CeCl3·7H2O-Catalyzed One-Pot Synthesis of Hantzsch 1,4-Dihydropyridines at Room Temperature

Article abstract of DOI:10.1080/00397910802656091

An efficient synthesis of a series of 1,4-dihydropyridines was accomplished at room temperature by the reaction of aldehydes with ammonium acetate and ethyl acetoacetate catalyzed by CeCl₃·7H₂O.

Full text of DOI:10.1080/00397910802656091

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CeCl3·7H2O-Catalyzed One-
Pot Synthesis of Hantzsch 1,4-
Dihydropyridines at Room
Temperature
Gowravaram Sabitha a , K. Arundhathi a , K.
Sudhakar a , B. S. Sastry b & J. S. Yadav a
a Organic Division I, Indian Institute of Chemical
Technology , Hyderabad, India
b University College of Pharmaceutical Sciences,
Andhra University , Vizag, India
Published online: 14 Jul 2009.
To cite this article: Gowravaram Sabitha , K. Arundhathi , K. Sudhakar , B. S. Sastry
& J. S. Yadav (2009) CeCl3·7H2O-Catalyzed One-Pot Synthesis of Hantzsch 1,4-
Dihydropyridines at Room Temperature, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 39:16, 2843-2851,
DOI: 10.1080/00397910802656091
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Synthetic Communications1, 39: 2843–2851, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802656091
CeCl3ꢀ7H2O-Catalyzed One-Pot Synthesis
of Hantzsch 1,4-Dihydropyridines
at Room Temperature
Gowravaram Sabitha,1 K. Arundhathi,1 K. Sudhakar,1 B. S. Sastry,2
and J. S. Yadav1
1Organic Division I, Indian Institute of Chemical Technology,
Hyderabad, India
2University College of Pharmaceutical Sciences, Andhra University,
Vizag, India
Abstract: An efficient synthesis of a series of 1,4-dihydropyridines was accom-
plished at room temperature by the reaction of aldehydes with ammonium acetate
and ethyl acetoacetate catalyzed by CeCl3ꢀ7H2O.
Keywords: CeCl3ꢀ7H2O, 1,4-dihydropyridines, multicomponent reactions (MCRs),
room temperature
INTRODUCTION
Hantzsch 1,4-dihydropyridines (1,4-DHPs) are the most important class
of calcium-channel modulators[1] and have been introduced as potential
drugs for the treatment of congestive heart failure.[2] Some of them, such
as nifedipine,[3] amlodipine, isradipine, and nimodipine, have been com-
mercialized and are known as vital drugs in the treatment of angina and
hypertension[4] because of their vasodilator properties. The DHP scaffold
is common in many vasodialator, bronchiodialator, antitumor, hepato-
protective, and antidiabetic agents.[5] 1,4-DHPs also exhibit several
Received October 18, 2008.
Address correspondence to Gowravaram Sabitha, Organic Division I, Indian
Institute of Chemical Technology, Hyderabad 500 007, India. E-mail: gowravar
amsr@yahoo.com
2843

2844
G. Sabitha et al.
Scheme 1.
medicinal applications, which include neuroprotectant[6a] and platelet
anti-aggregatory activity,[6b] and are important in Alzheimer’s disease
as anti-ischemic agents. The success of those calcium antagonists has
led to the development of novel synthetic strategies to improve their clas-
sical methods of preparation.[7] The first 1,4-DHPs were obtained more
than one century ago by Hantzsch.[5] This reaction involves heating a
mixture of an aldehyde, an ethyl or methyl acetoacetate, and ammonia
either in acetic acid or ethanol for a long time, producing poor yields.
Modified procedures involve the use of preformed Knoevenagel adducts
between the aldehyde and the ketoester or the use of preformed enami-
noesters. Recently, numerous modified methods under improved condi-
tions have been reported.[9] In addition to this, some procedures
require the use of an autoclave and microwave irradiation. However,
many of them suffer from drawbacks such as unsatisfactory yields, high
temperatures, and long reaction times. Pharmacological profiles of
Hantzsch 1,4-DHPs still attract much attention, and further development
of mild and simple methods for the synthesis of these compounds is in
great demand. To the best of our knowledge, the synthesis of 1,4-DHP
derivatives has not reported using CeCl3ꢀ7H2O.
In continuation of our interest in the application of CeCl3ꢀ7H2O for
development of useful synthetic methodologies, we report a simple, mild,
and efficient method for the preparation of 1,4-DHPs and their deriva-
tives at room temperature using CeCl3ꢀ7H2O as a cost-effective and
water-tolerant catalyst.
Thus, the reaction of 1 eq. of benzaldehyde 1a and 2 eq. of ethyl acet-
oacetate 2a with 1.5 eq. of ammonium acetate in the presence of a catalytic
amount of CeCl3ꢀ7H2O in acetonitrile at room temperature for 3 h
resulted in 1,4-dihydropyridine 3a in 80% yield (Scheme 1). Encouraged
by this result, a wide range of aromatic aldehydes were used for this
conversion. As indicated in Table 1, in all cases the reaction proceeds effi-
ciently at room temperature using a catalytic amount of catalyst, giving

