Laccase-catalyzed stereoselective oxidative ring opening of 2,5-dialkylfurans into 2-ene-1,4-diones using air as an oxidant

Article abstract of DOI:10.1039/c1gc15810d

The laccase-catalyzed ring opening of 2,5-dimethylfuran using air as an oxidant stereoselectively yields (Z)- or (E)-3-hexene-2,5-dione depending on the mediator employed: with TEMPO the (Z)-3-hexene-2,5-dione is formed, while a combination of TEMPO and violuric acid gives (E)-3-hexene-2,5-dione. The (Z)-selective ring cleavage was extended to a variety of symmetrical and unsymmetrical 2,5-dialkylfurans. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1gc15810d

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COMMUNICATION
Laccase-catalyzed stereoselective oxidative ring opening of
2,5-dialkylfurans into 2-ene-1,4-diones using air as an oxidant†
Chime`ne Asta, Ju¨rgen Conrad, Sabine Mika and Uwe Beifuss,†*
Received 7th July 2011, Accepted 3rd August 2011
DOI: 10.1039/c1gc15810d
The laccase-catalyzed ring opening of 2,5-dimethylfuran
using air as an oxidant stereoselectively yields (Z)- or (E)-
3-hexene-2,5-dione depending on the mediator employed:
with TEMPO the (Z)-3-hexene-2,5-dione is formed, while
a combination of TEMPO and violuric acid gives (E)-
3-hexene-2,5-dione. The (Z)-selective ring cleavage was
extended to a variety of symmetrical and unsymmetrical
2,5-dialkylfurans.
o-phenylenediamine with benzaldehydes provides access to
2-aryl-1-H-benzimidazoles.11
Over the last few years considerable effort has been devoted
to develop alternative liquid fuels for transportation as well as
alternative feedstocks for the chemical industry from renewable
resources.12 Several furans have been identified as platform
molecules that can be obtained from renewable resources.13 In
this respect, 2,5-dimethylfuran (1a) is of great interest as it
has been considered a serious candidate for the substitution
of fossil liquid fuel.14 In contrast to the chemistry of some of
these furans,15 only little is known about the reactions of 2,5-
dimethylfuran (1a).
The oxidative ring opening of furans provides the most
important synthetic access to unsaturated 1,4-diones.16 Due to
the high density of functional groups 2-ene-1,4-diones are highly
versatile substrates which can be transformed in multiple ways.
This is why the oxidative ring opening of furans to 2-ene-1,4-
dicarbonyls has been performed with a great number of reagents.
The selection of suitable oxidizing agents includes reagents as
different as Br2,17a meta-chloroperbenzoic acid (m-CPBA),17b
Cr(VI) reagents,17c 1O2,17d ceric ammonium nitrate (CAN),17e
dimethyldioxirane,17f H2O2/TS-1,17g NaClO2/NaH2PO4,17h and
magnesium monoperoxyphthalate (MMPP).17i So it is all the
more surprising that the ring opening of furans has not yet been
performed using air which is not only the cheapest and most
easily accessible oxidant but also allows for oxidations to be
performed in an environmentally friendly manner.
Laccases that belong to the blue multicopper oxidases are
widely distributed in nature.1 They have several biological
functions including wound response and lignification in plants
and lignin degradation in fungi. In laccases the four-electron
reduction of O2 to H2O is coupled with four concomitant
one-electron oxidations of organic substrate molecules. The
resulting substrate radicals can undergo oxidative couplings
with formation of dimers, oligomers and polymers, or can be
further oxidized. The use of air, which is the cheapest and
greenest oxidant available, the independence from cofactors and
the formation of H2O as the only by-product have stimulated
the development of laccase-catalyzed processes in the pulp and
paper, textile, food and cosmetic industries as well as in the field
of bioremediation.2
Due to their broad substrate spectrum, the possibility
to extend their redox potential by mediators, their high
stability and mild reaction conditions laccases have re-
ceived increasing attention in organic synthesis.3 Laccases
or laccase/mediator systems have been used to oxidize aro-
matic methyl groups,4 alcohols,5 ethers,6 benzyl amines and
hydroxylamines.7 The oxidation of catechols and hydroquinones
to the corresponding benzoquinones and related transforma-
tions are also known.8 We have reported on laccase-initiated
domino reactions between catechols and 1,3-dicarbonyls al-
lowing for the efficient preparation of heterocyclic systems.9
It has been shown that laccases catalyze the oxidative
coupling of phenolics.10 The laccase-catalyzed reaction of
We started with the observation that the laccase-catalyzed
oxidation of 2,5-dimethylfuran (1a) with air in the presence of
10 mol% TEMPO (4a) as a mediator (Fig. 1) yielded 47% (Z)-2-
hexene-2,5-dione (2a) (Z : E = 90 : 10) (Scheme 1). When instead
the oxidation was performed with violuric acid (5) (E)-2-hexene-
2,5-dione (3a) (E : Z = 88 : 12) was isolated with 35% yield.
