Pyrazolopyridines I: Synthesis of some pyrazolo[3,4-b]pyridine-4- carboxylates

Article abstract of DOI:10.14233/ajchem.2013.14573A

A series of ethyl 3-methyl-1-phenyl-6-aryl-1 H-pyrazolo[3,4-b]pyridine-4- carboxylates were prepared by the reaction of 5-amino-3-methyl-1-phenyl-1 H-pyrazole with various aromatic aldehydes and ethyl pyruvate. The structure elucidation of the prepared compounds was carried out by spectroscopic techniques (mass, IR, ¹H and ¹³C NMR).

Full text of DOI:10.14233/ajchem.2013.14573A

Asian Journal of Chemistry; Vol. 25, No. 14 (2013), 7715-7718
http://dx.doi.org/10.14233/ajchem.2013.14573A
Pyrazolopyridines I: Synthesis of Some Pyrazolo[3,4-b]pyridine-4-carboxylates
1
1
2,*
3
4
TAHIR MAQBOOL , MISBAHUL AIN KHAN , MUHAMMAD NAEEM KHAN , MARK C. ELLIOTT , MUNAWAR ALI MUNAWAR ,
1
1
4
5
MUHAMMAD NASRULLAH , ABDUL WAHEED BHATTI , AREESHA NAZEER and WHEI-OH LIN
1Department of Chemistry, The Islamia University of Bahawalpur, Pakistan
2Applied Chemistry Research Center, PCSIR Laboratories Complex, Lahore, Pakistan
3School of Chemistry, Cardiff University, Cardiff, Wales, CF10 3XQ, United Kingdom
4Institute of Chemistry, University of The Punjab, Lahore, Pakistan
5Instituto de Quimica, Universidade do Grande Rio, Duque de Caxias, RJ, Brasil
*Corresponding author: E-mail: changwani_1@yahoo.com
(Received: 14 September 2012;
Accepted: 8 July 2013)
AJC-13792
A series of ethyl 3-methyl-1-phenyl-6-aryl-1H-pyrazolo[3,4-b]pyridine-4-carboxylates were prepared by the reaction of 5-amino-3-
methyl-1-phenyl-1H-pyrazole with various aromatic aldehydes and ethyl pyruvate. The structure elucidation of the prepared compounds
was carried out by spectroscopic techniques (mass, IR, 1H and 13C NMR).
Key Words: Pyrazolopyridines, Doebner reaction, Schiff’s bases, Ethyl pyruvate.
MHz. High resolution mass spectra were recorded on a
Micromass Q-TOF Micro spectrometer.
INTRODUCTION
Fused heterocyclic ring systems containing a pyrazole
nucleus are well known for their wide range of biological and
medicinal significance1-5. For example allopurinol a pyrazolo-
pyrimidine, is in use for the treatment of gout6. Pyrazolo[3,4-
b]pyridines have also been evaluated for their diverse biologi-
cal and pharmacological applications such as antitubercular,
antibacterial, antiinflammatory, antipyretic, antileishminial and
Preparation of ethyl 3-methyl-1-phenyl-6-aryl-1H-pyrazolo-
[3,4-b]pyridine-4-carboxylates (2a-2n) (Table-1)
General procedure: A mixture of an equimolar amount
of 5-amino-3-methyl-1-phenyl-1H-pyrazole (1.73 mmol),
appropriate aromatic aldehydes (1.73 mmol) and one drop of
HCl in ethanol (5 mL) was heated under reflux for 1 h. Ethanol
was evaporated, followed by the successive addition of an
equimolar amount of ethyl pyruvate and 3 mL of glacial
acetic acid. The mixture was further heated for 1.5 h at 100 ºC
on an oil bath, allowed to cool to room temperature, filtered
and dried. The crude compounds were recrystallized from
ethanol.
protein kinase inhibiting agents7-16
.
Recently, some pyrazolo[3,4-b]pyridines have been
evaluated for antichagasic activity to establish a structure
activity relationship17.Also another publication deals with their
preparation both with classical and microwave methods18.
In continuation of our work on the chemistry of pyrazolo-
[3,4-b]pyridines19, we would now like to report the preparation
of various ethyl 1,6-diaryl-1H-pyrazolo[3,4-b]pyridine-4-
carboxylates.
In case of the liquid aldehydes (products 2a, 2b, 2c, 2f,
2g, 2k and 2m), the Schiff bases were prepared by heating the
mixture at 100 ºC without a solvent, followed by the same
general procedure as described above.
