Amberlite IR-120H: An efficient and recyclable heterogeneous catalyst for the synthesis of pyrrolo[1,2-a]quinoxalines and 5′H-spiro[indoline-3,4′-pyrrolo[1,2-a]quinoxalin]-2-ones

Article abstract of DOI:10.1016/j.tetlet.2015.11.003

A simple, highly efficient and environmentally benign method for the synthesis of pyrrolo[1,2-a]quinoxalines has been developed using a green and recyclable catalyst Amberlite IR-120H resin under solvent-free conditions. The method provides several advant

Full text of DOI:10.1016/j.tetlet.2015.11.003

Accepted Manuscript
Amberlite IR-120H: An efficient and recyclable heterogeneous catalyst for the
synthesis of pyrrolo[1,2-a]quinoxalines and 5’H-spiro[indoline-3,4’-pyrro-
lo[1,2-a]quinoxalin]-2-ones
Ahmed Kamal, Korrapati Suresh Babu, Jeshma Kovvuri, Vindravath Manasa,
A. Ravikumar, Abdullah Alarifi
PII:
S0040-4039(15)30311-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.11.003
TETL 46943
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
7 October 2015
28 October 2015
1 November 2015
Please cite this article as: Kamal, A., Babu, K.S., Kovvuri, J., Manasa, V., Ravikumar, A., Alarifi, A., Amberlite
IR-120H: An efficient and recyclable heterogeneous catalyst for the synthesis of pyrrolo[1,2-a]quinoxalines and
5’H-spiro[indoline-3,4’-pyrrolo[1,2-a]quinoxalin]-2-ones, Tetrahedron Letters (2015), doi: http://dx.doi.org/
10.1016/j.tetlet.2015.11.003
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Graphical Abstract
Amberlite IR-120H: An efficient and
recyclable heterogeneous catalyst for the
synthesis of pyrrolo[1,2-a]quinoxalines and
5'H-spiro[indoline-3,4'-pyrrolo[1,2-
a]quinoxalin]-2-ones
Ahmed Kamal, Korrapati Suresh Babu, Jeshma Kovvuri, Vindravath Manasa, A. Ravikumar, Abdullah Alarifi

