A facile preparation of imidazo[1,2- A]pyridin-3- Amine derivatives via a three-component reaction with β-cyclodextrin-so3h as catalyst

Article abstract of DOI:10.3987/COM-16-13508

Because heterogeneous catalysts have attracted great interest in organic chemistry, this paper reports facile β-cyclodextrin-SO₃H-catalyzed cyclization to form imidazo[1,2- A]pyridin-3- Amine derivatives via a three-component reaction. The main

Full text of DOI:10.3987/COM-16-13508

HETEROCYCLES, Vol. 92, No. 9, 2016, pp. -. © 2016 The Japan Institute of Heterocyclic Chemistry
Received, 26th May, 2016, Accepted, 8th July, 2016, Published online, 21st July, 2016
DOI: 10.3987/COM-16-13508
A FACILE PREPARATION OF IMIDAZO[1,2-a]PYRIDIN-3-AMINE
DERIVATIVES VIA A THREE COMPONENT REACTION WITH
β-CYCLODEXTRIN–SO3H AS CATALYST
Jian Wu,* Fang-Zhou Xu,# She-Lei Feng, Wei Xue, and Zhen-Zhen Wang
Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of
Education, Guizhou University; Research and Development Center for Fine
Chemicals, Guizhou University, Huaxi District, Guiyang 550025, P.R. China;
E-mail: jianwu2691@126.com, # Co-first author for the manuscript
Abstract – Because heterogeneous catalysts have attracted great interest in
organic chemistry, this paper reports facile β-cyclodextrin–SO3H–catalyzed
cyclization to form imidazo[1,2-a]pyridin-3-amine derivatives via
a
three-component reaction. The main advantages of this strategy include short
reaction time, practical simplicity, and high yield, and the catalyst can be
separated easily by filtration and reused at least four times.
INTRODUCTION
Imidazo[1,2-a]pyridine derivatives are an important class of fused heterocyclic compounds because of
their potential biological pharmacological activities.1–4 They have also been found in many drugs such as
zolpidem, alpidem, zolimidine, olprinone, saripidem, and necopidem.5,6 Therefore, preparation of
imidazo[1,2-a]pyridines has attracted increasing attention, and several effective strategies have been
developed for the construction of imidazo[1,2-a]pyridines.5–12 Condensation of 2-aminoazines, aldehydes,
and isocyanides is one convenient technique for synthesizing imidazo[1,2-a]pyridines. In past decades,
several catalysts, such as montmorillonite K10 (microwave),13 ZrCl,14 ZnCl2,15 SnCl2,16 RuCl3,17
LaCl3•7H2O,18 InCl3,19 cellulose sulfuric acid,20 p-toluenesulfonic acid,21 γ-Fe2O3@–SiO2–OSO3H,22
sulfide nanotubes,23 and bromodimethylsulfonium bromide24 have been developed for condensation of
2-aminoazines, aldehydes, and isocyanides. In addition, some catalyst-free and solvent-free protocols
have been developed.25 These methodologies give good results in many instances. However, some of the
synthetic strategies have high-temperature requirements and suffer from low yields of products, long
reaction times, high cost catalysts, tedious preparation of catalysts, difficult recovery of homogeneous
catalysts and separation of products. Hence, the development of efficient, simple, easy work-up protocol

for the synthesis of imidazo[1,2-a]pyridine derivatives is still desirable and in demand.
β-Cyclodextrin (β-CD), a macrocyclic oligosaccharide with a hydrophobic cavity and a hydrophilic
outer-surface, has attracted much attention and has been used widely.26,27 Recently, β-CD and its
derivatives have served as efficient catalysts in numerous organic reactions.19,20,27–35 Because β-CD–
SO3H is a powerful catalyst, it is used for the synthesis of 4-thiazolidinones,31 the esterification of
carboxylic acids with alcohols,36 and the preparation of 2,3-dihydroquinazolin-4(1H)-one derivatives in
water,34 and the catalytic property of β-CD–SO3H has attracted our interests in organic synthesis.
Continuing our work on β-CD–SO3H,34 we present a cyclodextrin-SO3H-catalyzed heterogeneous process
to prepare imidazo[1,2-a]pyridin-3-amine derivatives via ring closure of substituted pyridin-2-amines,
isocyanides, and aldehydes. To the best of our knowledge, this is a convenient means of preparing
imidazo[1,2-a]pyridin-3-amine derivatives by using cheap, recyclable, and easily available β-CD–SO3H
as a heterogeneous catalyst.
RESULTS AND DISCUSSION
β-CD–SO3H was prepared according to our previous work.34 Subsequently, the effects of solvent,
reaction time, and the amount of catalyst on the yields of products were investigated on the basis of the
one-pot reaction of pyridin-2-amine with cyclohexanecarbonitrile and benzaldehyde (Scheme 1). The
results are summarized in Table 1.
Scheme 1
Initially, the reaction was performed in aqueous media and catalyzed by β-CD–SO3H (Table 1, Entry 1);
unfortunately, this technique yielded only <35%. We then focused on screening organic solvents and
noted the following: non-polar solvents, such as toluene, gave moderate yield of the product (Table 1,
Entry 2); polar solvents gave much better yields than that of toluene (Table 1, Entries 3–7); both
acetonitrile (Table 1, Entry 6) and ethanol (Table 1, Entry 7) gave excellent yields; and ethanol gave a
slightly higher yield than that of acetonitrile. Reactions were conducted for determining the most-suitable
temperature, and the results showed that, at room temperature to 60 °C, moderate yields (56–80%) could
be obtained (Table 1, Entries 8–10), whereas the yield did not increase after reaction for 5 hours (Table 1,
Entry 10). These results indicate that temperature is a key factor for this reaction. The reaction time was
investigated further, and it was found that the yields were not obviously impacted when the reaction time

