An entry to polysubstituted furans via the oxidative ring opening of furan ring employing NBS as an oxidant

Article abstract of DOI:10.1016/j.tetlet.2012.12.085

A class of polysubstituted functionalized furans was synthesized efficiently starting from readily available furans involving the oxidative ring opening of the furan rings using NBS as an oxidant. The reaction proceeded through a sequence of oxidative dearomatization of the furan ring/spirocyclization/aromatization.

Full text of DOI:10.1016/j.tetlet.2012.12.085

Tetrahedron Letters 54 (2013) 1256–1260
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An entry to polysubstituted furans via the oxidative ring opening of furan
ring employing NBS as an oxidant
Huiyue Yu a, Weiqiang Zhong b, Tingyu He a, Wenxiang Gu a, Biaolin Yin b,
⇑
a Department of Applied Chemistry, College of Science, South China Agricultural University, Guangzhou 510642, PR China
b School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A class of polysubstituted functionalized furans was synthesized efficiently starting from readily avail-
able furans involving the oxidative ring opening of the furan rings using NBS as an oxidant. The reaction
Received 6 October 2012
Revised 17 November 2012
Accepted 21 December 2012
Available online 29 December 2012
proceeded through
aromatization.
a
sequence of oxidative dearomatization of the furan ring/spirocyclization/
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Furan
Dearomatization
Oxidation
Ring opening
The polysubstituted furans represent an important class of five-
membered heterocycles found to widely exist in a number of bio-
active natural products as well as in various pharmaceuticals and
agrochemicals.1 On the other hand, because furan rings can pro-
vide up to four unsaturated carbons and contain masked function-
alities of olefin, diene, enol ether, and 1,4-dicarbonyl, these
polysubstituted furans also represent valuable synthetic interme-
diates for the preparation of a series of important molecules. To
date, there are a number of different methods that, despite having
unequal scope, allow the dearomatization of the furan ring.2 For
example, the Birch-reduction of furan rings results in 2,5-
dihydrofurans,3 while cycloadditions of furans as dienes have been
widely used for the construction of different bicyclic or tricyclic
skeletons.4 Likewise, 2-furycarbinols are isomerized into cyclopen-
tenones upon treatment with acids in aqueous media.5 For these
reasons, the synthesis of polysubstituted furan and work toward
developing new synthetic methods using furans as the fundamen-
tal building blocks continue to attract the interest of many
synthetic chemists. Nevertheless, the development of a flexible
and predictable approach to polysubstituted furans starting from
inexpensive and readily available furans via the ring opening of
the furan ring has seldom been reported.6
furan rings begins with an electrophilic bromination of the furan
ring, thus revealing an electrophilic oxonium ion 1 (Scheme 1a).
Trapping of the oxonium ion 1 with nucleophiles leads to the pro-
duction of 2,5-dihydrofurans. This strategy has been successfully
applied to the synthesis of trioxadispiroketals from furans carrying
two nucleophilic side chains at the two
a
-positions.8 Recently, a
general approach to the synthesis of benzoannelated heterocyclic
compounds was developed based on the cyclization of ortho-
substituted benzylfurans under acidic conditions (Scheme 1b).9
This method involved the acetalization step using the furan ring
Br
NBS
a):
b):
Acid, HNu
+
Nu
O
Nu
O
O
1
2
R'
ROC
HX
X
R'
protonic acid
R
O
4
3
Due to the high electron-density of the furan ring, its dearoma-
tizing oxidation is easily achieved by using a range of oxidants such
as O2, Br2, NBS, m-CPBA, and so on, to construct complex mole-
cules.7 For example, the well-known NBS-mediated oxidation of
O
R
R
E
O
2
acidic and oxidative
conditions
O
R
c)
2
R
R
O
6
E
1
?
R
1
5
⇑
Corresponding author. Tel./fax: +86 20 87113735.
E-mail address: blyin@scut.edu.cn (B. Yin).
