Oxidative dimerization of (E)- and (Z)-2-propenylsesamol with O2 in the presence and absence of laccases and other catalysts: Selective formation of carpanones and benzopyrans under different reaction conditions

Article abstract of DOI:10.1021/jo300263k

The oxidative dimerization of 2-propenylsesamol to carpanone with O ₂ as the oxidant, which probably proceeds as a domino phenol oxidation/anti-β,β-radical coupling/intramolecular hetero Diels-Alder reaction, can be efficiently catalyzed by laccases. Experiments with laccases and other catalysts like a Co(salen) type catalyst and PdCl₂ clearly demonstrate that the diastereoselectivity of the carpanone formation does not depend on the catalyst but on the double-bond geometry of the substrate. With (E)-2-propenylsesamol as the substrate, carpanone and a so far unknown carpanone diastereoisomer are formed in a 9:1 ratio. When (Z)-2-propenylsesamol is used as starting material, carpanone is accompanied by two carpanone diastereoisomers unknown so far in a 5:1:4 ratio. All three carpanone diastereoisomers have been separated by HPLC, and their structures have been elucidated unambiguously by NMR spectroscopy, DFT calculations, and spin work analysis. When the oxidation of 2-propenylsesamol with O₂ is performed in the absence of any catalyst two diastereoisomeric benzopyrans are formed, probably as the result of a domino oxidation/intermolecular hetero Diels-Alder reaction. Under these conditions, carpanone is formed in trace amounts only.

Full text of DOI:10.1021/jo300263k

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pubs.acs.org/joc
Oxidative Dimerization of (E)- and (Z)-2-Propenylsesamol with O2 in
the Presence and Absence of Laccases and Other Catalysts: Selective
Formation of Carpanones and Benzopyrans under Different Reaction
Conditions
Mihaela-Anca Constantin,† Jurgen Conrad,† Elena Merisor,† Katja Koschorreck,‡ Vlada B. Urlacher,‡
̧
̈
and Uwe Beifuss†,*
†Bioorganische Chemie, Institut fur Chemie, Universitat Hohenheim, Garbenstraße 30, D-70599 Stuttgart, Germany
̈
̈
‡Institut fur Biochemie, Heinrich-Heine Universitat Dusseldorf, Universitatsstraße 1, D-40225 Dusseldorf, Germany
̈
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The oxidative dimerization of 2-propenylsesa-
mol to carpanone with O2 as the oxidant, which probably
proceeds as a domino phenol oxidation/anti-β,β-radical
coupling/intramolecular hetero Diels−Alder reaction, can be
efficiently catalyzed by laccases. Experiments with laccases and
other catalysts like a Co(salen) type catalyst and PdCl2 clearly
demonstrate that the diastereoselectivity of the carpanone
formation does not depend on the catalyst but on the double-
bond geometry of the substrate. With (E)-2-propenylsesamol as the substrate, carpanone and a so far unknown carpanone
diastereoisomer are formed in a 9:1 ratio. When (Z)-2-propenylsesamol is used as starting material, carpanone is accompanied by
two carpanone diastereoisomers unknown so far in a 5:1:4 ratio. All three carpanone diastereoisomers have been separated by
HPLC, and their structures have been elucidated unambiguously by NMR spectroscopy, DFT calculations, and spin work
analysis. When the oxidation of 2-propenylsesamol with O2 is performed in the absence of any catalyst two diastereoisomeric
benzopyrans are formed, probably as the result of a domino oxidation/intermolecular hetero Diels−Alder reaction. Under these
conditions, carpanone is formed in trace amounts only.
INTRODUCTION
■
Carpanone (1a) is the most well-known member of a small
group of naturally occurring benzoxanthenones which belong to
the lignans and are characterized by an O-containing tetracyclic
basic skeleton. The natural product with its highly oxygenated
hexacyclic ring system and five contiguous stereogenic centers
was isolated from the bark of the carpano tree (Cinnamonum sp.,
family Lauraceae) as a racemate (Figure 1).1a Aside from
carpanone (1a), isocarpanone and carpananone were isolated
from the carpano tree. Their structures show great similarities
with carpanone (1a) but have not been elucidated completely.1a
In addition, carpacin (2) was obtained from the same plant. This
styrene derivative might be considered as a biosynthetic
precursor of carpanone (1a).1
Later, some additional naturally occurring benzoxanthe-
nones were isolated. Among them are several sauchinones
from Saururus chinensis2 as well as the polemannones A−C
(4−6) from Polemannia Montana (Figure 2).3 As sauchinone
(3) exhibits various biological activities, such as hepato-
protective effects,2a antihypertension effects,2b immunosup-
pressive effects,2c and anti-inflammatory effects,2d the
synthesis of natural as well as unnatural benzoxanthenones
has become a field of growing interest in medicinal chemistry
in recent years.
Figure 1. Natural products isolated from the carpano tree
(Cinnamonum sp., family Lauraceae).
Most synthetic studies of both natural and unnatural
benzoxanthenones can be traced back to the fundamental
contribution from Chapman et al. who recognized that it is
possible to synthesize carpanone (1a) by an intramolecular
hetero-Diels−Alder reaction of the o-quinone methide 9, which
in turn is accessible from an anti-selective β,β-phenol coupling
of two molecules (E)-7 (Scheme 1).4
Indeed, when Chapman et al. reacted (E)-7 with 50 mol %
of PdCl2 under air they isolated 46% racemic carpanone (1a)
after crystallization. The structure of 1a was established by
Received: February 6, 2012
Published: March 30, 2012
© 2012 American Chemical Society
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oxidative dimerization of (E)-7 can be achieved with 10 mol %
of CuCl2/10 mol % of (−)-sparteine and air to deliver racemic
1a as the sole product in 91% yield.9 It was Poli et al. who
studied the effect of the double bond geometry of the substrate
on the course of the reaction for the first time.10 Unfortunately,
they did not use (E)-7 and (Z)-7 with the free phenolic OH
groups as starting materials. Instead, the TBS-protected phenols
(E)-10 and (Z)-10, which were deprotected in situ with TBAF,
were chosen as substrates (Figure 3). With (E)-10 as the
Figure 2. Some natural products with a benzoxanthenone skeleton.
Figure 3. Silyl ethers (E)-10 and (Z)-10 used as precursors for the
formation of carpanones; structure 1b as proposed by Poli et al.10
X-ray structure analysis.4 It is noteworthy that Chapman et al.
achieved the synthesis of a comparatively complex natural
product with five stereogenic centers in a highly elegant fashion
and in a single synthetic operation. The synthesis is also an
impressive example for the application of Diels−Alder reactions
in the total synthesis of natural products.5 Possibly, Chapman's
approach to carpanone also represents a biomimetic synthesis
of 1a because the potential biosynthesis precursor carpacin (2)
has also been isolated from the carpano tree. Meanwhile, the
preparation of racemic carpanone (1a) has been performed
successfully with a number of transition-metal catalysts and
reagents. Kuroda and Matsumoto reported that the synthesis
of racemic 1a can be achieved by reaction of (E)-7 using a
catalytic amount of transition metal complexes like Co(II)salpr,
Co(II)salen, Fe(II)salen, and Mn(II)salen as catalysts and O2
as the oxidant.6 With 2.7 mol % of Co(II)salen 94% of 1a was
isolated. However, they also reported the additional formation
of small amounts of a second compound, whose structure could
not be elucidated. Trivedi et al. used 2 equiv of Ag2O for the
oxidation of (E)-7 and observed the formation of racemic 1a in
48% yield.7 Ley et al. reported on the synthesis of racemic
carpanone (1a) employing different reagents.8 The best results
were obtained when a polymer-bound Co(II)salen complex
was used for the transformation of (E)-7. Even under these
conditions 1a was not produced as the sole product. Ley et al.
observed the additional formation of small amounts of another
compound, and it was assumed that this compound might be
an isomer of carpanone (1a).8a Lindsley et al. reported that the
substrate the reaction with 10 mol % of PdCl2/O2 exclusively
delivered racemic 1a in 82% yield. A similar result was obtained
with CuCl2/O2. The oxidative dimerization of (Z)-10 with
10 mol % of PdCl2/O2 also resulted in 1a as the sole product
(77% yield), but when the oxidation was performed in the
presence of 10 mol % of CuCl2/O2 as the catalyst 40% of a 63:37
mixture of carpanone (1a) and a carpanone diastereoisomer
whose structure was assigned to 1b was obtained.10 The obvious
discrepancies between the results of the different groups prompted
us to investigate the oxidative dimerization of diastereoisomerically
pure (E)-7 and (Z)-7 under different reaction conditions.
