INTRODUCTION:

Multicomponent reaction (MCR) is a chemical reaction where more than two reactants combine to form a single product. By using this strategy, wide range of functionally novel and complex heterocyclic molecules are prepared. MCRs are superior as compared to routine multistep synthesis ¹. Nowadays, Multi-component reactions (MCRs) used extensively in the field of synthetic and medicinal chemistry, because the strategies of MCR offer significant advantages over conventional linear-type synthesis such as shorter reaction time, higher yield of products, economical favorable and environmental friendly approach ^2,3.

Pyranopyrazoles are fused heterocyclic compounds, which are biologically important because it shows properties such as bactericidal ⁴, vasodilatory activities ⁵ and they act as anticancer agents ⁶. They also shows important application in pharmaceutical ingredients and biodegradable agrochemicals ⁷. Moreover, pyrano [2,3-c]pyrazoles also act as potential insecticidal ⁸ and molluscicidal agents ^9,10. Due to this potent biological activities have encouraged various chemists to prepare 1, 4-dihydropyrano [2, 3- c] pyrazole derivatives ¹¹. Condensed pyrazolo derivatives are also biologically potent compounds and their chemistry has recently received considerable attention ^12,13. The biological properties of 1,4-dihydropyrano[2,3-c]-pyrazole have attracted many synthetic chemists to explore different methods suitable for their synthesis, though there are several methods reported in the literature for the formation of four component reaction such as by using nano TiO₂¹⁴, nano-CuI ¹⁵, Fe3O4 nanoparticles ¹⁶, DABCO¹⁷, DBU ¹⁸, isonicotinic acid ¹⁹, disulfonic acid imidazolium chloroaluminate ²⁰ , piperidine and pyridine ²¹ , pyrrolidine ²² , iodine ²³ , cerium ammonium nitrate (CAN) ²⁴.

Microwave synthesis represents a major breakthrough in synthetic chemistry methodology, a dramatic change in the way chemical synthesis is performed and in the way it is perceived in the scientific community. Microwave provides a powerful way to do synthetic chemistry in green approach ^25-28. Chitosan (CS) is an example of a polysaccharide that is widely distributed in living organisms and act as solid base catalyst, due to this it is attractive polysaccharide for application in catalysis ^29,30. Chitosan is actually a heteropolymer containing both glucosamine units and acetyl glucosamine units. In various biopolymers, chitosan based materials have attracted great interest as support for catalytic applications. Chitosan, due to natural polymer, can be suitable for various homogeneous and heterogeneous catalytic activities ^31-35 .

In this work, we frame new eco-friendly synthetic approach for the syntheses of pyranopyrazoles by using reusable green catalyst under water as green solvent medium. Thus, it is reported as simple, efficient, and a one-pot four-component protocol for the synthesis of pyranopyrazole derivatives in water using chitosan hydrogel as a green, inexpensive, and efficient catalyst involved environmentally friendly procedure.

RESULTS AND DISCUSSION:

Pyranopyrazole derivatives 4a-4h was synthesized by using recyclable catalyst and environmentally friendly conditions and the results were presented in Table 3.

Catalyst Characterisation

The FT-IR spectrum for the catalyst is shown in Fig. 1. The peak at 1645cm⁻¹ indicated the presence of –NH2 group resulting from deacetylation of chitin. The band at 3284cm⁻¹ corresponds to the vibrational stretching of the hydroxyl groups. This wide peak also indicated that the hydroxyl groups are hydrogen-bonded. The band that appeared at 2873cm⁻¹ is due to the C–H stretching vibration of aliphatic CH groups and that at 1060cm⁻¹ to the C–O–C bond stretching vibrations ^36-40.

Fig. 1. FT-IR spectrum of the chitosan Hydrogel catalyst

Fig. 2. SEM image of the chitosan Hydrogel catalyst.

The surface morphology of the catalyst is shown in Fig. 2. From the SEM image it is clear that the catalyst surface has heterogeneous layered structure with flakes and voids present. Flakes of different sizes are formed due to this so many activation sites are present.

Optimization of reaction conditions

Optimization of catalyst loading

To find optimal loading of catalyst and reaction condition, a mixture of 4-chlorobenzaldehyde (1 mmol), malononitrile (2 mmol), ethyl acetoacetate (1 mmol), hydrazine hydrate (1.5 mmol) and Chitosan hydrogel as catalyst in water (3 ml) as solvent were irradiated in microwave synthesizer system at 630W (70^oC-75^oC) for 100 sec as model reaction.. In the absence of catalyst, the yield of the product was very low which indicate crucial role of catalyst. 25 mg of Chitosan hydrogel as catalyst was suitable to catalyze the reaction smoothly and results were given in Table 1.

