Palladium-catalyzed intramolecular hydroarylation of 6-benzofuranyl alkynoates

Article abstract of DOI:10.3987/COM-12-S(N)36

Intramolecular hydroarylation of 6-benzofuranyl alkynoates efficiently proceeded using Pd(OAc)₂ as catalyst in TFA and CH₂Cl ₂ at room temperature. This intramolecular hydroarylation gave a mixture of regioisomers of furocoumarins, i.e., psoralens and allopsorallens.

Full text of DOI:10.3987/COM-12-S(N)36

HETEROCYCLES, Vol. 86, No. 1, 2012
759
HETEROCYCLES, Vol. 86, No. 1, 2012, pp. 759 - 766. © 2012 The Japan Institute of Heterocyclic Chemistry
Received, 18th June, 2012, Accepted, 28th June, 2012, Published online, 6th July, 2012
DOI: 10.3987/COM-12-S(N)36
PALLADIUM-CATALYZED INTRAMOLECULAR HYDROARYLATION
OF 6-BENZOFURANYL ALKYNOATES
Tsugio Kitamura* and Kensuke Otsubo
Department of Chemistry and Applied Chemistry, Graduate School of Science
and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan
Abstract – Intramolecular hydroarylation of 6-benzofuranyl alkynoates
efficiently proceeded using Pd(OAc)2 as catalyst in TFA and CH2Cl2 at room
temperature. This intramolecular hydroarylation gave a mixture of regioisomers of
furocoumarins, i.e., psoralens and allopsorallens.
Furocoumarins are an important class of tricyclic aromatic compounds composing of a fused structure of
coumarin and furan nuclei. They have long attracted much attention because of their biological and
industrial applications. Furocoumarin derivatives are naturally occurring or synthetic compounds and are
known to possess a high photobiological activity, as discussed in many reviews.1
For the reasons mentioned above, many synthetic methods for furocoumarins have been extensively
developed so far.1a,1b,1g,2 There are two main approaches to construct the furocoumarin skeleton, namely,
furan-ring construction and pyrone-ring construction. Among these methods, the methodology fusing a
furan ring on a coumarin nucleus has been traditionally and widely applied for the synthesis of
furocoumarins.1a,1b On the other hand, hydroxybenzofuran derivatives have been used for the
furocoumarin synthesis via the pyrone-ring construction as starting materials, and the synthetic method
involves the Perkin condensation,3 the Pechmann reaction,4 the Wittig reaction,2d,5 and related reactions.6
Intramolecular hydroarylation of alkynes is an efficient and promising method for synthesizing
carbocyclic and heterocyclic aromatic compounds.7 Recently several coumarin derivatives have been
prepared by transition metal-catalyzed hydroarylation reactions.8 However, to the best of our knowledge,
there is only one report on synthesis of furocoumarins by the intramolecular hydroarylation
methodology.9 In this paper, we want to report another palladium-catalyzed intramolecular hydroarylation
yielding furocoumarin derivatives, as shown in Scheme 1.
Dedicated with respect to Dr. Ei-ichi Negishi on the occasion of his 77th birthday.

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HETEROCYCLES, Vol. 86, No. 1, 2012
Scheme 1
First, 6-hydroxybenzofurans were prepared by the Sonogashira-Hagihara coupling of 4-iodoresorcinol
with phenylacetylene as shown in Scheme 2. Iodination of resorcinol with ICl was examined according to
the literature method.10 The reaction of resorcinol with ICl at room temperature for 3 h gave
4-iodoresorcinol. The 1H NNR spectrum of the product showed the presence of 4-iodoresorcinol in a high
yield but purification by column chromatography on silica gel resulted in a large loss of the product due
to the contamination with unreacted resorcinol and diiodoresorcinol. Then, we decided to use the crude
4-iodoresorcinol for the Sonogashira-Hagihara coupling reaction. The coupling reaction of the crude
4-iodoresorcinol with phenylacetylene was conducted in the presence of PdCl2(PPh3)2 and CuI in
triethylamine and DMF at 60 °C for 12 h. After separation by column chromatography on silica gel,
6-hydroxy-2-phenylbenzofuran (1) was obtained in 45% yield based on the starting resorcinol.
Scheme 2
Next, to prepare benzofuranyl alkynoates, the condensation of 6-hydroxy-2-phenylbenzofuran (1) and
alkynoic acids (2) was conducted using DCC and DMAP in CH2Cl2 at room temperature (Scheme 3). The
results are given in Table 1. 6-Benzofuranyl alkynoates (3) was obtained in good to high yields.
Scheme 3

