Solvent-free microwave-assisted synthesis of substituted pyridines using NH4OAc as nitrogen source

Article abstract of DOI:10.2174/157017811799304296

New 1,5-dicarbonyl compounds were prepared, as versatile precursors to pyridine derivatives, by a tandem Claisen-Schmidt condensation/ Michael addition reaction, that is condensation between 5-chloro-3-methyl-1-phenyl-1H-pyrazole- 4-carbaldehyde and aceto

Full text of DOI:10.2174/157017811799304296

652
Letters in Organic Chemistry, 2011, 8, 652-655
Solvent-Free Microwave-Assisted Synthesis of Substituted Pyridines Using
NH4OAc as Nitrogen Source
Jorge Trilleras*,1, Pedro De La Torre1, Dency J. Pacheco1, Jairo Quiroga*,2, Manuel Nogueras3 and
Justo Cobo3
1Grupo de Investigación en Compuestos Heterocíclicos, Programa de Química, Universidad del Atlántico, Km 7 Vía
antigua Puerto Colombia, Barranquilla, Colombia
2Grupo de Investigación de Compuestos Heterocíclicos, Departamento de Química, Universidad del Valle, A. A. 25360
Cali, Colombia
3Departamento de Química Inorgánica y Orgánica, Universidad de Jaén, 23071 Jaén, España
Received May 09, 2011: Revised June 15, 2011: Accepted June 28, 2011
Abstract: New 1,5-dicarbonyl compounds were prepared, as versatile precursors to pyridine derivatives, by a tandem
Claisen-Schmidt condensation/ Michael addition reaction, that is condensation between 5-chloro-3-methyl-1-phenyl-1H-
pyrazole-4-carbaldehyde and acetophenones, and the formed adduct then reacts with a second molecule of acetophenone.
The preparation of substituted pyridines was achieved using NH4OAc as nitrogen source under solvent-free microwave
irradiation.
Keywords: Chalcone, 1,5-dicarbonyl compounds, cyclocondensation, microwave, NH4OAc, pyridines, solvent-free
procedure.
INTRODUCTION
On the other hand, chalcones are well-known precursors
to flavones and key intermediates for the combinatorial
assembly of different heterocyclic structures [11].
Cyclization of chalcones to afford pyrroles, pyrazoles,
pyridines and diazepines has been a developing field within
the realm of heterocyclic chemistry for the past several
years, because of their ready accessibility and diverse
biological activity [12].
Pyridine derivatives have attracted considerable attention
of organic and medicinal chemists because of their broad
spectrum of biological activity; such antitumor [1], antiviral
[2], antibacterial, antifungal [3], improvement in MCH1R
affinity [4], or potent CDK1 inhibitors [5]. These examples
clearly indicate the remarkable potential of pyridine
derivatives as a source of valuable drug candidates.
Due to our interest in the ꢀ,ꢁ-unsaturated ketones
(chalcones) and their application in the preparation of
heterocyclic compounds [13], we have now reported them as
precursors of polysubstituted pyridine derivatives bearing a
pyrazoles at C-4.
Numerous methods have been reported for the synthesis
of pyridine derivatives. The classical method involves
dicarbonyl compounds (The Hantzsch Synthesis) to give 1,4-
dihydropyridine derivatives [6]. Recently, several methods
have been reported comprising: i) application of microwave
irradiation along with one-pot reaction using efficient Lewis
acid catalysts such as InCl3 [7]; ii) multicomponent reactions
[8], iii) hetero-Diels-Alder reaction from Baylis–Hillman
adducts for the preparation of pyridylphosphonates [9] and
substituted pyridines [10]. However, those methodologies
present a strong limitation, that is the use of high
temperatures, expensive and environmentally harmful metal
catalysts, and longer reaction times. Therefore, the search for
better conditions reaction for the synthesis of substituted
pyridine derivatives, have such new catalyst, where the
absent solvents or the use of less hazardous solvents is still
of prime importance.
