Synthesis and structure of novel 6-(pyrazol-1-yl)-4-methyl-1H-pyrazolo[3,4- b]pyridin-3-ol derivatives

Article abstract of DOI:10.1007/s11164-012-0958-2

New 6-(pyrazol-1-yl)pyrazolo[3,4-b]pyridin-3-ol compounds were synthesized by cyclization reaction from 2,6-dichloro-4-methylnicotinonitrile. Their derivatives exist as the 3-hydroxy tautomer. The structure of the compound 1a of one of the resulting compounds was studied in detail by X-ray diffraction.

Full text of DOI:10.1007/s11164-012-0958-2

Res Chem Intermed
DOI 10.1007/s11164-012-0958-2
Synthesis and structure of novel 6-(pyrazol-1-yl)-4-
methyl-1H-pyrazolo[3,4-b]pyridin-3-ol derivatives
Ai Qun Wu
Li-Qun Shen
•
Yong Tang
•
Su-Yu Huang
•
Received: 25 August 2012 / Accepted: 27 November 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract New 6-(pyrazol-1-yl)pyrazolo[3,4-b]pyridin-3-ol compounds were syn-
thesized by cyclization reaction from 2,6-dichloro-4-methylnicotinonitrile. Their
derivatives exist as the 3-hydroxy tautomer. The structure of the compound 1a of
one of the resulting compounds was studied in detail by X-ray diffraction.
Keywords Pyrazol Á Pyrazolo[3,4-b]pyridine Á 3-Hydroxy tautomer Á X-ray
diffraction
Introduction
Pyrazolo[3,4-b]pyridines belong to an important class of heterocyclic compounds
which are attractive targets in organic synthesis due to their wide variety of
biological and pharmacological properties. A number of pyrazolo[3,4-b]pyridines
are well known for their properties as antitubercular [1], anxiolytic [2], antiviral [3],
hypnotic [4], inhibitors of xanthine oxidases [5], and analgesic [6], and for treatment
of Alzheimer’s disease, gastrointestinal diseases, and anorexia nervosa [7]. In
addition, they have been proved to be cholesterol formation-inhibiting compounds
[8]. More importantly, these compounds are also promising for inhibition of
glycogen synthase kinase-3 [9].
Since the mid-1990s, the chemistry and biology of pyrazolo[3,4-b]pyridines have
attracted the attention of many researchers. In order to synthesize a series of
pyrazolo[3,4-b]pyridine derivatives decorated with useful pharmacophores, some
research workers have established an array of new and innovative strategies to
prepare diverse pyrazolo[3,4-b]pyridine cores [10]. For example, Lipson and
A. Q. Wu Á Y. Tang Á S.-Y. Huang Á L.-Q. Shen (&)
Key Laboratory of Development and Application of Forest Chemicals of Guangxi, College of
Chemistry and Chemical Engineering, Guangxi University for Nationalities, Nanning 530006, China
e-mail: liqunshen@126.com
123

