A new copper(I)-catalyzed cycloetherification/acid-catalyzed allylic nucleophilic substitution for one-pot synthesis of 2-substituted benzofurans

Article abstract of DOI:10.1055/s-0031-1290767

A new copper(I)-catalyzed cycloetherification followed by an acid-catalyzed allylic nucleophilic substitution have been developed for the one-pot synthesis of 2-substituted benzofurans. This one-pot reaction proceeds efficiently under extremely mild conditions with simple and inexpensive catalysts, providing diversely substituted benzofurans in good to excellent yields. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0031-1290767

LETTER
▌1043
lAetteNr ew Copper(I)-Catalyzed Cycloetherification/Acid-Catalyzed Allylic
Nucleophilic Substitution for One-Pot Synthesis of 2-Substituted Benzofurans
One-Pot Synthesis of Benzofurans
Xin Li, Jijun Xue,* Rui Chen, Ying Li*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000,
P. R. of China
Fax +86(931)8912582; E-mail: xuejj@lzu.edu.cn; E-mail: liying@lzu.edu.cn
Received: 19.01.2012; Accepted after revision: 20.02.2012
Initially, our interest focused on the annulation of 2-(1-hy-
Abstract: A new copper(I)-catalyzed cycloetherification followed
droxyprop-2-ynyl)phenol derivatives. Although numer-
by an acid-catalyzed allylic nucleophilic substitution have been de-
ous cycloetherifications have been reported, to the best of
our knowledge, there are only three papers on the cycliza-
veloped for the one-pot synthesis of 2-substituted benzofurans. This
one-pot reaction proceeds efficiently under extremely mild condi-
tions with simple and inexpensive catalysts, providing diversely tion of these compounds, which was catalyzed by Pd, Au
and Ag, respectively.8a,9 This prompted us to develop a
substituted benzofurans in good to excellent yields.
new catalyst system. A model reaction was carried out
with 2-(1-hydroxyprop-2-ynyl)phenol (1a) in MeOH, us-
ing 5 mol% CuI and 10 mol% Cs2CO3 as the catalyst.
However, after 24 hours of reaction time, no cyclization
product was detected (Table 1, entry 1). The reaction mix-
ture was then heated to reflux for nine hours, after which
time the desired product 2a was obtained in 85% yield
(entry 2). To our pleasure, the addition of 5 mol% Ph3P ac-
celerated the reaction greatly and enabled the cycloether-
ification of 1a to proceed at room temperature with
excellent yield (entry 3). Control experiments suggested
that CuI, Cs2CO3 and Ph3P were all necessary for the re-
action (entries 4 and 5) and lower CuI or Cs2CO3 loading
both resulted in a decrease of the yield (entries 6 and 7).
Moreover, various copper catalysts and other metal salts
were examined. Among them, CuCl, CuBr, CuCl2,
Cu(OTf)2, PtCl2, AgNO3 and InCl3 were all less effective
than CuI (entries 8–14). FeCl3 and LaCl3 failed to promote
the reaction (entries 15 and 16). Interestingly, when a sim-
ple acid catalyst p-TsOH was employed, an undesired
etherification by-product 1g10 was formed in high yield
(entry 17).
Key words: benzofurans, cycloetherifications, allylic nucleophilic
substitution, copper, one-pot synthesis
Benzofurans are ubiquitous structural motifs in a wide
range of biologically active compounds which display
various pharmacological activities such as anti-HIV,1 an-
ticancer,2 antifungal,3 antioxidantive4 and anti-inflamma-
tory.5 Thus synthetic access to benzofurans is of
considerable interest, and numerous efficient methods
have been disclosed in the literature.6,7 Among them, tran-
sition-metal-catalyzed cycloetherification is regarded as
one of the most attractive approaches for its high atom-
economy and mild reaction conditions.7 Recently, a novel
and efficient two-step procedure synthesis of 2-substitut-
ed benzofurans was successfully developed by Gabriele
and co-workers.8 Compared with the traditional stepwise
approach, one-pot procedure has received more attention
because they could increase the reaction efficiency, avoid
the separation of intermediates, save the use of solvents,
and reduce the emission of pollutant waste. Herein, we
disclose a one-pot approach to the 2-substituted benzofu-
rans via an unprecedented copper-catalyzed cycloetherifi-
cation followed by an acid-catalyzed allylic nucleophilic
substitution (Scheme 1).
