Iodine-catalyzed synthesis of pyrazolo[4,3-f]quinoline derivatives via a highly regio-selective Povarov reaction

Article abstract of DOI:10.1007/s11164-012-0712-9

A highly regio-selective Povarov reaction of an aromatic aldehyde, 1H-indazol-5-amine, and methyl 3-oxobutanoate catalyzed by iodine is described. This novel reaction selectively gave 3H-pyrazolo[4,3-f]quinolin-9-yl acetates, in high yield, rather than 3H-pyrazolo[4,3-f]quinoline-8-carboxylate derivatives. This procedure has the advantages of mild reaction conditions, high yields, metal-free catalyst, and high regio-selectivity.

Full text of DOI:10.1007/s11164-012-0712-9

Res Chem Intermed (2013) 39:1781–1787
DOI 10.1007/s11164-012-0712-9
Iodine-catalyzed synthesis of pyrazolo[4,3-f]quinoline
derivatives via a highly regio-selective Povarov reaction
•
Mei-Mei Zhang Wei Wang Tuan-Jie Li
•
•
•
Chang-Sheng Yao Xiang-Shan Wang
Received: 28 February 2012 / Accepted: 27 June 2012 / Published online: 17 July 2012
Ó Springer Science+Business Media B.V. 2012
Abstract A highly regio-selective Povarov reaction of an aromatic aldehyde,
1H-indazol-5-amine, and methyl 3-oxobutanoate catalyzed by iodine is described.
This novel reaction selectively gave 3H-pyrazolo[4,3-f]quinolin-9-yl acetates, in
high yield, rather than 3H-pyrazolo[4,3-f]quinoline-8-carboxylate derivatives. This
procedure has the advantages of mild reaction conditions, high yields, metal-free
catalyst, and high regio-selectivity.
Keywords Pyrazolo[4,3-f]quinoline ꢀ 1H-indazol-5-amine ꢀ Regio-selectivity ꢀ
Iodine ꢀ Synthesis
Introduction
Pyrazoloquinoline and its derivatives are important heterocyclic compounds
because of their variety of medicinal properties, for example antimicrobial [1],
antifungal [2], antibacterial [3], and antileishmanial activity [4]. They are also
promising materials for optoelectronic applications [5–11]. Therefore, preparation
of pyrazoloquinolines by use of mild, efficient, and environmentally benign
methods is an attractive proposition.
The Povarov reaction [12] is an important method that provides convenient
access to quinoline derivatives by use of a Schiff base and an electron-rich
M.-M. Zhang
The Key Laboratory of Biotechnology on Medical Plant of Jiangsu Province, Jiangsu Normal
University, Xuzhou 221116, Jiangsu, People’s Republic of China
W. Wang ꢀ T.-J. Li ꢀ C.-S. Yao ꢀ X.-S. 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
e-mail: xswang1974@yahoo.com
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M.-M. Zhang et al.
dienophile as reactants. In 2006, this method was substantially improved by Wang
and coworkers who used iodine as catalyst [13] and an aliphatic aldehyde as starting
material. In this type of Povarov reaction, iodine is a novel metal-free Lewis-acid
catalyst which converts the aliphatic aldehyde to its enol form; this electron-rich
dienophile reacts with the Schiff base to give the desired quinolines.
It is well known that 3-oxobutanoate is in equilibrium with its enol form, which is
readily formed. Furthermore, the enol usually forms on the methylene side and is,
therefore, stabilized by conjugation with the ester carbonyl (Scheme 1). On the
basis of this assumption, 3H-pyrazolo[4,3-f]quinoline-8-carboxylate (4a0) might be
obtained in high yield by reaction of 4-chlorobenzaldehyde (1a), 1H-indazol-5-
amine (2), and methyl 3-oxobutanoate (3) as reactants. However, this reaction gave
methyl 2-(7-(4-chlorophenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl) acetate (4a) in 89 %
yield and with high regio-selectivity rather than methyl 7-(4-chlorophenyl)-9-
methyl-3H-pyrazolo[4,3-f]quinoline-8-carboxylate (4a0) (Scheme 1).
In connection with our continued research on the iodine-catalyzed Povarov
reaction [14–16], in this paper, we report an efficient and highly regio-selective
synthesis of methyl 2-(7-aryl-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate derivatives
catalyzed by iodine.
