A facile synthesis of 2-aryl-4-(indol-3-yl)-4H-chromenes using amberlyst-15 as an efficient recyclable heterogeneous catalyst

Article abstract of DOI:10.1007/s00706-014-1401-8

Starting from 2-hydroxychalcones and indoles, a facile synthesis of 2-aryl-4-(indol-3-yl)-4H-chromenes has been achieved by use of amberlyst-15, a sulfonated polystyrene resin, as a recyclable heterogeneous catalyst. The methodology involves a domino sequ

Full text of DOI:10.1007/s00706-014-1401-8

Monatsh Chem
DOI 10.1007/s00706-014-1401-8
ORIGINAL PAPER
A facile synthesis of 2-aryl-4-(indol-3-yl)-4H-chromenes using
amberlyst-15 as an efficient recyclable heterogeneous catalyst
•
Chayan Guha Nayim Sepay Asok K. Mallik
•
Received: 22 September 2014 / Accepted: 28 December 2014
Ó Springer-Verlag Wien 2015
Abstract Starting from 2-hydroxychalcones and indoles,
a facile synthesis of 2-aryl-4-(indol-3-yl)-4H-chromenes
has been achieved by use of amberlyst-15, a sulfonated
polystyrene resin, as a recyclable heterogeneous catalyst.
The methodology involves a domino sequence of Michael
addition, cyclization, and dehydration.
opener [7], insecticide [8], termiticide [9], etc. Again, some
chromene crystals are reported to exhibit photochromism
[10]. Such wide range of natural availability coupled with
their diverse utility has made these heterocycles popular
synthetic targets [11–17]. On the other hand, indole
derivatives are well-known for their versatile biological
activities [18], some of which include selective PPAR
gamma modulator [19], sodium/potassium exchanger
inhibitor [20], etc. It is observed that construction of
molecules by combination of two or more different bio-
logically active structural units often generates compounds
having enriched activities [5, 6]. So, we became interested
to undertake the synthesis of molecules containing both
chromene and indole moieties. A detailed survey of liter-
ature revealed that there are several reports describing
synthesis of such molecules [13, 21–24]. However, those
synthetic methodologies are found to involve use of
expensive reagents, complex starting materials, or non-
recyclable catalyst. Herein, we report a facile synthesis of
2-aryl-4-(indol-3-yl)-4H-chromenes 1 starting from mole-
cules as simple as 2-hydroxychalcones 2 and indoles 3 and
using amberlyst-15, a commercially available sulfonated
polystyrene resin, as a recyclable catalyst. The methodol-
ogy involves a domino sequence of Michael addition,
cyclization, and dehydration.
Graphical abstract
Keywords 2-Aryl-4-(indol-3-yl)-4H-chromenes Á
Heterogeneous catalysis Á Michael addition Á Cyclization
Introduction
Chromenes are often found to be present in the structural
motif of a good number of natural products, the important
ones of which include some alkaloids, flavonoids, toc-
opherols, and anthocyanins [1]. Many chromenes exhibit
important biological activities like anticancer [2, 3], anti-
bacterial [4], apoptosis inducer [5, 6], potassium channel
Results and discussion
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-014-1401-8) contains supplementary
material, which is available to authorized users.
In this study, our aim was to achieve a simple synthesis of
2-aryl-4-(indol-3-yl)-4H-chromenes 1 in three steps, the
first one being a Michael addition of indoles 3 with 2-hy-
droxychalcones 2 and the subsequent ones cyclization
followed by dehydration. Chalcones and their derivatives,
by virtue of possessing an a,b-unsaturated keto system,
C. Guha Á N. Sepay Á A. K. Mallik (&)
Department of Chemistry, Jadavpur University, Kolkata 700032,
India
e-mail: mallikak52@yahoo.co.in
123

C. Guha et al.
serve as good Michael acceptors [25]. On the other hand,
indoles are well-known for exhibiting electrophilic substi-
tution reactions at their C-3 position and thus serving as a
good source of carbon nucleophile of soft nature [26].
