Facile synthesis of imidazo[1,2-a]pyridines promoted by room-temperature ionic liquids under ultrasound irradiation

Article abstract of DOI:10.1007/s00706-018-2238-3

Abstract: A simple and efficient procedure for the synthesis of substituted imidazo[1,2-a]pyridines under ultrasound irradiation has been developed. The reactions were carried out using ionic liquids as catalyst. The reaction procedure demonstrated a broad substrate scope for both acetophenones and 2-aminopyridines, and provided convenient access to a wide variety of imidazo[1,2-a]pyridines. The present method offers several advantages compared to traditional heating methods such as higher yields, shorter reaction times, milder reaction conditions, and easier work-up procedure. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s00706-018-2238-3

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-018-2238-3(0123456789(). -, vo Vl
)
0
1
2
3
4
5
6
7
8
9
(
)
.
-
v
o
l
V
)
ORIGINAL PAPER
Facile synthesis of imidazo[1,2-a]pyridines promoted by room-
temperature ionic liquids under ultrasound irradiation
Piyawat Paengphua1 Sirirat Chancharunee1
•
Received: 3 October 2017 / Accepted: 23 May 2018
Ó Springer-Verlag GmbH Austria, part of Springer Nature 2018
Abstract
A simple and efficient procedure for the synthesis of substituted imidazo[1,2-a]pyridines under ultrasound irradiation has
been developed. The reactions were carried out using ionic liquids as catalyst. The reaction procedure demonstrated a
broad substrate scope for both acetophenones and 2-aminopyridines, and provided convenient access to a wide variety of
imidazo[1,2-a]pyridines. The present method offers several advantages compared to traditional heating methods such as
higher yields, shorter reaction times, milder reaction conditions, and easier work-up procedure.
Graphical abstract
Keywords Cyclization Á Ketones Á One-pot synthesis Á Heterocycles
Introduction
aminooxygenation [14], and hydroamination [15]. Of the
many methods which have been reported, the condensation
reactions of 2-aminopyridines with a-halogenoketones is
the most attractive approach compared to other synthetic
methods due to several advantages such as operational
simplicity, and reduction in waste generation [16]. There
are; however, some limitations of this method such as high
toxicity and lachrymatory property of a-halogenoketones
[17]. This limitation can be partly overcome by the use of
a-ketomethylpyridinium iodide salts directly without iso-
lation of the a-haloketone via Ortoleva–King reaction
[18, 19]. This approach; however, has significant draw-
backs such as long reaction times, high reaction tempera-
tures, and low yields.
In recent years, the synthesis of imidazo[1,2-a]pyridines
has drawn considerable attention due to their useful
antiviral [1–3], antimicrobial [4], antiulcer [5], and anti-
inflammatory therapeutic activities[6]. Additionally, their
application has recently expanded to include biological
imaging probes due to their remarkable fluorescence
properties [7].
Various methods have been published for the synthesis
of imidazo[1,2-a]pyridines. These methods have been
continually modified and improved. These included
oxidative coupling [8–12], oxidative amination [13],
In recently years, ionic liquids (ILs) have attracted
considerable attention for organic synthesis as alternative
reaction media to volatile organic solvents owing to their
distinctive physical properties such as low volatility, and
non-flammability. Furthermore, the physical and chemical
properties of the ionic liquids can be controlled by tailoring
their structures. Although, ionic liquids were initially
developed for use as solvents; recent studies have shown
that ionic liquids can be used in catalysis as catalyst
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s00706-018-2238-3) contains
supplementary material, which is available to authorized
users.
