Water-mediated selective synthesis of pyrazolo[1,5-a ]quinazolin-5(4 H)-ones and [1,2,4]triazolo[1,5-a ]quinazolin-5(4 H)-one via copper-catalyzed cascade reactions

Article abstract of DOI:10.1080/00397911.2015.1083032

A convenient and sustainable synthesis of pyrazolo[1,5-a]quinazolin-5(4H)-ones and [1,2,4]triazolo[1,5-a]quinazolin-5(4H)-one through copper-catalyzed cascade reactions of 2-bromobenzoates with 1H-pyrazol-5-amines or 1H-1,2,4-triazol-5-amine under ligand-

Full text of DOI:10.1080/00397911.2015.1083032

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Water-Mediated Selective Synthesis of
Pyrazolo[1,5-a]quinazolin-5(4H)-ones and
[1,2,4]Triazolo[1,5-a]quinazolin-5(4H)-one via
Copper-Catalyzed Cascade Reactions
Xinying Zhang, Lin Gao, Zhangxin Wang & Xuesen Fan
To cite this article: Xinying Zhang, Lin Gao, Zhangxin Wang & Xuesen Fan (2015): Water-
Mediated Selective Synthesis of Pyrazolo[1,5-a]quinazolin-5(4H)-ones and [1,2,4]Triazolo[1,5-
a]quinazolin-5(4H)-one via Copper-Catalyzed Cascade Reactions, Synthetic Communications
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Date: 17 September 2015, At: 20:35

Water-Mediated Selective Synthesis of Pyrazolo[1,5-a]quinazolin-5(4H)-ones and
1,2,4]Triazolo[1,5-a]quinazolin-5(4H)-one via Copper-Catalyzed Cascade Reactions
[
1
1
1
1
Xinying Zhang , Lin Gao , Zhangxin Wang , Xuesen Fan
1
School of Chemistry and Chemical Engineering, Collaborative Innovation Center of
Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green
Chemical Media and Reactions, Ministry of Education, Henan Key Laboratory for
Environmental Pollution Control, Henan Normal University, Xinxiang, Henan, P. R.
China
Correspondence authors: E-mails: xinyingzhang@htu.cn (Xinying Zhang),
xuesen.fan@htu.cn (Xuesen Fan)
Abstract
A convenient and sustainable synthesis of pyrazolo[1,5-a]quinazolin-5(4H)-ones and
1,2,4]triazolo[1,5-a]quinazolin-5(4H)-one through copper-catalyzed cascade reactions of
-bromobenzoates with 1H-pyrazol-5-amines or 1H-1,2,4-triazol-5-amine under
[
2
ligand-free conditions in water is presented. It is notable that aqueous medium turned out
to be crucial for the chemoselective formation of the title compounds. Compared with
literature protocols, this new method showed advantages such as simple and sustainable
procedure, commercially available starting materials, and convenient reuse of the reaction
medium together with the copper catalyst.
GRAPHICAL ABSTRACT
1

KEYWORDS: Pyrazolo[1,5-a]quinazolinone; [1,2,4]triazolo[1,5-a]quinazolinone;
Copper catalysis; Cascade reaction
INTRODUCTION
Pyrazoloquinazolinone is a privileged scaffold frequently found in compounds possessing
remarkable pharmacological activities including anti-allergic, anti-parasitic and
[
1-3]
anti-inflammatory.
In addition, some pyrazoloquinazolinone derivatives have been
[
4]
developed as potential cytokines TNF-α and IL-1β modulator, polymerase-1 inhibitor,
[
5]
and topoisomerase I inhibitor. Owing to their importance, a number of methods have
been developed for the preparation of pyrazoloquinazolinones. For example, Taliani
reported that this class of compounds could be obtained via the reaction of 5H-pyrazolo
[
5]
[
1,5-a][3,1]benzoxazin-5-one with amines. In another approach,
pyrazoloquinazolinones were synthesized through condensing α-cyanoketones with
[
2,6]
2
-hydrazinobenzoic acids.
pyrazoloquinazolinone and its aza-analogues were obtained via the reaction of
-halobenzamides with different N-heterocycles through a Cu(I)-catalyzed C-N
More recently, Bao developed an elegant strategy in which
2
[
7]
coupling/C-H activation/C-N formation cascade process. While these literature
protocols are generally efficient and reliable, some of them still suffer from delicate
substrates, tedious procedures and harsh reaction conditions. Therefore, new synthetic
methods starting from readily available substrates and realized in a more environmentally
sustainable manner are highly desirable.
In recent years, a rapidly growing interest has been devoted to water as an alternative
2

