Synthesis of pyrano[2,3-c]pyrazoles by ionic liquids under green and eco-safe conditions

Article abstract of DOI:10.1007/s11164-016-2648-y

Optimization of a green approach to the synthesis of pyrano[2,3-c]pyrazoles based on the one-pot, four-component condensation via a domino Knoevenagel/Michael/cyclization sequence was investigated. This method involved the evaluation of the activity of se

Full text of DOI:10.1007/s11164-016-2648-y

Res Chem Intermed
DOI 10.1007/s11164-016-2648-y
Synthesis of pyrano[2,3-c]pyrazoles by ionic liquids
under green and eco-safe conditions
1
1,2
•
Masoumeh Zakeri
3
•
Mohamed Mahmoud Nasef
1
1
•
Tina Kargaran Arshad Ahmad
4
Jahanbakhsh Asadi
•
•
Ebrahim Abouzari-Lotf
Received: 23 March 2016 / Accepted: 8 July 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract Optimization of a green approach to the synthesis of pyrano[2,3-
c]pyrazoles based on the one-pot, four-component condensation via a domino
Knoevenagel/Michael/cyclization sequence was investigated. This method involved
the evaluation of the activity of several ionic liquids (ILs) in various solvents. This
one-pot, four-component reaction revealed simplicity, higher yield and lower tox-
icity advantages over a corresponding three-component method. The effect of
reaction parameters including the type and amount of catalyst, type of solvent,
reaction temperature and time were studied with respect to yield of pyrano[2,3-
c]pyrazoles. Catalyst recyclability and time-saving aspects of the reaction suggest
that this method presents real alternatives over conventional reaction protocols.
Keywords Green chemistry Á Ionic liquids Á Pyrano[2,3-c]pyrazoles Á 1-Phenyl-3-
methyl pyrazolone
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2648-y)
contains supplementary material, which is available to authorized users.
&
&
1
Masoumeh Zakeri
ms.zakeri@gmail.com
Mohamed Mahmoud Nasef
mahmoudeithar@cheme.utm.my
Centre for Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia,
54100 Kuala Lumpur, Malaysia
2
Malaysia-Japan International Institute of Technology, International Campus, Universiti
Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia
3
Department of Toxicology and Pharmacology, Faculty of Pharmacy, Pharmaceutical Sciences
Branch, Islamic Azad University, Tehran, Iran
4
Metabolic Disordes Research Center, Golestan University of Medical Sciences, Gorgan, IR Iran
123

M. Zakeri et al.
Introduction
Green chemistry is an increasingly important aspect of chemical research devoted to
minimizing the use and generation of hazardous substances, organic solvents, and
toxic catalysts on the environment [1]. In this context, it is highly desirable to
develop environmentally benign processes that can be conducted in an aqueous
media due to the low-cost, simple operation and high efficiency in many organic
reactions [2]. However, using water is limited by the low solubility of organic
compounds. One of the most important strategies to overcome this problem is the
use of phase transfer catalysts such as ionic liquids (ILs). Increasing interest in ILs
is related to their possible usage as environmentally friendly catalysts and solvents
due to their negligible vapor pressure, thermal and chemical stability, solvating
ability and easy recyclability [3, 4]. Diverse liquid salts have been produced by the
combination of ammonium, pyridinium, or imidazolium cations with various
inorganic or organic anions that led to numerous possible applications in the field of
heterocyclic compounds synthesis, catalysis, biocatalysis, chemical engineering and
electrochemistry [5–7].
Pyrano[2,3-c]pyrazoles are an important class of heterocyclic compounds that
play an essential role as biologically active compounds and represent an interesting
template in medicinal chemistry. Many of these compounds are known for their
antimicrobial [8], insecticidal [9] and anti-inflammatory activities [10]. In recent
years, several methods for the synthesis of pyrano[2,3-c]pyrazoles via multicom-
ponent reactions have been reported. Synthesis of pyrano[2,3-c]pyrazole derivatives
have been catalyzed by per-6-amino-b-cyclodextrin [11], magnesium oxide [12],
piperidine [13], silica-supported tetramethylguanidine (SiO TMG) [14], meglumine
[15], isonicotinic acid [16], L-proline in ionic liquid media [17], D,L-proline [18]
magnetic Fe O nanoparticles [19] and catalyst-free [20, 21]. However, there is a
2
3
4
scope for further work towards the development of an inexpensive, cleaner and eco-
friendly procedure that has a short reaction time.
Biological activity of pyrano[2,3-c]pyrazoles and our experience in using ILs as
catalysts in the synthesis of heterocyclic compounds [22–24] encouraged us to
establish an efficient synthesis route for pyrano[2,3-c]pyrazoles via the four-
component reaction of hydrazine hydride (or phenyl hydrazine), ethyl acetoacetate,
aromatic aldehydes and malononitrile using specific ILs as catalysts (as shown in
Scheme 1). This one-pot, four-component reaction showed advantages over a one-
Scheme 1 The preparation of pyranopyrazoles using ILs as catalyst
1
23