& Sirirat Chancharunee
sirirat.ch@cmu.ac.th
1
Department of Chemistry, Faculty of Science, Chiang Mai
University, Chiang Mai 50200, Thailand
123

P. Paengphua, S. Chancharunee
Scheme 1
Table 1 Reaction conditions and yields for the synthesis of 2-phenylimidazo[1,2-a]pyridine (3a) using conventional method (CM) and ultra-
sound irradiation condition (US)
Entry
Condition
Solvent
Time/h
Yield/%a
Method
1
Benzene
CMb
CMb
CMb
CMb
CMb
CMb
CMc
USd
USd
USd
USd
USd
USd
24
56
55
50
52
70
73
69
55
58
59
60
73
72e
2
Solvent-free
[BMIM]Br
24
3
24
4
[BMIM]Cl
24
5
[BMIM]BF4
[BMIM]PF6
Benzene ? [BMIM]PF6
Benzene
24
6
24
7
24
8
2.5
2.5
2.5
2.5
2.5
2.5
9
Solvent-free
[BMIM]Br
10
11
12
13
[BMIM]Cl
[BMIM]BF4
[BMIM]PF6
Reaction conditions: 1a (0.51 mmol), 2a (1.17 mmol), iodine (0.61 mmol) under solvent-free or solvents 0.7 cm3, carried out under specified
conditions, followed by excess aqueous NaOH (45%), 100 °C, 1 h
aIsolated yield
bCM: conventional method performed by heating at 110 °C
cThe reaction was carried out in benzene containing 4% (w/v) of [BMIM]PF6
dUS: ultrasound irradiation method performed at a frequency of 40 kHz at 30–35 °C
eThe reaction under conventional heating for 2.5 h afforded 3a in 12%
[20–22], catalyst activator [23], and co-catalyst for various
reactions [24, 25].
In this paper, we wish to report an efficient method for
one-pot synthesis of imidazo[1,2-a]pyridines using ionic
liquids as catalyst with the assist of ultrasound irradiation.
The method provides a significant achievement in com-
parison to conventional methods in terms of reducing
reaction time, operational simplicity, and enhancing yield.
The application of ultrasonic waves as alternative source
of energy has received considerable attention over the past
few decades, and it has made a major contribution in the
field of organic synthesis. It is being used increasingly to
speed up various organic reactions and provides new pos-
sibilities to stimulate new reactions that are difficult to
synthesize by conventional methods. Recently, the sono-
chemical approach has been further extended to the syn-
theses of fused heterocyclics such as benzofuran [26],
pyrimidopyrans [27], pyridopyrimidines [28], pyrazolopy-
ridines [29], and dihydropyranopyrazoles [30].
Results and discussion
To probe whether the reaction can be influenced by ionic
liquids, we began our studies using acetophenone (1a) and
2-aminopyridine (2a) as a model reaction. The model
123

Facile synthesis of imidazo[1,2-a]pyridines promoted by room-temperature ionic liquids under…
explored the possibility of using the ultrasonication method
Table 2 Effect of [BMIM]PF6 concentrations on the synthesis of 3a
under ultrasound irradiation
to further improve the procedure. Hence, the model reac-
tion was further investigated under the influence of ultra-
sound irradiation at 30–35 °C for 2.5 h. It was revealed
that under ultrasound irradiation, in all cases, yields were
comparable to or better than using conventional methods in
shorter reaction time at lower temperature (Table 1, entries
8–13). Under ultrasound irradiation, higher yields, i.e., 58,
59, 60, and 73% (Table 1, entries 9, 10, 11, and 12) were
obtained in comparison with the same reactions performed
in the same reaction medium under conventional heating
(Table 1, entries 2, 3, 4, and 5, respectively). The higher
yield was obtained when [BMIM]BF4 and [BMIM]PF6
were used as compared to the reaction under solvent-free,
benzene, and other ionic liquids. These results revealed that
ionic liquids [BMIM]BF4 and [BMIM]PF6 could effec-
tively accelerate the reaction under ultrasound irradiation.
