Cascade synthesis of benzofuran derivatives via laccase oxidation-Michael addition

Article abstract of DOI:10.1016/j.tet.2007.08.066

The cascade synthesis of benzofuran derivatives was conducted from the reaction of catechols and 1,3-dicarbonyl compounds via oxidation-Michael addition in the presence of laccase and Sc(OTf)₃/SDS. This reaction was carried out under air at room temperature in aqueous medium, and provided benzofuran products in moderate to good yields. In addition, this reaction system showed recyclability.

Full text of DOI:10.1016/j.tet.2007.08.066

Tetrahedron 63 (2007) 10958–10962
Cascade synthesis of benzofuran derivatives
via laccase oxidation–Michael addition
*
Suteera Witayakran, Leslie Gelbaum and Arthur J. Ragauskas
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA
Received 7 July 2007; revised 15 August 2007; accepted 20 August 2007
Available online 25 August 2007
Abstract—The cascade synthesis of benzofuran derivatives was conducted from the reaction of catechols and 1,3-dicarbonyl compounds via
oxidation–Michael addition in the presence of laccase and Sc(OTf) /SDS. This reaction was carried out under air at room temperature in aque-
3
ous medium, and provided benzofuran products in moderate to good yields. In addition, this reaction system showed recyclability.
Published by Elsevier Ltd.
5
1. Introduction
and anti-HIV-1 activities. Therefore, the syntheses of ben-
zofuran derivatives have been extensively investigated.
6
In recent years, enzymes have been used in the development
of organic synthesis reactions due to their synthetic, eco-
nomical, and especially, environmental advantages. Lac-
Most of these synthetic methods involve the formation of
an annellated furan ring by the intramolecular cyclization
of benzene derivatives. These procedures involve either
multi-steps, rigorous reaction conditions, or expensive re-
agents. Recently, Nematollahi et al. and Bu et al. reported
the one-pot synthesis of polyhydroxylated benzofurans via
the oxidation of catechols by an electrochemical method or
sodium iodate, respectively, in the presence of 1,3-dicarbonyl
compounds. Nevertheless, using biocatalysis in the prepara-
tion of polyhydroxylated benzofurans has never been re-
ported. This study reports the first study at accomplishing
this synthesis via a biocatalyst.
1
case (benzenediol:oxygen oxidoreductase, EC 1.10.3.2),
a multi-copper-containing oxidoreductase enzyme, is one
of the enzymes that is being studied as a biocatalyst in or-
ganic synthesis due to its ability to catalyze the oxidation
of various substrates, specifically, phenols, o- and p-diphe-
nols, and lignin derivatives. This oxidation is accompanied
7
8
2
by the reduction of dioxygen to water. Moreover, the stabil-
ity of laccases in solution and their mild reaction condition
for phenol moieties provide promising synthetic opportuni-
ties. Indeed, a number of laccase-catalyzed reactions have
3
,4
been reported. Recently, laccase was examined in the field
of enzyme-initiated domino reaction chemistry. For exam-
ple, utilizing their well known propensity to oxidize pheno-
lics, Lalk et al. reported a laccase-catalyzed nuclear
In this procedure, o-quinones, generated in situ from the ox-
idation of catechols by laccase, underwent the Michael addi-
tion reaction with 1,3-dicarbonyl compounds, and then,
underwent intramolecular cyclization to benzofuran deriva-
tives. In addition, this study investigated the reaction system
in the presence of either Lewis acid or Lewis base to improve
reaction condition, and documented the recyclability of the
catalytic system.
3
a
animation tandem reaction, and recently, we reported the
cascade synthesis of naphthoquinone derivatives via Diels–
Alder reaction catalyzed by laccase. These studies have
4
demonstrated the synthetic research capabilities of this
oxidative enzyme.
