Three-step synthesis of novel 2-aryl-3-benzamidobenzofurans: Regiospecific reactions catalyzed by molybdate sulfuric acid

Article abstract of DOI:10.1002/jhet.1961

A solvent-free and synthetic pathway to novel benzofuran derivatives, starting from oxidation of phenyl ketones to arylglyoxals in three steps was developed. The molybdate sulfuric acid catalyzed the reaction of arylglyoxals with benzamide and phenols to afford 2-aryl-3-benzamidobenzofurans in high yield. The structures of the synthesized compounds were assigned on the basis of elemental analysis, IR, ¹H NMR, and ¹³C NMR spectral data. The present methodology offers several advantages such as non-hazardous reaction condition, simple operation, and work-up procedure.

Full text of DOI:10.1002/jhet.1961

322
Three-Step Synthesis of Novel 2-Aryl-3-benzamidobenzofurans:
Vol 51
Regiospecific Reactions Catalyzed by Molybdate Sulfuric Acid
a
Fatemeh Hashemi,a Bahador Karami,a,b and Saeed Khodabakhshi
*
aDepartment of Chemistry, Gachsaran Branch, Islamic Azad University, Gachsaran, Iran
bDepartment of Chemistry, Yasouj University, Yasouj 75918-74831, Iran
*
E-mail: karami@mail.yu.ac.ir
Additional Supporting Information may be found in the online version of this article.
Received November 2, 2012
DOI 10.1002/jhet.1961
Published online 4 November 2013 in Wiley Online Library (wileyonlinelibrary.com).
A solvent-free and synthetic pathway to novel benzofuran derivatives, starting from oxidation of phenyl
ketones to arylglyoxals in three steps was developed. The molybdate sulfuric acid catalyzed the reaction
of arylglyoxals with benzamide and phenols to afford 2-aryl-3-benzamidobenzofurans in high yield. The
1
structures of the synthesized compounds were assigned on the basis of elemental analysis, IR, H NMR,
and 13C NMR spectral data. The present methodology offers several advantages such as non-hazardous
reaction condition, simple operation, and work-up procedure.
J. Heterocyclic Chem., 51, 322 (2014).
INTRODUCTION
RESULTS AND DISCUSSION
Benzofurans are present in various biologically active
molecules [1] possessing anti-bacterial [2], anti-fungal [3],
anti-inflammatory [4], anti-depressant [5], anti-convulsant
[6], and so forth. For example, Benzbromaron is a uricosuric
agent used in the treatment of gout [7], cloridarol is a vasodi-
lator [8], oxetorone is an antimigraine agent [9], and
amiodarone is an antiarrhythmic agent [10] used for
various types of cardiac dysrhythmias, both ventricular and
atrial (Fig. 1).
The methods for synthesis of benzofuranes have been
reviewed by Kadieva and Oganesyan in 1997 [11]. The
traditional methods for the synthesis of benzofurans are
the preparation via O-alkylation of salicylaldehyde with
chloroacetic acid followed by dehydration of the resulting
ether [12] and via Perkin rearrangement in which a
coumarin is reacted with a hydroxide [13–15]. Recently,
new strategies were employed for synthesis of this
benzoheterocycle such as Claisen rearrangement and
ring-closing metathesis [16], condensation of benzil with
phenols and aryl ethers mediated by SnCl2 [17], reaction
of phenols, arylglyoxal monohydrates, and para
toluenesulfonamide [18].
Considering the aforementioned background and
our programmatic interest on the heterocyclic synthesis
[19–21], and catalyzed organic reactions [22–25], we
found that arylglyoxals 2, which in turn are obtained by
selenium dioxide oxidation of the corresponding phenyl
ketones 1, can participate in a one-pot and three-component
reaction with benzamide (3) and phenols 4 in the presence
of molybdate sulfuric acid (MSA) under solvent-free
conditions yielding novel benzofurans 5 (Scheme 1). It is
interesting to note that these reactions showed regiospecificity
toward benzofurans 5 because it was obtained as the only
isomer via heterocyclization reaction of intermediate 2;
and therefore, the oxazoles 6 were not formed. It should
be also noted that the intermediate 1 was in situ generated,
and the intermediate 2 was not isolable during the reaction
as previous report in literature [26] for aldehydes instead
of arylglyoxals.
