New synthesis of 3-aryl-2,5-dihydrofurans

Article abstract of DOI:10.3987/COM-06-10795

We present a straightforward synthesis of 3-aryl-2,5-dihydrofurans by ring contraction of 4-aryl-3,6-dihydro-2H-pyrans with the repeated treatment of MCPBA and BF₃-OEt₂. The building block 3-aryltetrahydrofuran-3-carboxylic acid with

Full text of DOI:10.3987/COM-06-10795

HETEROCYCLES, Vol. 68, No. 9, 2006
1941
HETEROCYCLES, Vol. 68, No. 9, 2006, pp. 1941 - 1948. © The Japan Institute of Heterocyclic Chemistry
Received, 26th May, 2006, Accepted, 10th July, 2006, Published online, 11th July, 2006. COM-06-10795
NEW SYNTHESIS OF 3-ARYL-2,5-DIHYDROFURANS
Meng-Yang Chang,*,a Chun-Yu Lin,b and Chun-Li Paib
aDepartment of Applied Chemistry, National University of Kaohsiung,
b
Kaohsiung 811, Taiwan. Department of Chemistry, National Sun Yat-Sen
University, Kaohsiung 804, Taiwan. Email: mychang@nuk.edu.tw
Abstract – We present a straightforward synthesis of 3-aryl-2,5-dihydrofurans by
ring contraction of 4-aryl-3,6-dihydro-2H-pyrans with the repeated treatment of
MCPBA and BF3-OEt2. The building block 3-aryltetrahydrofuran-3-carboxylic
acid with potential biological activities was also prepared.
1. INTRODUCTION
Many natural products and biological active compounds contain five-membered oxygen heterocyclic
system.1-2 Molecules incorporating either mono-, di-, or poly-substituted furans, dihydrofurans,
tetrahydrofurans and other related analogs are well documented in the literature and these often served as
key building blocks. A novel and facile preparation of a variety type of substituted furans, dihydrofurans,
and tetrahydrofurans remains a current challenge in the organic synthesis. Development of a general and
novel procedure for differently substituted five-membered oxygen heterocyclic system provides an
expedient entry as shown in Figure 1.
intramolecular
ring closure
ring-closing
metathesis
R
R
R
HO
OH
O
O
metal ion induced
cyclization
our work
R
R
H2C C
HO
O
Figure 1. Some synthetic methods of 3-substituted dihydrofurans
Basically, the synthetic methods of 3-substituted dihydrofurans and its related analogs can be summarized
in transition metal-promoted ring-closing cyclization and cycloisomerization or metathesis e.g.

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HETEROCYCLES, Vol. 68, No. 9, 2006
palladium,3 silver,4 rhodium,5 gold,6 copper,7 ruthenium,8 mercury,9 and zirconium.10 Here we describe a
straightforward strategy to 3-aryldihydrofurans by ring contraction of 4-aryl-3,6-dihydro-2H-pyrans with
m-chloroperoxybenzoic acid (MCPBA) and boron trifluoride etherate (BF3-OEt2).
2. RESULTS AND DISCUSSION
Commercially available tetrahydro-4H-pyran-4-one (1) was chosen as the starting material for the
preparation of 3-aryl-2,5-dihydrofurans. Some representative results are shown in Scheme 1 and Scheme
2. 3-Aryltetrahydrofuran-3-carbaldehydes (3a-3e) were prepared by two one-pot transformations. The
first one is the easy access to produce 4-aryl-3,6-dihydro-2H-pyrans (2a-2e) by the Grignard addition of
compound (1) with arylmagnesium bromides (a, Ar=C6H5; b, Ar=3-MeOC6H4; c, Ar=4-MeOC6H4; d,
Ar=3,4-CH2O2C6H3; e, Ar=4-C6H5C6H4) in tetrahydrofuran at -78 oC for 2 h and subsequent dehydration
of the resulting tertiary alcohols with boron trifluoride etherate for 15 min. The second step is a ring
contraction from 4-aryl-3,6-dihydro-2H-pyrans (2a-2e) to 3-aryltetrahydrofuran-3-carbaldehydes (3a-3e)
by epoxidation with MCPBA at rt for 3 h followed by rearrangement reaction of the resulting epoxides
with BF3-OEt2 for 15 min. Thus, 3-aryltetrahydrofuran-3-carbaldehyde (3) could be obtained as a sole
product in good yield via these two one-pot reactions, that is, Grignard reaction/dehydration and
epoxidation/ring contraction.
