A simple and practical synthesis of methyl benzo[b]furan-3-carboxylates

Article abstract of DOI:10.3987/COM-08-11458

A simple and practical two-step procedure for the preparation of 2-unsubstituted l-benzo[b]furan-3-carboxylic acid methyl esters is described. The procedure uses the copper-catalyzed intramolecular C-O bond formation and provides an efficient route to the title compounds in good to excellent yields.

Full text of DOI:10.3987/COM-08-11458

HETEROCYCLES, Vol. 75, No. 12, 2008
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HETEROCYCLES, Vol. 75, No. 12, 2008, pp. 2973 - 2980. © The Japan Institute of Heterocyclic Chemistry
Received, 3rd June, 2008, Accepted, 14th July, 2008, Published online, 17th July, 2008. COM-08-11458
A SIMPLE AND PRACTICAL SYNTHESIS
OF METHYL BENZO[b]FURAN-3-CARBOXYLATES
Ferdinand S. Melkonyan, Nikita E. Golantsov, and Alexander V. Karchava*
Department of Chemistry, M. V. Lomonosov Moscow State University, Moscow
119992, Russia, karchava@org.chem.msu.ru
Abstract – A simple and practical two-step procedure for the preparation of
2-unsubstituted 1-benzo[b]furan-3-carboxylic acid methyl esters is described. The
procedure uses the copper-catalyzed intramolecular C–O bond formation and
provides an efficient route to the title compounds in good to excellent yields.
INTRODUCTION
Benzo[b]furan derivatives are an important class of heterocyclic compounds widely presented in nature.1
Substituted benzo[b]furans are known to possess important biological properties and find application as
pharmaceuticals, agricultural chemicals and in other fields of chemistry.2 For this reason, numerous
methods for the synthesis of benzo[b]furan derivatives1-3 have been developed over the years and the
majority of methods for selectively constructing benzo[b]furan ring system includes the transition
metal-catalyzed transformations.4
1-Benzo[b]furan-3-carboxylic acids (1) and their derivatives have found significant use as building blocks
for
the
synthesis
of
pharmaceutically
important
molecules.5
Moreover,
lithium
2-lithiobenzo[b]furan-3-carboxylate which readily generated from acid 1 is a useful synthetic intermediate
for 2-substituted derivatives of acid 1 via reactions with a number of electrophiles.6 Although several
synthetic routes to benzo[b]furan-3-carboxylic acid (1) and its esters have been reported (Scheme 1), all of
these methods suffer from significant drawbacks which include: low-yielding or lengthy in terms of steps;
the use of expensive or hazardous starting materials or catalysts, reactions can be performed only on gram
to mg scale, drastic reaction conditions.
Historically, acid 1 was prepared by decarboxylation of benzo[b]furan-2,3-dicarboxylic acid7 or via
lithiation/carboxylation of 3-bromobenzo[b]furan.8 Both these methods are difficult to operate on a
multi-gram scale and complicated by side-reactions.9 Recently, the synthesis of ethyl
benzo[b]furan-3-carboxylate (3a) from salicylaldehyde and ethyl diazoacetate in the presence of
HBF4×Et2O as a catalyst has been reported.10 Although the method is high-yielding and simple to operate,