Herein, to the best of our knowledge, we present the first
laccase-catalyzed carbon–carbon bond formation via
oxidation–Michael addition for the cascade synthesis of ben-
zofuran derivatives. Benzofurans have attracted much atten-
tion due to their broad spectrum of pharmacological
activities, such as anticancer, antimicrobial, antioxidant,
2
. Results and discussion
In a preliminary study, the reaction of 3-methylcatechol (1a)
and acetylacetone (2a) in the presence of laccase was inves-
tigated. The reaction was carried out under air at room tem-
ꢀ
perature (23 C) in the aqueous buffer solution for 4 h. This
reaction system was chosen to be a model reaction for this
study because the product, 3-acetyl-5,6-dihydroxy-2,7-di-
methylbenzofuran (3a), gradually precipitated during the re-
action and was easy to recover by filtration after the reaction.
Keywords: Laccase; Oxidation–Michael reaction; Benzofurans; Quinone;
Green chemistry; Cascade reaction.
*
Corresponding author. Tel.: +1 404 894 9701; fax: +1 404 894 4778;
e-mail: arthur.ragauskas@chemistry.gatech.edu
0040–4020/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.tet.2007.08.066

S. Witayakran et al. / Tetrahedron 63 (2007) 10958–10962
10959
The effect of pH on this reaction system was initially stud-
ied. As summarized in Table 1, the optimal yields of 3a
were achieved when the reaction was conducted at pH 7.0.
At pH 4.5, no product formed because this low pH was not
basic enough to deprotonate alpha-proton from acetylace-
tone to facilitate the Michael addition reaction with the in
situ-generated o-quinone. At a higher pH value of 8.0,
only a small yield of 3a was obtained due to laccase activity
Table 2. The effect of Lewis bases on the reaction of 3-methylcatechol (1a)
and acetylacetone (2a)
OH
OH
O
O
O
OH
Laccase
+
Lewis base
Solvent
rt, 4 h
OH
O
1a
2a
3a
a
9
Entry Lewis bases Solvent
1a:2a:Lewis Yield of
base (equiv) 3a (%)
which was dramatically decreased at this pH. Therefore,
only a small amount of starting catechol was oxidized and
reacted subsequently with acetylacetone. Moreover, the ratio
of 1a and 2a also affected the yield of 3a. The result shows
that the yield of 3a increased when using 1a and 2a in 1:2
ratio (entry 2).
1
2
Pyridine
Pyridine
Water
1:2:0.5
33
40
0.1 M Phosphate 1:2:0.5
buffer pH 7.0
0.1 M Phosphate 1:2:1
buffer pH 7.0
0.1 M Phosphate 1:2:1
buffer pH 7.0
0.1 M Phosphate 1:2:1
buffer pH 7.0
3
4
5
Pyridine
DMAP
54
9
After this preliminary study, the next phase was to improve
the yield of the product by enhancing the Michael addition
step. Traditionally, Michael reactions are catalyzed by strong
DABCO
13
1
0
a
Isolated yield.
bases, such as alkali metal, alkoxides, or hydroxides.
However, these strongly basic conditions can lead to a
number of side reactions and subsequent reactions, and
especially for this reaction system, the in situ-generated
o-quinones are easily decomposed in the presence of hydr-
become an environmental concern. Studies by Kobayashi
et al. haveshowed that the rareearth metal triflates (Sc(OTf)3,
1
1
Yb(OTf) , etc.) can be used as Lewis acid catalyst in water-
3
oxides. Recently, Xia et al. reported the use of a Lewis
base to catalyze the Michael addition of azide ion to cyclic
1
4
containing solvents. Therefore, we examined a variety of
Lewis acids including the water-compatible Lewis acid,
Sc(OTf) , and Yb(OTf) for the synthesis of 3a, and also
1
2
enones in water. Herein, adding Lewis base to the cata-
lyzed Michael addition step was investigated. Table 2 re-
veals that the best yield of 3a was obtained when using
pyridine as the Lewis base in phosphate buffer pH 7.0, and
the ratio of 1a:2a:pyridine was 1:2:1. While the use of stron-
ger Lewis bases such as 4-dimethylaminopyridine (DMAP)
and 1,4-diazabicyclo[2.2.2]octane (DABCO) provided only
a low yield of the product. Although the use of pyridine gave
the best result for this reaction system, the yield of the prod-
uct (54%, entry 3) was still much lower than the yield of the
product (64%, Table 1, entry 2) accomplished without pyri-
dine. According to these results, adding basic reagents into
this reaction did not enhance the reaction efficiency, espe-
cially, when a strong base was used.