Amberlyst-15, known to be useful for Michael addition
[25, 27], was employed to catalyze the first step which was
performed under refluxing condition in toluene. Here, it
was observed with interest that instead of stopping in the
Michael addition step, the reaction proceeded further
rationalized. The overall process restores the resin in its
original form, which is used in the next cycle of reactions.
It has been observed that the resin can be recovered from
the mixture after completion of the reaction and can be
used further with no significant reduction of its activity
(Figs. 1, 2).
In fine, we report a very simple methodology for rapid
synthesis of 2-aryl-4-(indol-3-yl)-4H-chromenes 1 having
potential biological activities.
generating our target molecules
(Scheme 1).
1 in good yield
It was then our interest to develop the methodology
further. The reaction condition was optimized for this
purpose using different solvents and catalyst loads
(Table 1). Thus, use of only 30 mg of the catalyst for
1 mmol of each of the reactants under refluxing toluene for
3 h was found to be the optimum condition for synthesis of
1h. This protocol was then successfully applied to sixteen
combinations of reactants involving four indoles and seven
2-hydroxychalcones and one related compound (Table 2).
It was observed that the presence of an electron-with-
drawing substituent like Cl at either of the phenyl rings of
2-hydroxychalcone improved the yield whereas an elec-
tron-releasing substituent like OMe diminished the same.
The reaction with 2-hydroxybenzalacetone with indole did
not yield any product analogous to 1. Again, 2-methylin-
dole was found to give better yield than indole itself while
the yields were found to be gradually lower in cases of
5-methoxyindole and 5-bromoindole. Attempted synthesis
of the compounds analogous to 1 by involving 1-methyl-
indole and 2-phenylindole were, however, unsuccessful.
The plausible mechanism for formation of the indolyl-
chromene derivatives 1 is shown in Scheme 2. The acidic
resin first catalyzes the Michael addition to give the adduct
4 which undergoes acid catalyzed cyclization followed by
proton capture generating the protonated alcohol 5. The
latter on dehydration followed by deprotonation yields 1. It
is reasonable to expect that the conjugate acid of 2 will be
more reactive towards indole addition when that contains
electron withdrawing group at either of the phenyl rings,
and this was our observation. However, the effect of
incorporating a substituent in indole could not be fully
Experimental
Melting points were recorded on a Kofler block. Analytical
samples were dried in vacuo at room temperature. IR
spectra were recorded on a Perkin Elmer FT-IR Spectro-
photometer (Spectrum BX II) as KBr pellets. H and 13C
1
NMR spectra were recorded in CDCl3 on a Bruker AV-300
(300 MHz) and Bruker AVANCE IIIHD (400 MHz)
spectrometer using TMS as an internal standard. FAB MS
was measured with a JEOL JMS-700 spectrometer and
HRMS with a Waters Xevo G2QTof HRMS spectrometer.
Microanalytical data were recorded on two Perkin-Elmer
2400 Series II C, H, N analyzers. Column chromatography
was performed on neutral alumina using petroleum ether
(60–80 °C) and petroleum ether-ethyl acetate mixtures as
eluents. TLC was done with silica gel G. Amberlyst-15 was
manufactured by Fluka Chemika, Switzerland; its specifi-
cation and SEM picture are given as supplementary
material. 2-Hydroxychalcones and related compound used
in this study were obtained by simple Claisen-Schmidt
condensation of salicylaldehydes and aromatic methyl
ketones.
General procedure for synthesis of 2-aryl-4-(indol-3-
yl)-4H-chromenes 1
A mixture of 2-hydroxychalcone 2 (1 mmol), indole 3
(1 mmol), and 30 mg amberlyst-15 was refluxed in 10 cm3
dry toluene for 3 h. The resulting mixture was filtered
through cotton and washed thoroughly with diethyl ether.
The catalyst thus separated was preserved for recovery.
Scheme 1
123

A facile synthesis of 2-aryl-4-(indol-3-yl)-4H-chromenes
Table 1 Optimization of reaction conditions
4-(Indol-3-yl)-2-phenyl-4H-chromene (1a)
Dark red solid; m.p.: 104–106 °C (Ref. [24] 100–102 °C).
Entry Amount of
catalyst/mg
Solvent
Temp.
