Aromatization of hantzsch 1,4-dihydropyridines with Al(NO 3)39H2O and/or Fe(NO3)39H2O in the presence of silica sulfuric acid under mild and heterogeneous conditions

Article abstract of DOI:10.1080/00397910903262195

A simple and convenient method for the aromatization of Hantzsch 1,4-dihydropyridines to the corresponding pyridines is achieved via combination of aluminum or ferric nitrates and silica sulfuric acid as environmentally friendly and novel oxidizing media.

Full text of DOI:10.1080/00397910903262195

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Aromatization of Hantzsch 1,4-
Dihydropyridines with Al(NO3)3·9H2O
and/or Fe(NO3)3·9H2O in the Presence
of Silica Sulfuric Acid Under Mild and
Heterogeneous Conditions
Arash Ghorbani-Choghamarani a & Javad Zeinivand a
a Faculty of Science , Department of Chemistry, Ilam University ,
Ilam, Iran
Published online: 26 Jul 2010.
To cite this article: Arash Ghorbani-Choghamarani & Javad Zeinivand (2010) Aromatization of
Hantzsch 1,4-Dihydropyridines with Al(NO3)3·9H2O and/or Fe(NO3)3·9H2O in the Presence of
Silica Sulfuric Acid Under Mild and Heterogeneous Conditions, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:16, 2457-2463,
DOI: 10.1080/00397910903262195
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Synthetic Communications1, 40: 2457–2463, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903262195
AROMATIZATION OF HANTZSCH 1,4-
DIHYDROPYRIDINES WITH Al(NO3)3 ꢀ 9H2O
AND/OR Fe(NO3)3 ꢀ 9H2O IN THE PRESENCE OF
SILICA SULFURIC ACID UNDER MILD AND
HETEROGENEOUS CONDITIONS
Arash Ghorbani-Choghamarani and Javad Zeinivand
Faculty of Science, Department of Chemistry, Ilam University, Ilam, Iran
A simple and convenient method for the aromatization of Hantzsch 1,4-dihydropyridines to
the corresponding pyridines is achieved via combination of aluminum or ferric nitrates and
silica sulfuric acid as environmentally friendly and novel oxidizing media. This oxidation
was carried out in dichloromethane under heterogeneous conditions with good to excellent
yields.
Keywords: Aluminum nitrate [Al(NO3)3 ꢀ 9 H2O]; aromatization; 1,4-dihydropyridines; ferric nitrate
[Fe(NO3)3 ꢀ 9 H2O]; silica sulfuric acid [SiO2-OSO3H]
INTRODUCTION
Five- and six-membered heterocyclic compounds often play important roles in
biologically active natural products and synthetic compounds of medicines.[1] The
Hantzsch reaction, discovered in 1881 for the preparation of 1,4-dihydropyridine-
3,5-dicarboxylates (DHPs), continues to be of considerable interest.[2] Hantzsch
1,4-dihydropyridines (1,4-DHPs) are an important class of drugs for the treatment
of cardiovascular diseases such as hypertension and angina pectoris.[3] Reduced
nicotinamide adenine dinucleotides (NADHs) as 1,4-DHP derivatives act as the
electron source for the reduction of O2 to H2O in the respiratory chain.[4,5] They
process neuroprotective, platet antiaggregration, and antidiabetic activities.[6] Some
representatives, such as nifedipine, niguldipine, nicardipine, and amlodipine, are
widely used for treatment of hypertension.[7]
The oxidation of Hantzsch 1,4-DHPs, a class of model compounds of NADH
coenzymes,[8,9] has attracted continuing interest over the years.[10] Recently, it was
found that drugs such as nifedipine and niguldipine undergo redox processes due
to the catalysis of cytochrome P-450 in the liver during their metabolism.[11] There-
fore, the oxidation of 1,4-DHPs has attracted considerable attention, and there are
several reports on the oxidation of this compounds in the literature.[7,12–25] Despite
Received June 9, 2009.
Address correspondence to Arash Ghorbani-Choghamarani, Faculty of Science, Department of
Chemistry, Ilam University, P.O. Box 69315516, Ilam, Iran. E-mail: arashghch58@yahoo.com
2457