Bioorganische Chemie, Institut fu¨r Chemie, Universita¨t Hohenheim,
Garbenstr. 30, Stuttgart, D-70599, Germany.
Fig. 1 Structures of mediators used for laccase-catalyzed reactions.
E-mail: ubeifuss@uni-hohenheim.de; Fax: (+49) 711459 22951;
Tel: (+49) 711 459 22171
† Electronic supplementary information (ESI) available: Experimental
data. See DOI: 10.1039/c1gc15810d
Control experiments clearly showed that the oxidative ring
cleavage of 1a does not proceed in the absence of either laccase
from Trametes versicolor or a mediator (4a or 5). In all these cases
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Table 2 Optimization of the selective oxidation of 1a to 2aa
Entry
4 (mol%)
Buffer (M)
Yield (%)
2a:3ab
1
2
3
4
5
6
7
8
9
a (10)
a (10)
a (20)
b (10)
b (10)
b (20)
c (10)
c (10)
c (20)
0.1
57
61
72
73
79
74
75
73
72
98 : 2
98 : 2
94 : 6
97 : 3
99 : 1
100 : 0
94 : 6
99 : 1
90 : 10
0.01
0.01
0.1
0.01
0.01
0.1
Scheme 1 Initial results for the laccase-catalyzed ring opening of 1a
with air in the presence of different mediators.
0.01
0.01
Table 1 Control experiments with 2a and 3a
a 2 mmol of 1a were reacted. b Ratio determined by 1H NMR.
Entry Substrate U (Laccase) Mediator (mol%) t (h) Product (2a:3aa)
acetate buffer concentrations on both yield and stereoselectivity
(Table 2).
1
2
3
4
5
6
7
8
2a
2a
2a
2a
3a
3a
3a
3a
350
—
350
—
350
—
350
—
4a (10)
4a (10)
5 (35)
18 92 : 8
18 94 : 6
48 5 : 49
48 9 : 91
18 0 : 100
18 0 : 100
48 0 : 100
48 0 : 100
The best results were obtained when the laccase-catalyzed
transformation of 1a was performed with 10 mol% 4-hydroxy-
TEMPO (4b) in 0.01 M acetate buffer/n-octane (10 : 1).§ Under
these conditions 79% (Z)-2-hexene-2,5-dione (2a) (2a:3a = 99 : 1)
were isolated (Table 2, entry 5).¶ Yields and selectivity were in
the same range when 4-acetamido-TEMPO (4c) was employed
as a mediator (Table 2, entry 8). Now an enzyme-catalyzed
procedure for the oxidative ring opening of 1a with air is available
for the first time, which delivers (Z)-2-hexene-2,5-dione (2a) with
both high yield and outstanding (Z)-selectivity.
All oxidations presented so far were performed on a 2 mmol
scale. To demonstrate the usefulness of this laccase-catalyzed
reaction in preparative organic chemistry the transformation
was scaled up to the 10 mmol scale (Table 3). In these
experiments the mediator 4-hydroxy-TEMPO (4b) was replaced
by the cheaper TEMPO (4a). The yields of 2a strongly depend on
the amount of laccase used. With 350 U of laccase a maximum
of 40% product could be isolated (Table 3, entry 1); with 500 U
laccase the yield increased to 54% (Table 3, entry 2). A further
increase to 66 and 71% was achieved by extending the reaction
time from 18 to 36 h and 48 h, respectively (Table 3, entry 5 and
7). No further improvement could be achieved by increasing the
amount of the laccase to 700 U (Table 3, entry 6). Raising the
5 (35)
4a (10)
4a (10)
5 (35)
5 (35)
a Ratio determined by 1H NMR
only the substrate 1a was detected after 18 h at rt. With argon
instead of air only traces of 2a and 3a were formed. These results
demonstrate that this transformation requires the triple action
of a laccase, a mediator and air. When the amount of laccase
was reduced from 350 U to 250 U and 100 U, respectively, the
yields of 2a and 3a decreased substantially.