Time/h Yielda/%
4-(Indol-3-yl)-2-(4-methylphenyl)-4H-chromene
1
No catalyst Toluene
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
3
13
56
66
76
76
74
73
43
46
34
22
17
23
(1b, C24H19NO)
Dark red solid; m.p.: 94–96 °C; 1H NMR (300 MHz, CDCl3):
d = 2.35 (1H, s, –CH3), 5.16 (1H, d, J = 3.9 Hz, chromene
H-4), 5.65 (1H, d, J = 3.9 Hz, chromene H-3), 6.85–6.98 (m,
1H), 7.02–7.21 (m, 7H), 7.45 (d, 2H, J = 8.7 Hz), 7.63 (dd,
2H, J = 8.2, 2.4 Hz), 7.89 (d, 1H, J = 8.1 Hz), 8.14 (s, 1H,
indole N–H) ppm; 13C NMR (75 MHz, CDCl3): d = 21.2,
32.1, 100.2, 111.3, 116.5, 119.4, 119.6, 121.6, 122.1, 123.3,
123.5, 124.6, 126.4, 127.5, 128.9, 129.3, 129.5, 131.7, 136.7,
2
10
20
30
40
30
30
30
30
30
30
30
30
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
3
3
3
4
3
5
3
6
3.5
4
6
7
3
8
Ethanol
3
ꢀ
138.2, 147.8, 151.2 ppm; IR (KBr): m = 3,406 (N–H), 2,857,
9
Acetonitrile Reflux
3
1,726, 1,465, 1,233, 756 cm-1; FAB MS: m/z calcd. for
11
12
13
Dioxane
DMF
Reflux
Reflux
120 °C
3
C24H19NO (M?) 337.28, found 337.30.
3
DMF
3
4-(Indol-3-yl)-2-(4-methoxyphenyl)-4H-chromene (1c)
Dark red solid; m.p.: 98–100 °C (Ref. [24] 96–97 °C).
Reaction was carried out with 1 mmol 2-hydroxychalcone (2a) and
1 mmol 2-methylindole (3b)
a
2-(4-Chlorophenyl)-4-(indol-3-yl)-4H-chromene (1d)
Dark red solid; m.p.: 87–89 °C (Ref. [24] 85–86 °C).
Isolated yields
4-(Indol-3-yl)-2-(thiophen-2-yl)-4H-chromene (1e)
1
Dark red oil; H NMR was found to agree with the one
From the filtrate, solvent was distilled out under reduced
pressure and the residue was subjected to a rapid column
chromatography over neutral alumina to get 1 in pure state.
given in Ref. [24].
6-Chloro-4-(indol-3-yl)-2-phenyl-4H-chromene
Recovery of the catalyst
(1f, C23H16ClNO)
Dark red solid; m.p.: 76–78 °C; 1H NMR (300 MHz,
CDCl3): d = 5.14 (1H, d, J = 3.9 Hz, chromene H-4),
5.70 (1H, d, J = 3.9 Hz, chromene H-3), 7.03 (1H, d,
J = 2.1 Hz), 7.10–7.47 (9H, m), 7.66 (1H, d, J = 7.8 Hz),
7.77 (2H, d, J = 6.9 Hz), 7.98 (1H, s, indole N–H) ppm;
13C NMR (75 MHz, CDCl3): d = 62.3, 100.8, 111.5,
The catalyst separated from the reaction mixture was
washed thoroughly with ethyl acetate and acetone succes-
sively until the washings became colorless. It was then
dried in a hot air oven at 100 °C for 8 h. The dried catalyst
was used further.
Table 2 Synthesis of 2-aryl-4-
(indole-3-yl)-4H-chromenes (1)
from substituted
2-hydroxychalcones and indoles
(Scheme 1)
Entry
Prod.
