Magnetically recoverable nanocatalyst for the synthesis of pyranopyrazoles: CoFe2O4@SiO2-HClO4 European Journal of Chemistry 14 (3) (2023) 385-392 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2023 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. https://dx.doi.org/10.5155/eurjchem.14.3.385-392.2457 European Journal of Chemistry View Journal Online View Article Online Magnetically recoverable nanocatalyst for the synthesis of pyranopyrazoles: CoFe2O4@SiO2-HClO4 Nikita Vinod Thakare 1, Anand Shankar Aswar 2,* and Nilesh Govindrao Salunkhe 2 1 Department of Chemistry, Elphinstone College, Homi Bhabha State University, Mumbai, 400032, India 2 Department of Chemistry, Sant Gadge Baba Amravati University, Amravati, 444602, India * Corresponding author at: Department of Chemistry, Sant Gadge Baba Amravati University, Amravati, 444602, India. e-mail: aswaranand@gmail.com (A.S. Aswar). 10.5155/eurjchem.14.3.385-392.2457 Received: 30 May 2023 Received in revised form: 08 July 2023 Accepted: 02 August 2023 Published online: 30 September 2023 Printed: 30 September 2023 The multiheterocyclic ring system shows valuable pharmaceutical and biological activities. In the present study, a microwave-assisted three-component reaction between aryl aldehyde, malononitrile, and 5-methyl-2,4-dihydro-3H-pyrazole-3-one led to the synthesis of pyrano[2,3-c]pyrazoles has been described. The reaction was carried out under solvent- free conditions in the presence of a new magnetically recoverable nanocatalyst (CoFe2O4@SiO2-HClO4). The reported protocol offers several advantages such as being environmentally benign, being rapid, inexpensive, having high atom and step economy, and being facile. The simple method of catalyst preparation, easy magnetic recovery, and reusability of the catalyst for four runs are notable features of the nanocatalyst. Antibacterial activity of all synthesized compounds was tested against Escherichia coli and Staphylococcus aureus. All synthesized compounds showed promising biological activity and may be used as a potential antibacterial candidate in biological science. Solvent-free Pyranopyrazoles Microbial activity Microwave-assisted Multicomponent reaction Magnetically recoverable catalyst Cite this: Eur. J. Chem. 2023, 14(3), 385-392 Journal website: www.eurjchem.com 1. Introduction Nowadays, heterocyclic scaffolds like pyranopyrazoles play a very important role because of their potential application in pharmaceutical and biological activities, such as anticancer [1], antimicrobial [2], anti-inflammatory [3], insecticidal, mollusci- cidal [4] and also, they are identified as a screening kit for Chk1 kinase [5]. In addition, Qvortrup et al. reported the synthesis of dihydropyrano-[2,3-c]pyrazoles as a new class of peroxisome proliferator-activated receptor gamma (PPARγ) partial ago- nists [6]. These compounds are found to be very good pre- cursors in the field of medicinal chemistry [7,8]. Therefore, enormous efforts have been made to develop green and convenient routes for their high-yielding synthesis. For the synthesis of such an important class of heterocyclic scaffolds, various synthetic procedures have been applied, such as single- or multi-step reactions, as well as two- or multi- component reactions [9]. Otto proposed the first reported method for the synthesis of dihydropyrano[2,3-c] pyrazoles in 1974, through base-catalyzed cyclization of 4-arylidene-5- pyrazolone [10]. In a further report, Otto and Schmelz showed that weak bases can also be used for a Michael-type cyclization [11]. The first multicomponent approach to the synthesis of the pyrano[2,3-d]pyrazole motif was based on the reaction between tetracyanoethylene and 3-methyl-1H-pyrazolin-5- one, which provide products in good yields. Subsequently, many synthetic routes for development substituted derivatives of pyrano[2,3-c]pyrazoles [12], spiro-pyrano[2,3-c]pyrazoles [13] and dihydropyrano[2,3-c]pyrazoles [14] were also reported. Recently, the most common and convenient approach to synthesize a library of various pyrano[2,3-c]pyrazoles was reported by catalyzed multicomponent reactions [15]. A catalyst is a substance that enhances the rate of reaction by lowering the activation energy. Some of the catalysts reported for pyranopyrazole synthesis were triethanolamine [16], urea [17], and earth clay bleaching [18]. Recently, environmentally compatible catalysts have been developed such as BF3/MNP [19], Fe3-xTixO4@SO3H [20], Nd-SM [21], ZnO2 nanoparticles [22] sodium lactate [23] for the condensation of aldehyde, cyclic ketone, and malononitrile. In addition to the above, several strategies have been developed to synthesize pyrano[2,3-c] pyrazoles, such as microwave irradiation [24], electrocatalysis [25], ultrasonication [26] and reaction in different solvents, various temperature conditions and catalysts. Although the reported methods are quite satisfactory, some of them suffer from