In order to optimize the reaction conditions for the
synthesis of compounds 5, the reaction of phenylglyoxal
(2a), benzamide (3), and b-naphthol (4a) was selected as
a model (Scheme 2). We found that in the absence of the
catalyst, the reaction did not complete, even at long
reaction times in high temperatures. Through screening,
we found that this reaction was completed with MSA
(10 mol%) under solvent-free conditions at 120ꢀC.
To the best of our knowledge, there is no report on
the synthesis of 2-aryl-3-amido benzofurans via three-
component reaction of arylglyoxals, benzamide, and phenols
in literature. Therefore, we describe here a three-step
synthesis of novel class of 2-aryl-3-benzamidobenzofurans
by employing green reaction conditions.
To test the generality of the reaction, this thermal and
solvent-free procedure was employed for similar substrates
in the presence of MSA. The results have been shown in
© 2013 HeteroCorporation

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Three-Step Synthesis of Novel 2-Aryl-3-benzamidobenzofurans: Regiospecific
323
Reactions Catalyzed by Molybdate Sulfuric Acid
O
protons of NH groups also appeared as a singlet at
d = 10.77 (Fig. 2). The proton decoupled 13C NMR
spectrum of 5e also showed 24 distinct resonances in
agreement with the proposed structure. As can be seen in
Figure 2, the appearance of the signal for carbonyl of
amide group at 166.63 ppm can prove that the compounds
5e have been formed. Another evidence for the formation
of regioisomer 5e is the appearance of one peak in the
Me
O
O
I
Me
O
N
Me
O
I
N
Me
Me
Oxetorone
Amiodarone
Cl
infrared for secondary amide at 3200 cmÀ1
.
Br
O
HO
The nature of Ar groups showed no significant effect on
the yield or reaction rate. Minimizing catalyst loss and
avoidance of organic solvents during chemical reactions
require a fundamental understanding of green chemistry
factors. These concepts may be useful in the improvement
of organic syntheses and environmental and economic
problems. The advantages of solvent-free procedures
include cost savings, reduced energy consumption,
decreased reaction times, and a considerable reduction in
reactor size and, therefore, capital investment. These
attributes have inspired a substantial research effort
directed toward the development of solvent-free reactions
[27,28]. Solid acid catalysts are interesting alternatives
for the common liquid acids because of their simple
handling and isolation of the reaction products and reagent
[29,30]. A practical test of the recyclability of the MSA
was also examined for the model reaction, and this
catalyst was reused for three cycles during which a little
appreciable loss was observed in the catalytic activities.
O
Br
O
OH
Me
Cloridarol
Benzbromarone
Figure 1. Benzofuran derivatives as drug.
Scheme 1. Regiospecific formation of new benzofurans.
O
O
NH2
O
O
O
H
N
(i)
3
(ii)
Ar
Ar
Me
Ar
O
1
2
Int. 1
G
H
N
OH
O
G
O
O
Ar
O
4
HN
G
5
Ar
OH
(ii)
OH
N
Table 1
Synthesis of benzofuran derivatives using MSA at 120ꢀC under solvent-
free conditions.
O
Int. 2
G
Ar
6
Yielda
(%)
Mp
(ꢀC)
(i): SeO2, 1,4-dioxane/H2O
(ii): Molybdate sulfuric acid (10 mol%), Solvent-free, 120 oC
Entry
Ar
Phenol
Product
5a
C6H5
2-Naphthol
80
75
85
80
85
75
85
75
230–
232
Scheme 2. A model for the optimization of reaction conditions.