2458
A. GHORBANI-CHOGHAMARANI AND J. ZEINIVAND
these intensive efforts, most of the reported oxidation procedures require use of strong
and toxic oxidants, severe conditions, long reaction times, excess reagent, and
laborious workup, form by-products, and give poor yields of the target products.
RESULTS AND DISCUSSION
A series of 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylates were synthe-
sized to investigate their conversion to pyridines by the one-pot, three-component
condensation of an aliphatic or aromatic aldehyde, methyl acetoacetate, and
NH4OAc under neat conditions according to our recently reported communication
(Scheme 1).[26]
Herein, we report a new, environmentally benign, and effective method for
aromatization of 1,4-DHPs using silica sulfuric acid (SiO2-OSO3H) and Al(NO3)3 ꢀ
9H2O or Fe(NO3)3 ꢀ 9H2O at room temperature.
Silica sulfuric acid is a nontoxic, powerful, and solid acid that can be easily
prepared by the reaction of chlorosulfonic acid with silica gel (Scheme 2).[27]
Initially, we started our investigation with oxidation of dimethyl 4-(4-
flourophenyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate by silica sulfuric
acid and Fe(NO3)3 ꢀ 9H2O in a different solvent. From the solvent screening
(Table 1), dichloromethane (DCM) emerged as the most convenient because the
product was isolated in almost quantitative yield in the shortest reaction time.
With optimized conditions in hand, we decided to consider the scope and
limitation of this new heterogeneous method. Therefore, a variety of dimethyl
2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate derivatives were subjected to
aromatization via combination of silica sulfuric acid (SiO2 ꢀ OSO3H) and aluminum
nitrate [Al(NO3)3 ꢀ 9H2O] or ferric nitrate [Fe(NO3)3 ꢀ 9H2O] in DCM at room
temperature with good to excellent yields (Scheme 3, Table 2).
The oxidation reactions were carried out under completely heterogeneous con-
ditions in DCM at room temperature with excellent yields. The reaction procedure is
very simple, and the products are easily isolated from the reaction media because in
Scheme 1. Synthesis of Hantzsch 1,4-dihydropyridines.
Scheme 2. Preparation of silica sulfuric acid.

AROMATIZATION OF HANTZSCH 1,4-DIHYDROPYRIDINES
2459
Table 1. Oxidation of dimethyl 4-(4-flourophenyl)-2,6-dimethyl-1,4-dihydropyridine-
3,5-dicarboxylate using silica sulfuric acid and Fe(NO3)3 ꢀ 9H2O in different solvents at
room temperaturea
Entry
Solvent
Time
Yield (%)b
1
2
3
4
5
6
Acetone
21 min
21 min
8 min
48 h
99
98
99
Chloroform
Dichloromethane
n-Hexane
c
—
99
Methyl acetate
Ethanol
57 min
72 h
d
—
aDHP=Fe(NO3)3 ꢀ 9H2O=SiO2 ꢀ OSO3H ¼ 1: 1.5 mmol: 0.2 g.
bIsolated yield.
cNo reaction.
dReaction did not complete.
this heterogeneous system all reaction components are insoluble in DCM except sub-
strate and product. Therefore, products can be easily obtained by simple filtration
and evaporation of CH2Cl2. As is evident from Table 2, the oxidation reaction pro-
ceeded more rapidly and effectively with Fe(NO3)3 ꢀ 9H2O; therefore, ferric nitrate is
a stronger oxidant than aluminum nitrate for the aromatization of 1,4-DHPs.
To investigate the efficiency of this heterogeneous media, we compared our
obtained results for the aromatization of 2,6-dimethyl-(4-chlorophenyl)-1,4-dihydro-
pyridine-3,5-dicarboxylate with the best data from the literature, as shown in Table 3.
In summary herein, we report an efficient method for the selective oxidation
of 1,4-DHPs under mild and heterogeneous conditions. Also, the cheapness and
availability of the reagents, easy and clean workup, and good yields make this
method attractive for chemists.
Scheme 3. Aromatization of 1,4-dihydropyridines to the corresponding pyridines.

2460
A. GHORBANI-CHOGHAMARANI AND J. ZEINIVAND
Table 2. Oxidation of 1,4-dihydropyridines 1 to the corresponding pyridines 2
using SiO2 ꢀ OSO3H (I), Al(NO3)3 ꢀ 9H2O (II), or Fe(NO3)3 ꢀ 9H2O (III) in DCM at
room temperature
Substrate=Reagentsa
Entry Substrate Product
I
II
III
Time (min) Yield (%)b
1
2
1a
1a
1b
1b
1c
1c
1d
1d
1e
1e
1f
2a
2a
2b
2b
2c
2c
2d
2d
2e
2e
2f
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
1.5
—
—
1.5
—
35
13
270
70
20
8
99
98
99
99
98
99
91
98
90
97
98
98
99
98
91
98
98
81
99
99
98
99
3
1.5
—
4
1.5
—
5
1.5
—
6
1.5
—
7
1.5
—
10
6
8
1.5
—
9
1.5
—
50
10
20
11
65
17
35
8
10
11
12
13
14
15
16
17
18
19
20
21
22
1.5
—
1.5
—
1f
2f
1.5
—
1g
1g
1h
1h
1i
2g
2g
2h
2h
2i
1.5
—
1.5
—
1.5
—
1.5
—
1.5
—
65
30
60
7
1i
2i
1.5
—
1j
2j
1.5
—
1j
2j
1.5
—
1k
1k
2k
2k
1.5
—
85
40
1.5
aI refers to grams of silica sulfuric acid; II and III refer to mmol of aluminium
nitrate and ferric nitrate respectively.
bIsolated yield. All products were identified by comparison with authentic samples
(mp, IR, 1H NMR, and 13C NMR).
Table 3. Efficiency comparison of various oxidizing systems for the oxidation of
2,6-dimethyl-(4-chlorophenyl)-1,4-dihydropyridine-3,5-dicarboxylate
Entry
Conditions
Time (min) Yield (%)a Reference
1
2
3
4
5
6
7
Fe(NO3)3 ꢀ 9H2O=SiO2-OSO3H (rt)
Al(NO3)3 ꢀ 9H2O=SiO2-OSO3H (rt)
DMSO=O2 (70 ꢁC)
SiO2 ꢀ HNO3 (rt)
6
10
270
6
98
91
91
98
96
96
99
This work
This work
2
28
28
18
29
PVP ꢀ HNO3 (rt)
14
90
20
UHP=I2 (rt)
t-BuOOH=Fe(III) phthalocyanine
chloride (rt)
VOCl3 (rt)
8
9
60
210
92
75
30
31
t-BuOOH=H2O
aIsolated yield.