O
Ar
O
Ar
ArMgBr, THF
then BF3-OEt2
MCPBA, DCM
then BF3-OEt2
CHO
O
O
1
2a, Ar=C6H5(78%)
3a (85%)
3b (88%)
3c (87%)
3d (82%)
3e (90%)
2b, Ar=3-MeOC6H4 (82%)
2c, Ar=4-MeOC6H4 (76%)
2d, Ar=3,4-CH2O2C6H3 (80%)
2e, Ar=4-C6H5C6H4 (83%)
Scheme 1. Synthesis of 3-aryltetrahydrofuran-3-carbaldehydes (3a-3e)
Lewis acid-mediated ring contraction of six-membered ring framework was already investigated from the
literature reports.11-13 There are some different results between 4-aryl-3,4-epoxypiperidine and
1-aryl-1,2-epoxycyclohexane. According to Nagai’s11a and Lyle’s reports,11b different N-substitutents
could affect the distribution of several products in the reaction of 4-aryl-3,4-epoxypiperidine derivatives.
For the rearrangement reaction of 1-aryl-1,2-epoxycyclohexane, Schiketanz12a and Macchia12b-d reported
totally different results in the reactivity and the structures of products. These interesting differences
triggered our attention to examine the rearrangement of 4-aryl-3,4-epoxydihydropyran skeleton.13 With
enough amounts of compounds (3a-3e) in hand, synthesis of 3-aryl-2,5-dihydrofurans (4a-4e) and

HETEROCYCLES, Vol. 68, No. 9, 2006
1943
3-aryltetrahydrofuran-3-carboxylic acids (5a-5e) was further studied. Conversion of compounds (3a-3e)
into 4a-4e was achieved in good yield by the repeated combination of MCPBA and BF3-OEt2 via
Baeyer-Villiger oxidation and formic acid elimination. We also tried to prepare
3-methyl-2,5-dihydrofuran by repeated treatment of 4-methyl-3,6-dihydro-2H-pyran with the
combination of MCPBA and BF3-OEt2, but complex products were provided. Although the synthetic
application is decreased, the present work is complementary to existing methodology. While poring out
the related literature of tetrahydrofuran-3-carboxylic acid, we found that it was a useful building block in
the synthesis of various potential biological compounds.14 Compounds (5a-5e) were afforded in nearly
quantitative yield by oxidation of compounds (3a-3e) with sodium chlorite (NaClO2).15
Ar
Ar
Ar
1) MCPBA, DCM
2) BF3-OEt2
NaClO2
CO2H
CHO
O
O
O
5a (98%)
5b (98%)
5c (97%)
5d (99%)
5e (99%)
3
4a (70%)
4b (75%)
4c (78%)
4d (76%)
4e (80%)
Scheme 2. Synthesis of 3-aryl 2,5-dihydrofurans (4a-4e) and tetrahydrofuran-3-carboxylic acids (5a-5e)
3. CONCLUSION
In summary, we have developed an easy and straightforward synthesis of 3-aryl-2,5-dihydrofurans (4a-4e)
and 3-aryltetrahydrofuran-3-carboxylic acids (5a-5e) via the reaction of 4-aryl-3,6-dihydro-2H-pyrans
(2a-2e) with the repeated treatment of MCPBA and BF3-OEt2.
4. EXPERIMENTAL
4.1. General. Tetrahydrofuran (THF) was distilled prior to use from a deep-blue solution of
sodium-benzophenone ketyl. All other reagents were obtained from commercial sources and used without
further purification. Reaction was routinely carried out under an atmosphere of dry nitrogen with
magnetic stirring. Extract was dried with anhydrous MgSO4 before concentration in vacuo. Crude product
was purified using column chromatography on SiO2 (MN Kieselgel 60, 70~230 mesh).
4.2. A representative procedure for 4-aryl-3,6-dihydro-2H-pyrans (2a-2e) is as follows: A solution of
arylmagnesium bromide (0.5 M in THF, 0.6 mL, 1.2 mmol) was added to a stirred solution of
tetrahydro-4H-pyran-4-one (1) (100 mg, 1.0 mmol) in THF (10 mL) at -78 oC. The reaction mixture was
stirred at rt for 2 h. The total procedure was monitored by TLC until the reaction was completed. Without

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further purification, BF3-OEt2 (1 mL) was added to a stirred solution of the resulting reaction mixture at 0
oC. The reaction mixture was stirred at rt for 15 min. Saturated aqueous NaHCO3 solution (1 mL) was
added to the reaction mixture and the solvent was removed under reduced pressure. Water (2 mL) and
AcOEt (10 mL) was added to the residue and the mixture was filtered through a short plug of Celite. The
filtrate was concentrated under reduced pressure. The residue was extracted with AcOEt (3 x 30 mL). The
combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product.
