Silica gel-promoted convenient synthesis of 2-bromo-3-hydroxybenzoate derivatives

Article abstract of DOI:10.1016/j.tetlet.2014.07.084

A convenient silica gel-promoted synthesis of 2-bromo-3-hydroxybenzoate derivatives has been developed via the Diels-Alder reaction of furans with methyl 3-bromopropiolate, followed by a ring-opening aromatization. In addition, 2-bromo-3-methoxybenzoate, derived from 2-bromo-3-hydroxybenzoate with iodomethane, was found to be a good substrate for Pd-catalyzed cross-coupling reactions, despite its sterically hindered structure.

Full text of DOI:10.1016/j.tetlet.2014.07.084

Tetrahedron Letters 55 (2014) 5302–5305
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Silica gel-promoted convenient synthesis of
2-bromo-3-hydroxybenzoate derivatives
a
a
b
Hiroyuki Shinohara a, Motohiro Sonoda b, , Naoya Hayagane , Shota Kita , Shinji Tanimori ,
⇑
Akiya Ogawa a
a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuencho, Nakaku, Sakai, Osaka 599-8531, Japan
b Department of Bioscience and Informatics, Graduate School of Life and Environmental Science, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient silica gel-promoted synthesis of 2-bromo-3-hydroxybenzoate derivatives has been devel-
oped via the Diels–Alder reaction of furans with methyl 3-bromopropiolate, followed by a ring-opening
aromatization. In addition, 2-bromo-3-methoxybenzoate, derived from 2-bromo-3-hydroxybenzoate
with iodomethane, was found to be a good substrate for Pd-catalyzed cross-coupling reactions, despite
its sterically hindered structure.
Received 22 May 2014
Revised 16 July 2014
Accepted 23 July 2014
Available online 30 July 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Diels–Alder reaction
Ring-opening aromatization
2-Bromo-3-hydroxybenzoate
Silica gel
Furan
Hydroxybenzoates are ubiquitous compounds found in a vari-
ety of biologically active agents including natural products.1 They
are commonly used as starting materials or building blocks for
pharmaceuticals1,2 and functional materials.1b,3 Therefore, the
development of convenient synthesis of hydroxybenzoate deriva-
tives is still of current interest. In particular, bromohydroxybenzo-
ates are attractive from the perspective of further functionalization
possibilities via transition-metal–catalyzed cross-coupling reac-
tions.3b,c,4,5 Traditional syntheses of bromohydroxybenzoates have
typically involved the bromination of inexpensive hydroxybenzo-
ates.6 However, these methods have often suffered from the pro-
duction of byproducts, low regioselectivities, and low product
yields. Therefore, a highly regioselective, direct, and convenient
synthesis of bromohydroxybenzoates remains an attractive
challenge.
Recently, we reported the facile IrCl3ꢀ3H2O- or FeCl3-catalyzed
synthesis of 3-hydroxyphthalate derivatives7 using potentially
beneficial 7-oxanorborna-2,5-dienes8 derived from furans and
dimethyl acetylenedicarboxylate. Toward the development of a
convenient method for the direct synthesis of bromohydroxy-
benzoate derivatives, we focused on the Diels–Alder reaction of fur-
ans with bromoalkynes. The regioselective Diels–Alder reactions of
alkyl furans 1 with methyl 3-bromopropiolate 2 were found to
afford 1-bromo-7-oxanorborna-2,5-dienes 39 as major products
(Scheme 1). Herein, we wish to report the novel and metal-free
synthesis of sterically hindered 2-bromo-3-hydroxybenzoate deriv-
atives promoted by silica gel, and their further functionalizations
using Pd-catalyzed cross-coupling reactions.
Initially, we examined a one-pot reaction using pentylfuran 1a
and methyl 3-bromopropiolate 2. After the initial Diels–Alder reac-
tion of 1a (0.6 mmol) with 2 (0.5 mmol) in toluene at reflux for
24 h under nitrogen atmosphere, IrCl3ꢀ3H2O (10 mol %) was added
and the solution was stirred for 48 h to give 2-bromo-3-hydroxy-
benzoate 4a in 58% yield. Small amounts of Diels–Alder adducts
3a (10%) and 3a0 (trace) were also observed (Table 1, entry 1). With
the stronger Lewis acid FeCl3 as a catalyst, the desired product 4a
was obtained in 76% yield as a major product (entry 2). However,
regioisomer 4a0, which is difficult to separate from 4a, was also
generated in 5% yield.10 These results showed that FeCl3 was a suit-
able catalyst for the latter ring-opening aromatization reaction of
3, in contrast to the results of the IrCl3ꢀ3H2O-catalyzed reaction.
Reducing the reaction time required for the aromatization to 2 h
afforded nearly the same result as when FeCl3 was used as a cata-
lyst (entry 4), though, in the case of IrCl3ꢀ3H2O, the ring-opening
aromatization was incomplete (entry 3).
In order to optimize the reaction condition for the latter step,
we attempted to isolate the Diels–Alder adduct 3a by preparative
thin layer chromatography (PTLC). However, pure 3a could not
be obtained and a small to moderate amount of 4a was included.
This result showed that the latter step proceeded upon passage
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.tetlet.2014.07.084
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