With an optimal set of catalyst system selected, the effects
of solvents and bases were examined. These data reveal
that the cycloetherifications can be carried out in a broad
range of organic solvents at room temperature (Table 2,
entries 1–12) or even in water under reflux (entry 13). In
addition, a number of bases, including inorganic and or-
ganic, were efficient for the reaction (entries 14–21) but
pyridine was found to be totally inefficient. This is proba-
bly because copper strongly coordinates with pyridine, re-
sulting in the loss of its catalytic activity (entry 22).
R2 OH
R2
OR3
one-pot
R1
R1
O
OH
1
3
R2
OH
R1
With the optimal reaction conditions in hand, the scope of
this transformation was evaluated. As can be seen from
Table 3, internal alkynes are less active than terminal
ones. They required higher catalyst loading, higher tem-
perature and longer reaction time but afforded lower
yields (entries 1–3). In addition, phenol 1d substituted at
the terminal alkyne carbon with a TMS groups resulted in
desilylation (entry 3). It should be noted that the presence
of benzylic OH group plays a crucial role in the cyclo-
p-TsOH
CuI, Ph3P, Cs2CO3
R3OH, r.t.
O
R3OH, r.t.
2
Scheme
1
One-pot synthetic approach to 2-substituted benzofu-
rans
SYNLETT 204012, 237, 1043–1046
Advanced online publication: 05.04.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1290767; Art ID: ST-2012-W0058-L
© Georg Thieme Verlag Stuttgart · New York

1044
X. Li et al.
LETTER
Table 1 Screening of Catalyst
Table 2 Screenings of Solvent and Base
OH
OH
OH
OH
CuI (5 mol%), Ph3P (5 mol%)
catalyst
MeOH
OH
base (10 mol%), solvent
OH
O
O
1a
2a
1a
2a
Entry Catalyst
(mol%)
Condition
Product
(yield, %)
Entry
Solvent
Base
Conditions
Yield (%)
1
2
EtOH
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
r.t., 5 h
94
92
89
92
95
56
72
86
82
65
47
74
n.r.a
1
2
CuI/Cs2CO3 (5:10)
r.t., 24 h
reflux, 9 h
r.t., 4 h
i-PrOH
t-BuOH
CH2Cl2
CHCl3
DMF
r.t., 5 h
CuI/Cs2CO3 (5:10)
2a (85)
2a (98)
3
30 °C, 6 h
r.t., 6 h
3
CuI/Ph3P/Cs2CO3 (5:5:10)
Ph3P/Cs2CO3 (5:10)
4
n.r.a
4
r.t., 24 h
r.t., 24 h
r.t., 4 h
5
r.t., 8 h
n.r.a
5
CuI/Ph3P (5:5)
6
r.t., 24 h
r.t., 24 h
r.t., 24 h
r.t., 11 h
r.t., 14 h
r.t., 24 h
r.t., 24 h
6
CuI/Ph3P/Cs2CO3 (2:5:10)
CuI/Ph3P/Cs2CO3 (5:5:5)
CuCl/Ph3P/Cs2CO3 (5:5:10)
CuBr/Ph3P/Cs2CO3 (5:5:10)
CuCl2/Ph3P/Cs2CO3 (5:5:10)
Cu(OTf)2/Ph3P/Cs2CO3 (5:5:10)
PtCl2/Ph3P/Cs2CO3 (5:5:10)
AgNO3/Ph3P/Cs2CO3 (5:5:10)
InCl3/Ph3P/Cs2CO3 (5:5:10)
FeCl3/Ph3P/Cs2CO3 (5:5:10)
LaCl3/Ph3P/Cs2CO3 (5:5:10)
p-TsOH (5)
2a (89)
2a (79)
2a (92)
2a (91)
2a (77)
2a (83)
2a (72)
2a (59)
2a (7)
7
MeCN
benzene
1,4-dioxane
THF
7
r.t., 4 h
8
8
r.t., 4 h
9
9
r.t., 4 h
10
11
12
13
14
15
16
17
18
19
20
21
22
10
11
12
13
14
15
16
17
r.t., 4 h
DME
r.t., 4 h
hexane
H2O
r.t., 24 h
r.t., 24 h
r.t., 24 h
r.t., 24 h
r.t., 24 h
r.t., 7 h
