Ionic liquid as an efficient promoting medium for synthesis of bis-pyrazolo[3,4-b:4′,3′-e]pyridines

Article abstract of DOI:10.1002/jccs.200800113

The preparation of a series of bis-pyrazolo[3,4-b:4′,3′-e] pyridines by the reaction of 5-aminopyrazole with aldehydes in ionic liquid [bminiJBr is described. This new method has the advantages of easier work-up, milder reaction conditions, high yields and environmental friendliness compared with other methods.

Full text of DOI:10.1002/jccs.200800113

Journal of the Chinese Chemical Society, 2008, 55, 755-760
755
Ionic Liquid as an Efficient Promoting Medium for Synthesis of
Bis-pyrazolo[3,4-b:4¢,3¢-e]pyridines
Da-Qing Shia,b,c,* (
) and Fang Yangb (
)
aCollege of Chemistry and Chemical Engineering, Key Laboratory of Organic Synthesis of Jiangsu Province,
Suzhou University, Suzhou 215123, P. R. China
bCollege of Chemistry and Chemical Engineering, Xuzhou Normal University,
Xuzhou 221116, P. R. China
cKey Laboratory for Biotechnology on Medicinal Plants of Jiangsu Province,
Xuzhou 221116, P. R. China
The preparation of a series of bis-pyrazolo[3,4-b:4¢,3¢-e]pyridines by the reaction of 5-aminopyrazole
with aldehydes in ionic liquid [bmim]Br is described. This new method has the advantages of easier
work-up, milder reaction conditions, high yields and environmental friendliness compared with other
methods.
Keywords: Bis-pyrazolo[3,4-b:4¢,3¢-e]pyridine; 5-Aminopyrazole; Aldehyde; Ionic liquid.
INTRODUCTION
Room temperature ionic liquids, especially those
5-amino-3-methyl-1-phenylpyrazole and (5-chloro-3-meth-
yl-1-phenylpyrazol-4-yl)(4-aryl)methanone were treated
in DMF at 240~250 °C for 2~3 hours, products were formed
in 14~60% yield.21 Gonzales et al. reported the reaction be-
tween 5-amino-1,3-dimethylpyrazole and aromatic alde-
hydes catalyzed by benzamide; the products were bis-pyra-
zolo[3,4-b:4¢,3¢-e]pyridines (yield of 40%) and pyrazolo-
[3,4-d] pyrimidines (yield of 15%);22 Puchala et al. re-
ported that high yields of 42-76% were obtained in hot eth-
anol, but some reactions needed two steps;23 Joshi et al.
found bis-pyrazolo[3,4-b:4¢,3¢-e] pyridines can also be ob-
tained when 5-amino-3-methyl-1-phenylpyrazole and 2-
chlorobenzaldehyde or 2-fluorobenzaldehyde were treated
in xylene and acetum;24 Recently, Quiroga et al. described
the synthesis of a series of bis-pyrazolo[3,4-b:4¢,3¢-e]pyri-
dines in the reaction of amino-pyrazole and aldehydes un-
der microwave irradiation and solvent-free conditions,
with an over average yield.25
based on 1-alkyl-3-methylimidazolium cations, have shown
great promise as an attractive alternative to conventional
organic solvents, and more attention has been currently fo-
cused on organic reactions promoted by ionic liquids.1
They are nonvolatile, recyclable, nonexplosive, easily op-
erable, and thermally robust.2 There are many reports con-
cerning the applications of ionic liquids in organic reac-
tions, such as Friedel-Crafts reactions,3,4 Diels-Alder reac-
tions,5-7 Heck reactions,8,9 Pechmann condensations,10
Biginelli reactions,11 Beckmann rearrangements,12,13 and
other reactions.14,15
Bis-pyrazolo[3,4-b:4¢,3¢-e]pyridines have been at-
tractive for physicochemical applications since they ex-
hibit a high fluorescence in both solution and solid state un-
der exposure to white light,16 which makes them appropri-
ate in the design of electroluminescent materials, like or-
ganic light emitting diodes (OLEDs).16e-g In addition, the
pyrazole nucleus has long shown pharmacological proper-
ties, as in anti-anxiety,17 anti-pyretic, analgesic and anti-in-
flammatory agents,18 as well as for their anti-microbial
properties,19 especially anti-bacterial and anti-fungal activ-
ities.19a,c,20
However, the principal drawbacks are low yields,
toxic waste and the products can be difficult to separate
from the reaction mixture.