H. Shinohara et al. / Tetrahedron Letters 55 (2014) 5302–5305
5303
Br
Br
CO2Me
A
Br
H
O
O
R
O
R
O
R
Br
CO2Me
+
+
A
CO2Me
toluene
Br
O
4a
CO2Me
CO2Me
nC5H11
3a
nC5H11
1
2
3
3'
A : Lewis acid or H+
major
minor
Scheme 1. Previous regioselectivity of the Diels–Alder reactions.
Scheme 2. Effect of the bromo group in ring-opening aromatization.
through silica gel without FeCl3 or IrCl3ꢀ3H2O. Therefore, we exam-
ined the metal-based Lewis acid-free one-pot reaction using dry
silica gel as an additive for the latter ring-opening step of 3a. When
300 or 150 mg of silica gel was added instead of FeCl3 or IrCl3ꢀ3H2-
O, the desired product 4a was obtained in good yield (Table 2,
entries 1 and 2). Interestingly, in both cases, regioisomer 4a0 was
not formed, although a small amount of Diels–Alder adduct 3a0
was detected. These results imply that an electronic effect due to
the bromo substituent is important: a cationic intermediate,
formed by the ring-opening of oxanorbornadiene 3a by the aid of
Brønsted or Lewis acidity of silica gel,11 would be stabilized by a
resonance effect of the bromo group which is located on the termi-
nal carbon of an allyl cation moiety (Scheme 2).12 Reducing the
amount of silica gel (50 mg) resulted in an incomplete reaction
(entry 3). A similar result was observed when using 150 mg of sil-
ica gel deactivated with 6 wt % triethylamine (entry 4). Addition of
1 N aqueous HCl was not effective on the ring-opening reaction
(entry 5). On the basis of these results, the reaction conditions in
entry 2 were employed as the optimized conditions unless other-
wise noted.
The one-pot reactions of various furans (1a–m) with 2 were
examined, and the results are summarized in Table 3. The reaction
of 2-methoxyfuran (1b) with 2 proceeded to give the corresponding
2-bromo-3-hydroxybenzoate 4b in good yield (entry 2). 2-(p-
Methoxyphenyl)furan (1c), 2-(p-tolyl)furan (1d), and 2-phenylfu-
ran (1e) were applicable for this one-pot reaction to give 4c, 4d,
Table 1
Synthesis of 2-bromobenzoate 4a in the one-pot method catalyzed by Lewis acida
OH
OH
Br
Br
CO2Me
Br
nC5H11
O
Br
CO2Me
Br
cat. [M]
O
O
+
CO2Me
toluene
24 h
toluene
time
CO2Me
nC5H11
nC5H11
CO2Me
nC5H11
nC5H11
1a
2
3a
3a'
4a
4a'
Entry
Timeb (h)
Cat. [M]
Temp. (°C)
Yieldc (%)
3a
3a0
4a
4a0
1
2
3
4
48
48
2
IrCl3ꢀ3H2O (10 mol %)
FeCl3 (10 mol %)
Reflux
Reflux
Reflux
Reflux
10
—
74
—
Trace
—
9
58
(76)
14
—
5
—
6
IrCl3ꢀ3H2O (10 mol %)
2
FeCl3 (10 mol %)
—
(82)
a
b
c
Reaction conditions: 1a (0.6 mmol), 2 (0.5 mmol), toluene (2.0 mL), N2 atmosphere.
Time indicates reaction time for the latter ring-opening aromatization.
Determined by 1H NMR. Isolated yield in parentheses.
Table 2
Synthesis of 2-bromobenzoate 4a in the one-pot methoda
OH
OH
Br
CO2Me
Br
Br
nC5H11
O
Br
CO2Me
Br
O
additive
toluene
reflux, 2 h
O
+
CO2Me
toluene
CO2Me
nC5H11
4a
nC5H11
nC5H11
reflux, 24 h
CO2Me
nC5H11
4a'
1a
2
3a
3a'
Entry
Additive
Yieldb (%)
3a
3a0
4a
4a0
1
2
3
4
5
Silica gel (300 mg)
Silica gel (150 mg)
Silica gel (50 mg)
Silica gel (150 mgc)
HCl (aq) (1.5 equiv)
—
—
55
69
60
1
2
6
8
8
(79)
(82)
26
5
30
—
—
—
—
—
a
b
c
Reaction condition: 1a (0.6 mmol), 2 (0.5 mmol), toluene (2.0 mL), N2 atmosphere.
Determined by 1H NMR. Isolated yield in parentheses.
Treated with 6 wt % triethylamine.

