SO4 ?2/SnO2–catalyzed cyclocondensation for the synthesis of fully functionalized pyridines

Article abstract of DOI:10.1002/jhet.3806

An efficient and promising synthetic approach to assemble skeletons of multifunctionalized pyridine derivatives in presence of recyclable heterogeneous sulfated tin oxide (STO) catalyst has been evolved. The STO catalyst was used as a promoter for the cyc

Full text of DOI:10.1002/jhet.3806

Received: 9 May 2019
DOI: 10.1002/jhet.3806
Revised: 11 October 2019
Accepted: 14 October 2019
C O M M U N I C A T I O N
−2
SO4 /SnO2–catalyzed cyclocondensation for the synthesis of
fully functionalized pyridines
Ramesh Goud Koduri1 | Ramakanth Pagadala2
Ravi Varala1
| Sathyanarayana Boodida3 |
1 Department of Chemistry, RGUKT Basar
Abstract
(IIIT Basar), Nirmal, India
An efficient and promising synthetic approach to assemble skeletons of
multifunctionalized pyridine derivatives in presence of recyclable heterogeneous
sulfated tin oxide (STO) catalyst has been evolved. The STO catalyst was used as a
promoter for the cyclocondensation process in ethanol at 70°C. Overall perfor-
mance of this catalyst was attributed to the cooperative contribution of its Lewis
and Brønsted‐Lowry acidic sites. Nanosized STO catalyst was synthesized by
using sol‐gel process and characterized by Fourier transform infrared (FTIR)
spectroscopy, X‐ray diffraction (XRD), 1H‐NMR, and scanning electron micros-
copy (SEM). This catalyst tolerates most of the substrates, and protocol shows
precious capabilities consist of high yields, operational simplicity, less reaction
time, and eco‐friendly conditions. The newly synthesized heterogeneous catalyst
was easily separated and reused. All the reactions are carried out for subsequent
cycles without significant loss of catalytic activity and with good proficiency.
2 Chemistry Division, H&S Department,
CVR College of Engineering,
Ibrahimpatnam, Hyderabad, India
3 Department of Chemistry, JNTUH
College of Engineering, Jagityal, India
Correspondence
Sathyanarayana Boodida, Department of
Chemistry, JNTUH College of
Engineering, Jagityal, India.
Email: bsnarayana77@gmail.com
Ravi Varala, Department of Chemistry,
RGUKT Basar (IIIT Basar), Nirmal, India.
Email: ravivarala@gmail.com
1 | INTRODUCTION
high temperature or microwave support,[12] low yield,
and with toxic solvents.[13] In recent years, several rules
The pyridine ring is a structurally important motif in sev-
eral synthetic compounds because of its metabolic activi-
ties in pharmaceutical interest.[1] Pyridine ring has
noticeable applications in preparation of chiral ligands[2]
and preparation of new materials with electrochemical
and photochemical properties.[3] Natural products like
vitamin B, nicotinamide, and nicotinic acid with pyridine
skeleton plays an important role in biological and meta-
bolic activities.[4,5] 2‐Amino‐3‐cyanopyridine derivatives
have eminent significance for allowing access to many
demonstrated bioactive agents[6] such as novel IKK‐b
inhibitors,[7] A2A adenosine receptor antagonists,[8] and
potent inhibitor of HIV‐1 integrase.[9]
and legislations are made on environmental emission,
considering that efficient catalytical methods are much
desirable. The replacement of homogeneous Lewis acids
by heterogeneous solid super‐acids has predictable advan-
tages like easy separation from the reaction mixture,
noncorrosiveness, low cost, easy maintenance, tolerating
continuous operation, regeneration, and recycling with
the same efficiency.[14,15] Sulfated metal oxides were used
as solid super‐acid catalysts since they had Lewis and
Brønsted acid sites, and they were also nontoxic, atmo-
spherically stable, and thermally stable in nature.[16–19]
Sulfated tin oxide (STO) is used as a heterogeneous
catalyst for the last few years because of its large surface
area.[20,21] It is also found as an efficient catalyst in aldol
condensation, Mukaiyama reactions, benzoylation, and
trans‐esterification of keto esters.[22] Various gas phase
reactions including hydration, dehydration, alkylation,
isomerization, esterification, and polymerization,
Multiple component and one pot reactions have
established extensive attention for the synthesis of hetero-
cyclic compounds.[10] The most common and conven-
tional methods reported for the construction of pyridine
ring involved several steps,[11] long time consumption,
J Heterocyclic Chem. 2019;1–6.
wileyonlinelibrary.com/journal/jhet
© 2019 Wiley Periodicals, Inc.
1