Chapman's approach to the synthesis of benzoxanthenones
is not restricted to the synthesis of carpanone (1a) itself but
can also be applied to the construction of other benzox-
anthenones.9,11,12 As an example, Lindsley et al. have achieved
the preparation of diastereoisomerically pure polemannones
B (5) and C (6) by treatment of suitable substituted precursors
with 10 mol % of CuCl2/10 mol % of (−)-sparteine.12
Astonishingly, there is no report so far that the formation of
carpanone (1a) by domino phenol oxidation/anti-β,β-radical
coupling/intramolecular hetero-Diels−Alder reaction can be
catalyzed enzymatically. As it is generally believed that the
formation of 9 proceeds via radical dimerization of (E)-7,
it seemed reasonable to try the synthesis of carpanone (1a)
with enzymes known to be capable to initiate oxidative phenol
couplings. We expected laccases to be suitable for this purpose
Scheme 1. Synthesis of Racemic Carpanone (1a) According to Chapman et al.4
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as a number of oxidative phenol couplings have been catalyzed
successfully by this type of enzymes.
enzyme, which can result in improvements concerning activity,
selectivity, stability, expression, or redox potential. So far, fungal
laccases have been heterologous produced mainly in filamentous
fungi like Aspergillus niger, Aspergillus oryzae, and Trichoderma
reseii as well as in yeasts like Pichia pastoris, Saccharomyces
cerevisiae, Yarrowia lipolytica, and Klyveromyces lactis.38
The focus of this study was to explore whether the oxidative
dimerization of two molecules 2-propenylsesamol (7) to
carpanone (1a) can be performed with laccase as the catalyst
and O2 as the oxidant. The obvious discrepancies between the
results of different groups prompted us to study the influence
of the double bond geometry of 7 on the stereoselectivity of the
carpanone formation as well.
Laccases (benzenediol:oxygen oxidoreductase, EC 1.10.3.2)
are widespread in nature.13−16 Among the oxidoreductases there
are several groups of biocatalysts, including dehydrogenases,
oxidases, oxygenases, and peroxidases.17 The laccases belong
to the oxidases which catalyze the oxidation of a substrate using
O2 as the oxidant without incorporation of oxygen into the
substrate. The laccases are similar to the ascorbate oxidases from
plants and to the mammalian plasma protein ceruloplasmin.
Together they form the small group of the blue copper
oxidases.18 Laccases catalyze four one-electron oxidations of
organic substrate molecules to the corresponding four radicals
using O2 as the oxidant. Usually, the resulting radicals are highly
reactive and undergo radical couplings with formation of dimers,
oligomers, or polymers. The oxidation of the substrate is
coupled with the four-electron reduction of O2 to water.19 In
nature, laccases contribute to numerous processes. It is believed
that plant laccases are involved in the synthesis of lignin20 while
fungal laccases are known for the degradation of lignin.21
Because of their ability to oxidize organic substrates with O2,
laccases play an important role in industrial and biotechnological
processes including applications from the field of food, paper,
textile, and cosmetic industries. The application of laccases in
soil remediation is also important.22 Naturally occurring laccases
normally exhibit redox potentials in the range of between 0.5
and 0.8 V.13,20d,22a As a result, the number of substrates which
undergo laccase-catalyzed oxidation is limited. However, the
redox potential of laccases can be extended by using mediators,
allowing other classes of compounds to be oxidized. It has been
demonstrated that oxidations catalyzed by laccases and laccase/
mediator systems, respectively, can be employed to achieve a
number of selective transformations,17,22a,23 including the oxida-
tion of aromatic methyl groups,24 alcohols,25 ethers,26 benzyl-
amines, and hydroxyl amines.27 Laccases can also be used for the
oxidation of phenolic substrates and for phenolic couplings.
Examples for laccase-catalyzed oxidative couplings include the
dimerizations of penicillin X,28 bisphenol A,29 17-β-estradiol,30
isoeugenol,31 coniferyl alcohol,32 trans-resveratrol,33 and poly-
hydroxystilbene derivatives.34 One-pot reactions which combine
a laccase-catalyzed oxidation with a chemical reaction are highly
attractive.35 Recently, we have reported on laccase-initiated
domino reactions of catechols and hydroquinones with 1,3-
dicarbonyls.36 The combination of the laccase-catalyzed
oxidation of catechols and hydroquinones to the corresponding
benzoquinones with intermolecular Diels−Alder reactions with
normal electron demand is also known.23a The use of laccases
in organic synthesis offers numerous advantages: (a) they can
be easily isolated and some of them are commercially available,
(b) laccase-catalyzed processes use O2, which is the cheapest
and most abundant oxidant, and water is the only byproduct
formed, (c) laccases catalyze the transformations of a great
number of substrates and there is no need for a cofactor
and cofactor regeneration, (d) the redox potential of laccases
can be extended by mediators, (e) laccase-catalyzed oxidations
can be performed in aqueous solvent systems as well as mixtures
of aqueous and organic solvents,22a and (f) laccases can be
immobilized.37 The recombinant production of laccases allows
the targeted overexpression of specific laccases. On one hand,
this facilitates the isolation of a specific laccase compared with
its purification from a mixture of laccases, as formed by most
natural producers of fungi. On the other hand, the heterologous
production of laccase allows for the specific modification of the
RESULTS AND DISCUSSION
■
We started with the laccase-catalyzed reaction of an 8:2
mixture of (E)- and (Z)-2-propenylsesamol (7). This 8:2
mixture was obtained by column chromatography of the crude
7:3 mixture of (E)- and (Z)-7 (1H NMR) from the KOtBu-
mediated isomerization of 1-propenylsesamol (11) (Scheme 2).
Scheme 2. Synthesis of 2-Propenylsesamol (7)
1-Propenylsesamol (11) was prepared by Claisen rearrangement
of 12, which in turn can easily be synthesized by allylation of
13.4,39,40
Then, the laccase-catalyzed reaction of the (E)/(Z)-mixture of
7 was investigated. In a first experiment, it was found that the
reaction of 1 mmol 7 (E/Z = 8:2) with 28 U laccase from
Trametes versicolor in acetate buffer (0.1 M, pH = 5.0)/acetone
(9:1) under O2 at room temperature for 24 h delivered 46% of a
1
product mixture after column chromatography (Scheme 3). H
NMR analysis after column chromatography clearly demon-
strated that carpanone (1a) was the major, but not the only,
component of the product mixture. The structure elucidation
unequivocally established that the two other components formed
are the carpanone diastereoisomers 1c and 1d. This experiment
clearly demonstrated that the oxidative dimerization of 7 can
be catalyzed by a laccase. To study the influence of the double
bond geometry of 7 on the stereoselectivity of the oxidative
dimerization of 7 pure (E)-7 and (Z)-7 had to be prepared.
