Synthesis of benzofuro[2,3-c]pyridines via a one-pot three-component reaction

Article abstract of DOI:10.1039/c4ob00670d

A convenient one-pot reaction was conducted by mixing bromoacetophenone, a functionalized α,β-unsaturated ketone and potassium hydroxide in tetrahydrofuran at room temperature, ammonium acetate was added and heated to reflux, resulting in four chemical bonds forming from easily accessible substrates. This process provides a flexible and rapid synthetic route for the construction of polysubstituted benzofuro[2,3-c]pyridines in moderate to good yields. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00670d

Organic &
Biomolecular Chemistry
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PAPER
Synthesis of benzofuro[2,3-c]pyridines via a
one-pot three-component reaction†
Jiaxing Hu,a Zhihong Deng,b Xueqiong Zhang,a Fanglin Zhang*a,b and Hua Zheng*a
Cite this: Org. Biomol. Chem., 2014,
12, 4885
A convenient one-pot reaction was conducted by mixing bromoacetophenone, a functionalized α,β-un-
saturated ketone and potassium hydroxide in tetrahydrofuran at room temperature, ammonium acetate
was added and heated to reflux, resulting in four chemical bonds forming from easily accessible sub-
strates. This process provides a flexible and rapid synthetic route for the construction of polysubstituted
benzofuro[2,3-c]pyridines in moderate to good yields.
Received 30th March 2014,
Accepted 16th April 2014
DOI: 10.1039/c4ob00670d
www.rsc.org/obc
and alkynes via rhodium(III) catalysis in good yields.9 However,
the harsh reaction conditions (e.g. basic or anhydrous con-
Introduction
Heterocyclic compounds are abundant in nature. Benzofuro- ditions) required by those processes limited their further appli-
pyridines represent an important class of heterocyclic com- cations, and there has not been a systematic pathway for the
pounds that exhibit
a wide range of biological and synthesis of such a class of compounds. Therefore, it is urgent
pharmaceutical activities, including antitumor, antibacterial that an efficient and convenient method to construct benzo-
and antimalarial activities, among others.1 For example, furo[2,3-c]pyridines is developed.
research showed that ethyl benzofuro[2,3-c]pyridine-3-carboxy-
late (I) and its derivatives were able to be non-benzodiazepine
structural ligands, binding to benzodiazepine receptors in
anxiolytics, tranquilizers, and anticonvulsants.2 Benzofuro-
[2,3-c]pyridin-3-yl(quinuclidin-3-yl)methanone (II) has been
reported for its central nervous system activity.3 Benzofuro-
[2,3-c]pyridine and its derivatives (III) have been shown to be
useful as phosphodiesterase-10 inhibitors.4 Because of these
interesting biological activities, several synthetic routes have
been developed to produce benzofuropyridines.5 However,
Multicomponent reaction (MCR) is becoming an attractive
benzofuro[2,3-c]pyridine, considered as one of the most extre-
strategy for the facile construction of useful complex chemical
mely significant benzofuropyridine structures, could pre-
compounds and has been receiving considerable attention
viously only be obtained through a few processes.6–9 Dubbaka
with the increasing economic and ecological pressures.10 This
and Kienle synthesized benzofuro[2,3-c]pyridines in 62% yield
novel approach provides diverse molecules in a one-pot reac-
using functionalized alkynyl lithium and aryl magnesium
tion and proceeds in a highly efficient and atom-economical
reagents at −78 °C.6 Lai and co-workers prepared benzofuro-
manner to generate multiple new bonds, which saves time and
[2,3-c]pyridine in 63% yield using 3-fluoro-4-(2-methoxy-
energy by avoiding multistep purifications of various inter-
phenyl)-pyridine.7 Furthermore, Wen reported the synthesis of
mediates.11 Thus, developing a new MCR using simple and
benzofuro[2,3-c]pyridine utilizing α- and γ-activation of chloro-
easily available substrates is one of the most important
pyridines as well as palladium-mediated reactions.8 Todd
research topics in organic chemistry.
found that benzofuropyridines could be prepared from oximes
aDepartment of Pharmaceutical Engineering, School of Chemical Engineering,
Results and Discussion
Wuhan University of Technology, Wuhan, 430070, China. E-mail: hyvzfl@gmail.com,
zhenghua.whut@126.com; Fax: +8627 87859019; Tel: +86 27 87859019
Our recent studies have been focused on the development of
bKey Laboratory of Functional Small organic molecule, Ministry of Education, Jiangxi
new synthetic pathways for the preparation of cyclic com-
Normal University, Nanchang, 330022, China
pounds, which were based on the use of cascade or one-pot
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4ob00670d
reactions.12 In this paper, using bromoacetophenones,
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Table 1 Optimization of the reaction conditions for the synthesis of
benzofuro[2,3-c]pyridinea
Scheme 1 Synthesis of benzofuropyridines via a MCR.
