O-heterocycles via laccase-catalyzed domino reactions with O2 as the oxidant

Article abstract of DOI:10.1055/s-2005-922748

Laccase-catalyzed domino reactions of 4-hydroxy-6-methyl-2H-pyran-2-one or substituted 4-hydroxy-2H-chromen-2-ones with catechols using molecular oxygen as an oxidant afford coumestans and related O-heterocycles with yields ranging from 51% to 99%. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-922748

3126
LETTER
O-Heterocycles via Laccase-Catalyzed Domino Reactions with O2 as
the Oxidant
L
accase-Catalyzed
e
D
ominoR
i
eaction
k
s
o Leutbecher, Jürgen Conrad, Iris Klaiber, Uwe Beifuss*
Bioorganische Chemie, Institut für Chemie, Universität Hohenheim, Garbenstraße 30, 70599 Stuttgart, Germany
Fax +49(711)4592951; E-mail: ubeifuss@uni-hohenheim.de
Received 24 August 2005
quinoid systems, which can then further react in various
subsequent reactions.10
Abstract: Laccase-catalyzed domino reactions of 4-hydroxy-6-
methyl-2H-pyran-2-one or substituted 4-hydroxy-2H-chromen-2-
ones with catechols using molecular oxygen as an oxidant afford
coumestans and related O-heterocycles with yields ranging from
51% to 99%.
The coumestans are comprised of a group of natural prod-
ucts characterized by a 6H-benzofuro[3,2-c]chromen-6-
one skeleton.11 This group exhibits a number of interest-
ing biological activities, among them phytoestrogenic,
antibacterial, antifungal, antihepatotoxic and phytoalex-
ine effects.12 As a privileged scaffold the coumestans have
recently gained increased attention, and, in addition to the
already known ones,13 a number of novel synthetic meth-
ods have been developed for constructing the coumestan
and related ring systems.14 One of the most successful
methods has been the Wanzlick type of oxidative conden-
sation of 4-hydroxy-2H-chromen-2-one with catechols.15
These reactions are typically performed with inorganic
oxidants such as K3[Fe(CN)6],15,16 but can also be
accomplished both electrochemically17 and enzymatically
with tyrosinase.18
Key words: enzymes, catalysis, oxygen, heterocycles, domino
reactions
Enzyme-catalyzed reactions play an increasingly impor-
tant role in organic synthesis.1 Biotransformations have
proved to be particularly valuable for hydrolyzing esters
and amides, reducing aldehydes and ketones, oxidizing
alcohols and aldehydes as well as various C–C bond form-
ing reactions. Little investigation has been directed
though towards the application of enzymes for the synthe-
sis of heterocycles and for domino reactions.2
Laccases (benzenediol:O2 oxidoreductase E.C. 1.10.3.2)
are multicopper oxidases that are capable of oxidizing a
wide range of substrates while concomitantly reducing
O2.3 They contain a type 1 (T1) Cu center, a type 2 (T2)
Cu center and a type 3 (T3) Cu center; T2 and T3 form a
trinuclear Cu cluster. The oxidation of the substrate
occurs at the type 1 Cu center. The electrons are trans-
ferred to the trinuclear Cu cluster where O2 is reduced to
H2O.4
O
O
OH
O
cat. laccase, O2
pH 4.37, r.t.
OH
OH
OH
O
O
R1
R1
+
51–99%
+
3
R1
O
O
OH
Laccases are found in some plants, in the majority of
fungi, but also prokaryotes. Although they belong to the
longest known enzymes, they are still being intensively
investigated due to their great importance for the forma-
tion of plant lignins,5 their role as virulence factors
involved in fungal diseases and, in particular, their large
potential in various industrial oxidative processes such as
delignification, dye or stain bleaching, bioremediation
and plant fiber modification.6
1
2
OH
O
OH
4
Scheme 1
Table 1 Laccase-Catalyzed Domino Reactions of 1 and 2 in the
Presence of O2 as an Oxidant
Laccases still play a minor part in organic synthesis, but
recently the interest in laccase-catalyzed transformations
has been growing steadily.7 Apart from the oxidation of
benzyl alcohols to benzaldehydes performed in the
presence of a so-called mediator8 laccases have been used
in the oxidative coupling of phenolic substrates.9 Also,
they have been applied for the oxidative generation of
Entry
2
R1
Time (h) Product(s)
Yield (%)
1
2
3
4
5
a
b
c
H
3.5
3.5
4.5
7
3a
76
Me
3b
99
OMe
F
3c
51
d
e
3d, 4d
4e, 5
76a
85b
CO2Me
4
a Compounds 3d and 4d were obtained in a 1:1 ratio.
b Compounds 4e and 5 were obtained in a 7:3 ratio.
