A convenient ultrasound-promoted regioselective synthesis of fused polycyclic 4-aryl-3-methyl-4,7-dihydro-1H-pyrazolo[3,4-b]pyridines

Article abstract of DOI:10.1016/j.ultsonch.2009.08.001

Fused polycyclic 4-aryl-3-methyl-4,7-dihydro-1H-pyrazolo[3,4-b]pyridines were obtained in a three-component regioselective reaction of 5-amino-3-methyl-1H-pyrazole, 2H-indene-1,3-dione and arylaldehydes in ethanol under ultrasound irradiation. This rapid method produced the products in short reaction times (4-5 min) and excellent yields (88-97%).

Full text of DOI:10.1016/j.ultsonch.2009.08.001

Ultrasonics Sonochemistry 17 (2010) 301–305
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Short Communication
A convenient ultrasound-promoted regioselective synthesis of fused polycyclic
4-aryl-3-methyl-4,7-dihydro-1H-pyrazolo[3,4-b]pyridines
*
Mohammad Nikpassand, Manouchehr Mamaghani , Farhad Shirini, Khalil Tabatabaeian
Department of Chemistry, Faculty of Sciences, University of Guilan, P.O. Box 41335-1914, Rasht, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 June 2009
Received in revised form 2 August 2009
Accepted 4 August 2009
Available online 7 August 2009
Fused polycyclic 4-aryl-3-methyl-4,7-dihydro-1H-pyrazolo[3,4-b]pyridines were obtained in a three-
component regioselective reaction of 5-amino-3-methyl-1H-pyrazole, 2H-indene-1,3-dione and arylalde-
hydes in ethanol under ultrasound irradiation. This rapid method produced the products in short reaction
times (4–5 min) and excellent yields (88–97%).
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Regioselective
Ultrasound
Pyrazolopyridines
Multicomponent
1. Introduction
2. Results and discussion
Pyrazolo-fused compounds have been of interest due to their
wide range of biological and pharmacological properties [1–3]. In
particular pyrazolopyridines have attracted many interests in re-
cent years as possible anti-viral agent, [4,5] potent p38 kinase
inhibitors [6], HIV reverse transcriptase inhibitors [7], and inhibi-
tors of cGMP degradation, together with several herbicidal and
fungicidal activities [8,9]. Some derivatives exhibit potential anti-
malarial properties [10,11]. Others show intense fluorescence in
the bluegreen region and have been considered for applications
as fluorescence standards and luminophores in organic light emit-
ting diodes [12], bactericidal activity [13], and also used as vasodi-
lators [14] or evaluated for CCR1 antagonists and enzymatic
inhibitory activities [15,16]. Therefore several protocols for the
synthesis of pyrazolopyridines in recent years have been reported
respect to their different structures [17–19].
On the other hand, green chemistry has become a major driving
force for organic chemists to develop environmentally benign
routes for the preparation of organic compounds of synthetic and
biological values. For example, the possibility of performing reac-
tions under ultrasound irradiations to enhance the reaction effi-
ciency from both economic and ecological points of view has
given to this kind of procedures a remarkable synthetic value
and received a great attention [20].
As part of our program aimed at developing new selective and
environmentally friendly methodologies for the preparation of
heterocyclic compounds [21], we describe here an efficient
method for the regioselective synthesis of new derivatives of
pyrazolopyridines.
Multicomponent reactions of 1H-pyrazol-5-amines, aldehydes
and CH-acid compounds due to formation of different condensa-
tion products depending on the specific conditions have recently
attracted the interest of many chemists [22]. In this report we have
devised an efficient one-pot three-component reaction for the syn-
thesis of novel fused tetracyclic derivatives of pyrazolopyridines
(4a–l) from 5-amino-3-methyl-1H-pyrazole (1), 2H-indene-1,3-
dione (2) and aldehydes (3a–l) under ultrasound irradiation
(Scheme 1).