A. Q. Wu et al.
co-workers have reported the synthesis of pyrazolo[3,4-b]pyridin-6-ones by reaction
of 3-methyl-5-aminopyrazole with an equimolar amount of 2,2-dimethyl-1,3-
dioxane-4,6-dione (Meldrum’s acid) and ketones in methanol and in DMF [11].
Kolosov and co-workers [12] have reported the synthesis of pyrazolo[3,4-
b]pyridines by treatment of a-cyanochalcones with phenylhydrazine. Here, we
report the efficient methodology for the synthesis of several new 6-(pyrazol-1-yl)-4-
methyl-1H- pyrazolo[3,4-b] pyridine-3-ol derivatives.
Materials and methods
General
All chemicals were of reagent grade and used as commercially purchased without
further purification. All solvents were dried and distilled before use. For TLC
analysis, pre-coated plates of silica gel 60 F254 were used. Melting points were
determined using a WRS-1B apparatus and were uncorrected. The IR spectra were
-
1
recorded in the range of 400–4,000 cm with a Magna 550 FT-IR spectrometer
1
13
using KBr pellets. The H and C NMR spectra were recorded on a Bruker AV600
spectrometer. UV–Vis spectra were recorded a U-4100 (Hitachi). High resolution
mass (HRMS) spectra were obtained in ESI mode on a Finnigan MAT95XP HRMS
system (Thermo Electron).
Synthesis and characterization
2
,6-Dichloro-4-methylnicotinonitrile (2, 5.0 g, 26.7 mmol) in concentrated H SO4
2
(25 mL) was stirred at 80 °C for 4 h, then cooled to 25 °C, and poured into ice water
(100 mL). The suspension was filtered and the filter cake recrystallized from acetone
to obtain the product 3 (4.05 g, 80.9 %) as a white solid. m.p.: 173.5–175 °C (lit
1
1
69–173 °C) [13]. H NMR ((CD ) SO): d 8.10 (s,1H), 7.93 (s, 1H), 7.55 (s, 1H),
3
2
-1
2
.31 (s, 3H). IR (KBr): 3,444, 3,343, 3,169, 1,682, 1,577, 1,081, 917, 848 cm
.
6
-Hydrazino-4-methyl-1-H-pyrazolo[3,4-b]pyridine-3-ol (4)
2
,6-Dichloro-4-methylnicotinamid (5.000 g, 4.9 mmol) was added to 80 % hydra-
zine hydrate (5 mL) and stirred at 100 °C for 5 h, cooled to room temperature, and a
yellow solid formed. The yellow solid product was isolated filtration, washed with
1
9
5 % ethanol, and dried to give 4 (3.443 g, 71.8 %). m.p. [ 300 °C. H NMR
(
600 MHz, (CD ) SO): d 11.56 (s, 1H), 10.35 (s,1H), 7.97 (s,1H), 6.09 (s, 1H), 4.2
3
2
(
brs, 2H). 2.37 (s, 3H). IR (KBr): 3,395, 3,325, 3,207, 1,619, 1,505, 1,306, 1,015,
-
47 cm . The product was used without further purification.
1
7
6
-(3,5-Dimethyl-pyrazol-1-yl)-4-methyl-1H-pyrazolo[3,4-b]pyridin-3-ol (1a)
A mixture of the 6-hydrazino-4-methyl-1-H-pyrazolo[3,4-b]pyridine-3-ol (0.9 g,
.0 mmol) and 2,4-pentanedione (1 mL, 9.7 mmol) were stirred in ethanol (10 mL)
5
1
23

