Synthesis, characterization and investigation of catalytic activity of a highly sulfonated carbon solid acid in the synthesis of dihydropyrimidinones under solvent-free conditions

Article abstract of DOI:10.1007/BF03245891

A highly sulfonated carbon as an efficient, recyclable, nontoxic and green solid acid catalyst was synthesized readily by simultaneous sulfonation, dehydration and carbonization of sucrose C₁₂H₂₂O ₁₁ in sulfuric acid and w

Full text of DOI:10.1007/BF03245891

J. Iran. Chem. Soc., Vol. 8, No. 3, September 2011, pp. 608-615.
JOURIrNaAnLiaOnF THE
Chemical Society
Synthesis, Characterization and Investigation of Catalytic Activity of a Highly
Sulfonated Carbon Solid Acid in the Synthesis of Dihydropyrimidinones under
Solvent-Free Conditions
V. Mirkhani*, M. Moghadam, S. Tangestaninejad, I. Mohammadpoor-Baltork and M. Mahdavi
Chemistry Department, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran
(Received 2 October 2008, Accepted 11 June 2009)
A highly sulfonated carbon as an efficient, recyclable, nontoxic and green solid acid catalyst was synthesized readily by
simultaneous sulfonation, dehydration and carbonization of sucrose C12H22O11 in sulfuric acid and was characterized with Fourier
transform infrared spectroscopy (FT-IR), thermal gravimetric and differential thermal gravimetric analysis (TG-DTG), X-ray
diffraction (XRD), carbon-hydrogen-nitrogen-sulfur analysis (CHNS), neutralization potentiometric titration and scanning
electron microscopy (SEM). This new catalyst was used in one pot three-component condensation reaction of various aromatic
aldehydes with β-ketoester and urea under solvent-free conditions to afford the corresponding dihydropyrimidinones (DHPMs).
This catalyst can be reused several times without loss of its activity.
Keywords: Solid acid catalyst, Sulfonated carbon, Dihydropyrimidinones, Solvent-free conditions
INTRODUCTION
long reaction times and frequently low yields. In recent years,
new methods for preparation of dihydropyrimidinones have
been the subject of research for chemists. A number of
synthetic procedures based on the modifications of the
classical Biginelli’s approach have been developed during past
few years. Basically, these methods are all similar, using
different Lewis acid catalysts. The replacement of
environmental hazardous solvents (by solvent free conditions)
and catalysts (by the use of solid acid catalysts) are the
innovative trends.
In the other hand the preparation of cheap, recyclable and
nontoxic strong solid acids also is an attractive area in green
chemistry. This kind of solid acids must be as strong, active
and stable as sulfuric acid [9-14]. A good material for this
purpose should have high stability and numerous strong
protonic acid sites. Most commonly used Lewis acids are BF3-
OEt2/CuCl, InCl3, NbCl5, vanadium(III) chloride, cupric
chloride, LiBr, SnCl2-CdCl2, N-bromosuccinimide under
The dihydropyrimidinone (DHPMs) derivatives have
attracted considerable interest in recent years due to promising
activities as calcium channel blockers, and as
antihypertensive, antibacterial, antitumor and anti-
inflammatory agents [1-6]. Additionally, their particular
structure have been found in natural marine alkaloid
Batzelladine A and B, which are natural products reported in
the literature to inhibit the binding of HIV gp-120 to CD4
cells, so disclosing a new field towards the development of
AIDS therapy [7]. The original procedure for the preparation
of this type of compounds was reported by Biginelli [8],
involving one-pot condensation of ethyl acetoacetate, aromatic
aldehydes and urea under strongly acidic conditions.
However, this reaction suffers from the harsh conditions,
*Corresponding author. E-mail: mirkhani@sci.ui.ac.ir

Mirkhani et al.