RESULTS AND DISCUSSION
For the preparation of the chalcones we have carried out
a Claisen–Schmidt condensation reaction between 5-chloro-
3-methyl-1-phenyl-1H-pyrazole-4-carbaldehyde
1
and
acetophenones 2, so in ethanol with basic catalyst at room
temperature during 10-30 minutes, we obtained the
respective (E)-3-(5-chloro-3-methyl-1-phenyl-1H-pyrazol-4-
yl)-1-arylprop-2-en-1-one
3
[14]. Next, the pyrazole
chalcone analogs of 3 are subjected to a base-catalyzed
Michael type reaction addition with another equivalent of the
acetophenone to produce 1,5-dicarbonyl compounds 4
(Scheme 1).
*Address correspondence to these authors at the Grupo de Investigación en
Compuestos Heterocíclicos, Programa de Química, Universidad del
Atlántico, Km 7 Vía antigua Puerto Colombia, Barranquilla, Colombia; Tel:
+57 5 3599484; Fax: +57 5 3599481;
The reaction proceeded quite well in moderate yields,
both with electron donating and with electron-withdrawing
groups at acetophenones.
E-mail: jorgetrilleras@mail.uniatlantico.edu.co
Grupo de Investigación de Compuestos Heterocíclicos, Departamento de
Química, Universidad del Valle, A. A. 25360 Cali, Colombia; Tel: +57 2
3393248; Fax: +57 2 3392440; E-mail: jaiquir@univalle.edu.co
This process is favored under reactional conditions for
two and a half additional hours (Method A).
1875-6255/11 $58.00+.00
© 2011 Bentham Science Publishers

Solvent-Free Microwave-Assisted Synthesis of Substituted Pyridines
Letters in Organic Chemistry, 2011, Vol. 8, No. 9
653
Method A
O
O
O
Ph
O
Ar
N
N
H3C
H
H3C
Ar
CH3
Ar
CH3
2
2
Cl
Ar
CH3
Ar
N
NaOH/Ethanol
Cl
NaOH/Ethanol
N
N
Cl
N
O
Ph
O
Ph
1
3
4
O
Ar
CH3
2
Method B
2 equiv.
NaOH/Ethanol
Scheme 1.
Table 1. Heterocyclic Chalcones and 1,5-Dicarbonyl Derivatives by Conventional Method
Compound 3
Compound 4*
Compound 6
Yield (%)
Entry
Ar
M.p °C
Yield (%)
M.p °C
Yield (%)
M.p °C
MWI (min.)
a
b
c
d
e
f
C6H5
4-H3CC6H4
4-BrC6H4
116-118
113-115
155-157
153-155
125-127
98-100
65
63
60
61
50
70
70
60
120-123
132-136
140-145
>190 dec
192-196
105-107
156-158
>180 dec.
65
70
60
50
45
40
65
45
163-165
182-185
>240 dec.
214-217
>300
85
75
60
85
75
90
90
80
2
2
2.5
3
4-ClC6H4
4-FC6H4
2
4-H3COC6H4
3,4,5-tri-H3COC6H2
Thiophenyl
183-185
173-175
>300
2
g
h
140-142
> 300
2
3
* Yields obtained to compounds 4 by Method B.
The direct synthesis of 1,5-dicarbonyl compounds 4 from
1,5-Dicarbonyl
compounds
4
could
undergo
1
certainly proceeds through the formation of the
intramolecular cyclization with the chlorine atom through a
SNAr reaction, however, spectral analysis turned up no
evidence of the formation of derivative 5 under the reaction
conditions (see Scheme 2). Analytical and spectroscopic data
of compounds 6 are all consistent with the proposed
corresponding heterocyclic chalcone 3, which was isolated
detected during reaction monitoring. Yields obtained for 1,5-
dicarbonyl derivatives were through reaction using two
equivalents of acetophenone under similar reaction
conditions (Method B). These results are given in Table 1.
1
structures. All H NMR spectra showed a singlet for Hꢀ of
pyridine ring at around 7.9 ppm. As depicted from Table 1,
the yields of all products are ranging from good to excellent,
and no relationship was found between them and the nature
of the substituents at the aryl groups.
This approach is particularly attractive because of the
symmetry presented via method B, and the possibility of
asymmetric derivatives via method A, which pointed towards
simple routes to obtained 1,5-dicarbonyl asymmetric
derivatives and their corresponding pyridine derivatives. The
structures of compounds 4 were elucidated from their
CONCLUSION
1
analytical and spectroscopic data (IR, H and 13C NMR, and
In this work, we have reported the preparation of 1,5-
dicarbonyl compounds, and simple and efficient applications
of addition–cyclization reactions for the synthesis assisted by
microwave irradiation of substituted pyridines derivatives,.
