Silica-supported ceric ammonium nitrate (CAN): a simple, mild and solid-supported reagent for quickest oxidative aromatization of Hantzsch 1,4-dihydropyridines

Article abstract of DOI:10.1007/s11696-018-0666-5

An efficient and environmentally benign methodology for the oxidative aromatization of 1,4-dihydropyridines to their corresponding pyridine derivatives is developed. The oxidative aromatization of 1,4-dihydropyridines was explored using silica-supported ceric ammonium nitrate as catalyst in CH₃CN with or without sonication at room temperature. This supported catalyst acts as a more efficient oxidising reagent and offers several advantages over other reported reagents in terms of reaction time and yields. The supported reagent is found to be more efficient and selective when compared with its unsupported form. The Belousov–Zhabotinskii reaction was not observed in present reaction. The dealkylation observed in case of 4-n-alkyl/n-alkenyl with other oxidising agents is also not observed in the present case.

Full text of DOI:10.1007/s11696-018-0666-5

Chemical Papers
https://doi.org/10.1007/s11696-018-0666-5
ORIGINAL PAPER
Silica‑supported ceric ammonium nitrate (CAN): a simple, mild
and solid‑supported reagent for quickest oxidative aromatization
of Hantzsch 1,4‑dihydropyridines
Parvin Kumar1 · Kulbir Kadyan1 · Meenakshi Duhan1 · Jayant Sindhu2 · Khalid Hussain3 · Sohan Lal1
Received: 31 July 2018 / Accepted: 4 December 2018
© Institute of Chemistry, Slovak Academy of Sciences 2018
Abstract
An efcient and environmentally benign methodology for the oxidative aromatization of 1,4-dihydropyridines to their
corresponding pyridine derivatives is developed. The oxidative aromatization of 1,4-dihydropyridines was explored using
silica-supported ceric ammonium nitrate as catalyst in CH3CN with or without sonication at room temperature. This sup-
ported catalyst acts as a more efcient oxidising reagent and ofers several advantages over other reported reagents in terms
of reaction time and yields. The supported reagent is found to be more efcient and selective when compared with its unsup-
ported form. The Belousov–Zhabotinskii reaction was not observed in present reaction. The dealkylation observed in case
of 4-n-alkyl/n-alkenyl with other oxidising agents is also not observed in the present case.
Keywords Silica supported · CAN · 1,4-DHP · Ultrasonication · Oxidation
Introduction
antitubercular (Wächter et al. 1998), anti-infammatory (Bri-
ukhanov et al. 1994), and agrochemicals such as fungicide
(Hargreaves et al. 2000), pesticide (Wang et al. 2014), and
herbicide (Tawata et al. 2005) are associated with com-
pounds containing 1,4-DHP’s structural unit. The pharma-
cological activities associated with DHP’s derivatives were
attributed to the antioxidant protective efect due to its reac-
tivity with radical species (Mason et al. 1999). 1,4-DHP
systems are oxidatively transformed into their corresponding
pyridine derivatives during the redox processes and in the
course of drug metabolism.
Hantzsch 1,4-dihydropyridines chemistry (Hantzsch 1,4-
DHP) have been extensively expanded in synthesising
NADH mimics, exploring its oxidative reactions and utiliz-
ing it as a substrate in various synthetic reactions. A number
of compounds based on 1,4-DHP motif are used as calcium
entry blockers for the treatment of cardiovascular diseases.
Some of them are Amlodipine, Nifedipine and Niguldipine,
etc. (Boecker and Guengerich 1986). A wide range of phar-
macological properties such as antimalarial (Kumar et al.
2017c), antitumor (Kadayat et al. 2015), antihypertensive
(Loev et al. 1974), antiproliferative (Rassokhina et al. 2016),
Pyridine derivatives can be obtained easily by the oxida-
tion of readily accessible Hantzsch 1,4-DHP’s. Therefore,
organic chemists are involved in oxidation of 1,4-dihydro-
pyridines to develop newer and improved methods such as
microwave irradiation (Vanden Eynde and Mayence 2003),
solvent free (Maheswara et al. 2006), ultrasound irradia-
tion (Kumar et al. 2012) and solid supported (Ghorbani-
Choghamarani et al. 2009; Ghorbani-Choghamarani and
Zeinivand 2010). However, some reported methods suf-
fer from various limitations such as use of strong oxidants
[HNO3 (Boecker and Guengerich 1986), CrO3 (Grinshtein
et al. 1967), KMnO4 (Eynde et al. 1994)] and severe reaction
conditions (Pd/C dehydrogenations) (Kamal et al. 1964).
Although using CAN alone as an oxidant (Pfster 1990)
could ensure high chemical yields, it still presents a series
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-018-0666-5) contains
supplementary material, which is available to authorized users.
