Solvent-free combinatorial synthesis of tetrazolo[1,5-a]thiopyrano[3,4-d] pyrimidine derivatives

Article abstract of DOI:10.1007/s11164-012-0714-7

A series of novel tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidine derivatives were synthesized by reaction of aryl aldehyde, 2H-thiopyran-3,5(4H,6H)-dione, and 5-aminotetrazole under solvent-free conditions. The features of this procedure are mild reaction conditions, high yields, and operational simplicity.

Full text of DOI:10.1007/s11164-012-0714-7

Res Chem Intermed (2013) 39:1799–1806
DOI 10.1007/s11164-012-0714-7
Solvent-free combinatorial synthesis of tetrazolo[1,
5-a]thiopyrano[3,4-d]pyrimidine derivatives
•
Shide Shen Honghong Zhang Chenxia Yu
•
•
•
Changsheng Yao Tuanjie Li Bingbin Qin
•
•
•
Jun Lu Donglin Wang
Received: 1 May 2012 / Accepted: 30 June 2012 / Published online: 17 July 2012
Ó Springer Science+Business Media B.V. 2012
Abstract A series of novel tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidine deriva-
tives were synthesized by reaction of aryl aldehyde, 2H-thiopyran-3,5(4H,6H)-
dione, and 5-aminotetrazole under solvent-free conditions. The features of this
procedure are mild reaction conditions, high yields, and operational simplicity.
Keywords Tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidine Á
Solvent-free synthesis Á Multicomponent reaction
Introduction
Thiopyran and fused thiopyran derivatives are frequently found heterocyclic
compounds. They are important in agrochemical and medicinal chemistry because
of their extensive biological activity, for example herbicidal activity [1], pesticidal
activity [2], antiproliferative activity [3], fungicidal activity [4], and inhibition of
glycosidase [5]. Compounds containing the dihydrotetrazolo[1,5-a]pyrimidine
structure have a variety of biological and pharmaceutical activity, for example
antimicrobial activity [6], inhibition of human neutrophil elastase [7], and
antiproliferative activity [8]. Most of the methods developed for synthesis of these
S. Shen
Jiangsu Jianzhu Institute of Architectural Technology, Xuzhou 221116, Jiangsu, China
H. Zhang Á C. Yu Á T. Li Á B. Qin Á J. Lu Á D. Wang
School of Chemistry and Chemical Engineering, Jiangsu Key Laboratory of Green Synthetic
Chemistry for Functional Materials, Jiangsu Normal University,
Xuzhou 221116, Jiangsu, People’s Republic of China
C. Yao (&)
School of Chemistry and Chemical Engineering, Key Laboratory of Biotechnology on Medical
Plant, Jiangsu Normal University, Xuzhou 221116, Jiangsu, People’s Republic of China
e-mail: csyao@jsnu.edu.cn
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ArCHO
1
Scheme 1 Synthesis of
tetrazolo[1,5-a]thiopyrano
[3,4-d]pyrimidine derivatives
O
Ar
O
H
N
solvent-free, p-TSA
N
N
N
N
N
+
NH2
30 o
C
N
S
N
S
N
H
O
3
4
2
compounds involve reaction of 5-aminotetrazole with a,b-unsaturated carbonyl
compounds [9–11]. Because of their unique biological properties and interesting
photochemical properties, the development of more expeditious and environmen-
tally benign synthetic approaches from easily available precursors is highly
desirable. Recent progress in solvent-free reactions is attracting much attention
because of their high efficiency and selectivity, easy separation and purification,
mild reaction conditions, and eco-friendliness [12–15]. In addition, by virtue of their
atom-economy, simple procedures, and convergence, multi-component reactions
(MCRs) have been recognized and their utility in preparing compound libraries to
screen for functional molecules is well recognized [16, 17]. Many organic
transformations have been reported to proceed efficiently in multi-component
reactions. In continuation of our work on the multi-component synthesis of
heterocyclic compound libraries with high diversity [18–20], herein we report
efficient and solvent-free synthesis of tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidine
derivatives 4 by reaction of an aryl aldehyde 1, 2H-thiopyran-3,5(4H,6H)-dione 2,
and 5-aminotetrazole 3 (Scheme 1).