In order to avoid volatility and toxicity that many or-
ganic solvents inherently have, we describe a very simple,
green and efficient route to synthesize bis-pyrazolo[3,4-
b:4¢,3¢-e]pyridines in an ionic liquid [bmim]Br.
Brack has previously reported the synthesis of bis-
pyrazolo[3,4-b:4¢,3¢-e]pyridines, and described that when
To get bis-pyrazolo[3,4-b:4¢,3¢-e]pyridines 3, we re-
* Corresponding author. E-mail: dqshi@263.net

756 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008
Shi and Yang
ported a facile method consisting of 5-aminopyrazole 1 and
aldehydes 2 (in ratio 2:1) in an ionic liquid [bmim]Br at 90
°C for a few hours (Scheme I).
Table 1. Solvent optimization for the synthesis of 3b
Entry
Solvent
Temperature (°C) Time (h) Yield (%)
1
[bmim]Br
[bmim]BF4
[bmim]PF6
EtOH
90
90
3
4
5
5
4
5
5
5
5
5
93
76
2
3
90
69
Scheme I
4
80
40
R
5
DMF
1100
50
50
H3C
H3C
CH3
6
CH3COCH3
CH3COOEt
CHCl3
N. R.
N. R.
N. R.
Trace
N. R.
7
80
[bmim]Br
90oC
N
RCHO
+
N
N
8
60
N
9
CH3CN
80
N
N
NH2
N
Ph
10
H2O
1000
Ph
Ph
1
2a-n
3a-n
withdrawing groups (such as halide, nitro groups) or elec-
tron-donating groups (such as alkyl, hydroxyl groups), but
also to heterocyclic aldehydes, which highlighted the wide
scope of this reaction. Therefore, we concluded that the
electronic nature of the substituents of aldehydes has no
significant effect on this reaction.
RESULTS AND DISCUSSION
It is well-known that choosing an appropriate solvent
is crucially important for an efficient organic synthesis.
With the intention to search for the optimum solvent, the re-
action of 5-amino-3-methyl-1-phenylpyrazole 1 and 4-chlo-
robenzaldehyde was examined using some ionic liquids,
water and organic solvents. The corresponding results are
summarized in Table 1.
As new clean media, room temperature ionic liquids
are recyclable and can be reused. The reaction between 5-
amino-3-methyl-1-phenylpyrazole and 4-chlorobenzalde-
hyde was also investigated in the recycling reactions of
ionic liquids, and the corresponding results are listed in
Table 3.
It is shown in Table 1 that the ionic liquid [bmim]Br
as solvent resulted in the most excellent yield and shortest
reaction time. Therefore, [bmim]Br was chosen as the sol-
vent of this reaction.
From the reaction results, we found that after every
recycle, reaction yields are almost the same as before.
Although we have not yet established the mechanism,
a possible explanation is given in Scheme II.
Under these optimized reaction conditions, a series of
bis-pyrazolo[3,4-b:4¢,3¢-e]pyridines 3 were synthesized.
The results are summarized in Table 2.
In conclusion, a series of bis-pyrazolo[3,4-b:4¢,3¢-
e]pyridines were synthesized via reactions of 5-amino-3-
As shown in Table 2, this protocol could be applied
not only to the aromatic aldehydes with either electron-
Table 2. Synthesis of 3 in ionic liquid [bmim]Br
Entry
R
Time (h)
Yield (%)
m.p. (°C)
Lit. m.p. (°C)
a
b
c
d
e
f
4-BrC6H4
2
3
93
93
94
95
97
88
91
79
96
85
92
71
86
70
231-233
243-245
214-216
246-249
293-295
268-270
219-221
208-210
255-257
261-264
229-231
277-278
255-257
242-244
227-22825
240-24126
216-21825
4-ClC6H4
4-FC6H4
3
3,4-(CH3)2C6H3
3-ClC6H4
2
2
4-NO2C6H4
3.5
2
269-27126
g
h
i
3,4-(CH3O)2C6H3
3-NO2C6H4
4
213-21525
259-26025
4-CH3C6H4
2
j
3,4-Cl2C6H4
4-CH3OC6H4
4-HOC6H4
2
k
l
2
232-23327
3
m
n
3,4-(OCH2O)2C6H3
4-(CH3)2NC6H4
2.5
2.5
260-26125
244-24627

Synthesis of Bispyrazolopyridines in Ionic Liquid
Table 3. Recyclation of ionic liquid
J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 757
liquid [bmim]Br was then recovered for reuse by drying at
80 °C for several hours in a vacuum. The crude product was
purified by recrystallization from 95% EtOH to give 3.
