Synthesis of some bisindolyl analogs for in vitro cytotoxic and DNA cleavage studies

Article abstract of DOI:10.1007/s00044-012-0370-x

One-pot, three components, conventional and microwave-assisted synthesis of bisindolyl analogs is described. Michael addition of preformed 2,5-disubstituted indole-3-carboxaldehydes and 3-methyl-1H-pyrazol-5(4H)-one with 2,5-disubstituted indoles under solvent and catalyst-free conditions afforded the hitherto unreported 2,5-disubstituted bisindolyl analogs bearing a pyrazolone moiety in excellent yields. All the synthesized compounds were characterized using IR, ¹H NMR, mass spectral, and analytical data. The analogs were screened for in vitro cytotoxic and DNA cleavage studies. Among the screened compounds 4c, 4f, and 4h-4j have emerged as most potent cytotoxic and 4c and 4f-4h as active DNA cleavage analogs.

Full text of DOI:10.1007/s00044-012-0370-x

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res
DOI 10.1007/s00044-012-0370-x
ORIGINAL RESEARCH
Synthesis of some bisindolyl analogs for in vitro cytotoxic
and DNA cleavage studies
•
B. S. Sasidhar J. S. Biradar
Received: 31 July 2012 / Accepted: 14 November 2012
Ó Springer Science+Business Media New York 2012
Abstract One-pot, three components, conventional and
microwave-assisted synthesis of bisindolyl analogs is descri-
bed. Michael addition of preformed 2,5-disubstituted indole-
3-carboxaldehydes and 3-methyl-1H-pyrazol-5(4H)-one with
2,5-disubstituted indoles under solvent and catalyst-free
conditions afforded the hitherto unreported 2,5-disubstituted
bisindolyl analogs bearing a pyrazolone moiety in excellent
yields. All the synthesized compounds were characterized
Thus, small molecules that bind to genomic DNA have
proven to be effective antitumor, antiviral, and antibacterial
agents. Hence, DNA cleavage agents form an important
class of drugs in antitumor therapy (Brana et al., 2001;
Rangappa et al., 2010; Ramya et al., 2010).
Many natural and synthetic anticancer agents having the
ability to interact with DNA have been discovered. Among
them, indole derivatives have shown significant antitumor
activity by possessing inhibitory properties of topoiso-
merase I- and II-mediated DNA cleavage activity (Jean
et al., 1993). Alongside, bisindolyl analogs inhibit the
growth of bladder cancer, renal cell cancer, lung cancer,
colon cancer, prostate cancer, proliferation process in
breast tumor cells, and mammary tumors (Morteza et al.,
2010).
1
using IR, H NMR, mass spectral, and analytical data. The
analogs were screened for in vitro cytotoxic and DNA
cleavage studies. Among the screened compounds 4c, 4f, and
4h–4j have emerged as most potent cytotoxic and 4c and
4f–4h as active DNA cleavage analogs.
Keywords Michael addition ꢀ Bisindolyl analogs ꢀ
3-Methyl-1H-pyrazol-5(4H)-one ꢀ
Within the frame of green chemistry, the search of
innovative solutions for the reduction of chemical steps,
wastes, and energy in organic processes has become a
challenging task. In this context, multicomponent reactions
(MCRs) represent a fascinating tool to achieve this goal
(Virsodia et al., 2001; Atul et al., 2007; Kappe, 2004;
Kappe and Dallinger, 2009). MCRs generate high levels of
diversity, as they allow more than two building blocks to
be combined in practical, time saving one-pot operations.
Which gives rise to complex structures by simultaneous
formation of two or more bonds (Bagley et al., 2002; Cui
et al., 2005; Huang et al., 2005; Ramon and Yus, 2005;
Domling, 2006; Domling et al., 2007). Achieving organic
reactions under solvent- and catalyst-free conditions
(Kidwai et al., 2005; Mu et al., 2006; Zhao et al., 2006)
through a reaction cascade by two or more sequential
reactions set off by a single trigger implies an elegant
control over chemical transformations. On the other hand,
solvent-, and catalyst-free conditions are especially suitable
for microwave activation and several advantages are
Microwave-assisted synthesis ꢀ Cytotoxic activity
Introduction
Cancer is one of the main causes of deaths in the world
despite the major breakthroughs in modern medicine over
the past 100 years. Therefore, the development of new
drugs against cancer continues to be the priority of devel-
opment of science and fundamental research. Deoxyribo-
nucleic acid (DNA) damage, mutation, and altered gene
expression are key players in the process of carcinogenesis.
B. S. Sasidhar ꢀ J. S. Biradar (&)
Central Research Lab, Department of Chemistry,
Gulbarga University, Gulbarga 585 106, Karnataka, India
e-mail: jsbiradar@yahoo.com
B. S. Sasidhar
e-mail: nayakbs7@rediffmail.com
123

Med Chem Res
evident in this approach (Anastas and Warner, 1998; Pelle
et al., 2001; Cui et al., 2005; Ramon and Yus, 2005; Mu
et al., 2006; Zhao et al., 2006; Victoria et al., 2010).
Michael addition of indoles to a,b-unsaturated system is
an efficient approach to generate biologically potent indole
analogs (Bartoli et al., 2003; Lin et al., 2005; Renzetti
et al., 2008). The literature reveals several reports on
Michael addition by, the combination of cerium(III) chlo-
ride heptahydrate and sodium iodide supported on silica gel
(Bartoli et al., 2005), sulfamic acid catalyzed (An et al.,
2007), p-TsOH catalyzed in acetonitrile (Chakrabarty
et al., 2004), TiCl4/Et3N promoted (Renzetti et al., 2008),
N-bromosuccinimide catalyzed in dichloromethane (Kuo
et al., 2009), protic and Lewis acids (Ji and Wang, 2003;
Zhan et al., 2005) for the C-3 alkylation of indoles.
However, most of these protocols have significant draw-
backs such as long reaction times, low yields, harsh reac-
tion conditions, difficult workup and the use of expensive,
environmentally toxic catalysts, and reagents.
out in conventional heating. The products were obtained in
moderate yields in 40–50 min (Table 1). However, further
continuation of reaction ([60 min) has not shown any
improvement on the yields. The reaction has suffered by
moderate yields when carried out without energy-transfer
medium (AcOH).