It was found that (E)-7 and (Z)-7 can be obtained in
diastereoisomerically pure form by repeated column chroma-
tography (SiO2; dichloromethane/cyclohexane = 5:2) of the
mixture of (E)-7 and (Z)-7 obtained by isomerization of 11. 1H
NMR spectroscopy clearly indicated that (E)-7 and (Z)-7 were
diastereoisomerically pure. To avoid any isomerization and/or
oxidation the two diastereoisomers were reacted immediately.
First, we reacted (E)-7 under the reaction conditions reported
by Chapman et al.4 When (E)-7 was treated with 50 mol % of
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Scheme 3. Laccase-Catalyzed Oxidative Dimerization of an 8:2 Mixture of (E)- and (Z)-2-Propenylsesamol (7)
a
Table 1. Oxidative Dimerization of (E)-7 with Transition-Metal-Based Reagents
b
c
entry
reagent
T (°C)
time (h)
yield 1a + 1c (%)
1a:1c
d
e
1
2
3
50 mol % PdCl2
10 mol % PdCl2
38; rt
50
2; 1
4
42
47
79
9:1
9:1
9:1
f
2.6 mol % 14
rt
4
a
b
c
1 mmol of (E)-7 was reacted. Yields refer to yields after flash chromatography. The ratio 1a:1c was determined by 1H NMR spectroscopy after
d
e
flash chromatography. The reaction was performed in MeOH/H2O = 5:1 using air as the oxidant. The reaction was performed in MeOH/H2O =
1:1 using O2 as the oxidant. The reaction was run in CH2Cl2 and O2 was used as the oxidant.
f
PdCl2 under air, we did not observe the exclusive formation of
carpanone (1a) as reported by Chapman et al. but instead
isolated 42% of a mixture of two carpanone diastereoisomers in
a 9:1 ratio (Table 1, entry 1). Structure elucidation clearly
established that the major diastereoisomer was the expected
carpanone (1a) and that the minor diastereoisomer was 1c
(cf. Structure Elucidation). When the reaction was carried out
under Poli’s conditions using 10 mol % of PdCl2 and O2 as the
oxidant, again a 9:1 mixture of 1a and 1c was obtained. Under
these conditions, the two diastereoisomers were isolated in
47% yield (Table 1, entry 2). Finally, the reaction was run with
2.6 mol % of (R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)-
1,2-cyclohexanediaminocobalt(II) (14) and O2 as the oxidant
(Figure 4). In agreement with the results of Matsumoto and
T. versicolor produces several laccases which can be assigned
to four groups of isoenzymes α, β, γ, and δ.44 Commercially
available laccases of T. versicolor represent mixtures of these
isoenzymes. It has been shown that Lccβ is the most stable one
of these isoenzymes. In addition, Lccβ displays a high activity
toward a number of substrates.43 Because of the low solubility of
(E)-7 in sodium acetate buffer (0.1 M, pH 5.0), the reactions
were carried out in the presence of cosolvents such as acetone or
methanol. When (E)-7 was reacted with 28 U laccase and O2 as
the oxidant in a 9:1 mixture (v/v) of sodium acetate buffer
and acetone, 36% of a 9:1 mixture (1H NMR) of 1a and 1c
were isolated after 2 h (Table 2, entry 1). The reason for the low
yield can be attributed to incomplete consumption of the
starting material (TLC, 1H NMR spectrum of the crude
product, isolation). By increasing the reaction time from 2 to
6 h, the yield could be improved to 60%. Again, a 9:1 mixture
(1H NMR) of 1a and 1c was formed (Table 2, entry 3). A
further slight improvement of the yield to 62% could be achieved
by extending the reaction time to 24 h (Table 2, entry 4). In
addition, the influence of increasing the amount of acetone from
10 to 18 vol % in the solvent mixture was studied. With 18 vol %
of acetone, (E)-7 was consumed completely after 2 h and 68%
of a 9:1-mixture (1H NMR) of 1a and 1c were isolated (Table 2,
entry 5). Similar results were obtained when the reaction was run
with a laccase from A. bisporus and a recombinant Lccβ-laccase,
respectively. With the commercially available laccase from
A. bisporus as the catalyst 65% of a 9:1 mixture (1H NMR) of
1a and 1c were obtained (Table 2, entry 6). The reaction with
the Lccβ laccase from T. versicolor as the catalyst produced 53%
of a mixture of 1a and 1c (Table 2, entry 7). Replacement of
acetone with MeOH resulted in considerably lower yields of the
9:1 mixture (1H NMR) of 1a and 1c (Table 2, entries 8 and 9).
This is probably due to lower solubility of (E)-7 in this solvent
mixture. Separation of the two stereoisomers 1a and 1c was
achieved by HPLC (see the Supporting Information). Structure
elucidation (vide infra) clearly established that 1a is carpanone.
As in 1a, the orientation of the two methyl groups at C-8 and at
Figure 4. Structure of 14.
Kuroda,6 carpanone (1a) was not formed exclusively. Instead,
we isolated 79% of a 9:1 mixture of 1a and 1c (Table 1, entry 3).
From these experiments it is evident that the oxidative dimeriza-
tion of pure (E)-7 results in the formation of a mixture of two
diastereoisomeric benzoxanthenones, namely 1a and 1c. This
indicates that the oxidative dimerization of (E)-7 is not completely
diastereoselective as has been assumed previously by several
authors.4,7,9,10
Next, the laccase-catalyzed reaction of pure (E)-7 was studied.
Three laccases were used as catalysts, namely commercially
available laccases from T. versicolor,41 and A. bisporus42 as well as
a recombinant laccase from T. versicolor.43 The white-rot fungus
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a
Table 2. Optimization of the Laccase-Catalyzed Oxidative Dimerization of (E)-7
b
c
entry
laccase
cosolvent (vol %)
time (h)
yield 1a + 1c (%)
1a:1c
d
1
2
3
4
5
6
7
8
9
Tv
acetone (10)
acetone (10)
acetone (10)
acetone (10)
acetone (18)
acetone (18)
acetone (18)
MeOH (18)
MeOH (18)
2
2
36
22
60
62
68
65
53
31
32
9:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
e
Ab
a
Tv
6
d
Tv
24
2
d
Tv
e
Ab
2
f
Lccβ
d
2
Tv
2
e
Ab
2
a
b
c
1 mmol of (E)-7 was reacted. Yields refer to yields after flash chromatography. The ratio 1a:1c was determined by 1H NMR spectroscopy after
flash chromatography. Commercially available laccase from T. versicolor. The enzyme activity was determined using ABTS as the substrate.30
d
e
f
Commercially available laccase from A. bisporus. The enzyme activity was determined using ABTS as the substrate.30 Recombinant laccase Lccβ
from T. versicolor. The enzyme activity was determined using ABTS as the substrate.30
a
Table 3. Oxidative Dimerization of (E)-7 Using 4-Methoxy-TEMPO as the Catalyst
b
c
entry
4-methoxy-TEMPO (mol %)
2.7
time (h)
yield 1a + 1c (%)
1a:1c
1
2
12
4
56
79
9:1
9:1
27
a
b
c
1 mmol of (E)-7 was reacted. Yields refer to yields after flash chromatography. The ratio 1a:1c was determined by 1H NMR spectroscopy after
flash chromatography.
