Gold-catalyzed efficient synthesis of 2,4-disubstituted furans from aryloxy-enynes

Article abstract of DOI:10.1039/c2ob07173h

A series of (E)-1-aryloxy-1-en-3-ynes has been prepared by Sonogashira coupling of 2-bromo-3-aryloxypropenoates with terminal alkynes using Pd(PPh ₃)₄ and CuI as the catalysts in Et₃N. The resulting enynyl-aryl ethers are found to be highly applicable to the synthesis of 2,4-disubstituted furans with an ester group at C-4 position through an Au/Ag-catalyzed annulation reaction under extremely mild reaction conditions. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob07173h

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COMMUNICATION
Gold-catalyzed efficient synthesis of 2,4-disubstituted furans from aryloxy-
enynes†
Ende Li, Wenjun Yao, Xin Xie, Chengyu Wang, Yushang Shao and Yanzhong Li*
Received 24th December 2011, Accepted 23rd February 2012
DOI: 10.1039/c2ob07173h
A series of (E)-1-aryloxy-1-en-3-ynes has been prepared by
Sonogashira coupling of 2-bromo-3-aryloxypropenoates with
terminal alkynes using Pd(PPh3)4 and CuI as the catalysts in
Et3N. The resulting enynyl-aryl ethers are found to be highly
applicable to the synthesis of 2,4-disubstituted furans with an
ester group at C-4 position through an Au/Ag-catalyzed
annulation reaction under extremely mild reaction
conditions.
Furans are of great importance as core structures of many natural
products and pharmaceuticals.1 They are also useful building
blocks in synthetic organic chemistry.2 Accordingly, many syn-
Scheme 1
thetic approaches toward the synthesis of furan rings have been
disclosed.3 Among them, the intramolecular attack of a nucleo-
philic oxygen atom onto a C–C triple bond offers a straightfor-
ward access to furan derivatives. Generally, these reactions
include the electrophilic cyclization of functionally substituted
alkynes and the transition metal-catalyzed annulation. A wide
range of furans have been prepared using these strategies. The
precursors employed are acetylenic compounds containing a
pendant oxygen functionalities, such as enynones,4 enynols,5
alkynyl ketones,6 2-methoxypent-4-yn-1-ols,7 γ-alkynyl
ketones,8 alkynyl ketone enolates,9 allenyl compounds,10 and
propargyl ketones.11 In these reactions, the carbonyl oxygen or a
hydroxyl group served as a nucleophile to attack the activated
triple bonds. On the other hand, the heteroannulation reaction
utilizing the oxygen from an ether moiety as the nucleophile has
also proved to be efficient for the construction of furan rings via
the cleavage of a C–O bond. Most of the studies focus on the
methyl ethers bearing an ortho alkyne for the synthesis of benzo-
fused heterocycles through the demethylation reactions,12,13 such
as benzo[b]furans, furopyridinones, furopyrones, and furocou-
marins (Scheme 1, eqn (1)). However, there is no report for the
utility of aryl ethers for the preparation of furans. We assumed
that the utility of aryl ethers for the preparation of furans might
also be possible (Scheme 1, eqn (2)). During the course of our
ongoing study on the development of heterocycle forming proto-
cols using substrates with push-pull character,14 we found that
(E)-1-aryloxy-1-en-3-ynes could readily be converted to 2,4-dis-
ubstituted furans in the presence of an Au catalyst. Herein, we
would like to report these results.
The requisite (E)-1-aryloxy-1-en-3-ynes with an ester group
on C-2 (3) were synthesized via the Sonogashira coupling of
the corresponding 2-bromo-3-aryloxypropenoates and terminal
alkynes (Table 1). It is known that Sonogashira coupling usually
was carried out using both palladium and copper as catalysts in
Et3N. In fact, when the coupling reaction of 1a with phenylace-
tylene (2a) was performed using 5 mol% of Pd(PPh3)4 and
10 mol% of CuI in Et3N at 70 °C, the desired product 3a could
be obtained in 88% isolated yield (Table 1, entry 1). As can be
seen from Table 1, a variety of 2-halo-3-aryloxypropenoates
reacted with terminal alkynes smoothly at 70 °C in Et3N, and the
corresponding (E)-1-aryloxy-1-en-3-ynes were produced in high
yields in most cases. Aryl alkynes with both electron-donating
and electron-withdrawing groups at the para position of the aro-
matic ring gave satisfactory yields (Table 1, entries 2–5). The
substituents on terminal alkynes have also been expanded suc-
cessfully to alkenyl (Table 1, entry 7) and alkyl groups (Table 1,
entries 8–11). It is noteworthy that not only the 2-halo-3-arylox-
ypropenoates but also the 2-halo-3-alkyloxypropenoate deriva-
tives (Table 1, entries 12, 14) were compatible with the reaction
and gave the target products in good yields.
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, 3663 North
Zhongshan Road, Shanghai, 200062, People’s Republic of China.