was changed from 2 hours to 1 hour (Table 1, Entries 7 and 11); however, the yield decreased
substantially by shortening the reaction time to 30 min (Table 1, Entry 12). We next examined the effects
of the amount of catalyst on this reaction, and the results (Table 1, Entries 13–15) indicated that this
played an essential role in this reaction. Table 1, Entry 13, showed that the yield of the reaction was not
increased by extending reaction time under catalyst-free conditions but that the yields could be improved
by adding 5% of β-CD–SO3H (Table 1, Entry 14). By adding >10% of the catalyst, reaction occurred
smoothly and resulted in a higher yield (up to 93%) (Table 1, Entry 11); however, an increase in the
amount of catalyst cannot enhance the yield of the product (Table 1, Entry 15). Moreover, β-CD (Table 1,
Entry 16) was also used as catalyst, a hydrogen bonding formed between “-CH2OH” of β-CD and “O” at
aldehyde, this bonding can enhances the eletrophilicity of carbonyl and accelerate proceeding of the
reaction,37,38 but the yield was much lower than that of β-CD–SO3H. After completion of this reaction,
β-CD–SO3H could be isolated completely by filtration, and the organic phase was distilled in vacuum to
yield the product, which was washed with 2 mL of ethyl acetate and cyclohexane (1:3 in volume).
Table 1. Optimization for synthesis of imidazo[1,2-a]pyridin-3-amine derivtives (model reaction) a
Entry
1
Solvent
H2O
Time (h)
Temperature (°C)
Amount of the catalyst (mol%)
Yields (%) b
2
2
80
10 c
10 c
10 c
10 c
10 c
10 c
10 c
10 c
10 c
10 c
10 c
10 c
<35
65
45
70
75
90
94
80
74
56
93
84
35
75
94
70
2
toluene
CH2Cl2
THF
80
3
2
reflux
reflux
reflux
80
4
2
5
MeOH
MeCN
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
2
6
2
7
2
reflux
60
8
2
9
2
40
10
11
12
13
14
15
16
5
rt
1
reflux
reflux
reflux
reflux
reflux
reflux
0.5
4
d
—
1
5 c
20 c
10 e
1
1
a
All reactions were conducted with 2-aminobenzamide (1 mmol), benzaldehyde (1 mmol), and isocyanides (1
mmol); b Isolated yields; c β-cyclodextrin–SO3H; d Catalyst-free; e β-cyclodextrin.
With these optimized conditions accessible, the protocol was applied further to various substituted
pyridin-2-amine, isocyanides, and aldehydes. Generally, the reactions were performed by using 10 mol%
of β-CD–SO3H in ethanol or acetonitrile under refluxing conditions for 1 hour to achieve the desired yield,
the results are listed in Table 2. It was observed that most of the reactions proceeded smoothly. First,

different aromatic aldehydes were used to react with pyridin-2-amine and isocyanocyclohexane, which
afforded excellent yields of the products 4b to 4h (89–95%). Then, 2-isocyano-2-methylpropane was used
instead of isocyanocyclohexane (Table 2, compounds 4i to 4q), and the protocols were also performed
smoothly. The substituents on pyridin-2-amine (Table 2, compounds 4m to 4r) showed basically no
influence on the yields of products. Moreover, the reactions of the ethyl 2-isocyanoacetate with
pyridin-2-amine and aldehydes also gave excellent yields.
Table 2. β-Cyclodextrin–SO3H catalyzed synthesis of imidazo[1,2-a]pyridin-3-amine derivatives a
a
Reaction conditions: substituted pyridin-2-amine (1 mmol), arylaldehyde (1 mmol), isocyanide (1 mmol),
and β-cyclodextrin–SO3H (mmol 10%); b Reactions performed in acetonitrile; c Same catalyst used for each of
the four runs.

Subsequently, we turned our attention to catalyst recycling and reuse. After completing the reaction, the
reaction mixture turned clear, and the catalysts were deposited on the button, which had been recovered
by filtration, washed with CH2Cl2 and were reused in the next reaction. These results in Table 2
(compounds 4h, 4i, 4n, and 4q) indicated that the catalytic activity were maintained, even when the
catalyst was reused 4 times, with a slight decrease in product yields.
To compare the efficiency of β-CD–SO3H with the reported catalysts for the synthesis of
imidazo[1,2-a]pyridin-3-amine derivatives, we tabulated the results of these catalysts to promote the
synthesis of target compounds from substituted pyridin-2-amines, isocyanides, and aldehydes in Table 3.
The results showed that β-CD–SO3H is a better catalyst with respect to reaction. From the results of
sulfonated catalyst (Table 3, Entries 5–7), it can be concluded that cellulose sulfuric acid,20 sulfide
nanotubes,23 and β-CD–SO3H could give excellent yields of the products, but the biggest differences in
course of reaction are the reaction time and temperature, it concluded that β-CD–SO3H could give much
shorter reaction time than that of cellulose sulfuric acid; although the sulfide nanotubes could provide a
little superiority in reaction time, but the yields of products were certain decreased when compared that of
β-CD–SO3H; moreover, the preparation process of β-CD–SO3H34 is much convenient than that of sulfide
nanotubes.23
Table 3. Comparison of β-cyclodextrin–SO3H with the reported catalysts for synthesis of
imidazo[1,2-a]pyridin-3-amines
Entry
Catalyst
ZrCl4
Solvent
PEG-400
Temperature (°C)
Time
2–7 h
1 h a, 5 h b
Yield (%) Reference
1
2
3
4
5
6
7
50
reflux
40
72−90
9–75
14
ZnCl2
1,4-dioxane
15
c
RuCl3
—
1 h
87−93
73−96
89−98
71−91
89−96
17
18
c
LaCl3∙7H2O
cellulose sulfuric acid
sulfide nanotubes
β-cyclodextrin–SO3H
—
60
rt d
rt d
20–90 min
3 h
MeOH
MeOH
20
40 min
1 h
23
EtOH/ MeCN
reflux
This work
a Microwave; b Reflux; c Solvent-free; d Room temperature.
Some possible mechanisms for construction of imidazo[1,2-a]pyridin-3-amine derivatives via
three-component one-pot reaction have been reported previously.18,39 According to our experimental
observations and also other mechanisms reported in the literature,39 a possible mechanism of this reaction
is presented in Scheme 2. First, the aldehyde (1) was activated by β-CD–SO3H to give a positive ion, A,
−
and β-CD–SO3 . Then, the carbonyl unit of A was attacked by N-nucleophilic amine (2) to produce an
intermediate, B, which in turn gave an intermediate, C. After which, intermediate D was formed via
dewatering of intermediate C, which was attacked further by a carbonyl unit of isocyanide to generate