Entry
[BMIM]PF6/mol%
Isolated yield/%
1
2
3
4
5
6
None
700 (0.7 cm3)b
58a
72
100
30
74
72
80 (82)c
20
10
69
Reaction conditions: 1a (0.51 mmol), 2a (1.17 mmol), iodine
(0.61 mmol) in the presence of various [BMIM]PF6 concentrations,
ultrasound irradiation at 30–35 °C, 2.5 h, followed by excess aqueous
NaOH (45%), 100 °C, 1 h
aReaction conditions: 1a (0.51 mmol), 2a (1.17 mmol), iodine
(0.61 mmol) under solvent-free condition, ultrasound irradiation at
30–35 °C, 2.5 h, followed by excess aqueous NaOH (45%), 100 °C,
1 h
b[BMIM]PF6 0.7 cm3 (700 mol%) was used as solvent
cReaction conditions: 1a (0.51 mmol), 2a (1.17 mmol), iodine
(0.61 mmol) and 20 mol% [BMIM]PF6, ultrasound irradiation at
30–35 °C, 2.5 h, followed by excess aqueous K2CO3 (35%), ultra-
sound irradiation at 40–45 °C, 20 min
Effect of ionic liquids concentrations
Prompted by these encouraging results, we next investi-
gated the effect of ionic liquids concentrations on the
reaction as catalyst. The system with [BMIM]PF6 was
chosen for this investigation due to its water immiscible
property which provided an advantage over [BMIM]BF4 in
work-up process. The results are summarized in Table 2. It
was observed that, decreasing the amount of [BMIM]PF6 to
100 mol% led to an improvement of yield (74%) than that
was obtained using [BMIM]PF6 as solvent (Table 2, entries
2 and 3). However, decreasing the amount of [BMIM]PF6
to 30 mol% only provided the desired product with com-
parable yield. The best result was obtained when 20 mol%
of [BMIM]PF6 was employed (Table 2, entry 5). An
attempt to further decrease the amount of [BMIM]PF6
(10 mol%) resulted in a significantly lower yield (69%;
Table 2, entry 6). It was also observed that the second step
of this reaction can be accomplished under milder condi-
tions with slightly higher yield using aqueous K2CO3
(35%) under ultrasound irradiation at 40–45 °C for 20 min
instead of heating with aqueous NaOH (45%) at 100 °C for
1 h. These results demonstrated that the use of ionic liquids
in catalytic amount under ultrasound irradiation led to
higher yields and the reactions were markedly accelerated
using ultrasound irradiation.
reaction was carried out in the presence of iodine at 110 °C
for 24 h in three different systems, benzene (as reference
reaction medium), solvent-free and room temperature ionic
liquids, i.e., 1-butyl-3-methylimidazolium bromide
[(BMIM)Br],
1-butyl-3-methylimidazolium
chloride
[(BMIM)Cl], 1-butyl-3-methylimidazolium tetrafluorobo-
rate [(BMIM)BF4], and 1-butyl-3-methylimidazolium
hexafluorophosphate [(BMIM)PF6], followed by adding
excess amount of aqueous NaOH (45%) and heating at
100 °C for 1 h (Scheme 1). The results are summarized in
Table 1.
As indicated in Table 1, the corresponding product 3a
was obtained in moderate yield after heating at 110 °C for
24 h under solvent-free condition which was comparable to
the yield obtained in benzene, whereas slightly lower yields
were obtained when [BMIM]Br and [BMIM]Cl were
employed (Table 1, entries 1–4). However, when
[BMIM]BF4 and [BMIM]PF6 were used, higher yields
were obtained (Table 1, entries 5 and 6). In addition, a
significant increase of yield was also observed, when the
model reaction was carried out in benzene containing 4%
(w/v) of [BMIM]PF6 (Table 1, entry 7). These conditions
have proved that [BMIM]BF4 and [BMIM]PF6 played a
significant role in the model reaction. Although the reac-
tions in [BMIM]BF4 and [BMIM]PF6 proceeded more
smoothly and in the better yields, relatively heating at high
temperature (110 °C) and long reaction times were
required to reach completion (24 h). In an effort to increase
the efficiency and decrease the temperature, we then
Optimization the molar ratio of reagents
under ultrasound irradiation
Next, we examined the influence of the molar ratio of
reagents under ultrasound irradiation and the results of
these findings are presented in Table 3.