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HETEROCYCLES, Vol. 75, No. 12, 2008
the use of diazoacetate as a reagent makes this protocol less attractive for medium and large scale. Several
methods for the preparation of benzo[b]furan-3-carboxylic acid esters include the palladium-catalyzed
transformations: the intramolecular Heck reaction of (2-bromo(iodo)phenoxy)acrylate11 and carbonylation
of the triflates derived from 3-coumaranones (Scheme 1).12 Both these approaches use relatively expensive
(or not easily achievable) starting materials and/or catalysts. Several other reactions leading to acid 1 or the
corresponding esters have also been described,13 but all of these approaches cannot be regarded as a general
method. Thus, a simple and efficient route to benzo[b]furan-3-carboxylic acids is still of current interest to
synthetic organic chemists. Therefore, our efforts have been focused on the development of a simple and
efficient procedure for the preparation of acid 1 or the corresponding esters. Herein we report a practical
means of the medium-scale synthesis of methyl benzo[b]furane-3-carboxylates via a copper-catalyzed
intramolecular C–O bond-forming reaction.
Br
CO2H
1. BuLi, 2. CO2
ref. 8
t oC
CO2H
ref. 7
O
O
O
OR
O
Pd(OAc)2
Ph3P, NEt3
N2CHCO2Et
HBF4
O
ref. 11
X
CO2Et
OH
OTf
O
ref. 10
OH
CO2Et
X=Br, I
Pd(OAc)2, dppp
1 (R=H), 2a (R=Me),
3a (R=Et)
NEt3, MeOH, CO
ref. 12
X
O
Scheme 1. Literature routes to benzo[b]furan-3-carboxylic acid and corresponding esters.
RESULTS AND DISCUSSION
In the course of our ongoing medicinal chemistry programs, we required an operationally simple method
for the preparation of benzo[b]furan-3-carboxylic acid (1) in a multi-gram scale. In recent research, the
copper-catalyzed C–O bond formation reactions14 has been demonstrated to be among the most promising
ways for the construction of the benzo[b]furan ring system. This useful synthetic tool has been successfully
applied to the synthesis of a wide variety of 2-, 3- and 2,3-substituted benzo[b]furans15 via a ring closure of
2-haloaromatic ketones and esters 2-substituted benzo[b]furan-3-carboxylic acid from
1-bromo-2-iodobenzene and β-ketoesters.16 In recent years, the copper-catalyzed C–C and C–X (X = N, O,
S) bond formation reactions have widely been applied to the synthesis and transformation of various other
heterocyclic compounds as well.14, 17 Recently, we described a new protocol for the assembly of N-alkyl-
and N-aryl-1H-indole-3-carboxylic acid derivatives based on the copper(I)-catalyzed intramolecular
amination of aryl bromides.18 This catalytic process is simply performed and provides good to high yields
of the desired products even under an air atmosphere. We therefore focused our efforts on the development
a scaleable synthesis of methyl benzo[b]furan-3-carboxylate (2a) via the copper-catalyzed intramolecular

HETEROCYCLES, Vol. 75, No. 12, 2008
2975
C–O bond formation since we believed that this methodology could offer distinctive advantages over other
approaches to date.
Our synthetic pathway to methyl benzo[b]furan-3-carboxylate (2a) is depicted in Scheme 2 and based on an
elegant and highly efficient synthesis of benzo[b]furans via a copper-catalyzed ring closure of 2-halo
aromatic ketones reported by Chen and Dormer.15a The starting materials 5a-d were easily synthesized in
one step from corresponding methyl phenylacetates 4a-d by a known procedure.19 The following furan
ring-closure reaction was first investigated using copper (I) iodide (10 mol%) as the catalyst. The reaction
was performed in DMF as the solvent without any ligand added. Catalytic cyclization of substrate 5a turned
out to give 87% of ester 2a after as little as 1.5 h at 100 oC (bath temperature hereafter) with quantitative
conversion of the starting material (Table 1, entry 1). Advantageously, no column chromatography is
1
necessary for the product, after standard work-up it was obtained in high purity (≥95% by GC and H
NMR).
CO2Me
CO2Me
1. NaH (4 eq),
OH
HCO2Me
CO2Me
catalyst
R
R
R
2. H+, H2O
DMF, K2CO3
100 oC
O
X
X
4a - d
5a - d
2a-d
Scheme 2.
The reaction with 5 and even 1 mol% catalyst loadings gave essentially the same result for slightly longer
reaction times (entries 2 and 3). At the rt a complete conversion was reached after 12 h and product was
obtained with excellent yield (92%, entry 2). Other copper sources such as Cu(OAc)2 and CuSO4×5H2O
were demonstrated to be similarly effective, though with CuSO4×5H2O as the catalyst precursor ester 2a
was obtained in slightly lower yield (entry 5), probably because of the hydrolysis of the ester group.
We also investigated the use of FeCl3 as an alternative catalyst for the benzo[b]furan ring construction via
the catalytic C–O bond-forming reaction. Recently several new practical methods of the C–C, C–N and
C–O bond formations using iron salts as a catalyst have been described.20 Iron-catalyzed reactions are very
attractive because of extremely low price of iron and its environmentally benign features. Unfortunately, in
our case FeCl3 has proved to be much less effective than any copper catalyst used. The reaction with 10
mol% of FeCl3 gave ester 2a in only 51% yield with quantitative conversion of the starting material after 24
o
h at 100 C (entry 6). When a higher catalyst loading was employed no significant improvement was
achieved and ester 2a was obtained in only 53% yield after 10 h.
o
Using the optimal reaction conditions (5 mol% CuI, 100 C) we prepared methyl 5-methoxy- (2b) and
6-fluorobenzofuran-3-carboxylates (2c) in high yields (entries 8 and 9). Chloride 5d seemed to be much