Dihydropyridine Synthesis at Room Temperature
2845
Table 1. CeCl3ꢀ7H2O-catalyzed synthesis of Hantzsch 1,4-dihydropyridines
Entry
Aldehyde
Time (h)
Yield (%)
Mp (ꢁC)
3a
3
80
158
3b
3c
3d
3e
4
82
86
91
81
159
139
144
97
3.5
4
5
3f
5
86
95
3g
3h
3i
3.5
5.6
5
82
65
71
128–129
169
163
3j
5
3
72
82
196
115
3k
3l
6
61
92
94
89
142
3m
3n
3o
5.5
5
164–165
156–159
95–98
5
aAll products were characterized by spectral data and compared with the
authentic samples.
bIsolated pure products.

2846
G. Sabitha et al.
very good yields of products in 3–6 h. From Table 1, one can see that both
aromatic aldehydes with an electron-withdrawing group and those with an
electron-donating group can be employed in the synthesis of Hantzsch
esters. Aliphatic and heterocyclic aldehydes also reacted well at room tem-
perature to furnish the corresponding Hantzsch DHPs in good yields. The
synthesized Hantzsch esters are known compounds, and their identities
were confirmed by comparison of their spectral data and melting points
with those reported in the literature. The experimental procedure is simple
and mild and has the ability to tolerate a variety of functional groups such
as methoxy, nitro, hydroxy, and halides under the reaction conditions. The
advantages of the process are increased reactivity at room temperature,
associated with low cost, availability, and easy handling of the catalyst.
In summary, we have demonstrated a simple, efficient, and green
protocol for the one-pot synthesis of 1,4-DHPs by treatment of ethyl
acetoacetate and aldehydes with NH4OAc using CeCl3ꢀ7H2O as a cata-
lyst at room temperature. The simple workup, easy isolation of products,
good yields, mild reaction conditions, short reaction times, and nontoxic
catalyst are the salient features of this new procedure.
EXPERIMENTAL
Melting points were determined using a Buchi R-535 apparatus and are
uncorrected. All reactions were monitored by thin-layer chromatography
(TLC) using silica-coated plates and visualization under ultraviolet (UV)
light. Yields refer to pure products isolated by spectroscopically (1H, IR)
1
homogeneous material. H NMR spectra were recorded on Varian FT
200-MHz (Gemini) and Bruker UXNMR FT 300-MHz (Avance) instru-
ments in CDCl3. Chemical shift values were reported in parts per million
(d) relative to tetramethylsilane (TMS) (d 0.0) as an internal standard.
Mass spectra(MS) were recorded under electron impact at 70 eV on an
LC-MSD instrument (Agilent Technologies).
General Procedure
A mixture of aldehyde (1 mmol), ethyl acetoacetate (2 mmol), NH4OAc
(1.5 mmol), and CeCl3ꢀ7H2O (0.1 mmol) was stirred at room temperatue
in acetonitrile for an appropriate time (Table 1). After completion of the
reaction mixture as indicated by TLC, acetonitrile was removed, and
ethyl acetate was added to the residue. Ethyl acetate was washed with
water and brine solution, dried, and concentrated. The crude residue
was purified by column chromatography (EtOAc=hexane, 1:3) to afford