5304
H. Shinohara et al. / Tetrahedron Letters 55 (2014) 5302–5305
Table 3
Synthesis of 2-bromobenzoate 4 in the one-pot methoda
OH
Br
Br
R1
O
Br
silica gel
O
+
R2
toluene, reflux
2 h
toluene, reflux
24 h
R2
CO2Me
CO2Me
R2
R1
CO2Me
R1
4
1
2
3
Entry
Furans
R1
R2
Yieldb (%)
3
4
1
2
3
4
5
6
7
8
9
10
11
12
13
1a
1b
1c
1d
1e
1f
1g
1h
1i
nC5H11
OMe
4-MeO–C6H4
4-Me–C6H4
Ph
4-F–C6H4
4-Br–C6H4
4-CF3–C6H4
4-CN–C6H4
4-NO2–C6H4
CO2Me
H
H
H
H
H
H
H
H
H
H
H
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
—
—
—
—
—
—
—
4a
4b
4c
4d
4e
4f
4g
4h
4i
82
75
81
85
93
76
75
20c
40c
0c
0c
—
40c
Tracec
0c
1j
1k
1l
3j
3k
3l
4j
4k
4l
0c
Mesityl
Me
0c
1m
Me
3m
65c
4m
Tracec
a
b
c
Reaction conditions: 1 (0.6 mmol), 2 (0.5 mmol), toluene (2.0 mL), silica gel (150 mg), N2 atmosphere.
Isolated yield.
Determined by 1H NMR.
and 4e in good yields, respectively (entries 3–5). 2-Phenylfurans
with fluorine (1f) or bromine (1g) at the para-positions were also
good substrates for the reactions, giving 4f and 4g in good yields,
respectively (entries 6 and 7). In contrast, for 2-phenylfurans bear-
ing strong electron-withdrawing groups such as trifluoromethyl
(1h), cyano (1i), or nitro (1j), significant decreases in reactivity
were observed (entries 8–10). Methyl 2-furoate (1k) was an unfa-
vorable substrate as a diene for the Diels–Alder reaction (entry 11).
Unfortunately, the reaction of 2-mesitylfuran with 2 did not pro-
ceed due to steric factor (entry 12). In the case of 2,3-dimethylfu-
ran (1m), Diels–Alder adduct 3m was obtained in 65% yield as a
major product (entry 13).
Next, we examined the one-pot reaction of 1h, 1i, and 1m,
which showed the poor reactivity in the latter ring-opening aroma-
tization, using FeCl3. After the Diels–Alder reaction of 1h with 2 at
reflux for 24 h under nitrogen atmosphere, 10 mol % of FeCl3 was
added and stirred for 2 h at reflux to give the desired product 4h
and its regioisomer 4h0 in 54% and 3% yields, respectively (Table 4,
entry 1). In the same way, 1i was also converted to the correspond-
ing products 4i and 4i0 in 40% and 4% yields, respectively (entry 2).
In the case of disubstituted furan 1m, the desired product 4m was
obtained in 79% yield with its regioisomer 4m0 (11%) and a small
amount of 3m0 (4%). Although yields of desired products 4h, 4i,
and 4m were increased by the use of FeCl3 for latter ring-opening
aromatization, troublesome regioisomer 40, which is difficult to
isolate from 4, was included in case of entry 3.
Next, we planned to apply our products in Pd-catalyzed cross-
coupling reactions. First, we conducted the Suzuki–Miyaura cou-
pling reaction of 4e with phenylboronic acid in the presence of a
catalytic amount of Pd(PPh3)4. In spite of the sterically hindered
aryl bromide, the reaction proceeded successfully to afford the
desired product 6 in 43% yield (Table 5, entry 1). In this case, many
unidentified byproducts and the starting aryl bromide 4e were
detected by 1H NMR of the reaction mixture. We presumed that
these byproducts were caused by the presence of the phenolic
hydroxy group, and therefore, we examined the reaction of aryl
Table 5
Table 4
a
Suzuki–Miyaura cross-coupling reaction using 4e and its derivative 5
Synthesis of 2-bromobenzoates 4 using FeCl3
OR
B(OH)2
Br
R1
O
Br
+
+
3
+
3'
toluene
R2
CO2Me
CO2Me
2
reflux, 24 h
OR
Ph
Ph
1
cat. [Pd]
ligand
Cs2CO3
5.0 mol %
FeCl3
(10 mol %)
4
+
4'
20 mol %
2.0 equiv
toluene, reflux, 2 h
CO2Me
tolene, reflux, 24 h
Entry
R1
R2
Yieldb (%)
30
4
40
Entry
R
Cat. [Pd]
Ligand
Yielda (%)
1
2
3
1h
1i
1m
4-CF3–C6H4
4-CN–C6H4
Me
H
H
Me
Trace
Trace
4
54
40
79
3
4
11
1
2
3
4
H
4e
5
5
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd2(dba)3ꢀCHCl3
—
—
6
43
Trace
41
Me
Me
Me
7
7
7
P(Cy)3
a
Reaction conditions:
atmosphere.
1 (0.6 mmol), 2 (0.5 mmol), toluene (2.0 mL), N2
5
P(tBu)3
82(82)
Determined by 1H NMR. Isolated yield in parentheses.
a
Determined by 1H NMR spectra.
b