The effect of molar ratio of the reagents was investi-
gated under ultrasound irradiation at 30–35 °C in the
123

P. Paengphua, S. Chancharunee
which provided 5a in 48% (Scheme 2). The best condition
for synthesizing imidazo[1,2-a]pyridine (3a) was obtained
by carrying out the reaction under ultrasound irradiation
with 2.3:1.0:1.2 molar ratios of 2-aminopyridine (2a),
acetophenone (1a), and iodine in the presence of 20 mol%
[BMIM]PF6 at 30–35 °C for 2.5 h. When compared to the
conventional method, this approach has greatly improved
the total reaction time with milder conditions and easier
operation.
Table 3 Effect of substrate ratios
Entry
Ratio 2a:1a
Iodine vs. 1a/equiv
Isolated yield/%
1
1.2
1.5
2.0
2.3
2.5
2.7
3.0
2.3
2.3
2.3
2.3
1.2
1.2
1.2
1.2
1.2
1.2
1.2
0.5
1.0
1.5
2.0
41
52
69
82
83
83
80
29
67
68a
64b
2
3
4
5
6
7
Substrate scope of imidazo[1,2-a]pyridines
synthesis
8
9
10
11
Having established the optimal reaction conditions, the
substrate scope of [BMIM]PF6-catalyzed system under
sonication was then examined. A series of substituted
acetophenones 1 containing electron-withdrawing and
electron-donating groups were subjected to react with
2-aminopyridine or 2-amino-4-methylpyridine 2 using
20 mol% [BMIM]PF6 as catalyst under ultrasound irradi-
ation and compared with the conventional heating method
and work-up procedure (Table 4). In general, shorter
reaction time and highly significant differences in yields
were obtained among entries when the reaction was carried
out under ultrasound irradiation. It was also observed that,
under sonication condition, the substituted acetophenones,
such as p-bromo, p-methoxy, m-nitro, and m-methoxy
groups on the aryl ring, reacted with 2a efficiently and gave
the desired products 3b, 3c, 3e, and 3f in high yields of
70–76% (Table 4, entries 2, 3, 5, and 6).
Reaction conditions: 20 mol% [BMIM]PF6, ultrasound irradiation at
30–35 °C, 2.5 h, followed by excess aqueous K2CO3 (35%), ultra-
sound irradiation at 40–45 °C, 20 min
a4a was isolated in 7%
b4a and 5a were isolated in 19 and 10%, respectively
presence of 20 mol% [BMIM]PF6. The molar ratio of
2-aminopyridine/acetophenone/iodine
ranging
from
1.2:1.0:1.2 to 3.0:1.0:1.2 was varied (Table 3). The results
revealed that increasing the molar ratio of 2-aminopyridine
from 1.2 to 2.3 resulted in higher yields (Table 3, entries
1–4). However, no significant change in product yield was
observed, as the molar ratio of 2-aminopyridine was further
increased to 3.0. Additionally, the reaction was also opti-
mized by varying the amount of iodine. The yield of the
product significantly decreased with increased the mole
ratio of iodine from 1.2 to 2.0 (Table 3, entries 10 and 11).
This was due to the formation of the side reaction products,
6-iodo-2-phenylimidazo[1,2-a]pyridine (4a) and 3-iodo-2-
phenylimidazo[1,2-a]pyridine (5a). The observed minor
side reaction product 4a resulted from iodination of
2-aminopyridine which then further reacted with ace-
tophenone (1a). On the other hand, 5a was formed pre-
sumably by further iodination of the desired product 3a.