* Parvin Kumar
parvinchem@kuk.ac.in; parvinjangra@gmail.com
1
2
3
Department of Chemistry, Kurukshetra University,
Kurukshetra 136119, Haryana, India
Department of Chemistry, K. M. Government College,
Narwana 126116, Haryana, India
Chemistry Department, Mewat Engineering College,
Palla, Nuh 122107, Haryana, India
Vol.:(0123456789)
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Chemical Papers
of drawbacks such as excessive catalyst loading, whereas
with sodium bromate as an co-oxidant, low selectivity in
terms of Belousov–Zhabotinskii reaction (Franck 1978) was
observed. The environmental viability of this strategy was
also limited due to issues related with catalyst recycling and
product separation. Therefore, it is a matter of great interest
to develop alternatives to overcome the drawbacks, which
are more sustainable, for instance through the design of suit-
able supported catalyst (Kumar et al. 2015, 2017a).
in 96% yield. The scope of this reaction was further explored
using diferent solvents, i.e., CH3CN, acetone, CH3COOH,
C2H5OH and CHCl3 (Table 1). It has been revealed from a
set of experiments that silica-supported CAN (containing
1.2 equivalent of CAN) in CH3CN is the optimal reaction
condition for the oxidative aromatisation of 1a (Method A).
The scope of this reaction was also explored by attempting
the similar reaction under ultrasonic irradiation (Method B).
The reaction was found to be complete in 30 s as evident
from TLC and resulted in 100% conversion of 1,4-DHP to
their corresponding pyridine derivative, i.e., 2a (Table 1).
It has been revealed from Table 1 that using silica-sup-
ported CAN (containing 1.2 equivalent of CAN) as cata-
lyst and CH3CN as solvent resulted into rapid and complete
conversion of 1,4-DHP (1a) into their corresponding pyri-
dine derivative (2a). It has been reported that only traces of
expected pyridine derivatives were observed when a com-
bination of CAN (0.1 equivalent) and sodium bromate (2
equivalent) were used as catalyst in aqueous acetone at room
temperature (Pfster 1990). This behaviour was explained
by the occurrence of Belousov–Zhabotinskii reaction in
which acetone would be acting as a reducing agent, forming
bromoacetone, and thereby consuming the oxidants. How-
ever, no change in time and yield of oxidation reaction was
reported when acetone was replaced by acetonitrile. Moreo-
ver, along with small amount of expected pyridine (16%)
two other by-products were also reported. The formation
of these products was also rationalized in terms of Belou-
sov–Zhabotinskii reaction.
As a part of our interest in heterocyclic synthesis (Kumar
et al. 2017b, 2018) and oxidation of 1,4-DHPs, we have
reported a number of reagents for this organic transforma-
tion (Kumar 2009, 2010; Kumar et al. 2012, 2014, 2017d;
Kumar and Kumar 2010). We herein report the use of sil-
ica-supported CAN alone and in the presence of NaBrO3
as a mild and highly efcient catalyst for the oxidation of
1,4-dihydropyridine derivatives to their corresponding pyri-
dine derivatives using CH3CN as solvent with conventional
and non-conventional source of energy, i.e., ultrasonication.
Results and discussion
Chemistry
1,4-DHP required for the accomplishment of our aim of
developing an efcient methodology for their oxidation
was synthesised by well-known Hantzsch-dihydropyridine
synthesis. For this, a three-component reaction of ethyl
acetoacetate (2.00 mmol), benzaldehyde (1.00 mmol) and
ammonium acetate (2.00 mmol) was attempted using EtOH
as solvent under refux for 8 h. The completion of the reac-
tion was monitored using TLC. The reaction mixture was
allowed to stand at room temperature and the solid thus
obtained was fltered and recrystallised from aqueous etha-
nol to yield diethyl 2,6-dimethyl-4-phenyl-1,4-dihydropyri-
dine-3,5-dicarboxylate (1a) in excellent yield (Kumar et al.
2017c).
Keeping in mind the report on oxidation of 1,4-DHP
using CAN and sodium bromate catalytic system. We have
decided to explore the efect of using sodium bromate as co-
oxidant with silica-supported CAN instead of CAN alone.
It was found that exclusively pyridine derivatives were
obtained in excellent yield without any side product when
the same optimized reaction was attempted in acetone. The
reaction was completed in 2 min and resulted in 100% con-
version when explored using silica-supported CAN (0.05
equivalent of CAN) and sodium bromate (2.0 equivalent) in
acetone without the formation of any side product. Similar
results were obtained when the solvent system of the reac-
tion was changed to CH3CN instead of acetone at room tem-
perature (Method A) (Table 1). The same optimized reaction
was also explored using ultrasonic irradiation (Method B). It
has been concluded from above that silica-supported CAN
was used as a catalyst instead of oxidant and also proved
its superiority over CAN due to formation of exclusively
single product (Pfster 1990). The oxidising potential of
this supported catalyst was also compared with some of the
well established methods reported for the oxidation of 1a
(Table 2).