R
microwave irradiation, polymer-supported ytterbium(II)
R
OEt
CH3
reagent, polyphosphate ester, Mn(OAc)3-2H2O, LaCl3-7H2O,
BiCl3, ZrCl4, La(OTf)3, NiCl2-6H2O, FeCl3-6H2O, Bi(OTf)3,
natural clays, HEU-type zeolite and HY-zeolite, lithium
perchlorate, Ag3PW12O40, RuCl3, ferric chloride/tetraethyl
orthosilicate, MgCl2-6H2O, iodine-alumina, trimethylchloro-
silane, heteropoly acids (HPAs), silica gel supported-sodium
hydrogen sulfate, silica sulfuric acid, KHSO4 as well as
Bronsted acids, such as p-toluene sulfonic acid,
montmorilonite KSF, and methanesulfonic acid [15-55].
Obviously, many of these catalysts are not acceptable in the
context of green synthesis. Although many inorganic-oxide
solid acids (such as zeolites and niobic acid that provide acidic
OH groups) and organic acids (such as cationic exchange
resins and perfluorosulfonated ionomers with SO3H groups as
strong acidic sites) have been studied extensively but these
materials are expensive and the activity of acidic sites are
much lower than liquid inorganic acids. Therefore, the
adequate performance in acid-catalyzed reactions, in aqueous
media, has not been obtained [56-60].
Highly sulfonated carbon material is a promising solid acid
catalyst. However, sulfonation of carbon in concentrated
sulfuric acid is very difficult and finally mellitic acid,
[C6(COOH)6], is produced. Therefore, the sulfonation of
carbon nanotubes has been investigated for preparation of
sulfonated carbon, but carbon nanotubes are very expensive
and the degree of sulfonation is very low [9]. Okamura et al.
used D-glucose for preparation of sulfonated carbon as a solid
acid. First, they pyrrolyzed the D-glucose and polycyclic
aromatic carbon sheets were obtained by heating and then
sulfonation was carried out in sulfuric acid under N2. This
process produces a low density of sulfonic acid groups on
carbon sheets [10]. Hara et al. used sulfonated naphthalene as
starting material and then carbonized the naphthalene sulfonic
acid under N2. This process is very tedious and time
consuming and releases a lot of SO2 as by-product [11].
Here, we introduce a highly sulfonated carbon material by
simultaneous sulfonation, dehydration and carbonization of
sucrose C12H22O11 in one step. The obtained catalyst is a
highly sulfonated carbon solid acid (CH0.43O0.65S0.22) with an
amorphous structure.
O
C-SO3H
O
O
O
+
+
H2N
NH2
EtO
H3C
NH
CHO
N
H
O
Scheme 1. Synthesis of dihydropyrimidinones
using new catalysts has been under attention. Here, we wish to
report the use of a highly sulfonated carbon material as a
promising alternate solid acid catalyst for preparation of
dihydropyrimidinones in the Biginelli-type reactions in one-
pot condensation of ethyl acetoacetate, benzaldehyde and urea
under solvent free reactions (Scheme 1).
EXPERIMENTAL
Instruments
Melting points were measured on an Elecrtothermal 9100
apparatus. 1H NMR spectra were recorded on a BRUKER
DRX-300 AVANCE spectrometer using TMS as an internal
standard (CDCl3 solution). Fourier transform infrared spectra
(FT-IR) were recorded on a Nicolet IMACT 400D instrument.
Thermal gravimetric and differential thermal gravimetric
analyses (TG-DTG) were carried out using a METTLER TG
50 instrument. X-ray diffraction patterns (XRD) were obtained
on a Bruker D8 ADVANCE instrument. Scanning electron
microscopy images (SEM) were taken on SEM Philips XL30.
The specific surface area and mean pore diameter (dp) were
calculated from adsorption isotherms of nitrogen at 77 °K
using the standard Brunauer-Emmett-Teller (BET) equation
and were obtained on a NOVA 2200, Quantachrome
Corporation, Version 7.11 instrument.
Typical Procedure for Preparation of Solid Acid
In a typical procedure for the preparation of highly
sulfonated carbon solid acid, 25 g sucrose was ground and
gradually added to 75 ml of concentrated H2SO4 (98 wt%).