Equimolar amounts of reactants 1, 2 and 3 (Table 2 entries 1–3,
0.5 eq.) in ethanol were placed into Pyrex-glass open vessel and
irradiated at 60 °C by ultrasound (45 kHz) to produce desired pyr-
azolopyridine derivatives (4a–l) in 4–5 min with 88–97% yields
(Tables 1 and 2). A control reaction on substrates 3a, 3e and 3j in
refluxing ethanol, using the same ratio of the reactants furnished
the related pyrazolopyridines in lower yields and much longer
reaction times (60–75 min) (Tables 1 and 2).
Interestingly this one-pot multicomponent approach also affor-
ded an efficient protocol for the synthesis of bis-pyrazolopyridines
4j–l (Fig. 1) in high yields (90–92%) (Table 2). The structures of all
the products were fully characterized by spectroscopic (IR, 1H
NMR, 13C NMR) and elemental analyses. In this study the main
* Corresponding author. Tel.: +98 1317270899; fax: +98 1313233262.
E-mail address: mchem41@gmail.com (M. Mamaghani).
1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2009.08.001

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M. Nikpassand et al. / Ultrasonics Sonochemistry 17 (2010) 301–305
Ar
CH3
N
O
O
H3C
ultrasound
EtOH, 60 o
+
N
+ ArCHO
3a-l
NH2
N
C
N
N
H
H
H
O
4a-l
1
2
Ar = o-NO2C6H4, m-NO2C6H4, p-NO2C6H4, m-BrC6H4, m-ClC6H4, p-ClC6H4, 2,4-Cl2C6H3,
3-indoyl, 5-NO2-2-furyl, etc.
Scheme 1. Synthesis of fused polycyclic 4-aryl-3-methyl-4,7-dihydro-1H-pyrazolo[3,4-b] pyridines.
Table 1
N
N
CH3
H3C
Synthesis of 4-aryl-3-methyl-4,7-dihydro-1H-pyrazolo[3,4-b]pyridines (4a–i) under
ultrasound irradiation.
HN
NH
Entry
Reagents
Time (min)
Product
Yield (%)a,b
HN
NH
1
2
3
4
5
6
7
8
9
1 + 2 + 2-NO2C6H4CHO
1 + 2 + 3-NO2C6H4CHO
1 + 2 + 4-NO2C6H4CHO
1 + 2 + 3-BrC6H4CHO
1 + 2 + 2-ClC6H4CHO
1 + 2 + 3-ClC6H4CHO
1 + 2 + 4-ClC6H4CHO
1 + 2 + 2,4-Cl2C6H3CHO
1 + 2 + 5-NO2–furyl–CHO
4 (60)c
5
4a
4b
4c
4d
4e
4f
4g
4h
4i
97 (85)c
93
96
O(CH2)nO
n= 4, 6
4
5
88
5 (65)c
91 (78)c
88
O
O
5
5
4
4
89
95
94
Fig. 1. Bis-pyrazolopyridines 4j–l.
a
b
c
Isolated yields.
Identified by spectroscopic analyses (IR, 1H NMR,13C NMR).
Equimolar amounts of 5-amino-3-methyl-1H-pyrazole
1
(10 mmol), 2H-
Mechanistically the formation of the products (4a–l) can be
visualized by initial Knoevenagel condensation of aryl aldehyde
(3) and 2H-indene-1,3-dione (2), followed by a Michael type nucle-
ophilic addition of C-4 of the pyrazole ring to the enone intermedi-
ate (5) and subsequent cyclodehydration to furnish the desired
compounds (Scheme 2).
