Palladium-catalyzed one-pot synthesis of polysubstituted furans from alkynoates and 2-yn-1-ols

Article abstract of DOI:10.1016/j.tet.2011.12.033

Easily accessible propargyl vinyl ethers are found to be acyclic precursors to construct furan derivatives. According to the reactive behavior of the substrates and the regulation of reaction conditions, three synthetic routes to polysubstituted furans vi

Full text of DOI:10.1016/j.tet.2011.12.033

Tetrahedron 68 (2012) 3135e3144
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Palladium-catalyzed one-pot synthesis of polysubstituted furans from alkynoates
and 2-yn-1-ols
*
Huawen Huang, Huanfeng Jiang , Hua Cao, Jinwu Zhao, Dabin Shi
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Easily accessible propargyl vinyl ethers are found to be acyclic precursors to construct furan derivatives.
According to the reactive behavior of the substrates and the regulation of reaction conditions, three
synthetic routes to polysubstituted furans via palladium-catalyzed reactions of propargyl vinyl ethers in
situ-prepared from alkynoates and 2-yn-1-ols are presented herein. These methods could have potential
application in synthetic and pharmaceutical chemistry for its facilitation and accessible starting
materials.
Received 19 September 2011
Received in revised form 6 December 2011
Accepted 13 December 2011
Available online 7 February 2012
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Propargyl vinyl ethers
Furan derivatives
Palladium-catalyzed
Alkynoates
2-Yn-1-ols
1. Introduction
reacted with other different reagents, and then was cyclized to
multifarious products. For examples, Toste9a reported cascade se-
The preparation of furans has always been of importance be-
cause furan is one of the most useful class of heterocyclic com-
pounds,1 incorporated in the key structural units of numerous
natural products and pharmaceutical compounds.2 Furans have
also been used as building blocks for the synthesis of more elabo-
rate heterocyclic compounds.3 On the other hand, transition metal
catalysts play an essential role in the synthesis of heterocyclic
chemistry due to their great usefulness as a powerful tool,4 by
which many facile methodologies have been developed to provide
countless useful compounds for human beings.5 During the last
decade, transition metal-catalyzed synthesis of polysubstituted
furans from acyclic precursors, such as allenyl ketones,6aef 3-alkyn-
1-ones,6g,h 2-(1-alkynyl)-2-alken-1-ones,6ien propargylic esters,6o,p
has been intensively reported. However, facile synthetic method to
construct highly functionalized polysubstituted furans remains
a challenging target in synthetic and pharmaceutical chemistry.
Propargyl vinyl ether7 has been broadly explored as an acyclic
precursor to construct variable heterocyclic compounds, such as
furans,8 dihydropyrans,9 pyrroles,10 and dihydropyridines (Scheme
1).11 Of these procedures, the derivation of the intermediate pro-
duced through propargyl-Claisen rearrangement has been the most
extensively studied. The regiocontrolled intermediate regularly
quences of propargyl vinyl ethers to afford dihydropyrans, in which
the remarkable nucleophilic addition to the proposed oxocarbe-
nium intermediate is a central feature of the proposed mechanism.
Kirsch10b,11b and Xu11c described Au- or Ag-catalyzed procedures to
access pyrrole and pyridine derivatives with a crucial step of amine
condensation of allenylcarbonyl intermediates. As a complemen-
tary strategy, the formation of new functionalized groups (FG) after
rearrangement and cyclization of propargyl vinyl ethers has exactly
same important function.
Scheme 1. Accessing heterocyclic compounds from propargyl vinyl ethers: (1) for-
mation of FG; (2) intermediate derivation.
Very recently, our group developed facile methods for the direct
oxidative synthesis of furan aldehydes/ketones from propargyl vi-
nyl ethers in the presence of copper12a or iron12b catalysts. By using
* Corresponding author. Tel./fax: þ86 20 87112906; e-mail address: jianghf@
scut.edu.cn (H. Jiang).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.12.033

3136
H. Huang et al. / Tetrahedron 68 (2012) 3135e3144
silver as the catalyst, the reaction afforded highly substituted furans
via direct 5-exo-dig cycloisomerization.12c Further investigation
showed that regio-isomeric furans were observed when
substituted aryl alkynyl ketones were employed. Taken together
this may suggest that highly functionalized furans are selectively
accessed due to different reaction behavior of propargyl vinyl
ethers or different catalytic systems. In view of our long-standing
attention on palladium-catalyzed activation or functionalization
of multiple bonds,13 and the fact that palladium was generally
considered as an effect kind of catalysts for Claisen rearrangment,14
we were encouraged to screen the nature of palladium catalysts to
propargyl vinyl ethers. As a continuing interest in expanding the
synthetic application of propargyl vinyl ethers, we herein describe
our investigatory results on palladium-catalyzed one-pot synthesis
of polysubstituted furans.
We presumed that the reaction forming furan 4aa was carried
out through a -hydride elimination process, in which base and
b
oxidant were usually essential.16 When base, such as Li2CO3,
Na2CO3, K2CO3, was added alone, 20 mol % of Na2CO3 increased the
yield of 4aa to 57% (Table 1, entry 3), however, Li2CO3 or K2CO3 was
unexpectedly disappointing (Table 1, entries 2 and 4). CuCl2$2H2O
(50 mol %) was employed alone as oxidant, and only trace of 4aa
was detected (Table 1, entry 5). By combining oxidant and base, we
found that the addition of Na2CO3 and CuCl2$2H2O could better the
yield of the desired product (entry 6). As for the effect of oxidant,
CuCl2$2H2O was outstandingly superior to Cu(OAc)2$H2O or CuCl
(Table 1, entries 6e8). The suitable amount of base and oxidant is
30 mol % of Na2CO3 and 50 mol % of CuCl2$2H2O (Table 1, entries
9e12). Different solvents and reaction temperature were also
tested (Table 1, entries 13e17), providing negative results com-
pared with that foregoing. We also found that the reaction was
carried out with 20 mol % of Na2CO3 and 20 mol % of CuCl2$2H2O as
the additive under nitrogen, 16% of 4aa was isolated (Table 1, entry
18). Finally, the optimal conditions for this reaction are: 5 mol % of
PdCl2, 30 mol % of Na2CO3, 50 mol % of CuCl2$2H2O in DMF at 80 ꢀC
for 14 h under the atmosphere.
2. Results and discussion
2.1. Synthesis of 5-vinyl-substituted furans
In accordance with the previous reports,7a,10b,c we expected that
the adducts of prop-2-yn-1-ol to electron-deficient alkynes would
fail to afford the corresponding furans by using palladium as the
catalyst, because of the poisoning of palladium to terminal alkynes.
Thus we initially chose diethyl but-2-ynedioate (1a, 0.2 mmol) and
1-phenylpent-1-yn-3-ol (2a, 0.2 mmol) as the starting materials to
investigate the transformation under palladium-catalyzed condi-
tions. Surprisingly, 5-vinyl-substituted furan 4aa was obtained
when 5 mol % of PdCl2 was employed in DMF at 80 ꢀC under air
(Table 1, entry 1). To the best of our knowledge, there are few re-
ported examples of the direct synthesis of 5-vinyl-substituted fu-
rans from acyclic precursors in a one-pot manner.15 Thus we
determined to optimize the reaction conditions of this protocol.
We next studied the generality of the reaction under optimized
conditions to synthesize 5-vinyl-substituted furans. Various 3-
arylpent-2-yn-1-ols, with alkyl, halogen or methoxyl on the aro-
matic ring or 3-heterocyclicpent-2-yn-1-ol, reacted successfully
with diethyl or dimethyl but-2-ynedioate (1a or 1b) to afford the
desired products in moderate to good yields (4aae4ai, 46e85%,
Table 2) with high stereoselectivities (determined by GC). Gener-
ally, electron-rich methoxyl and thiophen exhibited lower reaction
efficiencies (4ah and 4ai), while halogen on the aromatic ring fa-
vored the stereoselectivity of the products (4ade4ag). 3-Alkyl-
substituted-2-yl-1-ols were also effective substrates for the re-
action (4bj, 4bk). The reaction of substrate 2le2o afforded the re-
spective furans stereoselectively with almost singular isomer (E/Z
>98/2), probably due to the steric hindrance effect of R3. 1-
Isopropyl (2p) and cyclohexyl-2-yl-1-ol (2q) smoothly gave re-
spective 5-vinyl-substituted furans 4bp and 4bq. The structure of
Table 1
Optimization of reaction conditionsa
Table 2
Synthesis of 5-vinyl-substituted furans
Entry
Additive (mol %)
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
d
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Toluene
DMSO
DMA
DMF
DMF
DMF
42
21
57
37
Trace
79
20
<10
82
84
85 (97/3)
81
45
23
65
66
82
16
Li2CO3 (20)
Na2CO3 (20)
K2CO3 (20)
CuCl2$2H2O (50)
Entry
1 (R1)
2 (R2, R3, R4)
Product (yield, E/Z)a
1
2
3
4
5
6
7
8
1a (Et)
1b (Me)
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1a
1b
1a
1a
2a (Ph, Me, H)
2a (Ph, Me, H)
4aa (85%, 93/7)
4ba (77%, 93/7)
4ab (80%, 97/3)
4ac (75%, 90/10)
4ad (84%, >99/1)
4ae (78%, >99/1)
4af (68%, >99/1)
4ag (72% (97/3)
4ah (51%, 90/10)
4ai (46%, 98/2)
4bj (60%, 92/8)
4bk (77%, 95/5)
4bl (78%, >99/1)
4al (85%, >99/1)
4bm (82%, 98/2)
4an (72%, >99/1)
4ao (58%, >99/1)
4bp (84%)
Na2CO3 (20), CuCl2$2H2O (20)
Na2CO3 (20), Cu(OAc)2$H2O (20)
Na2CO3 (20), CuCl (20)
Na2CO3 (20), CuCl2$2H2O (30)
Na2CO3 (20), CuCl2$2H2O (50)
Na2CO3 (30), CuCl2$2H2O (50)
Na2CO3 (100), CuCl2$2H2O (100)
Na2CO3 (30), CuCl2$2H2O (50)
Na2CO3 (30), CuCl2$2H2O (50)
Na2CO3 (30), CuCl2$2H2O (50)
Na2CO3 (30), CuCl2$2H2O (50)
Na2CO3 (30), CuCl2$2H2O (50)
Na2CO3 (20), CuCl2$2H2O (20)
2b (4-MeeC6H4, Me, H)
2c (3-MeeC6H4, Me, H)
2d (4-FeC6H4, Me, H)
2e (3-FeC6H4, Me, H)
2f (4-CleC6H4, Me, H)
2g (4-BreC6H4, Me, H)
2h (4-MeOeC6H4, Me, H)
2i (Thiophen-3-yl, Me, H)
2j (Et, Me, H)
2k (n-Pr, Me, H)
2l (Ph, Ph, H)
2l (Ph, Ph, H)
2m (4-MeeC6H4, Ph, H)
2n (4-CleC6H4, Me, H)
2o (4-EtOeC6H4, Me, H)
2p (Ph, Me, Me)
2q (Ph, eCH2CH2CH2CH2CH2e)
2r (H, Me, H)
9
10
11
12
13
14
15
16c
17d
18e
9
10
11
12
13
14
15
16
17
18
19
20
a
Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol), DABCO (10 mol %), PdCl2
(5 mol %), additive as specified. Rearrangement cyclization proceeded at 80 ꢀC for
14 h.
1b
1b
1b
b
Isolated yield of the combined E&Z isomers and the ratio determined by GC in
4bq (70%)
4br (0)
the parenthesis.
c
At 60 ꢀC.
d
a
At 100 ꢀC.
Isolated yield of the combined E&Z isomers and the ratio determined by GC were
e
Under N2.
given in the parenthesis.

H. Huang et al. / Tetrahedron 68 (2012) 3135e3144
3137
4bp was corroborated by the X-ray crystallographic analyses
(Fig. 1), which also provided the basis for the assignment of the
relative configuration of other products. However, pent-1-yn-3-ol
(2r) failed to afford the desired product in this system.
Scheme 3. Proposed pathway to afford 3-vinyl-substituted furan.
rearrangement and direct 5-exo-dig cycloisomerization of propargyl
vinyl ethers to provide polysubstituted furans have not been re-
ported, it is believed necessary to present these results in this paper.