AROMATIZATION OF HANTZSCH 1,4-DIHYDROPYRIDINES
2461
EXPERIMENTAL
General
Chemicals were purchased from Fluka, Merck, and Aldrich chemical compa-
nies. The oxidation products were characterized by comparison of their spectral
1
[infrared (IR), H NMR, and 13C NMR] and physical data with authentic samples.
General Procedure for Synthesis of Silica Sulfuric Acid
A 500-mL suction flask containing silica gel (60 g) was equipped with a
constant-pressure dropping funnel containing chlorosulfonic acid (23.3 g, 0.2 mol)
and gas inlet tube to conduct HCl gas into water. Chlorosulfonic acid was added
dropwise over a period of 30 min at room temperature. HCl gas evolved from the
reaction vessel immediately. Then the mixture was shaken for 30 min. A white solid
(silica sulfuric acid) of 76.0 g was obtained.[27]
Aromatization of Dimethyl-4-(4-bromophenyl)-2,6-dimethyl-1,4-
dihydropyridine-3,5-dicarboxylate (b) with Fe(NO3)3 ꢀ 9H2O in the
Presence of SiO2 ꢀ OSO3H as a Typical Procedure
Fe(NO3)3 ꢀ 9H2O (0.472 g, 1.5 mmol) was added at one time to a solution of
compound 1b (0.380 g, 1.0 mmol) in CH2Cl2 (5 mL), and after that SiO2 ꢀ OSO3H
(0.2 g) was added. The reaction mixture was stirred at room temperature for
70 min [the reaction progress was monitored by thin-layer chromatography (TLC)]
and then filtered. The residue was washed with CH2Cl2 (4 ꢂ 5 mL). Finally, solvents
were removed, and product 2b was obtained in 99% yield (0.374 g). Pale yellow solid;
1
mp 136–140 ꢁC; H NMR (90 MHz, CDCl3): d ¼ 7.47 (d, 2H), 7.16 (d, 2H), 3.57
(s, 2OCH3), 2.59 (s, 2CH3) ppm; IR (KBr): V ¼ 2917, 1732, 1595, 1462, 1377,
1242, 1212, 1038 cmꢃ1
.
Spectral Data for Selected Products
Compound 2a. Pale yellow solid; mp 123–126 ꢁC; 1H NMR (90 MHz,
CDCl3): d ¼ 7.37 (m, 5H), 3.52 (s, 2OCH3), 2.59 (s, 2CH3) ppm; IR (KBr):
V ¼ 2854, 1733, 1590, 1456, 1377, 1243, 1213, 1037 cmꢃ1
.
Compound 2d. Pale yellow solid; mp 140–142 ꢁC; 1H NMR (400 MHz,
DMSO): d ¼ 7.51–7.54 (d, 2H, J ¼ 11.30 Hz), 7.20–7.17 (d, 2H, J ¼ 11.29 Hz),
3.55 (s, 6H), 2.49 (s, 6H) ppm; 13C NMR (100 MHz, DMSO): d ¼ 167.8, 155.6,
144.6, 135.0, 134.2, 129.8, 129.1, 126.6, 52.9, 23.0 ppm; IR (KBr): V ¼ 1731, 1597,
1573, 1557, 1492, 1434, 1374, 1240, 1211, 113, 1097, 968, 856, 832, 804, 761 cmꢃ1
.
1
Compound 2h. Pale yellow solid; mp 78–80 ꢁC; H NMR (90 MHz, CDCl3):
d ¼ 3.93 (s, 6H), 2.51 (s, 6H), 2.25 (s, 3H) ppm; IR (KBr): V ¼ 2853, 1731, 1569,
1457, 1377, 1222, 1103 cmꢃ1
.

2462
A. GHORBANI-CHOGHAMARANI AND J. ZEINIVAND
ACKNOWLEDGMENT
The authors thank the research facilities of Ilam University, Ilam, Iran, for
financial support of this research project.
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