Ar
R1
R2
R3
R4
Time/h
Yield/%a
1
1a
1b
1c
1d
1e
1f
C6H5
H
H
H
H
H
H
Cl
Cl
H
H
H
H
Cl
Cl
H
H
H
H
H
3
73
69
65
74
53
74
68
76
71
78
69
77
70
65
55
61
2
4-CH3–C6H4
4-CH3O–C6H4
4-Cl–C6H4
2-Thienyl
C6H5
H
H
H
3
3
H
H
H
3.5
3
4
H
H
H
5
H
H
H
4
6
H
H
H
3
7
1g
1h
1i
4-CH3O–C6H4
C6H5
H
H
H
3
8
H
CH3
CH3
CH3
CH3
CH3
CH3
H
H
3
9
4-CH3O–C6H4
4-Cl–C6H4
C6H5
H
H
3
10
11
12
13
14
15
16
1j
H
H
3
1k
1l
CH3O
H
H
3.5
3
C6H5
H
1m
1n
1o
1p
4-CH3O–C6H4
C6H5
H
H
3
H
CH3O
Br
Br
3.5
4
Reaction was performed under
optimized condition
4-CH3–C6H4
4-Cl–C6H4
H
H
H
H
4
a
Isolated yields
123

C. Guha et al.
Scheme 2
118.0, 119.3, 119.8, 120.6, 122.3, 124.8, 125.2, 126.2,
127.7, 128.1, 128.4, 128.6, 128.8, 129.2, 134.1, 136.8,
75
70
65
60
55
50
ꢀ
147.7, 149.8 ppm; IR (KBr): m = 3,406 (N–H), 3,057,
1,719, 1,598, 1,485, 1,230, 1,060 cm-1; HRMS: m/z calcd.
for C23H16ClNO (M ? H)? 358.10, found 358.08.
6-Chloro-4-(indol-3-yl)-2-(4-methoxyphenyl)-4H-chro-
mene (1g, C24H18ClNO2)
Dark red solid; m.p.: 90–92 °C; 1H NMR (300 MHz,
CDCl3): d = 3.86 (3H, s, –OCH3), 5.09 (1H, d,
J = 3.9 Hz, chromene H-4), 5.54 (1H, d, J = 3.9 Hz,
chromene H-3), 6.88 (2H, d, J = 8.4 Hz), 6.92–7.25 (m,
6H), 7.36 (1H, d, J = 8.1 Hz), 7.59–7.72 (3H, m), 8.06
(1H, s, indole N–H) ppm; 13C NMR (75 MHz, CDCl3):
d = 32.1, 55.2, 99.4, 111.2, 113.7, 116.5, 119.4, 119.6,
121.8, 122.0, 122.1, 123.3, 123.5, 126.1, 126.4, 127.2,
Fresh Run 1 Run 2 Run 3 Run 4 Run 5 Run 6
Catalytic cycle
Fig. 1 Plot of isolated yield against catalytic cycle
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
127.4, 129.5, 136.7, 147.6, 151.2, 159.8 ppm; IR (KBr):
-1
ꢀ
m = 3,423 (N–H), 2,924, 1,639, 1,501, 1,238, 1,172 cm
;
HRMS: m/z calcd. for C24H18ClNO2 (M ? H)? 388.10,
found 388.09.
4-(2-Methylindol-3-yl)-2-phenyl-4H-chromene (1h)
Pale brown solid; m.p.: 149–151 °C (Ref. [24]
146–147 °C).
2-(4-Methoxyphenyl)-4-(2-methylindol-3-yl)-4H-chromene
0.0
(1i, C25H21NO2)
Fresh Run 1 Run 2 Run 3 Run 4 Run 5 Run 6
1
Pale brown solid; m.p.: 70–72 °C; H NMR (400 MHz,
Catalytic cycle
CDCl3): d = 2.39 (3H, s, –CH3), 3.85 (3H, s, –OCH3),
5.18 (1H, d, J = 3.6 Hz, chromene H-4), 5.45 (1H, d,
Fig. 2 Plot of reaction time against catalytic cycle
123

A facile synthesis of 2-aryl-4-(indol-3-yl)-4H-chromenes
J = 3.6 Hz, chromene H-3), 6.86–6.97 (5H, m), 7.05–7.17
(4H, m), 7.45 (1H, d, J = 8.0 Hz), 7.65 (2H, dd, J = 7.0,
1.8 Hz), 7.81 (1H, s, indole N–H) ppm; 13C NMR
(75 MHz, CDCl3): d = 11.9, 30.9, 55.3, 99.3, 110.2,
113.7, 116.1, 116.3, 118.6, 119.4, 120.9, 123.3, 123.5,
126.0, 127.2, 127.4, 127.9, 129.4, 131.4, 135.2, 147.5,
151.3, 159.8 ppm; IR (KBr): mꢀ = 3,404 (N–H), 2,858,
2,354, 1,731, 1,465, 1,241 cm-1; FAB MS: m/z calcd. for
C25H21NO2 (M?) 367.17, found 367.2.