Fig. 1. X-ray structures of compound 4bp.
2.3. Synthesis of polysubstituted furans via direct 5-exo-
cyclization
Furthermore, propargyl vinyl ethers 3017 were subjected to the
reaction (Scheme 2). The corresponding endocyclic furan de-
rivatives (4a and 4b) were stereoselectively obtained in moderated
yields. It shows potentiality of this method for synthesis of natural
products since endocyclic furan derivatives are common units of
many natural products.18
In a typical procedure, 1a (0.5 mmol), 2a0 (0.5 mmol), and
DABCO (10 mol %) in CH2Cl2 (2 mL) were stirred at room temper-
ature for 10 min, and then 5 mol % of PdCl2 were added. The direct
5-exo-cyclization product 6ba0 was isolated in 70% yield after 5 h
(Table 4, entry 1). When Pd(OAc)2 and Pd(PPh3)4 were employed
instead of PdCl2, the reaction gave the product in 45% and 23%
yields, respectively (Table 4, entries 2 and 3). Additives, such as
Bu4NþBrꢁ, Bu4NþClꢁ, and Bu4NþFꢁ, significantly improved the yield
of 6ba0 in a relatively short reaction time (2 h), and Bu4NþFꢁ was
the best additive (Table 4, entries 4e6). Then we tried several dif-
ferent solvents in the cyclization process (Table 4, entries 7e11),
and found DMF was the most effective media (93% yield). No target
product was detected without palladium catalyst, and 0.1 mol % of
PdCl2 afforded 6ba0 in a lower yield (Table 4, entries 12 and 13).
Increment of the reaction temperature could not promote this re-
action (Table 4, entry 14). Interestingly, hex-3-yn-2-ol instead of 1-
phenylhex-2-yn-1-ol generated no corresponding product, and
almost 100% of the propargyl vinyl ether was reclaimed (Table 4,
entry 15), indicating that the substituent groups in 1-position of 2-
yn-1-ols are quite important to the reaction effectivity. Compared
with reported Au-,8b Ag-catalyzed12c or microwave-assisted8c
procedures, this via palladium-catalyzed Claisen-rearrangement
and 5-exo-cyclization of propargyl vinyl ether suffers the limitation
of unusual substrates. However, moderate reaction conditions and
high efficiency still make the methodology meaningful.
Next, to explore the scope of this reaction, a series of 1-aryl-2-
yn-1-ols were employed for synthesis of furans (Table 5). Gener-
ally, all of the 1-aryl-2-yn-1-ols, regardless of their electronic or
steric properties, proceeded smoothly to afford the expected tet-
rasubstituted furans in moderate to good yields (Table 5, 66e94%).
Electron-withdrawing groups (eF, eCl, eBr, eCF3) on the aryl (Ar)
were well tolerated leading to excellent yields (Table 5, entries
4e10). 2-Yn-1-ols with heterocyclic aryls or aryls with electron-rich
groups also afforded the corresponding furans, however, in lower
yields (Table 5, entries 11e13). Notably, 3-trimethylsilyl furan (6bt0)
was successfully formed from corresponding alkynol (Table 5, entry
22), which indicated that trisubstituted furans or more diverse
furan derivatives can be conveniently prepared by this synthetic
methodology.20
Scheme 2. Preparation of endocyclic furan derivatives.
2.2. Construction of 4-vinyl-substituted furans
As known in literature, allenyl compounds can collapse into 1,3-
dienes via palladium catalysis.19 As to the product of propargyl-
Claisen rearrangement, we believe that allenyl intermediate A prob-
ablyconverts to 1,3-dieneI, II, orIII(Scheme 3).For1,3-dieneIor II, the
final cyclization product seems to be the same with that mentioned
above. However, for 1,3-diene III, oxidative cyclization of III0, the iso-
mer of III by s-bond rotation, may afford 3-vinyl-substituted furan 5.
To this end, we employed 1b and 2j as the starting materials and
attempted various additives to optimize the reaction conditions.
Investigation showed that the additive Bu4NþFꢁ (20 mol %) was
indispensable for the regioselective construction of 4-vinyl-
substituted furan 5bj, however, addition of a base (30 mol % of
Na2CO3) almost quenched the formation of 5bj. Several 2-yl-1-ols
were subjected to this reaction, and respective 4-vinyl-substituted
furans were smoothly prepared in moderate yields with good ster-
eoselectivity (Table 3, entries 1e7). When but-2-yn-1-ol was
employed as substrate, the reaction afforded the desired furan de-
rivative in less than 10% yield (Table 3, entry 8). When R4 were aryls,
such as 4-FeC6H4e, the target furan 5bc0 was isolated in 22% yield
(Table 3, entry 9). Moreover, we tried our best to optimize the re-
action conditions, but the 5-exo product of allenyl intermediate was
still favored. Since palladium-catalyzed propargyl-Claisen

3138
H. Huang et al. / Tetrahedron 68 (2012) 3135e3144
Table 4
Screening the reaction conditions
Table 3
Palladium-catalyzed synthesis of 4-vinyl-substituted furans
Entry Catalyst (0.2 mol %) Additive (2 mol %) Solvent
Time (h) Yielda (%)
1b
2b
3b
4
5
6
7
8
9
PdCl2
Pd(OAc)2
Pd(PPh3)4
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
d
d
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
Toluene
DCE
1,4-Dioxane 2
DMF
DMF
DMF
DMF
DMF
5
5
5
2
2
2
2
2
2
70
45
23
74
77
82
35
56
58
30
93
0
d
d
Entry
1
1
2
Product (yield, E/Z)a
Bu4NþBrꢁ
Bu4NþClꢁ
Bu4NþFꢁ
Bu4NþFꢁ
Bu4NþFꢁ
Bu4NþFꢁ
Bu4NþFꢁ
Bu4NþFꢁ
Bu4NþFꢁ
Bu4NþFꢁ
Bu4NþFꢁ
Bu4NþFꢁ
2j
1b
10
11
12c
5bj (76%, 87/13)
2
2
5
2
5
13d PdCl2
77
90
0
14e PdCl2
15f
PdCl2
2
1a
2j
a
Isolated yield.
Using 5 mol % of catalyst.
Without catalyst.
PdCl2 (0.1 mol %) was used.
The reaction was carried out at 50 ꢀC.
b
c
d
e
f
5aj (68%, 91/9)
5bs (65%, 98/2)
2s
Hex-3-yn-2-ol was used as the substrate instead of 1-phenylhex-2-yn-1-ol, and
3
4
1b
1a
5 mol % of PdCl2 were used.
Table 5
Palladium-catalyzed one-pot synthesis of furans 6
2s
5as (58%, 96/4)
2t
5
6
1b
1a
Entry
1
2 (R2/Ar)
Product
Yield (%)
5bt (72%, 97/3)
5at (69%, 94/6)
1
2
3
4
5
6
7
8
1b
1a
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1a
1a
1a
1a
1a
1a
1a
1b
2a0 (Pr/Ph)
6ba0
6aa0
6bb0
6bc0
6bd0
6be0
6bf0
6bg0
6bh0
6bi0
93
94
88
92
88
81
85
88
86
89
79
70
66
90
87
91
80
83
87
90
86
79
2a0 (Pr/Ph)
2b0 (Pr/4-MeeC6H4)
2c0 (Pr/4-FeC6H4)
2d0 (Pr/4-CleC6H4)
2e0 (Pr/2,6-Cl2eC6H3)
2f0 (Pr/4-BreC6H4)
2g0 (Pr/3-BreC6H4)
2h0 (Pr/2-BreC6H4)
2i0 (Pr/4eCF3eC6H4)
2j0 (Pr/4-MeOeC6H4)
2k0 (Pr/furan-2-yl)
2l0 (Pr/thiophen-2-yl)
2m0 (Ph/Ph)
2t
9
2u
10
11
12
13
14
15
16
17
18
19
20
21
22
6bj0
7
8
1b
6bk0
6bl0
5bu (55%, >99/1)
5bv (<10)
6bm0
6am0
6an0
6ao0
6ap0
6aq0
6ar0
6as0
6bt0
2m0 (Ph/Ph)
2n0 (4-BreC6H4/Ph)
2o0 (4-MeOeC6H4/Ph)
2p0 (4-CO2MeeC6H4/Ph)
2q0 (4-EteC6H4/Ph)
2r0 (3-MeeC6H4/Ph)
2s0 (2-MeeC6H4/Ph)
2t0 (Trimethylsilyl/Ph)
2v
1b
1b
2c′
2.4. Plausible reaction mechanism
9
During the course of search on the palladium-catalyzed reaction
of 3bj, we discovered an interesting regioreversal of the cyclization
controlled by different additives. PdCl2-catalyzed reaction of 3bj by
using Na2CO3 and CuCl2$2H2O as additives produces 5-vinyl-
5bc′(22%, >99/1)
a
Isolated yield of the combined E&Z isomers. The ratio was determined by GC.

H. Huang et al. / Tetrahedron 68 (2012) 3135e3144
3139
substituted furan 4bj in good yield, whereas the reaction using
Bu4NþFꢁ as additives affords 4-vinyl-substituted furan 5bj regio-
selectively, in which two different procedures of isomerization of
allene intermediate and cyclization most probably proceed to give
distinct final products.19a,e,f
were recorded using a Bruker Avance 400 MHz NMR spectrometer.
The chemical shifts are referenced to signals at 7.26 and 77.0 ppm,
respectively, and chloroform is solvent with TMS as the internal
standard. Mass spectra were recorded on a Shimadzu GCeMS-
QP5050A spectrometer at an ionization voltage of 70 eV equipped
with a DB-WAX capillary column (internal diameter: 0.25 mm,
length: 30 m). Elemental analyses were performed with a Vario EL
elemental analyzer. IR spectra were obtained as potassium bromide
pellets or as liquid films between two potassium bromide pellets
with a Bruker Vector 22 spectrometer. High-resolution mass
spectra were obtained with Shimadazu LCMS-IT-TOF mass spec-
trometer. TLC was performed by using commercially prepared
100e400 mesh silica gel plates (GF254) and visualization was
effected at 254 nm.
According to the experimental results, the mechanism of
palladium-catalyzed domino reactions to form three kinds of furans
fromalkynoates and2-yn-1-ols was proposed (Scheme 4). Propargyl
vinyl ether 3, in situ-formed via DABCO-catalyzed nucleophilic ad-
dition of propargyl alcohol 2 to alkynoate 1,21 undergo propargyl-
Claisen rearrangement to furnish allene intermediate A.
Palladium-mediated oxypalladationand 5-exo-cyclization convertB
into intermediate C. Subsequently, protonolysis of C at ambient
temperature efficiently regenerates polysubstituted furan 6 and the
active catalyst species Pd(II) when R3 are aryls, otherwise
b-hydride
elimination of C will readily occur to give 5-vinyl-substituted furan 4
at elevated temperature and more importantly with a suitable base
in the reaction system. On the other hand, Bu4NþFe as the additive
may promote isomerization of allene intermediate A to 1,3-diene D.
4.2. General procedure for the synthesis of furans 4
The mixture of alkynoate (1, 0.2 mmol), 2-yn-1-ol (2, 0.2 mmol),
and DABCO (10 mol %) was stirred in CH2Cl2 (1 mL) at room tem-
perature for 10 min. And then the solution was evaporated to
dryness under reduced pressure. Subsequently, PdCl2 (5 mol %),
Na2CO3 (30 mol %), CuCl2$2H2O (50 mol %), and DMF (1.5 mL) were
added at 80 ꢀC. After completion of the reaction (as monitored by
TLC), 5 mL water was added. The aqueous solution was extracted
with diethyl ether (3ꢂ5 mL) and the combined extract was dried
with anhydrous MgSO4. The solvent was removed and the crude
product was separated by column chromatography to give the pure
product.
Palladium-mediated cyclization of D and b-hydride elimination of E
readily provide 4-vinyl-substituted furan 5. Both procedures leading
vinyl-substituted furans release Pd(0) and HCl at the end. Cu(II) and
oxygen as the oxidant can oxidize Pd(0) to Pd(II).22,23
4.2.1. (E)-Diethyl 4-phenyl-5-(prop-1-enyl)furan-2,3-dicarboxylate
(4aa). Yellowish viscous oil. IR (KBr) v 2982, 1734, 1561, 1453,
1378, 1304, 1192, 1059, 862, 768, 702 cmꢁ1 1H NMR (400 MHz,
.
