Molecular iodine: A green and inclusive catalyst for the synthesis of highly functionalized 1,3,5-trisubstituted pyrazoles in aqueous medium

Article abstract of DOI:10.1039/c3ra44315a

A library of 1,3,5-trisubstituted pyrazoles was synthesized in excellent yields from the reaction of aldehyde, phenylhydrazine and alkynes in the presence of iodine via an eco-friendly, simple process performed in aqueous medium. This single pot transformation generates new CN, C-C, C-N bonds and proceeds by a domino reaction involving the formation of Schiff base, Mannich addition and finally intramolecular cyclisation. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3ra44315a

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Molecular iodine: a green and inclusive catalyst for
the synthesis of highly functionalized 1,3,5-
trisubstituted pyrazoles in aqueous medium†
Cite this: RSC Adv., 2014, 4, 779
Received 12th August 2013
Accepted 14th October 2013
a
a
b
a
Pratibha Rai, Madhulika Srivastava, Jaya Singh and Jagdamba Singh*
DOI: 10.1039/c3ra44315a
www.rsc.org/advances
A library of 1,3,5-trisubstituted pyrazoles was synthesized in excellent moiety are shown in Fig. 1. Celecoxib is a non-steroidal, anti-
yields from the reaction of aldehyde, phenylhydrazine and alkynes in inammatory drug useful in the treatment of arthritis, and
1
3
14
the presence of iodine via an eco-friendly, simple process performed acute pain (A). Fipronil is an insecticide (B), and Lonazolac is
15
in aqueous medium. This single pot transformation generates new used as an NSAID (C). With the above background we have
C]N, C–C, C–N bonds and proceeds by a domino reaction involving tried multicomponent reaction (MCR) methods to achieve our
the formation of Schiff base, Mannich addition and finally intra- important target molecule using an efficient approach. Reac-
molecular cyclisation.
tions with multicomponents have gained popularity in recent
years because the important product is formed in a single step
and several products can be formed by varying the reacting
agent of multiple compounds. Rapid organic synthesis and
functional group transformation with the help of a multicom-
Introduction
Heterocyclic compounds with a nitrogen atom are found ponent reaction can be proven to be an efficient approach for
16,17
extensively in biological active pharmaceuticals and natural producing useful compounds for science and humanity.
products such as hormones, vitamins and alkaloids and are also Multicomponent reactions show a number of benets from the
1
present in ne chemicals for increasing the quality of life. The environmental point of view for example simple automation,
pyrazole nucleus is one of the most interesting and promising operational simplicity, minimum number of by-products,
18
heterocyclic scaffolds. They are an elite class of compound simple extraction, purication and work-up steps.
which show a broad spectrum of biological activities such as comparison to traditional multistep synthesis multicomponent
In
2
3
4
5
-a-reductase inhibitor,
antiparasitic,
antiproliferative,
reactions dramatically reduce the amounts of reactants,
5
6,7
8
19
herbicides, antiprotozoal, and anti-inammatory etc. Almost solvents, absorbents, and save energy. MCR which takes place
9
a
every treatment of Alzheimer's disease, inammation and in aqueous media has received little attention in the develop-
inammation related disorders, such as arthritis uses the pyr- ment of bioactive heterocycles. In order to eliminate organic
20
9b,c
azole nucleus.
applications in agriculture because of their fungicidal and is to develop an environmentally benign reaction media.
herbicidal activity; hence they have been used as insecticides Freely available in nature, water has huge potential as a medium
In addition, they are also used widely in solvents in chemical processes the main priority of researchers
21
10a
and pesticides.
Success of a pyrazole COX-II inhibitor in for chemical reactions. Any other chemical medium has the
recent years has further increased the value of these heterocyclic possibility to cause side effects for natural resources and human
10b–h
scaffolds in medicinal chemistry.
Among them the most
important are aryl substituted pyrazoles reported to have the
11
non-nucleoside HIV-1 reverse transcriptase inhibitory activity.
Several 1,3,4-trisubstituted pyrazole derivatives have displayed
À4
cytotoxic effects in a primary tumor cell line test at 10 M drug
12
concentration. Some important drugs having a pyrazole
a
Department of Chemistry, University of Allahabad, Allahabad, Uttar Pradesh, India.