Novel 6-(pyrazol-1-yl)-4-methyl-1H-pyrazolo[3,4-b]pyridin-3-ol derivatives
at room temperature for 30 min. Then the reaction mixture was heated to 80 °C for
3
h. The yellow solution was evaporated in vacuum, the residue was isolated by
filtration and recrystallized from ethanol to give 1a as a white solid (1.0 g, 75.8 %).
m.p.: 270.1–271.9 °C. IR(KBr) 3,638, 3,141, 3,089, 2,929, 1,602, 1,567, 1,553,
-
1 1
1
,390, 1,289, 903, 827 cm . H NMR (600 MHz, (CD ) SO): d 11.89 (s, 1H),
3
2
13
1
0.94 (s,1H), 7.32 (s,1H), 6.11 (s,1H), 2.61 (s, 3H), 2.49 (s, 3H), 2.19 (s,3H).
C
NMR (150 MHz, (CD ) SO): d 157.1, 153.0, 151.8, 149.0, 146.2, 141.2, 109.3,
3
2
107.9, 102.3, 17.8, 14.5, 13.5. HRMS (m/z): Calcd. for C H N O: 243.1120
12 13 5
Found: 243.1117.
6
-(5-Hydroxy-3-methyl-pyrazol-1-yl)-4-methyl-1H-pyrazolo[3,4-b]pyridin-3-ol (1b)
A mixture of the 6-hydrazino-4-methyl-1-H-pyrazolo[3,4-b]pyridine-3-ol (0.9 g,
.0 mmol) and ethyl acetoacetate (1.2 mL, 9.5 mmol) in ethanol (15 mL) with the
addition of catalytic amounts conc. HCl was refluxed gently for 4 h. Cooling the
5
1
reaction mixture gave a yellow solid 1b (0.67 g, 54.5 %). m.p. [ 300 °C. H NMR
(
600 MHz, (CD ) SO): 12.76 (s, 1H), 11.59 (s,1H), 11.08 (s,1H), 7.56 (s,1H), 6.54
3 2
1
3
(
s,1H), 2.61 (s, 3H), 2.41 (s,3H). C NMR (150 MHz, (CD ) SO): d 157.0, 156.3,
3
2
1
53.0, 149.6, 144.2, 141.0, 110.0, 106.3, 103.1, 18.1, 14.7. IR(KBr) 3,746, 3,228,
-
1
2,929, 1,619, 1,522, 1,407, 1,049, 1,015 cm
C H N O : 245.0913 Found: 245.0917.
HRMS (m/z): Calcd. for
1
1 11 5 2
5
3
-Hydroxy-1-(3-hydroxy-4-methyl-1H-pyrazolo[3,4-b]pyridine-6-yl)-1H-pyrazole-
-carboxylic acid methyl ester (1c)
Dimethyl acetylenedicarboxylate (DMADC) (6.00 mL, 48.9 mmol) in methanol
60 mL) was added dropwise to a solution of 6-hydrazino-4-methyl-1-H-pyrazol-
(
o[3,4-b]pyridine-3-ol (4) (5.00 g, 34.8 mmol) in methanol (50 mL) and the mixture
was stirred vigorously for 5 h at 0 °C. The resulting suspension was filtered and the
filter cake was washed thoroughly with cold methanol to obtain 2-[(3-hydroxy-4-
methyl-1H-pyrazolo[3,4-b]pyridine-6-yl)-hydrazono]-succinic acid dimethyl ester
1
(
5) (8.12 g, 82.7 %) as a yellow solid. H NMR (CDCl ): d 11.23 (s, 1H), 10.92
3
(
s, 1H), 10.71 (s, 1H), 6.82 (s, 1H), 3.82 (s, 2H), 3.75 (s, 3H), 3.62 (s, 3H), 2.50
s, 3H). IR (KBr): 3,329, 3,169, 2,957, 1,723, 1,710, 1,605, 1,445, 1,400, 1,195,
(
-
1
8
34 cm
To a solution of 2-[(3-hydroxy-4-methyl-1H-pyrazolo[3,4-b]pyridine-6-yl)-hy-
drazono]-succinic acid dimethyl ester (5) (5.00 g, 17.5 mmol) in methanol
100 mL) was added sodium methoxide (1 mL). The reaction was refluxed for
h, then cooled to 0 °C, and quenched with water. The suspension was filtered and
.
(
4
the filter cake was washed thoroughly with cold methanol. The filter cake
recrystallized from acetone to obtain the product (1.95 g, 43.3 %) as a light red
1
solid m.p. [ 300 °C. H NMR (pyridine-d5): d 7.71 (s, 1H), 6.38 (s, 1H), 3.88
1
3
(
s, 3H), 2.81 (s, 3H). C NMR (150 MHz, pyridine-d5): d 162.9, 157.0, 156.9,
1
2
2
53.1, 150.0, 148.6, 144.2, 106.3, 104.5, 90.1, 51.7, 18.3. IR (KBr): 3,603, 3,223,
-
,967, 1,720, 1,612, 1,261, 1,048 cm . HRMS (m/z): Calcd. for C H N O :
1
1
2 11 5 4
89.0811 Found: 289.0823.
123

A. Q. Wu et al.
Crystallography
The crystal was obtained from an ethanol solution by slow evaporation. Diffraction
˚
experiments for 1a were carried out on with Mo Ka radiation (k = 0.71073 A)
using a Bruker SMART APEX CCD diffractometer at 296 K. Raw frame data were
integrated with the SAINT program. The structures were solved by direct methods
which gave the positions of all non-hydrogen atoms and refined with full-matrix
2
least-squares on F using SHELXS-97 and SHELXL-97 [14]. The hydrogen atoms
were set in the calculated positions and refined by riding model. The crystallo-
graphic and refinement data of 1a are listed in Table 1.
Results and discussion
Synthesis
The synthesis of compounds 1a, 1b, and 1c reported in this article was started with
2
,6-dichloro-4-methylnicotinonitrile (2). The compound 2 on reaction with
Table 1 Crystal data and refinement details for 1a
Compound reference
Empirical formula
Formula weight
Crystal system
1a
12 13 5
C H N O
243.27
Monoclinic
Space group
P2(1)/n
˚
Unit cell dimensions
a = 7.648(7) A, a = 90.00
˚
b = 7.992(7))A, b = 92.482(11)
˚
c = 19.393(18) A, c = 90.00
3
˚
1,184.3(19) A
Volume
Z
4
3
Density (calculated)
Absorption coefficient
F(000)
1.364 g/cm
-
1
0.0935 mm
512
3
Crystal size
0.20 9 0.18 9 0.17 mm
2.10–25.00°
5,924
Theta range for data collection (°)
Reflections collected
Completeness to h max
Data/restraints/parameters
0.997 %
2,078/0/167
1.052
2
Goodness-of-fit on F
a,b
Final R indices [I [ 2r(I)]
R
R
1
= 0.0447, wR
= 0.0597, wR
2
2
= 0.1151
= 0.1301
R indices (all data)
1
-3
˚
0.268 and -0.200 e A
Largest diff. peak and hole
P
P
||Fo|-|Fc||/ |Fo|
P
a
1
R =
P
b
wR
2
= [ w(Fo2-Fc2)2/ w(Fo2)2]1/2, w = [2(Fo)2 ? (0.1(max(0, Fo2) ? 2Fc2)/3)2]-1
1
23

