Ligand Free One-Pot Synthesis of Pyrano[2,3-c]pyrazoles in Water Extract of Banana Peel (WEB): A Green Chemistry Approach

Recent advances in metal-free catalysts for the synthesis of N-heterocyclic frameworks focusing on 5- and 6-membered rings: a review

The tremendous potential of transition metal-free multi-component reactions (MCR) in the synthesis of N-heterocyclic frameworks is examined in this review, offering a complete overview of this subject matter. The discussion on the mechanistic rationale of the reaction routes and intermediates provides profound insights into the underlying changes, encouraging deeper investigation into various molecular frameworks. This review serves as a doorway to study the practicality of exploiting these reactions for the efficient and uncomplicated synthesis of specific nitrogen heterocycles. Specifically, we reveal the potential of transition metal-free catalysts in this field. Because of their extensive scope and diversity, these reactions enable the synthesis of various heterocycles that contain nitrogen, which include 5-membered (carbazole, pyrimidines, and pyrroles) and 6-membered rings (piperidine, pyridine, quinoline, diazinane, pyrazine, quinoxaline, and 1,2,3-triazine). In addition, the compatibility of transition metal-free catalysts with various functional groups and substrates not only increases the synthetic value of these compounds but also broadens their relevance in the domains of medical chemistry, materials science, and other relevant areas of study. To motivate future study and development in this field, the successful examples described in this review highlight the potential of transition metal-free catalysts as powerful instruments for the quick and efficient synthesis of nitrogen heterocycles. In general, this review provides a thorough and insightful examination of transition metal-free catalysts, highlighting the relevance of these compounds in contemporary organic synthesis and their potential to revolutionize the field of nitrogen heterocycle synthesis.

Review on advancements of pyranopyrazole: synthetic routes and their medicinal applications

Pyranopyrazoles are among the most distinguished, biologically potent, and exciting scaffolds in medicinal chemistry and drug discovery. Synthesis and design of pyranopyrazoles using functional modifications via multicomponent reactions (MCRs) are thoroughly found in synthetic protocols by forming new C-C, C-N, and C-O bonds. This review aims to focus on the biological importance of pyranopyrazoles as well as on a diverse synthetic approach for their synthesis using various catalytic systems such as acid-catalyzed, base-catalyzed, ionic liquids and green media-catalyzed, nano-particle-catalyzed, metal oxide-supported catalysts, and silica-supported catalysts. In this review, we have summarized data on the advancements in synthesizing pyranopyrazole from the last two decades to the mid-2023 and research papers describing the importance of these scaffolds. This review will be significant for synthetic organic chemists and researchers working in organic chemistry.

Visible-light-induced organophotocatalytic and singlet oxygen-initiated domino construction of 1,4-dihydropyridines, C-3 functionalized spiro[indoline-3,4'-pyridines] and C-11 functionalized spiro[indeno-[1,2-b]quinoxaline-11,4'-pyridines]

A highly efficient pot, atom, and step economical method for the construction of pharmacologically potent structurally functionalized 1,4-dihydropyridines, quaternary centered C-3 functionalized spiro[indoline-3,4'-pyridines], and C-11 functionalized spiro[indeno[1,2-b]quinoxaline-11,4'-pyridines] via rose bengal photoredox catalysis under blue LED irradiation in an aqueous medium at room temperature has been developed. The products were isolated in excellent yields within a short reaction time for a variety of functional groups under transition metal- and ligand-free energy-efficient conditions in a green solvent system with high reaction mass efficiency and process mass intensity, which are the key advantages of the current work.

Brønsted acid catalyzed mechanochemical domino multicomponent reactions by employing liquid assisted grindstone chemistry

Here, we have demonstrated a metal-free energy-efficient mechanochemical approach for expedient access to a diverse set of 2-amino-3-cyano-aryl/heteroaryl-4H-chromenes, tetrahydrospiro[chromene-3,4'-indoline], 2,2'-aryl/heteroarylmethylene-bis(3-hydroxy-5,5-dimethylcyclohex-2-enone) as well as tetrahydro-1H-xanthen-1-one by employing the reactivity of 5,5-dimethylcyclohexane-1,3-dione/cyclohexane-1,3-dione with TsOH⋅H₂O as Brønsted acid catalyst under water-assisted grinding conditions at ambient temperature. The ability to accomplish multiple C-C, C=C, C-O, and C-N bonds from readily available starting materials via a domino multicomponent strategy in the absence of metal-catalyst as well as volatile organic solvents with an immediate reduction in the cost of the transformation without necessitates complex operational procedures, features the significant highlights of this approach. The excellent yield of the products, broad functional group tolerances, easy set-up, column-free, scalable synthesis with ultralow catalyst loading, short reaction time, waste-free, ligand-free, and toxic-free, are other notable advantages of this approach. The greenness and sustainability of the protocol were also established by demonstrating several green metrics parameters.