2-hydroxyphenyl functionalized α,β-unsaturated ketones and
ammonium acetate (Scheme 1), we report a one-pot three-com-
ponent tandem procedure for the synthesis of benzofuro[2,3-c]-
pyridines under mild and metal-free conditions in good yields.
To the best of our knowledge, this process was established
for the first time for the tandem construction of
benzofuropyridines.
Entry
Base (eq.)
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
DMAP (1.5 eq.)
Et3N (1.5 eq.)
Na2CO3 (1.5 eq.)
NaHCO3 (1.5 eq.)
K2CO3 (1.5 eq.)
KOH (1.5 eq.)
KOH (1.2 eq.)
KOH (1.4 eq.)
KOH (2.0 eq.)
KOH (1.5 eq.)
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
11
9
12
Trace
25
74
26
61
55
73
The first step of the cascade reaction was critical, including
a substitution reaction of the halides (1) with the 2-hydroxy-
phenyl functionalized α,β-unsaturated ketones (2), and the for-
mation of a new C–O bond.13 Then, a C–C bond was produced
through an intramolecular Michael addition reaction, yielding
the intermediate products (4) that contained 1,5-dicarbonyl
scaffolds.14 After the addition of ammonium acetate, pyridine
ring systems were generated by the formation of two bonds,
from [5 + 1] atom fragments between the 1,5-dicarbonyl con-
structions and the ammonium acetate.15 The dihydropyridines
(5) were formed first by a dehydration reaction after the
ammonium acetate was added, and the processes of dehydro-
genation and oxidation followed closely in air from the dihy-
dropyridines to the pyridines without additional oxidants. The
final products (3) were obtained as a pattern of benzofuro-
[2,3-c]pyridines. This new one-pot reaction provided an
efficient method for synthesizing complex fused pyridine
systems by simultaneous formation of four bonds from three
readily accessible substrates (Scheme 2).
9
10c
a The first steps of the reactions were carried out using 2-
bromoacetophenone (0.9 mmol) and 4-(2-hydroxyphenyl)but-3-en-2-
one (0.75 mmol), in the presence of the catalyst in THF (15 mL) at
room temperature. After the addition of ammonium acetate (4 mmol)
and 5 mL C2H5OH, the reactions were refluxed for 1 hour. b Yield of
the isolated product after chromatography on silica gel. c 6 mmol
ammonium acetate was added to the reaction in THF.
KOH, K2CO3, Na2CO3 and NaHCO3, were investigated for the
cascade reaction in THF. As summarized in Table 1 (entries
1–10), the choice of base significantly affected the yield. Most
of the bases could not promote this process efficiently. Reac-
tions with DMAP or Et3N gave 3a in less than 15% yield
(entries 1 and 2) and a group of inorganic bases such as
K2CO3, Na2CO3 and NaHCO3 also yielded unsatisfactory
results (entries 3–5). KOH was found to be the most suitable
base for the first step, with a yield of 74% (entry 6). Then the
loading of KOH was also tested (entries 7–9), and 1.5 eq. of
KOH gave the highest yield. The reaction was also performed
in the presence of 6 mmol ammonium acetate, which had
little influence on the yield of the product (entry 10).
In order to further improve the efficiency of this procedure,
different solvents, including toluene, dichloromethane, 1,2-
dichloroethane, methanol, ethanol, acetonitrile, tetrahydro-
furan and water were tested. The results displayed in Table 2
indicate that no expected product was found when toluene was
chosen as the reaction solvent (entry 1). On the other hand,
dichloromethane and 1,2-dichloroethane resulted in poor
yields of around 10% (entries 2–3). Moderate yields could be
obtained in alcohols and acetonitrile (entries 4–6). In order to
further improve the yield, we tested smaller volumes of solvent
and a lower temperature for this reaction. The results showed
that the yield of the reaction increased to 77% in the presence
of 5 mL THF, and the first-step reaction time also decreased to
30 min (entry 9). A longer reaction time in a lower temperature
did not significantly improve the yield (entry 10), and the
To optimize the reaction conditions, we investigated the
optimal conditions regarding both the base and the solvent. In
particular, 2-bromoacetophenone 1a (0.9 mmol), 4-(2-hydroxy-
phenyl)but-3-en-2-one 2a (0.75 mmol) and ammonium acetate
(4 mmol) were selected as the model reaction. Initially, organic
bases such as DMAP and Et3N, and inorganic bases such as
corresponding product was obtained with
a 75% yield.