Synthesis of pyrano[2,3-c]pyrazoles by ionic liquids…
pot, three-component method of 1-phenyl-3-methyl pyrazolone, aromatic aldehydes
and malononitrile in its simple procedure, high yield and low toxicity.
Experimental
Materials and methods
All common reagents and solvents were used as obtained from commercial
suppliers without further purifications. Three Brønsted acidic ILs (cat. 1), (cat. 2)
and (cat. 3) were prepared according to the procedure reported in the literature
[
25, 26]. The rest of the ILs ([95 %) were obtained from Shanghai Cheng Jie
Chemical, China. Melting points were measured using the capillary tube method
with an electrothermal 9200 apparatus. IR spectra were recorded on a JASCO-FT-
1
13
IR model 5300 spectrometer. H NMR and C NMR spectra were recorded on a
Bruker AQS AVANCE-300 spectrometer using TMS as an internal standard.
Elemental analysis performed by a Perkin Elmer 2004 (II) CHN analyzer.
Typical procedure for the four-component synthesis of multi-substituted
pyranopyrazole derivatives
A mixture of hydrazine hydrate 80 % or phenyl hydrazine (1.2 mmol), ethyl
acetoacetate (1 mmol), aromatic aldehydes (1 mmol), malononitrile (1.2 mmol) and
1
,3-dimethyl-2-oxo-1,3-bis(4-sulfobutyl) imidazolidine-1,3-diium hydrogen sulfate
[
DMDBSI]Á2HSO (10 mol%) in 10 mL of H O was stirred at 60 °C for an
4
2
appropriate period of time. After completion as monitored by TLC (n-hexane/ethyl
acetate: 7/3), the reaction mixture was cooled to room temperature and the resulting
solid was collected by filtration, washed with water (3 9 10 mL) and then dried
under vacuum at 80 °C overnight.
Typical procedure for the three-component synthesis of multi-substituted
pyranopyrazole derivatives
[
DMDBSI]Á2HSO4 (10 mol%) was added to a mixture of 1-phenyl-3-methyl
pyrazolone (1 mmol), aromatic aldehyde (1 mmol) and malononitrile (1.2 mmol) in
0 mL of EtOH/H O (2/1). The reaction mixture was heated at 60 °C for an
1
2
appropriate period of time. After the reaction was completed, the mixture was
cooled to room temperature and the resulting solid was collected by filtration,
washed with water (3 9 10 mL) and then dried under vacuum at 80 °C overnight.
Typical procedure for the synthesis of the multi-substituted
bispyranopyrazole derivatives
A mixture of hydrazine hydrate or phenyl hydrazine (2.5 mmol), ethyl acetoacetate
(
[
2 mmol), terephthalaldehyde (1 mmol), malononitrile (2.5 mmol) and
DMDBSI]Á2HSO4 (10 mol%) in 10 mL of H O was stirred at 60 °C for an
2
123

