SCMNPs@Uridine/Zn: An efficient and reusable heterogeneous nanocatalyst for the rapid one-pot synthesis of tricyclic fused pyrazolopyranopyrimidine and 3-methyl carboxylate substituted pyrano[2,3-c]pyrazole derivatives under solvent-free conditions

Abstract SCMNPs@Uridine/Zn is utilized as an environmental-friendly and efficient heterogeneous nanocatalyst for two one-pot four-component condensation reactions, containing hydrazine hydrate, arylaldehyde, ethyl acetoacetate, and barbituric acid to yield tricyclic fused pyrazolopyranopyrimidine derivatives (5a-q), and hydrazine hydrate, arylaldehyde, malononitrile, and dimethyl acetylenedicarboxylate/diethyl acetylenedicarboxylate to yield 3-methyl carboxylate substituted pyrano[2,3-c]pyrazole derivatives (8a-y) under solvent-free conditions with high to excellent yields. The main advantages of this process are easy work-up, short reaction times, no chromatographic purifications, and recyclability of the catalyst for a minimum of six runs without any significant decrease in yields of the products. Also, the prepared catalyst SCMNPs@Uridine/Zn was synthesized and fully characterized by various techniques including Fourier transform infrared spectroscopy (FT-IR), energy dispersive X-ray (EDX), thermogravimetric analysis (TGA), X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Raman spectroscopy.


INTRODUCTION
Catalyst plays a signifi cant role in the industry as it can supply the highest yield of the desired product in the reduced time and temperature. A lot of systematic information about the catalytic properties has been established and accumulated from the literature. One of the most important information obtained relates to the limitations of catalytic reaction to separation and distribution 1-3 . Functionalization of the heterogeneous magnetic nanoparticles (MNPs) due to their unique features such as biocompatibility, simple modifi cation, large surface area to volume ratios, and non-toxicity as the bridge with homogenous catalysts can also provide benefi ts of them including high activity and selectivity 4 . In addition to the above-mentioned features, easy separation from the reaction medium and reused of MNPs by an external magnetic fi eld due to their paramagnetic nature make them the useful catalyst for sustainable and green chemistry. In order to protect the MNPs surface from oxidation and agglomeration due to the strong dipole-dipole attraction, and increase the dispersibility, effi ciency, and colloidal stability of this type of magnetic nanoparticles, their surface has to protect by an organic (surfactants, polymers) or inorganic (carbon, silica, calcium hydroxyapatite) materials 5- 10 .
Pyrazolopyranopyrimidine and molecules containing this considerable unit represent a variety of pharmaceutical and biological activities, such as hypoglycemic 26 , antidepressant 27 , hypotensive 28 , hepatoprotective 29 , antitumor 30 , antibronchitic 31 , and antimicrobial 32  , Oleic acid 37 , and Meglumine 38 . Most of these strategies suffer from disadvantages, such as requiring hazardous organic solvents, formation of byproduct, large concentration of the catalyst, long reaction times, and diffi culty in separation of the catalyst from the reaction solution. Therefore, the efforts for improving the reaction conditions for the preparation of pyrazolopyranopyrimidines using effective and reusable heterogeneous catalysts in the absence of solvent is a real challenge for synthetic chemists. scanning electron microscope. Magnetic susceptibility measurements were performed with a PPMS 6000 vibrating sample magnetometer (VSM) at the magnetic fi eld range of -10000 Oe to 10000 Oe at room temperature. The X-ray diffraction (XRD) studies were accomplished with a Siemens D-500 X-ray diffractometer (Munich, Germany). Thermogravimetric analysis (TGA) was carried out on a TGA thermoanalyzer (PerkinElmer) instrument. Elemental analysis was conducted using a Carlo-Erba EA1110CNNO-S. Raman analysis was conducted using a Witec Alpha 300 confocal Raman imaging microscope.