In conclusion, this one-pot three component protocol under
ultrasound irradiation provides a regioselective, fast and practical
method for the preparation of fused polycyclic pyrazolo[3,4-b]pyr-
idines from 5-amino-3-methyl-1H-pyrazole, 2H-indene-1,3-dione
indene-1,3-dione 2 (10 mmol) and aldehyde 3 (10 mmol) were refluxed in EtOH
(15 mL) in the absence of sonication.
features of pyrazolodihydropyridine formation are the appearance
of two separate singlets for NH protons in downfields and charac-
teristic CH signal (1H NMR: 4.82–5.56 ppm; 13C NMR: 26.2–
35.2 ppm). In this method no trace of pyridine products which
could have been the result of oxidation of the formed pyra-
zolodihydropyrindes (4a–l) could be detected.
Table 2
Synthesis of bis-pyrazolopyridines 4j–l under ultrasound irradiation.
Entry
Reagents
Time (min)
5 (75)c
Product
Yield (%)a,b
92 (80)
1
4j
CHO
CHO
CHO
O(CH2)4O
1 + 2 +
2
3
5
5
4k
4l
90
91
CHO
O(CH2)6O
1 + 2 +
1 + 2 +
CHO
CHO
O - (CH2)6-O
a
b
c
Isolated yields.
Identified by spectroscopic analyses (IR, 1H NMR,13C NMR).
A solution of 5-amino-3-methyl-1H-pyrazole 1 (10 mmol), 2H-indene-1,3-dione 2 (10 mmol) and aldehyde 3 (5 mmol) were refluxed in EtOH (15 mL) in the absence of
sonication.

M. Nikpassand et al. / Ultrasonics Sonochemistry 17 (2010) 301–305
303
H3C
O
O
O
N
H
NH2
N
H
1
+
ArCHO
Ar
O
5
3
2
Ar
Ar
CH3
N
O
CH3
N
O
-H2O
N
H
N
N
H
H
H2N
O
6
4
Scheme 2. Mechanism of formation of pyrazolopyridines 4a–l.
and various aryl aldehydes in short reaction times and in excellent
yields, which pave the way for assessment of pharmalogical activ-
ities of these novel pyrazolopyridine derivatives.
3.1.2. 4b: red solid
mp 300–302 °C; IR (KBr, cmÀ1
)
mmax 3392, 3188, 3064, 1695,
1558, 1488, 1541, 1340; 1H NMR (500 MHz, CDCl3) dH: 1.82 (3H,
s), 5.15 (1H, s), 7.15 (1H, d, J = 6.96 Hz), 7.27–7.29 (1H, t,
J = 7.23 Hz), 7.35–7.38 (1H, t, J = 7.56 Hz), 7.49–7.52 (1H, t,
J = 7.92 Hz), 7.64 (1H, d, J = 7.22 Hz), 7.67 (1H, d, J = 7.76 Hz), 7.97
(1H, dd, J = 8.10, 1.44 Hz), 8.02 (1H, s), 11.32 (1H, s), 12.17 (1H,
s); 13C NMR (125 MHz, CDCl3) dC: 190.3, 158.1, 149.0, 148.5,
148.0, 137.6, 137.1, 135.6, 135.4, 132.0, 131.1, 130.5, 122.9,
122.0, 120.6, 120.1, 105.0, 104.0, 34.9, 10.4 ppm. Anal. Calc. for
C20H14N4O3: C, 67.04; H, 3.94; N, 15.63. Found: C, 67.20; H, 3.80;
N, 15.55.
3. Experimental
3.1. General
Melting points were measured on an Electrothermal 9100 appa-
ratus. For the ultrasound reactions, ultrasound apparatus Astra 3D
(9.5 L, 45 kHz frequency, input power with heating, 305 W, num-
ber of transducers, 2) from TECNO-GAZ was used. IR spectra were
determined on a Shimadzo IR-470 spectrometer. 1H NMR and 13C
NMR spectra were recorded on a 500 MHz Bruker DRX-500 in
CDCl3 as solvent and TMS as an internal standard. Chemicals were
purchased from Merck and Fluka. Elemental analyses were done on
a Carlo-Erba EA1110CNNO-S analyser and agreed with the calcu-
lated values. All solvents used were dried and distilled according
to standard procedures.