Purification by column on SiO2 (hexane/AcOEt = 6/1) afforded 4-aryl-3,6-dihydro-2H-pyrans (2a-2e).
For 2a: 1H NMR (300 MHz, CDCl3) δ 7.45-7.22 (m, 5H), 6.16 (br s, 1H), 4.18 (br s, 2H), 3.96 (t, J = 7.5
Hz, 2H), 2.52 (br s, 2H); HRMS (ESI) m/z calcd for C11H13O (M++1) 161.0966, found 161.0968; Anal.
1
Calcd for C11H12O: C, 82.46; H, 7.55. Found: C, 82.60; H, 7.38. For 2b: H NMR (300 MHz, CDCl3) δ
7.28 (t, J = 8.1 Hz, 1H), 6.96 (d, J = 8.1 Hz, 1H), 6.95 (br s, 1H), 6.82 (dd, J = 2.4, 8.1 Hz, 1H), 6.18 (br s,
1H), 4.35 (br s, 2H), 3.91 (t, J = 7.5 Hz, 2H), 3.81 (s, 3H), 2.50 (br s, 2H); HRMS (ESI) m/z calcd for
C12H15O2 (M++1) 191.1072, found 191.1076; Anal. Calcd for C12H14O2: C, 75.76; H, 7.42. Found: C,
75.61; H, 7.36. For 2c: 1H NMR (300 MHz, CDCl3) δ 7.37 (d, J = 8.4 Hz, 2H), 6.88 (d, J = 8.4 Hz, 2H),
6.02 (br s, 1H), 4.30 (br s, 2H), 3.93 (t, J = 7.5 Hz, 2H), 3.79 (s, 3H), 2.46 (br s, 2H); HRMS (ESI) m/z
calcd for C12H15O2 (M++1) 191.1072, found 191.1078; Anal. Calcd for C12H14O2: C, 75.76; H, 7.42.
Found: C, 75.94; H, 7.66. For 2d: 1H NMR (300 MHz, CDCl3) δ 6.89 (d, J = 1.8 Hz, 1H), 6.76 (d, J = 8.1
Hz, 1H), 6.72 (dd, J = 1.8, 8.1 Hz, 1H), 5.99 (br s, 1H), 5.95 (s, 2H), 4.31 (br s, 2H), 3.91 (br s, 2H), 2.44
(br s, 2H); HRMS (ESI) m/z calcd for C12H13O3 (M++1) 205.0865, found 205.0870; Anal. Calcd for
1
C12H12O2: C, 70.57; H, 5.92. Found: C, 70.21; H, 5.66. For 2e: H NMR (300 MHz, CDCl3) δ 7.62-7.58
(m, 4H), 7.50-7.39 (m, 4H), 7.36-7.32 (m, 1H), 6.10 (br s, 1H), 4.38 (br s, 2H), 3.97 (t, J = 7.2 Hz, 2H),
2.58 (br s, 2H); HRMS (ESI) m/z calcd for C17H17O (M++1) 237.1280, found 237.1283; Anal. Calcd for
C17H16O: C, 86.40; H, 6.82. Found: C, 86.11; H, 6.47.
