AlCl3 induced C-N bond formation followed by Pd/C-Cu mediated coupling-cyclization strategy: Synthesis of pyrrolo[2,3-b]quinoxalines as anticancer agents

Article abstract of DOI:10.1016/j.tetlet.2012.08.119

AlCl₃ facilitated C-N bond forming reaction between 2,3-dichloroquinoxaline and anilines affording a convenient method for the preparation of N-aryl substituted 3-chloroquinoxalin-2-amines. A related N-benzyl derivative, however, was prepared via a conventional method. These N-alkyl/aryl substituted 3-chloroquinoxalin-2-amines on coupling with terminal alkynes in toluene under Pd/C-Cu catalysis afforded a range of 1,2-disubstituted pyrrolo[2,3-b]quinoxalines within 3-5 h in good to excellent yields. Some of the compounds synthesized showed promising anti-proliferative properties when tested in vitro against two cancer cell lines. Docking studies indicated that these molecules interact well with human Akt in silico.

Full text of DOI:10.1016/j.tetlet.2012.08.119

Tetrahedron Letters 53 (2012) 6059–6066
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
AlCl3 induced C–N bond formation followed by Pd/C–Cu mediated
coupling–cyclization strategy: synthesis of pyrrolo[2,3-b]quinoxalines as
anticancer agents
Bagineni Prasad a, K. Shiva Kumar a, P. Vijaya Babu a, K. Anusha a, D. Rambabu a, Ajit Kandale a,
G. R. Vanaja b, Arunasree M. Kalle b, Manojit Pal a,
⇑
a Institute of Life Sciences, University of Hyderabad Campus, Gachibowli, Hyderabad 500046, India
b Department of Animal Sciences, School of Life Sciences, University of Hyderabad, Hyderabad 500046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2012
Revised 27 August 2012
Accepted 28 August 2012
Available online 1 September 2012
AlCl3 facilitated C–N bond forming reaction between 2,3-dichloroquinoxaline and anilines affording a
convenient method for the preparation of N-aryl substituted 3-chloroquinoxalin-2-amines. A related
N-benzyl derivative, however, was prepared via a conventional method. These N-alkyl/aryl substituted
3-chloroquinoxalin-2-amines on coupling with terminal alkynes in toluene under Pd/C–Cu catalysis
afforded a range of 1,2-disubstituted pyrrolo[2,3-b]quinoxalines within 3–5 h in good to excellent yields.
Some of the compounds synthesized showed promising anti-proliferative properties when tested in vitro
against two cancer cell lines. Docking studies indicated that these molecules interact well with human
Akt in silico.
Keywords:
2,3-Dichloroquinoxaline
Pyrrolo[2,3-b]quinoxaline
AlCl3
Ó 2012 Elsevier Ltd. All rights reserved.
Palladium
While a broad spectrum of biological activities is known for
quinoxaline and its derivatives1 only limited pharmacological
properties have been documented for the pyrroloquinoxaline class
of compounds. These include the study of pyrrolo[1,2-a]quinoxa-
lines as potent and selective 5-HT3 receptor ligands2 and inhibitors
of Akt kinase.3 Due to their key role in tumor cell survival/prolifer-
ation and their over expression/activation in many human cancers
Akt, serine/threonine protein kinase [also known as protein kinase
B (PKB)] represents an attractive and potential target for therapeu-
tic intervention. Thus, pyrrolo[1,2-a]quinoxaline (A, Fig. 1) based
compounds were tested for their in vitro ability to inhibit the
proliferation of the human leukemic cell lines K562, U937, and
HL60, and the breast cancer cell line MCF7.3a Notably, three of
these human cell lines (K562, U937, and MCF7) exhibited an active
phosphorylated Akt form. A follow up study focusing on the SAR of
new pyrrolo[1,2-a]quinoxalines indicated the importance of sub-
stitution at the C-4 position of the pyrroloquinoxaline ring and
the need for a functionalization on the pyrrole ring.3b All these
observations and our continuing interest on quinoxaline deriva-
tives4 prompted us to examine the anti cancer properties of a series
of compounds based on regioisomeric pyrrolo[2,3-b]quinoxaline
scaffold (C, Fig. 1). The structure C was reached from A via B by
(i) dissecting the C–N bond of the 5-membered ring of A and
connecting the carbon end to the C-4 to create a new pyrrole ring
(ii) further functionalization of this newly created pyrrole ring.
