Synthesis of imidazo[1,2a]pyridines via three-component reaction of 2-aminopyridines, aldehydes and alkynes

Article abstract of DOI:10.1016/j.tetlet.2010.05.139

A novel three-component reaction towards the synthesis of imidazo[1,2a]pyridines was independently developed based on 2-aminopyridines, aldehydes and alkynes, and thereby imidazo[1,2a]pyridines were obtained in acceptable yields by the CuSO₄/TsOH catalyzed three-component reaction.

Full text of DOI:10.1016/j.tetlet.2010.05.139

Tetrahedron Letters 51 (2010) 4605–4608
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of imidazo[1,2a]pyridines via three-component reaction
of 2-aminopyridines, aldehydes and alkynes
b,
Ping Liu a,b, Li-song Fang a, Xinsheng Lei a, , Guo-qiang Lin
*
*
a School of Pharmacy, Fudan University, 826 Zhangheng Road, Shanghai 201203, China
b Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel three-component reaction towards the synthesis of imidazo[1,2a]pyridines was independently
developed based on 2-aminopyridines, aldehydes and alkynes, and thereby imidazo[1,2a]pyridines were
obtained in acceptable yields by the CuSO4/TsOH catalyzed three-component reaction.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 16 April 2010
Revised 23 May 2010
Accepted 28 May 2010
Available online 1 June 2010
Keywords:
Multicomponent reaction
Imidazol[1,2a]pyridines
Catalysis
Heterocycles
During the past decades, multicomponent reactions (MCRs) to
form several bonds in a single-step-cascade synthesis manner1 and
to integrate several different fragments into one molecule attracted
much attention and achieved great progress.2 In view of atom-eco-
nomics, step-efficiency, and structural diversity in terms of green
chemistry and drug discovery, developing new MCRs, especially
those toward ‘bias’ scaffolds remains very active in current process
chemistry and medicinal chemistry. As a ‘bias’ scaffold, imi-
dazo[1,2a]pyridine unit has widely occurred in various bioactive
molecules and clinically applied drugs,3 such as Zolpidem to treat
insomnia, Alpidem as an anxiolytic agent, Olprinone to treat acute
heart failure, and Minodronic acid to treat osteoporosis. To date, sev-
eral synthetic approaches to imidazo[1,2a]pyridines have been re-
ported,4 and among them the popular approaches utilized in
medicinal chemistry are: (1) coupling reaction of 2-aminopyridine
surrogate the alkyne, and thereby the novel approach to 3-alkyl
imidazo[1,2a]pyridines could become possible with 2-aminopyri-
dine, aldehydes, and alkynes (Fig. 1).
If so, the simplified process should possibly include: (1) conden-
sation of 2-aminopyridine and aldehyde to form imine; (2) nucle-
ophilic attack of alkyne to imine to form propargyl amine; (3)
intramolecular nucleophilic attack of nitrogen in pyridine ring to
the triple bond possibly in 5-exo-dig (attack to C-a) or 6-endo-dig
way (attack to C-b); (4) aromatic isomerization of the cyclic inter-
mediate (Fig. 2).
Recently, a number of methodologies have been disclosed on
transitional-metal-mediated formationof chiral orachiralpropargyl
amines with amines, aldehydes, and alkynes,7 or cycloisomerization
toward heterocycles with alkynes tethering intramolecular
nucleophiles;8 meanwhile, in principle the 5-exo-dig cyclic interme-
diate produced in step 3 could be preferably isomerized into the
thermodynamic stable heteroarenes, whereas the 6-endo-dig one
could not; and thus we reason that the designed approach could be
feasible. Herein, we are willing to present our preliminary effort
and complementary information on this issue, although most re-
cently an elegant method based on the similar strategy has been
developed by Chernyak and Gevorgyan in the course of our
research.9
According to the designed route toward the synthesis of imi-
dazo[1,2a]pyridines, we initially screened the different transitional
metal catalysts in the model reaction, including Cu, Fe, In, Pd, Zn,
Ag, etc., most of which occurred in alkyne-participated formation
of propargyl amines or cycloisomerization.7,8 Consequently, we
found Cu or Ag might be the promising catalyst for the reaction.
with
a-halocarbonyl compounds; (2) one-pot condensation of 2-
aminopyridines, aldehydes, and isonitriles, since a variety of starting
materials are either commercially available or easily synthesized.
However, both of them remain limited either to 3-amino imi-
dazo[1,2a]pyridines,5 or to stepwise available precursors and long
reaction time (up to 10 days).6 Therefore, it is highly desirable to ex-
plore a novel approach to imidazo[1,2a]pyridines, especially in a sin-
gle-step-cascade synthesis manner with multiple components.
Logically speculating along the known three-component
approach to the 3-amino imidazo[1,2a]pyridines, we hypothesize
whether or not the isonitrile could be replaced by its isosteric
* Corresponding authors. Tel./fax: +86 21 51980128.
E-mail address: leixs@fudan.edu.cn (X. Lei).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.139

