Nano sulfated titania as a heterogeneous solid acid catalyst for the synthesis of pyrroles by clauson-kaas condensation under solvent-free conditions

Article abstract of DOI:10.1007/s10593-014-1425-3

A new and environmentally benign method for the preparation of N-substituted pyrroles from one-pot condensation reaction of 2,5-dimethoxytetrahydrofuran with amines and diamines in the presence of nano sulfated titania under solvent-free conditions is presented. This new protocol features simple operation, easy availability, stability, and eco-friendliness of catalyst, as well as high to excellent yields of products. In addition, we report for the first time an alternative method for the synthesis of pyrroles from aromatic amines containing the β-lactam fragment using nano sulfated titania as catalyst.

Full text of DOI:10.1007/s10593-014-1425-3

Chemistry of Heterocyclic Compounds, Vol. 49, No. 12, March, 2014 (Russian Original Vol. 49, No. 12, December, 2013)
NANO SULFATED TITANIA AS A HETEROGENEOUS
SOLID ACID CATALYST FOR THE SYNTHESIS OF
PYRROLES BY CLAUSON-KAAS CONDENSATION
UNDER SOLVENT-FREE CONDITIONS
1
1
1
1
*
M. Hosseini-Sarvari , S. Najafvand-Derikvandi , A. Jarrahpour , and R. Heiran
A new and environmentally benign method for the preparation of N-substituted pyrroles from one-pot
condensation reaction of 2,5-dimethoxytetrahydrofuran with amines and diamines in the presence of
nano sulfated titania under solvent-free conditions is presented. This new protocol features simple
operation, easy availability, stability, and eco-friendliness of catalyst, as well as high to excellent yields
of products. In addition, we report for the first time an alternative method for the synthesis of pyrroles
from aromatic amines containing the β-lactam fragment using nano sulfated titania as catalyst.
Keywords: 2,5-dimethoxytetrahydrofuran, nano sulfated titania, N-substituted pyrroles, Clauson-Kaas
condensation.
Heterocyclic small molecules play an important role in the search for new therapeutic and drug
candidates. Pyrroles are an important class of heterocyclic compounds with remarkable biological activities and
are useful intermediates and structural units found in a vast array of natural products (such as porphyrins, bile
pigments, coenzymes, and alkaloids) [1-3], synthetic materials, and bioactive molecules, such as heme, vitamin
B12, and cytochromes [4, 5]. Consequently, an enormous number of procedures have been developed for the
construction of pyrroles. Conjugate addition reactions [6], transition metal-mediated reactions [7], reductive
couplings [8], aza-Wittig reactions [9], Hantzsch synthesis [10], multicomponent and multistep reactions [11,
12], and Paal–Knorr and Clauson-Kaas condensation reactions [13-37] have been performed for the synthesis of
pyrroles. Among them, the Paal–Knorr and Clauson-Kaas condensation reactions remain the most commonly
used protocol for the construction of the pyrrole framework.
Therefore, many catalysts and protocols have been developed for the synthesis of pyrroles by Paal–
Knorr and Clauson-Kaas condensation, such as TfOH [25], montmorillonite K-10 [26], p-chloropyridine
hydrochloride [27], FeCl3·7H2O [28], Bi(NO3)3·5H2O [29], MgI2·(Et2O)n [30], BiCl3–SiO2 [31], silica sulfuric
acid [32], Sc(OTf)3 [33], as well as ultrasound-assisted Bi(NO3)3-induced condensation [34]. However, despite
the potential utility of these catalysts, several of them often suffer from certain drawbacks such as low yields,
_______
*To whom correspondence should be addressed, e-mail: hossaini@shirazu.ac.ir.
1Department of Chemistry, Shiraz University, Shiraz 71454, I. R. Iran.
_________________________________________________________________________________________
Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1870-1877, December, 2013.
Original article submitted September 6, 2013.
1732
0009-3122/14/4912-1732©2014 Springer Science+Business Media New York

prolonged reaction time, the use of hazardous and volatile organic solvents and toxic and detrimental metal
precursors as catalyst, the use of relatively expensive reagents and tedious work-up, leading to the generation of
large amount of toxic metal-containing waste. In addition, the main disadvantage of almost all exiting methods
is that the catalysts are destroyed in the work-up procedure and their recovery and reuse is often impossible.