[
8]
solvent for organic reactions. It has been well demonstrated that a wide range of
reactions can be realized efficiently in water. In addition, industry is increasingly
adopting water as a solvent. The strong motive behind this trend is mostly attributed to
the urgent desire for “greener” and “cleaner” chemistry. In addition to being safe, cheap,
and environmentally sustainable, water as solvent also offers better selectivity, faster
reaction rates, and higher yields in numerous cases.
Compared with noble metal catalysts, copper compounds are cheaper and easier to handle.
In recent years, copper-catalyzed coupling between aryl halides and nitrogen
[
9, 10]
nucleophiles have been widely utilized in the preparation of N-heterocycles.
regard, we have disclosed a straightforward synthetic pathway toward
pyrazolo[1,5-a]quinazolines via copper-catalyzed cascade reactions of
In this
[
11]
2
-bromobenzaldehydes with aminopyrazoles. Following this success, we presumed
that under similar conditions, 2-bromobenzoate would react with aminopyrazole to give
pyrazolo[1,5-a]quinazolin-5(4H)-one via an amide formation and C-N coupling cascade
process. Experimental studies showed that the reaction pattern of 2-bromobenzoate was,
however, more complicated than that of 2-bromobenzaldehyde and it was largely
dependent on the nature of the reaction medium. Among various reaction medium studied,
water was found to be the most suitable for the formation of
pyrazolo[1,5-a]quinazolin-5(4H)-ones from the reaction of 2-bromobenzoates with
aminopyrazoles.
3

RESULTS AND DISCUSSION
Initially, methyl 2-bromobenzoate (1a) was treated with 1H-pyrazol-5-amine (2a) in the
[
11]
2 3
presence of K CO , CuI, and en in DMF at 110 °C for 5 h. From this reaction, the
desired pyrazolo[1,5-a]quinazolin-5(4H)-one (3a) was obtained but in low yield (8%,
Table 1, entry 1). Along with 3a, the unexpected 5-methoxypyrazolo[1,5-a]quinazoline
(
4), a by-product, was obtained in a yield of 10% (entry 1). In order to improve the
reaction in terms of efficiency and chemo-selectivity, L-proline or 1,10-phen was tried as
an additive. However, they were found to be no more effective than en (entries 2 and 3).
2 3 2 3
When Cs CO was used to replace K CO as the basic promoter, the yields of 3a and 4
increased obviously. However, addition of en, L-proline, or 1,10-phen did not give better
yields (entries 4-7). Next, switching the solvent from DMF to DMSO, THF, or i-PrOH
did not give satisfying results (entries 8-10). Gratifyingly, when a mixed solvent of DMF
and water was used, the yield of 3a increased along with the growing ratio of water
(entries 11-14). When only water was used as the reaction medium, 3a was obtained in a
yield of 61% while 4 was formed only in a trace amount (entry 15). Under a nitrogen
atmosphere, the yield of 3a could be further improved to 70% (entry 16). Further attempt
to improve the reaction by using en, L-proline, 1,10-phen, DMEDA, or TMEDA as an
additive (entries 17-21), and CuBr or Cu(OAc)
2
as a catalyst turned out to be not
productive (entries 22-23). In summary of the optimization study, treating 1a and 2a with
0
.2 equiv of CuI, 2 equiv of Cs
2
CO
3
2
in water under N at 100 ºC for 6 h afforded 3a in a
yield of 70%.
4