To explore the oxidation of 1,4-DHP using SiO2-CAN, 1a
has been chosen as representative substrate. Initially, a reac-
tion of 1a was attempted using SiO2-CAN (containing 0.6
equivalent of CAN) as catalyst in CH2Cl2 at room tempera-
ture. The progress of the reaction was monitored using TLC
[ethyl acetate: petroleum ether (30:70%, v/v)]. The reaction
was incomplete even after 1 h as evident from TLC and
yields the corresponding diethyl 2,6-dimethyl-4-phenylpyr-
idine-3,5-dicarboxylate (2a) in 66% yield. The scope of this
reaction was explored further by changing the catalyst load-
ing in the reaction. The same reaction was explored using
diferent amount of catalyst (Table 1). It has been observed
that reaction was completed in 10 min when attempted using
SiO2-CAN (containing 1.2 equivalent of CAN) as catalyst in
CH2Cl2 and yield the corresponding pyridine derivative (2a)
The oxidising capabilities of the supported catalyst
were explored further using the differently substituted
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Chemical Papers
Table 1 Optimization of reaction condition for the oxidation of diethyl 2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylate (1a)
S. no.
Solvent
Catalyst
Amount of CAN (mole
Time (min)
Yield (%)
equivalent)
1
2
3
4
5
6
7
8
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
Acetone
CH3COOH
C2H5OH
CHCl3
CH3CN
Acetone
CH3CN
CH3CN
SiO2-CAN
SiO2-CAN
SiO2-CAN
SiO2-CAN
SiO2-CAN
SiO2-CAN
SiO2-CAN
SiO2-CAN
SiO2-CAN
SiO2-CAN
SiO2-CAN/NaBrO3
SiO2-CAN/NaBrO3
SiO2-CAN/NaBrO3
0.6
0.8
1.0
1.2
1.2
1.2
1.2
1.2
1.2
1.2
0.05
0.05
0.05
60
40
20
10
2
6
12
5
10
0.5
2
2
0.5
66a
78a
82
96
100
98
96
98
96
9
10
11
12
13
100b
100
100
100b
aIncomplete reaction
bReaction performed using ultrasonic irradiation
4-alkyl and aryl-1,4-dihydropyridines as substrates.
All the reactions proceeded with less reaction time and
resulted exclusively into the corresponding pyridine
derivatives in good to excellent yields (96–100% yield)
at room temperature (Method A) and also under ultrasonic
irradiation (Method B) (Table 3) (Scheme 1) without the
formation of any dealkylation product.
Characterization of catalyst by scanning electron
microscopy (SEM)
The distribution of elements on the surface was determined
using a scanning electron microscope (SEM, JEOL JSM-
6510LV, SAI labs, Thaper technology campus, Patiala, Pun-
jab, India) coupled with a Princeton Gamma-Tech energy
dispersive X-ray spectrometer (SEM–EDX). The coating
of gold particles on reagent was made before recording the
micrograms. Creating a conductive layer of metal on the
sample improve the imaging of samples and also useful to
prevent any damages by the electron beam.
The plausible mechanism for the oxidation of 1,4-
DHP’s to their corresponding pyridine derivatives is
shown in scheme 2. The oxidative aromatisation of 1a
proceeded with a single electron transfer mechanism
(Yadav et al. 2001).
The results obtained using silica-supported CAN in
CH3CN were also compared with unsupported CAN
(Pfister 1990) to compare the oxidising efficiency of the
supported catalyst. The comparison was done by choos-
ing differently substituted 1,4-DHP as substrates. It has
been observed that silica-supported CAN was proved to
be highly efficient catalyst as compared to unsupported
CAN in terms of catalyst loading, reaction time and yield
of products (Table 4).
Figure 1 shows the SEM image of silica-supported CAN.
The image shows that particles are uniformly distributed
throughout on the silica. EDX analysis was carried out to
confrm the presence of CAN in silica. The weight per-
centage of the elements present was found; N = 5.41%,
O = 55.9%, Si = 27.51%, Ce = 11.12%. The morphology
shows the distribution of CAN over silica. The observed
enhanced reactivity of CAN supported on silica material can
be possibly attributed to the CAN and support interactions,
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Chemical Papers
Table 2 Efciency comparison of various oxidising systems for the oxidation of diethyl 2, 6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicar-
boxylate (1a)
Oxidant
Solvent/reaction conditions
Amount of oxidant Time (min) Yield (%)
(mole equivalent)
CAN (Pfster 1990)
FeCl3 (Xia and Wang 2005)
KMnO4 (Xia and Wang 2005)
Fe(ClO4)3 (Heravi et al. 2005)
HgO/I2 (Montazerozohori et al. 2006)
H2O2 CO(NH2)2/I2 (Filipan-Litvić et al. 2008)
Hydroxytosyloxy iodobenzene (HTIB) (Kumar 2009)
Iodobenzene diacetate (IBD) (Kumar et al. 2012)
Iodobenzene diacetate (IBD) (Kumar et al. 2012)
SeO2 (Cai et al. 2005)
Aq. acetone, r.t.