Preparation of Fe 3 O 4 magnetic nanoparticles (MNPs)
Magnetic nanoparticles of Fe 3 O 4 were achieved by an improved chemical co-precipitation procedure. Firstly, 7.3 g of FeCl 3 . 6H 2 O and 3.2 g of FeCl 2 . 4H 2 O salts were poured into a 250 mL fl ask reaction and dissolved in 150 mL of deionized water. Subsequently, 40 mL of ammonia solution was added dropwise into the reaction fl ask with vigorous stirring under a continuous fl ow of nitrogen gas at 70 o C for 50 min. Then the reaction solution was stirred and heated for another 1 h. The black precipitate MNPs was isolated by an appropriate magnetic fi eld and rinsed with an equal mixture of deionized water and ethanol several times and then dried under vacuum at 60 o C.

Preparation of Magnetite/Silica (SCMNPs) Core-Shell Nanoparticles
The SCMNPs core-shell structures were obtained by modifying the Stöber strategy. At the beginning, 1 g of freshly prepared MNPs were poured into a 100 mL fl ask reaction, and then 7 ml of distilled water, 40 mL of ethanol and 4.0 mL of aqueous ammonia solution (25 wt%) was added to the reaction fl ask. In the following, the reaction mixture was sonicated for 35 min at room temperature. After this period of time, 0.5 mL of tetraethylorthosilicate (TEOS) was added dropwise into the reaction fl ask and sonicated for another 20 min. The reaction solution was stirred for 4h at room temperature. The resultant was extracted by an external magnetic fi eld and washed with an equal mixture of de-ionized water and ethanol three times, and then dried under vacuum for 8 h.

Preparation of SCMNPs bonded 3-chloropropyltriethoxysilane (SCMNPs@Pc)
1g of the prepared SCMNPs was dispersed in 100 mL of dry toluene and mixed with 2 mL of 3-chloropropyltriethoxysilane (CPTCSi), and the resultant mixture was refl uxed for 24 h under a continuous fl ow of nitrogen gas. The precipitates of core-shell SCMNPs@Pc nanoparticles were separated using a powerful external magnet and washed with 3×10 mL of distilled water to eliminate the unreacted organic groups and next dried in a vacuum oven for 15 h.

Preparation of SCMNPs@Pc/TSC
1 g of core-shell SCMNPs@Pc nanoparticles was poured in 50 mL ethanol and dispersed with the aid of ultrasonication for 30 min. Next, 5 mL of thiosemicarbazide (TSC) was added into the reaction solution, and the contents of the reaction vessel were refl uxed for 24 h under the nitrogen atmosphere. After that, the precipitates were isolated from the solution by a powerful magnet and then rinsed three times with an equal mixture of ethanol and distilled water. Finally, the resulted product (SCMNPs@Pc/TSC) was dried in a vacuum oven for 12 h.

Preparation of SCMNPs@Uridine
1 g of the prepared SCMNPs@Pc/TSC was dispersed in 100 mL of an equal mixture of ethanol and water and mixed with 0.9 g of uridine. The resultant mixture was stirred at 90 o C for 10 h. The resultant solid (SCMNPs@ Uridine) was isolated using an external magnet and rinsed three times to remove the unreacted organic groups and dried in a vacuum oven for 15 h.

Preparation of SCMNPs@Uridine/Zn
1 g of SCMNPs@Uridine and 0.2 g of ZnCl 2 was added in 30 mL of acetonitrile and then was stirred for 24 h under nitrogen atmosphere. After the specifi ed time, the precipitate of SCMNPs@Uridine/Zn formed was isolated by magnetic decantation, rinsed three times with 3×10 mL of ethanol to eliminate unreacted metal precursors. Last of all, the resultant dried under vacuum oven for 15 h to achieve the pure product. All stages of the SCMNPs@Uridine/Zn synthesis are shown in Scheme 1. Scheme 1. All stages of the SCMNPs@Uridine/Zn synthesis along Fe-O bonds of magnetic Fe 3 O 4 nanoparticles. In addition, the bands at 345 (E g ), 478 (T 2g ), and 695 (A 1g ) cm -1 related to the maghemite can be seen. These results confi rmed that the magnetic nanocatalyst is successfully composed of the maghemite and magnetite phases of iron oxide.