Results and discussion
Reaction of aromatic aldehydes 1a-l, 1H-indazol-5-amine 2, and methyl 3-oxobut-
anoate 3 in THF in the presence of 5 mol% iodine under reflux conditions afforded
the corresponding methyl 2-(7-aryl-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate deriva-
tives 4a-l in high yields (Scheme 2).
In our initial study, several conditions, including type and amount of catalyst,
reaction temperature, and solvents, were screened for this reaction. The model
reaction was conducted using 4-chlorobenzaldehyde (1a), 1H-indazol-5-amine, and
methyl 3-oxobutanoate. The results obtained from screening of the reaction are
summarized in Table 1. When different Lewis acids were used, iodine gave the best
N
HN
OH
O
CO2CH3
3
3
OCH3
CHO
H
N
N
I2
N
+
Cl
THF
4a'
H2N
Cl
N
HN
CH2CO2CH3
OH
O
2
1a
OCH3
O
O
N
3
=
OCH3
Cl
4a
Scheme 1 The highly regio-selective model reaction
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Iodine-catalyzed synthesis of pyrazolo[4,3-f]quinoline derivatives
1783
H
O
O
N
HN
CH2CO2CH3
N
I2
+
N
+
Ar CHO
OCH3
THF
H2N
N
Ar
1
2
3
4
Scheme 2 Reaction of 1, 2, and 3
results. Except for TsOH the other Lewis acid catalysts tested did not promote this
reaction successfully (Table 1, entries 11–15). Subsequently, 1, 5, and 10 mol%
iodine were used to mediate the reaction, 5 mol% I2 under reflux in THF is
sufficient to initiate the reaction (Table 1, Entries 4–6). To find the optimum
reaction temperature, the reaction was carried out with 5 mol% of I2 at room
temperature, 50 °C, and reflux temperature, resulting in the isolation of 4a in a trace
amount, and 67 % and 89 % yields, respectively (Table 1, entries 2, 3, and 5).
In addition, CH3CN, benzene, DMF, CHCl3 (Table 1, entries 7–10) were also tested as
the solvents. Under these conditions, product 4a was formed in slightly lower yields.
Using these optimized conditions, a variety of aromatic aldehydes 1a-l were then
reacted with 2 and 3 to generate (7-aryl-3H-pyrazolo [4,3-f]quinolin-9-yl)acetate
derivatives 4a-l (Table 2). For aldehydes 1a-l, the yields of 4a-l were not sensitive
to the electronic properties of the aromatic ring in the presence of electron-
withdrawing groups (for example halide and nitro) or electron-donating groups
(for example alkyl or alkoxy) (Table 2).
Table 1 Optimization of yield of 4a under different conditions
Entry
Temp. (°C)
Cat. (mol%)
Solvent
Yields (%)a
1
Reflux
r.t.
–
THF
0
2
I2(5)
THF
Trace
67
3
50
I2(5)
THF
4
Reflux
Reflux
Reflux
Reflux
Reflux
80
I2(1)
THF
82
5
I2(5)
THF
89
6
I2(10)
I2(5)
THF
89
7
CH3CN
Benzene
DMF
CHCl3
THF
83
8
I2(5)
78
9
I2(5)
76
10
11
12
13
14
15
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
I2(5)
80
CuI(5)
TsOH(5)
Yb(OTf)3(5)
Sc(OTf)3(5)
AgOTf(5)
Trace
67
THF
THF
Trace
Trace
Trace
THF
THF
Reagents and conditions: 4-chlorobenzaldehyde 1a (0.281 g, 2.0 mmol), 2 (0.266 g, 2.0 mmol),
3 (0.232 g, 2.0 mmol), solvent (10 mL)
a
Isolated yields
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M.-M. Zhang et al.
Yield (%)a
Table 2 Results from synthesis of 4a-l catalyzed by iodine in THF
Entry
Ar
Product
Time (h)
1
4-ClC6H4
4a
4b
4c
4d
4e
4f
12
14
12
10
16
16
16
12
12
15
12
10
89
92
82
86
78
87
90
79
84
90
85
88
2
4-BrC6H4
3
4-FC6H4
4
3,4-Cl2C6H3
4-MeC6H4
4-MeOC6H4
3,4-(MeO)2C6H3
3-ClC6H4
5
6
7
4g
4h
4i
8
9
2,4-Cl2C6H3
3,4-Me2C6H3
2-Thienyl
10
11
12
4j
4k
4l
4-NO2C6H4
Reagents and conditions: 1a-l (2.0 mmol), 2 (0.266 g, 2.0 mmol), 3 (0.232 g, 2.0 mmol), I2 (0.1 mmol,
0.026 g), THF (10 mL)
b
Isolated yields
1
When 4a-l were characterized by use of H NMR, IR, and HRMS, the results
were in good agreement with the proposed structures. For instance, for compounds
1
4a (Ar = 4-ClC6H4), only a methyl group was observed in H NMR; this was
identified as a methoxy group at 3.66 ppm (3H), and the corresponding methylene
was observed at 4.52 ppm (2H). In addition, total of eight rather than seven
aromatic protons also confirmed the structure of 4a.