2
KODURI ET AL.
biodiesel production, β‐acetamido ketones, aryl
dibenzo[a.j]xanthenes, 2,4,5‐triaryl‐1H‐imidazole, and
2,4‐diphenyl‐4,6,7,8‐tetrahydro chromen‐5‐one are also
being catalyzed.[23–31] STO has not been explored as a cat-
alyst for the synthesis of multifunctionalized pyridine
derivatives.
The synthesis of fully substituted pyridines is an
important in synthetic organic chemistry because these
compounds find significant attention as powerful inhib-
itors of HIV‐1.[32] The development of a greatly profi-
cient process for the preparation of fully substituted
pyridines is of great attention. Earlier, many researchers
have shown synthetic routes for the preparation of fully
substituted pyridines and reported that most of the
strategies suffer from many important limitations.[33]
In such a scenario, the development of economically
friendly methodology to deliver fully substituted pyri-
dine derivatives will provide a green conventional
approach.
2 | RESULTS AND DISCUSSION
To identify the feasibility of this route, we commenced
our work with the development of a less expensive and
green process for the synthesis of 2‐amino‐3‐
cyanopyridine derivatives with simply accessible starting
materials by using STO catalyst (Scheme 1). We initially
started a model reaction of benzaldehyde, malononitrile,
cycloheptanone, and ammonium hydroxide with differ-
ent catalysts and solvents (Table 1). Keeping in view of
environmental concern, water was used as the solvent
in reaction without and with catalyst, but it was con-
firmed that the reaction did not progress in water
(Table 1, entries 2 and 3). In order to establish the real
effectiveness of the solvent and catalyst for the synthesis
of fully substituted pyridines, a test reaction was per-
formed with and without catalyst using ethanol at high
temperature. It was found that only 25% of product was
obtained in ethanol without catalyst at 80°C even after
8 hours (Table 1, entry 4). To increase the yield of the tar-
get compounds, the same test reaction was performed
with pTSA (Table 1, entry 5), SnO2 (Table 1, entry 6),
and STO (Table 1, entry 7). Significantly, when the reac-
tion was conducted in the presence of STO (10 mg), 92%
Hence, we have chosen to develop multisubstituted
pyridines using STO because of its wide catalytic applica-
tions. STO can be easily regenerated and repeatedly used
for another fresh reaction with the same catalytical
efficiency.
SCHEME 1 Four‐component reaction
for the synthesis of 2‐amino‐3‐
cyanopyridines (6a‐g and 7a‐d). [Color
figure can be viewed at wileyonlinelibrary.
com]
TABLE 1 Screening of cyclocondensation reaction
Entry
Product No.
Catalyst
Amount
Solvent
Temperature, °C
Time, h
Yield,a %
1
2
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
‐
‐
Water
Water
Water
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
Rt
80
80
80
70
70
70
Rt
70
70
8.0
6.0
6.0
8.0
6.0
5.0
2.0
8.0
1.5
1.5
b
‐
‐
b
3
STO
‐
20.0 mg
‐
b
4
25
c
5
PTSA
SnO2
STO
STO
STO
STO
20.0 mg
20.0 mg
10.0 mg
10.0 mg
20.0 mg
30.0 mg
6
40
92
30
98
98
7
8
9
10
aIsolated yields.
bProduct was not found.
cTrace.

