Synthesis of Pyranopyrazoles using a Magnetically Separable Modified Preyssler Heteropoly Acid

Full text of DOI:10.1080/00304948.2016.1206424

Organic Preparations and Procedures International
The New Journal for Organic Synthesis
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Synthesis of Pyranopyrazoles using a Magnetically
Separable Modified Preyssler Heteropoly Acid
Ali Javid, Amir Khojastehnezhad, Hossein Eshghi, Farid Moeinpour, Fatemeh
F. Bamoharram & Javad Ebrahimi
To cite this article: Ali Javid, Amir Khojastehnezhad, Hossein Eshghi, Farid Moeinpour, Fatemeh
F. Bamoharram & Javad Ebrahimi (2016) Synthesis of Pyranopyrazoles using a Magnetically
Separable Modified Preyssler Heteropoly Acid, Organic Preparations and Procedures
International, 48:5, 377-384, DOI: 10.1080/00304948.2016.1206424
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Date: 23 September 2016, At: 06:45

Organic Preparations and Procedures International, 48:377–384, 2016
Copyright Ó Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2016.1206424
Synthesis of Pyranopyrazoles using a Magnetically
Separable Modified Preyssler Heteropoly Acid
Ali Javid,1 Amir Khojastehnezhad,2 Hossein Eshghi,3
Farid Moeinpour,4 Fatemeh F. Bamoharram,5
and Javad Ebrahimi2
1Department of Chemistry, College of Science, Ahvaz Branch, Islamic Azad
University, Ahvaz, Iran
2Young Researchers and Elite Club, Mashhad Branch, Islamic Azad University,
Mashhad, Iran
3Department of Chemistry, Faculty of Sciences, Islamic Azad University of
Ahvaz and Mashhad, Mashhad, Iran
4Department of Chemistry, College of Sciences, Bandar Abbas Branch, Islamic
Azad University, Bandar Abbas 7915893144, Iran
5Department of Chemistry, Mashhad Branch, Islamic Azad University,
Mashhad, Iran
The design and preparation of recyclable catalysts are of great economic and environmen-
tal importance in the chemical and medicinal industries. Immobilization of homogeneous
catalysts on various insoluble supports can lead to simplified catalyst recycling by filtra-
tion or centrifugation.1 Nanoparticles (NPs) are attractive candidates as solid supports for
the immobilization of homogeneous catalysts2 and because of their large surface area,
which can bear a large amount of catalytically active species, these supported catalysts
display very high activity under mild conditions. Among these NPs, much attention has
been directed toward the production of magnetic nanoparticles (MNPs),3–5 because these
NPs can be well dispersed in reaction mixtures without magnetic field thus providing
large surface for ready access to substrate molecules. More importantly, after completion
of the reactions, the MNP catalysts may be isolated efficiently from the reaction mixture
through a simple magnetic separation process.6,7
Preyssler heteropoly acid (HPA, H14NaP5W30O120) is an important acid catalyst
because it is safe and has significant other advantages such as the availability of fourteen
acidic hydro-gens, high thermal stability and high hydrolytic stability (pH 0–12).8,9
Because their low surface area (7–10 m2/g) and high solubility in polar solvents, it usu-
ally preferable to use them in a supported form such as on acid-neutral solids, such as sil-
ica and alumina, activated carbons, zeolites, acidic ion-exchange resins.8–10
Multi-component reactions (MCRs) have emerged as an efficient and powerful
method in modern organic chemistry because the synthesis of complex organic molecules
Received March 12, 2016; in final form June 12, 2016.