Thus, a milder, selective, nonhazardous, inexpensive, recyclable, and eco-friendly catalyst is still in demand.
Herein we report a simple and highly chemoselective method for the synthesis of N-substituted pyrroles
starting from commercially available 2,5-dimethoxytetrahydrofuran and various amines by using nano sulfated
titania under solvent-free conditions. This reaction produces pyrroles in excellent yields and within a short
period of time.
In our previous papers [38-40], we have reported the preparation and characterization of nano sulfated titania
and its catalytic activity for the synthesis of fatty amides and Beckmann rearrangement. Hence, the textural
properties of nano sulfated titania used in the present study are given in brief. Analysis of the SEM and TEM images
of this catalyst reveals that the particles are uniformly distributed in spherical shape. Hence, SO42– ion modification
preferably retarded the aggregation. This facilitates the organic transformations. It is found from XRD and TEM
analyses that the catalyst exists in the form of nanoparticles (40-50 nm). The anatase and rutile phases of TiO2 have
been confirmed and sulfated modification does not change the phases. The surface area was found to be 218 m2/g,
and the acidity of the catalyst was measured as Ho = 1.916. As determined by thermogravimetric analysis, the
standard portion of the catalyst (0.2 g), used in 1 mmol loading of the reactants, corresponds to 0.015 mol% of the
sulfated active centers.
Because of the biological activities of heterocyclic derivatives, we became interested in the synthesis of
pyrroles by using nano sulfated titania. Since the Pall–Knorr method requires 1,4-dicarbonyl compounds and
Lewis acids as catalyst, we envision that our work using nano sulfated titania may prove useful for the synthesis
of pyrroles under mild conditions if we select 2,5-dimethoxytetrahydrofuran (2) as one of the reactants. Our
hypothesis has been tested by reacting aniline (1a) and compound 2 as model compounds in the presence of
nano sulfated titania as catalyst in various reaction conditions (Table 1).
Nano sulfated TiO2
Ph NH2
+
Ph N
OMe
MeO
O
1a
2
3a
TABLE 1. Optimization Reaction Conditions for the Synthesis of
N-Phenylpyrrole (3a) in the Presence of Nano Sulfated TiO2*
Entry
Solvent
Temperature, °C
Time, h
Isolated yield, %
1
2
3
4
5
6
7
8
9
DMF
DMSO
EtOH
H2O
None
None
None
None
None
None
None
120
120
Reflux
Reflux
Room temp.
60
5
1.5
3
3
5
5
3
2
95
97
Trace
0
0
0
50
90
98
98
98
80
100
120
130
0.5
0.5
0.5
10
11
160
_______
*Reaction conditions: 1.0 mmol of aniline (1a), 1.0 mmol of compound 2,
and 0.2 g of nano sulfated titania.
1733

As shown in Table 1, some commonly used solvents such as DMF, DMSO, ethanol, and water were
tested at elevated temperature (entries 1-4). It is evident that DMF and DMSO are suitable reaction media
entries 1, 2), whereas EtOH and water did not afford any desired product (entries 3, 4). It is interesting to note
that the reaction proceeded smoothly also under solvent-free condition to generate the corresponding pyrrole in
excellent yields in short reaction time. So, to avoid the use of volatile solvent and to reduce environmental
pollution, all the reactions were performed under solvent-free conditions. A range of temperatures (25-160°C)
was also examined. As expected, the yield of product increases from 0 to 98% by increasing the temperature
from 25 to 160°C (entries 5-11). This indicates that the catalyst is quite stable and the reaction is free from the
diffusion limitation at higher temperature. Consequently, the best reaction temperature, which was optimized, is
120°C (entry 9).
To define the scope and generality of this new protocol, we tried various aromatic amines 1b-p under
optimized reaction conditions. The reaction conditions are very general and can easily be varied for different
substrates. As shown in Table 2, aromatic amines with electron-donating groups or electron-withdrawing groups
are both effective in the Clauson-Kaas reaction, giving desired pyrroles 3b-p. Moreover, the reaction of aromatic
diamine 1m with compound 2 was selectively conducted to produce monopyrrolyl derivative 3m (entry 12). We
also studied the condensation of aliphatic amines with 2,5-dimethoxytetrahydrofuran. Unfortunately, aliphatic
amines are inert in the presence of nano sulfated titania.