Novel 6-(pyrazol-1-yl)-4-methyl-1H-pyrazolo[3,4-b]pyridin-3-ol derivatives
concentrated H SO furnished 2,6-dichloro-4-methylnicotinamide (3) in 80.9 %
2
4
yield. The cyclization of compound 3 and hydrazine hydrate produced compound 4.
The formation of pyrazolo ring on pyridine moiety in compound 4 was achieved
by the condensation reaction. 6-(3,5-dimethyl-pyrazol-1-yl)-4-methyl-1H-pyrazol-
o[3,4-b]pyridin-3-ol (1a) could be obtained in 75.8 % yield, by reaction of substrate
4
with pentane-2,4-dione and ethanol as the solvent. 6-(5-hydroxy-3-methyl-
pyrazol-1-yl)-4-methyl-1H-pyrazolo[3,4-b]pyridin-3-ol (1b) was prepared from the
heterocyclization of compound 4 with acetoacetic ester by intermediate hydrazones.
When the hydrazone (5), prepared from the compound 4 and dimethyl acetylene-
dicarboxylate (DMADC), was heated in methanol containing sodium methoxide the
5
-hydroxy-1-(3-hydroxy-4-methyl-1H-pyrazolo [3,4-b]pyridine-6-yl)-1H-pyrazole-
-carboxylic acid methyl ester (1c) was obtained. The structures 1a–c were
3
1
13
confirmed by IR, H NMR, C NMR, and HRMS (Scheme 1). These data revealed
the appearance of 3-hydroxy tautomer which have only rarely been reported,
especially cases involving C=O ? C=N tautomerism [15, 16].
Until now, 6-substituted pyrazole-1H-pyrazolo[3,4-b]pyridine derivatives have
not been described, nor has a simple procedure been found to obtain substituted
1H-pyrazolo[3,4-b]pyridine at positions 6 of the pyridine ring. This makes the
investigation of 6-hydrazino-4-methyl-1H-pyrazolo[3,4-b]pyridine-3-ol in reactions
OH
CN
Cl
CONH2
Cl
con H SO
NH NH2
.
H O
2
4
2
2
N
Cl
N
Cl
N
2
H NHN
N
N
H
2
3
4
OH
N
CH COCH COCH
3
3
2
+
C H OH,H , reflux
2
5
N
N
H
N
N
1
a
OH
N
CH COCH COOC H
2 5
3
2
+
C H OH,H , reflux
2
5
N
N
H
N
N
OH
1
b
OH
N
OH
CH OH,CH ONa
DMADC
3
3
N
N
N
H COOC
N
N
N
H
N
N
N
3
H
H
H COOC
3
COOCH3
OH
5
1c
Scheme 1 Synthesis of compounds 1a, 1b and 1c
123