one or other drawbacks such as the absence of a green chemistry approach, the use of volatile and hazardous organic ABSTRACT RESEARCH ARTICLE KEYWORDS https://dx.doi.org/10.5155/eurjchem.14.3.385-392.2457 https://www.eurjchem.com/ https://dx.doi.org/10.5155/eurjchem.14.3.385-392.2457 mailto:aswaranand@gmail.com http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.14.3.385-392.2457&domain=pdf&date_stamp=2023-09-30 386 Thakare et al. / European Journal of Chemistry 14 (3) (2023) 385-392 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.385-392.2457 Scheme 1. Preparation of magnetic recoverable nanoparticles-CoFe2O4@SiO2-HClO4. solvents, low yields, extended reaction time, high temperature, and a tedious procedure for the preparation of catalysts. Therefore, we develop a new CoFe2O4@SiO2-HClO4 nano- catalyst that bridges the gap between homogeneous and heterogeneous catalysis. An easy method of preparation and magnetic recovery of nanoparticles follows the green chemistry principle. Previously developed and successfully characterized CoFe2O4@SiO2-HClO4 as magnetically recoverable nano- particles exhibited its effectiveness against the microwave- assisted synthesis of 8-phenyl-7H-acenaphtho[1,2-d]imida- zoles [27]. Furthermore, we explored the potential of CoFe2O4@ SiO2-HClO4 magnetically recoverable nanoparticles against the synthesis of pyrano[2,3-c]pyrazoles via tandem reaction of aldehyde, malononitrile, and 5-methyl-2,4-dihydro-3H-pyra- zol-3-one under solvent-free condition and microwave irra- diation. The CoFe2O4@SiO2-HClO4 nanocatalyst brought notable organic transformations due to its ability to enhance the rate of organic reactions, high catalytic activity, higher yield of products, magnetic recovery, and reusability of the catalyst. 2. Experimental 2.1. Materials and instrumentations All solvents and chemicals were of analytical grade and purchased from Sigma-Aldrich and used as received. 1H and 13C NMR spectra were recorded on a Bruker Advance spectrometer using DMSO-d6 as solvent. A Fourier transform infrared spectrum was recorded on the Shimadzu FT-IR-8400 spectrometer. The microwave-assisted reaction was exhibited in a Scientific Ragatech microwave oven (2450 MHz). This system is fitted with a temperature and power feedback control switch and measures the temperature via a highly sensitive IR sensor. 2.2. Preparation of magnetically recoverable nanocatalyst- CoFe2O4@SiO2-HClO4 Previously developed and successfully characterized CoFe2O4@SiO2-HClO4 as magnetically recoverable nano- particles used as nanocatalysts in the synthesis of pyrano- pyrazoles [27]. In the initial stage of preparation, CoFe2O4 nanoparticles were prepared by the known hydrothermal method by mixing stoichiometric proportions of Co(NO3)2·5H2O (1 mmol) and Fe(NO3)3·9H2O (2 mmol) and NaOH (1 mmol) dissolved in distilled water. To avoid agglomeration and to get chemical stability, the core nanoparticles of CoFe2O4 were wrapped with silica shells by slow addition of tetraethyl- orthosilicate (TEOS). The suspension of the formed CoFe2O4@ SiO2 nanoparticles was centrifuged on a centrifuge machine; the filtrate was discarded and the precipitate of the nanoparticles was first washed with water and then with ethanol. The obtained core-shell nanoparticles were dried in an oven overnight at 70 °C. The dried magnetic core-shell NPs were separated using an external magnet. In the next stage of preparation, the functionalization of the core-shell nano- particles (NPs) was carried out by refluxing CoFe2O4@SiO2 magnetic nanoparticles in diethyl ether and 0.03 mmol of HClO4 of 70% aqueous solution under vacuum. The mixture was concentrated and the residue was heated for 72 h at 70 °C under vacuum to obtain functionalized CoFe2O4@SiO2-HClO4 magnetic nanoparticles. The surface-modified perchloric acid core-shell CoFe2O4@SiO2 magnetic nanoparticles were characterized by XRD, SEM-EDX, TEM, VSM, BET, TG-DTA, and FT-IR analysis. Successfully synthesized and characterized magnetic nano- particles (MNPs) employed in organic transformation as catalysts. This protocol is depicted in Scheme 1. 2.3. General procedure for the synthesis of pyranopyrazoles using magnetic recoverable nanocatalyst- CoFe2O4@SiO2- HClO4 (4a-4o) A mixture of 5-methyl-2,4-dihydro-3H-pyrazol-3-one (1.0 mmol), aldehyde (1.0 mmol), malononitrile (1.0 mmol), and previously developed CoFe2O4@SiO2-HClO4 (10 Wt%) was placed in 50 mL round bottom flask and the reaction mixture was irradiated in the microwave (560 W at 100 °C) for the desired time. The progress of the reaction was monitored by thin-layer chromatography. After completion of the reaction, the reaction mixture was diluted with dichloromethane: methanol mixture (1:1, v/v) (20 mL) and the catalyst was separated by an external magnet. The decanted solution was concentrated under reduced pressure to obtain a solid residue. The solid product was washed with water and recrystallized with DMF and water mixture (1:1, v/v). The recovered catalyst was washed with dichloromethane:methanol mixture (1:1, v/v) and dried at 70 °C and reused for the next cycle (Scheme 2). All purified products were characterized by IR, 1H-NMR, 13C-NMR, and MS and compared with the literature data. 2.4. Antimicrobial activity In the present protocol, we tested the bactericidal activity of the synthesized organic compounds using the disc diffusion method described by Kirby-Bauer [28,29]. Mueller-Hinton agar was used in the Kirby-Bauer method for rapid growth of aerobic organisms. The medium in the plates was sterilized and the depth of the medium was kept at about 4 mm. Pure culture was used as an inoculum. 