6-Chloro-2-(4-methoxyphenyl)-4-(2-methylindol-3-yl)-4H-
chromene (1m, C25H20ClNO2)
1
Pale brown solid; m.p.: 142–144 °C; H NMR (300 MHz,
CDCl3): d = 2.42 (3H, s, –CH3), 3.83 (3H, s, –OCH3),
5.11 (1H, d, J = 3.3 Hz, chromene H-4), 5.41 (1H, d,
J = 3.3 Hz, chromene H-3), 6.85 (d, 3H, J = 8.4 Hz),
6.89–7.29 (m, 6H), 7.43 (d, 1H, J = 7.8 Hz), 7.63 (d, 1H,
J = 8.7 Hz),7.84 (1H, s, indole N–H) ppm; 13C NMR
(75 MHz, CDCl3): d = 11.9, 26.9, 55.5, 99.3, 110.2,
113.7, 116.1, 116.3, 118.6, 119.4, 120.9, 123.3, 123.5,
126.0, 127.2, 127.4, 127.9, 129.4, 131.4, 135.2, 147.5,
2-(4-Chlorophenyl)-4-(2-methylindol-3-yl)-4H-chromene
(1j, C24H18ClNO)
ꢀ
151.3, 159.8 ppm; IR (KBr): m = 3,453 (N–H), 2,865,
Dark brown solid; m.p.: 106–108 °C; 1H NMR (300 MHz,
CDCl3): d = 2.40 (3H, s, –CH3), 5.18 (1H, d, J = 3.6 Hz,
chromene H-4), 5.55 (1H, d, J = 3.9 Hz, chromene H-3),
6.87–6.98 (2H, m), 7.05–7.22 (4H, m), 7.33–7.43 (4H, m),
7.65 (2H, d, J = 7.2 Hz), 7.83 (1H, s, indole N–H) ppm;
13C NMR (75 MHz, CDCl3): d = 11.9, 31.1, 101.6, 110.4,
116.0, 116.4, 118.7, 119.6, 121.2, 123.5, 123.6, 124.8,
127.6, 127.9, 128.4, 129.6, 131.6, 134.5, 135.2, 147.8,
2,347, 1,822, 1,466, 1,088 cm-1; HRMS: m/z calcd. for
C25H20ClNO2 (M ? H)? 402.1283, found 402.1296.
4-(5-Methoxyindol-3-yl)-2-phenyl-4H-chromene
(1n, C24H19NO2)
1
Dark red oil; H NMR (300 MHz, CDCl3): d = 3.66 (3H,
s, –OCH3), 5.15 (1H, d, J = 3.9 Hz, chromene H-4), 5.70
(1H, d, J = 3.9 Hz, chromene H-3), 6.85 (dd, 1H, J = 8.7,
2.1 Hz), 6.97 (t, 2H, J = 6.9 Hz), 7.06–7.74 (m, 7H),
7.75–7.77 (m, 3H), 7.98 (1H, s, indole N–H) ppm; 13C
NMR (75 MHz, CDCl3): d = 32.2, 55.8, 101.0, 101.2,
111.9, 112.3, 116.5, 121.3, 122.8, 123.4, 124.7, 126.8,
127.5, 128.3, 128.6, 129.5, 131.8, 134.4, 147.9, 151.2,
153.9 ppm; FAB MS: m/z calcd. for C24H19NO2 (M?)
353.14, found 353.3.
ꢀ
151.3 ppm; IR (KBr): m = 3,413 (N–H), 3,024, 1,666,
1,582, 1,458, 1,326, 1,230, 1,181 cm-1; HRMS: m/z calcd.
for C24H18ClNO (M ? H)? 372.13, found 372.14.