Overall our design was aimed toward the linear molecular shape
of pyrroloquinoxaline as derivatives possessing this geometry,
which were reported to be of potential pharmacological interest
earlier.1f Herein we report our preliminary results on in vitro phar-
macological evaluation of 1,2-disubstituted pyrrolo[2,3-b]quinoxa-
lines as potential anti cancer agents.
The synthesis of pyrrolo[2,3-b]quinoxalines has previously been
reported by the reaction of 2-alkynyl-3-trifluoroacetamidoquinox-
alines with aryl and vinyl halides or triflates.5 The methodology
however required protecting and deprotecting steps to synthesize
the necessary alkynylaminoquinoxalines. A more straightforward
method was reported in 2010 that involved the reaction of N-al-
kyl-3-chloroquinoxaline-2-amine with terminal alkynes in the
presence of PdCl2–PPh3–CuI as a catalyst system, K2CO3, and a
surfactant, for example, lauryl sulfate in water.6 In addition to
N
N
N
N
N
4
N
3
N
H
2
C
B
1
A
⇑
Corresponding author. Tel.: +91 40 6657 1500; fax: +91 40 6657 1581.
E-mail address: manojitpal@rediffmail.com (M. Pal).
Figure 1. Design of pyrrolo[2,3-b]quinoxaline scaffold C from the regioisomeric
pyrrolo[1,2-a]quinoxaline template A.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.119

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B. Prasad et al. / Tetrahedron Letters 53 (2012) 6059–6066
Pd/C, PPh3
CuI, Et3N
N
Cl
N
EtOH
N
N
Cl
4
PhCH2NH2
+
R
+
R
reflux
N
N
H
Ph
N
R'
N
3
5h, 88%
NHR'
Toluene
1f
95-100 0
3-5 h
C
2
1
Scheme 2. Preparation of N-benzyl-3-chloroquinoxalin-2-amine (1f).
Scheme
1. Pd/C-mediated
synthesis
of
1,2-disubstituted
pyrrolo[2,3-
b]quinoxalines.
Table 2
Effect of reaction conditions on Pd/C-mediated coupling of 1a with 2aa
Pd/C, PPh3
CuI, Et3N
N
N
Table 1
N
N
Cl
AlCl3 mediated synthesis of N-aryl substituted 3-chloroquinoxalin-2-amine (1)a
Ph
Ph
+
N
AlCl3
Solvent
Heat
N
H
N
N
Cl
N
N
Cl
Cl
ClCH2CH2Cl
+
R'NH2
1a
2a
80 0
C
3a
NHR'
10-12h
4
5
1
Entry
Pd-catalyst
Solvent
Timeb (h)
Yieldc (%)
Entry
1
N-Aryl-3-chloroquinoxalin-2-amine (1)
Time (h)
Yieldb (%)
1
2
3
4
5
6
10% Pd/C–PPh3
10% Pd/C–PPh3
10% Pd/C–PPh3
10% Pd/C–PPh3
10% Pd/C–PPh3
10% Pd/C–PPh3
Toluene
Toluene
Toluene
Ethanol
Methanol
CH3CN
3
6
3
6
6
6
92
82
N
N
Cl
d
40
60e
60e
65e
10
75
N
H
1a
a
All reactions were carried out by using 1a (1.0 mmol), 2a (1.8 mmol), 10% Pd/C
(0.028 mmol), PPh3 (0.15 mmol), CuI (0.052 mmol), Et3N (2.5 mmol) in a solvent
(5 mL) at 95–100 °C under N2.
N
N
Cl
OCH3
2
3
4
11
12
10
72
70
72
N
H
b
After adding 2a.
Isolated yields.
The reaction was carried out without the CuI.
The reaction was carried out at refluxing temp.
c
1b
d
N
N
Cl
F
e
N
H
1c
lines. More importantly, the methodology was expected to provide
us access to our targeted library of molecules based on C (Fig. 1).
The Pd/C being an inexpensive, easily separable, and recyclable cat-
alyst was the obvious choice10 for the development of our required
methodology. Indeed, we observed that Pd/C–Cu mediated cou-
pling of N-alkyl/aryl substituted 3-chloroquinoxalin-2-amine (1)
with terminal alkynes (2) in toluene afforded 1,2-disubstituted
pyrrolo[2,3-b]quinoxalines (3) within 3–5 h (Scheme 1).
