One-pot, three component tandem reaction of 2-aminopyridines, acetophenones and aldehydes: Synthesis of 3-aroylimidazo[1,2-a]pyridines

Article abstract of DOI:10.1039/c4ra13056a

A facile synthesis of 3-aroylimidazo[1,2-a]pyridine derivatives has been achieved through the one-pot, three-component tandem reaction of acetophenones, arylaldehydes and 2-aminopyridines in the presence of a catalytic amount of copper(ii) chloride and air as the sole oxidant. The developed one-pot method is atom-economical and utilizes readily available precursors to offer highly functionalized N-fused imidazoles in moderate to good yields (26-82%). The presented tandem process is expected to proceed via crossed aldol condensation, Michael addition, copper catalyzed oxidative cyclization and subsequent aromatization.

Full text of DOI:10.1039/c4ra13056a

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PAPER
One-pot, Three Component Tandem Reaction of 2-Aminopyridines,
Acetophenones and Aldehydes: Synthesis of 3-Aroylimidazo[1,2-
a]pyridines
a
a
a
a
Pinku Kaswan, Kasiviswanadharaju Pericherla, Hitesh Kumar Saini, and Anil Kumar*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A facile synthesis of 3-aroylimidazo[1,2-a]pyridine derivatives
has been achieved through the one-pot, three-component
tandem reaction of acetophenones, arylaldehydes and 2-
aminopyridines in the presence of catalytic amount of
copper(II) chloride and air as a sole oxidant. The developed
one-pot method is atom-economical and utilizes readily
available precursors to offer highly functionalized N-fused
imidazoles in moderate to good yields (26-82%). The
presented tandem process is expected to proceed via crossed
aldol condensation, Michael addition, copper catalyzed
oxidative cyclization and subsequent aromatization.
desirable.
1
0
1
5
Introduction
Construction of bioactive fused heterocycles by exploiting
transition metal catalyzed coupling reactions is highly
challenging and attractive task in organic synthesis.
Multicomponent reactions (MCRs) together with tandem
sequences have been recognized as a powerful tool in modern
5
0
Fig. 1 Bioꢀactive molecules containing imidazo[1,2ꢀa]pyridine skeleton
2
2
3
3
4
4
0
5
0
5
0
5
1
2ꢀAminopyridine is the most common precursor for the syntheses
of imidazo[1,2ꢀa]pyridines. Recently, some interesting results
have been obtained when 2ꢀaminopyridine was reacted with
organic chemistry for the synthesis of fused heterocycles with
carbonyl compounds in the presence of copper catalysts (Scheme
2
8
diverse substitutions. The basic concepts of disconnections are 55 1). Several groups including ours have independently shown that
generally overruled for the synthesis of molecules following the
combination of MCRs and tandem processes. Several innovative
strategies have been witnessed in last decade where coupling
2ꢀarylimidazo[1,2ꢀa]pyridines are obtained by the reaction of 2ꢀ
aminopyridine with methyl ketones in presence of catalytic
amount of copper via imine formation followed by intramolecular
3
9
reactions have been amalgamated with MCRs/tandem reactions.
oxidative cyclization (Scheme 1) and Su group reported that 2ꢀ
In recent years, synthesis of imidazo[1,2ꢀa]pyridines have 60 alkenylimidazo[1,2ꢀa]pyridines were obtained when 2ꢀ
gained great interest because of their significance in medicinal
aminopyridine was reacted with methyl vinyl ketones in the
4
10
chemistry, material science and organometallics. Several drugs
presence of copper (Scheme 1).
11
such as alpidem, zolpidem, zolimidine, saripidem, and necopidem
Very recently, Hajra and our group independently reported
the copper catalyzed oxidative cyclization of chalcones and 2ꢀ
(Figure 1) contain imidazo[1,2ꢀa]pyridine skeleton as a core with
slight variations in substitutions. In addition, several novel 65 aminopyridines for the synthesis of 3ꢀaroylimidazo[1,2ꢀ
molecules with imidazo[1,2ꢀa]pyridine as a key structure have
a]pyridines. However, to the best of our knowledge there is no
report available for the synthesis of 3ꢀaroylimidazo[1,2ꢀ
a]pyridines via oneꢀpot, threeꢀcomponent tandem approach. With
our continuous interest in synthesis and functionalizations of
been synthesized and studied for their activity against various
5
biological targets. Among them, 3ꢀaroylimidazo[1,2ꢀa]pyridines
6
are the interesting structures with anticancer activities (Figure 1).
9b, 12
It has been reported that direct Cꢀ3 aroylation of imidazo[1,2ꢀ 70 imidazo[1,2ꢀa]pyridines,
herein we wish to report an efficient
a]pyridine nucleus is unsuccessful and desired 3ꢀ
aroylimidazo[1,2ꢀa]pyridines were obtained in fourꢀsteps from 2ꢀ
synthesis of 3ꢀaroylimidazo[1,2ꢀa]pyridines via oneꢀpot threeꢀ
component copper catalyzed tandem reaction of acetophenones,
arylaldehydes and 2ꢀaminopyridines with air as a sole oxidant
(Scheme 1).
6a
aminopyridine. Aroyl functionality has also been reported to be
highly responsible for the elevated biological applications of
7
various other heterocycles. In this context, direct methods
75
toward the synthesis of 3ꢀaroylimidazo[1,2ꢀa]pyridines is highly
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product 6a could be reduced if rate of reaction between 1a and 2a
to give 7a is faster than the rate of reaction between 1a and 3a.
We succeeded in eliminating formation of 6a in the reaction by
keeping 1a as a limiting agent with other substrates (2a and 3a) in
3
4
4
5
5
5
0
5
0
5
1
.2 equivalents. Addition of K CO to the reaction keeping other
2 3
parameters same resulted in slight improvement in the yield of 4a
(entry 2, Table 1). A smooth enhancement in yield of 4a was
observed with DMF as solvent (entry 3, Table 1). With the
encouraging result in hand, we next increased the reaction
temperature to 150 °C keeping other parameters constant.