Results and discussion
We initially examined the catalytic activity of p-toluenesulfonic acid (p-TSA) with
2H-thiopyran-3,5(4H,6H)-dione 2 (1 mmol), 5-aminotetrazole 3 (1 mmol), and
4-bromobenzaldehyde 1a (1 mmol) as model substrates in EtOH at 30 °C for 8 h. It
was found that the three-component reaction, in the presence of 0.1 mmol p-TSA,
proceeded smoothly and gave the expected product 4a in 65 % yield (Table 1, entry
1). Other results showed the activity of other acids in this reaction was poor under
the same conditions (Table 1, entries 2–8).
We then investigated the optimum solvent for the reaction using 10 % mol
p-TSA as catalyst (Table 2, entries 1–7). It was found that reaction under solvent-
free conditions (Table 2, entry 1) resulted in higher yield than the other conditions.
To determine a practical temperature for the synthesis, the reaction was
conducted under solvent-free conditions at 20, 25, 30, 35, and 40 °C. It was very
surprising that the solvent-free reaction at 30 °C proceeded in excellent yield
(Table 3, entry 3).
On the basis of these optimized reaction conditions, a series of tetrazolo[1,
5-a]thiopyrano[3,4-d]pyrimidine derivatives were synthesized successfully. As
shown in Table 4, reaction at 30 °C under solvent-free conditions gave the
corresponding products in moderate to good yields. Aryl aldehydes bearing either
electron-donating (for example methoxy) or electron-withdrawing (for example
123

Solvent-free combinatorial synthesis
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Table 1 Selection of catalyst for synthesis of 4a
Entry
Catalyst
Temp. (°C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
p-TSA
NH2SO3H
FeCl3
30
30
30
30
30
30
30
30
8
8
8
8
8
8
8
8
65
53
57
50
42
60
45
40
AlCl3
FeSO4
HCl
H2SO4
HNO3
Table 2 Solvent effect on the synthesis of 4a
Entry
Solvent
Temp. (°C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
Solvent-free
AcOH
30
30
30
30
30
30
30
8
8
8
8
8
8
8
84
72
65
43
50
52
46
EtOH
CH3CN
DMF
MeOH
CHCl3
Table 3 Temperature optimization for synthesis of 4a
Entry
Temp. (°C)
Time (h)
Yield (%)
1
2
3
4
5
20
25
30
35
40
8
8
8
8
8
50
75
84
70
65
nitro) groups afforded high yields of compounds 4 in the multi-component reaction
under the same conditions. So, we concluded safely that the electronic nature of
substituents of the aromatic aldehyde had no obvious effect on the reaction.
However, when aliphatic and heteroaromatic aldehydes were used in this reaction
the expected product was not obtained.
On the basis of a previous report [21], a plausible reaction mechanism was
proposed for formation of 4 (Scheme 2). The enamination reaction between 2H-
thiopyran-3,5(4H,6H)-dione 2 and 5-aminotetrazole 3 generated the intermediate 5;
condensation of this with aryl aldehyde 1, furnished the final products 4.
1
All of the compounds were characterized by HRMS, FTIR, and H NMR.
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S. Shen et al.
Table 4 Synthesis of compound 4
Entry
Compound
Ar
Time (h)
Yield (%)
M.p. (°C)
1
4a
4b
4c
4d
4e
4f
4-BrC6H4
4-ClC6H4
4-CH3C6H4
4-FC6H4
8
8.5
9
84
76
75
80
70
72
75
70
73
65
75
80
260–262
261–262
262–263
209–210
285–281
[300
2
3
4
7
5
2-ClC6H4
3-FC6H4
8.5
7
6
7
4g
4h
4i
2-FC6H4
7.5
8
246–248
[300
8
2,3-Cl2C6H3
3,4-Cl2C6H3
3-NO2C6H4
2-BrC6H4
4-CH3OC6H4
9
9
214–215
281–282
286–288
205–206
10
11
12
4j
10
8
4k
4l
7
H
N
N
O
O
Ar
NH2
O
O
H
Ar
N
ArCHO
1
N
3
N
N
N
N
N
HN
- H2O
N
N
N
N
S
S
S
S
N
N
N
N
p-TSA
p-TSA
p-TSA
N
H
O
H
H
HO
2
5
9
4
Scheme 2 Plausible reaction mechanism for the three-component synthesis
Conclusion
We have developed a simple and efficient method for synthesis of a series of
tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidine derivatives via one-pot multi-compo-
nent reaction under solvent-free conditions. The features of this procedure are mild
reaction conditions, generally good to excellent yields, readily available starting
materials, and operational simplicity (only one pot).