Experimental
Melting points were determined in open capillaries and are uncorrected. IR spectra
1
were recorded on a Tensor 27 spectrometer, as KBr pellets. H NMR spectra were
obtained from solutions in DMSO-d6 or CDCl3, with Me4Si as internal standard, by
use of a Bruker-400 spectrometer. HRMS analysis was performed with a Bruker-
micro-TOF-Q-MS analyzer.
General procedure for synthesis of methyl 2-(7-aryl-3H-pyrazolo[4,3-f]quinolin-
9-yl)acetate derivatives 4a-l
A dry 50 mL flask was charged with aromatic aldehyde (2.0 mmol), 1H-indazol-5-
amine (0.266 g, 2.0 mmol), methyl 3-oxobutanoate (0.232 g, 2.0 mmol), I2 (0.026 g,
0.1 mmol), and THF (10 mL). The reaction mixture was stirred at reflux for 10–16 h.
After completion of the reaction, as indicated by TLC, small amounts of DMF were
added to the mixture until all the yellow solid had dissolved. The mixture was then
cooled to room temperature and the powder obtained was isolated by filtration to give
the pure product 4.
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Iodine-catalyzed synthesis of pyrazolo[4,3-f]quinoline derivatives
1785
Methyl 2-(7-(4-chlorophenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4a
1
mp: 217–218 °C; H NMR (DMSO-d6, 400 MHz): dH 3.66 (s, 3H, CH3), 4.52 (s,
2H, CH2), 7.63 (d, J = 8.0 Hz, 2H, ArH), 7.95–8.02 (m, 2H, ArH), 8.25 (s, 1H,
ArH), 8.30 (d, J = 8.4 Hz, 2H, ArH), 8.43 (s, 1H, ArH), 13.78 (s, 1H, NH). IR
(KBr, cm-1): m 3188, 2998, 2949, 1739, 1580, 1566, 1495, 1423, 1334, 1278, 1204,
1185, 1156, 1097, 1012, 991, 934, 837, 793, 746. HRMS (ESI, m/z): Calcd for
C19H15ClN3O2 [M ? H]? 352.0853, found 352.0850.
Methyl 2-(7-(4-bromophenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4b
1
mp: 249–251 °C; H NMR (DMSO-d6, 400 MHz): dH 3.66 (s, 3H, CH3), 4.52 (s,
2H, CH2), 7.77 (d, J = 7.6 Hz, 2H, ArH), 7.95–8.01 (m, 2H, ArH), 8.22–8.24 (m,
3H, ArH), 8.34 (s, 1H, ArH), 13.78 (s, 1H, NH). IR (KBr, cm-1): m 3192, 2949,
1736, 1662, 1578, 1563, 1536, 1492, 1422, 1363, 1333, 1276, 1204, 1185, 1099,
1006, 990, 934, 834, 746, 723. HRMS (ESI, m/z): Calcd for C19H15BrN3O2
[M ? H]? 396.0348, found 396.0345.
Methyl 2-(7-(4-fluorophenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4c
1
mp: 194–197 °C; H NMR (DMSO-d6, 400 MHz): dH 3.66 (s, 3H, CH3), 4.51 (s,
2H, CH2), 7.40 (t, J = 8.8 Hz, 2H, ArH), 7.97 (d, J = 9.6 Hz, 2H, ArH), 8.21 (s,
1H, ArH), 8.30–8.31 (m, 3H, ArH), 13.76 (s, 1H, NH). IR (KBr, cm-1): m 3166,
2954, 1742, 1664, 1601, 1576, 1509, 1435, 1364, 1333, 1281, 1221, 1197, 1158,
1101, 1010, 934, 841, 795, 734. HRMS (ESI, m/z): Calcd for C19H15FN3O2
[M ? H]? 336.1148, found 336.1151.