This was confirmed by reacting 3a with 1 equivalent of
iodine in [BMIM]PF6 under the same condition for 2.5 h
On the other hand, p-nitro and o-hydroxy acetophenones
reacted with 2a, to provide the expected product 3d and 3g
in 53-58% (Table 4, entries 4 and 7). Whereas, the
2-amino-4-methylpyridine (2b) reacted efficiently with
acetophenone and substituted acetophenone, such as p-
bromo, p-methoxy, p-nitro, and o-hydroxy groups on the
aryl ring, and gave the desired products 3h–3k and 3m in
high-to-excellent yields of 72–86% (Table 4, entries 8–11
and 13). Similarly, the product 3l was obtained in moderate
Scheme 2
123

Facile synthesis of imidazo[1,2-a]pyridines promoted by room-temperature ionic liquids under…
Table 4 One-pot synthesis of imidazo[1,2-a]pyridines 3 using conventional method (CM) and ultrasound irradiation condition (US)
Entry
R1
R2
Product
Isolated yield/% (Time/h)
M.p. (lit. m.p.)/°C
CMa
USb
1
H
H
3a
3b
3c
3d
3e
3f
55 (24)
30 (24)
42 (24)
37 (24)
51 (24)
62 (4)
80 (2.5)
70 (2)
135–136 (136–137 [33])
213–215 (214–216 [34])
133–134 (133–134 [35])
265–267 (266–267 [36])
201–202 (206–208 [37])
60–62 (61–62 [38])
2
4-Br
H
3
4-OMe
4-NO2
3-NO2
3-OMe
2-OH
H
H
70 (2)
4
H
53 (1.5)c
73 (3)c
76 (1)c
58 (2.5)
72 (1.5)
77 (2.5)
74 (1.5)
86 (2.5)
67 (1)
5
H
6
H
7
H
3g
3h
3i
51 (12)
51 (24)
50 (6)
143–145 (142–143 [39])
170–172 (174 [34])
8
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
H
9
4-Br
203–205 (202–204 [40])
160–162 (160–162 [41])
215–217 (214–216 [41])
77–79 (76–77 [41])
10
11
12
13
14
4-OMe
4-NO2
3-OMe
2-OH
4-SO2Me
3j
42 (12)
36 (12)
40 (6)
3k
3l
3m
3n
31 (12)
53 (24)
77 (2.5)
72 (3)
138–139 (136–137 [11])
244–245 (242 [42])
Reaction conditions: acetophenone 1 (0.51 mmol), 2-aminopyridine 2 (1.17 mmol), iodine (0.61 mmol); reactions were carried out under
specified conditions
aCM: conventional method performed by heating at 110 °C under solvent-free condition, followed by excess aqueous NaOH (45%), 100 °C, 1 h
bUS: ultrasound irradiation method performed at a frequency of 40 kHz at 30–35 °C in the presence of 20 mol% [BMIM]PF6, followed by
excess aqueous K2CO3 (35%), ultrasound irradiation at 40–45 °C, 20 min
cUS: ultrasound irradiation method performed at a frequency of 40 kHz at 40–45 °C in the presence of 20 mol% [BMIM]PF6, followed by excess
aqueous K2CO3 (35%), ultrasound irradiation at 40–45 °C, 20 min
yield (67%) when m-methoxyacetophenone was used
Experimental
(Table 4, entry 12).
Moreover, by applying this method, we were able to
synthesize the marketed drug zolimidine using metal-free
one-pot synthesis in 72% (Table 4, entry 14) with milder
condition and shorter reaction time (3 h) than the previ-
ously reported methods [4, 11, 31, 32].
All reagents were obtained from various commercial sup-
pliers. They were further carefully purified before using.
The melting points were determined using a MEL-TEMP
from Laboratory device. The 1H NMR (400 MHz) and 13
C
NMR (100 MHz) spectra were recorded on Bruker
AVANCE 400 spectrometer using the solvent peak as
internal reference and CDCl3 or DMSO-d6 as solvents.
Sonication was carried out using a HIQ-LAB ultrasonic
cleaner Model ULA-4L (with a frequency of 40 kHz and a
nominal power 150 W).
Conclusion
In summary, a convenient, milder, and efficient one-pot
procedure for the synthesis of imidazo[1,2-a]pyridines using
catalytic amount of [BMIM]PF6 assisted by sonication has
been developed. This new method offers very attractive
features such as reduced reaction times, higher yields and
operational simplicity than the conventional method. The
generality of the method was demonstrated by its application
to various kinds of 2-aminopyridines and acetophenones.
