DABCO catalyzed highly regioselective synthesis of fused imidazo-heterocycles in aqueous medium

Article abstract of DOI:10.1016/j.tetlet.2013.06.123

An efficient regioselective synthesis of 3-aryl imidazo[1,2-a]pyridines, 5-aryl imidazo[2,1-b]thiazoles, and 3-aryl benzo[d]imidazo[2,1-b][1,3]thiazoles is described by the reaction of phenacyl bromide with heterocyclic amine in the presence of DABCO unde

Full text of DOI:10.1016/j.tetlet.2013.06.123

Accepted Manuscript
DABCO catalyzed highly regioselective synthesis of fused imidazo-heterocy‐
cles in aqueous medium
Vikas M. Bangade, B. Chennakesava Reddy, Pramod B. Thakur, B. Madhu
Babu, H. M. Meshram
PII:
S0040-4039(13)01108-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.06.123
TETL 43176
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
25 March 2013
25 June 2013
26 June 2013
Please cite this article as: Bangade, V.M., Chennakesava Reddy, B., Thakur, P.B., Madhu Babu, B., M. Meshram,
H., DABCO catalyzed highly regioselective synthesis of fused imidazo-heterocycles in aqueous medium,
Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.06.123
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DABCO catalyzed highly regioselective synthesis
of fused imidazo-heterocycles in aqueous medium
Vikas M. Bangade, B. Chennakesava Reddy,
Pramod B. Thakur, B. Madhu Babu and H. M. Meshram*
O
Br
N
N
via
NH2
Present Work
Previous Report
inorganic base
organic solvent
N
N
O
N
H
DABCO
H2O
N
+
N
Br
H
via
NH2
Br
25 examples
N
O

Tetrahedron Letters
1
TETRAHEDRON
LETTERS
Pergamon
DABCO catalyzed highly regioselective synthesis of fused
imidazo-heterocycles in aqueous medium
Vikas M. Bangade, B. Chennakesava Reddy, Pramod B. Thakur,
B. Madhu Babu and H. M. Meshram*,
Medicinal Chemistry and Pharmacology Division,
Indian Institute of Chemical Technology, Hyderabad – 500 607, India
Tel.: +91-40-27191642, Fax: +91-40-27193275, hmmeshram@yahoo.com
Abstract— An efficient regioselective synthesis of 3-aryl imidazo[1,2-a]pyridines, 5-aryl imidazo[2,1-b]thiazoles and 3-aryl
benzo[d]imidazo[2,1-b][1,3]thiazoles is described by the reaction of phenacyl bromide with heterocyclic amine in presence of DABCO
under aqueous medium. The method is applicable for a variety of substituted phenacyl bromides as well as 2-amino-heterocycles. An
aqueous reaction medium, regioselectivity, mild reaction condition and high yield of products are the important features of this protocol.
Keywords: phenacyl bromide, 2-aminopyridine, regioselectivity, fused imidazo-heterocycles, DABCO, water.
Fused imidazo-heterocycles possesses significant biological
activities like antiviral, antitumor, antimicrobial, herbicidal
and immunosuppressive agents.1 Additionally imidazo-
heterocycles like imidazo-pyridines and imidazo-thiazoles
constitute the core structure in some of the pharmaceutically
active molecules.2 Recently, imidazo-heterocycles has been
reported as a ligand for detecting β-amyloid (Aβ) plaques in
the brain, whose production is a pivotal event in the
pathology of Alzheimer’s disease.3 In particular, the bioactive
3-aryl substituted imidazo-heterocycles, TP-003 (I, Figure 1)
is an anxiolytic drug with anticonvulsant activity,4 compound
II is a ligand for GABA receptors used in treatment of
adverse condition of the central nervous system including
anxiety, convulsions and cognitive disorders.5 Compound III
also is a potent antimicrobial agent6 (Figure 1). Due to
distinct pharmaceutical properties of imidazo-heterocycles, it
is desirable to develop an efficient and convenient method for
the synthesis of these molecules.
There are scanty literature for the synthesis of imidazo-
heterocycles.7 However, the success of reported methods
mostly depend on the use of organic solvents and inorganic
bases.8 Commonly, it is prepared by the condensation of
heterocyclic amine with α-halo arylketone in ethanol at higher
temperatures. Though the reported methods are satisfactory,
they have some drawbacks such as use of expensive and
harmful solvents, inorganic bases, higher temperature and
longer reaction time (18-24 h). Moreover, the condensation
reaction between heterocyclic amine with α-halo-arylketone
in the presence of an inorganic base leads to the formation of
2-aryl imidazo[1,2-a]pyridines as a product (Scheme 1).9
The use of aqueous medium is a fascinating step in organic
reactions which has attracted the increasing interest of
chemist because of environmental and economical issue.10 1,
4-Diazabicyclo[2.2.2]octane (DABCO) has emerged as an
efficient organic base which has been successfully used for
various organic reactions like, Baylis–Hillman reaction,11
cross-coupling reactions,12 cyclization of o-alkynylaryl
isocyanides.13 However, to the best of our knowledge, present
reaction have not yet been studied using organic base i.e.
DABCO.
S
N
F
F
N
N
N
HO
N
O
N
N
N
N
N
HN
F
TP-003
CF3
F
In continuation of our ongoing research on DABCO promoted
reactions,14 we observed an exciting result during our study
which encouraged us to explore the effect of DABCO in order
to understand the mechanistic aspects. Herein we wish to
report an efficient and environmentally benign method for the
regioselective synthesis of fused imidazo-heterocyclic system
II
III
I
Figure 1. Representative examples of bioactive 3-aryl
substituted imidazo-heterocyclic motifs.