5b
5c
5d
5e
5f
4-MeO-
C6H4
2-Naphthol
2-Naphthol
2-Naphthol
2-Naphthol
2-Naphthol
239–
241
O
O
HO
H
O
NH2
4-Br-
C6H4
286–
288
+
+
O
O
HN
3
2a
4a
4-Cl-
C6H4
270–
272
5a
3-MeO-
C6H4
198–
200
2-
220–
222
Table 1. In regard of thin layer chromatography experi-
ments during the reaction progresses, we found that only
one regioisomer has been formed. The regiospecificity of
these reactions was proved by NMR and IR spectroscopy.
As a representative sample, the 1H NMR (DMSO-d6,
400 MHz) spectrum of 5e exhibited a sharp singlet
identified as a methyl (d = 3.74) protons. The signals
(d = 8.25–6.99) corresponded to the aromatic protons. The
Naphthyl
5g
5h
C6H5
1,2,3-(OH)3
C6H3
248–
250
4-Cl-
C6H4
1,2-(OH)2
C6H4
225–
227
MSA, molybdate sulfuric acid.
aIsolated yield.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

324
F. Hashemi, B. Karami, and S. Khodabakhshi
Vol 51
Scheme 3. Suggested mechanism for the formation of benzofuran 5 using
molybdate sulfuric acid (MSA).
O
H
O
O
H
-2 H2O
HA
H
O
N
A-
Ar
Ar
+
O
O
H
H
2
H2N
Ar
Int. 1
O
G
HO
2
3
OH
Ar
H
H
O
Ar
O
HO
O
G
A-
-H2O
H
H
NH
O
NH
G
Int. 3
Int. 2
Ar
G
O
O
NH
+
HA
5
O O
O
HO
O
S
Mo
S
HA:
O
OH
O
O O
from Merck and Aldrich. The reactions were monitored by TLC
(silica gel 60 F254, hexane/EtOAc). IR spectra were recorded on
a FT-IR JASCO-680, and the NMR spectra were obtained on a
Bruker-Instrument DPX-400 MHz Avance III model. The Vario
EL CHNS at the Isfahan Industrial University was used for
elemental analysis.
General procedure for the synthesis of benzofurans 5.
A
mixture of arylglyoxal (1 mmol), benzamide (1 mmol), and MSA
(0.1 mmol) was stirred and heated at 120ꢀC in a preheated oil
bath for 30 min. Then the phenolic substrate was added, and the
mixture was stirred for an appropriate time (120–240 min).
After completion of the reaction as indicated by TLC (EtOAc/
hexan, 1:2), the reaction mixture was dissolved in hot EtOH,
and the catalyst was separated by filtration. The solvent was
evaporated and the products 5 were purified by recrystallization
Figure 2. The NMR spectra for compound 5e.
in EtOH.
1-Benzamido-2-phenylnaphtho[2,1-b]furan (5a). IR (KBr)
v: 3160, 3110, 2089, 1640, 1485, 1260, 1040, 800, 700 cmÀ1; 1H
NMR (DMSO-d6, 400 MHz): 10.78 (s, 1H), 8.26 (d, 1H, J = 7.6),
8.21 (d, 2H, J = 7.2 Hz), 8.09 (d, 1H, J = 8.0 Hz), 7.90–8.04 (m,
4H), 7.74–7.65 (m, 3H), 7.53, 7.57 (2d, 4H, J = 8.0, 7.6 Hz),
7.43 (t, 1H, J = 7.6 Hz); 13C NMR (DMSO-d6, 100 MHz):
167.21, 150.86, 149.10, 134.07, 132.72, 130.92, 129.83,
129.54, 129.47, 129.38, 129.25, 128.27, 127.66, 127.18,
126.87, 125.84, 125.41, 122.67, 121.40, 117.26, 113.05; Anal.