3
3,
examined the recyclability of the optimal Lewis acid for this
reaction as described later in this paper. The reaction was
carried out under the optimal condition in the preliminary
study (Table 1, entry 2) but Lewis acids were varied. The re-
sults of this Lewis acid study is summarized in Table 3. The
results show that the water-stable Lewis acid, Sc(OTf) and
3
Yb(OTf) can enhance the Michael addition step for this re-
3
action system and provided a very good yield of 3a. Sc(OTf)3
showed better result than Yb(OTf) . However, we have to use
3
0.2 equiv of Sc(OTf) to obtain the highest yield of 3a (74%,
entry 2) because using only 0.1 equiv of Sc(OTf) did not
3
3
have any effect on the reaction yield (63%, entry 1) when
compared to the reaction without Sc(OTf) (64%, Table 1,
entry 2).
3
In order to circumvent the alkaline conditions mentioned
above, we decided to investigate the reaction in the presence
of a Lewis acidas an alternative method. Lewis acid-catalyzed
Michael reactions have been developed, allowing the reac-
tion to be carried out under milder conditions with high effi-
As we conducted the reaction in aqueous medium, the main
drawback was the low solubility of organic substances. To
overcome this problem, a small amount of surfactant, so-
dium dodecyl sulfate (SDS, 20 mol %) was used to improve
1
3
ciency. Our studies focus on the use of water as the reaction
medium to avoid the use of organic solvents which have
Table 3. The effect of Lewis acids on the reaction of 3-methylcatechol (1a)
and acetylacetone (2a)
Table 1. The effect of pH on the reaction of 3-methylcatechol (1a) and
acetylacetone (2a)
OH
OH
OH
O
O
O
OH
OH
+
OH
OH
O
O
O
Laccase, Lewis acid
OH
+
Laccase
Solvent
rt, 4 h
Phosphate buffer pH 7
rt, 4 h
O
1a
2a
3a
O
1a
2a
3a
a
1a:2a:Lewis acid (equiv) Yield of 3a (%)
Entry Lewis acid
a
Yield of
3a (%)
Entry
Solvent
1a:2a
equiv)
1
2
3
4
5
6
Sc(OTf)
Sc(OTf)
Sc(OTf)
3
3
3
1:2:0.1
1:2:0.2
/SDS 1:2:0.2
63
74
76
72
71
49
(
1
2
3
4
0.1 M Phosphate buffer pH 7.0
0.1 M Phosphate buffer pH 7.0
0.1 M Acetate buffer pH 4.5
0.1 M Phosphate buffer pH 8.0
1:1
1:2
1:2
1:2
46
64
0
Yb(OTf)
3
1:2:0.2
1:2:0.2
1:2:0.2
InCl
CuCl
3
2
$4H O
6
2
a
a
Isolated yield.
Isolated yield.

10960
S. Witayakran et al. / Tetrahedron 63 (2007) 10958–10962
the solubility, and the result shows a small increase of prod-
uct yield from 74% to 76% (Table 3, entry 3). This result
agrees with Kobayashi’s work on the study of surfactant-
0.1 M phosphate buffer (pH 7.0), in the presence of laccase,
20 mol % of Sc(OTf) , and 20 mol % of SDS under air at
room temperature. The proposed reaction pathway of this
catalytic system is illustrated in Scheme 1, and the result
of the reaction of various catechols and 1,3-dicarbonyl com-
pounds are summarized in Table 4.
3
1
5
aided Lewis acid catalysis in aqueous aldol reaction.