E-mail: dr.jdsau@gmail.com
b
L R PG College, Department of Chemistry, Sahibabad, Uttar Pradesh, India
†
Electronic supplementary information (ESI) available: See DOI:
0.1039/c3ra44315a
1
Fig. 1 Drugs containing a pyrazole moiety.
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a
beings. Water as a medium for a reaction is non hazardous, Table 1 Optimization of different types of catalyst
environment friendly, non inammable, inexpensive and easily
available worldwide and most importantly it is non exhaust-
22
ible. It is noticeable that in recent literature of chemical
research shows that organic reactions performed in water as a
23
solvent are faster and display novel reactivity proles.
Continuing with this theme we herein wish to report a synthesis
of trisubstituted pyrazoles catalyzed by iodine three component
reactions for trisubstituted pyrazoles.
b
Entry
Catalyst (mol%)
Time (hours)
Yield (%)
1
2
3
4
5
6
7
8
9
None
TFA
TFOH
Oxalic acid
12
12
12
12
12
12
12
12
12
12
Trace
25
In eco-efficient organic synthesis, molecular iodine has
achieved considerable attention from the synthetic community
due to its benecial, cheap, easily available, green, and eco-
efficient properties etc. Iodine has enticed great curiosity for
some time by its advantages of catalytic activity, mild Lewis
acidity, ease of handling and reusability. This protocol
demonstrates that molecular iodine is an excellent catalyst in
aqueous medium. Our deep interest in green chemistry,
simultaneously with the advantages of applying molecular
iodine as a reaction promoter we herein report new method-
ology for the synthesis of 1,3,5-trisubstituted pyrazole.
28
65
60
55
50
35
46
75
FeCl
CuSO
3
4
Boric acid
b-Cyclodextrin
Vitamin B1
Iodine
10
a
Reaction condition: benzaldehyde 1a (1 mmol), phenylhydrazine 2a
1 mmol), phenylacetylene 3a (1 mmol) and catalyst in water (5 ml) at
(
6
ꢀ
b
0 C for 12 h. Isolated yield of products.
Several methods already exist for the synthesis of 1,3,5-
trisubstituted pyrazoles. These include a three component
24
reaction catalyzed by using variety of catalyst PTSA, copper chloride as the reaction promoter for 12 hours which provided
25
25b
iodide and NaOH/PTC/water. The existing methods have 65% and 60% of the product, respectively (entry 4 and 5 of
many drawbacks such as long reaction time, use of organic Table 1). In the presence of copper sulfate and boric acid the
solvents, and the requirement for a metal catalyst, all of which yield of product is low (entry 6 and 7 of Table 1). Iodine has the
26
may promote side effects on the environment and to human ability to substitute acidic catalysts and that is why we used
beings, it may also involve tedious work up, operational iodine as the catalyst, which promoted the multicomponent
complexity and generation of waste. Herein we report a novel reaction and increased the product yield signicantly (Entry10
method for the synthesis of 1,3,5-trisubstituted pyrazole by of Table 1). In the last decade iodine has attracted the attention
using benzaldehyde (1) phenylhydrazine (2) and alkynes (3) of organic chemists as a cost effective, easily available, green,
2
7
(
Scheme 1).