C-8′ in the carpanone diastereoisomer 1c is trans. However, the
two diastereoisomers differ in the linkage of the dihydropyran
and the cyclohexene rings (ring C and ring D) at C-7′ and C-2.
In contrast to carpanone (1a) the linkage between the two rings
in 1c is trans. A control experiment clearly demonstrated that 1a
is stable under the reaction conditions of the laccase-catalyzed
oxidative dimerization; i.e., no conversion of 1a into 1c was
observed. Despite of a number of attempts8a,9 the enantiose-
lective synthesis of carpanone has not been achieved so far. To
study the enantioselectivity of the laccase-catalyzed dimerization
of 7 the separation of the enantiomers of 1a was studied by
HPLC using a chiral phase. It was found that Chiralpak IB as a
chiral phase is suitable for the separation of the enantiomers of
1a as well as that of 1c. Determination of the enantioselectivity
of the laccase-catalyzed reaction of (E)-7 revealed that 1a as
well as 1c were formed in racemic form; i.e., the oxidative
dimerization proceeds without enantioselectivity. This holds true
for all three laccases under study as well as for the reaction with
14. In summary, all laccase-catalyzed dimerizations of (E)-7
proceed with exclusive formation of the two diastereoisomers 1a
and 1c in a 9:1 ratio (Table 2). The chemical yield, however,
depends on the laccase employed, the type and the amount of
the cosolvent, and the reaction time.
Subsequently, it was studied whether the oxidative dimeriza-
tion of (E)-7 can also be catalyzed by nitroxides. When (E)-7
was treated with 2.7 mol % of 4-methoxy-TEMPO at 70 °C,
56% of a 9:1 mixture (1H NMR) of 1a and 1c was isolated after
12 h. In addition to 1a and 1c, approximately 18% of phenol-
3,4-methylenedioxy-6-carbaldehyde (15) (1H NMR of the
crude product), which can arise through oxidative cleavage
of the C,C-double bond in (E)-7, was formed (Figure 5).
Figure 5. Structure of 15.
Further experiments revealed that the yield of 1a,c can be
controlled by the amount of 4-methoxy-TEMPO. By increasing
the amount of 4-methoxy-TEMPO from 2.7 to 27 mol %, the
yield of the 9:1 mixture (1H NMR) of 1a and 1c could be
improved to 79%. The diastereoselectivity of the dimerization,
however, could not be influenced by the amount of the nitroxide.
To summarize, the oxidative dimerization of (E)-7 as well as
the mixture of (E)- and (Z)-7 can not only be achieved by
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a
Table 4. Oxidative Dimerization of (Z)-7
b
c
entry
catalyst
time (h)
solvent
yield 1a + 1c + 1d (%)
1a:1c:1d
d
1
2
28 U laccase
2
4
acetate buffer pH = 5.0
40
43
5:1:4
5:1:4
2.6 mol % 14
CH2Cl2
a
b
c
1
1 mmol of (Z)-7 was reacted. Yields refer to yields after flash chromatography. The ratio 1a:1c:1d was determined by H NMR spectroscopy
d
after flash chromatography. Commercially available laccase from T. versicolor.
PdCl2 and Co complex 14 but can also be catalyzed by laccases
and 4-methoxy-TEMPO using O2 as the oxidant. It is
remarkable to note that irrespective of the catalyst and the
oxidant used, in all cases the carpanone diastereoisomers 1a and
1c were formed in a 9:1 ratio. The advantage of the laccase-
catalyzed reactions is that (a) there is no need for the use of a
transition-metal catalyst or the use of nitroxides like 4-methoxy-
TEMPO, (b) the reactions can be run in aqueous solvent
systems, and (c) O2 can be used as the oxidant.
as the catalyst (Table 4, entry 2). It is noteworthy that in both
cases the three carpanone diastereoisomers 1a, 1c, and 1d were
formed in a 5:1:4 ratio. These results also support the view that
irrespective of the catalyst used the same reaction mechanism
applies. The three diastereoisomers could be separated by
HPLC (see the Supporting Information). In this way, it was also
possible to obtain sufficient amounts of 1d for structure
elucidation. In contrast to 1a and 1c, the methyl groups at
C-8 and C-8′ in 1d are cis-oriented. The structure elucidation
also revealed that the dihydropyran and the cyclohexene rings in
1d are trans-fused as they are in 1c.
It is proposed that regardless of the geometry of the olefinic
double bond of 7 the radical 16 is formed which in turn
undergoes dimerization and finally an intermolecular hetero-
Diels−Alder reaction. The dimerization can happen in two
ways: either as an anti-β,β-coupling or as a syn-β,β-coupling. It
is assumed that the transformation of (Z)-7 into 1a and 1c
follows the same path than the reaction of (E)-7 (Scheme 4).
This means that in the first step 9 is formed by an anti-β,β-
coupling of two radicals 16 which react either via the endo-E-syn
transition state 17 to 1a or via the exo-E-anti transition state 18
to 1c. The formation of 1d, however, requires 20 as an inter-
mediate which can result from a syn-β,β-coupling of two
radicals 16 (Scheme 5). Subsequently, the syn-β,β-structure 20
undergoes an intermolecular hetero-Diels−Alder reaction
through the exo-E-anti transition state 21 to yield 1d. The
formation of 1e that would result from the corresponding endo-
E-syn transition state 22 was not observed.
It is remarkable that starting with (E)-7 the Diels−Alder
products 1a and 1c are formed exclusively. With this substrate
not even a trace of 1d could be detected. Product 1d is formed
only when (Z)-7 was used as the substrate. The different
results obtained using (Z)-7 and (E)-7 are unexpected as both
starting materials are assumed to form radical 16 as a common
intermediate and so far no satisfactory explanation for the
different behavior of the two isomers can be offered.
Finally, some control experiments were performed to study
the effect of the laccase on the outcome of the reaction in
greater detail. For this purpose, pure (E)-7 was stirred in
acetate buffer (pH = 5.0)/acetone = 82:18 at room temperature
under O2 in the absence of any catalyst for 2 h. It came as a big
surprise when apart from 56% of unreacted (E)-7 and 3% of a
carpanone-containing fraction 21% of an unknown compound
(m/z 356) were isolated (see the Supporting Information).
It is assumed that the reaction of (E)-7 proceeds as a domino
phenol oxidation/anti-β,β-radical coupling/intramolecular
hetero-Diels−Alder reaction (Scheme 4). In the first step,
(E)-7 undergoes one-electron oxidation to the resonance-
stabilized radical 16. Subsequently, dimerization of 16 takes
place in the sense of a diastereoselective anti-β,β-coupling with
formation of intermediate 9. In the last step, an intramolecular
hetero-Diels−Alder reaction with inverse electron demand
occurs, and the products 1a and 1c are formed. When the
cycloaddition proceeds via the endo-E-syn transition state 17, the
cycloadduct 1a with a cis-linkage of the dihydropyran and the
cyclohexene rings is formed. If, however, the cycloaddition goes
through the exo-E-anti transition state 18 the formation of
cycloadduct 1c with a trans-linkage of dihydropyran and
cyclohexene rings takes place. The preferred formation of 1a
is probably due to the fact that the endo-E-syn transition state 17
is sterically less demanding than the exo-E-anti transition state
18. It should be mentioned that alternative pathways for the
oxidative dimerization of 2-propenylsesamol (7) have been
proposed. In case of the Pd(II)-catalyzed/mediated process, the
β,β-coupled intermediate 9 may also be formed via a
carbopalladative reaction based on a two-electron process.