3-4 similar colonies were selected and transferred to 5 mL of suitable broth such as tryptone soya broth. Incubation was kept at 35 °C for 2-8 hours until light to moderate turbidity developed. The turbidity was adjusted to yield a uniform suspension in the range of 1×105 and 1×106 cells/mL. A sterile non-toxic cotton swabbed dip into the standardized inoculum (turbidity adjusted to obtain confluent growth on the Petri plate), and rotated the soaked swab firmly against the upper wall of the tube to express excess fluid. We streaked the entire agar surface of the plate with the swab three times, turning the plate at 60 ° angles between each streaking. We allowed the inoculum to dry for 5-15 minutes with the lid in place. The sterilization of forceps was carried out by dipping them in alcohol and heating. A paper disc was taken using sterilized forceps and the disc was soaked in the chemical (2000 µg/mL antibacterial solution) to be tested. Thakare et al. / European Journal of Chemistry 14 (3) (2023) 385-392 387 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.385-392.2457 Table 1. Effect of solvent on the synthesis of pyrano[2,3-d]pyrazole a. Entry Solvent Reaction condition Time (min) Yield (%) b 1 Methanol Reflux 180 44 2 Ethanol Reflux 180 62 3 THF Reflux 180 56 4 Acetonitrile Reflux 180 30 5 Water Reflux 180 67 6 1,4-Dioxane Reflux 180 64 7 Solvent-free 90 °C/MW 15 87 a Reaction conditions: 5-Methyl-2,4-dihydro-3H-pyrazol-3-one: benzaldehyde: malononitrile (1:1:1), CoFe2O4@SiO2-HClO4 (10 Wt %). b Isolated yield. Compound 1/4 R Compound 1/4 R a H i 4-CH3 b 3-NO2 j 4-(CH3)2CH c 4-Cl k 4-(CH3)2N d 4-OH l 2-Furyl e 3-OC2H5, 4-OH m 2-Thiophene f 4-OCH3 n 3-Indolyl g 3,4-OCH3 o Terephthalaldehyde h 3,4,5-OCH3 Scheme 2. Synthesis of pyranopyrazoles using magnetic recoverable nanocatalyst-CoFe2O4@SiO2-HClO4 (4a-4o). Scheme 3. Model reaction for the synthesis of pyrano[2,3-d]pyrazoles. The excess chemical was drained by touching the disc on the sides of the tube. The disc was inserted into the center of the Petri dish in aseptic condition. After incubation of the Petri dish at 37 °C, the results were examined after 18-20 hours. Measure- ment of the zone showing complete inhibition was performed and data was recorded in diameter (mm). 3. Results and discussion In continuation of our research, successfully synthesized and characterized nanoparticles were applied as a catalyst in multicomponent reactions to synthesize pyranopyrazoles and their derivatives. In the first stage of the investigation, we considered a three-component reaction of the 5-methyl-2,4- dihydro-3H-pyrazol-3-one (1 mmol), benzaldehyde (1 mmol) and malononitrile (1 mmol) system as a model reaction. Various reaction parameters such as solvent, catalyst amount, temperature, and microwave irradiation frequency have been optimized for the model reaction (Scheme 3). 3.1. Effect of solvent Initially, we carried out screening of solvents for pyropyazole synthesis. We tested the reaction in some protic and aprotic solvents such as ethanol, methanol, THF, 1,4- dioxane, water, and acetonitrile in the presence of CoFe2O4@ SiO2-HClO4 nanocatalyst. It was observed that the reaction in the protic solvent had a satisfactory performance, but in the aprotic solvent (acetonitrile), the efficiency was poor, even after 24 h. Afterward, the reaction was carried out under solvent-free conditions. Because of solvent-free reaction conditions, the productivity of the reaction was better, and additionally, the reaction time also decreased. Related results are presented in Table 1. Therefore, in the next step of the investigation, we used solvent-free conditions for the synthesis of substituted pyranopyrazoles. 3.2. Effect of catalyst concentration In the next stage of the investigation, we checked the best catalytic amount for the model reaction. The yield of products without or with different catalytic concentrations, under solvent-free conditions at 90 °C, is reported in Figure 1. The 10 Wt% of the catalyst shows good activity towards synthesis, afterward increasing the catalyst amount there were no remarkable changes observed in the yield of the product. Progress of the reaction was monitored with TLC after regular intervals. The resulting results are summarized in Figure 1. 