An aqueous, catalyst-free and three-component synthesis of 6-amino-3-(trifluoromethyl)-1,4-dihydro-1-phenyl-4-arylpyrano[2,3-c] pyrazole-5-carbonitriles

Article abstract of DOI:10.1007/s11164-013-1058-7

A concise synthesis of 6-amino-4-aryl-3-(trifluoromethyl)-1,4-dihydro-1- phenylpyrano[2,3-c]pyrazole-5-carbonitriles was performed effectively in aqueous media without catalyst by the reaction of aryl aldehydes, malononitrile, and 1-phenyl-3-(trifluoromethyl)-1H-pyrazol-5(4H)-one. This method has the advantages of mild condition, avoidance of the use of catalysts, high yields, and environmentally benign procedure.

Full text of DOI:10.1007/s11164-013-1058-7

Res Chem Intermed
DOI 10.1007/s11164-013-1058-7
An aqueous, catalyst-free and three-component
synthesis of 6-amino-3-(trifluoromethyl)-1,4-dihydro-
1-phenyl-4-arylpyrano[2,3-c]pyrazole-5-carbonitriles
•
•
•
Chenxia Yu Changsheng Yao Tuanjie Li
Xiangshan Wang
Received: 6 January 2013 / Accepted: 15 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A concise synthesis of 6-amino-4-aryl-3-(trifluoromethyl)-1,4-dihydro-
1-phenylpyrano[2,3-c]pyrazole-5-carbonitriles was performed effectively in aqueous
media without catalyst by the reaction of aryl aldehydes, malononitrile, and 1-phenyl-
3-(trifluoromethyl)-1H-pyrazol-5(4H)-one. This method has the advantages of mild
condition, avoidance of the use of catalysts, high yields, and environmentally benign
procedure.
Keywords Aqueous synthesis Á Multi-component reaction Á Catalyst-free
synthesis Á Trifluoromethylated heterocycles Á Pyrano[2,3-c]pyrazole
Introduction
The organofluorine compounds are of significant importance to the pharmaceutical
and agrochemical industries, due to their often unique biological properties, such as
increased membrane permeability, enhanced hydrophobic binding, and stability
against metabolic oxidation [1–5]. The trifluoromethyl group (CF3) is an important
structural moiety in diverse classes of bioactive organic molecules. It has a bigger
van der Waals radius than that of a CH3 group and the same electronegativity
as oxygen [6]. Thus, much effort has been exerted in the introduction of a
C. Yu Á T. Li Á X. Wang
School of Chemistry and Engineering, Jiangsu Key Laboratory of Green Synthetic Chemistry
for Functional Materials, Jiangsu Normal University, Xuzhou 221116, Jiangsu, People’s Republic
of China
C. Yao (&)
School of Chemistry and Chemical Engineering, Key Laboratory of Biotechnology on Medical
Plant, Jiangsu Normal University, Xuzhou 221116, Jiangsu, People’s Republic of China
e-mail: csyao@jsnu.edu.cn
123

C. Yu et al.
trifluoromethyl group into organic molecules to tune their physical, chemical, and
physiological properties [7–11].
The chemistry of condensed pyranopyrazoles has received considerable attention
for their derivatives which display a broad range of bioactivities including anti-
microbial, antibacterial, antifungal, antitubercular, anti-inflammatory, and cytotoxic
[12–16]. Thus, the synthesis and biological investigation of fluorinated condensed
pyranopyrazoles arouses great research interest in the fields of organic chemistry,
medicinal chemistry, and agrochemistry. Very recently, Bhavanarushi et al. [17]
reported the cytotoxic and DNA binding studies of fused trifluoromethylated
pyranopyrazoles. This led us to pay much attention to the efficient synthesis of these
biologically significant heterocyclic compounds.
Recently, aqueous multi-component reaction (MCR), which couples the advan-
tages of MCR with that of aqueous synthesis, has become an attractive protocol for
the diversity-oriented synthesis of heterocycles due to its prominent benefits
including atom-economy, efficiency in bond formation, straightforward reaction
design, and generation of products in an environmentally benign way [18–22]. To
continue our work on the aqueous synthesis of heterocyclic compounds via MCR
[23–25], we here disclose a catalyst-free synthesis of 6-amino-3-(trifluoromethyl)-
1,4-dihydro-1-phenyl-4-arylpyrano[2,3-c]pyrazole-5-carbonitriles via the reaction
of aryl aldehydes, malononitrile, and 1-phenyl-3-(trifluoromethyl)-1H-pyrazol-
5(4H)-one in water (Scheme 1).
Results and discussion
Initially, the effect of temperature on the three-component synthesis was investi-
gated using benzaldehyde 1a, malononitrile 2, and 1-phenyl-3-(trifluoromethyl)-1H-
pyrazol-5(4H)-one 3 as model substrates (Table 1). When the reaction was
performed below 70 °C, no obvious transformation of the starting material was
observed, as indicated by TLC. Later, the reaction proceeded well and gave the
desired product in moderate to good yields at elevated temperatures. The final
results showed 90 °C to be preferable.
Inspired by the abovementioned result, we next examined the generality of this
method using several aryl aldehydes as substrates. The results are summarized in
Table 2. The protocol showed good tolerance to the aryl aldehydes bearing both
electron-withdrawing (Table 2, entries 3–4) and electron-donating (Table 2, entry 7)
F3C
Ar
CF3
NC
H2O
N
O
ArCHO + CH2(CN)2 +
N
N
90 oC
N
H2N
O
2
3
4
1
Scheme 1 Catalyst-free aqueous synthesis trifluoromethylated pyrano[2,3-c]pyrazoles
123

Aqueous, catalyst-free and three-component synthesis
groups, since they were subjected to the reaction and in nearly all cases furnished the
expected trifluoromethylated heterocycles in excellent yields under the same conditions
despite the elimination of a molecule of water from the reaction system. To our delight,
terephthalaldehyde could also be involved in the three-component condensation and
provided the product 4h successfully incorporating two trifluoromethylated fused
pyranopyrazole frameworks. Thus, we concluded safely that the electronic nature of the
substituents on the aryl ring had no significant influence on the reaction yields and this
highlighted the wide scope of the catalyst-free aqueous strategy.
1
All of the products were characterized by elemental analysis, FTIR, and H
NMR.
According to our results, the possible mechanism to account for the reaction was
proposed (Scheme 2). One molecule of aryl aldehyde 1 was first condensed with
malononitrile 2 to afford a-cyanocinnamonitrile derivative 5, which can be regarded
as a fast knoevenagel condensation. The active methylene of 3 reacted with the
electrophilic C–C double bond to give the intermediate 6, which underwent
cyclization by the intramolecular nucleophilic attack of OH group on the cyano
(CN) moiety to afford the intermediate 7. Finally, the expected products 4 were
obtained through isomerization (7 ? 8 ? 4). To confirm this reaction mechanism,
2-(4-bromobenzylidene)malononitrile was synthesized and reacted with 3 in 2 h,
which gave the corresponding product in similar yield as entry 2.
Table 1 Optimization of
solvent and temperature for the
three-component synthesis
Entry
Temp. (°C)
Time (h)
Yield (%)
1
2
3
4
5
6
70
80
6
56
62
68
79
78
79
5
85
5
90
4
95
3.5
3.5
100
Table 2 Aqueous three-
component synthesis of
trifluoromethylated
Entry Compound Ar
Time (h) Yield (%)
1
4a
4b
4c
4d
4e
4f
C6H5
4.0
3.5
3.0
3.5
5.0
4.0
79
86
82
90
87
81
84
84
78
87
81
pyrano[2,3-c]pyrazoles
2
4-BrC6H4
3-NO2C6H4
4-NO2C6H4
4-ClC6H4
4-FC6H4
3
4
5
6
7
4g
4h
4i
3,4,5-(MeO)3C6H2 3.5
8
4-HCOC6H4
3-FC6H4
4.0
5.0
3.0
4.5
9
10
11
4j
2-FC6H4
4k
4-HO-3-NO2C6H3
123

C. Yu et al.
CN
CN
Ar
ArCHO +
CH2(CN)2
H
1
2
5
Ar
F3C
CN
CN
CN
F3C
F3C
Ar
CN
N
O
N
H2O
N
N
N
H
OH
O
N
3'
6
Ar
O
3
Ar
CN
Ar
O
CF3
F3C
CF3
N
NC
H2N
NC
HN
N
N
N
OH
N
N
N
8
7
4
Scheme 2 A proposed plausible reaction pathway for the aqueous synthesis
Conclusion
In summary, we have developed an efficient, economical, safe, and eco-friendly
method to synthesize 6-amino-4-aryl-3-(trifluoromethyl)-1,4-dihydro-1-phenylpyr-
ano[2,3-c]pyrazole-5-carbonitrile by the catalyst-free reaction of aryl aldehydes,
malononitrile, and 1-phenyl-3-(trifluoromethyl)-1H-pyrazol-5(4H)-one in aqueous
media. The result showed that our strategy for the syntheses of these trifluorom-
ethylated heterocyclic molecules is of great importance in both the organofluorine
and organic synthetic fields.
Experimental
Common reagents and materials were purchased from commercial sources and
purified by recrystallization or distillation. The starting material, 1-phenyl-3-
trifluoromethylpyrazol-5-one 3, was prepared according to Ref. [26]. Infrared (IR)
spectra were recorded on a TENSOR 27 spectrophotometer in KBr pellet and are
reported in terms of frequency of absorption (cm-1). NMR spectra were recorded on
123

Aqueous, catalyst-free and three-component synthesis
a Bruker AV-400; data for 1H are reported as follows: chemical shift (ppm),
multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; and br, broad),
coupling constant (Hz) and integration. Elemental analyses were performed on
Yanaco-CHN CORDER elementary analyzer.
General procedure for the synthesis of 6-amino-3-(trifluoromethyl)-1,4-dihydro-
1-phenyl-4-arylpyrano[2,3-c]pyrazole-5-carbonitrile (4a–4k)
Aryl aldehydes (1.0 mmol), malononitrile (1.0 mmol; for compound 4h, 2.0 mmol
was used.), 1-phenyl-3-(trifluoromethyl)-1H-pyrazol-5(4H)-one (1.0 mmol; for
compound 4h, 2.0 mmol was used.), and 3 mL water were added to a 25-mL
round-bottom flask and were stirred at 90 °C for several hours (monitored by TLC).
Then the mixture was cooled and filtered. The solid was further recrystallized from
ethanol to give pure products.
6-Amino-3-(trifluoromethyl)-1,4-dihydro-1,4-diphenylpyrano[2,3-c]pyrazole-5-
carbonitrile (4a)
M.p.: 190–191 °C (Lit. [27].: 185–187 °C); IR (potassium bromide) (v, cm-1):
3,325, 2,200, 1,663, 1,595, 1,518, 1,481, 1,456, 1,389, 1,231, 1,184, 1,145, 1,036,
1
759, 697; H NMR (400 MHz, DMSO-d6): 4.79 (s, 1H, C4-H), 7.25–7.40 (m, 7H,
ArH ? NH2), 7.48 (t, J = 7.6 Hz, 1H, ArH), 7.58 (t, J = 7.6 Hz, 2H, ArH), 7.85
(d, J = 8.0 Hz, 2H, ArH); Anal. calcd. For C20H13F3N4O: C, 62.83; H, 3.43; N,
14.65. Found: C, 62.75; H, 3.54; N, 14.74.
6-Amino-4-(4-bromophenyl)-3-(trifluoromethyl)-1,4-dihydro-1-phenylpyrano[2,3-
c]pyrazole-5-carbonitrile (4b)
M.p.: 188–189 °C; IR (potassium bromide) (v, cm-1): 3,441, 3,324, 3,207, 2,198,
1,658, 1,595, 1,578, 1,520, 1,483, 1,458, 1,396, 1,310, 1,277, 1,233, 1,200, 1,144,
1,074, 1,038, 1,024, 1,011, 912, 893, 827, 807, 759, 734, 696; 1H NMR (400 MHz,
DMSO-d6): 4.81 (s, 1H, C4-H), 7.22 (d, J = 8.4 Hz, 2H, ArH), 7.38 (s, 2H, NH2),
7.45–7.59 (m, 5H, ArH), 7.82 (d, J = 8.4 Hz, 2H, ArH); Anal. calcd. For
C20H12BrF3N4O: C, 52.08; H, 2.62; N, 12.15. Found: C, 52.15; H, 2.48; N, 12.24.
6-Amino-3-(trifluoromethyl)-1,4-dihydro-4-(3-nitrophenyl)-1-phenylpyrano[2,3-
c]pyrazole-5-carbonitrile (4c)
M.p.: 192–193 °C; IR (potassium bromide) (v, cm-1): 3,355, 2,194, 1,655, 1,595,
1,530, 1,482, 1,458, 1,394, 1,353, 1,312, 1,233, 1,188, 1,146, 1,038, 1,023, 763,
1
687; H NMR (400 MHz, DMSO-d6): 5.12 (s, 1H, C4-H), 7.50 (s, 2H, NH2), 7.52
(s, 1H, ArH), 7.58 (t, J = 8.0 Hz, 2H, ArH), 7.65 (t, J = 7.8 Hz, 1H, ArH), 7.82
(d, J = 8.4 Hz, 1H, ArH), 7.86 (d, J = 7.6 Hz, 2H, ArH), 8.16 (d, J = 8.4 Hz, 1H,
123

C. Yu et al.
ArH), 8.21 (s, 1H, ArH); Anal. calcd. For C20H12F3N5O3: C, 56.21; H, 2.83; N,
16.39. Found: C, 56.15; H, 2.68; N, 16.34.
6-Amino-3-(trifluoromethyl)-1,4-dihydro-4-(4-nitrophenyl)-1-phenylpyrano[2,3-
c]pyrazole-5-carbonitrile (4d)
M.p.: 195–196 °C; IR (potassium bromide) (v, cm-1): 3,425, 3,328, 3,213, 2,202,
1,666, 1,596, 1,582, 1,520, 1,483, 1,458, 1,397, 1,350, 1,311, 1,233, 1,196, 1,145,
1
1,039, 1,025, 817, 760; H NMR (400 MHz, DMSO-d6): 5.06 (s, 1H, C4-H), 7.51
(s, 3H, NH2 ? ArH), 7.59 (t, J = 8.4 Hz, 4H, ArH), 7.85 (d, J = 7.6 Hz, 2H, ArH),
8.21 (d, J = 8.4 Hz, 2H, ArH); Anal. calcd. For C20H12F3N5O3: C, 56.21; H, 2.83;
N, 16.39. Found: C, 56.18; H, 2.78; N, 16.46.
6-Amino-4-(4-chlorophenyl)-3-(trifluoromethyl)-1,4-dihydro-1-phenylpyrano[2,3-
c]pyrazole-5-carbonitrile (4e)
M.p.: 215–217 °C (Lit. [27].: 210–211 °C); IR (potassium bromide) (v, cm-1):
3,448, 3,324, 3,208, 2,202, 1,661, 1,595, 1,580, 1,521, 1,482, 1,458, 1,396, 1,311,
1
1,233, 1,200, 1,145, 1,090, 1,038, 1,024, 830, 810, 760, 735; H NMR (400 MHz,
DMSO-d6): 4.85 (s, 1H, C4-H), 7.31 (d, J = 8.4 Hz, 2H, ArH), 7.39–7.41 (m, 4H,
NH2 ? ArH), 7.50 (t, J = 7.2 Hz, 1H, ArH), 7.58 (t, J = 8.0 Hz, 2H, ArH), 7.84
(d, J = 7.6 Hz, 2H, ArH); Anal. calcd. For C20H12ClF3N4O: C, 57.64; H, 2.90; N,
13.44. Found: C, 57.58; H, 2.88; N, 13.36.
6-Amino-3-(trifluoromethyl)-4-(4-fluorophenyl)-1,4-dihydro-1-phenylpyrano[2,3-
c]pyrazole-5-carbonitrile (4f)
M.p.: 218–219 °C; IR (potassium bromide) (v, cm-1): 3,459, 3,327, 3,213, 2,205,
1,666, 1,597, 1,583, 1,523, 1,510, 1,483, 1,459, 1,393, 1,311, 1,278, 1,228, 1,200,
1
1,145, 1,038, 1,026, 816, 761, 735, 687; H NMR (400 MHz, DMSO-d6): 4.84
(s, 1H, C4-H), 7.14 (t, J = 8.4 Hz, 2H, ArH), 7.31–7.35 (m, 2H, ArH), 7.36 (s, 2H,
NH2), 7.48 (t, J = 7.4 Hz, 1H, ArH), 7.58 (t, J = 7.8 Hz, 2H, ArH), 7.84
(d, J = 8.4 Hz, 2H, ArH); Anal. calcd. For C20H12F4N4O: C, 60.00; H, 3.02; N,
14.00. Found: C, 60.08; H, 2.88; N, 13.86.
6-Amino-3-(trifluoromethyl)-1,4-dihydro-4-(3,4,5-trimethoxyphenyl)-1-
phenylpyrano[2,3-c]pyrazole-5-carbonitrile (4g)
M.p.: 149–150 °C; IR (potassium bromide) (v, cm-1): 3,394, 3,325, 3,215, 2,193,
1,654, 1,593, 1,518, 1,482, 1,469, 1,458, 1,427, 1,394, 1,324, 1,311, 1,231, 1,149,
1
1,130, 1,038, 1,023, 813, 761, 736, 686; H NMR (400 MHz, DMSO-d6): 3.66
(s, 3H, CH3O), 3.73 (s, 6H, 2 9 CH3O), 4.78 (s, 1H, C4-H), 6.54 (s, 2H, ArH), 7.32
(s, 2H, NH2), 7.47 (t, J = 7.4 Hz, 1H, ArH), 7.57 (t, J = 7.8 Hz, 2H, ArH), 7.86
(d, J = 8.0 Hz, 2H); Anal. calcd. For C23H19F3N4O4: C, 58.48; H, 4.05; N, 11.86.
Found: C, 58.38; H, 3.98; N, 11.76.
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Aqueous, catalyst-free and three-component synthesis
6-Amino-3-(trifluoromethyl)-1,4-dihydro-4-(4-(6-amino-5-cyano-3-
(trifluoromethyl)-1,4-dihydro-1-phenylpyrano[2,3-c]pyrazol-4-yl)phenyl)-1-
phenylpyrano[2,3-c]pyrazole-5-carbonitrile (4h)
M.p.: 132–133 °C; IR (potassium bromide) (v, cm-1): 3,475, 3,326, 2,200, 1,655,
1,596, 1,522, 1,482, 1,458, 1,389, 1,310, 1,233, 1,185, 1,143, 1,038, 1,022, 801,
759, 685; 1H NMR (400 MHz, DMSO-d6): 4.76 (s, 2H, 2 9 C4-H), 7.20
(d, J = 5.2 Hz, 4H, ArH), 7.33 (s, 4H, 2 9 NH2), 7.49–7.51 (m, 3H, ArH),
7.58–7.62 (m, 4H, ArH), 7.82 (t, J = 8.4 Hz, 4H); Anal. calcd. For C34H20F6N8O2:
C, 59.48; H, 2.94; N, 16.32. Found: C, 59.38; H, 2.98; N, 16.47.
6-Amino-3-(trifluoromethyl)-4-(3-fluorophenyl)-1,4-dihydro-1-phenylpyrano[2,3-
c]pyrazole-5-carbonitrile (4i)
M.p.: 211–212 °C; IR (potassium bromide) (v, cm-1): 3,476, 3,335, 3,216, 2,198,
1,661, 1,613, 1,594, 1,581, 1,522, 1,483, 1,458, 1,393, 1,329, 1,309, 1,277, 1,233,
1,198, 1,146, 1,132, 1,119, 1,077, 1,039, 1,024, 943, 890, 835, 795, 783, 763, 733,
1
695, 679; H NMR (400 MHz, DMSO-d6): 4.86 (s, 1H, C4-H), 7.08–7.19 (m, 3H,
ArH), 7.36–7.40 (m, 3H, NH2 ? ArH), 7.48 (t, J = 7.4 Hz, 1H, ArH), 7.58
(t, J = 7.6 Hz, 2H, ArH), 7.85 (d, J = 8.0 Hz, 2H); Anal. calcd. For C20H12F4N4O:
C, 60.00; H, 3.02; N, 14.00. Found: C, 60.15; H, 3.17; N, 14.06.
6-Amino-3-(trifluoromethyl)-1,4-dihydro-4-(4-hydroxy-3-nitrophenyl)-1-
phenylpyrano[2,3-c]pyrazole-5-carbonitrile (4j)
M.p.: 196–198 °C; IR (potassium bromide) (v, cm-1): 3,416, 3,329, 2,196, 1,662,
1,630, 1,583, 1,483, 1,458, 1,432, 1,396, 1,313, 1,278, 1,233, 1,177, 1,145, 1,076,
1
1,039, 912, 886, 825, 763, 735, 698, 682, 601; H NMR (400 MHz, DMSO-d6):
5.10 (s, 1H, C4-H), 7.16–7.21 (m, 2H, ArH), 7.31 (q, J = 7.6 Hz, 2H, ArH), 7.44
(s, 2H, NH2), 7.48 (t, J = 7.4 Hz, 1H, ArH), 7.58 (t, J = 7.8 Hz, 2H, ArH), 7.83
(d, J = 8.0 Hz, 2H, ArH); Anal. calcd. For C20H12F4N4O: C, 60.00; H, 3.02; N,
14.00. Found: C, 59.88; H, 3.08; N, 14.17.
6-Amino-3-(trifluoromethyl)-4-(2-fluorophenyl)-1,4-dihydro-1-phenylpyrano[2,3-
c]pyrazole-5-carbonitrile (4k)
M.p.: 212–214 °C; IR (potassium bromide) (v, cm-1): 3,484, 3,335, 2,201, 1,657,
1,615, 1,596, 1,583, 1,522, 1,490, 1,458, 1,393, 1,311, 1,279, 1,232, 1,190, 1,144,
1
1,102, 1,076, 1,038, 1,021, 912, 896, 863, 757, 741, 697; H NMR (400 MHz,
DMSO-d6): 4.90 (s, 1H, C4-H), 7.09 (d, J = 8.8 Hz, 1H, ArH), 7.41 (s, 2H, NH2),
7.47–7.51 (m, 2H, ArH), 7.57 (t, J = 8.0 Hz, 2H, ArH), 7.84 (d, J = 8.0 Hz, 3H,
ArH), 11.01 (s, 1H, OH); Anal. calcd. For C20H12F3N5O4: C, 54.18; H, 2.73; N,
15.80. Found: C, 54.05; H, 2.87; N, 15.68.
Acknowledgments We are grateful for financial support by A Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions, and the Major Basic Research Project of
123

C. Yu et al.
the Natural Science Foundation of the Jiangsu Higher Education Institutions (09KJA430003). This
project was also sponsored by Qing Lan Project (10qlg005).
References
´
1. J.P. Begue, D. Bonnet-Delpon, J. Fluor. Chem. 127, 992 (2006)
2. K.L. KirK, J. Fluor. Chem. 127, 1013 (2006)
´
3. R.D. Chambers, D. Holling, G. Sandforda, A.S. Batsanov, J.A.K. Howard, J. Fluor. Chem. 125, 661
(2004)
4. K. Uneyama, Organofluorine Chemistry (Blackwell, Oxford, 2006)
5. P. Kirsch, Modern Fluoroorganic Chemistry: Synthesis, Reactivity, Applications (Wiley, Weinheim,
2004)
6. M.A. McClinton, D.A. McClinton, Tetrahedron 48, 6555 (1992)
7. S.E. Lopez, J. Restrepo, J. Salazar, Curr. Org. Chem. 7, 414 (2010)
8. S. Rozen, Adv. Synth. Catal. 352, 2691 (2010)
9. J. Nie, H. Guo, D. Cahard, J. Ma, Chem. Rev. 111, 455 (2011)
10. A.T. Parsons, S. Buchwald, Nature 480, 184 (2011)
11. A. Studer, Angew. Chem. Int. Ed. 51, 8950 (2012)
12. N.J. Thumar, M.P. Patel, ARKIVOC 13, 363 (2009)
13. R. Sakhuja, S.S. Panda, L. Khanna, S. Khurana, S.C. Jain, Bioorg. Med. Chem. Lett. 21, 5465 (2011)
14. J.A. Makawana, D.C. Mungra, M.P. Patel, R.G. Patel, Bioorg. Med. Chem. Lett. 21, 6166 (2011)
15. P.T. Mistry, N.R. Kamdar, D.D. Haveliwala, S.K. Patel, J. Heterocycl. Chem. 49, 349 (2012)
16. S.R. Mandha, S. Siliveri, M. Alla, V.R. Bommena, M.R. Bommineni, S. Balasubramanian, Bioorg.
Med. Chem. Lett. 22, 5272 (2012)
17. S. Bhavanarushi, V. Kanakaiah, E. Yakaiah, V. Saddanapu, A. Addlagatta, J. V. Rani, Med. Chem.
Res. ASAP. (2012). doi:10.1007/s00044-012-0239-z
18. K. Kumaravel, G. Vasuki, Curr. Org. Chem. 13, 1820 (2009)
19. C. Nitsche, C.D. Klein, Tetrahedron Lett. 53, 5197 (2012)
20. M. Rumum Rohman, H. Mecadon, A.T. Khan, B. Myrboh, Tetrahedron Lett. 53, 5261 (2012)
21. J. Liu, M. Lei, L. Hu, Green Chem. 14, 2534 (2012)
22. A. Khalafi-Nezhad, S. Sarikhani, E.S. Shahidzadeh, F. Panahi, Green Chem. 14, 2876 (2012)
23. X. Wang, M. Zhang, Q. Li, C. Yao, S. Tu, Tetrahedron 63, 5265 (2007)
24. X. Wang, Q. Li, J. Wu, M. Zhang, Synth. Commun. 39, 3069 (2009)
25. X. Wang, J. Zhou, K. Yang, M. Zhang, Synth. Commun. 40, 1065 (2010)
26. C.H. Mao, Y.F. Lu, M.Z. Huang, L. Huang, C. Chen, X.M. Ou, Chin. J. Syn. Chem. 10, 167 (2002)
27. H.F. Zohdi, A.H.H. Elghandour, N.M. Rateb, M.M.M. Sallam, J. Chem. Res. 396, 3015 (1992)
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