intermediate E. Finally, intermediate G was formed via rearrangement and proton transfer, which in turn
–
affords the target product 4. The generated β-CD–SO3 subsequently became β-CD–SO3H by receiving a
proton, which could then be used in the next catalytic cycle.
Scheme 2
To check the applicability of this protocol, a gram scale-up reaction for the synthesis of ethyl
2-((8-methyl-2-(5-methylfuran-2-yl)imidazo[1,2-a]pyridin-3-yl)amino)acetate was performed (Scheme 3),
which resulted in a 90% yield (2.65 g). Because both carboxylic acid and acylhydrazine can be derived
from compounds containing an ester and can form further by obtaining molecules such as amide and
hydrazone to develop biologically active molecules, our continuing research focuses on deriving and
screening for biological activity based on these ester-containing compounds.
Scheme 3

EXPERIMENTAL
The substituted pyridin-2-amines were obtained from TCI (Shanghai, China), isocyanides aromatic
aldehydes were purchased from Accela ChemBio Co., Ltd (Shanghai, China). Melting points were
uncorrected and determined on a WRX-4 monocular microscope (Shanghai Yice Apparatus & Equipment
1
Co., Ltd, China). The H-NMR and 13C-NMR spectra were recorded on a JEOL ECX 500 NMR
1
spectrometer (JEOL Ltd, Japan) at room temperature operating at 500 MHz for H-NMR and 125 MHz
for 13C-NMR by using CDCl3, DMSO-d6 or CD3OD as solvents and TMS as an internal standard; infrared
spectra were recorded in KBr on a IR Pristige-21 spectrometer (Shimadzu corporation, Japan),
absorbencies are reported in cm-1; HR-MS were recorded on a Orbitrap LC-MS instrument (Q-Exative,
Thermo Scientific™, American). The course of the reactions was monitored by TLC; analytical TLC was
performed on silica gel GF 254.
Preparation of sulfonated β-cyclodextrin. To a well stirred mixture of β-cyclodextrin (10.0 g, 4.5
mmol) in CH2Cl2 (50 mL), chlorosulfonic acid (2.00 g, 10 mmol) was added slowly at 0 °C during 3 h.
The resulting mixture was stirred for another 2 h to remove HCl from the reaction vessel. Then, the
mixture was filtered and washed with MeOH and dried at room temperature to obtain
β-cyclodextrin-SO3H as a white powder. 1H NMR (500 MHz, DMSO-d6) δ 5.70 (d, J = 6.9 Hz, 7H), 5.65
(d, J = 2.4 Hz, 7H), 4.79 (d, J = 3.6 Hz, 7H), 4.43 (t, J = 5.7 Hz, 7H), 3.74-3.47 (m, 28H), 3.35-3.31 (m,
7H), 3.28-3.22 (m, 7H); 13C NMR (125 MHz, DMSO-d6) δ 102.48, 82.06, 73.57, 72.94, 72.57, 60.43. The
–SO3H content was measured by titration method and showed 0.52 mequiv./g.34
General procedure for the preparation of imidazo[1,2-a]pyridin-3-amines. To a mixture of
2-aminopyridines (1 mmol), aromatic aldehydes (1 mmol) and isocyanides in EtOH (or MeCN) was
added β-cyclodextrin-SO3H (10 mol%). The reaction mixture was then allowed to stir for 1 h under 80 °C.
After complication of this reaction, the resulting mixture was cooled and the β-cyclodextrin-SO3H was
removed by filtration, the organic phase was concentrated under reduced pressure. Afterwards the residue
were washed with EtOAc and cyclohexane (1:3) and dried to give the product.
N-Cyclohexyl-2-phenylimidazo[1,2-a]pyridin-3-amine (4a): White solid; mp 199.2-201.3 °C, lit.20
200-202 °C; IR 3257, 2957, 2925, 2852, 1630, 1554, 1503, 1447, 1342, 1226, 1197, 1099, 1017, 889, 840,
1
752, 734, 605 cm-1; H NMR (500 MHz, CDCl3) δ 1.12-1.25 (m, 5H), 1.68-1.83 (m, 5H), 2.95-3.0 (m,
1H), 3.12 (br, 1H), 6.76 (t, J = 5 Hz, 1H), 7.2 (t, J = 5 Hz, 1H), 7.32 (t, J = 10 Hz, 1H), 7.43-7.46 (m, 2H),
13
7.54 (d, J = 5 Hz, 1H), 8.02-8.04 (m, 2H), 8.10 (d, J = 5 Hz, 1H); C NMR (125 MHz, CDCl3) δ 24.9,
25.8, 34.3, 57.0, 111.6, 117.3, 122.8, 123.9, 127.1, 127.3, 128.6, 134.6, 141.7, 141.8; HR-MS (ESI+) m/z
Calcd for C19H21N3 [M+H]+ 292.18082; Found 292.18054.
N-Cyclohexyl-2-(thiophen-2-yl)imidazo[1,2-a]pyridin-3-amine (4b): Grey white solid; mp
164.3-165.4 °C; IR 3305, 2967, 2927, 2854, 1648, 1556, 1502, 1344, 1226, 1197, 1099, 1019, 901, 845,