Nano sulfated TiO2
Ar
N
ArNH2
+
2
1b–m
3b–m
Nano
sulfated
TiO2
NH2
N
O
O
O
Ar
Ar
+
2
N
N
O
OMe
OMe
1n–p
3n–p
TABLE 2. Synthesis of Pyrroles 3b-p Using Nano Sulfated TiO2 under
Solvent-free Conditions*
Entry
Starting amine
Ar
Pyrrole
Time, min
Isolated yield, %
1
2
3
4
5
6
7
8
9
10
11
12
13
14
3-MeC6H4
4-MeC6H4
2-EtC6H4
25
120
25
60
60
60
65
60
45
20
90
90
45
60
60
98
90
98
95
90
95
95
95
95
95
95
90
90
85
80
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3-MeOC6H4
2-HOC6H4
3-HOC6H4
4-HOC6H4
2-HO-5-MeC6H4
4-NCC6H4
2-F3CC6H4
4-BrC6H4
4-H2NC6H4
2,4-Cl2C6H4
4-ClC6H4
__1_5____
Ph
*Reaction conditions: 1.0 mmol of amine 1b-p, 1.0 mmol of 2,5-di-
methoxytetrahydrofuran (2), and 0.2 g of nano sulfated titania; heating on
an oil bath at 120°C.
1734

The 2-azetidinone (β-lactam) scaffold is extremely versatile and has featured in a number of clinically
used drugs. 2-Azetidinone derivatives have a high level of biological activity, such as antimicrobial, antifungal,
antitubercular, antidiabetic, anticancer, antiparkinsonian, anti-inflammatory, and analgesic activity. These
compounds also act as cholesterol absorption inhibitors and thrombin inhibitors [41]. Due to the biological
activities of β-lactams, we were interested in applying our newly developed procedure for the synthesis of
pyrroles to the amines 1n-p of a newly synthesized β-lactam series (Table 2, entries 13-15). It is suspected that
these new β-lactams 3n-p, containing pyrrol fragments may potentially have some unique bioactivities.
The recyclability of the catalyst was investigated using a model reaction between aniline (1a) and
compound 2 in the presence of nano sulfated TiO2. After the completion of the reaction, the mixture was diluted
by EtOAc, and the catalyst was centrifuged, washed with EtOAc, and reused for another three successive test
runs without any further activation. It was found that its activity does not show any significant decrease after
three runs (Table 3).
A plausible mechanism for the synthesis of N-substituted pyrroles in the presence of nano sulfated
titania is shown in the scheme below. Usually, in sulfated metallic oxides, there may be many as Lewis acid
sites as Brønsted acid sites. The superacidity of these materials is attributed to the Brønsted acid sites, created or
already existing, whose acidity is increased by the presence of neighboring strong Lewis acid sites. The strength
of these Lewis acid sites is due to an inductive effect exercised by sulfate on the metallic cation, which becomes
more deficient in electrons [42], as seen in the scheme. Thus, nano sulfated titania represent a novel type of
Lewis acid catalyst. The mechanism probably involves initial formation of the reactive intermediate I, which
reaction with amines 1 can lead to pyrroles 3 via a nucleophilic addition and subsequent expulsion of MeOH,
dehydration, and aromatization steps.
Brønsted
acid site
Lewis
acid site
O
O
O
O
O
O
O
O
S
S
OH
Ti
Ti
Ti
Ti
Ti
O
O
O
O
O O
Surface of sulfated titania
Ar
N
H
OMe
OMe
OMe
O
O
MeO
.
.
.
.
.
O
2
.
.
.
.
+
ArNH2
.
O
O
O
O
O
O
O
O
O
O
O
O
.
1
H
H
H
S
S
S
O
–MeOH
O
O
Ti
Ti
Ti
Ti
Ti
Ti
O
O O
O
O
O
O
O O
I
–MeOH
H
Nano
sulfated
Nano
sulfated
TiO2
H
TiO2
.
