One-pot and convenient conversion of 5-azidopyrazole-4-carboxaldehyde to pyrazolo[3,4-b]pyridines

Article abstract of DOI:10.1080/00397910600588868

A one-pot and convenient synthesis of multisubstituted pyrazolo[3, 4-b]pyridines in moderate to high yields has been achieved by a two-step reaction: diazo-transfer of 5-azido-1-phenylpyrazole-4-carboxaldehydes to ketones in ethanolic KOH to give 5-amino-

Full text of DOI:10.1080/00397910600588868

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One‐Pot and Convenient Conversion of
5‐Azidopyrazole‐4‐carboxaldehyde to
Pyrazolo[3,4‐b]pyridines
a
a
a
Airong Zheng , Wei Zhang & Jinhui Pan
a
State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou, China
Published online: 21 Aug 2006.
To cite this article: Airong Zheng , Wei Zhang & Jinhui Pan (2006): One‐Pot and Convenient Conversion of
5‐Azidopyrazole‐4‐carboxaldehyde to Pyrazolo[3,4‐b]pyridines, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 36:11, 1549-1556
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Synthetic Communications , 36: 1549–1556, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600588868
One-Pot and Convenient Conversion
of 5-Azidopyrazole-4-carboxaldehyde
to Pyrazolo[3,4-b]pyridines
Airong Zheng, Wei Zhang, and Jinhui Pan
State Key Laboratory of Applied Organic Chemistry,
Lanzhou University, Lanzhou, China
Abstract: A one-pot and convenient synthesis of multisubstituted pyrazolo[3,
4-b]pyridines in moderate to high yields has been achieved by a two-step reaction:
diazo-transfer of 5-azido-1-phenylpyrazole-4-carboxaldehydes to ketones in
ethanolic KOH to give 5-amino-1-phenylpyrazole-4-carboxaldehyde and subsequent
Friedlaender reaction of 5-amino-1-phenylpyrazole-4-carboxaldehyde with ketones.
Keywords: 5-Azido-1-phenylpyrazole-4-carboxaldehyde, diazo-transfer, Friedlaender
reaction, pyrazolo[3,4-b]pyridines
INTRODUCTION
Pyrazolo[3,4-b]pyridine derivatives are well known for their wide range
of biological and pharmacological activities. Many of these are associated
with antihypertensive, anti-inflammatory, antitumor, antiviral, antimalarial,
[
1–8]
and antipyretic activities.
of the pyrazolo[3,4-b]pyridine core have been developed in recent
Hence, a variety of methods for the synthesis
[1,2,6–11]
years.
The usual synthetic routes to the pyrazolo[3,4-b]pyridine
core are ring closure of the pyridine ring of a functionized pyrazole or
ring closure of the pyrazole ring of a functionized pyridine, for example,
Friedlaender condensation of 5-amino-pyrazole-4-carboxaldehyde, which
Received in Japan September 25, 2005
Address correspondence to Wei Zhang, State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China. Fax: þ86(931)-8625657;
E-mail: zhang_wei6275@yahoo.com
1
549