To obtain the optimized conditions for the reaction, it was and is used to synthesise the corresponding product in high
examined by applying a series of catalysts and solvents with the yield in a short time. In addition its moderate Lewis acidity and
and eco-friendly catalyst for many organic transformations,
28
hope of minimizing the reaction time and maximizing the yield its associated water tolerance has further increased its use in
of product. In our initial attempt we investigated the model organic transformations. Iodine as a mild oxidizer shows
reaction of benzaldehyde (1a) (1 mmol), phenylhydrazine (2a) tolerance to a wide range of functional groups and exhibits
(1 mmol), and phenyl acetylene (3a) (1 mmol) which were stirred polarizability, which is an unique quality of iodine. Under all
at room temperature in presence of solvent under catalyst free conditions, in dilute, concentrated and even in solvent free
26
conditions, we found that the reaction even aer 12 hours had conditions, iodine displays high catalytic activity. Aer this
only a trace amount of product formed (entry 1 of Table 1). To optimization we further investigated the potential for better
enhance the yield of product and to optimize the reaction reaction conditions. In this continuation, we examined the
conditions, in our next step a number of alternative catalysts reaction results using a different amount of iodine. The
preferably non-metals were investigated (Table 1). Further the increase in the amount of iodine from 10 to 20 mol% not only
reaction was carried out in the presence of TFA, TFOH, shortens the reaction time but also enhances the yield of
b-cyclodextrin and vitamin B1 for 12 hours but the yield of the product from 75% to 90%. Similarly, using a greater amount of
product was not satisfactory (entry 2, 3, 8 and 9 of Table 1). The catalyst had no effect on the yield of product and reaction time
reactions were also restrained by using oxalic acid and ferric (entry 3, 4 and 5 of Table 2). The effect of using solvents for
2 2 2
example H O (entry 1, of Table 3), CH Cl (entry 4 of Table 3),
CH OH (entry 6, of Table 3) and toluene (entry 7 of Table 3) was
3
also examined and it was observed that reaction gave the best
result in presence of water as a solvent.
Under optimized conditions we further used a range of
aldehydes, hydrazines and alkynes for the synthesis of 1,3,5-
trisubstituted pyrazoles. The results obtained when we
performed the reaction under optimized conditions are
summarized in Table 4. We found that aldehyde with an
Scheme 1 Synthesis of 1,3,5-trisubstituted pyrazole by using aldehyde
1), phenylhydrazine (2) and alkyene (3).
(
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Table 2 Optimization table of iodine for synthesis of 1,3,5-trisubsti- Table 4 Iodine promoted three component reaction for synthesis of
a
a
tuted pyrazoles
functionalized pyrazole derivatives
b
Entry
Iodine (mol%)
Yield (%)
Time (hours)
b
Entry Aldehyde (R
1
)
Hydrazine (R
2
)
Alkynes (R
3
; R
4
)
Yield (%)
1
2
3
4
5
10
20
30
40
50
75
90
90
90
90
4
3
3
3
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1a: H
2a: H
2b: H
2c: H
2d: H
2e: Ph
2f: H
2g: H
2h: 4-MeO
2i: H
2j: H
2k: H
2l: H
2m: H
2n: H
2o: H
3a: H; H
3b: 4-MeO; H
3c: H; H
3d: H; Ph
3e: Ph; H
3f: H; TMS
3g: H; H
3h: F; H
3i: H; H
3j: H; H
3k: 4-Me; H
3l: n-Bu; H
3m: F; H
3n: 4-Br; H
3o: 4-Me; H
90
92
90
0
1b: 4-NO
1c: 4-Cl
1d: H
1e: thienyl
1f: H
1g: CH3
1h: H
1i: 4-MeO
1j: NO
1k: H
1l: H
1m: H
1n: H
1o: H
2
86
75
80
81
79
91
84
83
71
81
87
a
Reaction condition: benzaldehyde 1a (1 mmol), phenylhydrazine
a (1 mmol), phenylacetylene 3a (1 mmol) and iodine in water
2
b
(
5 ml). Isolated yield of products.
0
1
2
3
4
5
2
a
Table 3 Effect of solvent in presence of iodine as the catalyst
a
Reaction condition: benzaldehyde 1 (1 mmol), phenylhydrazine 2 (1
mmol), phenylacetylene 3 (1 mmol) and iodine in water (5 ml) at
ꢀ
b
6
0 C for the stipulated time period. Isolated yield of pure product.
b
Entry
Solvent
Time (hours)
Yield (%)
1
2
3
4
5
6
7
H
2
O
3
4
5
5
6
6
6
90
88
86
70
72
86
40
donor atoms O and N (ref. 29) which leads to Mannich addition.
Finally it promotes intramolecular cyclization to afford the
desired product (4) in excellent yield. (Scheme 2).
