Rapid one-pot, four component synthesis of pyranopyrazoles using heteropolyacid under solvent-free condition

Article abstract of DOI:10.5012/bkcs.2011.32.11.3963

A series of pyranopyrazoles, was efficiently synthesized via one-pot, four component reaction of ethyl acetoacetate, hydrazine hydrate, aldehydes and malononitrile in the presence of catalytic amount silicotungstic acid under solvent free condition. NOE e

Full text of DOI:10.5012/bkcs.2011.32.11.3963

Rapid One-pot, Four Component Synthesis of Pyranopyrazoles
Bull. Korean Chem. Soc. 2011, Vol. 32, No. 11 3963
http://dx.doi.org/10.5012/bkcs.2011.32.11.3963
Rapid One-pot, Four Component Synthesis of Pyranopyrazoles Using
Heteropolyacid Under Solvent-free Condition
*
Hemant V. Chavan, Santosh B. Babar, Rahul U. Hoval, and Babasaheb P. Bandgar
Medicinal Chemistry Research Laboratory, School of Chemical Sciences, Solapur University, Solapur-413 255, India
*
E-mail: bandgar_bp@yahoo.com
Received August 26, 2011, Accepted September 11, 2011
A series of pyranopyrazoles, was efficiently synthesized via one-pot, four component reaction of ethyl
acetoacetate, hydrazine hydrate, aldehydes and malononitrile in the presence of catalytic amount silicotungstic
acid under solvent free condition. NOE experiments confirmed that the product exist exclusively in the 2H
form. The present protocol offers the advantages of clean reaction, short reaction time, high yield, easy
purification and economic availability of the catalyst.
Key Words : Pyranopyrazoles, Multicomponent, Heteropolyacid, Solvent-free, NOE
Introduction
active heterocycles. They are reported to possess a multi-
12a
plicity of pharmacological properties including anticancer,
12b 12c
antimicrobial, antiinflammatory, insecticidal, and mol-
An increase in regulatory restrictions on the use, manu-
facture and disposal of organic solvents has focused atten-
tion on the development of non-hazardous alternatives such
as solvent-free syntheses, multicomponent reactions and
reusable heterogeneous catalysts for the sustainable develop-
ment of chemical enterprise. These organic reactions possess
advantages over traditional reactions in organic solvents.
For example, solvent-free, multicomponent reactions with
reusable heterogeneous catalysts reduce the consumption of
environmentally hazardous solvents and utilize scaled-down
reaction vessels. In the recent years, the use of solid acids as
heterogeneous catalyst has received considerable attention in
12d,e
luscicidal activities.
They are also potential inhibitors of
12f
human Chk1 kinase. They also find applications as
13
pharmaceutical ingredients and biodegradable agrochemicals.
In a view of great importance of pyranopyrazoles various
methods for synthesis of 6-amino-5-cyanodihydro-pyrano-
[2,3-c]pyrazoles has been reported. These compounds may
be readily obtained from the reaction of 4-arylmethylene-5-
pyrazolone and malononitrile, or 2-pyrazolin-5-ones and
14
benzylidenemalononitriles. The first reported pyranopyra-
zole was synthesized from the reaction between 3-methyl-1-
15
phenylpyrazolin-5-one and tetracyanoethylene. Various 6-
1
different areas of organic synthesis. Heteropolyacids (HPAs)
amino-5-cyanodihydro-pyrano[2,3-c]pyrazoles were synthe-
sized by the reaction of arylidienemalononitrile with 3-
methylpyrazoline-5-ones or by the condensation of 4-
have several advantages, including high flexibility on modi-
fication of the acid strength, ease of handling, environmental
2
16
compatibility, non toxicity and experimental simplicity.
arylidienepyrazoline-5-one with malononitrile.