1
550
A. Zheng, W. Zhang, and J. Pan
Scheme 1.
was obtained by reduction of 5-azidopyrazole-4-carboxaldehyde with hydrogen
[13]
[12]
sulphide or sodium dithionite, with ketones or with malononitrile. In view
of these reports and as a continuation of our studies on the synthesis of hetero-
[14–16]
cyclic structures with biological activities,
we report herein a new
synthesis of the pyrazolo[3,4-b]pyridine derivatives based on a convenient
one-pot reaction of 5-azido-1-phenylpyrazole-4-carboxaldehydes (1a, b) with
acetophenones (2a–c), acetone (2d), acetylacetone (2e), benzoylacetone (2f),
cyclopentanone (2g), cyclohexanone (2h), dimedone (2i), and 1-tetralone (2j)
in a solution of ethanolic KOH as depicted in Scheme 1.
DISCUSSION
It was observed from the results listed in Table 1 that 5-azido-1-phenylpyrazole-
4-carboxaldehydes could react with both aromatic ketones and aliphatic ketones
smoothly at 508C for half an hour and then reflux for several hours to afford
pyrazolo[3,4-b]pyridines in moderate to high yields, except for the 1,3-dicarbo-
1
13
nyls. The structures of pyrazolo[3,4-b]pyridines were assigned by H, C, and
D NMR spectra as well as Nuclear Overhauser Effect (NOE) experiments.
NOE difference experiment showed that the methyl group at C-3 and
2
pyridine H-4 interacted through space in 3a. Irradiating the methyl group at d
2.63ppm enhanced the pyridine H-4 signal at d 8.03ppm. All the NMR
spectra indicated that the products contained the pyridine ring. This conclusion
is based on the observation of typical pyridine-ring shift values of 8.03 (H-4,
J ¼ 8.1 Hz) and 7.63 (H-5, J ¼ 8.1 Hz) in 3a and 7.78 (H-4, J ¼ 8.1 Hz) and
6
.93 (H-5, J ¼ 8.1 Hz) in 3d, respectively. The presence of the doublet signal
at 8.42ppm (2H) in 3a and 8.32ppm (2H) in 3d, which is derived from ortho
3
0
0
protons of 1-phenyl with the coupling constants J (H2 -H 3 ) ¼ 8.1 Hz,
confirms that the pyrazole ring is connected with the pyridine ring in (3,4-b)
position.
This one-pot synthesis of pyrazolo[3,4-b]pyridines was a unexpected
discovery resulting from a proposed base-catalyzed Knoevenagel conden-
sation of 5-azido-1-phenylpyrazole-4-carboxaldehyde (1a) with acetophenone
(2a) at room temperature in aqueous ethanolic NaOH. Actually, the

Table 1. Reaction of 5-azidopyrazole-4-carboxaldehyde (1) with ketones (2)
Substrates
a
Time
(h)
2
3
b
Yield of 3 (%)
Entry
R1
R
R
Mp (8C)
128–129
1
2
3
4
5
6
7
8
9
CH3
H
H
Ph
p-MeC
3
3
3a
88
86
80
78
70
75
87
68
15
75
68
60
73
78
92
75
65
CH
CH
3
3
6
H
4
3b
3c
3d
3d
3a
3e
3f
84–85
102–103
H
p-MeOC
Me
6
H
4
3
[
[
8i]
8i]
CH3
H
2
48–49 (43–46
48–49 (43–46
128–129
)
)
CH
CH
3
3
Ac
Ac
CH
Ph
3
2
3
CH3
22 (CH ) 22
2 3
2
114–116
76–77 (71–72
[
6g]
CH
CH
3
3
22 (CH
22 CO(CH
22 (CH ) C H 22
2
)
4
22
3
)
c
[6g]
2
CMe
2
CH
2
)22
12
4
3g
3h
3i
128–129 (130
156–157
135–146
)
10
11
12
13
14
15
16
17
CH3
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2 2
6 4
H
H
H
H
Ph
p-MeC
3
6
H
4
3
3j
162–163
178–179
p-MeOC
Me
6
H
4
4
3k
3l
[
8i]
3
124–125 (123–125
149–150
)
22 (CH
22 (CH
2
)
)
3
22
22
3
3m
3n
3o
[
6g]
2
4
4
138–139 (140
165–166
)
22 (CH ) C H 22
2 2 6 4
4
a
b
c
Total reaction time.
Isolated yield based on 1.
2
Products obtained in 5% NaOH-H O-EtOH at room temperature.