Additionally, this method is a fast and efficient method to
synthesize the marketed drug zolimidine.
General procedure for the synthesis
of imidazo[1,2-a]pyridines under conventional
method
A mixture of acetophenone 1 (0.51 mmol), 2-aminopy-
ridine 2 (1.17 mmol), and iodine (0.61 mmol) were added
into a 5-cm3 dried flat-bottom capped-vial equipped with
123

P. Paengphua, S. Chancharunee
´
¨
6. Hieke M, Rodl CB, Wisniewska JM, la Buscato E, Stark H,
Schubert-Zsilavecz M, Steinhilber D, Hofmann B, Proschak E
(2012) Bioorg Med Chem Lett 22:1969
magnetic bar. The mixture was stirred at 110 °C and
monitored by TLC. To complete the cyclization step, the
excess aqueous NaOH (45%) was added and subsequently
stirred at 100 °C for 1 h. Then, the reaction mixture was
cooled to room temperature and diluted with CHCl3. The
mixture was neutralized with 3.5 M HCl. The organic
phase was separated and aqueous layer was extracted with
CHCl3. The combined organic extract was dried over
Na2SO4 and evaporated under reduced pressure to afford
the crude product which was purified by column chro-
matography using ethyl acetate/hexane (1:9–2:8).
7. Hodgkiss RJ, Middleton RW, Parrick J, Rami HK, Wardman P,
Wilson GD (1992) J Med Chem 35:1920
8. Donohoe TJ, Kabeshov MA, Rathi AH, Smith IED (2012) Org
Biomol Chem 10:1093
9. Zeng J, Tan YJ, Leow ML, Liu X-W (2012) Org Lett 14:4386
10. Cao H, Liu X, Liao J, Huang J, Qiu H, Chen Q, Chen Y (2014) J
Org Chem 79:11209
11. Bagdi AK, Rahman M, Santra S, Majee A, Hajra A (2013) Adv
Synth Catal 355:1741
12. Meng X, Wang Y, Yu C, Zhao P (2014) RSC Adv 4:27301
13. Monir K, Bagdi AK, Ghosh M, Hajra A (2014) Org Lett 16:4630
14. Wang H, Wang Y, Liang D, Liu L, Zhang J, Zhu Q (2011)
Angew Chem Int Ed 50:5678
15. Chioua M, Soriano E, Infantes L, Jimeno ML, Marco-Contelles J,
Samadi A (2013) Eur J Org Chem 2013:35
16. Patil SV, Gaikwad ND, Bobade VD (2016) Arab J Chem 9:S1649
17. Karade NN, Kondre JM, Gampawar SV, Shinde SV (2009) Synth
Commun 39:2279
General procedure for the synthesis
of imidazo[1,2-a]pyridines derivatives using
ultrasonic irradiation
A mixture of acetophenone 1 (0.51 mmol), 2-aminopy-
ridine 2 (1.17 mmol), iodine (0.61 mmol), and 20 mol%
[BMIM]PF6 (0.10 mmol) were added into a 5-cm3 dried
flat-bottom capped-vial equipped with magnetic bar and
irradiated at 30–45 °C at a frequency of 40 kHz for 1–3 h.
The reaction temperature of ultrasonic bath was controlled
manually by addition or removal of small amounts water.
Then, the excess aqueous K2CO3 (35%) was subsequently
added and further irradiated at the same frequency at
40–45 °C for 20 min to accomplish the cyclization. After
completion, the mixture was diluted with CHCl3 and neu-
tralized using 3.5 M HCl. The organic layer was separated
and the aqueous layer was extracted with CHCl3. The
combined organic layer was dried with Na2SO4 and con-
centrated under reduced pressure. The crude product was
further purified by column chromatography using ethyl
acetate/hexane (1:9–2:8) to give the desired product.