N
N
Cl
N
H
1d
N
N
Cl
5
N
H
10
55c
OH
The preparation of key starting material, for example, N-aryl
substituted 3-chloroquinoxalin-2-amine (1) required for our study
was an initial challenge as unlike aliphatic amines6 the nucleo-
philic substitution of 2,3-dichloroquinoxaline (4) with aromatic
amines (5) did not proceed well. While the reaction proceeded in
the presence of a base, for example, Et3N a mixture of products,
that is, compound 1 along with the corresponding N2,N3-diarylqui-
noxaline-2,3-diamines were isolated in this case. Finally, the
compound 1a–e was prepared in acceptable yield via the reaction
of 4 with 5 in the presence of AlCl3 in 1,2-dichloroethane (Table 1).
While, the present strategy of AlCl3 mediated C–N bond forming
reaction was not known earlier the methodology however did
not work when an aliphatic amine was used perhaps due to its
complexation with AlCl3. Thus, N-benzyl-3-chloroquinoxalin-2-
amine (1f) was prepared via the reaction of 4 with benzyl amine
in EtOH (Scheme 2).6 The use of 2,3-dibromoquinoxaline prepared
according to the reported method5 was also explored in our pres-
ent strategy. While the reaction proceeded well when benzyl
amine was used (in EtOH under refluxing condition for 5 h to give
the corresponding N-benzyl-3-bromoquinoxalin-2-amine in 69%
yield), it afforded a complex mixture of products when 4-methy-
laniline was used in the presence of AlCl3 under the conditions
presented in Table 1.
1e
a
All reactions were carried out using 2,3-dichloroquinoxaline (1, 1.0 mmol), an
appropriate amine (2, 1.0 mmol) and AlCl3 (1.1 mmol) in 1,2-dichloroethane (5 mL)
at 80 °C under nitrogen.
b
Isolated yields.
c
Formation of a side product, for example, 3-chloro-N-(2-(3-chloroquinoxalin-2-
yloxy)phenyl)quinoxalin-2-amine was observed.
the requirement of longer reaction time (20 h) the study was lim-
ited to the use of N-alkyl-3-chloroquinoxaline-2-amine only and
no corresponding N-aryl derivatives were examined. A subsequent
study on the use of propargyl bromide as an alkyne coupling part-
ner in the presence of (PPh3)2PdCl2, CuI, and aqueous morpholine
was also limited to the use of N-alkyl derivatives.7 Additionally,
the alkynes employed in both the cases lacked variations. Very
recently, inspired by our success of using Pd/C–CuI as a catalytic
system for the efficient Sonogashira coupling8 a Pd/C-catalyzed,
multicomponent reaction of 1,2-dichloroquinoxaline with hydra-
zine hydrate, phenyl acetylene, and a variety of aldehydes have
been reported to afford N-substituted pyrrolo[2,3-b]quinoxalines
in water.9 While the methodology appeared to be interesting the
study once again was limited to the use of phenyl acetylene. It
was therefore necessary to develop a more versatile, faster, and
inexpensive approach for the synthesis of pyrrolo[2,3-b]quinoxa-
Having prepared the required starting materials we then chose
to examine the coupling of compound 1a with phenyl acetylene
(2a) in the presence of 10% Pd/C–PPh3–CuI and Et3N in a range of

B. Prasad et al. / Tetrahedron Letters 53 (2012) 6059–6066
6061
Table 3
Pd/C–Cu mediated synthesis of 1,2-disubstituted pyrrolo[2,3-b]quinoxalines (3) (Scheme 1)a
Entry
Chloro compound (1)
Alkyne; R= (2)
Product (3)
Timeb (h)
Yieldc (%)
N
N
N
–Ph
2a
1
1a
3
92
82
79
75
3a
N
N
N
N
OH
–CMe2OH
2b
2
3
4
1a
1a
1a
3.2
3b
N
N
OH
–CH2CH2OH
2c
4
3c
HO
N
N
HO
N
2d
5
3d
CN
N
N
N
–(CH2)3CN
2e
5
1a
4.2
90
3e
N
N
N
–(CH2)4CH3
6
1a
3
92
2f
3f
N
N
N
–C(CH3)3
7
1a
4.5
83
2g
3g
N
N
N
–C6H4CH3-p
8
1a
3.5
89
2h
3h
(continued on next page)

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B. Prasad et al. / Tetrahedron Letters 53 (2012) 6059–6066
Table 3 (continued)
Entry
Chloro compound (1)
Alkyne; R= (2)
Product (3)
Timeb (h)
Yieldc (%)
N
N
N
–(CH2)3CH3
9
1a
3.2
84
2i
3i
N
N
N
OH
–CH(OH)CH3
10
1a
4
90
2j
3j
N
N
N
–(CH2)5CH3
2k
11
1a
3.5
85
3k
N
N
–(CH2)9CH3
2l
N
12
1a
3
87
3l
N
N
N
13
1b
2i
5
82
OMe
3m
N
N
N
14
1c
2i
4
89
F
3n
N
N
N
15
1d
2i
4
78
3o

B. Prasad et al. / Tetrahedron Letters 53 (2012) 6059–6066
6063
Table 3 (continued)
Entry
Chloro compound (1)
Alkyne; R= (2)
Product (3)
Timeb (h)
Yieldc (%)
N
N
N
OH
16
1e
2i
5
74
62
3p
N
N
N
17
1f
2i
5
3q
a
All reactions were carried out by using 1 (1.0 mmol), 2 (1.8 mmol), 10% Pd/C (0.028 mmol), PPh3 (0.15 mmol), CuI (0.052 mmol), Et3N (2.5 mmol) in toluene (5 mL) at
95–100 °C under N2.
