Tandem approach for the synthesis of imidazo[1,2-a]pyridines from alcohols

Article abstract of DOI:10.1016/j.tetlet.2012.10.112

Propylphosphonic anhydride (T3P) is shown to be an effective and mild reagent for the one-pot synthesis of imidazo[1,2-a]pyridines from a variety of alcohols. Alcohols are oxidized in situ to aldehydes under mild conditions, which in turn undergo a three-component reaction with various 2-aminopyridines and isocyanides to afford imidazo[1,2-a]pyridines in excellent yields. T3P acts as an activator for both DMSO in oxidation reaction and the Schiff base in nucleophilic addition reaction with isocyanides.

Full text of DOI:10.1016/j.tetlet.2012.10.112

Tetrahedron Letters 54 (2013) 95–100
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Tetrahedron Letters
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Tandem approach for the synthesis of imidazo[1,2-a]pyridines from alcohols
Ajjahalli B. Ramesha, Goravanahalli M. Raghavendra, Kebbahalli N. Nandeesh,
⇑
Kanchugarakoppal S. Rangappa, Kempegowda Mantelingu
Department of Studies in Chemistry, Manasagangotri, University of Mysore, Mysore 570006, India
a r t i c l e i n f o
a b s t r a c t
Propylphosphonic anhydride (ÒT3P) is shown to be an effective and mild reagent for the one-pot synthe-
sis of imidazo[1,2-a]pyridines from a variety of alcohols. Alcohols are oxidized in situ to aldehydes under
mild conditions, which in turn undergo a three-component reaction with various 2-aminopyridines and
isocyanides to afford imidazo[1,2-a]pyridines in excellent yields. ÒT3P acts as an activator for both DMSO
in oxidation reaction and the Schiff base in nucleophilic addition reaction with isocyanides.
Ó 2012 Elsevier Ltd. All rights reserved.
Article history:
Received 4 July 2012
Revised 23 October 2012
Accepted 25 October 2012
Available online 2 November 2012
Keywords:
ÒT3P
Imidazo[1,2-a]pyridines
Alcohols
Heterocycles
Three component reactions
The imidazo[1,2-a]pyridine scaffold is present in a large number
of compounds showing an impressive variety of biological proper-
ties,1 such as antibacterial, antifungal, anthelmintic, antiviral, anti-
protozoal, anti-inflammatory, anticonvulsant, anxiolytic, hypnotic,
gastrointestinal, antiulcer, and immunomodulatory activities.2,3
Furthermore, imidazo[1,2-a]pyridine scaffold forms the core struc-
ture of several drug molecules4 like zolpidem used in the treat-
ment of insomnia, alpidem, as an anxiolytic agent, olprinone for
the treatment of acute heart failure, minodronic acid useful for
the treatment of osteoporosis, and zolimidine used for the treat-
ment of peptic ulcer (Fig. 1).
Several methods are available for the preparation of 2- or
3-substituted imidazo[1,2-a]pyridines.5a–g Among these, the
three-component coupling of 2-aminopyridine, aldehyde, and al-
kyne is one of the most common methods for the synthesis of imi-
dazo[1,2-a]pyridines. Another approach is the three-component
reaction (3-CR) of aromatic amidines with aromatic aldehyde and
isocyanide.6 Numerous catalysts are being used over the years such
as Sc(OTf)37, solid supported p-toluenesulfonic acid,8a glyoxylic
acid,8b ZnCl2,9 and ionic liquid.10 Recently, Adib et al. (2011) re-
ported the synthesis of imidazo[1,2-a]pyridine scaffolds by a mod-
ified three-component coupling (3-CR) of aromatic amidines,
benzyl halides, and isocyanides in the presence of DMSO and
K2CO3 (Scheme 1).11a
Some of the methods entail such demerits as the requirement of
expensive and excess amounts of catalysts, prolonged reaction
times, difficulties in work-up procedure, and harsh reaction condi-
tions. Though these protocols are quite useful, there is still a need
to develop new methodologies which might work under milder
reaction conditions. So far, there is no report on the one-pot
synthesis of imidazo[1,2-a]pyridines directly from alcohols.
Recently, propylphosphonic anhydride (ÒT3P) has received in-
creased attention as a coupling agent and as a water scavenger,12
offering several advantages such as high yields, purity, low toxicity,
broad functional group tolerance, and easy work-up when com-
pared to traditional reagents.12,13 Though, ÒT3P has been identified
as a mild water scavenger, the wider scope and synthetic utility of
this reagent for oxidization and cyclodehydration have not been
explored.
In continuation of our work on the development of the useful
synthetic methodologies,14–17 we have recently reported, a novel
and straight forward approach for the one-pot synthesis of benzim-
idazoles and benzothiazoles using ÒT3P/DMSO.18 In the present
work, the method used involves an in situ oxidation–cycloconden-
sation sequence starting from alcohols. Thus, under mild oxidative
conditions, alcohols reacting with propylphosphonicanhydride and
DMSO in ethyl acetate as a solvent at room temperature get con-
verted into the corresponding aldehydes (Scheme 2). Subsequently,
the in situ formed aldehyde is condensed with 2-aminopyridine
followed by treatment with isocyanide at room temperature which
resulted in the corresponding imidazo[1,2-a]pyridines.
Adib et al. (2007) reported a catalyst-free, simple, and environ-
mentally friendly approach for the synthesis of imidazo[1,2-a]pyr-
idines11b using aldehydes as one of the substrates in water at 70 °C.
In order to explore the scope and optimal conditions (Table 1)
for ÒT3P/DMSO catalyzed synthesis of imdiazo[1,2-a]pyridine,
alcohol 1b was selected as a model. With reaction of 1b (1.0 equiv),
⇑
Corresponding author. Tel.: +91 821 2412191/2419661; fax: +91 821 2412191.
E-mail address: kmantelingu@yahoo.com (K. Mantelingu).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.112