To further corroborate these findings a number of control
experiments were conducted with 2a and 3a. When the pure
Z-isomer 2a was reacted with 350 U laccase‡ and 10 mol%
TEMPO (4a), 2a and 3a were isolated in a 92 : 8 ratio (Table 1,
entry 1). A similar result was obtained when 2a was stirred in
the presence of 10 mol% 4a alone (Table 1, entry 2). When 2a
was reacted with violuric acid (5) as a mediator, isomerization
to 3a (which is considered to be thermodynamically more stable
than 2a) was observed to a more or less extensive degree (Table
1, entry 3 and 4). Interestingly, isomerization to 3a is nearly
complete with 5 alone (Table 1, entry 4). In the presence of
laccase and 5, 2a undergoes only partial isomerization (Table
1, entry 3). We concluded that violuric acid (5a) was the
responsible agent. Control experiments with the pure E-isomer
3a and the mediators 4a or 5 in the presence/absence of laccase
demonstrated that an isomerization of 3a to 2a does not occur in
any case (Table 1, entries 5–8). These observations confirmed the
assumption that 3a is thermodynamically more stable than 2a.
With these results in hand we were encouraged to optimize
the (Z)-selective ring opening of furan 1a. Experiments with
a number of organic solvents demonstrated that a two-phase
system consisting of acetate buffer and n-octane (10 : 1) was
the most suitable. Further experiments studied the influence of
different concentrations of the mediators 4a–c as well as different
Table 3 Scale-up experimentsa
Entry
U (Laccase)
T (◦C)
t (h)
Yield (%)
2a:3ab
1
2
3
4
5
6
7
350
500
500
500
500
700
500
r.t.
r.t.
40
18
18
18
18
36
36
48
40
54
46
9
66
65
71
96 : 4
97 : 3
95 : 5
92 : 8
98 : 2
98 : 2
97 : 3
60
r.t.
r.t.
r.t.
a 10 mmol of 1a were reacted. b Ratio determined by 1H NMR.
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reaction temperature from rt to 40 ◦C and 60 ◦C led to decreasing
yields (Table 3, entry 3 and 4). To sum up, the stereoselective
transformation of 1a into 2a can easily be scaled up to the 10
mmol scale by appropriately adjusting the reaction conditions.
The next set of experiments was devoted to find out whether
the laccase-catalyzed (Z)-selective ring opening of 1a can
be extended to other 2,5-disubstituted furans. The required
substrates 1b–g (Fig. 2) were obtained by deprotonation of
the corresponding 2-alkylfurans with n-BuLi and subsequent
alkylation with the corresponding alkyl iodides.18
Table 5 Optimization of the selective oxidation of 1a to 3aa
Entry
4
5 (mol%)
T/◦C
t (isom., h) Yield (%)
2a:3ab
1
2
3
4
5
6
7
8
b
b
b
a
a
a
a
a
a
a
50
50
—
50
37.5
25
—
50
25
10
25
50
50
25
25
25
50
50
50
50
42
4
4
42
42
48
6
6
6
15
50
50
70
67
62
52
—
61
70
60
0 : 100
0 : 100
99 : 1
0 : 100
0 : 100
0 : 100
94 : 6c
0 : 100
0 : 100
0 : 100
9
10
1
a 2 mmol of 1a were reacted. b Ratio determined by H NMR. c Crude
product.
Fig. 2 Structures of 2,5-dialkylfurans 1b–g.
individual transformations indicates that decreasing yields of 2
are observed when the length of the side chains of the furans
increases. The only substrate that could not be oxidized at all
was 2,5-dibutylfuran (1g) (Table 4, entry 6).
The resulting 2,5-dialkylfurans 1b–g were then reacted with
350 U laccase in the presence of 10 mol% 4b as the mediator
under the conditions given in Table 2, entry 5, resulting in
the exclusive formation of the (Z)-substituted enediones 2b–
f (Table 4). Not even traces of the (E)-substituted enediones
could be detected. The results clearly demonstrate that both sym-
metrically and asymmetrically substituted dialkylfurans can be
successfully ring-opened to yield the (Z)-substituted enediones
2b–f in a highly diastereoselective manner. A comparison of the
Finally, our focus was on the (E)-selective oxidative ring
opening of 1a into 3a.ꢀ We clearly needed to improve the
transformation presented in Scheme 1 with respect to both yield
and selectivity. On the basis of the results given in Tables 1 and
2, we assumed that this goal could be achieved by combining
the laccase-catalyzed (Z)-selective oxidative ring opening of 1a
to 2a with 4 as a mediator and the isomerization of 2a to
3a using violuric acid (5). This assumption was confirmed by
the experiments summarized in Table 5. The best results were
obtained when 1a was first reacted using 350 U laccase, 10
mol% of 4a and air f◦or 18 h at rt and the reaction mixture was
stirred for 6 h at 50 C after adding 25 mol% of 5. Using this
protocol, stereoisomerically pure (E)-2-hexene-2,5-dione (3a)
could be isolated with 70% yield (Table 5, entry 9). Decreasing
the amount of mediator 5 to 10 mol%, isomerically pure 3a
could be isolated with 60% yield (Table 5, entry 10). Control
experiments established that the selective formation of 3a is
impossible when 5 is absent (Table 5, entries 3,7).