17
Sharanin et al., have developed a three-component
They are known to possess a strong purely Brönsted acidity,
and found to be very efficient in catalyzing reactions that
conventionally use Lewis acids. Thus, the use of HPAs as a
catalyst makes the process convenient and environmentally
benign.
reaction between pyrazolone, an aldehyde and malononitrile
in ethanol using triethylamine as the catalyst. Vasuki and co-
workers reported an efficient four-component reaction
protocol for the synthesis of pyranopyrazole derivatives in
the presence of bases such as piperidine, pyrrolidine,
Heteropolyacids found to exhibit excellent catalytic pro-
3 4
perties in the dehydration of diols, rearrangements, tetra-
18
morpholine and triethylamine at ambient temperature.
5
hydropyranylation of alcohols, Friedel-Craft alkylation,
6
Kappusami et al. have developed solvent-free multicompo-
nent synthesis of pyranopyrazoles using per-6-amino-β-
7 8
Prins reaction, synthesis of dihydroquinolines, pyrimidine
9
synthesis, Biginelli reaction and Dakin-West reaction.
10
11
19
cyclodextrin as a catalyst. More recently Myrboh et al.
Pyranopyrazoles are an important class of biologically
reported the synthesis of pyranopyrazoles using L-proline
20
and γ-alumina as catalyst. As a part of ongoing program on
Figure 1. Observed NOE enhancements between CH (CH3) and
2' 2
N H as well as CH (CH3) and C H.
Scheme 1

3964 Bull. Korean Chem. Soc. 2011, Vol. 32, No. 11
Hemant V. Chavan et al.
21
the development of novel methods in organic synthesis, we
report herein a simple, rapid and high yielding one pot four-
component reaction protocol for the synthesis of pyrano-
pyrazole derivatives employing environmentally friendly
silicotungstic acid (H4[SiW12O40]) as a catalyst under solv-
ent free condition (Scheme 1).
(s, 1H), 7.10 (s, 2H), 7.16 (d, J = 8.0 Hz, 1H), 7.37 (d, J =
13
8.0 Hz, 2H), 12.17 (s, 1H); C NMR (100 MHz, CDCl3): δ
10.3, 36.4, 59.8, 118.9, 119.6, 128.7, 134.2, 137.8, 143.5,
+
148.9, 148.6, 178.2; MS (ESI): m/z = 321 (M ); Anal. calcd
for C14H10Cl2N4O: C, 52.36; H, 3.14; Cl, 22.08; N, 17.45; O,
4.98; Found: C, 52.08; H, 2.95; Cl, 21.98; N, 17.30; O, 4.88.
6-Amino-4-(2,4-difluorophenyl)-2,4-dihydropyrano[2,3-
c]pyrazole-5-carbonitrile (5k): Yello solid, mp 176-178
°C, yield 90%; IR (KBr): ν 3427, 3242, 2191, 1612, 1428,
Experimental
−1 1
1060 cm ; H NMR (300 MHz, CDCl3): δ 1.81 (s, 3H), 4.86
(s, 1H), 7.09 (s, 2H), 7.22 (d, J = 8.2 Hz, 1H), 7.39 (d, J =
General Procedure for the Synthesis of the Pyrano-
pyrazoles (5a-t). In a 25 mL round-bottom flask, hydrazine
hydrate 96% (0.107 g, 2 mmol), ethyl acetoacetate (0.26 g, 2
mmol), 3-nitro benzaldehyde (0.30 g, 2 mmol) malononitrile
(0.13 g, 2 mmol) and silicotungstic acid (2 mol %) were
13
8.2 Hz, 1H), 7.88 (s, 1H), 12.15 (s, 1H); C NMR (100
MHz, CDCl3): δ 10.0, 36.5, 58.2, 118.9, 120.6, 127.4, 128.8,
133.7, 136.7, 137.6, 143.0, 148.9, 150.1, 178.7; MS (ESI):
o
taken. The reaction mixture was heated at 60 C under
+
m/z = 288 (M ).
solvent free conditions for 10 min. After completion, the
reaction mixture was cooled to room temperature, and
acetonitrile was added and shaken well for 5 min, then
poured over crushed ice, stirred for 10 min, precipitated
product was filtered, washed with water, dried and re-
crystallized from methanol.