1
753, 737, 602 cm-1; H NMR (500 MHz, CDCl3) δ 1.12-1.36 (m, 6H), 1.60-1.88 (m, 4H), 3.04-3.08 (m,
1H), 3.10 (br, 1H, NH), 6.78 (t, J= 10 Hz, 1H), 7.12-7.17 (m, 2H), 7.32 (d, J = 4.5 Hz, 1H), 7.54 (d, J =
13
10 Hz, 1H), 7.62 (d, J = 5 Hz, 1H), 8.10 (d, J = 5 Hz, 1H); C NMR (125 MHz, CDCl3) δ 25.0, 25.8,
34.4, 57.2, 112.1, 116.9, 122.9, 124.1, 124.3, 124.8, 124.9, 125.0, 127.8, 141.5; HR-MS (ESI+) m/z Calcd
for C17H19N3S [M+H]+ 298.13724; Found 298.13699.
N-Cyclohexyl-2-(thiophen-3-yl)imidazo[1,2-a]pyridin-3-amine (4c): Yellow solid; mp 170.8-171.6 °C;
IR 3309, 3301, 2955, 2863, 1632, 1549, 1458, 1342, 1226, 1197, 1099, 1017, 889, 840, 752, 734, 605
1
cm-1; H NMR (500 MHz, CDCl3) δ 1.16-1.37 (m, 5H), 1.70-1.85 (m, 5H), 2.97-3.02 (m, 1H), 3.09 (br,
1H), 6.77 (t, J = 6.5 Hz, 1H), 7.12 (t, J = 7.5 Hz, 1H), 7.38 (dd, J = 3 Hz, 5 Hz, 1H), 7.54 (d, J = 9 Hz,
1H), 7.84 (dd, J = 3 Hz, 1 Hz, 1H), 8.09 (d, J = 7 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 25.0, 25.8, 34.4,
57.2, 112.0, 117.0, 120.0, 121.9, 122.9, 124.3, 124.6, 125.8, 126.6; HR-MS (ESI+) m/z Calcd for
C17H19N3S [M+H]+ 298.13724; Found 298.13687.
N-Cyclohexyl-2-(1H-imidazol-4-yl)imidazo[1,2-a]pyridin-3-amine
(4d):
Yellow
solid;
mp
1
184.2-185.7 °C; IR 3286, 2959, 2932, 2857, 1633, 1559, 1513, 1467, 889, 843, 756, 714, 625 cm-1; H
NMR (500 MHz, CDCl3) δ 1.13-1.34 (m, 5H), 1.70-1.86 (m, 5H), 2.96-3.00 (m, 1H), 4.19 (br, 1H), 6.85
(t, J = 10 Hz, 1H), 7.20 (t, J = 5 Hz, 1H), 7.61 (d, J = 10 Hz, 1H), 7.65 (s, 1H), 7.74 (s, 1H), 8.08 (d, J = 5
Hz, 1H), 9.81 (br, 1H); 13C NMR (125 MHz, CDCl3) δ 25.09, 25.77, 34.14, 56.66, 112.82, 115.89, 118.53,
123.04, 125.51, 125.54, 125.82, 127.05, 130.70, 135.51, 139.31, 140.30; HR-MS (ESI+) m/z Calcd for
C16H19N5 [M+H]+ 282.17132; Found 282.17108.
N-Cyclohexyl-2-(5-methylfuran-2-yl)imidazo[1,2-a]pyridin-3-amine (4e): White solid; mp
89.1-90.5 °C; IR 3305, 2968, 2945, 2856, 1645, 1545, 1543, 1450, 1354, 1226, 1098, 1013, 885, 837, 755
cm-1; 1H NMR (500 MHz, CDCl3): 1.14-1.32 (m, 6H), 1.71-1.89 (m, 4H), 2.39 (s, 1H), 2.91-2.98 (m, 1H),
3.50 (br, 1H), 6.09-6.10 (m, 1H), 6.72 (d, J = 3.1 Hz, 1H), 6.75 (td, J = 6.7, 0.8 Hz, 1H), 7.09 (ddd, J =
13
9.0, 6.7, 1.0 Hz, 1H), 7.49 (d, J = 9.0 Hz, 1H), 8.05 (dd, J = 6.8, 1.1 Hz, 1H); C NMR (125 MHz,
CDCl3): δ 13.9, 25.1, 25.9, 34.3, 57.3, 107.4, 107.6, 111.6, 117.2, 122.8, 123.9, 125.0, 141.9, 148.5,
151.5; HR-MS (ESI+) m/z Calcd for C18H21N3O [M+H]+ 296.17574; Found 296.17548.
2-(6-Bromopyridin-3-yl)-N-cyclohexylimidazo[1,2-a]pyridin-3-amine (4f): Yellow solid; mp
156.4-158.3 °C; IR 3287, 3007, 2946, 2872, 1645, 1564, 1521, 1453, 1374, 1246, 1213, 1106, 1034, 886,
844, 755, 745, 632 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.13-1.33 (m, 6H), 1.67-1.81 (m, 4H), 2.92-2.97
(m, 1H), 3.15 (br, J = 5 Hz, 1H), 6.82 (t, J = 5 Hz), 7.18 (t, J = 5 Hz), 8.06 (dd, J = 7 Hz, 1 Hz, 1H), 8.31
(d, J = 2.5 Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 24.9, 25.7, 34.3, 57.0, 112.4, 117.6, 120.00, 120.01,
122.9, 125.1, 125.5, 128.1, 129.9, 133.0, 137.0, 140.5, 142.2, 148.3; HR-MS (ESI+) m/z Calcd for
C18H19BrN4 [M+H]+ 371.08659; Found 371.08649.
N-Cyclohexyl-2-(furan-2-yl)imidazo[1,2-a]pyridin-3-amine (4g): Yellow solid; mp 121.3-123.2 °C,