Table 1 Optimization of catalyst loading to synthesize Pyranopyrazoles^a

Entry	Catalyst mg	Time (Hr)	Yield ^b (%)
1	0	1	15
2	5	1	40
3	10	1	55
4	15	1	65
5	20	1	80
6	25	1	90
7	30	1	88

^a Reaction conditions: aromatic aldehyde (1 mmol), malononitrile (1 mmol), ethyl acetate (1 mmol), hydrazine hydrate (1.5mmol)

^b Isolated yield

From the green chemistry point of view, efficient recovery and reuse of the catalyst was highly desirable, thus the recovery and reusability of Chitosan hydrogel was investigated. After the reaction completed, Chitosan hydrogel as the catalysts was isolated from the reaction mixture by simple filtration and reused again after washing with EtOH. The reusability of Chitosan hydrogel was examined efficiently (without any activation) by using 4- chloro benzaldehyde as a model substrate. The recovered Chitosan hydrogel was reused directly for four consecutive cycles and all the results were tabulated in Table2 and graphically represented in Fig 3.

Table 2 Catalyst Reusability for synthesis of Pyranopyrazoles^a

No. Of cylcles	Catalyst Recovery	Yield ^b (%)
Cycle 1	95	90
Cycle 2	90	82
Cycle 3	85	74
Cycle 4	80	70

^a Reaction conditions: aromatic aldehyde (1 mmol), malononitrile (1 mmol), ethyl acetate(1 mmol),hydrazine hydrate (1.5 mmol)

^b Isolated yield

Experimental Section

All reactions were performed in the borosil round bottom flask, volume 25 mL. Analytical thin layer chromatography was performed using thin layer chromatography (TLC) pre-coated silica gel 60 F254 Merck (20 × 20 cm). TLC plates were visualized by exposing to ultraviolet light. Microwave reactions were carried out in a Microwave Synthesizer System (850W power; Cata R System). Melting points were taken in an open capillary and are uncorrected. SEM image was obtained on JEOL JSM-6360. ¹H nuclear magnetic resonance (NMR) and ¹³C spectra were recorded with AV 400 Bruker 400 MHz NMR instrument. Chemical data for protons are reported in parts per million (ppm, scale) downfield from tetramethylsilane and are referenced to the residual proton in the NMR solvent (DMSO: δ 2.5).

Preparation of Chitosan Hydrogel

The chitosan catalyst was synthesized by using the procedure reported in literature ^40-43. Low-molecular weight chitosan (0.32 g) obtained from shrimp shells was dissolved at room temperature in 20 ml 0.1M HCl solution and magnetically stirred up to its complete dissolution. The completely dissolved chitosan solution was poured dropwise into 0.1M NaOH solution (300 mL). The resulting chitosan hydrogel was kept as it is at room temperature for 1 hr without stirring and then filtered. After filtration chitosan found in the form of hydrogel was washed with excess distilled water until the filtrate became neutral. The neutrality of the filtrate was checked by phenolphthalein indicator. The obtained chitosan hydrogel was dried at 80 ^◦C and then was powdered by using a mortar and pestle. FT-IR spectra of the catalyst were recorded using KBr pellets in the wavelength range of 400–4000cm⁻¹. Surface morphology was characterized using scanning electron microscopy (JEOL JSM-6360).

Thermal Method for the preparation of Substituted Pyranopyrazoles Derivatives:

A mixture of aromatic aldehyde (1mmol), malononitrile (1mmol),ethyl acetoacetate ( 1mmol) , hydrazine hydrate (1.5 mmol) and chitosan hydrogel (25 mg) as catalyst in 5 ml water was mixed and magnetically stirred at 90^oC temperature for appropriate reaction time as specified in Table 1.The progress of the reaction was monitored by TLC (30 % EA : n-Hexane). After completion of the reaction as monitored by TLC, the reaction mass cooled, filtered off and washed with hot ethanol (5 ml) to separate the product from the catalyst. The ethanol from the filtrate was allowed to evaporate at room temperature to afford the pure products (Scheme 1).

Microwave Irradiation for the preparation of Substituted Pyranopyrazoles Derivatives:

A mixture of aromatic aldehyde (1mmol), malononitrile (1mmol), ethyl acetoacetate (1mmol) , hydrazine hydrate (1.5 mmol) and chitosan hydrogel (25 mg) as catalyst in 5 ml water was mixed and were irradiated in microwave synthesizer system at 630W (70^oC-75^oC) for 100 sec. Work up was done as per reported in thermal method (Scheme 1).

Spectral data of a representative compounds

6-amino-4-(4-chlorophenyl)-3-methyl-1,4-dihydropyrano [2,3-c]pyrazole-5- carbonitrile (4a)

Off-white solid, IR (KBr) cm^-1: 3477, 3391 (NH2), 3230 (–NH–), 3096, 2193 (–CN), 1490 (–NH–), 1047 (Ar–Cl), 796 (Para-Cl).