Originally, this pathway has been suggested by Chapman et al.4
Recently, this idea has been picked up again by Poli et al.10,47
For the oxidative dimerization of 7 with PhI(OAc)2 as the
oxidant a third mechanistic alternative has been proposed.11,48
Subsequently, the oxidative dimerization of (Z)-7 was studied.
The study focused on experiments with laccase/O2 and Co
complex 14/O2. It was found that the laccase-catalyzed oxidation
of (Z)-7, which was carried out under the same reaction
conditions than the reaction of (E)-7, gave 40% of a mixture of
1a, 1c, and 1d in a ratio of 5:1:4 (1H NMR) (Table 4, entry 1).
Almost identical results were observed when the oxidation of
(Z)-7 was performed with 2.6 mol % of (R,R)-(−)-N,N′-bis(3,5-
di-tert-butylsalicylidene)-1,2-cyclohexanediaminocobalt(II) (14)
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Scheme 4. Proposed Reaction Mechanism for the Formation
of 1a and 1c by Laccase-Catalyzed Domino Phenol
Oxidation/Anti-β,β-Radical Coupling/Intramolecular
Hetero-Diels−Alder Reaction of (E)-7
Scheme 5. Proposed Reaction Mechanism for the Formation
of 1d by Laccase-Catalyzed Domino Phenol Oxidation/
syn-β,β-Radical Coupling/Intramolecular Hetero-Diels−
Alder Reaction of (Z)-7
Because of this surprising result, the reaction of (E)-7 with O2
was studied in greater detail (Scheme 6). When the reaction of
(E)-7 with O2 in acetate buffer (pH = 5.0/acetone = 82:18) at
room temperature was run for 19 h the yield of the unknown
product increased to 73%. The structure elucidation revealed
that the new compound was a 3:2 mixture of the two
diastereoisomeric benzopyrans 26a,b, which can be regarded as
the products of an intermolecular/hetero Diels−Alder reaction.
The benzopyrans 26a and 26b could be completely separated
by column chromatography and were characterized. In both
diastereoisomers, the substituents at C-2 and C-3 are trans-
oriented. Diastereoisomers 26a and 26b differ only in the
cis- and trans-arrangement of the substituents at C-3 and C-4
of the dihydrochromene ring. It was also found that both 26a
and 26b were formed as racemates. That was established by
determination of the optical rotation of 26a and by separation
of the enantiomers of the acetate of 26b using HPLC on a
chiral phase (see the Supporting Information). A detailed
analysis revealed that traces of 26a,b are already formed during
preparation of the mixture of (E)- and (Z)-7 by base-catalyzed
isomerization of 1-propenylsesamol (11). For the preparation
of pure (E)- and (Z)-7, the Diels−Alder products 26a,b
were completely removed by column chromatography. In this
way, a contamination of (E)-7 and (Z)-7 with 26a,b could be
excluded.
Scheme 6. Intermolecular Oxidative Dimerization of (E)-7
It is assumed that 26a,b are formed by a domino oxidation/
intermolecular hetero-Diels−Alder reaction (Scheme 7).45 In
contrast to the laccase-catalyzed oxidation of (E)-7 with O2 to
the radical 16, the oxidation of (E)-7 with O2 in the absence of
a laccase results in the formation of the o-quinone methide 23
which undergoes an intermolecular inverse-electron-demand
hetero-Diels−Alder reaction with (E)-7 as the dienophile to
produce the benzopyrans 26a,b. The cycloadduct 26a is formed
via exo transition state 24 while 26b results from the
corresponding endo transition state 25.
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best fit to the NMR data of 1a (most probable 1aA, 1aB), 1c
and 1d were recomputed at the B3LYP/6-31+G(d,p) level of
theory and compared with the experimental NMR data sets.
For evaluation purposes, we first applied our methodology to
the determination of the already known relative stereochemistry
Scheme 7. Proposed Reaction Mechanism for the Formation
of 26a and 26b by Domino Oxidation/Intermolecular
Hetero-Diels−Alder Reaction of (E)-7
3
of 1a because the medium-sized coupling constants J(7′-H,
3
3
8′-H) = 2.6 Hz, J(2-H, 7′-H) = 7.3 Hz, and J(8-H, 8′-H)
∼ 1 Hz (see Table 5) did not allow an unambiguous assignment
1
Table 5. Important Experimental H NMR Coupling
Constants for 1a, 1c, and 1d
entry
1
3J2,7′ (Hz)
3J7′,8′ (Hz)
3J8,8′ (Hz)
3J7,8 (Hz)
1
2
3
a
c
d
7.3
10.4
10.7
2.6
11.7
10.0
∼1.0
3.7
5.2
6.4
3.8
9.7
of the relative configuration at C-2, C-8, C-7′, and C-8′.
However, strong ROESY correlations between 7′-H and 2-H,
2-H and 9′-CH3, 7′-H and 8′-H, and 7′-H and 9′-CH3 indicate a
cis-orientation of 2-H, 7′-H, and 9′-CH3 as well as a trans-located
proton 8′-H with respect to the reference plane of the molecule.
Because of the complex multiplet pattern (caused by additional
long-range couplings) of 8-H and 8′-H the vicinal coupling
constants could not easily be deduced. Along with a weak
ROESY correlation between 9-CH3 and 9′-CH3 two possible
configurations at C-8 can be discussed (Figure 6). Selective
Structure Elucidation. Using optimized HPLC conditions
(see the Supporting Information) the separation of the three
diastereoisomers 1a, 1c, and 1d could be achieved. Overall, the
1H and 13C NMR spectra showed high similarity except for
the signals of 7′-H, 8′-H, 8-H, and 2-H of the cyclohexene ring
in compounds 1a, 1c, and 1d.
Figure 6. Stereochemistry and numbering of carpanone (1a). (A)
Dihedral angles between 8-H and 8′-H (left) and 7-H and 8-H (right)
in trans-orientation of the methyl groups. (B) Dihedral angles
concerning 8-H and 8′-H (left) and 7-H and 8-H (right) in cis-
orientation of the methyl groups.
The major compound was identified as carpanone (1a) on
the basis of a comparison of its fully assigned 1H and 13C NMR
data with literature data of carpanone (1a).4,10 Since repeated
HPLC afforded only a few milligrams of each of the compounds
1c and 1d single crystals could not be obtained. This is why for
the determination of the relative stereochemistry of 1c and 1d
an alternative approach comprising analysis of experimental
JH,H values and ROESY data in combination with calculated
coupling constants and dihedral angles obtained by ab initio
DFT calculations was applied. Thus, all possible diaster-
eoisomers of carpanone (1a) which can be differentiated by
NMR (16 diastereoisomers due to five stereogenic centers)
were subsequently optimized at the AM1 level followed by the
RHF/3-21G level and finally by the B3LYP/6-31G(d) level of
theory within the Gaussian 03 package (for references see the
homoband decoupling of the resonances of both methyl groups
at C-9 and C-9′ (see the Supporting Information) revealed a
small coupling of ∼1 Hz, which accounts for a dihedral angle
of ∼90° between 8-H and 8′-H according to the Karplus
relationship. Moreover, an experimental coupling constant of
3J(7-H, 8-H) = 5.2 Hz which fits quite nicely with a dihedral
angle of ∼50° (calcd 51° and 5.6 Hz) establishes substructure A
in carpanone (1a). On the contrary, one would expect coupling
constants of 3J(8-H, 8′-H) ∼ 5 Hz (calculated +5.2 Hz and 49°)
3
and J(7-H, 8-H) ∼1−2 Hz (calculated 3.2 Hz and 82°) in
1
Supporting Information). In the next step the H,1H NMR
the case of substructure B. The ROESY correlations of 10-H
with 2-H and 10-H with 7′-H indicate the position of the
coupling constants of those optimized geometries showing the
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3,4-methylenedioxycyclohexenone ring below the reference
groups 9 and 9′. As for 1c, the weak ROESY correlation of 10-H
to 2-H indicates the 3,4-methylenedioxycyclohexenone ring above
the reference plane of molecule 1d. The calculated coupling
constants obtained by DFT methods were in agreement with the
experimental data (see the Supporting Information).
plane of the molecule 1a.
