New efficient protocol for the production of hantzsch 1,4-dihydropyridines using RuCl3

Article abstract of DOI:10.1080/00397910802622762

Hantzsch 1,4-dihydropyridines are synthesized in a few minutes and in good to excellent yields from aldehydes, 1,3-dicarbonyl compounds, and ammonium acetate. The reaction is performed in solvent-free conditions, and 5mol% of RuCl3 is enough for this protocol.

Full text of DOI:10.1080/00397910802622762

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New Efficient Protocol for the
Production of Hantzsch 1,4-
Dihydropyridines Using RuCl3
Suresh a , Dhruva Kumar a & Jagir S. Sandhu a
a Department of Chemistry, Punjabi University,
Patiala, Punjab, India
Available online: 27 Apr 2009
To cite this article: Suresh, Dhruva Kumar & Jagir S. Sandhu (2009): New Efficient
Protocol for the Production of Hantzsch 1,4-Dihydropyridines Using RuCl3 , Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 39:11, 1957-1965
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Synthetic Communications1, 39: 1957–1965, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802622762
New Efficient Protocol for the Production of
Hantzsch 1,4-Dihydropyridines Using RuCl3
Suresh, Dhruva Kumar, and Jagir S. Sandhu
Department of Chemistry, Punjabi University, Patiala, Punjab, India
Abstract: Hantzsch 1,4-dihydropyridines are synthesized in a few minutes and in
good to excellent yields from aldehydes, 1,3-dicarbonyl compounds, and ammo-
nium acetate. The reaction is performed in solvent-free conditions, and 5 mol% of
RuCl3 is enough for this protocol.
Keywords: Aldehydes, ammonium acetate, 1,3-dicarbonyl compound, Hantzsch
1,4-dihydropyridines, RuCl3, solvent free
INTRODUCTION
Hantzsch 1,4-dihydropyridines (DHPs) (A), though first reported in
1881,[1] remained unattended untill the early seventies. Since their resem-
blance to reduced nicotinamide adenine dinucleotide (NADH) (B) was
reported, they started attracting the attention of chemists.[2] This
intensive research followed the discovery of their excellent pharma-
cological activities. As a result of these efforts, more than a dozen
drugs having a Hantzsch pyridine scaffold are in clinical use as anti-
hypertensive agents.[3] They are also shown to possess activities such as
antithrombiotic,[4] anti-inflammatory,[5] antitumor,[6] antitubercular,[7]
analgesic,[8] and anticonvulsant[9] activity.
Received September 24, 2008.
Address correspondence to Jagir S. Sandhu, Department of Chemistry,
Punjabi University, Patiala 147 002, Punjab, India. E-mail: j_sandhu2002@
yahoo.com
1957

1958
Suresh, D. Kumar, and J. S. Sandhu
These excellent pharmacological properties as well as DHPs’ (B) use as
organo-reducing agents[10,11] in organic synthesis have prompted several
efforts to achieve higher efficacy in their production procedures. The ori-
ginal procedure by Hantzsch had several limitations. To produce these
molecules economically in high yields, additives=catalysts continue to
be reported. In these improvements, several catalysts and conditions
include use of microwaves,[12] ionic liquid,[13] and Lewis acids such as
Yb(OTf)3,[14] ceric ammonium nitrate (CAN),[15] and K7[PW11CoO40].[16]
Obviously there is scope for further improvement in the production pro-
cesses of DHPs, as several of these procedures are either very slow or do
not offer very good yields.
Application of RuCl3 in organic synthesis is of current interest
because of its various attractive properties such as mildness, selectivity,
and solubility in organic solvents. RuCl3 has been successfully used for
the deoxygenation of organic N-oxides,[17] cleavage of epoxides to yield
trans-diols,[18] and selective protection of aldehydes[19] as acylals. Use
of RuCl3 has also been extended to other important named reactions
of organic chemistry such as the Biginelli reaction,[20] Prins reaction,[21]
aldol reaction,[22] Strecker synthesis of a-amino nitriles,[23] synthesis of
1,5-benzodiazepines[24] and the synthesis of bis-indolylmethanes.[25]
Because of these promising applications and other attractive features
of this mild Lewis acid, and also in continuation of our own work,[17,19,24]
we were prompted to use RuCl3 in the synthesis of DHPs (Scheme 1).
Scheme 1. Synthesis of Hantzsch 1,4-dihydropyridines.