1
Tetrahedron Letters
Amberlite IR-120H: An efficient and recyclable heterogeneous catalyst for the
synthesis of pyrrolo[1,2-a]quinoxalines and 5'H-spiro[indoline-3,4'-pyrrolo[1,2-
a]quinoxalin]-2-ones
Ahmed Kamal*a,b,c Korrapati Suresh Babu,a Jeshma Kovvuri,a Vindravath Manasa,c A. Ravikumara,c and
Abdullah Alarifib
aMedicinal Chemistry and Pharmacology, CSIR-Indian Institute of Chemical Technology, Hyderabad-500007, India.
bCatalytic Chemistry Chair, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia.
cDepartment of Medicinal Chemistry, National Institute of Pharmaceutical Education & Research (NIPER), Hyderabad 500 037, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A simple, highly efficient and environmentally benign method for the synthesis of pyrrolo[1,2-
a]quinoxalines has been developed using a green and recyclable catalyst Amberlite IR-120H
resin under solvent-free conditions. The method provides several advantages such as mild
conditions, no use of oxidant, environmentally compatible and inexpensive catalyst. Moreover,
the catalyst can be recovered after completion of the reaction and can be reused as the catalytic
property of the resin is not effected even up to five cycles.
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved.
pyrrolo[1,2-a]quinoxaline
Amberlite IR-120H
Solvent free,
Recyclability
In view of their wide range of applications, numerous synthetic
methods have been reported in the literature for the preparation
Introduction
of
4,5-dihydropyrrolo[1,2-a]quinoxalines,9
pyrrolo[1,2-
5'H-spiro[indoline-3,4'-pyrrolo[1,2-
Quinoxalines represent as an important class of biologically
active compounds that are known to possess antimicrobial,1
a]quinoxalines,10
and
anticancer,2
antiviral,3
antidiabetic,4
antifungal5
and
a]quinoxalin]-2-ones.11 Even though some of these methods are
effective, they often suffer from one or more disadvantages such
as long reaction times, use of metal catalyst, use of oxidant, harsh
reaction conditions, unsatisfactory product yields and use of
hazardous organic solvents. To overcome these problems we
have recently developed a convenient and greener approach for
the synthesis of 4,5-dihydropyrrolo[1,2-a]quinoxalines.12
antidepressant activities.6 In these, particularly pyrrolo[1,2-
a]quinoxaline scaffold is of considerable importance in medicinal
chemistry and is present in various biologically and medicinally
useful molecules,7 and act as anticancer (I),8a,b as well as anti-
HIV agents (II),8c antimycobacterial agents,8d (III) cannabinoid
receptor antagonists (IV),8e PARP-1 inhibitors (V)8f and also
important intermediates for the construction of 5-HT3 receptor
agonists (VI) (Fig. 1).8g,h
On the other hand, heterogeneous catalysis has played an
emerging role in various organic transformations. Heterogeneous
solid acids like ion-exchange resins are superior to the
conventional homogeneous acid catalysts due to their low cost,
operational simplicity, low toxicity and environmental
compatibility. Moreover, they can be easily recovered from
reaction mixtures by simple filtration and can be reused with or
without activation, making the process economically viable.
Amberlite-IR 120 acidic resin is one of the best examples of
heterogeneous catalyst that is comprised of polystyrene
divinylbenzene polymer with supported –SO3H functional group
and is an efficient heterogeneous catalyst for various chemical
transformations.13
.
In the course of our interest to develop environmentally benign
synthetic methodes for useful organic transformations,14 we
herein report a convenient and greener protocol for the synthesis
Figure 1. Biologically active pyrrolo[1,2-a]quinoxalines and their
derivatives.
of
a series of 4-phenylpyrrolo[1,2-a]quinoxalines, 4-(1,3-
diphenyl-1H-pyrazol-4-yl)pyrrolo[1,2-a]quinoxalines and 5'H-