lit.14 121-122 °C; IR 3308, 2978, 2944, 2867 1645, 1538, 1533, 1467, 1354, 1226, 1209, 1109, 1065, 878,
843, 759, 738, 635 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.12-1.31 (m, 6H), 1.71-1.91 (m, 4H), 2.91-2.97
(m, 1H), 3.61 (d, J = 6.7 Hz, 1H ), 6.52 (dd, J = 3.4, 1.8 Hz, 1H), 6.76 (td, J = 6.8, 1.1 Hz, 1H), 6.86 (dd,
J = 3.4, 0.7 Hz, 1H), 7.11 (ddd, J = 9.1, 6.6, 1.3 Hz, 1H), 7.49 (ddt, J = 3.7, 1.9, 1.0 Hz, 2H), 8.04 (dt, J =
6.9, 1.2 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 25.1, 25.8, 34.2, 57.2, 106.5, 111.6, 111.8, 117.3, 122.9,
124.1, 125.6, 128.0, 141.6, 141.9, 150.4; HR-MS (ESI+) m/z Calcd for C17H19N3O [M+H]+ 282.16009;
Found 282.16000.
2-(5-Bromopyridin-3-yl)-N-cyclohexylimidazo[1,2-a]pyridin-3-amine (4h): Yellow solid; mp
158.8-159.6 °C; IR 3273, 2987, 2955, 2892, 1639, 1564, 1576, 1457, 1335, 1246, 1209, 1089, 1027, 892,
1
844, 752, 734 cm-1; H NMR (500 MHz, CDCl3) δ 1.12-1.31 (m, 6H), 1.68-1.82 (m, 4H), 2.92-2.98 (m,
1H), 3.07 (d, J = 4.5 Hz, 1H ), 6.82 (td, J = 6.8, J = 1.0 Hz, 1H), 7.18 (ddd, J = 9.0, 6.7, 1.3 Hz, 1H), 7.53
(dd, J = 9.1, 1.0 Hz, 1H), 8.07 (dd, J = 6.9, 1.1 Hz, 1H), 8.63 (t, J = 2.1 Hz, 1H), 8.63 (t, J = 2.1 Hz, 1H),
9.26 (d, J = 1.8 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 24.8, 25.7, 34.3, 57.1, 112.4, 112.7, 117.7, 121.7,
122.9, 125.0, 125.9, 132.3, 132.6, 136.8, 142.3, 146.0, 149.0; HR-MS (ESI+) m/z Calcd for C18H19BrN4
[M+H]+ 371.08659; Found 371.08646.
2-(5-Bromopyridin-2-yl)-N-(tert-butyl)imidazo[1,2-a]pyridin-3-amine (4i): Yellow solid; mp
124.1-124.9 °C; IR 3299, 2987, 2945, 2878, 1656, 1564, 1523, 1452, 1352, 1233, 1207, 1102, 1027, 894,
854, 755, 739, 645 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.14 (s, 9H), 5.18 (br, 1H), 6.75 (t, J = 5 Hz, 1H),
7.18 (t, J = 6.5 Hz, 1H), 8.06 (dd, J = 9 Hz, 6.5 Hz, 1H), 7.52 (d, J = 9 Hz, 1H), 8.09 (d, J = 8.5 Hz, 1H),
13
8.27 (d, J = 7 Hz, 1H), 8.27 (d, J = 2 Hz, 1H); C NMR (125 MHz, CDCl3) δ 30.2, 57.4, 111.8, 117.3,
118.5, 122.7, 124.4, 124.9, 129.1, 133.4, 139.3, 141.6, 149.4, 153.1; HR-MS (ESI+) m/z Calcd for
C16H17BrN4 [M+H]+ 345.07094; Found 345.07098.
N-(tert-Butyl)-2-phenylimidazo[1,2-a]pyridin-3-amine (4j): White solid; mp 166.2-167.9 °C, lit.38
169-170 °C; IR 3320, 2989, 2965, 2862, 1645, 1554, 1523, 1468, 1342, 1226, 1197, 1099, 1017, 840, 752,
1
734 cm-1; H NMR (500 MHz, CDCl3) δ 1.02 (s, 9H), 5.64 (br, 1H), 6.79 (dd, J = 9.3, 3.9 Hz, 1H),
7.33-7.28 (m, 1H), 7.40-7.43 (m, 2H), 7.58 (d, J = 8.6 Hz, 1H), 7.89 (s, 1H), 7.90 (s, 1H), 8.25 (d, J = 6.8
Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 30.4, 56.6, 111.8, 117.0, 123.7, 124.8, 127.7, 128.3, 128.4, 134.6,
138.9, 141.7; HR-MS (ESI+) m/z Calcd for C17H19N3 [M+H]+ 266.16517; Found 266.16495.
N-(tert-Butyl)-2-(thiophen-2-yl)imidazo[1,2-a]pyridin-3-amine
(4k):
Yellow
solid;
mp
132.5-134.3 °C; IR 3305, 2968, 2945, 2856, 1645, 1564, 1523, 1457, 1340, 1236, 1230, 1109, 1031, 889,
1
832, 738 cm-1; H NMR (500 MHz, CDCl3) δ 1.18 (s, 9H), 3.12 (br, 1H), 6.77 (td, J = 6.8, 1.0 Hz, 1H),
7.09 (dd, J = 5.0, 3.6 Hz, 1H), 7.14 (ddd, J = 9.0, 6.7, 1.2 Hz, 1H), 7.31 (dd, J = 5.1, 1.1 Hz, 1H), 7.53 (d,
J = 9.0 Hz, 1H), 7.60 (dd, J = 3.6, 1.1 Hz, 1H), 8.21 (dt, J = 6.9, 1.1 Hz, 1H); 13C NMR (125 MHz,
CDCl3): δ ppm = 30.6, 56.9, 111.8, 113.1, 117.0, 123.0, 123.7, 124.9, 125.1, 127.5, 134.4, 137.1, 142.0;