With the optimized conditions in hand, the scope and generality of this novel synthesis of
pyrazolo[1,5-a]quinazolin-5(4H)-one was studied. As a first aspect, 2-bromobenzoates
(
1) with different substituents on the phenyl ring were reacted with 5-aminopyrazole (2a).
1
The results listed in Table 2 showed that the R unit of 1 can be hydrogen (entry 1),
chloro (entry 2), the electron-donating methoxy (entry 3), or the electron-withdrawing
nitro group (entries 4-5), without showing obvious electronic or steric effects.
Furthermore, diversely substituted aminopyrazoles (2) were tested for this reaction. The
results listed in Table 3 showed that methyl, cyclopropyl, phenyl, or 2-thienyl substituted
aminopyrazoles reacted with 1a smoothly to afford 3f-3i with reasonable efficiency
(Table 3, entries 1-4). To our delight, 4-cyanoaminopyrazole, an aminopyrazole with a
strong electron-withdrawing substituent on the pyrazole ring, could also take part in this
reaction albeit the yield was lower (entry 5). In following study, various
2
-bromobenzoates 1 bearing different functional groups on the phenyl ring were allowed
to react with different aminopyrazoles 2. It turned out that all the substrates studied were
compatible for this cascade reaction to give 3k-3t in moderate yields (entries 6-15).
Having established a simple and general method for the preparation of
pyrazolo[1,5-a]quinazolines, we were then interested in using this strategy to prepare
another kind of fused N-heterocycle, [1,2,4]triazolo[1,5-a]quinazolin-5(4H)-one, which
[
12]
has been proved to be biologically significant. Thus, methyl 2-bromobenzoate (1a)
was treated with 1H-1,2,4-triazol-5-amine (5) under the optimum reaction conditions for
the preparation of 3a (Table 1, entry 16). We were pleased to found that the expected
[1,2,4]triazolo[1,5-a]quinazolin-5(4H)-one (6) could be successfully obtained from this
reaction in a yield of 51% (Scheme 1).
5

Finally, it is notable that the reaction medium together with the catalyst used for the
preparation of pyrazoloquinazolinone could be conveniently recovered and efficiently
reused. Thus, upon completion of the reaction of 1a and 2a under the catalysis of CuI in
water, the resulting mixture was extracted with ethyl acetate to collect product 3a and
remove possible side-products. The remaining aqueous phase containing the copper
catalyst was directly reused. It turned out that the reaction of 1a and 2a using this
recycled catalytic system afforded 3a in a yield of 63%.
Based on the above results, the mechanism for the formation of 3a is proposed in Scheme
2
. The initial nucleophilic addition of 2a onto 1a gives intermediate A. Elimination of
O from A would give intermediate B, which then undergoes an intramolecular C-N
H
2
coupling under the catalysis of CuI to afford 4 (pathway I). Alternatively, elimination of
MeOH from A would give rise to intermediate C, from which 3a is formed (pathway II).
When the reaction is run in DMF, the two pathways are offered with similar opportunity
and the selectivity is only marginal. When it is run in water, on the other hand,
elimination of MeOH is preferred to that of water as it is suppressed by the presence of
2
an excess amount of H O.
The plausible mechanism shown in Scheme 2 was supported in part by the following
o
control experiments. When 1a was treated with 2a in H
Cs CO
1%. Addition of 0.2 equiv of CuI and stirring of the resulting mixture for 2 h led to the
formation of 3a in a total yield of 65%.
2
O at 100 C in the presence of
2
3
for 4 h, it afforded 2-bromo-N-(1H-pyrazol-5-yl)benzamide (C) in a yield of
7
6

EXPERIMENTAL
All the starting materials were commercially available reagents and used without further
purification. Melting points were recorded with a micro melting point apparatus and
1
13
uncorrected. H and C NMR spectra were recorded at 400 and 100 MHz, respectively.
Chemical shifts were reported in ppm from tetramethylsilane (TMS) as internal standard
in DMSO-d
triplet), m (multiplet), dd (doublet of doublets), td (triplet of doublets), br s (broad
singlet), etc., and coupling constants were given in Hz. High-resolution mass spectra
HRMS) were obtained via ESI mode by using a MicrOTOF mass spectrometer. All
reactions were monitored by thin-layer chromatography (TLC) using silica gel plates
silica gel 60 F254 0.25 mm).
6
solution. Multiplicity was indicated as follows: s (singlet), d (doublet), t
(
(
(
Typical Procedure For The Preparation Of Pyrazolo[1,5-A]Quinazolin-5(4H)-One
3a)
To a solution of Cs
1a, 1 mmol), 1H-pyrazol-5-amine (2a, 1.2 mmol), and CuI (0.2 mmol). The mixture was
(
2
3 2
CO (2 mmol) in H O (5 mL) were added methyl 2-bromobenzoate
(
stirred at 100 °C under nitrogen until a complete conversion as indicated by TLC. The
mixture was then cooled to room temperature and extracted with ethyl acetate. The
2 4
combined organic phase was dried over anhydrous Na SO , and concentrated under
vacuum. The crude product was purified by column chromatography eluting with
petroleum ether/ethyl acetate (3:1) to give 3a. Products 3b-3t and 6 were obtained in a
similar manner.
7