The reaction has been explored for variety of 2,5-
disubstituted indole-3-carboxaldehydes and substituted
indoles with 3-methyl-1H-pyrazol-5(4H)-one to look at the
scope of the cascade Michael addition under optimized
conditions. The representative results are shown in
Table 1. The IR spectrum of 4-(bis(5-methyl-2-phenyl-1H-
indol-3-yl)methyl)-3-methyl-1H-pyrazol-5(4H)-one (4a) has
shown strong absorption at 3,327 cm-1 corresponding to
indole NH and absorption at 3,070 cm-1 corresponds to
pyrazolone NH. Absorptions at 1,655 and 1,578 cm-1
1
correspond to C=O and C=N stretching, respectively. H
NMR spectrum of 4a has exhibited a singlet at d 11.50 due
to indole NH (s, 1H, NH) and peak at d 12.30 assigned to
pyrazolone NH (s, 1H, NH) are D2O exchangeable. A
multiplet between d 7.00–7.80 (m, 16H, Ar–H) integrating
for 16 aromatic protons. A singlet at d 1.90 and at d 2.50 is
for CH3 protons in the pyrazolone and indoles, respec-
tively. 13C NMR spectrum of 4a has displayed a downfield
signals at d 167 for C=O carbon and less deshielded peak
at d 157 integrated for the C=N of the pyrazolone ring.
Upfield signals at d 18 and 22 correspond to the CH3
carbon of pyrazolone and indoles, respectively. The mass
spectrum of compound 4a has shown molecular ion peak
at m/z 522 [M]?.which corresponds to its molecular
weight.
Hence, as a part of our enduring efforts on the devel-
opment of new routes for the synthesis of antitumor and
DNA cleavage bioactive indole analogs by economic and
environmentally benign techniques (Biradar et al., 2008,
2010, 2011; Biradar and Sasidhar, 2011). Herein, we report
bisindolyl analogs via one-pot Michael addition of indoles.
Results and discussion
Chemistry
The present method encompasses, synthesis of bisindolyl
analogs by conventional and microwave irradiation using
acetic acid as an energy-transfer medium and homogenizer to
increase reaction temperature (Biradar et al., 2008; Biradar
and Sasidhar, 2011). An equimolar mixture of 2,5-disubsti-
tuted indole-3-carboxaldehydes (1a–1c) and 3-methyl-1H-
pyrazol-5(4H)-one (2) undergo Knoevenagel condensation to
form an intermediate I. 2,5-Disubstituted indoles (3a–3d) as
nucleophile underwent Michael reaction with intermediate
I to produce bisindolyl analogs (4a–4k). A reasonable
mechanism for the formation of bisindolyl analogs is outlined
in Scheme 1.
Biological evaluations
In vitro cytotoxic studies
As a preliminary study, antitumor cytotoxicity for the
synthesized compounds was performed against three tumor
cell lines, A-549 (lung carcinoma), HEp-2 (laryngeal car-
cinoma), and HeLa (cervical carcinoma) by MTT assay
(Skehan et al., 1990) with doxorubicin as positive refer-
ence. The results are presented in Table 2. As can be seen
from Table 2, of all the synthesized compounds, 4f, 4h,
and 4j have shown very effective cytotoxicity against all
three cell lines. Compounds 4c against A-549 (IC50
1.80 lM), HEp-2 (IC50 3.50 lM), and 4i against A-549
(IC50 1.60 lM), HeLa (IC50 3.80 lM) have shown effec-
tive cytotoxicity compared with the standard drug. Com-
pounds 4e and 4 k have also shown profound cytotoxic
activity against all three cell lines. Interestingly, com-
pounds 4c and 4f with 5-OCH3 substitution and compounds
4h–4k with 5-H substitution have their IC50 values falling
In microwave-assisted synthesis, under solvent- and
catalyst-free conditions, the title compounds (4a–4k)
obtained in excellent yields with high purity (Scheme 2).
First, the Michael addition was optimized for both neat and
utilizing acetic acid as energy-transfer medium and
homogenizer. A controlled experiment without AcOH, has
resulted in the formation of Michael adduct only in trace
amounts due to the fractional conversion of reactants.
When it is carried out with AcOH, it has produced excel-
lent yields with high purity. The same reaction was carried
123

Med Chem Res
Scheme 1 Feasible mechanism for the Michael addition of indoles
Scheme 2 Michael addition of
2,5-disubstituted indoles
R3
R3
(3a-d)
HN
O
HN
R2
R2
NH
N
+
O
NH
H
40-50 Mins
O
N
C
R1
MW
7 mins
R1
R
R
2
N
H
N
H
(1a-c)
(4a-k)
R= Ph, H R1= Me,Cl, H R2= Ph R3= Me, Cl, OMe, H
Table 1 Comparative data of conventional and MW methods for the synthesis of bisindolyl analogs (4a–4k)
Entry
Substituents
Conventional method
Microwave method
Mp (°C)
R
R1
R2
R3
Time (min)
Yield (%)
Time (min)
Power (%)
Yield (%)
4a
4b
4c
4d
4e
4f
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
Me
Me
Me
Me
Cl
Cl
Cl
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Cl
40–50
40–50
40–50
40–50
40–50
40–50
40–50
40–50
40–50
40–50
40–50
60
63
57
57
60
55
60
54
64
55
60
7
7
7
7
7
7
7
7
7
7
7
70
70
70
70
70
70
70
70
70
70
70
87
90
80
86
85
79
87
76
80
83
85
209–210
240–242
190–192
226–228
236–238
178–180
233–234
268–270
201–202
220–221
206–207
OMe
H
Cl
OMe
H
4g
4h
4i
Me
Cl
H
H
4j
H
H
OMe
H
4k
H
H
DNA cleavage activity
in the range of 3.20–6.70 with better activity than that of
the drug doxorubicin (IC50 8.70 lM) against HEp-2. In
contrast, compounds 4a, 4b, 4g have showed least activity,
and 4d has failed to show any tendency against all the three
cell lines. The results clearly signify compounds 4c, 4f, and
4h–4k with OCH3 and H substitution, respectively, at the
fifth position of the indole ring has improved and showed
markedly strong cytotoxicity against all the three cell lines.
Whereas, the ‘‘Cl’’ substitution has shown some tendency
to improve the activity when compared with ‘‘CH3’’
substituted bisindolyl analogs.
DNA cleavage activity was determined using gel electro-
phoresis by Sambrook et al. (1989). According to cell
biology, DNA is the primary target molecule for most of
the anticancer and antiviral therapies. Investigations of the
interaction of DNA with small molecules are basic work in
the design of new types of pharmaceutical molecules.