.
.
Nano
sulfated
TiO2
+
O
+
O
–H2O
N
N
N
H
H
Ar
3
Ar
Ar
Finally, to show the advantage of this work in comparison with previously reported procedures,
synthesis of pyrrole 3a was considered as a representative example. As shown in Table 4, in addition to the
general advantages attributed to heterogeneous catalysts, our present method is better or comparable with others
in terms of yield and reaction time.
1735

TABLE 3. Recycle Studies of Nano Sulfated Titania*
Cycle
Isolated yield, %
Fresh
1st
2nd
3rd
98
97
95
92
_______
*Reaction conditions: 1.0 mmol of aniline (1a), 1.0 mmol of 2,5-di-
methoxytetrahydrofuran (2), and 0.2 g of nano sulfated titania; heating on an
oil bath at 120°C for 30 min.
TABLE 4. Comparison of Activity of Nano Sulfated Titania with Other
Catalysts in Synthesis of Pyrrole 3a
Catalyst
Temperature, °C
Solvent
Time, min
Isolated yield, %
Nano sulfated TiO2
MgI2·(Et2O)n
Acid free
120
80
150*
125
75*
140*
None
MeCN
H2O
PhMe
THF
30
360
30
45
15
98
85 [30]
90 [35]
87 [37]
90 [29]
88 [36]
P2O5
Bi(NO3)3·5H2O
Nano organocatalyst
H2O
20
_______
*Heated by microwave irradiation.
In conclusion, nano sulfated titania is introduced as a novel heterogeneous catalyst for the Clauson-Kaas
pyrrole cyclocondensation. In most cases, the catalyst showed good to excellent activity for the condensation
reaction. The procedure is superior to the other Lewis acid-mediated synthesis of N-substituted pyrroles in terms
of yield of the products (more than 80-98%) and time of the reaction (0.5-2.0 h). The described synthetic
protocol allows for the preparation of a variety of pyrroles without the use of expensive or sensitive reagents.
Based on the simplicity of the procedure, the products can be isolated very easily. The method as reported
herein may find applications in other areas of research.
EXPERIMENTAL
1
IR spectra were recorded on an impact 400D Nicolet FT-IR spectrophotometer in KBr pellets. H and
13C NMR spectra were recorded on a Bruker Avance DPX FT 250 apparatus (250 and 63 MHz, respectively) in
CDCl3, internal standard TMS. Elemental analyses were performed on a Thermo Finnigan Flash EA 1112 series
microanalyzer. Melting points were determined on Buchi 510 apparatus and are uncorrected. TLC monitoring
was performed on silica gel Polygram SIL G/UV 254 plates, eluent hexane–EtOAc, 7:3. Chemicals were
purchased from Fluka, Merck, B.D.H., and Aldrich.
Catalyst Preparation. Sulfated TiO2 nano powder was prepared and characterized by the sol-gel
process as previously reported [38-40]. Titanium isobutoxide (98%, Fluka) was used as the source of TiO2. A
portion of Ti(O-i-Bu)4 (14.3 ml) was hydrolyzed in H2O (150 ml) containing HNO3 (65% Merck, 1.25 ml), the
aqueous solution was stirred continuously at room temperature for 2 h to form a highly dispersed sol, and then
the sol was concentrated and dried at 60°C. Sulfation was performed using 2 ml of 0.5 M H2SO4 per 1 g of the
catalyst. This suspension was stirred for 2 h at 110°C until dried and then calcined at 500°C for 5 h.
1736

Synthesis of Pyrroles 3a-p Using Nano Sulfated Titania (General Method). A mixture of amine
1a-p (1 mmol), 2,5-dimethoxytetrahydrofuran (2) (0.132 g, 1 mmol), and nano sulfated titania (0.200 g) was
stirred at 120°C on an oil bath. The reaction was monitored by TLC. After the appropriate time indicated in
Tables 1 and 2, the reaction mixture was diluted with EtOAc (15 ml), centrifuged to remove the catalyst, and
evaporated to obtain the crude product. The crude product was purified by short column chromatography on
silica gel (100% petroleum ether) to afford the pure pyrrole derivatives. The yields of pyrroles 3a-p are given in
Tables 1 and 2. Compounds 3a-e,h,j,l,m are well known; their spectral data correspond to those reported in the
literature [21-37, 42-44], and elemental analysis confirms the proposed structures. The spectral and elemental
data of the newly synthesized compounds are given below.