The solution was stirred violently and then was heated for 2 h
in 523 K. Some of the carbon atoms in hot sulfuric acid were
oxidized to CO2 and a lot of foam was made during heating of
mixture, but heating was necessary for decomposition of C-O-
SO3H esters which were produced during the catalyst
Due to the importance of Biginelli reaction products, the
discovery and introduction of better and milder conditions
609

Synthesis, Characterization and Investigation of Catalytic Activity
SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
OH
HO
OH
OH
OH
OH
SO3H
SO3H
-H2O
SO3H
O
O
+ H2SO4
SO3H
SO3H
SO3H
SO3H
SO3H
OH
HO
OH
SO3H
C12H22O11
SO3H
SO3H
SO3H
Scheme 2. Preparation of sulfonated carbon
preparation. After then, the product was dried in an oven at
373 K and then heated at 623 K (350 °C) for removing trapped
sulfuric acid. After cooling to room temperature, the mixture
was filtered on a sintered glass and the solid was washed with
distilled water until no sulfate ion was detected in the filtrates
by BaCl2. The product was dried in an oven at 373 °K and
then heated at 623 °K (350 °C) for removing any trapped
sulfuric acid. The highly sulfonated carbon solid acid was an
amorphous material. The sample was characterized with FT-
IR spectrum, TG-DTG analysis, neutralization potentiometric
titration, carbon-hydrogen-nitrogen-sulfur analysis (CHNS). A
speculative structure of sulfonated carbon sheets have been
proposed in Scheme 2.
afford the pure product.
RESULTS AND DISCUSSION
Characterization of Highly Sulfonated Carbon
The FT-IR spectrum of H2SO4-treated sucrose (Fig. 1)
shows a broad band at 3300-3500 cm-1, which is assigned to
OH groups (3442 cm-1). The strong and sharp bands at 1719,
1613 cm-1 and broad band at 1164 cm-1 are attributed to the
stretching vibration modes of sulfate groups. The strong and
sharp bands indicate that sucrose can be easily sulfonated,
dehydrated and carbonizated for attaching -SO3H groups to
geraphen rings with a high density of acid sites.
The density of -SO3H groups on sulfonated carbon was
determined by potentiometric titration by NaOH (0.01 N) and
it was in agreement with CHNS analysis (CH0.43O0.65S0.22).
The neutralization titration showed that the amount of SO3H
which is attached to carbon is 7.23 mmol g-1. The BET was
9.25 m2 g-1 and the dp was 56 A°. The thermal stability of
sulfonated carbon was examined by TG and DTG under a flow
of nitrogen at a heating rate of 10 K min-1 (Figs. 2 and 3). The
results showed that the sample weight decreases as
temperature increases. The weight loss of sample is about 94%
in the temperature range of 673-804 °K (400-531 °C, peak
temperature at 421 °C). These results indicate that the weight
loss can be attributed to thermal decomposition of sulfonated
General Procedure for Preparation of DHPMs under
Solvent-Free Conditions
In the solvent-free procedure, a mixture of aromatic
aldehyde (10 mmol), ethyl acetoacetate (10 mmol), urea (15
mmol) and sulfonated carbon (50 mg) was heated and stirred
at 120 °C in an oil bath for appropriate time according to
Table 1. The progress of the reaction was monitored by TLC.
After completion of the reaction, hot ethanol (10 ml) was
added and the reaction mixture was filtrated. The filtrate was
poured onto crushed ice and a yellow to white solid was
produced, the solid was filtered, washed with cold water and
finally crystallizated from EtOH or EtOAc:petroleum ether to
610

Mirkhani et al.
Table 1. Highly Sulfonated Carbon Catalyzed Synthesis of Dihydropyrimidin-2(1H)-ones
(DHPMs) under Solvent Free Conditions
R
R
OEt
O
C-SO3H
O
O
O
+
+
H2N
NH2
EtO
H3C
NH
CHO
CH3
N
H
O
Entry
R
Time
(min)
Yield (%)a
m.p. (°C)
Recorded
204
Reported
202-203
213-215
209-211
218-220
259-260
207-209
201-203
214-215
213-215
185-186
1
H
25
30
30
30
25
30
25
25
30
30
95
96
95
94
96
95
95
96
94
93
2
4-Cl
211
3
4-NO2
2- NO2
2-MeO
3-MeO
4-MeO
4-Me
210
4
218
5
258
6
207
7
203
8
213
9
4-Br
214
10
4-F
184
aIsolated yields.