2
Tetrahedron Letters
groups (Table 1, entries c-e) reacted slowly and gave
comparatively less yield of product.
by the reaction of phenacyl bromide with heterocyclic amine
in presence of DABCO under aqueous medium.
Table 1: Synthesis of fused 3-aryl imidazo[1,2-a]pyridines.a
N
R2
R1
N
Entry
Phenacyl bromide (1)
O
Yield %b
Amines (2)
Product (3)
Time (h)
2-aryl imidazo[1,2-a]pyridine
Br
NH2
O
N
N
R1
Br
R2
NH2
+
3a7a
1
N
a
93
N
N
1
2
N
R2
N
N
NH2
N
,,
3b8b
b
2.5
86
O2N
R1
O2N
N
N
O
NH2
NH2
Br
3c15a
82
84
83
c
3
3-aryl imidazo[1,2-a]pyridine
N
N
MeO
OMe
OMe
N
N
,,
,,
3
4
d
e
3d
3e
Scheme 1. Regioselective synthesis of 3-aryl imidazo[1,2-
N
N
a]pyridine derivatives
NH2
NH2
N
N
Initially, we attempted the reaction of phenacyl bromide (1a,
1mmol) with 2-aminopyridine (2a, 1mmol) in water (5 mL) in
the presence of 10 mol % of DABCO. Though the reaction
was sluggish at room temperature, it did improve at 60-70 °C
affording 3-phenyl imidazo[1,2-a]pyridine (3a) as a sole
product in 93% yield. The structure of product 3a was
confirmed by the spectral data.15 Though the reaction of
phenacyl bromide and 2-aminopyridine was reported9 to give
2-phenyl imidazo[1,2-a]pyridine, the present protocol gave
regioselectively 3-phenyl imidazo[1,2-a] pyridine. After
extensive screening of different mole % of DABCO, we
observed that 10 mol% of DABCO was sufficient for
maximum conversion of product. On the basis of the above
study, phenacyl bromide (1mmol), 2-aminopyridine (1mmol),
DABCO (10 mol %) in water (5 mL) at 60-70 °C (Scheme 2,
Table 1, entry a) were chosen as a set of optimized
conditions16 for the synthesis of 3-phenyl imidazo[1,2-
a]pyridine.
O
OMe
N
N
Br
f
92
94
3f
2
3
O2N
F
O2N
F
N
N
NH2
,,
3g
N
g
Me
Me
N
F
O
NH2
NH2
Br
N
Cl 3h
Cl
2.5
91
95
N
N
O2N
h
Cl
O2N
Cl
N
N
,,
3
3i
Cl
Cl
i
Me
Me
N
N
O
NH2
NH2
Br
3j21
1
96
92
N
N
j
O2N
NO2
N
N
,,
3k
4
O2N
k
O2N
NO2
O
N
N
Br
NH2
NH2
3l21
l
2.5
92
83
N
N
N
N
m
,,
3
3m
O2N
O2N
a Reaction condition: phenacyl bromide (1 mmol), Amine (1 mmol) and DABCO (10 mol%) in water (5 ml) at 60 O
b isolated Yields
C
Scheme 2. Reaction of phenacyl bromide with 2-
aminopyridine
Inspired by the results obtained from the reactions of
phenacyl bromides with 2-aminopyridines, we extended our
efforts to study the reaction of phenacyl bromides with 2-
aminothiazoles and 2-aminobenzothiazoles under similar
reaction condition. Accordingly, the reaction of phenacyl
bromide (1a) with 2-aminothiazole (4a) in water (5 mL) in
the presence of catalytic amount of DABCO (10 mol %)
proceeded smoothly and resulted into corresponding 5-phenyl
imidazo[2,1-b]thiazole (5a) in high yield (Scheme 3, Table 2,
entry a).
Next, we studied the scope of the reaction with respect to a
variety of functionlized phenacyl bromides and substituted 2-
aminopyridines under optimized reaction condition and the
results are summarized in Table 1. Interestingly, a range of
phenacyl bromides and substituted 2-aminopyridines
participated well in this conversion to afford corresponding 3-
aryl imidazo[1,2-a]pyridines (3) in good to excellent yield
(Table 1, entries b-k). From Table 1, it is evident that,
electronic factors of aryl ring on phenacyl bromides had
shown some effect on the conversion. In general, substituted
phenacyl bromides bearing electron-withdrawing groups
(Table 1, entries d, e, h, i) reacted faster and furnish high
yields whereas phenacyl bromide bearing electron-donating
Likewise, the reaction of phenacyl bromide (1a) with 6-
methoxy benzo[d]thiazol-2-amine gave the corresponding 3-