96
A. B. Ramesha et al. / Tetrahedron Letters 54 (2013) 95–100
Table 1
N
Optimization of reaction conditions
Cl
N
N
N
Cl
N
No.
T3Pa (equiv)
Time (h)
Temperature
Yield (%)
O
1
2
3
4
5
6
7
1.0
1.5
2.0
2.5
3.0
2.5
2.5
24
24
8
4.5
4.0
3
RT
RT
RT
RT
RT
35 °C
45 °C
Trace
44
68
72
69
O
N
Zolpidem
Alpidem
68
62
3
O
P
a
CN
HO
HO
O
O
50% solution in ethyl acetate.
OH
S
O
OH
HN
P
O
A variety of aromatic alcohols containing electron donating and
N
HO
N
N
N
N
electron withdrawing substituents such as p-methoxy (table 2, en-
try 8b), p-isopropyl (Table 2, entry 2b), p-bromo (Table 2, entry 4b),
and p-cyano (Table 2, entry 5b), p-fluoro (Table 2, entry 14b), and
p-nitrobenzyl alcohol (Table 2, entry 10b), participate well in this
three component reaction. Notably, even sterically hindered
alcohols such as (Table 2, entry 11b) and heterocyclic alcohols
(Table 2, entry 6b, 7b, 12b, 13b) proved to be effective to afford
the respective imidazo[1,2-a]pyridines. In addition, sensitive func-
tionalities like Boc (Table 2, entry 1b), chloro (Table 2, entry 2a, 5a,
and 7a), cyano (Table 2, entry 5b) etc. are tolerated. The scope and
generality of this procedure are demonstrated for various isocya-
nides as well t-butyl isocyanide, benzyl isocyanide, n-pentylisocy-
anide participate in the three-component reaction to give the
corresponding imidazo[1,2-a]pyridines. Then this protocol pro-
vides an opportunity for further chemical elaborations in getting
various functionalized products. Several therapeutically active
moieties can be incorporated into N-fused imidazo[1,2-a]pyridines
at ease by this method.
N
Zolimidine
Olprinone
Minodronic acid
Figure 1. Examples of imidazo[1,2-a]pyridine containing drugs and biologically
active compounds.
DMSO (2.0 equiv), 2-amino-5-fluoropyridine 1a (1.0 equiv), isocya-
nide 1c (1.5 equiv), ÒT3P (1.0 equiv, 50% solution in EtOAc), in 10
volumes of EtOAc at room temperature for 24 h, the desired prod-
uct was obtained in a trace yield. Upon increasing the equivalent of
ÒT3P, the formation of the desired product was improved (Table 1,
entry 2). At 2.5 equiv of ÒT3P, the yield was 72% (Table 1, entry 4).
These results suggested that ÒT3P played a vital role in various
stages of the reaction. The effect of temperature was the next
parameter to be investigated (Table 1, entries 6–7), and we found
that the room temperature was the most effective. The role of
DMSO and of the solvents on the synthesis of imidazo[1,2-a]
pyridine 1d, was then studied. We found that the reaction of alco-
hol 1b (1.0 equiv), 2-amino-5-fluoropyridine 1a (1.0 equiv), isocy-
anide 1c (1.5 equiv), ÒT3P (2.5 equiv) without DMSO, did not take
place both at room temperature and under reflux condition. This
makes the obvious point that DMSO is needed as an oxidizing
agent to convert alcohol into aldehyde. However, increasing the
amount of DMSO with respect to ethyl acetate in the ratio
(EtOAc/DMSO) 1:1, 1:2, 1:3, and 1:4 did not improve the yield. Sol-
vents other than ethyl acetate such as THF, DMF, toluene, acetoni-
trile, acetone, and benzene did not make a difference. Then ethyl
acetate was chosen as the preferred solvent.
All synthesized compounds were adequately characterized by
the physical and spectral data which are in good agreement with
the structures of the products.
A possible mechanism of the oxidative and dehydrative cycliza-
tion to get imidazo [1,2-a]pyridine is suggested in Scheme 3. DMSO
1 gets activated by T3P 2 to give an electrophilic sulfur species 3,
which then reacts with alcohol 4 to give an aryloxysulfonium salt
6. The hydrolyzed T3P obtained as byproduct 5 acts as a base to pull
out H of aryloxysulfonium salt 6 to form aldehyde 7. P,P0,P00-Tripro-
pyltriphosphonic acid 5a and dimethyl sulfide formed as the
byproduct. The second step in the probable mechanism is the T3P
catalyzed condensation of aminopyridine with aldehyde to afford
aminol intermediate 9, which undergoes elimination to form the
imine intermediate 10. The byproduct, P,P0,P00-tripropyltriphos-
phonic acid 5a protonates the imine to give the activated form 11.
The stage is now set for nucleophilic addition of isocyanide 12 to
Using the optimized reaction conditions (Table 1, entry 4), we
subsequently explored the reaction scope by using aminopyri-
dines, aromatic or heterocyclic alcohols, and isocyanides to afford
imidazo[1,2-a]pyridine products in 72–94% yields (Table 2).
N
Br
K2CO3/DMSO
N
+
NH2
R
N
C
+
N
90 °C/6h
ref 11a
NH
R
Scheme 1. Synthesis of imidazo[1,2-a]pyridines from benzyl halides.
N
N
Boc
F
HO
N
T3P/DMSO
Ethyl acetate
F
N
C
N
Boc
NH
N
NH2
1a
1b
1c
1d
Scheme 2. Synthesis of imidazo[1,2-a]pyridines from alcohols.