2
Tetrahedron
spiro[indoline-3,4'-pyrrolo[1,2-a]quinoxalin]-2-ones using 1-(2-
Table 2. Optimization of temperature and amount of catalyst
aminophenyl)pyrrole, different benzaldehydes, 1,3-diphenyl-1H-
pyrazole-4-carbaldehydes and isatins catalyzed by Amberlite IR-
120H.
required.
Entry
Temperature (°C)
Amount of catalyst (mg)
Yielda (%)
35
1
110
110
110
110
110
110
60
50
Results and discussion
2
58
100
150
200
250
300
250
250
250
250
250
250
250
3
76
4
90
5
92
6
92
Scheme 1. Model Reaction
7
<30
48
Initially,
a model reaction was performed using 1-(2-
8
70
aminophenyl)pyrrole, 4-nitrobenzaldehyde and Amberlite IR-
120H (200 mg) under solvent and solvent-free conditions at room
temperature (Scheme 1). It was observed that the reaction did not
proceed even until 12 hours (Table1, entry 1-5). These results
indicated that room temperature conditions are not suitable for
this transformation with Amberlite IR-120H. The same reaction
was carried out under refluxing conditions, using various
solvents such as CH3CN, MeOH, EtOH and water, it was
observed that the 4-(4-nitrophenyl)-4,5-dihydropyrrolo[1,2-
a]quinoxaline (3) was obtained in yields 42%, 45%, 48% and
60% respectively along with the oxidized form of quinoxaline 4a
(in 22%, 28%, 30% and 34% yield as shown in Table 1, entries
6-9). Later the reaction was performed under solvent-free
conditions at 110°C for 2 hours, products 3 and 4a were obtained
in 45% and 48% yields respectively (Table 1, entry 10). When
the reaction was continued for another 2 hours, the dihydro
product is completely oxidized to give 90% of 4a (Table 1, entry
11).
9
80
70
10
11
12
13
90
85
100
110
120
92
92
92
aisolated yields
To explore the scope and generality of the present protocol, the
reaction was performed with different substituents on the
benzaldehyde (Scheme 2) and the results are summarized in
Table 3. From the results it can be concluded that the presence of
electron withdrawing groups on the aldehyde, provided excellent
yields of the corresponding products (Table 3, entries 4a–4c).
While, the presence of electron donating groups on the aldehyde,
such as methoxy (Table 3, entries 4e and 4f), the corresponding
products were obtained in lower yields. Whereas, the presence of
both electron withdrawing and electron donating groups on the
aldehyde also lowered the yields marginally (Table 3, entry 4d).
Table 1. Optimization of reaction conditions for the synthesis
of 3 and 4a
Entry
Solvent
Temperature (°C)
Time (h)
3a
--
4a
--
Scheme 2. Amberlite IR-120H catalysed synthesis of 4-
phenylpyrrolo[1,2-a]quinoxaline derivatives.
1
2
--
RT
RT
RT
RT
RT
80
12
12
12
12
12
4
CH3CN
--
--
3
--
--
Methanol
Ethanol
Water
4
--
--
5
--
--
6
CH3CN
42
45
48
60
45
--
22
28
30
34
48
90
Table 3. Amberlite IR-120H catalysed synthesis of 4-
phenylpyrrolo[1,2-a]quinoxaline derivatives.
7
80
4
Methanol
Ethanol
Water
--
8
80
4
Entry
R
Yielda (%)
92
9
100
110
110
6
4-NO2
4a
10
2
4-CN
89
85
78
70
65
4b
4c
11
--
4
2-Cl,5-NO2
2-OMe,4-NO2
3,4-DiOMe
3,4,5-Tri OMe
aisolated yields
ball the reations carried out with Amberlite IR-120H (200 mg).
4d
4e
After obtaining the desired product in excellent yields, the
amount of catalyst and the temperature required for this reaction
were examined (Table 2). It was observed that upon increasing
the amount of Amberlite IR-120 from 50 mg to 100, 150, 200,
250 and 300 mg, the yields enhanced from 35% to 58%, 76%,
90%, 92% and 92%, respectively (Table 2, entries 1–6). Later the
reaction was conducted at different temperatures; it was observed
that upon increasing the temperatures from 60 to 70, 80, 90, 100,
110 and 120 °C the yields also enhanced from <30 % to 48%,
70%, 85%, 92%, 92% and 92% respectively (Table 2, entries 7–
13). Therefore, it could be concluded that 250 mg of Amberlite
IR-120 at 100 °C for 4 hours are the optimum conditions for this
transformation (Table 2, entry 11).
4f
aisolated yields
Based on the interesting results obtained for the synthesis of 4-
phenylpyrrolo[1,2-a]quinoxaline derivatives (4a-4f), it was
considered to also perform the reaction using different 1,3-diaryl
pyrazole carboxaldehydes (Scheme 3). In this case, when the
model reaction (Scheme 3) was conducted under the above
optimized conditions, it was slower as compared to the aryl
aldehydes. Therefore this reaction was performed under different
reaction conditions and the optimized condition for this reaction
requires 6 hours duration at 120°C (Table 4, entry 6). Further to