Experimental
Common reagents and materials were purchased from commercial sources and were
purified by recrystallization or distillation. Infrared (IR) spectra were recorded on a
Tensor 27 spectrophotometer in KBr pellets and are reported in terms of frequency
of absorption (cm-1). NMR spectra were recorded on a Bruker DPX 400; data for
1H are reported as follows: chemical shift (ppm), multiplicity (s, singlet; d, doublet;
t, triplet; q, quartet; m, multiplet; and br, broad), coupling constant (Hz), and
integration. HRMS (ESI) were determined by use of a microTOF-QIIHRMS/MS
instrument (Bruker).
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Solvent-free combinatorial synthesis
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General procedure for synthesis of 9-aryl-7,9-dihydro-4H-tetrazolo[1,5-a]thio-
pyrano[3,4-d]pyrimidin-8(5H)-ones (4a–4l)
To a 5 mL flask were sequentially added p-TSA (0.1 mmol), aryl aldehyde 1
(1 mmol), 2H-thiopyran-3,5(4H,6H)-dione 2 (1 mmol), and 5-aminotetrazole 3
(1 mmol). The reaction vessel was placed in an oil bath at 30 °C and the mixture
was stirred until the substrates had been completely consumed. The reaction mixture
was poured into water. The solid was recrystallized from DMF to give pure product
4. A similar procedure was used to prepare the following compounds.
9-(4-Bromophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,
4-d]pyrimidin-8(5H)-one (4a)
IR (KBr, m, cm-1): 3084, 3024, 2946, 2887, 2851, 2776, 1664, 1617, 1566, 1488,
1428, 1407, 1361, 1304, 1214, 1183, 1071, 1010, 864, 846, 519; 1H NMR
(400 MHz, DMSO-d6) (d, ppm): 11.96 (s, 1H, NH), 7.55 (d, J = 8.4 Hz, 2H, ArH),
7.30 (d, J = 8.4 Hz, 2H, ArH), 6.70 (s, 1H, CH), 3.93 (d, J = 17.2, 1H, CH2), 3.69
(d, J = 16.8 Hz, 1H, CH2), 3.51 (d, J = 16.0 Hz, 1H, CH2), 3.23 (d, J = 16.0 Hz,
1H, CH2). HRMS (ESI): m/z Calcd. for: C13H10BrN5NaOS [M ? Na]? 385.9687,
found: 385.9664.
9-(4-Chlorophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,
4-d]pyrimidin-8(5H)-one (4b)
IR (KBr, m, cm-1): 3026, 2945, 2887, 2850, 2776, 1645, 1618, 1551, 1490, 1430,
1361, 1303, 1233, 1213, 1183, 1134, 1090, 1013, 864, 847, 804, 521; H NMR
1
(400 MHz, DMSO-d6) (d, ppm): 11.99 (s, 1H, NH), 7.42 (d, J = 8.4 Hz, 2H, ArH)
7.37 (d, J = 8.4 Hz, 2H, ArH), 6.71 (s, 1H, CH), 3.94 (d, J = 16.8 Hz, 1H, CH2),
3.69 (d, J = 17.2 Hz, 1H, CH2), 3.52 (d, J = 16.0 Hz, 1H, CH2), 3.24 (d,
J = 16.0 Hz, 1H, CH2). HRMS (ESI): m/z Calcd. for: C13H9ClN5OS [M - H]-
318.0211, found: 318.0241.