M. Zakeri et al.
appropriate period of time. After completion of the reaction, the mixture was cooled
to room temperature and the resulting solid was collected by filtration, washed with
water (3 9 10 mL) and then dried under vacuum at 80 °C overnight.
Results and discussion
Recently, we observed that the use of [BSO HPy]HSO as a catalyst resulted in a
3
4
significant rate acceleration with a high yield in the synthesis of tetraquinazoline-2-
amine [27]. Bearing this in mind, we evaluated the catalyst activity of
[
BSO HPy]HSO in the four-component reaction between a mixture of phenylhy-
3 4
drazine, ethyl acetoacetate, benzaldehyde and malononitrile to prepare pyrano[2,3-
c]pyrazole (Scheme 1). The reaction occurred to offer the desired products in
medium yields of 60 and 48 % in water and ethanol media, respectively (Table 1,
entries 1 and 2). In order to find an effective catalyst and optimize the reaction, a
series of ILs with respect to both cation and anion counterparts were screened
(
Fig. 1).
It was observed that almost all of the investigated ILs were capable of catalyzing
the desired products, but the model reaction proceeded faster and gave the product
in higher yield with [DMDBSI]Á2HSO (Table 1, entry 6). It can also be observed
4
that the sulfonic acid ILs effectively catalyze the synthesis of pyrano[2,3-c]pyrazole
5
g as indicated from the superior results obtained. [BzMIM]Cl and [AMIM]Br did
not significantly increase the yield of the desired product (Table 1, entries 18 and
1).
To adjust the amount of [DMDBSI]Á2HSO , different experiments were carried
2
4
out, and it was found that, by increasing the amount of catalyst for the 5g from 3 to
, 7 and 10 mol%, the yields increased from 70 to 75, 82 and 85 %, respectively
Table 1, entries 3–6). The use of 10 mol % IL was found to be sufficient to push
5
(
this reaction forward and a higher catalyst amount (12 and 15 mol%) did not
improve the yields of the reaction (Table 1, entries 7 and 8).
Choosing an appropriate solvent has crucial importance for the successful
synthesis. To search for the optimal solvent, the model reaction was investigated in
the presence of 10 mol% of [DMDBSI]Á2HSO using various solvents including
4
ethanol, water, acetonitrile and toluene at 60 °C and also at 120 °C under solvent-
free conditions to determine which gave the best results. The results of these
comparative experiments are summarized in Table 1. Polar protic solvents such as
water and ethanol enhance the rate of reaction (Table 1, entries 6 and 9) while, in
the case of non-polar solvent such as toluene, the rate of reaction decreased
(
Table 1, entry 12). In comparison with organic solvents, water was found to give
the best result. This solvent can have substantial effects in realizing the goals of
green chemistry.
To optimize the reaction temperature, the reaction was carried out at different
temperatures ranging from room temperature to reflux condition in water
(30–100 °C). Consequently, 60 °C was found to be the most suitable reaction
temperature (Table 2).
1
23

Synthesis of pyrano[2,3-c]pyrazoles by ionic liquids…
Table 1 Effect of different catalysts and solvents on synthesis of compound 5g
a
Yield (%)
Entry
Reaction conditions
Time (min)
1
2
3
4
5
6
7
8
9
[BSO
3
3
HPy]HSO
4
4
(3 mol%), water
20
20
15
15
15
15
15
15
15
15
30
30
15
15
15
15
15
40
15
25
40
60
48
70
75
82
85
85
85
72
55
70
35
72
77
80
78
71
34
53
54
25
[BSO
HPy]HSO
(3 mol%), ethanol
[DMDBSI]Á2HSO
[DMDBSI]Á2HSO
[DMDBSI]Á2HSO
[DMDBSI]Á2HSO
[DMDBSI]Á2HSO
[DMDBSI]Á2HSO
[DMDBSI]Á2HSO
[DMDBSI]Á2HSO
[DMDBSI]Á2HSO
[DMDBSI]Á2HSO
4
(3 mol%),water
4
4
4
4
4
4
4
4
4
(5 mol%), water
(7 mol%), water
(10 mol%), water
(12 mol%), water
(15 mol%), water
(10 mol%), ethanol
(10 mol%), acetonitrile
(10 mol%), solvent-free, 120 °C
(10 mol%), toluene
10
11
12
13
14
15
16
17
18
19
20
21
a
[BSO
[BSO
[BSO
[BSO
3
3
3
3
HMIM]HSO
HPy]HSO (10 mol%), water
HMIM]CF SO (10 mol%), water
HPy]CF SO (10 mol%), water
4
(10 mol%), water
4
3
3
3
3
[BMIM]Cl (10 mol%), water
[BzMIM]Cl (10 mol%), water
[BMIM]Br (10 mol%), water
[BMIM]BF
4
(10 mol%), water
[AMIM]Br (10 mol%), water
Isolated yields
These findings confirm the establishment of an efficient and optimized green
approach for preparation of pyrano[2,3-c]pyrazole in 85 % isolated yield through a
one-pot reaction involving a mixture of phenyl hydrazine, ethyl acetoacetate,
malononitrile with benzaldehyde in the presence of 10 mol% [DMDBSI]Á2HSO at
4
6
0 °C for 15 min.
In comparison to the four-component and one-pot reaction, the three-component
reaction including 1-phenyl-3-methyl pyrazolone, aromatic aldehydes and malonon-
itrile underwent the same conditions to yield the target compound (Scheme 2).
Similar to the above optimization, different solvents and temperatures in the
presence of 10 mol% [DMDBSI]Á2HSO4 were used for the three-component
123

M. Zakeri et al.
Fig. 1 ILs tested in this study
reaction for the synthesis of compound 5g (Table 2). It is important to note tha,t
when the reaction was carried out in H O using [DMDBSI]Á2HSO as the catalyst,
2
4
the yield of product 5g in the four-component reaction reached 85 %. However, the
yield of compound 5g in the three-component reaction decreased when the reaction
was attempted in water using the same catalyst. All of the compounds prepared via
the four-component one-pot reaction demonstrated the higher yields, simple
procedure and atom efficiency for this method compared with three-component
reaction.
This optimized protocol was then extended to a variety of hydrazines, and
different aldehydes including either electron-withdrawing or electron-donating
groups via the four-component reaction. It was found that all these aromatic
aldehyde derivatives are suitable for this reaction, giving the desired products in the
range of good to excellent yields under water media conditions (Table 3).
In addition, the optimized reaction conditions were tested for the synthesis of
multi-substituted bispyrano[2,3-c]pyrazole derivatives via the four-component, one-
pot synthesis route using methyl acetoacetate, hydrazine (or phenylhydrazine),
terephthalaldehyde, and malononitrile at [DMDBSI]Á2HSO in H O at 60 °C that
4
2
1
23

Synthesis of pyrano[2,3-c]pyrazoles by ionic liquids…
Table 2 Optimization of reaction time and temperature in presence of 10 mol% [DMDBSI]Á2HSO
4
c
Yield (%)
Entry
Temperature (°C)
Time (min)
a
b
B
A
1
2
3
4
5
6
7
8
9
30
40
50
60
70
100
60
60
60
60
10
10
10
10
10
10
12
15
18
20
40
54
68
80
80
80
82
85
85
85
35
50
61
75
75
77
78
80
80
80
1
a
0
Yields for the four-component one-pot reaction
Yields for the three-component one-pot reaction
Isolated yields
b
c
Scheme 2 Three-component reaction for the synthesis of pyranopyrazoles
products 5m and 5n were obtained in good yields (Scheme 3). In this study, the
1
13
obtained products were characterized using melting point, IR, H NMR, C NMR
and CHN (supporting information).
The formation of the desired product 5g can be explained by the sequence of
Knoevenagel condensation and Michael addition followed by cyclization, as shown
in Scheme 4. Initially, ethyl acetoacetate is activated by [DMDBSI]Á2HSO . A
4
123

M. Zakeri et al.
wT aa tb el re m3 e dS i ya n at th e6 s0i s° Co f pyrano[2,3-c]pyrazole derivatives in presence of 10 mol% [DMDBSI]Á2HSO
4
in
5
a, 90 %, 10 min
5b, 90 %, 12 min
m.p.: 234–236
5c, 82 %, 15 min
m.p.: 248–250
5d, 85 %, 10 min
m.p.: 241–243
m.p.: 212
Lit.: [243–244] [15]
Lit.: [234–235] [15]
Lit.: [248–249] [15]
Lit.: [210–212] [15]
5
e, 80 %, 15 min
5f, 78 %, 15 min
m.p.: 244–246
5g, 85 %, 15 min
m.p.: 168–69
5h, 88 %, 15 min
m.p.: 173–175
m.p.: 219–222
Lit.: [223–224] [17]
Lit.: [244–245] [15]
Lit.: [168–170] [26]
Lit.: [177–178] [25]
5
i, 85 %, 15 min
5j, 86 %, 12 min
m.p.:178–180
5k, 80 %, 15 min
m.p.:196–199
5l, 76 %, 15 min
m.p.: 240
m.p.: 170–172
Lit.: [173] [28]
Lit.: [178–180] [29]
Lit.: [193–195] [29]
Lit.: [235–237] [29]
5
m, 78 %, 15 min
5n, 74 %, 15 min
m.p.: [300
m.p.: 238
Lit.:[238–240] [30]
Lit.: [[300] [30]
1
23