Scheme 1. Strategy to polysubstituted furans 6.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.12.085

H. Yu et al. / Tetrahedron Letters 54 (2013) 1256–1260
1257
Table 1
O
R
E
2
Yb (OTf) (10 mol %)
3
E
Optimization of the reaction conditionsa,b,c
R
1
DCM, reflux
R
O
+
R
Entry
Conditions
Yieldd (%)
O
5
COR
8
OH
2
R
7
1
1
AcOH/PhI(OAc)2 /DCE/rt, 3 h
NDe
ND
ND
ND
NRf
Trace
41
61
58
51
61
71
63
54
55
21
38
41-91% yields
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
AcOH/m-CPBA/DCE/rt, 3 h
AcOH/Br2/DCE, rt, 3 h
AcOH/BQ/DCE, rt, 3 h
AcOH/Ag2CO3/DCE, 100 °C, 24 h
R = H, Me, Ph
= H, Et, Cyclohexyl, Ph, p-MeO-Ph,
R
1
p-Me-Ph, p-NO2-Ph, p-F-Ph, 5-metyl-furan-2-yl
AcOH/NBS/DCE, rt, 0.5 h/100 °C 20 h
AcOH/NBS/THF-H2O, rt, 0.5 h/80 °C, 15 h
PTSA/NBS/THF-H2O, rt, 0.5 h/80 °C, 20 h
AgF/NBS/THF-H2O, rt, 0.5 h/80 °C, 15 h
PPTS/NBS/THF-H2O, rt, 0.5 h/80 °C, 24 h
Cu(OTf)2/AcOH/NBS/THF-H2O, rt, 0.5 h/80 °C, 24 h
Cu(OAc)2/AcOH/ NBS/THF-H2O, rt, 0.5 h/80 °C, 15 h
FeCl2/NBS/THF-H2O, rt, 0.5 h/80 °C, 10 h
ZnCl2/NBS/THF-H2O, rt, 0.5 h/80 °C, 10 h
FeCl3/NBS/THF-H2O, rt, 0.5 h/80 °C, 10 h
Cu(OAc)2/NBS/THF, rt, 0.5 h/then 80 °C, 10 h
Cu(OAc)2/AcOH/NBS/MeCN, rt, 0.5 h/80 °C, 3 h
MeCOCH COMe, MeCOCH COOEt, PhCOCH COPh
2
2
2
8 =
PhCOCH CN, PhCOCH CONHPh, CH (COOEt)
2
2
2
2
Scheme 2. The synthesis of the precursors 5 for recyclization.
as the source of carbonyl group and a subsequent aromatizing ring-
opening step. Inspired by these two reactions, we further consid-
ered the fact that chemical properties of the enolic hydroxyl group
are similar to those of the phenolic group. Working from this idea,
we envisioned that a general method for the synthesis of polysub-
stituted furans 6 from furans 5 could be developed under both
acidic and oxidative reaction conditions. With our interest in the
synthesis of heterocyclic compounds based on the dearomatization
of the furan ring,10,11 we herein discuss a practical and concise
route to synthesizing both polysubstituted and functionalized fur-
ans from furans via the acid-catalyzed oxidative ring opening of
the furan ring.
a
All reactions were carried out on a 0.3 mmol scale.
b
Unless otherwise noted, 0.1 equiv of the acid and 1.1 equiv of the oxidant were
used.
c
V(THF)/V(H2O) = 3/1.
Isolated yield.
ND: not detected.
NR: no reaction.
d
e
f
(entries 2–4). The weak oxidant Ag2CO3 gave rise to no reaction
(entry 5). Treatment of 5a with NBS (1.1 equiv) in the presence
of 0.1 equiv of acetic acid in DCE at room temperature produced
a new and unstable compound with a larger polarity than 5a
according to the TLC. After about 0.5 h, the starting material 5a
had been consumed completely. Heating the reaction mixture at
110 °C for 20 h produced a trace amount of 6a (entry 6). When
the solvent was switched to THF/H2O (3/1), the reaction of 5a with
NBS at room temperature also produced the same unstable com-
pound as that in DCE. After heating the resulting reaction mixture
at 80 °C for 15 h, 6a was formed in 41% yield (entry 7). Encouraged
by this outcome, we examined different acids and found that
Cu(OAc)2 (0.1 equiv)/AcOH (0.1 equiv) system was proved to be
the best, giving rise to 6a in 71% yield (entries 8–15). Notably,
when anhydrous THF or MeCN was used as the solvent, the yields
were less than 40% and a considerable amount of by-product 11a
was produced (entries 16 and 17). We conjectured that using
H2O as the solvent would be helpful in dissolving of the acids
and facilitating the opening of the furan ring.