H2O, refux
H2O, refux
CH3COOH, r.t.
CH2Cl2, r.t.
EtOAc, r.t.
2.0
2.0
2.0
0.2
1.5
2.0
1.1
10
240
180
90
108
60
7
10
60
30
35
100
55
74
93
88
98
90
95
78
96
99
Solvent free, r.t.
CH2Cl2,))))b (ultrasound irradiation) 1.2
CH2Cl2, r.t.
CH3COOH, r.t.
CH2Cl2, r.t.
1.2
1
1.5
Fe(NO3)3 9H2O/SiO2–OSO3H (Ghorbani-Choghamarani
and Zeinivand 2010)
Al(NO3)3.9H2O/SiO2–OSO3H (Ghorbani-Choghamarani
and Zeinivand 2010)
KBrO3/SnCl4 5H2O (Zeynizadeh et al. 2005)
SiO2–HNO3 (Ghorbani-Choghamarani et al. 2009)
BaMnO4 (Memarian et al. 2001)
SiO2-CAN
SiO2-CAN
SiO2-CAN/NaBrO3
SiO2-CAN/NaBrO3
CH2Cl2, r.t.
1.5
13
98
CH3CN, r.t.
CH2Cl2, r.t.
1
40
44
75
2
0.5
2
90
99
94
100
100
100
100
0.15a
2.0
1.2
Benzene, refux
CH3CN, r.t.
CH3CN,))))b (ultrasound irradiation) 1.2
CH3CN, r.t.
0.05
CH3CN,))))b (ultrasound irradiation) 0.05
0.5
a In grams, b)))) → ultrasound irradiation
Table 3 Synthesis of diferently
S. no.
Product
Time (min)
Method Aa
Yield (%)
Method Aa
M. pt.
(oC)
substituted pyridines by
Method Bb
Method Bb
oxidative aromatization of
diferent 1,4-DHPs with or
without sonication in CH3CN
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
2g
2h
2i
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
~ 100
98
~ 100
95
96
98
~ 100
98
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
98
63–64
50–52
110–111
160–162
Oil
170–172
65–67
68–70
9
96
115–117
105–108
136–138
101–103
128–131
105–107
97–99
113–116
100–104
104–107
103–106
Oil
10
11
12
13
14
15
16
17
18
19
20
2j
2k
2l
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
~ 100
97
2m
2n
2o
2p
2q
2r
2s
2t
98
98
97
~ 100
98
98
aStirring at room temperature
bUltrasonic irradiation at room temperature
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Chemical Papers
Scheme 1 Oxidative aromatization of 1,4-DHPs using SiO2-CAN under diferent conditions
Scheme 2 Proposed mechanism for CAN catalysed oxidative aromatization of 1,4-DHPs
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Chemical Papers
Table 4 Comparison of oxidation efciency of silica-supported CAN
system overcomes the limitation of previously reported
CAN system. Therefore, the present methodology could be
a constructive step towards the development of green reac-
tion methodologies in organic synthesis.
over CAN oxidation
Entry Reactant Silica-supported CANa CANb
Time (min) Yield (%) Time (min) Yield (%)
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
2
2
2
2
2
2
2
100
98
100
98
100
98
100
10
12
10
10
14
10
13
100
97
98
96
98
96
98
Experimental
General
All the chemicals used were purchased from Spectrochem,
Avra and Sigma-Aldrich and were used as received. Silica
gel 60 F254 (precoated aluminium plates) from Merck was
used to monitor reaction progress. Melting points were
determined on Buchi Melting Point M-560 apparatus
and found uncorrected. IR (KBr) spectra were recorded
on Perkin Elmer FTIR spectrophotometer and the values
1g
aAll reactions were performed using CH3CN as solvent
bAll reactions were performed using acetone–water as solvent
and the resultant changes in surface properties of the reac-
tive sites.
are expressed as νmax cm−1. The H and 13C spectra were
1
recorded on Bruker top spin and Jeol JNM ECX-400P at
400 MHz and 100 MHz, respectively. The chemical shift
values are recorded on δ scale and the coupling constants (J)
are in Hertz. Sonication was performed in a Rivotak ultra-
sound cleaner with a frequency of 30 kHz through manual
adjustment and an output power of 250 W. The reaction fask
was located at the centre of the cleaner, and the surface of
the reactants was placed slightly lower than the level of the
water. The addition or removal of water controlled the tem-
perature of the water bath. The temperature of the water bath
was controlled at 25–30οC.