After successfully conducting the optimization experiments
described above, we chose to conduct further synthesis of
benzofuran derivatives by introducing 1 mmol of substituted
catechols and 2 mmol of 1,3-dicarbonyl compounds in
The data in Table 4 show that the reaction depends on the
reactivity of the in situ-generated o-quinones. The very
O
(TfO)3Sc
O
O
OH
O
O
O
OH
2a
1a
Sc(OTf)3
Laccase (red)
Laccase (ox)
O
O
O
O
(
TfO)3Sc
(TfO)3Sc
O2 for Air
H2O
O
O
O
O
O
O
O
O
O
HO
HO
O
HO
HO
HO
Aromatization
O
OH
O
O
HO
3a
3
Scheme 1. Proposed mechanism of laccase/Sc(OTf) catalytic system for the synthesis of 3-acetyl-5,6-dihydroxy-2,7-dimethylbenzofuran (3a).
Table 4. The reaction of catechols and 1,3-dicarbonyl compounds
R1
OH
OH
OH
O
O
O
R1
R2
OH
Laccase, 0.2 equiv Sc(OTf)3
.1M Phosphate buffer pH 7
.2 equiv SDS, rt, 4 h
R3
R5
+
R3
R5
0
R4
0
O
a
Entry
Catechol
1,3-Dicarbonyl compound
Product (% yield)
R1
OH
O
O
OH
OH
R1
R2
OH
O
R3
R5
R3
R5
R4
2
O
1
3
1
2
3
4
5
6
7
8
9
1a: R
1a
1a
1b: R
1b
1b
1
¼Me, R
2
¼H
2a: R
2b: R
3
¼R
¼R
¼Me, R
5
¼Me, R
¼Me, R
¼Cl, R
4
¼H
¼Cl
¼OEt
3a (76)
3a (79)
3b (48)
3c (68)
3c (66)
3d (46)
No product formed
No product formed
3a (9)
b
b
3
5
4
2c: R
2a
3
4
5
1
¼R ¼H
2
b
b
2b
2c
2a
2a
2a
2a
1c: R
1d: R
1
¼OMe, R
¼F, R ¼H
¼H, R ¼Cl
¼H, R ¼COOH
2
¼H
1
2
1e: R
1f: R
1
2
10
1
2
3a (11)
a
Isolated yield.
Reaction time is 1 h.
b

S. Witayakran et al. / Tetrahedron 63 (2007) 10958–10962
10961
reactive quinones, such as 3-methoxy-1,2-benzoquinone and
-fluoro-1,2-benzoquinone, which have rich electron donat-
ing group (OMe) and a strong electron withdrawing group
F), respectively, did not provide any desired products (en-
Table 6. Recycling of the catalytic system for the synthesis of 3-acetyl-
5
,6-dihydroxy-3,7-dimethylbenzofuran (3a)
3
OH
OH
(
tries 7 and 8). This reactivity pattern may be caused by
side reactions of these highly reactive quinones. In contrast,
the reaction of catechols, such as 3-methylcatechol and cat-
echol with laccase generated moderately reactive quinones
that gave excellent yields of the corresponding benzofuran
products as shown in entries 1–6. Moreover, the reactivity
of 1,3-dicarbonyl compounds also has an effect on the reac-
tion. When we used 1,3-dicarbonyl compounds that had an
electron withdrawing group (Cl) at alpha-position, the reac-
tion time was only 1 h. The shorter reaction time caused by
the increase of alpha-proton acidity of these 1,3-dicarbonyl
compounds make them easier to deprotonate and ready to re-
act with in situ-generated o-quinone in the reaction. Besides
OH
OH
O
1a
Laccase, 0.2 equiv Sc(OTf)3
+
0
.1M Phosphate buffer pH 7
O
O
0.2 equiv SDS, rt, 4 h
3a
O
2a
a
Yield of 3a (%)
Run
1
2
3
76
62
51
a
Isolated yield.
3
4
-substituted catechols, 4-substituted catechols, such as
-chlorocatechol and 3,4-dihydroxy benzoic acid, can also
3. Conclusions
be used for the synthesis of polyhydroxylated benzofurans
entries 9 and 10). However, the yield of the product is low.