100.0
99.0
98.0
97.0
96.0
95.0
94.0
93.0
92.0
91.0
4000
3500
3000
2500
2000
1500
1000
500
(cm-1)
Fig. 1. FT-IR spectrum of sulfonated carbon.
611

Synthesis, Characterization and Investigation of Catalytic Activity
Fig. 2. TG of sulfonated carbon.
Fig. 4. XRD pattern of sulfonated carbon.
Fig. 3. DTG of sulfonated carbon.
Fig. 5. SEM of sulfonated carbon.
C + SO2 + H2O+ CO
C - SO3H
Scheme 3. Thermal decomposition of sulfonated carbon
Typical Procedure for Preparation of DHPMs in the
Presence of Highly Sulfonated Carbon
carbon to C, SO2, H2O and CO as shown in Scheme 3.
The XRD pattern (Fig. 4) exhibits two broad and weak
diffraction peaks (2θ = 10-30°, 35-50°), which is attributed to
amorphous carbon. The X-ray diffractogram is similar to char
because the sulfonic acid groups were attached to edges of
isotropic carbon sheets and the solid acid exhibits broad
diffraction peaks of amorphous carbon.
The SEM image of H2SO4-treated sucrose (Fig. 5) reveals
the morphology and structure of amorphous carbon. Sample
was gold coated prior to scanning.
First, the catalytic activity of highly sulfonated carbon for
synthesis of DHPMs was investigated in the reaction of
benzaldehyde with ethyl acetoacetate and urea under solvent
free conditions. So a mixture of benzaldehyde (10 mmol),
ethyl acetoacetate (10 mmol), urea (15 mmol) and sulfonated
carbon (50 mg) was prepared and heated at 120 °C in an oil
bath for appropriate time according to Table 1. The progress
of the reaction was monitored by TLC. After completion of
the reaction, hot ethanol (10 ml) was added and the reaction
mixture was filtrated. The filtrate was poured onto crushed ice
612

Mirkhani et al.
R
R
OH
O
and a yellow to white solid was produced, the solid was
R
O
C-SO3H
-H2O
C-SO3H
NH2
filtered, washed with cold water and finally crystallized from
EtOH or EtOAc: petroleum ether to afford the pure product.
The results showed that the reaction was completed in 25 min
and in the presence of 50 mg of catalyst at 120 °C. No product
was obtained when the reaction was carried out at room
temperature for a long time. In the absence of catalyst, the
reaction did not proceed even at high temperature. Therefore,
the reaction of different aromatic aldehydes bearing either
electron-donating or electron-withdrawing substituents with
ethyl acetoacetate and urea was carried out and the
corresponding DHPMs were obtained in excellent yields
(Table 1). The results showed that the nature of substituent has
no significant effect on the product yield. However, when the
aliphatic aldehydes were used as the starting materials,
DHPMs were produced in lower yield. A proposed reaction
pathway is shown in Scheme 4.
R
NH
NH2
N
NH2
NH
+
H2N
O
CHO
O
O
O
R
O
R'
H3C
NH
C-SO3H
-H2O
R'
H3C
Me
R'
O
O
H2N
N
H
O
Scheme 4. Suggested mechanismꢀ
ꢀ
ꢀ
Table 2. The Reusability of the Highly Sulfonated Carbon
Catalyst in Solvent-Free Synthesis of DHPMsa
Recycling No.
Yield (%)b
1
2
3
4
95
95
94
95
1
The products were identified by H NMR, and physical
aReaction conditions: benzaldehyde (10 mmol), ethyl
acetoacetate (10 mmol), urea (15 mmol), catalyst (50 mg),
time (24 min). bIsolated yield.
data with those reported in the literature [14,15,17,22,24,25,
26-31,52].
In order to show the versatility and generality of this
method, acetylacetone was used instead of ethyl acetoacetate.
The results showed that the corresponding DHPMs were
obtained in high yield (Table 1, entries 12 and 13).