At the outset of this study, the precursors 5 were prepared via
the reaction of 2-furylcarbinols 7 with activated methylene com-
pounds 8 in DCM in the presence of Yb(OTf)3 (10 mol %) as the cat-
alyst (Scheme 2). The success of the cyclization of 5 into 6 relies on
the careful selection of the substrate 5, followed by choosing the
suitable reaction conditions to suppress the direct dehydrogena-
tion of 5 to form
a,b-unsaturated carbonyl compound 9, the bro-
mination of the
a-position of the carbonyl group to form side
product 10, and the direct bromination of the furan ring to form
side product 11 (Scheme 3). With these considerations, furan 5a
was used as the substrate under the influence of various Lewis
acids and oxidants to optimize the reaction conditions, with results
summarized in Table 1. Initially, 5a was treated with a series of
oxidants such as PhI(OAc)2, m-CPBA, Br2, Ag2CO3, and BQ in the
presence of AcOH in DCE. It was found that these systems were
not suitable for this conversion and none of the product 6a was
observed (entries 1–5). Specifically, PhI(OAc)2 resulted in the for-
mation of 10a exclusively (entry 1). The oxidants of m-CPBA, Br2,
and BQ (benzoquinone) produced complicated reaction systems
With the optimized reaction conditions in hand, a variety of fur-
ans 5 with different R, R1, R2, and E groups were tested to investi-
gate the reaction scope.12 Results of this effort are provided in
Table 2. When R2 was a Me or Ph group, and E was an electron-
withdrawing group such as acetyl, benzoyl, and ethoxyacyl, these
reactions tend to proceed smoothly. The desired 6 was produced
in moderate to good yields. (entries 1–3). Due to the formation
of a certain amount of by-product 9c, the yield of 6c was relative
low (45%). The failure of 5d–5f could be attributed to the low acid-
ity of the proton of the activated methylene group which led to the
low concentration of enol under these reaction conditions. The
scope of R1 group was then tested. We were delighted to find that
a series of 6 were produced in moderate to good yields with a var-
iable R1 group including alkyl, hetero-aryl, and phenyl groups with
electron-donating or electron-withdrawing substituents. The low-
er yield of 6l might result from the sterically bulky R2 group, which
O
R2
E
O
R2
Br
E
R
R
O
9
O
R1
R1
10([O] = NBS
O
R2
E
[O]/acid
O
+
R
O
R1
5
Br
O
O
R2
O
has a tendency to impede attacking of the carbonyl group on the a-
R2
R
position of the furan ring. The reaction system of 5r showed to be
relatively complicated due to the instability of the furan ring in
acidic system, thus leading to a very low yield of 6r (28%).
Based on the experimental results shown above, a tentative
mechanism for the synthesis of 6 was proposed in Scheme 4.
R
E
E
R1
R1
6
11([O] = NBS
Scheme 3. Potential challenging for the oxidative recyclization of 5 into 6.

1258
H. Yu et al. / Tetrahedron Letters 54 (2013) 1256–1260
Table 2
Table 2 (continued)
The synthesis of 6a
Entry
12
5
6
Yieldb (%)
Cu(OAc)2 (0.1equiv), AcOH (0.1 equiv),
NBS (1.1 equiv), THF/H2O (3/1),
p
-NO2-Ph
O
Bu-t
O
O
O
O
R2
E
O
Bu-t
rt, 0.5 h, then 80 °, 16 h
R
O
R2
Me
Me
Me
Me
Me
Me
Me
Me
Me
31
R
O
O
OEt
p-NO -Ph
COOEt
R1
E
2
R1
5l
5
6
6l
p
-F-Ph
Ph
O
O
O
O
O
Ph
O
Entry
1
5
6
Yieldb (%)
13
14
15
16
60
74
75
72
O
5m
Ph
COPh
Me
O
O
O
O
p
-F-Ph
6m
O
O
O
O
Me
O
O
Me
Me
Me
Me
p
-F-Ph
Me
O
Me
70
O
Me
O
O
OEt
Me
6a
6b
COOEt
Me
5a
5b
5c
O
Me
COMe
p
-F-Ph
5n
O
O
O
O
6n
Me
Me
Me
2
3
75
45
p
-OMePh
Me
O
O
O
O
Me
Ph
COMe
Ph
O
Me
O
O
Me
p
-OMe-Ph
COMe
O
5o
6o
6c
Ph
COPh
OEt
p
-MePh
Me
O
O
O
Me
O
OEt
Me
O
O
Me
4
ND
COMe
Me
p
-Me-Ph
5p
6d
6p
O
OEt
Me
COOEt
5d
5e
OHC
O
O
O
O
O
Me
Me
O
COMe
O
Me
Me
17
18
28
57
5
6
ND
ND
O
O
O
O
NHPh
Ph
6r
6e
6f
CONHPh
O
Me
Ph Me
5r
Me
O
O
O
Ph
CN
O
O
O
Me
Me
O
Me
O
Me
Me
O
Ph
Ph
CN
5f
Et
O
5q
OEt
Ph
COOEt
Me
6q
Me
a
O
All reactions were carried out on a 0.3 mmol scale.