b
After adding 2.
Isolated yields.
c
SiMe3
Si
Pd/C, PPh3
CuI, Et3N
N
N
N
N
N
N
N
N
Cl
4 h
2 h
NH
N
NH
Toluene,
95-100 0C
2 h
NH
1a
1y
Scheme 3. Coupling of 1a with trimethylsilyl acetylene under Pd/C–Cu catalysis.
3r
1x
AlCl3
AlCl3
N
Cl
Cl
N
N
Cl
AlCl3
5
R'NH2 ( )
4
1
N
NHR'
Cl Al Cl
Cl
Al
Cl
HCl
Cl
Cl
N
Cl
5
R'NH2 ( )
N
N
NHR'
NHR'
X
N
Cl Al Cl
Cl
NHR'
Scheme 4. Proposed mechanism for the AlCl3 induced C–N bond forming reaction
between 4 and 5 leading to the 3-chloroquinoxaline derivative 1.
yield significantly (Table 2, entry 3) indicating the need of CuI for
the reaction to proceed smoothly. The use of other solvents, for
example, EtOH, MeOH, and CH3CN was also examined and found
to be less effective in terms of product yield (Table 2, entries 4,
5, and 6). Thus a combination of Pd/C–PPh3–CuI and Et3N in tolu-
ene was found to be optimum for the present coupling–cyclization
reaction.
Figure 2. ORTEP representation of the compound 3g (thermal ellipsoids are drawn
at 50% probability level).
Having established the optimized reaction conditions we then
examined the generality and scope of the present synthesis of pyr-
rolo[2,3-b]quinoxaline. Thus, compounds 1a–f were treated with a
variety of terminal alkynes under the conditions of entry 1 of Table
2 and the results are summarized in Table 3. Both aromatic (Table
3, entries 1 and 8) and aliphatic alkynes (Table 3, entries 2–7 and
9–17) participated well in this reaction affording the desired prod-
ucts in good yields. Various groups such as primary, secondary, and
solvents (Table 2). We observed that the reaction proceeded
smoothly in toluene at 95–100 °C affording the desired compound
3a as the only product (Table 2, entry 1). The reaction was carried
out for 3 h and a longer reaction time did not improve the product
yield (Table 2, entry 2). The absence of CuI decreased the product

6064
B. Prasad et al. / Tetrahedron Letters 53 (2012) 6059–6066
1
B = Et3N
N
N
Cl
R'
N
N
N
Pd Cl
Pd
BH+
Pd(0)-PPh3
complex in
solution
PPh3
leaching
Pd in
R'
N
Pd/C
solution
E-1
B
Generation of actual
catalytic species
The catalytic
cycle
2
BH+
CuI/Et3N
Precipitation of
Pd on C at the end
of catalytic cycle
Cu
R
Pd(0)
+
R
Cu
N
N
N
N
Pd
N
R
BH+I-
CuI, BH+
R
3
R'
N
R'
N
N
N
R'
BH+I-
CuI, B
E-2
Intramolecular
cyclization
Scheme 5. Proposed mechanism for the synthesis of 1,2-disubstituted pyrrolo[2,3-b]quinoxalines (3) under the catalysis of Pd/C–CuI–PPh3.
Ph
Pd/C, PPh3
CuI, Et3N
N
N
CuI, Et3N
N
N
Cl
N
N
N
toluene
95-100 oC, 3h
79%
Toluene,
95-100 0C, 1h
50%
NH
NH
1a
6
3a
Scheme 6. Cu-mediated cyclization of compound 6.
tertiary alcohol, CN, tBu etc present in the terminal alkynes (2) em-
ployed were well tolerated. Similarly, both N-aryl and N-alkyl
substituted 3-chloroquinoxaline-2-amines participated well in
the present reaction. The reactions were completed within 3–5 h.