(
In conclusion, this study provides an efficient green chemis-
try synthesis of benzofuran derivatives from the reaction of
catechols and b-dicarbonyl compounds using a catalytic sys-
In addition, we observed that the reactions summarized in
Table 4 gave only one isomer from other potential products
that could form. This could be explained by the existence of
a substituent at the C-3 position of catechols that probably
causes the Michael acceptors, in situ-generated o-quinones,
to be attacked by 1,3-dicarbonyl compounds only at less hin-
dered C-5 position to yield the observed product (see
Scheme 1). Most of the products from this study are known
compounds. Only product 3b is unknown. Therefore, the
tem of laccase and Sc(OTf) in surfactant aqueous medium.
3
This reaction is regioselective providing only one isomer
product and the first example of a two-component catalytic
system employing laccase and a lanthanide Lewis acid cat-
alyst. The yield of the products from reaction depended on
both the reactivity of catechols and b-dicarbonyl com-
pounds. For this reaction system, catechols with moderate
reactivity yield benzofuran products in excellent yield. In
addition, the newly developed catalytic system could also
be recycled and reused for two additional runs, with only
a minor drop in product yields.
1
13
structure of 3b was confirmed by the H NMR, C NMR
and HMBC correlations as shown in Table 5.
Next, we examined the recyclability of the two-component
catalytic system, laccase/Sc(OTf) , for the synthesis of ben-
3
zofuran 3a by the reaction of 1a and 2a in 0.10 M phosphate
buffer pH 7.0 and 20 mol % SDS. Due to the product 3a pre-
cipitated during the reaction, we can directly reuse this cat-
alytic system after product filtration. The results shown in
Table 6 demonstrate that this catalytic system was readily
recyclable for three runs, with approximately a 10% drop
of the product yield/reaction.
4. Experimental
4.1. General
All chemicals were obtained from Aldrich and used as re-
ceived without further purification. Laccase (EC 1.10.3.2)
from Trametes Villosa was donated by Novo Nordisk
1
13
Biochem, North Carolina. H and C NMR spectra were re-
corded on a Bruker-400 spectrometer operating at 400 MHz
1
13
for H and 100 MHz for C. For HMBC correlations, the ex-
periment was carried out on a Bruker-DRX 500 spectrome-
ter. Column chromatography was performed on Combiflash
Companion instrument (Teledyne Isco Company) using Re-
diSep normal-phase flash columns. TLC was performed on
aluminum sheets precoated with silica gel 60 F254 (EMD
Chemicals). Mass spectra were carried out in The Georgia
Institute of Technology Bioanalytical Mass Spectrometry
Facility.
1
Table 5. H and C assignments and HMBC correlations for compound 3b
13
a
8
1
O
7
7
a
6
5
OH
OH
2
a
2
3
a
3
1
'
4
3'
2'
O
O
3
b
1
3
1
1 13
H– C correlations
Carbon
C (d)
H (d)
2
4
8
2
3
a
14.2
102.8
8.9
59.8
14.2
2.66, s, 3H
7.13, s, 1H
2.25, s, 3H
4.29, q, 2H (7)
1.35, t, 3H (7)
9.31, s, 1H
C2, C3
C3, C5, C6, C7a
C6, C7, C7a
4
.2. Enzyme assay
0
0
0
0
0
Laccase activity was determined by oxidation of 2,2 -azino-
C1 , C3
C2
1
6
0
bis-(3-ethylbenzyl thiozoline-6-sulfonate) (ABTS). The
assay mixture contained 25 mM ABTS, 0.10 M sodium ace-
tate (pH 5.0), and a suitable amount of enzyme. The oxida-
tion of ABTS was followed by an absorbance increase at
420 nm (3420¼3.6ꢁ10 M cm ). Enzyme activity was
OH (5)
OH (6)
C4, C5
C6, C7
8.42, s, 1H
a
13
1
6
Measured in DMSO-d at 125 MHz ( C) or 500 MHz ( H, J (Hz) values
in parentheses). Chemical shifts are expressed in d (ppm).