Isolated yield.
b
7
8
36
41
O
OEt
Et
COOEt
5g
6g
O
O
Initially, NBS oxidation of the double bond of the furan ring of 5
resulted in the bromonium ion 12. The ring opening of the
bromonium ion with the intramolecular carbonyl group led to the
intermediate 13, which was deprotonated to 14. Compound
14 then underwent protonation and the subsequent ring opening
step to produce the carbocation 16. Compound 16 was finally
transformed into 6 through three successive steps of deprotonation,
elimination of HBr, and geometric isomerization of the double
bond.
In summary, we have developed a practical method for the
preparation of polysubstituted and functionalized furans. This
method is based on the oxidative ring opening of the furan rings
using NBS as the oxidant. The reaction proceeded through a se-
quence of oxidative dearomatization of the furan ring/spirocycliza-
tion/aromatization. This protocol possesses the advantages of
readily available starting materials, mild reaction conditions, and
short steps. Further studies on the synthesis of polysubstituted
and functionalized thiophenes and pyrroles using this protocol
are under way in our laboratory and the results will be reported
in due course.
Me
Me
Me
O
O
O
Me
Me
COOEt
6h
O
OEt
5h
5i
Ph Me
O
O
Me
O
Me
9
58
62
65
O
OEt
Ph
Ph
COOEt
6i
Ph Me
O
O
O
Me
O
Me
Me
O
Me
Me
10
11
O
Me
Me
COMe
5j
6j
p
-NO Ph
2
O
Me
O
O
O
Me
p
-NO -Ph
COMe
2
5k
6k

H. Yu et al. / Tetrahedron Letters 54 (2013) 1256–1260
1259
Br
O
R
E
4556–4559; (h) Dong, J.-Q.; Wong, H. N. C. Angew. Chem., Int. Ed. 2009, 48,
2351–2354.
8. McDermott, P. J.; Stockman, R. A. Org. Lett. 2005, 7, 27–29.
9. (a) Butin, A. V.; Stroganova, T. A.; Lodina, I. V.; Krapivin, G. D. Tetrahedron Lett.
2001, 42, 2031–2033; (b) Butin, A. V.; Smirnov, S. K.; Stroganova, T. A.; Bender,
W.; Krapivin, G. D. Tetrahedron 2006, 63, 474–491.
10. (a) Gao, Y.; Wu, W.-L.; Ye, B.; Zhou, R.; Wu, Y.-L. Tetrahedron Lett. 1996, 37,
893–896; (b) Yin, B.-L.; Yang, Z.-M.; Hu, T.-S.; Wu, Y.-L. Synthesis 2003, 1995–
2000; (c) Chen, L.; Xu, H.-H.; Yin, B.-L.; Xiao, C.; Hu, T.-S.; Wu, Y.-L. J. Agric. Food
Chem. 2004, 52, 6719–6723; (d) Yin, B.-L.; Lai, J.-Q.; Zhang, Z.-R.; Jiang, H.-F.
Adv. Synth. Catal. 2011, 353, 1961–1965; (e) Yin, B.; Huang, L.; Zhang, X.; Ji, F.;
Jiang, H. J. Org. Chem. 2012, 77, 6365–6370.