When the compound is made to interacted with DNA could
induce the breakage of DNA strands by appropriate
methods. The gel after electrophoresis (Fig. 1) clearly
123

Med Chem Res
Experimental section
Table 2 In vitro cytotoxic studies of bisindolyl analogs (4a–4k)
Compounds IC50 (lM)
Chemistry
A-549 (lung
carcinoma)
HEp-2 (laryngeal HeLa (cervical
carcinoma)
carcinoma)
All the chemicals and reagents were purchased from
MERCK, Himedia and SD fine chemical companies and
are used without further purification. Melting points of the
synthesized compounds are determined in open capillaries
and are uncorrected. Reactions are monitored by thin-layer
chromatography (TLC) on silica gel 60 F254 aluminium
sheets (MERCK). The mobile phase was chloroform and
benzene (3:1) and detection was made using UV light and
iodine. IR spectra are recorded in KBr on Perkin-Elmer
and FTIR spectrophotometer (mmax in cm-1). 1H NMR and
13C NMR spectra recorded on Bruker Avance II 400-MHz
NMR spectrometer (chemical shift in d ppm down field
from TMS as an internal reference). The mass spectra are
recorded on LC-MSD-Trap-SL instrument. The elemental
analysis was determined on FLASH EA 1112 SERIES
instrument. All the compounds gave C, H, and N analysis
within 0.4 % of the theoretical values. Microwave reac-
tions carried out in Onida 20STP21 800 W multimode
microwave oven.
4a
4b
4c
4d
4e
4f
37.0
35.0
1.80
22.1
7.00
1.10
31.3
1.30
1.60
2.70
5.50
NA
32.7
NA
32.4
3.50
NA
NA
NA
14.6
6.20
41.7
3.20
NA
9.50
1.20
29.8
2.40
3.80
1.70
7.70
0.71
4g
4h
4i
4j
5.30
6.70
8.70
4k
Doxorubicin 0.70
NA not active and having IC50 [ 100 lM
M
4c 4e
4f 4g 4h 4i
4j 4k
C
Typical experimental procedure for the synthesis
22.226
7.421
5.804
3.530
1.220
of 2,5-disubstituted indole-3-carboxaldehydes (1a–1c)
The precursors 2,5-disubstituted indole-3-carboxaldehydes
(1a–1c) were obtained from the Vilsmeier–Haack formy-
lation reaction of 2,5-disubstituted indoles (Biradar, 1982).
Synthesis of 3-methyl-1H-pyrazol-5(4H)-one (2)
Fig. 1 DNA cleavage analysis of bisindolyl analogs (4c and 4e–4k).
Where M standard DNA molecular weight marker and C control
E. coli DNA (untreated sample)
Ethylacetoacetate (0.01 mol) in ethanol (10–15 ml) was
cyclized with hydrazine hydrate (0.01 mol) by stirring at
room temperature for about 2–3 h. Solid separated out was
filtered washed with ethanol and recrystallized from etha-
nol (Biradar and Sasidhar, 2011).
revealed that, the compounds tested did act on DNA as
tailing in the bands can be observed in treated samples. The
difference was observed in bands of all the compounds
compared to the control DNA (C). This shows that the
control DNA alone does not show any apparent cleavage as
the compounds did. Compounds 4c and 4f–4h have shown
more intense streaks, indicating the significant DNA
cleavage ability of the molecules. However, the mechanism
and the nature of the intermediates involved in the DNA
cleavage by the compounds have not been clear. But, the
results indicate the importance of bisindolyl analogs in
these isolated DNA cleavage reactions. As the compounds
4c and 4f–4h were observed to cleave the DNA, it can be
concluded that the compound inhibits the growth of the
pathogenic organism by cleaving the genome.
General procedure for the synthesis of bisindolyl
analogs (4a–4k)
Conventional method
A mixture of 2,5-disubstituted indole-3-carboxaldehyde
(1a–1c) (0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-one(2)
(0.5 mmol), and 2,5-disubstituted indole (3a–3d) (0.5 mmol)
in acetic acid (2 ml) were stirred for 40–50 min at 160–
170 °C. After completion [TLC, chloroform–benzene (3:1)]
the reaction mixture was allowed to cool at room temperature,
washed with aqueous ethanol and filtered off to get the crude
123

Med Chem Res
Synthesis of 4-((5-chloro-2-phenyl-1H-indol-3-yl)(5-
methyl-2-phenyl-1H-indol-3-yl)methyl)-3-methyl-1H-
pyrazol-5(4H)-one (4b)
product. On further crystallization with ethanol will produce
4a–4k.
Microwave-assisted synthesis
A mixture of 5-methyl-2-phenyl-1H-indole-3-carboxaldehyde
(1a) (0.117 g, 0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-one (2)
(0.050 g, 0.5 mmol), and 5-chloro-2-phenyl-1H-indole (3b)
(0.113 g, 0.5 mmol) in acetic acid (2 ml) were made to paste,
introduced into open borosil glass tube and irradiated for 7 min
at 160–170 °C under microwave power (Table 2). After
completion [TLC, chloroform–benzene (3:1)] the reaction
mixture was brought to room temperature, washed with
aqueous ethanol and filtered off to get the crude product. On
further crystallization with ethanol will produce compound 4b
as pale yellow solid. Yield: 90 %; m.p.: 240–242 °C; IR (KBr)
Neat reaction A mixture of 2,5-disubstituted indole-3-
carboxaldehyde (1a–1c) (0.5 mmol), 3-methyl-1H-pyra-
zol-5(4H)-one (2) (0.5 mmol), and 2,5-disubstituted indole
(3a–3d) (0.5 mmol) were mixed, introduced into open
borosil glass tube and irradiated for 7 min at 160–170 °C
under microwave power (Table 2). After completion [TLC,
chloroform–benzene (3:1)] the reaction mixture was
brought to room temperature, washed with aqueous ethanol
and filtered off to get the titled compounds (4a–4k).