3
explore the scope, the reaction was performed using four
different 1,3-diaryl pyrazole carboxaldehydes (Scheme 4) and the
results are summarized in Table 5.
withdrawing as well as electron-donating substituents under
these optimized reaction conditions (Scheme 6). As evident from
the results summarized in Table 7, isatin containing electron
donating substituents (Table 7, entries 8d and 8e) afforded
products in slightly lower yields than isatin containing electron
withdrawing substituents (Table 7, entries 8b, 8c and 8f).
Similarly, excellent yields of products were observed in the
absence of substituents on isatin (Table 7, entry 8a).
Scheme 3. Model Reaction for synthesis of 4-(1,3-diphenyl-1H-
pyrazol-4-yl)pyrrolo[1,2-a]quinoxaline.
Scheme 5. Model Reaction for synthesis of 5'H-
spiro[indoline-3,4'-pyrrolo[1,2-a]quinoxalin]-2-one
derivatives.
Table 4. Optimization of reaction conditions for the synthesis
of
4-(1,3-diphenyl-1H-pyrazol-4-yl)pyrrolo[1,2-
a]quinoxaline.
Entry
Temperature (°C)
Time (in h)
6a
1
100
100
110
110
120
120
120
130
4
6
4
6
4
6
8
8
40
48
48
54
62
72
72
72
Table 6. Optimization of reaction conditions for the synthesis
2
of
5'H-spiro[indoline-3,4'-pyrrolo[1,2-a]quinoxalin]-2-one
derivatives.
3
Entry
Solvent
Temperature (°C)
Time (h)
8aa
4
1
2
3
4
5
6
7
--
--
100
110
120
80
6
6
35
42
45
65
77
82
96
5
6
--
6
7
CH3CN
Methanol
Ethanol
Water
4
8
aisolated yields
80
4
80
4
Scheme 4. Amberlite IR-120H catalysed synthesis of 4-(1,3-
diphenyl-1H-pyrazol-4-yl)pyrrolo[1,2-a]quinoxaline derivatives.
100
2.5
aisolated yields
Scheme 6. Amberlite IR-120H catalysed synthesis of 5'H-
spiro[indoline-3,4'-pyrrolo[1,2-a]quinoxalin]-2-ones.
Table 5. Amberlite IR-120H catalysed synthesis of 4-(1,3-
diphenyl-1H-pyrazol-4-yl)pyrrolo[1,2-a]quinoxalines.
Entry
R1
Yielda (%)
72
H
6a
Table 7. Amberlite IR-120H catalysed synthesis of 5'H-
spiro[indoline-3,4'-pyrrolo[1,2-a]quinoxalin]-2-ones.
4-F
70
70
65
6b
Entry
X
Yielda (%)
96
4-Cl
6c
H
8a
4-OMe
6d
F
95
95
90
88
96
8b
aisolated yields
Cl
8c
Me
OMe
NO2
8d
After the synthesis of 4-phenylpyrrolo[1,2-a]quinoxalines and 4-
(1,3-diphenyl-1H-pyrazol-4-yl)pyrrolo[1,2-a]quinoxalines we
8e
turned our attention towards the synthesis of 5'H- spiro[indoline-
3,4'-pyrrolo[1,2-a]quinoxalin]-2-one derivatives by reacting
different isatin substituents with 1-(2-aminophenyl)pyrrole. In
this regard, when the model reaction was conducted under
solvent free conditions (Scheme 5), the yield of the product was
low (Table 6, entries 1-3). When the same reaction was
performed in the presence of different solvents under refluxing
conditions, excellent yields of the product were obtained (Table
6, entries 4-7). Notably, water was found to be a suitable solvent
providing 96% of the product (Table 6, entry 7). The reaction
was performed using isatins possessing both electron-
8f
aisolated yields
The plausible mechanism of Amberlite IR-120H catalyzed 4-
phenylpyrrolo[1,2-a]quinoxaline formation is depicted in Figure
2. The reaction is initiated with the formation of imine which gets
protonated in the presence of Amberlite IR-120H and is readily
attacked by the electron rich 2nd position of pyrrole to afford the
dihydropyrrolo[1,2-a]quinoxaline, which is oxidised in air to
give the desired product. This is evident that, when the reaction
was carried out under nitrogen (absence of oxygen), the