Calcd. for C25H17NO2: C, 82.63; H, 4.72; N, 3.85. Found: C,
On the basis of the general mechanistic pathway for the
formation of compounds 5, Scheme 3 shows a reasonable
mechanism. The reaction is thought to take place in three
steps. It is reasonable to assume that the initial event
involves the generation of intermediate 1 via condensation
of the amide and arylglyoxal. In the next step, intramolecular
cyclization of intermediate 2 gives intermediate 3 followed
by dehydration to form corresponding product 5.
82.90; H, 4.65; N, 3.77.
1-Benzamido-2-(4-methoxyphenyl)naphtho[2,1-b]furan
(5b).
IR (KBr) v: 3165, 3110, 2950, 1640, 1510, 1475,
EXPERIMENTAL
1260, 1040, 800, 700 cmÀ1
;
1H NMR (DMSO-d6,
Arylgloxals were synthesized in accord with report by Riley
and Gray [31]. Molybdate sulfuric acid was prepared in accord
with our previous literature [32]. All chemicals were purchased
400 MHz): 10.70 (s,1H), 8.22, 8.24 (2d, 3H, J = 8.0,
6.8 Hz), 8.07 (d, 1H, J = 8.0 Hz), 7.89 (t, 4H, J = 8.4 Hz),
7.74–7.64 (m, 3H), 7.56–7.50 (m, 2H), 3.81 (s, 3H), 7.11
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Three-Step Synthesis of Novel 2-Aryl-3-benzamidobenzofurans: Regiospecific
325
IR
Reactions Catalyzed by Molybdate Sulfuric Acid
(d, 2H); 13C NMR (DMSO-d6, 100 MHz): 167.20, 160.10,
150.46, 149.46, 134.11, 132.66, 130.89, 129.40, 129.35,
128.25, 127.58, 127.45, 126.99, 126.20, 125.28, 122.65,
122.36, 121.54, 115.61, 115.05, 112.95, 55.77; Anal.
Calcd. for C26H19NO3: C, 79.37; H, 4.87; N, 3.56. Found:
C, 79.51; H, 4.80; N, 3.45.
1-Benzamido-2-(4-bromophenyl)naphtho[2,1-b]furan
(5c). IR (KBr) v: 3200, 1650, 1490, 1395, 1270, 1090, 800,
690 cmÀ1; 1H NMR (DMSO-d6, 400 MHz): 10.78 (s, 1H), 8.22
(d, 1H, J = 8.4 Hz), 8.17 (d, 2H, J = 7.6 Hz), 8.09 (d, 1H,
J = 8.0 Hz), 7.96 (d, 1H, J = 8.8 Hz), 7.91–7.87 (m, 3H), 7.76
(dd, 2H, J = 6.8, 2.0 Hz), 7.72 (d, 1H, J = 7.1 Hz), 7.66 (t, 2H,
J = 7.6 Hz), 7.57–7.53 (m, 2H); 13C NMR (DMSO-d6,
100 MHz): 167.10, 151.00, 148.13, 133.90, 132.79, 132.59,
130.93, 129.51, 129.39, 128.94, 128.28, 127.68, 127.58,
127.29, 127.25, 125.53, 122.65, 122.48, 121.24, 117.67,
113.03; Anal. Calcd. for C25H16BrNO2: C, 67.89; H, 3.65; N,
3.17. Found: C, 68.01; H, 3.48; N, 3.02.
1-Benzamido-2-(4-chlorophenyl)naphtho[2,1-b]furan
(5d). IR (KBr) v: 3205, 1640, 1485, 1395, 1280, 1090, 800,
710, 690 cmÀ1; 1H NMR (DMSO-d6, 400 MHz): 10.78 (s, 1H),
8.22 (d, 1H, J = 7.6 Hz), 8.17 (d, 2H, J = 8.0 Hz), 8.09 (d, 1H,
J = 7.8 Hz), 7.97–7.89 (m, 4H), 7.74–7.61 (m, 5H), 7.58–7.51
(m, 2H); 13C NMR (DMSO-d6, 100 MHz): 166.63, 150.47,
147.50, 133.43, 133.25, 132.26, 130.41, 129.19, 129.00,
128.87, 128.11, 127.78, 127.08, 126.94, 126.77, 126.70,
125.00, 122.13, 120.72, 117.25, 112.52; Anal. Calcd. for
C25H16ClNO2: C, 75.47; H, 4.05; N, 3.52. Found: C, 75.55;
H, 3.95; N, 3.31.