1
mmax (cm-1): 3438, 3410, 3190, 3013, 1670, 1600; H NMR
(DMSO-d6 ? CDCl3) d (ppm): 11.90 (s, 1H, indole NH),
11.20 (s, 1H, indole NH), 12.24 (s, 1H, pyrazolone NH),
6.80–7.50 (m, 16H, Ar–H), 4.40 (m, 2H, 2-CH), 2.10 (s, 3H,
CH3), 1.70 (s, 3H, CH3); 13C NMR (DMSO-d6 ? CDCl3) d
(ppm): 162, 151, 146, 145, 144, 143, 140, 138, 137, 136, 134,
133, 130, 129, 127, 125, 125, 118, 117, 50, 27, 25, 20; MS:
m/z = 543 [M]?.; Anal. calcd. for C34H27ClN4O: C, 75.20; H,
5.01; N, 10.32. Found: C, 75.10; H, 5.11; N, 10.18.
With AcOH A mixture of 2,5-disubstituted indole-3-car-
boxaldehyde (1a) (0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-
one (2) (0.5 mmol), and 2,5-disubstituted indole (3a)
(0.5 mmol) in acetic acid (2 ml) were made to paste,
introduced into open borosil glass tube and irradiated for
7 min at 160–170 °C under microwave power (Table 2).
After completion [TLC, chloroform–benzene (3:1)] the
reaction mixture was brought to room temperature, washed
with aqueous ethanol and filtered off to get 4a–4k with high
purity (TLC).
Synthesis of 4-((5-methoxy-2-phenyl-1H-indol-3-yl)(5-
methyl-2-phenyl-1H-indol-3-yl)methyl)-3-methyl-1H-
pyrazol-5(4H)-one (4c)
Synthesis of 4-(bis(5-methyl-2-phenyl-1H-indol-3-
yl)methyl)-3-methyl-1H-pyrazol-5(4H)-one (4a)
A mixture of 5-methyl-2-phenyl-1H-indole-3-carboxaldehyde
(1a) (0.117 g, 0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-one
(2) (0.050 g, 0.5 mmol), and 5-methoxy-2-phenyl-1H-indole
(3c) (0.111 g, 0.5 mmol) in acetic acid (2 ml) were made to
paste, introduced into open borosil glass tube and irradiated
for 7 min at 160–170 °C under microwave power (Table 2).
After completion [TLC, chloroform–benzene (3:1)] the
reaction mixture was brought to room temperature, washed
with aqueous ethanol and filtered off to get the crude product.
On further crystallization with ethanol will produce com-
pound 4c as pale yellow solid. Yield: 80 %; m.p.: 190–
192 °C; IR (KBr) mmax (cm-1): 3300, 3121, 3020, 3013, 1651,
A mixture of 5-methyl-2-phenyl-1H-indole-3-carboxaldehyde
(1a) (0.117 g, 0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-one
(2) (0.050 g, 0.5 mmol), and 5-methyl-2-phenyl-1H-indole
(3a) (0.103 g, 0.5 mmol) in acetic acid (2 ml) were made to
paste, introduced into open borosil glass tube and irradiated
for 7 min at 160–170 °C under microwave power (Table 2).
After completion [TLC, chloroform–benzene (3:1)] the
reaction mixture was brought to room temperature, washed
with aqueous ethanol and filtered off to get the crude product.
On further crystallization with ethanol will produce com-
pound 4a as pale yellow solid. Yield: 87 %; m.p.: 209–
210 °C; IR (KBr) mmax (cm-1): 3327, 3187, 3070, 1655, 1578;
1H NMR (DMSO-d6 ? CDCl3) d (ppm): 11.50 (s, 2H, indole
NH), 12.30 (s, 1H, pyrazolone NH), 7.00–7.80 (m, 16H, Ar–
H), 4.30 (m, 2H, 2-CH), 2.50 (s, 3H, CH3), 1.90 (s, 6H,
2-CH3); 13C NMR (DMSO-d6 ? CDCl3) d (ppm):167, 157,
147, 146, 145, 144, 140, 138, 137, 136, 130, 129, 127, 122,
118, 116, 48, 35, 25, 22; MS: m/z = 522 [M]?.; Anal. calcd.
for C35H30N4O: C, 80.43; H, 5.79; N, 10.72. Found: C, 80.37;
H, 5.82; N, 10.69.
1
1600. H NMR (DMSO-d6 ? CDCl3) d (ppm): 9.30 (s, 1H,
indole NH), 10.00 (s, 1H, indole NH), 12.23 (s, 1H, pyrazo-
lone NH), 7.00–8.00 (m, 16H, Ar–H), 4.70 (m, 2H, 2-CH),
3.20 (s, 3H, OCH3), 2.42 (s, 3H, CH3) 2.10 (s, 3H, CH3); 13
C
NMR (DMSO-d6 ? CDCl3) d (ppm): 165, 155, 148, 147,
146, 145, 138, 137, 136, 134, 133, 131, 130, 129, 125, 124,
123, 121, 52, 47, 32, 23, 20; MS: m/z = 538 [M]?.; Anal.
calcd. for C35H30N4O2: C, 78.04; H, 5.61; N, 10.40. Found:
C, 77.92; H, 5.66; N, 10.33.
123

Med Chem Res
Synthesis of 3-methyl-4-((5-methyl-2-phenyl-1H-indol
-3-yl)(2-phenyl-1H-indol-3-yl)methyl)-1H-pyrazol-
5(4H)-one (4d)
Synthesis of 4-((5-chloro-2-phenyl-1H-indol-3-yl)(5-
methoxy-2-phenyl-1H-indol-3-yl)methyl)-3-methyl-1H-
pyrazol-5(4H)-one (4f)
A mixture of 5-methyl-2-phenyl-1H-indole-3-carboxalde-
hyde (1a) (0.117 g, 0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-
one (2) (0.050 g, 0.5 mmol), and 2-phenyl-1H-indole (3d)
(0.096 g, 0.5 mmol) in acetic acid (2 ml) were made to
paste, introduced into open borosil glass tube and irradiated
for 7 min at 160–170 °C under microwave power (Table 2).
After completion [TLC, chloroform–benzene (3:1)] the
reaction mixture was brought to room temperature, washed
with aqueous ethanol and filtered off to get the crude
product. On further crystallization with ethanol will produce
compound 4d as colorless solid. Yield: 86 %; m.p.: 226–
228 °C; IR (KBr) mmax (cm-1): 3367, 3271, 3100, 2941,
A mixture of 5-chloro-2-phenyl-1H-indole-3-carboxaldehyde
(1b) (0.127 g, 0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-one
(2) 0.050 g, (0.5 mmol), and 5-methoxy-2-phenyl-1H-indole
(3c) (0.111 g, 0.5 mmol) in acetic acid (2 ml) were made to
paste, introduced into open borosil glass tube and irradiated
for 7 min at 160–170 °C under microwave power (Table 2).