A. B. Ramesha et al. / Tetrahedron Letters 54 (2013) 95–100
97
yield the isonitrilium intermediate 13, which cyclizes to the key
intermediate 14. Prototrophic tautomerization of 14 yields the final
imidazo[1,2-a]pyridine 15.
In conclusion, we have developed an efficient and simple one-
pot strategy for the synthesis of imidazo[1,2-a]pyridines by
in situ oxidation of alcohols to aldehydes followed by the three-
Table 2
Synthesis of imidazo[1,2-a]pyridines from alcohols via 3 cc reaction
Amino pyridine
Alcohol
Isocyanide
Product
Yield (%)
72
Time (h)
4.5
F
N
N
C
Boc
N
N
Boc
N
OH
F
1c
N
NH2
NH
1d
1b
2b
1a
Cl
N
Cl
N
N
OH
N
C
C
N
N
1c
82
85
5
5
NH
2d
NH2
2a
N
O
O
O
OH
N
N
N
N
O
1c
N
NH2
3b
3a
NH
3d
N
N
N
OH
OH
N
N
N
C
C
Br
Br
1c
NH2
89
94
4
4
4b
NH
3a
4d
Cl
Cl
N
N
N
CN
N
N
NC
1c
5b
NH
5d
N
NH2
2a
N
N
N
N
N
N
C
C
OH
N
1c
6b
84
80
5
5
NH
6d
N
NH2
4a
Cl
N
N
N
N
N
N
OH
N
Cl
1c
N
5a
NH2
7b
NH
7d
OH
C
N
N
N
N
NH2
N
O
6a
2c
2c
8b
78
4
NH
O
8d
OH
C
N
N
NH2
N
6a
2b
81
4
NH
9d
(continued on next page)

