Catalyst-free synthesis of coumarin-, quinolone- and pyridine-annulated oxazole derivatives

Article abstract of DOI:10.1016/j.tetlet.2012.01.015

An efficient approach for the synthesis of coumarin-, quinolone- and pyridine-annulated oxazole derivatives has been achieved by direct base-mediated intramolecular carbon-oxygen bond formation in a transition metal catalyst-free protocol. Intramolecular

Full text of DOI:10.1016/j.tetlet.2012.01.015

Tetrahedron Letters 53 (2012) 1553–1557
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Tetrahedron Letters
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Catalyst-free synthesis of coumarin-, quinolone- and pyridine-annulated
oxazole derivatives
K.C. Majumdar a,b, , Sintu Ganai a, Raj Kumar Nandi a, Krishanu Ray a
⇑
a Department of Chemistry, University of Kalyani, Kalyani 741 235, West Bengal, India
b Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784 028, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient approach for the synthesis of coumarin-, quinolone- and pyridine-annulated oxazole deriv-
atives has been achieved by direct base-mediated intramolecular carbon–oxygen bond formation in a
transition metal catalyst-free protocol. Intramolecular cyclization of o-bromoamides in DMSO in the
presence of Cs2CO3 at 130 °C affords heterocycle-annulated oxazole derivatives in high yields via a nucle-
ophilic addition of amide to form the C–O bond.
Received 28 September 2011
Revised 3 January 2012
Accepted 5 January 2012
Available online 13 January 2012
Ó 2012 Published by Elsevier Ltd.
Keywords:
Oxazole
Coumarin
Quinolone
Pyridine
Cs2CO3
DMSO
The oxazole moieties have a broad spectrum of biological activ-
ity1 and are found in a large number of natural products2 and func-
tional materials.3 For example, the benzoxazole core structure is
found in a variety of cytotoxic natural products.2c,d,4 Recently ther-
apeutic applications showed benzoxazole moieties are important
targets in drug discovery5 and in herbicides such as Fenoxaprop.
On the other hand arylated oxazoles are of great interest not only
due to their biological properties but also due to their importance
as organic materials such as scintillant compounds and fluorescent
dyes, for example, bis-benzoxazolyl ethylenes and arenes.6 More-
over, coumarin, quinolone, and pyridine scaffolds found in various
natural products are responsible for their biological activity and
exhibit antifungal, antibacterial, antiviral, and antimicrobial prop-
erties.7 Among them oxazolopyridine moiety has antihypertensive
effect8 and other medicinal relevance.9 The oxazolopyridine moi-
ety might offer some advantages in medicinal chemistry due to
better water solubility of the pyridine fragment by offering an
additional site for protonation and salt formation or it might
enhance intermolecular interactions with a target protein by the
formation of an additional hydrogen bond.10g
of benzoxazoles, these are associated with a number of drawbacks
such as requiring the use of highly toxic reagents, strong acids
and, in some cases, harsh reaction conditions.11 Batey group has
reported12 the synthesis of benzoxazoles via domino inter/
intramolecular copper-catalyzed cross-coupling reaction using
1,2-dihaloarenes and primary amides. According to Bolm et al.13a
KOH in dimethyl sulfoxide forms a superbasic medium that allows
one to access cross-coupling products from reactions between aryl
halides with various sulfur-, oxygen-, and nitrogen-based nucleo-
philes under transition metal-free conditions. Recently, Peng group
has reported13b the base-mediated reaction of o-haloanilides to af-
ford benzoxazole derivatives under mild conditions. The recent
program on the synthesis of bioactive heterocyclic compounds14
prompted us to undertake a study to synthesize potentially bioac-
tive coumarin-, quinolone-, and pyridine-annulated oxazole deriv-
atives by extending the scope of Peng’s protocol. Herein we report
our results.
Results and discussion
The conventional method for the synthesis of 2-substituted
benzoxazoles is via the condensation of o-aminophenols and alde-
hydes or carboxylic acid derivatives under oxidative conditions.10
Although these transformations are widely used in the preparation
The precursors 3a–i required for our present study were pre-
pared in 78–87% yields by the treatment of 1a–d with benzoyl
chlorides 2a–d under phase transfer catalysis condition using
TBAHS as a catalyst and potassium carbonate as a base (Scheme 1).
Initially, we have used 3a as a model substrate for the intramo-
lecular o-arylation reaction. At first, intramolecular C–O coupling
reaction of 3a was examined by using potassium carbonate
⇑
Corresponding author. Tel.: +91 33 2582 7521; fax: +91 33 25828282.
E-mail address: kcm_ku@yahoo.co.in (K.C. Majumdar).
0040-4039/$ - see front matter Ó 2012 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2012.01.015