reflux, 12 h 71
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
r.t., 4 h
r.t., 4 h
r.t., 4 h
r.t., 12 h
r.t., 4 h
r.t., 4 h
r.t., 4 h
r.t., 14 h
r.t., 24 h
91
98
94
74
95
97
97
62
n.r.a
K3PO4
n.r.a
LiOH·H2O
Et3N
n.r.a
1g (95)
i-Pr2NH
piperidine
TMG
a No reaction.
etherifications. When the OH group is replaced by a
hydrogen atom or oxidized to a carbonyl, no cycloetheri-
fication product was detected even after refluxing the re-
action mixture for 24 hours (entries 4 and 5). Furthermore,
when the OH group is protected by methyl, higher catalyst
loading and higher temperature were required but poor
yield was obtained (entry 6). With respect to the terminal
alkynes, their cycloetherifications completed in 4–8 hours
at ambient temperature with high yields (entries 7–14) and
the electron-donating, electron-withdrawing, and halogen
substitutions on the aryl ring or alkyl, aryl substitutions at
the benzylic position were all tolerated. These data clearly
suggest that the presence of a benzylic OH group and a
terminal alkyne are both necessary for this copper(I)-cat-
alyzed cycloetherification but the electronic effect of the
substituents on the aryl rings and the steric hindrance had
little impact on the efficiency of our method.
DABCO
pyridine
a No reaction.
the reaction mixture from the cycloetherification process
with 1.1 equivalents of acid at room temperature cleanly
provided the desired benzofuran in excellent yield (Table
4, entries 1–4). Among these acid tested, p-TsOH was
found to be most effective (entry 3). Encouraged by the
above success, the one-pot procedure was carried out with
other alcohols and moderate to good yields were obtained
(entries 5–9). These data (Table 1, entry 3, Table 2, entries
1–3 and Table 4, entries 3 and 5–7) showed that steric fac-
tor did has deleterious effects on the allylic nucleophilic
substitution: primary alcohols underwent the reaction
more effectively in terms of lower reaction temperatures
and higher yields, as compared to the secondary alcohols,
which in turn reacted more efficiently than their tertiary
Having established the cycloetherifications of phenols 1,
we then turned our efforts to realize the one-pot process
mentioned in Scheme 1. To our delight, the treatment of
Synlett 2012, 23, 1043–1046
© Georg Thieme Verlag Stuttgart · New York

LETTER
One-Pot Synthesis of Benzofurans
1045
Table 3 Copper-Catalyzed Cycloetherifications of 2-(1-Hydroxyprop-2-ynyl)phenols11
R2
R2 R3
R3
R1
R1
R4
CuI, Ph3P, Cs2CO3
MeOH
R4
O
OH
2
1
Entry
Substrates
Conditions
Product (yield, %)
1a
2a
1b (R1 = R2 = H, R3 = OH, R4 = Ph)
40 °C, 12 h
40 °C, 12 h
40 °C, 12 h
reflux, 24 h
reflux, 24 h
50 °C, 12 h
r.t., 4 h
2b (68)
2c (52)
2a (64)
n.r.b
1c (R1 = NO2, R2 = H, R3 = OH, R4 = Ph)
1d (R1 = R2 = H, R3 = OH, R4 = TMS)
1e (R1 = R2 = R3 = R4 = H)