KODURI ET AL.
3
of yield was produced. Gratifyingly, the yield of the
desired target compound was further improved up to
98% by conducting the reaction with STO catalyst amount
of 20 mg (Table 1, entry 9).
hydroxide and was followed by intramolecular cycliza-
tion, yielding pyridines. The resulting fully substituted
title compounds were characterized by Fourier transform
1
infrared (FTIR) spectroscopy, H NMR, 13C NMR, and
STO was used as catalyst and ethanol was chosen as
the solvent for this one‐pot cyclocondensation transfor-
mation. After optimizing the conditions, all the reactions
were carried out under identical conditions and were
examined by the reaction of several substituted
arylaldehyde using STO in ethanol at 80°C; the results
are shown in Table 2. The STO could be regenerated
and activated by calcinations at 400°C to 500°C tempera-
ture for 1 hour and reused for the next five cycles without
significant loss of activity (Table 3). A straightforward
and simple catalytic cyclocondensation mechanism for
the pyridine formation is depicted in Scheme 2. STO
catalyst acts as both Lewis and Brownsted acids. In the
presence of STO, Knoevenagel condensation between
the malononitrile and aryl aldehydes generated
arylidenemalononitrile intermediate, which undergo
Michael addition with ketone and form Michael adduct.
This Michael adduct further condensed with ammonium
mass spectrometry. All the synthesized compounds were
further identified by 15N NMR (GHSQC) to confirm the
presence of the
─NH2 group in their structure
(Supporting Information).
The IR spectra of the STO catalyst shows that the pres-
ence of sulfated group appeared at 1381, 1190, 1150, and
1075 cm−1, and the acidic nature of the catalyst was stud-
ied by proton NMR spectra (Supporting Information).
2.1 | Powder XRD
In order to understand the structural properties of STO
sample, X‐ray diffraction (XRD) analyses of STO sample
were carried out in the 20° to 80° range using CuKα radi-
ation. XRD shows a pattern of STO sample that was
found to display well‐distinguished XRD peaks at 26.6°,
33.8°, 37.9°, 51.7°, 54.8°, etc, and they are the diffractions
of (1 1 0), (1 0 1), (2 0 0), and (2 1 1) planes corresponding
to characteristic tetragonal phase of SnO2, which match
well with the reported JCPDS file no. 41‐1445 of SnO2,
confirming its tetragonal nature. This clearly indicates
that the presence of sulfate (SO4−2) and the calcination
temperature also stabilizes the tetragonal phase structure
of SnO2, and such stabilization of SnO2 and other oxides
TABLE 2 Multicomponent reaction for the synthesis of
multisubstituted pyridines (6a‐g and7a‐d)
Entry
Product No.
R
Time, h
Yield,a %
1
6a
6b
6c
6d
6e
6f
H
2.0
2.5
2.0
1.5
2.0
3.0
2.0
3.0
3.5
3.0
3.0
95.0
91.0
92.0
93.0
88.0
85.0
90.0
88.0
80.0
84.0
82.0
−2
2
4‐OCH3
4‐Br
in presence of SO4 has been reported in the litera-
ture.[34,35] The addition of SO4 causes a decrease in
−2
3
crystalline size and prevents their glomeration during
annealing. This can be confirmed by the gradual decrease
in intensity of bands in graph.
4
2‐Cl
5
2‐Br
6
2‐OCH3
4‐N (CH3)2
H
7
6g
7a
7b
7c
7d
2.2 | Scanning electron micrograph
8
9
4‐OCH3
4‐Br
Figure 1 depicts the scanning electron micrograph (SEM)
of STO nanopowder annealed at 600°C that reveals the
presence of irregularly shaped particles. Larger particles
observed in the figure are aggregates of small particles
ranging in the sizes of 100 to 190 nm. Interestingly, the
STO catalyst morphologies as indicated with uniform
size, reflecting the homogeneity in shapes and high
crystallinity.
10
11
2‐Cl
aIsolated yields.
TABLE 3 Recyclability of STO catalyst tested for compound (6a)
Entry
Catalyst
Time, h
Yield,a %
1
2
3
4
5
Fresh
2.0
2.0
2.0
2.0
2.0
98.0
98.0
97.0
97.0
96.0
3 | EXPERIMENTAL
Second run
Third run
Fourth run
Fifth run
3.1 | Procedure for the preparation of STO
catalyst
A total of 22.56 g of stannous chloride was weighed, dis-
solved in 200 mL of distilled water, and a clear solution
aIsolated yields.

4
KODURI ET AL.
SCHEME 2 Plausible reaction
mechanisms for the formation of (6a‐g)
[Color figure can be viewed at
wileyonlinelibrary.com]
FIGURE 1 Scanning electron
micrograph (SEM) images of the STO
catalyst
was prepared. To the clear solution, 25 mL of aqueous
ammonium hydroxide was added drop by drop with con-
tinues stirring magnetically until the pH of the solution
reaches to 8. The obtained yellow precipitate was filtered
and washed well with deionized water. The precipitate
was dried at 120°C for 12 hours in hot air oven to get
17.8 g of stannous hydroxide. Five grams of stannous
hydroxide was mixed with 25 mL of 2N H2SO4 and evap-
orated to dryness on water bath carefully and then
crushed into powder form. The powder was calcinated

KODURI ET AL.
5
at 600°C for 3 hours in a desiccator to get STO and stored
in the closed bottle until it was used.
Jagityal, and RGUKT Basar (IIIT Basar), Nirmal, India,
for support and encouragement during this research
work.
3.2 | Preparation of fully functionalized
pyridine derivatives
ORCID
Ramakanth Pagadala
1846
https://orcid.org/0000-0002-9248-
Benzaldehyde (1 mmol), ethanol (3 mL), malononitrile
(1 mmol), cyclohexanone or methyl ethyl ketone
(2 mmol), and ammonium hydroxide (4 mL) were added
to a round‐bottom flask in the same sequence, refluxed
with a condenser and magnetically stirred. The reaction
mixture was heated at 70°C for the appropriate time men-
tioned in the Table 2. The progress of the reaction was
monitored by TLC (ethylacetate/n‐hexane = 4:6). After
completion of the reaction, the mixture was cooled and
extracted in ethyl acetate, filtered, and washed with
water, and STO was collected. The filtrate was dried over
anhydrous sodium sulfate, and the solvent was
completely removed. The obtained precipitate was recrys-
tallized in ethanol to get more pure aimed compound
(6a). The recycled STO was activated and reused for the
next cycle. Compound 6a; Light yellow solid: mp 226°C
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How to cite this article: Koduri RG, Pagadala R,
Boodida S, Varala R. SO4−2/SnO2–catalyzed
cyclocondensation for the synthesis of fully
functionalized pyridines. J Heterocyclic Chem.
2019;1–6. https://doi.org/10.1002/jhet.3806
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