SYNLETT 2005, No. 20, pp 3126–3130
Advanced online publication: 28.11.2005
1
9
.1
2
.2
0
0
5
DOI: 10.1055/s-2005-922748; Art ID: G26705ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Laccase-Catalyzed Domino Reactions
3127
R3
R2
O
O
R3
R2
O
O
R1
cat. laccase, O2
buffer, r.t.
R3
R2
O
O
OH
OH
OH
OH
OH
OH
+
+
61–99%
O
O
R1
OH
R1
6
2
7
8
Scheme 2
OH
Here we report for the first time on laccase-catalyzed
domino reactions between 4-hydroxy-6-methyl-2H-py-
ran-2-one or 4-hydroxy-2H-chromen-2-ones and cate-
chols using molecular oxygen as the oxidating agent.
O
O
– 2e, – 2H
OH
2a
10a
First, the laccase-catalyzed reaction between 4-hydroxy-
6-methyl-2H-pyran-2-one (1) and catechol (2a, R1 = H)
was investigated in the presence of O2. We used commer-
cially available laccase of Trametes versicolor as the
enzyme.
O
O
O
O
O
1,4-addition
+
OH
OH
O
OH
OH
It was found that the transformation is best run at room
temperature in an acetate buffer at pH 4.37.19 After 3.5
hours 7,8-dihydroxy-3-methyl-1H-pyrano[4,3-b]benzo-
furan-1-one (3a) was isolated as the single product in a
yield of 76% (Scheme 1; Table 1, entry 1). Correspond-
ingly, the enzyme-catalyzed reaction with 4-hydroxy-2H-
chromen-2-one (6a) exclusively produced 8,9-dihydroxy-
6H-benzofuro[3,2-c]chromen-6-one (7a) in 85% yield
(Scheme 2; Table 2, entry 1).
1
10a
11a
O
O
O
O
O
OH
OH
– 2e, – 2H
1,4-addition
OH
O
O
3a
12a
We assume that in the first step of the reaction sequence a
laccase-catalyzed oxidation of catechol (2a) with oxygen
takes place to give o-benzoquinone 10a, which then reacts
with the nucleophilic 4-hydroxy-6-methyl-2H-pyran-2-
one (1) in an intermolecular 1,4-addition leading to non-
isolable 11a (Scheme 3). After a second laccase-catalyzed
oxidation of 11a to 12a an intramolecular 1,4-addition
occurs giving the heterocycle 3a. In other words, the
entire domino process consists of two oxidations and two
1,4-additions.
Scheme 3
Then the domino reactions were performed with 3-substi-
tuted catechols. Reactions of 1 and the donor-substituted
catechols 3-methyl catechol (2b) and 3-methoxy catechol
(2c) produced heterocycles 3b and 3c, respectively, each
as the single product (Scheme 1; Table 1, entries 2 and 3).
Corresponding results were obtained in transformations
with 4-hydroxy-2H-chromen-2-one (6a), where 7b and 7c
Table 2 Laccase-Catalyzed Domino Reactions of 2 and 6 in the Presence of O2 as the Oxidant
Entry
2
a
b
c
e
a
c
a
c
R1
6
a
a
a
a
b
b
c
R2
H
R3
H
pH
Time (h)
Product(s)
Yield (%)
1
2
3
4
5
6
7
8
H
4.37a
4.37a
4.37a
4.37a
6.0b
7
3
7a
85
99
61
89
96
94
99
88
Me
H
H
7b
7c
OMe
CO2Me
H
H
H
5
H
H
4
8d, 9c
7e
Me
Me
H
H
49
20
20
20
OMe
H
H
6.0b
7f
OMe
OMe
6.0b
7g
OMe
c
H
6.0b
7h
a The reaction was performed using the laccase of Trametes versicolor.
b The reaction was performed using the laccase of Agaricus bisporus.
c Compounds 8d and 9 were obtained in a 71:29 ratio.