Gratifyingly, excellent yield of 4a was observed at 150 °C (entry
4, Table 1). Among other solvents, good yields of 4a were
obtained in polar solvents like DMA and DMSO (entries 6ꢀ7,
Table 1) while moderate yields of 4a was obtained in water (entry
5
, Table 1). Desired product 4a was not obtained when 1,4ꢀ
Scheme 1 Reaction of 2ꢀaminopyridines with various carbonyl partners
in the presence of copper
dioxane and ethanol were used as solvents (entries 9ꢀ10, Table 1).
Simultaneously, various bases were screened for the model
Results and Discussion
reaction and the use of K
CO gave the optimum result for the
2 3
tandem process among other bases like Cs CO , K PO and KOH
2
3
3
4
5
0
5
0
5
In our initial study, acetophenone (1a), benzaldehyde (2a) and 2ꢀ
aminopyridine (3a) were chosen as model substrates for the
screening of reaction conditions. Based on literature survey,
several competing reactions could be expected with the present
set of substrates and other parameters which includes 1,3ꢀ
which also offered good yields of tandem products (entries 10
and 12ꢀ13, Table 1). However, no product was noticed in case of
NaHCO (entry 11, Table 1). Attempts were failed to replace the
3
copper catalyst to facilitate higher yields of tandem products
(entries 14ꢀ17, Table 1).
11
1
1
2
2
diphenylꢀ3ꢀ(pyridinꢀ2ꢀylamino)propanꢀ1ꢀone
(5a),
chalcone (7a), 2,4ꢀ
(Scheme 2). With these
2ꢀ
a
9
b
Table 1 Optimization of reaction conditions
phenylimidazo[1,2ꢀa]pyridine (6a),
1
3
diphenylꢀ1,8ꢀnaphthyridine (8a)
concerns in mind, 1a (1.0 mmol) was treated with 2a (1.0 mmol)
and 3a (1.0 mmol) in presence of CuCl .2H O in toluene for 12 h
2
2
at reflux. As envisaged, moderate yields of expected product,
phenyl(2ꢀphenylimidazo[1,2ꢀa]pyridinꢀ3ꢀyl)methanone (4a) was
obtained along with other byꢀproducts such as 6a and 7a in
minor quantities (entry 1, Table 1). However, Michael adduct
b
entry
catalyst
base
ꢀ
solvent
toluene
toluene
DMF
DMF
water
DMA
DMSO
1,4ꢀdioxane
EtOH
DMF
temp (°C) yield (%)
c
1
2
3
4
5
6
7
8
9
CuCl
CuCl
CuCl
2
2
2
.2H
.2H
.2H
2
2
2
O
O
O
120
120
120
150
reflux
150
35
38
45
81
55
72
63
K
K
2
2
CO
CO
3
(
5a) and naphthyridine (8a) were not detected in the reaction
3
mass. The structure of 4a was characterized by spectral data (IR,
MS, and NMR). In the IR spectrum of 4a, a strong peak appeared
CuCl
2
.2H
2
O
O
O
O
O
O
O
O
O
O
K
2
CO
3
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
2
2
2
2
2
2
2
2
2
.2H
.2H
.2H
.2H
.2H
.2H
.2H
.2H
.2H
2
K
K
2
K
2
K
2
2
CO
CO
CO
CO
CO
3
ꢀ
1
1
at 1597 cm for C=O stretching. In the H NMR spectrum of 4a,
2
2
2
2
2
2
2
2
3
3
3
3
150
a doublet appeared at δ 9.60 ppm for highly deshielded C ꢀH
5
d,e
reflux
reflux
150
ꢀ
ꢀ
along with other protons at their respective positions. The ketonic
d,e
K
2
carbon of 4a appeared at δ 187.34 along with all other expected
1
0
Cs
2
CO
3
52
13
d,e
carbons in the C NMR spectrum. The peak at 299.1162 for
11
NaHCO
KOH
3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
150
ꢀ
+
1
1
1
1
1
1
2
3
4
5
6
7
150
150
150
150
46
64
38
66
[
M+H] ion in the HRMS mass spectrum of 4a further confirmed
K
3
PO
4
its structure.
Cu(OAc)
2
.H
2
O
K
K
2
K
2
K
2
2
CO
CO
CO
CO
3
3
3
3
Cu(OTf)
CuBr
CuI
2
d,e
150
ꢀ
150
48
a
60
Reagents and conditions: 1a (1.0 mmol), 2a (1.2 mmol), 3a (1.2 mmol),
b
catalyst (20 mol %), base (2.0 mmol), solvent (4 mL), 12 h, air. Isolated
yields. No base was present in the reaction and 1a, 2a, and 3a were used
1
c
d
e
mmol each. Starting materials were recovered. 4a not detected.
65
With the optimized conditions in hand (entry 4, Table 1),
substrate scope for cascade process was evaluated and the results
are summarized in Table 2. In early experiments, diversely
substituted 2ꢀaminopyridines were tested which offered moderate
to good yields of 3ꢀaroylimidazo[1,2ꢀa]pyridines (entries 2ꢀ4 and
30
Scheme 2 Competing reactions with selected set of precursors
To enhance the yields of tandem product, we turned our focus 70 15, Table 2). It is worth to mention that highly sensitive bromo
on minimization of unwanted products by varying the molar
ratios of precursors. It was observed that the formation of byꢀ
substitution, towards transition metal catalyzed transformations
was well tolerated under the optimized conditions and afforded
2
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good yields of tandem product (entry 15, Table 1). Aryl
aldehydes substituted with electronꢀwithdrawing groups such as
ꢀCl, 2ꢀF and 4ꢀNO2 offered high yields of tandem products
entries 7ꢀ8 and 10ꢀ16, Table 2). On the other hand, reactions
involving aryl aldehydes with electron rich substituents produced
4
(
5
0
5
0
5
0
5
0
5
2
1
ꢀarylimidazo[1,2ꢀa]pyridines instead of tandem product (entry
7, Table 2). This may be due to the fact that under these reaction
conditions, reaction of acetophenones and 2ꢀaminopyridines is
faster as compared to the tandem reaction to give corresponding
9
1
1
2
2
3
3
4
4
2ꢀarylimidazo[1,2ꢀa]pyridines. To our delight, heterocyclic
ketones and aldehydes such as 2ꢀacetylthiophene and thiopheneꢀ
2
ꢀcarbaldehyde reacted smoothly under the optimized conditions
to give the corresponding tandem products in moderate yields
(entries 6 and 9, Table 2).