4.3. A representative procedure for 3-aryltetrahydrofuran-3-carbaldehydes (3a-3e) is as follows:
MCPBA (255 mg, 75%, 1.1 mmol) was added to a mixture of 3-aryltetrahydrofuran-3-carbaldehydes
(2a-2e) (0.5 mmol) and Na2CO3 (130 mg, 1.2 mmol) in CH2Cl2 (10 mL). The reaction mixture was stirred
at rt for 3 h. The total procedure was monitored by TLC until the reaction was completed. Saturated
aqueous Na2CO3 solution (5 mL) was added to the reaction mixture and the solvent was removed under
reduced pressure. The residue was extracted with AcOEt (3 x 20 mL). The combined organic layers were
washed with brine, dried, filtered and evaporated to afford crude product. Without further purification,
BF3-OEt2 (1 mL) was added to a stirred solution of crude product in CH2Cl2 (10 mL) at rt. The reaction
mixture was stirred at rt for 15 min. Saturated aqueous NaHCO3 solution (5 mL) was added to the
reaction mixture and the solvent was concentrated under reduced pressure. The residue was extracted with

HETEROCYCLES, Vol. 68, No. 9, 2006
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AcOEt (3 x 20 mL). The combined organic layers were washed with brine, dried, filtered and evaporated
to afford crude product. Purification by column on SiO2 (hexane/AcOEt = 6/1~4/1) afforded
1
3-aryltetrahydrofuran-3-carbaldehydes (3a-3e). For 3a: H NMR (300 MHz, CDCl3) δ 9.45 (s, 1H),
7.32-7.23 (m, 3H), 7.18-7.10 (m, 2H), 4.59 (d, J = 8.4 Hz, 1H), 3.95-3.86 (m, 2H), 3.81 (d, J = 8.4 Hz,
1H), 2.83-2.75 (m, 1H), 2.21-2.10 (m, 1H); HRMS (ESI) m/z calcd for C11H13O2 (M++1) 177.0916, found
1
177.0915; Anal. Calcd for C11H12O2: C, 74.98; H, 6.86. Found: C, 75.11; H, 6.59. For 3b: H NMR (300
MHz, CDCl3) δ 9.56 (s, 1H), 7.30 (t, J = 8.1 Hz, 1H), 6.86 (d, J = 8.1 Hz, 1H), 6.78 (dd, J = 2.1, 8.1 Hz,
1H), 6.71 (br s, 1H), 4.61 (d, J = 8.4 Hz, 1H), 3.96-3.83 (m, 3H), 3.83 (s, 3H), 2.88-2.80 (m, 1H),
2.28-2.09 (m, 1H); HRMS (ESI) m/z calcd for C12H15O3 (M++1) 207.1021, found 207.1023; Anal. Calcd
1
for C12H14O3: C, 69.88; H, 6.84. Found: C, 70.03; H, 7.05. For 3c: H NMR (300 MHz, CDCl3) δ 9.48 (s,
1H), 7.09 (d, J = 8.4 Hz, 2H), 6.89 (d, J = 8.4 Hz, 2H), 4.61 (d, J = 8.1 Hz, 1H), 3.96-3.84 (m, 3H), 3.85
(s, 3H), 2.86-2.78 (m, 1H), 2.24-2.06 (m, 1H); HRMS (ESI) m/z calcd for C12H15O3 (M++1) 207.1021,
1
found 207.1025; Anal. Calcd for C12H14O3: C, 69.88; H, 6.84. Found: C, 69.61; H, 6.59. For 3d: H NMR
(300 MHz, CDCl3) δ 9.43 (br s, 1H), 6.78 (d, J = 1.2 Hz, 1H), 6.62-6.45 (m, 2H), 5.98 (s, 2H), 4.59 (d, J
= 8.4 Hz, 1H), 3.98-3.83 (m, 2H), 3.80 (d, J = 8.4 Hz, 1H), 2.84-2.74 (m, 1H), 2.23-2.08 (m, 1H); HRMS
(ESI) m/z calcd for C12H13O4 (M++1) 221.0814, found 221.0816; Anal. Calcd for C12H12O4: C, 65.45; H,
1
5.49. Found: C, 65.68; H, 5.28. For 3e: H NMR (300 MHz, CDCl3) δ 9.58 (s, 1H), 7.61-7.58 (m, 4H),
7.50-7.41 (m, 4H), 7.38-7.34 (m, 1H), 4.63 (d, J = 8.4 Hz, 1H), 4.03-3.96 (m, 2H), 3.92 (d, J = 8.4 Hz,
1H), 2.98-2.83 (m, 1H), 2.38-2.22 (m, 1H); HRMS (ESI) m/z calcd for C17H17O2 (M++1) 253.1229, found
253.1233; Anal. Calcd for C17H16O2: C, 80.93; H, 6.39. Found: C, 81.25; H, 6.50.