388 Thakare et al. / European Journal of Chemistry 14 (3) (2023) 385-392 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.385-392.2457 Table 2. Optimization of temperature for the synthesis of pyrano[2,3-d]pyrazole a. Entry Temperature (°C) / Condition Time (min) Yield (%) b 1 30 / Solvent free 180 - 2 80 / Solvent free 15 74 3 90 / Solvent free 12 87 4 100 / Solvent free 4 96 5 110 / Solvent free 4 96 a Reaction conditions: 5-Methyl-2,4-dihydro-3H-pyrazol-3-one: benzaldehyde: malononitrile (1:1:1) in microwave and CoFe2O4@SiO2-HClO4 (10 Wt%) as catalyst. b Isolated yield. Table 3. Optimization of microwave irradiation for the synthesis of pyrano[2,3-d]pyrazole a. Entry MW (watts) Time (min) Yield (%) b 1 140 4 15 2 240 4 34 3 350 4 67 4 450 4 86 5 560 4 96 6 700 4 95 a Reaction conditions: 5-Methyl-2,4-dihydro-3H-pyrazol-3-one: benzaldehyde: malononitrile (1:1:1) in microwave and CoFe2O4@SiO2-HClO4 (10 Wt%) catalyst at 100 °C in solvent-free conditions. b Isolated yield. Figure 1. Optimization of the amount of catalyst for the synthesis of pyrano[2,3-d]pyrazole, reaction conditions: 5-Methyl-2,4-dihydro-3H-pyrazol-3-one: benzaldehyde: malononitrile (1:1:1) under microwave irradiation and solvent-free condition at 90 °C for 15 min, isolated yield. 3.3. Effect of temperature In the next stage of the investigation, a test of the model reaction for temperature was carried out. It was observed that at room temperature 30 °C the reaction did not take place, even a trace amount of the product was not observed on TLC. Further increasing the temperature to 80°C and 90°C in a microwave reactor gives a yield of 74% and 87% of the desired product, respectively. The reaction was accelerated in the presence of CoFe2O4@SiO2-HClO4 (10 Wt%) at 100 °C and the completion of the reaction occurred in just 4 min with a yield of 96% yield (Table 2, Entry 4). Furthermore, no increase in yield was observed when the reaction mixture was heated to 110°C in the microwave (Table 2, entry 5). 3.4. Effect of microwave irradiation To optimize the reaction in microwave irradiation, we first exposed the reaction mixture below 350 W. At that condition, the reaction proceeded slowly and gives a relatively low yield. Although no significant improvement in product yield was observed above 560 W. All subsequent studies were carried out under solvent-free conditions with a 10% Wt catalyst at 100 °C for microwave irradiation (560 W) and the results are reported in Table 3. 3.5. Scope for substrate With optimized reaction conditions, we started to synthesize derivatives of pyranopyrazoles in microwave under solvent-free conditions with CoFe2O4@SiO2-HClO4 (10 Wt%) as a catalyst (Scheme 2). Next, the substrate scope was examined with various substituted aldehydes that afforded the corres- ponding pyranopyrazoles with different yields mentioned in Table 4. As evident in Table 4, all reactions preceded comfortably with good to excellent yield. The reaction was sensitive to steric hindrance on aromatic aldehydes, therefore yield of the product decreases in Table 4 (Entries 5 and 8). The structure of all synthesized products was confirmed using spectroscopic techniques that included 1H-NMR, 13C-NMR, IR, and mass spectrometry. The structural variations in the aldehydes did not have a significant effect on the yields. Using aldehydes bearing functional groups such as Cl, OH, and OCH3, the reaction proceeded smoothly to provide the corresponding products in good yields, Table 4 (Entries 3, 4, and 6). The catalyst also worked well even with heterocyclic aldehydes such as furfural, 2-thiophene, and 3-Indole without leading to the formation of any side products. It was observed that a group such as -NO2, which has a strong electron-withdrawing nature as well as shows a mesomeric effect, gives an excellent yield of the desired product (Table 4, Entry 2). The electron-with- drawing group makes the carbonyl more vulnerable to nucleophilic attack. Terephthalaldehyde has also been used successfully to provide the corresponding bis nuclear pyrano [2,3-d]pyrazole in a good yield. Thakare et al. / European Journal of Chemistry 14 (3) (2023) 385-392 389 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.385-392.2457 Table 4. Synthesis of substituted pyranopyrazoles using magnetically recoverable nanocatalyst-CoFe2O4@SiO2-HClO4 a. Entry Product b R Time (min) Yield (%) c M.p. (°C) Found Reported [Reference] 1 4a H 4 96 262-264 264-266 [30] 2 4b 3-NO2 4 95 245-247 244-246 [30] 3 4c 4-Cl 6 82 244-245 245-247 [30] 4 4d 4-OH 5 90 222-225 218-220 [31] 5 4e 3-OC2H5, 4-OH 8 86 231-233 232-234 [32] 6 4f 4-OCH3 5 92 212-215 210-212 [30] 7 4g 3,4-OCH3 8 88 196-198 192-194 [33] 8 4h 3,4,5-OCH3 12 84 211-212 209-211 [28] 9 4i 4-CH3 5 92 205-207 206-208 [31] 10 4j 4-(CH3)2CH 7 90 188-190 181-182 [34] 11 4k 4-(CH3)2N 4 94 227-229 227-229 [30] 12 4l 2-Furyl 10 82 