Recent advances and prospects in the organocatalytic synthesis of quinazolinones

Quinazolinone, a bicyclic compound, comprises a pyrimidine ring fused at 4´ and 8´ positions with a benzene ring and constitutes a substantial class of nitrogen-containing heterocyclic compounds on account of their frequent existence in the key fragments of many natural alkaloids and pharmaceutically active components. Consequently, tremendous efforts have been subjected to the elegant construction of these compounds and have recently received immense interest in synthetic and medicinal chemistry. The domain of synthetic organic chemistry has grown significantly over the past few decades for the construction of highly functionalized therapeutically potential complex molecular structures with the aid of small organic molecules by replacing transition-metal catalysis. The rapid access to this heterocycle by means of organocatalytic strategy has provided new alternatives from the viewpoint of synthetic and green chemistry. In this review article, we have demonstrated a clear presentation of the recent organocatalytic synthesis of quinazolinones of potential therapeutic interests and covered the literature from 2015 to date. In addition to these, a clear presentation and understanding of the mechanistic aspects, features, and limitations of the developed reaction methodologies have been highlighted.

A New Pathway for the Preparation of Pyrano[2,3-c]pyrazoles and molecular Docking as Inhibitors of p38 MAP Kinase

We report a new pathway to synthesize pyrano[2,3-c]pyrazoles and their binding mode to p38 MAP kinase. Pyrano[2,3-c]pyrazole derivatives have been prepared through a four-component reaction of benzyl alcohols, ethyl acetoacetate, phenylhydrazine, and malononitrile in the presence of sulfonated amorphous carbon and eosin Y as catalysts. All products were characterized by melting point, ¹H and ¹³C NMR, and HRMS (ESI). The products were screened in silico for their binding activities to both the ATP-binding pocket and the lipid-binding pocket of p38 MAP kinase, using a structure-based flexible docking provided by the engine ADFR. The results showed that eight synthesized compounds had a higher affinity to the lipid pocket than to the other target site, which implied potential applications as allosteric inhibitors. Finally, the most biologically active compound, 5, had a binding affinity comparable to those of other proven lipid pocket inhibitors, with affinity to the target pocket reaching -10.9932 kcal/mol, and also had the best binding affinity to the ATP-binding pockets in all of our products. Thus, our research provides a novel pathway for synthesizing pyrano[2,3-c]pyrazoles and bioinformatic evidence for their biological capability to block p38 MAP kinase pockets, which could be useful for developing cancer or immune drugs.

Ultrasound-assisted transition-metal-free catalysis: a sustainable route towards the synthesis of bioactive heterocycles

Heterocycles of synthetic and natural origin are a well-established class of compounds representing a broad range of organic molecules that constitute over 60% of drugs and agrochemicals in the market or research pipeline. Considering the vast abundance of these structural motifs, the development of chemical processes providing easy access to novel complex target molecules by introducing environmentally benign conditions with the main focus on improving the cost-effectiveness of the chemical transformation is highly demanding and challenging. Accordingly, sonochemistry appears to be an excellent alternative and a highly feasible environmentally benign energy input that has recently received considerable and steadily increasing interest in organic synthesis. However, the involvement of transition-metal-catalyst(s) in a chemical process often triggers an unintended impact on the greenness or sustainability of the transformation. Consequently, enormous efforts have been devoted to developing metal-free routes for assembling various heterocycles of medicinal interest, particularly under ultrasound irradiation. The present review article aims to demonstrate a brief overview of the current progress accomplished in the ultrasound-assisted synthesis of pharmaceutically relevant diverse heterocycles using transition-metal-free catalysis.

Sonochemistry in an organocatalytic domino reaction: an expedient multicomponent access to structurally functionalized dihydropyrano[3,2-b]pyrans, spiro-pyrano[3,2-b]pyrans, and spiro-indenoquinoxaline-pyranopyrans under ambient conditions

A highly convenient and sustainable one-pot approach for the diversely-oriented synthesis of a variety of medicinally privileged amino-substituted 4,8-dihydropyrano[3,2-b]pyran-3-carbonitriles, and spiro[indoline-3,4'-pyrano[3,2-b]pyran]-3-carbonitrile/carboxylate derivatives on the basis of a domino three-component reaction of readily available carbonyl compounds including aryl aldehydes or isatins, active methylene compounds, and kojic acid as a Michael donor using secondary amine catalyst l-proline under ultrasound irradiation in aqueous ethanolic solution at ambient temperature has been developed. This methodology can involve the assembly of C-C, C[double bond, length as m-dash]C, C-O, C-N bonds via a one-pot operation, and following this protocol, a series of novel amino-substituted spiro[indeno[1,2-b]quinoxaline-11,4-pyrano[3,2-b]pyran]-3-carbonitrile/carboxylates have been synthesized. The practical utility of this method was found to be very efficient for scale-up reaction and other useful transformations. The methodology provides significant advantages including mild reaction conditions, energy-efficiency, short reaction time, fast reaction, simple work-up procedure, broad functional group tolerances, utilization of reusable catalyst, green solvent system, being metal-free, ligand-free, waste-free, inexpensive, etc. Excellent chemical yields have been achieved without using column chromatography. To address the issues of green and more sustainable chemistry, several metrics including Atom Economy (AE), Reaction Mass Efficiency (RME), Atom efficiency, E-factor, Process Mass Intensity (PMI), and Carbon Efficiency (CE) have been quantified for the present methodology that indicates the greenness of the present protocol.