´
18. Stasyuk AJ, Banasiewicz M, Cyranski MK, Gryko DT (2012) J
Org Chem 77:5552
19. Kielesinski L, Tasior M, Gryko DT (2015) Org Chem Front 2:21
20. Ranu BC, Jana R (2006) Eur J Org Chem 16:3767
21. Wu X, Han X, Zhou L, Li A (2012) Indian J Chem 51A:791
22. Chakraborti AK, Roy SR, Kumar D, Chopra P (2008) Green
Chem 10:1111
23. Wasserscheid P, Eichmann M (2001) Catal Today 66:309
ˇ
ˇ
24. Toma S, Sebesta R (2012) Ionic liquids as (co-) solvents and co-
catalysts for organocatalytic reactions. In: Ionic liquids in bio-
transformations and organocatalysis. Wiley, Hoboken, p 331
25. Choi J, Benedetti TM, Jalili R, Walker A, Wallace GG, Officer
DL (2016) Chem Eur J 22:14158
26. Palimkar SS, More VS, Srinivasan KV (2008) Ultrason Sono-
chem 15:853
27. Jin TS, Liu LB, Zhao Y, Li TS (2005) J Chem Res 3:162
28. Mosslemin MH, Nateghi MR (2010) Ultrason Sonochem 17:162
29. Nikpassand M, Mamaghani M, Shirini F, Tabatabaeian K (2010)
Ultrason Sonochem 17:301
30. Zou Y, Wu H, Hu Y, Liu H, Zhao X, Ji H, Shi D (2011) Ultrason
Sonochem 18:708
31. Santra S, Bagdi AK, Majee A, Hajra A (2013) Adv Synth Catal
355:1065
32. Yu J, Jin Y, Zhang H, Yang X, Fu H (2013) Chem Eur J 19:16804
33. Zhu DJ, Chen JX, Liu MC, Ding JC, Wu HY (2009) J Braz Chem
Soc 20:482
Acknowledgements The first author would like to thank the Science
Achievement Scholarship of Thailand for financial support. In addi-
tion, we wish to thank the Department of Chemistry, Faculty of
Science, Chiang Mai University, Thailand, for facilities supporting
this research.
34. Adib M, Mohamadi A, Sheikhi E, Ansari S, Bijanzadeh HR
(2010) Synlett 11:1606
35. Xie YY, Chen ZC, Zheng QG (2002) Synthesis 11:1505
36. Ueno M, Togo H (2004) Synthesis 16:2673
37. Samanta S, Jana S, Mondal S, Monir K, Chandra SK, Hajra A
(2016) Org Biomol Chem 14:5073
38. Kour D, Khajuria R, Kapoor KK (2016) Tetrahedron Lett
57:4464
39. Mutai T, Tomoda H, Ohkawa T, Yabe Y, Araki K (2008) Angew
Chem Int Ed 47:9522
40. Sundberg RJ, Dahlhausen DJ, Manikumar G, Mavunkel B, Bis-
was A, Srinivasan V, King F, Waid P (1988) J Heterocycl Chem
2:129
References
1. Gudmundsson KS, Williams JD, Drach JC, Townsend LB (2003)
J Med Chem 46:1449
2. Gueiffier A, Mavel S, Lhassani M, Elhakmaoui A, Snoeck R,
Andrei G, Chavignon O, Teulade JC, Witvrouw M, Balzarini J,
De Clercq E, Chapat JP (1998) J Med Chem 41:5108
3. Lhassani M, Chavignon O, Chezal JM, Teulade JC, Chapat JP,
Snoeck R, Andrei G, Balzarini J, De Clercq E, Gueiffier A (1999)
Eur J Med Chem 34:271
41. Wang FJ, Xu H, Xin M, Zhang Z (2016) Mol Divers 20:659
42. Habibi YA, Pourbasheer E, Danandeh JM (2008) Bull Korean
Chem Soc 29:833
4. Chandra MD, Reddy DR, Nageswara RS, Adimurthy S (2013)
Adv Synth Catal 355:2217
5. Starrett JE, Montzka TA, Crosswell AR, Cavanagh RL (1989) J
Med Chem 32:2204
123