4-(4-Bromophenyl)-3,5-dimethyl-1,7-diphenyl-1H,7H-
bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3a)
IR (KBr) n (cm-1): 3070, 3050, 2985, 2955, 2911,
1597, 1578, 1510, 1498, 1487, 1458, 1437, 1412, 1379,
1328, 1239, 1113, 1091, 1068, 1029, 1012, 904, 883, 849,
821, 754, 692; 1H NMR (DMSO-d6) d: 8.34 (d, J = 8.0 Hz,
4H, ArH), 7.85 (d, J = 8.4 Hz, 2H, ArH), 7.65-7.59 (m, 6H,
ArH), 7.38-7.33 (m, 2H, ArH), 2.06 (s, 6H, 2 ´ CH3);
HRMS: Calcd for C27H2079BrN5 493.0902, found 493.0888.
4-(4-Chlorophenyl)-3,5-dimethyl-1,7-diphenyl-1H,7H-
bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3b)
Entry
Temperature (°C)
Time (h)
Yield (%)
1
2
3
4
5
90
90
90
90
90
3
3
3
3
3
93
92
93
92
91
methyl-1-phenylpyrazole and aldehydes in ionic liquid
[bmim]Br. The advantages of this method are easier work-
up, milder reaction conditions, high yields and environ-
mentally benign procedure.
EXPERIMENTAL SECTION
IR (KBr) n (cm-1): 3043, 2921, 1598, 1577, 1509,
Melting points were determined in open capillaries
and are uncorrected. IR spectra were recorded on a Tensor
1
27 spectrometer. H NMR spectra were measured on a
1458, 1437, 1411,1378, 1327, 1276, 1239, 1175, 1091,
1
1015, 1006, 904, 883, 850, 823, 794, 755, 740, 694; H
NMR (DMSO-d6) d: 8.34 (d, J = 8.0 Hz, 4H, ArH), 7.72-
7.70 (m, 4H, ArH), 7.64-7.60 (m, 4H, ArH), 7.35 (t, J = 7.2
Hz, 2H, ArH), 2.07 (s, 6H, 2 ´ CH3); HRMS: Calcd for
C27H2035ClN5 449.1407, found 449.1425.
Bruker DPX-400 M Hz spectrometer using TMS as inter-
nal standard, DMSO-d6 as solvent. High resolution mass
spectra were obtained using a TOF-MS instrument.
General Procedure for the Synthesis of 4-Aryl-3,5-
dimethyl-1,7-diphenyl-1H,7H-bispyrazolo[3,4-b:4¢,3¢-
e]pyridines (3a-n)
4-(4-Fluorophenyl)-3,5-dimethyl-1,7-diphenyl-1H,7H-
bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3c)
IR (KBr) n (cm-1): 3067, 2924, 1595, 1577, 1497,
1459, 1438, 1413, 1379, 1327, 1275, 1237, 1226, 1159,
1114, 1091, 1016, 905, 883, 857, 832, 796, 755, 692, 648;
1H NMR (DMSO-d6) d: 8.35 (d, J = 8.4 Hz, 4H, ArH),
7.75-7.72 (m, 2H, ArH), 7.63 (t, J = 8.0 Hz, 4H, ArH), 7.49
(t, J = 8.8 Hz, 2H, ArH), 7.36 (t, J = 7.2 Hz, 2H, ArH), 2.07
(s, 6H, 2 ´ CH3); HRMS: Calcd for C27H2019FN5 433.1703,
A dry 50 mL flask was charged with amino-pyrazole
1 (2 mmol) and the corresponding aldehyde 2 (1 mmol) and
ionic liquid [bmim]Br (2 mL). The mixture was stirred at
90 °C for 2-4 h to complete the reaction (monitored by
TLC), then cooled to room temperature. The yellow solid
was filtered off and washed with water. The filtrate of ionic
Scheme II

758 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008
Shi and Yang
found 433.1701.