A. Q. Wu et al.
with pentane-2,4-dione and their analogues to obtain the new 6-(pyrazol-1-
yl)pyrazolo [3,4-b]pyridin-3-ol 1a–1c compounds in (22–44 %) overall yield.
X-ray crystal structure
The molecular structure of compound 1a determined in the crystalline phase with
atomic numbering scheme is illustrated in Fig. 1. The molecular structure 1a can be
described as being built from planar pyrazolo[3,4-b]pyridin fragments linking one
pyrazole ring. Compound 1a exhibits three possible tautomeric form as shown in
Scheme 2. Crystals of 1a shows a possible tautomerism of this compound. The C1–
˚
O1 bond length of 1.332(3) A is close to that of a nearly pure C–O single bond
(
3
average length of C–O sp –O single bond is 1.34 A), in accordance with recently
˚
˚
reported pyrazole-OH bond length (1.331(2) A) [17]. The X-ray crystal structural
research proves conclusively that the tautomer present in the crystal is the
tautomeric form A.
The fused core pyrazolo[3,4-b]pyridin fragment lies approximately in one planar
˚
with a maximum mean plane deviation of -0.031 A for atom N1. The dihedral
angles between the substituted pyrazolyl ring plane and pyridine plane is 38.3°, as
seen for similar cases [18]. Substituted, the nitrogen of the pyrazolyl rings is
oriented away from the pyridine nitrogen, in order to reduce the nitrogen lone pairs
of the electronic repulsion.
All bond distances in the pyrazole and pyrazolopyridine rings exhibit partial
double bond character, which means that a delocalized p-electronic system is in the
entire ring. The significant bond distances and angles of the compounds 1a are: C1–
˚
˚
˚
˚
N1 1.317(3)A, N1–N2 1.387(2)A, N2–C2 1.352(3)A, C2–N3 1.347(3)A, N4–N5
˚
˚
1
.375(2)A, N5–C12 1.329(3)A, C1–N1–N2 105.87(17)°, C2–N2–N1 110.53(17)°,
C7–N3–C2 113.16(18)°, C9–N4–N5 111.60(17)°, and C12–N5–N4 105.15(17)°.
These bond length and angles are all comparable with the similar literature data
Fig. 1 The molecular structure of the compound 1a with atom-numbering scheme; displacement
ellipsoids are drawn at the 30 % probability level
1
23

Novel 6-(pyrazol-1-yl)-4-methyl-1H-pyrazolo[3,4-b]pyridin-3-ol derivatives
OH
O
OH
N
NH
NH
N
N
N
N
H
N
N
N
N
N
N
N
N
H
Tautomer A
Scheme 2 Tautomeric forms of compounds 1a
Tautomer B
Tautomer C
reported for 2,6-bis(3,5-dimethyl-N-pyrazolyl) pyridine [18] and parent pyrazol-
o[3,4-b]pyridine reported in the CSD [16].
UV–Vis absorption spectral
The UV–Vis optical absorption of compounds 1a–c were measured in tetrahydro-
-
5
furan solution with the concentration of 2.5 9 10 M, respectively, as shown in
Fig. 2.
As can be seen, the three compounds displayed two major absorption spectra
band: one at kmax 247–251 nm and second absorption band with kmax 296–313 nm,
corresponding to p ? p* transition depending on the consequence of the extended
aromaticity of the pyrazolopyridine rings. The three compounds showed values of
the molar absorptivity coefficient (e) that exceeded 10,000; the long wavelength
bands at 300 nm also displayed values of e [ 10,000.
UV–Vis absorption spectra of the compound 1a in tetrahydrofuran, methanol,
-
5
ethanol, acetonitrile, and dimethylsulfoxide with the concentration of 2.5 9 10 M
are given in Fig. 3. The UV–Vis absorption spectra of compound 1a in protic
solvents are not very different. In protic solvents like methanol and ethanol, the
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1
1
1
a
c
b
225 240 255 270 285 300 315 330 345 360 375 390 405 420
Wavelength(nm)
Fig. 2 UV–Vis spectra of 1a–c in THF solution
123

A. Q. Wu et al.
0
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
.0
1
a-CH3OH
1a-THF
1
1
a-CH3CH2OH
a-CH3CN
2
25
250
275
300
325
350
375
400
425
Wavelength(nm)
Fig. 3 UV–Vis spectra of 1a in different solvents
absorption spectra of 1a have a slight decrease in absorbance in second absorption
band. This is caused by the interactions between the solutions and solute molecules.
Conclusions
In this study, we have described an efficient method for the synthesis of three
6
-(pyrazol-1-yl)-pyrazolo[3,4-b]pyridin-3-ol novel derivatives that are important
building blocks in the fields of pharmacy. Their structures were experimentally
characterized by means of NMR, IR, and HRMS, and typical compound 1a was also
determined by X-ray diffraction. The UV–Vis absorption spectra of compounds of
1
a–c in THF were observed to be similar.
Crystallographic data for compound 1a in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supplementary publication number
CCDC 888113. Copies of the data can be obtained, free of charge, on application to
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK [fax: ? 44(1223)336033 or
e-mail: deposit@ccdc.cam.ac.uk].
Acknowledgments This work was supported by National Natural Science Foundation of China (No.
2
2
1062002), and the Scientific Research Program of Guangxi University for Nationalities (Project No.
010QD018 and 2010ZD010).
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