Dihydropyridine Synthesis at Room Temperature
2847
the pure product. The structure of the products was confirmed by
spectral data (1H NMR, IR, mass).
Spectral Data
Compound 3a
1
Mp 158ꢁC; IR (KBr): 3355, 2980, 1682, 1504, 855, 677 cmꢂ1; H NMR
(300 MHz, CDCl3): 0.95 (t, J ¼ 8.32 Hz, 6H), 2.31 (s, 6H), 4.06 (q,
J ¼ 8.32 Hz, 4H), 4.61 (s, 1H), 5.63 (brs, 1H), 7.06–7.21 (m, 5H); ESI
MS: m=z 329 (Mþ).
Compound 3b
Mp 159ꢁC; IR (KBr): 3342, 2923, 2856, 1692, 1649, 1493, 831, 746, 680,
1
592 cmꢂ1; H NMR (300 MHz, CDCl3): 1.06 (t, J ¼ 7.5 Hz, 6H), 2.34 (s,
6H), 3.78 (s, 3H), 4.01 (q, J ¼ 8.32 Hz, 4H), 4.81 (s, 1H), 5.41 (brs, 1H),
7.16 (m, 4H); ESI MS: m=z 360 (Mþ þ 1).
Compound 3c
Mp 139ꢁC; IR (KBr): 3337, 2980, 2930, 1682, 1504, 855, 753, 677 cmꢂ1
;
1H NMR (200 MHz, CDCl3): 1.91–2.78 (m, 12H, 6CH2), 4.50 (s, 1H,
CH), 6.93 (d, J ¼ 8.4 Hz, 1H, ArH), 7.20 (d, J ¼ 2.4 Hz, 1H, ArH), 7.32
(dd, J ¼ 8.4 Hz, J ¼ 2.4 Hz, 1H, ArH), 10.71 (s, 1H, OH); ESI MS: m=z
347 (Mþ).
Compound 3d
Mp 144ꢁC: IR (KBr): 3337, 2926, 2830, 1682, 1487, 855, 753, 677 cmꢂ1
;
1H NMR (200 MHz, CDCl3): 1.2 (t, J ¼ 7.5 Hz, 6H), 2.60 (s, 6H), 4.12
(q, J ¼ 8.2 Hz, 4H), 4.68 (s, 1H), 5.78 (brs, 1H), 7.20–7.25 (m, 2H), 7.41
(d, J ¼ 9.06 Hz, 1H), 7.51 (d, J ¼ 8.3 Hz, 1H); ESI MS: m=z 386
(Mþ þ Na).
Compound 3e
Mp 97ꢁC: IR (KBr): 3355, 2926, 2856, 1702, 1682, 1484, 856, 681 cmꢂ1
;
1H NMR (300 MHz, CDCl3): 1.31 (t, J ¼ 8.2 Hz, 6H), 1.41 (s, 6H), 2.32
(s, 6H), 2.60 (s, 6H), 2.89 (m, 1H), 4.12 (q, J ¼ 8.4 Hz, 4H), 4.68

2848
G. Sabitha et al.
(s, 1H), 5.81 (brs, 1H), 7.01 (d, J ¼ 8.36 Hz, 2H), 7.21 (d, J ¼ 8.3 Hz, 2H);
ESI MS: m=z 291 (Mþ).
Compound 3f
Mp 95ꢁC; IR (KBr): 3340, 2962, 2869, 1727, 1695, 1487, 680, 556 cmꢂ1
;
1H NMR (200 MHz, CDCl3): 1.01 (s, 1H), 1.12 (s, 3H), 1.21 (t,
J ¼ 8.2 Hz, 6H), 2.31 (s, 6H), 4.01 (q, J ¼ 8.4 Hz, 4H), 4.67 (s, 1H), 5.71
(brs, 1H), 7.01–7.20 (m, 3H), 7.41–7.50 (m, 1H); ESI MS: m=z 385 (Mþ).
Compound 3g
Mp 128–129ꢁC; IR (KBr): 3322, 2927, 2856, 1702, 1647, 1486, 859, 753,
1
601 cmꢂ1; H NMR (300 MHz, CDCl3): 0.95 (t, J ¼ 8.2 Hz, 6H), 2.34 (s,
6H), 4.07 (q, J ¼ 8.4 Hz, 4H), 4.72 (s, 1H), 5.76 (brs, 1H), 7.41 (d,
J ¼ 8.56 Hz, 2H), 8.10 (d, J ¼ 8.56 Hz, 2H); ESI MS: m=z 374 (Mþ).
Compound 3h
Mp 169ꢁC; IR (KBr): 3355, 2926, 2855, 1659, 1485, 847, 756, 620,
1
585 cmꢂ1; H NMR (300 MHz, CDCl3): 1.01 (t, J ¼ 8.3 Hz, 6H), 2.41
(s, 6H), 4.12 (q, J ¼ 8.4 Hz, 4H), 4.68 (s, 1H), 5.79 (brs, 1H), 7.12–7.25
(m, 4H); ESI MS: m=z 374 (Mþ).
Compound 3i
1
Mp 178ꢁC; IR (KBr): 3358, 2925, 2854, 1646, 1484, 808, 663 cmꢂ1; H
NMR (200 MHz, CDCl3): 0.98 (t, J ¼ 8.3 Hz, 6H), 2.31 (s, 6H), 4.06 (q,
J ¼ 8.4 Hz, 4H), 4.67 (s, 1H), 5.79 (brs, 1H), 6.98–7.21 (m, 4H); ESI
MS: m=z 374 (Mþ).
Compound 3j
Mp 199ꢁC; IR (KBr): 3342, 2978, 2929, 1725, 1692, 1488, 860, 756 cmꢂ1
;
1H NMR (300 MHz, CDCl3): 0.96 (t, J ¼ 8.2 Hz, 6H), 2.30 (s, 6H), 4.01
(q, J ¼ 8.4 Hz, 4H), 5.61 (brs, 1H), 5.72 (s, 1H), 7.21–7.45 (m, 4H), 7.51
(d, J ¼ 8.6 Hz, 1H), 7.62 (d, J ¼ 8.6 Hz, 1H), 8.12 (d, J ¼ 8.6 Hz, 1H);
ESI MS: m=z 380 (Mþ).