Compared to the reported methods, our method is
convenient, safe, and can be performed under ambient
conditions with easy isolation procedures. A remarkable
achievement is the possibility of incorporating diversity and
structural complexity in reaction to yield compounds 4 and
so to develop a library of pyridine derivatives. The
advantages of this method over existing methods are the
short reaction times, solvent-free conditions, low cost, and
that the last aromatization step is accomplished under mild
conditions without additional dehydration or oxidant
MS spectra). Single crystal X-ray diffraction analysis of
compound 4a and 4h was used to corroborate the postulated
structures [15].
Then, we explored the reactivity of the 3-(5-chloro-3-
methyl-1-phenyl-1H-pyrazol-4-yl)-1,5-diarylpentane-1,5-
dione derivatives 4 towards ammonium acetate. Thus, 1,5-
dicarbonyl derivatives 4 (1 mmol) with an excess of
ammonium acetate (1.3 mmol) were exposed to MWI, using
a focused microwave reactor (CEM Discover TM) at 200 °C
with a maximum power of 300W during 2-3 minutes,
without solvent to get the efficient synthesis of substituted
pyridines derivatives 6, that were isolated in good yields by
simple crystallization of the reaction mixture from ethanol
(Scheme 2) [16].

654 Letters in Organic Chemistry, 2011, Vol. 8, No. 9
Trilleras et al.
O
Ar
Ar
H3C
N
N
N
Ph
N
N
5
Cl
Ar
CH3
Ar
X
NH4OAc
MW 2-3 minutes
O
O
4
N
N
Cl
CH3
Ar
N
Ar
6
Scheme 2.
reagents. Methodology is addressed towards green and high-
throughput chemistry.
General Procedure for the Synthesis of 3-(5-chloro-3-
methyl-1-phenyl-1H-pyrazol-4-yl)-1,5-diarylpentane-1,5-
dione 4a-h
Method B: A solution of (E)-3-(5-chloro-3-methyl-1-
ACKNOWLEDGEMENTS
phenyl-1H-pyrazol-4-yl)-1-arylprop-2-en-1-one
3
(1.0
The authors thanks to “Centro de Instrumentación
Científico-Técnica” of Universidad de Jaén and the staff for
data collection, and the Consejería de Innovación, Ciencia y
Empresa (Junta de Andalucía, Spain), Universidad del Valle,
Universidad del Atlántico and COLCIENCIAS for financial
support.
mmol) and respectively acetophenone 2 (2.0 mmol) in
ethanol (10 mL) with an amount of NaOH (1 pellet) was
stirred during two and a half hours at room temperature. The
resulting precipitate was filtered, washed with ethanol, and
recrystallized from ethanol.
General Procedure for the Synthesis of 4-(5-chloro-3-
methyl-1-phenyl-1H-pyrazol-4-yl)-2,6-diarylpyridine 6a-
h
EXPERIMENTAL SECTION
Melting points were determined in a Buchi Melting Point
Apparatus and are reported uncorrected. The H and 13C
1
A mixture of 3-(5-chloro-3-methyl-1-phenyl-1H-pyrazol-
4-yl)-1,5-diarylpentane-1,5-dione 4 (1 mmol), and an excess
of ammonium acetate (1.3 mmol) were subjected to
microwave irradiation (maximum power 300W during 2-3
minutes under solvent-free conditions at a controlled
temperature of 473 K) using a focused microwave reactor
(CEM Discover). The solid products were isolated by simple
crystallization of the reaction mixture from ethanol.
NMR spectra were measured on a Bruker Avance 400
spectrometer operating at 400 and 100 MHz, respectively,
and using DMSO-d6 as solvent and tetramethylsilane as
internal standard. The mass-spectra were scanned on a
Hewlett Packard HP Engine-5989 spectrometer (equipped
with a direct inlet probe) which was operating at 70 eV. The
elemental analyses have been obtained using a LECO
CHNS-900 and Thermo Finnigan FlashEA1112 CHNS-O
(STIUJA) elemental analyzers.