113.35, 113.82, 120.25, 120.37, 121.97, 125.74, 125.83,
129.63, 139.54, 144.97, 148.68, 149.84, 150.41; HRMS:
Calcd for C29H25N5O2 475.2008, found 475.2007.
3,5-Dimethyl-4-(3,4-dimethylphenyl)-1,7-diphenyl-
1H,7H-bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3d)
IR (KBr) n (cm-1): 3058, 3036, 2982, 2920, 1591,
1577, 1514, 1499, 1458, 1435, 1377, 1328, 1272, 1240,
1202, 1112, 1093, 1067, 1028, 1006, 900, 871, 820, 805,
793, 757, 738, 693, 670, 650; 1H NMR (DMSO-d6) d: 8.35
(d, J = 8.0 Hz, 4H, ArH), 7.62 (t, J = 8.0 Hz, 4H, ArH),
7.43-7.33 (m, 5H, ArH), 2.40 (s, 3H, CH3), 2.37 (s, 3H,
CH3), 2.07 (s, 6H, 2 ´ CH3); 13C NMR (CF3COOD) d:
13.93, 19.78, 19.88, 115.93, 126.75, 127.77, 129.09,
130.24, 131.92, 132.14, 133.36, 135.03, 140.59, 143.41,
150.62, 150.67, 157.77; HRMS: Calcd for C29H25N5
443.2110, found 443.2109.
3,5-Dimethyl-4-(3-nitrophenyl)-1,7-diphenyl-1H,7H-
bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3h)
IR (KBr) n (cm-1): 3050, 2963, 2911, 1592, 1580,
1535, 1498, 1460, 1437, 1378, 1348, 1326, 1270, 1238,
1110, 871, 758, 723, 712, 690, 649; 1H NMR (DMSO-d6) d:
8.57 (s, 1H, ArH), 8.53 (d, J = 8.0 Hz, 1H, ArH), 8.35 (d, J
= 8.0 Hz, 4H, ArH), 8.18 (d, J = 8.0 Hz, 1H, ArH), 7.96 (t, J
= 8.0 Hz, 1H, ArH), 7.64 (t, J = 8.0 Hz, 4H, ArH), 7.37 (t, J
= 7.6 Hz, 2H, ArH), 2.06 (s, 6H, 2 ´ CH3); HRMS: Calcd
for C27H20N6O2 460.1648, found 460.1644.
3,5-Dimethyl-4-(4-methylphenyl)-1,7-diphenyl-1H,7H-
4-(3-Chlorophenyl)-3,5-dimethyl-1,7-diphenyl-1H,7H-
bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3i)
bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3e)
IR (KBr) n (cm-1): 3029, 2921, 1591, 1578, 1517,
1498, 1458, 1436, 1412, 1378, 1326, 1277, 1241, 1182,
1114, 1091, 1022, 1004, 898, 881, 812, 792, 755, 691, 670,
648; 1H NMR (DMSO-d6) d: 7.35 (d, J = 8.0 Hz, 4H, ArH),
7.62 (t, J = 8.0 Hz, 4H, ArH), 7.53 (d, J = 8.0 Hz, 2H, ArH),
7.45 (d, J = 8.0 Hz, 2H, ArH), 7.35 (t, J = 7.2 Hz, 2H, ArH),
2.49 (s, 3H, CH3), 2.06 (s, 6H, 2 ´ CH3); HRMS: Calcd for
C28H23N5 429.1953, found 429.1958.
IR (KBr) n (cm-1): 3047, 2963, 2911, 1600, 1592,
1579, 1563, 1498, 1459, 1435, 1414, 1378, 1327, 1274,
1238, 1144, 1089, 1028, 1007, 963, 868, 799, 788, 749,
731, 689; 1H NMR (DMSO-d6) d: 8.34 (d, J = 8.0 Hz, 4H,
ArH), 7.83 (s, 1H, ArH), 7.74 (d, J = 7.6 Hz, 1H, ArH),
7.70-7.60 (m, 6H, ArH), 7.36 (t, J = 7.2 Hz, 2H, ArH), 2.07
(s, 6H, 2 ´ CH3); 13C NMR (CF3COOD) d: 14.05, 115.79,
127.58, 127.72, 129.70, 132.02, 132.54, 133.36, 133.49,
133.81, 135.28, 138.24, 149.96, 151.22, 153.60; HRMS:
Calcd for C27H2035ClN5 449.1407, found 449.1400.