9-(p-Tolyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidin-
8(5H)-one (4c)
IR (KBr, m, cm-1): 3222, 3155, 3081, 3051, 2963, 2904, 1622, 1556, 1427, 1350,
1306, 1271, 1234, 1211, 1182, 1148, 1135, 1092, 849, 514; H NMR (400 MHz,
1
DMSO-d6) (d, ppm): 11.87 (s, 1H, NH), 7.21 (d, J = 8.4 Hz, 2H, ArH), 7.14 (d,
J = 8.0 Hz, 2H, ArH), 6.64 (s, 1H, CH), 3.94 (d, J = 16.8 Hz, 1H, CH2), 3.68 (d,
J = 17.2 Hz, 1H, CH2), 3.52 (d, J = 16.0 Hz, 1H, CH2), 3.22 (d, J = 16.0 Hz, 1H,
CH2), 2.25 (s, 3H, CH3). HRMS (ESI): m/z Calcd. for: C14H12N5OS [M - H]-
298.0757, found: 298.0783.
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S. Shen et al.
9-(4-Fluorophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidin-
8(5H)-one (4d)
1
IR (KBr, m, cm-1): 3366, 3180, 2976, 1603, 1500, 1047, 795, 744, 670, 544; H
NMR (400 MHz, DMSO-d6) (d, ppm): 9.01 (s, 1H, NH), 7.36–7.42 (m, 2H, ArH),
7.18–7.22 (m, 2H, ArH), 6.70 (s, 1H, CH), 3.72 (d, J = 10.8 Hz, 1H, CH2), 3.66 (d,
J = 12.0 Hz, 1H, CH2), 3.55 (d, J = 13.6 Hz, 1H, CH2), 2.98 (d, J = 14.0 Hz, 1H,
CH2). HRMS (ESI): m/z Calcd. for: C13H9FN5OS [M - H]- 302.0506, found:
302.0509.
9-(2-Chlorophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,
4-d]pyrimidin-8(5H)-one (4e)
IR (KBr, m, cm-1): 3090, 3046, 2952, 2890, 2774, 1641, 1580, 1472, 1432, 1354,
1305, 1258, 1209, 1099, 1048, 757, 736, 713, 658; 1H NMR (400 MHz, DMSO-d6)
(d, ppm): 12.0 (s, 1H, NH), 7.47-7.44 (m, 2H, ArH), 7.35-7.31 (m, 2H, ArH), 6.98
(s, 1H, CH), 3.92 (d, J = 16.8, 1H, CH2), 3.71 (d, J = 16.8 Hz, 1H, CH2), 3.48 (d,
J = 16.0 Hz, 1H,CH2), 3.22 (d, J = 16.0 Hz, 1H, CH2). HRMS (ESI): m/z Calcd.
for: C13H9ClN5OS [M - H]- 318.0211, found: 318.0234.
9-(3-Fluorophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidin-
8(5H)-one (4f)
IR (KBr, m, cm-1): 3369, 3065, 2908, 1717, 1602, 1450, 1085, 1045, 768, 759, 514;
1H NMR (400 MHz, DMSO-d6) (d, ppm): 9.03 (s, 1H, NH), 7.15–7.24 (m, 4H,
ArH), 6.71 (s, 1H, CH), 3.75 (d, J = 11.2 Hz, 1H, CH2), 3.67 (d, J = 12.0 Hz, 1H,
CH2), 3.54 (d, J = 14.0 Hz, 1H,CH2), 2.80 (d, J = 12.0 Hz, 1H, CH2). HRMS
(ESI): m/z Calcd. for: C13H9FN5OS [M - H]- 302.0506, found: 302.0499.
9-(2-Fluorophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidin-
8(5H)-one (4g)
IR (KBr, m, cm-1): 3040, 2947, 2886, 2788, 1645, 1613, 1548, 1489, 1433, 1360,
1
1339, 1326, 1301, 1224, 1136, 876, 759, 517; H NMR (400 MHz, DMSO-d6) (d,
ppm): 11.96 (s, 1H, NH), 7.45 (t, J = 7.2 Hz, 1H, ArH), 7.35–7.40 (m, 1H, ArH),
7.17–7.21 (m, 2H, ArH), 6.86 (s, 1H, CH), 3.92 (d, J = 16.8 Hz, 1H, CH2), 3.68 (d,
J = 17.2 Hz, 1H, CH2), 3.50 (d, J = 16.0 Hz, 1H, CH2), 3.22 (d, J = 16.0 Hz, 1H,
CH2). HRMS (ESI): m/z Calcd. for: C13H9FN5OS [M - H]- 302.0506, found:
302.0532.