Synlett 2005, No. 20, 3126–3130 © Thieme Stuttgart · New York

3128
H. Leutbecher et al.
LETTER
were isolated in 99% and 61%, respectively (Scheme 2; Subsequently, we investigated domino processes of 4-hy-
Table 2, entries 2 and 3). The selectivity of these transfor- droxy-6-methyl-2H-chromen-2-one (6b) and 4-hydroxy-
mations may be explained by assuming that the first 1,4- 7-methoxy-2H-chromen-2-one (6c), which are both
addition exclusively occurs at the more electrophilic poorly water-soluble at pH 4.37, with catechol (2a) and 3-
carbon atom at C-5 of the respective o-benzoquinones 10. methoxy catechol (2c). When using laccase of Trametes
versicolor under different conditions no reaction
occurred. After several experiments we found that trans-
When reacting 1 with methyl 2,3-dihydroxybenzoate (2e)
a mixture of 4e20 and 5 (Figure 1)21 was isolated in a 70:30
formations are catalyzed by laccase of Agaricus bisporus
ratio and 85% yield and separated by HPLC (Scheme 1;
Table 1, entry 5). We assume that the formation of both
products starts with the selective 1,4-addition of 1 at the
more electrophilic carbon atom at C-4 of the o-benzo-
in a phospate buffer at pH 6.0. Thus heterocycles 7e–h22
were obtained selectively with yields between 88% and
99% (Scheme 2; Table 2, entries 5–8).
quinone 10e and yields the non-isolable intermediate 11e 8,9-Dihydroxy-3-methoxy-6H-benzofuro[3,2-c]chromen-
(Scheme 4). The OH group of the enolized 1,3-dicarbonyl 6-one (7g), which is available in 99% yield, can easily be
system may then either react directly with the ester group reacted with bromochloromethane to give the natural
in a lactonization reaction of 11e to yield 5 or, following product flemichapparin C (13) (Scheme 5). The synthesis
oxidation of 11e to give 12e, undergo an intramolecular of flemichapparin C was thus effected in just two steps
1,4-addition leading to 4e. Correspondingly, 2e was and a total yield of 43%. Flemichapparin C was first
reacted with 6a, giving a 71:29 mixture of tetracycles 8d isolated from the roots of Flemingia chappar23 and has
and 9 (Figure 1) with 89% yield (Scheme 2; Table 2, entry been synthesized with total yields ranging from 4% to
4). Here, the reaction products could also by separated by 19%.14d,24
HPLC.
MeO
O
O
O
O
O
O
O
O
OH
OH
O
7g
OH
OH
O
OH
O
OH
BrCH2Cl
Cs2CO3, DMF
3 h, 120 °C
5
9
MeO
O
O
O
Figure 1
O
O
44%
flemichapparin C (13)
O
O
O
O
O
CO2Me
OH
Scheme 5
lactonization
OH
OH
OH
Laccase-catalyzed synthesis of medicagol (15) was then
tackled. This natural product was first isolated from
Medicago sativa25 and has been synthesized repeated-
ly.12a,14d,26 To effect synthesis of medicagol (15) a laccase-
catalyzed reaction between 7-benzyloxy-4-hydroxy-2H-
chromen-2-one (6d) and 2a was attempted. It turned out,
though, that neither laccase of Trametes versicolor nor
laccase of Agaricus bisporus were able to catalyze this
transformation. The reasons remain unclear. But we
succeeded to construct 7i according to Wanzlick15 by
reacting 6d and 2a with 6.5 equivalents of K3[Fe(CN)6] in
THF–H2O with 92% yield (Scheme 6). Subsequent
introduction of the methylenedioxy group by reacting 7i
with bromochlormethane in 66% yield and the final
debenzylation of 14 with H2, Pd/C led to the formation of
the natural product. Starting from 2a and 6d the synthesis
of medicagol (15) was thus effected in 3 steps and a total
yield of 55%. Compound 7i can also be used to form – via
debenzylation with H2, Pd/C – trihydroxy coumestan (16),
another compound demonstrating interesting biological
activity (Scheme 7).12d
O
OH
11e
5
oxidation
O
O
O
O
CO2Me
CO2Me
OH
O
1,4-addition
OH
O
O
OH
12e
4e
Scheme 4
Quite unexpectedly, the laccase-catalyzed transformation
between 1a and 3-fluorocatechol (2d) proceeds unselec-
tively, yielding approximately equal amounts of 3d and
4d (Scheme 1; Table 1, entry 4).