E-mail: yzli@chem.ecnu.edu.cn; Fax: (+86) 021-62233969
†Electronic supplementary information (ESI) available: Experimental
details, spectroscopic characterization of all new compounds and X-ray
crystallography of compound 3d.20 CCDC 850260 (3d). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2ob07173h
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Table 1 Synthesis of (E)-1-aryloxy-1-en-3-ynes
Entry
1
Vinyl bromide
Alkyne
Product
Yield (%)a
88
3a
2
1a
3b
86
3
1a
1a
1a
1a
1a
1a
1a
1a
1a
3c
3d
3e
3f
92
4
92
5
89
6
54b
78c
43d
68d
60d
89
7
3g
3h
3i
8
9
10
11
3j
3k
12
2a
2b
3l
29d
13
3m
71
14
15
2a
2a
3n
3o
73
94
a Isolated yields. Unless noted, all the reactions were carried out with the ratio of 1 : 2 = 1 : 2 at 70 °C for 2–24 h. b 1-Naph is 1-naphthyl. c 1-CHex is
1-cyclohexenyl. d 3.0 equiv. of alkyne was used.
In recent years, the use of gold catalysis has emerged as a
powerful synthetic method for the construction of C–C and C–X
bonds15 under mild reaction conditions, especially for the C–O
bond formation. Excellent work on furan formation catalyzed by
gold has been reported.4–11 We envisioned that the thus formed
enynyl–aryl ethers would undergo an Au-catalyzed intramolecu-
lar C–O bond forming reaction toward furan derivatives, and the
nucleophilic oxygen may either come from the carbonyl oxygen
or the ether moiety. Initially, the reaction of (E)-1-phenoxy-1-en-
3-yne with an ethyl ester group on C-2 (3a) was carried out
using PPh3AuCl (5 mol%) as the catalyst in THF at room temp-
erature. However, no desired product was detected (Table 2,
entry 1). It is interesting that when PPh3AuCl (5 mol%) and
AgOTf (5 mol%) were employed, the desired 2,4-disubstituted
furan derivative 4a16 was formed in 92% yield (Table 2, entry
2). Lowering the catalyst loading to 2 mol% gave similar yield
to that of 5 mol% (Table 2, entry 3). Changing the solvent to
dichloromethane or toluene, 4a were produced in 71% and 56%
yields, respectively (Table 2, entries 5, 9). The use of only
AgOTf (5 mol%) resulted in no reaction (Table 2, entry 4).
Other Ag additives such as AgBF4 or AgSbF6 resulted in trace
or lower yield of 4a (Table 2, entries 6, 7). Therefore, the opti-
mized reaction condition was to use 2 mol% of PPh3AuCl and
2 mol% of AgOTf as the catalysts, THF as the solvent at room
temperature.
With the optimized reaction conditions in hand, we next
examined the substrate scope of this catalytic method for the
synthesis of 2,4-disubstituted furans using a variety of (E)-1-
aryloxy-1-en-3-ynes (Table 3). Firstly, we investigated the elec-
tronic effects of the aromatic substituents on triple bond. It was
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Table 2 Optimization studies for the formation of furan 4a
found that electron-donating aryl groups such as –Me or –OMe
afforded the corresponding products 4b, 4c in 77% and 85%
yields, respectively (Table 3, entries 2, 3). The trimethoxyl aryl
substitute also gave the desired 4d in 72% yield (Table 3, entry
4). An electron-withdrawing (–Cl) aryl group afforded the corre-
sponding furan 4e in 84% yield (Table 3, entry 5). The naphthyl
substituted one (3f) gave a good result (Table 3, entry 6). The
cyclohexenyl substituted precursor worked nicely, producing the
corresponding furan 4g in 44% yield (Table 3, entry 7). The sub-
stituents on the triple bond could also be alkyl groups, such as
n-butyl (3h), n-pentyl (3i), and n-hexyl (3j), furnishing 4h, 4i17
and 4j in 76%, 87% and 90% yields, respectively (Table 3,
entries 8–10). 3k with a phenylethyl group on the triple bond
reacted successfully under the optimal conditions, leading to the
corresponding 4k in 82% yield (Table 3, entry 11). When the
4-methoxyphenoxyl substituted enynyl ether 3m was employed,
it furnished 4b in 85% yield (Table 3, entry 13). It is noteworthy
that the aryloxyl substituent of 3 can be changed to alkyloxyl
Entry
Catalyst (mol%)
Solvent
Time (h)
Yielda
1
2
3
4
5
6
7
8
9
PPh3AuCl (5)
THF
THF
THF
THF
DCM
THF
THF
THF
Toluene
7
6
7
7
7
24
8
8
24
N.R.
92%
91%
N.R.
71%
Trace
68%
69%
56%
PPh3AuCl/AgOTf (5)
PPh3AuCl/AgOTf (2)
AgOTf (5)
PPh3AuCl/AgOTf (2)
PPh3AuCl/AgBF4 (2)
PPh3AuCl/AgSbF6 (2)
PPh3AuNTf2 (2)
PPh3AuNTf2 (2)
a Isolated yield.