238-240 240-244 [32] 13 4m 2-Thiophene 10 86 245-246 246-248 [30] 14 4n 3-Indolyl 10 78 210-212 205-206 [34] 15 4o Terephthalaldehyde d 8 87 261-263 258-260 [35] a Reaction conditions: 5-Methyl-2,4-dihydro-3H-pyrazol-3-one (1 mmol): aldehyde (1 mmol): malononitrile (1 mmol) in the presence of CoFe2O4@SiO2-HClO4 (10 Wt%) catalyst in microwave irradiation (560 W) at 100 °C under solvent-free conditions. b All products are known and were identified by their melting point, IR, and 1H and 13C NMR spectra according to literature. c Isolated yield. d Reaction conditions: 5-Methyl-2,4-dihydro-3H-pyrazol-3-one (1 mmol); terephthalaldehyde (0.5 mmol); malononitrile (1 mmol) in the presence of CoFe2O4@SiO2-HClO4 (10 Wt%) catalyst at 100 °C under solvent-free conditions. Table 5. Comparative study with different catalysts. Entry Catalyst Amount Condition / Solvent Time (min) / Yield (%) a Reference 1 [(CH2)4SO3HMIM][HSO4] 75 Wt% RT / Solvent-free 30 / 80 [37] 2 H4[W12SiO40] 28 Wt% 60 °C / Solvent free 10 / 95 [38] 3 [bmim]OH 30 Wt% 60 °C / Solvent free 5 / 90 [39] 4 γ-Alumina 29 Wt% Reflux / EtOH 50 / 80 [33] 5 ZrO2 12 Wt% RT / EtOH: H2O 5 / 95 [40] 6 β-Cyclodextrin 36 Wt% 60 °C / EtOH: H2O 35 / 87 [31] 7 CTACl 60 Wt% 90 °C / H2O 240 / 81 [41] 8 Isonicotinic acid 12 Wt% 85 °C / Solvent free 10 / 90 [30] 9 CoFe2O4@SiO2-HClO4 10 Wt% Microwave (100 °C) / Solvent-free 4 / 96 This work 3.6. Plausible mechanism Based on the results presented above and previous studies [36], a plausible mechanism can reasonably be proposed for the synthesis of pyranopyrazole 4a from 5-methyl-2,4-dihydro-3H- pyrazol-3-one (1 mmol), benzaldehyde (1 mmol), malononitrile (1 mmol) in the presence of CoFe2O4@SiO2-HClO4 (10 Wt%) catalyst in microwave irradiation (560 W) at 100 °C in solvent- free conditions (Scheme 4). The presence of the HClO4 group in the structure of CoFe2O4@SiO2-HClO4 catalyst plays an important role in its promotion activity for the formation of the Knoevenagel adduct (7) from the condensation of benz- aldehyde and malononitrile. Initially, the catalyst can activate the carbonyl groups of aldehydes (1) by decreasing the energy of the transition state and abstracting an acidic proton from malononitrile (2). This results in the formation of the nitrile anion (6). Finally, the intermediate arylidene nitrile (Knoeve- nagel adduct-7) is formed by the Knoevenagel condensation reaction of the intermediate nitrile anion (6) with the transition state of aldehyde (5), 5-methyl-2,4-dihydro-3H-pyrazol-3-one which undergoes tautomerism from an enolized compound. Subsequently, the enolizable compound condenses with the Knoevenagel adduct (7) followed by Michael addition, resulting in the formation of the intermediate in situ (Michael adduct-9). Finally, the intermediate (Michael adduct-9) underwent intramolecular cyclization by nucleophilic addition to provide the desired compound 4. Noticeably, -H+ plays a significant synergic effect in the overall sequence of the mechanism. Therefore, by using this CoFe2O4@SiO2-HClO4 magnetically recoverable nanocatalyst, better catalytic activity can be achieved. 3.7. Catalyst comparison The efficacy of CoFe2O4@SiO2-HClO4 nanocatalyst was evaluated and compared together with other solid catalysts for the preparation of pyrano[2,3-d]pyrazoles. Data of comparison are presented in Table 5. Among solid catalysts [(CH2)4SO3 HMIM][HSO4], H4[W12SiO40], [bmim]OH, γ-alumina, β-cyclo- dextrin, ZrO2, CTACl, isonicotinic acid and CoFe2O4@SiO2-HClO4 was found to be superior in terms of catalyst amount as well as yield and reaction time. 3.8. Reusability of catalyst The reusability of CoFe2O4@SiO2-HClO4 catalyst was studied by choosing the model reaction of benzaldehyde, 5- methyl-2,4-dihydro-3H-pyrazol-3-one and malononitrile under solvent-free conditions. Upon completion of the reaction, the solid mixture was dissolved in a dichloromethane: methanol mixture (1:1, v/v) and the catalyst was easily separated and recovered from the reaction mixture by an external magnet, followed by decantation of the reaction solution. The remaining catalyst was washed with dichloromethane: methanol (1: 1, v/v) solvent mixture to remove the residual product and dried under vacuum and reused in a subsequent reaction. 390 Thakare et al. / European Journal of Chemistry 14 (3) (2023) 385-392 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.385-392.2457 Scheme 4. Plausible mechanism for the synthesis of pyranopyrazoles. Thakare et al. / European Journal of Chemistry 14 (3) (2023) 385-392 391 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.385-392.2457 The recovered catalyst was reused for four cycles under the same reaction conditions for the preparation of products. The relationship between the number of reactions cycles and the catalytic activity in terms of product yields is presented in Figure 2. Figure 2. Recyclability of magnetically recoverable nanocatalyst- CoFe2O4@SiO2-HClO4. 