11. (a) Yin, B.; Zeng, G.; Cai, C.; Ji, F.; Huang, L.; Li, Z.; Jiang, H. Org. Lett. 2012, 14,
616–619; (b) Yin, B.; Cai, C.; Zeng, G.; Zhang, R.; Li, X.; Jiang, H. Org. Lett. 2012,
14, 1098–1101.
2
O
R
E
O
R
2
+
2
NBS
Br
+
R
O
R
R
H
O
O
E
R
1
5
R
R
13
1
1
12
Br
Br
Br
O
+
O
O
R
H
R
2
R
+
2
+
2
R
O
R
R
R
O
OH
R
H
R
14
E
1
R
E
1
E
1
15
H
16
Br
O
O
R
2
2
12. Typical procedure for the synthesis of 6: To a 25 mL flask was added 5 (0.3 mmol,
1 equiv), THF (3 mL), H2O (1 mL), AcOH (0.03 mmol, 0.1 equiv), and NBS
(0.33 mmol, 1.1 equiv). The mixture was stirred for 30 min at room
temperature until the disappearance of 5 according to the TLC. The reaction
mixture then was heated to 80 °C for 15 h. After cooled to room temperature,
the reaction mixture was added saturated Na2S2O3 solution (5 mL) and
extracted with EtOAc (3 Â 10 mL). The combined organic layers were washed
with saturated NaHCO3 solution and brine, dried over Na2SO4. After removal of
the solvent, the residue was purified by flash column chromatography to afford
6.
6
R
R
OH
R
17
O
E
R
18
E
1
1
Scheme 4. The proposed mechanism for the formation of 6.
Acknowledgments
Compound 6a: 1H NMR (400 MHz, CDCl3) d 7.17 (d, J = 15.9 Hz, 1H), 6.88 (s, 1H),
6.58 (d, J = 15.9 Hz, 1H), 4.28 (q, J = 7.1 Hz, 2H), 2.61 (s, 3H), 2.31 (s, 3H), 1.34 (t,
J = 7.1 Hz, 3H); 13C NMR (101 MHz, CDCl3) d 197.5, 163.1, 161.6, 148.5, 128.6,
124.4, 116.4, 116.1, 60.4, 28.0, 14.3, 14.1.
This work was supported by the Fundamental Research Funds
for the Central Universities (2012ZZ043), the National Natural
Science Foundation of China (21072062, 21272078), and the
Natural Science Foundation of Guangdong Province, China
(10351064101000000).
Compound 6b: 1H NMR (400 MHz, CDCl3) d 7.21 (d, J = 15.9 Hz, 1H), 6.87 (s, 1H),
6.62 (d, J = 15.9 Hz, 1H), 2.65 (s, 3H), 2.43 (s, 3H), 2.34 (s, 3H); 13C NMR
(101 MHz, CDCl3) d 197.4, 193.3, 161.0, 148.5, 128.4, 124.6, 123.7, 115.5, 29.0,
28.0, 14.6.
Compound 6c: 1H NMR (400 MHz, CDCl3) d 7.84 (d, J = 7.5 Hz, 2H), 7.75 (dd,
J = 6.4, 2.9 Hz, 2H), 7.54 (t, J = 7.4 Hz, 1H), 7.41 (t, J = 7.7 Hz, 2H), 7.37–7.30 (m,
3H), 7.27 (d, J = 4.9 Hz, 1H), 6.92 (s, 1H), 6.77 (d, J = 15.9 Hz, 1H), 2.37 (s, 3H);
13C NMR (101 MHz, CDCl3) d 197.4, 190.9, 157.3, 149.1, 137.5, 133.35, 129.9,
129.7, 128.9, 128.5, 128.4, 127.7, 126.9, 125.5, 123.2, 118.6, 28.0.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
12.085.
Compound 6g: 1H NMR (400 MHz, CDCl3)
d 7.33–7.20 (m, 1H), 6.56 (d,
J = 15.6 Hz, 1H), 4.28 (q, J = 7.1 Hz, 2H), 2.72 (q, J = 7.4 Hz, 2H), 2.57 (s, 3H), 2.29
(s, 3H), 1.33 (t, J = 7.1 Hz, 3H), 1.13 (t, J = 7.4 Hz, 3H); 13C NMR (101 MHz,
CDCl3) d 197.5, 163.5, 162.0, 145.0, 134.0, 126.3, 122.4, 115.2,, 60.2, 28.4, 17.8,
15.4, 14.6, 14.1.