Address correspondence to Amir Khojastehnezhad, Young Researchers and Elite Club, Mash-
had Branch, Islamic Azad University, Mashhad, Iran. E-mail: Akhojastehnezhad@yahoo.com
377

378
Khojastehnezhad et al.
from simple and readily available substrates can be achieved in a very rapidly and in an
efficient manner without the isolation of intermediates.11–14 MCRs contribute to the prin-
ciples of an environmentally friendly process by reducing the number of steps, energy
consumption and production of waste. Therefore, the discovery for new MCRs and
improvement of previously known MCRs with recyclable catalysts are of considerable
interest.15–17
Pyranopyrazoles constitute an emerging class of heterocycles,18,19 which is exten-
sively explored as an important core for emerging drugs that display numerous biological
properties including potential inhibitor of human Chk1 kinase,20 anti-inflammatory,21
anti-cancer, analgesic, molluscicidal22–24 and antimicrobial activity.25 Therefore, consid-
erable efforts have been expended to explore new simple and direct approaches towards
the construction of the pyranopyrazole skeleton. Most of these methods employ various
catalysts such as Fe3O4 NPs,26 b-cyclodextrin,27 L-proline,28 acidic ionic liquid,29 tetra
(n-butyl)ammonium bromide,30 P2O5-SiO2,31 CeCl3 and NH4H2PO4/Al2O3.33 Though
32
these catalysts are quite useful, most of them have limitations, such as cost, long reaction
times, use of toxic organic solvents and of harsh reaction conditions. Thus, the develop-
ment of simple, efficient, clean and environ-mentally friendly catalysts for the preparation
of these compounds in high yields is an im-portant goal. In continuation of our previous
studies,34–42 the present article describes the results of an investigation of the activity of
the Preyssler HPA supported onto the silica coated NiFe2O4 MNPs (NiFe2O4@SiO2-
Preyssler, abbreviated NFS-PRS) as an efficient and green catalyst for the synthesis of
pyranopyrazole derivatives.
To the best of our knowledge, there are no examples of the use of NFS-PRS as a cata-
lyst for the preparation of pyranopyrazoles from the condensation of aromatic aldehydes,
hydrazine hydrate, ethyl acetoacetate and malononitrile at room temperature in water.
(Scheme 1).
a) R = C6H5; b) R = 4-Cl-C6H4; c) R = 4-CH3- C6H4; d) R = 4-CH3O-C6H4; e) 4-Br-C6H4;
f) R = 4-NO2-C6H4; g) R = 4-F-C6H4; h) 4-OH-C6H4; i) 2-CH3O-C6H4; j) R = 2-Cl-C6H4;
k) R = 2-NO2-C6H4; l) R = 3-Br-C6H4; m) 3-NO2-C6H4; n) R= 3-Cl-C6H4; o) R = 3-CH3-C6H4
Scheme 1
In order to explore the catalytic activity of catalyst (NFS-PRS), prepared according to
our previous work,43 the four-component reaction of benzaldehyde, hydrazine hydrate,
ethyl acetoacetate and malononitrile was carried out as a model reaction. Solvent-free
conditions as well as the use of various solvents such as CH2Cl2, EtOH, MeOH and H2O
(Table 1) were investigated; no product was obtained in the absence of the catalyst (Entry
1). The yields of the reaction were better in polar solvents than in non-polar solvents and
under solvent-free conditions. It is possible that polar solvents facilitates the dissociation
of the HPAs and generate protons; the formation hydrogen-bond between the HPAs and
the solvent may also assist the process (Entries 5–7).44 Water gave the best results from
among (Entry 7). Increasing the reaction time and temperature did not improve the yields

Synthesis of Pyranopyrazoles
379
Table 1
Comparison of Different Solvents and Catalysts for the Synthesis of Pyranopyrazole 5a
Entry
Catalyst
Solvent
H2O
Time (min) Temperature
Yield (%)
1
None
NFS-PRS
120
15
15
15
15
15
15
15
60
15
15
15
rt
rt
No reaction
2
solvent-free
solvent-free
CH2Cl2
CH3OH
C2H5OH
H2O
48
56
3
NFS-PRS
NFS-PRS
100
rt
4
Trace
70
83
5
NFS-PRS
NFS-PRS
rt
6
rt
7
NFS-PRS
NFS-PRS
NFS-PRS
rt
98
98
8
H2O
H2O
ref
ref
rt
9
97
90
10
11
12
H14[NaP5W30O110
H3[PW12O40]
H4[SiW12O40]
]
H2O
H2O
rt
82
79
H2O
rt
(Entries 8 and 9). In addition, the model reaction carried out in presence of Preyssler and
others HPAs with Keggin structures (H3[PW12O40], H4[SiW12O40])45 the same time and
conditions led to lower yields than with NFS-PRS as a catalyst (Entries 10–12).
