Nafion-H-catalyzed synthesis of polyhydroquinolines via the Hantzsch multicomponent reaction

Article abstract of DOI:10.1007/s00706-012-0742-4

A facile and efficient one-pot, four-component synthesis of polyhydroquinoline derivatives via the Hantzsch reaction using Nafion-H as heterogeneous catalyst in PEG 400-water solvent system is described herein. The present methodology offers several advan

Full text of DOI:10.1007/s00706-012-0742-4

Monatsh Chem (2012) 143:1675–1680
DOI 10.1007/s00706-012-0742-4
ORIGINAL PAPER
Nafion-HÒ-catalyzed synthesis of polyhydroquinolines
via the Hantzsch multicomponent reaction
•
Mazaahir Kidwai Ritika Chauhan Divya Bhatnagar
•
•
•
Abhay K. Singh Biswajit Mishra Sharmistha Dey
•
Received: 28 February 2011 / Accepted: 26 February 2012 / Published online: 12 April 2012
Ó Springer-Verlag 2012
Abstract A facile and efficient one-pot, four-component
synthesis of polyhydroquinoline derivatives via the Han-
tzsch reaction using Nafion-HÒ as heterogeneous catalyst
in PEG 400–water solvent system is described herein. The
present methodology offers several advantages such as
excellent yields, simple procedure, shorter reaction times,
and milder conditions with remarkable recyclability.
especially to generate compound libraries for screening
purposes.
In recent years, much attention has been directed
towards synthesis of 1,4-dihydropyridyl (1,4-DHP) com-
pounds (Fig. 1) due to a wide range of biological activities
associated with this heterocyclic nucleus [6–11]. 1,4-Di-
hydropyridyl compounds are well known as calcium
channel blockers. Cardiovascular agents such as nifedipine
[12], nicardipine [13], amlodipine [14], and other related
derivatives are 1,4-dihydropyridyl compounds, which are
effective for treatment of hypertension [15]. They have
been reported to act as vasodilators [16] and bronchodila-
tors [17], and to possess antiatherosclerotic [18], antitumor
[19], geroprotective [20], hepatoprotective [21], and anti-
diabetic [22] activities. Furthermore, current literature has
revealed that 1,4-DHPs exhibit several medicinal applica-
tions [23–26]. For these reasons, polyhydroquinolines
(PHQs) not only have attracted the attention of chemists to
synthesize these compounds but also represent an inter-
esting research challenge.
Keywords Nafion-HÒ Á Polyethylene glycol Á
Polyhydroquinolines Á Recyclability Á
Hantzsch reaction
Introduction
In recent decades, multicomponent reactions (MCRs) have
gained wide applicability in the field of synthetic organic
chemistry. MCRs are one-pot processes that combine
three or more substrates simultaneously [1–5]. Such pro-
cesses are of great interest in diversity-oriented synthesis,
In view of the importance of polyhydroquinoline
derivatives, numerous methods have been developed for
their synthesis [27–30]. Experimentally, preparation of 1,4-
DHPs was first reported in 1882 by Hantzsch, involving
one-pot, three-component coupling of an aldehyde with
ethyl acetoacetate and ammonia in acetic acid or in re-
fluxing alcohol [31, 32]. However, these methods suffered
from drawbacks such as long reaction time, excess of
organic solvent, low yields, and harsh refluxing conditions.
Therefore, there is scope for further improvement towards
milder reaction conditions and higher yields, and progress
in this area is remarkable, including the recent use of
microwaves [33], TMSCl [34], ionic liquid [35], polymer
[36, 37], molecular I2 [38], CAN [39], Yb(OTf)3 [40],
HClO4-SiO2 [41], heteropolyacid [42], and MCM-41 [43].
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-012-0742-4) contains supplementary
material, which is available to authorized users.