Recovery and reuse of silica
One property of the catalyst is reusability of silica. After
reaction, the reaction mixture was fltered. Then residue was
dried to recover silica which was reused for making silica-
supported CAN.
Conclusion
We have developed an efcient method for the oxidation of
1,4-DHPs to their corresponding pyridine derivatives using
silica-supported CAN. The supported catalyst serves various
advantages over other reported oxidising catalyst in terms of
lesser reaction time, formation of exclusively single product
without any side product, i.e., dealkylation at the 4-position
of 1,4-DHPs, mild reaction conditions, ease of preparation
of the catalyst, and ease of work-up. Using only catalytic
amount of SiO2-CAN in the presence of sodium bromate,
only the corresponding pyridine was obtained in high yield
and short reaction time with both conventional and uncon-
ventional methods (A and B). Hence, SiO2-CAN catalytic
Synthesis of substituted 1,4‑dihydropyridines
(1,4‑DHPs): a general procedure
A mixture of aldehyde (1 mmol), ethyl acetoacetate
(2 mmol) and ammonium acetate (2 mmol) was dissolved
in ethanol (30 ml) in a 250-ml round bottom fask. This reac-
tion mixture was refuxed for about 6–8 h as reported in the
literature (Kumar et al. 2017c). Progress of the reaction was
monitored by TLC. After the completion of reaction, the
reaction mixture was cooled to room temperature and the
Fig. 1 SEM images of silica-supported CAN at diferent resolution
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Chemical Papers
solid thus obtained was fltered and purifed further to obtain
dihydropyridine derivatives (1) in good to excellent yield.
General procedure for oxidation of 1,4‑DHPs using
silica gel‑supported ceric ammonium nitrate
(catalytic amount) in the presence of sodium
bromate
Preparation of silica gel‑supported ceric ammonium
nitrate
1,4-DHP derivatives were dissolved in acetonitrile (20 ml)
in a 50-ml round bottomed fask. The content of the reac-
tion was stirred at room temperature for some time till a
clear solution was obtained. Then, catalytic amount of silica-
supported CAN (containing 0.05 equivalent of CAN) and
two equivalent of sodium bromate were added, and reaction
mixture was stirred at room temperature for 2 min to get the
corresponding pyridine derivative. The progress of the reac-
tion was monitored by TLC using ethyl acetate: petroleum
ether (30:70%, v/v). After completion of the reaction, the
reaction mixture was fltered through a sintered glass funnel.
The solvent was then evaporated to get the crude product.
The crude product was then recrystallized with ethanol. The
above reaction was also carried out in the presence of ultra-
sonic irradiation and reaction completes within 30 s only.
Work-up was done by the same procedure given above. All
products are known compounds, and their identity was con-
Silica gel-supported ceric ammonium nitrate was prepared
by the previously reported method (Hwu et al. 2000) using
acetonitrile as solvent. The reaction mixture was then stirred
for 4 h and the evaporation of acetonitrile under reduced
pressure (0.1 Torr) gave a dry yellowish powder, which con-
tained 20% (by weight) of CAN. This reagent was found
active for at least 6 months by storage in a well-capped bot-
tle. The reagent was characterized by SEM analysis.
General procedure for oxidation of 1,4‑DHPs using
silica gel‑supported ceric ammonium nitrate
(method A)
1,4-DHP derivatives were dissolved in acetonitrile (20 ml)
in a 50-ml round bottomed fask. The content of the reac-
tion was stirred at room temperature for some time till a
clear solution was obtained. Then, silica-supported CAN
(containing 1.2 equivalent of CAN) was added and reaction
mixture was stirred at room temperature for the stipulated
period of time as indicated in Table 3. The progress of the
reaction was monitored by TLC using ethyl acetate: petro-
leum ether (30:70%, v/v) as solvent system. After completion
of the reaction, the reaction mixture was fltered through a
sintered glass funnel. The solvent was then evaporated to get
the crude product. The crude product was then recrystallized
with ethanol to aford pyridine derivatives (2) in good to
excellent yield.
frmed by comparison of their spectroscopic data (1H, 13
C
NMR and IR) with previously reported data.
Characterization and spectral data for selected
products
Diethyl‑4‑phenyl‑2,6‑dimethylpyridine‑3,5‑dicarboxy‑
late (2a) IR (KBr) υmax: 3014, 2986, 1723, 1591, 1498,
1
1302, 1250, 1170, 791, 760 cm−1; H-NMR (CDCl3, δ
ppm, 400 MHz) δ 1.06 (t, J=7.40 Hz, 6H, CH3), 4.34 (q,
J = 7.40 Hz, 4H, OCH2), 2.76 (s, 6H, CH3), 6.91(m, 2H),
7.25 (m, 3H); Anal. Calcd. for C19H21NO4: C 69.71; H 6.47;
N 4.28. Found: C 69.83; H 6.38; N 4.32.