HR-MS (ESI+) m/z Calcd for C15H17N3S [M+H]+ 272.12159; Found 272.12128.
N-(tert-Butyl)-2-(thiophen-3-yl)imidazo[1,2-a]pyridin-3-amine (4l): White solid; mp 145.2-147.1 °C;
IR 3308, 2970, 2947, 2878, 1655, 1556, 1543, 1467, 1342, 1236, 1233, 1116, 1041, 892, 834, 738 cm-1;
1H NMR (500 MHz, CDCl3) δ 1.18 (s, 9H), 3.09 (s, 1H), 6.79 (t, J = 6.7 Hz, 1H) , 7.14 (dd, J = 8.3, 7.4
Hz, 1H), 7.39 (dd, J = 5.0, 3.0 Hz, 1H), 7.55 (d, J = 9.0 Hz, 1H), 7.74 (dd, J = 5.0, 1.2 Hz, 1H) 7.85 (dd, J
13
= 2.9, 1.1 Hz, 1H), 8.10 (d, J = 6.8 Hz, 1H); C NMR (125 MHz, CDCl3): δ 34.4, 57.2, 112.0, 117.0,
121.9, 122.9, 124.3, 124.6, 125.8, 126.6, 133.3, 135.0, 141.4; HR-MS (ESI+) m/z Calcd for C15H17N3S
[M+H]+ 272.12159; Found 272.12140.
N-(tert-Butyl)-2-(5-methylfuran-2-yl)imidazo[1,2-a]pyridin-3-amine (4m): White solid; mp
114.3-115.4 °C; IR 3289, 2947, 2955, 2876, 1636, 1556, 1523, 1487, 1363, 1230, 1221, 1123, 894, 845,
756, 734 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.16 (s, 9H), 2.37 (s, 3H), 3.48 (br, 1H), 6.09 (m, 1H), 6.74
(td, J = 6.8, 1.0 Hz, 1H), 6.78 (d, J = 3.2 Hz, 1H), 7.12 (ddd, J = 8.9, 6.7, 1.2 Hz, 1H), 7.52-7.47 (m, 1H),
8.22 (dd, J = 6.0, 0.9 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 13.9, 30.2, 56.7, 107.6, 108.5, 111.5, 116.8,
123.6, 123.7, 124.7, 130.6, 142.1, 148.2, 151.5; HR-MS (ESI+) m/z Calcd for C16H19N3O [M+H]+
270.16009; Found 270.15982.
N-(tert-Butyl)-5-methyl-2-(5-methylfuran-2-yl)imidazo[1,2-a]pyridin-3-amine (4n): Brown solid; mp
1
72.3-73.2 °C; IR 3303, 2967, 2935, 2862, 1642, 1567, 1523, 1465, 1342, 1230, 889, 825, 752 cm-1; H
NMR (500 MHz, CDCl3) δ 1.14 (s, 9H), 2.19 (br, 1H), 2.38 (s, 3H), 2.60 (s, 3H), 6.08 (dd, J = 3.2, 1.0 Hz,
1H), 6.67 (t, J = 6.8 Hz, 1H), 6.79 (d, J = 3.2 Hz, 1H), 6.91 (dt, J = 6.75 Hz, 1.15 Hz, 1H), 8.13-8.08 (d, J
= 6.85 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 13.85, 16.77, 30.18, 56.52, 107.50, 108.58, 111.44,
113.55, 121.65, 123.32, 124.18, 126.72, 142.55, 148.40, 151.30; HR-MS (ESI+) m/z Calcd for C17H21N3O
[M+H]+ 284.17574; Found 284.17548.
N-(tert-Butyl)-8-methyl-2-(5-methylfuran-2-yl)imidazo[1,2-a]pyridin-3-amine (4o): Brown sticky
1
liquid; IR 3278, 2987, 2975, 2878, 2854, 1635, 1564, 1554, 1457, 1343, 1230, 1210, 845, 734 cm-1; H
NMR (500 MHz, DOCD3) δ 0.94 (s, 9H), 2.36 (s, 1H), 2.96 (s, 3H), 6.12 (dd, J = 3.2, 1.0 Hz, 1H),
6.62-6.57 (m, 1H), 6.69 (d, J = 3.2 Hz, 1H), 7.13 (dd, J = 9.0, 6.8 Hz, 1H), 7.25 (dd, J = 5.3, 4.3 Hz, 1H);
13C NMR (125 MHz, DOCD3) δ 12.25, 19.55, 28.25, 55.74, 107.17, 108.99, 111.89, 113.67, 114.34,
125.76, 131.82, 137.92, 143.11, 147.52, 151.72; HR-MS (ESI+) m/z Calcd for C17H21N3O [M+H]+
284.17574; Found 284.17542.
2-(5-Bromopyridin-2-yl)-N-(tert-butyl)-8-methylimidazo[1,2-a]pyridin-3-amine (4p): Yellow solid;
mp 101.5-103.4 °C; IR 3301, 2967, 2945, 2864, 2874, 1654, 1545, 1523, 1454, 1343, 1226, 1214, 1108,
895, 845, 757 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.13 (s, 9H), 2.61 (s, 3H), 5.16 (br, 1H), 6.66 (dt, J =
11.6, 5.8 Hz, 1H), 6.91 (dd, J = 6.6, 1.0 Hz, 1H), 7.87 (dt, J = 8.5, 2.7 Hz, 1H), 8.14 (d, J = 6.9 Hz, 1H),
13
8.18 (dd, J = 8.6, 3.5 Hz, 1H), 8.60 (d, J = 2.2 Hz, 1H); C NMR (125 MHz, CDCl3) δ 16.74, 30.17,