Synthesis of pyrano[2,3-c]pyrazoles by ionic liquids…
Scheme 3 Synthesis of multi-substituted bispyrano[2,3-c]pyrazole derivatives
Scheme 4 Investigation of a possible reaction mechanism for 5g
123

M. Zakeri et al.
Table 4 Comparison of different conditions for the synthesis of 5a
Catalyst
Reaction condition
[Bmim]BF
, 50 °C
Time (min) Yield (%) References
L-Proline (10 mol%)
4
10
35
30
10
15
240
10
10
15
10
90
90
96
83
65
79
88
90
95
90
[17]
c-Alumina (30 mol%)
Solvent-free
[29]
SiO
Pipiridine (5 mol%)
Et N (20 mol%)
Catalyst-free
Bmim]OH (20 mol%)
2
TMG (10 mol%)
Solvent-free, 100 °C
[14]
H
2
O, room temperature (r.t.)
EtOH, reflux
O, 90 °C
[13]
3
[31]
H
2
[20]
[
50–60 °C
[32]
Isonicotinic acid (10 mol%)
Meglumine (10 mol%)
Solvent-free, 85 °C
EtOH/H O (9:1), r.t.
[16]
2
[15]
[DMDBSI]Á2HSO
4
(10 mol%)
H
2
O, 60 °C
This work
Table 5 Reusability of catalyst in synthesis of 5g
Entry
Cycle
Time (min)
Yield (%)
1
2
3
4
1st run
2st run
3st run
4st run
15
18
20
25
85
82
78
71
hydrogen bond between the hydrogen atom of [DMDBSI]Á2HSO and the carbonyl
4
group of ethyl acetate can expedite the pyrazolone synthesis (intermediate A). In the
next step, formation of the arylidene malononitrile (intermediate B) occurs via
Knoevenagel condensation between malononitrile and benzaldehyde, followed by
loss of water molecules. The next step involves the formation of 5g via Michael
addition of pyrazolone (intermediate A) to arylidene malononitrile (intermediate B),
followed by cyclization to give the pyrano[2,3-c]pyrazole.
Finally, in order to show the efficiency of the proposed method,
DMDBSI]Á2HSO catalyst was compared with other catalysts reported earlier for
[
4
the synthesis of 5a. As shown in Table 4, the use of [DMDBSI]Á2HSO leads to a
4
comparable protocol in terms of compatibility with environment, reaction time and
yield.
In another study, recyclability of the catalyst was examined upon the
condensation of phenyl hydrazine, ethyl acetoacetate, malononitrile and benzalde-
hyde. After completion of the reaction, the catalyst was extracted by water and
separated from the reaction mixture. Subsequently, the water was evaporated and
the catalyst was dried in a vacuum at 70 °C overnight. The results showed that the
IL could be used at least 4 times without any significant loss in the yield of the
reaction (Table 5).
1
23

Synthesis of pyrano[2,3-c]pyrazoles by ionic liquids…
Conclusions
A simple, expeditious, and green method has been established and optimized for the
generation of pyrano[2,3-c]pyrazoles via four-component reactions in the presence
of ILs in water media. The optimized results demonstrated that the yields of
7
4–90 % could be expected with a reaction time of 10–15 min, and a catalyst
amount of 10 mol% using water media in a temperature of 60 °C. Based on such
optimized conditions, diverse derivatives were obtained in high yields with a very
simple purification procedure avoiding the use of usual column chromatography.
The avoidance of using organic solvents and using reusable catalysts together with
the facile and fast features of this method provide an alternative route to
conventional reaction protocols.
Acknowledgment The authors wish to acknowledge the financial support from the Research University
Grant (vot. # 02E67) for Universiti Teknologi Malaysia.
References
1
2
3
4
5
. P.T. Anastas, J.B. Zimmerman, Environ. Sci. Technol. 37, 94A (2003)
. M.-O. Simon, C.-J. Li, Chem. Soc. Rev. 41, 1415 (2012)
. N. Karimi, H. Oskooi, M. Heravi, M. Saeedi, M. Zakeri, N. Tavakoli, Chin. J. Chem. 29, 321 (2011)
. H. Jun, Y. Ouchi, D. Kim, J. Mol. Liq. 179, 54 (2013)
. M. Zakeri, M.M. Heravi, M. Saeedi, N. Karimi, H.A. Oskooie, N. Tavakoli-Hoseini, Chin. J. Chem.
29, 1441 (2011)
6
7
8
9
. M.M. Heravi, M. Zakeri, N. Karimi, M. Saeedi, H.A. Oskooie, N. Tavakoli-Hosieni, Synth. Com-
mun. 40, 1998 (2010)
. D. Kogelnig, A. Stojanovic, F. Kammer, P. Terzieff, M. Galanski, F. Jirsa, R. Krachler, T. Hofmann,
B.K. Keppler, Inorg. Chem. Commun. 13, 1485 (2010)
. A. Balbi, M. Anzaldi, C. Maccio, C. Aiello, M. Mazzei, R. Gangemi, P. Castagnola, M. Miele, C.
Rosano, M. Viale, Eur. J. Med. Chem. 46, 5293 (2011)
. A. Dandia, R. Singh, S. Khaturia, C. Merienne, G. Morgant, A. Loupy, Bioorg. Med. Chem. 14, 2409
(
2006)
0. M.E. Zaki, H.A. Soliman, O.A. Hiekal, A.E. Rashad, Zeitschrift fur Naturforschung. C. J. Biosci. 61,
(2006)
1
1
1
1
1
1
1
1. K. Kanagaraj, K. Pitchumani, Tetrahedron Lett. 51, 3312 (2010)
2. M. Babaie, H. Sheibani, Arabian J. Chem. 4, 159 (2011)
3. G. Vasuki, K. Kumaravel, Tetrahedron Lett. 49, 5636 (2008)
4. A.B. Atar, J.T. Kim, K.T. Lim, Y.T. Jeong, Synth. Commun. 44, 2679 (2014)
5. R.-Y. Guo, Z.-M. An, L.-P. Mo, S.-T. Yang, H.-X. Liu, S.-X. Wang, Z.-H. Zhang, Tetrahedron 69,
9931 (2013)
16. M.A. Zolfigol, M. Tavasoli, A.R. Moosavi-Zare, P. Moosavi, H.G. Kruger, M. Shiri, V. Khakyzadeh,
RSC Adv. 3, 25681 (2013)
17. J.M. Khurana, B. Nand, S. Kumar, Synth. Commun. 41, 405 (2011)
18. S.-B. Guo, S.-X. Wang, J.-T. Li, Synth. Commun. 37, 2111 (2007)
19. M.A. El-Remaily, E. Aleem, A. Ali, Tetrahedron 70, 2971 (2014)
20. C. Yu, C. Yao, T. Li, X. Wang, Res. Chem. Intermed. 40, 1537 (2014)
21. H.M. Al-Matar, K.D. Khalil, A.Y. Adam, M.H. Elnagdi, Molecules 15, 6619 (2010)
22. Y.S. Beheshtiha, M.M. Heravi, M. Saeedi, N. Karimi, M. Zakeri, N. Tavakoli-Hossieni, Synth.
Commun. 40, 1216 (2010)
2
3. M. Zakeri, M.M. Nasef, E. Abouzari-Lotf, H. Haghi, Res. Chem. Intermed. 41, 10097 (2015)
4. M. Zakeri, M.M. Nasef, E. Abouzari-Lotf, A. Moharami, M.M. Heravi, J. Indust. Eng. Chem. 29, 273
2
(2015)
25. H.-F. Liu, F.-X. Zeng, L. Deng, B. Liao, H. Pang, Q.-X. Guo, Green Chem. 15, 81 (2013)
123

M. Zakeri et al.
2
2
2
2
3
3
6. Z. Chen, Q. Zhu, W. Su, Tetrahedron Lett. 52, 2601 (2011)
7. M. Zakeri, M.M. Nasef, E. Abouzari-Lotf, J. Mol. Liq. 199, 267 (2014)
8. F. Lehmann, M. Holm, S. Laufer, J. Comb. Chem. 10, 364 (2008)
9. H. Mecadon, MdR Rohman, M. Rajbangshi, B. Myrboh, Tetrahedron Lett. 52, 2523 (2011)
0. K. Ablajan, L.J. Wang, Z. Maimaiti, Y.T. Lu, Monatsh. Chem. 145, 491 (2014)
1. Y.M. Litvinov, A.A. Shestopalov, L.A. Rodinovskaya, A.M. Shestopalov, J. Comb. Chem. 11, 914
(2009)
32. J.M. Khurana, A. Chaudhary, Green Chem. Lett. Rev. 5, 633 (2012)
1
23