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HETEROCYCLES, Vol. 75, No. 12, 2008
less reactive under these reaction conditions and the corresponding ester 2d was isolated in a poor yield
(entries 10 and 11), though the conversion in both cases was nearly quantitative.
Table 1. Synthesis of benzo[b]furan-3-carboxylic acid derivatives
#
1
2
3
4
5
6
7
8
9
R
H
X Y
Yield, %
Catalyst (mol%)
CuI (10)
Time, h
Yield, %a
Br CO2Me
5a
90
1.5
2 (12b)
2a
87
88 (92b)
87
CuI (5)
CuI (1)
4
Cu(OAc)2 (10)
CuSO4×5H2O (10)
FeCl3 (10)
FeCl3 (20)
CuI (5)
2
88
2
75
24
10
2
51
53
5-MeO Br CO2Me
5b
5c
5d
85
82
86
2b
2c
2d
78
4-F
Br CO2Me
Cl CO2Me
2
82
CuI (5)
10 4-Cl
11
24
24
33c
31c
CuI (10)
Cu(OAc)2 (10)
a Yields are given for isolated products with purity ≥95% (GC and 1H NMR). b Reaction was carried out at rt.
c Isolated yield after column chromatography.
CONCLUSION
In conclusion, we have provided a simple and practical means for the preparation of methyl
benzo[b]furan-3-carboxylates starting from methyl o-bromophenylacetates using a copper-catalyzed
intramolecular C–O bond-forming reaction as the key step. The procedure can be performed under an air
atmosphere and allows synthesizing the title compounds with high yields and purity even without column
chromatography purification. This process is significantly improved compared to the literature methods
mentioned above.
EXPERIMENTAL
General. All chemicals and solvents were purchased from commercial suppliers and used as received. All
reactions were performed under an air atmosphere. 1H and 13C NMR spectra were obtained at 400 and 100
MHz correspondingly with CDCl3 or DMSO-d6 as a solvent and internal standard. Chemical shift values
are expressed in δ and J values are in Hz. Splitting patterns are indicated as s (singlet), d (doublet), dd
(doublet of doublets), t (triplet), td (triplet of doublets), m (multiplet), b (broad signal). Compounds
described in the literature were characterized by comparing their 1H and 13C NMR spectra to the previously

HETEROCYCLES, Vol. 75, No. 12, 2008
2977
reported data and all new compounds were further characterized. IR spectra were recorded for neat
compounds (liquid) or for suspension in Nujol (solid) and only noteworthy absorptions are listed. Mass
spectra were recorded in electron impact mode at 70 eV. Melting points were determined by open capillary
method and are uncorrected. Analytical samples were prepared by flash chromatography on silica gel
(230-400 mesh) using a mixture of hexane/EtOAc (20:1) as eluent. TLC was carried out on silica gel 60 F254
plates, and the spots were located with UV light.
Methyl 2-(2-bromophenyl)-2-formylacetate (5a).19
To a stirred solution of methyl 2-bromophenylacetate (71.1 g, 0.31 mol) in methyl formate (500 mL) a
suspension of sodium hydride (60%, 49.89 g, 1.24 mol) was slowly added over a period of 1 h at 10-15 oC.
After the mixture was stirred for an additional hour it was treated with iced water (600 mL) and two layers
were separated. The aqueous layer was acidified with 10% HCl and then extracted (3 x 300 mL) with
EtOAc. The organic layers were combined, washed successively with water (2 x 300 mL), saturated
aqueous NaHCO3 (2 x 300 mL) and saturated brine and then dried over anhydrous Na2SO4. Filtration and
evaporation gave the title compound as the only tautomer (71.1 g, 90%). Mp 123-125 oC (hexane). 1H NMR
(CDCl3): δ 3.78 (s, 3H), 7.19 (s, 1H), 7.20–7.26 (m, 2H), 7.33 (td, J = 8.0, 1.4 Hz, 1H), 7.63 (dd, J = 8.0, 1.2
Hz, 1H), 11.91 (d, J = 12.8 Hz). 13C NMR (CDCl3): δ 51.9, 108.4, 126.6, 127.4, 129.4, 132.3, 132.6, 134.7,
163.4, 171.3. MS, m/z (I, %): 258, 256 (M+; 5, 5), 228, 230 (8, 8), 224, 226 (6, 6), 177 (43), 145 (100), 134
(10), 121 (17), 105 (7), 88 (23), 89 (80), 77 (15), 63 (25), 50 (10), 39 (15). IR (film, ν, cm-1): 1615, 1670,
1740, 2935.
The above procedure was followed to afford the following compounds.
Methyl 2-(2-bromo-5-methoxyphenyl)-2-formylacetate (5b). Yield 85% as the only tautomer. Pale
yellow solid. Mp 110-115 oC (toluene/hexane). 1H NMR (CDCl3): δ 3.78 (s, 3H), 3.81 (s, 3H), 6.75–6.85
(m, 2H), 7.19 (d, J = 12.9 Hz, 1H), 7.49 (d, J = 8.6 Hz, 1H), 11.89 (d, J = 12.9 Hz). 13C NMR (DMSO-d6):
δ 51.5, 55.8, 109.9, 115.1, 115.9, 118.8, 133.0, 136.2, 156.9, 158.7, 167.5. MS, m/z (I, %): 288, 286 (M+; 3,
3), 207 (100), 175 (42), 119 (26), 77 (24), 76 (20), 75 (17), 65 (16), 63 (18), 62 (15), 51 (27), 50 (25). IR
(film, ν, cm-1) 1620, 1680, 1745, 2930. Anal. Calcd for C11H11BrO4: C, 46.02, H, 3.86. Found: C, 46.03, H,
3.84%.
Methyl 2-(2-bromo-4-fluorophenyl)-2-formylacetate (5c). Yield 82%. Pale yellow amorphous solid.
This compound was obtained as a mixture of three tautomers. 1H NMR (CDCl3): δ 3.73 (s, 0.9H), 3.77 (s,
1.8H), 3.83 (s, 0.3H), 5.04 (s, 0.1H), 6.76 (bs, 0.3H), 6.99–7.45 (m, 3.6H), 7.78 (bs, 0.3H), 9.99 (s, 0.1H),
11.96 (d, J = 12.9 Hz, 0.6H). 13C NMR (DMSO-d6): δ 51.5, 108.9, 114.9 (d, JC-F = 20.5 Hz), 119.6 (d, JC-F
=
24.2 Hz), 125.7 (d, JC-F = 10.3 Hz), 131.7 (d, JC-F = 3.7 Hz), 134.4 (d, JC-F = 8.1 Hz), 157.3, 161.4 (d, JC-F
=
247.4 Hz), 167.4. MS, m/z (I, %): 276, 274 (M+; 3, 3), 244, 242 (4, 4), 203, 201 (4, 4), 195 (19), 163 (55),