3.1.3. 4c: red solid
mp 304–306 °C; IR (KBr, cmÀ1
) mmax 3380, 3193, 3064, 1670,
1616, 1566, 1527, 1348; 1H NMR (500 MHz, CDCl3) dH: 1.82 (3H,
s), 5.10 (1H, s), 7.15 (1H, d, J = 6.94 Hz), 7.27 (1H, t, J = 7.6 Hz),
7.35 (1H, t, J = 7.5 Hz), 7.7 (2H, d, J = 8.54 Hz), 7.66 (1H, d,
J = 7.13 Hz), 8.08 (2H, d, J = 8.52 Hz), 11.32 (1H, s), 12.17 (1H, s);
13C NMR (125 MHz, CDCl3) dC: 190.2, 158.2, 154.4, 147.9, 146.6,
137.6, 137.2, 135.6, 131.9, 131.1, 129.8, 124.3, 120.5, 120.1,
104.9, 104.0, 35.2, 10.4 ppm. Anal. Calc. for C20H14N4O3: C, 67.04;
H, 3.94; N, 15.63. Found: C, 67.12; H, 3.85; N, 15.70.
General Procedure for the synthesis of Fused polycyclic 4-aryl-
3-methyl-4,7-dihydro-1H-pyrazolo[3,4-b]pyridines under ultra-
sound irradiation.
A solution of 5-amino-3-methyl-1H-pyrazole 1 (10 mmol), 2H-
indene-1,3-dione 2 (10 mmol) and aldehyde 3 (10 mmol) (Table
2, entries 1–3, 5 mmol) in EtOH (15 mL) were placed into Pyrex-
glass open vessel and irradiated in a water bath under silent condi-
tion by ultrasound (45 kHz) at 60 °C for the required reaction times
(Tables 1 and 2). When the irradiation was stopped, the amount of
solvent was reduced to a minimum under vacuum, to afford 4a–l as
crystalline products in 88–97% yields (Tables 1 and 2).
3.1.4. 4d: red solid
mp 240–242 °C; IR (KBr, cmÀ1
) mmax 3317, 3244, 3040, 1701,
1620, 1550, 1506, 1080; 1H NMR (500 MHz, CDCl3) dH: 1.67 (3H,
s), 4.92 (1H, s), 6.99–7.04 (2H, m), 7.05–7.08 (1H, td, J = 6.74,
2.00 Hz), 7.17–7.20 (1H, t, J = 7.90 Hz), 7.36 (1H, d, J = 7.10 Hz),
7.50 (1H, d, J = 7.02 Hz), 7.74 (1H, d, J = 8.16 Hz), 7.85 (1H, s),
10.66 (1H, s), 11.42 (1H, s); 13C NMR (125 MHz, CDCl3) dC: 192.8,
144.3, 140.4, 138.9, 137.9, 137.0, 136.9, 133.7, 132.8, 132.2,
132.2, 129.1, 131.4, 127.9, 124.2, 124.0, 123.3, 106.1, 105.6, 34.7,
12.0 ppm. Anal. Calc. for C20H14BrN3O: C, 61.24; H, 3.59; N,
10.71. Found: C, 61.32; H, 3.69; N, 10.78.
3.1.1. 4a: purple solid
mp 247–250 °C; IR (KBr, cmÀ1
) mmax 3320, 3168, 3056, 1680,
1616, 1558, 1551, 1350; 1H NMR (500 MHz, CDCl3) dH: 1.85 (3H,
s), 5.56 (1H, s), 7.10 (1H, d, J = 6.96 Hz), 7.24–7.26 (2H, t,
J = 7.71 Hz), 7.30–7.35 (2H, m), 7.47–7.50 (1H, m), 7.62 (1H, d,
J = 7.16 Hz), 7.76 (1H, d, J = 7.60 Hz), 11.31 (1H, s), 12.20 (1H, s);
13C NMR (125 MHz, CDCl3) dC: 190.0, 157.6, 149.5, 148.4, 140.4,
137.2, 135.5, 133.9, 132.5, 131.9, 131.1, 128.0, 124.2, 120.5,
120.1, 105.4, 103.6, 29.8, 10.2 ppm. Anal. Calc. for C20H14N4O3: C,
67.04; H, 3.93; N, 15.63. Found: C, 67.15; H, 3.88; N, 15.75.