Table 3 Synthesis of various of 2,4-disubstituted furans
Entry
1
Enynyl ether
Time (h)
Product
Yield (%)a
91
7
4
3
4a
4b
4c
2
3
77
85
4
5
4d
72
5
6
7
4e
4f
84
12
71b
7
8
9
10
3
4g
4h
4i
44
76
87
7
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Table 3 (Contd.)
Entry
Enynyl ether
Time (h)
7
Product
Yield (%)a
90c
10
11
4j
6
4k
82
12
13
21
4
4a
4b
60
85
14
15
6
4
4a
4l
88
90
a Isolated yields. Unless noted, all the reactions were carried out using 2 mol% of PPh3AuCl and AgOTf in THF. b 5 mol% of PPh3AuCl and AgOTf
were used. c Along with 42% of phenol were obtained.
Scheme 2 Isotopic experiment.
groups, providing the desired 4a in good to high yields (Table 3,
entries 12, 14). The ester group on C-2 could also be methyl
ester, and the corresponding furan 4l was produced in 90% yield
(Table 3, entry 15). Very interestingly, a mixture of 3a (E : Z =
1 : 1) could also afford the desired 4a in high yield under the
optimal conditions (eqn (1)). This result may further broaden the
substrate scope of this method.
was also isolated in a yield of 42% (Table 3, entry 10). One
possibility is that a dearylated O-annulation process occurs, and
the aryl part of the vinyl–aryl ether moiety is transformed to
phenol. Another possibility is that the aryl–enol–ether is hydro-
lyzed to the corresponding carbonyl group by H2O, which may
come from the air, and then undergo an O-cyclization. It is note-
worthy that when H2O (1–3 equiv.) was added to the reaction
mixture of 3a, the desired 4a could be obtained in around 80%
yield. More H2O resulted in a much lower yield. The addition of
4 Å MS gave only a trace amount of the desired product. It indi-
cated that certain amount of H2O may play an important role in
the reaction. To prove this point, we carried out an isotopic label-
ing experiment using 3b with 2 equiv. of H218O, and found that
61% of 18O was incorporated in the final furan ring, whereas,
phenol was formed without clear 18O incorporation (Scheme 2).
On the basis of the above observations, a possible reaction
mechanism is proposed in Scheme 3. First, coordination of
CvO double bond of compound 3 to Au+ facilitates the
ð1Þ
In order to understand the reaction mechanism, we examined
the reaction of (E)-ethyl 2-(phenoxymethylene)dec-3-ynoate (3j)
under the optimal conditions carefully. It was found that not only
the desired furan 4j was obtained in 90% yield after 7 h, phenol
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In conclusion, we have shown that (E)-1-aryloxy-1-en-3-ynes
can be efficiently prepared by the Sonogashira coupling reaction
using 2-bromo-3-aryloxypropenoates and terminal alkynes. The
resulting (E)-1-aryloxy-1-en-3-ynes are successfully applied to
the synthesis of 2,4-disubstituted furans with an ester group at
C-4 position through Au/Ag-catalyzed annulation reaction. Aryl,
alkenyl and alkyl substituents on the acetylene terminus are com-
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Experimental section
A typical procedure for the Au-catalyzed formation of ethyl 5-
phenylfuran-3-carboxylate
(E)-Ethyl 2-(phenoxymethylene)-4-phenylbut-3-ynoate (58 mg,
0.2 mmol), THF (2 mL), PPh3AuCl (2.0 mg, 2 mol%), AgOTf
(0.1 mL, 0.04 M in THF, 2 mol%) are added to a Schlenk tube.
The resulted solution was stirred at room temperature. After the
reaction was complete as monitored by thin-layer chromato-
graphy, the solvent was evaporated under the reduced pressure
and the residue was purified by chromatography on silica gel to
afford the 2,4-disubstituted furan derivatives 4a (33 mg, 76%) as
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1
a yellow solid; m.p. 45 °C. H NMR (400 MHz, CDCl3, Me4Si)
δ 1.36 (t, J = 6.8 Hz, 3H), 4.33 (q, J = 7.2 Hz, 2H), 6.97 (s, 1H),
7.30–7.32 (m, 1H), 7.37–7.41 (m, 2H), 7.66–7.68 (m, 2H), 8.02
(s, 1H); 13C NMR (100.6 MHz, CDCl3, Me4Si) δ 14.28, 60.49,
104.44, 121.24, 123.97, 128.09, 128.73, 129.82, 146.67, 155.08,
163.10; HRMS (EI) for C13H12O3: calcd 216.0786, found
216.0787.
Acknowledgements
We thank the National Natural Science Foundation of China
(Grant No. 20872037) for financial support.
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this paper.
This journal is © The Royal Society of Chemistry 2012
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