Dihydropyridine Synthesis at Room Temperature
2849
Compound 3k
Mp 115ꢁC; IR (KBr): 3358, 2925, 1696, 1651, 1487, 849, 786, 610 cmꢂ1
;
1H NMR (200 MHz, CDCl3): 1.21 (t, J ¼ 8.3 Hz, 6H), 2.01 (s, 1H), 2.41
(s, 6H), 4.04 (q, J ¼ 8.4 Hz, 4H), 5.34 (s, 1H), 5.78 (brs, 1H), 6.99 (d,
J ¼ 8.08 Hz, 2H), 7.16 (d, J ¼ 7.3 Hz, 2H); ESI MS: m=z 344 (Mþ þ 1).
Compound 3l
Mp 142ꢁC; IR (KBr): 3342, 2923, 2856, 1692, 1649, 1493, 831, 784,
1
680 cmꢂ1; H NMR (200 MHz, CDCl3): 0.98 (t, J ¼ 8.3 Hz, 6H), 2.29
(s, 6H), 3.78 (s, 3H), 3.99 (q, J ¼ 8.4 Hz, 4H), 4.78 (s, 1H), 5.71 (brs,
1H), 7.16–7.23 (m, 4H); ESI MS: m=z 409 (Mþ þ 1).
Compound 3m
1
Mp 164ꢁC; IR (KBr): 3350, 2981, 1682, 1503, 860, 756 cmꢂ1; H NMR
(200 MHz, CDCl3): 1.26 (t, J ¼ 8.2 Hz, 6H), 2.31 (s, 6H), 4.26 (q,
J ¼ 8.2 Hz, 4H), 4.96 (s, 1H), 5.63 (brs, 1H), 6.93–7.12 (m, 3H); ESI
MS: m=z 319 (Mþ ꢂ 1).
Compound 3n
Mp 156–159ꢁC; IR (KBr): 3340, 2980, 1652, 1486 cmꢂ1 1H NMR
;
(200 MHz, CDCl3): 1.29 (t, J ¼ 8.8 Hz, 6H), 2.21 (s, 6H), 4.10–4.26
(q, J ¼ 8.0 Hz, 4H), 5.30 (brs, 1H), 5.93 (brs, 1H), 6.73 (brs, 1H), 6.78
(t, J ¼ 8.8 Hz, 1H), 7.0 (d, J ¼ 8.8 Hz, 1H); ESI MS: m=z 334 (Mþ).
Compound 3o
1
Mp 96ꢁC; IR (KBr): 3358, 2952, 1681, 1515 cmꢂ1; H NMR (200 MHz,
CDCl3): 0.93 (d, J ¼ 7.2 Hz, 6H), 1.23 (t, J ¼ 8.2 Hz, 6H), 1.51 (m, 1H),
2.31 (s, 6H), 3.92 (d, J ¼ 7.2 Hz, 1H), 4.17 (q, J ¼ 8.2 Hz, 4H), 5.69
(brs, 1H); ESI MS: m=z 296 (Mþ þ 1).
ACKNOWLEDGMENT
K. S. thanks the University Grants Commission, New Delhi, for the
award of a fellowship.

2850
G. Sabitha et al.
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