1
2-(1H-Pyrrol-1-yl)phenol (3f). Brown oil. H NMR spectrum, δ, ppm (J, Hz): 7.33-7.27 (2H, m, H Ar);
7.10-7.01 (2H, m, H Ar); 6.91 (2H, t, J = 2.1, H pyrrole); 6.42 (2H, t, J = 2.1, H pyrrole); 5.60 (1H, br. s, OH). 13C
NMR spectrum, δ, ppm: 150.2; 128.9; 128.2; 126.7; 121.9; 120.9; 116.8; 110.3.
1
3-(1H-Pyrrol-1-yl)phenol (3g). Yellow oil. H NMR spectrum, δ, ppm (J, Hz): 7.28-7.21 (1H, m,
H Ar); 7.06 (2H, t, J = 2.2, H pyrrole); 6.96 (1H, d, J = 8.0, H Ar); 6.85 (1H, s, H Ar); 6.69 (1H, d, J = 8.0,
H Ar); 6.33 (2H, t, J = 2.2, H pyrrole); 5.19 (1H, s, OH). 13C NMR spectrum, δ, ppm: 156.4; 142.0; 130.5;
119.3; 112.8; 112.5; 110.4; 107.7.
1
4-Methyl-2-(1H-pyrrol-1-yl)phenol (3i). Brown oil. H NMR spectrum, δ, ppm (J, Hz): 6.84 (2H, d,
J = 5.4, H Ar); 6.72-6.69 (1H, m, H Ar); 6.64 (2H, t, J = 2.0, H pyrrole); 6.15 (2H, t, J = 2.0, H pyrrole);
13
5.00-4.81 (1H, br. s, OH); 2.10 (3H, s, CH3). C NMR spectrum, δ, ppm: 147.8; 130.3; 129.3; 127.8; 126.9;
121.8; 116.5; 110.2; 20.4.
1-[2-(Trifluoromethyl)phenyl]-1H-pyrrole (3k). Pale-yellowish oil. 1H NMR spectrum, δ, ppm (J, Hz):
7.61 (1H, d, J = 7.6, H Ar); 7.45–7.42 (1H, m, H Ar); 7.37-7.34 (1H, m, H Ar); 7.28-7.25 (1H, m, H Ar); 6.71
13
(2H, t, J = 2.0, H pyrrole); 6.18 (2H, t, J = 2.1, H pyrrole). C NMR spectrum, δ, ppm: 139.6; 132.6; 129.5;
127.9; 127.1; 127.0; 125.4; 123.3; 121.1; 109.3.
3-(2,4-Dichlorophenoxy)-1-(4-methoxyphenyl)-4-[3-(1H-pyrrol-1-yl)phenyl]azetidin-2-one
(3n).
White solid, mp 145-148°C. IR spectrum, ν, cm-1: 1759; 1608. H NMR spectrum, δ, ppm (J, Hz): 7.40-7.15
(7H, m, H Ar); 7.12-7.09 (2H, m, H Ar); 7.02 (2H, t, J = 2.0, H pyrrole); 6.84 (2H, d, J = 8.9, H Ar); 6.33 (2H,
t, J = 2.0, H pyrrole); 5.55 (1H, d, J = 4.3, CH lactam); 5.43 (1H, d, J = 4.3, CH lactam); 3.75 (3H, s, OCH3).
13C NMR spectrum, δ, ppm: 161.4; 156.7; 151.3; 140.9; 134.1; 130.0; 129.7; 127.6; 127.4; 125.3; 124.1; 121.1;
120.9; 120.1; 119.3; 118.8; 116.4; 114.5; 110.6; 81.5; 61.1; 55.4. Found, %: C 65.20; H 4.32; N 5.92.
C26H20Cl2N2O3. Calculated, %: C 65.15; H 4.21; N 5.84.