4
ꢂ1
ꢂ1

10962
S. Witayakran et al. / Tetrahedron 63 (2007) 10958–10962
expressed in units (U¼micromoles of ABTS oxidized per
219–226; (c) Ciecholewski, S.; Hammer, E.; Manda, K.; Bose,
G.; Nguyen, V. T. H.; Langer, P.; Schauer, F. Tetrahedron 2005,
minute).
6
1, 4615–4619; (d) Kurisawa, M.; Chung, J. E.; Uyama, H.;
4
.3. General procedure for benzofuran derivatives
Kobayashi, S. Biomacromolecules 2003, 4, 1394–1399; (e)
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(f) Hosny, M.; Rosazza, J. J. Agric. Food Chem. 2002, 50,
5539–5545; (g) Sch €a fer, A.; Specht, M.; Hetzheim, A.;
Francke, W.; Schauer, F. Tetrahedron 2001, 57, 7693–7699;
(h) Manda, K.; Hammer, E.; Mikolasch, A.; Niedermeyer, T.;
Dec, J.; Jones, A. D.; Benesi, A. J.; Schauer, F.; Bollag, J.-M.
J Mol. Catal., B: Enzym. 2005, 35, 86–92; (i) Leutbecher, H.;
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A. J. Green Chem. 2007, 9, 475–480.
In a 250-mL round-bottom flask, 30 mL of 0.10 M phos-
phate buffer pH 7.0 and catechol (1 mmol) were mixed to-
gether. Next, 100 U of laccase was added to the reaction
mixture and then, 1,3-dicarbonyl compound (2 mmol),
Sc(OTf)3 (0.2 mmol, 0.0984 g), and SDS (0.2 mmol,
0
.0576 g) were added. The reaction was then stirred under
air at room temperature for 1–4 h. After the reaction was fin-
ished, the reaction mixture was then filtered and washed with
water to collect the precipitated product. If the product did
not precipitate, the reaction mixture was extracted by
EtOAc. The organic phase was combined, dried over
MgSO , and evaporated. The resulting crude products
4
5. (a) McCallion, G. D. Curr. Org. Chem. 1999, 3, 67–76; (b)
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were purified by silica column chromatography, using ethyl
acetate and petroleum ether as the eluent to obtain the ben-
zofuran product. Products were characterized by H NMR,
1
1
3
C NMR, and MS. Products 3a, 3c, and 3d are known com-
pounds and their H and C NMR data are consistent with
those in the literature.
1
13
4
carboxylate (3b). Mp: 183–185 C; H NMR (DMSO-d ,
.3.1. Ethyl-5,6-dihydroxy-2,7-dimethyl-3-benzofuran
ꢀ
1
6
4
2
00 MHz) d 1.4 (t, J¼7 Hz, 3H, CH ), 2.3 (s, 3H, CH ),
3
3
.7 (s, 3H, CH ), 4.3 (q, J¼7 Hz, 2H, CH ), 7.1 (s, 1H,
3 2
1
3
Ar–H), 8.4 (s, 1H, OH), 9.3 (s, 1H, OH); C NMR
DMSO-d , 100 MHz) d 8.9, 14.2, 14.2, 59.8, 102.8,
(
6
1
07.2, 108.3, 115.9, 141.9, 143.0, 146.7, 161.0, 163.9; MS
EI) m/z 250 (M , 98%), 221 (100), 176 (11), 93 (4), 43
+
(
(
7); HRMS (EI) 250.08453 (C H O requires 250.08412).
13 14 5
Acknowledgements
7. (a) Nematollahi, D.; Forooghi, Z. Tetrahedron 2002, 58, 4949–
4
953; (b) Nematollahi, D.; Habibi, D. J. Org. Chem. 2004, 69,
The authors acknowledge financial support from
the IPST@GT student fellowship and the Royal Thai
Government Scholarship.
2637–2640; (c) Nematollahi, D.; Rafiee, M. J. Electroanal.
Chem. 2004, 566, 31–37; (d) Nematollahi, D.; Rafiee, M.
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8
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