EXPERIMENTAL
Ethyl 3-methyl-1,6-diphenyl-1H-pyrazolo[3,4-
b]pyridine-4-carboxylate (2a):Yield: 51 %, m.p. 134 ºC; lit.17
m.p. 123 ºC; IR (KBr, νmax, cm-1): 3040-2930 (C-H); 1734
The chemicals used were commercially available from
Merck or Fluka and were used as such. However when needed
were purified using normal techniques. The solvents used were
distilled and dried. FTIR spectra were recorded on Bruker
Tensor-27. Melting points were taken on a GallenKamp
melting point apparatus and are uncorrected. The 1H and 13C
NMR spectra were taken on Bruker DPX instrument at 400
1
(C=O). H NMR (CDCl3) δ: 1.55 (t, 3H, CH3CH2, J = 7.15
Hz), 2.80 (s, 3H, CH3), 4.55 (q, 2H, CH3CH2, J = 7.16 Hz),
8.15 (s, 1H, H-5), 7.32-8.39 (m, 10H,ArH), 13C NMR (CDCl3):
δ: 4.34 (CH3CH2), 16.45 (CCH3), 62.14 (CH2), 112.01, 115.21,
121.30, 125.77, 127.55, 128.94, 129.02, 129.90, 134.26,

7716 Maqbool et al.
Asian J. Chem.
138.30, 139.35, 142.58, 152.09, 156.62, 165.69 (C=O). HRMS
(ESI): calcd. (%) for (C22H20N3O2) [M + H]+: 358.1556; found
(%): 358.1553.
139.74, 142.59, 151.98,156.32,160.08, 165.67 (C=O). HRMS
(ESI): calcd. (%) for (C23H22N3O3) [M + H]+: 388.1661; found
(%): 388.1660.
Ethyl 6-(2'-chlorophenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2b): Yield: 33 %,
m.p. 125 ºC, IR (KBr, νmax, cm-1): 3020-2960 (C-H); 1730
(C=O); 1572 (C=C) 1H NMR (CDCl3) δ: 1.45 (t, 3H, CH3CH2,
J = 7.15 Hz), 2.75(s, 3H, CH3), 4.45 (q, 2H, CH3CH2, J = 7.12
Hz), 7.95 (s, 1H, H-5), 7.21-8.25 (m, 9H, ArH), 13C NMR
(CDCl3) δ: 14.29 (CH3CH2), 16.42 (CCH3), 62.15 (CH2),
112.00, 119.29, 121.36, 125.92, 127.11, 129.06, 130.19,
130.42, 131.97, 132.62, 133.54, 138.43, 139.17, 142.63,
151.63, 156.34, 165.48 (C=O). HRMS (ESI): calcd. (%) for
(C22H19N3O2Cl) [M + H]+: 392.1166; found (%): 394.1150.
Ethyl 6-(3'-chlorophenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2c): Yield: 48 %,
m.p. 146 ºC, IR (KBr, νmax, cm-1): 3040-2930 (C-H); 1732
(C=O); 1596 (C=C) 1H NMR (CDCl3) δ: 1.45 (t, 3H, CH3CH2,
J = 7.18 Hz), 2.75 (s, 3H, CH3), 4.45 (q, 2H, CH3CH2, J = 7.16
Hz), 8.05 (s, 1H, H-5), 7.20-8.24 (m, 9H, ArH) 13C NMR
(CDCl3): δ 14.34 (CH3CH2), 16.4 (CCH3), 62.26 (CH2), 112.37,
115.05, 121.36, 125.63, 125.95, 127.57,129.07, 129.81,
130.16, 134.47, 134.97, 139.16, 140.06, 142.6, 151.8, 155.00,
165.46 (C=O). HRMS (ESI): calcd. (%) for (C22H19N3O2Cl)
[M + H]+: 392.1166; found (%): 394.1152.
Ethyl 6-(4'-methoxyphenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2g): Yield: 37 %,
m.p. 120 ºC. lit.17 m.p. 122 ºC; IR (KBr, νmax, cm-1): 3040-
2930 (C-H); 1733 (C=O); 1591 (C=C). 1H NMR (CDCl3) δ:
1.45 (t, 3H, CH3CH2, J = 7.12 Hz), 2.75 (s, 3H, CH3), 4.45 (q,
2H, CH3CH2, J = 7.14 Hz), 7.95 (s, 1H, H-5), 6.94-8.26 (m,
9H, ArH), 3.80 (s, 3H, OCH3) 13C NMR (CDCl3): δ 14.33
(CH3CH2), 16.44 (CCH3), 55.44 (OCH3), 62.08 (CH2), 111.46,
114.27, 114.64, 121.27, 125.66, 128.92, 128.98, 130.83,
134.10, 139.41, 142.5, 152.11, 156.29, 161.19, 165.79 (C=O).