8-Methoxy-4-(2-methylindol-3-yl)-2-phenyl-4H-chromene
(1k, C25H21NO2)
Dark red solid; m.p.: 66–68 °C; 1H NMR (400 MHz,
CDCl3): d = 2.40 (3H, s, –CH3), 3.86 (3H, s, –OCH3),
5.17 (1H, d, J = 3.9 Hz, chromene H-4), 5.44 (1H, d,
J = 3.9 Hz, chromene H-3), 6.86–6.98 (5H, m), 7.05–7.17
(4H, m), 7.45 (1H, d, J = 8.4 Hz), 7.65 (2H, d,
J = 8.1 Hz), 7.79 (1H, s, indole N–H) ppm; 13C NMR
(75 MHz, CDCl3): d = 11.9, 30.9, 55.3, 99.2, 110.2,
113.6, 116.0, 116.3, 118.6, 119.3, 120.9, 123.3, 123.4,
126.0, 127.1, 127.3, 127.8, 129.4, 131.4, 135.2, 147.4,
4-(5-Bromoindol-3-yl)-2-(4-methylphenyl)-4H-chromene
(1o, C24H18BrNO)
Dark red solid; m.p.: 64–66 °C; 1H NMR (300 MHz,
CDCl3): d = 2.39 (3H, s, –CH3), 5.14 (1H, d, J = 3.9 Hz,
chromene H-4), 5.61 (1H, d, J = 3.9 Hz, chromene H-3),
6.97 (t, 1H, J = 7.8 Hz), 7.05–7.29 (m, 7H), 7.64 (d, 2H,
J = 8.1 Hz), 7.81 (s, 1H, indole H-4), 8.07 (1H, s, indole
N–H) ppm; 13C NMR (75 MHz, CDCl3): d = 21.2, 31.8,
99.8, 102.2, 112.5, 112.8, 116.7, 121.5, 121.8, 123.1,
123.4, 124.7, 125.0, 125.4, 128.1, 129.3, 131.5, 135.2,
ꢀ
151.2, 159.7 ppm; IR (KBr): m = 3,413 (N–H), 2,918,
1,589, 1,468, 1,308, 1,248, 753 cm-1; FAB MS: m/z calcd.
for C25H21NO2 (M?) 367.17, found 367.3.
ꢀ
138.4, 148.1, 151.2, 162.4 ppm; IR (KBr): m = 3,400
6-Chloro-4-(2-methylindol-3-yl)-2-phenyl-4H-chromene
(1l, C24H18ClNO)
(N–H), 2,928, 1,479, 1,317, 1,247, 743 cm-1; FAB MS: m/
z calcd. for C24H18BrNO (M?) 414.16 and 416.16, found
414.2 and 416.2.
Dark red solid; m.p.: 82–84 °C; 1H NMR (300 MHz,
CDCl3): d = 2.42 (3H, s, –CH3), 5.14 (1H, d, J = 3.3 Hz,
chromene H-4), 5.54 (1H, d, J = 3.3 Hz, chromene H-3),
6.91 (s, 1H), 6.97 (1H, t, J = 7.3 Hz), 7.04–7.12 (3H, m),
7.29–7.44 (5H, m), 7.70 (2H, d, J = 7.5 Hz), 7.86 (1H, s,
indole N–H) ppm; 13C NMR (75 MHz, CDCl3): d = 11.9,
29.7, 31.1, 100.5, 110.3, 115.4, 117.8, 118.5, 119.6, 121.2,
124.7, 125.0, 127.6, 128.1, 128.3, 128.5, 129.0, 131.5,
4-(5-Bromoindol-3-yl)-2-(4-chlorophenyl)-4H-chromene
(1p)
Dark red solid; m.p.: 72–74 °C (Ref. [24] 75–77 °C).
Acknowledgments Financial assistance from the UGC-CAS and
DST-PURSE programs, Department of Chemistry is gratefully
acknowledged. The authors also acknowledge the DST-FIST program
to the Department of Chemistry, Jadavpur University for providing
the NMR spectral data. CG and NS are thankful to the UGC, New
Delhi for their Research Fellowships.