After completion [TLC, chloroform–benzene (3:1)] the reac-
tion mixture was brought to room temperature, washed with
aqueous ethanol and filtered off to get the crude product. On
further crystallization with ethanol will produce compound 4f
as yellow solid. Yield: 79 %; m.p.: 178–180 °C. IR (KBr)
1
mmax (cm-1): 3309, 3171, 2993, 2923, 1665, 1581; H NMR
1
1647, 1590; H NMR (DMSO-d6 ? CDCl3) d (ppm): 9.70
(DMSO-d6 ? CDCl3) d (ppm): 10.00 (s, 1H, indole NH),
10.70 (s, 1H, indole NH), 11.40 (s, 1H, pyrazolone NH),
6.30–8.60 (m, 16H, Ar–H), 4.30 (m, 2H, 2-CH), 3.50 (s, 3H,
OCH3), 2.20 (s, 3H, CH3); 13C NMR (DMSO-d6 ? CDCl3) d
(ppm): 162, 153, 146, 145, 144, 137, 136, 134, 132, 130, 125,
124, 47, 40, 27, 21; MS: m/z = 561 [M?2]?; Anal. calcd. for
C34H27ClN4O2: C, 73.05; H, 4.87; N, 10.02. Found: C, 73.27;
H, 4.92; N, 10.28.
(s, 1H, indole NH), 10.50 (s, 1H, indole NH), 11.90 (s, 1H,
pyrazolone NH), 7.10–8.20 (m, 17H, Ar–H), 4.50 (m, 2H,
2-CH), 2.40 (s, 3H, CH3), 2.00 (s, 3H, CH3); 13C NMR
(DMSO-d6 ? CDCl3) d (ppm): 161, 151, 147, 146, 145,
144, 143, 138, 137, 136, 135, 134, 133, 131, 130, 129, 125,
124, 123, 120, 118, 117, 58, 29, 25, 18; MS: m/z = 509
[M?1]?.; Anal. calcd. for C34H28N4O: C, 80.29; H, 5.55; N,
11.02. Found: C, 80.19; H, 5.59; N, 10.88.
Synthesis of 4-((5-chloro-2-phenyl-1H-indol-3-yl)(2-
phenyl-1H-indol-3-yl)methyl)-3-methyl-1H-pyrazol-
5(4H)-one (4g)
Synthesis of 4-(bis(5-chloro-2-phenyl-1H-indol-3-
yl)methyl)-3-methyl-1H-pyrazol-5(4H)-one (4e)
A mixture of 5-chloro-2-phenyl-1H-indole-3-carboxaldehyde
(1b) (0.127 g, 0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-one
(2) (0.050 g, 0.5 mmol), and 5-chloro-2-phenyl-1H-indole
(3b) (0.113 g, 0.5 mmol) in acetic acid (2 ml) were made to
paste, introduced into open borosil glass tube and irradiated
for 7 min at 160–170 °C under microwave power (Table 2).
After completion [TLC, chloroform–benzene (3:1)] the
reaction mixture was brought to room temperature, washed
with aqueous ethanol and filtered off to get the crude product.
On further crystallization with ethanol will produce com-
pound 4e as yellow solid. Yield: 85 %; m.p.: 236–238 °C; IR
A mixture of 5-chloro-2-phenyl-1H-indole-3-carboxalde-
hyde (1b) (0.127 g, 0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-
one (2) (0.050 g, 0.5 mmol), and 2-phenyl-1H-indole (3d)
(0.096 g, 0.5 mmol) in acetic acid (2 ml) were made to paste,
introduced into open borosil glass tube and irradiated for
7 min at 160–170 °C under microwave power (Table 2).
After completion [TLC, chloroform–benzene (3:1)] the
reaction mixture was brought to room temperature, washed
with aqueous ethanol and filtered off to get the crude product.
On further crystallization with ethanol will produce com-
pound 4g as colorless solid. Yield: 87 %; m.p.: 233–234 °C;
IR (KBr) mmax (cm-1): 3222, 3137, 3007, 2901, 1643, 1597;
1H NMR (DMSO-d6 ? CDCl3) d (ppm): 10.40 (s, 1H, indole
NH), 10.90 (s, 1H, indole NH), 11.70 (s, 1H, pyrazolone NH),
7.30–8.70 (m, 17H, Ar–H), 4.10 (m, 2H, 2-CH), 2.70 (s, 3H,
CH3); 13C NMR (DMSO-d6 ? CDCl3) d (ppm): 161, 153,
144, 143, 137, 136, 135, 132, 130, 131, 123, 122, 120, 118,
43, 27, 23; MS: m/z = 530 [M]?.; Anal. calcd. for
C33H25ClN4O: C, 74.92; H, 4.76; N, 10.59. Found: C, 74.98;
H, 4.66; N, 10.87.
1
(KBr) mmax (cm-1): 3321, 3162, 3097, 3037, 1679, 1602; H
NMR (DMSO-d6 ? CDCl3) d (ppm): 9.90 (s, 2H, indole
NH), 12.50 (s, 1H, pyrazolone NH), 7.20–8.10 (m, 16H, Ar–
H), 4.00 (m, 2H, 2-CH), 2.60 (s, 3H, CH3); 13C NMR
(DMSO-d6 ? CDCl3) d (ppm): 167, 155, 147, 146, 145, 144,
138, 137, 136, 135, 133, 132, 130, 125, 124, 123, 120, 118,
55, 27, 24; MS: m/z = 564 [M?1]?.; Anal. calcd. for
C33H24Cl2N4O: C, 70.34; H, 4.29; N, 9.94. Found: C, 70.23;
H, 4.40; N, 9.82.