M. Kidwai (&) Á R. Chauhan Á D. Bhatnagar
Green Chemistry Research Laboratory,
Department of Chemistry, University of Delhi,
Delhi 110007, India
e-mail: kidwai.chemistry@gmail.com
Present Address:
M. Kidwai
Jiwaji University, Gwalior, Madhya Pradesh, India
A. K. Singh Á B. Mishra Á S. Dey
Department of Biophysics, All India Institute of Medical
Sciences, New Delhi, India
123

1676
M. Kidwai et al.
Fig. 1 General structure of 1,4-
dihydropyridyl (1,4-DHP)
compounds
benign alternative solvent in organic synthesis. Its use as a
reaction medium in organic reactions is relatively recent
[55–57]. As a part of our ongoing research program to
devise cheap synthetic methodologies using PEG 400 and
exploration of catalytic potential of Nafion-HÒ [58, 59],
herein we report a simple and efficient protocol for one-pot
synthesis of polyhydroquinolines using Nafion-HÒ as cat-
alyst coupled with aqueous PEG 400 medium (Scheme 2).
R"
R'
R
R '
N
H
R
Results and discussion
Each of the above methods for the Hantzsch reaction has its
own merits, while some of the methods are plagued by
limitations of poor yield, longer reaction time, tedious
workup, and effluent pollution. Moreover, the main dis-
advantage of almost all existing methods is that the
catalysts are destroyed in the workup procedure and cannot
be recovered or reused. Moreover, there are a relatively
limited number of reports on synthesis of polyhydroquin-
olines compared with synthesis of four-substituted 1,4-
DHPs. Therefore, the search continues for a better catalyst
and an ecofriendly approach for generating polyhydro-
quinolines in terms of operational simplicity, reusability,
economic viability, and greater selectivity.
Over the years, Nafion-HÒ, a superacidic perfluorinated
resin sulfonic acid, has enjoyed immense popularity as a
solid acid catalyst for a wide variety of synthetic trans-
formations [44]. The estimated Hammett (H0) value for
Nafion-HÒ is comparable to that of 96–100% H2SO4
(H0 = -12.0) [45]. The high catalytic activity, its selec-
tivity, recyclability, its superior chemical and thermal
stability, ease of separation from the reaction mixture, and
ecofriendly nature as compared with other available het-
erogeneous acid catalysts [46–48] place it in a unique
position as an attractive and promising heterogeneous acid
catalyst for organic synthesis. In recent years, replacement
of hazardous substances with environmentally benign sol-
vents [49, 50] has been one of the key areas of green
chemistry [51]. While use of water as a solvent is probably
the most desirable approach, this is often not possible due
to the hydrophobic nature of the reactants and sensitivity of
many catalysts to aqueous conditions. Recently, PEG and
its solutions have been introduced as interesting green
solvent systems [52–54]. These have replaced many other
‘‘neoteric solvents’’ such as ionic liquids, supercritical
carbon dioxide, and micellar systems, whose toxicological
properties, short- and long-term hazardous nature, and
biodegradability have not been established completely.
Low cost, reduced flammability and toxicity, recyclability,
completely nonhalogenated composition, easy degradabil-
ity, and miscibility with a wide variety of organic solvents
are some of the prominent features that render PEG a
First, a model reaction was carried out using 1 equivalent
each of benzaldehyde (1a), ethyl acetoacetate (2), dime-
done (3a), and ammonium acetate (4). These were stirred at
ambient temperature in ethanol. After 5 h, only 54% of the
expected product 5a was obtained. To improve the yield
and optimize the reaction conditions, the same reaction was
carried out in presence of Nafion-HÒ as catalyst. Surpris-
ingly, a significant improvement was observed and the
yield dramatically increased to 84% after stirring the
mixture for only 2 h. Thus, the catalytic efficiency of
Nafion-HÒ was definitely identified.
To increase the product yield further, we used polyeth-
ylene glycol as solvent system for the same representative
reaction scheme. The yield was better and increased to
92% after stirring the mixture for only 1.5 h. Prompted by
these promising outcomes, we further investigated the best
reaction conditions by using different ratios of the
PEG 400–water solvent system. A decrease in the amount
of PEG 400 from 100% to 60% increased the product yield
slightly from 92% to 96%. However, a further decrease in
the ratio of PEG 400 reduced the product yield, which is
attributed to loss of solubility of the reactants. Also, the
model reaction was examined in various solvents com-
monly used in organic synthetic procedures (Table 1).
Polar solvents such as ethanol and acetonitrile were much
better than nonpolar solvents such as toluene in terms of
better yields and shorter reaction times, due to the
enhanced solubility of reactants and the ability of the sol-
vent to swell Nafion-HÒ, which resulted in its high
catalytic activity. Nafion-HÒ, being nonporous, relies on
solvation of the ionic groups by an appropriate polar sol-
vent to form solvent channels and clusters. Low yields are
observed in case of nonpolar solvent due to failure of the
substrate to be able to access the catalyst [60].
The temperature of the reaction was kept at 50 °C unlike
in the reactions which used HClO4–SiO2 [41], hetero-
polyacid [42], and MCM-41 [43] as catalyst. These
reactions occurred at refluxing temperatures. It was also
observed that, on increasing the temperature, the reaction
time decreased, but we continued to use this time to
maintain the ambient conditions of the reaction.
123

Nafion-HÒ-catalyzed synthesis of polyhydroquinolines
1677
Table 1 Effect of different solvents on the synthesis of polyhydro-
quinolines
Table 2 Nafion-HÒ-catalyzed synthesis of polyhydroquinoline
derivatives
Entry
Solvent
Time/h
Yield/%a
Entry Ar
R
Product Time/ Yield/ M.p./°C
a
h
%
1
2
3
4
5
6
7
EtOH
2.0
2.0
2.5
2.5
3.5
3.0
1.5
84
90
78
60
63
68
92
1
2
3
4
5
C6H5
Me 5a
Me 5b
Me 5c
Me 5d
Me 5e
Me 5f
1.5
96
220–224
[42]
MeCN
THF
4-MeO-C6H4
4-Cl-C6H4
3-NO2-C6H4
Piperonyl
1.5
1.6
1.6
1.5
96
90
88
94
260–262
[63]
Toluene
DMF
246–250
[43]
DMSO
PEG 400
PEG 400:H2O
90:10
178–180
[62]
210–212
[61]
8
1.8
1.8
1.6
1.5
2.0
2.0
94
94
94
96
84
72
9
80:20
6
7
3-HO-C6H4
C6H5
1.5
1.5
92
94
130–134
10
11
12
13
70:30
H
H
H
H
H
H
5g
5h
5i
242–244
[62]
60:40
50:50
8
2-Thienyl
1.6
1.7
1.5
1.5
1.5
92
90
92
94
94
230–234
[39]
40:60
9
4-NO2-C6H4
204–206
[39]
Reaction conditions: benzaldehyde (1 mmol), ethyl acetoacetate
(1 mmol), dimedone (1 mmol), ammonium acetate (1 mmol); catalyst
Nafion-HÒ (30 mg); 5 cm3 solvent; 50 °C
10
11
12
2-HO-3-MeO-
C6H3
5j
218–222
a
Isolated yields
2-Furanyl
5k
5l
208–210
[39]
1-Naphthyl
236–240
The scope and generality of this four-component, one-
pot synthesis of polyhydroquinoline derivatives through the
Hantzsch reaction was illustrated with different aldehydes,
and the results are summarized in Table 2. It is worthwhile
to mention that the electronically and structurally diverse
aldehydes do not show any obvious effect on this conver-
sion, because the desired products were obtained in good
to excellent yields in relatively short reaction times
(Scheme 2). However, aliphatic substrates did not undergo
any reaction under similar conditions.
Reaction conditions: aromatic aldehyde (1 mmol), 1,3-cyclohexane-
dione or dimedone (1 mmol), ethyl acetoacetate (1 mmol),
ammonium acetate (1 mmol); catalyst Nafion-HÒ (30 mg); 5 cm3
PEG 400-water (60:40); 50 °C
a
Isolated yields
Next, we investigated the reusability and recyclability of
Nafion-HÒ (Fig. 2) for the model reaction including
1 equiv. each of benzaldehyde (1a), ethyl acetoacetate (2),
dimedone (3a), and ammonium acetate (4) giving product
5a under the optimized conditions (Scheme 1). The cata-
lyst was removed by simple filtration after completion of
the reaction [64] and was washed with acetone and dried
overnight at 105 °C. The catalyst was reused as such for
subsequent reactions (four runs) with fresh substrates under
the same conditions. The catalyst showed excellent recy-
clability in this reaction, as the reaction times and yield
remained almost the same without loss of catalytic activity.
Fig. 2 Reusability of the catalyst for synthesis of polyhydroquinolines
The DIFABS absorption correction was not applied due to
the small size of the crystals (0.2 9 0.4 9 0.2 mm3).
In conclusion, this paper describes a simple and efficient
method for synthesis of polyhydroquinoline derivatives via
improved Hantzsch condensation using Nafion-HÒ as
reusable catalyst in PEG 400–water solvent system. The
mildness of the conversion, experimental simplicity, com-
patibility with various functional groups, inexpensive
X-ray crystallography
Compound 5l was recrystallized from its solution in etha-
nol. The packing pattern of the compound is shown in
Fig. 3. The details of crystal data, intensity data collection,
and refinement are given in the Supplementary Material.
123

1678
M. Kidwai et al.
Scheme 1
O
Ph
O
COOEt
O
O
®
Nafion-H , 50 °C
+
+
+
NH4OAc
PhCHO
Me
Me
Me
Me
PEG 400-water
(60:40)
OEt
Me
N
H
O
1a
2
3a
4
5a
Scheme 2
O
Ar
O
COOEt
Me
O
O
®
Nafion-H , 50 °C
+
+
+
NH4OAc
ArCHO
R
R
PEG 400-water
(60:40)
OEt
N
H
O
R
R
1a-1k
2
3a, 3b
4
5a-5l
standard. Chemical shift values are recorded on the d scale,
and coupling constant values are in Hertz (Hz). Elemental
analysis was performed on a Heraeus CHN rapid analyzer.
The temperature of the reaction mixture was measured
through a noncontact infrared mini gun thermometer
(AZ minigun type, model 8868).
General procedure for synthesis of polyhydroquinolines
A 50-cm3 round-bottomed flask was charged with 1 equiv.
each of aldehydes 1a–1k (1 mmol), ethyl acetoacetate (2),
dimedone (3a) or 1,3-cyclohexanedione (3b), ammonium
acetate (4), and Nafion-HÒ (one bead, i.e., 30 mg) along
with 5 cm3 PEG 400–water (60:40) solvent system. The
mixture was then stirred at 50 °C until the reaction was
complete. The completion of the reaction was monitored
through TLC. After the reaction was complete, the reaction
mixture was allowed to cool to room temperature, the
catalyst Nafion-HÒ was removed by simple filtration, and
the product was extracted with ethyl acetate (3 9 5 cm3).
The combined organic layer was dried over anhydrous
sodium sulfate and filtered. The solvent was evaporated
under vacuo to give the crude product. After extraction
with ethyl acetate, the remaining solution of PEG 400 and
water was concentrated to recover pure PEG 400, which
was then reused for the subsequent reactions [65]. The
crude products obtained were subjected to purification via
recrystallization or through column chromatography on
silica gel (100–200 mesh size) using 25% ethyl acetate in
petroleum ether as eluent to yield polyhydroquinolines
5a–5l. The structures of all products were established on
the basis of spectral analysis (IR, 1H NMR, and 13C
NMR), elemental analysis, and melting point determina-
tion. X-ray analysis was carried out for compound 5l
(Figs. 3, 4).
Fig. 3 Packing pattern of compound 5l in the triclinic crystal lattice
reagents, high yields, short reaction times, and easy workup
procedure make this protocol very attractive to synthesize a
variety of these derivatives.
Experimental
All chemicals were purchased from Sigma-Aldrich and
were used without further purification. All reactions and
purity of polyhydroquinolines were monitored by thin-layer
chromatography (TLC) using aluminum plates coated with
silica gel F254 (Merck) using 30% ethyl acetate and 70%
petroleum ether as eluent. The spots were detected either
under ultraviolet (UV) light or by placing in an iodine
chamber. Melting points were determined using a Thomas
Hoover melting point apparatus. Infrared (IR) spectra were
recorded on a PerkinElmer FTIR-1710 spectrophotometer
Regeneration of catalyst
1
using Nujol film. H and 13C nuclear magnetic resonance
After filtration, the catalyst was washed successively with
(NMR) spectra were recorded on a JEOL JNM-ECX 400P
FT NMR system using tetramethylsilane (TMS) as internal
acetone and deionized water and dried overnight at 105 °C.
123

Nafion-HÒ-catalyzed synthesis of polyhydroquinolines
1679
125.4, 125.6, 126.9, 127.9, 143.2, 146.5, 150.5, 167.8,
196.4 ppm.
X-ray crystallography
X-ray diffraction data were collected using an Enraf–No-
nius CAD4 diffractometer. The structure of compound 5l
was determined by a direct method using the program
SHELXS 97 and difference Fourier calculation. The
coordinates of nonhydrogen atoms were refined aniso-
tropically using the program SHELXL 97 [66]. The
positions of hydrogen atoms were determined from dif-
ference Fourier maps and were included in the final cycles
of refinement using isotropic temperature factors of the
nonhydrogen atoms to which they were attached. The final
R-factor for observed 7,518 reflections [I C 2r(I)] was
0.1824. The atomic scattering factors used in these calcu-
lations were those of Cromer and Mann [67] for
nonhydrogen atoms and of Stewart et al. [68] for hydrogen
atoms.
Fig. 4 ORTEP plot of compound 5l at the 50% ellipsoidal
probability
The obtained catalyst had the same catalytic activity as the
fresh catalyst.
Ethyl 1,4,5,6,7,8-hexahydro-4-(3-hydroxyphenyl)-2,7,7-tri-
methyl-5-oxoquinoline-3-carboxylate (5f, C21H25NO4)
IR (Nujol): m = 3,245, 2,959, 1,600, 1,453, 1,379, 1,267,
Crystallographic data for the structure 5l have been
deposited with the Cambridge Crystallographic Data Cen-
tre as supplementary publication number CCDC 796332.
1,149, 707 cm-1
;
1H NMR (DMSO-d6, 400 MHz):
d = 0.90 (s, 3H, CH3), 1.02 (s, 3H, CH3), 1.16 (t, 3H,
J = 7.04 Hz, CH3), 2.13–2.29 (m, 4H, 2 9 CH2), 2.32 (s,
3H, CH3), 4.02 (q, 2H, J = 7.4 Hz, CH2), 4.96 (s, 1H, CH–
Ar), 5.85 (br s, 1H, NH), 6.75–6.99 (m, 4H, Ar–H), 7.81 (s,
1H, OH) ppm; 13C NMR (DMSO-d6, 100 MHz): d = 13.4,
18.4, 19.8, 27.3, 28.7, 32.3, 35.6, 50.9, 58.8, 100.4, 105.1,
112.0, 112.9, 114.9, 119.3, 128.2, 144.3, 148.5, 149.3,
167.4, 194.9 ppm.
Acknowledgments Ritika Chauhan is grateful to UGC (University
Grants Commission) for providing junior research fellowship. We
express our thanks to the Director of University Science and Instru-
mentation Centre, University of Delhi, Delhi, for providing the
spectral data and also acknowledge the Head of the Biophysics
Department, All India Institute of Medical Sciences, New Delhi, for
carrying out the X-ray studies.
Ethyl 1,4,5,6,7,8-hexahydro-4-(2-hydroxy-3-methoxy-
phenyl)-2-methyl-5-oxoquinoline-3-carboxylate
(5j, C20H23NO5)
References
1. Guillena G, Ramon DJ, Yus M (2007) Tetrahedron Asymmetry
18:693
2. Domling A (2006) Chem Rev 106:17
3. Simon C, Constantieux T, Rodrigeuz J (2004) Eur J Org Chem
4957
4. Domling A, Ugi I (2000) Angew Chem Int Ed 39:3168
5. Ramon DJ, Yus M (2005) Angew Chem Int Ed 44:1602
6. Mauzerall D, Westheimer FH (1955) J Am Chem Soc 77:2261
7. Baraldi PG, Budriesi R, Cacciari B, Chiarini A, Garuti L,
Giovanninetti G, Leoni A, Roberti M (1992) Collect Czech Chem
Commun 57:169
IR (Nujol): m = 3,342, 2,946, 1,733, 1,693, 1,607, 1,523,
1,390, 1,078 cm-1; 1H NMR (CDCl3, 400 MHz): d = 1.21
(t, 3H, J = 7.32 Hz, CH3), 1.88–2.27 (m, 6H, 3 9 CH2),
2.47 (s, 3H, CH3), 3.85 (s, 3H, OCH3), 4.10 (q, 2H,
J = 7.3 Hz, CH2), 5.17 (s, 1H, CH–Ar), 6.14 (br s, 1H,
NH), 6.69–6.79 (m, 3H, Ar–H), 9.26 (s, 1H, OH) ppm; 13
C
NMR (CDCl3, 100 MHz): d = 14.0, 19.5, 21.2, 25.4, 27.7,
29.7, 30.3, 36.2, 43.6, 55.8, 60.3, 81.2, 110.2, 120.8, 140.3,
148.7, 158.7, 169.9, 194.9 ppm.
8. Di Stilo A, Visentin S, Cena C, Gasco AM, Ermondi G, Gasco A
(1998) J Med Chem 41:5393
Ethyl 1,4,5,6,7,8-hexahydro-4-(1-naphthyl)-2-methyl-5-
oxoquinoline-3-carboxylate (5l, C23H23NO3)
9. Kawase M, Shah A, Gaveriya H, Motohashi N, Sakagami H,
Varga A, Molnar J (2002) Bioorg Med Chem 10:1051
10. Suarez M, Verdecia Y, Illescas B, Martinez-Alvarez R, Alvarez
A, Ochoa E, Seoane C, Kayali N, Martin N (2003) Tetrahedron
59:9179
11. Shan R, Velazquez C, Knaus EE (2004) J Med Chem 47:254
12. Bossert F, Meyer H, Wehinger E (1981) Angew Chem Int Ed
20:762
IR (Nujol): m = 3,292, 3,082, 2,952, 1,696, 1,609, 1,487,
1,379, 1,222 cm-1; 1H NMR (CDCl3, 400 MHz): d = 0.87
(t, 3H, J = 7.32 Hz, CH3), 1.86–2.27 (m, 6H, 3 9 CH2),
2.40 (s, 3H, CH3), 3.88 (q, 2H, J = 7.08 Hz, CH2), 5.84 (s,
1H, CH–Ar), 6.21 (br s, 1H, NH), 7.33–7.73 (m, 7H,
naphthyl) ppm; 13C NMR (CDCl3, 100 MHz): d = 14.1,
19.1, 21.0, 27.1, 31.8, 37.1, 59.8, 107.7, 114.5, 125.3,
13. Takenaka T, Usuda S, Nomura T, Maeno H, Sado T (1976)
Arzneim Forsch 26:2172
14. Julius S (1988) J Cardiovasc Pharmacol 12:S27
123

1680
M. Kidwai et al.
15. Liang J-C, Yeh J-L, Wang C-S, Liou S-F, Tsai C-H, Chen I-J
(2002) Bioorg Med Chem 10:719
16. Boschi D, Caron G, Visentin S, Di Stilo A, Rolando B, Fruttero
R, Gasco A (2001) Pharm Res 18:987
42. Heravi MM, Bakhtiari K, Javadi NM, Bamoharram FF, Saeedi M,
Oskooie HA (2007) J Mol Catal A Chem 264:50
43. Nagarapu L, Kumari MD, Kumari NV, Kantevari S (2007) Catal
Commun 8:1871
17. Godfraind T, Miller R, Wibo M (1986) Pharmacol Rev 38:321
18. Fleckenstein-Gruen G, Thimm F, Czirfuzs A, Matyas S, Frey M
(1994) J Cardiovasc Pharmacol 24:75
44. Olah GA, Iyer PS, Prakash GKS (1986) Synthesis 513
45. Olah GA, Prakash GKS, Sommer J (1985) Superacids. Wiley
Interscience, New York
19. Manpadi M, Uglinskii PY, Rastogi SK, Cotter KM, Wong Y-SC,
Anderson LA, Ortega AJ, Van Slambrouck S, Steelant WFA,
Rogelj S, Tongwa P, Antipin MY, Magedov IV, Kornienko A
(2007) Org Biomol Chem 5:3865
20. Emanuel NM, Obukhova LK, Dubur GJ, Tirzit GD, Uldrikis JR
(1985) Dokl Akad Nauk SSSR 284:1271
21. Bird GLA, Prach AT, McMahon AD, Forrest JAH, Mills PR,
Danesh BJ (1998) J Hepatol 28:194
22. Ogawa AK, Willoughby CA, Bergeron R, Ellsworth KP, Geissler
WM, Myers RW, Yao J, Harris G, Chapman KT (2003) Bioorg
Med Chem Lett 13:3405
46. Arata K (1990) Adv Catal 37:165
47. Yeo SC, Eisenberg A (1977) J Appl Polym Sci 21:875
48. Yeager HL, Eisenberg A (1982) In: Eisenberg A, Yeager HL
(eds), Perflourinated ionomer membranes. ACS Symposium ser-
ies, vol. 180, American Chemical Society, Washington, DC, p 1
49. Polshettiwar V, Varma RS (2008) Acc Chem Res 41:629
50. Polshettiwar V, Varma RS (2008) Chem Soc Rev 37:1546
51. Anastas PT, Warner JC (1998) Green chemistry: theory and
practice. Oxford University Press, Oxford
52. Chen J, Spear SK, Huddleston JG, Rogers RD (2005) Green
Chem 7:64
23. Klusa V (1995) Drugs Future 20:135
24. Bretzol RG, Bollen CC, Maester E, Federlin KF (1993) Am J
Kidney Dis 21:53
25. Bretzel RG, Bollen CC, Maeser E, Federlin KF (1992) Drugs
Future 17:465
26. Boer R, Gekeler V (1995) Drugs Future 20:499
27. Eisner U, Kuthan J (1972) Chem Rev 72:1
28. Stout DM, Meyers AI (1982) Chem Rev 82:223
29. Kutney JP (1977) Heterocycles 7:593
30. Widdowson DA (1978) In: Van Tamelen EE (ed) Bioorganic
chemistry, Series 4. Academic, New York, p 239
31. Hantzsch A (1882) Justus Liebigs Ann Chem 215:1
32. Loev B, Snader KM (1965) J Org Chem 30:1914
33. Tu S-J, Zhou J-F, Deng X, Cai P-J, Wan H, Feng J-C (2001) Chin
J Org Chem 21:313
34. Sabitha G, Reddy GSKK, Reddy CS, Yadav JS (2003) Tetrahe-
dron Lett 44:4129
35. Ji S-J, Jiang Z-Q, Lu J, Loh T-P (2004) Synlett 5:831
36. Breitenbucher JG, Figliozzi G (2000) Tetrahedron Lett 41:4311
37. Dondoni A, Massi A, Minghini E, Bertolasi V (2004) Tetrahe-
dron 60:2311
38. Ko S, Sastry MNV, Lin C, Yao C-F (2005) Tetrahedron Lett
46:5771
39. Ko S, Yao C-F (2006) Tetrahedron 62:7293
40. Wang L-M, Sheng J, Zhang L, Han J-W, Fan Z-Y, Tian H, Qian
C-T (2005) Tetrahedron 61:1539
53. Zhang Z-H, Yin L, Wang Y-M, Liu J-Y, Li Y (2004) Green
Chem 6:563
54. Kumar R, Chaudhary P, Nimesh S, Chandra R (2006) Green
Chem 8:356
55. Winter A, van den Berg AMJ, Hoogenboom R, Kickelbick G,
Schubert US (2006) Synthesis 2873
56. Han W, Liu C, Jin Z-L (2007) Org Lett 9:4005
57. Smith CB, Raston CL, Sobolev AN (2005) Green Chem 7:650
58. Kidwai M, Bhatnagar D, Chauhan R (2011) J Heterocycl Chem.
doi:10.1002/jhet.1037
59. Kidwai M, Chauhan R, Bhatnagar D (2011) J Sulfur Chem 32:37
60. Seen AJ (2001) J Mol Catal A Chem 177:105
61. Kumar S, Sharma P, Kapoor KK, Hundal MS (2008) Tetrahedron
64:536
62. Mobinikhaledi A, Foroughifar N, Fard MAB, Moghanian H,
Ebrahimi S, Kalhor M (2009) Synth Commun 39:1166
63. Bandgar BP, More PE, Kamble VT, Totre JV (2008) Arkivoc
15:1
64. Lingaiah BV, Ezikiel G, Yakaiah T, Reddy GV, Rao PS (2006)
Synlett 15:2507
65. Zhu D, Chen J, Xiao H, Liu M, Ding J, Wu H (2009) Synth
Commun 39:2895
66. Sheldrick GM (1997) SHELXL 97, A program for the determi-
nation of crystal structure. Anorganisch-Chemisches Institut der
¨
¨
Universitat Gottingen, Germany
67. Cromer DT, Mann JB (1968) Acta Cryst A24:321
68. Stewart RF, Davidson ER, Simpson WT (1965) J Chem Phys
42:3175
41. Maheswara M, Siddaiah V, Damu GLV, Rao CV (2006) Arkivoc
2:201
123