Iodine proved to be a highly active, eco-efficient and
selective catalyst for the synthesis of 1,3,5-trisubstituted pyr-
azoles. Iodine has an unique quality to replace strong acids as a
3
CH CN
C
2
H
5
OH
CH
2
Cl
2
30
THF
CH OH
Toluene
3
a
Reaction condition: benzaldehyde 1a (1 mmol), phenylhydrazine
2
a (1 mmol), phenylacetylene 3a (1 mmol) and iodine in solvent
b
(
5 ml). Isolated yield of products.
electron withdrawing group present at the para position gave
,3,5-trisubstituted pyrazole in better yield (entry 2, 3, 10 of
1
Table 4) in comparison to aldehydes having an electron
donating group (entry 9 and 7 of Table 4). In addition aldehyde
with thienyl and methyl were also suitable for the reaction
resulting in 70–80% yield (entry 5 and 7 of Table 4). The
difference in reactivity was not very high. Presumably the three
component reaction seems to occur following the mechanistic
pathway demonstrated in Scheme 2. The important feature of
the mechanism is that iodine catalyzed the reaction in each
step. First of all iodine polarizes and activates the carbonyl
27a
group of aldehyde as it behaves as a mild Lewis acid. In the
rst step iodine binds with the carbonyl group of aldehyde to
give an iodine–aldehyde complex and thus increases the elec-

27b
trophilic character of the carbonyl carbon of aldehyde. (1) and
facilitate dehydration which leads the formation of Schiff base
(
A), which results in the formation of new (C]N) bond in
28
second step. It further activates the C]N bond of Schiff base
Scheme 2 Plausible mechanism of the synthesis of pyrazole in the
(
B) with the formation of two halogen bonds with both electron presence iodine in aqueous medium.
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catalyst as we have shown. This method has several advantages 11 M. J. Genin, C. Biles, B. J. Keiser, S. M. Poppe, S. M. Swaney,
over other methods because the reaction method is very simple,
the synthesis took place under mild, green reaction conditions,
W. G. Tarpley, Y. Yagi and D. L. Romero, J. Med. Chem., 2000,
43, 1034.
in the presence of water as a solvent without any costly, exotic 12 (a) M. J. Graneto, R. G. Kurumbail, M. L. Vazquez,
and highly sensitive catalysts. In addition it also displayed
important green chemistry parameters. In summary we have
developed a green, cost effective and efficient method by using
aldehyde, phenylhydrazine and phenyl acetylene in aqueous
medium, which is unique and promising for future research.
H. S. Shieh, J. L. Pawlitz, J. M. Williams, W. C. Stallings,
L. Geng, A. S. Naraian, F. J. Koszyk, M. A. Stealey, X. D. Xu,
R. M. Weier, G. J. Hanson, R. J. Mourey, R. P. Compton,
S. J. Mnich, G. D. Anderson, J. B. Monahan and R. Devraj,
J. Med. Chem., 2007, 50, 5712; (b) S. Kuettel, A. Zambon,
M. Kaiser, R. Brun, L. Scapozza and R. Perozzo, J. Med.
Chem., 2007, 50, 5833.
Acknowledgements
1
3 (a) A. H. Abadi, A. A. Eissa and G. S. Hassan, Chem. Pharm.
Bull., 2003, 51, 838; (b) R. J. Flower, Nat. Rev. Drug
Discovery, 2003, 2, 179.
4 F. E. Chamberlain, C. Wang, H. Shi, D. C. Admas and Y. Ma,
J. Agric. Food Chem., 2010, 58, 6895.
5 G. R. Bebernitz, G. Argentieri, B. Battle, C. Brennan,
B. Balkan, B. F. Burkey, M. Eckhardt, J. Gao, P. Kapa,
R. J. Strohschein, H. F. Schuster, M. Wilson and D. D. Xu,
J. Med. Chem., 2001, 44, 2601.
We thank the University Grant Commission for the generous

nancial support. We also thank SAIF, Punjab University,
1
Chandigarh for providing all the spectroscopic and analytical
data.
1
References
1
M. Srivastava, J. Singh, S. B. Singh, K. Tiwari, K. V. Pathak 16 (a) Multicomponent reactions, ed. J. Zhu and H. Bienayme,
and J. Singh, Green Chem., 2012, 14, 901.
A. N. Amr AEI GES, L. Abdel and M. M. Abdlla, Acta Pharm.,
Wiley-VCH, Weinheim, Germany, 2005; (b) H. Bienayme,
C. Hulme, G. Oddon and P. Schmitt, Chem.–Eur. J., 2000,
6, 3321; (c) A. Domling and I. Ugi, Angew. Chem., Int. Ed.,
2000, 39, 3168.