1
3-(4-Chlorophenoxy)-1-(4-methoxyphenyl)-4-[3-(1H-pyrrol-1-yl)phenyl]azetidin-2-one
(3o).
1
White solid, mp 123-125°C. H NMR spectrum, δ, ppm (J, Hz): 7.35-7.21 (6H, m, H Ar); 7.13-7.10 (2H, m,
H Ar); 6.99 (2H, d, J = 1.9, H pyrrole); 6.83-6.72 (4H, m, H Ar); 6.33 (2H, d, J = 1.9, H pyrrole); 5.55 (1H, d,
J = 4.1, CH lactam); 5.39 (1H, d, J = 4.1, CH lactam); 3.74 (3H, s, OCH3). 13C NMR spectrum, δ, ppm: 161.9;
156.7; 155.2; 140.8; 134.2; 130.0; 129.2; 127.3; 125.2; 120.9; 119.8; 119.1; 118.8; 116.8; 114.5; 110.7; 107.6;
81.0; 61.5; 55.4. Found, %: C 69.95; H 4.87; N 6.43. C26H21ClN2O3. Calculated, %: C 70.19; H 4.76; N 6.30.
1-(4-Methoxyphenyl)-3-phenoxy-4-[3-(1H-pyrrol-1-yl)phenyl]azetidin-2-one (3p). White solid, mp
1
157–160°C. H NMR spectrum, δ, ppm (J, Hz): 7.27-7.23 (5H, m, H Ar); 7.19-7.18 (1H, m, H Ar); 7.16-7.00
(2H, m, H Ar); 6.88 (2H, t, J = 1.7, H pyrrole); 6.81-6.70 (5H, m, H Ar); 6.24 (2H, t, J = 1.7, H pyrrole); 5.53
(1H, d, J = 4.7, CH lactam); 5.31 (1H, d, J = 4.7, CH lactam); 3.68 (3H, s, OCH3). 13C NMR spectrum, δ, ppm:
161.1; 156.6; 140.7; 134.5; 130.2; 129.6; 129.3; 125.2; 122.3; 120.8; 120.1; 119.2; 118.8; 115.5; 115.4; 114.4;
112.6; 81.0; 61.7; 55.4. Found, %: C 76.32; H 5.59; N 6.77. C26H22N2O3. Calculated, %: C 73.05; H 5.27;
N 7.16.
We gratefully acknowledge the support of this work by the Shiraz University Research Council.
1737

REFERENCES
1.
2.
D. L. Boger, C. W. Boyce, M. A. Labroli, C. A. Sehon, and Q. Jin, J. Am. Chem. Soc., 121, 54 (1999).
J. T. Gupton, Top. Heterocycl. Chem., 2, 53 (2006).
3.
H. Fan, J. Peng, M. T. Hamann, and J. F. Hu, Chem. Rev., 108, 264 (2008).
F. Aydogan, M. Basarir, C. Yolacan, and A. S. Demir, Tetrahedron, 63, 9746 (2007).
R. Paolesse, S. Nardis, F. Sagone, and R. G. Khoury, J. Org. Chem., 66, 550 (2001).
R. K. Dieter and H. Yu, Org. Lett., 2, 2283 (2000).
M. Yamagishi, K. Nishigai, T. Hata, and H. Urabe, Org. Lett., 13, 4873 (2011).
Furstner, H. Weintritt, and A. Hupperts, J. Org. Chem., 60, 6637 (1995).
Freifeld, H. Shojaei, and P. Langer, J. Org. Chem., 71, 4965 (2006).
Eftekhari-Sis, M. Zirak, and A. Akbari, Chem. Rev., 113, 2958 (2013).
E. Ghabraie, S. Balalaie, M. Bararjanian, H. R. Bijanzadeh, and F. Rominger, Tetrahedron, 67, 5415
(2011).
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
V. Estévez, M. Villacampa, and J. C. Menéndez, Chem. Soc. Rev., 39, 4402 (2010).
A. Jafari, H. Mahmoudi, and B. F. Mirjalili, J. Iran. Chem. Soc., 8, 851 (2011).
P. Mundy, M. G. Ellerd, and F. G. Favaloro, Jr., Name Reactions and Reagents in Organic Synthesis,
2nd ed., John Wiley & Sons, Inc. (2005), p. 476.