The yield declined rapidly with the addition of a small
Scheme 2 The proposed process for the tandem reaction.
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Table 2 Optimization of the reaction solvent conditions for the syn-
thesis of benzofuro[2,3-c]pyridinea
Table 3 The cascade reactions of various substrates for the synthesis
of benzofuro[2,3-c]pyridine derivativesa
Entry
R1
R2
R3
Yieldb (%)
Entry
Base (eq.)
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
H
3-Br
4-Br
4-OCH3
2-CH3
3-CH3
4-CH3
4-tert-Butyl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Ph
4-Br-Ph
3-Br-Ph
2-Cl-Ph
4-Cl-Ph
4-CH3O-Ph
4-CH3-Ph
77 (3a)
46 (3b)
42 (3c)
55 (3d)
74 (3e)
59 (3f)
68 (3g)
71 (3h)
46 (3i)
44 (3j)
71 (3k)
78 (3l)
57 (3m)
63 (3n)
32 (3o)
75 (3p)
81 (3q)
55 (3r)
63 (3s)
65 (3t)
83 (3u)
1
2
3
4
5
6
7
KOH (1.5 eq.)
KOH (1.5 eq.)
KOH (1.5 eq.)
KOH (1.5 eq.)
KOH (1.5 eq.)
KOH (1.5 eq.)
KOH (1.5 eq.)
KOH (1.5 eq.)
KOH (1.5 eq.)
KOH (1.5 eq.)
PhCH3
CH2Cl2
ClCH2CH2Cl
C2H5OH
CH3CN
CH3OH
H2O
THF
THF
THF
Trace
9
11
33
52
41
Trace
31
77
75
8c
9d
10e
9
5-Cl
5-Br
5-tert-Butyl
3.5-Di-tert-butyl
3.5-Di-methyl
4.5-Di-methyl
H
H
H
H
H
H
H
10
11
12
13
14
15
16
17
18
19
20
21
a The first steps of the reactions were carried out using
2-bromoacetophenone (0.9 mmol) and 4-(2-hydroxyphenyl)but-3-en-2-
one (0.75 mmol), in the presence of 1.5 eq. KOH (1.125 mmol), with
different solvents (15 mL) at room temperature. After the addition of
ammonium acetate (4 mmol) and 5 mL C2H5OH, the reactions were
refluxed for
1
hour. b Yield of the isolated product after
chromatography on silica gel. c The reaction was carried out in 12 mL
THF and 3 mL H2O. d The reaction was carried out in 5 mL THF. e The
first step of the reaction was carried out at 0 °C for 6 hours.
a Reaction conditions: the first steps of the reactions were carried out
using bromoacetophenone derivatives (0.9 mmol), 2-hydroxyphenyl
functionalized α,β-unsaturated ketone derivatives (0.75 mmol), and 1.5
eq. KOH (1.125 mmol) in 5 mL THF at room temperature. After the
addition of ammonium acetate (4 mmol) and 5 mL C2H5OH, the
reactions were refluxed. b Yield of the isolated product after
chromatography on silica gel.
amount of water, while no products were gained in pure water
(entries 7–8).
We further probed the scope of the two substrates for the
cascade reaction under the optimal conditions, and it was
found that all of the reactions of various bromoacetophenones
and 2-hydroxyphenyl functionalized α,β-unsaturated ketones
were suitable, and led to the corresponding benzofuro[2,3-c]-
pyridine derivatives in good yields (Table 3).