7-Hydroxy-2-phenyl-3-benzamidobenzofuran (5h).
(KBr) v: 3410–3200, 3050–3000, 1668–1615, 1540–1498,
1100–1098, 740–689; 1H NMR (DMSO-d6, 400 MHz): 10.74
(s, 1H), 9.31 (d, 1H, J = 8 Hz), 8.00–8.02 (m, 2H), 7.90–7.95
(m, 3H), 7.48–7.72 (m, 6H), 6.97 (d, 2H, J = 1H); 13C NMR
(DMSO-d6, 100 MHz): 166.56, 166.48, 138.78, 133.61,
132.81, 132.37, 131.32, 130.46, 130.03, 129.38, 128.89,
128.03, 126.03, 122.12, 120.01; Anal. Calcd. for C21H14ClNO3:
C, 69.33; H, 3.88; N, 3.85. Found: C, 69.55; H, 3.71; N, 3.70.
CONCLUSION
In summary, the reaction between arylglyoxals, phenols,
and benzamide in the presence of molybdate sulfuric acid
provides a simple one-pot entry for the synthesis of novel
2-aryl-3-benzamidobenzofurans of potential synthetic and
pharmaceutical interest. The use of a green and recyclable
catalyst (MSA) under solvent-free conditions, high yield of
pure products, and a simple workup procedure make the
present method a valid contribution in accord with green
chemistry principles. It is worthwhile to note that the
presence of transformable functionalities in the products
makes them potentially valuable for further synthetic
manipulations.
Acknowledgments. Partial financial support from the Islamic
Azad University, Gachsaran Branch is gratefully acknowledged.
1-Benzamido-2-(3-methoxyphenyl)naphtho[2,1-b]furan
(5e).
IR (KBr) v: 3200, 20100, 1650, 1509, 1390, 1250,
;
700 cmÀ1
1H NMR (DMSO-d6, 400 MHz): 10.77 (s, 1H),
8.24 (d, 1H, J = 8.0 Hz), 8.19 (d, 2H, J = 7.6 Hz), 8.09 (d, 1H,
J = 7.6 Hz), 7.94, 7.91 (2d, 2H, J = 9.2, 8.8 Hz), 7.71–7.63 (m,
3H), 7.56–7.43 (m, 5H), 7.00 (dd, 1H, J = 8.2, 2.4 Hz), 3.74
(s, 3H); 13C NMR (DMSO-d6, 100 MHz): 166.63, 159.43,
150.29, 148.35, 133.45, 132.25, 130.45, 130.39, 130.22,
128.96, 128.86, 127.71, 127.123, 126.711, 126.44, 124.93,
122.14, 120.86, 117.72, 116.95, 114.54, 112.54, 110.488,
55.043; Anal. Calcd. for C26H19NO3: C, 79.37; H, 4.87; N,
3.56. Found: C, 79.48; H, 4.70; N, 3.50.
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IR
1
(KBr) v: 3190, 2200, 1650, 1500, 1395, 1010, 790; H NMR
(DMSO-d6, 400 MHz): 10.86 (s, 1H), 8.28 (d, 1H, J = 7.6 Hz),
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1H NMR (DMSO-d6, 400 MHz): 10.81 (s, 1H), 9.26 (s, 1H),
8.99 (s, 1H), 7.93,7.98 (2d, 2H, J = 8, 7.6 Hz), 7.44–7.61 (m,
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