In order to determine the optimum quantity of NFS-PRS to be used, the model reac-
tion was carried out under the previously mentioned conditions using different amounts
of catalyst (Table 2). As already been mentioned, no product was obtained in the absence
of the catalyst (Entry 1). Increasing the amount of the catalyst improved the yield of 5a
(Entries 2–5), with the use of 0.03 g of catalyst resulted in the highest yield in 15 min
(Entry 5). Further increase of the amount of the catalyst failed to affect the yield notice-
ably (Entries 6 and 7).
The optimized conditions having been determined, the scope and efficiency of the
reaction were investigated for the preparation of a variety of substituted 6-amino-4-aryl-
3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles in the presence of NFS-PRS
using a number aromatic aldehydes bearing both electron-donating and electron-with-
drawing substi-tutents. Excellent yields were obtained as illustrated in Table 3.
After the completion of the model reaction, the nano-magnetic catalyst was separated
from the reaction mixture by placing an external magnet and decantation of the solution
Table 2
Comparison of the Amount of NFS-PRS and Yields for the Synthesis of Pyranopyrazole
5a
Entry
Conditions
Catalyst amount (g)
Time (min)
Yield (%)
1
2
3
4
5
6
7
Room temperature/ H2O
Room temperature/ H2O
Room temperature/ H2O
Room temperature/ H2O
Room temperature/ H2O
Room temperature/ H2O
Room temperature/ H2O
None
0.003
0.007
0.015
0.030
0.050
0.100
120
30
20
15
15
15
15
None
39
70
84
98
97
98

380
Khojastehnezhad et al.
Table 3
Synthesis of Pyranopyrazole (5a-o) using of NFS-PRS as Catalysta
Entry
Product
Time (min)
Yield (%)
Found Mp (ꢀC)
Lit. Mp (ꢀC)
1
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
15
15
20
20
20
15
25
25
30
25
20
25
20
30
30
98
97
95
96
96
99
95
94
96
94
92
95
94
90
93
238–240
233–234
198–201
207–209
180–183
248–249
173–175
225–226
250–252
145–147
240–242
220–222
191–192
180–182
171–173
244–24646
234–23646
205–20846
210–21246
181–18347
248–25046
171–17246
223–22546
252–25347
146–14846
242–24347
223–22446
190–19346
175–17846
171–17346
2
3
4
5
6
7
8
9
10
11
12
13
14
15
aAromatic aldehydes 1 (1.0 mmol), hydrazine hydrate 2 (0.03 g, 1.0 mmol), ethyl acetoacetate 3
(0.14 g, 1.0 mmol), malononitrile 4 (0.07 g, 1.0 mmol) and NFS-PRS (0.03 g) at room temperature
in water.
(containing the product).48 The separated catalyst was washed with acetone and dried at
100ꢀC under vacuum for 2 h and reused five times for the same reaction to afford 98%,
98%, 97%, 95% and 94% yields, respectively.
In summary, NFS-PRS has been used as an efficient, re-usable and green solid acid
catalyst for the one-pot synthesis of pyranopyrazole derivatives. Excellent yields, short
reaction times, simplicity of operation and easy separation and re-usability of catalyst are
some advantages of this method.
Experimental Section
All materials and reagents were purchased from Merck and Aldrich and used without fur-
ther purification. Mps were determined on an Electrothermal type 9100 melting point
apparatus and are uncorrected. The IR spectra were recorded on a Thermo Nicolet AVA-
1
TAR-370 FT-IR spectrophotometer and H NMR spectra were obtained on a Bruker
DRX400 spectrometer.