Tetrahedron Letters
3
phenyl benzo[d]imidazo[2,1-b][1,3]thiazole (5b) as a product
(Table 2, entry b). The scope of the methodology is
illustrated in Table 2 with respect to other phenacyl bromides
and 2-aminothiazoles/2-aminobenzothiazoles. We found that,
various substituted 2-aminothiazoles (Table 2, entries e, f, g,
h, and j) and 2-aminobenzothiazoles (Table 2, entries c, d, i,
k, and l) also reacted smoothly with phenacyl bromides
resulting in the formation of respective 5-aryl imidazo[2,1-
b]thiazole and 3-aryl benzo[d]imidazo[2,1-b][1,3]thiazole (5)
derivatives with high yields.
Table 2: Synthesis of fused 5-aryl imidazo[2,1-b]thiazole and
3-aryl benzo[d]imidazo[2,1-b][1,3]thiazole derivatives.a
Scheme 3. Reaction of phenacyl bromide with 2-
aminothiazole.
The efficiency of the protocol is demonstrated by the
successful reaction of bicyclic phenacyl bromide. For
example 2-bromo-1-(naphthalen-2-yl)ethanone also reacted
smoothly
with
2-aminothiazole
and
6-methoxy-2-
aminobenzothiazole (Table 2, entries h and i respectively)
under optimized condition and gave corresponding product in
good yield. This method provides the access for the synthesis
of novel compounds (3e-3g, 3i, 3k, and 5c-5l) which are not
prepared earlier.
Generally the condensation of 2-aminopyridine and phenacyl
bromide is sustained by the most favorable attack of ring
nitrogen of pyridine due to the formation of highly stable
pyridinium ion intermediate.17 But technically the formation
of stable pyridinium ion has been inhibited, which is a key
step for the formation of 2-phenyl imidzo[1,2-a]pyridine by
inserting DABCO as a catalyst. This catalyst forms a stable
water soluble quaternary salt (6) (scheme 3) with phenacyl
bromide.18 The tentative mechanistic pathway for the
synthesis of 3-phenyl imidazo[1,2-a]pyridine (3a) is proposed
and shown in Scheme 4. We reasoned that initially phenacyl
bromide reacts with DABCO to form quaternary salt (6).
Further it reacts with amine to furnish intermediate (7)19
which on cyclization followed by dehydration20 leads to the
desired product (3a). This regioselective formation of product
(3a) may be attributed to DABCO which forms quaternary
salt (6) and diverts the attack of primary amine on carbon
near to quaternary nitrogen instead of carbonyl carbon
(Scheme 4).
In conclusion, we have demonstrated DABCO catalyzed
convenient and efficient method exclusively for the synthesis
of 3-phenyl imidazo[1,2-a]pyridine in high yield. The
aqueous medium makes the procedure more eco-friendly.
This procedure is suitable for substituted heterocyclic amines
and substituted phenacyl bromides. Moreover the method
provides an access for the regioselective synthesis of 3-aryl
imidazo[1,2-a]pyridine as a single isomeric product. We
believe that the present method may be an attractive
alternative over the earlier methods.
Scheme 4: Plausible reaction pathway.