tert-butyl group on the aromatic rings worked better in the
First, the scope of the substrates on the bromoacetophe- process, and resulted in 71% and 78% yields (Table 3, entries
none was explored.16 The results of reactions of various bro- 11 and 12), respectively. However, substrates with other elec-
moacetophenones with 4-(2-hydroxyphenyl)but-3-en-2-one and tron-donating groups, such as methyl groups, afforded lower
ammonium acetate showed that the procedure could tolerate yields (Table 3, entries 13 and 14). The cascade reactions not
aromatic bromoketones with electronically different substitu- only worked with alkyl 2-hydroxyphenyl functionalized α,β-un-
ents, and it was observed that the substituents on the aromatic saturated ketones, but also with less reactive aromatic systems
rings had some influence on the yields of the products. No at both ends, with high yields (Table 3, entries 15–21).18 It was
matter whether bromoacetophenone with electron-withdraw- interesting to find that functionalized α,β-unsaturated ketones
ing groups (Table 3, entries 2 and 3), or electron-donating with aromatic groups on the carbonyl could consistently
groups, such as methyl, methoxyl and tert-butyl groups produce moderate to excellent yields in the reactions. It was
(Table 3, entries 4–8), on the aromatic rings were used, the noteworthy that either substrates with electron-withdrawing
reactions proceeded to give slightly lower yields. Electron- (Table 3, entries 16–19) or electron-donating (Table 3, entries
donating substrates gave superior yields (55%–74%), while 20 and 21) groups on the aromatic rings could offer higher
electron-withdrawing substrates only took place in 42%–46% yields than an electron-neutral group (entry 15). A higher yield
isolated yields.
occurred when an aromatic group with a meta substituent was
Then, the scope of the substitutional 2-hydroxyphenyl func- present on the carbonyl, than with para-, or ortho-substituted
tionalized α,β-unsaturated ketones with a methyl group on the aromatic rings (Table 3, entries 16–19) present on the carbo-
carbonyl17 were extended, and
a similar reactivity was nyl. The best result was obtained in 83% yield (Table 3, entry
observed. In most cases, adducts were obtained in moderate 21) when the functionalized α,β-unsaturated ketone substrate
yields (Table 3, entries 9–14). A 5-tert-butyl group and a 3,5-di- with a 4-CH3-Ph group on the carbonyl was investigated.
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To get a preliminary understanding of the reaction mechan- mixture and heated to reflux. The reaction mixture was cooled
ism, we tried to identify the intermediates in the reaction. As a to room temperature, the solvent was removed under reduced
1,5-dicarbonyl construction was proposed as intermediate 4a, pressure and the residue was purified by column chromato-
we trapped it by mixing 2-bromoacetophenone 1a, 4-(2-hydro- graphy to afford the desired product 3.
xyphenyl)but-3-en-2-one 2a, and potassium hydroxide in tetra-
Synthesis of substituted 1-(2-benzoyl-2,3-dihydrobenzofuran-
hydrofuran at room temperature. It was observed in a 95% 3-yl)propan-2-one 4a. Compound 1a (179 mg, 0.9 mmol) and
yield after purification by column chromatography, and NMR compound 2a (122 mg, 0.75 mmol) were dissolved in dry THF
spectroscopy was carried out to confirm the structure of inter- (5 mL). KOH (63 mg, 1.125 mmol) was slowly added and the
mediate 4a.
mixture was stirred at room temperature until the reaction was
The following experiments were then conducted to confirm complete (monitored by TLC). The reaction mixture was then
that 1-(2-benzoyl-2,3-dihydrobenzofuran-3-yl)propan-2-one 4a adsorbed onto silica gel, loaded on a silica column, and eluted
could be converted into the final product under the standard with a mixture of ethyl acetate and petroleum ether. The com-
conditions: when intermediate 4a was applied under the reac- pound obtained after column chromatography was generally
tion conditions with 10 eq. of ammonium acetate, in 5 mL the pale yellow solid 4a, 200 mg in 95% yield.
C2H5OH, 69% of the desired product 3a was obtained, which
confirmed that 4a was indeed an intermediate.
Acknowledgements
This work was supported by National Natural Science Foun-
dation of China (51273156, 20902030) and the Fundamental
Conclusions
Research Funds for the Central Universities (2012-IV-032). Z.H.D.
also thanks the Open Project Program of the Key Laboratory of
Functional Small Organic Molecule, Ministry of Education,
Jiangxi Normal University (No. KLFS-KF-201210).
In conclusion, we have developed a one-pot procedure for the
synthesis of polysubstituted benzofuro[2,3-c]pyridines in good
yields. The sequential reaction process contained the con-
venient substitution–Michael addition cascade reactions that
formed a 1,5-dicarbonyl scaffold from the easily accessible bro-
moacetophenone and 2-hydroxyphenyl functionalized α,β-un-
saturated ketones, followed by dehydration-cyclization and
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