HETEROCYCLES, Vol. 86, No. 1, 2012
761
Table 1. Preparation of 6-benzofuranyl alkynoates (3)a
Entry
Alkynoic acid 2
Product 3
Yield (%)b
1
2
3
4
2a (R = Ph)
3a
3b
3c
3d
89
89
93
68
2b (R = n-C5H11)
2c (R = Me)
2d (R = H)
a Conditions: 1 (2 mmol), 2 (2 mmol), DCC (2 mmol), DMAP (0.2 mmol), CH2Cl2 (2 mL), rt, 3 h.
b Isolated yield.
Intramolecular hydroarylation was performed using Pd(OAc)2 catalyst in a mixed solvent of TFA and
CH2Cl2. The outline was shown in Scheme 4. The results are given in Table 2. When 6-benzofuranyl
phenylpropynoate (3a) was allowed to react with 1 mol% Pd(OAc)2 at room temperature for 1 h, the
1
desired furocoumarin derivatives were obtained in 90% yield (Entry 1). However, H NMR spectrum of
the product showed that it was a 23:77 regioisomeric mixture of furocoumarins 4a and 5a. The ratio was
determined by the integral of the characteristic vinylic proton at the 3 position. The elemental analysis of
the regioisomeric mixture was consistent with the proposed furocoumarins. In addition, the major isomer
was obtained by repeated column chromatography and recrystallization and assigned to 5a based on a
characteristic AB pattern of the aromatic protons:  7.34 (d, J = 8.6 Hz) and 7.73 (d, J = 8.6 Hz).
Under the similar conditions the reaction of 6-benzofuranyl octynoate (3b) gave furocoumarin derivatives
4b and 5b by the ratio of ca. 40:60 in 68-88% yields (Entries 2 and 3). 6-Benzofuranyl butynoate (3c)
also showed a similar behavior to 3b, giving furocoumarins 4c and 5c in 56-58% yields. 6-Benzofuranyl
propynoate (3d) afforded a mixture of furocoumarins 4d and 5d by the ratio of 50:50 in 27% yield due to
the side-reactions. Then, to suppress the side-reactions, the reaction of 3d was conducted with an
increased amount of solvent. Although the use of large amount of solvent improved the yield of 4d and
5d, the best result was 47% (Entries 7-9).
The intramolecular hydroarylation of 6-benzofuranyl alkynoates (3) has two processes. The cyclization at
the 5 position of the benzofuran ring gives furocoumarins 4 (psoralens), while the cyclization at the 7
position affords 5 (allopsoralens).
Scheme 4

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HETEROCYCLES, Vol. 86, No. 1, 2012
Table 2. Intramolecular hydroarylation of 3 with Pd(OAc)2 catalysta
CH2Cl2
/mL
0.5
0.5
0.75
0.5
0.75
0.5
12
30
45
TFA
/mL
0.5
0.5
0.25
0.5
0.25
0.5
4
Time
/h
1
1
1
1
1
1
Yield
/%b
90
68
88
56
58
27
44
Ratioc
4 : 5
Entry
R
Product
1
2
3
4
5
6
7
8
9d
Ph
4a and 5a
4b and 5b
4b and 5b
4c and 5c
4c and 5c
4d and 5d
4d and 5d
4d and 5d
4d and 5d
23:77
38:62
41:59
33:67
41:59
50:50
45:55
49:51
48:52
C5H11
C5H11
CH3
CH3
H
H
H
H
24
24
24
10
15
47
42
a Conditions: 3 (1 mmol), Pd(OAc)2 (0.01 mmol), TFA, CH2Cl2, rt.
b Isolated yield.
c Determined by 1H NMR.
d At 50 °C.
As discussed in the previous paper,9 it is considered that the intramolecular hydroarylation of
6-benzofuranyl alkynoates also proceeds as follows. First, TFA undergoes ligand exchange of Pd(OAc)2
with acetate ion to generate a more reactive Pd(OCOCF3)2. Thus, the resulting reactive palladium species
coordinates with the triple bond to induce intramolecular electrophilic aromatic substitution leading to a
furanocoumaryl palladium complex. This intramolecular cyclization occurs both at the 5 and 7 positions
of the benzofuran ring. The resulting furanocoumaryl palladium complexes undergo protonation with
TFA to give furocoumarins and the palladium species is regenerated.
In conclusion, we have demonstrated an efficient palladium-catalyzed intramolecular hydroarylation of
6-benzofuranyl alkynoates, which gives a mixture of regioisomers of furocoumarins, i.e., psoralens and
allopsorallens.
EXPERIMENTAL
All solvents and starting materials were used during the research work as received without further
1
purification unless otherwise indicated. H and 13C NMR were recorded on a JEOL JNMR-AL 300
FT-NMR spectrometer (TMS as an internal standard). Melting points were measured with a Yanaco
micro melting point apparatus and are uncorrected. Elemental analysis was performed by the Service
Center of the Elemental Analysis of Organic Compounds, Faculty of Science, Kyushu University,
Fukuoka, Japan. High resolution mass spectra were measured by the Analytical Center, Institute for
Materials Chemistry and Engineering, Kyushu University. Column chromatographic separation was
carried out using Silica Gel 60, spherical (Kanto Chemical Co.).

