), all reactions were performed using microwave radiation under solvent-free conditions, and an overall yield of up to 63% was obtained. Photophysical properties of three representative 1,7-dipyridinyl-BPs (PBPs, 6a-6c) substituted at position 4 with different donor (D) or acceptor (A) groups were investigated. Compounds exhibited large Stokes shift in different solvents and strong blue light emission in both solution and solid state, and quantum yields were as high as 88% for some of them; thus, a twisted intramolecular charge transfer (TICT) fluorescence mechanism characteristic of the 1,4,7-triaryl-BPs was confirmed. The 4-phenyl-substituted probe (Ph-PBP, 6b) was used successfully in the detection of some metals (Cu2+, Co2+, Ni2+, and Hg2+) by fluorescence quenching phenomena, which could be reversed in the presence of ethylenediamine. This probe showed a greater sensitivity toward Cu2+ in concentrations as low as 26 nM, and in the process of "on-off-on" for this fluorescent molecular switch, only 1 equiv of the analyte was used.

5-Amino-pyrazoles: potent reagents in organic and medicinal synthesis

5-Amino-pyrazoles have proven to be a class of fascinating and privileged organic tools for the construction of diverse heterocyclic or fused heterocyclic scaffolds. This review presents comprehensively the applications of 5-amino-pyrazoles as versatile synthetic building blocks in the synthesis of remarkable organic molecules with an emphasis on versatile functionalities. Following a brief introduction of synthesis methods, planning strategies to construct organic compounds, particularly diverse heterocyclic scaffolds, such as poly-substituted heterocyclic compounds and fused heterocyclic compounds via 5-amino-pyrazoles, have been summarized. Fused heterocycles are classified as bicyclic, tricyclic, tetracyclic, and spiro-fused pyrazole derivatives. These outstanding compounds synthesized via wide variety of approaches include conventional reactions, one-pot multi-component reactions, cyclocondensation, cascade/tandem protocols, and coupling reactions. 5-Amino-pyrazoles represent a class of promising functional reagents, similar to the biologically active compounds, highlighted with diverse applications especially in the field of pharmaceutics and medicinal chemistry. Notably, this critical review covers the articles published from 1981 to 2018.

3-Formylpyrazolo[1,5- a]pyrimidines as Key Intermediates for the Preparation of Functional Fluorophores

A one-pot route for the regioselective synthesis of 3-formylpyrazolo[1,5- a]pyrimidines 4a-k in good yields through a microwave-assisted process is provided. The synthesis proceeds via a cyclocondensation reaction between β-enaminones 1 with NH-3-aminopyrazoles 2, followed by formylation with an iminium salt moiety (Vilsmeyer-Haack reagent). These N-heteroaryl aldehydes 4 were successfully used as strategic intermediates for the preparation of novel functional fluorophores with yields up to 98%. The structures of the products obtained and regioselectivity of the reactions were determined on the basis of NMR measurements and X-ray diffraction analysis. Since pyrazolo[1,5- a]pyrimidines (PPs) 3 have shown an important fluorescence, photophysical properties of four 2-methylderivatives substituted at position 7 with different acceptor (A) or donor (D) groups were investigated. The compounds evaluated exhibited large Stokes shift in different solvents, but only the substituted p-methoxyphenyl (4-An) showed a strong fluorescence intensity with quantum yields up to 44% due to its greater ICT. Therefore, hybrid systems based on pyrazolo[1,5- a]pyrimidines could be used as fluorescent probes to detect biologically or environmentally relevant species.