4.4. A representative procedure for 3-aryl-2,5-dihydrofurans (4a-4e) is as follows: MCPBA (255 mg,
75%, 1.1 mmol) was added to a mixture of 3-aryltetrahydrofuran-3-carbaldehydes (3a-3e) (0.5 mmol) and
Na2CO3 (130 mg, 1.2 mmol) in CH2Cl2 (10 mL). The reaction mixture was stirred at rt for 3 h. The total
procedure was monitored by TLC until the reaction was completed. Saturated aqueous Na2CO3 solution
(5 mL) was added to the reaction mixture and the solvent was removed under reduced pressure. The
residue was extracted with AcOEt (3 x 20 mL). The combined organic layers were washed with brine,
dried, filtered and evaporated to afford crude product. Without further purification, BF3-OEt2 (1 mL) was
added to a stirred solution of crude product in CH2Cl2 (10 mL) at rt. The reaction mixture was stirred at rt
for 15 min. Saturated aqueous NaHCO3 solution (5 mL) was added to the reaction mixture and the solvent
was concentrated under reduced pressure. The residue was extracted with AcOEt (3 x 20 mL). The
combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product.
Purification by column on SiO2 (hexane/AcOEt = 10/1~8/1) afforded 3-aryl-2,5-dihydrofurans (4a-4e).
For 4a: 1H NMR (500 MHz, CDCl3) δ 7.38-7.27 (m, 5H), 6.24 (t, J = 2.0 Hz, 1H), 5.02 (dd, J = 2.0, 5.0

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HETEROCYCLES, Vol. 68, No. 9, 2006
13
Hz, 2H), 4.86 (dd, J = 2.0, 5.0 Hz, 2H); C NMR (125 MHz, CDCl3) δ 138.44, 132.46, 128.60 (2x),
127.99, 125.74 (2x), 120.43, 76.75, 75.35; HRMS (ESI) m/z calcd for C10H11O (M++1) 147.0810, found
1
147.0813; Anal. Calcd for C10H10O: C, 82.16; H, 6.89. Found: C, 82.44; H, 6.68. For 4b: H NMR (500
MHz, CDCl3) δ 7.27 (t, J = 8.0 Hz, 1H), 6.94 (d, J = 8.0 Hz, 1H), 6.88 (br s, 1H), 6.85 (dd, J = 2.5, 8.0
Hz, 1H), 6.23 (t, J = 2.0 Hz, 1H), 5.00 (dd, J = 2.0, 5.0 Hz, 2H), 4.85 (dd, J = 2.0, 5.0 Hz, 2H), 3.83 (s,
3H); 13C NMR (125 MHz, CDCl3) δ 159.70, 138.37, 133.81, 129.60, 120.90, 118.32, 113.23, 111.58,
76.71, 75.38, 55.24; HRMS (ESI) m/z calcd for C11H13O2 (M++1) 177.0916, found 177.0916; Anal. Calcd
for C11H12O2: C, 74.98; H, 6.86. Found: C, 74.61; H, 6.70. For 4c: 1H NMR (500 MHz, CDCl3) δ 7.28 (d, J
= 9.0 Hz, 2H), 6.94 (d, J = 8.5 Hz, 2H), 6.09 (t, J = 2.0 Hz, 1H), 4.98 (dd, J = 2.0, 5.0 Hz, 2H), 4.85 (dd,
13
J = 2.0, 5.0 Hz, 2H), 3.80 (s, 3H); C NMR (125 MHz, CDCl3) δ 159.35, 137.84, 129.98 (2x), 125.27,
118.23, 113.99 (2x), 76.79, 75.43, 55.28; HRMS (ESI) m/z calcd for C11H13O2 (M++1) 177.0916, found
1
177.0917; Anal. Calcd for C11H12O2: C, 74.98; H, 6.86. Found: C, 74.73; H, 6.62. For 4d: H NMR (500
MHz, CDCl3) δ 6.90 (d, J = 1.5 Hz, 1H), 6.78 (d, J = 8.0 Hz, 1H), 6.74 (dd, J = 1.5, 8.0 Hz, 1H), 6.07 (t,
13
J = 2.0 Hz, 1H), 5.97 (s, 2H), 4.95 (dd, J = 2.0, 5.0 Hz, 2H), 4.84 (dd, J = 2.0, 5.0 Hz, 2H); C NMR
(125 MHz, CDCl3) δ 147.94, 147.41, 137.96, 126.77, 119.49, 118.98, 108.27, 106.04, 101.13, 76.71,
75.44; HRMS (ESI) m/z calcd for C11H11O3 (M++1) 191.0708, found 191.0709; Anal. Calcd for C11H10O3:
1
C, 69.46; H, 5.30. Found: C, 69.68; H, 4.98. For 4e: H NMR (500 MHz, CDCl3) δ 7.62-7.59 (m, 4H),
7.48-7.41 (m, 4H), 7.38-7.35 (m, 1H), 6.28 (t, J = 2.0 Hz, 1H), 5.06 (dd, J = 2.0, 5.0 Hz, 2H), 4.89 (dd, J
= 2.0, 5.0 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 140.73, 140.46, 138.07, 131.41, 128.82 (2x), 127.46,
127.26 (2x), 126.93 (2x), 125.17 (2x), 120.55, 76.81, 75.36; HRMS (ESI) m/z calcd for C16H15O (M++1)
223.1123, found 223.1124; Anal. Calcd for C16H14O: C, 86.45; H, 6.35. Found: C, 86.68; H, 6.20.