6-Amino-4-(4-methoxyphenyl)-3-methyl-2,4-dihydro-
pyrano[2,3-c]pyrazole-5-carbonitrile (5n): White solid,
mp 210-212 °C, yield 95%; IR (KBr): ν 3485, 3258, 2185,
−1
1
1608, 1442 cm ; H NMR (300, CDCl3): δ 1.77 (s, 3H),
3.82 (s, 3H), 4.62 (s, 1H), 6.88 (d, J = 8.2 Hz, 2H), 6.88 (d, J
13
= 8.2 Hz, 2H), 7.15 (s, 2H), 12.10 (s, 1H); C NMR (100
Spectral and Analytical Data of Some Representative
Compounds.
MHz, CDCl3): δ 10.0, 36.2, 55.1, 57.9, 107.4, 113.4, 114.2,
118.5, 136.8, 144.2, 155.8, 156.3, 160.5; MS (ESI): m/z =
+
175 (M ); Anal. Calcd. for C15H14N4O2: C, 63.82; H, 5.10;
6-Amino-3-methyl-4-phenyl-2,4-dihydropyrano[2,3-c]-
pyrazole-5-carbonitrile (5a): White powder, mp 244-246
°C, yield 95%; IR (KBr): ν 3371, 3248, 2192, 1613, 1445
N, 19.85. Found: C, 63.42; H, 4.98; N, 19.79.
6-Amino-4-(2,4-dimethoxyphenyl)-2,4-dihydropyrano-
[2,3-c]pyrazole-5-carbonitrile (5o): White solid, mp 222-
224 °C, yield 92%; IR (KBr): ν 3382, 3240, 2168, 1608,
−1 1
cm ; H NMR (300, CDCl3): δ 1.79 (s, 3H), 4.85 (s, 1H),
13
6.91 (s, 2H), 7.18-7.35 (m, 5H), 12.13 (s, 1H); C-NMR
−1
1
(100 MHz, CDCl3): δ 10.1, 36.8, 58.1, 121.0, 126.9, 127.9,
1450 cm ; H NMR (300 MHz, CDCl3): δ 1.78 (s, 3H), 3.76
(s, 3H), 3.77 (s, 3H), 4.77 (s, 1H), 6.46 (s, 1H), 6.69 (d, J =
8.0 Hz, 1H), 6.85 (s, 2H), 6.99 (d, J = 8.0 Hz, 1H), 12.06 (s,
+
128.7, 136.5, 146.5; MS (ESI): m/z = 252 (M ); Anal. Calcd.
for C14H12N4O: C, 66.65; H, 4.79; N, 22.21. Found: C,
66.54; H, 4.68; N, 22.13.
+
1H); MS (ESI): m/z = 312 (M ).
6-Amino-3-methyl-4-(3-nitro-phenyl)-2,4,dihydro-pyrano-
[2,3-c]pyrazole-5-carbonitrile (5c): Brown solid, mp 214-
216 °C, yield 96%; IR (KBr): ν 3385, 3278, 2189, 1622,
6-Amino-4-(2,4,6-trimethoxyphenyl)-2,4-dihydropyrano-
[2,3-c]pyrazole-5-carbonitrile (5p): White solid, mp 228-
230 °C, yield 90%; IR (KBr): ν 3381, 3246, 2170, 1610,
−1
1
1456 cm ; H NMR (300, CDCl3): δ 1.83 (s, 3H), 4.98 (s,
1H), 6.95 (s, 2H), 7.84 (s, 1H), 7.78 (d, J = 8.4 Hz, 1H),
−1 1
1455 cm ; H NMR (300 MHz, CDCl3): δ 1.76 (s, 3H), 3.71
(s, 3H), 3.72 (s, 6H), 4.74 (s, 1H), 6.31 (s, 2H), 6.84 (s, 2H),
13
7.42-7.45 (m, 2H), 12.16 (s, 1H); C-NMR (100 MHz,
+
12.03 (s, 1H); MS (ESI): m/z = 342 (M ).