ꢀ
134.0, 135.2, 147.7, 149.9 ppm; IR (KBr): m = 3,425
(N–H), 2,879, 2,347, 1,598, 1,472, 1,248 cm-1; FAB MS:
m/z calcd. for C24H18ClNO (M?) 371.16, found 371.2.
123

C. Guha et al.
´ ´
13. Bello D, Ruiz-Rodrıguez J, Albericio F, Ramon R, Lavilla R
(2010) Eur J Org Chem 28:5373
References
14. Kharrat SE, Laurent P, Blancou H (2006) J Org Chem 71:8637
15. Liu Y, Qian J, Lou S, Zhu J, Xu Z (2010) J Org Chem 75:1309
16. Shi YL, Shi M (2007) Org Biomol Chem 5:1499
17. Malakar CC, Schmidt D, Conrad J, Beifuss U (2011) Org Lett
13:1972
18. Sharma V, Kumar P, Pathak D (2010) J Heterocycl Chem 47:491
19. Lamotte Y, Martres P, Faucher N, Laroze A, Grillot D, Ancellin
N, Saintillan Y, Beneton V, Gampe RT (2010) Bioorg Med Chem
Lett 20:1399
20. Kitano M, Kojima A, Nakano K, Miyagishi A, Noguchi T, Oh-
ashi N (1999) Chem Pharm Bull 47:1538
21. Shanthi G, Perumal PT (2007) Tetrahedron Lett 48:6785
22. Chen W, Cal Y, Fu X, Liu X, Lin L, Feng X (2011) Org Lett
13:4910
23. Pang W, Han JW, Zhao JW, Zhu SZ (2011) Synthesis 2011:3364
24. Yin G, Fan L, Ren T, Zheng C, Tao Q, Wub A, She N (2012) Org
Biomol Chem 10:8877
25. Guha C, Mondal R, Pal R, Mallik AK (2013) J Chem Sci
125:1463
26. Jackson AH, Smith AE (1965) Tetrahedron 21:989 and references
cited therein
27. Pal R, Sarkar T, Khasnobis S (2012) Arkivoc (i):570
1. Iacobucci GA, Sweeny JG (1983) Tetrahedron 39:3005
2. Shestopalov AM, Litvinov YM, Rodinovskaya LA, Malyshev
OR, Semenova MN, Semenov VV (2012) ACS Comb Sci 14:484
3. Tripathi AK, Mukherjee D, Koul S, Taneja SC, Agrawal SK,
Sharma PR, Saxena AK (2011) Ind J Chem 50B:1619
4. Kidwai M, Saxena S, Khan MKR, Thukral SS (2005) Bioorg Med
Chem Lett 15:4295
5. Kemnitzer W, Drewe J, Jiang S, Zhang H, Zhao J, Crogan-
Grundy C, Xu L, Lamothe S, Gourdeau H, Denis R, Tseng B,
Kasibhatla S, Cai SX (2007) J Med Chem 50:2858
6. Kemnitzer W, Drewe J, Jiang S, Zhang H, Crogan-Grundy C,
Denis L, Bubenick M, Attardo G, Denis R, Lamothe S, Gourdeau
H, Tseng B, Shailaja K, Cai SX (2008) J Med Chem 51:417
7. Gadwood RC, Kamdar BV, Dubray LAC, Wolfe ML, Smith MP,
Watt W, Mizsak SA, Groppit VE (1993) J Med Chem 36:1480
8. Towers GHN, Rodriguez E (1983) J Nat Prod 46:331
9. Meepagala KM, Osbrink W, Burandt C, Laxb A, Dukea SO
(2011) Pest Manag Sci 67:1446
10. Hobley J, Malatesta V, Millini R, Giroldini W, Wis L, Goto M,
Kishimoto M, Fukumura H (2000) Chem Commun 36:1339
11. Zhang X, Zhang S, Wang W (2010) Angew Chem Int Ed 49:1481
12. Yang G, Luo C, Mu X, Wang T, Liu XY (2012) Chem Commun
48:5880
123