15.
16.
17.
18.
19.
T. Maehara, R. Kanno, S. Yokoshima, and T. Fukuyama, Org. Lett., 14, 1946 (2012).
G. Dou, C. Shi, and D. Shi, J. Comb. Chem., 10, 810 (2008).
H. Veisi and S. Hemmati, J. Chin. Chem. Soc., 56, 240 (2009).
S. Handy and K. Lavender, Tetrahedron Lett., 54, 4377 (2013).
L. Gao, L. Bing, Z. Zhang, H. Kecheng, H. Xiaoyun, and K. Deng, J. Organomet. Chem., 735, 26
(2013).
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
B. S. Gourlay, P. P. Molesworth, J. H. Ryan, and J. A. Smith, Tetrahedron Lett., 47, 799 (2006).
C. Miles, S. M. Mays, B. K. Southerland, T. J. Auvil, and D. M. Ketcha, ARKIVOC, xiv, 181 (2009).
R. Bharadwaj and K. A. Scheidt, Org. Lett., 6, 2465 (2004).
C. Silveira, M. P. Fortes, and S. R. Mendes, Curr. Org. Chem., 16, 1540 (2012).
F. Aydogan and C. Yolacan, J. Chem. (2013). http://dx.doi.org/10.1155/2013/976724.
M. Abid, L. Teixeira, and B. Török, Tetrahedron Lett., 48, 4047 (2007).
M. Abid, S. M. Landge, and B. Török, Org. Prep. Proced. Int., 38, 495 (2006).
C. Rochais, V. Lisowski, P. Dallemagne, and S. Rault, Tetrahedron Lett., 45, 6353 (2004).
N. Azizi, A. Khajeh-Amiri, H. Ghafuri, M. Bolourtchian, and M. R. Saidi, Synlett, 2245 (2009).
S. Rivera, D. Bandyopadhyay, and B. K. Banik, Tetrahedron Lett., 50, 5445 (2009).
X. Zhang and J. Shi, Tetrahedron, 67, 898 (2011).
Aghapoor, L. Ebadi-Nia, F. Mohsenzadeh, M. M. Morad, Y. Balavar, and H. R. Darabi, J. Organomet.
Chem., 708, 25 (2012).
H. Veisi, Tetrahedron Lett., 51, 2109 (2010).
J. Chen, H. Wu, Z. Zheng, C. Jin, X. Zhang, and W. Su, Tetrahedron Lett., 47, 5383 (2006).
D. Bandyopadhyay, S. Mukherjee, J. C. Granados, J. D. Short, and B. K. Banik, Eur. J. Med. Chem., 50,
209 (2012).
32.
33.
34.
35.
36.
37.
38.
39.
40.
A. Wilson, G. Filzen and G. S. Welmaker, Tetrahedron Lett., 50, 4807 (2009).
V. Polshettiwar and R. S. Varma, Tetrahedron, 66, 1091 (2010).
Y. Fang, D. Leysen, and H. C. J. Ottenheijm, Synth. Commun., 25, 1857 (1995).
Hosseini-Sarvari, E. Sodagar, and M. M. Doroodmand, J. Org. Chem., 76, 2853 (2011).
Hosseini-Sarvari and E. Safari, J. Sulfur Chem., 32, 463 (2011).
M. Hosseini-Sarvari, E. Safari, A. Jarrahpour, and R. Heiran, C. R. Chim., 15, 980 (2012).
1738

41.
42.
M. Zarei, A. Jarrahpour, E. Ebrahimi, M. Aye, and S. A. Torabi Badrabady, Tetrahedron, 68, 5505
(2012).
L. K. Noda, R. M. de Almeida, L. F. D Probst, and N. S. Goncalves, J. Mol. Catal. A: Chem., 225, 39
(2005).
43.
44.
X. Song and A. Sayari, Catal. Rev. Sci. Eng., 38, 329 (1996).
H.-C. Ma and X.-Z. Jiang, J. Org. Chem., 72, 8943 (2007).
1739