3,5-Dimethyl-4-(4-nitrophenyl)-1,7-diphenyl-1H,7H-
bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3f)
IR (KBr) n (cm-1): 3049, 2980, 2923, 2853, 1592,
1576, 1519, 1499, 1459, 1438, 1413, 1380, 1348, 1326,
1283, 1241, 1113, 1103, 1089, 1015, 1007, 850, 837, 756,
720, 694; 1H NMR (DMSO- d6) d: 8.48 (d, J = 8.4 Hz, 2H,
ArH), 8.34 (d, J = 8.0 Hz, 4H, ArH), 7.99 (d, J = 8.4 Hz,
2H, ArH), 7.63 (t, J = 8.0 Hz, 4H, ArH), 7.36 (t, J = 7.2 Hz,
2H, ArH), 2.04 (s, 6H, 2 ´ CH3); HRMS: Calcd for
C27H20N6O2 460.1648, found 460.1656.
4-(3,4-Dichlorophenyl)-3,5-dimethyl-1,7-diphenyl-
1H,7H-bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3j)
IR (KBr) n (cm-1): 3052, 2897, 1591, 1578, 1549,
1511, 1498, 1459, 1437, 1412, 1379, 1365, 1327, 1278,
1239, 1115, 1090, 1033, 1006, 869, 823, 755, 691, 670,
650; 1H NMR (DMSO-d6) d: 8.34 (d, J = 8.0 Hz, 4H, ArH),
8.09 (d, J = 2.0 Hz, 1H, ArH), 7.93 (d, J = 8.0 Hz, 1H,
ArH), 7.71 (dd, J1 = 2.0 Hz, J2 = 8.4 Hz, 1H, ArH), 7.63 (t, J
= 8.0 Hz, 4H, ArH), 7.36 (t, J = 7.2 Hz, 2H, ArH), 2.11 (s,
6H, 2 ´ CH3); 13C NMR (CF3COOD) d: 14.13, 115.63,
127.55, 128.68, 131.59, 131.80, 131.90, 133.18, 133.29,
135.24, 136.58, 138.73, 149.54, 151.28, 151.91; HRMS:
Calcd for C27H1935Cl2N5 483.1018, found 483.1029.
4-(4-Methoxyphenyl)-3,5-dimethyl-1,7-diphenyl-
1H,7H-bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3k)
4-(3,4-Dimethoxyphenyl)-3,5-dimethyl-1,7-diphenyl-
1H,7H-bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3g)
IR (KBr) n (cm-1): 3070, 2999, 2954, 2931, 2834,
1600, 1590, 1577, 1516, 1498, 1460, 1438, 1412, 1378,
1330, 1251, 1232, 1159, 1140, 1099, 1028, 903, 851, 814,
755, 695, 670; 1H NMR (DMSO-d6) d: 8.35 (d, J = 8.0 Hz,
4H, ArH), 7.62 (t, J = 8.0 Hz, 4H, ArH), 7.35 (t, J = 8.0 Hz,
2H, ArH), 7.25 (s, 1H, ArH), 7.22-7.14 (m, 2H, ArH), 3.90
(s, 3H, CH3O), 3.81 (s, 3H, CH3O), 2.12 (s, 6H, 2 ´ CH3);
13C NMR (DMSO-d6) d: 16.15, 56.02, 56.22, 111.54,
IR (KBr) n (cm-1): 3042, 2963, 2923, 2838, 1598,
1578, 1517, 1499, 1458, 1436, 1411, 1378, 1327, 1290,
1249, 1171, 1115, 1104, 1091, 1030, 1009, 901, 879, 854,
827, 794, 758, 693, 671, 649; 1H NMR (DMSO-d6) d: 8.34
(d, J = 8.0 Hz, 4H, ArH), 7.61 (t, J = 7.6 Hz, 4H, ArH), 7.55
(d, J = 8.4 Hz, 2H, ArH), 7.34 (t, J = 7.6 Hz, 2H, ArH), 7.18
(d, J = 8.4 Hz, 2H, ArH), 3.90 (s, 3H, OCH3), 2.06 (s, 6H, 2
´ CH3); HRMS: Calcd for C28H23N5O 445.1903, found

Synthesis of Bispyrazolopyridines in Ionic Liquid
J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 759
Chem. Int. Ed. 2003, 42, 148. (d) Seddon, K. R. J. Chem.