HRMS (ESI): calcd. (%) for (C23H22N3O3) [M + H]+: 388.1661;
found (%): 388.1667.
Ethyl 3-methyl-6-(2'-nitrophenyl)-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2h): Yield: 49 %,
m.p. 134 ºC. IR (KBr, νmax, cm-1): 3040-2930 (C-H); 1709
(C=O); 1596 (C=C); 1528, 1343 (NO2). 1H NMR (CDCl3) δ:
1.45 (t, 3H, CH3CH2, J = 7.13 Hz), 2.70 (s, 3H, CH3), 4.45 (q,
2H, CH3CH2, J = 7.16 Hz), 7.85 (s, 1H, H-5), 7.19-8.00 (m,
9H, ArH). 13C NMR (CDCl3) δ: 14.30 (CH3CH2), 16.44
(CCH3), 62.32 (CH2), 112.53, 116.91, 121.65, 124.64, 126.31,
129.09, 129.83, 131.49, 132.32, 134.19, 134.79, 139.66,
142.67, 149.78, 151.26,154.17, 165.16(C=O). HRMS (ESI):
calcd. (%) for (C22H19N4O4) [M + H]+: 403.1406; found (%):
403.1401.
Ethyl 6-(4'-chlorophenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2d): Yield: 45 %,
m.p. 127 ºC. lit.17 m.p. 123 ºC; IR (KBr, νmax, cm-1): 3040-
Ethyl 3-methyl-6-(3'-nitrophenyl)-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2i): Yield: 35 %,
m.p. 165 ºC. IR (KBr, νmax, cm-1): 3040-2930 (C-H); 1735
(C=O); 1594 (C=C); 1526, 1347 (NO2). 1H NMR (CDCl3) δ:
1.45 (t, 3H, CH3CH2, J = 7.16 Hz), 2.75(s,3H, CH3), 4.45 (q,
2H, CH3CH2, J = 7.16 Hz), 8.05 (s, 1H, H-5), 7.24-8.90 (m,
9H,ArH). 13C NMR (CDCl3) δ: 14.35(CH3CH2), 16.42 (CCH3),
62.43 (CH2), 112.81, 114.82, 121.38, 122.29, 124.34, 126.16,
129.13, 129.97, 133.26, 134.89, 139.01, 139.95, 142.76,
148.82, 151.52, 153.70, 165.25 (C=O). HRMS (ESI):
calcd. (%) for (C22H19N4O4) [M + H]+: 403.1406; found (%):
403.1420
1
2930 (C-H); 1732 (C=O); 1596 (C=C) H NMR (CDCl3) δ:
1.44 (t, 3H, CH3CH2, J = 7.12 Hz), 2.75 (s, 3H, CH3), 4.45 (q,
2H, CH3CH2, J = 7.15 Hz), 7.95 (s, 1H, H-5), 7.20-8.23 (m,
9H, ArH), 13C NMR (CDCl3): δ 14.33 (CH3CH2), 16.65
(CCH3), 62.23 (CH2), 112.13, 114.86, 121.36, 125.91, 128.76,
129.13, 129.04, 129.13, 134.43, 136.08, 136.69, 139.20,
142.64, 151.95, 155.31, 165.55 (C=O). HRMS (ESI): calcd.
(%) for (C22H19N3O2Cl) [M + H]+ : 392.1166; found (%):
394.1212.
Ethyl 6-(2'-methoxyphenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2e): Yield: 44 %,
m.p. 126 ºC, IR (KBr, νmax, cm-1): 3040-2920 (C-H); 1731
(C=O); 1592 (C=C). 1H NMR (CDCl3) δ: 1.45 (t, 3H, CH3CH2,
J = 7.18 Hz), 2.75 (s, 3H, CH3), 4.45 (q, 2H, CH3CH2, J = 7.14
Hz), 8.15 (s, 1H, H-5), 6.95-8.25 (m, 9H, ArH), 3.80 (s, 3H,
-OCH3) 13C NMR (CDCl3) δ: 14.30 (CH3CH2), 16.41 (CCH3),
55.71 (OCH3), 61.93 (CH2), 111.44, 111.59, 120.01, , 121.18,
121.24, 125.62,128.29, 128.99, 130.84, 131.63, 132.99,
139.47, 142.46, 151.93, 155.89, 157.51, 165.99 (C=O). HRMS
(ESI): calcd. (%) for (C23H22N3O3) [M + H]+: 388.1661; found
(%): 388.1643
Ethyl 3-methyl-6-(4'-nitrophenyl)-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2j): Yield: 52 %,
m.p. 144 ºC. lit.17 m.p. 138; IR (KBr, νmax, cm-1): 3040-2950
(C-H); 1734 (C=O); 1594 (C=C); 1501, 1347 (NO2). 1H NMR
(CDCl3) δ: 1.45 (t, 3H, CH3CH2, J = 7.18 Hz), 2.70 (s, 3H,
CH3), 4.45 (q, 2H, CH3CH2, J = 7.15 Hz), 8.10(s, 1H, H-5),
7.25-8.30 (m, 9H,ArH) 13C NMR (CDCl3) δ: 14.34 (CH3CH2),
16.44 (CCH3), 62.43 (CH2), 112.87, 115.35, 121.38, 124.12,
126.18, 128.27, 129.12, 134.76, 139.01, 142.76, 144.07,
148.51, 151.85,153.69, 165.22(C=O). HRMS (ESI): calcd. (%)
for (C22H19N4O4) [M + H]+: 403.1406; found (%): 403.1392.