Notably, the coupling of 1a with trimethylsilyl acetylene under
the reaction conditions employed provided 1-p-tolyl-1H-pyrrol-
o[2,3-b]quinoxaline (3r) via a stepwise formation of intermediate
1x followed by 1y (Scheme 3). Both the intermediates were iso-
lated and well characterized. This observation helped in proposing
a probable mechanism for the present coupling–cyclization under
Pd/C–Cu catalysis. All the 1,2-disubstituted pyrrolo[2,3-b]quinoxa-
lines synthesized were characterized by spectral (NMR, IR, and MS)
data and this was supported by the molecular structure of a repre-
sentative compound 3g being confirmed unambiguously by single
crystal X-ray diffraction study (Fig. 2).11 Interestingly, the N-aryl
group of 3g was found to be in a perpendicular plane to that of
the core pyrrolo[2,3-b]quinoxaline ring.
Mechanistically, the amination of 2,3-dichloroquinoxaline (4)
seems to proceed through the complexation of AlCl3 with one of
the ring nitrogens12a of 4 followed by nucleophilic attack by the
amine 5 and finally release of AlCl3 affording the desired product
1 (Scheme 4). A second nucleophilic attack on 1 was disfavored
perhaps due to the preferential complexation of AlCl3 with the
N-1 (aided by the adjacent NHR0 group) over N-4 of 1. Notably, un-
like phenols the aromatic amines did not participate in the C–C
bond forming reactions12b–f under the conditions employed due
to the higher nucleophilicity of –NH2 group over phenolic –OH
moiety. Nevertheless, based on the outcome of Scheme 3 a proba-
ble mechanism for the Pd/C–Cu mediated coupling–cyclization
step is presented in Scheme 5. The reaction proceeds through the
generation of an active Pd(0) species from the minor portion of
the bound palladium (Pd/C) via a Pd leaching process into the solu-
tion.10 The leached Pd then interacts with phosphine ligands to
give a dissolved Pd(0)–PPh3 complex which actually catalyzes the
C–C bond forming reaction in solution. Once generated the active
Pd(0) species undergoes oxidative addition with 1 to give the org-
ano-Pd(II) species E-1. The species E-1 then facilitates the stepwise
formation of C–C bond via (i) trans organometallation with copper
acetylide generated in situ from CuI and the terminal alkyne (2)
followed by (ii) reductive elimination of Pd(0) (which is re-precip-
itated on the surface of the charcoal) to afford the internal alkyne
E-2. The alkyne E-2 thus formed subsequently undergoes Cu-med-
iated intramolecular ring closure13 to give the desired product 3.
This was further supported by the fact that the compound 6 (ob-
tained via Pd/C–Cu mediated reaction of 1a with phenyl acetylene
for 1 h) underwent intramolecular cyclization when treated with
CuI in the presence of Et3N to give the corresponding pyrrol-
o[2,3-b]quinoxaline derivative 3a (Scheme 6).
Having prepared a variety of 1,2-disubstituted pyrrolo[2,3-
b]quinoxalines we aimed for structural elaboration of a representa-
tive compound, for example, 3g. Thus, 3-bromo-2-tert-butyl-1-p-
tolyl-1H-pyrrolo[2,3-b]quinoxaline 7 prepared via bromination of
3g by using N-bromo succinamide (see Supplementary data) was
amenable for further derivatization through transition metal med-
iated various C–C and C–N bond forming reactions such as Sono-
gashira, Heck, Suzuki or Buchwald reactions.

B. Prasad et al. / Tetrahedron Letters 53 (2012) 6059–6066
6065
Table 4
N-aryl group was in a perpendicular plane to that of the core pyr-
rolo[2,3-b]quinoxaline ring. Some of the compounds synthesized
showed promising anti-proliferative properties when tested
in vitro against two cancer cell lines. Docking studies indicated that
these molecules interact well with human Akt in silico. Overall, our
effort has led to the identification of pyrrolo[2,3-b]quinoxaline
based small molecules that could be useful for the potential treat-
ment of leukemia or breast cancer.
The % inhibition of growth of cancer cell lines by compound 3a
Compounds
K-562 (leukemia)
10
MDA-MB-231 (breast)
100
l
M
l
M
1
lM
100
l
M
10
lM
1 lM
3b
3c
3d
3e
3f
37.8
32.3
54.9
58.7
40.2
42.5
38.8
29.2
27.1
50.4
49.0
29.9
39.5
28.7
23.1
27.8
46.6
43.4
27.0
35.6
26.6
57.5
75.0
70.1
61.3
54.6
49.5
52.6
55.0
55.8
60.2
50.2
49.5
45.3
47.7
43.8
43.5
45.7
48.6
42.5
38.1
37.1
3g
3j
Acknowledgments
a
Data presented are the average of three experiments.
The authors thank Professor J. Iqbal for encouragement and
support. M.P. thanks the DST, New Delhi, India for financial support
(Grant No. SR/S1/OC-53/2009). K.S. thanks the UGC, New Delhi, In-
dia for a Dr. D.S. Kothari Post doctoral fellowship [No. F.4–2/
2006(BSR)/13–24/2010(BSR)]. B. Prasad thanks the CSIR, New Del-
hi, India for a Senior Research Fellowship.
Some of the compounds14 (3) synthesized were tested in vitro
for their anti-proliferative properties against leukemia (K-562)
and breast (MD-AMB-231) cancer cell lines in a MTT assay. Har-
mine, a member of beta-carboline family of compounds showed
cytotoxicity against HL60 and K562 cell lines15 was used as a ref-
erence compound in this assay.16 The results of active molecules
identified by this assay are presented in Table 4. While 3d and
3e showed good activity against leukemia cells (Table 4), most of
the compounds, for example, 3b, 3c, 3d, 3e, and 3f were found to
be effective against breast cancer and 3e being the best among
them (Table 4). The compound 3e showed significant anti-prolifer-
ative properties at all the concentrations tested and maintained the
same at low concentrations. Notably, IC50 value of Harmine was
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.08.
119.
References and notes
1. (a) Smits, R. A.; Lim, H. D.; Hanzer, A.; Zuiderveld, O. P.; Guaita, E.; Adami, M.;
Coruzzi, G.; Leurs, R.; DeEsch, I. J. P. J. Med. Chem. 2008, 51, 2457; (b) Rong, F.;
Chow, S.; Yan, S.; Larson, G.; Hong, Z.; Wu, J. Bioorg. Med. Chem. Lett. 2007, 17,
1663; (c) Gao, H. L.; Yamasaki, E. F.; Chan, K. K.; Shen, L. L.; Snapka, R. M. Mol.
Pharmacol. 2003, 63, 1382; (d) Carta, A.; Loriga, M.; Paglietti, G.; Mattana, A.;
Fiori, P. L.; Mollicotti, P.; Sechi, L.; Anetti, S. Eur. J. Med. Chem. 2004, 39, 195; (e)
Hui, X.; Desrivot, J.; Bories, C.; Loiseau, P. M.; Franck, X.; Hocquemiller, R.;
Figadere, B. Bioorg. Med. Chem. Lett. 2006, 16, 815; (f) Andreichikov, Yu. S.;
Pitirimova, S. G.; Saraeva, R. F.; Goleneva, A. F. Pharm. Chem. J. 1979, 13, 1141.
http://dx.doi.org/10.1007/BF00778088.
2. (a) Campiani, G.; Cappelli, A.; Nacci, V.; Anzini, M.; Vomero, S.; Hamon, M.;
Cagnotto, A.; Fracasso, C.; Uboldi, C.; Caccia, S.; Consolo, S.; Mennini, T. J. Med.
Chem. 1997, 40, 3670; (b) Campiani, G.; Morelli, E.; Gemma, S.; Nacci, V.; Butini,
S.; Hamon, M.; Novellino, E.; Greco, G.; Cagnotto, A.; Goegan, M.; Cervo, L.;
Dalla Valle, F.; Fracasso, C.; Caccia, S.; Mennini, T. J. Med. Chem. 1999, 42, 4362.
3. (a) Desplat, V.; Geneste, A.; Begorre, M. A.; Fabre, S. B.; Brajot, S.; Massip, S.;
Thiolat, D.; Mossalayi, D.; Jarry, C.; Guillon, J. J. Enzyme Inhib. Med. Chem. 2008,
23, 648; (b) Desplat, V.; Moreau, S.; Gay, A.; Fabre, S. B.; Thiolat, D.; Massip, S.;
Macky, G.; Godde, F.; Mossalayi, D.; Jarry, C.; Guillon, J. J. Enzyme Inhib. Med.
Chem. 2010, 25, 204.
4. (a) Kumar, K. S.; Rambabu, D.; Sandra, S.; Kapavarapu, R.; Krishna, G. R.; Rao, M.
V. B.; Chatti, K.; Reddy, C. M.; Misra, P.; Pal, M. Bioorg. Med. Chem. 2012, 20,
1711; (b) Kumar, K. S.; Adepu, R.; Kapavarapu, R.; Rambabu, D.; Krishna, G. R.;
Reddy, C. M.; Priya, K. K.; Parsa, K. V. L.; Pal, M. Tetrahedron Lett. 2012, 53, 1134;
(c) Kumar, K. S.; Rambabu, D.; Prasad, B.; Mujahid, M.; Krishna, G. R.; Rao, M. V.
B.; Reddy, C. M.; Vanaja, G. R.; Kalle, A. M.; Pal, M. Org. Biomol. Chem. 2012, 10,
4774.
5. Arcadi, A.; Cacchi, S.; Fabrizi, G.; Parisi, L. M. Tetrahedron 2004, 45, 2431.
6. Keivanloo, A.; Bakherad, M.; Rahimi, A.; Taheri, S. A. N. Tetrahedron Lett. 2010,
51, 2409.
found to be 45 and 54 lM when tested against K-562 and MDA-
MB 231 cell lines in our MTT assay. The compound 3e therefore ap-
peared to be a promising and potential anticancer agent of further
interest. These findings also suggest that pyrrolo[2,3-b]quinoxaline
framework could be a new template for the design and identifica-
tion of small molecules that could be useful for the potential treat-
ment of leukemia or breast cancer.
Prompted by the reported Akt kinase inhibitory properties of
pyrrolo[1,2-a]quinoxaline class of compounds3 we decided to as-
sess the Akt kinase inhibitory potential of compounds 3d and 3e
in silico. Thus, these molecules were docked in human Akt (PDB
3MVH). The interactions of compound 3d with Akt kinase protein
were mainly contributed by (i) H-bond between OH of 3d and
the asn 279 and (ii) Pi-cation interaction of benzene ring of 3d with
lys179 (see Supplementary Fig. 1). Similarly, (i) a H-bond between
the nitrile of 3e with asp 439 and (ii) Pi-cation interaction of the
benzene ring of 3e with arg4 was observed in the other case (see
Supplementary Fig. 2). The compound 3d showed a dock score of
ꢀ4.18 whereas that of compound 3e was ꢀ5.46. The major contrib-
uting factor for these scores was lipophilic energy. While both the
molecules filled the active site partially however this partial filling
of active site gave good lipophilic scores. Thus, both the com-
pounds showed good interactions with human Akt in silico. The
observed anticancer properties of these molecules therefore could
be due to their potential Akt kinase inhibitory properties.
In conclusion, N-aryl substituted 3-chloroquinoxalin-2-amines
were elegantly prepared via AlCl3 induced C–N bond forming reac-
tion between 2,3-dichloroquinoxaline and aromatic amines afford-
ing a new method for the preparation of this class of compounds. A
related N-benzyl derivative however was prepared via a conven-
tional method as the AlCl3 induced method did not work in this
case. These N-alkyl/aryl substituted 3-chloroquinoxalin-2-amines
on coupling with terminal alkynes in toluene under Pd/C–Cu catal-
ysis afforded a range of 1,2-disubstituted pyrrolo[2,3-b]quinoxa-
7. Keivanloo, A.; Bakherad, M.; Rahimi, A. Synthesis 2010, 10, 1599.
8. (a) Raju, S.; Kumar, P. R.; Mukkanti, K.; Annamalai, P.; Pal, M. Bioorg. Med. Chem.
Lett. 2006, 16, 6185; (b) Reddy, E. A.; Barange, D. K.; Islam, A.; Mukkanti, K.; Pal,
M. Tetrahedron 2008, 64, 7143; (c) Pal, M.; Subramanian, V.; Yeleswarapu, K. R.
Tetrahedron Lett. 2003, 44, 8221; (d) Batchu, V. R.; Subramanian, V.;
Parasuraman, K.; Swamy, N. K.; Kumar, S.; Pal, M. Tetrahedron 2005, 61,
9869; (e) Novak, Z.; Szabo, A.; Repasi, J.; Kotschy, A. J. Org. Chem. 2003, 68, 3327.