CDCl3):
d
7.32e7.42 (m, 5H), 6.56e6.65 (m, 1H), 6.24 (d, J¼16.0 Hz,
1H), 4.36 (q, J¼7.2 Hz, 2H), 4.25 (q, J¼7.2 Hz, 2H), 1.87 (d, J¼6.8 Hz,
3H), 1.36 (t, J¼6.8 Hz, 3H), 1.21 (t, J¼6.8 Hz, 3H). 13C NMR (100 MHz,
CDCl3):
d 163.8, 157.9, 151.8, 139.4, 131.7, 130.5, 129.3, 128.6, 127.9,
127.2, 121.9, 117.2, 61.7, 61.3, 18.6, 14.2, 13.9. HRMS (ESI) calcd
C19H20O5 [MþH]þ: 329.1389, found: 329.1379.
4.2.2. (E)-Dimethyl 4-phenyl-5-(prop-1-enyl)furan-2,3-
dicarboxylate (4ba). Yellowish viscous oil. IR (KBr) v 2955, 1737,
1562, 1444, 1306, 1215, 1160, 1063, 857, 774, 703 cmꢁ1 1H NMR
.
(400 MHz, CDCl3):
d 7.32e7.43 (m, 5H), 6.57e6.66 (m, 1H), 6.26 (d,
Scheme 4. Plausible reaction mechanism.
J¼15.6 Hz, 1H), 3.91 (s, 3H), 3.79 (s, 3H), 1.87 (d, J¼6.8 Hz, 3H). 13C
NMR (100 MHz, CDCl3):
d 164.3, 158.3, 151.9, 139.3, 131.9, 130.4,
129.2, 128.7,128.0, 127.1,122.0,117.2, 52.7, 52.3, 18.7. MS (EI) m/z: 77,
89, 115, 153, 253, 268, 300. Anal. Calcd for C17H16O5: C 67.99, H 5.37,
found: C 67.87, H 5.40.
3. Conclusion
In summary, palladium-catalyzed transformation of propargyl
vinyl ethers in situ-prepared from alkynoates and 2-yn-1-ols to
polysubstituted furans with high regioselectivity was developed. It
was found that the regioselectivity leading three kinds of poly-
substituted furans was dependent on both the reaction behavior of
the substrates and the reaction conditions. That methodology has
diversified the chemistry of propargyl vinyl ethers, and shows po-
tentiality for synthetic and pharmaceutical chemistry for its facili-
tation and accessible starting materials. Further investigation on
the mechanisms, especially the role of the additive Bu4NþFꢁ, is
underway in our laboratory.
4.2.3. (E)-Diethyl 5-(prop-1-enyl)-4-p-tolylfuran-2,3-dicarboxylate
(4ab). Yellowish viscous oil. IR (KBr) v 2981, 1732, 1555, 1376,
1311, 1171, 1056, 822, 779 cmꢁ1 1H NMR (400 MHz, CDCl3):
.
d
7.19e7.24 (m, 4H), 6.55e6.64 (m, 1H), 6.24 (d, J¼16.0 Hz, 1H), 4.36
(q, J¼7.2 Hz, 2H), 4.25 (q, J¼7.2 Hz, 2H), 2.37 (s, 3H), 1.86 (d,
J¼7.2 Hz, 3H), 1.36 (t, J¼7.2 Hz, 3H), 1.23 (t, J¼7.2 Hz, 3H). 13C NMR
(100 MHz, CDCl3):
d 163.9, 157.9, 151.7, 139.3, 137.8, 131.4, 129.3,
129.1, 127.5, 127.3, 121.9, 117.3, 61.7, 61.3, 21.2, 18.6, 14.2, 14.0. MS (EI)
m/z: 77, 91, 115, 152, 196, 253, 281, 297, 342. Anal. Calcd for
C20H22O5: C 70.16, H 6.48, found: C 70.10, H 6.53.
4. Experimental section
4.1. General methods
4.2.4. (E)-Diethyl 5-(prop-1-enyl)-4-m-tolylfuran-2,3-dicarboxylate
(4ac). Yellowish viscous oil. IR (KBr) v 2981, 1735, 1592, 1459,
1379, 1305, 1191, 1060, 859, 785, 704 cmꢁ1 1H NMR (400 MHz,
.
CDCl3): d 7.24 (s, 1H), 7.09e7.17 (m, 3H), 6.54e6.63 (m, 1H), 6.22 (d,
€
Melting points were measured with a BUCHI B-545 melting
J¼15.6 Hz, 1H), 4.34 (q, J¼7.2 Hz, 2H), 4.23 (q, J¼7.2 Hz, 2H), 2.35 (s,
point instrument and were uncorrected. 1H and 13C NMR spectra
3H), 1.85 (d, J¼6.8 Hz, 3H), 1.33 (t, J¼7.2 Hz, 3H), 1.20 (t, J¼7.2 Hz,

3140
H. Huang et al. / Tetrahedron 68 (2012) 3135e3144
3H). 13C NMR (100 MHz, CDCl3):
d
163.9, 157.9, 151.8, 139.3, 138.2,
1571, 1456, 1383, 1303, 1190, 1059, 1018, 860, 791 cmꢁ1
.
1H NMR
131.5,129.8,128.5,127.3,126.3,122.0,117.3, 61.7, 61.3, 21.4,18.7,14.2,
14.0. MS (EI) m/z: 77, 91, 114, 128, 152, 196, 253, 281, 342. Anal. Calcd
for C20H22O5: C 70.16, H 6.48, found: C 70.05, H 6.52.
(400 MHz, CDCl3): 7.36e7.38 (m, 1H), 7.30e7.31 (m, 1H), 7.11e7.13
d
(m,1H), 6.56e6.65 (m,1H), 6.31 (d, J¼15.6 Hz,1H), 4.36 (q, J¼7.2 Hz,
2H), 4.29 (q, J¼7.2 Hz, 2H), 1.89 (d, J¼7.2 Hz, 3H), 1.35 (t, J¼7.2 Hz,
3H), 1.27 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 164.0, 157.8,
4.2.5. (E)-Diethyl 4-(4-fluorophenyl)-5-(prop-1-enyl)furan-2,3-
dicarboxylate (4ad). Yellowish viscous oil. IR (KBr) v 2983, 1733,
151.9, 139.3, 131.8, 130.3, 128.2, 127.0, 126.0, 124.1, 117.2, 116.9, 61.8,
61.3, 18.7, 14.2, 14.0. MS (EI) m/z: 77, 83, 115, 152, 167, 209, 253, 281,
297, 342. Anal. Calcd for C17H18O5S: C 61.06, H 5.43, found: C 60.62,
H 5.47.
1600, 1506, 1458, 1378, 1304, 1164, 1056, 844, 789 cmꢁ1 1H NMR
.
(400 MHz, CDCl3):
d 7.29e7.32 (m, 2H), 7.07e7.11 (m, 2H),
6.56e6.65 (m, 1H), 6.18 (d, J¼16.0 Hz, 1H), 4.36 (q, J¼7.2 Hz, 2H),
4.25 (q, J¼7.2 Hz, 2H), 1.87 (d, J¼7.2 Hz, 3H) 1.36 (t, J¼7.2 Hz, 3H),
4.2.11. (E)-Diethyl 4-ethyl-5-(prop-1-enyl)furan-2,3-dicarboxylate
1.22 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
163.7, 162.6,
(4bj). Yellowish viscous oil. IR (KBr) v 2961, 2884, 1736, 1599,
157.8, 151.8, 139.5, 132.0, 131.1, 127.0, 126.5, 120.9, 116.9, 115.6, 61.8,
61.4, 18.7, 14.2, 14.0. HRMS (ESI) calcd C19H19FO5 [MþH]þ: 347.1295,
found: 347.1296.
1554, 1446, 1294, 1212, 1095, 1041, 970, 862, 799 cmꢁ1 1H NMR
.
(400 MHz, CDCl3):
d
6.42e6.51 (m, 1H), 6.22 (d, J¼16.0 Hz, 1H), 3.90
(s, 3H), 3.87 (s, 3H), 2.48 (q, J¼7.2 Hz, 2H), 1.90 (d, J¼6.8 Hz, 3H) 1.10
(t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 164.4, 158.3, 151.5,
4.2.6. (E)-Diethyl 4-(3-fluorophenyl)-5-(prop-1-enyl)furan-2,3-
dicarboxylate (4ae). Yellowish viscous oil. IR (KBr) v 2984, 1735,
139.8, 130.0, 126.6, 123.1, 116.7, 52.5, 52.1, 18.7, 16.8, 15.0. MS (EI) m/
z: 77, 91, 105, 134, 173, 220, 252. Anal. Calcd for C13H16O5: C 61.90, H
6.39, found: C 61.62, H 6.43.
1580, 1447, 1378, 1305, 1190, 1060, 864, 788, 693 cmꢁ1 1H NMR
.
(400 MHz, CDCl3):
d 7.34e7.39 (m, 1H), 7.04e7.12 (m, 3H),
6.59e6.68 (m, 1H), 6.23 (d, J¼16.0 Hz, 1H), 4.37 (q, J¼7.2 Hz, 2H),
4.2.12. (E)-Dimethyl 4-propyl-5-styrylfuran-2,3-dicarboxylate
(4bk). Yellowish viscous oil. IR (KBr) v 2955, 2872, 1731, 1590,
1531,1444,1316,1253,1202,1098,1042, 960, 804, 755, 695 cmꢁ1. 1H
4.27 (q, J¼7.2 Hz, 2H), 1.88 (d, J¼6.8 Hz, 3H) 1.36 (t, J¼7.2 Hz, 3H),
1.23 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 163.6, 162.7,
157.8, 151.9, 139.6, 132.6, 132.5, 130.1, 126.9, 125.1, 120.6, 116.8, 116.3,
115.0, 61.8, 61.4, 18.7, 14.2, 13.9. MS (EI) m/z: 77, 109, 133, 171, 213,
157, 285, 301, 346. Anal. Calcd for C19H19FO5: C 65.89, H 5.53, found:
C 65.97, H 5.48.
NMR (400 MHz, CDCl3):
d
7.51 (d, J¼7.6 Hz, 2H), 7.29e7.39 (m, 4H),
6.86 (d, J¼16.0 Hz, 1H), 3.93 (s, 3H), 3.92 (s, 3H), 2.56 (t, J¼7.6 Hz,
2H), 1.52e1.62 (m, 2H) 0.94 (t, J¼7.6 Hz, 3H). 13C NMR (100 MHz,
CDCl3):
d 164.2, 158.3, 152.0, 140.3, 136.3, 131.2, 128.8, 128.5, 127.0,
126.8, 124.0, 112.9, 52.5, 52.3, 25.5, 23.7, 13.7. MS (EI) m/z: 77, 91,
115, 153, 179, 207, 239, 267, 296, 328. Anal. Calcd for C19H20O5: C
69.50, H 6.14, found: C 69.46, H 6.17.
4.2.7. (E)-Diethyl 4-(4-chlorophenyl)-5-(prop-1-enyl)furan-2,3-
dicarboxylate (4af). Yellowish viscous oil. IR (KBr) v 2981,1734,
1594, 1485, 1380, 1306, 1192, 1095, 1057, 836, 782 cmꢁ1 1H NMR
.
(400 MHz, CDCl3):
d
7.36e7.38 (d, J¼8.4 Hz, 2H), 7.27 (d, J¼8.4 Hz,
4.2.13. (E)-Dimethyl 4-phenyl-5-styrylfuran-2,3-dicarboxylate
(4bl). White viscous solid, mp: 99e101 ꢀC. IR (KBr) v 2948, 1734,
1589, 1537, 1442, 1331, 1208, 1157, 1061, 968, 851, 762, 700 cmꢁ1. 1H
2H), 6.56e6.65 (m, 1H), 6.18 (d, J¼15.6 Hz, 1H), 4.36 (q, J¼7.2 Hz,
2H), 4.25 (q, J¼7.2 Hz, 2H), 1.87 (d, J¼6.8 Hz, 3H), 1.35 (t, J¼7.2 Hz,
3H), 1.22 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
163.6, 157.8,
NMR (400 MHz, CDCl3):
d
7.28e7.47 (m, 11H), 6.88 (d, J¼16.4 Hz,
151.9, 139.7, 134.1, 132.3, 130.6, 129.0, 128.9, 126.9, 120.7, 116.8, 61.8,
61.4, 18.7, 14.2, 14.0. MS (EI) m/z: 77, 115, 152, 209, 253, 273, 287, 317,
362. Anal. Calcd for C19H19ClO5: C 62.90, H 5.28, found: C 62.76, H
5.30.