Pyrazolo[1,5-A]Quinazolin-5(4H)-One (3a):
1
white solid, mp 284-286 °C; H NMR (400 MHz, DMSO-d
6
) δ: 5.89 (d, J
7
1
.46 (t, J = 7.2 Hz, 1H), 7.77 (d, J = 2.0 Hz, 1H), 7.84-7.88 (m, 1H), 8.06 (d, J = 8.4 Hz,
13
1 2
H), 8.12 (dd, J = 8.0 Hz, J = 1.2 Hz, 1H), 12.21(s,1H); C NMR (100 MHz,
DMSO-d
6
) δ: 88.9, 114.8, 116.5, 125.7, 128.6, 135.50, 137.8, 139.0, 142.6, 158.9. HRMS
+
calcd for C10
H
8
N
3
O: 186.0667 [M+H] , found: 186.0674.
CONCLUSION
In conclusion, we have developed a simple and sustainable synthesis of diversely
substituted pyrazoloquinazolinones via copper-catalyzed cascade reactions of
2
-bromobenzoates with 5-aminopyrazoles. Using similar strategy,
1,2,4]triazolo[1,5-a]quinazolin-5(4H)-one could be obtained from the reaction of
-bromobenzoate with 1H-1,2,4-triazol-5-amine. Compared with previous methods for
[
2
the preparation of pyrazoloquinazolinones, the protocol reported herein has the
advantages such as commercially available starting materials, simple synthetic
procedures, mild reaction conditions, environmentally benign nature and recyclability of
the reaction medium and catalyst.
FUNDING
We are grateful to the National Natural Science Foundation of China (NSFC) (grant
numbers 21172057, 21272058), Program for Science and Technology Innovation Talents
in Universities of Henan Province (15HASTIT005), Program for Innovative Research
8

Team in Science and Technology in University of Henan Province (15IRTSTHN003),
and PCSIRT (IRT 1061) for financial support.
SUPPLEMENTAL MATERIAL
1
13
Full experimental detail, H and C NMR spectra, and HRMS for this article can be
accessed on the publisher’s website
REFERENCES
1
. (a) Alexander, E. J. US4105764, Appl No 826, 162, 1978; (b) Alexander, E. J.,
US4105766, Appl No 826, 163, 1978.
. Orvieto, F.; Branca, D.; Giomini, C.; Jones, P.; Koch, U.; Ontoria, J. M.; Palumbi, M.
C.; Rowley, M.; Toniatti, C.; Muraglia, E. Bioorg. Med. Chem. Lett. 2009, 19, 4196.
. (a) Rajput, R.; Mishra, A. P. Int. J. Res. Pharm. Bio. Sci. 2012, 3, 82; (b) Rajput, R.;
2
3
Mishra, A. P. Int. J. Pharm. Pharm. Sci. 2012, 4, 66; (c) Selvam, T. P.; Kumar, P. V. Res.
Pharm. 2011, 1, 1.
4
. Cirillo, P. F.; Hickey, E. R.; Regan, J. R.; Zhang, L. H. WO 00/50425 (A1), PCT Int.
Appl. 2000.
. Taliani, S.; Pugliesi, I.; Barresi, E.; Salerno, S.; Marchand, C.; Agama, K.; Simorini, F.;
5
La Motta, C.; Marini, A. M.; Di Leva, F. S.; Marinelli, L.; Cosconati, S.; Novellino, E.;
Pommier, Y.; Di Santo, R.; Da Settimo, F. J. Med. Chem. 2013, 56, 7458.
6
. Vasquez Jr., T. E.; Nixey, T.; Chenera, B.; Gore, V.; Bartberger, M. D.; Sun, Y.;
Hulme, C. Mol. Divers. 2003, 7, 161.
9