Catalyst Preparation
Preparation of NiFe2O4 Nanoparticles
A solution of the two aqueous metallic salts (1 M of FeCl3, 160 mL and 1 M of NiCl2,
40 mL) was poured as quickly as possible into one liter of a boiling aqueous solution of
1M NaOH under vigorous stirring (700 rpm) using a mechanical stirrer. Then, the solu-
tion was cooled to room temperature and stirred continuously for 90 min. The resulting

Synthesis of Pyranopyrazoles
381
precipitate was then purified by four repeated centrifugation (4000–6000 rpm, 20 min)
with water (500 mL) and decantation.
Preparation of NiFe2O4@SiO2 Core-shell
The NiFe2O4 NP (2.0 g, 8.57nbsp;mmol) was ultrasonically dispersed (Crest Ultrasonic
Bath Model CP2600T) in ethanol (25 mL) for 2 h at 60ꢀC and then a 25% aqueous
ammonia (10 mL) was added to the mixture and stirred at 60ꢀC for 40 min. Then tet-
raethyl orthosilicate (TEOS) (1.0 mL) was added (as the silica source) to the mixture and
stirring was continued at the same temperature for 24 h. The suspended silica-coated
MNP was separated from the solution by placing an external magnet on the outside of the
flask and the remaining solution was decanted. The MNP was washed three times with
methanol and dried in vacuum for 48 h. The resulting MNP (NiFe2O4@SiO2) were then
calcinated at 800ꢀC for 4 h.
Preparation of Preyssler HPA (H14[NaP5W30O120])
The Preyssler HPA (H14[NaP5W30O120]) was prepared as follows: A solution of 33 g
(0.1 mole) Na2WO4.2H2O in 45 mL of water was stirred (200 rpm) at 45ꢀC for 30 min.
Then it was cooled to room temperature followed by the addition of 25 mL of 85% phos-
phoric acid and the resulting yellow solution was heated at reflux for 5 h. The solution
was allowed to cool to room temperature, diluted with 50 mL of water and 10 g of potas-
sium chloride was added with stirring (stir bar, 200 rpm). The mixture was stirred for
30 min at room temperature and then evaporated (rotary evaporator) to dryness (90ꢀC).
The solid obtained was dissolved in 75 mL of warm water (90ꢀC) and upon cooling to
room temperature, white crystals (15.4 g) of the potassium salt of Preyssler
(K14[NaP5W30O120]) were formed. The free acid of Preyssler HPA (H14[NaP5W30O120])
was obtained by passage of a solution of 11.4 g of the potassium salt of Preyssler
(obtained above) in 20 mL of water through a column (50 cm £ 1 cm) of acidic Dowex
50W £ 8 followed by evaporation of the eluate to dryness in vacuum overnight.49–51
Preparation of NiFe2O4@SiO2-Preyssler (NFS-PRS)
The Preyssler HPA was immobilized on the NiFe2O4@SiO2as follows. To a suspension
of NiFe2O4@SiO2 (1.0 g) in water (50 mL) was added dropwise a solution of Preyssler
HPA (0.75 g) in water (5 mL) and the mixture was stirred (stir bar, 200 rpm) for 12 h at
room temperature under N2 atmosphere.Then the solvent was evaporated and the sup-
ported catalyst was collected and dried in vacuum overnight and the supported nano-cata-
lyst (NFS-PRS) was calcinated at 250ꢀC for 2 h.43
General Procedure for the Synthesis of Pyranopyrazole Derivatives (5a–o)
A mixture of the aromatic aldehyde (1.0 mmol), ethyl acetoacetate (0.14 g, 1.0 mmol),
malononitrile (0.07 g, 1.0 mmol), hydrazine hydrate (100%, 0.03 g, 1.0 mmol) and NFS-
PRS (0.03 g) in water (10 ml) was stirred (stir bar, 100 rpm) at room temperature for 15–
30 min. Upon completion of the reaction, the nano-magnetic catalyst was separated from
the reaction mixture using an external magnet (placed on the outside wall of the reaction
vessel), washed with acetone (50 ml) and dried at 100ꢀC for 2 h. It was re-used at least

382
Khojastehnezhad et al.
five times without loss of activity. The solvent was evaporated from the decanted solution
obtained from the isolation of the catalyst to give the pure products 5a–o (see Table 3).