123

Med Chem Res
Synthesis of 4-((1H-indol-3-yl)(5-methyl-2-phenyl-1H-
indol-3-yl)methyl)-3-methyl-1H-pyrazol-5(4H)-one (4h)
mmol), and 5-methoxy-2-phenyl-1H-indole (3c) (0.111 g,
0.5 mmol) in acetic acid (2 ml) were made to paste, intro-
duced into open borosil glass tube and irradiated for 7 min at
160–170 °C under microwave power (Table 2). After com-
pletion [TLC, chloroform–benzene (3:1)] the reaction mixture
was brought to room temperature, washed with aqueous
ethanol and filtered off to get the crude product. On further
crystallization with ethanol will produce compound 4j as pale
yellow solid. Yield: 83 %; m.p.: 220–221 °C; IR (KBr) mmax
(cm-1): 3323, 3277, 3024, 2930, 1685, 1581; 1H NMR
(DMSO-d6 ? CDCl3) d (ppm): 10.10 (s, 1H, indole NH),
10.80 (s, 1H, indole NH), 11.60 (s, 1H, pyrazolone NH),
6.50–7.50 (m, 13H, Ar–H), 4.20 (m, 2H, 2-CH), 3.00 (s, 3H,
OCH3), 2.50 (s, 3H, CH3); 13C NMR (DMSO-d6 ? CDCl3) d
(ppm): 162, 153, 146, 145, 144, 137, 136, 134, 132, 130, 125,
124, 50, 45, 27, 22; MS: m/z = 450 [M?2]?.; Anal. calcd. for
C28H24N4O2: C, 74.98; H, 5.39; N, 12.49. Found: C, 74.92;
H, 5.47; N, 12.65.
A mixture of indole-3-carboxaldehyde (1c) (0.073 g, 0.5mmol),
3-methyl-1H-pyrazol-5(4H)-one (2) (0.050 g, 0.5 mmol), and
5-methyl-2-phenyl-1H-indole (3a) (0.103 g, 0.5 mmol) in
acetic acid (2 ml) were made to paste, introduced into open
borosil glass tube and irradiated for 7 min at 160–170 °C under
microwave power (Table 2). After completion [TLC, chloro-
form–benzene (3:1)] the reaction mixture was brought to room
temperature, washed with aqueous ethanol and filtered off to get
the crude product. On further crystallization with ethanol will
produce compound 4h as pale yellow solid. Yield: 76 %; m.p.:
268–270 °C; IR (KBr) mmax (cm-1): 3343, 3211, 3180, 2907,
1697, 1598; 1H NMR (DMSO-d6 ? CDCl3) d (ppm): 9.90 (s,
1H, indole NH), 10.40 (s, 1H, indole NH), 11.00 (s, 1H, pyr-
azolone NH), 7.50–8.60 (m, 13H, Ar–H), 4.20 (m, 2H, 2-CH),
2.70 (s, 3H, CH3), 2.10 (s, 3H, CH3); 13C NMR (DMSO-
d6 ? CDCl3) d (ppm): 167, 157, 146, 145, 144, 136,135, 134,
133, 126, 125, 124, 120, 118, 117, 45, 37, 25, 20; MS: m/
z = 432 [M]?.; Anal. calcd. for C28H24N4O: C, 77.75; H, 5.59;
N, 12.95. Found: C, 77.62; H, 5.76; N, 12.75.
Synthesis of 4-((1H-indol-3-yl)(2-phenyl-1H-indol-3-yl)
methyl)-3-methyl-1H-pyrazol-5(4H)-one (4k)
A mixture of indole-3-carboxaldehyde (1c) (0.073 g, 0.5
mmol), 3-methyl-1H-pyrazol-5(4H)-one (2) (0.050 g, 0.5
mmol), and 2-phenyl-1H-indole (3d) (0.096 g, 0.5 mmol) in
acetic acid (2 ml) were made to paste, introduced into open
borosil glass tube and irradiated for 7 min at 160–170 °C
under microwave power (Table 2). After completion [TLC,
chloroform–benzene (3:1)] the reaction mixture was brought
to room temperature, washed with aqueous ethanol and fil-
tered off to get the crude product. On further crystallization
with ethanol will produce compound 4k as colorless solid.
Yield: 85 %; m.p.: 206–207 °C; IR (KBr) mmax (cm-1): 3287,
3171, 2993, 2923, 1665, 1581; 1H NMR (DMSO-
d6 ? CDCl3) d (ppm): 9.80 (s, 1H, indole NH), 10.40 (s, 1H,
indole NH), 11.00 (s, 1H, pyrazolone NH), 6.60–7.30 (m,
14H, Ar–H), 4.10 (m, 2H, 2-CH), 2.60 (s, 3H, CH3); 13C
NMR (DMSO-d6 ? CDCl3) d (ppm): 160, 154, 144, 143,
142,137, 136, 135, 134, 132, 130, 125, 124, 123, 120, 119, 44,
33, 25; MS: m/z = 420 [M?2]?.; Anal. calcd. for C27H22
N4O: C, 77.49; H, 5.30; N, 13.39. Found: C, 77.54; H, 5.21;
N, 13.45.
Synthesis of 4-((5-chloro-2-phenyl-1H-indol-3-yl)(1H-
indol-3-yl)methyl)-3-methyl-1H-pyrazol-5(4H)-one (4i)
A mixture of indole-3-carboxaldehyde (1c) (0.073 g, 0.5
mmol), 3-methyl-1H-pyrazol-5(4H)-one (2) (0.050 g,
0.5 mmol), and 5-chloro-2-phenyl-1H-indole (3b) (0.113 g,
0.5 mmol) in acetic acid (2 ml) were made to paste, introduced
into open borosil glass tube and irradiated for 7 min at
160–170 °C under microwave power (Table 2). After com-
pletion [TLC, chloroform–benzene (3:1)] the reaction mixture
was brought to room temperature, washed with aqueous etha-
nol and filtered off to get the crude product. On further crys-
tallization with ethanol will produce compound 4i as yellow
solid. Yield: 80 %; m.p.: 201–202 °C; IR (KBr) mmax (cm-1):
3323, 3200, 3101, 2911, 2923, 1667, 1583; 1H NMR (DMSO-
d6 ? CDCl3) d (ppm): 10.70 (s, 1H, indole NH), 11.70 (s, 1H,
indole NH), 12.30 (s, 1H, pyrazolone NH), 7.00–8.30 (m, 13H,
Ar–H), 4.70 (m, 2H, 2-CH), 2.40 (s, 3H, CH3); 13C NMR
(DMSO-d6 ? CDCl3) d (ppm): 163, 150, 145, 144, 143, 136,
135, 134, 132, 131, 125, 124, 123, 122, 119, 52, 37, 25; MS: m/
z = 452 [M]?.; Anal. calcd. for C27H21ClN4O: C, 71.60; H,
4.67; N, 12.37. Found: C, 71.37; H, 4.72; N, 12.55.
In vitro cytotoxic studies
Synthesis of 4-((1H-indol-3-yl)(5-methoxy-2-phenyl-1H-
indol-3-yl)methyl)-3-methyl-1H-pyrazol-5(4H)-one (4j)
Cell lines and culture condition
A-549 (lung carcinoma), HEp-2 (laryngeal carcinoma), and
HeLa (cervical carcinoma) cells were cultured in RPMI
1640 (Himedia, India) medium supplemented with 10 %
A mixture of indole-3-carboxaldehyde (1c) (0.073 g,
0.5 mmol), 3-methyl-1H-pyrazol-5(4H)-one (2) (0.050 g, 0.5
123

Med Chem Res
FCS, containing penicillin 100 U/ml and streptomycin
bromophenol blue dye at 1:1 ratio) was loaded carefully
into the electrophoresis chamber wells along with standard
DNA marker containing TAE buffer (4.84 g tris base, pH
8.0, 0.5 M EDTA/1 l) and finally loaded on agarose gel
and passed the constant 50 V of electricity for 30 min.
Removing the gel and stained with 10.0 mg/ml ethidium
bromide for 10–15 min, the bands were observed under
Vilber Lourmat gel documentation system and then pho-
tographed to determine the extent of DNA cleavage. The
results are compared with standard DNA marker.
100 lg/ml at 37 °C in a CO2 incubator with 5 % CO2.
MTT assay
Briefly, the test compounds were diluted in DMSO
(0–100 lg/ml) and cytotoxic activity of the compounds on
A-549, HEp-2, and HeLa cells (1 9 105 cells/well) were
tested by using the cell quantity MTT cell viability assay
kit. The wells with only culture medium served as control
and the graph is plotted with cell viability against the time
period in hours at increasing concentrations of secondary
metabolite. The IC50 values were calculated by nonlinear
regression analysis from three independent experiments
(Skehan et al., 1990).
Conclusions
In conclusion, we have demonstrated the synthesis of bis-
indolyl analogs via one-pot, three-component reactions in
solvent- and catalyst-free condition. In our preliminary
cytotoxic studies, 5-OCH3 and 5-H-substituted bisindolyl
analogs 4c, 4f and 4h–4k, respectively, have shown pro-
found cytotoxic potentialities. In DNA cleavage activity,
compounds 4c and 4f–4h have shown promising ability in
DNA cleavage. The above-mentioned Michael addition has
provided new types of potential anticancer compounds
originating from indole. Consequently, this method will
provide a great impact on organic chemists and biochem-
ists for further investigations in the innovation of economic
and greener methodologies in synthesizing ‘‘drug-like’’
molecules. Based on results, selected compounds are being
screened for in vivo anticancer activity which will be
reported in due course.
DNA cleavage activity
Preparation of culture media
DNA cleavage experiments were done according to the
literature (Sambrook et al., 1989). Nutrient broth [peptone
10, yeast extract 5, NaCl 10, in (lg/l)] was used for cul-
turing of Escherichia coli. 50 ml media was prepared,
autoclaved for 15 min at 121 °C under 15 lb pressures. The
autoclaved media were inoculated for 24 h at 37 °C.
Isolation of DNA
Acknowledgments Sasidhar B.S. thanks the Council of Scientific
and Industrial Research, New Delhi (India), for the award of a Senior
Research Fellowship (Ref. No. 9/450(0032)2K10-EMR-I). We also
thank the Director of IISc (Bangalore) and IIT (madras) for spec-
troscopic analysis and BIOGENICS, Hubli, for their assistance in
carrying out biological activities.
The fresh bacterial culture (1.5 ml) is centrifuged to obtain
the pellet which is then dissolved in 0.5 ml of lysis buffer
(100 mM tris pH 8.0, 50 mM EDTA, 10 % SDS). To this
0.5 ml of saturated phenol was added and incubated at
55 °C for 10 min, then centrifuged at 10,000 rpm for
10 min and to the supernatant, equal volume of chloro-
form:isoamyl alcohol (24:1) and 1/20th volume of 3 M
sodium acetate (pH 4.8) was added. Centrifuge at
10,000 rpm for 10 min and to the supernatant, three vol-
umes of chilled absolute alcohol is added. The precipitated
DNA was separated by centrifugation and the pellet was
dried and dissolved in TAE buffer (10 mM tris pH 8.0,
1 mM EDTA) and stored in cold condition.
References
An LT, Zou JP, Zhang LL, Zhang Y (2007) Sulfamic acid-catalyzed
Michael addition of indoles and pyrrole to electron-deficient
nitroolefins under solvent-free condition. Tetrahedron Lett
48:4297–4300
Anastas PT, Warner JC (1998) Green chemistry: theory and practice.
Oxford university press, Oxford
Atul M, Pravin B, Vijay V, Rupesh K, Anamik S (2007) Microwave-
assisted and Zn[l-proline]2 catalyzed tandem cyclization undersolvent
free conditions: a rapid synthesis of chromeno[4,3-c]pyrazol-4-ones.
J Mol Catal A Chem 275:148–152
Agarose gel electrophoresis
Bagley MC, Dale JW, Bower J (2002) A new one-pot three-
component condensation reaction for the synthesis of 2,3,4,6-
tetrasubstituted pyridines. Chem Commun 16:1682–1683
Bartoli G, Bartolacci M, Bosco M, Foglia G, Giulani A, Mareantoni
E, Sambri L, Torregiani E (2003) The Michael addition of
indoles to a,b-unsaturated ketones catalyzed by CeCl3ꢀ7H2O/NaI
combination supported on silica gel. J Org Chem 68:4594–4597
Cleavage products were analyzed by agarose gel electro-
phoresis method. Test samples (1 lg/ml) were prepared in
DMF. The samples (25 lg) were added to the isolated
DNA of E. coli. The samples were incubated for 2 h at
37 °C and then 20 ml of DNA sample (mixed with
123

Med Chem Res
Bartoli G, Bosco M, Giuli S, Giuliani A, Lucarelli L, Marcantoni E,
Sambri L, Torregiani E (2005) Efficient preparation of 2-indolyl-
1-nitroalkane derivatives employing nitroalkenes as versatile
Michael acceptors: new practical linear approach to alkyl 9H-b-
carboline-4-carboxylate. J Org Chem 70:1941–1944
Biradar JS (1982) Studies in the indole field. Ph.D. Thesis, Gulbarga
University
Kuo CW, Wang CC, Fang HL, Raju BR, Kavala V, Habib PM, Yao
CF (2009) An efficient method for the N-bromosuccinimide
catalyzed synthesis of indolyl-nitroalkanes. Molecules 14:
3952–3963
Lin C, Hsu J, Sastry MNV, Fang H, Tu Z, Ju-Tsung L, Ching-Fa Y
(2005) I2-catalyzed Michael addition of indole and pyrrole to
nitroolefins. Tetrahedron 61:11751–11757
Biradar JS, Sasidhar BS (2011) Solvent-free, microwave assisted
Knoevenagel condensation of novel 2,5-disubstituted indole
analogues and their biological evaluation. Eur J Med Chem
46:6112–6118
Biradar JS, Praveen SM, Sharanbasappa B, Sasidhar BS (2008)
Improved synthesis of some pyrazolines under microwave.