General procedure for oxidation of 1,4‑DHPs using
silica gel‑supported ceric ammonium nitrate
(method B)
Diethyl‑4‑(4‑methoxyphenyl)‑2,6‑dimethylpyridine‑3,5‑di‑
carboxylate (2b) IR (KBr) υmax: 3030, 2973, 1729, 1614,
1557, 1291, 1107, 857, 835, 779 cm−1; 1H-NMR (CDCl3,
δ ppm, 400 MHz) δ 1.00 (t, J=7.32 Hz, 6H, CH3), 4.06 (q,
J = 7.32 Hz, 4H, OCH2), 2.59 (s, 6H, CH3), 3.82 (s, 3H,
OCH3), 6.91 (d, J=8.2 Hz, 2H, Ar–H), 7.20 (d, J=8.2 Hz,
2H, Ar–H); Anal. Calcd. for C20H23NO5: C 67.21; H 6.49;
N 3.92. Found: C 67.05; H 6.40; N 3.88.
1,4-DHP derivatives were dissolved in acetonitrile (20 ml)
in a 50-ml round bottomed fask. The content of the reaction
was stirred at room temperature for some time till a clear
solution was obtained. Then silica-supported CAN (con-
taining 1.2 equivalent of CAN) was added and the reaction
mixture was irradiated by ultrasound waves under normal
conditions at room temperature. Sonication was continued
for 30 s and the progress of reaction was monitored by TLC
using ethyl acetate: petroleum ether (30:70%, v/v). After
completion of the reaction, the reaction mixture was fltered
through a sintered glass funnel. The solvent was then evapo-
rated to get the crude product. The crude product was then
recrystallized with ethanol to aford pyridine derivatives (2)
in good to excellent yield.
Diethyl‑4‑(4‑nitrophenyl)‑2,6‑dimethylpyridine‑3,5‑dicarbo‑
xylate (2c) IR (KBr) υmax: 3012, 2977, 1723, 1557, 1518,
1349, 1116, 865, 843, 745 cm−1; 1H-NMR (CDCl3, δ ppm,
400 MHz) δ 1.01 (t, J = 7.18 Hz, 6H, CH3), 2.88 (s, 6H,
CH3), 4.11 (q, J=7.18 Hz, 4H, OCH2), 7.52 (d, J=8.0 Hz,
2H, Ar–H), 8.34 (d, J=8.0 Hz, 2H, Ar–H); Anal. Calcd. for
C19H20N2O6: C 61.29; H 5.41; N 7.53. Found: C 61.31; H
5.36; N 7.50.
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Chemical Papers
Diethyl 2,6‑dimethyl‑4‑cinnamylpyridine‑3,5‑dicarboxylate
(2d) IR (KBr) υmax: 3067, 2966, 2835, 1729, 1588, 1552,
1466, 1240, 1230, 1108, 1035, 962, 835, 739 cm−1; 1H-NMR
(CDCl3, δ ppm, 400 MHz) δ 1.30 (t, J=7.24 Hz, 6H, CH3),
4.40 (q, J=7.24 Hz, 4H, OCH2), 2.83 (s, 6H, CH3), 6.98 (d,
J = 15.8 Hz, 1H); 7.13 (d, J = 15.8 Hz, 1H); 7.25 (m, 2H,
Ar–H), 7.43 (m, 3H,Ar–H); Anal. Calcd. for C21H23NO4:
C 71.37; H 6.56; N 3.96. Found: C 71.41; H 6.66; N 4.11.
Diethyl 4‑(3‑(4‑methylphenyl)‑1‑phenyl‑1H‑pyra‑
zol‑4‑yl)‑2,6‑dimethylpyridine‑3,5‑dicarboxylate (2j) IR
(KBr) υmax: 3067, 3008, 2991, 1741, 1617, 1590, 1445,
851, 719 cm−1, 1H-NMR (CDCl3, δ ppm, 400 MHz): 0.96
(t, 6H), 2.12 (s, 3H), 2.87 (s, 6H), 3.95–4.14 (m, 4H), 7.42
(d, 2H), 7.49–7.57 (m, 3H), 7.74 (d, 2H), 7.91 (d, 2H,), 8.04
(s, 1H); Anal. Calcd for C29H29N3O4: C 72.03; H 6.04; N
8.69. Found: C 71.91; H 5.98; N 8.81.
Diethyl 2,6‑dimethyl‑4‑ethylpyridine‑3,5‑dicarboxylate
Diethyl 4‑(3‑(4‑methoxyphenyl)‑1‑phenyl‑1H‑pyra‑
zol‑4‑yl)‑2,6‑dimethylpyridine‑3,5‑dicarboxylate (2k) IR
(KBr) υmax: 3051, 2994, 1737, 1611, 1588, 1452, 832 cm−1,
1H-NMR (CDCl3, δ ppm, 400 MHz): 1.01 (t, 6H), 2.88 (s,
6H), 3.99 (s, 3H), 4.05–4.19 (m, 4H), 7.12 (d, 2H), 7.28
(m, 1H), 7.425 (m, 2H); 7.75 (m, 2H), 7.80 (m, 2H) 8.02 (s,
1H); Anal. Calcd for C29H29N3O5: C 69.72; H 5.85; N 8.41.