HETEROCYCLES, Vol. 86, No. 1, 2012
763
Preparation of 4-Iodoresorcinol. A solution of ICl (1 mmol) in ether (40 mL) was added to resorcinol (1
mmol) and the mixture was stirred at room temperature for 3 h. The reaction mixture was quenched with
aqueous sodium sulfite and extracted with Et2O. The ethereal solution was washed with water, then with
brine, and dried over anhydrous sodium sulfate. The crude 4-iodoresorcinol was used for the
Sonogashira-Hagihara coupling reaction without further purification.
1
4-Iodoresorcinol:10 a colorless oil; H NMR (300 MHz, CDCl3)  5.53 (s, 1H, OH), 5.77 (s, 1H, OH),
6.25 (dd, J = 2.3, 8.6, 1H, aryl), 6.52 (d, J = 2.3 Hz, 1H, aryl), 7.43 (d, J = 8.6 Hz, 1H, aryl); 13C NMR (75
MHz, CDCl3)  74.51, 102.72, 110.55, 138.46, 155.50, 157.36.
Preparation of 6-Hydroxy-2-phenylbenzofuran (1). To the crude 4-iodoresorcinol (ca 1 mmol)
prepared above, were added successively phenylacetylene (2 mmol), PdCl2(PPh3)2 (0.035 mmol), CuI
(0.0525 mmol), Et3N (2 mL), and DMF(2.5 mL). The mixture was stirred at 60 °C for 12 h under argon
atmosphere. The reaction mixture was acidified with diluted HCl and extracted with Et2O. The ethereal
extract was washed with water, then with brine, and dried with anhydrous sodium sulfate. After
concentration in vacuo, the product was separated by column chromatography on silica gel using
CH2Cl2/hexane as eluent.
1
6-Hydroxy-2-phenyl-benzofuran (1):11 white solids; mp 177−179 °C; H NMR (300 MHz, acetone-d6)
 6.86 (dd, J = 2.4, 8.3 Hz, 1H, aryl), 7.06−7.13 (m, 2H, aryl), 7.29−7.46 (m, 4H, aryl), 7.83−7.86 (m, 2H,
aryl), 8.54 (s, 1H, OH); 13C NMR (75 MHz, acetone-d6)  98.50, 102.36, 113.32, 122.06, 122.61, 125.01,
128.79, 129.63, 131.60, 155.30, 156.74, 156.91.
General Procedure for Preparation of 2-Phenylbenzofuran-6-yl Alkynoates (3). A mixture of
6-hydroxy-2-phenylbenzofuran (1) (2 mmol), alkynoic acid (2) (2 mmol), and 4-(dimethylamino)pyridine
(DMAP) (0.2 mmol) was stirred at 0 °C for 5 min using an ice/water bath and then
N,N-dicyclohexylcarbodiimide (DCC) (2 mmol) was added. The mixture was stirred at 0 °C for 5 min and
then at room temperature for 3 h. After filtration of the resulting solids, the mother liquor was treated with
diluted HCl and extracted with CH2Cl2. The CH2Cl2 extract was washed with water and dried over
anhydrous sodium sulfate. Evaporation of the solvent in vacuo, the product was separated by column
chromatography on silica gel using CH2Cl2/hexane as eluent.
1
2-Phenylbenzofuran-6-yl Phenylpropynoate (3a): white solids; mp 140−142 °C; H NMR (300 MHz,
CDCl3)  7.01 (s, 1H, aryl), 7.08 (dd, J = 1.8, 8.4 Hz, 1H, aryl), 7.33−7.51 (m, 7H, aryl), 7.57 (d, J = 8.4
13
Hz, 1H, aryl), 7.62−7.65 (m, 2H, aryl), 7.83−7.86 (m, 2H, aryl); C NMR (75 MHz, CDCl3)  80.26,
88.85, 101.01, 105.06, 116.90, 119.24, 120.94, 124.89, 127.59, 128.66, 128.75, 128.81, 130.09, 131.04,
133.19, 147.22, 152.60, 154.55, 157.27. HRMS Calcd for C23H14O3: M, 338.0943. Found: m/z 338.0942.
1
2-Phenylbenzofuran-6-yl 2-octynoate (3b): white solids; mp 94−96 °C; H NMR (300 MHz, CDCl3) 