1
Contrary to 1a, the H NMR spectra of 1c and 1d showed
partially overlapped and/or complex multiplet patterns for 7′-H,
2-H, 8′-H, and 8-H of the cyclohexene rings (see the Supporting
Information) preventing stereochemical assignments based on
coupling constants. Therefore, selective homoband decoupling
In 2009, Poli et al.10 reported on the CuCl2-catalyzed synthesis
of carpanone. They obtained an inseparable mixture of
carpanone (1a) and a diastereoisomer 1b whose stereostructure
was elucidated by NOESY as shown in Figure 10 of their
contribution. However, comparison of the whole NMR data set
of isolated 1d with the NMR spectra of the mixture of 1a and 1b
obtained by Poli et al. (1H and 13C of 1a and 1b, see the
Supporting Information)10 discloses that both data sets are
identical suggesting that either our structural assignment of 1d or
the assignment of 1b proposed by Poli et al.10 has to be
1
methods as well as computational H spin pattern analysis
(SpinWorks 3.1)46 were applied to extract individual coupling
constants for assignment purposes (see the Supporting Informa-
tion). An observed coupling constant of 11.7 Hz between 7′-H and
8′-H in diastereoisomer 1c compared to 2.6 Hz in 1a (Table 5) and
significant 13C chemical shifts differences of C-2 (1a: δ 35.2 ppm/
1c: δ 44.3 ppm), C-7′(1a: δ 33.5 ppm/1c: δ 46.1 ppm), C-8′ (1a: δ
35.5 ppm/1c: δ 37.6 ppm), and C-8 (1a: δ 36.2 ppm/1c: δ 40.6
ppm) (Table 6) reflect important evidence for an altered relative
1
incorrect. Evaluation of the NOESY and H NMR spectra of
the mixture of 1a and 1b published by Poli et al.10 reveals that
the positions of 7′-H and 9-CH3 have to be corrected for the
following reasons: a large coupling constant of ∼11 Hz between
2-H and 7′-H indicates a dihedral angle close to 0° or 180°. In
the NOESY spectrum, a strong correlation peak between the
aforementioned protons suggests a cis-orientation. However,
detailed inspection of the cross peak gives rise to a COSY type
artifact (zero-quantum coherence) due to its anti phase pattern
and not to a NOESY correlation. Moreover, in the case of a cis-
arrangement between 2-H and 7′-H one must also observe
NOEs between 7′-H and 8′-H as well as between 7′-H and 8-H
or 9-CH3 in 1b. Strong NOESYs between 2-H and 8′-H as well
as 9-CH3 and 9′-CH3 along with a coupling constant of 9.7 Hz
between 8-H and 8′-H (dihedral angle close to 0) clearly
establish a cis-orientation of both methyl groups at C-9 and C-9′.
Furthermore a NOE between 7′-H and 9′-CH3 proved a cis-
arrangement of 7′-H and 9′-CH3. Unfortunately, NOESY
correlations concerning protons 8-H and 8′-H could not be
evaluated for an unambiguous assignment of stereostructure 1b
because of the partial overlap of these signals with the signals of
1a in the investigated mixture of 1a and 1b. In summary, the
NMR arguments presented here justify the conclusion that the
minor diastereoisomer of Poli’s product mixture has not
structure 1b as previously assigned, but structure 1d (Figure 10).
Table 6. Chemical Shift Differences of 1a, 1c, and 1d in 13C
NMR Spectra
entry
1
C-7′ (ppm)
C-8′ (ppm)
C-2 (ppm)
C-8 (ppm)
1
2
3
a
c
d
33.5
46.1
46.9
35.5
37.6
32.0
35.2
44.3
44.0
36.2
40.6
33.6
stereochemistry in 1c. Another large coupling constant between
7′-H and 2-H (10.4 Hz) along with a strong ROESY between 2-H
and 8′-H indicates an alternating trans-diaxial orientation of these
protons (2-H, 7′-H and 7′-H, 8′-H; Figure 7). An additional
Figure 7. Chemical structures and numbering of 1c. Important
ROESY correlations (green arrow, weak ROESY correlations; red
arrow, strong ROESY correlations).
CONCLUSIONS
■
The laccase-catalyzed oxidative dimerization of (E)-propenylse-
samol (7) which can be performed in aqueous solvent systems
and under mild reaction conditions offers a new and efficient
access to carpanone (1a). To study the influence of the
stereochemistry of the olefinic double bond in 7 on the outcome
of the carpanone formation, (E)-7 and (Z)-7 were separated and
reacted in diastereoisomerically pure form using different
catalysts. In contrast to the results from previous studies it was
established that the reaction of pure (E)-7 results in the
formation of a mixture of carpanone (1a) and its diastereoisomer
1c. Irrespective of the catalyst used, a 9:1 mixture of 1a and 1c
was formed. The structure of diastereoisomer 1c, which was
unknown so far, was unambiguously elucidated by means of
NMR spectroscopy, DFT calculations, and spin work analysis.
Both 1a and 1c are the products of a domino phenol oxidation/
anti-β,β-radical coupling/intramolecular hetero-Diels−Alder re-
action. In the first step of the reaction, the oxidation of (E)-7
takes place to yield radical 16, which dimerizes diastereose-
lectively to give 9 as the product of an anti-β,β-coupling reaction.
Subsequently, 9 undergoes an intramolecular hetero-Diels−Alder
reaction to 1a via the endo-E-syn transition state 17 and to 1c via
ROESY between 2-H and 9-CH3 as well as 8′-H and 9-CH3
unequivocally arrange 2-H, 9-CH3 and 8′-H above the cyclo-
hexene ring. Finally, the weak ROESY correlation between 2-H to
10-H reveals the position of the 3,4-methylenedioxycyclohex-
enone ring above the reference plane of molecule 1c. Coupling
constants obtained by DFT calculations were close to experi-
mental values (see the Supporting Information).
As for 1c, the coupling constants of the complex multiplet
patterns of 1d, e.g., 8′-H and 8-H (Figure 8) were analyzed and
extracted by spin analysis and decoupling experiments (see the
Supporting Information).
Similarly to 1c the protons 2-H, 7′-H, and 8′-H of 1d show
an alternating trans-axial arrangement as evidenced by vicinal
coupling constants of ∼10 Hz and ROESY correlations
between 2-H and 8′-H as well as 7′-H and 9′-CH3 (Figure 9).
Besides considerably different 13C chemical shifts values of C-8′
and C-8 (see Table 6) compared to 1c, strong ROESYs between
2-H and 8-H as well as 9′-CH3 and 9-CH3 clearly establish the
axial position of 8-H and the cis-configuration of the methyl
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Figure 8. Experimental (lower) and iterated (upper) multiplet pattern of 8′-H (δ 2.50 ppm) and 8-H (δ 2.55 ppm) in 1d.
different products. The oxidation of (E)-7 with O2 in the absence
of any catalyst came as a surprise as the formation of two
diastereoisomeric benzopyrans was observed. The formation of the
benzopyrans can be understood in such a way that the oxidation
of (E)-7 in the absence of any catalyst results in the formation of an
o-quinone methide which undergoes an intermolecular hetero-
Diels−Alder as a diene with (E)-7 as the dienophile.