9. Bakherad, M.; Keivanloo, A.; Jajarmi, S. Tetrahedron 2012, 68, 2107.
10. Pal, M. Synlett 2009, 2896.
11. Crystal data of 3g: Molecular formula = C21H21N3, formula weight = 315.41,
crystal system = triclinic, space group = P-1, a = 9.017 (15) Å, b = 9.324 (16) Å,
c = 10.768 (18) Å, V = 821.4 (2) Å 3, T = 296 K, Z = 2, Dc = 1.283 Mg mꢀ3
,
l
(Mo-
) = 0.08 mmꢀ1, 10844 reflections measured, 3582 independent reflections,
2881 observed reflections [I > 2.0 (I)], R1_obs = 0.028, goodness of fit = 1.03.
Ka
r
Crystallographic data (excluding structure factors) for 3g have been deposited
with the Cambridge Crystallographic Data Center as supplementary
publication number CCDC 876150.
12. (a) This was further supported by the fact that 2-chloro-3-p-chloroanilino
quinoxaline was obtained by refluxing an aqueous suspension of 2,3-
dichloroquinoxaline and p-chloroaniline in the presence of very dilute
hydrochloric acid, see: Haworth, R. D.; Robinson, S. J. Chem. Soc. 1948, 777.
The reaction seems to proceed through the protonation of one of the ring
lines within 3–5 h in good to excellent yields.
A probable
mechanism of the C–N bond forming reaction and the coupling–
cyclization process has been presented. The single crystal X-ray
diffraction study of a representative compound indicated that the

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B. Prasad et al. / Tetrahedron Letters 53 (2012) 6059–6066
nitrogen of quinoxaline moiety in this case thereby activation of the adjacent
Veeramaneni, V. R.; Pal, M. Eur. J. Med. Chem. 2011, 46, 4887; (c) Mulakayala,
N.; Rambabu, D.; Raja, M. R.; Chaitanya, M.; Kumar, C. S.; Kalle, A. M.; Krishna,
G. R.; Reddy, C. M.; Rao, M. V. B.; Pal, M. Bioorg. Med. Chem. 2012, 20, 759.
15. Jahaniani, F.; Ebrahimi, S. A.; Rahbar-Roshandel, N.; Mahmoudian, M.
Phytochemistry 2005, 66, 1581.
16. MTT assay: Cell viability was determined by (3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyltetrazolium bromide (MTT) assay. Cells (5 ꢁ 103 cells/well) were
seeded to 96-well culture plate and cultured with or without compounds at 1,
C–Cl bond for nucleophilic attack by the p-chloroaniline employed. Notably,
according to this report the reaction of 2,3-dichloroquinoxaline with aniline
under a variety of conditions yielded 2,3-dianilinoquinoxalins and attempts to
isolate 2-chloro-3-anilinoquinoxaline failed.; (b) Pal, M.; Batchu, V. R.;
Parasuraman, K.; Yeleswarapu, K. R. J. Org. Chem. 2003, 68, 6806; (c) Pal, M.;
Batchu, V. R.; Dager, I.; Swamy, N. K.; Padakanti, S. J. Org. Chem. 2005, 70, 2376;
(d) Kodimuthali, A.; Nishad, T. C.; Prasunamba, P. L.; Pal, M. Tetrahedron Lett.
2009, 50, 354; (e) Kodimuthali, A.; Chary, B. C.; Prasunamba, P. L.; Pal, M.
Tetrahedron Lett. 2009, 50, 1618; (f) Kumar, K. S.; Chamakuri, S.; Vishweshwar,
P.; Iqbal, J.; Pal, M. Tetrahedron Lett. 2010, 51, 3269.
10, and 100
treatment, the medium was removed and 20
added to the fresh medium. After 2 h incubation at 37 °C, 100
l
M concentration for 24 h in a final volume of 200
l of MTT (5 mg/ml in PBS) was
l of DMSO was
ll. After
l
l
13. For Cu(I)-mediated similar cyclization process; see: Patil, N. T.; Yamamoto, H.;
Wu, Y. J. Org. Chem. 2005, 70, 4531.
14. For our earlier effort on identification of novel anticancer agents, see: (a)
Swamy, N. K.; Tatini, L. K.; Babu, J. M.; Annamalai, P.; Pal, M. Chem. Commun.
2007, 1035; (b) Rajitha, C.; Dubey, P. K.; Sunku, V.; Piedrafita, F. J.;
added to each well and plates were agitated for 1 min. Absorbance was read at
570 nm on a multi-well plate reader (Synergy Mx, Biotek Inc., USA). Percent
inhibition of proliferation was calculated as a fraction of the control (without
compound).