Technol. Biotechnol. 1997, 68, 351. (e) Wilkes, J. S. Green
Chem. 2002, 4, 73. (f) Zhao, D.; Wu, M.; Kou, Y.; Min, E.
Catal. Today. 2002, 74, 157.
445.1924.
4-(4-Hydroxyphenyl)-3,5-dimethyl-1,7-diphenyl-
1H,7H-bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3l)
IR (KBr) n (cm-1): 3177, 2925, 1592, 1573, 1517,
1500, 1458, 1435, 1381, 1327, 1276, 1238, 1208, 1171,
1117, 1103, 1028, 1011, 882, 856, 830, 755, 690, 673, 648;
1H NMR (DMSO-d6) d: 9.93 (s, 1H, OH), 8.34 (d, J = 8.0
Hz, 4H, ArH), 7.61 (t, J = 7.6 Hz, 4H, ArH), 7.42 (d, J = 8.4
Hz, 2H, ArH), 7.34 (t, J = 7.2 Hz, 2H, ArH), 7.00 (d, J = 8.4
Hz, 2H, ArH), 2.09 (s, 6H, 2 ´ CH3); 13C NMR (CF3COOD)
d: 14.19, 114.71, 116.18, 117.16, 118.24, 125.27, 127.84,
131.65, 132.04, 133.47, 135.13, 150.39, 150.82; HRMS:
Calcd for C27H21N5O 431.1746, found 431.1739.
2. (a) Welton, T. Chem. Rev. 1999, 99, 2071. (b) Gordon, K. R.
Pure Appl. Chem. 2000, 72, 1391. (c) Dupont, J.; de Souza,
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3,5-Dimethyl-4-(3,4-methylenedioxyphenyl)-1,7-diphen-
yl-1H,7H-bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3m)
IR (KBr) n (cm-1): 3065, 2888, 2779, 1591, 1573,
1508, 1499, 1485, 1458, 1438, 1378, 1365, 1327, 1278,
1238, 1185, 1103, 1082, 1033, 1006, 966, 934, 901, 854,
806, 791, 752, 718, 688, 651; 1H NMR (DMSO-d6) d: 8.34
(d, J = 7.6 Hz, 4H, ArH), 7.62 (t, J = 7.6 Hz, 4H, ArH), 7.35
(t, J = 7.6 Hz, 2H, ArH), 7.27 (d, J = 1.2 Hz, 1H, ArH), 7.17
(d, J = 7.6 Hz, 1H, ArH), 7.08 (dd, J1 = 1.2 Hz, J2 = 7.6 Hz,
1H, ArH), 6.21 (s, 2H, OCH2O), 2.13 (s, 6H, 2 ´ CH3);
HRMS: Calcd for C28H21N5O2 459.1695, found 459.1700.
3,5-Dimethyl-4-(4-(dimethylamino)phenyl)-1,7-diphen-
yl-1H,7H-bispyrazolo[3,4-b:4¢,3¢-e]pyridine (3n)
IR (KBr) n (cm-1): 3043, 2910, 2800, 1591, 1575,
1522, 1497, 1458, 1435, 1410, 1395, 1377, 1360, 1326,
1276, 1239, 1204, 1164, 1112, 1092, 1066, 1029, 1005,
953, 899, 876, 815, 756, 691, 649; 1H NMR (DMSO-d6) d:
8.35 (d, J = 8.0 Hz, 4H, ArH), 7.61 (t, J = 7.6 Hz, 4H, ArH),
7.42 (d, J = 8.0 Hz, 2H, ArH), 7.34 (t, J = 7.6 Hz, 2H, ArH),
6.92 (d, J = 8.0 Hz, 2H, ArH), 3.05 (s, 6H, N(CH3)2), 2.14
(s, 6H, 2 ´ CH3); HRMS: Calcd for C29H26N6 458.2219,
found 458.2213.
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ACKNOWLEDGEMENT
We are grateful to the Foundation of the Key Labora-
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