Found: C 69.86; H 5.99; N 8.51.
(2e) IR (KBr) υmax: 2992, 2879, 1731, 1576, 1438, 1286,
1
1112, 1045, 923, 847, 751 cm−1; H-NMR (CDCl3, δ
ppm, 400 MHz) δ 1.28 (t, J = 7.5 Hz, 3H, CH3), 1.44 (t,
J=7.16 Hz, 6H, CH3), 2.80 (q, J=7.5 Hz, 2H, CH2), 2.83
(s, 6H, CH3), 4.50 (q, J=7.16 Hz, 4H, OCH2); Anal. Calcd.
for C15H21NO4: C 64.50; H 7.58; N 5.01. Found: C 64.78;
H 7.78; N 4.94.?
Diethyl 4‑(4‑hydroxyphenyl)‑2,6‑dimethylpyridine‑3,5‑di‑
Diethyl 4‑(3‑(4‑chlorophenyl)‑1‑phenyl‑1H‑pyra‑
zol‑4‑yl)‑2,6‑dimethylpyridine ‑3,5‑dicarboxylate
(2l) IR(KBr) υmax: 3055, 2989, 1747, 1620, 1597, 1461,
carboxylate (2f) IR (KBr) υmax: 3300, 1740, 1540, 1230,
1
1100, 1030 cm−1; H-NMR (CDCl3, δ ppm, 400 MHz) δ
1
1.10 (t, J = 6.8 Hz, 6H, CH3), 2.62 (s, 6H, CH3), 4.12 (q,
J=6.8 Hz, 4H, OCH2), 7.20 (d, J=8.2 Hz, 2H, Ar–H), 7.52
(d, J = 8.2 Hz, 2H, Ar–H), 8.26 (s, OH); Anal. Calcd. for
C19H21NO5: C 66.46; H 6.16; N 4.08. Found: C 66.31; H
6.10; N 4.14.
1322, 1087, 1002, 952, 850, 836, 698 cm−1, H-NMR
(CDCl3, δ ppm, 400 MHz): 0.97 (t, 6H), 2.87 (s, 6H), 4.03–
4.17 (m, 4H), 7.32 (m, 2H); 7.46–7.53 (m, 5H), 7.74 (d, 2H),
7.98 (s, 1H); Anal. Calcd. for C28H26ClN3O4: C 66.73; H
5.20; N 8.34. Found: C 61.66; H 5.29; N 8.26.
Diethyl 4‑(4‑chlorophenyl)‑2,6‑dimethylpyridine‑3,5‑di‑
Diethyl 4‑(2‑chloroquinolin‑3‑yl)‑2,6‑dimethylpyri‑
dine‑3,5‑dicarboxylate (2m) IR (KBr) υmax: 1720 cm−1
(− COOEt); 1H-NMR (CDCl3, δ ppm, 400 MHz): 8.08 (s,
1H, C4′–H), 7.97 (dd, Jo=7.2 Hz, Jm=2.4 Hz, 1H, C8′–H),
7.84 (m, 2H, C5′ and C7′–H), 7.65 (ddd, Jo = 6.9, 5.7 Hz,
Jm=2.4 Hz, 1H, C6′–H), 4.06 (q, J=7.2 Hz, 4H, −OCH2–
CH3), 2.79 (s, 6H, C2and C6–CH3), 0.83 (t, J=7.2 Hz, 6H,
− CH2–CH3); Anal. Calcd. for C22H21ClN2O4: C = 64.00,
H=5.13, Cl=8.59, N=6.79, O=15.50; found C=64.02,
H=5.09, Cl=8.59, N=6.81, O=15.48.
carboxylate (2g) IR (KBr) υmax: 3025, 2984, 1729, 1580,
1
1231, 1104, 1044, 858, 658 cm−1; H-NMR (CDCl3,
δ ppm, 400 MHz) δ 1.01 (t, J = 7.28 Hz, 6H, CH3), 4.12
(q, J = 7.28 Hz, 4H, OCH2), 2.87 (s, 6H, CH3), 7.24 (d,
J = 8.62 Hz, 2H, Ar–H), 7.47 (d, J = 8.62 Hz, 2H, Ar–H);
Anal. Calcd. for C19H20ClNO4: C 63.07; H 5.57; N 3.87.
Found: C 62.97; H 5.44; N 4.03.