CDCl3): δ 9.78, 32.5, 59.6, 120.2, 121.4, 124.7, 127.6,
6-Amino-4-(furan-2-yl)-3-methyl-2,4-dihydropyrano-
[2,3-c]pyrazole-5-carbonitrile (5s): White solid; mp 218-
220 °C, yield 88%; IR (KBr): ν 3362, 3275, 2180, 1610,
130.0, 134.1, 135.3, 141.8, 146.2, 148.8, 175.9; MS (ESI):
+
m/z = 297 (M ); Anal. Calcd. for C14H11N5O3: C, 56.56; H,
−1
1
3.73; N, 23.56. Found: C, 56.14; H, 3.62; N, 23.35.
1450 cm ; H NMR (300, CDCl3): δ 1.80 (s, 3H), 4.77 (s,
1H), 6.14 (s, 2H), 6.22 (d, J = 1.8 Hz, 1H), 6.42 (dd, J = 1.8,
6-Amino-4-(2,4-dichlorophenyl)-2,4-dihydropyrano[2,3-
c]pyrazole-5-carbonitrile (5f): White solid, mp 196-198
°C, yield 95%; IR (KBr): ν 3422, 3240, 2190, 1610, 1424,
13
J = 3.4 Hz, 1H), 7.53 (d, J = 3.4 Hz, 1H), 12.15 (s, 1H); C
NMR (100 MHz, CDCl3): δ 10.23, 30.11, 54.06, 95.18,
−1 1
1062 cm ; H NMR (300 MHz, CDCl3): δ 1.82 (s, 3H), 4.80
(s, 1H), 7.05 (s, 2H), 7.18 (d, J = 8.4 Hz, 1H), 7.33 (d, J = 8.4
105.34, 111.23, 120.66, 136.28, 146.22, 154.82, 155.92,
+
162.18; MS (ESI): m/z = 242 (M ); Anal. Calcd. for
13
Hz, 1H), 7.86 (s, 1H), 12.18 (s, 1H); C NMR (100 MHz,
C12H10N4O2: C,59.50; H, 4.16; N, 23.13. Found: C, 59.40;
H, 4.05; N, 23.07.
CDCl3): δ 10.2, 35.2, 58.6, 118.5, 119.8, 126.9, 128.3, 133.1,
135.8, 136.3, 142.6, 148.4, 149.9, 178.1; MS (ESI): m/z =
+
321 (M ); Anal. calcd for C14H10Cl2N4O: C, 52.36; H, 3.14;
Results and Discussion
Cl, 22.08; N, 17.45; O, 4.98; Found: C, 52.02; H, 2.99; Cl,
21.92; N, 17.38; O, 4.83.
Our initial experiments were focused on the one pot, four
component reactions of ethyl acetoacetate, hydrazine hydrate,
3-nitro benzaldehyde and malononitrile using different
catalysts under solvent free conditions, and the results are
listed in Table 1.