1H), 3.95 (s, 3H), 3.82 (s, 3H). 13C NMR (100 MHz, CDCl3):
d
164.1,
158.2, 152.0, 149.8, 139.8, 136.0, 132.9, 130.2, 129.3, 128.9, 128.7,
128.3, 127.3, 127.0, 124.3, 113.3, 52.8, 52.5. MS (EI) m/z: 77, 105, 115,
165, 215, 244, 270, 331, 362. Anal. Calcd for C22H18FO5: C 72.92, H
5.01, found: C 72.67, H 5.07.
4.2.8. (E)-Diethyl 4-(4-bromophenyl)-5-(prop-1-enyl)furan-2,3-
dicarboxylate (4ag). Yellowish viscous oil. IR (KBr) v 2982, 1734,
4.2.14. (E)-Diethyl 4-phenyl-5-styrylfuran-2,3-dicarboxylate
(4al). Yellowish viscous oil. IR (KBr) v 2983, 1729, 1587, 1536,
1487, 1373, 1319, 1182, 1057, 860, 763, 701 cmꢁ1. 1H NMR (400 MHz,
1592, 1480, 1304, 1190, 1060, 1014, 830, 782 cmꢁ1 1H NMR
.
(400 MHz, CDCl3):
d
7.53 (d, J¼7.2 Hz, 2H), 7.21 (d, J¼7.6 Hz, 2H),
6.57e6.66 (m, 1H), 6.18 (d, J¼16.0 Hz, 1H), 4.36 (q, J¼7.2 Hz, 2H),
CDCl3):
d
7.28e7.46 (m, 11H), 6.88 (d, J¼16.0 Hz, 1H), 4.41 (q,
4.25 (q, J¼7.2 Hz, 2H), 1.87 (d, J¼6.8 Hz, 3H), 1.35 (t, J¼6.8 Hz, 3H),
J¼7.2 Hz, 2H), 4.28 (q, J¼7.2 Hz, 2H), 1.40 (t, J¼7.2 Hz, 3H), 1.23 (t,
1.23 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
163.6, 157.8,
J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 163.7,157.9, 151.9, 139.9,
151.8, 139.7, 132.3, 131.8, 130.9, 129.5, 126.8, 122.3, 120.7, 116.8, 61.8,
61.4, 18.7, 14.2, 14.0. MS (EI) m/z: 76, 87, 115, 153, 181, 209, 253, 281,
347, 361, 406. Anal. Calcd for C19H19BrO5: C 56.03, H 4.70, found: C
55.88, H 4.71.
136.1, 132.8, 130.4, 129.3, 128.8, 128.7, 128.6, 128.3, 127.4, 127.0,
124.2, 113.4, 61.8, 61.5, 14.3, 14.0. MS (EI) m/z: 77, 103, 115, 139, 202,
215, 244, 271, 315, 345, 390. Anal. Calcd for C24H22O5: C 73.83, H
5.68, found: C 73.60, H 5.75.
4.2.9. (E)-Diethyl 4-(4-methoxyphenyl)-5-(prop-1-enyl)furan-2,3-
4.2.15. (E)-Dimethyl 5-styryl-4-p-tolylfuran-2,3-dicarboxylate
dicarboxylate (4ah). Yellowish viscous oil. IR (KBr) v 2981, 1732,
(4bm). White solid, mp: 104e106 ꢀC. IR (KBr) v 2951, 1735, 1589,
1608, 1508, 1458, 1377, 1301, 1248, 1178, 1053, 837, 782 cmꢁ1
.
1H
1501, 1443, 1332, 1155, 1059, 966, 815, 753, 697 cmꢁ1 1H NMR
.
NMR (400 MHz, CDCl3):
d
7.25 (d, J¼7.2 Hz, 2H), 6.93 (d, J¼7.2 Hz,
(400 MHz, CDCl3):
3.94 (s, 3H), 3.82 (s, 3H), 2.40 (s, 3H). 13C NMR (100 MHz, CDCl3):
164.2, 158.2, 151.9, 139.7, 138.2, 136.1, 132.7, 129.6, 129.1, 128.8,
128.7, 127.4, 127.2, 127.0, 124.3, 113.5, 52.8, 52.4, 21.3. MS (EI) m/z:
77, 103, 153, 202, 215, 230, 258, 287, 344, 376. Anal. Calcd for
C23H20O5: C 73.39, H 5.36, found: C 73.50, H 5.34.
d
7.24e7.45 (m, 10H), 6.88 (d, J¼16.0 Hz, 1H),
2H), 6.53e6.62 (m, 1H), 6.22 (d, J¼15.6 Hz, 1H), 4.35 (q, J¼7.2 Hz,
2H), 4.25 (q, J¼7.2 Hz, 2H), 3.83 (s, 3H),1.86 (d, J¼6.8 Hz, 3H),1.35 (t,
J¼7.2 Hz, 3H), 1.23 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
d
164.0, 159.4, 157.9, 151.7, 139.2, 131.3, 130.5, 127.3, 122.7, 121.7,
117.3, 114.0, 61.7, 61.3, 55.3, 18.6, 14.2, 14.0. MS (EI) m/z: 77, 89, 114,
128, 184, 212, 269, 297, 358. Anal. Calcd for C20H22O6: C 67.03, H
6.19, found: C 67.12, H 6.09.
4.2.16. (E)-Diethyl 4-(4-chlorophenyl)-5-styrylfuran-2,3-
dicarboxylate (4an). Yellowish viscous oil. IR (KBr) v 2983, 1730,
1587,1538, 1483, 1380,1316,1176,1057, 835, 760, 697 cmꢁ1. 1H NMR
4.2.10. (E)-Diethyl 5-(prop-1-enyl)-4-(thiophen-3-yl)furan-2,3-
dicarboxylate (4ai). Yellowish viscous oil. IR (KBr) v 2982, 1732,
(400 MHz, CDCl3):
d
7.28e7.46 (m, 10H), 6.81 (d, J¼16.0 Hz, 1H),

H. Huang et al. / Tetrahedron 68 (2012) 3135e3144
3141
4.41 (q, J¼7.2 Hz, 2H), 4.28 (q, J¼7.2 Hz, 2H), 1.39 (t, J¼7.2 Hz, 3H),
product was separated by column chromatography to give the
pure product.
1.25 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 163.4, 157.8,
152.0, 140.2, 135.9, 134.5, 133.2, 130.7, 129.0, 128.9, 128.8, 128.7,
127.1, 127.0, 123.0, 113.0, 61.9, 61.6, 14.2, 14.0. HRMS (ESI) calcd
C24H21ClO5 [MþH]þ: 425.1156, found: 425.1162.
4.3.1. (E)-Dimethyl 4-(but-1-enyl)-5-methylfuran-2,3-dicarboxylate
(5bj). Yellowish viscous oil. IR (KBr) v 2967, 1734, 1555, 1446,
1284, 1211, 1095, 1041, 802 cmꢁ1 1H NMR (400 MHz, CDCl3):
.
4.2.17. (E)-Diethyl 4-(4-methoxyphenyl)-5-styrylfuran-2,3-
dicarboxylate (4ao). Yellowish viscous oil. IR (KBr) v 2981, 1730,
1605,1503, 1377, 1304,1241, 1184, 1051, 835, 760, 697 cmꢁ1. 1H NMR
d
6.02e6.04 (d, J¼16.0 Hz, 1H), 5.77e5.84 (m, 1H), 3.86 (s, 3H), 3.82
(s, 3H), 2.33 (s, 3H), 2.09e2.16 (m, 2H), 0.97e1.01 (t, J¼7.2 Hz, 3H).
13C NMR (100 MHz, CDCl3):
164.8, 158.2, 152.5, 139.4, 136.1, 124.9,
d
(400 MHz, CDCl3):
d
7.27e7.46 (m, 8H), 6.96 (d, J¼8.8 Hz, 2H), 6.86
119.5, 116.6, 52.6, 52.2, 26.4, 13.4, 12.7. MS (EI) m/z: 77, 91, 105, 133,
160, 188, 205, 220, 252. Anal. Calcd for C13H16O5: C 61.90, H 6.39,
found: C 62.03, H 6.36.
(d, J¼16.4 Hz, 1H), 4.40 (q, J¼7.2 Hz, 2H), 4.28 (q, J¼7.2 Hz, 2H), 4.08
(q, J¼7.2 Hz, 2H), 1.45 (t, J¼6.8 Hz, 3H), 1.39 (t, J¼7.2 Hz, 3H), 1.25 (t,
J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 163.9, 159.0, 157.9,151.7,
139.7, 136.2, 132.3, 130.6, 128.8, 128.6, 127.5, 126.9, 124.0, 122.4,
114.7, 113.6, 63.5, 61.8, 61.5, 14.8, 14.3, 14.0. HRMS (ESI) calcd
C26H26O6 [MþH]þ: 435.1808, found: 435.1813.
4.3.2. (E)-Diethyl 4-(but-1-enyl)-5-methylfuran-2,3-dicarboxylate
(5aj). Yellowish viscous oil. IR (KBr) v 2982, 1742, 1557, 1460,
1254, 1026, 860, 777 cmꢁ1 1H NMR (400 MHz, CDCl3):
. d 6.08 (d,
J¼16.0 Hz, 1H), 5.82e5.90 (m, 1H), 4.30e4.39 (m, 4H), 2.36 (s, 3H),
4.2.18. Dimethyl 5-(2-methylprop-1-enyl)-4-phenylfuran-2,3-
dicarboxylate (4bp). White solid, mp: 98e100 ꢀC. IR (KBr) v 2989,
1742, 1577, 1500, 1332, 1162, 997, 819, 702 cmꢁ1. 1H NMR (400 MHz,
2.13e2.19 (m, 2H), 1.31e1.38 (m, 6H), 1.03 (t, J¼7.6 Hz, 3H). 13C NMR
(100 MHz, CDCl3):
d 164.5, 157.9, 152.4, 139.5, 135.8, 125.0, 119.3,
116.7, 61.7, 61.2, 26.4, 14.2, 14.1, 13.4, 12.7. MS (EI) m/z: 77, 91, 105,
134, 162, 175, 188, 206, 219, 234, 280. Anal. Calcd for C15H20O5: C
64.27, H 7.19, found: C 64.11, H 7.26.
CDCl3):
d
7.31e7.40 (m, 5H), 5.99 (s, 1H), 3.91 (s, 3H), 3.80 (s, 3H),
164.5, 158.3,
2.12 (s, 3H), 1.89 (s, 3H). 13C NMR (100 MHz, CDCl3):
d
152.8, 143.1, 138.9, 130.9, 129.2, 128.6, 127.9, 126.8, 122.9, 111.3, 52.7,
52.2, 27.6, 20.8. HRMS (ESI) calcd C18H18O5 [MþH]þ: 315.1232,
found: 315.1237.
4.3.3. (E)-Dimethyl 4-(but-1-enyl)furan-2,3-dicarboxylate
(5bs). Yellowish viscous oil. IR (KBr) v 2961, 1739, 1444, 1242,
1100, 1044, 804, 613 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d 7.51 (s, 1H),
4.2.19. Dimethyl 5-(cyclohexylidenemethyl)-4-phenylfuran-2,3-
6.15 (d, J¼15.6 Hz, 1H), 6.00e6.07 (m, 1H), 3.93 (s, 3H), 3.90 (s, 3H),
dicarboxylate (4bq). White solid, mp: 112e114 ꢀC. IR (KBr) v 2968,
2.14e2.21 (m, 2H), 1.05 (t, J¼7.6 Hz, 3H). 13C NMR (100 MHz, CDCl3):
1738, 1601, 1554, 1327, 1158, 1009, 820, 699 cmꢁ1
.