57.26, 118.25, 122.31, 123.01, 123.37, 127.21, 129.42, 139.17, 139.18, 142.12, 149.35, 153.56; HR-MS
(ESI+) m/z Calcd for C17H19BrN4 [M+H]+ 359.08659; Found 359.08624.
2-(4-Bromopyridin-3-yl)-N-(tert-butyl)-8-methylimidazo[1,2-a]pyridin-3-amine (4q): White solid;
mp 163.1-164.5 °C; IR 3297, 2959, 2945, 2854, 1645, 1564, 1524, 1446, 1354, 1230, 1208, 1114, 902,
1
854, 755 cm-1; H NMR (500 MHz, CDCl3) δ 1.06 (s, 9H), 2.61 (s, 3H), 6.73 (t, J = 6.8 Hz, 1H),
7.04-6.88 (m, 1H), 7.58-7.46 (m, 1H), 8.04 (d, J = 6.9 Hz, 1H), 8.25 (dd, J = 8.3, 2.4 Hz, 1H), 9.03 (dd, J
13
= 2.5, 0.7 Hz, 1H); C NMR (125 MHz, CDCl3) δ 16.64, 30.59, 56.61, 112.08, 121.36, 123.72, 124.44,
127.42, 127.87, 130.99, 135.05, 138.11, 140.38, 143.03, 149.43; HR-MS (ESI+) m/z Calcd for
C17H19BrN4 [M+H]+ 359.08659; Found 359.08643.
Ethyl 2-((5-methyl-2-(thiophen-3-yl)imidazo[1,2-a]pyridin-3-yl)amino)acetate (4r): Brown sticky
liquid; IR 3307, 2998, 2965, 2867, 1715, 1647, 1563, 1543, 1467, 1362, 1226, 1197, 1103, 1054, 895,
1
847, 752 cm-1; H NMR (500 MHz, CDCl3) δ 1.25 (t, J = 7.1 Hz, 3H), 2.91 (s, 3H), 3.75 (d, J = 4.9 Hz,
2H), 3.80 (br, 1H), 4.21 (q, J = 7.2 Hz, 2H), 6.47 (dd, J = 6.8, 1.0 Hz, 1H), 7.05 (dd, J = 8.9, 6.9 Hz, 1H),
7.37-7.33 (m, 1H), 7.42 (d, J = 9.3 Hz, 1H), 7.71 (dt, J = 5.0, 1.1 Hz, 1H), 7.87 (d, J = 2.9 Hz, 1H); 13C
NMR (125 MHz, CDCl3) δ 14.21, 19.70, 51.41, 61.47, 114.17, 115.07, 125.64, 122.46, 125.97, 125.77,
126.60, 134.22, 136.63, 142.81, 171.17; HR-MS (ESI+) m/z Calcd for C16H17N3O2S [M+H]+ 316.11142;
Found 316.11105.
Ethyl 2-((2-(5-bromopyridin-2-yl)imidazo[1,2-a]pyridin-3-yl)amino)acetate (4s): Brown solid, mp
1
91.7-93.5 °C; H NMR (500 MHz, CDCl3) δ 1.21 (t, J = 7.2 Hz, 3H), 3.88 (s, 2H), 4.17 (q, J = 7.2 Hz,
2H), 6.26 (br, 1H, NH), 6.84 (t, J = 6.7 Hz, 1H), 7.22-7.14 (m, 1H), 7.60 (d, J = 9.1 Hz, 1H), 7.88 (dd, J =
8.5, 2.4 Hz, 1H), 8.11 (t, J = 8.5 Hz, 2H), 8.64 (d, J = 2.3 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 14.17,
48.61, 61.38, 112.78, 117.67, 118.35, 121.97, 122.94, 124.96, 130.57, 139.46, 140.72, 149.91, 170.73;
HR-MS (ESI+) m/z Calcd for C16H15BrN4O2 [M+H]+ 375.04511; Found 375.04483.
Ethyl 2-((8-methyl-2-(5-methylthiophen-2-yl)imidazo[1,2-a]pyridin-3-yl)amino)acetate (4t): Brown
sticky liquid; 1H NMR (500 MHz, CDCl3) δ 1.29 (t, J = 7.1 Hz, 3H), 2.50 (s, 3H), 2.87 (s, 3H), 3.57 (t, J
= 5.1 Hz, 1H), 3.83 (d, J = 5.2 Hz, 2H), 4.26 (q, J = 7.1 Hz, 2H), 6.41 (d, J = 6.8 Hz, 1H), 6.74 (d, J = 3.0
13
Hz, 1H), 6.99 (dd, J = 8.8, 7.0 Hz, 1H), 7.36 (t, J = 6.1 Hz, 2H); C NMR (125 MHz, CDCl3) δ 4.26,
15.44, 15.45, 19.43, 51.31, 61.41, 113.55, 115.54, 124.20, 124.36, 124.87, 126.04, 134.38, 135.01, 136.09,
139.88, 143.54, 171.05; HR-MS (ESI+) m/z Calcd for C17H19N3O2S [M+H]+ 330.12707; Found
330.12665.
Ethyl 2-((8-methyl-2-(5-methylfuran-2-yl)imidazo[1,2-a]pyridin-3-yl)amino)acetate (4u): Brown
sticky liquid; 1H NMR (500 MHz, CDCl3) δ 1H NMR (500 MHz, CDCl3) δ 1.26 (t, J = 7.2 Hz, 3H), 2.38
(s, 3H), 2.59 (s, 3H), 3.84 (d, J = 5.7 Hz, 2H), 4.00 (t, J = 5.7 Hz, 1H), 4.21 (q, J = 7.2 Hz, 2H), 6.09 (dd,
J = 2.1, 0.9 Hz, 1H), 6.70 (t, J = 6.8 Hz, 1H), 6.74 (d, J = Hz, 2H), 6.92 (dd, J = 6.8, 1.0 Hz, 1H), 8.04 (dd,