4
Tetrahedron Letters
6. Farag, A. M.; Mayhoub, A. S.; Barakat, S. E.; Bayomi,
Acknowledgments
A. H. Bioorg. Med. Chem. 2008, 16, 4569.
The authors V M B, B C K R, P B T and B M B thank CSIR-
UGC for the award of a fellowship and Dr. (Ms.) M. Lakshmi
Kantam, Director IICT, for her support and encouragement.
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16. General procedure: The mixture of phenacyl bromide
(1 mmol), 2-aminopyridine (1 mmol) and DABCO (10
mol %) was stirred in water (5 mL) at 60-70 °C for 1 h.
The reaction was monitored by TLC. After the
completion of the reaction, it was extracted with ethyl
acetate (3x5 mL). The combined organic layer washed
with brine solution, dried over anhydrous Na2SO4 and
concentrated under reduced pressure. The crude products
were purified by silica gel column chromatography (ethyl
acetate/hexane, 1:10). All compounds were characterized
by M.P., NMR, Mass, HRMS and IR spectral data.
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Tetrahedron Letters
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Spectral data for representative compounds:
Compound 3d: White solid. mp 152–154 °C; 1H NMR (300
MHz, CDCl3+DMSO-d6): δ = 7.92 (s, 1H), 7.56-7.49 (m,
3H), 7.45 (s, 1H), 7.19 (d, J = 8.31 Hz, 1H), 7.12 (d, J = 7.36
Hz, 2H), 3.84 (s, 3H), 2.28 (s, 3H) ppm. 13C NMR (75 MHz,
CDCl3+DMSO d6): δ = 159.82, 146.02, 143.93, 137.67,
132.04, 127.28, 125.35, 125.02, 117.14, 113.65, 55.28, 20.05
ppm. IR (KBr) ν = 3123, 1562, 1245, 1423, 715 cm–1; MS
(ESI) m/z 261(M+Na)+. HRMS (ESI): m/z calcd. For
C15H14N2NaO[M+Na]+ = 261.09983, found 261.09960.
1
Compound 3e: White solid. mp 145–147 °C; H NMR (300
MHz, CDCl3+DMSO-d6): δ = 7.98 (d, J = 8.86 Hz, 1H), 7.50
(s, 1H), 7.48-7.29 (m, 3H), 7.15 (d, J = 7.17 Hz, 2H), 6.54 (d,
J = 8.31 Hz, 1H), 3.86 (s, 3H), 2.28 (s, 3H) ppm. 13C NMR
(75 MHz, CDCl3+DMSO d6): δ = 159.90, 145.95, 138.67,
136.23, 134.19, 127.26, 125.24, 124.13, 119.14, 113.47,
55.28, 20.05 ppm. IR (KBr) ν = 3119, 1570, 1435, 1259, 722
cm–1; MS (ESI) m/z 261(M+Na)+. HRMS (ESI): m/z calcd.
For C15H14N2NaO[M+Na]+ = 261.09983, found 261.09960.

8
Tetrahedron Letters
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DABCO catalyzed highly regioselective synthesis
of fused imidazo-heterocycles in aqueous medium
Vikas M. Bangade, B. Chennakesava Reddy,
Pramod B. Thakur, B. Madhu Babu and H. M. Meshram*
altered.