4606
P. Liu et al. / Tetrahedron Letters 51 (2010) 4605–4608
O
O
?
H
R1
H
R1
R2
Known approach
A=NH
Designed approach
R2
R2
N
N
N
NH2
N
NH2
A=CH2
A
R3
R1
N
R3
R3
Figure 1. The designed approach for imidazo[1,2-a]pyridines.
O
H
R1
R2
R2
4
N
N
N
R2
N
N
NH2
H
R1
R1
R3
R3
R3
1
R2
R3
H
R2
N
N
R2
N
NH
R1
H
N
N
R1
3
2
a
R1
R3
b
R3
Figure 2. The simplified process for synthesis of imidazo[1,2-a]pyridines.
Table 1
Screening the catalysts in the two- or three-component reactiona
N
NH2
N
N
N
N
O
H
Entry
Catalysts (mol%)
Yieldb (%)
2C Reaction
3C Reaction
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
CuSO4
Cu(OTf)2
CuCN
AgOTf
Ag(O2CCF3)
Ag2O
AgNO3
AgBF4
CuI
CuCl
CuBr
Cu2O
CuOAc
CuBr2
30
42
20
34
Trace
35
9
12
9
Trace
6
7
Trace
Trace
Trace
Trace
4
8
Cu(acac)2
Trace
a
Two-component reaction (2CR) conditions: N-benzylidenepyridin-2-amine (0.5 mmol), acetylene (0.5 mmol), catalyst (10 mol %),
toluene (2 mL), 18 h; Three-component reaction (3CR) condition: 2-aminopyridine (1.0 mmol), benzaldehyde (1.0 mmol), acetylene
(1.0 mmol), catalyst (10 mol %), toluene (5 mL), 110 °C, 18 h.
b
Isolated yield.
Next, we examined in detail the different Cu or Ag salts in the
two- (N-benzylidenepyridin-2-amine and 1-ethynylbenzene: 2CR)
or three-component reaction (aminopyridine, benzaldehyde, and
ethynylbenzene: 3CR) and the results are shown in Table 1. In