Next, we performed some control experiments to understand
the synthetic sequence and plausible mechanism of the reported
protocol (Scheme 3). In the absence of copper catalyst, no
product formation was observed (eq. 1, Scheme 3) which
confirmed the crucial role of copper catalyst for the successful
Scheme 3 Control experiments
3a, 11a
Based on literature reports
and findings from the control
formation of 4. When the model reaction was performed under 50 experiments, the mechanism of the tandem process for the
nitrogen atmosphere, only catalytic amount of product was
formed which confirms the necessity of aerobic conditions for the
current transformation (eq. 2, Scheme 3). Reaction performed in
the presence of radical scavenger, (2,2,6,6ꢀtetramethylpiperdinꢀ1ꢀ
synthesis of 3ꢀaroylimidazo[1,2ꢀa]pyridines is proposed as shown
in Scheme 4. It is believed that initially chalcone (7) is generated
by the crossed aldol condensation of 1 and 2 which then
undergoes Michael addition with 3 to afford intermediate 5.
yl)oxy (TEMPO), produced both tandem product 4a and 6a in 55 Interaction of 5 with copper salt through pyridinium nitrogen and
1
1a
4
6% and 24% isolated yields, respectively (eq. 3, Scheme 3). It
enolic carbon simultaneously affords intermediate 9. Oxidation
9a, 14
was concluded that both 4a and 6a are formed via a nonꢀradical
of copper (II) to copper (III)
gives intermediate 10, which on
9
pathway as reported earlier for 6a. Two synthetic routes could be
reductive elimination may result in the formation dihydro
imidazopyridine 11. Rapid aromatization of intermediate 11
under aerobic conditions affords the desired product 4. Oxidation
of Cu(I) to Cu(II) in the presence of air completes the catalytic
possible to attain the tandem product 4 from corresponding
substrates via chalcone intermediate and b) via imine
intermediate. However, formation of imine was not observed
from the reaction of 2a with 3a in the absence of 1a under these
conditions (eq. 5, Scheme 3). This excludes the possibility of
imine as intermediate in this tandem process. In other experiment
1a and 2a were reacted under the same standard reaction
condition to result chalcone (7a) which confirm the given
reaction path (eq. 6, Scheme 3). Thus, it is believed that the
probable pathway for this transformation is via chalcone
intermediate which is formed through the crossed aldol
condensation between 1a and 2a. This was further supported by
the fact that reaction of preꢀsynthesized chalcone 7a with 3a
under similar conditions afforded good yield of tandem product
6
0
15
cycle. Reaction of 2ꢀaminopyridine (3) with methylketones (1)
in presence of copper results in the formation of 2ꢀ
9
arylimidazo[1,2ꢀa]pyridines (6).
4
4
a (eq. 4, Scheme 3). It is important to mention that the yield of
a was higher from tandem reaction than that of stepꢀwise
65
approach under these conditions.
Scheme 4 Plausible mechanism for the synthesis of 3ꢀaroylimdazo[1,2ꢀ
a]pyridines
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a
Table 2 Substrate scope for oneꢀpot, threeꢀcomponent tandem reaction for the synthesis of 3ꢀaroylimidazo[1,2ꢀa]pyridines
1
2
b
Entry
Ar
Ar
R
H
Product (4)
Yield (%)
1
.
C
6
H
5
5
C
6
H
5
5
81
2
.
C
6
H
C
6
H
3ꢀMe
62
3
4
.
.
C
C
6
6
H
5
5
C
C
6
6
H
5
5
4ꢀMe
5ꢀMe
46
63
H
H
5
.
3,4ꢀ(OMe)
2
C
6
H
3
C
C
6
6
H
5
5
H
47
6
7
8
9
1
.
2ꢀthienyl
H
H
H
36
52
76
46
82
.
C
C
C
C
6
6
6
6
H
5
H
5
H
5
H
5
4ꢀClC
6
H
4
.
4ꢀNO
2
C
6
H
4
H
.
2ꢀthienyl
H
0.
4ꢀClC
4ꢀClC
6
6
H
4
4
5ꢀMe
1
1.
4ꢀClC
6
H
4
H
H
63
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1
1
2.
3.
4ꢀClC
4ꢀClC
6
H
4
4ꢀClC
6
H
4
5ꢀMe
75
45
6
H
4
2ꢀFC
2ꢀFC
2ꢀFC
6
H
4
H
1
1
4.
5.
4ꢀClC
4ꢀClC
6
H
4
6
H
4
4ꢀMe
5ꢀBr
42
6
H
4
6
H
4
51
44
1
1
6.
7.
4ꢀOMeC
6
H
4
4ꢀNO
2
C
6
H
4
H
H
c,d
4ꢀMeC
6
H
4
4ꢀMeC
6
H
4
ꢀ
a
b
Reagents and conditions: 1a (1.0 mmol), 2a (1.2 mmol), 3a (1.2 mmol), CuCl
Isolated yields. 4q was not formed. 2ꢀ(pꢀTolyl)imidazo[1,2ꢀa]pyridine was isolated in 56% yield.
2
.2H
2
O (20 mol %), K
2
CO
3
(2.0 mmol), DMF (4 mL), 150 °C, 12 h, air.
c
d
chromatography (EtOAc: Hexanes) to afford 4a in 81% (241 mg)
yield.