2
2006, 56, 1203.
3
4
A. T. Henry, Ann. Trop. Med. Parasitol., 1988, 82, 257.
S. Chimichi, M. Boccalini, M. M. Hassan, G. Viola, 17 F. E. Chamberlain, C. Wang, H. Shi, D. C. Admas and Y. Ma,
F. Dall'Acqua and M. Curini, Tetrahedron, 2006, 62, 90.
J. Agric. Food Chem., 2010, 58, 6895.
K. Hisafumi, K. Motto and N. Fumio, Chem. Abstr., 1989, 106, 18 P. Gunasekaran, K. Balamurugan, S. Sivakumar, S. Perumal
97008/g.
and C. J. Men ´e ndez, Green Chem., 2012, 14, 750.
(a) A. M. Hantoon, Minn. Med., 2001, 84, 102; (b) 19 A. Dandia, A. k. Laxkar and R. Singh, Tetrahedron Lett., 2012,
M. Srivastava, P. Rai, J. Singh and J. Singh, RSC Adv., 2013, 53, 3012.
, 16994; (c) A. Hasaninejad and S. Firoozi, Mol. Diversity, 20 P. Gunasekaran, K. Balamurugan, S. Sivakumar, S. Perumal
5
6
2
3
2013, 17, 459; (d) P. Rai, M. Srivastava, J. Singh and
and C. J. Men ´e ndez, Green Chem., 2012, 14, 750.
J. Singh, RSC Adv., 2013, 3, 18775.
X. Zhang, X. Li, F. G. Allan and T. Sbriscia, J. Med. Chem.,
21 (a) P. Prasanna, S. Perumal and J. C. Men ´e ndez, Green Chem,
2013, 15, 1292; (b) Y. Gu, Green Chem., 2012, 14, 2091; (c)
J.-N. Tan, H. Li and Y. Gu, Green Chem., 2010, 12, 1772; (d)
G. Williams, A. Cullen, A. Fourie, H. Henning, M. Lawton,
W. Mommsen, P. Nangu, J. Parker and A. Renison, Green
Chem., 2010, 12, 1919; (e) A. Solhy, A. Elmakssoudi,
R. Tahir, M. Karkouri, M. Larzek, M. Bousmina and
M. Zahouily, Green Chem., 2010, 12, 2261.
7
2007, 50, 3857.
8
9
A. N. Richard, M. Meghan, et al., J. Med. Chem., 1993, 36, 134.
(a) M. Rowley, H. B. Broughton, I. Collins, R. Baker, F. Emms,
R. Marwood, S. Patel, C. I. Ragan, S. B. Freedman and
P. D. Leeson, J. Med. Chem., 1996, 39, 1943; (b)
S. J. Wittenberger, J. Org. Chem., 1996, 61, 356; (c)
G. Dannhardt, P. Dominiak and S. Laufer, Arzneim. Forsch., 22 (a) R. Breslow and U. Maitra, Tetrahedron Lett., 1984, 25,
1
993, 43, 441.
0 (a) E. M. Pinho and M. V. D. Teresa, Curr. Org. Chem., 2005, 9,
25; (b) F. Colliot, K. A. Kukorowski, D. W. Hawkins and
D. A. Roberts, Brighton Crop Prot. Conf.–Pests Dis., 1992, 1,
9; (c) H. S. Chen, Z. M. Li and Y. F. Han, J. Agric. Food
1239; (b) R. Breslow, U. Maitra and D. Rideout, Tetrahedron
Lett., 1983, 24, 1901; (c) D. C. Rideout and R. Breslow, J.
Am. Chem. Soc., 1980, 102, 7816; (d) S. Narayan,
J. Muldoon, M. G. Finn, V. V. Fokin, H. C. Kolb and
K. B. Sharpless, Angew. Chem., Int. Ed., 2005, 44, 3275; (e)
Y. S. Jung and R. A. Marcus, J. Am.Chem. Soc., 2007, 129,
5492.