ꢀ
CONCLUSIONS
In comparison with other solid acid catalysts, this
sulfonated carbon showed very high density of acidic sites on
the amorphous carbon. Our procedure is very easy and simple
that produces stable, cheap, insoluble, recyclable and
environmentally friendly strong solid acid. The sulfonated
carbon has a good potential replacement for sulfuric acid in
catalytic reactions which can be attributed to high density of
SO3H groups with strong acidity. In the other hand, carbon-
based solid acid is superior in term of toxicity [LD50]. The
LD50 (Lethal Dose, 50%) for carbon-based solid acid is
comparable with cationic exchange resins with sulfonic acid
functional groups. This catalyst can be an effective catalyst for
the formation of DHPMs with excellent yields. The uses of
this inexpensive and easily available catalyst under solvent
free conditions make this protocol practical and economically
attractive. The simple workup procedure, mild reaction
conditions, excellent yields make our methodology a valid
contribution to the existing processes in the field of DHPMs
derivatives synthesis.
Reusability of the Catalyst
We found that carbon-based solid acid shows high
catalytic activity and could be recovered and recycled several
times without significant loss of activity. At the end of each
reaction, the catalyst was filtered, washed with hot ethanol and
diethyl ether, successively, dried at 180 °C for 1 h, and reused
in another reaction. The results showed that this solid acid
catalyst can be reused several times (5 times was checked)
without loss of its catalytic activity or leaching (Table 2).
Experimental studies about leaching in the beginning were
done by boiling carbon-based solid acid in various organic
solvents and hot filtration after boiling. In all of these
experiments no weight loss or leaching were observed and we
confided the carbon-based solid acid is insoluble in organic
solvents and no catalyst leaching was detected.
Comparison of this highly sulfonated carbon solid with
previously reported acid catalysts in the synthesis of DHPMs
showed that the present method is superior in terms of reaction
times or product yields and catalyst reusability (Table 3).
613

Synthesis, Characterization and Investigation of Catalytic Activity
Table 3. Comparison of Catalytic Activity of other Lewis Acids with Carbon-Based Solid Acid under
Various Conditionsa
Lewis acid
Time (h)
Yield (%)b
Solvent
Ref.
-
-
Carbon-based solid acid
p-Toluenesulfonic acid
Sr(NO3)2
0.45
0.05
6
95
93
77.8
Present work
[34]
AcOH
[24]
-
HBF4
0.6
6
5
24
18
5
95
91
75
50
87
80
75
95
[50]
[51]
[48]
[54]
[16]
[55]
[20]
[39]
Silica sulfuric acid
Natural HEU Zeolite
HCl
Phenylboronic acid
12-Molybdophosphoric acid
12-Tungstophosphoric acid
Iodine
EtOH
AcOH
EtOH
Acetonitrile
AcOH
AcOH
Toluene
6
4
Nano CeO2/Vinylpyridine
Anchored sulfonic acid on SiO2
FeCl3/Si(OEt)4
4.5
8
92
90
88
96
H2O
[36]
[50]
[41]
[48]
Acetonitrile
Isopropanol
Acetonitrile
3
Nafion NR-50
3
aReaction mixture: benzaldehyde, ethyl acetoacetate, urea and catalyst. bIsolated yield.
REFERENCES
Pharmacol. 26 (1995) 289.
[7]
A.D. Patil, N.V. Kumar, W.C. Kokke, M.F. Bean, A.J.
Freyer, C. Debrossi, S. Mai, A. Truneh, D.J. Faulkner,
B. Carte, A.L. Breen, R.P. Hertzberg, R.K. Johnson,
J.W. Westly, B.C. M. Potts, J. Org. Chem. 60 (1995)
1182.
P. Biginelli, Gazz. Chim. Ital. 23 (1983) 360.
F. Peng, L. Zhang, H. Wang, P. Lv, H. Yu, Carbon. 43
(2005) 2397.
[1]
[2]
P.G. Biginelli, Chim. Ital. 23 (1893) 360.
H. Cho, M. Ueda, K. Shima, A. Mizuno, M.
Hayashimatsu, Y. Ohnaka, Y. Takeuchi, M.
Hamaguchi, K. Aisaka, T. Hidaka, M. Kawai, M.