Synthesis of bioactive scaffolds catalyzed by agro-waste-based solvent medium

The backbone of synthetic organic chemistry is the formation of carbon–carbon and carbon–heteroatom bonds. Scientists are actively working to develop new methods of bond-forming reactions because it is one of the most useful tools for the development of structurally diverse molecular entities. On the other hand, scientists are constantly discovering chemical processes to make them more sustainable in order to avoid the ever-increasing chemical emission associated with hazards to the environment. Thus, the development of greener catalytic reactions demonstrated a massive uptick in the ability to carry out carbon–carbon and carbon–heteroatom bond-forming reactions under environmentally friendly and simple reaction conditions. Various approaches are demonstrated, namely, solvent-free, microwave irradiation, ionic liquids, ball milling, ultrasound, one-pot, and aqueous-mediated methods under green chemistry protocol. Agro-waste is the postharvest part or agricultural residues derived from various agricultural activities, which has diverse scope and applications. The use of this agro-waste is an eco-friendly and cost-effective process of waste management. Appropriate and optimal utilization of these waste by-products is one of the major challenges in the present days. The recent trend around the globe is to transform waste into wealth concepts to achieve various applications. Agro-waste-derived ashes and extracted medium are successfully studied recently as a heterogeneous- or homogenous-based catalyst in various organic transformations. Agro-waste-derived catalysts are easily available, cost-effective, simple to prepare, nontoxic, easy to handle, biodegradable, and more environmentally benign. This article focuses more on a few instances of agro-waste-based homogeneous and heterogeneous organic synthesis, especially those used in the construction of bioactive molecule synthesis via C–C and C–X bond formation reactions are discussed. The compiled literature in this article is based on keywords used in the search engine on “agro-waste-based catalyst for organic transformations”, and review articles published on this topic, future scope, and summary are discussed.

Recent advances in the transition-metal-free synthesis of quinoxalines

Quinoxalines, also known as benzo[a]pyrazines, constitute an important class of nitrogen-containing heterocyclic compounds as a result of their widespread prevalence in natural products, biologically active synthetic drug candidates, and optoelectronic materials. Owing to their importance and chemists' ever-increasing imagination of new transformations of these products, tremendous efforts have been dedicated to finding more efficient approaches toward the synthesis of quinoxaline rings. The last decades have witnessed a marvellous outburst in modifying organic synthetic methods to create them sustainable for the betterment of our environment. The exploitation of transition-metal-free catalysis in organic synthesis leads to a new frontier to access biologically active heterocycles and provides an alternative method from the perspective of green and sustainable chemistry. Despite notable developments achieved in transition-metal catalyzed synthesis, the high cost involved in the preparation of the catalyst, toxicity, and difficulty in removing it from the final products constitute disadvantageous effects on the atom economy and eco-friendly nature of the transformation. In this review article, we have summarized the recent progress achieved in the synthesis of quinoxalines under transition-metal-free conditions and cover the reports from 2015 to date. This aspect is presented alongside the mechanistic rationalization and limitations of the reaction methodologies. The scopes of future developments are also highlighted.

Natural Feedstock in Catalysis: A Sustainable Route Towards Organic Transformations

Catalysts are the jewel in the crown of the chemical industry, accelerating reaction kinetics and augmenting the efficiency of desired reaction paths. Natural feedstock is a renewable resource capable of providing valuable functional products; in addition, it confers an opportunity to create catalysts. As an alternative to stoichiometric reagents, and as a part of a sustainable approach, the implications of using natural feedstocks as a source of new catalysts has attracted considerable interest. Natural feedstock-derived catalysts can promote chemical transformations more efficiently. Recent reports have highlighted the significant role of these biogenic, cost-effective, innocuous, biodegradable materials as catalysts in many biologically and pharmacologically important protocols. This review outlines the decisive organic transformations for which feedstock-derived catalysts have been employed effectively and successfully, along with their economic and environmental benefits over traditional catalytic systems.

Highly efficient catalyst-free domino conjugate addition, decarboxylation and esterification/amidation of coumarin carboxylic acid/esters with pyrazolones: a green chemistry approach

Tandem conjugate addition, decarboxylation and esterification/amidation of coumarin 3-carboxylic acid derivatives with pyrazolones have been developed.