Experimental Section
30
General:
Phenyl(2-phenylimidazo[1,2-a]pyridin-3-yl)methanone (4a):
Yield 81%; Colorless solid; mp 124ꢀ127 °C; H NMR (300 MHz,
1
5
0
5
Melting points were determined in open capillary tubes on a EZꢀ
Melt Automated melting point apparatus and are uncorrected.
Reactions were monitored by using thin layer chromatography
CDCl ) δ 9.60 (d, J = 7.0 Hz, 1H), 7.86 (d, J = 8.9 Hz, 1H), 7.64
3
–
7.49 (m, 3H), 7.37 (dd, J = 7.8, 1.4 Hz, 2H), 7.35 – 7.30 (m,
13
3
5
1H), 7.22 – 7.08 (m, 6H); C NMR (75 MHz, CDCl ) δ 187.3,
3
(TLC) on 0.2 mm silica gel F254 plates (Merck). The chemical
1
55.0, 147.4, 138.7, 134.0, 131.7, 130.2, 129.5, 129.2, 128.3,
structures of final products were determined by their NMR
1
13
128.2, 127.7, 120.0, 117.5, 114.6; IR (KBr): 3070, 1597, 1388,
1
spectra ( H and C NMR). Chemical shifts are reported in parts
per million (ppm) using deuterated solvent peak or
tetramethylsilane as an internal standard. The HRMS data were
recorded on a mass spectrometer with electrospray ionization and
TOF mass analyzer. IR spectra were recorded on a FTIR
ꢀ
1
1
326, 1218 cm ; HRMS (ESI) calcd for C H N O 299.1179
20 15 2
+
found 299.1162 [M+H] .
40
(8-Methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)(phenyl)
methanone (4b): Yield 62%; Colorless solid; mp 140ꢀ143 °C; H
1
ꢀ
1
1
spectrophotometer and the νmax is expressed in cm . All
chemicals were obtained from the commercial suppliers and used
without further purification.
NMR (400 MHz, CDCl ) δ 9.43 (d, J = 4.9 Hz, 1H), 7.51 (d, J =
3
6
7
.5 Hz, 2H), 7.38 – 7.32 (m, 3H), 7.30 – 7.22 (m, 1H), 7.15 –
.06 (m, 5H), 7.04 – 6.98 (m, 1H), 2.75 (s, 3H); C NMR (101
13
4
5
MHz, CDCl ) δ 187.5, 154.6, 147.7, 138.9, 134.4, 131.7, 130.4,
3
General procedure for synthesis of 4a:
1
29.6, 128.2, 127.8, 127.8, 127.6, 126.1, 120.6, 114.7, 17.2; IR
ꢀ
1
An ovenꢀdried 10 mL RB flask was charged with acetophenone
(120 mg, 1.0 mmol), benzaldehyde (122 mg, 1.2 mmol), 2ꢀ
aminopyridine (113 mg, 1.2 mmol), K CO (276 mg, 2.0 mmol),
(KBr): 3063, 1606, 1466, 1388, 1250, 910, 702 cm ; HRMS
+
2
0
(ESI) calcd for C H N O 313.1335 found 313.1352 [M + H] .
21
17
2
2
3
(
7-Methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)(phenyl)
CuCl .2H O (34 mg, 20 mol %) and DMF (4 mL). The resulting
2
2
1
50
methanone (4c): Yield 26%; Offꢀwhite solid; mp 137ꢀ140 °C; H
solution was stirred at 150 °C for 12 h. On completion of the
reaction, the reaction mass was allowed to cool to ambient
temperature and then diluted with water (10 mL). The mixture
was extracted with EtOAc (2 × 10 mL). The combined organic
layer was dried over anhydrous Na SO and evaporated to
NMR (400 MHz, CDCl ) δ 9.52 (d, J = 3.0 Hz, 1H), 7.63 (s, 1H),
3
7
5
.56 (d, J = 6.2 Hz, 2H), 7.41 – 7.28 (m, 3H), 7.20 – 7.11 (m,
H), 7.01 (d, J = 0.9 Hz, 1H), 2.59 (s, 3H); C NMR (101 MHz,
2
5
13
CDCl ) δ 187.1, 155.4, 147.9, 140.9, 138.8, 134.2, 131.6, 130.2,
3
2
4
55
129.1, 128.2, 127.7, 127.7, 127.5, 117.1, 116.1, 21.6; IR (KBr):
dryness. The crude residue so obtained was purified by column
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ꢀ
1
3
060, 1605, 1466, 1396, 918, 694 cm ; HRMS (ESI) calcd for 55 (2-Phenylimidazo[1,2-a]pyridin-3-yl)(thiophen-2-yl)
+
1
C H N O 313.1335 found 313.1321 [M + H] .
methanone (4i): Yield 46%; Offꢀwhite solid; mp 118ꢀ120 °C; H
NMR (400 MHz, CDCl ) δ 9.30 (d, J = 7.0 Hz, 1H), 7.70 (d, J =
2
1
17
2
3
(
6-Methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)(phenyl)
methanone (4d): Yield 63%; Colorless solid; mp 156ꢀ158 °C H
NMR (400 MHz, CDCl ) δ 9.38 (s, 1H), 7.71 (d, J = 8.9 Hz, 1H),
.51 (d, J = 7.2 Hz, 2H), 7.38 (d, J = 8.9 Hz, 1H), 7.31 (d, J = 6.7
Hz, 2H), 7.29 – 7.22 (m, 1H), 7.14 – 7.04 (m, 5H), 2.45 (s, 3H);
C NMR (101 MHz, CDCl ) δ 187.4, 154.9, 146.5, 138.8, 134.2,
32.2, 131.7, 130.2, 129.6, 128.2, 127.8, 127.8, 126.2, 124.6,
119.9, 116.7, 18.6; IR (KBr): 3063, 1605, 1466, 1389, 903, 733
9
7
.0 Hz, 1H), 7.62 – 7.57 (m, 2H), 7.46 – 7.40 (m, 1H), 7.37 –
.28 (m, 1H), 7.21 – 7.19 (m, 1H), 7.18 – 7.16 (s, 1H), 7.17 –
1
5
3
6
0
7.15 (m, 1H), 6.98 (td, J = 6.9, 1.2 Hz, 1H), 6.63 – 6.58 (m, 1H),
7
13
6
1
1
1
.56 – 6.54 (m, 1H); C NMR (101 MHz, CDCl ) δ 187.1, 147.5,
3
47.2, 138.6, 135.8, 132.3, 130.1, 129.5, 129.3, 128.2, 128.0,
27.7, 127.2, 117.3, 114.6, 100.0; IR (KBr): 3078, 2947, 1603,
1
3
3
1
ꢀ
1
466, 1389, 1227, 748, 756 cm ; HRMS (ESI) calcd for
10
+
65
C H N OS 305.0743 found 305.0746 [M + H] .
18 13 2
ꢀ
1
cm ; HRMS (ESI) calcd for C H N O 313.1335 found
2
1
17
2
+
3
13.1348 [M + H] .
(2-(4-Chlorophenyl)-6-methylimidazo[1,2-a]pyridin-3-
yl)(phenyl)methanone (4j): Yield 82%; Offꢀwhite solid; mp
(
3,4-Dimethoxyphenyl)(2-phenylimidazo[1,2-a]pyridin-3-
1
1
48ꢀ150 °C; H NMR (400 MHz, CDCl ) δ 9.35 (s, 1H), 7.70 (d,
1
3
yl)methanone (4e): Yield 47%; Gummy mass; H NMR (400
MHz, CDCl ) δ 9.29 (d, J = 7.0 Hz, 1H), 7.72 (d, J = 8.9 Hz, 1H),
7
8
6
3
148.3, 147.3, 134.2, 131.1, 130.1, 128.7, 128.4, 128.0, 127.9,
1
3
J = 9.0 Hz, 1H), 7.50 (d, J = 7.2 Hz, 2H), 7.39 (dd, J = 9.1, 1.5
Hz, 1H), 7.33 (t, J = 7.4 Hz, 1H), 7.24 (d, J = 8.4 Hz, 2H), 7.14
15
3
7
0
.44 – 7.39 (m, 1H), 7.34 (dd, J = 7.5, 1.7 Hz, 2H), 7.19 (dd, J =
.4, 1.8 Hz, 1H), 7.11 – 7.06 (m, 3H), 7.05 (d, J = 1.8 Hz, 1H),
.98 (t, J = 6.9 Hz, 1H), 6.52 (d, J = 8.4 Hz, 1H), 3.74 (s, 3H),
1
3
(t, J = 7.7 Hz, 2H), 7.05 (d, J = 8.4 Hz, 2H), 2.45 (s, 3H);
C
NMR (101 MHz, CDCl ) δ 187.1, 153.4, 146.4, 138.7, 134.3,
3
1
1
32.7, 132.3, 131.9, 131.3, 129.6, 127.9, 126.1, 124.8, 119.9,
16.7, 18.5; IR (KBr): 3089, 2947, 1612, 1458, 1389, 1227, 1088,
13
.58 (s, 3H); C NMR (101 MHz, CDCl ) δ 186.0, 153.4, 152.5,
3
20
ꢀ
1
75
795, 733 cm ; HRMS (ESI) calcd for C H ClN O 347.0946
21 16 2
found 347.0956 [M + H] .
24.5, 119.9, 117.4, 114.3, 112.4, 110.0, 56.0, 55.7; IR (KBr):
+
ꢀ
1
073, 2947, 1605, 1466, 1389, 1227, 913, 733 cm ; HRMS (ESI)
+
calcd for C H N O 359.1390 found 359.1385 [M + H] .
(4-Chlorophenyl)(2-(4-chlorophenyl)imidazo[1,2-a]pyridin-3-
22
19
2
3
yl)methanone (4k): Yield 63%; Offꢀwhite solid; mp 192ꢀ194 °C;
Phenyl(2-(thiophen-2-yl)imidazo[1,2-a]pyridin-3-yl)
1
H NMR (400 MHz, CDCl ) δ 9.51 (dt, J = 7.0, 1.1 Hz, 1H), 7.81
3
25
methanone (4f): Yield 36%; pale yellow solid; mp 122ꢀ124 °C;
8
0
– 7.79 (m, 1H), 7.61 – 7.53 (m, 1H), 7.49 – 7.43 (m, 2H), 7.30 –
1
H NMR (400 MHz, CDCl ) δ 9.17 (d, J = 7.0 Hz, 1H), 7.71 (d, J
3
13
7
.24 (m, 2H), 7.17 – 7.09 (m, 5H); C NMR (101 MHz, CDCl3)
=
9.0 Hz, 1H), 7.45 (dd, J = 6.6, 2.9 Hz, 2H), 7.43 – 7.36 (m,
δ 185.7, 153.6, 147.5, 138.5, 136.9, 134.9, 132.4, 131.4, 130.9,
2
6
H), 7.18 – 7.12 (m, 3H), 7.02 (dd, J = 3.7, 0.7 Hz, 1H), 6.98 –
.92 (m, 1H), 6.63 – 6.55 (m, 1H); C NMR (101 MHz, CDCl3)
1
1
29.6, 128.2, 128.2, 119.9, 117.6, 115.0; IR (KBr): 3086, 1612,
1
3
ꢀ1
496, 1404, 1335, 1227, 795, 756 cm ; HRMS (ESI) calcd for
30
δ 178.8, 153.2, 147.2, 143.6, 134.5, 134.2, 133.1, 130.0, 128.8,
+
85
C H Cl N O 367.0399 found 367.0387 [M + H] .
2
0
13
2
2
1
2
28.5, 128.1, 127.8, 127.4, 119.8, 117.5, 114.3; IR (KBr): 3078,
947, 1589, 1466, 1389, 1227, 795, 756, 733 cm ; HRMS (ESI)
ꢀ
1
(4-Chlorophenyl)(2-(4-chlorophenyl)-6-methylimidazo[1,2-
+
calcd for C H N OS 305.0743 found 305.0749 [M + H] .
a]pyridin-3-yl)methanone (4l): Yield 75%; Offꢀwhite solid; mp
18
13
2
1
1
1
54ꢀ156 °C; H NMR (400 MHz, CDCl ) δ 9.32 (d, J = 1.8, Hz,
3
(2-(4-Chlorophenyl)imidazo[1,2-a]pyridin-3-
H), 7.71 (dd, J = 9.1, 0.5 Hz, 1H), 7.48 – 7.45 (m, 1H), 7.45 –
35
yl)(phenyl)methanone (4g): Yield 52%; Offꢀwhite solid; mp
90
7.39 (m, 2H), 7.28 – 7.23 (m, 2H), 7.15 – 7.10 (m, 4H), 2.46 (s,
1
1
32ꢀ134 °C; H NMR (400 MHz, CDCl ) δ 9.55 (dt, J = 7.0, 1.1
13
3
3
1
1
1
H); C NMR (101 MHz, CDCl ) δ 185.6, 153.5, 146.5, 138.3,
3
Hz, 1H), 7.82 (d, J = 9.0, 1H), 7.59 – 7.54 (m, 1H), 7.54 – 7.52
m, 1H), 7.51 (d, J = 1.3 Hz, 1H), 7.38 – 7.32 (m, 1H), 7.28 (t, J
1.5 Hz, 1H), 7.27 – 7.25 (m, 1H), 7.18 – 7.12 (m, 3H), 7.10 –
37.2, 134.7, 132.6, 132.5, 131.3, 130.9, 128.2, 128.1, 126.1,
25.0, 119.7, 116.8, 18.5; IR (KBr): 3063, 2955, 1605, 1466,
381, 1335, 1242, 957, 764 cm ; HRMS (ESI) calcd for
(
=
ꢀ
1
1
3
40
7.08 (m, 1H), 7.07 – 7.05 (m, 1H); C NMR (101 MHz, CDCl3)
δ 187.2, 153.5, 147.4, 138.5, 134.5, 132.5, 132.0, 131.4, 129.4,
+
95
C H Cl N O 381.0556 found 381.0551 [M + H] .
21 15 2 2
1
1
29.4, 128.3, 182.0, 120.1, 117.5, 114.8; IR (KBr): 3070, 1605,
(4-Chlorophenyl)(2-(2-fluorophenyl)imidazo[1,2-a]pyridin-3-
ꢀ
1
489, 1466, 1389, 1327, 1227, 933, 748 cm ; HRMS (ESI) calcd
yl)methanone (4m): Yield 45%; Offꢀwhite solid; mp 144ꢀ146
+
1
for C H ClN O 333.0789 found 333.0795 [M + H] .
°C; H NMR (400 MHz, CDCl ) δ 9.58 (d, J = 6.6 Hz, 1H), 7.87
20
14
2
3
(
d, J = 8.8 Hz, 1H), 7.64 – 7.53 (m, 2H), 7.48 (d, J = 8.0 Hz, 2H),
4
5
(2-(4-Nitrophenyl)imidazo[1,2-a]pyridin-3-yl)(phenyl)
1
00 7.25 (d, J = 5.4 Hz, 1H), 7.21 – 7.05 (m, 4H), 6.74 (t, J = 9.0 Hz,
1
Hz), 148.4, 147.7, 137.8, 136.9, 131.2 (d, J = 2.4 Hz), 130.9,
1
1
05 (KBr): 3055, 2924, 1612, 1481, 1389, 1227, 1080, 764 cm ;
HRMS (ESI) calcd for C H Cl N O 381.0556 found 381.0551
[
3
methanone (4h): Yield 76%; pale yellow solid; mp 241ꢀ243 °C;
13
H); C NMR (101 MHz, CDCl ) δ 185.75, 159.11 (d, J = 248.7
3
1
H NMR (400 MHz, DMSOꢀd ) δ 9.42 (d, J = 6.9 Hz, 1H), 7.98 –
6
7
.89 (m, 3H), 7.78 – 7.69 (m, 1H), 7.57 (d, J = 8.7 Hz, 2H), 7.49
30.9 (d, J = 8.3 Hz), 129.3, 128.2, 127.7, 124.2 (d, J = 3.5 Hz),
1
3
(d, J = 7.2 Hz, 2H), 7.41 – 7.28 (m, 2H), 7.19 – 7.16 (m, 2H);
C
22.8, 122.7, 120.9, 117.6, 115.2 (d, J = 22.0 Hz), 115.0; IR
50
NMR (101 MHz, DMSOꢀd ) δ 186.7, 151.7, 147.2, 147.1, 141.2,
6
ꢀ1
1
1
1
1
3
38.9, 132.5, 131.6, 130.7, 130.6, 129.9, 128.4, 128.4, 123.0,
17.9, 116.1; IR (KBr): 3078, 1605, 1512, 1466, 1389, 1250,
2
1
15
2
2
+
M+H] . HRMS calcd for C H ClFN O 351.0695 found
20 13 2
ꢀ
1
227, 856, 748 cm ; HRMS (ESI) calcd for C H N O
20
14
3
3
+
51.0698 [M + H] .
+
44.1030 found 344.1025 [M + H] .
(4-Chlorophenyl)(2-(2-fluorophenyl)-7-methylimidazo[1,2-
110 a]pyridin-3-yl)methanone (4n): Yield 42%; Offꢀwhite solid; mp
6
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

RSC Advances
Page 8 of 9
View Article Online
DOI: 10.1039/C4RA13056A
1
1
1
2
1
47ꢀ149 °C; H NMR (400 MHz, CDCl ) δ 9.45 (d, J = 7.1 Hz,
Notes and references
a
3
H), 7.59 (s, 1H), 7.49 (td, J = 7.4, 1.7 Hz, 1H), 7.45 – 7.40 (m,
H), 7.26 – 7.19 (m, 1H), 7.12 – 7.04 (m, 3H), 7.00 – 6.96 (m,
H), 6.75 – 6.65 (m, 1H), 2.54 (s, 3H); C NMR (101 MHz,
CDCl ) δ 185.6, 159.2 (d, J = 248.7 Hz), 148.8, 148.2, 141.1,
3
37.6, 137.0, 131.2 (d, J = 2.5 Hz), 130.8 (d, J = 8.3 Hz), 130.5,
27.6, 127.5, 124.1 (d, J = 3.5 Hz), 123.0, 122.8, 120.6, 117.5,
16.2, 115.2 (d, J = 22.0 Hz), 21.6; IR(KBr): 3055, 2965, 1612,
481, 1227, 1076, 764 cm ; HRMS calcd for C H ClFN O
365.0851 found 365.0835 [M + H] .
Department of Chemistry, Birla Institute of Technology and Science,
Pilani, 333031, India.
5
6
5
0
1
3
Fax: 91-1596-244183; Tel: 91-1596-515652;
E-mail: anilkumar@pilani.bits-pilani.ac.in
5
†
Electronic Supplementary Information (ESI) available: Characterization
1
1
1
1
1 13
data for 6a, copies of H NMR and C NMR spectra for compounds 4a-p
and 6a are available. See DOI: 10.1039/b000000x/
ꢀ
1
2
1
15
2
1. (a) I. Nakamura and Y. Yamamoto, Chem. Rev., 2004, 104, 2127ꢀ
+
2
198; (b) S. E. Allen, R. R. Walvoord, R. PadillaꢀSalinas and M. C.
1
0
Kozlowski, Chem. Rev., 2013, 113, 6234ꢀ6458; (c) J. Koubachi, S.
El Kazzouli, M. Bousmina and G. Guillaumet, Eur. J. Org. Chem.,
(
6-Bromo-2-(2-fluorophenyl)imidazo[1,2-a]pyridin-3-yl)(4-
65
70
75
2
014, 5119ꢀ5138.
(a) W. Ge, X. Zhu and Y. Wei, Eur. J. Org. Chem., 2013, 6015ꢀ
020; (b) T.ꢀf. Niu, M. Sun, M.ꢀf. Lv, W.ꢀb. Yi and C. Cai, Org.
chlorophenyl)-methanone (4o): Yield 51%; Offꢀwhite solid; mp
2
.
1
1
73ꢀ175 °C; H NMR (400 MHz, CDCl ) δ 9.71 (s, 1H), 7.73 (d,
3
6
J = 9.3 Hz, 1H), 7.63 (d, J = 9.4 Hz, 1H), 7.53 (t, J = 6.9 Hz, 1H),
7.45 (d, J = 7.4 Hz, 2H), 7.24 (d, J = 5.4 Hz, 1H), 7.15 – 7.05 (m,
H), 6.71 (t, J = 9.1 Hz, 1H); C NMR (101 MHz, CDCl ) δ
85.7, 159.1 (d, J = 249.1 Hz), 148.3, 146.1, 138.2, 136.4, 136.4,
32.7, 131.2 (d, J = 6.4 Hz), 131.1, 130.4, 128.3, 127.8, 124.3 (d,
J = 3.5 Hz), 122.3 (d, J = 14.0 Hz), 120.9, 118.1, 115.3 (d, J =
22.0 Hz), 109.9; IR (KBr): 3101, 3055, 2924, 1612, 1481, 1389,
227, 1080, 764 cm ; HRMS calcd for C H BrClFN O
20 12 2
28.9800 found 428.9822 [M+H] .
Biomol. Chem., 2013, 11, 7232ꢀ7238; (c) K. Pericherla, A. Kumar
and A. Jha, Org. Lett., 2013, 15, 4078ꢀ4081; (d) A. Dömling, W.
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Wan and Y. Liu, Synthesis, 2010, 2010, 3943ꢀ3953; (f) R. C. Cioc,
E. Ruijter and R. V. A. Orru, Green Chem., 2014, 16, 2958ꢀ2975;
(g) J.ꢀP. Wan and Y. Liu, RSC Adv., 2012, 2, 9763ꢀ9777; (h) Y. Liu,
ARKIVOC, 2014, 1ꢀ20; (i) J.ꢀP. Wan, Y. Lin and Y. Liu, Curr. Org.
Chem., 2014, 18, 687ꢀ699.
1
5
13
3
1
1
3
2
0
3
.
(a) H. Sun, H. Zhou, O. Khorev, R. Jiang, T. Yu, X. Wang, Y. Du,
Y. Ma, T. Meng and J. Shen, J. Org. Chem., 2012, 77, 10745ꢀ
10751; (b) A. El Akkaoui, M.ꢀA. Hiebel, A. Mouaddib, S. Berteinaꢀ
Raboin and G. Guillaumet, Tetrahedron, 2012, 68, 9131ꢀ9138; (c)
H. Yan, R. Yan, S. Yang, X. Gao, Y. Wang, G. Huang and Y.
Liang, Chem. Asian J., 2012, 7, 2028ꢀ2031; (d) A. Yang, R. Jiang,
O. Khorev, T. Yu, Y. Zhang, L. Ma, G. Chen, J. Shen and T. Meng,
Adv. Synth. Catal., 2013, 355, 1984ꢀ1988; (e) V. Tyagi, S. Khan, V.
Bajpai, H. M. Gauniyal, B. Kumar and P. M. S. Chauhan, J. Org.
Chem., 2012, 77, 1414ꢀ1421; (f) Y. Wang, B. Frett and H.ꢀy. Li,
Org. Lett., 2014, 16, 3016ꢀ3019; (g) H. Yan, Y. Wang, C. Pan, H.
Zhang, S. Yang, X. Ren, J. Li and G. Huang, Eur. J. Org. Chem.,
ꢀ
1
1
4
+
8
8
9
9
0
5
0
5
(
4-Methoxyphenyl)(2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-
yl)methanone (4p): Yield 44%; pale yellow solid; mp 199ꢀ201
°C H NMR (400 MHz, CDCl ) δ 9.38 (s, 1H), 8.07 (d, J = 6.9
Hz, 2H), 7.87 (d, J = 8.7 Hz, 1H), 7.67 – 7.57 (m, 5H), 7.18 –
.16 (m, 1H), 6.70 (d, J = 6.8 Hz, 2H), 3.80 (s, 3H); C NMR
101 MHz, CDCl ) δ 185.6, 163.4, 150.3, 147.2, 147.2, 140.7,
32.0, 130.9, 130.8, 129.2, 127.9, 123.0, 120.6, 117.7, 114.9,
113.5, 55.5; IR (KBr): 3109, 3070, 2222, 1612, 1512, 1478, 1342,
1
2
5
3
13
7
(
1
3
2
014, 2754ꢀ2763.
3
0
4. (a) J. Wan, C.ꢀJ. Zheng, M.ꢀK. Fung, X.ꢀK. Liu, C.ꢀS. Lee and X.ꢀ
H. Zhang, J. Mater. Chem., 2012, 22, 4502ꢀ4510; (b) C. Enguehardꢀ
Gueiffier and A. Gueiffier, Mini-Rev. Med. Chem., 2007, 7, 888ꢀ
ꢀ
1
1
3
026, 756 cm ; HRMS calcd for C H N O 374.1135 found
21 16 3 4
74.1123 [M + H] .
+
8
1
99; (c) A. John, M. M. Shaikh and P. Ghosh, Dalton Trans., 2009,
0581ꢀ10591; (d) G. Song, Y. Zhang and X. Li, Organometallics,
Conclusions
2008, 27, 1936ꢀ1943; (e) N. Chernyak and V. Gevorgyan, Angew.
Chem. Int. Ed., 2010, 49, 2743ꢀ2746.
In summary, we have developed a straightforward method for the
synthesis of 3ꢀaroylimidazo[1,2ꢀa]pyridines through oneꢀpot,
threeꢀcomponent tandem reaction. This protocol makes the use of
simple and readily available precursors like acetophenones,
aldehydes and 2ꢀaminopyridines to deliver highly functionalized
bioꢀactive imidazo[1,2ꢀa]pyridines in single step. Atom and stepꢀ
economy, use of economically attractive and readily available
precursors, simple isolation procedures, moderate to good yields
of tandem products, air as a sole oxidant, and good functional
group tolerance are the salient features of the method.
5
.
(a) S. Kang, R. Y. Kim, M. J. Seo, S. Lee, Y. M. Kim, M. Seo, J. J.
Seo, Y. Ko, I. Choi, J. Jang, J. Nam, S. Park, H. Kang, H. J. Kim, J.
Kim, S. Ahn, K. Pethe, K. Nam, Z. No and J. Kim, J. Med. Chem.,
2014, 57, 5293ꢀ5305; (b) F. Couty and G. Evano, in Comprehensive
Heterocyclic Chemistry III, eds. A. R. Katritzky, C. A. Ramsden, E.
F. V. Scriven and R. J. K. Taylor, Elsevier, Oxford, 2008, pp. 409ꢀ
3
5
1
1
1
1
1
00
05
10
4
99; (c) M. Lhassani, O. Chavignon, J.ꢀM. Chezal, J.ꢀC. Teulade, J.ꢀ
4
0
P. Chapat, R. Snoeck, G. Andrei, J. Balzarini, E. De Clercq and A.
Gueiffier, Eur. J. Med. Chem., 1999, 34, 271ꢀ274; (d) M. H. Fisher
and A. Lusi, J. Med. Chem., 1972, 15, 982ꢀ985; (e) N. Masurier, E.
Debiton, A. Jacquemet, A. Bussière, J.ꢀM. Chezal, A. Ollivier, D.
Tétégan, M. Andaloussi, M.ꢀJ. Galmier, J. Lacroix, D. Canitrot, J.ꢀ
C. Teulade, R. C. Gaudreault, O. Chavignon and E. Moreau, Eur. J.
Med. Chem., 2012, 52, 137ꢀ150.
(a) Y.ꢀS. Tung, M. S. Coumar, Y.ꢀS. Wu, H.ꢀY. Shiao, J.ꢀY. Chang,
J.ꢀP. Liou, P. Shukla, C.ꢀW. Chang, C.ꢀY. Chang, C.ꢀC. Kuo, T.ꢀK.
Yeh, C.ꢀY. Lin, J.ꢀS. Wu, S.ꢀY. Wu, C.ꢀC. Liao and H.ꢀP. Hsieh, J.
Med. Chem., 2011, 54, 3076ꢀ3080; (b) 2006.
6
.
4
5
Acknowledgement
Authors sincerely acknowledge Council of Scientific and
Industrial Research (CSIR), New Delhi [02(0115)/13/EMRꢀII]
and Department of Science and Technology, New Delhi [DSTꢀ
FIST, CSIꢀ174/2008] for the financial support. PK and HKS are
thankful to CSIR, New Delhi for junior research fellowship. KP
is thankful to UGC, New Delhi for senior research fellowship.
15 7. (a) C. Hamdouchi, J. de Blas, M. del Prado, J. Gruber, B. A. Heinz
and L. Vance, J. Med. Chem., 1998, 42, 50ꢀ59; (b) L. Hu, J.ꢀd.
Jiang, J. Qu, Y. Li, J. Jin, Z.ꢀr. Li and D. W. Boykin, Bioorg. Med.
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5
0
8
.
L. Albrecht, A. Albrecht, L. K. Ransborg and K. A. Jorgensen,
Chem. Sci., 2011, 2, 1273ꢀ1277.
20
9
.
(a) A. K. Bagdi, M. Rahman, S. Santra, A. Majee and A. Hajra, Adv.
Synth. Catal., 2013, 355, 1741ꢀ1747; (b) K. Pericherla, P. Kaswan,
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