Methyl 2-(7-(3,4-dichlorophenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4d
1
mp: 263–264 °C; H NMR (DMSO-d6, 400 MHz): dH 3.66 (s, 3H, CH3), 4.51 (s,
2H, CH2), 7.82 (d, J = 8.4 Hz, 1H, ArH), 7.95–8.03 (m, 2H, ArH), 8.26 (d,
J = 8.4 Hz, 1H, ArH), 8.31 (s, 1H, ArH), 8.35 (s, 1H, ArH), 8.51 (s, 1H, ArH),
13.79 (s, 1H, NH). IR (KBr, cm-1): m 3286, 2956, 1730, 1577, 1560, 1537, 1473,
1434, 1400, 1356, 1325, 1292, 1207, 1166, 1099, 1026, 989, 936, 898, 846, 815,
739, 715. HRMS (ESI, m/z): Calcd for C19H14Cl2N3O2 [M ? H]? 386.0463, found
386.0441.
Methyl 2-(7-(4-methylphenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4e
1
mp: 247–249 °C; H NMR (DMSO-d6, 400 MHz): dH 2.40 (s, 3H, CH3), 3.66
(s, 3H, CH3), 4.50 (s, 2H, CH2), 7.38 (d, J = 8.0 Hz, 2H, ArH), 7.97 (s, 2H, ArH),
8.16–8.18 (m, 3H, ArH), 8.35 (s, 1H, ArH), 13.72 (s, 1H, NH). IR (KBr, cm-1):
m 3147, 3097, 2921, 1740, 1605, 1580, 1514, 1434, 1367, 1329, 1279, 1229, 1201,
1163, 1008, 985, 931, 895, 800, 729. HRMS (ESI, m/z): Calcd for C20H18N3O2
[M ? H]? 332.1399, found 332.1396.
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M.-M. Zhang et al.
Methyl 2-(7-(4-methoxyphenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4f
1
mp: 212–214 °C; H NMR (DMSO-d6, 400 MHz): dH 3.66 (s, 3H, CH3), 3.86 (s,
3H, OCH3), 4.50 (s, 2H, CH2), 7.12 (d, J = 8.0 Hz, 2H, ArH), 7.93–7.99 (m, 2H,
ArH), 8.16 (s, 1H, ArH), 8.23 (d, J = 8.0 Hz, 2H, ArH), 8.32 (s, 1H, ArH), 13.70 (s,
1H, NH). IR (KBr, cm-1): m 3308, 2955, 2837, 1718, 1605, 1570, 1513, 1458, 1429,
1364, 1337, 1282, 1254, 1212, 1178, 1161, 1026, 981, 930, 903, 837, 795, 744.
HRMS (ESI, m/z): Calcd for C20H18N3O3 [M ? H]? 348.1348, found 348.1354.
Methyl 2-(7-(3,4-dimethoxyphenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4g
1
mp: 238–240 °C; H NMR (DMSO-d6, 400 MHz): dH 3.66 (s, 3H, CH3), 3.85 (s,
3H, OCH3), 3.92 (s, 3H, OCH3), 4.50 (s, 2H, CH2), 7.13 (d, J = 8.4 Hz, 1H, ArH),
7.83 (d, J = 7.6 Hz, 1H, ArH), 7.90 (s, 1H, ArH), 7.90–8.00 (m, 2H, ArH), 8.20 (s,
1H, ArH), 8.32 (s, 1H, ArH), 13.71 (s, 1H, NH). IR (KBr, cm-1): m 3299, 2956,
1719, 1597, 1572, 1519, 1459, 1421, 1361, 1319, 1275, 1219, 1174, 1119, 1021,
983, 927, 866, 811, 747, 714. HRMS (ESI, m/z): Calcd for C21H20N3O4 [M ? H]?
378.1454, found 378.1459.
Methyl 2-(7-(3-chlorophenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4h
mp: 242–245 °C;1H NMR (DMSO-d6, 400 MHz): dH 3.66 (s, 3H, CH3), 4.52 (s,
2H, CH2), 7.55–7.61 (m, 2H, ArH), 8.00 (s, 2H, ArH), 8.23 (d, J = 6.8 Hz, 1H,
ArH), 8.29 (s, 1H, ArH), 8.33 (s, 1H, ArH), 8.37 (s, 1H, ArH), 13.78 (s, 1H, NH). IR
(KBr, cm-1): m 3286, 3069, 2953, 1733, 1578, 1568, 1540, 1474, 1433, 1363, 1328,
1290, 1259, 1203, 1166, 1101, 1009, 988, 935, 900, 848, 797, 779, 711. HRMS
(ESI, m/z): Calcd for C19H15ClN3O2 [M ? H]? 352.0853, found 352.0833.