3.1.5. 4e: orange-brown solid
mp 230–232 °C; IR (KBr, cmÀ1
) mmax 3220, 3182, 3060, 1690,
1614, 1568, 1080; 1H NMR (500 MHz, CDCl3) dH: 1.79 (3H, s),
5.33 (1H, s), 7.25–7.64 (8H, m), 11.22 (1H, s), 12.06 (1H, s); 13C
NMR (125 MHz, CDCl3) dC: 190.0, 157.3, 148.0, 145.1, 137.4,
137.2, 136.6, 135.7, 132.4, 132.4, 131.1, 130.2, 128.7, 128.4,
120.4, 119.9, 105.7, 104.2, 33.2, 10.2 ppm. Anal. Calc. for

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M. Nikpassand et al. / Ultrasonics Sonochemistry 17 (2010) 301–305
C20H14ClN3O: C, 69.07; H, 4.05; N, 12.08. Found: C, 69.19; H, 4.18;
N, 12.16.
136.7, 136.0, 135.1, 131.6, 130.7, 130.3, 127.9, 121.1, 120.2,
119.5, 112.9, 106.1, 105.7, 68.7, 26.8, 26.4, 19.4, 10.1 ppm. Anal.
Calc. for C46H40N6O4: C, 74.58; H, 5.44; N, 11.34. Found: C, 74.67;
H, 5.32; N, 11.22.
3.1.6. 4f: orange solid
mp 235–237 °C; IR (KBr, cmÀ1
) mmax 3309, 3200, 1710, 1535,
1506, 1080; 1H NMR (500 MHz, CDCl3) dH: 1.84 (3H, s), 4.95 (1H,
s), 7.00–7.88 (8H, m), 9.95 (1H, s), 11.23 (1H, s); 13C NMR
(125 MHz, CDCl3) dC: 192.9, 158.0, 153.6, 149.4, 143.8, 137.4,
137.3, 135.7, 135.0, 132.0, 131.4, 129.8, 122.8, 122.5, 120.5,
120.0, 105.4, 104.4, 35.0, 10.4 ppm. Anal. Calc. for C20H14ClN3O:
C, 69.07; H, 4.05; N, 12.08. Found: C, 69.19; H, 4.12; N, 12.14.
3.1.12. 4l: light orange solid
mp > 350 °C; IR (KBr, cmÀ1
) mmax 3317, 3244, 3040, 1701, 1620,
1550, 1506; 1H NMR (500 MHz, CDCl3) dH: 1.74 (4H, s), 1.81 (6H, s),
1.92 (4H, s), 3.95 (4H, s), 4.82 (2H, s), 6.71 (4H, d, J = 8.5 Hz), 7.05
(4H, d, J = 8.5 Hz), 7.12 (4H, d, J = 6.89 Hz), 7.22 (2H, t,
J = 7.05 Hz), 7.31 (2H, t, J = 7.28 Hz), 7.36 (2H, m), 11.02 (2H, s),
12.02 (2H, s); 13C NMR (125 MHz, CDCl3) dC: 190.2, 160.2, 157.6,
148.2, 139.2, 137.4, 135.8, 131.8, 131.6, 130.8, 129.4, 120.31,
119.7, 114.8, 106.5, 105.4, 67.8, 34.4, 26.4, 26.3, 10.4 ppm. Anal.