XRD analysis of SCMNPs@Uridine/Zn
The structure of MNPs, SCMNPs, and SCMNPs@ Uridine/Zn were characterized by X-ray diffraction (XRD) spectroscopy (Fig. 4). XRD analysis of the MNPs exhibited close patterns to the patterns of spinel ferrites explained in the literature (JCPDS NO. 88-0315). Also, the same peaks were achieved for the SCMNPs, and SCMNPs@Uridine/Zn XRD patterns compared to the MNPs, indicating retention of the spinel phase of magnetic iron oxide nanoparticles during the silica--coating process.

FTIR analysis of SCMNPs@Uridine/Zn
The FT-IR spectra of MNPs, SCMNPs, SCMNPs@Pc, SCMNPs@Pc/TSC and SCMNPs@Uridine, SCMNPs@ Uridine/Zn were shown in Fig. 1. In the FTIR spectrum of the MNPs, the peak at 593 and 459 cm -1 were attributed to the characteristic Fe-O-Fe vibration of the magnetite phase, and the peak at 3356 cm -1 was attributed to the characteristic O-H stretching vibration. The absorption peaks of the SCMNPs at 1095 and 951 cm -1 are connected to the stretching vibration of Si-O-Si and Si-OH groups, respectively. The absorption peaks of the aliphatic group in the SCMNPs@Pc at 2983 cm -1 can be ascribed to stretching vibrations of C-H bond. The spectra of SCMNPs@Pc/TSC showed the characteristic bending vibration of N-H, stretching vibration of C=S, and stretching vibration of N-H at 1476, 1305, and 3312 cm -1 , respectively. As can be seen from the spectra of SCMNPs@Uridine/Zn, the functionalization of SC-MNPs@Pc/TSC with uridine can be confi rmed by bands at 1576 and 1411 cm -1 resulting from the C=C group.

VSM analysis of SCMNPs@Uridine/Zn
The magnetic features of the MNPs, SCMNPs, and SC-MNPs@Uridine/Zn were appointed at room temperature by using a vibrating sample magnetometer (VSM) (Fig. 2). The saturation magnetizations (Ms) for MNPs, SCMNPs, and SCMNPs@Uridine/Zn were discovered to be 61.42, 40.15, and 33.59 emu/g, respectively. Compared with the MNPs, a clear decrease in the saturation magnetization of the SCMNPs confi rms the formation infl uence of the silica layers on the surface of the magnetic nanoparticles. Furthermore, a decrease in the saturation magnetization of the SCMNPs@Uridine/Zn, confi rms functionalization of the MNPs with the organic moieties

Raman analysis of SCMNPs@Uridine/Zn
In order to discern both phases of iron oxides, the Raman microscope was applied to collect the spectra of MNPs, SCMNPs, and SCMNPs@Uridine/Zn at several regions (Fig. 3). In all of the samples, the main band around 654 cm -1 is observed. This band can be attributed to the intrinsic stretching vibrations of oxygen atoms are shown in Fig. 5. In all of the samples, initial weight loss was achieved up to 150 o C due to the loss of the physisorbed and chemisorbed hydroxyl groups and solvents on the support and silica layers. In the case of SCMNPs@Pc, a weight loss appears below 320 o C due to the decomposition of the attached organic molecule of 3-chloropropyltriethoxysilane on the Fe 3 O 4 MNPs surface. Moreover, the TGA curves of SCMNPs@Pc/TSC and SCMNPs@Uridine/Zn, several weight loss in the range from 200 to 600 o C, resulting from the decomposition of the covalently attached organic moieties In this research, we reported our outcomes for the rapid and effi cient synthesis of tricyclic fused pyrazolopyranopyrimidine and 3-methyl carboxylate substituted pyrano[2,3-c]pyrazole derivatives using SCMNPs@Uridine/ Zn as an effective heterogeneous magnetic nanocatalyst under solvent-free conditions (Scheme 2).