Methyl 2-(7-(2,4-dichlorophenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4i
1
mp: 224–226 °C; H NMR (DMSO-d6, 400 MHz): dH 3.66 (s, 3H, CH3), 4.52 (s,
2H, CH2), 7.61 (d, J = 8.4 Hz, 1H, ArH), 7.75 (d, J = 8.0 Hz, 1H, ArH), 7.81 (s,
1H, ArH), 7.87 (s, 1H, ArH), 7.96–8.02 (m, 2H, ArH), 8.39 (s, 1H, ArH), 13.80 (s,
1H, NH). IR (KBr, cm-1): m 3157, 2961, 1736, 1588, 1576, 1537, 1474, 1435, 1337,
1282, 1230, 1152, 1100, 1047, 1010, 989, 936, 898, 814, 798, 756. HRMS (ESI,
m/z): Calcd for C19H14Cl2N3O2 [M ? H]? 386.0463, found 386.0465.
Methyl 2-(7-(3,4-dimethylphenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4j
1
mp: 262–265 °C; H NMR (DMSO-d6, 400 MHz): dH 2.31 (s, 3H, CH3), 2.36 (s,
3H, CH3), 3.66 (s, 3H, CH3), 4.50 (s, 2H, CH2), 7.33 (d, J = 7.2 Hz, 1H, ArH),
7.94–7.98 (m, 3H, ArH), 8.07 (s, 1H, ArH), 8.18 (s, 1H, ArH), 8.33 (s, 1H, ArH),
13.73 (s, 1H, NH). IR (KBr, cm-1): m 3161, 3096, 3053, 2949, 1738, 1579, 1506,
1437, 1372, 1331, 1278, 1234, 1197, 1169, 1009, 927, 893, 835, 802, 726. HRMS
(ESI, m/z): Calcd for C21H20N3O2 [M ? H]? 346.1556, found 346.1559.
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1787
Methyl 2-(7-(2-thienyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4k
1
mp: [300 °C; H NMR (DMSO-d6, 400 MHz): dH 3.66 (s, 3H, CH3), 6.77 (s, 2H,
CH2), 7.44 (s, 1H, ArH), 7.68 (s, 1H, ArH), 8.14 (d, J = 9.6 Hz, 2H, ArH), 8.25 (m,
3H, ArH), 13.75 (s, 1H, NH). IR (KBr, cm-1): m 3125, 2923, 1644, 1594, 1541, 1465,
1375, 1337, 1303, 1227, 1187, 1156, 1124, 1083, 1017, 947, 930, 853, 795, 768, 734.
HRMS (ESI, m/z): Calcd for C17H14N3O2S [M ? H]? 324.0827, found 324.0800.
Methyl 2-(7-(4-nitrophenyl)-3H-pyrazolo[4,3-f]quinolin-9-yl)acetate 4l
mp: 269–271 °C; 1H NMR (DMSO-d6, 400 MHz): dH 3.67 (s, 3H, CH3), 4.55 (s, 2H,
CH2), 7.99–8.06 (m, 2H, ArH), 8.37 (s. 2H, ArH), 8.41 (s. 2H, ArH), 8.52–8.56 (m,
2H, ArH), 13.83 (s, 1H, NH). IR (KBr, cm-1): m 3111, 2952, 1737, 1663, 1593, 1575,
1511, 1433, 1389, 1344, 1289, 1166, 1108, 1027, 1009, 941, 892, 851, 793, 733.
HRMS (ESI, m/z): Calcd for C19H15N4O4 [M ? H]? 363.1093, found 363.1094.
Conclusion
In conclusion, we have found a mild and highly regio-selective method for synthesis of
3H-pyrazolo[4,3-f]quinolin-9-yl)acetate derivatives via three-component reaction of an
aromatic aldehyde, 1H-indazol-5-amine, and methyl 3-oxobutanoate, catalyzed by
iodine. The features of this procedure are mild reaction conditions, high yields,
operational simplicity, metal-free catalyst, and high regio-selectivity.
Acknowledgments We are grateful to a Project Funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions, Qing Lan Project (10QLD008) and Graduate
Foundation (CXLX12_0984) of Jiangsu Education Committee for financial support.
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