3.9. Antimicrobial activity All synthesized derivatives of pyrano[2,3-c]pyrazoles were tested for antimicrobial activities in vitro using the disc diffusion method. The antimicrobial activity of the synthesized compounds was monitored against Escherichia coli (Gram negative) (MTCC443) and Staphylococcus aureus (Gram positive) (MTCC96) bacterial strains using streptomycin as standard. The initial screening of the prepared products and standard drugs was carried out using 2000 μg/mL as a fixed concentration. The zone of inhibition was measured at the end of 24 h for bacteria at a temperature of 35 °C. The results of the screening are summarized in Figure 3 and the code of compounds mentioned in Table 4. From the above biological activities, it is found that compounds 4a and 4c is active against E. coli. Although compound 4c is found to be fairly active against S. aureus, it may be due to the presence of a chlorine atom. The remaining compounds show moderate to very less activity against different strains. Figure 3. Antibacterial activity of pyrano[2,3-c]pyrazoles (4a-4o) compounds. 4. Conclusions In this work, we have developed a new magnetically recoverable CoFe2O4@SiO2-HClO4 nanocatalyst via the simple known method. It has been proven that the catalyst manifests high catalytic performance during the synthesis of pyrano- pyrazoles and their derivatives under microwave irradiation conditions. The notable features of this protocol include mild reaction conditions, high activity, easy work-up, moderate to excellent yield, and reusability of a catalyst. Along with this, the synthesized compounds showed satisfactory antimicrobial activity. All these features of the present protocol make it a green alternative. Concerning the acceptable catalytic proper- ties observed of CoFe2O4@SiO2-HClO4 nanocatalyst, we look further at its use in the investigation of other magnetically recoverable nanocatalysts. Acknowledgments We thank Sant Gadge Baba Amravati University for providing laboratory facilities for the present research work. Disclosure statement The authors declare that they have no conflict of interest. Ethical approval: All ethical guidelines have been adhered to. Sample availability: Samples of the compounds are available from the author. CRediT authorship contribution statement Conceptualization: Anand Shankarrao Aswar; Methodology: Nikita Vinod Thakare, Nilesh Govindrao Salunkhe; Software: Nikita Vinod Thakare, Nilesh Govindrao Salunkhe; Validation: Anand Shankarrao Aswar; Formal Analysis: Nikita Vinod Thakare, Nilesh Govindrao Salunkhe; Investigation: Nikita Vinod Thakare, Nilesh Govindrao Salunkhe; Resources: Nikita Vinod Thakare, Nilesh Govindrao Salunkhe; Data Curation: Anand Shankarrao Aswar; Writing - Nikita Vinod Thakare, Nilesh Govindrao Salunkhe; Writing - Review and Editing: Nikita Vinod Thakare, Nilesh Govindrao Salunkhe; Visualization: Nikita Vinod Thakare, Nilesh Govindrao Salunkhe; Funding acquisition: Nikita Vinod Thakare, Nilesh Govindrao Salunkhe; Supervision: Anand Shankarrao Aswar; Project Administration: Anand Shankarrao Aswar. ORCID and Email Nikita Vinod Thakare nikitathakre@elphinstone.ac.in https://orcid.org/0009-0005-9583-110X Anand Shankar Aswar aswaranand@gmail.com https://orcid.org/0000-0003-4368-496X Nilesh Govindrao Salunkhe ngsalunkhe@gmail.com https://orcid.org/0000-0001-7897-2721 References [1]. Sadeghian, Z.; Bayat, M.; Safari, F. Synthesis and in vitro anticancer activity evaluation of spiro[indolo[2,1-b]quinazoline-pyrano[2,3- c]pyrazole] via sequential four-component reaction. J. Mol. Struct. 2022, 1250, 131759. [2]. Reddy, G. M.; Garcia, J. R.; Zyryanov, G. V.; Sravya, G.; Reddy, N. B. Pyranopyrazoles as efficient antimicrobial agents: Green, one pot and multicomponent approach. Bioorg. Chem. 2019, 82, 324–331. [3]. Faidallah, H. M.; Rostom, S. A. F. Synthesis, anti-inflammatory activity, and COX-1/2 inhibition profile of some novel non-acidic polysubstituted pyrazoles and pyrano[2,3-c ]pyrazoles: Anti- inflammatory pyrazoles and pyrano[2,3-c ]pyrazoles. Arch. Pharm. (Weinheim) 2017, 350, 1700025. [4]. Abdelrazek, F. M.; Metz, P.; Metwally, N. H.; El-Mahrouky, S. F. Synthesis and molluscicidal activity of new cinnoline and pyrano [2,3- c]pyrazole derivatives. Arch. Pharm. (Weinheim) 2006, 339, 456–460. [5]. Foloppe, N.; Fisher, L. M.; Howes, R.; Potter, A.; Robertson, A. G. S.; Surgenor, A. E. Identification of chemically diverse Chk1 inhibitors by receptor-based virtual screening. Bioorg. Med. Chem. 2006, 14, 4792– 4802. [6]. Kimata, A.; Nakagawa, H.; Ohyama, R.; Fukuuchi, T.; Ohta, S.; Suzuki, T.; Miyata, N. New series of antiprion compounds: Pyrazolone derivatives have the potent activity of inhibiting protease-resistant prion protein accumulation. J. Med. Chem. 2007, 50, 5053–5056. [7]. Chougala, B. M.; Samundeeswari, S.; Holiyachi, M.; Shastri, L. A.; Dodamani, S.; Jalalpure, S.; Dixit, S. R.; Joshi, S. D.; Sunagar, V. A. Synthesis, characterization and molecular docking studies of substituted 4-coumarinylpyrano[2,3-c]pyrazole derivatives as potent antibacterial and anti-inflammatory agents. Eur. J. Med. Chem. 2017, 125, 101–116. [8]. Abdelgaleil, S. A. M.; Badawy, Y. M. Herbicidal, insecticidal and structure-activity relationship studies on pyranopyrazole and oxinobispyrazole derivatives. Alex. Sci. Exch. J. Int. Q. J. Sci. Agric. Environ. 