EDX analysis of SCMNPs@Uridine/Zn
The EDX result of SCMNPs@Uridine/Zn sample is depicted in Fig. 6, indicating the presence of Fe, Si, N, O, C, S and Zn in the prepared catalyst. The existence of C, O, N, and S elements demonstrates the successful loading of uridine onto the surface of the silica-coated magnetic nanoparticles. Moreover, the presence of Zn signals indicates the coordination of Zn with the organic molecules on the surface of the material. According to these results, it can be expected that SCMNPs@Uridine/ Zn catalyst had been successfully prepared.
For our initial investigation and in order to optimize the reaction conditions, the reaction of hydrazine hydrate (1 mmol), benzaldehyde (1 mmol), ethyl acetoacetate (1 mmol), and barbituric acid (1 mmol) was accomplished using different amounts of catalyst with numerous solvents in the absence and presence of temperature. The results are summarized in Table 1 Entries 8 and 14-15). As is evident, by increasing the amount of catalyst (20 to 25 mg), the yield of the product was not improved (Table 1, Entry 14) and also by decreasing it to 15 mg, the product quantity was changed from excellent to good yield (Table 1, Entry 15). In the fi nal section of the study, in order to determine the ability of SCMNPs@Uridine/Zn in preparation of tricyclic fused pyrazolopyranopyrimidine derivatives, the effi ciency of SCMNPs@Uridine/Zn for the reaction of hydrazine hydrate, aldehyde, ethylacetoacetate, and barbituric acid was compared with previously reported catalysts in the literature (Table 1, Entries 16-21). Thus, this procedure with SCMNPs@Uridine/Zn as the novel heterogeneous nanocatalyst seems superior to other recently reported synthetic methods.
Having optimized reaction conditions at hand, a highly effi cient one-pot four-component synthesis of tricyclic fused pyrazolopyranopyrimidine derivatives has also been developed applying the same methodology (Table 2). To determine the limitation of the reaction, the reaction of barbituric acid with various aromatic aldehydes was performed according to the general optimized experimental procedure. The corresponding products are tabulated in this table. The yields of most products are higher than 89%. Thus, the aldehydes bearing electron-withdrawing groups and also electron-donating substituents (2a-q) gave the corresponding products with hydrazine hydrate (1), ethyl acetoacetate (3), and barbituric acid (4) under the optimized conditions (Table 2, Entries 1-16). As shown, in the presence of SCMNPs@Uridine/Zn as a catalyst, excellent yields of the product were achieved in shorter reaction times under solvent-free conditions.
To optimize the reaction conditions, condensation of hydrazine hydrate (1 mmol), 4-nitrobenzaldehyde (1 mmol), malononitrile (1 mmol), and dimethyl acetylenedicarboxylate (7a) /diethyl acetylenedicarboxylate (7b) (1 mmol) was selected as a model reaction ( Table 3). The above fourcomponent reaction was performed at different temperatures and solvents in the various amounts of the catalyst to establish the real effi cacy of the SCMNPs@Uridine/Zn catalyst. To compare the effects of solvent on the reaction, various solvents such as H 2 O, EtOH, H 2 O/EtOH, CH 2 Cl 2 , CH 3 CN, and under solvent-free conditions were utilized ( Table 3, Entries 1-6). Among the solvents mentioned above, H 2 O was found to be the best one, generating the high yield of the product (Table 3, Entry 1). Moreover, when the reaction was performed under solvent-free conditions, the target product was obtained with excellent yield and short reaction time (Table 3, Entry 6). Moreover, the model reaction was tested in the presence of various molar ratios of SCMNPs@Uridine/Zn at 25 o C under solvent-free conditions (Table 3, Entries 6, and 13-14). These entries indicate that an excellent yield and a short reaction time was achieved when the reaction was performed in the presence of 15 mg of the catalyst in the absence of solvent; under Table 1. Optimization of reaction condition using different conditions a Table 2. Synthesis of tricyclic fused pyrazolopyranopyrimidine derivatives a these conditions, the corresponding 3-methyl carboxylate substituted