6-Amino-4-(2,6-dichlorophenyl)-2,4-dihydropyrano[2,3-
c]pyrazole-5-carbonitrile (5g): White solid, mp 188-190
°C, yield 92%; IR (KBr): ν 3428, 3250, 2195, 1614, 1422,
−1 1
1066 cm ; H NMR (300 MHz, CDCl3): δ 1.83 (s, 3H), 4.85

Rapid One-pot, Four Component Synthesis of Pyranopyrazoles
Bull. Korean Chem. Soc. 2011, Vol. 32, No. 11 3965
Table 1. Screening of catalysts for one-pot condensation of ethyl
Table 3. Four-component synthesis of Pyranopyrazoles
acetoacetate, hydrazine hydrate, 3-nitro benzaldehyde and malono-
a
Entry
R
Product
Yield (%)
a
nitrile
1
2
C6H5
5a
5b
5c
5d
5e
5f
95
94
96
94
94
95
92
96
96
93
90
94
95
95
92
90
90
91
88
86
58
65
b
Entry
Catalyst
none
Catalyst (mol %) Time (min) Yield (%)
2'-NO2-C6H4
3'-NO2-C6H4
4'-NO2-C6H4
3'-Cl-C6H4
1
2
3
4
5
6
7
8
9
-
2
180
10
10
10
10
10
10
10
10
10
-
3
FeCl3
20
38
25
32
15
96
34
68
95
4
SnCl4
2
5
ZnCl2
2
6
2'-Cl,4'-Cl-C6H4
2'-Cl,6'-Cl-C6H3
3'-Br-C6H4
P2O5
2
7
5g
5h
5i
CAN
2
8
H4[SiW12O40]
H4[SiW12O40]
H4[SiW12O40]
2
9
4'-Br-C6H4
0.5
1
10
11
12
13
14
15
16
17
18
19
20
21
21
4'-F-C6H4
5j
2'-F,4'-F-C6H3
4'-Me-C6H4
5k
5l
10 H4[SiW12O40]
5
a
Reaction conditions: ethyl acetoacetate (2.0 mmol), hydrazine hydrate
4'-Me2N-C6H4
4'-MeO-C6H4
2'-MeO,4'-MeO-C6H3
2',4',6'-(MeO)3-C6H2
2'-OH-C6H4
4'-OH-C6H4
2'-Furanyl
5m
5n
5o
5p
5q
5r
5s
(2.0 mmol), 3-nitro benzaldehyde (2.0 mmol), and malononitrile (2.0
b
mmol), solvent free, 60 °C (oil bath). Isolated yield.
Table 2. Effect of solvent on the reaction of ethyl acetoacetate,
hydrazine hydrate, 3-nitro benzaldehyde and malononitrile cataly-
sed by H4[SiW12O40]
o
Temp ( C) Time (min) Yield (%)
Entry
Solvent
acetone
2'-Thiophenyl
Propyl
5t
1
2
3
4
5
6
7
reflux
reflux
reflux
reflux
reflux
reflux
60
60
60
60
60
60
60
10
35
27
36
40
52
68
96
5u
5v
dichloromethane
tetrahydrofuran
ethanol
n-Hexyl
a
Isolated yields.
methanol
To study the generality of this protocol, we extended our
acetonitrile
none
study with different aromatic and aliphatic aldehydes to
prepare a series of pyranopyrazoles (5a-v, Table 3). In case
of aromatic aldehydes, products were obtained in good to
excellent yields while aliphatic aldehydes results poor
yields. All the synthesized compounds were characterized
It was found that silicotungstic acid (H4[SiW12O40]) show-
ed better catalytic activity among other catalysts such as
FeCl3, SnCl4, ZnCl2, P2O5 and ceric ammonium nitrate
(CAN). When 2 mol % silicotungstic acid was used, the
reaction proceeded smoothly and gave the product 5c in
96% yield (Table 1, entry 7). Moreover, we found that the
yields were obviously affected by the amount of silicotung-
stic acid loaded. When 0.5 mol %, 1 mol % and 5 mol % of
silicotungstic acid were used, the yields were 34%, 68%, and
95%, respectively (Table 1, entries 8-10). Therefore, 2 mol %
of silicotungstic acid was sufficient to push the reaction
forward, and further increasing the amount of silicotungstic
acid did not increase the yields. In addition, no desired
product was detected in the absence of the catalyst (Table 1,
entry 1). The above results showed that silicotungstic acid
was essential in the reaction, and the best results were
obtained when the reaction was carried out with 2 mol % of
silicotungstic acid under solvent free conditions at 60 °C.
Then, we examined the effect of solvents for above reac-
tion. The results of Table 2 indicate that solvents affected the
efficiency of the reaction. Yields were poor in dichloro-
methane, acetone, tetrahydrofuran and ethanol (Table 2,
entries 1-4). Better yields were obtained in acetonitrile
and methanol (Table 2, entry 5 and 6). However, the best
results were obtained under solvent free conditions (Table 2,
entry 7).