Hantzsch 1,4-Dihydropyridines
1959
Table 1. Synthesis of substituted 1,4-dihydropyridines using 5 mol % RuCl3
under solvent-free conditions
Time
(min)
Yield
(%)b
Entry
R1
R2
Producta
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
H
Me
C6H5
3-(NO2)-C6H4
2-Furyl
H
Me
OEt
OEt
OEt
OEt
4a
4b
4c
4d
4e
4f
6
9
11
12
7
90
79
85
80
88
91
85
85
82
80
88
91
81
78
83
OEt
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
8
4g
4h
4i
13
10
13
10
12
9
12
12
12
C6H5
3-(NO2)-C6H4
4-(OMe)-C6H4
4-(Cl)-C6H4
2-Furyl
4-(OH)-C6H4
2-(Cl)-C6H4
4-(Me)-C6H4
4j
4k
4l
4m
4n
4o
aThe products were characterized by comparison of their melting points and
1
spectral (IR, H NMR) data with those of authentic samples.
bIsolated yields after recrystallization.
We initiated our investigation with pilot experiments using para-
formaldehyde, ethylacetoacetate, ammonium acetate, and RuCl3
(1:2:2:0.05) by thoroughly mixing=stirring the contents at 60ꢀC under
solvent-free conditions for 6 min to obtain Hantzsch 1,4-dihydropyri-
dines in 90% yields. To check the efficacy and applicability of this cata-
lyst, we could successfully obtain a range of desired products in 78–91%
yields (Table 1). In summary, the present method employing RuCl3 is a
mild, efficient, and environmentally benign protocol for the synthesis
of DHPs. Products are obtained in excellent yields in short reaction
times. Operational simplicity and solvent-free conditions are the other
attractive features that make this procedure superior to the existing ones.
Needless to say, during workup no hazardous=unsafe by-products were
formed with this new method.
EXPERIMENTAL
Melting points were determined in open capillaries and are uncorrected.
Reagent-grade chemicals were purchased from a commercial source and

1960
Suresh, D. Kumar, and J. S. Sandhu
used without further purification. Infrared (IR) spectra were recorded in
KBr discs on a Perkin-Elmer 240C analyzer. 1H NMR spectra were
recorded on a Varian Gemini 300 (300-MHz) spectrometer using tetra-
methylsilane (TMS) as an internal standard. The progress of the reaction
was monitored by thin-layer chromatography (TLC) using silica gel G
(Merck).
General Procedure for Preparation of Hantzsch 1,4-Dihydropyridines
The mixture of aldehydes (10 mmol), 1,3-dicarbonyl compound (20 mmol),
ammonium acetate (20 mmol), and RuCl3 (0.05 mmol) was stirred at 60ꢀC
for the time given in Table 1. After completion of the reaction (monitored
via TLC), it was diluted with ethyl acetate (20 mL). The organic layer was
washed with saturated NaHCO3 solution (3ꢁ 15mL) and then with brine.
It was dried over anhydrous Na2SO4 and concentrated under reduced
pressure to give the crude product, which was purified by recrystallization
from an ethyl acetate–ether mixture.
Representative Spectral Data
Diethyl 2,6-Dimethyl-1,4-dihydropyridine-3,5-dicarboxylate (4a)
Mp 171–172ꢀC. IR (KBr): 3351, 3108, 2987, 2898, 1693, 1654, 1507 cmꢂ1
.
1H NMR (CDCl3): 1.30 (t, 6H, J ¼ 3.48), 2.19 (s, 2H), 3.26 (s, 6H), 4.43
(q, 4H, J ¼ 7.17), 8.70 (s, 1H).
Diethyl 2,4,6-Trimethyl-1,4-dihydropyridine-3,5-dicarboxylate (4b)
1
Mp 129–131ꢀC. IR (KBr): 3344, 2962, 1697, 1642, 1492 cmꢂ1. H NMR
(CDCl3): 0.96 (d, 3H, J ¼ 6.45), 1.27 (t, 6H, J ¼ 7.05), 2.26 (s, 6H), 3.82
(q, 1H, J ¼ 6.45), 4.12 (m, 4H), 5.45 (s, 1H). MS: (m=z) 267.14 (Mþ),
265, 252, 236, 224, 220, 208, 196, 192, 178,164, 150, 77, 43.
Diethyl 2,6-Dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylate (4c)
1
Mp 158–160ꢀC. IR (KBr): 3341, 2981, 1687, 1650, 1488 cmꢂ1. H NMR
(CDCl3): 1.19 (t, 6H), 2.33 (s, 6H), 4.04 (m, 4H), 4.98 (s, 1H), 5.52 (s, 1H),
7.11–7.28 (m, 5H). MS: (m=z) 329.16 (Mþ), 327, 282, 254, 252, 236, 224,
210, 196, 180, 139.