1
552
A. Zheng, W. Zhang, and J. Pan
product obtained was not azido-substituted a,b-unsaturated ketone but
,6-diphenyl-3-methylpyrazolo-[3,4-b]pyridine (3a). It was noticed by thin-
1
layer chromatography (TLC) that a reduction product, 5-amino-1-phenylpyr-
azole-4-carboxaldehyde, which was separated from the mixture and assigned
1
by H NMR spectra and mass spectra, was produced from the beginning of the
reaction, accompanied by the formation of a little 3a, and both could not
disappear after a long time stirring when the molar ratio of 1a and 2a was
1:1. Only when another equivalent acetophenone was added could the
reaction of 5-amino-1-phenylpyrazole-4-carboxaldehyde finish to afford 3a
after several hours. It was also noticed that no reduction product 5-amino-1-
phenylpyrazole-4-carboxaldehyde was detected in the absence of acetophe-
none. This indicated acetophenone was responsible for the formation of
5
-amino-1-phenylpyrazole-4-carboxaldehyde. A rational explanation for
theses observations was the diazo-transfer from 5-azido-1-phenylpyrazole-
-carboxaldehyde to acetophenone, resulting in the formation of 5-amino-1-
phenylpyrazole-4-carboxaldehyde, and subsequent Friedlaender reaction of
4
5
1
-amino-1-phenylpyrazole-4-carboxaldehyde with acetophenone to give
,6-diphenyl-3-methylpyrazolo[3,4-b]pyridine (3a). Although there had no
report for the diazo-transfer reaction of 1a, b to ketones before, that other
azido heterocycles were reported to be efficiently converted to amino
[
17]
compounds by diazo-transfer to acetylacetone
support to the proposal.
Alcoholic KOH was more suitable for the synthesis of compounds 3 than
could be a potential
aqueous alcoholic NaOH, and the temperature rise could accelerate the
the reactions were carried
[18]
reaction greatly. To avoid decomposition of 1a,
firstly at 508C for 1 h and then under refluxed for several hours to promote
Friedlaender reactions. Comparatively, 1,3-dicarbonyls behaved a little differ-
ently than the other ketones. The products obtained from reaction of 1a with
acetylacetone or benzoylacetone in ethanolic KOH were the same as those
from reaction with acetone or acetophenone as shown in Table 1, which
implied that the decomposition of acetylacetone or benzoylacetone took place
before the Friedlaender reaction. In the case of dimedone, the diazo-transfer
reaction with 1a was complete within half an hour even at room temperature
but the Friedlaender reaction was rather slow as compared with other ketones.
According to the results obtained, we propose the following mechanistic
rationale to explain the formation of the pyrazolo[3,4-b]pyridines (Scheme 2).
The first step is the formation of 5-amino-1-phenylpyrazole-4-carboxaldehyde
from the diazo-transfer reaction of 5-azido-1-phenylpyrazole-4-carboxalde-
hyde to ketones followed by Friedlaender reaction of 5-amino-1-phenylpyra-
zole-4-carboxaldehyde with ketones.
In summary, a convenient one-pot route for preparation of pyrazolo[3,4-
b]pyridine derivatives has been developed by the reaction of 5-azido-1-phe-
nylpyrazole-4-carboxaldehyde with ketones in ethanolic KOH at 508C and
then refluxing. The formation of pyrazolo[3,4-b]pyridine can be rationalized
to proceed via diazo-transfer reaction from 5-azidopyrazole-4-carboxaldehyde

5-Azidopyrazole-4-carboxaldehyde
1553
Scheme 2.
to ketones and subsequent Friedlander reaction of 5-amino-1-phenylpyrazole-
-carboxaldehyde with ketones.
4
EXPERIMENTAL
1 13
H and C NMR spectra were recorded on a Bruker AM-400 NMR or a
Bruker DRX-300 NMR spectrometers in CDCl with TMS as an internal
3
standard. EI-MS were recorded with a HP 5988 A mass spectrometer.
Melting points were determined on a Yanagimoto melting-point apparatus
and uncorrected. Elementary analyses were carried out on a Perkin-Elmer
2
400 II analyzer. Organic solvents were distilled prior to use.
General Procedure for Synthesis of Pyrazolo[3,4-b]pyridines
To a 25-ml alcoholic solution of 5-azidopyrazole-4-carboxaldehyde (1a, b,
[
19]
2
mmol)
and ketones (2a–j, 3.5–4 mmol), 25 ml of 10% ethanolic KOH