2016, 37, 572–580. mailto:nikitathakre@elphinstone.ac.in https://orcid.org/0009-0005-9583-110X mailto:aswaranand@gmail.com https://orcid.org/0000-0003-4368-496X mailto:ngsalunkhe@gmail.com https://orcid.org/0000-0001-7897-2721 392 Thakare et al. / European Journal of Chemistry 14 (3) (2023) 385-392 2023 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.14.3.385-392.2457 [9]. Maggi, R.; Ballini, R.; Sartori, G.; Sartorio, R. Basic alumina catalysed synthesis of substituted 2-amino-2-chromenes via three-component reaction. Tetrahedron Lett. 2004, 45, 2297–2299. [10]. Otto, H.-H. Darstellung einiger 4H-Pyrano[2.3-c]pyrazolderivate. Arch. Pharm. (Weinheim) 1974, 307, 444–447. [11]. Otto, H.-H.; Schmelz, H. Heterocyclen durch Michael-Reaktionen, 5. Mitt. Nucleophile Additionen an 4-Aryliden-pyrazolone. Arch. Pharm. (Weinheim) 1979, 312, 478–486. [12]. Ramtekkar, R.; Kumarvel, K.; Vasuki, G.; Sekar, K.; Krishna, R. Computer-aided drug design of pyranopyrazoles and related compounds for checkpoint kinase-1. Lett. Drug Des. Discov. 2009, 6, 579–584. [13]. Amiri, Z.; Bayat, M. A catalyst-free approach to synthesis of spiroacenaphthylene-pyranopyrazole derivatives in water media. Mol. Divers. 2021, 25, 121–129. [14]. Dehghani Tafti, A.; Mirjalili, B. B. F.; Bamoniri, A.; Salehi, N. Rapid four- component synthesis of dihydropyrano[2,3-c]pyrazoles using nano- eggshell/Ti(IV) as a highly compatible natural based catalyst. BMC Chem. 2021, 15, 6. [15]. Yavari, I.; Sheykhahmadi, J. TFA-mediated synthesis of functionalized pyrano[2,3-c]pyrazoles from pyrazol-3-ones, active carbonyl compounds and tert-BuOH. Mol. Divers. 2022, 26, 879–890. [16]. Litvinov, Y. M.; Shestopalov, A. A.; Rodinovskaya, L. A.; Shestopalov, A. M. New convenient four-component synthesis of 6-amino-2,4- dihydropyrano[2,3-c]pyrazol-5-carbonitriles and one-pot synthesis of 6’-aminospiro[(3H)-indol-3,4’-pyrano[2,3-c]pyrazol]-(1H)-2-on- 5’-carbonitriles. J. Comb. Chem. 2009, 11, 914–919. [17]. Li, W.; Ruzi, R.; Ablajan, K.; Ghalipt, Z. One-pot synthesis of highly functionalized pyrano[2,3-c]pyrazole-4,4′-diacetate and 6-oxo- pyrano[2,3-c]pyrazole derivatives catalyzed by urea. Tetrahedron 2017, 73, 164–171. [18]. Kamble, R. D.; Dawane, B. S.; Yemul, O. S.; Kale, A. B.; Patil, S. D. Bleaching earth clay (pH 12.5): a green catalyst for rapid synthesis of pyranopyrazole derivatives via a tandem three-component reaction. Res. Chem. Intermed. 2013, 39, 3859–3866. [19]. Abdollahi-Alibeik, M.; Moaddeli, A.; Masoomi, K. BF3 bonded nano Fe3O4 (BF3/MNPs): an efficient magnetically recyclable catalyst for the synthesis of 1,4-dihydropyrano[2,3-c]pyrazole derivatives. RSC Adv. 2015, 5, 74932–74939. [20]. Azarifar, D.; Abbasi, Y. Sulfonic acid–functionalized magnetic Fe3- xTixO4 nanoparticles: New recyclable heterogeneous catalyst for one- pot synthesis of tetrahydrobenzo[b]pyrans and dihydropyrano[2,3- c]pyrazole derivatives. Synth. Commun. 2016, 46, 745–758. [21]. Rather, R. A.; Siddiqui, Z. N. Synthesis, characterization and application of Nd-Salen schiff base complex Immobilized Mesoporous Silica in solvent free synthesis of pyranopyrazoles. J. Organomet. Chem. 2018, 868, 164–174. [22]. Sachdeva, H.; Saroj, R. ZnO nanoparticles as an efficient, heterogeneous, reusable, and ecofriendly catalyst for four-component one-pot green synthesis of pyranopyrazole derivatives in water. ScientificWorldJournal 2013, 2013, 1–8. [23]. Sonar, J. P.; Pardeshi, S. D.; Dokhe, S. A.; Bhavar, G. M.; Tekale, S. U.; Zine, A. M.; Thore, S. N. One pot synthesis of pyranopyrazole using sodium lactate as an efficient catalyst. Eur. Chem. Bull. 2019, 8, 207– 221. [24]. Agrwal, A.; Pathak, R. K.; Kasana, V. Molecular docking and antibacterial studies of pyranopyrazole derivatives synthesized using [pap-glu@chi] biocatalyst through a greener approach. Arab. J. Sci. Eng. 2022, 47, 347–363. [25]. Elinson, M. N.; Dorofeev, A. S.; Miloserdov, F. M.; Nikishin, G. I. Electrocatalytic multicomponent assembling of isatins, 3-methyl-2- pyrazolin-5-ones and malononitrile: facile and convenient way to functionalized spirocyclic [indole-3,4’-pyrano[2,3-c]pyrazole] system. Mol. Divers. 2009, 13, 47–52. [26]. Maddila, S.; Gorle, S.; Shabalala, S.; Oyetade, O.; Maddila, S. N.; Lavanya, P.; Jonnalagadda, S. B. Ultrasound mediated green synthesis of pyrano[2,3-c]pyrazoles by using Mn doped ZrO2. Arab. J. Chem. 2019, 12, 671–679. [27]. Thakare, N. V.; Aswar, A. S.; Salunkhe, N. G. CoFe2O4@SiO2–HClO4 magnetic nanoparticles: synthesis and its application in catalysis. Emergent Mater. 