1H NMR
d 164.1, 158.1, 142.7, 141.3, 136.4, 125.4, 123.5, 115.7, 52.7, 52.4, 26.1,
(400 MHz, CDCl3):
3.80 (s, 3H), 2.58e2.69 (m, 2H), 2.18e2.21 (m, 2H), 1.58e1.74 (m,
6H). 13C NMR (100 MHz, CDCl3):
164.6, 158.3, 152.7, 151.1, 138.8,
d
7.32e7.41 (m, 5H), 5.92 (s, 1H), 3.90 (s, 3H),
13.2. MS (EI) m/z: 77, 91, 120, 148, 175, 206, 238. Anal. Calcd for
C12H14O5: C 60.50, H 5.92, found: C 60.27, H 6.10.
d
130.9, 129.2, 128.6, 127.8, 126.8, 123.3, 108.2, 52.7, 52.2, 38.1, 30.8,
28.6, 27.7, 26.3. HRMS (ESI) calcd C21H22O5 [MþH]þ: 355.1545,
found: 355.1543.
4.3.4. (E)-Diethyl 4-(but-1-enyl)furan-2,3-dicarboxylate
(5as). Yellowish viscous oil. IR (KBr) v 2984, 1765, 1540, 1461,
1243, 856, 772, 608 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d 7.48 (s, 1H),
6.12 (d, J¼15.6 Hz, 1H), 5.98e6.05 (m, 1H), 4.30e4.39 (m, 4H),
4.2.20. (E)-3-Phenyl-2-(prop-1-enyl)-6,7-dihydrobenzofuran-4(5H)-
one (4a). White solid, mp: 148e150 ꢀC. IR (KBr) v 2961, 1670, 1443,
1240, 1172, 1058, 1002, 169, 701 cmꢁ1. 1H NMR (400 MHz, CDCl3):
2.11e2.18 (m, 2H), 1.31e1.37 (m, 6H), 1.01 (t, J¼7.2 Hz, 3H). 13C NMR
(100 MHz, CDCl3):
d 163.8, 157.8, 142.7, 141.2, 136.2, 125.2, 123.6,
115.8, 61.8, 61.5, 26.1,14.2,14.1,13.2. MS (EI) m/z: 77, 91,118,146,174,
192, 221, 266. Anal. Calcd for C14H18O5: C 63.15, H 6.81, found: C
63.20, H 6.76.
d
7.30e7.40 (m, 5H), 6.20e6.35 (m, 2H), 2.92 (t, J¼6.4 Hz, 2H), 2.49
(t, J¼6.4 Hz, 2H), 2.16e2.22 (m, 2H), 1.85 (d, J¼6.4 Hz, 3H). 13C NMR
(100 MHz, CDCl3):
d 194.0, 166.3, 149.2, 131.2, 130.2, 127.9, 127.4,
126.8, 120.2, 119.0, 117.8, 38.7, 23.8, 22.4, 18.5. HRMS (ESI) calcd
4.3.5. (E)-Dimethyl 5-methyl-4-(prop-1-enyl)furan-2,3-
C17H16O2 [MþH]þ: 253.1229, found: 253.1238.
dicarboxylate (5bt). Yellowish viscous oil. IR (KBr) v 2956, 1743,
1556, 1444, 1248, 1050, 789 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d 6.09
4.2.21. (E)-6,6-Dimethyl-3-phenyl-2-(prop-1-enyl)-6,7-
dihydrobenzofuran-4(5H)-one (4b). White solid, mp: 151e154 ꢀC. IR
(KBr) v 2958, 1679, 1450, 1239, 1169, 1048, 975, 758, 699 cmꢁ1. 1H
(d, J¼16.0 Hz, 1H), 5.76e5.83 (m, 1H), 3.89 (s, 3H), 3.84 (s, 3H), 2.35
(s, 3H), 1.81 (d, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 164.8,
158.2, 152.4, 139.4, 129.2, 124.8, 119.5, 118.8, 52.7, 52.2, 18.9, 12.7. MS
(EI) m/z: 77, 91, 118, 146, 174, 204, 238. Anal. Calcd for C12H14O5: C
60.50, H 5.92, found: C 60.38, H 6.06.
NMR (400 MHz, CDCl3):
d 7.32e7.43 (m, 5H), 6.22e6.33 (m, 2H),
2.79 (s, 2H), 2.38 (s, 2H), 1.86 (d, J¼5.6 Hz, 3H), 1.17 (s, 6H). 13C NMR
(100 MHz, CDCl3):
d 193.4, 165.5, 149.5, 131.2, 130.2, 127.9, 127.3,
126.8, 119.1, 118.9, 117.8, 53.0, 37.8, 34.9, 26.8, 18.5. MS (EI) m/z: 77,
83, 115, 127, 153, 167, 181, 223, 265, 280. Anal. Calcd for C19H20O2: C
81.40, H 7.19, found: C 81.17, H 7.22.
4.3.6. (E)-Diethyl 5-methyl-4-(prop-1-enyl)furan-2,3-dicarboxylate
(5at). Yellowish viscous oil. IR (KBr) v 2984, 1739, 1556, 1376,
1191, 860, 777 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d
6.10 (d, J¼16.0 Hz,
1H), 5.79e5.88 (m, 1H), 4.29e4.39 (m, 4H), 2.36 (s, 3H), 1.82 (d,
4.3. General procedure for the synthesis of furans 5
J¼6.4 Hz, 3H), 1.31e1.38 (m, 6H). 13C NMR (100 MHz, CDCl3):
d
164.5, 157.8, 152.3, 139.5, 129.0, 124.9, 119.3, 118.8, 61.7, 61.2, 18.9,
The mixture of alkynoate (1, 0.2 mmol), 2-yn-1-ol (2,
0.2 mmol), and DABCO (10 mol %) was stirred in CH2Cl2 (1 mL) at
room temperature for 10 min. And then the solution was evapo-
rated to dryness under reduced pressure. Subsequently, PdCl2
(5 mol %), Bu4NþFꢁ (20 mol %), and DMF (1.5 mL) were added at
60 ꢀC. After completion of the reaction (as monitored by TLC),
5 mL water was added. The aqueous solution was extracted with
diethyl ether (3ꢂ5 mL) and the combined extract was dried with
anhydrous MgSO4. The solvent was removed and the crude
14.2, 14.1, 12.7. MS (EI) m/z: 28, 43, 65, 77, 91, 120, 148, 175, 193, 220,
266. Anal. Calcd for C14H18O5: C 63.15, H 6.81, found: C 63.08, H
6.96.
4.3.7. (E)-Dimethyl 4-(but-1-enyl)-5-phenylfuran-2,3-dicarboxylate
(5bu). Yellowish viscous oil. IR (KBr) v 2956, 1744, 1555, 1438,
1264, 1016, 856, 765 cmꢁ1 1H NMR (400 MHz, CDCl3):
. d 7.71 (d,
J¼7.2 Hz, 2H), 7.39e7.46 (m, 3H), 6.33 (d, J¼16.0 Hz, 1H), 5.99e6.06
(m, 1H), 3.95 (s, 3H), 3.91 (s, 3H), 2.16e2.24 (m, 2H), 1.06 (t,

3142
H. Huang et al. / Tetrahedron 68 (2012) 3135e3144
J¼7.6 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
164.9, 158.3, 152.2,
2H), 1.38e1.47 (m, 2H), 0.85 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz,
139.7, 137.8, 129.4, 129.2, 128.7, 127.3, 126.6, 119.7, 117.1, 52.8, 52.3,
26.5, 13.3. MS (EI) m/z: 77, 91, 105, 139, 152, 165, 195, 223, 267, 282,
314. Anal. Calcd for C18H18O5: C 68.78, H 5.77, found: C 68.91, H 5.67.
CDCl3):
d
164.4, 161.7 (d, J¼244 Hz), 158.2, 153.7, 140.7, 132.5, 129.8
(d, J¼8.0 Hz), 126.2, 122.4, 115.5 (d, J¼21 Hz), 52.5, 52.1, 31.6, 25.5,
23.4, 13.8. MS (EI) m/z: 28, 59, 77, 83, 91, 109, 133, 159, 185, 213, 243,
259, 274, 302, 334. Anal. Calcd for C18H19FO5: C 64.66, H 5.73,
found: C 64.39, H 5.78.
4.3.8. (E)-Dimethyl 5-ethyl-4-(4-fluorostyryl)furan-2,3-
dicarboxylate (5bc0). Yellowish viscous oil. IR (KBr) v 2976, 1743,
1438, 1279, 997, 842 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d
7.37e7.41
4.4.5. Dimethyl 5-(4-chlorobenzyl)-4-propylfuran-2,3-dicarboxylate
(6bd0). Yellowish viscous oil. IR (KBr) v 2959, 2828, 1728, 1554,
1491,1440,1319,1202,1165,1096,1045, 805, 769, 517 cmꢁ1. 1H NMR
(m, 2H), 7.01e7.05 (m, 2H), 6.75 (d, J¼16.8 Hz, 1H), 6.66 (d,
J¼16.8 Hz, 1H), 3.95 (s, 3H), 3.90 (s, 3H), 2.84 (q, J¼7.6 Hz, 2H), 1.32
(t, J¼7.6 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
164.8, 162.5, 158.5,
(400 MHz, CDCl3):
d
7.25 (d, J¼8.4 Hz, 2H), 7.11 (d, J¼8.4 Hz, 2H),
149.6, 137.9, 133.1, 130.4, 127.9, 126.1, 124.5, 118.4, 115.6, 52.8, 52.3,
22.0, 13.5. MS (EI) m/z: 77, 91, 115, 152, 184, 214, 271, 285, 301, 332.
Anal. Calcd for C18H17FO5: C 65.05, H 5.16, found: C 65.21, H 5.09.
3.96 (s, 2H), 3.90 (s, 3H), 3.85 (s, 3H), 2.42 (t, J¼7.6 Hz, 2H),
1.39e1.49 (m, 2H), 0.86 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz,
CDCl3):
d 164.4, 158.2, 153.3, 140.7, 135.3, 129.7, 128.8, 128.0, 126.1,
122.5, 52.5, 52.2, 31.7, 25.4, 23.4, 13.8. MS (EI) m/z: 28, 59, 77, 89,
112, 125, 139, 152, 164, 193, 230, 275, 290, 318, 350. Anal. Calcd for
C18H19ClO5: C 61.63, H 5.46, found: C 61.88, H 5.38.
4.4. General procedure for the synthesis of furans 6
The mixture of alkynoate (1, 0.5 mmol), 2-yn-1-ol (2, 0.5 mmol),
and DABCO (10 mol %) was stirred in CH2Cl2 (2 mL) at room tem-
perature for 10 min. And then the solution was evaporated to
dryness under reduced pressure. Subsequently, PdCl2 (0.2 mol %),
Bu4NþFꢁ (2 mol %), and DMF (2 mL) were added at room temper-
ature. After completion of the reaction (as monitored by TLC), 8 mL
water was added. The aqueous solution was extracted with diethyl
ether (3ꢂ8 mL) and the combined extract was dried with anhy-
drous MgSO4. The solvent was removed and the crude product was
separated by column chromatography to give a pure sample.
4.4.6. Dimethyl 5-(2,6-dichlorobenzyl)-4-propylfuran-2,3-
dicarboxylate (6be0). Yellowish viscous oil. IR (KBr) v 2958, 2872,
1727, 1612, 1554, 1438, 1321, 1201, 1096, 1044, 944, 807, 768 cmꢁ1
.
1H NMR (400 MHz, CDCl3):
d
7.31 (d, J¼8.0 Hz, 2H), 7.15 (t, J¼7.6 Hz,
1H), 4.33 (s, 2H), 3.88 (s, 3H), 3.82 (s, 3H), 2.37 (t, J¼7.6 Hz, 2H),
1.34e1.43 (m, 2H), 0.82 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz,
CDCl3):
d 164.4, 158.2, 150.9, 140.5, 136.1, 132.4, 128.9, 128.4, 126.2,
122.5, 52.4, 52.0, 28.9, 25.4, 23.3, 13.8. MS (EI) m/z: 28, 59, 77, 89,
105, 125, 139, 159, 193, 207, 229, 263, 289, 324, 353, 384. Anal. Calcd
for C18H18Cl2O5: C 56.12, H 4.71, found: C 55.90, H 4.78.
4.4.1. Dimethyl 5-benzyl-4-propylfuran-2,3-dicarboxylate
(6ba0). Yellowish viscous oil. IR (KBr) v 2958, 2872, 1740, 1609,
4.4.7. Dimethyl 5-(4-bromobenzyl)-4-propylfuran-2,3-dicarboxylate
(6bf0). Yellowish viscous oil. IR (KBr) v 2959, 2832, 1730, 1554,
1491, 1439, 1319, 1200, 1165, 1096, 1045, 810, 769, 523, 499 cmꢁ1. 1H
1553, 1441, 1321, 1200, 1098, 1044, 769, 727, 569 cmꢁ1 1H NMR
.