1
554
A. Zheng, W. Zhang, and J. Pan
was added. The resulted mixture was stirred first at 508C for 1 h, then refluxed
for 1–3 h. The progress of the reaction was tracked by TLC. The reaction
mixture was poured into cold water, and the solid was collected. After
drying the solids were separated on a silica-gel column and recrystallized
from ethanol to afford the desired pyrazolo[3,4-b]pyridines.
Data
1
3
1
a: H NMR (300 MHz, CDCl ) d 2.64 (s, 3H, CH ), 7.27 (t, J ¼ 7.2 Hz,
3
3
0
H, H-4 ), 7.42–7.55 (m, 5H), 7.63 (d, J ¼ 8.1 Hz, 1H, H-5), 8.03
00
(
d, J ¼ 8.1 Hz, 1H, H-4), 8.15 (d, J ¼ 7.2 Hz, 2H, H-2 ) 8.43
0
3 3
13
(
d, J ¼ 8.1 Hz, 2H, H-2 ). C NMR (75 MHz, CDCl ) d 12.47 (CH ),
0 0
00
114.23 (C-5), 115.74 (C-3a), 120.46 (2C, C-2 ), 125.14 (C-4 ), 127.51 (2C,
C-2 ), 128.76 (2C, C-3 ), 128.92 (2C, C-3 ), 129.37 (C-4 ), 129.88
0
0
0
00
0
0
0
(
(
C-4),139.15 (C-7a), 139.76 (C-1 ), 142.50 (C-1 ), 150.98 (C-3), 156.61
þ
C-6). MS m/z 285 (M , 100), 270 (25), 243 (9), 140 (22), 77 (87). Anal.
calcd. for C H N : C, 79.97; H, 5.30; N, 14.73. Found: C, 79.88; H, 5.42;
1
9 15 3
N, 14.67%.
1
d: H NMR (300 MHz, CDCl ) d 2.55 (s, 3H, CH ), 2.65 (s, 3H, CH ), 6.93
3
(
3
3
3
0
d, J ¼ 8.1 Hz, 1H, H-5), 7.22 (t, J ¼ 7.2 Hz, 1H, H-4 ), 7.47 (t, J ¼ 8.1 Hz,
0
0
2
H, H-3 ), 7.78 (d, J ¼ 8.1 Hz, 1H, H-4), 8.31 (d, J ¼ 8.4 Hz, 2H, H-2 ).
1
3
C NMR (75 MHz, CDCl ) d 12.27 (CH ), 24.95 (CH ), 114.67 (C-3a),
3
3
3
0
1
17.04 (C-4 ), 120.44 (2C), 125.0 (C-5), 128.79 (2C), 129.03 (C-4),139.69
þ
0
(
C-7a), 142.19 (C-1 ), 150.71 (C-3), 158.55 (C-6). MS m/z 223 (M , 100),
208 (26), 181 (19), 105 (13), 77 (37). Anal. calcd. for C H N : C, 75.31;
H, 5.87; N, 18.82. Found: C, 75.25; H, 5.96; N, 18.74%.
14 13 3
1
f: H NMR (300 MHz, CDCl ) d 1.84–1.96 (m, 4H, 2CH ), 2.58 (s, 3H,
3
3
2
CH ), 2.91 (t, J ¼ 6.3 Hz, 2H, CH ), 3.07 (t, J ¼ 6.3 Hz, 2H, CH ), 7.23
3
2
2
0
0
(
t, J ¼ 7.8 Hz, 1H, H-4 ), 7.48 (t, J ¼ 7.8 Hz, 2H, H-3 ), 7.65 (s, 1H, H-4),
0
13
.32 (d, J ¼ 8.1 Hz, 2H, H-2 ), C NMR (75 MHz, CDCl ) d 12.45 (CH ),
8
2
3
3
3.09 (2C, 2CH ), 29.20 (CH ), 33.68 (CH ), 115.68 (C-3a), 120.18 (2C,
2 2 2
0
00
C-2 ), 124.78 (C-4), 125.95 (C-5), 128.88 (2C, C-3 ), 139.86 (C-4a), 141.85
þ
0
(
C-1 ), 149.67 (C-3), 158.31 (C-8a). MS m/z 263 (M , 100), 248 (12), 235
(
17), 220 (8), 193 (7), 77 (41). Anal. calcd. for C H N : C, 77.53; H,
1
7 17 3
6.51; N, 15.96. Found: C, 77.46; H, 6.61; N, 15.95%.
1
3
3
7
h: H NMR (300 MHz, CDCl ) d 2.63 (s, 3H, CH ), 2.94–3.01 (m, 2H, CH ),
3 3 2
.06–3.11 (m, 2H, CH ), 7.23–7.28 (m, 3H, ArH), 7.33–7.47(m, 2H, ArH),
2
0
.53 (t, J ¼ 7.8 Hz, 2H, H-2 ), 7.81 (s, 1H, H-4), 8.45 (d, J ¼ 7.8 Hz, 2H,
0
13
H-2 ), 8.48 (d, J ¼ 7.8 Hz, 1H, H-10). C NMR (75 MHz, CDCl ) d 12.41
3
0
(
CH ), 28.54 (CH ), 29.60 (CH ), 116.03 (C-3a), 120.18 (2C, C-2 ), 124.87
3
2
2
0
(
C-9), 125.95 (C-4 ), 127.16 (C-8), 127.83 (2C, C-7, C-10), 128.86 (2C,