4
Tetrahedron
dihydropyrrolo[1,2-a]quinoxaline (3) was formed exclusively.
monitored by TLC. After completion of the reaction, the mixture
was cooled to room temperature, water and ethyl acetate were
added and stirred for a while. The organic layer was separated
and dried over anh. Na2SO4, and concentrated under reduced
pressure. The crude solid obtained was recrystallized using
ethanol or by short column chromatography on silica gel using
ethyl acetate–petroleum ether as the eluent. The catalyst was
quantitatively recovered by washing with water (3 times) and
dried in oven then it was reused for another cycle.
This indicates the necessity of air which serves as a source of
oxygen.
General procedure for the synthesis of 5'H-spiro[indoline-
3,4'-pyrrolo[1,2-a]quinoxalin]-2-ones (8a-8f)
In a 25 mL round bottom flask 1-(2-aminophenyl)pyrrole (1
mmol), corresponding isatin (1 mmol) and Amberlite IR-120H
(250 mg) in H2O were refluxed. The progress of the reaction was
monitored by TLC. After completion of the reaction, the reaction
mixture was cooled and ice-cold water was added and stirred for
a while. The solid product obtained along with catalyst was
filtered and washed with water. The crude product was extracted
using chloroform and was purified by crystallisation using
ethanol to afford the products in good yields.
Figure 2. Plausible mechanism for the formation of 4-
phenylpyrrolo[1,2-a]quinoxaline
The recyclability of the Amberlite IR-120H is one of the
important advantages of this protocol that makes it useful for
commercial applications. Thus in this process the catalyst used
was recovered, after completion of the reaction (model reaction)
as mentioned in the experimental procedure and the catalyst was
reused for the same reaction. The use of catalyst for the first time
gave a yield of 92% and when the same catalyst was repeatedly
re-used upto five times under similar conditions, the yields
obtained were 91%, 90%, 88%, 87% and 85% respectively
(Table 8, entries 1-6). The catalyst showed excellent recyclability
in all these reactions wherein the reaction times and yields
remained almost same without any loss of catalytic activity.
Acknowledgements
K.S.B. thanks CSIR, New Delhi for the award of senior research
fellowship and J.K thanks DST for the award of fellowship under
DST-Inspire Scheme. We acknowledge funding received from
the project entitled “Affordable Cancer Therapeutics (ACT)”
(CSC0301) under XIIth five year plan. This project was also
supported by King Saud University, Deanship of Scientific
Research, Research Chair.
Table 8. Recycling and reuse of Amberlite IR-120H.
Entry
Reaction cycle
First (fresh run)
Second cycle
Third cycle
Yielda (%)
1
2
3
4
5
6
92
91
90
88
87
85
References and notes
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aisolated yields
Conclusion
In conclusion, we have developed a mild, efficient and
environmentally benign synthetic method for the synthesis of
pyrrolo[1,2-a]quinoxalines
and
5'H-spiro[indoline-3,4'-
pyrrolo[1,2-a]quinoxalin]-2-ones using Amberlite IR-120H resin,
a commercially available green and recyclable catalyst. The
advantages of this method over previous reports include its
simplicity of operation, mild reaction conditions and no usage of
external oxidant. Moreover, the resin is inexpensive, stable,
noncorrosive and easy to handle. The reusability of the resin
makes it economically viable thereby reducing the waste. The
acidic ion-exchange-resin catalyst could be easily separated and
directly reused for five times without any reactivation, while
showing no significant loss of activity.
8.
General procedure for the synthesis of 4-phenylpyrrolo[1,2-
a]quinoxaline derivatives (4a-4f) and 4-(1,3-diphenyl-1H-
pyrazol-4-yl)pyrrolo[1,2-a]quinoxaline derivatives (6a-6d)
9.
In a 25 mL round bottom flask 1-(2-aminophenyl)pyrrole (1
mmol), corresponding aldehyde (1 mmol) and Amberlite IR-
120H (250 mg) were heated. The progress of the reaction was

5
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