98
A. B. Ramesha et al. / Tetrahedron Letters 54 (2013) 95–100
Table 2 (continued)
Amino pyridine
Alcohol
Isocyanide
Product
Yield (%)
Time (h)
OH
C
N
N
N
N
N
NH2
2c
6a
9b
NH
86
4
N
10d
O
O
OH
C
C
N
N
N
N
NH2
N
N
3b
4b
2c
2c
6a
6a
81
4.5
N
H
O
O
11d
OH
NH2
N
N
Br
90
93
92
4
NH
Br
12d
OH
C
C
N
N
N
O2N
N
N
NH2
N
10b
2c
2c
6a
6a
4
NH
NO2
13d
OH
N
NH2
N
NC
5b
3.5
NH
CN
14d
O
O
OH
C
N
N
N
N
NH2
Br
Br
11b
2c
6a
87
76
75
5
5
5
NH
O
O
15d
Cl
Cl
C
N
OH
OH
N
N
N
N
5a
NH2
3c
3c
12b
13b
NH
N
16d
C
N
N
N
N
NH2
N
4a
NH
N
17d

A. B. Ramesha et al. / Tetrahedron Letters 54 (2013) 95–100
99
Table 2 (continued)
Amino pyridine
Alcohol
Isocyanide
Product
Yield (%)
Time (h)
C
OH
N
Cl
Cl
F
3c
N
N
N
7a
NH2
14b
92
3.5
NH
18d
F
Ar
OH
4
O
P
R
O
P
R
O
P
R
O
O
+
_
O
P
O
O
P
R
_
O
R
P
O
P
R
O
P
R
O
O
O
5
H
HO
_
O
P
R
S +
_
S
+
O
+
O
H
O
O
Ar
O
6
S
+
1
R
O
O
P
R
O
P
R
O
P
R
3
2
O
OH
O
HO
-
S
5a
O
O
O
P
P
OH
O
O
P R
O
R
R
Ar
O
R
P
R R
O
+
NH2
H
O
O
N
N
+
H
P
Ar
P
N
N
N
H
O
O
P
R
O
P
R
O
P
R
O
10
R
O
-
8
O
OH
2
O
7
HO
9
H+
5a
H
H
H
H
+
Ar
N
Ar
N
N
N
_
C
+
Ar
Ar
N
+
N
R1
N
N
N
+N
R2
12
NH
N
11
R1
R1
15
13
14
Scheme 3.
component coupling of aldehyde, 2-aminopyridine, and isocya-
nide. Excellent yield, tandem nature, mild conditions, simple and
convenient work-up are the highlights of this reaction. The method
provides an easy access to a diverse array of medicinally-relevant
N-fused heterocycles.
4.83 (s, 1H), 0.98 (s, 9H); 13C NMR (100 MHz, DMSO-d6):
d = 143.15, 139.57, 137.10, 134.19, 131.55, 130.24, 128.93,
126.27, 121.46, 117.75, 56.77, 30.43; MS (ES) m/z (M+2) = 346.1;
Compound 11d: 1H NMR (400 MHz, DMSO-d6): d = 8.13 (d,
J = 6.31, 1H), 7.68 (s, 2H), 7.08–7.40 (m, 7H), 6.96 (d, J = 8.19, 1H),
6.75 (t, J = 6.2, 1H), 6.03 (s, 2H), 5.26 (s, 1H), 4.05 (d, J = 5.44,
2H); 13C NMR (100 MHz, DMSO-d6): d = 147.86, 146.70, 140.70,
140.23, 134.83, 129.24, 128.64, 127.47, 126.14, 124.15, 123.49,
120.76, 116.96, 111.56, 108.76, 107.28, 101.32, 51.62; MS (ES) m/
z (M+1) = 344.3; Compound 14d: 1H NMR (400 MHz, DMSO-d6):