Hantzsch 1,4-Dihydropyridines
1961
Diethyl 2,6-Dimethyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,
5-dicarboxylate (4d)
Mp 159–161ꢀC. IR (KBr): 3344, 3090, 2989, 2932, 1705, 1645, 1524,
1
1487 cmꢂ1. H NMR (CDCl3): 1.19 (t, 6H), 2.36 (s, 6H), 4.03 (m, 4H),
5.09 (s, 1H), 5.66 (s, 1H), 7.34 (m, 1H), 7.62 (m, 1H), 7.98 (m, 1H),
8.11 (m, 1H). MS: (m=z) 374.14 (Mþ), 355, 327, 299, 281, 252, 224,
196, 150, 127, 77, 43.
Diethyl 4-Furan-2-yl-2,6-dimethyl-1,4-dihydropyridine-
3,5-dicarboxylate (4e)
1
Mp 166–168ꢀC. IR (KBr): 3345, 2982, 1699, 1650, 1488 cmꢂ1. H NMR
(CDCl3): 1.23 (t, 6H, J ¼ 7.11), 2.33 (s, 6H), 4.07 (m, 4H), 5.19 (s, 1H),
5.69 (s, 1H), 5.94 (d, 1H, J ¼ 3.18), 6.20 (dd, 1H, J ¼ 1.86, 1.86), 7.21
(d, 1H, J ¼ 1.62). MS: (m=z) 319.14 (Mþ), 317, 290, 262, 246, 245, 218,
214, 174, 144, 115.
Dimethyl 2,6-Dimethyl-1,4-dihydropyridine-3,5-dicarboxylate (4f)
Mp 209–210ꢀC. IR (KBr): 3345, 3107, 3005, 2949, 1697, 1646, 1505, 1431,
1383 cmꢂ1. 1H NMR (CDCl3): 2.85 (s, 6H), 3.72 (s, 2H), 3.93 (s, 6H), 8.70
(s, 1H). MS: (m=z) 225.10 (Mþ), 224, 223, 210, 192, 165, 164, 132, 120,
104, 77, 43.
Dimethyl 2,4,6-Trimethyl-1,4-dihydropyridine-3,5-dicarboxylate (4g)
1
Mp 152–154ꢀC. IR (KBr): 3341, 3266, 3108, 2949, 1680, 1649 cmꢂ1. H
NMR (CDCl3): 0.94 (d, 3H, J ¼ 6.30), 2.27 (s, 6H), 3.71 (s, 6H), 3.77
(q, 1H, J ¼ 6.51), 5.52 (s, 1H). MS: (m=z) 239.11 (Mþ), 224, 208, 192,
178, 164, 149, 120, 43.
Dimethyl 2,6-Dimethyl-4-phenyl-1,4-dihydropyridine-3,
5-dicarboxylate (4h)
1
Mp 195–197ꢀC. IR (KBr): 3343, 3081, 3026, 2950, 1699, 1649 cmꢂ1. H
NMR (CDCl3): 2.34 (s, 6H), 3.64 (s, 6H), 5.00 (s, 1H), 5.58 (s, 1H),
7.12–7.25 (m, 5H). MS: (m=z) 301.13 (Mþ), 299, 267, 236, 224, 210,
192, 164, 77.