Compound 6h: 1H NMR (400 MHz, CDCl3) d 7.47 (d, J = 15.5 Hz, 1H), 6.58 (d,
J = 15.5 Hz, 1H), 4.30 (q, J = 7.1 Hz, 2H), 3.19–3.02 (m, 1H), 2.54 (s, 3H), 2.31 (s,
3H), 1.84–1.68 (m, 7H), 1.43–1.29 (m, 6H); 13C NMR (101 MHz, CDCl3) d 197.5,
163.9, 161.4, 144.8, 136.8, 128.1, 122.6, 115.5, 60.3, 35.8, 32.6, 28.6, 27.0, 25.9,
14.9.
References and notes
1. For selected reviews, see: (a) Keay, B. A.; Hopkins, J. M.; Dibble, P. W. In
Comprehensive Heterocyclic Chemistry I; Jones, G., Ramsden, C. A., Eds.;
Elsevier: Amsterdam, 2008; Vol. 3, p 571; (b) d’lschia, M.; Napolitano, A.;
Pezzella, A. In Comprehensive Heterocyclic Chemsitry I; Jones, G., Ramsden, C.
A., Eds.; Elsevier: Amsterdam, 2008; Vol. 3, p 353; (c) Lipshutz, B. H. Chem. Rev.
1986, 86, 795–820; (d) Nakanishi, K. Natural Products Chemistry; Kodansha:
Tokyo, 1974.
2. For reviews concerning the dearomatization of furan ring, see: (a) Bach, T.
Angew. Chem., Int. Ed. Engl. 1996, 35, 729–730; (b) Bur, S. K.; Padwa, A. Chem.
Rev. 2004, 104, 2401–2432.
3. (a) Gribble, G. W. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I.,
Eds.; Pergamon: Oxford, 1991; Vol. 8, p 603; (b) Donohoe, T. J.; Garg, R.;
Stevenson, C. A. Tetrahedron: Asymmetry 1996, 7, 317–344; (c) Donohoe, T. J.;
Calabrese, A.; Stevenson, A. C. A.; Ladduwahetty, T. J. Chem. Soc., Perkin Trans. 1
2000, 3724–3731.
4. (a) Hosomi, A.; Tominaga, Y. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Paquette, L. A., Eds.; Pergamon: Oxford, 1991; Vol. 5, p 593; (b)
Chen, C.-H.; Rao, P. D.; Liao, C.-C. J. Am. Chem. Soc. 1998, 120, 13254–13255; (c)
Domingo, L. R.; Aurell, M. J. J. Org. Chem. 2002, 67, 959–965; (d) Xiong, H.;
Hsung, R. P.; Berry, C. R.; Rameshkumar, C. J. Am. Chem. Soc. 2001, 123, 7174–
7175; (e) Davies, H. M. L.; Ahmed, G.; Churchill, M. R. J. Am. Chem. Soc. 1996,
118, 10774–10782; (f) Kappe, C. O.; Murphree, S. S.; Padwa, A. Tetrahedron
1997, 53, 14179–14233; (g) Liu, L.; Gao, Y.; Che, C.; Wu, N.; Wang, D. Z.; Li, C.-
C.; Yang, Z. Chem. Commun. 2009, 662–664.
5. (a) Veits, G. K.; Wenz, D. R.; de Alanz, J. R. Angew. Chem., Int. Ed. 2010, 49, 9484–
9487; (b) Palmer, L. I.; de Alaniz, J. R. Angew. Chem., Int. Ed. 2011, 50, 7167–
7170; (c) Dygos, J. H.; Adamek, J. P.; Babiak, K. A.; Behling, J. R.; Medich, J. R.; Ng,
J. S.; Wieczorek, J. J. J. Org. Chem. 1991, 56, 2549–2552; (d) Rodriguez, A.;
Nomen, M.; Spur, B. W.; Godfroid, J. Eur. J. Org. Chem. 1999, 2655–2662; (e) Yin,
B. L.; Wu, Y.-K.; Wu, Y.-L. J. Chem. Soc., Perkin Trans. 1 2002, 1746–1747.
6. The example concerning the synthesis of furan via the ring opening of the
furans, see: Richard, T.; Cummings, R. T.; DiZio, J. P.; Krafft, G. A. Tetrahedron
Lett. 1988, 29, 69–72.
7. (a) Georgiou, T.; Tofi, M.; Montagnon, T.; Vassilikogiannakis, G. Org. Lett. 2006,
8, 1945–1948; (b) McDermott, P. J.; Stockman, R. A. Org. Lett. 2005, 7, 27–29; (c)
Jackson, K. L.; Henderson, J. A.; Morris, J. C.; Motoyoshi, H.; Phillips, A. J.
Tetrahedron Lett. 2008, 49, 2939–2941; (d) Abrams, J. N.; Babu, R. S.; Guo, H.; Le,
D.; Le, J.; Osbourn, J. M.; O’Doherty, G. A. J. Org. Chem. 2008, 73, 1935–1940; (e)
Bi, J.; Aggarwal, V. K. Chem. Commun. 2008, 120–122; (f) Cong, X.; Liu, K.-G.;
Liao, Q.-J.; Yao, Z.-J. Tetrahedron Lett. 2005, 46, 8567–8571; (g) Pavlakos, E.;
Georgiou, T.; Tofi, M.; Montagnon, T.; Vassilikogiannakis, G. Org. Lett. 2009, 11,
Compound 6i: 1H NMR (400 MHz, CDCl3) d 7.42–7.37 (m, 3H), 7.30–7.26 (m,
2H), 7.07 (d, J = 15.9 Hz, 1H), 6.65 (d, J = 15.9 Hz, 1H), 4.13 (q, J = 7.1 Hz, 2H),
2.67 (s, 3H), 2.24 (s, 3H), 1.09 (t, J = 7.1 Hz, 3H); 13C NMR (101 MHz, CDCl3) d
197.5, 163.2, 161.5, 146.0, 131.8, 131.1, 130.0, 128.1, 127.9, 127.6, 124.5, 115.7,
60.2, 28.0, 14.5, 13.8.
Compound 6j: 1H NMR (400 MHz, CDCl3) d 7.43 (d, J = 6.6 Hz, 3H), 7.25 (dd,
J = 7.6, 2.0 Hz, 2H), 6.99 (d, J = 15.8 Hz, 1H), 6.63 (d, J = 15.8 Hz, 1H), 2.60 (s,
3H), 2.21 (s, 3H), 1.91 (s, 3H); 13C NMR (101 MHz, CDCl3) d 197.4, 195.2, 160.4,
145.8, 131.4, 130.9, 129.7, 128.8, 128.7, 127.3, 124.7, 124.5, 30.7, 28.2, 14.6.
Compound 6k: 1H NMR (400 MHz, CDCl3) d 8.30 (d, J = 8.7 Hz, 2H), 7.46 (d,
J = 8.7 Hz, 2H), 6.93 (d, J = 15.7 Hz, 1H), 6.70 (d, J = 15.7 Hz, 1H), 2.65 (s, 3H),
2.25 (s, 3H), 2.08 (s, 3H); 13C NMR (101 MHz, CDCl3) d 197.2, 193.7, 160.6,
147.8, 146.2, 138.4, 130.8, 128.4, 126.0, 125.7, 124.3, 123.8, 30.8, 28.5, 14.9.
Compound 6l: 1H NMR (400 MHz, CDCl3) d 8.28 (d, J = 8.7 Hz, 2H), 7.46 (d,
J = 8.7 Hz, 2H), 7.02 (d, J = 15.8 Hz, 1H), 6.69 (d, J = 15.8 Hz, 1H), 4.15–4.09 (m,
2H), 2.28 (s, 3H), 1.47 (s, 9H), 1.06 (t, J = 7.1 Hz, 3H); 13C NMR (101 MHz, CDCl3)
d 197.2, 167.5, 163.8, 147.6, 144.7, 138.4, 130.5, 129.6, 128.8, 126.4, 125.7,
123.5, 123.3, 115.6, 61.2, 35.1, 28.5, 28.3, 13.7.
Compound 6m: 1H NMR (400 MHz, CDCl3) d 7.79 (d, J = 7.3 Hz, 2H), 7.64 (dd,
J = 6.6, 2.9 Hz, 2H), 7.44 (t, J = 7.4 Hz, 1H), 7.36–7.19 (m, 8H), 6.99 (t, J = 8.6 Hz,
2H), 6.90 (d, J = 15.8 Hz, 1H), 2.34 (s, 3H); 13C NMR (101 MHz, CDCl3) d 197.4,
192.5, 164.0, 161.5, 153.92, 146.3, 136.8, 133.9, 132.1, 131.2, 131.2, 129.7,
129.7, 128.8, 128.6, 128.6, 127.1, 126.8, 126.4, 125.5, 123.3, 116.0, 115.8, 28.3.
Compound 6n: 1H NMR (400 MHz, CDCl3)
d 7.34–7.25 (m, 2H), 7.17 (t,
J = 8.6 Hz, 2H), 6.99 (d, J = 15.8 Hz, 1H), 6.67 (d, J = 15.8 Hz, 1H), 2.64 (s, 3H),
2.26 (s, 3H), 1.98 (s, 3H); 13C NMR (101 MHz, CDCl3) d 197.3, 194.8, 164.2,
161.7, 160.5, 146.0, 131.6, 131.5, 129.7, 127.4, 127.4, 126.9, 124.7, 124.6, 116.0,
115.8, 30.8, 28.3, 14.7.
Compound 6o: 1H NMR (400 MHz, CDCl3) d 7.18 (d, J = 8.7 Hz, 2H), 7.02 (d,
J = 15.8 Hz, 1H), 6.97 (d, J = 8.7 Hz, 2H), 6.62 (d, J = 15.8 Hz, 1H), 3.85 (s, 3H),
2.60 (s, 3H), 2.23 (s, 3H), 1.95 (s, 3H); 13C NMR (101 MHz, CDCl3) d 197.45,
195.39, 160.33, 159.90, 145.82, 130.91, 130.77, 127.43, 124.74, 124.16, 123.39,
114.26, 55.29, 30.69, 28.16, 14.63.
Compound 6p: 1H NMR (400 MHz, CDCl3) d 7.19 (q, J = 8.0 Hz, 4H), 7.08 (d,
J = 15.9 Hz, 1H), 6.63 (d, J = 15.9 Hz, 1H), 4.15 (q, J = 7.1 Hz, 2H), 2.66 (s, 3H),
2.39 (s, 3H), 2.24 (s, 3H), 1.13 (t, J = 7.1 Hz, 3H); 13C NMR (101 MHz, CDCl3) d
197.5, 163.2, 161.4, 146.0, 137.9, 132.0, 129.9, 128.6, 128.0, 127.8, 124.3, 115.7,
60.2, 27.9, 21.2, 14.5, 13.9.

1260
H. Yu et al. / Tetrahedron Letters 54 (2013) 1256–1260
Compound 6r: 1H NMR (400 MHz, CDCl3) d 9.61 (d, J = 7.9 Hz, 1H), 7.34 (d,
Compound 6q: 1H NMR (400 MHz, CDCl3) d 8.01 (d, J = 7.2 Hz, 2H), 7.56–7.31
(m, 10H), 4.15 (q, J = 7.1 Hz, 2H), 2.72 (s, 3H), 1.10 (t, J = 7.1 Hz, 3H); 13C NMR
(101 MHz, CDCl3) d 189.3, 163.3, 161.5, 146.9, 138.2, 132.71, 132.2, 131.1,
130.1, 129.1, 128.5, 128.3, 128.1, 127.9, 119.4, 115.9, 60.2, 14.6, 13.8.
J = 15.6 Hz, 1H), 6.63 (dd, J = 15.7, 7.9 Hz, 1H), 6.51 (d, J = 3.1 Hz, 1H), 6.15 (d,
J = 3.2 Hz, 1H), 2.60 (s, 3H), 2.38 (s, 3H), 2.23 (s, 3H); 13C NMR (101 MHz, CDCl3)
d 195.0, 192.6, 160.9, 154.2, 145.8, 142.2, 135.8, 126.6, 123.7, 123.0, 121.3,
113.4, 107.9, 30.0, 14.6, 13.7.