Calc. for C46H40N6O4: C, 74.58; H, 5.44; N, 11.34. Found: C, 74.69;
H, 5.35; N, 11.26.
3.1.7. 4g: red solid
mp 342–344 °C; IR (KBr, cmÀ1
) mmax 3390, 3189, 3056, 1670,
1625, 1540, 1490, 1080; 1H NMR (500 MHz, CDCl3) dH: 1.83 (3H,
s), 4.93 (1H, s), 7.15 (1H, d, J = 6.95 Hz), 7.20 (2H, d, J = 8.5 Hz),
7.23 (2H, d, J = 8.5 Hz), 7.25–7.28 (1H, t, J = 7.92 Hz), 7.33–7.36
(1H, t, J = 7.26 Hz), 7.62 (1H, d, J = 7.15 Hz), 11.20 (1H, s), 12.09
(1H, s); 13C NMR (125 MHz, CDCl3) dC: 190.3, 157.9, 147.9, 145.9,
137.3, 135.7, 131.9, 131.2, 131.0, 130.3, 128.8, 120.4, 119.9,
105.7, 104.7, 34.7, 10.4 ppm. Anal. Calc. for C20H14ClN3O: C,
69.07; H, 4.05; N, 12.08. Found: C, 69.18; H, 4.01; N, 12.01.
Acknowledgements
The authors are grateful to the Research Council of University of
Guilan for the partial support of this research work and also to Dr.
N.O. Mahmoodi for the aldehydes 3j–l.
3.1.8. 4h: red solid
References
mp 302–304 °C; IR (KBr, cmÀ1
) mmax 3236, 3186, 3060, 1670,
1612, 1558, 1535, 1068; 1H NMR (500 MHz, CDCl3) dH: 1.80 (3H,
s), 5.32 (1H, s), 7.13 (1H, d, J = 6.95 Hz), 7.14–7.24 (2H, m), 7.26
(1H, t, J = 7.2 Hz), 7.33 (1H, t, J = 7.2 Hz), 7.43 (1H, s), 7.64 (1H, d,
J = 7.17 Hz), 11.27 (1H, s), 12.10 (1H, s); 13C NMR (125 MHz, CDCl3)
dC: 190.0, 158.4, 148.0, 142.3, 141.5, 137.3, 137.2, 135.7, 133.2,
132.0, 131.9, 131.0, 129.1, 128.5, 120.4, 120.0, 104.8, 104.1, 33.1,
10.2 ppm. Anal. Calc. for C20H13Cl2N3O: C, 62.84; H, 3.42; N,
10.99. Found: C, 62.72; H, 3.50; N, 10.83.
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3.1.9. 4i: light red solid
mp > 350 °C; IR (KBr, cmÀ1
) mmax 3415, 3254, 1668, 1616, 1580,
1488, 1545, 1350, 1240; 1H NMR (500 MHz, CDCl3) dH: 2.07 (3H, s),
5.23 (1H, s), 6.57 (1H, d, J = 3.72 Hz), 7.24 (1H, d, J = 6.93 Hz), 7.32
(1H, t, J = 7.4 Hz), 7.38 (1H, t, J = 7.23 Hz), 7.53 (1H, d, J = 3.70 Hz),
7.65 (1H, d, J = 7.12 Hz) 11.40 (1H, s), 12.27 (1H, s); 13C NMR
(125 MHz, CDCl3) dC: 190.0, 162.1, 159.0, 151.5, 147.8, 138.2,
137.0, 135.6, 132.1, 131.3, 120.8, 120.3, 115.3, 110.9, 100.5,
100.4, 29.4, 10.3 ppm. Anal. Calcd. for C18H12N4O4: C, 62.07; H,
3.47; N, 16.09. Found: C, 62.16; H, 3.35; N, 16.01.
3.1.10. 4j: red solid
mp 262–264 °C; IR (KBr, cmÀ1
) mmax 3419, 3244, 3060, 1680,
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