7
6
8
. Chen, D.; Chen, Q.; Liu, M.; Dai, S.; Huang, L.; Yang, J.; Bao, W. Tetrahedron 2013,
9, 6461 and references cited therein.
. (a) Fu, X.-P.; Liu, L.; Wang, D.; Chen, Y.-J.; Li, C.-J. Green Chem. 2011, 13, 549; (b)
Wang, K.; Bi, X.-H.; Xing, S.-X.; Liao, P.-Q.; Fang, Z.-X.; Meng, X.-Y.; Zhang, Q.; Liu,
Q.; Ji, Y. Green Chem. 2011, 13, 562; (c) Butler, R. N.; Coyne, A. G. Chem. Rev. 2010,
1
10, 6302.
. For selected reviews, see: (a) Liu, Y.; Wan, J. P. Chem. Asian. J. 2012, 7, 1488; (b) Rao,
H.; Fu, H. Synlett 2011, 745; (c) Zhang, C.; Tang, C.; Jiao, N. Chem. Soc. Rev. 2012, 41,
464; (d) Beletskaya, I. P.; Cheprakov, A.V. Organometallics 2012, 31, 7753; (e) Evano,
9
3
G.; Blanchard, N.; Toumi, M. Chem. Rev. 2008, 108, 3054; (f) Ma, D.; Cai, Q. Acc. Chem.
Res. 2008, 41, 1450.
1
0. For selected examples, see: (a) Besandre, R.; Jaimes, M.; May, J. A. Org. Lett. 2013,
5, 1666; (b) Gogoi, A.; Guin, S.; Rout, S. K.; Patel, B. K. Org. Lett. 2013, 15, 1802; (c)
1
Wang, Z.; Yang, F.; Lv, X.; Bao, W. J. Org. Chem. 2011, 76, 967; (d) Truong, V. L.;
Morrow, M. Tetrahedron Lett. 2010, 51, 758; (e) Lv, Y.; Li, Y.; Xiong, T.; Pu, W.; Zhang,
H.; Sun, K.; Liu, Q.; Zhang, Q. Chem. Commun. 2013, 6439; (f) Yang, X.; Luo, Y.; Jin, Y.;
Liu, Y.; Jiang, Y.; Fu, H. RSC Adv. 2012, 2, 8258; (g) Guo, S.; Wang, J.; Fan, X.; Zhang,
X.; Guo, D. J. Org. Chem. 2013, 78, 3262; (h) Fan, X.; Li, B.; Guo, S.; Wang, Y.; Zhang,
X. Chem. Asian J. 2014, 9, 739; (i) Zhang, X.; Guo, X.; Fan, X. Chem. Asian J. 2015, 10,
1
06; (j) Fan, X.; Zhang, J.; Li, B.; Zhang, X. Chem. Asian J. 2015, 10, 1281; (k) Guo, S.;
Wang, J.; Li, Y.; Fan, X. Tetrahedron 2014, 70, 2383; (l) Li, B.; Guo, S.; Zhang, J.;
Zhang, X.; Fan, X. J. Org. Chem. 2015, 80, 5444; (m) Kong, L.; Zhou, Y.; Huang, H.;
10

Yang, Y.; Liu, Y.; Li, Y. J. Org. Chem. 2015, 80, 1275; (n) Yuan, X.; Xu, X.; Zhou, X.;
Yuan, J. Mai, L.; Li, Y. J. Org. Chem. 2007, 72, 1510.
1
1. Gao, L.; Song, Y.; Zhang, X.; Guo, S.; Fan, X. Tetrahedron Lett. 2014, 55, 4997.
2. Alagarsamy, V.; Giridhar, R.; Yadav, M. R. Bioorg. Med. Chem. Lett. 2005, 15, 1877.
1
11