9-(2,3-Dichlorophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,
4-d]pyrimidin-8(5H)-one (4h)
IR (KBr, m, cm-1): 3049, 2950, 2885, 2776, 1641, 1580, 1561, 1425, 1353, 1302,
1247, 1208, 1152, 1098, 767, 737; 1H NMR (400 MHz, DMSO-d6) (d, ppm): 12.13
(s, 1H, NH), 7.62 (d, J = 8.0 Hz, 1H, ArH), 7.44 (d, J = 7.2 Hz, 1H, ArH), 7.36 (t,
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Solvent-free combinatorial synthesis
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J = 8.0 Hz, 1H, ArH), 7.04 (s, 1H, CH), 3.91 (d, J = 17.2 Hz, 1H, CH2), 3.72 (d,
J = 16.8 Hz, 1H, CH2), 3.48 (d, J = 16.0 Hz, 1H, CH2), 3.24 (d, J = 16.0 Hz, 1H,
CH2). HRMS (ESI): m/z Calcd. for: C13H8Cl2N5OS [M - H]- 351.9821, found:
351.9820.
9-(3,4-Dichlorophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,
4-d]pyrimidin-8(5H)-one (4i)
1
IR (KBr, m, cm-1): 3401, 3198, 2911, 1711, 1604, 1031, 790, 730, 583, 513; H
NMR (400 MHz, DMSO-d6) (d, ppm): 9.07 (s, 1H, NH), 7.69 (d, J = 2.0 Hz, 1H,
ArH), 7.65 (d, J = 8.4 Hz, 1H, ArH), 7.39–7.42 (m, 1H, ArH), 6.74 (s, 1H, CH),
3.76 (d, J = 11.2 Hz, 1H, CH2), 3.66 (d, J = 12.4 Hz, 1H, CH2), 3.52 (d,
J = 14.0 Hz, 1H, CH2), 2.99 (d, J = 14.0 Hz, 1H, CH2). HRMS (ESI): m/z Calcd.
for: C13H8Cl2N5OS [M - H]- 351.9821, found: 351.9819.
9-(3-Nitrophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,4-d]pyrimidin-
8(5H)-one (4j)
IR (KBr, m, cm-1): 3200, 3099, 2751, 1639, 1556, 1515, 1456, 1355, 1269, 736,
515; 1H NMR (400 MHz, DMSO-d6) (d, ppm): 9.13 (s, 1H, NH), 8.17–8.25 (m, 3H,
ArH), 7.89 (d, J = 8.0 Hz, 1H, ArH), 6.78 (s, 1H, CH), 3.86 (d, J = 11.6 Hz, 1H,
CH2), 3.65 (d, J = 12.0 Hz, 1H, CH2), 3.56 (d, J = 10.0 Hz, 1H, CH2), 2.81 (d,
J = 12.0 Hz, 1H, CH2). HRMS (ESI): m/z Calcd. for: C13H9N6O3S [M - H]-
329.0451, found: 329.0454.
9-(2-Bromophenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,
4-d]pyrimidin-8(5H)-one (4k)
IR (KBr, m, cm-1): 3235, 3165, 2885, 1643, 1579, 1304, 1098, 755, 650, 518, 507;
1H NMR (400 MHz, DMSO-d6) (d, ppm): 11.99 (s, 1H, NH), 7.63 (d, J = 8.0 Hz,
1H, ArH), 7.34-7.40 (m, 2H, ArH), 7.24 (t, J = 7.6 Hz, 1H, ArH), 6.97 (s, 1H, CH),
3.91 (d, J = 16.8 Hz, 1H, CH2), 3.72 (d, J = 16.8 Hz, 1H, CH2), 3.48 (d,
J = 16.0 Hz, 1H, CH2), 3.23 (d, J = 16.0 Hz, 1H, CH2). HRMS (ESI): m/z Calcd.
for: C13H9BrN5OS [M - H]- 361.9711, found: 361.9712.