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HETEROCYCLES, Vol. 75, No. 12, 2008
108 (23), 107 (100), 81 (20), 57 (32). IR (film, ν, cm-1) 1610, 1670, 1735, 2940. Anal. Calcd for
C10H8BrFO3: C, 43.66, H, 2.93. Found: C, 43.70, H, 2.94 %.
Methyl 2-(2,4-dichlorophenyl)-2-formylacetate (5d).21 Yield 86%. Yellow liquid. This compound was
obtained as a mixture of three tautomers with more than 80% of major tautomer. 1H NMR (CDCl3, major
tautomer): δ 3.78 (s, 3H), 7.16 (d, J = 8.2, 1H), 7.19 (d, J = 12.8 Hz, 1H), 7.26 (dd, J = 8.2, 2.2 Hz, 1H), 7.45
(d, J = 2.2 Hz, 1H), 11.95 (d, J = 12.8 Hz, 1H). 13C NMR (DMSO-d6): δ 51.5, 106.8, 127.4, 128.9, 132.3,
133.0, 134.6, 135.5, 157.7, 164.3. MS, m/z (I, %): 248, 246 (M+; 8, 14), 216, 214 (16, 26), 181 (41), 179
(100), 160 (24), 159 (35), 158 (36), 125 (55), 124 (20), 123 (93), 111 (25), 97 (29), 89 (35), 87 (4), 86 (29),
85 (26), 75 (41), 74 (22), 73 (48), 63 (45), 62 (43), 61 (41), 57 (27), 55 (27), 50 (25), 43 (40), 41 (28), 39
(24). IR (film, cm-1) 1610, 1670, 1735, 2940. Anal. Calcd for C10H8Cl2O3: C, 48.61, H, 3.26. Found: C,
48.63, H, 3.26.
Methyl
1-benzofuran-3-carboxylate
(2a).12
To
a
stirred
solution
of
methyl
2-(2-bromophenyl)-2-formylacetate (5a) (71.1 g, 0.275 mol) in DMF (300 mL) CuI (21.7 g, 0.114 mol, 5
mol%) and K2CO3 (130.9 g, 0.55 mol) were added. The reaction mixture was heated at 100oC (bath
temperature) for 2 h and allowed to cool to rt. Solvent was evaporated to dryness under reduced pressure.
Water (300 mL) was added to the residue, the mixture was extracted (3 x 150 mL) with EtOAc. The
combined organic layers were washed with brine and dried over anhydrous Na2SO4. The solvent was
removed in vacuo to yield 42.6 g (88%) of the crude product as pale yellow liquid with purity ≥95%. 1H
NMR (CDCl3): δ 3.96 (s, 3H), 7.36–7.41 (m, 2H), 7.52–7.58 (m, 1H), 8.07–8.12 (m, 1H), 8.28 (s, 1H). 13C
NMR (CDCl3): δ 51.6, 111.7, 114.5, 122.0, 124.2, 124.6, 125.3, 151.0, 155.6, 163.4. MS, m/z (I, %): 176
(M+; 37), 145 (50), 90 (19), 89 (100), 88 (22), 87 (21), 86 (20), 77 (25), 72 (16), 63 (99), 62 (97), 61 (43), 53
(16), 51 (40), 50 (56), 45 (16), 44 (17), 43 (22), 39 (97), 38 (40), 37 (29). IR (film, ν, cm-1) 1060, 1130,
1730.
The above procedure was followed to afford the following compounds.
Methyl 5-methoxy-1-benzofuran-3-carboxylate (2b). Yield 78%. Tan liquid.1H NMR (CDCl3): δ 3.89 (s,
3H), 3.93 (s, 3H), 6.96 (dd, J = 8.8, 2.5 Hz, 1H), 7.41 (d, J = 9.1 Hz, 1H), 7.51 (d, J = 2.5 Hz, 1H), 8.21 (s,
1H). 13C NMR (CDCl3): δ 51.5, 55.9, 103.7, 112.2, 114.3, 114.6, 125.3, 150.5, 151.5, 157.0, 163.8. MS, m/z
(I, %): 206 (M+; 78), 191 (21), 175 (100), 147 (19), 120 (16), 119 (38), 104 (19), 91 (16), 89 (18), 88 (17),
87 (27), 86 (17), 82 (32), 77 (37), 76 (71), 75 (37), 74 (35), 65 (58), 63 (40), 62 (44), 61 (17), 59 (47), 54
(42), 53 (31), 51 (26), 50 (87), 39 (27), 38 (21). IR (film, ν, cm-1) 1060, 1180, 1725. Anal. Calcd for
C11H10O4: C, 64.07, H, 4.89. Found: C, 64.10, H, 4.85.
Methyl 6-fluoro-1-benzofuran-3-carboxylate (2c). Yield 82%. Of-white solid. Mp 63-65 oC
(toluene/hexane). 1H NMR (CDCl3): δ 3.94 (s, 3H), 7.07–1.19(m, 1H), 7.21–7.30 (m, 1H), 7.95–8.05 (m,