Table 4 Laccase-catalyzed ring opening of 2,5-dialkylfurans 1b–g to
(Z)-enediones 2b–fa
Entry
1
1
b
t (h)
Product
Yield (%)
52
Ratio 2:3
72
100 : 0b
Due to their molecular symmetry the magnetically equivalent
vinylic protons of 2a and 3a resonate as a singlet each. Thus
it was impossible to directly determine the E/Z-configuration
of the double bonds on the basis of the size of the vicinal
coupling constants. Replacement of one olefinic 12C by 13C (ap-
prox. 1% natural abundance), however, abolishes the magnetic
equivalence, and an ABX spin system with two doublets of
doublets is formed (Fig. 3). The size of 3JHH in these isotopomers
can be easily derived from the outer doublets which show no
2
3
c
72
96
59
33
100 : 0b
100 : 0b
d
4
5
6
e
f
72
72
96
65
22
100 : 0b
100 : 0b
1
signal overlap with the central proton singlet because of JCH
2
ꢁ JCH. Vicinal coupling constants of 11.7 Hz and 16.5 Hz
unambiguously establish the double bond configuration to be Z
for 2a and E for 3a, respectively.
In summary, combinations of a laccase and a mediator
were demonstrated to catalyze the efficient and sustainable
oxidative ring opening of 2,5-dialkyl furans to the corresponding
enediones using air as an oxidant. Depending on the mediator
g
—
a 2 mmol of 1 were reacted. b In the H NMR no traces of 3 could be
detected.
1
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¶ Selected analytical data for (Z)-3-hexene-2,5-dione (2a): Rf 0.19
(diethyl ether/n-pentane = 1 : 1); dH (300 MHz; CDCl3) 2.28 (s, 6H,
1-H3 and 6-H3), 6.29 (s, 2H, 3-H and 4-H); dC (75 MHz; CDCl3) 29.72
(C-1 and C-6), 135.67 (C-3 and C-4), 200.40 (C-2 and C-5).
ꢀ Selected analytical data for (E)-3-hexene-2,5-dione (3a): Rf 0.29
(diethyl ether/n-pentane = 1 : 1); dH (300 MHz; CDCl3) 2.36 (s, 6H,
1-H3 and 6-H3), 6.78 (s, 2H, 3-H and 4-H); dC (75 MHz; CDCl3) 28.00
(C-1 and C-6), 137.81 (C-3 and C-4), 198.46 (C-2 and C-5).
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Acknowledgements
Financial support from the German Research Foundation
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Notes and references
‡ Determination of laccase activity according to ref. 19: A 0.1 M solution
of ABTS (0.3 mL) in 0.01 M acetate buffer (pH = 4.5) was diluted with
0.01 M acetate buffer (2.6 mL, pH = 4.5) and treated with a solution
of laccase in the same buffer (0.1 mL). The change in absorption was
followed via UV-spectroscopy (l = 414 nm). One unit was defined as the
amount of laccase (Trametes versicolor, Fluka) that converts 1 mmol of
ABTS per minute at pH = 4.5 at rt.
§ General procedure for the synthesis of enediones 2a–f: A 250 ml round
bottomed flask with a magnetic stirrer bar was charged with 2 mmol
2,5-dialkylfuran 1, 1 mL n-octane, 10 mL 0.01 M acetate buffer pH
4.5, 0.2 mmol 4-hydroxy-TEMPO (4b) and 350 U laccase (Trametes
versicolor, Fluka) and closed with a septum. The mixture was stirred at
room temperature for the time given in Table 4. The aqueous phase was
saturated with 1 g NaCl and extracted with CH2Cl2 (4 ¥ 25 mL). The
combined organic phases were dried over Na2SO4. After removal of the
solvents in vacuo the crude product was purified by flash chromatography
on SiO2 (diethyl ether/n-pentane = 1 : 1).
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