pyrano[2,3-c]pyrazole 8f was produced in 97% yield within 15 min (Table 3, Entry 6). Increasing the amount of the catalyst to 20 mg illustrated no substantial improvement in the yield of product (Table 3, Entry 13), whereas the yield diminished by decreasing the amount of catalyst to 10 mg (Table 3, Entry 14). Also, the effect of temperature on the transformation was examined, and the results obtained were summarized in Table 3. It is obvious that at 25 o C, a high yield of product was obtained (Table 3, Entry 6). With an increase in the temperature from 40 o C to 90 o C, the product was achieved without any increase in the yield (Table 3, Entries 7-12). In order to show the effi ciency of this procedure, we have compared our results from the synthesis of 3-methyl carboxylate substituted pyrano[2,3-c]pyrazoles using SCMNPs@Uridine/Zn as the catalyst with some of the previously reported methods (Table  3, Entries 15-20). As it is clear from these entries, the use of SCMNPs@Uridine/Zn leads to an improved protocol in the product yields, reaction times, compatibility with the environment, and the amount of the catalyst.
After optimization of the reaction conditions, a wide range of structurally-diverse aldehydes (2) containing electron-withdrawing as well as electron-donating groups such as Cl, F, Br, NO 2 , OCH 3 , and OH in the ortho, meta, and para positions on the benzene ring were reacted with hydrazine hydrate (1), malononitrile (4), dimethyl acetyle-nedicarboxylate (7a)/diethyl acetylenedicarboxylate (7b), in the presence of SCMNPs@Uridine/Zn at 25 o C to provide the corresponding products in high yields and short reaction times (Table 4, Entries 1-25).
On the basis of the above result, a plausible mechanism for the formation of tricyclic fused pyrazolopyranopyrimidine derivatives from hydrazine hydrate, aldehyde, ethylacetoacetate and barbituric acid was described in Scheme 3.  The reusability of the SCMNPs@Uridine/Zn in the preparation of 3-methyl-4-(4-nitrophenyl)-4,8-dihydropyrazolo[4',3':5,6]pyrano[2,3-d]pyrimidine-5,7(1H,6H)-dione (5k) and methyl 6-amino-5-cyano-4-(4-nitrophenyl)-1,4--dihydropyrano[2,3-c]pyrazole-3-carboxylate (8f) under the optimized reaction conditions was also studied for six runs. To achieve this goal, after the completion of the reaction, the achieved solid product was dissolved in a hot ethanol. After that, the SCMNPs@Uridine/ Zn was extracted using a permanent magnetic fi eld. The separated catalyst was rinsed with deionized water, dried, and used for six runs with a negligible decrease in catalytic activity (Table 5). Table 6. Comparison of the current procedure with several of the previously reported strategies for synthesizing pyrazolopyranopyrimidine and 3-methyl carboxylate substituted pyrano[2,3-c]pyrazole derivatives Table 6 illustrates the effi ciency of SCMNPs@Uridine/ Zn as the novel and effective magnetic nanocatalyst in the synthesis of pyrazolopyranopyrimidine and 3-methyl carboxylate substituted pyrano[2,3-c]pyrazole derivatives compared with several of the previously reported catalysts. It is clear that a suitable strategy in terms of product yield, reaction time, and compatibility with the environment in the presence of a low amounts of the SCMNPs@Uridine/Zn has been developed.

CONCLUSION
Magnetically recoverable SCMNPs@Uridine/Zn was found to be a highly effi cient and economically sustainable heterogeneous nanocatalyst for preparing a broad range of substituted tricyclic fused pyrazolopyranopyrimidine and 3-methyl carboxylate substituted pyrano[2,3-c]pyrazole derivatives through a one-pot four-component condensation reaction. The considerable features of this procedure are good-to-excellent product yield, short reaction time, green and mild reaction conditions, as well as the use of an easily magnetically recyclable catalyst. This procedure suggests various merits, including lower loading of the catalyst, the usage of a green catalyst, no hazardous solvent, and easy work-up. The catalyst was completely characterized using various techniques, including Fourier transform infrared spectroscopy (FT-IR), energy dispersive X-ray (EDX), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM).