Synthesis and characterization of Imidazo [1,2-a]pyridines and Zolimidine

Article abstract of DOI:10.14233/ajchem.2017.20533

A convenient and efficient synthesis, without any organic solvent or catalyst is described for the preparation of imidazo [1,2-a] pyridine derivatives. The reaction has been done between 2-aminopyridines and α-bromoacetone derivatives in good to excellent yields in water. In several cases the isolation does not require any chromatography for purification. The reaction thus developed has been applied for quick synthesis of zolimidine with 91 % yield.

Full text of DOI:10.14233/ajchem.2017.20533

Asian Journal of Chemistry; Vol. 29, No. 7 (2017), 1495-1498
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2017.20533
Synthesis and Characterization of Imidazo[1,2-a]pyridines and Zolimidine
1
2
1
NARVA DESHWAR KUSHWAHA1,2,*, SOMNATH MONDAL , RENU SHARMA and NARESH KUMAR
1Jubilant Chemsys Ltd., B-34, Sector-58, Noida-201 301, India
2NIMS University, Shobha Nagar, Jaipur-303 121, India
*Corresponding author: E-mail: narvadeshwarkushwaha@gmail.com
Received: 23 January 2017;
Accepted: 9 March 2017;
Published online: 13 May 2017;
AJC-18381
A convenient and efficient synthesis, without any organic solvent or catalyst is described for the preparation of imidazo[1,2-a]pyridine
derivatives. The reaction has been done between 2-aminopyridines and α-bromoacetone derivatives in good to excellent yields in water.
In several cases the isolation does not require any chromatography for purification. The reaction thus developed has been applied for
quick synthesis of zolimidine with 91 % yield.
Keywords: Imidazo[1,2-a]pyridine, 2-Aminopyridines, α-Bromoacetone, Zolimidine.
Three component coupling reaction are available using
2-amino pyridines, aldehyde/alkyl halide and alkynes/
isocyanide [7,8]. Adib et al. [3] reports such reaction in a
catalyst-free reaction in water at 70 °C.
Two component systems are also done with condensation
of 2-aminopyridine with α-haloketones [4,5,9]. Most of the
reactions uses polar organic solvent (particularly ethanol or
methanol) in presence/absence of different bases (Scheme-I).
The isolation and purification has been done by using column
chromatography to obtained moderate to good yields [4,5,10].
INTRODUCTION
Organic chemists are continuously searching for the
chemical reaction in non-hazardous conditions to minimize
the generation of hazardous substances in the environment
[1]. Water is the ideal solvent for chemical reaction as it is
safe and environmental friendly [1,2]. But, in general, its use
is limited in chemical reactions because most of the organic
compounds are either insoluble or unstable [3]. Herein we
report synthesis of imidazo[1,2-a]pyridine derivatives in water
with high yields.
Imidazo[1,2-a]pyridine derivatives are an important class
of molecules and have wide variety of biological activities
such as anti-inflammatory, antiviral, antibacterial, antifungal,
antiprotozoal, immunomodulatory, gastrointestinal, anxiolytic,
hypnotic and antiulcer activities [4,5].
R1
O
N
EtOH
reflux
+
Br
R1
R2
R2
N
N
NH2
Scheme-I: Preparation of imidazo-pyridines in ethanol
Imidazo[1,2-a]pyridine moieties also known in certain
drug molecules such as zolpidem (I, treatment of insomnia),
alpidem (II, an anxiolytic agent), minodronic acid (III, use
for osteoporosis treatment) and zolimidine (IV, for peptic ulcer
treatment) [6] (Fig. 1).
Syntheses have also been carried out under microwave
irradiation in the presence of Montmorillonite K10 [11],
Sc(OTf)3 [12], etc. It has also been performed using a non-
polar solvent [13], or in the presence of an ionic liquid [14].