(400 MHz, CDCl3):
d 7.17e7.30 (m, 5H), 3.99 (s, 2H), 3.89 (s, 3H),
3.84 (s, 3H), 2.42 (t, J¼8.0 Hz, 2H), 1.39e1.48 (m, 2H), 0.86 (t,
NMR (400 MHz, CDCl3):
d
7.35 (d, J¼8.0 Hz, 2H), 7.01 (d, J¼8.4 Hz,
J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
164.5, 158.3, 154.0,
2H), 3.90 (s, 2H), 3.85 (s, 3H), 3.81 (s, 3H), 2.38 (t, J¼7.6 Hz, 2H),
140.5, 136.8, 128.7, 128.4, 126.8, 126.2, 122.4, 52.4, 52.1, 32.4, 25.5,
23.4,13.8. MS (EI) m/z: 28, 59, 77, 91,115,141,167,195, 225, 256, 284,
316. Anal. Calcd for C18H20O5: C 68.34, H 6.37, found: C 68.08, H
6.46.
1.35e1.45 (m, 2H), 0.81 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
164.3, 158.2, 153.2, 140.7, 135.8, 131.7, 130.1, 126.1, 122.5, 120.7,
52.4, 52.1, 31.8, 25.4, 23.4, 13.8. MS (EI) m/z: 28, 59, 77, 91, 128, 138,
168, 193, 223, 254, 274, 334, 362, 394. Anal. Calcd for C18H19BrO5: C
54.70, H 4.85, found: C 54.73, H 4.88.
4 . 4 . 2 . D i e t h y l 5 - b e n z y l - 4 - p r o p y l f u r a n - 2 , 3 -
dicarboxylate(6aa0). Yellowish viscous oil. IR (KBr) v 2964, 2872,
1723, 1609, 1552, 1454, 1370, 1312, 1242, 1183, 1095, 1043, 829, 768,
4.4.8. Dimethyl 5-(3-bromobenzyl)-4-propylfuran-2,3-dicarboxylate
(6bg0). Yellowish viscous oil. IR (KBr) v 2958, 2865, 1728, 1554,
725, 698 cmꢁ1
.
1H NMR (400 MHz, CDCl3):
d
7.17e7.30 (m, 5H),
1456, 1320, 1201, 1089, 1044, 809, 768 cmꢁ1 1H NMR (400 MHz,
.
4.30e4.39 (m, 4H), 4.00 (s, 2H), 2.41 (t, J¼7.6 Hz, 2H), 1.39e1.49 (m,
CDCl3): d 7.33e7.39 (m, 2H), 7.09e7.16 (m, 2H), 3.96 (s, 2H), 3.89 (s,
2H), 1.31e1.38 (m, 6H), 0.86 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz,
3H), 3.84 (s, 3H), 2.43 (t, J¼7.6 Hz, 2H), 1.40e1.50 (m, 2H), 0.86 (t,
CDCl3):
d
164.2, 158.0, 153.9, 140.5, 136.9, 128.7, 128.4, 126.8, 126.4,
J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 164.3, 158.1, 152.9,
122.1, 61.5, 61.2, 32.5, 25.5, 23.4, 14.2, 14.1, 13.8. MS (EI) m/z: 28, 43,
65, 77, 91, 105, 115, 141, 167, 207, 224, 241, 252, 269, 298, 344. Anal.
Calcd for C20H24O5: C 69.75, H 7.02, found: C 69.69, H 7.11.
140.8, 139.1, 131.3, 130.2, 129.9, 127.0, 126.1, 122.7, 122.6, 52.4, 52.1,
31.9, 25.4, 23.4, 13.8. MS (EI) m/z: 28, 59, 77, 91, 128, 139, 168, 193,
226, 254, 274, 334, 362, 394. Anal. Calcd for C18H19BrO5: C 54.70, H
4.85, found: C 54.49, H 4.95.
4.4.3. Dimethyl 5-(4-methylbenzyl)-4-propylfuran-2,3-dicarboxylate
(6bb0). Yellowish viscous oil. IR (KBr) v 2957, 2871, 1731, 1553,
4.4.9. Dimethyl 5-(2-bromobenzyl)-4-propylfuran-2,3-dicarboxylate
1440, 1318, 1242, 1162, 1097, 1045, 768, 478 cmꢁ1
.
1H NMR
(6bh0). Yellowish viscous oil. IR (KBr) v 2959, 2867, 1738, 1622,
(400 MHz, CDCl3):
d
7.06e7.11 (m, 4H), 3.96 (s, 2H), 3.90 (s, 3H),
1557, 1442, 1321, 1201, 1087, 1046, 944, 811, 769 cmꢁ1 1H NMR
.
3.85 (s, 3H), 2.42 (t, J¼7.6 Hz, 2H), 2.30 (s, 3H), 1.39e1.49 (m, 2H),
(400 MHz, CDCl3):
d
7.54 (d, J¼8.4 Hz, 1H), 7.22 (t, J¼7.2 Hz, 1H),
0.86 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d
164.6, 158.3,
7.06e7.11 (m, 2H), 4.13 (s, 2H), 3.91 (s, 3H), 3.85 (s, 3H), 2.42 (t,
J¼7.6 Hz, 2H), 1.39e1.48 (m, 2H), 0.85 (t, J¼7.2 Hz, 3H). 13C NMR
154.3, 140.5, 136.4, 133.7, 129.4, 128.3, 126.3, 122.2, 52.4, 52.1, 32.1,
25.5, 23.5, 21.0, 13.8. MS (EI) m/z: 65, 77, 91, 105, 115, 128, 141, 153,
171, 181, 193, 210, 223, 239, 255, 269, 287, 298, 330. Anal. Calcd for
C19H22O5: C 69.09, H 6.71, found: C 68.87, H 6.79.
(100 MHz, CDCl3):
d 164.4, 158.2, 152.4, 140.1, 136.2, 132.8, 130.2,
127.7, 126.2, 124.1, 123.2, 52.5, 52.3, 32.7, 25.5, 23.4, 13.8. MS (EI) m/
z: 28, 59, 77, 90, 115, 139, 168, 193, 223, 251, 274, 283, 334, 362, 394.
Anal. Calcd for C18H19BrO5: C 54.70, H 4.85, found: C 54.53, H 4.92.
4.4.4. Dimethyl 5-(4-fluorobenzyl)-4-propylfuran-2,3-dicarboxylate
(6bc0). Yellowish viscous oil. IR (KBr) v 2959, 2873, 1728, 1604,
1554, 1510, 1440, 1320, 1223, 1161, 1098, 1045, 814, 774, 537,
4.4.10. Dimethyl 4-propyl-5-(4-(trifluoromethyl)benzyl)furan-2,3-
dicarboxylate (6bi0). Yellowish viscous oil. IR (KBr) v 2958, 2872,
1727,1612,1573,1438,1319,1201,1046, 952, 808, 772 cmꢁ1. 1H NMR
493 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d
7.11e7.15 (m, 2H), 6.96 (t,
J¼8.4 Hz, 2H), 3.96 (s, 2H), 3.89 (s, 3H), 3.85 (s, 3H), 2.40 (t, J¼7.6 Hz,
(400 MHz, CDCl3):
d
7.55 (d, J¼8.0 Hz, 2H), 7.31 (d, J¼8.0 Hz, 2H),

H. Huang et al. / Tetrahedron 68 (2012) 3135e3144
3143
4.07 (s, 2H), 3.91 (s, 3H), 3.87 (s, 3H), 2.45 (t, J¼7.6 Hz, 2H),
1.42e1.51 (m, 2H), 0.87 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
7H), 4.35 (q, J¼7.2 Hz, 2H), 4.25 (q, J¼7.2 Hz, 2H), 4.02 (s, 2H),
1.34 (t, J¼7.2 Hz, 3H), 1.22 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz,
d
164.3, 158.2, 152.7, 140.9, 140.8, 129.2 (q, J¼64 Hz), 128.7, 126.1,
CDCl3): d 163.6, 157.7, 154.2, 140.6, 136.6, 131.8, 130.8, 129.5,
125.6 (q, J¼8.0 Hz), 124.2 (CF3, J¼541 Hz), 122.8, 52.4, 52.2, 32.1,
25.4, 23.4, 13.7. MS (EI) m/z: 59, 109, 159, 193, 264, 324, 352, 384.
Anal. Calcd for C19H19F3O5: C 59.37, H 4.98, found: C 59.50, H 4.91.
128.8, 128.3, 126.9, 126.2, 122.6, 122.4, 61.8, 61.4, 32.6, 14.2, 14.0.
MS (EI) m/z: 29, 65, 91, 115, 126, 165, 191, 202, 231, 258, 312, 339,
385, 410, 456. Anal. Calcd for C23H21BrO5: C 70.21, H 4.63, found:
C 70.42, H 4.75.
4.4.11. Dimethyl 5-(4-methoxybenzyl)-4-propylfuran-2,3-
dicarboxylate (6bj0). Yellowish viscous oil. IR (KBr) v 2962, 2872,
4.4.17. Diethyl 5-benzyl-4-(4-methoxyphenyl)furan-2,3-
dicarboxylate (6ao0). Yellowish viscous oil. IR (KBr) v 2983, 2838,
1739, 1605, 1562, 1455, 1371, 1370, 1329, 1155, 1056, 1025, 836, 773,
1739, 1608, 1559, 1442, 1322, 1212, 1096, 949, 810, 774 cmꢁ1 1H
.
NMR (400 MHz, CDCl3):
d
7.08 (d, J¼8.4 Hz, 2H), 6.81 (d, J¼8.8 Hz,
2H), 3.92 (s, 2H), 3.88 (s, 3H), 3.84 (s, 3H), 3.75 (s, 3H), 2.40 (t,
729, 609, 536 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d 7.15e7.29 (m, 7H),
J¼7.6 Hz, 2H), 1.38e1.47 (m, 2H), 0.84 (t, J¼7.2 Hz, 3H). 13C NMR
6.89e6.91 (d, J¼8.8 Hz, 2H), 4.34 (q, J¼7.2 Hz, 2H), 4.26 (q, J¼7.2 Hz,
(100 MHz, CDCl3):
d 164.5, 158.5, 158.3, 154.4, 140.4, 129.4, 128.8,
2H), 4.03 (s, 2H), 3.79 (s, 3H), 1.34 (t, J¼7.2 Hz, 3H), 1.23 (t, J¼7.2 Hz,
126.3, 122.1, 114.1, 55.2, 52.4, 52.1, 31.6, 25.5, 23.4, 13.8. MS (EI) m/z:
28, 59, 77, 91, 108, 115, 121, 128, 141, 171, 187, 197, 227, 255, 285, 314,
346. Anal. Calcd for C19H22O6: C 65.88, H 6.40, found: C 65.72, H
6.44.
3H). 13C NMR (100 MHz, CDCl3):
d 164.0, 159.4, 157.8, 153.9, 140.1,
137.0, 130.3, 128.7, 128.4, 126.8, 123.3, 122.7, 114.1, 61.7, 61.3, 55.2,
32.5, 14.2, 14.0. MS (EI) m/z: 29, 65, 77, 91, 105, 115, 135, 144, 165,
189, 202, 221, 234, 262, 290, 305, 335, 363, 408. Anal. Calcd for
C24H24O6: C 70.57, H 5.92, found: C 70.42, H 5.85.
4.4.12. Dimethyl 5-(furan-2-ylmethyl)-4-propylfuran-2,3-
dicarboxylate (6bk0). Yellowish viscous oil. IR (KBr) v 2958, 2875,
4.4.18. Diethyl 5-benzyl-4-(4-(methoxycarbonyl)phenyl)furan-2,3-
dicarboxylate (6ap0). Yellowish viscous oil. IR (KBr) v 2985, 1740,
1611, 1561, 1439, 1280, 1184, 1195, 1108, 1056, 861, 771, 730, 702,
1727, 1554, 1439, 1319, 1238, 1096, 1044, 944, 807, 588 cmꢁ1 1H
.
NMR (400 MHz, CDCl3):
d
7.31 (d, J¼5.6 Hz, 1H), 6.29 (t, J¼2.0 Hz,
1H), 6.07 (d, J¼3.2 Hz,1H), 4.01 (s, 2H), 3.90 (s, 3H), 3.87 (s, 3H), 2.41
500 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d
8.06 (d, J¼8.4 Hz, 2H), 7.39
(t, J¼7.6 Hz, 2H), 1.41e1.49 (m, 2H), 0.87 (t, J¼7.2 Hz, 3H). 13C NMR
(d, J¼8.4 Hz, 2H), 7.14e7.27 (m, 5H), 4.36 (q, J¼7.2 Hz, 2H), 4.26 (q,
(100 MHz, CDCl3):
d 164.4, 158.2, 151.1, 149.8, 141.8, 140.7, 126.2,
J¼7.2 Hz, 2H), 4.06 (s, 2H), 3.91 (s, 3H), 1.35 (t, J¼7.2 Hz, 3H), 1.21 (t,
122.8, 110.5, 106.9, 52.5, 52.2, 25.7, 25.3, 23.4, 13.7. MS (EI) m/z: 28,
59, 77, 91, 103, 131, 159, 187, 215, 245, 274, 306. Anal. Calcd for
C16H18O6: C 62.74, H 5.92, found: C 62.80, H 5.81.
J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 166.6, 163.5, 157.6,
154.5, 140.7, 136.5, 135.3, 129.9, 129.7, 129.1, 128.8, 128.3, 126.9,
126.2, 122.8, 61.8, 61.5, 52.2, 32.6, 14.2, 13.9. MS (EI) m/z: 29, 59, 65,
77, 91, 115, 129, 163, 191, 202, 231, 249, 259, 290, 317, 333, 363, 390,
405, 436. Anal. Calcd for C25H24O7: C 68.80, H 5.54, found: C 68.58,
H 5.59.
4.4.13. Dimethyl 5-(thiophen-2-ylmethyl)-4-propylfuran-2,3-
dicarboxylate (6bl0). Yellowish viscous oil. IR (KBr) v 2956, 2877,
1727,1552,1438,1316,1247, 1165, 1096, 1044, 807, 697, 589 cmꢁ1. 1H
NMR (400 MHz, CDCl3):
d
7.15 (dd, J¼5.2 Hz, 1H), 6.91 (dd, J¼4.8 Hz,
4.4.19. Diethyl 5-benzyl-4-(4-ethylphenyl)furan-2,3-dicarboxylate
(6aq0). Yellowish viscous oil. IR (KBr) v 2971, 2933, 1732, 1605,
1560, 1507, 1454, 1370, 1327, 1173, 1056, 997, 836, 771, 727, 698,
1H), 6.83e6.84 (m, 1H), 4.17 (s, 2H), 3.90 (s, 3H), 3.87 (s, 3H), 2.43 (t,
J¼7.6 Hz, 2H), 1.42e1.51 (m, 2H), 0.88 (t, J¼7.2 Hz, 3H). 13C NMR
(100 MHz, CDCl3):
d
164.4, 158.2, 152.7, 140.7, 138.8, 127.0, 126.1,
534 cmꢁ1. 1H NMR (400 MHz, CDCl3):
d 7.16e7.29 (m, 9H), 4.35 (q,
125.7, 124.5, 122.4, 52.5, 52.2, 26.9, 25.5, 23.4, 13.8. MS (EI) m/z: 45,
59, 77, 97, 115, 134, 147, 175, 193, 203, 231, 262, 290. 322. Anal. Calcd
for C16H18O5S: C 59.61, H 5.63, found: C 59.39, H 5.71.
J¼7.2 Hz, 2H), 4.26 (q, J¼7.2 Hz, 2H), 4.05 (s, 2H), 2.66 (q, J¼15.2 Hz,
2H), 1.34 (t, J¼7.2 Hz, 3H), 1.20e1.26 (m, 6H). 13C NMR (100 MHz,
CDCl3):
d 164.0, 157.9, 154.0, 144.1, 140.2, 137.0, 129.0, 128.7, 128.4,
128.1, 127.7, 126.8, 126.7, 123.7, 61.7, 61.3, 32.6, 28.6, 15.4, 14.2, 14.0.
MS (EI) m/z: 65, 77, 91,115,127,155,189, 202, 231, 259, 288, 331, 361,
406. Anal. Calcd for C25H26O5: C 73.87, H 6. 45, found: C 73.70, H
6.51.
4.4.14. Dimethyl 5-benzyl-4-phenylfuran-2,3-dicarboxylate
(6bm0). White solid. IR (KBr) v 3030, 2952, 1734, 1556, 1493, 1441,
1337, 1290, 1203, 1157, 1061, 999, 761, 699 cmꢁ1. 1H NMR (400 MHz,
CDCl3):
d
7.26e7.39 (m, 8H), 7.15 (d, J¼7.2 Hz, 2H), 4.05 (s, 2H), 3.88
(s, 3H), 3.78 (s, 3H). 13C NMR (100 MHz, CDCl3):
d
164.3,158.2,154.3,
4.4.20. Diethyl 5-benzyl-4-m-tolylfuran-2,3-dicarboxylate
140.3, 136.8, 131.8, 130.4, 129.0, 128.8, 128.7, 128.4, 128.1, 126.9,
123.8, 52.7, 52.3, 32.6. MS (EI) m/z: 28, 59, 77, 91, 129, 203, 231, 260,
318, 350. Anal. Calcd for C21H18O5: C 71.99, H 5.18, found: C 72.13, H
5.10.
(6ar0). Yellowish viscous oil. IR (KBr) v 2983, 2932, 1740, 1554,
1453, 1371, 1328, 1184, 1059, 1025, 851, 728, 701 cmꢁ1 1H NMR
.
(400 MHz, CDCl3):
d
7.10e7.28 (m, 9H), 4.34 (q, J¼7.2 Hz, 2H), 4.25
(q, J¼7.2 Hz, 2H), 4.04 (s, 2H), 2.33 (s, 3H), 1.33 (t, J¼7.2 Hz, 3H), 1.21
(t, J¼7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3):
d 163.9, 157.8, 154.1,
4.4.15. Diethyl 5-benzyl-4-phenylfuran-2,3-dicarboxylate
140.2, 138.2, 137.0, 130.4, 129.8, 128.8, 128.7, 128.5, 128.4, 126.8,
126.7, 126.1, 123.8, 61.7, 61.3, 32.6, 21.4, 14.2, 14.0. MS (EI) m/z: 29,
65, 78, 91, 105, 115, 128, 143, 165, 189, 202, 218, 231, 246, 274, 289,
317, 346, 392. Anal. Calcd for C24H24O5: C 73.45, H 6.16, found: C
73.19, H 6.22.
(6am0). Yellowish viscous oil. IR (KBr) v 3035, 2958, 1728, 1556,
1493, 1454, 1321, 1279, 1184, 1060, 996, 770, 672 cmꢁ1 1H NMR
.
(400 MHz, CDCl3):
(q, J¼7.2 Hz, 2H), 4.05 (s, 2H), 1.34 (t, J¼7.2 Hz, 3H), 1.20 (t, J¼7.2 Hz,
3H). 13C NMR (100 MHz, CDCl3):
163.9, 157.8, 154.1, 140.3, 136.9,
d
7.15e7.39 (m, 10H), 4.35 (q, J¼7.2 Hz, 2H), 4.24
d
130.6, 129.1, 128.7, 128.6, 128.4, 128.1, 126.8, 126.6, 123.7, 61.7, 61.4,
32.5, 14.2, 14.0. MS (EI) m/z: 51, 65, 77, 91, 105, 129, 165, 191, 202,
231, 259, 275, 305, 332, 378. Anal. Calcd for C23H22O5: C 73.00, H
5.86, found: C 72.88, H 5.91.
4.4.21. Diethyl 5-benzyl-4-o-tolylfuran-2,3-dicarboxylate
(6as0). Yellowish viscous oil. IR (KBr) v 2979, 2933, 1738, 1559,
1443, 1370, 1327, 1176, 1046, 839, 726, 698 cmꢁ1. 1H NMR (400 MHz,
CDCl3):
d
7.11e7.28 (m, 7H), 7.05e7.07 (d, J¼7.2 Hz, 2H), 4.35 (q,
J¼7.2 Hz, 2H), 4.12 (q, J¼7.2 Hz, 2H), 3.84 (q, J¼13.0 Hz, 2H), 2.08 (s,
4.4.16. Diethyl 5-benzyl-4-(4-bromophenyl)furan-2,3-dicarboxylate
3H), 1.34 (t, J¼7.2 Hz, 3H), 1.05 (t, J¼7.2 Hz, 3H). 13C NMR (100 MHz,
(6an0). Yellowish viscous oil. IR (KBr) v 2983, 1737, 1594, 1555,
CDCl3): d 163.2, 157.9, 154.3, 140.7, 137.8, 136.5, 130.7, 129.9, 129.8,
1488, 1370, 1327, 1174, 1056, 996, 830, 772, 729, 702, 526 cmꢁ1
.
128.6, 128.5, 126.8, 126.7, 125.6, 123.1, 61.4, 61.3, 32.7, 19.9, 14.2, 13.7.
MS (EI) m/z: 29, 65, 77, 91, 105, 115, 128, 155, 189, 202, 215, 227, 245,
1H NMR (400 MHz, CDCl3):
d
7.50 (d, J¼8.4 Hz, 2H), 7.12e7.28 (m,

3144
H. Huang et al. / Tetrahedron 68 (2012) 3135e3144
2010, 49, 6669; (m) Liu, Y. H.; Zhou, S. Org. Lett. 2005, 7, 4609; (n) Patil, N. T.;
Wu, H.; Yamamoto, Y. J. Org. Chem. 2005, 70, 4531; (o) Barluenga, J.; Riesgo, L.;
Vicente, R.; Lopez, L. A.; Tomas, M. J. Am. Chem. Soc. 2008, 130, 13528 Zhan, Z. P;
(p) Wang, S. P.; Cai, X. B.; Liu, H. J.; Yu, J. L.; Cui, Y. Y. Adv. Synth. Catal. 2007, 349,
2097.
7. For recent examples of propargyl vinyl ethers, see: (a) Sherry, B. D.; Toste, F. D. J.
Am. Chem. Soc. 2004, 126, 15978; (b) Kim, H.; Lee, C. J. Am. Chem. Soc. 2006, 128,
6336; (c) Mauleon, P.; Krinsky, J. L.; Toste, F. D. J. Am. Chem. Soc. 2009, 131, 4513;
(d) Tejedor, D.; Mendez-Abt, G.; Cotos, L.; Ramirez, M. A.; Garcia-Tellado, F.
Chem.dEur. J. 2011, 17, 3318.
273, 301, 346, 375, 392. Anal. Calcd for C24H24O5: C 73.45, H 6.16,
found: C 73.32, H 6.25.
4.4.22. Dimethyl 5-benzyl-4-(trimethylsilyl)furan-2,3-dicarboxylate
(6bt0). 1H NMR (400 MHz, CDCl3):
d
7.01e7.10 (m, 3H), 6.92e6.94
(d, J¼6.8 Hz, 2H), 3.89 (s, 2H), 3.69 (s, 3H), 3.64 (s, 3H), 0.03 (s, 9H).
13C NMR (100 MHz, CDCl3):
166.4, 162.7, 158.6, 141.3, 137.5, 131.6,
d
129.1, 128.6, 127.2, 116.9, 52.9, 52.6, 35.0, 0.1. MS (EI) m/z: 28, 45, 59,
77, 89, 105, 127, 155, 183, 229, 257, 288, 331, 346.
8. (a) Suhre, M. J.; Reif, M.; Kirsch, S. F. Org. Lett. 2005, 7, 3925; (b) Gille, A.;
Rehbein, J.; Hiersemann, M. Org. Lett. 2011, 13, 2122; (c) David, T.; Leandro, C.;
Fernando, G. T. Org. Lett. 2011, 13, 4422.
9. (a) Sherry, B. D.; Maus, L.; Laforteza, B. N.; Toste, F. D. J. Am. Chem. Soc. 2006, 128,
8132; (b) Menz, H.; Kirsch, S. F. Org. Lett. 2006, 8, 4795; (c) Pastine, S. J.; Youn, S.
W.; Sames, D. Org. Lett. 2003, 5, 1055.
10. (a) Tejedor, D.; Gonzalez-Cruz, D.; Garcia-Tellado, F.; Marrero-Tellado, J. J.;
Rodriguez, M. L. J. Am. Chem. Soc. 2004, 126, 8390; (b) Jorg, T. B.; Kirsch, S. F. Org.
Lett. 2006, 8, 2151; (c) Saito, A.; Oda, S.; Fukaya, H.; Hanzawa, Y. J. Org. Chem.
2009, 74, 1517.
11. (a) Tejedor, D.; Mendez-Abt, G.; Garcia-Tellado, F. Chem.dEur. J. 2010, 16, 428;
(b) Tobias Harschneck, T.; Kirsch, S. F. J. Org. Chem. 2011, 76, 2145; (c) Wei, H.;
Wang, Y.; Yue, B.; Xu, P. F. Adv. Synth. Catal. 2010, 352, 2450.
Acknowledgements
We thank the National Natural Science Foundation of China
(20932002), National Basic Research Program of China (973 Pro-
gram) (2011CB808600), the Doctoral Fund of the Ministry of Edu-
cation of China (20090172110014), and Guangdong Natural Science
Foundation (10351064101000000) for financial support.
12. (a) Cao, H.; Jiang, H. F.; Yao, W. J.; Liu, X. H. Org. Lett. 2009, 11, 1931; (b) Jiang, H.
F.; Yao, W. J.; Cao, H.; Huang, H. W.; Cao, D. R. J. Org. Chem. 2010, 75, 5347; (c)
Cao, H.; Jiang, H. F.; Mai, R. H.; Zhu, S. F.; Qi, C. R. Adv. Synth. Catal. 2010, 352,
143.
13. (a) Wang, A. Z.; Jiang, H. F.; Chen, H. J. J. Am. Chem. Soc. 2009, 131, 3846; (b)
Zhou, P.; Huang, L. B.; Jiang, H. F.; Wang, A. Z.; Li, X. W. J. Org. Chem. 2010, 75,
8279; (c) Chen, H. J.; Jiang, H. F.; Cai, C. B.; Dong, J.; Fu, W. Org. Lett. 2011, 13,
992; (d) Jiang, H. F.; Liu, X. H.; Zhou, L. Chem.dEur. J. 2008, 14, 11305.
14. (a) Lutz, R. P. Chem. Rev. 1984, 84, 205; (b) Ziegler, F. E. Chem. Rev. 1988, 88, 1423;
(c) Martín Castro, A. M. Chem. Rev. 2004, 104, 2939.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2011.12.033. These data in-
clude MOL files and InChiKeys of the most important compounds
described in this article.
References and notes
15. (a) Katritzky, A. R.; Fali, C. N.; Li, J. Q. J. Org. Chem. 1997, 62, 8205; (b) Mross, G.;
Holtz, E.; Langer, P. J. Org. Chem. 2006, 71, 8045; (c) Zhang, X. B.; Lu, Z.; Fu, C. L.;
Ma, S. M. J. Org. Chem. 2010, 75, 2589.
16. (a) Hay, M. B.; Wolfe, J. P. J. Am. Chem. Soc. 2005, 127, 16468; (b) Konnick, M. M.;
Stahl, S. S. J. Am. Chem. Soc. 2008, 130, 5753; (c) Mueller, J. A.; Goller, C. P.;
Sigman, M. S. J. Am. Chem. Soc. 2004, 126, 9724; (d) Schweizer, S.; Song, Z. Z.;
Meyer, F. E.; Parsons, P. J.; de Meijere, A. Angew. Chem., Int. Ed. 1999, 38, 1452; (e)
Min, G. K.; Hernandez, D.; Lindhardt, A. T.; Skrydstrup, T. Org. Lett. 2010, 12,
4716.
1. (a) Lipshutz, B. H. Chem. Rev. 1986, 86, 795; (b) Bur, S. K.; Padwa, A. Chem. Rev.
2004, 104, 2401; (c) Wang, L.; Carrasco, C.; Kumar, A.; Stephens, C. E.; Bailly, C.;
Boykin, D. W.; Wilson, W. D. Biochemistry 2001, 40, 2511.
2. (a) Katritzky, A. R.; Tala, S. R.; Lu, H.; Vakulenko, A. V.; Chen, Q. Y.; Sivapackiam,
J.; Pandya, K.; Jiang, S. B.; Debnath, A. K. J. Med. Chem. 2009, 52, 7631; (b)
Racane, L.; Tralic-Kulenovic, V.; Pavelic, S. K.; Ratkaj, I.; Peixoto, P.; Nhili, R.;
Depauw, S.; Hildebrand, M. P.; David-Cordonnier, M. H.; Pavelic, K.; Karminski-
Zamola, G. J. Med. Chem. 2010, 53, 2418; (c) Rawal, R. K.; Prabhakar, Y. S.; Katti,
S. B.; De Clercq, E. Bioorg. Med. Chem. 2005, 13, 6771; (d) Flynn, B. L.; Hamel, E.;
Jung, M. K. J. Med. Chem. 2002, 45, 2670; (e) Kanokmedhakul, S.;
Kanokmedhakul, K.; Kantikeaw, I.; Phonkerd, N. J. Nat. Prod. 2006, 69, 68.
3. For recent examples, see: (a) Fructos, M. R.; Alvarez, E.; Diaz-Requejo, M. M.;
Perez, P. J. J. Am. Chem. Soc. 2010, 132, 4600; (b) Lohse, A. G.; Krenske, E. H.;
Antoline, J. E.; Houk, K. N.; Hsung, R. P. Org. Lett. 2010, 12, 5506; (c) Mao, Y. L.;
Mathey, F. Org. Lett. 2010, 12, 3384; (d) Noutsias, D.; Kouridaki, A.; Vassiliko-
giannakis, G. Org. Lett. 2011, 13, 1166; (e) Singh, R. P.; Foxman, B. M.; Deng, L. J.
Am. Chem. Soc. 2010, 132, 9558.
17. Mansilla, H.; Afonso, M. M. Synth. Commun. 2008, 38, 2607.
18. For examples for endocyclic furan derivatives, see: (a) Plouvier, B.; Beatch, G. N.;
Jung, G. L.; Zolotoy, A.; Sheng, T.; Clohs, L.; Barrett, T. D.; Fedida, D.; Wang, W.
Q.; Zhu, J. J.; Liu, Y. Z.; Abraham, S.; Lynn, L.; Dong, Y.; Wall, R. A.; Walker, M. J. A.
J. Med. Chem. 2007, 50, 2818; (b) Shimada, I.; Maeno, K.; Kazuta, K. I.; Kubota, H.;
Kimizuka, T.; Kimura, Y.; Hatanaka, K. I.; Naitou, Y.; Wanibuchi, F.; Sakamoto, S.;
Tsukamoto, S. I. Bioorg. Med. Chem. 2008, 16, 1966; (c) Yadav, P. P.; Gupta, P.;
Chaturvedi, A. K.; Shukla, P. K.; Maurya, R. Bioorg. Med. Chem. 2005, 13, 1497.
19. For palladium-catalyzed reactions of allenes to 1,3-dienes, see: (a) Zimmer, R.;
Dinesh, C. U.; Nandanan, E.; Khan, F. A. Chem. Rev. 2000, 100, 3067; (b) Coulson,
D. R. J. Org. Chem. 1973, 38, 1483; (c) Moriya, T.; Furuuchi, T.; Miyaura, N.;
4. (a) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. Rev. 2002, 102, 3615;
(b) Roucoux, A.; Schulz, J.; Patin, H. Chem. Rev. 2002, 102, 3757.
ꢀ
Suzuki, A. Tetrahedron 1994, 50, 7961; (d) Ahmar, M.; Cazes, B.; Gore, J. Tetra-
hedron Lett. 1984, 25, 4505; (e) Shimizu, I.; Sugiura, I.; Tsuji, J. J. Org. Chem. 1985,
50, 537; (f) Al-Masum, M.; Yamamoto, Y. J. Am. Chem. Soc. 1998, 120, 3809; (g)
Chang, H. M.; Cheng, C. H. J. Org. Chem. 2000, 65, 1767.
5. For recent examples, see (a) Rakshit, S.; Patureau, F. W.; Glorius, F. J. Am. Chem.
Soc. 2010, 132, 9585; (b) Aubin, Y.; Fischmeister, C.; Thomas, C. M.; Renaud, J. L.
Chem. Soc. Rev. 2010, 39, 4130; (c) Houlden, C. E.; Hutchby, M.; Bailey, C. D.;
Ford, J. G.; Tyler, S. N. G.; Gagne, M. R.; Lloyd-Jones, G. C.; Booker-Milburn, K. I.
Angew. Chem., Int. Ed. 2009, 48, 1830; (d) Stuart, D. R.; Bertrand-Laperle, M.;
Burgess, K. M. N.; Fagnou, K. J. Am. Chem. Soc. 2008, 130, 16474.
6. For selected recent cycloisomerization approaches to substituted furans, see (a)
Dudnik, A. S.; Gevorgyan, V. Angew. Chem., Int. Ed. 2007, 46, 5195; (b) Sromek,
A. W.; Rubina, M.; Gevorgyan, V. J. Am. Chem. Soc. 2005, 127, 10500; (c) Ma, S.
M.; Yu, Z. Angew. Chem., Int. Ed. 2002, 41, 1775; (d) Ma, S. M.; Zhang, J. L.; Lu, L.
Chem.dEur. J. 2003, 9, 2447; (e) Kim, J. T.; Kel’in, A. V.; Gevorgyan, V. Angew.
Chem., Int. Ed. 2003, 42, 98; (f) Schwier, T.; Sromek, A. W.; Yap, D. M. L.;
Chernyak, D.; Gevorgyan, V. J. Am. Chem. Soc. 2007, 129, 9868; (g) Sromek, A. W.;
Kel’in, A. V.; Gevorgyan, V. Angew. Chem., Int. Ed. 2004, 43, 2280; (h) Kel’in, A.
V.; Gevorgyan, V. J. Org. Chem. 2002, 67, 95; (i) Yao, T. L.; Zhang, X. X.; Larock, R.
C. J. Am. Chem. Soc. 2004, 126, 11164; (j) Yao, T. L.; Zhang, X. X.; Larock, R. C. J.
Org. Chem. 2005, 70, 7679; (k) Xiao, Y. J.; Zhang, J. L. Angew. Chem., Int. Ed. 2008,
47, 1903; (l) Liu, F.; Qian, D. Y.; Li, L.; Zhao, X. L.; Zhang, J. L. Angew. Chem., Int. Ed.
20. (a) Francesco, B.; Stefania, R. C.; Gianluca, M. F.; Linda, C. L.; Francesco, N.; Vita,
P. Chem. Commun. 2007, 3756; (b) Yim, H. K.; Liao, Y.; Wong, H. N. C. Tetrahedron
2003, 59, 1877; (c) Yick, C. Y.; Wong, H. N. C. Tetrahedron 2001, 57, 6935.
21. Fan, M. J.; Li, G. Q.; Liang, Y. M. Tetrahedron 2006, 62, 6782.
22. For reviews, see: (a) Tsuji, J. Organic Synthesis with Palladium Compounds;
Springer-Veerlag: Berlin, 1980; (b) Tsuji, J. Palladium Reagents and Catalysts:
Innovations in Organic Synthesis; Wiley: New York, NY, 1995; (c) Stahl, S. S.
Angew. Chem., Int. Ed. 2004, 43, 3400.
23. For recent examples for palladium-catalyzed reactions with oxygen, see: (a)
Stahl, S. S. Science 2005, 309, 1824; (b) Aouf, C.; Thiery, E.; Le Bras, J.; Muzart, J.
Org. Lett. 2009, 11, 4096; (c) Han, W.; Mayer, P.; Ofial, A. R. Angew. Chem., Int. Ed.
2011, 50, 2178; (d) Li, B. H.; Tian, S. L.; Fang, Z.; Shi, Z. H. Angew. Chem., Int. Ed.
2008, 47, 1115; (e) Shi, Z. Z.; Zhang, C.; Li, S.; Pan, D. L.; Ding, S. T.; Cui, Y. X.; Jiao,
N. Angew. Chem., Int. Ed. 2009, 48, 4572; (f) Wang, D. H.; Wasa, M.; Giri, R.; Yu, J.
Q. J. Am. Chem. Soc. 2008, 130, 7190.