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HETEROCYCLES, Vol. 86, No. 1, 2012
0.92 (t, J = 7.2 Hz, 3H, CH3), 1.31−1.46 (m, 4H, CH2), 1.56−1.66 (m, 2H, CH2), 2.41 (t, J = 7.2 Hz, 2H,
CH2), 7.00 (s, 1H, aryl), 7.02 (dd, J = 2.1, 8.4 Hz, 1H, aryl), 7.33−7.38 (m, 2H, aryl), 7.42−7.45 (m, 2H,
aryl), 7.54 (d, J = 8.4 Hz, 1H, aryl), 7.82−7,85 (m, 2H, aryl); 13C NMR (75 MHz,CDCl3)  13.84, 18.81,
22.09, 27.13, 30.97, 72.71, 92.64, 101.00, 105.06, 116.92, 120.87, 124.88, 127.47, 128.72, 128.80, 130.13,
147.26, 152.34, 154.55, 157.18. Anal. Calcd for C22H20O3: C, 79.50; H, 6.06. Found: C, 79.47; H, 6.04.
1
2-Phenylbenzofuran-6-yl 2-butynoate (3c): white solids; mp 113-118 °C; H NMR (300 MHz, CDCl3)
 2.08 (s, 3H, CH3), 7.00−7.04 (m, 2H, aryl), 7.34−7.47 (m, 4H, aryl), 7.54 (d, J = 8.4 Hz, 1H, aryl),
7.82−7.85 (m, 2H, aryl); 13C NMR (75 MHz,CDCl3)  3.72, 72.06, 88.19, 100.92, 104.87, 116.75, 120.78,
124.74, 127.37, 128.59, 128.67, 129.98, 147.15, 152.02, 154.43, 157.08. Anal. Calcd for C18H12O3: C,
78.25; H, 4.38. Found: C, 78.25; H, 4.33.
2-Phenylbenzofuran-6-yl propynoate (3d): white solids; mp 218-220 °C; 1H NMR (300 MHz, CDCl3) 
3.09 (s, 1H, CH), 7.00−7.05 (m, 2H, aryl), 7.33−7.47 (m, 4H, aryl), 7.55 (d, J = 8.4 Hz, 1H, aryl),
7.82−7.85 (m, 2H, aryl); 13C NMR (75 MHz,CDCl3)  74.30, 76.90, 77.20, 100.98, 104.90, 116.64,
120.97, 124.93, 127.75, 128.82, 130.07, 146.91, 151.18, 154.51, 157.43. HRMS Calcd for C17H10O3: M,
262.0630. Found: m/z 262.0632.
General Procedure for Intramolecular Hydroarylation of 3. A solution of benzofuran-6-yl alkynoate 3
(1 mmol) and Pd(OAc)2 (0.01 mmol) in CH2Cl2 (0.5 mL) and TFA (0.5 mL) was stirred at room
temperature for 30 min. The reaction mixture was neutralized with aqueous NaHCO3 and extracted with
CH2Cl2. The CH2Cl2 extract was washed with water and dried over anhydrous sodium sulfate. After
concentration in vacuo, the product was separated by column chromatography on silica gel using
CH2Cl2/hexane as eluent.
A Mixture of 2,5-Diphenyl-7H-furo[3,2-g][1]benzopyran-7-one (4a) and 2,9-Diphenyl-7H-
furo[2,3-f][1]benzopyran-7-one (5a): white solids; mp 137−161 °C; 1H NMR (300 MHz, CDCl3)  6.32
(s, vinyl, 4a), 6.38 (s, vinyl, 5a), 6.95 (s, aryl), 7.27−7.60 (m, aryl), 7.69 (d, J = 8.1 Hz, aryl), 7.81 (d, J =
7.2 Hz, aryl); 13C NMR (75 MHz, CDCl3)  99.70, 100.39, 100.64, 105.90, 113.05, 113.26, 115.65,
115.81, 118.59, 124.00, 124.33, 124.87, 126.01, 126.38, 128.03, 128.14, 128.38, 128.52, 128.56, 128.79,
128.97, 129.10, 129.33, 129.37, 129.52, 135.73, 137.13, 149.33, 151.88, 152.12, 153.29, 156.06, 156.18,
156.50, 157.91, 160.40, 160.88. One carbon is overlapped. Anal. Calcd for C23H14O3: C, 81.64; H, 4.17.
Found: C, 81.64; H, 4.18. 5a: mp 148-150 °C; 1H NMR (300 MHz, CDCl3)  6.41 (s, 1H, vinyl), 6.98 (s,
1H, aryl), 7.25−7.32 (m, 5H, aryl), 7.34 (d, J = 8.6 Hz, 1H, aryl), 7.53−7.64 (m, 5H, aryl), 7.73 (d, J = 8.6
Hz, 1H, aryl).
A Mixture of 5-Pentyl-2-phenyl-7H-furo[3,2-g][1]benzopyran-7-one (4b) and 9-Pentyl-2-phenyl-7H-
1
furo[2,3-f][1]benzopyran-7-one (5b): white solids; mp 115−128 °C; H NMR (300MHz, CDCl3) 