[a]
Table 1. Optimization study for the synthesis of 3a
[
b]
[c]
Entry Catalyst
Solvent
Base
Ligand
en
T
t (h)
Yield(%)
o
(
C)
3
8
9
a
4
1
2
3
CuI
CuI
CuI
DMF
DMF
DMF
K
2
CO
CO
CO
3
3
3
110
5
5
5
10
11
10
K
2
L-proline 110
K
2
1,10-phe 110
n
12
4
5
6
CuI
CuI
CuI
DMF
DMF
DMF
Cs
2
CO
CO
CO
3
3
3
en
110
5
5
5
33
31
34
35
28
36
Cs
2
L-proline 110
Cs
2
1,10-phe 110
n
7
8
9
CuI
CuI
CuI
DMF
DMSO
THF
Cs
2
CO
CO
CO
3
3
3
-
-
-
110
110
6
6
6
36
20
32
20
Cs
2
Cs
2
reflu
x
trace trace
1
0
CuI
i-PrOH
Cs
2
CO
3
-
reflu
6
12
11
12

x
11
12
13
14
CuI
CuI
CuI
CuI
V
(DMF)
/
Cs
2
CO
CO
CO
CO
3
3
3
3
-
-
-
-
100
6
6
6
6
44
48
56
58
8
V(H2O)=2:1
V
(DMF)
/
Cs
2
100
100
100
8
V(H2O)=1:1
V
(DMF)
/
Cs
2
trace
trace
V(H2O)=1:2
V
(DMF)
/
Cs
2
V(H2O)=1:3
1
1
1
1
1
5
6
7
8
9
CuI
CuI
CuI
CuI
CuI
H
H
H
H
H
2
2
2
2
2
O
O
O
O
O
Cs
2
CO
CO
CO
CO
CO
3
3
3
3
3
-
100
100
100
6
6
6
6
6
61
70
71
65
64
trace
trace
trace
trace
trace
[
[
[
[
d]
d]
d]
d]
Cs
2
-
Cs
2
en
Cs
2
L-proline 100
Cs
2
1,10-phe 100
n
[
[
[
d]
d]
d]
2
2
2
0
1
2
CuI
H
H
H
2
2
2
O
O
O
Cs
2
CO
CO
CO
3
3
3
DMEDA 100
TMEDA 100
6
6
6
62
58
55
trace
trace
trace
CuI
Cs
2
CuBr
Cs
2
-
100
13

[
d]
2
3
Cu(OAc)
H
2
O
Cs
2
CO
3
-
100
6
57
trace
2
[
a]
Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), base (1 mmol), catalyst (0.1
[
b]
mmol), solvent (3 mL). Ligand: ethylenediamine (en), L-proline, 1,10-phenanthroline
1,10-phen), N, N'-dimethylethylenediamine (DMEDA), and N, N, N',
N'-tetramethylethylene diamine (TMEDA). The amount of ligand is 0.2 equiv. Isolated
(
[
c]
[
d]
yield. Under nitrogen atmosphere.
14

[a]
Table 2. Substrate scope for the synthesis of 3 (I)
[b]
Entry
1
1
3
Yield (%)
70
3
a
2
3
4
5
56
62
58
65
3
b
3
c
3
d
3
e
[a]
2 3
Reaction conditions: 1 (1 mmol), 2 (1.2 mmol), CuI (0.2 mmol), Cs CO (2 mmol),
o
[b]
2 2
H O (5 mL), 100 C, N , 6 h. Isolated yield.
15

[a]
Table 3. Substrate scope for the synthesis of 3 (II)
[b]
Entry
1
2
3
Yield (%)
75
1
3
f
2
3
62
61
3
g
3
h
4
54
3
i
5
6
45
62
3
j
3
l
k
7
8
57
64
3
3
m
16

9
1
51
3
3
n
o
0
52
64
11
3
p
1
2
3
63
70
3
q
1
3
r
1
4
5
52
3
s
1
42
3
t
[a]
Reaction conditions: 1 (1 mmol), 2 (1.2 mmol), CuI (0.2 mmol), Cs
2
CO
3
(2 mmol),
o
[b]
2 2
H O (5 mL), 100 C, N , 6 h. Isolated yield.
17

Scheme 1. Synthesis of [1,2,4]triazolo[1,5-a]quinazolin-5(4H)-one (6)
18

Scheme 2. Plausible mechanism for the formation of 3a
19