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K. C. Majumdar et al. / Tetrahedron Letters 53 (2012) 1553–1557
O
Br
Br
R
NH2
+
HN
O
(i)
Cl
R
O
X
O
X
3a, X = NMe, R = p-MeC6H4
3b, X = NMe, R = o-ClC6H4
3c, X = NMe, R = -Me
1a, X= NMe
1b, X= NEt
1c, X= O
2a, R = C6H5
2b, R = p-MeC6H4
2c, R =o-ClC6H4
2d, R = Me
3d, X = NEt, R = C6H5
3e, X = O, R = C6H5
3f, X = O, R = p-MeC6H4
3g, X = O, R = o-ClC6H4
H
O
Br
NH2
Br
N
R
(i)
+
Cl
R
O
N
Br
Br
N
2a, R = C6H5
3h, R = C6H5
1d
3i, R = p-MeC6H4
2b, R = p-MeC6H4
Scheme 1. Reagents and conditions: (i) 1 equiv compound 1, 1.5 equiv compound 2, 1.5 equiv K2CO3, TBASH, CHCl3–H2O (1:1), rt, 5 h.
(2.0 equiv) as a base in DMSO at 140 °C for 12 h. Even after allow-
ing a long reaction time we obtained a product in low yield. The
spectral analysis of the compound revealed the formation of the
desired product 4a.15 Different experiments were performed in
an attempt to improve the yield of the products by varying the
bases (Table 1). Moderate yield was obtained in the case of Na2CO3
but the organic bases DABCO, Et3N failed to give any product. NaO-
Ac and KOAc also gave poor yields of the products. However the
desired product benzoxazole 4a was smoothly obtained in a 93%
yield when the reaction was carried out with a minimum of
1.5 equiv Cs2CO3 in DMSO at 130 °C in 5 h (entry 2, Table 1). To ex-
plore the possibility of metal impurities in the commercially
available cesium carbonate that may potentially induce this trans-
formation,16 we analyzed Cs2CO3 using inductively coupled plasma
spectroscopy (ICP) where 0.19 ppm of Co, 5.39 ppm of Cu, and
0.52 ppm of Fe were detected in the base. However, no reaction oc-
curred when 15 mol % CoBr2, CuI, or FeCl3 was added to the reac-
tion mixture in the absence of any Cs2CO3. The reaction efficiency
was reduced when the reaction was performed using Cs2CO3 in
the presence of these salts. Based on the above experiments, we
conclude that the presence of trace metal impurities does not pro-
mote this carbon–oxygen bond formation reaction. No significant
improvement was observed by increasing the amount of base or
reaction temperature. Furthermore, the effect of solvent on the
reaction was examined for a range of solvents. Water as well as
non-polar solvents such as toluene, dioxane were found to be inef-
fective and no product was detected (entries 4–6, Table 1). A poor
yield of 4a was obtained when DMF was used as the solvent. DMSO
was found to be the best choice for this reaction. The optimized
condition is: 1.5 equiv Cs2CO3 in 5 mL DMSO, 130 °C. The other
substrates 3b–i were treated under the above optimized reaction
condition to afford the products (4b–i) in excellent yields except
for 3c which was obtained in only 68% yield. The results are sum-
marized in Table 2.
The formation of oxazolopyridine 4h–i from 3h–i may be ratio-
nalized following the Peng’s13 mechanism, that is, an initial forma-
tion of a benzyne-like intermediate (3C) in the presence of a base
followed by intramolecular nucleophilic addition of amide to the
intermediate 3C may afford the final products (Scheme 2). But it
is worthy to note that no such benzyne like intermediate can be
formed in cases of the coumarin or quinolone substrates (3a–g).
However, the formation of products 4a–g from 3a–g may be ex-
plained by considering the intermediates 3A that are formed by
the abstraction of a proton from the amide nitrogen in the presence
of a base (Cs2CO3). The intermediates 3A are resonance stabilized
through the keto group (conformers 3B). An intramolecular aro-
matic nucleophilic displacement of bromine of the aromatic ring
by the amide oxyanions of 3B may finally furnish the products
4a–g. This alternative pathway offers a general plausible mecha-
nism for the reaction (of 3a–g) as this path can also rationalize
the formation of some of the Peng’s products. This mechanism
seems to be more likely to operate as the formation of benzyne
usually requires a relatively stronger base.
Table 1
Optimization of the intramolecular cyclizationa
Bolm and coworker have reported17 the synthesis of 2-aryl
substituted benzoxazole derivatives via iron-catalyzed intramolec-
ular o-arylation using FeCl3, Cs2CO3, TMHD in DMF at 120 °C.
Heravi group has reported18 a one-pot synthesis of 2-arylbenzox-
azoles from the reaction of 2-aminophenol with benzoic acids
using heteropolyacids catalyst under refluxing condition. But the
transition metal catalyst-free protocol of such potentially biologi-
cally active compounds is a fundamental goal in organic synthesis
so as to avoid the use of expensive reagents.
In summary, the preparation of potentially bioactive coumarin-,
quinolone-, and pyridine-annulated oxazole derivatives has been
efficiently achieved starting from readily available starting materi-
als. The reaction condition is very simple and affords the products
in good yields in the absence of a transition metal catalyst. This is
an environmentally benign protocol, suitable for large-scale
Entry
Base
Solvent
Temp (°C)
Time (h)
Yieldd (%)
b
1
2
3
4
5
6
7
8
K2CO3
DMSO
DMSO
DMF
Toluene
Dioxane
H2O
DMSO
DMSO
DMSO
DMSO
DMSO
140 °C
130 °C
130 °C
120 °C
120 °C
120 °C
140 °C
140 °C
140 °C
140 °C
140 °C
12
5
9
57
93
45
0
0
0
c
Cs2CO3
c
c
c
c
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
24
24
24
24
16
18
24
18
DABCOb
0
b
Na2CO3
NaOAcb
Et3Nb
52
17
0
9
10
11
K0Acb
24
a
b
c
All the reactions were carried out using 3a.
Base used 2 equiv.
Base used 1.5 equiv.
d
Isolated yields.