1 13
by IR, H NMR, C NMR, LCMS and elemental analysis
1
13
data. H NMR and C NMR indicated the presence of
product 5 (the 2H form). In addition, the regiochemistry of
compound 5 was also determined by an NOE study (Figure
1). NOE experiments could readily confirm the predomin-
ance of product 5 as irradiating the CH3 signal at 1.79 δ,
Scheme 2

3966 Bull. Korean Chem. Soc. 2011, Vol. 32, No. 11
Hemant V. Chavan et al.
7. Molnar, A.; Keresszegi, C. S.; Beregszászi, T.; Torok, B.; Bartok,
M. Catalysis of Organic Reactions; Marcell Dekker: New York,
1998; p 507.
resulted in an enhanced NH signal at 12.13 δ along with aryl
proton enhancement. Thus, we conclude that the product 5
formed in predominant form and this is also supported by
8. Kamakshi, R.; Reddy, B. S. R. Catalysis Commun. 2007, 8, 825.
9. Heravi, M. M.; Sadjadi, S.; Oskooie, H. A.; Shoar, R. H.;
Bamoharram, F. F. Tetrahedron Lett. 2009, 50, 662.
18
the previous X-ray crystallographic investigations.
Mechanistically, the reaction occurs via initial formation
of arylidenemalononitrile 7 by the Knoevenagel conden-
sation between 3 and 4, and pyrazolone 6 by the reaction
between 1 and 2. Finally, Michael addition of pyrazolone 7
to arylidenemalononitrile 7, followed by cyclization and
tautomerization yield pyranopyrazole 5 (Scheme 2).
10. Rafiee, E.; Shahbhazi, F. J. Mol. Cat A: Chemical. 2006, 250, 57.
11. Rafiee, E.; Shahbhazi, F.; Joshaghani, M.; Tork, F. J. Mol. Cat A:
Chemical. 2005, 242, 129.
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S. M.; Croce, C. M.; Alnemri, E. S.; Huang, Z. Proc. Natl. Acad.
Sci. U.S.A. 2000, 97, 7124. (b) El-Tamany, E. S.; El-Shahed, F. A.;
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A.; Soliman, H. A.; Hiekal, O. A.; Rashad, A. E. Z. Naturforsch.
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El-Mahrouky, S. F. Arch. Pharm. 2006, 339, 456. (e) Abdelrazek,
F. M.; Metz, P.; Kataeva, O.; Jager, A.; EI-Mahrouky, S. F. Arch.
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Chem. 2006, 14, 4792.
Conclusion
In conclusion, we have developed a highly efficient hetero-
polyacid catalyzed, one-pot, four component protocol for the
synthesis of pyranopyrazoles via condensation of ethyl
acetoacetate, hydrazine hydrate, aldehyde and malononitrile.
The advantages of this method are clean reaction, short
reaction time, high yield, easy purification and economic
availability of the catalyst.
13. (a) Sosnovskikh, V. Y.; Barabanov, M. A.; Usachev, B. I.; Irgashev,
R. A.; Moshkin, V. S. Russ. Chem. Bull., Int. Ed. 2005, 54, 2846.
(b) El-Assiery, S. A.; Sayed, G. H.; Acta Pharm. 2004, 54, 143. (c)
Guard, J. A. M.; Steel, P. J. ARKIVOC 2001, vii, 32. (d) Rodinovskaya,
L. A.; Gromova, A. V.; Shestopalov, A. M.; Nesterov, V. N. Russ.
Chem. Bull., Int. Ed. 2003, 52, 2207.
Acknowledgments. The author H. V. C. thankful to
Council of Scientific and Industrial Research (CSIR), New
Delhi, for financial support in the form of JRF.
14. Otto, H. H. Arch. Pharm. 1974, 307, 444. (b) Otto, H. H.;
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