1
9
2
Chem., 2000, 48, 5312; (d) C. B. Vicentini, C. Romagnoli,
E. Reotti and D. Mares, J. Agric. Food Chem., 2007, 55,
1
0331; (e) C. B. Vicentini, D. Mares, A. Tartari, M. Manfrini 23 O. M. Simon and J. C. Li, Chem. Soc. Rev., 2012, 41, 1415.
and G. Forlani, J. Agric. Food Chem., 2004, 52, 1898; (f) 24 P. Liu, M. Y. Pan, Y. Xu and L. S. H. Wang, Org. Biomol.
J. Elguero, in Comprehensive Heterocyclic Chemistry II, ed. A.
Chem., 2012, 10, 4696.
R. Katritzky, C. W. Rees and E. F. V. Scriven, Pergamon, 25 (a) J. C. Li, C. Ma, Y. R. Yan, Z. Ren and G. Huang, Synlett,
Oxford, 1996, vol. 3, p. 1; (g) C. Lamberth, Heterocycles,
007, 71, 1467; (h) T. Eicher, S. Hauptmann and
2011, 1321; (b) J. Wen, Y. Fu, R.-Y. Zhang, J. Zhang,
S.-Y. Chen and X.-Q. Yu, Tetrahedron, 2011, 67, 9618.
2
A. Speicher, The Chemistry of Heterocycles, Wiley & Sons, 26 (a) For examples: K. M. Kim and E. K. Ryu, Tetrahedron Lett.,
New York, 2nd edn, 2011, vol. 2, p. 635.
1996, 37, 1441; (b) H. Firouzabadi, N. Iranpoor and
782 | RSC Adv., 2014, 4, 779–783
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
H. Hazarkhani, J. Org. Chem., 2001, 66, 7527; (c)
K. Ramalinga, P. Vijayalakshmi and T. N. B. Kaimal,
Tetrahedron Lett., 2002, 43, 879; (d) H. Firouzabadi,
N. Iranpoor and S. Sobhani, Tetrahedron Lett., 2002, 43,
Tetrahedron Lett., 2004, 45, 4785; (m) J. Sun, Y. Dong,
X. Wang, S. Wang and Y. Hu, J. Org. Chem., 2004, 69, 8932;
(n) R. S. Bhosale, S. V. Bhosale, T. Wang and
P. K. Zubaidha, Tetrahedron Lett., 2004, 45, 9111; (o) B. Ke,
Y. Qin, Q. He, Z. Huang and F. Wang, Tetrahedron Lett.,
2005, 46, 1751; (p) B. K. Banik, M. Fernandez and
C. Alvarez, Tetrahedron Lett., 2005, 46, 2479.
3653; (e) J. S. Yadav, B. V. S. Reddy, M. S. Reddy and
A. R. Prasad, Tetrahedron Lett., 2002, 43, 9703; (f)
B. P. Bandgar and K. A. Shaikh, Tetrahedron Lett., 2003, 44,
1959; (g) B. Das, J. Banerjee, R. Ramu, R. Pal, 27 (a) M. Kidwai, V. Bansal, P. Mothsra, et al., J. Mol. Catal. A:
N. Ravindranath and C. Ramesh, Tetrahedron Lett., 2003,
4, 5465; (h) R. Saeeng, U. Sirion, P. Sahakitpichan and
Chem., 2007, 265, 177; (b) M. Kidwai, V. Bansal,
P. Mothsra, et al., J. Mol. Catal. A: Chem., 2007, 268, 76.
4
M. Isobe, Tetrahedron Lett., 2003, 44, 6211; (i) S.-J. Ji, 28 X.-F. Lin, L.-S. Cui and G.-Y. Wang, Tetrahedron Lett., 2006,
S.-Y. Wang, Y. Zhang and T.-P. Loh, Tetrahedron, 2004, 60, 47, 3127.
051; (j) J. S. Yadav, B. V. S. Reddy, S. Shubashree and 29 R. G. Reddy, R. T. Reddy, S. C. Joseph, S. K. Reddy and M. Pal,
2
K. Sadashiv, Tetrahedron Lett., 2004, 45, 2951; (k)
RSC Adv., 2012, 2, 3387.
P. Phukan, J. Org. Chem., 2004, 69, 4005; (l) P. Phukan, 30 H. Song, Y. Liu and Q. Wang, Org. Lett., 2013, 15, 3274.
This journal is © The Royal Society of Chemistry 2014
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