EXPERIMENTAL SECTION
■
General Remarks. All commercially available reagents were used
without further purification. Solvents used for extraction and
purification were distilled prior to use. Reaction temperatures are
reported as bath temperatures. Melting points were obtained on a
melting point apparatus with open capillary tubes and are uncorrected.
IR spectra were measured on a FT-IR spectrometer. UV/vis spectra
were recorded with a spectrophotometer. Optical rotations were
determined using a polarimeter. 1H and 13C NMR spectra were
recorded at 500 (125) and 300 (75) MHz, and the NMR chemical
shifts were referenced to CDCl3 as solvent signals (δ = 7.26 ppm in 1H
spectra and δ = 77.00 in 13C spectra) relative to TMS. HSQC, HMBC,
NOESY, ROESY, HSQMBC, and COSY spectra were recorded on a
spectrometer at 500 and 300 MHz. Coupling constants J (Hz) were
directly taken from the spectra and are not averaged. Splitting patterns
are designated as s (singlet), d (doublet), t (triplet), q (quartet), m
(multiplet), and br (broad). 1D and 2D homonuclear NMR spectra
were measured with standard pulse sequences. Low-resolution impact
mass spectra were obtained at 70 eV. The intensities are reported as
percentages relative to the base peak (I = 100%). Thin-layer
chromatography (TLC) was performed on TLC silica gel 60 F254.
Products were purified by column chromatography on silica gel MN 60,
0.063−0.20 mm, or by flash column chromatography on silica gel MN
60, 0.04−0.053 mm. Analytical HPLC was performed using a HPLC
pump equipped with a PDA detector. Laccases from T. versicolor and A.
bisporus were commercially available (EC 1.10.3.2), Lccβ: β-laccase
from T. versicolor was heterologously expressed in P. pastoris X-33
cells.43 Recombinant cells were cultured in a 7.5 L bioreactor (Infors)
in basal salt medium (5 L) [K2SO4 (9.1 g), MgSO4·7H2O (7.5 g),
KOH (6.2 g), CaSO4·2H2O (2.47 g), 85% H3PO4 (13.35 mL), glycerol
(50 g), antifoam solution 286 (0.1 mL), biotin (0.87 mg), and PTM1
(4.35 mL) trace salts per liter]. One liter of PTM1 contains CuSO4·
5H2O (6 g), NaI (0.08 g), MnSO4·H2O (3.0 g), CoCl2 (0.5 g), ZnCl2
Figure 9. Chemical structures and numbering of 1d. Important
ROESY correlations (green arrow, weak ROESY correlations; red
arrow, strong ROESY correlations).
Figure 10. Structure of 1b as proposed by Poli et al.,10 and structure of 1d.
exo-E-anti transition state 18. With pure (Z)-7 as the substrate,
carpanone (1a) and the two carpanone diastereoisomers 1c and
1d were formed in a 5:1:4 ratio. The diastereoisomers were
separated by HPLC, and the structure of 1d was elucidated by
combination of NMR spectroscopy, DFT calculations, and spin
work analysis. For the formation of 1d it is assumed that the
dimerization of 16 proceeds as a syn-β,β-coupling reaction and
subsequent intramolecular hetero-Diels−Alder reaction of the
coupling product 20 via an exo-E-anti transition state. So far,
it remains unclear why the reactions of (E)- and (Z)-7 yield
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(20.0 g), H3BO3 (0.02 g), Na2MoO4·2H2O (0.2 g), FeSO4·7H2O
(65.0 g), biotin (0.2 g), and H2SO4 (5 mL). Cells were grown at 30 °C
(pH 5.0, aeration 10 L/min, agitation 1000 rpm) during the glycerol
phase. After depletion of glycerol, the methanol-fed batch phase was
started, CuSO4 (0.3 mM) was added, and the temperature was decreased
to 18 °C. A dissolved O2 spike controlled feeding method was applied for
automatic addition of depleted methanol [20 g, with 1.2% (v/v) PTM1
0.5% (v/v)]. After 10 days of cultivation, cells were harvested (10.000 g,
20 min), and Lccβ-containing solution was concentrated by cross-flow
ultrafiltration [Millipore, 10 kDa cutoff membrane (Pall)]. The activities
of the enzymes were determined according to a modified procedure taken
from ref 30 using a solution of ABTS [(2,2′-azinobis(3-ethylbenzothiazo-
line-6-sulfonic acid) diammonium salt] as substrate. The change in absorp-
tion was followed via UV spectroscopy (λ = 414 nm; ε414 = 31100 L
mol−1 cm−1). One U is equivalent to the amount of enzyme that catalyzes
the conversion of 1 μmol ABTS per minute at 24 °C and pH 5.0.
5-Allyloxy-1,3-benzodioxole (12).39
IR (ATR) ν 3244 (OH), 2907 (OCH2O), 1636 (CHCH2), 1457
̅
(arom CC), 1167 (ArOCH), 1030 (ArOCH), 909 (CCHH),
830(arom C−H) cm−1; UV (MeOH) λmax (log ε) 235 (4.53), 302 (3.67)
1
nm; H NMR (300 MHz, CDCl3) δ 3.30−3.32 (m, 2H, 7-H), 5.12−5.18
(m, 2H, 9-H), 5.92−6.01 (m, 1H, 8-H), 5.98 (s, 2H, 10-H), 6.43 (s, 1H,
4-H), 6.58 (s, 1H, 1-H); 13C NMR (75 MHz, CDCl3) δ 34.9 (C-7), 98.6
(C-4), 100.9 (C-10), 109.5 (C-1), 116.3 (C-9), 116.9 (C-6), 136.4 (C-8),
141.4 (C-2), 146.7 (C-3), 148.5 (C-5); MS (EI, 70 eV) m/z 178 (39) [M]+,
151 (8) [M − C2H3]+, 28 (100).
Synthesis of 6-(Prop-1-enyl)-1,3-benzodioxol-5-ol (7) and Sepa-
ration of (E)-7 and (Z)-7.4
To a solution of 6-allyl-1,3-benzodioxol-5-ol (11) (2.5 g, 14 mmol) in dry
DMSO (25 mL) was added potassium tert-butoxide (1.7 g, 15 mmol),
and the apparatus was flushed with argon. The reaction mixture was
heated at 100 °C for 90 min under argon. The solution was cooled using
an ice−water bath, water (100 mL) was added, and the mixture was
acidified with 5% aq HCl (about 20 mL). The solution was extracted with
EtOAc (3 × 150 mL). The combined organic layers were dried over
Na2SO4, filtered, and evaporated under reduced pressure to give 3 g of a
brown liquid. The crude product was purified by column chromatography
over silica gel (dichloromethane/cyclohexane = 5:2) to afford (E)-6-
(prop-1-enyl)-1,3-benzodioxol-5-ol [(E)-7] in 52% yield (1.3 g, 7.30 mmol)
and (Z)-6-(prop-1-enyl)-1,3-benzodioxol-5-ol [(Z)-7] in 19% yield
(0.475 g, 2.70 mmol).