3a
4
1f (R1 = H, R2 = R3 = O, R4 = H)
–
c
5
6a
7
1g (R1 = R2 = H, R3 = OMe, R4 = H)
1h (R1 = MeO, R2 = H, R3 = OH, R4 = H)
1i (R1 = NO2, R2 = H, R3 = OH, R4 = H)
1j (R1 = Cl, R2 = H, R3 = OH, R4 = H)
1k (R1 = Br, R2 = H, R3 = OH, R4 = H)
1l (R1 = Me, R2 = H, R3 = OH, R4 = H)
1m (R1 = H, R2 = Me, R3 = OH, R4 = H)
1n (R1 = H, R2 = Et, R3 = OH, R4 = H)
1o (R1 = H, R2 = Ph, R3 = OH, R4 = H)
2g (59)
2h (92)
2i (85)
2j (89)
2k (86)
2l (92)
2m (88)
2n (86)
2o (82)
8
r.t., 6 h
9
10
11
12
13
14
r.t., 8 h
r.t., 8 h
r.t., 6 h
r.t., 8 h
r.t., 8 h
r.t., 8 h
a CuI (10 mol%), Ph3P (10 mol%) and Cs2CO3 (20 mol%) were used.
b No reaction.
c Decomposition was observed.
counterparts (entries 5–7). Finally, the generality of this
one-pot procedure was examined. As expected, a series of
diversely substituted benzofurans were obtained with sat-
isfactory yields (entries 10–17). However, halogen substi-
tutions caused an obvious decrease in the yield (entries 12
and 13), although there is no reasonable explanation for
the results.
References and Notes
(1) Dai, J. R.; Hallock, J. H.; Boyd, M. J. J. Nat. Prod. 1998, 61,
351.
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(3) Zacchino, S.; Rodrigues, G.; Pezzenatti, G.; Orellana, G.
J. Nat. Prod. 1997, 60, 659.
In summary, we have developed a new type of copper(I)-
catalyzed cycloetherifications and an acid-catalyzed allyl-
ic nucleophilic substitution, which can be carried out in
one-pot under mild conditions. This method provides an
efficient, straightforward and wide-scope route to diverse-
ly substituted benzofurans in high yields. Further studies
for construction of other biologically important heterocy-
cles using this method are underway.
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Acknowledgment
We are grateful for the financial support of the National Natural Sci-
ence Foundation of China (Grant No. 21072084).
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.onmorinftIangoimnuSpinorttfoIangiStupor
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1043–1046

1046
X. Li et al.
LETTER
Table 4 One-Pot Synthesis of 2-Substituted Benzofurans12
Kotschy, A. Tetrahedron 2003, 59, 7509. (r) Hashmi, A. S.
K.; Wölfle, M. Tetrahedron 2009, 65, 9021. (s) Kim, I.; Lee,
S.; Lee, S. Tetrahedron Lett. 2008, 49, 6579. (t) Katritzky,
A. R.; Ji, Y.; Fang, Y.; Prakash, Y. J. Org. Chem. 2001, 66,
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6964. (v) Luca, L. D.; Giacomelli, G.; Nieddu, G. J. Org.
Chem. 2007, 72, 3955. (w) Lu, B.; Wang, B.; Zhang, Y.; Ma,
D. J. Org. Chem. 2007, 72, 5337. (x) Huang, X.; Liu, Y.;
Liang, Y.; Pi, S.; Wang, F.; Li, J. Org. Lett. 2008, 10, 1525.
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131, 17387. (z) Liang, Z.; Hou, W.; Du, Y.; Zhang, Y.; Pan,
Y.; Mao, D.; Zhao, K. Org. Lett. 2009, 11, 4978.