5-Azidopyrazole-4-carboxaldehyde
1555
0
0
C-3 ), 129.57 (C-4), 133.75 (C-11a), 134.76 (C-4a),139.24 (C-6a), 139.96 (C-
þ
0
1
0a), 142.10 (C-1 ), 150.46 (C-3), 158.62 (C-6). MS m/z 311 (M , 100), 296
8), 269 (10), 223 (38), 77 (81). Anal. calcd. for C H N : C, 81.01; H, 5.50;
(
N, 13.50. Found: C, 79.93; H, 5.61; N, 13.42%.
2
1 17 3
1
3
j: H NMR (300 MHz, CDCl ) d 2.43 (s, 3H, CH ), 7.33 (d, J ¼ 7.8 Hz, 2H,
3
3
000
0
H-3 ), 7.33 (t, J ¼ 7.2 Hz, 1H, H-4 ), 7.51–7.58 (m, 3H), 7.71 (d, J ¼ 8.7 Hz,
0
000
1
H, H-5), 8.05–8.09 (m, 4H, H-2 , H-2 ), 8.40 (d, J ¼ 8.7 Hz, 1H, H-4), 8.53
0
3 3
13
(
d, J ¼ 8.1 Hz, 2H, H-2 ). C NMR (75 MHz, CDCl ) d 21.33 (CH ), 113.94
0 0 000
0 00 000 00
(
C-3a), 115.05 (C-5), 121.08 (2C, C-2 ), 125.60 (C-4 ), 127.02 (2C,C-2 ),
1
C-3 ), 130.89 (C-4), 133.09 (C-4 ), 136.2 (C-1 ), 139.71 (C-1 ), 139.84
27.08 (2C,C-3 ), 128.05 (C-4 ), 128.82 (4C,C-3 , C-2 ), 129.58 (2C,
00
00
000
000
0
þ
(
7a), 144.08 (C-1 ), 151.64 (C-3), 156.74 (C-6). MS m/z 361 (M , 20), 284
(4), 240 (12), 229 (6), 188 (9), 119 (35), 77 (100). Anal. calcd. for
C H N : C, 83.07; H, 5.30; N, 11.63. Found: C, 82.97; H, 5.37; N, 11.65%.
2
5
19
3
1
m: H NMR (300 MHz, CDCl ) d 2.20–2.30 (m, 2H, CH ), 3.05
3
3
2
(
t, J ¼ 6.9 Hz, 2H, CH ), 3.14 (t, J ¼ 7.2 Hz, 2H, CH ), 7.29 (t, J ¼ 7.2 Hz,
2
2
0
00
1
H, H-4 ), 7.45 (t, J ¼ 6.9 Hz, 1H, H-4 ), 7.50–7.55 (m, 4H), 8.03–8.06
00 0
13
(
m, 3H, H-4, H-2 ), 8.43 (d, J ¼ 8.7 Hz, 1H, H-2 ). C NMR (75 MHz,
CDCl ) d 24.14 (CH ), 30.20 (CH ), 34.53 (CH ), 114.18 (3a), 121.33 (2C,
2
3
2
2
0
0
00
00
C-2 ), 125.01 (C-4 ), 125.65 (C-4 ), 127.31 (2C, C-2 ), 128.39 (C-4),
1
0 00 00
0 0
28.85 (2C, C-3 ), 128.97 (2C, C-3 ), 132.15 (C-4a), 133.20 (C-1 ), 139.77
þ
(
C-1 ), 143.93 (C-1 ), 151.85 (C-3), 167.32 (C-7a). MS m/z 311 (M , 31),
2
85 (4), 249 (4), 234 (9), 105 (14), 77 (100). Anal. calcd. for C H N : C,
21 17 3
8
1.01; H, 5.50; N, 13.49. Found: C, 79.96; H, 5.58; N, 13.42%.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China (20472027 and
0021001) for financial support.
2
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