Takeda, T. Ishihara, K. Funahashi, F. Satah, M. Morita,
T.J. Noguchi, Med. Chem. 32 (1989) 2399.
[8]
[9]
[3]
[4]
K.S. Atwal, G.C. Rovnyak, S.D. Kimball, D. M. Floyd,
S. Moreland, B.N. Swanson, J.Z. Gougoutas, J.
Schwartz, K.M. Smillie, M.F.J. Malley, Med. Chem. 33
(1990) 2629.
G.C. Rovnyak, S.D. Kimball, B. Beyer, G. Cucinotta,
J.D. Dimarco, J. Gougoutas, A.O. Hedberg, M. Malley,
J.P. McCarthy, R. Zhang, S. Moreland, J. Med. Chem.
38 (1995) 119.
[10] M. Okamura, A. Takagaki, M. Toda, J.N. Kondo, K.
Domen, T. Tatsumi, M. Hara, S. Hayashi, Chem. Mater.
18 (2006) 3039.
[11] M. Hara, T. Yoshida, A. Takagaki, T. Takata, J.N.
Kondo, S. Hayashi, K. Domen, Angew. Chem. Int. Ed.
43 (2004) 2955.
[12] P.T. Anastas, M.M. Kirchhoff, Acc. Chem. Res. 35
(2002) 686.
[13] J.H. Clark, Acc. Chem. Res. 35 (2002) 791.
[14] T. Okuhara, Chem. Rev. 102 (2002) 3641.
[15] E.H. Hu, D.R. Sidler, U.H. Dolling, J. Org. Chem. 63
(1998) 3454.
[5]
[6]
K.S. Atwal, B.N. Swanson, S.E. Unger, D.M. Floyd, S.
Moreland, A.O. Hedberg, B.C. Reilly, J. Med. Chem.
34 (1991) 806.
G.J. Grover, S. Dzwonczyk, D.M. McMullen, C.S.
Normadinam, P.G. Sleph, S.J. Moreland, J. Cardiovasc.
614

Mirkhani et al.
[16] B.C. Ranu, A. Hajra, U.J. Jana, Org. Chem. 65 (2000)
[40] I. Cepanec, M. Litvic, A. Bartolincic, M. Lovric,
Tetrahedron 61 (2005) 4275.
6270.
[17] J.S. Yadav, B.V.S. Reddy, J.J. Naidu, K. Sadashiv,
Chem. Lett. 33 (2004) 926.
[18] B. Gangadasu, S. Palaniappan, V. Jayathirtha Rao,
Synlett 7 (2004) 1285.
[19] D. Russowsky, F.A. Lopes, V.S.S. Silva, K.F.S. Canto,
M.G. Montes D’Oca, M.N. Godoi, J. Braz. Chem. Soc.
(2004) 165.
[41] G.L. Zhang, H.X. Cai, Synth. Commun. 35 (2005) 829.
[42] I. Saxena, D.C. Borah, J.C. Sarma, Tetrahedron Lett. 46
(2005) 1159.
[43] Y.L. Zhu, Y.J. Pan, S.L. Huang, Synth. Commun. 34
(2004) 3167.
[44] M.A. Chari, K.J. Syamasundar, J. Mol. Catal. A: Chem.
221 (2004) 137.
[20] A.V. Narsaiah, B.K. Nagaiah, Synthesis 8 (2004) 1253.
[21] H. Hazarkhani, B. Karimi, Synthesis 8 (2004) 1239.
[22] A. Dondoni, A. Massi, Tetrahedron Lett. 42 (2001)
7975.
[45] T.S. Jin, H.X. Wang, C.Y. Xing, X.L. Li, T.S. Li,
Synth. Commun. 34 (2004) 3009.
[46] G. Salitha, G.S.K. Reddy, K.B. Reddy, J.S. Yadav,
Tetrahedron Lett. 44 (2003) 6497.
[23] C.O. Kappe, S.F. Falsone, Synlett (1998) 718.
[24] K.A. Kumar, M. Kasthuraiah, C.S. Reddy, D. Reddy,
Tetrahedron Lett. 42 (2001) 7873.