R2 OH
R2
1. CuI, Ph3P, Cs2CO3
R3OH, r.t., 7 h
R1
R1
OR3
2. acid (1.1 equiv)
R3OH, r.t., 13 h
O
OH
3
1
R1
R2
R3
Entry
Acid
Product
(yield, %)
1
2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
Et
Ph
Me
Me
Me
Me
Et
CSA
3a (79)
3a (82)
3a (95
(7) (a) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev. 2004,
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2005, 127, 15024. (f) Liang, Y.; Tang, S.; Zhang, X.; Mao,
L.; Xie, Y.; Li, J. Org. Lett. 2006, 8, 3017. (g) Zanardi, A.;
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Y.; Zhang, Y.; Yang, Z.; Fathi, R. J. Org. Chem. 2002, 67,
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2008, 49, 3437. (o) Kabalka, G. W.; Wang, L.; Pagni, R. M.
Tetrahedron 2001, 57, 8017. (p) Konno, T.; Chae, J.;
Ishihara, T.; Yamanaka, H. Tetrahedron 2004, 60, 11695.
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2008, 73, 7336. (b) Gabriele, B.; Raffaella, M.; Salemo, G.
Eur. J. Org. Chem. 2010, 3459.
H
PPTS
3
H
p-TsOH
Amberlysta
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
p-TsOH
4
H
3a (85)
3b (84)
3c (62)
3d (49)
3e (88)
3f (83)
3g (86)
3h (83)
3i (47)
3j (41)
3k (87)
3l (78)
3m (80)
3n (76)
5
H
6b
7c
H
i-Pr
t-Bu
c-Hex
Bn
H
8b,d
9d
10
11
12
13
14
15
16
17
H
H
OMe
NO2
Cl
Br
Me
H
Me
Me
Me
Me
Me
Me
Me
Me
H
(9) (a) Harkat, H.; Blanc, A.; Weibel, J.; Pale, P. J. Org. Chem.
2008, 73, 1620. (b) Yu, M.; Skouta, R.; Zhou, L.; Jiang, H.;
Yao, X.; Li, C. J. Org. Chem. 2009, 74, 3378.
H
a Amberlyst (100 mg) was used.
(10) See Table 3, entry 6 for detailed structure.
b Reactions were performed at 35 °C.
c Reactions were performed at 45 °C.
(11) General Procedure for Copper(I)-Catalyzed
Cycloetherification: CuI (0.005 mmol, 1.0 mg), Ph3P
(0.005 mol, 1.3 mg) and Cs2CO3 (0.01 mmol, 3.3 mg) were
added to a solution of phenol 1 (0.1 mmol) in MeOH (0.5
mL). The mixture was stirred at r.t. until the reaction was
complete (monitored by TLC). After that the solvent was
removed and the residue was purified by flash silica gel
chromatography using n-hexane–EtOAc (4:1) as eluent to
give the desired cycloetherification products.
(12) General Procedure for One-Pot Synthesis of 2-
Substituted Benzofurans: CuI (0.005 mmol, 1.0 mg), Ph3P
(0.005 mol, 1.3 mg) and Cs2CO3 (0.01 mmol, 3.3 mg) were
added to a solution of phenol 1 (0.1 mmol) in alcohol (0.5
mL). The mixture was stirred at r.t. for 4–6 h. After this time,
p-TsOH (0.11 mmol, 20.9 mg) was added and the mixture
was stirred at r.t. until the starting material was consumed.
After that the mixture was diluted with Et2O and washed
with H2O. The organic phase was separated, dried over
Na2SO4, then concentrated and purified by silica gel
chromatography using n-hexane–EtOAc (16:1) as eluent
to give the desired benzofurans.
d Reactions were performed in CHCl3–alcohol (1:1) as solvent.
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Synlett 2012, 23, 1043–1046
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