Diethyl 4‑(3‑hydroxyphenyl)‑2,6‑dimethylpyridine‑3,5‑dicar‑
boxylate (2h) IR (KBr) υmax: 3300, 1735, 1540, 1230, 1100,
1040 cm−1; 1H-NMR (CDCl3, δ ppm, 400 MHz) δ 0.91 (t,
J=7.2 Hz, 6H, CH3), 2.86 (s, 6H, CH3), 4.06 (q, J=7.2 Hz,
4H, OCH2), 7.26–7.50 (m, 4H, Ar–H), 4.19 (s, OH); Anal.
Calcd. for C19H21NO5: C 66.46; H 6.16; N 4.08. Found: C
66.29; H 6.14; N 4.10.
Diethyl 4‑(2‑chloro‑6′‑methylquinolin‑3‑yl)‑2,6‑dimethylp
yridine‑3,5‑dicarboxylate (2n) IR (KBr) υmax: 1728 cm−1
(− COOEt): 1H-NMR (CDCl3, δ ppm, 400 MHz): 7.99–7.63
(m, 4H, C4′,5′,7′,8′–H), 4.23–3.89 (q, 4H, − OCH2–CH3), 2.90
(s, 6H, C2 and C6–CH3), 2.58 (s, 3H, C6′–CH3), 1.97-1.81 (t,
6H, − CH2–CH3); Anal. Calcd. for C23H23ClN2O4: 64.71,
H = 5.43, Cl = 8.30,N = 6.56, O = 14.99; found C = 64.73,
H=5.41, Cl=8.27, N=6.59, O=14.98.
Diethyl 4‑(1,3‑diphenyl‑1H‑pyrazol‑4‑yl)‑2,6‑dimethylpyr‑
idine‑3,5‑dicarboxylate (2i) IR (KBr) υmax: 3055, 3033,
1
2989, 1737, 1618, 1591, 1470, 1212, 747 cm−1, H-NMR
(CDCl3, δ ppm, 400 MHz) 0.92 (t, 6H), 2.61 (s, 6H), 3.90–
4.07 (m, 4H), 7.29 (m, 2H), 7.31 (m, 2H), 7.47 (m, 2H),
7.53 (m, 2H), 7.75 (m, 2H), 7.92 (s, 1H); Anal. Calcd. for
C28H27N3O4: C 71.62; H 5.80; N 8.95. Found: C 71.77; H
5.98; N 9.09.
Diethyl 4‑(2‑chloro‑7′‑methylquinolin‑3‑yl)‑2,6‑dim
ethylpyridine‑3,5‑dicarboxylate (2o) IR (KBr) υmax
:
1720 cm−1(− COOEt); 1H-NMR (CDCl3, δ ppm, 400 MHz):
7.91 (s, 1H, C4′–H), 7.84 (d, Jm=2.4 Hz, 1H, C8′–H), 7.71 (d,
J=8.1 Hz, 1H, C5′–H), 7.45 (dd, J=8.1 Hz, Jm=2.4 Hz, 1H,
C6′–H), 4.16 (q, J=6.9 Hz, 4H, − OCH2–CH3), 2.71 (s, 6H,
1 3

Chemical Papers
C2 and C6–CH3), 2.61 (s, 3H, C7′–CH3), 0.89 (t, J=8.4 Hz,
6H, − CH2–CH3); Anal. Calcd. for C23H23ClN2O4:
C=64.71, H=5.43, Cl=8.30, N=6.56, O=14.99; found
C=64.68, H=5.45, Cl=8.26, N=6.54, O=14.99.
N = 4.10, O = 18.75; found C = 70.36, H = 6.79, N = 4.10,
O=18.75.
Acknowledgements Kulbir Kadyan thanks UGC for providing junior
research fellowship (JRF).
Diethyl 4‑(2‑chloro‑8‑methylquinolin‑3‑yl)‑2,6‑dimeth‑
ylpyridine‑3,5‑dicarboxylate (2p): IR (KBr) υmax
:
Compliance with ethical standards
1720 cm−1(− COOEt); 1H-NMR (CDCl3, δ ppm, 400 MHz):
7.93 (s, 1H, C4′–H), 7.65 (m, 2H, C5′ and C7′–H), 7.51 (dd,
J=7.8 Hz and 7.2 Hz, 1H, C6′–H), 4.12 (q, J=7.2 Hz, 4H,
-OCH2-CH3), 2.93 (s, 3H, C8′–CH3), 2.80 (s, 6H, C2and C6–
CH3), 0.89 (t, J = 6.6 Hz, 6H, − CH2–CH3); Anal. Calcd.
for C23H23ClN2O4: C=64.71, H=5.43, Cl=8.30, N=6.56,
O=14.99; found C=64.71, H=5.46, Cl=8.27, N=6.56,
O=14.98.
Conflict of interest On behalf of all authors, the corresponding author
states that there is no confict of interest.
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