HETEROCYCLES, Vol. 86, No. 1, 2012
765
0.92−1.01 (m, CH3), 1.41−1.55 (m, CH2), 1.69−1.77 (m, CH2), 2.77 (t, J = 7.4 Hz, CH2, 4b), 3.14 (t, J =
7.4 Hz, CH2, 5b), 6.20 (s, vinyl, 4b), 6.26 (s, vinyl, 5b), 6.99 (s, aryl), 7.00 (s, aryl), 7.20 (d, J = 8.4Hz,
aryl), 7.35−7.81 (m, aryl); 13C NMR (75 MHz, CDCl3)  13.85, 13.93, 22.36, 22.47, 27.72, 28.19, 31.56,
31.74, 32.02, 34.88, 99.61, 100.60, 101.08, 106.75, 111.98, 113.21, 113.79, 115.57, 115.91, 123.43,
124.48, 124.88, 125.69, 126.31, 128.49, 128.81, 128.88, 129.06, 129.47, 129.68, 149.76, 151.79, 152.03,
155.30, 156.03, 156.47, 156.65, 157.85, 160.73, 161.15. Anal. Calcd for C22H20O3: C, 79.50; H, 6.06.
Found: C, 79.55; H, 5.97.
A Mixture of 5-Methyl-2-phenyl-7H-furo[3,2-g][1]benzopyran-7-one (4c) and 9-Methyl-2-phenyl-
1
7H-furo[2,3-f][1]benzopyran-7-one (5c): white solids; mp 174−188 °C; H NMR (300MHz, CDCl3) 
2.44 (s, CH3, 4c), 2.78 (s, CH3, 5c), 6.20 (s, vinyl, 4c), 6.23 (s, vinyl, 5c), 6.97 (s, aryl), 7.17 (d, J = 8.4
Hz, aryl), 7.36−7.79 (m, aryl); 13C NMR (75 MHz, CDCl3)  18.81, 21.63, 99.06, 100.32, 100.70, 106.75,
112.69, 113.02, 114.50, 115.67, 116.31, 123.25, 124.23, 124.61, 125.35, 126.08, 128.64, 128.67, 128.72,
128.93, 129.15, 129.30, 149.86, 150.94, 151.24, 151.38, 152.39, 155.81, 156.45, 157.58, 160.31, 160.70.
Anal. Calcd for C18H12O3: C, 78.25; H, 4.38. Found: C, 78.08; H, 4.35.
A
Mixture
of
2-Phenyl-7H-furo[3,2-g][1]benzopyran-7-one
(4d)
and
2-Phenyl-7H-
1
furo[2,3-f][1]benzopyran-7-one (5d): yellow solids; mp 242-261 °C; H NMR (300 MHz, CDCl3) 
6.36 (d, J = 9.6 Hz, vinyl, 4d), 6.51 (d, J = 9.6 Hz, vinyl, 5d), 7.00 (s, aryl), 7.03 (s, aryl), 7.20−7.25 (m,
aryl), 7.38−7.46 (m, aryl), 7.60−7.67 (m, aryl), 7.74−7.84 (m, aryl and vinyl), 8.24 (d, J = 9.6 Hz, vinyl,
5d); 13C NMR (75 MHz, CDCl3)  99.61, 100.46, 101.41, 105.45, 112.51, 114.49, 115.52, 116.29, 119.20,
123.75, 124.41, 124.74, 125.00, 125.03, 126.70, 128.68, 128.91, 128.93, 129.21, 129.48, 129.74, 136.84,
144.01, 149.79, 151.88, 152.04, 156.39, 157.13, 158.08, 160.67. HRMS Calcd for C17H10O3: M,
262.0630. Found: m/z 262.0634.
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