Ethyl 6-(2'-bromophenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2k):Yield: 35.02 %,
m.p. 122 ºC. IR (Kbr, νmax, cm-1): 3040-2920 (C-H); 1731
(C=O); 1572 (C=C). 1H NMR (CDCl3) δ: 1.45(t, 3H, CH3CH2,
J = 7.15 Hz), 2.75 (s, 3H, CH3), 4.45 (q, 2H, CH3CH2, J = 7.17
Hz), 7.95 (s, 1H, H-5), 7.20-8.25 (m, 9H, ArH), 13C NMR
(CDCl3) δ: 14.29 (CH3CH2), 16.42 (CCH3), 62.16 (CH2),
112.01, 119.22, 121.42, 122.05,122.52, 125.95, 127.64,
Ethyl 6-(3'-methoxyphenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2f): Yield: 48 %,
m.p. 96 ºC, IR (KBr, νmax, cm-1): 3030-2940 (C-H); 1715
(C=O); 1594 (C=C). 1H NMR (CDCl3) δ: 1.45 (t, 3H, CH3CH2,
J = 7.13 Hz), 2.75 (s, 3H, CH3), 4.45 (q, 2H, CH3CH2, J = 7.15
Hz), 8.05 (s, 1H, H-5), 6.95-8.27 (m, 9H, ArH), 3.80 (s, 3H,
OCH3), 13C NMR (CDCl3) δ: 14.33 (CH3CH2), 16.44 (CCH3),
62.15(CH2), 55.40 (OCH3), 112.11, 112.98, 115.30, 115.47,
119.99, 121.29, 125.77, 128.98, 129.93, 134.24, 139.33,

Vol. 25, No. 14 (2013)
Pyrazolopyridines I: Synthesis of Some Pyrazolo[3,4-b]pyridine-4-carboxylates 7717
CO2C2H5
127.85, 129.06, 129.80, 130.53, 132.88, 133.50, 133.53,
139.72, 140.38, 142.64, 151.49, 157.67, 165.45 (C=O). HRMS
(ESI): calcd. (%) for (C22H19N3O2Br) [M + H]+: 436.0661;
found (%): 438.0675.
H3C
H3C
N
(i)XC6H4CHO,EtOH,HCl
(ii) Ethylpyruvate,acetic acid,
N
N
NH2
N
Ethyl 6-(4'-bromophenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2l): Yield: 28 %,
m.p. 126 ºC, IR (KBr, νmax, cm-1): 3040-2920 (C-H); 1723
(C=O); 1575 (C=C). 1H NMR (CDCl3) δ: 1.45 (t, 3H, CH3CH2,
J = 7.16 Hz), 2.75 (s, 3H, CH3), 4.45 (q, 2H, CH3CH2, J =
7.12 Hz), 7.25-8.25 (m, 10H,ArH), 13C NMR (CDCl3) δ: 14.33
(CH3CH2), 16.42 (CCH3), 62.24 (CH2), 112.20, 114.83,
121.37, 124.50, 125.92, 129.05, 132.11, 134.48, 137.19,
139.20, 142.65, 151.98, 155.39, 165.54 (C=O). HRMS (ESI):
calcd. (%) for (C22H19N3O2Br) [M + H]+: 436.0677; found (%):
438.0729.