Indian J Heterocycl Chem 18:141–144
Biradar JS, Sasidhar BS, Parveen R (2010) Synthesis, antioxidant and
DNA cleavage activities of novel indole derivatives. Eur J Med
Chem 45:4074–4078
Biradar JS, Parveen R, Sasidhar BS (2011) Synthesis of new indolyl
benzodiazepins and their DNA cleavage and antimicrobial
activities. Indian J Heterocycl Chem 20:181–182
Brana MF, Cacho M, Gradillas A, Pascual-Teresa BD, Ramos A
(2001) Intercalators as anticancer drugs. Curr Pharm Des
7:1745–1780
Chakrabarty M, Basak R, Ghosh G, Harigaya Y (2004) Michael
reaction of indoles with 3-(20-nitrovinyl)indole under solvent-
free conditions and in solution. An efficient synthesis of 2,2-
bis(indolyl)nitroethanes and studies on their reduction. Tetrahe-
dron 60:1941–1949
Cui SL, Lin XF, Wang YG (2005) Parallel synthesis of strongly
fluorescent polysubstituted 2,6-dicyanoanilines via microwave-
promoted multicomponent reaction. J Org Chem 70:2866–2869
Domling A (2006) Recent progress in isocyanide based multicom-
ponent reaction chemistry. Chem Rev 106:17–89
Domling A, Achatz J, Beck B (2007) Novel anti-tuberculosis agents
from MCR libraries. Biorg Med Chem Lett 17:5483–5486
Huang YJ, Yang FY, Zhu CJ (2005) Highly enantioselective Biginelli
reaction using a new chiral ytterbium catalyst: asymmetric
synthesis of dihydropyrimidines. J Am Chem Soc 127:16386–
16387
Morteza S, Mohammad AZ, Hendrik GK, Zahra T (2010) Bis- and
trisindolylmethanes (BIMs and TIMs). Chem Rev 110:2250–
2293
Mu XJ, Lei MY, Zou JP, Zhang W (2006) Microwave-assisted
solvent-free and catalyst-free Kabachnik–Fields reactions for a-
amino phosphonates. Tetrahedron Lett 47:1125–1127
Pelle L, Jason T, Bernard W, Jacob W (2001) Microwave assisted
organic synthesis—a review. Tetrahedron 57:9225–9283
Ramon DJ, Yus M (2005) Asymmetric multicomponent reactions
(AMCRs): the new frontier. Angew Chem Int Ed 44:1602–1634
Ramya VS, Kallappa MH, Rangappa SK, Mallinath HH (2010)
Derivatives of benzimidazole pharmacophore: synthesis, anti-
convulsant, antidiabetic and DNA cleavage studies. Eur J Med
Chem 45:1753–1759
Rangappa SK, Kallappa MH, Ramya VS, Mallinath HH (2010)
Analgesic, anti-pyretic and DNA cleavage studies of novel
pyrimidine derivatives of coumarin moiety. Eur J Med Chem 45:
2597–2605
Renzetti A, Dardennes E, Fontana A, De Maria P, Sapi J, Gerard S
(2008) TiCl4/Et3N-promoted three-component condensation
between aromatic heterocycles, aldehydes, and active methylene
compounds. J Org Chem 73:6824–6827
Sambrook J, Fritsch EF, Maniatis T (1989) Molecular cloning: a
laboratory manual, 2nd edn. Cold spring harbor laboratory, Cold
spring harbor
Skehan P, Storeng R, Scudiero D, Monks A, McMahon J, Visitica D,
Warren TJ, Bokesch H, Kenny S, Boyd RM (1990) New
colorimetric cytotoxicity assay for anticancer-drug screening.
J Natl Cancer Inst 82:1107–1112
Victoria GM, Andres M, Ester V, de Antonio LH, Ana IA, Angel
Diaz O (2010) Synergy between microwave irradiation and
heterogeneous catalysis in an environmentally friendly self-
condensation of hydroxybenzene derivatives. ARKIVOC
3:264–273
Virsodia V, Manvar A, Upadhyay K, Loriya R, Karia D, Jaggi M,
Singh A, Mukherjee R, Yadav JS, Abraham S, Reddy BVS,
Sabitha G (2001) Indium trichloride-catalyzed conjugate addi-
tion of indoles with electron-deficient olefins. Synthesis
14:2165–2169
Jean FR, Philippe F, Chi HN, Annette KL, Marie CB, Lucile G, Jean
MS, Emile B, Francois L (1993) Inloplicine (RP 60475), and its
derivatives, a new class of antitumor agents inhibiting both
topoisomerase I and II activities. Cancer Res 15(53):5987–5993
Ji SJ, Wang SY (2003) Ultrasound-accelerated Michael addition of
indole to a, b unsaturated ketones catalyzed by ceric ammonium
nitrate (CAN). Synlett 13:2074–2076
Kappe CO (2004) Controlled microwave heating in modern organic
synthesis. Angew Chem Int Ed 43:6250–6284
Kappe CO, Dallinger D (2009) Controlled microwave heating in
modern organic synthesis: highlights from the 2004–2008
literature. Mol Divers 13:71–193
Kidwai M, Mothsra P, Mohan R, Biswas S (2005) 1-Aryl-4,6-
diamino-1,2-dihydrotriazine as antimalarial agent: a new syn-
thetic route. Bioorg Med Chem Lett 15:915–917
Zhan ZP, Yang RF, Lang K (2005) Samarium triiodide-catalyzed
conjugate addition of indoles with electron-deficient olefins.
Tetrahedron Lett 46:3859–3862
Zhao JL, Liu L, Zhang HB, Wu YC, Wang D, Chen YJ (2006) Three-
component Friedel–Crafts reaction of indoles, glyoxylate, and
amine under solvent-free and catalyst free conditions—synthesis
of (3-indolyl)glycine derivatives. Synlett 1:96–98
123