d = 8.30 (d, J = 8.05 Hz, 2H), 8.17 (d, J = 6.66 Hz, 1H), 7.84 (d,
J = 7.92 Hz, 2H), 7.44 (d, J = 8.87 Hz, 1H), 7.15–7.42 (m, 6H), 6.80
(t, J = 6.44 Hz, 1H), 5.53 (d, J = 5.73 Hz, 1H), 4.08 (d, J = 5.82 Hz,
2H); 13C NMR (100 MHz, DMSO-d6): d = 141.27, 139.93, 139.68,
132.75, 132.67, 128.71, 128.68, 127.58, 127.25, 125.12, 123.85,
119.67, 117.50, 112.12, 109.25, 51.75; MS (ES) m/z (M+1) = 325.2.
Typical procedure for the synthesis of (4d): To the solution of
alcohol 4b (0.50 g, 2.67 mmol) and DMSO (0.41 g, 5.34 mmol) in
ethyl acetate (5 ml) was added ÒT3P (1.84 g, 6.68 mmol, 2.5 equiv,
50% solution in ethyl acetate) at 0 °C. The resulting mixture was al-
lowed to warm to RT and stirred for 1 h. Pyrazine-2-amine 3a
(0.254 g, 2.67 mmol) was added to the above mixture and stirred
for 15 min, which was followed by the addition of isocyanide 1c
(0.33 g, 4.01 mmol) at room temperature and stirring for 4 h. Pro-
gress of the reaction was monitored by TLC. After the reaction was
complete, the reaction mixture was diluted with ethyl acetate and
neutralized with aqueous sodium bicarbonate solution. The organ-
ic layer was separated and the aqueous layer was extracted with
ethyl acetate (25 mL ꢀ 2), the combined organic phases were
washed with water, brine solution, dried over anhydrous sodium
sulfate, and concentrated under vacuum to afford a crude product,
which was purified on silica gel using ethyl acetate and petroleum
ether.
Acknowledgements
K.M. thanks the Ministry of Human Resource Development
(MHRD) and the University Grant Commission (UGC), New Delhi,
India, for recognizing the University of Mysore as the Institute of
Excellence and for the financial assistance. We also thank Dr. Prah-
ladaRao S. for valuable discussion during the preparation of this
manuscript.
Analytical data for some representative compounds: Compound
4d: 1H NMR (400 MHz, DMSO-d6): d = 8.92 (s, 1H), 8.40 (d, J = 3.30,
1H), 8.15 (d, J = 8.13, 2H), 7.85 (d, J = 4.08, 1H), 7.60 (d, J = 8.12, 2H),

100
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