P. Liu et al. / Tetrahedron Letters 51 (2010) 4605–4608
4607
Table 2
Screening the co-catalysts in the two- or three-component reactiona
Table 3
Synthesis of some imidazo[1,2a]pyridinesa
Entry
Catalysts
Additive
Yieldb (%)
3CR
R2
2CR
CuSO4 (0.1eq.)
TsOH (0.1eq.)
Toluene
R2
O
1
2
3
4
5
6
7
8
9
10
11
12
13
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
CuSO4
CuSO4
CuSO4
CuSO4
CuSO4
42
54
65
63
38
35
45
50
R3
N
TsOH
N
H
R1
N
NH2
PhCO2H
4-HO–C4H6CO2H
BINOL
4A MS
CF3SO3H
Ti(i-OPr)4
Reflux
R3
18 h
R3
31
47
39
Entry
R1
Ph
R2
R3
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
60
56
58
55
66
62
64
n.d.
n.d.
59
64
49
55
n.d.
57
46
60
68
55
53
28
n.d.
n.d.
n.d.
30
Trace
24
4-CH3–C6H4
4-OCH3–C6H4
4-OMOM–C6H4
4-Cl–C6H4
4-F–C6H4
4-CF3–C6H4
4-HO–C6H4
4-Me2N–C6H4
3-Cl–C6H4
3-CF3–C6H4
3-NO2–C6H4
2-Cl–C6H4
2-Br–C6H4
Ph
PhCO2H
TsOH
CF3SO3H
60c
45
54
D-camphor acid
14
15
16
CuCN
CuI
AgOTf
TsOH
TsOH
TsOH
39
30
15
a
Two-component reaction (2CR) conditions: N-benzylidenepyridin-2-amine
(0.5 mmol), acetylene (0.5 mmol), catalyst (10 mol %), additive (10 mol %) if
required, toluene (2 mL), 18 h; Three-component reaction (3CR) condition: 2-ami-
nopyridine (1.0 mmol), benzaldehyde (1.0 mmol), acetylene (1.0 mmol), catalyst
(10 mol %), additive (10 mol %) if required, toluene (5 mL), 110 °C, 18 h.
4-CH3–C6H4
4-OCH3–C6H4
4-F–C6H4
Ph
Ph
Ph
Ph
Ph
CONMe2
CO2Et
b
Isolated yield.
Ph
Ph
Ph
Ph
c
When TsOH (0.02 mmol) used, yield: 45%; solvent optimized for the 3CR
H
(dioxane: 51%; DMF: 41%; EtOH: 40%); the ratio for the acetylene verus aldehyde or
amine (1 equiv, 60%; 1.2 equiv, 57%; 1.5 equiv, 52%).
5-CH3
3-F
H
H
H
cyclo-C6H10
Et2CH
the case of two-component reaction, CuII, CuI, or AgI has been dem-
onstrated to catalyze the reaction to some extent (20–42%, entries
1–4 in Table 1) and resulted in the desired product, which was
identified by the routine physical spectrum and monocrystal
X-ray diffraction.10 In our case, CuII salts appeared to display better
than CuI salts or AgI salts in the formation of propargyl amine and
cycloisomerization.9 Despite a rewarding yield (Cu(OTf)2: 42%) in
hand, several attempts failed to improve the reaction by enhancing
activities of the metal catalyst with some mono-, bi-, or tri-dentate
ligands containing nitrogen or phosphorus atom. Additionally, the
efforts to expand those catalysts from 2CR to 3CR have also been
proved to be disappointing, and in 3CR most of CuII, CuI, or AgI salts
dramatically hampered the reaction and led to extremely low
yields except Cu(OTf)2 (35%).
Considering that both the nonbeneficial electronic effect of 2-
aminopyridine and the soft acidity of the copper or silver salt do
not favor the formation of the imine, the reason for lower yields
obtained in 3CR than in 2CR is that the in situ formation of the
imine is possibly difficult under the above-mentioned condition.
Thus additives to possibly benefit the formation of the imine were
examined, and the results are shown in Table 2. In order to avoid
the additives interfering the subsequential processes, the two-
component reaction was investigated prior to the three-compo-
nent reaction.
CH3CH2CH2
Ha
H
H
Hc
a
Three-component reaction (3CR) condition: 2-aminopyridine (1.0 mmol),
aldehydes (1.0 mmol), acetylenes (1.0 mmol), CuSO4/TsOH (10/10 mol %), toluene
(5 mL), 110 °C, 18 h.
b
Isolated yield; n.d. means not determined.
Paraformaldehyde is used.
c
Lewis acids, or drying agents as a co-catalyst did not significantly
improve the 3CR (entries 1 and 6–8 in Table 2). Fortunately, CuSO4,
instead of Cu(OTf)2, combined with Bransted acids such as TsOH,
improved the 3CR dramatically with a yield of 60% versus trace
in the 2CR (entries 9–13 in Table 2). Such a synergeric effect of
TsOH was also observed with CuI salts but not AgI salt in 3CR (en-
tries 14–16 in Table 2). Having screened the sorts and amounts of
several other acids, the ratio of the reagents and solvents, we found
that the product was obtained in 60% yield under the optimized
condition for the 3CR (see entries 10–13 and footnote c in Table 2).
Under the optimized condition, the reaction mixtures showed,
by TLC analysis, that the residues included the product, the unreac-
tive imine, and benzaldehyde as well as several minor unidentified
residues. Furthermore, among the minor residues, the possible
intermediates such as the propargyl amine or the allene amine15
could not be found by LC–MS analysis. Prolonging the reaction
time or increasing the amount of CuSO4/TsOH did not ameliorate
the reaction, but decreasing the amount of the catalyst
indeed led to a lower yield. Notwithstanding, considering that
the single-step-cascade synthesis includes at least three steps
(condensation for imine, nucleophilic addition of alkyne, and
cycloisomerization), the yield is acceptable (average 85%/per step).
With the optimized condition in hand, the scope and limitation
of the reaction were finally examined12 and the results are shown
in Table 3. In most cases of aromatic aldehydes and aromatic al-
kynes, the three-component reaction proceeded smoothly under
the condition and the corresponding products were offered in
moderate yields (46–68%, entries 1–19, Table 3), and the condi-
tion was compatible with various function groups including
Bransted acids, such as TsOH or benzoic acid11, were observed
to co-catalyze the two-component reaction with Cu(OTf)2 and led
to significantly increased yields, especially benzoic acid could en-
able the yield up to 65% versus 42% (entries 1 and 3 in Table 2),
however, the weaker Bransted acid such as BINOL had no beneficial
effect on the transformation (entries 1 and 5 in Table 2). It sug-
gested that the strong Bransted acid might, to some extent, acti-
vate nucleophilic addition of the alkyne to the imine due to
protonation of the imine14 or inhibition of coordination of the pyr-
idine nitrogen to copper, whereas the weaker acids could not.
Unexpectedly, when the strong Bransted acid combined with
Cu(OTf)2 in the three-component reaction, only a slightly lower
yield (45% vs 54% for TsOH; 50% vs 65% for PhCO2H, entries 2–3
in Table 2) was offered. Screening other strong Bransted acids,