13
J = 6.8, 0.5 Hz, 1H). C NMR (125 MHz, CDCl3) δ 13.89, 14.24, 16.78, 49.82, 51.31, 61.36, 107.57,
107.90, 108.70, 111.96, 120.57, 123.17, 127.15, 142.36, 147.94, 151.96, 171.56; HR-MS (ESI+) m/z
Calcd for C17H19N3O3 [M+H]+ 319.14992; Found 319.14951.
Ethyl 2-((2-(2-chlorophenyl)-5-methylimidazo[1,2-a]pyridin-3-yl)amino)acetate (4v): Light brown
1
sticky liquid; H NMR (500 MHz, CDCl3) δ 1.15 (t, J = 7.1 Hz, 3H), 2.94 (s, 3H), 3.49 (d, J = 5.7 Hz,
2H), 3.69 (t, J = 5.7 Hz, 1H), 4.01 (q, J = 7.1 Hz, 2H), 6.48 (d, J = 6.7 Hz, 1H) 7.04 (dd, J = 8.8, 6.9 Hz,
1H), 7.32 (dd, J = 6.2, 2.8 Hz, 2H), 7.39 (d, J = 9.0 Hz, 1H), 7.58 (dd, J = 5.9, 3.3 Hz, 1H), 7.48-7.45 (m,
13
1H); C NMR (125 MHz, CDCl3) δ 14.14, 19.94, 52.32, 61.10, 113.34, 113.64, 115.97, 124.66, 126.95,
129.58, 129.61, 132.43, 133.34, 133.72, 136.75, 137.10, 143.35, 170.89; HR-MS (ESI+) m/z Calcd for
C18H18ClN3O2 [M+H]+ 344.11603; Found 344.11572.
Ethyl 2-((2-(2-fluorophenyl)-5-methylimidazo[1,2-a]pyridin-3-yl)amino)acetate (4w): Light brown
sticky liquid; 1H NMR (500 MHz, CDCl3) δ 1.17 (t, J = 7.1 Hz, 3H), 2.95 (s, 3H), 3.57 (d, J = 5.8 Hz, 2H),
3.81 (br, 1H), 4.04 (q, J = 7.2 Hz, 2H), 6.48 (d, J = 6.8 Hz, 1H), 7.04 (dd, J = 8.9, 6.9 Hz, 1H), 7.17 (dd, J
= 10.5, 8.6 Hz, 1H), 7.24-7.27 (m, 1H), 7.33-7.38 (m, 1H), 7.42 (d, J = 9.0 Hz, 1H), 7.79 (td, J = 7.6, 1.7
13
Hz, 1H); C NMR (125 MHz, CDCl3) δ 14.16, 19.87, 52.45, 61.12, 113.66, 115.72, 115.91, 124.62,
124.72, 129.74, 129.81, 131.72, 131.75, 133.88, 136.78, 143.83, 170.93; HR-MS (ESI+) m/z Calcd for
C18H18FN3O2 [M+H]+ 328.14558; Found 328.14526.
ACKNOWLEDGEMENTS
Financial support from the National Natural Science Foundation of China (Nos. 21302025, 21562012,
21142006), the Special Funds for Training Object of Outstanding Young Scientific & Technological
Talents in Guizhou Province (No. 2015-15#), and the Science & Technology Foundation of Guizhou
Province (J[2014]2056#).
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