H. Shinohara et al. / Tetrahedron Letters 55 (2014) 5302–5305
5305
3. For recent examples of functional materials, see: (a) Song, C.; Zhao, P.; Hu, Z.;
Shi, S.; Gui, Y.; Chang, J. Bioorg. Med. Chem. 2010, 20, 2297–2298; (b) Altemöller,
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M.; Katsumata, H.; Manabe, K. J. Org. Chem. 2013, 78, 9270–9281; (b) Huang, J.;
Foyle, D.; Lin, X.; Yang, J. J. Org. Chem. 2013, 78, 9166–9173; (c) Schmidt, B.;
Riemer, M.; Karras, M. J. Org. Chem. 2013, 78, 8680–8688; (d) He, Y.; Zu, J.; Yu,
Z.-H.; Gunawan, A. M.; Wu, L.; Wang, L.; Zhang, Z.-Y. J. Med. Chem. 2013, 56,
832–842; (e) Moreno-Sanz, G.; Duranti, A.; Melzig, L.; Fiorelli, C.; Ruda, G. F.;
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cat. [Pd] 2.5 mol %
P(tBu)3 20 mol %
OMe
CuI
10 mol %
+
5
NEt3, reflux, 24 h
CO2Me
76%
1.0 equiv
1.0 equiv
Ph
8
cat. [Pd] 2.5 mol %
P(tBu)3 20 mol %
OMe
CO2Me
NEt3
2.0 equiv
CO2Me
5
+
CH3CN, reflux, 24 h
CO2Me
90%
1.0 equiv 1.0 equiv
Ph
9
Scheme 3. Sonogashira and Heck reactions using 5.
bromide 5, derived from 4e by methylation using iodomethane and
K2CO3. However, only a trace amount of coupling product 7 was
detected by 1H NMR and almost all of the starting 5 was recovered
(entry 2). In order to facilitate the oxidative addition step, the reac-
tion was conducted using Pd2(dba)3ꢀCHCl3 with P(Cy)3 to afford the
desired coupling product 7 in 41% yield (entry 3). Enhancement of
the reactivity was observed by the use of the more basic P(tBu)3 to
give 7 in 82% yield (entry 4). With these results in hand, we exam-
ined the Sonogashira coupling reaction of 5 with phenylacetylene
in the presence of CuI as a co-catalyst. The reaction proceeded
smoothly to give the corresponding coupling product 8 in 76%
yield. To our pleasure, the Heck reaction of 5 with methyl acrylate
also proceeded effectively to afford the desired product 9 in 90%
yield (Scheme 3).
In summary, we have developed a novel and efficient method
for the synthesis of 2-bromobenzoates via the Diels–Alder reaction
of furans with methyl 3-bromopropiolate 2 and the subsequent
ring-opening aromatization reaction promoted by silica gel in
one-pot. Moreover, the obtained 2-bromobenzoates were good
substrates for Pd-catalyzed cross-coupling reactions, including
the Suzuki–Miyaura coupling reaction, Sonogashira coupling reac-
tion, and Heck reaction. Further studies of these transformations
are now in progress.
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Acknowledgment
This work was financially supported by JSPS KAKENHI Grant
Number 25410051.
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10. The presence of regioisomer 4a0 was confirmed clearly by the 1H NMR spectra.
The hydroxy proton signal for 4a was observed at d 5.82 ppm. In contrast, the
hydroxy proton signal for 4a0 appeared at lower field (d 9.86 ppm) due to a
contribution of hydrogen-bonding interaction between the hydroxyl group and
the adjacent carbonyl group. These assignments were also supported by the
hydroxy proton signals for 2-bromophenol (d 5.51 ppm) and for methyl
salicylate (d 10.75 ppm).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.07.
084.
References and notes
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