4.5. A representative procedure for 3-aryltetrahydrofuran-3-carboxylic acids (5a-5e) is as follows:
A solution of 3-aryl-2,5-dihydrofurans (4a-4e) (0.3 mmol) and 2-methyl-2-butene (chlorine scavenger)
(0.5 mL) in t-BuOH (10 mL) was treated with a solution of NaClO2 (80%, 110 mg, 1.0 mmol) and
KH2PO4 (95 mg, 0.7 mmol) in water (5 mL) at rt. The mixture was stirred for an additional 30 min and
the solvent was removed under reduced pressure. The residue was extracted with AcOEt (3 x 20 mL). The
combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product.
Purification by column on SiO2 (hexane/AcOEt = 1/1) afforded 3-aryltetrahydrofuran-3-carboxylic acids
(5a-5e). For 5a: 1H NMR (300 MHz, CDCl3) δ 7.37-7.25 (m, 5H), 4.74 (d, J = 8.1 Hz, 1H), 4.02-3.97 (m,
2H), 3.89 (d, J = 8.1 Hz, 1H), 3.03-2.95 (m, 1H), 2.21-2.03 (m, 1H); HRMS (ESI) m/z calcd for C11H13O2
(M++1) 193.0865, found 193.0868; Anal. Calcd for C11H12O3: C, 68.74; H, 6.29. Found: C, 68.92; H,
1
6.51. For 5b: H NMR (300 MHz, CDCl3) δ 6.92-6.90 (m, 4H), 4.70 (d, J = 8.1 Hz, 1H), 4.06-3.97 (m,
2H), 3.88 (d, J = 8.1 Hz, 1H), 3.83 (s, 3H), 3.03-2.94 (m, 1H), 2.31-2.06 (m, 1H); HRMS (ESI) m/z calcd

HETEROCYCLES, Vol. 68, No. 9, 2006
1947
for C12H15O4 (M++1) 223.0970, found 233.0977; Anal. Calcd for C12H14O4: C, 64.85; H, 6.35. Found: C,
65.02; H, 6.53. For 5c: 1H NMR (300 MHz, CDCl3) δ 7.27 (d, J = 8.7 Hz, 2H), 6.87 (d, J = 8.7 Hz, 2H),
4.71 (d, J = 8.4 Hz, 1H), 4.02-3.99 (m, 2H), 3.86 (d, J = 8.4 Hz, 1H), 3.80 (s, 3H), 3.02-2.94 (m, 1H),
2.29-2.19 (m, 1H), 1.92 (br s, 1H); HRMS (ESI) m/z calcd for C12H15O4 (M++1) 223.0970, found
233.0971. For 5d (rotamer): 1H NMR (300 MHz, CDCl3) δ 6.82-6.63 (m, 3H), 5.88 (br s, 2H), 4.60 (br s,
1H), 3.88-3.74 (m, 3H), 2.89-2.80 (m, 1H), 2.30-2.20 (m, 1H); HRMS (ESI) m/z calcd for C12H13O5
1
(M++1) 237.0763, found 237.0765. For 5e: H NMR (300 MHz, CDCl3) δ 7.66-7.60 (m, 4H), 7.52-7.30
(m, 5H), 4.79 (d, J = 8.1 Hz, 1H), 4.08-3.94 (m, 2H), 3.90 (d, J = 8.1 Hz, 1H), 3.11-2.95 (m, 1H),
2.19-2.11 (m, 1H); HRMS (ESI) m/z calcd for C17H17O3 (M++1) 269.1178, found 269.1180; Anal. Calcd
for C17H16O3: C, 76.10; H, 6.01. Found: C, 76.31; H, 6.28.
ACKNOWLEDGEMENTS
The authors would like to thank the National Science Council of the Republic of China for financial
support.
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