Synlett 2005, No. 20, 3126–3130 © Thieme Stuttgart · New York

LETTER
Laccase-Catalyzed Domino Reactions
3129
6.5 equiv K3[Fe(CN)6]
NaOAc
BrCH2Cl
Cs2CO3, DMF
2.5 h, 120 °C
BnO
O
O
THF / H2O
5 h, r.t.
BnO
O
O
OH
OH
+
OH
OH
66%
92%
O
OH
O
6d
2a
7i
BnO
O
HO
O
O
O
H2, Pd/C
THF
24 h, r.t.
O
O
O
91%
O
O
14
medicagol (15)
Scheme 6
(6) (a) Schneider, P.; Caspersen, M. B.; Mondorf, K.; Halkier,
T.; Skov, L. K.; Ostergaard, P. R.; Brown, K. M.; Brown, S.
H.; Xu, F. Enzyme Microb. Technol. 1999, 25, 502.
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S.; Jurasek, L. J. Pulp Pap. Sci. 1995, 21, J280.
BnO
O
O
OH
OH
O
7i
HO
O
O
H2, Pd/C
THF
23 h, r.t.
OH
(f) Bourbonnais, R.; Paice, M. G.; Reid, I. D.; Lanthier, P.;
Yaguchi, M. Appl. Environ. Microb. 1995, 61, 1876.
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Microb. Technol. 1990, 12, 846. (i) Reid, I. D.; Paice, M.
G.; Ho, C.; Jurasek, L. Tappi J. 1990, 73, 149.
100%
O
OH
16
Scheme 7
(7) Burton, S. G. Curr. Org. Chem. 2003, 7, 1317.
(8) (a) Fabbrini, M.; Galli, C.; Gentili, P.; Macchitella, D.
Tetrahedron Lett. 2001, 42, 7551. (b) Potthast, A.; Rosenau,
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The structures of all the products described in this paper
have been elucidated unambiguously by NMR spectro-
scopic methods.
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Synlett 2005, No. 20, 3126–3130 © Thieme Stuttgart · New York

3130
H. Leutbecher et al.
LETTER
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(20) Selected data for 4e: IR (ATR): 3405, 3119, 1703, 1440,
1296, 1220, 1050, 839 cm–1. UV (MeCN): lmax (lg e) = 233
(4.28), 335 nm (4.18). 1H NMR (300 MHz, DMSO-d6):
d = 2.36 (s, 3 H, CH3), 3.83 (s, 3 H, OCH3), 6.95 (s, 1 H, 4-
H), 7.20 (s, 1 H, 6-H), 9.51 (br s, 2 H, 7-OH, 8-OH). 13
C
NMR (75 MHz, DMSO-d6): d = 20.51 (CH3), 52.44 (OCH3),
96.36 (C-4), 100.04 (C-6), 103.02 (C-9b), 110.94 (C-9a),
114.45 (C-9), 142.02, 146.40, 148.43 (C-5a, C-7 or C-8),
158.72 (C-1), 162.76 (C-3), 164.31 (C-4a), 166.73 (C=O).
MS (EI, 70 eV): m/z (%) = 290 (20) [M+], 258 (100) [M+ –
CH4O], 229 (4), 202 (16), 174 (11), 69 (7), 43 (19). HRMS:
m/z calcd for C14H10O7: 290.04266; found: 290.04251.