K. C. Majumdar et al. / Tetrahedron Letters 53 (2012) 1553–1557
1555
Table 2
Summarized results of the intramolecular cyclization reactiona
Entry
Substrate
Product
O
MeN
Yieldb (%)
O
Me
Br
O
N
NH
Me
1
93
86
O
N
4a
3a
Me
Cl
O
MeN
Cl
O
O
N
Br
Br
Br
NH
3b
2
4b
O
O
O
N
Me
O
O
MeN
Me
O
N
NH
3c
Me
3
4
68
90
N
4c
Me
O
O
EtN
O
NH
3d
N
N
4d
4e
Et
O
O
O
Br
Br
O
N
NH
3e
5
6
87
92
O
O
O
O
O
O
O
Me
O
N
NH
Me
4f
3f
Cl
O
O
Cl
O
O
N
Br
7
8
82
85
NH
4g
O
O
N
3g
O
N
N
Br
Br
NH
Br
O
4h
3h
Me
O
N
Me
N
Br
9
91
Br
NH
Br
4i
O
N
3i
a
1 equiv compound 3, DMSO, 1.5 equiv Cs2CO3 ,130 °C, 5 h.
Isolated yield.
b
synthesis, providing a valuable synthetic process for industrial
application. The examples of coumarin and quinolone substrates
described here shed further insight into the mechanism of the
base-mediated reaction. Further application of this methodology

1556
K. C. Majumdar et al. / Tetrahedron Letters 53 (2012) 1553–1557
R
N
R1
N
R1
NH
O
Br
O
Br
NH
R
O
O
N
3h-i
Br
O
X
N
Br
O
X
3a-g
4h-i
4a-g
-HBr Base
R
N
R1
O
R1
O
R
O
Br
O
Br
N
NH
N
Base
O
X
O
X
N
Br
N
Br
3A
3B
3C
3D
Scheme 2. Rationalization for the formation of products 4.
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Acknowledgements
We thank CSIR (New Delhi) and DST (New Delhi) for financial
assistance. Three of us (S.G., R.K.N. and K.R) are grateful to the CSIR
(New Delhi) for their research fellowships. We also thank the DST
(New Delhi) for providing Bruker NMR (400 MHz), Perkin–Elmer
CHN Analyser, FTIR and UV–vis spectrometer. We thank the direc-
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.01.015.
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15. Procedure for the preparation of compound 4a: To a solution of the compound 3a
(185 mg, 0.50 mmol) in DMSO (5 mL), 0.75 mmol Cs2CO3 (244.4 mg) was added.
The reaction mixture was stirred at 130 °C for 5 h. After completion of the
reaction (monitored by TLC), the reaction mixture was diluted with DCM
(15 mL Â 3). The combined organic layer was washed with water (10 mL Â 3)
and dried over Na2SO4. The filtrate was concentrated and the crude product mass
was purified by column-chromatography over silica-gel (60–120 mesh) using
petroleum ether and ethyl acetate (1:1) as eluent to give a colorless solid 4a.
Compound 4a: Yield 93%, colorless solid, mp 234 °C; IR (KBr): 2916, 1661,
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Evindar, G.; Batey, R. A. J. Org. Chem. 2006, 71, 1802; (b) Tully, D. C.; Liu, H.;
1530 cmÀ1 1H NMR (CDCl3, 400 MHz): dH = 2.42 (s, 3H, –CH3), 3.72 (s, 3H, NCH3),
;

K. C. Majumdar et al. / Tetrahedron Letters 53 (2012) 1553–1557
1557
6.80 (d, 1H, J = 12 Hz, ArH), 7.25 (d, 1H, J = 8.8 Hz, ArH), 7.30 (d, 2H, J = 7.6 Hz,
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ArH), 7.81 (d, 1H, J = 8.8 Hz, ArH), 8.06 (t, 3H, J = 8.4 Hz, ArH); 13C NMR (CDCl3,
100 MHz): dC = 21.7, 30.1, 106.4, 111.3, 121.6, 122.3, 123.8, 127.3, 129.6, 131.1,
136.3, 137.8, 142.2, 147.0, 161.9, 163.1; MS: m/z = 291 [M++H]; Anal. Calcd. For:
Bamoharram, F. F. J. Chinese Chem. Soc. 2008, 55, 890.
C
18H14N2O2: C, 74.47; H, 4.86; N, 9.65%; Found: C, 74.07; H, 4.76; N, 9.55%.