HETEROCYCLES, Vol. 75, No. 12, 2008
2979
1H), 8.24 (s, 1H). 13C NMR (CDCl3): δ 51.7, 99.4 (d, JC-F = 27.1 Hz), 112.6 (d, JC-F = 24.2 Hz), 114.4, 120.8,
122.5 (d, JC-F = 10.3 Hz), 151.4 (d, JC-F = 3.7 Hz), 155.4 (d, JC-F = 13.2 Hz), 161.2 (d, JC-F = 243.7 Hz), 163.5.
MS, m/z (I, %): 194 (M+; 25), 163 (61), 107 (85), 81 (33), 63 (23), 62 (27), 57 (100), 50 (23), 43 (20), 41
(21). IR (Nujol, cm-1) 1060, 1140, 1735. Anal. Calcd for C10H7FO3: C, 61.86, H, 3.63. Found: C, 61.85, H,
3.61.
Methyl 6-chloro-1-benzofuran-3-carboxylate (2d). Yield 33% (after chromatography purification).
Yellow solid. Mp 67 oC. 1H NMR (CDCl3): δ 3.95 (s, 3H), 7.34 (dd, J = 8.3, 1.8 Hz, 1H), 7.54 (d, J = 1.3 Hz,
1H), 7.97 (d, J = 8.4 Hz, 1H), 8.23 (s, 1H). 13C NMR (CDCl3): δ 51.7, 112.2, 114.4, 122.6, 123.3, 125.0,
131.3, 151.4, 155.5, 163.3. MS, m/z (I, %): 212, 210 (M+; 8, 25), 181, 179 (15, 46), 125 (19), 123 (40), 111
(21), 97 (37), 95 (23), 87 (20), 83 (40), 81 (29), 75 (18), 73 (20), 71 (52), 70 (22), 69 (60), 67 (30), 63 (20),
62 (25), 57 (92), 56 (33), 55 (90), 43 (100), 42 (25), 41 (97), 39 (29). IR (Nujol, cm-1) 1060, 1130, 1730.
Anal. Calcd for C10H7ClO3: C, 57.03, H, 3.35. Found: C, 57.01, H, 3.34.
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