A series of (E)-5-(arylideneamino)-1-tert-butyl-1H-pyrrole-3-carbonitriles and their reduction products to secondary amines: syntheses and X-ray structural studies

An efficent access to a series of N-(pyrrol-2-yl)amines, namely (E)-1-tert-butyl-5-[(4-chlorobenzylidene)amino]-1H-pyrrole-3-carbonitrile, C16H16ClN3, (7a), (E)-1-tert-butyl-5-[(2,4-dichlorobenzylidene)amino]-1H-pyrrole-3-carbonitrile, C16H15Cl2N3, (7b), (E)-1-tert-butyl-5-[(pyridin-4-ylmethylene)amino]-1H-pyrrole-3-carbonitrile, C15H16N4, (7c), 1-tert-butyl-5-[(4-chlorobenzyl)amino]-1H-pyrrole-3-carbonitrile, C16H18ClN3, (8a), and 1-tert-butyl-5-[(2,4-dichlorobenzyl)amino]-1H-pyrrole-3-carbonitrile, C16H17Cl2N3, (8b), by a two-step synthesis sequence (solvent-free condensation and reduction) starting from 5-amino-1-tert-butyl-1H-pyrrole-3-carbonitrile is described. The syntheses proceed via isolated N-(pyrrol-2-yl)imines, which are also key synthetic intermediates of other valuable compounds. The crystal structures of the reduced compounds showed a reduction in the symmetry compared with the corresponding precursors, viz. Pbcm to P\overline{1} from compound (7a) to (8a) and P21/c to P\overline{1} from compound (7b) to (8b), probably due to a severe change in the molecular conformations, resulting in the loss of planarity observed in the nonreduced compounds. In all of the crystals, the supramolecular assembly is controlled mainly by strong (N,C)—H...N hydrogen bonds. However, in the case of (7a)–(7c), C—H...Cl interactions are strong enough to help in the three-dimensional architecture, as observed in Hirshfeld surface maps.

Synthesis of Dicyanovinyl-Substituted 1-(2-Pyridyl)pyrazoles: Design of a Fluorescent Chemosensor for Selective Recognition of Cyanide

A fluorescence "turn-off" probe has been designed and successfully applied to detect cyanide (CN^-) based on a Michael-type nucleophilic addition reaction and intramolecular charge transfer (ICT) mechanism. For this research, a family of 3-aryl-4-(2,2-dicyanovinyl)-1-(2-pyridinyl)pyrazoles as donor-π-acceptor (D-π-A) systems have been synthesized in 58-66% overall yield, by a three-step synthesis sequence starting from p-substituted acetophenones. The substituted p-methoxyphenyl showed good fluorescence emission and large Stokes shifts in different solvents due to its greater ICT. Likewise, this probe evidenced high selectivity and sensitivity and fast recognition for CN^- with a detection limit of 6.8 μM. HRMS analysis, ¹H NMR titration experiments, and TD-DFT calculations were performed to confirm the mechanism of detection and fluorescence properties of the chemodosimeter of CN^-. Additionally, fluorescent test paper was conveniently used to detect cyanide in aqueous solution.

One-Step Synthesis of Fully Functionalized Pyrazolo[3,4-b]pyridines via Isobenzofuranone Ring Opening

A novel series of fully substituted pyrazolo[3,4-b]pyridines 4 has been prepared in a regioselective manner by the microwave-assisted reaction between N-substituted 5-aminopyrazoles 1 and 3-(3-oxo-2-benzofuran-1(3H)-ylidene)pentane-2,4-dione (2). This is the second reported example of a cyclocondensation reaction using substrate 2 as a 1,3-bis-electrophilic reagent. Remarkably, this synthesis offers functionalized products with acetyl and carboxyl groups in one step, in good yields, and with short reaction times. Additionally, the cyclization intermediate 3 was isolated, allowing us to postulate a mechanism for this reaction, which is initiated via isobenzofuranone ring opening of 2 in a Michael-type reaction. The structures of the products and regioselectivity of the reactions were determined on the basis of NMR measurements and X-ray diffraction. For this new reaction using substrate 2, the optimal reaction conditions and its scope were investigated.

Simple access toward 3-halo- and 3-nitro-pyrazolo[1,5-a]pyrimidines through a one-pot sequence

Herein, a regioselective, time-efficient and one-pot route for the synthesis of diversely substituted 3-halo- and 3-nitropyrazolo[1,5-a]pyrimidines in good to excellent yields through a microwave-assisted process is provided.