N
N
N
Cl
N
HO
P
N
O
Cl
N
OH
O
N
O
O
S
OH
HO
P
N
N
O
N
HO O
Minodronic acid (III
Fig. 1. Few examples of imidazo[1,2-a]pyridine containing drugs
Zolimidine (IV
)
Zolpidem (I)
Alpidem (II
)
)

1496 Kushwaha et al.
Asian J. Chem.
In recent years several improved conditions have been
reported [15]. Lei et al. [16] published the results on synthesis
of imidazo[1,2-a]pyridines using silver carbonate in dioxane
at 110 °C, with moderate yields [16] and Cai et al. [17] reported
synthesis of imidazo[1,2-a]pyridines using I2/CuO in methanol.
Though these methods are useful, but use of water as solvent
for organic reactions is the most interesting and attracted step
because of its economic and environmental friendly nature.
2-(tert-Butyl)imidazo[1,2-a]pyridine (3a): m.p.: 81-82
°C (80 °C) [1]; 1H NMR (400 MHz, DMSO-d6): δ 8.51 (d, J =
6.8 Hz, 1H), 7.77 (s, 1H), 7.55 (d, J = 9.2 Hz, 1H), 7.34-7.30
(m, 1H), 6.96-6.93 (m, 1H), 1.33 (s, 9H). MS, m/z: 175.04
(M+H+).
2-(tert-Butyl)-7-methylimidazo[1,2-a]pyridine (3b):
m.p.: 94-96 °C (lit. 96-98 °C) [2]. 1H NMR (400 MHz, CDCl3):
δ 7.90 (d, J = 6.8 Hz, 1H), 7.32 (s, 1H), 7.24 (s, 1H), 6.53 (d,
J = 6.8 Hz, 1H), 2.35 (s, 3H), 1.38 (s, 9H). 13C NMR (100
MHz, CDCl3): δ 156.97, 145.46, 134.59, 124.60, 115.64,
114.16, 106.04, 32.22, 30.22, 21.32. MS, m/z: 189.13 (M+H+).
2-(tert-Butyl)-7-chloroimidazo[1,2-a]pyridine (3c):
m.p.: 136-138 °C; 1H NMR (400 MHz, CDCl3): δ 7.95 (d, J =
7.2 Hz, 1H), 7.56 (s, 1H), 7.31 (s, 1H), 6.70 (dd, J = 6.8 Hz, J
= 1.6 Hz, 1H), 1.38 (s, 9H). 13C NMR (100 MHz, CDCl3): δ
158.36, 144.78, 130.17, 125.58, 116.12, 113.23, 106.95, 32.35,
30.11. MS, m/z: 209.0 (M+H+).
EXPERIMENTAL
Unless otherwise noted all reactions were carried out at
room temperature. Solvents and reagents were purchased at
the highest commercial quality and used without further drying
and purification respectively. Reactions were monitored by
thin-layer chromatography (TLC) analysis using TLC
aluminum sheets silica gel 60 F254.
1H NMR and 13C NMR spectra were recorded in CDCl3
and DMSO-d6 at 400 MHz. The chemical shifts are reported
in ppm downfield to TMS (δ = 0) for 1H NMR and 13C NMR.
The water was prepared from tap water after sand filter-carbon
filter-ionic exchange resin-reverse osmosis-UV-micro filter by
passing through pre-packed cartridges.
General procedure and representative analytical data
of imidazo[1,2-]pyridines:A solution of 2-aminopyridine (1
eq) and α-bromoketone (1 eq) in water was stirred at room tempe-
rature for 8 h (Scheme-II). For some insoluble 2-amino-
pyridine, it was dissolved by heating up to 60-70 °C and then
added α-bromoketones. The reactions were performed at room
temperature for 8 h (Scheme-III). After completion of the
reaction, the reaction mixture was basified (pH = 9) with 10
% NaOH aqueous solution. In many cases product was
precipitated out, so it was filtered to get pure compound. If
product was not solid, it was extracted with pentane to get
product in pure form.
1
2-Ethylimidazo[1,2-a]pyridine (3d): H NMR (400
MHz, CDCl3): δ 8.03 (d, J = 6.8 Hz, 1H), 7.52 (d, J = 9.2 Hz,
1H), 7.33 (s, 1H), 7.12-7.08 (m, 1H), 6.73-6.71 (m, 1H), 2.82
(q, J = 7.6 Hz, 2H), 1.34 (t, J = 7.6 Hz, 3H). MS, m/z: 146.99
(M+H+).
2-Ethyl-7-methylimidazo[1,2-a]pyridine (3e): 1H NMR
(400 MHz, CDCl3): δ 7.90 (d, J = 6.8 Hz, 1H), 7.25 (d, J =
8.0 Hz, 2H), 6.54 (d, J = 6.8 Hz, 1H), 2.79 (q, J = 7.6 Hz,
2H), 2.36 (s, 3H), 1.33 (t, J = 7.6 Hz, 3H). MS, m/z: 161.07
(M+H+).
7-Chloro-2-ethylimidazo[1,2-a]pyridine (3f): 1H NMR
(400 MHz, CDCl3): δ 7.95 (d, J = 7.2 Hz, 1H), 7.51 (s, 1H),
7.31 (s, 1H), 6.71 (dd, J = 7.2 Hz, J = 2.0 Hz 1H), 2.80 (q, J
= 7.6 Hz, 2H), 1.33 (t, J = 7.6 Hz, 3H). MS, m/z: 181.09
(M+H+).
1
2-Cyclohexylimidazo[1,2-a]pyridine (3g): H NMR
(400 MHz, DMSO-d6): δ =8.43 (d, J = 6.8 Hz, 1H), 7.64 (s,
1H), 7.43 (d, J = 8.8 Hz, 1H), 7.16-7.12 (m, 1H), 6.81-6.78
(m, 1H), 2.65 (bs, 1H), 2.02-1.99 (m, 2H), 1.78-1.67 (m, 3H),
1.58-1.32 (m, 4H), 1.27-1.23 (m, 1H). MS, m/z: 201.20
(M+H+).
R
H2O
rt
O
R
N
2-Cyclohexyl-7-methylimidazo[1,2-a]pyridine (3h): 1H
NMR (400 MHz, DMSO-d6): δ 8.30 (d, J = 6.8 Hz, 1H), 7.53
(s, 1H), 7.20 (s, 1H), 6.64 (d, J = 6.8 Hz, 1H), 2.61 (bs, 1H),
2.31 (s, 3H), 2.06-2.01 (m, 2H), 1.77-1.61 (m, 3H),1.45-1.31
(m, 4H), 1.08-1.00 (m, 1H). MS, m/z: 215.21 (M+H+).
7-Chloro-2-cyclohexylimidazo[1,2-a]pyridine (3i): 1H
NMR (400 MHz, CDCl3): δ 7.95 (d, J = 7.2 Hz, 1H), 7.52 (s,
1H), 7.28 (s, 1H), 6.71 (d, J = 7.2 Hz, 1H), 2.74-2.72 (m, 1H),
2.12-2.09 (m, 2H), 1.85-1.82 (m, 2H), 1.76-1.30 (m, 1H), 1.52-
1.37 (m, 4H), 1.35-1.26 (m, 1H). MS, m/z: 235.11 (M+H+).
Br
R1
R
1
N
N
NH2
1
2
3
R= H, Me, Cl
R1= Et, t-Bu, Ph,Cyclohexyl
Scheme-II
Synthesis of zolimidine: The suspension of 2-bromo-1-
(4-(methylsulfonyl)phenyl)ethanone (1 eq) and pyridin-2-
amine (3 eq) in water was stirred at room temperature for 18 h
(Scheme-IV). The heterogeneous reaction mixture was directly
filtered and washed with water, followed by diethyl ether to
get yellow solid zolimidine (91 %).
1
2-Phenylimidazo[1,2-a]pyridine (3j): H NMR (400
MHz, CDCl3): δ 8.13 (d, J = 6.8 Hz, 1H), 7.98-7.96 (m, 2H),
7.87 (s, 1H), 7.64 (d, J = 9.2 Hz, 1H), 7.46-7.42 (m, 2H),
7.35-7.31 (m, 1H), 7.19-7.15 (m, 1H), 6.80-6.77 (m, 1H). MS,
m/z: 195.00 (M+H+).
R
4
O
N
3
H2O
rt
5
R
1
Br
7-Methyl-2-phenylimidazo[1,2-a]pyridine (3k): H
N
N
NH2
NMR (400 MHz, CDCl3): δ 7.99 (d, J = 6.8 Hz, 1H), 7.93 (d,
J = 7.6 Hz, 2H), 7.78 (s, 1H), 7.44-7.30 (m, 3H), 7.26 (s, 1H),
6.6 (d, J = 6.8 Hz, 1H), 2.39 (s, 3H). MS, m/z: 209.08 (M+H+).
4
5
6
R=4-bromo,3-cyano, 3-chloro,5-chloro,5-bromo
Scheme-III: Reaction of different substituted 2 amino pyridine and 1-
1
7-Chloro-2-phenylimidazo[1,2-a]pyridine (3l): H
bromo-3,3-dimethylbutan-2-one
NMR (400 MHz, CDCl3): δ 8.05 (d, J = 7.2 Hz, 1H), 7.93 (d,

Vol. 29, No. 7 (2017)
Synthesis and Characterization of Imidazo[1,2-a]pyridines and Zolimidine 1497
J = 7.2 Hz, 2H), 7.84 (s, 1H), 7.64 (s, 1H), 7.46-7.42 (m, 2H),
7.36-7.33 (m, 1H), 6.78 (d, J = 5.2 Hz, 1H). MS, m/z: 229.04
(M+H+).
pyridines: no substitution, 4-methyl and 4-chloro have been
selected to represent variety. These are made to react with
α-bromoacyl derivatives with bulky group t-butyl, ethyl, cyclo-
hexyl and phenyl. All the 12 products are obtained in good to
excellent yields (64-91 %).
7-Bromo-2-(tert-butyl)imidazo-[1,2-a]pyridine (6a): 1H
NMR (400 MHz, CDCl3): δ 7.90 (d, J = 7.2 Hz, 1H), 7.75 (s,
1H), 7.31(s, 1H), 6.81 (dd, J = 7.2 Hz, J = 1.6 Hz,1H), 1.38
(s, 9H). MS, m/z: 253.02 (M+H+).
The isolation of the product needs basification of the
reaction mixture with aq. sodium hydroxide to precipitate out
required compounds or to extract the compounds in pentane.
Some of the substituted 2-amino pyridines (Scheme-III)
showed insolubility in water, so were warmed up to 60-70 °C
for making a clear solution. The reaction proceeded at room
temperature and produced the required product in good yields
(72-80 %), except for 6b (40 %). We believe the steric may
not be the reason for such low yields. As the 3-chloro amino
pyridine gave good yield (80 %), so the poor yield may be
due to complication by participation of CN group during
product formation (Table-2). All the compounds are purified
without column chromatography.
The water plays a role of reaction medium only and does
not play any role in the reaction mechanism. This was evident
by performing a reaction in heavy water (example 3b). The
reaction gave similar high yield as in water (91 % vs. 88 %).
This result is in contrast to the results obtained in literature
[18]. The NMR data of the product 3b obtained in heavy water
was same as the one obtained in water.
2-(tert-Butyl)imidazo[1,2-a]-pyridine-8-carbonitrile
(6b): 1H NMR (400 MHz, CDCl3): δ 8.23 (d, J = 7.2 Hz, 1H),
7.55 (d, J = 7.2 Hz, 1H), 7.45 (s, 1H), 6.80-6.76 (m, 1H), 1.41
(s, 9H). MS, m/z: 200.06 (M+H+).
2-(tert-Butyl)-8-chloroimidazo[1,2-a]pyridine (6c): 1H
NMR (400 MHz, CDCl3): δ 7.97 (d, J = 6.4 Hz, 1H), 7.39 (s,
1H), 7.17 (d, J = 7.2 Hz, 1H), 6.66-6.62 (m, 1H), 1.41 (s, 9H).
MS, m/z: 209.04 (M+H+).
2-(tert-Butyl)-6-chloroimidazo[1,2-a]pyridine (6d): 1H
NMR (400 MHz, CDCl3): δ 8.08 (s, 1H), 7.50 (d, J = 8.8 Hz,
1H), 7.31 (s, 1H), 7.07 (dd, J = 9.6 Hz, J = 1.6 Hz, 1H), 1.39
(s, 9H). MS, m/z: 209.06 (M+H+).
6-Bromo-2-(tert-butyl)imidazo[1,2-a]pyridine (6e): 1H
NMR (400 MHz, CDCl3): δ 8.18 (s, 1H), 7.46 (d, J = 9.6 Hz,
1H), 7.31 (s, 1H), 7.16 (d, J = 9.6 Hz, 1H), 1.39 (s, 9H). MS,
m/z: 254.95 (M+H+).
2-[4-(Methylsulfonyl)phenyl]imidazo[1,2-a]pyridine
(IV): m.p.: 222-224 °C; 1H NMR (400 MHz, CDCl3): δ 8.15
(d, J = 8.8 Hz, 3H), 8.09 (d, J = 9.6 Hz, 3H), 7.65 (d, J = 9.2
Hz, 1H), 7.22 (d, J = 8.4 Hz, 1H), 6.84 (t, J = 6.8 Hz, 1H),
3.09 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 145.98, 143.57,
139.27, 127.92, 126.61, 125.86, 125.56, 117.86, 113.10,
109.69, 44.63. MS, m/z: 273.15 (M+H+).
The developed reaction was applied to synthesize the
zolimidine (IV) as per Scheme-IV. It gave the required product
in good yields (91 %) and high purity (> 97 %) and was isolated
with simple filtration. In recent paper, it has been synthesized
over two steps (overall yield 40 %) [16]. The high yields and
quick isolation makes it ideal route for scale up and manu-
facturing of zolimidine.
RESULTS AND DISCUSSION
While pursuing synthesis of some specific imidazo[1,2-
a]pyridines, we hypothesized this to be ideal reaction for trying
in water as solvent. This has been further developed as reaction
without use of any base/catalyst and isolating without any
chromatography. The reaction produces good to excellent
isolated yields at room temperature (Table-1). Three amino
O
N
H2O
Br
+
S O
O
N
N
NH2
rt, 18h
91%
S
O
IV
O
Scheme-IV: Synthesis of zolimidine in water
TABLE-1
YIELD OF IMIDAZO[1,2-a]PYRIDINES FROM 2-AMINO PYRIDINE AND α-BROMOACETONE
Compd. No.
Structure
Yield (%)
Compd. No.
Structure
Yield (%)
78
N
N
3a
84
3g
N
N
N
N
3b
3c
91
90
84
86
3h
3i
67
64
75
73
N
N
N
Cl
Cl
Cl
N
N
N
N
N
3d
3e
3j
N
N
N
N
3k
N
N
N
N
Cl
N
3f
75
3l
75
N

1498 Kushwaha et al.
Asian J. Chem.
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TABLE-2
YIELD OF COMPOUNDS (6a-6e)
Compd. No.
Structure
Yield (%)
72
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N
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6a
N
CN
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N
40
80
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6b
6c
N
Cl
N
N
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N
77
72
6d
6e
N
Cl
Br
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N
Conclusion
In summary, an easy, catalyst free, efficient and environ-
mentally friendly synthesis of imidazo[1,2-a]pyridines from
2-aminopyridines with α-bromo acyl derivatives is reported. The
reactions are done in water as medium and at room temperature.
Several products are isolated without any column chromato-
graphy with good to excellent yields. The drug like zolimidine
has been synthesized using the developed procedure in high yield
and purity using water as reaction medium.
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ACKNOWLEDGEMENTS
Sincere thanks to Jubilant Chemsys Management in
facilitating this research work and also to the analytical team
for providing the spectral data.
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