4608
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NO2, Cl). The electronic effect seemed to have a slight influence
on the reaction since either the electron-withdrawing or the elec-
tron-donating groups on the different aromatic ring resulted in the
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inactivation (Me2N or OH with CuII). The reaction with aliphatic
aldehydes, instead of aromatic aldehydes, seems to be promising
because cyclohexanecarbaldehyde resulted in 53% yield, while
some low boiling-point aliphatic aldehydes appeared to be prob-
lematic and led to lower yields (entries 21–22 in Table 3). An
attempt to employ 2-aminopyridine and paraformaldehyde with
aliphatic alkynes, such as N,N-dimethylpropiolamide or ethyl pro-
piolate (entries 23–24 in Table 3) aiming to the synthesis of Zolpi-
dem4e or Mindronic acid,13 failed to obtain the desired products,
that indicates the disadvantage of the current method.
In conclusion, we have independently explored a novel ap-
proach to the synthesis of imidazo[1,2a]pyridines via three-com-
ponent reaction of 2-aminopyridines, aldehydes, and alkynes, and
imidazo[1,2a]pyridines are obtained in acceptable yields by the
CuSO4/TsOH-catalyzed three-component reaction, and the reaction
could tolerate a variety of functional groups on the aromatic moi-
eties but remains incompatible with hydroxy, dialkylamino, and
bromo existing in the substrates.
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Acknowledgments
We thank National Natural Foundation of China(20872019) and
Fudan University for the research financial support and we are
grateful to Shanghai Institute of Organic Chemistry for recording
ESI-MS, HR-MS, 1H NMR, and 13C NMR spectra, respectively.
9. Chernyak, N.; Gevorgyan, V. Angew. Chem., Int. Ed. 2010, 49, 2743.
10. Crystallographic data for 3-benzyl-2-phenyl-imidazo[1,2-a]pyridine: CCDC
771862.
Supplementary data
11. Jun, C.-H.; Lee, D.-Y.; Lee, H.; Hong, J.-B. Angew. Chem., Int. Ed. 2000, 39, 3070.
12. General procedure: To a 25 mL flask were sequentially added 2-aminopyridine
(94 mg, 1.0 mmol), p-TsOHÁH2O (19 mg, 0.1 mmol, 10 mol %)?and CuSO4
(16 mg, 0.1 mmol, 10 mol %) (note: aldehyde was added at the time only if
solid). And then dry toluene (5 mL, 0.2 M) was added under Ar atmosphere,
followed by benzaldehyde (0.102 mL, 1.0 mmol) and phenylacetylene
(0.110 mL, 1.0 mmol). The reaction mixture was then stirred at 110 °C until
2-aminopyridine had been consumed completely (ca. 18 h) and then cooled to
room temperature and filtered through Celite with the aid of CH2Cl2. After
evaporation of the solvent, the residue was purified by flash chromatography
(petroleum ether/AcOEt/Et3N = 80:10:1) to afford the desired product (171 mg,
60% yield). The spectral data of imidazo[1,2a]pyridines have been presented in
the Supporting Information free of charge.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.05.139.
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