Tetrabutylammonium hexatungstate [TBA]2[W6O 19]: Novel and reusable heterogeneous catalyst for rapid solvent-free synthesis of polyhydroquinoline via unsymmetrical Hantzsch reaction

Article abstract of DOI:10.1016/S1872-2067(12)60547-6

A novel, efficient, and environmentally friendly method for the synthesis of polyhydroquinoline derivatives by a one-pot, four-component unsymmetrical Hantzsch condensation of dimedone, aldehydes, ethyl acetoacetate, and ammonium acetate in the presence of a catalytic amount of tetrabutylammonium hexatungstate [TBA]₂[W₆O₁₉] under solvent-free conditions has been developed. The results showed that this heterogeneous catalyst has high catalytic activity and the desired products were obtained in good to high yields. Moreover, the catalyst was found to be reusable and considerable catalytic activity was still achieved after the fifth run.

Full text of DOI:10.1016/S1872-2067(12)60547-6

Chinese Journal of Catalysis 34 (2013) 1173–1178
ava i l a b l e a t w w w. s c i e n c ed i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e /c h n j c
Article
Tetrabutylammonium hexatungstate [TBA]2[W6O19]: Novel and
reusable heterogeneous catalyst for rapid solvent‐free synthesis
of polyhydroquinolines via unsymmetrical Hantzsch reaction
Abolghasem DAVOODNIA*, Maryam KHASHI, Niloofar TAVAKOLI‐HOSEINI
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel, efficient, and environmentally friendly method for the synthesis of polyhydroquinoline
derivatives by a one‐pot, four‐component unsymmetrical Hantzsch condensation of dimedone,
aldehydes, ethyl acetoacetate, and ammonium acetate in the presence of a catalytic amount of tet‐
rabutylammonium hexatungstate [TBA]2[W6O19] under solvent‐free conditions has been developed.
The results showed that this heterogeneous catalyst has high catalytic activity and the desired
products were obtained in good to high yields. Moreover, the catalyst was found to be reusable and
considerable catalytic activity was still achieved after the fifth run.
Received 29 December 2012
Accepted 29 January 2013
Published 20 June 2013
Keywords:
Heterogeneous catalysis
Polyhydroquinolines
© 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Tetrabutylammonium hexatungstate
1. Introduction
4‐aryl‐1,4‐DHPs are well known as calcium‐channel blockers
and have emerged as one of the most important classes of
Multicomponent reactions (MCRs) have attracted much in‐
terest and are highly regarded in modern organic synthesis and
medicinal chemistry because they are one‐pot processes that
bring together three or more components and show high atom
economy and high selectivity [1–4]. MCRs have been widely
used in the convergent synthesis of complex and important
organic molecules from simple and readily available starting
materials, and have emerged as powerful tools for drug dis‐
covery [5,6]. The development of new MCRs and improvement
of known MCRs are therefore areas of considerable current
interest. One such reaction is the synthesis of polyhydroquino‐
lines via the unsymmetrical Hantzsch reaction.
The 1,4‐dihydropyridine (1,4‐DHP) core is found in a range
of compounds exhibiting a broad spectrum of biological activi‐
ties [7–9]. Some of the representative compounds of this class
possess antimicrobial [10], antitubercular [11], insecticidal
[12], and neuroprotectant [13] activities. In particular,
drugs for the treatment of cardiovascular diseases [14,15].
Recently, 1,4‐DHPs have been used as organocatalysts for
asymmetric reactions such as hydrogenation of quinolines in
the synthesis of alkaloids [16], asymmetric reductive amination
of aldehydes [17], and hydrogenation of α,β‐unsaturated alde‐
hydes and ketones [18,19].
Polyhydroquinolines are a class of fused 1,4‐DHPs that have
received less attention than other fused 1,4‐DHPs, and com‐
paratively few methods for their preparation have been re‐
ported. Polyhydroquinolines are generally synthesized by an
unsymmetrical Hantzsch reaction, which involves the one‐pot,
four‐component condensation of dimedone, aldehydes, ethyl
acetoacetate, and ammonium acetate, using various promoting
agents such as montmorillonite K‐10 [20], ionic liquids [21], I2
[22], FeF3 [23], Sc(OTf)3 (OTf = trifluoromethanesulfonate)
[24], Yb(OTf)3 [25], cerium(IV) ammonium nitrate [26], and
trifluoroethanol [27]. Syntheses of these compounds using mi‐
*Corresponding author. Tel: +98‐511‐8435000; Fax: +98‐511‐8424020; E‐mail: adavoodnia@mshdiau.ac.ir, adavoodnia@yahoo.com
This work was supported by Islamic Azad University, Mashhad Branch, Iran.
DOI: 10.1016/S1872‐2067(12)60547‐6 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 34, No. 6, June 2013

AbolghasemDAVOODNIA et al. / Chinese Journal of Catalysis 34 (2013) 1173–1178
crowave irradiation [28,29], solar thermal energy [30], and
grinding [31] have also been reported.
ml) and 12 mol/L HCl (18 ml) in DMF (50 ml) was then added
drop‐wise over a period of time with stirring, and the resulting
mixture was filtered to remove the undissolved white solids. A
solution of tetrabutylammonium bromide (15.1 g, 0.047 mol) in
methanol (50 ml) was then added to the filtrate with rapid stir‐
ring to give a white precipitate. The resulting suspension was
stirred for 5 min and the product was then collected by filtra‐
tion. Recrystallization from a minimum amount of hot dimethyl
sulfoxide gave the product as colorless diamond‐shaped crys‐
tals [55].
However, most of these methodologies suffer from disad‐
vantages such as unsatisfactory yields, toxic organic solvents,
harsh reaction conditions, long reaction times, and the use of
relatively expensive reagents. These findings prompted us to
perform investigations to find new catalysts that will enable the
synthesis of polyhydroquinoline derivatives using simple ex‐
perimental set‐ups and eco‐friendly conditions.
The development of environmentally benign, efficient, and
economical methods for the synthesis of biologically interesting
compounds [32–35] remains a significant challenge in synthet‐
ic chemistry. Green chemistry emphasizes the need for envi‐
ronmentally clean synthesis, which involves improvements in
selectivity, elimination of hazardous solvents, and easy
work‐up, using reusable catalysts. As a result, attempts to re‐
place homogeneous catalysts by non‐toxic, non‐corrosive, easy
to handle, and environmentally friendly heterogeneous cata‐
lysts have increased rapidly in recent years [36–39]. Reactions
under solvent‐free conditions have also continued to attract the
attention of researchers both from academia and industry
[40,41]. This is because solvent‐free reactions usually need
shorter reaction times and have simpler work‐up procedures.
As a result of our interest in the synthesis of heterocyclic
compounds [42–45], and as part of our research on the devel‐
opment of environmentally friendly methods for the synthesis
of organic compounds using reusable catalysts [46–50], we
recently investigated the use of tetrabutylammonium hex‐
atungstate, [TBA]2[W6O19], as a catalyst in the Knoevenagel
condensation [51], the Biginelli reaction [52], and in the syn‐
theses of biscoumarins [53] and 1,8‐dioxodecahydroacridines
[54]. This new reusable heterogeneous catalyst performed well
and showed high catalytic activity in these transformations.
These facts encouraged us to explore the use of this catalyst in
the synthesis of polyhydroquinoline derivatives under sol‐
vent‐free conditions.
2.2. General procedure for synthesis of polyhydroquinolines
5a–5m
A mixture of dimedone 1 (1 mmol), an aldehyde 2a–2m (1
mmol), ethyl acetoacetate 3 (1 mmol), ammonium acetate 4 (1
mmol), and [TBA]2[W6O19] (0.07 mmol, 7 mol%) was heated in
an oil bath at 110 °C for 20–30 min. The reaction was moni‐
tored using thin‐layer chromatography. On completion of the
transformation, the reaction mixture was cooled to room tem‐
perature and hot ethanol was added. This resulted in precipita‐
tion of the catalyst, which was collected by filtration. The prod‐
uct was collected from the filtrate after cooling to room tem‐
perature and recrystallized from ethanol to give compounds
5a–5m in high yields (Scheme 1). The melting points were
recorded using a Stuart SMP3 melting‐point apparatus. The IR
spectra of the products were obtained with KBr disks, using a
1
Tensor 27 Bruker spectrophotometer. The H NMR (400 and
500 MHz) spectra were recorded using Bruker 400 and 500
spectrometers.
Ethyl
2,7,7‐trimethyl‐5‐oxo‐4‐phenyl‐1,4,5,6,7,8‐hexahy‐
droquinoline‐3‐carboxylate (5a). 1H NMR (500 MHz, CDCl3): 
0.97 (s, 3H, CH3), 1.11 (s, 3H, CH3), 1.23 (t, 3H, J = 7.1 Hz, CH3),
2.15–2.39 (m, 4H, 2CH2), 2.41 (s, 3H, CH3), 4.09 (q, 2H, J = 7.1
Hz, OCH2), 5.01 (s, 1H, CH), 6.18 (s br., 1H, NH), 7.12 (t, 1H, J =
7.3 Hz, arom‐H), 7.22 (t, 2H, J = 7.5 Hz, arom‐H), 7.33 (d, 2H, J =
7.7 Hz, arom‐H); IR (KBr, cm–1): ν 3289, 3219, 3082, 2963,
1699, 1644, 1611, 1485, 1381, 1213, 1072, 761, 698, 530.
Ethyl 4‐(4‐bromophenyl)‐2,7,7‐trimethyl‐5‐oxo‐1,4,5,6,7,8‐
2. Experimental
1
2.1. Synthesis of [TBA]2[W6O19]
hexahydroquinoline‐3‐carboxylate (5b). H NMR (400 MHz,
CDCl3):  0.95 (s, 3H, CH3), 1.09 (s, 3H, CH3), 1.22 (t, 3H, J = 7.2
Hz, CH3), 2.14–2.37 (m, 4H, 2CH2), 2.39 (s, 3H, CH3), 4.08 (q, 2H,
J = 7.2 Hz, OCH2), 5.04 (s, 1H, CH), 6.21 (s br., 1H, NH), 7.21 (d, J
= 8.4 Hz, 2H, arom‐H), 7.34 (d, J = 8.4 Hz, 2H, arom‐H); IR (KBr,
cm–1): ν 3275, 3206, 3076, 2958, 1703, 1649, 1604, 1487, 1381,
A mixture of sodium tungstate dihydrate (Na2WO4·2H2O,
99%, 33 g, 0.1 mol), acetic anhydride (40 ml), and
N,N‐dimethylformamide (DMF, 30 ml) was heated at 100 °C for
3 h to obtain a white cream. A solution of acetic anhydride (20
R
O
H
2a-2m
O
O
O
R
O
[TBA]2[W6O19
]
OEt
OEt
+
Solvent-free
110 oC
O
O
N
H
3
1
5a-5m
NH4OAc
4
Scheme 1. [TBA]2[W6O19]‐catalyzed synthesis of polyhydroquinoline derivatives.

AbolghasemDAVOODNIA et al. / Chinese Journal of Catalysis 34 (2013) 1173–1178
1280, 1215, 1073, 843, 534.
Ethyl 4‐(4‐chlorophenyl)‐2,7,7‐trimethyl‐5‐oxo‐1,4,5,6,7,8‐
hexahydroquinoline‐3‐carboxylate (5d). H NMR (400 MHz,
CDCl3):  0.95 (s, 3H, CH3), 1.10 (s, 3H, CH3), 1.22 (t, 3H, J = 7.2
Hz, CH3), 2.14–2.39 (m, 4H, 2CH2), 2.40 (s, 3H, CH3), 4.08 (q, 2H,
J = 7.2 Hz, OCH2), 5.05 (s, 1H, CH), 6.08 (s br., 1H, NH), 7.18 (d, J
= 8.4 Hz, 2H, arom‐H), 7.26 (d, J = 8.4 Hz, 2H, arom‐H); IR (KBr,
cm–1): ν 3275, 3207, 3077, 2959, 1706, 1649, 1605, 1489, 1382,
1280, 1214, 1071, 844, 535.
droquinoline‐3‐carboxylate (5m). 1H NMR (400 MHz, CDCl3): 
0.85 (t, 3H, J = 7.2 Hz, CH3), 1.11 (s, 6H, 2CH3), 1.16–1.28 (m,
2H, CH2), 1.31 (t, 3H, J = 7.2 Hz, CH3), 1.34–1.47 (m, 2H, CH2),
2.17‐2.38 (m, 4H, 2CH2), 2.33 (s, 3H, CH3), 4.04 (t, 1H, J = 5.6 Hz,
CH), 4.11–4.27 (m, 2H, OCH2), 5.91 (s br., 1H, NH); IR (KBr,
cm–1): ν 3278, 3209, 2957, 1699, 1640, 1602, 1489, 1393, 1279,
1216, 1066, 740.
1
3. Results and discussion
Ethyl 4‐(4‐methoxyphenyl)‐2,7,7‐trimethyl‐5‐oxo‐1,4,5,6,7,
8‐hexahydroquinoline‐3‐carboxylate (5g). 1H NMR (400 MHz,
CDCl3):  0.96 (s, 3H, CH3), 1.09 (s, 3H, CH3), 1.23 (t, 3H, J = 7.2
Hz, CH3), 2.14–2.36 (m, 4H, 2CH2), 2.38 (s, 3H, CH3), 3.75 (s, 3H,
OCH3), 4.09 (q, 2H, J = 7.2 Hz, OCH2), 5.02 (s, 1H, CH), 6.27 (s br.,
1H, NH), 6.76 (d, J = 8.8 Hz, 2H, arom‐H), 7.24 (d, J = 8.8 Hz, 2H,
arom‐H); IR (KBr, cm–1): ν 3277, 3207, 3078, 2958, 1701, 1648,
1606, 1495, 1380, 1281, 1216, 850, 537.
To optimize the reaction conditions, a selected model reac‐
tion was carried out with dimedone
1
(1 mmol),
4‐chlorobenzaldehyde 2d (1 mmol), ethyl acetoacetate 3 (1
mmol), and ammonium acetate 4 (1 mmol) under different sets
of reaction conditions. The results are summarized in Table 1.
The results clearly show that the catalyst is essential, and that
the catalytic activity of [TBA]2[W6O19] is high, giving 5d in high
yield in a short reaction time. Using [TBA]2[W6O19] as the cata‐
lyst, we evaluated the reaction in various solvents and under
solvent‐free conditions. The product yield in refluxing H2O was
low, even after reaction for 240 min (Table 1, entry 15),
whereas relatively good yields were obtained in refluxing
CH2Cl2, CH3CN, or EtOH (Table 1, entries 16–18). However, the
best results in terms of yield, as well as reaction time, were
obtained under solvent‐free conditions (Table 1, entry 12). It
was also found that the yield of compound 5d was strongly
affected by the catalyst amount and reaction temperature un‐
der solvent‐free conditions. No product was observed when the
reaction was carried out under solvent‐free conditions without
any catalyst, even after a long reaction time (entry 1). The best
result was obtained when the reaction was conducted at 110 °C
in the presence of 7 mol% of the [TBA]2[W6O19] catalyst (Table
1, entry 12).
Ethyl 4‐(4‐methylphenyl)‐2,7,7‐trimethyl‐5‐oxo‐1,4,5,6,7,8‐
1
hexahydroquinoline‐3‐carboxylate (5h). H NMR (400 MHz,
CDCl3):  0.97 (s, 3H, CH3), 1.09 (s, 3H, CH3), 1.24 (t, 3H, J = 7.2
Hz, CH3), 2.13–2.34 (m, 4H, 2CH2), 2.27 (s, 3H, CH3), 2.37 (s, 3H,
CH3), 4.09 (q, 2H, J = 7.2 Hz, OCH2), 5.04 (s, 1H, CH), 6.41 (s
br., 1H, NH), 7.02 (d, J = 7.6 Hz, 2H, arom‐H), 7.21 (d, J = 7.6 Hz,
2H, arom‐H); IR (KBr, cm–1): ν 3276, 3207, 3077, 2959, 1702,
1648, 1605, 1493, 1380, 1282, 1216, 1072, 849, 531.
Ethyl 2,7,7‐trimethyl‐4‐(4‐nitrophenyl)‐5‐oxo‐1,4,5,6,7,8‐
hexahydroquinoline‐3‐carboxylate (5j). 1H NMR (500 MHz,
CDCl3):  0.94 (s, 3H, CH3), 1.12 (s, 3H, CH3), 1.21 (t, 3H, J = 7.1
Hz, CH3), 2.16–2.44 (m, 4H, 2CH2), 2.45 (s, 3H, CH3), 4.08 (q, 2H,
J = 7.1 Hz, OCH2), 5.19 (s, 1H, CH), 6.00 (s br., 1H, NH), 7.51 (d, J
= 8.5 Hz, 2H, arom‐H), 8.11 (d, J = 8.5 Hz, 2H, arom‐H); IR (KBr,
cm–1): ν 3276, 3189, 3075, 2966, 1703, 1649, 1607, 1519, 1493,
1378, 1345, 1280, 1216, 1072, 867, 831, 533.
Ethyl
2,7,7‐trimethyl‐5‐oxo‐4‐propyl‐1,4,5,6,7,8‐hexahy‐
To determine the generality of this method, the scope of the
Table 1
Effect of [TBA]2[W6O19] catalyst amount, solvent, and temperature on model reaction.
Entry
Catalyst (mol%)
Solvent
Temperature (°C)
Time (min)
Yield* (%)
1
2
3
4
5
—
3
3
3
3
5
5
5
5
7
7
7
7
8
7
7
7
7
—
—
—
—
—
120
60
90
110
130
60
120
40
40
20
20
—
54
66
70
71
55
72
77
76
60
81
95
94
85
40
63
65
75
6
7
—
—
40
20
90
8
9
—
—
—
—
—
—
110
130
60
20
20
40
20
20
20
10
11
12
13
14
15
16
17
18
90
110
130
110
reflux
reflux
reflux
reflux
—
20
H2O
CH2Cl2
CH3CN
EtOH
240
240
240
240
Reaction conditions: dimedone 1 (1 mmol), 4‐chlorobenzaldehyde 2d (1 mmol), ethyl acetoacetate 3 (1 mmol), and ammonium acetate 4 (1 mmol).
*Isolated yields.

AbolghasemDAVOODNIA et al. / Chinese Journal of Catalysis 34 (2013) 1173–1178
Table 2
[TBA]2[W6O19]‐catalyzed synthesis of polyhydroquinoline derivatives 5a–5m.
Melting point (oC)
Reported
R
Producta
Time (min)
Yieldb (%)
Found
C6H5
5a
5b
5c
5d
5e
5f
5g
5h
5i
20
20
30
20
20
20
20
20
30
20
20
40
40
93
90
85
95
86
91
82
88
83
83
88
69
67
214–216
259–260
206–208
246–248
225–227
239–241
257–259
257–260
181–183
245–247
240–243
163–165
165–167
209–210 [22]
254–255 [24]
206–208 [28]
244–246 [26]
218–220 [28]
237–238 [22]
258–259 [24]
260–261 [25]
178–180 [26]
244–246 [28]
245–247 [26]
145–146 [25]
147–148 [25]
4‐BrC6H4
2‐ClC6H4
4‐ClC6H4
3‐HOC6H4
4‐HOC6H4
4‐MeOC6H4
4‐MeC6H4
3‐O2NC6H4
4‐O2NC6H4
2‐Furyl
5j
5k
5l
Et
n‐Pr
5m
Reaction conditions: dimedone 1 (1 mmol), an aldehyde 2a–2m (1 mmol), ethyl acetoacetate 3 (1 mmol), ammonium acetate 4 (1 mmol),
[TBA]2[W6O19] (0.07 mmol, 7 mol%), 110 oC, solvent‐free.
a All the products were characterized by their IR spectral data and a comparison of their melting points with those of authentic samples. The struc‐
tures of some products were also confirmed by 1H NMR analysis.
b Isolated yields.
reaction was investigated using a number of aromatic and ali‐
phatic aldehydes under the optimized reaction conditions; the
results are presented in Table 2. The results show that the pro‐
tocol is useful for different aromatic aldehydes bearing elec‐
tron‐withdrawing and electron‐donating substituents in the
aromatic rings, giving high yields of the products. Aliphatic
aldehydes, however, afforded the corresponding polyhydro‐
quinolines in moderate yields.
The principle advantage of the use of heterogeneous solid
catalysts in organic transformations is their reusability. The
catalyst was readily recovered from the reaction mixture using
the procedure outlined in the experimental section. The sepa‐
rated catalyst was washed with cold ethanol and then dried at
60 °C under vacuum for 1 h before being reused in a similar
reaction. We found that the catalyst could be used at least five
times with only a slight reduction in activity (Fig. 1). Further‐
more, retention of the structure of the catalyst was confirmed
by comparing the FT‐IR spectra of the recovered catalysts (Fig.
2(2)–(5)) with that of the fresh catalyst (Fig. 2(1)) for the mod‐
el reaction. As shown, these spectra are almost identical.
A mechanistic rationalization for this reaction is provided in
Scheme 2. On the basis of our previous reports [51–54], it is
reasonable to assume that [TBA]2[W6O19] can play a dual role.
We therefore propose that the tetrabutylammonium ion
[(n‐Bu)4N+] induces polarization of the carbonyl groups,
whereas the terminal oxygen atoms or the bridging oxygen
atom in the polyoxometalate anion, W6O192−, are slightly basic
and can promote the necessary reactions. [TBA]2[W6O19] can
therefore activate the reactants and also the intermediates in
this reaction. As shown in Scheme 2, polyhydroquinolines can
be formed either through path A or through path B. It is pro‐
posed that intermediate I, in path A, is obtained from the reac‐
tion of dimedone with an aldehyde, whereas in path B the in‐
termediate is formed from the reaction of ethyl acetoacetate
with an aldehyde. Intermediate II in paths A and B is obtained
from the reaction of ammonium acetate with ethyl acetoacetate
(5)
(4)
100
80
60
40
20
0
95
(3)
93
91
91
90
(2)
(1)
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm1)
0
1
2
3
4
5
6
Fig. 2. FT‐IR spectra of fresh catalyst ((1), first run), and recovered
catalysts ((2–5), second to fifth run, respectively) for synthesis of com‐
pound 5d.
Reaction cycle
Fig. 1. Reusability of [TBA]2[W6O19] for synthesis of compound 5d.

AbolghasemDAVOODNIA et al. / Chinese Journal of Catalysis 34 (2013) 1173–1178
-
2
[W6O19 ][(n-Bu)4N+]2
R
O
Y
R
O
O
Y
O
Y
O
H
OH
-H2O
X
X
X
H
O
H
O
R
H
I
1 or 3
2a-2m
O
O
O
H
H N
-H2O
X'
X'
X'
NH4OAc
H2N
H2N
Y'
II
Y'
O
Y'
H
HO
4
1 or 3
-
2
[W6O19 ][(n-Bu)4N+]2
O
O
O
O
O
O
X
R
O
R
R
X'
X
Y
X
X'
Y'
X'
Y'
HN
H
Y'
Y
Y
O
NH
OH HN
I
II
H
-
2
[(n-Bu)4N+]2[W6O19
]
-
2
[(n-Bu)4N+]2[W6O19
]
O
Y
R
O
O
Y
R
O
-H2O
X
X'
X
X'
Path A: XY = -(CH2CMe2CH2)-, X' = CO2Et, Y' = Me
Path B: X'Y' = -(CH2CMe2CH2)-, X = CO2Et, Y = Me
N
Y'
N
H
Y'
H
HO
5a-5m
Scheme 2. Plausible mechanism for formation of polyhydroquinolines in the presence of [TBA]2[W6O19] as catalyst.
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intermediates subsequently react together to give the final
products 5a–5m. Under the conditions used, attempts to iso‐
late intermediates I and II failed, even after careful monitoring
of the reactions.
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4. Conclusions
A simple, convenient, and practical method was reported for
the synthesis of polyhydroquinolines through a one‐pot,
four‐component, unsymmetrical Hantzsch condensation of
dimedone, aldehydes, ethyl acetoacetate, and ammonium ace‐
tate using [TBA]2[W6O19] as a catalyst under solvent‐free con‐
ditions. The method offers several significant advantages, in‐
cluding short reaction times, high yields of products, easy
work‐up, and the absence of any hazardous organic solvents.
The simplicity of the procedure, combined with the ease of
recovery, reusability, and stability of the catalyst, makes this
method an economical, environmentally benign, and waste‐free
chemical process for the synthesis of polyhydroquinolines.
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AbolghasemDAVOODNIA et al. / Chinese Journal of Catalysis 34 (2013) 1173–1178
Graphical Abstract
Chin. J. Catal., 2013, 34: 1173–1178 doi: 10.1016/S1872‐2067(12)60547‐6
Tetrabutylammonium hexatungstate [TBA]2[W6O19]: Novel and reusable heterogeneous catalyst for rapid solvent‐free
synthesis of polyhydroquinolines via unsymmetrical Hantzsch reaction
Abolghasem DAVOODNIA*, Maryam KHASHI, Niloofar TAVAKOLI‐HOSEINI
Mashhad Branch, Islamic Azad University, Iran
A simple and efficient method for the synthesis of polyhydroquinolines by a one‐pot, four‐component reaction of dimedone, aldehydes,
ethyl acetoacetate, and ammonium acetate, using tetrabutylammonium hexatungstate, [TBA]2[W6O19], as a recyclable catalyst under sol‐
vent‐free conditions is described. The catalyst has high activity and the desired products were obtained in good to high yields.
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