(21) Selected data for 5: IR (ATR): 3440, 3092, 2500, 1693,
1636, 1273, 1252, 1046, 832 cm–1. UV (MeCN): lmax (lg e) =
225 (4.00), 244 (4.08), 347 nm (4.04). 1H NMR (300 MHz,
acetone-d6): d = 2.42 (s, 3 H, CH3), 6.53 (s, 1 H, 4-H), 7.49
(d, 3J9-H, 10-H = 8.7 Hz, 1 H, 9-H or 10-H), 8.39 (d, 3J10-H, 9-H
=
8.7 Hz, 1 H, 9-H or 10-H), 8.74 (br s, 1 H, OH), 10.87 (br s,
1 H, OH). 13C NMR (75 MHz, acetone-d6): d = 19.27 (CH3),
98.95 (C-4), 99.72 (C-10b), 106.14 (C-6a or C-10a), 116.58
(C-9 or C-10), 124.46 (C-9 or C-10), 124.50 (C-6a or
C-10a), 145.42 (C-7 or C-8), 149.31 (C-7 or C-8), 160.13
(C-1, C-4a or C-6), 160.15 (C-1, C-4a or C-6), 163.15 (C-3),
164.33 (C-1 or C-6). MS (EI, 70 eV): m/z (%) = 260 (100)
[M+], 245 (4) [M+ – CH3], 232 (4), 189 (5), 176 (11), 161 (6),
120 (15), 43 (25). HRMS: m/z calcd for C13H8O6:
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1303.
(19) General Procedure for Laccase-Catalyzed Domino
Reactions.
260.03207; found: 260.03162.
(22) Selected data for 7f: IR (ATR): 3504, 3207, 1685, 1459,
1318, 1254, 1085, 848, 817 cm–1. UV (MeCN): lmax (lg e) =
215 (4.57), 250 (4.14), 343 nm (4.21). 1H NMR (300 MHz,
DMSO-d6): d = 2.43 (s, 3 H, CH3), 4.11 (s, 3 H, OCH3), 7.06
(s, 1 H, 7-H), 7.44 (s, 2 H, 3-H, 4-H), 7.82 (s, 1 H, 1-H), 9.38
(br s, 2 H, 8-OH, 9-OH). 13C NMR (75 MHz, DMSO-d6):
d = 21.01 (CH3), 61.35 (OCH3), 100.04 (C-7), 106.12 (C-6a
or C-6b), 112.66 (C-11b), 115.01 (C-6a or C-6b), 117.49
(C-3 or C-4), 121.51 (C-1), 133.05 (C-3 or C-4), 134.16
(C-10), 135.16 (C-2), 138.16, 142.41, 146.55 (C-8, C-9 or
C-10a), 151.32 (C-4a), 158.29 (C-6 or C-11a), 158.67 (C-6
or C-11a). MS (EI, 70 eV): m/z (%) = 312 (100) [M+], 297
(45) [M+ – CH3], 269 (17), 185 (7), 156 (9), 139 (11), 128
(16), 77 (12). Anal. Calcd for C17H12O6 (312.27): C, 65.39;
H, 3.87. Found: C, 65.11; H, 4.10.
4-Hydroxy-2H-pyran-2-one (1, 1 equiv, 3.0 mmol), 4-
hydroxy-2H-chromen-2-one (6, 1 equiv, 3.0 mmol), and
catechol 2 (1.1 equiv, 3.4 mmol) were dissolved in 200 mL
acetate buffer (pH 4.37, 0.2 M) and vigorously stirred under
air at r.t. in the presence of 25 mg laccase (19 U/mg) of
Trametes versicolor until the substrates had been fully
consumed, as judged by TLC. The reaction mixture was
saturated with NaCl and filtered on a Buchner funnel. The
filter cake was washed with a solution of 200 mL 15% NaCl
and 10 mL H2O. The crude products obtained after drying
exhibited a purity of 90–95% (NMR). Analytically pure
products could be obtained by recrystallization. Trans-
formations with laccase (320 U/mg) of Agaricus bisporus
were performed in a phosphate buffer (pH 6.0, 0.2 M).
(23) (a) Adityachaudhury, N.; Gupta, P. K. Phytochemistry 1973,
12, 425. (b) Adityachaudhury, N.; Gupta, P. K. Chem. Ind.
(London) 1970, 1113.
(24) (a) Farkas, L.; Antus, S.; Nogradi, M. Acta Chim. Acad. Sci.
Hung. 1974, 82, 225. (b) Fukui, K.; Nakayama, M.; Sesita,
H. Bull. Chem. Soc. Jpn. 1964, 37, 1887.
(25) Livingston, A. L.; Witt, S. C.; Lundin, R. E.; Bickoff, E. M.
J. Org. Chem. 1965, 30, 2353.
(26) Jurd, L. J. Pharm. Sci. 1965, 54, 1221.
Synlett 2005, No. 20, 3126–3130 © Thieme Stuttgart · New York