Tavakoli-Hoseini et al.
FULL PAPER
Scheme 1 Synthesis of pyrazolo[3,4-d]pyrimidin-4-ones cata-
7.9 Hz, 2H, arom-H), 8.25 (s, 1H, CH of pyrazole ring),
lyzed by Brønsted-acidic ILs
12.-31 (s, 1H, NH); IR (KBr) ν: 3444 (NH), 1682 (CO)
1
cm .
6-Ethyl-1-phenyl-1,5-dihydro-4H-pyrazolo[3,4-
d]pyrimidin-4-one (3c) 1H NMR (DMSO-d6, 500
MHz) δ: 1.26 (t, J=7.5 Hz, 3H, CH3), 2.70 (q, J=7.5
Hz, 2H, CH2), 7.38 (t, J=7.4 Hz, 1H, arom-H), 7.56 (t,
J=7.7 Hz, 2H, arom-H), 8.09 (d, J=7.9 Hz, 2H,
arom-H), 8.26 (s, 1H, CH of pyrazole ring), 12-.28 (s,
1
1H, NH); IR (KBr) ν: 3449 (NH), 1678 (CO) cm .
3-Methyl-1-phenyl-1,5-dihydro-4H-pyrazolo[3,4-
d]pyrimidin-4-one (3d) 1H NMR (DMSO-d6, 500
MHz) δ: 2.53 (s, 3H, CH3), 7.36 (t, J=7.3 Hz, 1H,
arom-H), 7.53 (t, J=7.8 Hz, 2H, arom-H), 8.02 (d, J=
7.9 Hz, 2H, arom-H), 8.13 (s, 1H, CH of pyrimidine
ring), 12.33 (s. br., 1H, NH); IR (KBr) ν: 3454 (NH),
-1
1675 (CO) cm .
3,6-Dimethyl-1-phenyl-1,5-dihydro-4H-pyrazolo-
[3,4-d]pyrimidin-4-one (3e) 1H NMR (DMSO-d6,
500 MHz) δ: 2.38 (s, 3H, CH3), 2.49 (s, DMSO & CH3),
7.34 (t, J=7.3 Hz, 1H, arom-H), 7.52 (t, J=7.8 Hz, 2H,
arom-H), 8.02 (d, J=7.7 Hz, 2H, arom-H), 12.22 (s, 1H,
apparatus. The IR spectra were obtained using a Tensor
1
27 Bruker spectrophotometer as KBr disks. The H
NMR (400 & 500 MHz) and 13C NMR (100 MHz)
spectra were recorded on Bruker 400 & 500 spectrome-
ters.
-1
NH); IR (KBr) ν: 3449 (NH), 1676 (CO) cm .
General procedure for the synthesis of pyrazolo[3,4-
d]pyrimidin-4-ones 3a— 3g catalyzed by Brønsted-
acidic ILs
6-Ethyl-3-methyl-1-phenyl-1,5-dihydro-4H-pyra-
zolo[3,4-d]pyrimidin-4-one (3f) 1H NMR (DMSO-d6,
500 MHz) δ: 1.24 (t, J=7.5 Hz, 3H, CH3), 2.50 (s,
DMSO & CH3), 2.66 (q, J=7.5 Hz, 2H, CH2), 7.33 (t,
J=7.3 Hz, 1H, arom-H), 7.52 (t, J=7.7 Hz, 2H,
arom-H), 8.07 (d, J=8.1 Hz, 2H, arom-H), 12.19 (s, 1H,
A mixture of 5-amino-1H-pyrazole-4-carboxamides
(1a—1c) (2 mmol), triethyl orthoesters+ (2a—2c) (2.5
mmol), and Brønsted-acidic IL, [MIM (CH2)4SO3H]-
+
-1
-
-
NH); IR (KBr) ν: 3448 (NH), 1682 (CO) cm .
[ HSO4 ] (IL1) or [Et3N (CH2)4SO3H][ HSO4 ] (IL2),
(0.1 mmol, 5 mol%) was heated in the oil bath at 80 ℃
for the indicated time. After the completion of the reac-
tion, the reaction mixture was cooled to room tempera-
ture and water was added. The precipitated solid was
isolated by filtration, washed with water, and recrystal-
lized from ethanol to give pure products 3a—3g in high
yields.
1-(2,4-Dinitrophenyl)-1,5-dihydro-4H-pyrazolo-
[3,4-d]pyrimidin-4-one (3g) 1H NMR (DMSO-d6,
400 MHz) δ: 8.21—8.29 (m, 2H, arom-H & CH of
pyrimidine ring), 8.51 (s, 1H, CH of pyrazole ring), 8.75
(dd, J=8.0, 4.0 Hz, 1H, arom-H), 8.92 (d, J=4.0 Hz,
1H, arom-H), 12.70 (s, 1H, NH); 13C NMR (DMSO-d6,
100 MHz) δ: 108.08, 121.81, 128.96, 129.33, 134.38,
138.96, 143.80, 146.71, 150.52, 153.83, 157.29; IR
-1
Recycling and reusing of the catalyst
(KBr) ν: 3441 (NH), 1682 (CO) cm ; MS m/z: 302
(M+). Anal. calcd for C11H6N6O5: C 43.72, H 2.00, N
27.81; found C 43.93, H 2.12, N 27.62.
The catalysts are soluble in water and could there-
fore be recycled as the filtrate. The catalysts were re-
covered by evaporation of the water, washed with di-
ethyl ether, dried at 50 ℃ under vacuum for 1 h, and
reused in another reaction with only slight reduction in
the catalytic activity.
Results and discussion
Solvent-free conditions are especially important for
providing an eco-friendly system. The number of publi-
cations reporting solvent-free conditions for the hetero-
cyclic synthesis has increased rapidly in recent years.47
One advantage of solvent-free reactions, in comparison
to the reaction in molecular solvents, is that the com-
pounds formed are often sufficiently pure to circumvent
extensive purification using chromatography. Therefore,
due to the increasing demand in modern organic proc-
esses for avoiding expensive purification, we decided to
investigate the efficiency of Brønsted-acidic ILs as
catalyst in the synthesis of pyrazolo[3,4-d]pyrimidin-4-
ones under solvent-free conditions.
Spectral data for compounds 3a— 3g
Phenyl-1,5-dihydro-4H-pyrazolo[3,4-d]pyri-
midin-4-one (3a) 1H NMR (DMSO-d6, 500 MHz) δ:
7.40 (t, J=7.5 Hz, 1H, arom-H), 7.56 (t, J=7.8 Hz, 2H,
arom-H), 8.03 (d, J=8.3 Hz, 2H, arom-H), 8.19 (s, 1H,
CH of pyrimidine ring), 8.32 (s, 1H, CH of pyrazole
ring), 12.44 (s. br., 1H, NH); IR (KBr) ν: 3449 (NH),
-1
1680 (CO) cm .
6-Methyl-1-phenyl-1,5-dihydro-4H-pyrazolo[3,4-
d]pyrimidin-4-one (3b) 1H NMR (DMSO-d6, 500
MHz) δ: 2.41 (s, 3H, CH3), 7.38 (t, J=7.3 Hz, 1H,
arom-H), 7.55 (t, J=7.6 Hz, 2H, arom-H), 8.04 (d, J=
For our investigations, two Brønsted-acidic ILs,
2422
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 2421— 2426