1H NMR (400 MHz, DMSO-d6): (ppm) 1.78 (s, 3H), 4.62 (s, 1H), 6.89 (s, 2H, NH2), 7.16–7.19 (d, 2H, J = 8.40 Hz, Ar–H), 7.35–7.37 (d, 2H, J = 8.40 Hz, Ar–H), 12.10 (s, 1H, NH).

6-amino-4-(4-bromophenyl)-3-methyl-1,4-dihydropyrano [2,3-c]pyrazole-5- carbonitrile (4b)

Yellow solid, IR (KBr) cm^-1: 3477, 3391 (NH2), 3226 (–NH–), 3095(Aromatic), 2191 (–CN), 1489 (–NH–), 1049 (Ar–Br), 792 (Para-Br).

1H NMR (400 MHz, DMSO-d6): (ppm) 1.78 (s, 3H), 4.60 (s, 1H), 6.89 (s, 2H, NH2), 7.11–7.13 (d, 2H, J = 8 Hz, Ar–H), 7.49–7.51 (d, 2H, J = 8 Hz, Ar–H), 12.11 (s, 1H, NH).

6-amino-4-(4-hydroxyphenyl)-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5carbonitrile (4c)

White solid, IR (KBr) cm^-1: 3433, 3371 (NH2), 3302 (–NH–), 3124 (Aromatic), 2171 (–CN), 1489 (–NH–), 1039 (Ar–Cl), 808 (Para).

1H NMR (400 MHz, DMSO-d6): (ppm) 1.77 (s, 3H), 4.45 (s, 1H), 6.66–6.68 (d, 2H, J = 8.40 Hz, Ar–H), 6.74 (s, 2H, NH2), 6.92–6.95 (d, 2H, J = 8.40 Hz, Ar–H), 9.23(s, 1H, OH) 12.10 (s, 1H, NH). ¹³C NMR (100 MHz, DMSO-d6): (ppm) 9.69, 35.45, 57.79, 98.02, 115.06, 120.82, 128.37, 134.71, 135.45, 154.71, 155.97, 160.58.

6-amino-4-(4-methylyphenyl)-3-methyl-1,4-dihydropyrano [2, 3-c] pyrazole-5- carbonitrile (4d)

White solid, IR (KBr) cm^-1: 3398, 3379 (NH2), 3312 (–NH–), 3186 (Aromatic), 2191 (–CN), 1485 (–NH–), 1039 (Ar–Cl), 871 (Para).

1H NMR (400 MHz, DMSO-d6): (ppm) 1.77 (s, 3H), 2.25 (s, 3H), 4.52 (s, 1H), 6.79 (s, 2H, NH2) 7.02–7.04 (d, 2H, J = 8.0 Hz, Ar–H), 7.09–7.11 (d, 2H, J = 8.0 Hz, Ar–H), 12.04 (s, 1H, NH). ¹³C NMR (100 MHz, DMSO-d6): (ppm) 9.70, 20.57, 35.83, 57.40, 97.69, 120.74, 127.31, 128.94, 135.49, 135.67, 141.4, 154.74, 160.74.

6-amino-4-(4-methoxyphenyl)-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4h)

Light yellow solid, IR (KBr) cm^-1: 3483, 3402 (NH2), 3248 (–NH–), 3097 (Aromatic), 2189 (–CN), 1490 (–NH–), 802 (Para-NO2).

1H NMR (400 MHz, DMSO-d6): (ppm) 1.71 (s, 3H,CH3), 3.71 (s, 3H, OCH3), 4.52 (s, 1H, 4H), 6.77 (s, 2H, NH2), 6.83–6.87 (d, 2H,J = 8.4 Hz, Ar–H), 7.04–7.08 (d, 2H, J = 8.4 Hz, Ar–H), 12.04 (s, 1H, NH). 13C NMR (100 MHz, DMSO-d6): (ppm) 9.69, 35.41, 54.94, 97.84, 113.71, 114.35, 120.76, 128.43, 129.92, 135.49, 136.44, 154.72, 157.92, 160.64.

CONCLUSION:

In summary, we have reported efficient one-pot four component protocol for the preparation of various 6-amino-1,4-dihydropyrano[2,3-c]-pyrazole-5-carbonitriles using Chitosan Hydrogel as a green and reusable catalyst. Easy preparation of the catalyst, low reaction times was the main advantages of this method. Satisfactory yield of products and easy workup make this a useful protocol for green synthesis of this class of compounds.

ACKNOWLEDGEMENTS:

This research was financially supported by UGC, New Delhi [File No. 47-1160/14 (WRO) dated: 28^th Dec. 2015].We are grateful for analytical help of SIF, VIT university, Vellore and SAIF IISc, Bangalore.

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Received on 13.01.2018 Modified on 28.02.2018

Asian J. Research Chem. 2018; 11(2):477-484.

DOI:10.5958/0974-4150.2018.00087.1