9-(4-Methoxyphenyl)-7,9-dihydro-4H-tetrazolo[1,5-a]thiopyrano[3,
4-d]pyrimidin-8(5H)-one (4l)
1
IR (KBr, m, cm-1): 3178, 2907, 1604, 1260, 1036, 891, 547; H NMR (400 MHz,
DMSO-d6) (d, ppm): 8.96 (s, 1H, NH), 7.24 (d, J = 8.0 Hz, 2H, ArH), 6.91 (d,
J = 7.6 Hz, 2H, ArH), 6.64 (s, 1H, CH), 3.95 (d, J = 17.2 Hz, 1H, CH2), 3.75 (s,
3H, OCH3), 3.66 (d, J = 10.8 Hz, 1H, CH2), 3.56 (d, J = 14.0 Hz, 1H, CH2), 2.96
(d, J = 13.6 Hz, 1H, CH2). HRMS (ESI): m/z Calcd. for: C14H12N5O2S [M - H]-
314.0712, found: 314.0728.
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Acknowledgments We are grateful for financial support by A Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions and the Major Basic Research Project of
the Natural Science Foundation of the Jiangsu Higher Education Institutions (09KJA430003).
References
1. J.N. Scutt, C.J. Mathews, WO patent No. 150094 (2009)
2. C.J. Mathews, J.N. Scutt, M. Govenkar, M. Muehlebach, WO patent No. 150095 (2009)
3. L. Dalla Via, S. Marciani Magno, O. Gia, A.M. Marini, F. Da Settimo, S. Salerno, C. La Motta, F.
Simorini, S. Taliani, A. Lavecchia, C. Di Giovanni, G. Brancato, V. Barone, E. Novellino, J. Med.
Chem. 52, 5429 (2009)
4. H. Xie, N.Y. Huang, M.W. Ding, Phosphorus Sulfur Silicon Relat. Elem. 184, 480 (2009)
5. H. Yuasa, M. Izumi, H. Hashimoto, Curr. Top. Med. Chem. 9, 76 (2009)
6. V.L. Gein, T.M. Zamaraeva, A.A. Kurbatova, E.V. Voronina, M.I. Vakhrin, Pharm. Chem. J. 44, 366
(2010)
7. F. Von Nussbaum, D. Karthaus, S. Anlauf, M. Delbeck, V.M. J. Li, D. Meibom, K. Lustig, WO
patent No. 078953 (2010)
8. A.M. Hussein, O.M. Ahmed, Bioorg. Med. Chem. 18, 2639 (2010)
9. V.A. Chebanov, S.M. Desenko, Y.I. Sakhno, E.S. Panchenko, V.E. Saraev, V.I. Musatov, V.F.
Konev, Fiziol. Akt. Rechovini 2, 10 (2002)
10. S.M. Desenko, E.S. Gladkov, S.A. Komykhov, O.V. Shishkin, V.D. Orlov, Chem. Heterocycl.
Compd. 37, 747 (2001)
11. M.A. Metwally, M.S. El-Hussiny, F.Z. El-Ablak, A.M. Khalil, Pharmazie 44, 261 (1989)
12. A. Domling, I. Ugi, Angew. Chem. Int. Ed. 39, 3168 (2000)
13. A. Shaabani, M. Seyyedhamzeh, A. Maleki, M. Behnam, F. Rezazadeh, Tetrahedron Lett. 50, 2911
(2009)
14. M. Adib, E. Sheibani, H.R. Bijanzadeh, L.G. Zhu, Tetrahedron 64, 10681 (2008)
15. J.D. Sunderhaus, S.F. Martin, Chem. Eur. J. 15, 1300 (2009)
16. F. Eiden, P. Gmeiner, Arch. Pharm. 321, 397 (1988)
17. Z.J. Ren, W.G. Cao, W.Q. Tong, Synth. Commun. 32, 3475 (2002)
18. C.S. Yao, S. Lei, C.H. Wang, C.X. Yu, S.J. Tu, J. Heterocycl. Chem. 45, 1609 (2008)
19. C.S. Yao, C.H. Wang, B. Jiang, S.J. Tu, J. Comb. Chem. 12, 472 (2010)
20. T.J. Li, X.D. Feng, C.S. Yao, C.X. Yu, B. Jiang, S.J. Tu, Bioorg. Med. Chem. Lett. 21, 453 (2011)
21. L.Y. Zeng, C. Cai, J. Comb. Chem. 12, 35 (2010)
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