N
C6H5
C6H5
X
(1)
(2a-2n)
Scheme-I
TABLE-1
ETHYL 3-METHYL-1-PHENYL-6-ARYL-1H-
PYRAZOLO[3,4-b]PYRIDINE-4-CARBOXYLATES (2a-2n)
Compound
X
Yield (%)
51
m.p. (ºC)
H
2a
2b
2c
2d
2e
2f
134∗
125
146
127∗∗
126
96
120+
134
165
144+ +
122
126
138
142
2-Cl
3-Cl
4-Cl
33
Ethyl 6-(2'-hydroxyphenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2m): Yield: 46 %,
m.p. 138 ºC, IR (KBr, νmax, cm-1): 3020-2920 (C-H); 1714
(C=O); 1595 (C=C); 3370(OH). 1H NMR (CDCl3) δ: 1.45 (t,
3H, CH3CH2, J = 7.17 Hz), 2.75 (s, 3H, CH3), 4.45 (q, 2H,
CH3CH2, J = 7.16 Hz), 8.15 (s, 1H, H-5), 6.85-7.90 (m, 9H,
ArH), 13.1 (br.s, 1H, OH), 13C NMR (CDCl3): δ 14.32
(CH3CH2), 16.29 (CCH3), 62.42 (CH2), 111.67 , 114.25,
118.66, 118.68, 119.43, 122.71, 127.33, 127.59, 129.56,
129.54, 132.51, 135.31, 138.16, 143.28, 148.84, 157.01,
159.51, 165.14 (C=O). HRMS (ESI): calcd. (%) for
(C22H20N3O3) [M + H]+: 374.1505; found (%): 374.1520.
Ethyl 6-(3'-hydroxyphenyl)-3-methyl-1-phenyl-1H-
pyrazolo[3,4-b]pyridine-4-carboxylate (2n): Yield: 42 %,
m.p. 142 ºC. IR (KBr, νmax, cm-1): 3020-2920 (C-H); 1733
(C=O); 1596 (C=C); 3380 (OH). 1H NMR (CDCl3) δ: 1.45 (t,
3H, CH3CH2, J = 7.22 Hz), 2.75 (s, 3H, CH3), 4.45 (q, 2H,
CH3CH2, J = 7.19 Hz), 8.0 (s, 1H, H-5), 7.25-8.25 (m, 9H,
ArH), 9.9 (br.s, 1H, OH). 13C NMR (CDCl3) δ: 14.33
(CH3CH2), 16.44 (CCH3), 62.15 (CH2), 112.11, 112.98,
115.30, 115.47, 119.99, 121.29, 125.77, 128.98, 129.93,
134.24, 139.33, 139.74, 142.59, 151.98, 156.32, 160.08,
165.67 (C=O). HRMS (ESI): calcd. (%) for (C22H20N3O3) [M
+ H]+ : 374.1505; found (%): 374.1499.
48
45
2-OCH3
3-OCH3
4-OCH3
2-NO2
3-NO2
4-NO2
2-Br
44
48
37
2g
2h
2i
49
35
52
2j
35
2k
2l
4-Br
28
2-OH
46
2m
2n
3-OH
42
*lit.17 m.p. 123; **lit.17 m.p. 123; +lit.17 m.p. 122; ++lit.17 m.p. 138
of 1735-1709 cm-1 was due to C=O group of the ester at 4th
1
position. In the H NMR spectra, the ester group signals
appeared as typical triplets and quartets at 1.45 and 4.45 ppm,
respectively with J value of around 7.14 Hz. The H-5 of the
pyridine ring showed a neat singlet in the range of 6.85-8.15
ppm. The singlet at 2.70-2.75 ppm was due to methyl group
on the pyrazole ring while the rest of the aromatic protons
signals appeared between δ 6.85-8.90 ppm. In all the 13C NMR
spectra the carbonyl carbon of the ester at 4 position gave a
discrete signal in the range of 165.14-165.79 ppm. High reso-
lution mass spectrometry results also confirmed the structures
of the prepared compounds.
RESULTS AND DISCUSSION
REFERENCES
Ethyl 3-methyl-1-phenyl-6-aryl-1H-pyrazolo[3,4-
b]pyridine-4-carboxylates (2a-2n) were prepared as per
Scheme-I from 5-amino-3-methyl-1-phenyl-1H-pyrazole20,
aromatic aldehydes and ethyl pyruvate. 5-Aminopyrazole was
made to react with various aromatic aldehydes to obtain
respective Schiff's bases which were, without isolation,
subjected to condensation with ethyl pyruvate to give ethyl
3-methyl-1-phenyl-6-aryl-1H-pyrazolo[3,4-b]pyridine-4-
carboxylates (2a-2n). The yields of the recrystallized products
are in the range of 28-51 %; the crude yields are although
much higher. No attempts, at this stage, were made to optimize
them. Products (2a-2n) (Table-1) obtained as fine crystals, all
are stable compounds at room temperature and were charac-
terized by IR, 1H, 13C NMR spectra and high resolution mass
spectrometry.
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