[25] J.S. Yadav, B.V. Subba Reddy, R. Srinivas, C.
Venugopal, T. Ramalingam, Synthesis (2001) 1341.
[26] K. Ramalinga, P. Vijayalakshmi, T.N.B. Kaimal,
Synlett (2001) 863.
[27] C.V. Reddy, M. Mahesh, P.V.K. Raju, R. Babu, V.V.N.
Reddy, Tetrahedron Lett. 43 (2002) 2657.
[28] A.S. Paraskar, G.K. Dewkar, A. Sudalai, Tetrahedron
Lett. 44 (2003) 3305.
[29] A. Shaabani, A. Bazgir, F. Teimouri, Tetrahedron Lett.
44 (2003) 857.
[30] N.-Y. Fu, Y.-F. Yuan, Z. Cao, S.-W. Wang, J.-T. Wang,
C. Peppe, Tetrahedron 58 (2002) 4801.
[31] K. Singh, J. Singh, P.K. Deb, H. Singh, Tetrahedron 55
(1999) 12873.
[32] Y. Ma, C. Qian, L. Wang, M. Yang, J. Org. Chem. 65
(2000) 3864.
[33] J. Lu, Y. Bai, Synthesis (2002) 466.
[34] J. Peng, Y. Deng, Tetrahedron Lett. 42 (2001) 5917.
[35] R. Varala, M.M. Alam, S.R. Adapa, Synlett 1 (2003)
67.
[47] M. Gohain, D. Prajapati, J.S. Sandhu, Synlett (2004)
235.
[48] G. Maiti, P. Kundu, C. Guin, Tetrahedron Lett. 44
(2003) 2757.
[49] X. Jing, Z. Li, X. Pan, Y. Shi, C. Yan, J. Iran. Chem.
Soc. 6 (2009) 514.
[50] T. Jin, S. Zhang, T. Li, Synth. Commun. 32 (2002)
1847.
[51] P. Salehi, M. Dabiri, M.A. Zolfigol, M.A.B. Fard,
Tetrahedron Lett.44 (2003) 2889.
[52] S. Tu, F. Fang, S.L. Zhu, T.X. Zhang, Q. Zhuang,
Synlett (2004) 537.
[53] F. Bigi, S. Carloni, B. Frullanti, R. Maggi, G. Sartori,
Tetrahedron Lett. 40 (1999) 3465.
[54] M. Tajbakhsh, B. Mohajerani, M.M. Heravi, A.N.
Ahmadi, J. Mol. Catal. A: Chem. 236 (2005) 216.
[55] S. Besoluk, M. Kucukislamoglu, M. Nebioglu, M.
Zengin, M. Arslan, J. Iran. Chem. Soc. 5 (2008) 62.
[56] S. Ghassamipour, A.R. Sardarian, J. Iran. Chem. Soc. 7
(2010) 237.
[57] M.M. Heravi, K. Bakhtiari, F.F. Bamoharram, Catal.
Commun. 7 (2006) 373.
[58] J.B. Lee, Y.K. Park, O.B. Yong, Y. Kang, K.W. Jun,
Y.J. Lee, H.Y. Kim, K.H. Lee, W.C. Choi, J. Power
Sources 158 (2006) 1251.
[59] E. Cano-Serrano, J.M. Campos-Martin, J.L.G. Fierro,
Chem. Commun. ( 2003) 246.
[60] A. Takagaki, M. Toda, M. Okamura, J.N. Kondo, S.
Hayashi, K. Domen, M. Hara, Catal. Today 116 (2006)
157.
[36] A.K. Mitra, K. Banerjee, Synlett 10 (2003) 1509.
[37] V.R. Rani, N. Srinivas, M.R. Kishan, S.J. Kulkarni,
K.V. Raghavan, Green Chem. 3 (2001) 305.
[38] J.S. Yadav, B.V.S. Reddy, P. Sridhar, J.S.S. Reddy, K.
Nagaiah, N. Lingaiah, P.S. Saiprasad, Eur. J. Org.
Chem. (2004) 552.
[39] S.K. De, R.A. Gibbs, Synthesis 11 (2005) 1748.
615