1962
Suresh, D. Kumar, and J. S. Sandhu
Dimethyl 2,6-Dimethyl-4-(3-nitrophenyl)-1,
4-dihydropyridine-3,5-dicarboxylate (4i)
1
Mp 210–212ꢀC. IR (KBr): 3349, 3005, 2953, 1704, 1650, 1528 cmꢂ1. H
NMR (CDCl3): 2.37 (s, 6H), 3.64 (s, 6H), 5.10 (s, 1H), 5.71 (s, 1H),
7.25 (t, 1H, J ¼ 7.89), 7.61 (m, 1H), 7.98 (m, 1H), 8.08 (t, 1H, J ¼ 1.84).
MS: (m=z) 346.11 (Mþ), 344, 328, 327, 313, 297, 281, 224, 192, 152,
115, 43.
Dimethyl 4-(4-Methoxyphenyl)-2,6-dimethyl-1,
4-dihydropyridine-3,5-dicarboxylate (4j)
1
Mp 186–188ꢀC. IR (KBr): 3348, 2948, 1696, 1650, 1625 cmꢂ1. H NMR
(CDCl3): 2.33 (s, 6H), 3.64 (s, 6H), 3.75 (s, 3H), 4.93 (s, 1H), 5.56 (s, 1H),
6.74 (d, 2H, J ¼ 6.67). 7.16 (d, 2H, J ¼ 6.83). MS: (m=z) 331.14 (Mþ), 329,
298, 272, 266, 265, 238, 224, 210, 196, 115, 77, 43.
Dimethyl 4-(4-Chlorophenyl)-2,6-dimethyl-1,
4-dihydropyridine-3,5-dicarboxylate (4k)
Mp 197–198ꢀC. IR (KBr): 3314, 3102, 2945, 1699, 1651, 1486 cmꢂ1
.
1H NMR (CDCl3): 2.38 (s, 6H), 3.64 (s, 6H), 4.96 (s, 1H), 5.62 (s,
1H), 7.15–7.21 (m, 4H). MS: (m=z) 335.09 (Mþ), 304, 270, 224,
192, 164, 43.
Dimethyl 4-Furan-2-yl-2,6-dimethyl-1,4-dihydropyridine-3,5-
dicarboxylate (4l)
1
Mp 192–194ꢀC. IR (KBr): 3344, 2951, 1698, 1651, 1492, 1434 cmꢂ1. H
NMR (CDCl3): 2.34 (s, 6H), 3.70 (s, 6H), 5.19 (s, 1H), 5.70 (s, 1H),
5.92 (d, 1H, J ¼ 3.21), 6.20 (dd, 1H, J ¼ 1.90, 1.70), 7.21 (d, 1H,
J ¼ 1.66).
Dimethyl 4-(4-Hydroxyphenyl)-2,6-dimethyl-1,4-
dihydropyridine-3,5-dicarboxylate (4m)
Mp 231–233ꢀC. IR (KBr): 3329, 3001, 2953, 1681, 1650, 1611, 1589 cmꢂ1
.
1H NMR (CDCl3): 2.33 (s, 6H), 3.64 (s, 6H), 4.70 (s, 1H), 4.92 (s, 1H),
5.57 (s, 1H), 6.65 (d, 2H, J ¼ 6.60), 7.10 (d, 2H, J ¼ 6.66).

Hantzsch 1,4-Dihydropyridines
1963
Dimethyl 4-(2-Chlorophenyl)-2,6-dimethyl-1,4-
dihydropyridine-3,5-dicarboxylate (4n)
Mp 194–196ꢀC. IR (KBr): 3302, 3233, 3095, 2947, 1702, 1682, 1649 cmꢂ1
.
1H NMR (CDCl3): 2.31 (s, 6H), 3.60 (s, 6H), 5.39 (s, 1H), 5.61 (s, 1H),
7.03–7.38 (m, 5H). MS: (m=z) 335.09 (Mþ), 304, 298, 224, 192, 164, 127.
Dimethyl 2,6-Dimethyl-4-(4-methylphenyl)-1,4-
dihydropyridine-3,5-dicarboxylate (4o)
1
Mp 174–176ꢀC. IR (KBr): 3314, 3105, 2942, 1697, 1655, 1495 cmꢂ1. H
NMR (CDCl3): 2.27 (s, 3H), 2.33 (s, 6H), 3.64 (s, 6H), 4.96 (s, 1H),
5.62 (s, 1H), 7.00 (d, 2H, J ¼ 8.01), 7.14 (d, 2H, J ¼ 8.07). MS: (m=z)
315.14 (Mþ), 313, 281, 250, 224, 192, 152, 115, 43.
ACKNOWLEDGMENTS
The authors are thankful to the Council of Scientific and Industrial
Research, New Delhi, India, for financial assistance and to the Indian
National Science Academy, New Delhi, India, for additional financial
support for this research project.
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