2023, https://doi.org/10.1007/s42247-023-00502- 2. [28]. Leung, Y. L. Staphylococcus aureus. In Encyclopedia of Toxicology; Elsevier, 2014; pp. 379–380. [29]. Pisano; Kumar; Medda; Gatto; Pal; Fais; Era; Cosentino; Uriarte; Santana; Pintus; Matos Antibacterial activity and molecular docking studies of a selected series of hydroxy-3-arylcoumarins. Molecules 2019, 24, 2815. [30]. Zolfigol, M. A.; Tavasoli, M.; Moosavi-Zare, A. R.; Moosavi, P.; Kruger, H. G.; Shiri, M.; Khakyzadeh, V. Synthesis of pyranopyrazoles using isonicotinic acid as a dual and biological organocatalyst. RSC Adv. 2013, 3, 25681. [31]. Tayade, Y. A.; Dalal, D. S. β-Cyclodextrin as a Supramolecular Catalyst for the Synthesis of 1H-Pyrazolo[1,2-b]phthalazine-5,10-dione Derivatives in Water. Catal. Letters 2017, 147, 1411–1421. [32]. Badiger, K. B.; Giddaerappa; Hanumanthappa, R.; Sannegowda, L. K.; Kamanna, K. An Agro-waste based Eco-friendly synthesis, electrochemical behavior and anti-oxidant properties evaluation of pyrano[2,3- c ]pyrazole and pyrazolyl-4 H -chromenes derivatives. ChemistrySelect 2022, 7, e202104033. [33]. Mecadon, H.; Rohman, M. R.; Rajbangshi, M.; Myrboh, B. γ-Alumina as a recyclable catalyst for the four-component synthesis of 6-amino-4- alkyl/aryl-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-5- carbonitriles in aqueous medium. Tetrahedron Lett. 2011, 52, 2523– 2525. [34]. Zhou, C.-F.; Li, J.-J.; Su, W.-K. Morpholine triflate promoted one-pot, four-component synthesis of dihydropyrano[2,3-c]pyrazoles. Chin. Chem. Lett. 2016, 27, 1686–1690. [35]. Aliabadi, R. S.; Mahmoodi, N. O. Green and efficient synthesis of pyranopyrazoles using [bmim][OH−] as an ionic liquid catalyst in water under microwave irradiation and investigation of their antioxidant activity. RSC Adv. 2016, 6, 85877–85884. [36]. Patel, K. G.; Misra, N. M.; Vekariya, R. H.; Shettigar, R. R. One-pot multicomponent synthesis in aqueous medium of 1,4- dihydropyrano[2,3-c]pyrazole-5-carbonitrile and derivatives using a green and reusable nano-SiO2 catalyst from agricultural waste. Res. Chem. Intermed. 2018, 44, 289–304. [37]. Ebrahimi, J.; Mohammadi, A.; Pakjoo, V.; Bahramzade, E.; Habibi, A. Highly efficient solvent-free synthesis of pyranopyrazoles by a Brønsted-acidic ionic liquid as a green and reusable catalyst. J. Chem. Sci. (Bangalore) 2012, 124, 1013–1017. [38]. Chavan, H. V.; Babar, S. B.; Hoval, R. U.; Bandgar, B. P. Rapid one-pot, four component synthesis of pyranopyrazoles using heteropolyacid under solvent-free condition. Bull. Korean Chem. Soc. 2011, 32, 3963– 3966. [39]. Khurana, J. M.; Chaudhary, A. Efficient and green synthesis of 4H- pyrans and 4H-pyrano[2,3-c] pyrazoles catalyzed by task-specific ionic liquid [bmim]OH under solvent-free conditions. Green Chem. Lett. Rev. 2012, 5, 633–638. [40]. Saha, A.; Payra, S.; Banerjee, S. One-pot multicomponent synthesis of highly functionalized bio-active pyrano[2,3-c]pyrazole and benzylpyrazolyl coumarin derivatives using ZrO2 nanoparticles as a reusable catalyst. Green Chem. 2015, 17, 2859–2866. [41]. Wu, M.; Feng, Q.; Wan, D.; Ma, J. CTACl as catalyst for four-component, one-pot synthesis of pyranopyrazole derivatives in aqueous medium. Synth. Commun. 2013, 43, 1721–1726. Copyright © 2023 by Authors. This work is published and licensed by Atlanta Publishing House LLC, Atlanta, GA, USA. The full terms of this license are available at http://www.eurjchem.com/index.php/eurjchem/pages/view/terms and incorporate the Creative Commons Attribution-Non Commercial (CC BY NC) (International, v4.0) License (http://creativecommons.org/licenses/by-nc/4.0). By accessing the work, you hereby accept the Terms. This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution, or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (http://www.eurjchem.com/index.php/eurjchem/pages/view/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). http://www.eurjchem.com/index.php/eurjchem/pages/view/terms http://creativecommons.org/licenses/by-nc/4.0 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms 1. Introduction 2. Experimental 2.1. Materials and instrumentations 2.2. Preparation of magnetically recoverable nanocatalyst-CoFe2O4@SiO2-HClO4 2.3. General procedure for the synthesis of pyranopyrazoles using magnetic recoverable nanocatalyst- CoFe2O4@SiO2-HClO4 (4a-4o) 2.4. Antimicrobial activity 3. Results and discussion 3.1. Effect of solvent 3.2. Effect of catalyst concentration 3.3. Effect of temperature 3.4. Effect of microwave irradiation 3.5. Scope for substrate 3.6. Plausible mechanism 3.8. Reusability of catalyst 3.9. Antimicrobial activity 4. Conclusions Acknowledgments Disclosure statement CRediT authorship contribution statement ORCID and Email References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField16: PrintField17: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: PrintField26: PrintField27: