Ultrasound-assisted L-proline catalyzed synthesis of novel derivatives of azo-linked dihydropyridines

Article abstract of DOI:10.1007/s00706-011-0575-6

The ultrasound-assisted multicomponent synthesis of novel derivatives of azo-linked dihydropyridines from azo-linked salicylaldehydes, dimedone, and ammonium acetate in the presence of an efficient inexpensive catalyst, L-proline, in high yield and a short reaction time is reported.

Full text of DOI:10.1007/s00706-011-0575-6

Monatsh Chem (2012) 143:289–293
DOI 10.1007/s00706-011-0575-6
ORIGINAL PAPER
Ultrasound-assisted L-proline catalyzed synthesis of novel
derivatives of azo-linked dihydropyridines
•
Mohammad Nikpassand Leila Zare
•
Maryam Saberi
Received: 18 January 2011 / Accepted: 12 July 2011 / Published online: 16 August 2011
Ó Springer-Verlag 2011
Abstract The ultrasound-assisted multicomponent syn-
thesis of novel derivatives of azo-linked dihydropyridines
from azo-linked salicylaldehydes, dimedone, and ammo-
nium acetate in the presence of an efficient inexpensive
catalyst, L-proline, in high yield and a short reaction time is
reported.
hypnotic, and anti-inflammatory agents [8, 9]. DHPs are
commercially used as calcium channel blockers for the
treatment of cardiovascular diseases, including hypertension
[10].
1,4-DHPs are generally synthesized by the Hantzsch
method [11]. This reaction involves the one-pot condensa-
tion of an aldehyde with ethyl acetoacetate and ammonium
acetate refluxing in alcohol. The Hantzsch method was
developed more than a century ago; since then, several other
efficient methods for the synthesis of 1,4-DHPs—involving
the use of microwaves [12], ionic liquid [13], refluxing
solvent at high temperature [14], TMSCl-NaI [15], metal
triflates [16], I2 [17], and CAN [18]—have been reported.
However, the need for high temperatures, harsh reaction
conditions, expensive metal precursors, or longer reaction
times, or the occurrence of several side products, or indeed
difficulties with recovering and reusing the catalyst limit
these methods. Thus, the development of a simple, efficient,
and versatile method for the preparation of 1,4-dihydro-
pyridine derivatives is an active area of research, and there
is scope for further improvements in terms of milder reac-
tion conditions and higher product yield.
Keywords Multicomponent Á Azo-linked
dihydropyridines Á Ultrasound Á L-Proline
Introduction
Azo dyes are compounds that contain azo groups linked to
methane or an aromatic sp2-hybridized C-atom. Azo dyes
are a well-known class of organic photoactive materials
due to their excellent optical switching properties, good
chemical stabilities, and high solution processing abilities
[1, 2]. These materials are widely used in the heat transfer
printing and textile industries [3, 4], in optical data storage
[5], and in the photorefractive polymer industry [6].
1,4-Dihydropyridines (1,4-DHPs) and their derivatives
are an important class of bioactive molecules in the field of
drugs and pharmaceuticals [7]. The heterocyclic ring in DHP
is a common feature of various bioactive compounds, such as
anticonvulsant, antidiabetic, antianxiety, antidepressive,
antitumor, analgesic, sedative, vasodilator, bronchodilator,
Results and discussion
Recent developments in dihydropyridine chemistry and our
continued interest in the development of efficient and
environmentally friendly procedures for the synthesis of
pharmaceutical compounds [19–21] prompted us, in this
paper, to report an efficient procedure for the synthesis of
novel derivatives of azo-linked dihydropyridines in the
presence of a catalytic amount of L-proline (1) in refluxing
EtOH, and (2) under ultrasound irradiation, as shown in
Scheme 1.
M. Nikpassand (&) Á M. Saberi
Institute of Chemistry, Islamic Azad University of Rasht,
Rasht, Iran
e-mail: nikpassand@iaurasht.ac.ir
L. Zare
Young Researcher Club, Islamic Azad University of Rasht,
Rasht, Iran
123

290
M. Nikpassand et al.
Scheme 1
H
N
CHO
OH
O
O
+
O
NH4OAc
L-Proline
O
OH
N
N
N
N
R
R
1
2
3
Method A: refluxing in EtOH, 80 °C
Method B: ultrasound irradiation at 60 °C
In an initial endeavor, 1a, dimedone (2), and ammonium
acetate were refluxed in 10 cm3 EtOH. After 7 h, only 45%
of the expected product 3a was obtained after purification.
To improve the product yields and to optimize the reaction
conditions, the same reaction was carried out in the pres-
ence of various catalytic amounts of L-proline (either 5, 10,
15, or 20 mol%). The results are summarized in Table 1. A
15 mol% loading of L-proline was sufficient to push the
reaction forward, but 5 or 10 mol% of L-proline was not
enough. Higher amounts of L-proline did not lead to a
significant change in the reaction yield.
environmentally friendly, clean procedure and its excellent
results, the simple product isolation and the possibility of
recycling the catalyst offer significant advantages. When
the reaction was complete, the solvent was removed under
vacuum and the residue was treated with 10 cm3 water.
Because L-proline is soluble only in H2O, and the desired
products are insoluble in H2O, the products can be directly
separated by filtering. The remaining reaction solution can
be recycled. Studies using 3a as a model substrate showed
that the recovered reaction solution could be successively
recycled in six rounds. The catalytic efficiency of L-proline
in the reaction solution had not decreased afteras many as
seven cycles.
Based on the above observations, we conducted the
same reaction using various azo-linked salicylaldehydes
(1a–1i), 5,5-dimethyl-1,3-cyclohexanedione (2), and
ammonium acetate in the presence of 15 mol% L-proline
under reflux in EtOH (80 °C). As expected, satisfactory
results were observed, and the results are summarized in
Table 2. All compounds were characterized by spectro-
scopic methods (IR and NMR) and elemental analysis.
In recent years, there has been increasing interest in
reactions that proceed under ultrasound irradiation, due to
their reduced pollution, low cost, and simple processing
and handling. This process is green, environmentally
friendly, clean, and fast, and can be carried out easily at a
mild temperature (60 °C) without incurring undesirable
side reactions. As a continuation of our study, we con-
centrated on the synthesis of azo-linked dihydropyridines
in green conditions, replacing the organic solvent with H2O
under ultrasound irradiation and using 15 mol% of L-pro-
line. This gave better results in terms of both yield and
reaction time (Table 2). Apart from the green,
Following these results, we further investigated the
potential applicability of L-proline to the selective synthesis
of dihydropyridines of different types of azo-linked sali-
cylaldehydes. The results showed that L-proline is able to
discriminate between aromatic compounds containing
electron-donating and electron-withdrawing groups, a
transformation that is difficult to accomplish via conven-
tional methods (Scheme 2).
We propose a possible mechanism for the L-proline
catalyzed synthesis of azo-linked dihydropyridine deriva-
tives 3 (Scheme 3). We suggest that L-proline can form an
enamine with dimedone. This enamine can react through
an aldol reaction with the aldehyde. After the elimination
of L-proline, intermediate 6 may be produced. Then another
enamine (L-proline with dimedone) can participate in a
Michael reaction, and finally, after the hydrolysis of proline
from the iminium intermediate, the desired product 7
is obtained. Upon intramolecular cyclization and dehy-
dration with ammonium acetate, this product would give
rise to 3.
Table 1 The effect of the amount of L-proline on the synthesis of of
3a at 80 °C
In conclusion, we developed an efficient and convenient
procedure for the synthesis of novel derivatives of azo-
linked 1,4-dihydropyridines through the three-component
synthesis of azo-linked salicylaldehydes, dimedone, and
ammonium acetate by L-proline. This procedure offers
advantages such as reduced reaction time, mild reaction
conditions, better productivity and a higher yield, ease of
execution, and economic viability of the catalyst. This
simple process combined with the use of a natural catalyst
Entry
Catalyst/mol%
Reaction time/min
Isolated yield/%
1
2
3
4
5
None
5
420
300
180
120
120
45
50
57
66
61
10
15
20
123

Ultrasound-assisted L-proline catalyzed synthesis
291
Table 2 Synthesis of fused 1,4-dihydropyridine derivatives 3a–3i using L-proline
NH
R
O
N N
O
OH
Compound
R
Method A
Time/min
Method B
Time/min
Yield/%a
Yield/%a
3a
3b
3c
3d
3e
3f
H
120
165
160
90
66
60
58
80
78
74
75
70
72
10
15
15
5
73
68
64
92
90
82
84
78
79
4-Me
4-Et
4-NO2
4-Cl
3-Cl
4-Br
2-Cl
4-I
100
110
100
120
90
5
5
3g
3h
3i
5
5
5
a
Isolated yield
Scheme 2
NH
O
N
N
O2N
CHO
OH
O2N
N
N
O
OH
(1 mmol)
73%
O
O
L-Proline
NH4OAc
(1 mmol)
+
+
60 °C, ultrasound,
10 min
NH
CHO
OH
(2 mmol)
O
Me
N
N
N
N
Me
O
OH
(1 mmol)
0%
make this procedure an economic, benign, and waste-free
chemical process for the synthesis of dihydropyridines.
Chemicals were purchased from Merck and Fluka. Ele-
mental analyses were done on a Carlo-Erba EA1110CNNO-
S analyzer and agreed with the calculated values. All
solvents used were dried and distilled according to standard
procedures.
Experimental
Melting points were measured on an Electrothermal 9100
apparatus. IR spectra were determined on a Shimadzo IR-
470 spectrometer. NMR spectra were recorded on a
500 MHz Bruker DRX-500 in CDCl3 or DMSO-d6 as
solvent and TMS as internal standard. For the ultrasound
reactions, an ultrasound apparatus (Astra 3D: 9.5 L,
45 kHz frequency, input power with heating: 305 W,
number of transducers: 2) from Tecno-Gaz was used.
General procedure for the synthesis of 3a–3i
in the presence of L-proline in EtOH (method A)
A mixture of aldehyde (1 mmol), dimedone (2 mmol),
NH4OAc (1 mmol), and a catalytic amount of catalyst
(15 mol %) in 10 cm3 EtOH was refluxed for the required
reaction time (Table 2). The progress of the reaction was
monitored by TLC (EtOAc:petroleum ether 1:4). After the
123

292
M. Nikpassand et al.
1.96–2.38 (m, 8H), 4.74 (s, 1H), 7.15 (d, J = 8.6 Hz,
1H), 7.44 (t, J = 7.2 Hz, 1H), 7.47 (t, J = 5.6 Hz, 2H),
7.62 (d, J = 2.1 Hz, 1H), 7.77 (dd, J = 2.26, 8.6 Hz, 1H),
7.83 (d, J = 7.3 Hz, 2H) ppm; 13C NMR (125 MHz,
DMSO-d6, 25 °C): d = 27.9, 30.0, 34.4, 40.1, 51.7, 113.1,
119.2, 121.6, 123.5, 126.2, 127.0, 129.5, 130.0, 147.6,
149.3, 150.8, 157.3, 196.3 ppm; IR (KBr): vꢀ = 3,396,
O
CO2H
O
O
N
O
Ar
H
1
2
5
CO2H
OH
N
3,016, 2,995, 1,649, 1,224, 1,022 cm-1
.
CO2H
N
H
4
Ar
3,4,6,7,9,10-Hexahydro-9-[2-hydroxy-5-[(4-methyl-
phenyl)diazenyl]phenyl]-3,3,6,6-tetramethyl-
O
CO2H
Ar
N
O
acridine-1,8(2H,5H)-dione (3b, C30H33N3O3)
Light orange solid, m.p.: 160–162 °C; 1H NMR (500 MHz,
DMSO-d6, 25 °C): d = 0.84 (s, 6H), 0.94 (s, 6H),
1.99–2.52 (m, 11H), 5.11 (s, 1H), 7.09 (d, J = 8.6 Hz,
1H), 7.32 (d, J = 8.1 Hz, 2H), 7.46 (s, 1H), 7.60 (dd,
J = 2.1, 10.8 Hz, 1H), 7.68 (d, J = 8.1 Hz, 2H) ppm; 13C
NMR (125 MHz, DMSO-d6, 25 °C): d = 23.0, 26.6, 31.1,
32.3, 41.8, 52.5, 113.5, 118.2, 121.2, 124.0, 125.5, 128.7,
131.2, 140.7, 146.4, 153.3, 156.2, 160.2, 198.2 ppm; IR
(KBr): vꢀ = 3,394, 2,999, 1,649, 1,431, 1,313, 1,228,
6
O
O
O
Ar
O
O
O
2
7
1,020 cm-1
.
O
Ar
O
NH4OAc
9-[5-[(4-Ethylphenyl)diazenyl]-2-hydroxyphenyl]-
3,4,6,7,9,10-hexahydro-3,3,6,6-tetramethylacridine-
1,8(2H,5H)-dione (3c, C31H35N3O3)
N
H
Dark orange solid, m.p.: 129–131 °C; 1H NMR (500 MHz,
DMSO-d6, 25 °C): d = 0.85 (s, 6H), 1.00 (s, 6H), 1.18 (t,
J = 7.2 Hz, 3H), 2.03–2.56 (m, 8H), 2.65 (q, J = 7.5 Hz,
2H), 5.11 (s, 1H), 7.10 (d, J = 8.6 Hz, 1H), 7.35 (d,
J = 8.2 Hz, 2H), 7.46 (s, 1H), 7.61 (dd, J = 8.6, 2.2 Hz,
1H), 7.71 (d, J = 8.2 Hz, 2H) ppm; 13C NMR (125 MHz,
DMSO-d6, 25 °C): d = 17.6, 26.4, 29.3, 31.5, 35.0, 42.6,
51.0, 114.4, 119.6, 121.2, 124.5, 124.6, 128.5, 131.5,
145.9, 146.4, 154.2, 157.0, 159.4, 196.9 ppm; IR (KBr):
3
Scheme 3
required reaction time, the mixture was concentrated under
vacuum and the organic compound was purified by column
chromatography to produce the 1,4-dihydropyridine
derivatives 3a–3i as pure crystalline products.
General procedure for the ultrasound-assisted synthesis
of 3a–3i in the presence of L-proline (method B)
vꢀ = 3,436, 2,997, 1,658, 1,433, 1,313, 1,228, 1,053 cm-1
.
3,4,6,7,9,10-Hexahydro-9-[2-hydroxy-5-[(4-nitro-
phenyl)diazenyl]phenyl]-3,3,6,6-tetramethylacridine-
1,8(2H,5H)-dione (3d, C29H30N4O5)
A mixture of aldehyde (1 mmol), dimedone (2 mmol),
NH4OAc (1 mmol), and a catalytic amount of catalyst
(15 mol%) in 10 cm3 EtOH was placed into a Pyrex glass
open vessel and irradiated in a water bath under silent
conditions by ultrasound (45 kHz) at 60 °C for the required
reaction time (Table 2). When the irradiation was stopped,
the solvent was evaporated under vacuum, and then the
catalyst was separated by adding 10 cm3 water. The
organic residue was then purified by column chromatog-
raphy to produce the 1,4-dihydropyridine derivatives 3a–3i
as pure crystalline products.
Dark orange solid, m.p.: 248–250 °C; 1H NMR (500 MHz,
DMSO-d6, 25 °C): d = 0.89 (s, 6H), 1.00 (s, 6H),
2.03–2.36 (m, 8H), 4.88 (s, 1H), 6.87 (d, J = 8.6 Hz, 1H),
7.62 (s, 1H), 7.64 (m, 1H), 7.90 (d, J = 8.9 Hz, 2H), 8.35 (d,
J = 8.9 Hz, 2H) ppm; 13C NMR (125 MHz, DMSO-d6,
25 °C): d = 28.0, 31.2, 33.6, 42.7, 50.2, 114.9, 121.2, 123.5,
123.8, 129.8, 129.9, 132.0, 133.4, 146.8, 149.7, 154.1, 157.9,
198.8 ppm; IR (KBr): vꢀ = 3,272, 3,197, 3,064, 2,927, 1,612,
1,519, 1,481, 1,371, 1,228 cm-1
.
3,4,6,7,9,10-Hexahydro-9-[2-hydroxy-5-(phenyl-
diazenyl)phenyl]-3,3,6,6-tetramethylacridine-
1,8(2H,5H)-dione (3a, C29H31N3O3)
Yellow solid, m.p.: 186–188 °C; 1H NMR (500 MHz,
DMSO-d6, 25 °C): d = 1.04 (s, 6H), 1.14 (s, 6H),
9-[5-[(4-Chlorophenyl)diazenyl]-2-hydroxyphenyl]-
3,4,6,7,9,10-hexahydro-3,3,6,6-tetramethylacridine-
1,8(2H,5H)-dione (3e, C29H30ClN3O3)
Orange solid, m.p.: 165–167 °C; 1H NMR (500 MHz,
DMSO-d6, 25 °C): d = 0.85 (s, 6H), 1.19 (s, 6H),
123

Ultrasound-assisted L-proline catalyzed synthesis
293
2.01–2.54 (m, 8H), 5.12 (s, 1H), 7.13 (d, J = 8.6 Hz, 1H),
7.48 (s, 1H), 7.60 (d, J = 8.7 Hz, 2H), 7.65 (d, J = 8.6 Hz,
1H), 7.81 (d, J = 8.6 Hz, 2H), 10.31 (br s, 1H) ppm; 13C
NMR (125 MHz, DMSO-d6, 25 °C): d = 27.3, 30.1, 33.9,
41.0, 51.5, 113.9, 120.1, 122.0, 123.8, 126.2, 128.2, 129.4,
133.4, 144.8, 147.8, 152.9, 158.2, 197.0 ppm; IR (KBr):
vꢀ = 3,436, 3,274, 3,070, 2,937, 1,656, 1,377, 1,228,
IR (KBr): vꢀ = 3,436, 3,274, 3,070, 2,941, 1,631, 1,375,
1,226, 1,026 cm-1
.
3,4,6,7,9,10-Hexahydro-9-[2-hydroxy-5-
[(4-iodophenyl)diazenyl]phenyl]-3,3,6,6-
tetramethylacridine-1,8(2H,5H)-dione (3i, C29H30IN3O3)
Dark orange solid, m.p.: 148–150 °C, 1H NMR (500 MHz,
DMSO-d6, 25 °C): d = 0.94 (s, 6H), 1.00 (s, 6H),
2.00–2.46 (m, 8H), 5.01 (s, 1H), 6.88–6.95 (m, 4H), 7.05
(t, J = 8.3 Hz, 1H), 7.57 (d, J = 9.0 Hz, 1H), 7.89 (d,
J = 8.5 Hz, 1H), 10.25 (br s, 1H) ppm; 13C NMR
(125 MHz, DMSO-d6, 25 °C): d = 27.1, 31.3, 35.0, 42.0,
52.0, 100.3, 114.1, 119.3, 123.4, 123.6, 126.8, 128.8,
137.7, 145.7, 146.7, 152.9, 158.2, 197.8 ppm; IR (KBr):
1,026 cm-1
.
9-[5-[(3-Chlorophenyl)diazenyl]-2-hydroxyphenyl]-
3,4,6,7,9,10-hexahydro-3,3,6,6-tetramethylacridine-
1,8(2H,5H)-dione (3f, C29H30ClN3O3)
Yellow solid, m.p.: 265–267 °C; 1H NMR (500 MHz,
DMSO-d6, 25 °C): d = 0.85 (s, 6H), 1.08 (s, 6H),
2.33–2.52 (m, 8H), 5.12 (s, 1H), 7.11 (d, J = 8.7 Hz,
1H), 7.51 (s, 1H), 7.50–7.57 (m, 2H), 7.64 (d, J = 8.7 Hz,
1H), 7.75–7.77 (m, 2H) ppm; 13C NMR (125 MHz,
DMSO-d6, 25 °C): d = 26.7, 30.1, 33.8, 41.4, 51.6,
113.9, 119.4, 122.5, 126.3, 126.7, 128.2, 130.4, 131.4,
144.8, 147.8, 152.9, 158.9, 196.9 ppm; IR (KBr):
vꢀ = 3,620, 3,458, 3,020, 2,925, 1,710, 1,614, 1,519,
vꢀ = 3,442, 3,070, 2,950, 1,649, 1,379, 1,232, 1,027 cm-1
.
Acknowledgments We thank the Research Committee of Islamic
Azad University of Rasht and the Young Researchers Club of Islamic
Azad University of Rasht for their support of this study.
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.
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9-[5-[(4-Bromophenyl)diazenyl]-2-hydroxyphenyl]-
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1,8(2H,5H)-dione (3g, C29H30BrN3O3)
Orange solid, m.p.: 171–173 °C; 1H NMR (500 MHz,
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52.5, 114.7, 119.6, 122.6, 123.3, 125.5, 127.5, 128.9,
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1,022 cm-1
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9-[5-[(2-Chlorophenyl)diazenyl]-2-hydroxyphenyl]-
3,4,6,7,9,10-hexahydro-3,3,6,6-tetramethylacridine-
1,8(2H,5H)-dione (3h, C29H30ClN3O3)
Light brown solid, m.p.: 167–169 °C; 1H NMR (500 MHz,
DMSO-d6, 25 °C): d = 0.87 (s, 6H), 1.25 (s, 6H),
2.04–2.46 (m, 8H), 5.07 (s, 1H), 7.16 (d, J = 8.6 Hz,
1H), 7.42 (t, J = 7.3 Hz, 1H), 7.49 (t, J = 8.0 Hz, 1H),
7.53 (br s, 1H), 7.57 (d, J = 8.0 Hz, 1H), 7.65 (t,
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51.6, 113.8, 119.5, 123.5, 127.7, 127.0, 128.8, 131.9,
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