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Solvent-Free Microwave-Assisted Synthesis of Substituted Pyridines
Letters in Organic Chemistry, 2011, Vol. 8, No. 9
655
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methoxyphenyl)prop-2-en-1-one (3f): White solid, 70%. M.p. 98-
1
100 ºC; IR (KBr cm-1): 3058, 1662. NMR H (400 MHz, DMSO-
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Hm, aryl, J = 9.0 Hz), 7.56-759 (7H, m, CH phenyl, Hꢀ, Hꢁ), 8.05
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(OCH3), 113.8 (C4), 114.1 (Cm, aryl), 120.6 (Cꢀ), 125.0 (Cm),
128.9 (Cp), 129.3 (Co), 130.7 (Co, aryl), 131.5 (Cꢁ), 133.0 (Ci,
aryl), 137.2 (Ci), 141.9 (C5), 149.5 (C3), 163.2 (Cp, aryl), 187.0
(C=O). MS IE m/z: 317 (M+ - Cl, 100). Elemental Analysis for
C20H17ClN2O2 C: 68.09, H: 4.86, N: 7.94. Found C: 67.91, H: 4.99,
N: 8.23.
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(a) Selective spectral data for 3-(5-chloro-3-methyl-1-phenyl-1H-
pyrazol-4-yl)-1,5-bis(4-methoxyphenyl)pentane-1,5-dione (4f):
White solid, 40%. M.p. 105-107 °C; IR (KBr cm-1): 3065, 1677.
1
NMR H (400 MHz, DMSO-d6) ꢀ (ppm): 2.22 (3H, s, CH3), 3.42-
3.46 (4H, m, CH2), 3.82 (6H, s, OCH3), 3.91-3.95 (1H, m, CH),
7.01 (4H, d, Ho, aryl, J = 9.0 Hz), 7.37 (1H, t, Hp, J = 7.0 Hz), 7.39
(2H, d, Ho, J = 7.0 Hz), 7.45 (2H, t, Hm, J = 7.3 Hz), 7.93 (4H, d,
Hm, aryl, J = 9.0 Hz). NMR 13C ꢀ (ppm): 12.9 (CH3), 32.1 (CH),
41.7 (CH2), 55.5 (OCH3), 113.8 (Co, aryl), 114.9 (Cm, aryl), 118.5
(C4, hetaryl), 123.6 (C5 hetaryl), 124.4 (Co), 127.8 (Cp), 129.0
(Cm), 130.2 (Ci, aryl), 137.7 (Ci), 148.1 (C3 hetaryl), 163.1 (Cp,
aryl), 196.8 (C=O). MS: (70 eV) m/z = 503/501 (3/7 M+2/M+),
355/353 (13.45/41.15), 317 (18.1), 135 (100). Elemental Analysis
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[16]
Selective spectral data for 4-(5-chloro-3-methyl-1-phenyl-1H-
pyrazol-4-yl)-2,6-bis(4-methoxyphenyl)pyridine (6f): White
1
solid, 90%. M.p. 183-185 ºC; IR (KBr cm-1): 3050, 1596. NMR H
(400 MHz, DMSO-d6, 100 °C) ꢀ (ppm): 2.43 (3H, s, CH3), 3.83
(6H, s, OCH3), 7.09 (4H, d, Ho, aryl, J = 8.8 Hz), 7.52 (1H, t, Hp, J
= 7.0 Hz), 7.58 – 7.64 (4H, m, phenyl), 7.85 (2H, s, CH, pyridine),
8.19 (4H, d, Hm, aryl, J = 8.8 Hz). NMR 13C ꢀ (ppm): 13.2 (CH3),
55.2 (OCH3), 114.1 (Co, aryl), 116.9 (C4, hetaryl), 125.1 (C3
pyridine), 128.1 (Cp), 128.6 (Cm), 129.3 (Ci, aryl), 131.1 (Cm,
aryl), 137.6; 138.0; 140.6; 147.8 (C3 hetaryl), 156.0 (C4, pyridine),
160.3 (C2, pyridine). MS: (70 eV) m/z = 481/482 (34/100,
M+2/M+). Elemental Analysis for C29H24ClN3O2 C: 72.27, H: 5.02,
N: 8.72. Found C: 72.35, H: 4.98, N: 8.68.
[13]