Procedure for Larger Scale Preparation of 5a
A mixture of the benzaldehyde (2.1 g, 20 mmol), ethyl acetoacetate (2.8 g, 20 mmol),
malononitrile (1.4 g, 20 mmol), hydrazine hydrate (100%, 0.6 g, 20 mmol) and NFS-
PRS (0.6 g) in water (200 ml) was stirred at room temperature for 15–30 min. Upon com-
pletion of the reaction, the nano-magnetic catalyst was separated from the reaction mix-
ture using an external magnet as described above, washed with acetone (50 ml) and dried
at 100ꢀC for 2 h and re-used. The decanted solution obtained from the isolation of the cat-
alyst was evaporated to give the pure product (5a) in 98% yield.
Analytical Data for Selected Compounds
6-Amino-3-methyl-4-phenyl-1,4-dihydropyrano[2,3c]pyrazole-5-carbonitrile
1
(5a): IR: 3450, 3370, 2195, 1645, 1610, 1605, 1446; H NMR (500 MHz, DMSO-d6):
d 1.91 (s, 3H), 4.74 (s, 1H), 6.98 (s, 2H), 7.45–7.16 (m, 5H), 11.98 (s, 1H); 13C NMR
(500 MHz, DMSO-d6): d 11.5, 25.1, 70.8, 112.0, 126.2, 127.4, 130.1, 131, 140.2, 144.1
152.3, 160.5.
6-Amino-3-methyl-4-(4-chlorophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carb
1
onitrile (5b): IR: 3380, 3281, 2193, 1622, 1454; H NMR (500 MHz, DMSO-d6): d 1.80
(s, 3H), 4.59 (s, 1H), 6.7 (s, 2H), 7.20 (d, 2H), 7.31 (d, 2H), 12.1 (s, 1H); 13C NMR
(500 MHz, DMSO-d6): d 11.5, 24.3, 70.2, 112, 126.8, 127.3, 130.2, 134.4, 141.3, 142,
153.5, 160.6.
6-Amino-3-methyl-4-(4-methoxyphenyl)-1,4-dihydropyrano[2,3c]pyrazole-5-car
bonitrile (5d): IR: 3480, 3259, 2181, 1610, 1442; 1H NMR (500, CDCl3): d 1.78 (s, 3H),
3.85 (s, 3H), 4.60 (s, 1H), 6.88 (d, 2H), 6.89 (d, 2H), 7.20 (s, 2H), 12.08 (s, 1H); 13C
NMR (500 MHz, CDCl3): d 10.1, 36.3, 55, 57.6, 107.7, 113.5, 114.3, 119, 136.8, 144.4,
156, 156.2, 160.5.
6-Amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbo
nitrile (5f): IR: 3439, 3378, 2180, 1651, 1597, 1516, 1456; 1H NMR (500 MHz, DMSO-
d6): d 1.98 (s, 3H), 4.71 (s, 1H), 6.94 (s, 2H), 7.49 (d, 2H), 8.14 (d, 2H), 11.65 (s, 1H);
13C NMR (500 MHz, DMSO-d6): d 11.4, 23.5, 70.8, 112.8, 128, 129, 130.1, 135.5, 141.8,
150.2, 154.2, 160.8.
6-Amino-3-methyl-4-(3-nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbo
1
nitrile (5m): IR: 3388, 3280, 2185, 1622, 1455; H NMR (500 MHz, CDCl3): d 1.82 (s,
3H), 4.99 (s, 1H), 6.97 (s, 2H), 7.85 (s, 1H), 7.78 (d, 1H), 7.42–7.45 (m, 2H), 12.16 (s,
1H); 13C NMR (500 MHz, CDCl3): d 9.8, 32.6, 59.8, 120.5, 121.6, 124.9, 128, 130.2,
134.3, 135.5, 142, 146.2, 148.8, 175.8.
Acknowledgments
The authors are grateful to Ferdowsi University of Mashhad for financial support.
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