A solution of sesamol (13) (10 g, 72 mmol) in acetone
(50 mL) was refluxed for 20 min over dried K2CO3 (10 g, 70.2
mmol). The mixture was cooled to room temperature, allyl bromide
(10 mL, 0.11 mol) was added, and the resulting mixture was refluxed at
70 °C for 5 h under argon. The reaction mixture was cooled to room
temperature, and H2O (500 mL) was added. The organic layer was
removed, and the aqueous phase was extracted with EtOAc (3 × 200
mL). The combined organic phases were washed with 2 N NaOH (2 ×
100 mL) and saturated NaCl solution (2 × 100 mL) and dried over
Na2SO4, and the volatiles were removed under reduced pressure. The
residue thus obtained was purified by column chromatography over
silica gel (petroleum ether/EtOAc = 10:1) to afford 5-allyloxy-1,3-
benzodioxole (12) as a colorless oil in 84% yield (10.7 g, 60.11 mmol):
(E)-6-(Prop-1-enyl)-1,3-benzodioxol-5-ol [(E)-7].
Rf = 0.54 (petroleum ether/EtOAc = 10:1); IR (ATR) ν 2886
̅
(OCH2O), 1630 (CHCH2), 1484 (arom CC), 1178 (ArOCH),
1036 (ArOCH), 922 (CCHH), 814 (arom CH) cm−1; UV
1
(MeOH) λmax (log ε) 235 (3.63), 296 (3.66) nm; H NMR (300
mp 84−85 °C (87−88 °C);4 Rf = 0.20 (dichloromethane/cyclohexane
MHz, CDCl3) δ 4.46 (dt, 3J (7-H, 8-H) = 5.2 Hz, 4J (7-H, 9a-H) = 1.5
= 5:2); IR (ATR) ν 3319 (OH), 2898 (OCH2O), 1636 (CC), 1441
̅
Hz, 4J (7-H, 9b-H) = 1.5 Hz, 2H, 7-H), 5.27 (dq, 3J (8-H, 9b-H) = 10.5
(arom CC), 1163 (ArOCH), 1031 (ArOCH), 928 (CCH), 833
(arom CH) cm−1; UV (MeOH) λmax (log ε) 258 (3.60), 326 (3.46)
nm; 1H NMR (300 MHz, CDCl3) δ 1.88 (dd, 3J (8-H, 9-H) = 6.6 Hz,
4J (7-H, 9-H) = 1.7 Hz, 3H, 9-H), 4.79 (s, 1H, OH), 5.88 (s, 2H, 10-
2
3
Hz, J (9a-H, 9b-H) = 1.5 Hz, 1H, 9b-H), 5.42 (dq, J (8-H, 9a-H) =
17.2 Hz, 2J (9a-H, 9b-H) = 1.5 Hz, 1H, 9a-H), 5.91 (s, 2H, 10-H), 6.03
(ddd, 3J (7-H, 8-H) = 5.1 Hz, 3J (8-H, 9b-H) = 10.3 Hz, 3J (8-H, 9a-H)
= 17.2 Hz, 1H, 8-H), 6.34 (dd, 3J (1-H, 6-H) = 8.5 Hz, 4J (4-H, 6-H) =
3
3
H), 6.03 (dq, J (7-H, 8-H) = 15.7 Hz, J (8-H, 9-H) = 6.6 Hz, 1H,
4
2.5 Hz, 1H, 6-H), 6.52 (d, J (4-H, 6-H) = 2.5 Hz, 1H, 4-H), 6.70 (d,
3
4
8-H), 6.39 (s, 1H, 4-H), 6.50 (dq, J (7-H, 8-H) = 15.7 Hz, J (7-H,
9-H) = 1.50 Hz, 1H, 7-H), 6.76 (s, 1H, 1-H); 13C NMR
(75 MHz, CDCl3) δ 18.7 (C-9), 98.2 (C-4), 101.0 (C-10), 105.9
(C-1), 117.3 (C-6), 124.9 (C-8), 126.6 (C-7), 141.8 (C-2), 147.0 (C-3),
147.2 (C-5).
3J (1-H, 6-H) = 8.5 Hz, 1H, 1-H); 13C NMR (75 MHz, CDCl3) δ 69.8
(C-7), 98.3 (C-4), 101.1 (C-10), 106.0 (C-6), 107.9 (C-1), 117.6
(C-9), 133.4 (C-8), 141.7 (C-2), 148.2 (C-3), 154.1 (C-5); MS (EI,
70 eV) m/z 178 (100) [M]+, 137 (100) [M − C3H5]+, 107 (69), 79
(8), 28 (100).
(Z)-6-(Prop-1-enyl)-1,3-benzodioxol-5-ol [(Z)-7].
6-Allyl-1,3-benzodioxol-5-ol (11).40
A round-bottomed flask equipped with a reflux condenser was charged
with 5-allyloxy-1,3-benzodioxole (12) (10 g, 56.18 mmol) and heated
at 180 °C for 90 min under argon. The mixture was cooled to room
temperature, and the residue obtained was purified by column
chromatography over silica gel (petroleum ether/EtOAc = 10:1) to afford
6-allyl-1,3-benzodioxol-5-ol (11) as colorless needles in 90% yield (9.3 g,
52.24 mmol): mp 75−76 °C; Rf = 0.38 (petroleum ether/EtOAc = 10:1);
Rf = 0.18 (dichloromethane/cyclohexane = 5:2); IR (ATR) ν 3479
̅
(OH), 2890 (OCH2O), 1627 (CC), 1439 (arom CC), 1165
(ArOCH), 1034 (ArOCH), 932 (CCH), 845 (arom CH) cm−1; UV
1
(MeOH) λmax (log ε) 256 (3.88), 312 (3.85) nm; H NMR (300
MHz, CDCl3) δ 1.71 (dd, 3J (8-H, 9-H) = 6.9 Hz, 4J (7-H, 9-H) = 1.8
3
Hz, 3H, 9-H), 4.83 (s, 1H, OH), 5.90 (s, 2H, 10-H), 5.94 (dq, J
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ratio of 1a:1c in the crude product was 9:1 (1H NMR). The ratio of
1a:1c after flash chromatography was 9:1 (1H NMR).
(7-H, 8-H) = 11.1 Hz, 3J (8-H, 9-H) = 6.9 Hz, 1H, 8-H), 6.28 (dq, 3J
(7-H, 8-H) = 11.1 Hz, 4J (7-H, 9-H) = 1.5 Hz, 1H, 7-H), 6.48 (s, 1H,
4-H), 6.56 (s, 1H, 1-H); 13C NMR (75 MHz, CDCl3) δ 14.5 (C-9),
97.5 (C-4), 101.0 (C-10), 108.3 (C-1), 114.9 (C-6), 124.0 (C-8),
6. Oxidative Dimerization of (E)-7 Using a Lccβ Laccase from T.
versicolor as a Catalyst. Reaction of (E)-7 (189 mg, 1.06 mmol) with
laccase (8.2 mL, 28 U) according to the general procedure gave a
mixture of 1a and 1c as a white solid. The reaction mixture was then
lyophilized. The residue was refluxed for 90 min in dichloromethane
(50 mL). After filtration the solvent was removed under reduced
pressure. The crude product obtained was purified by flash chromato-
graphy over silica gel (dichloromethane/cyclohexane = 5:2) to afford a
mixture of 1a and 1c as a white solid in 53% yield (97 mg, 0.27 mmol).
The ratio of 1a:1c in the crude product was 9:1 (1H NMR). The ratio of
1a:1c after flash chromatography was 9:1 (1H NMR).
7. Determination of the Enantiomeric Excess of 1a, 1c, and 1d
by Chiral HPLC. The oxidative dimerization of (E)-7 was carried out
with three different laccases (commercially available laccase from
T. versicolor, commercially available laccase from A. bisporus, genetically
modified Lccβ laccase from T. versicolor). The crude product was
purified by column chromatography on SiO2 to give a mixture of 1a
and 1c. Compounds 1a and 1c were separated by analytical HPLC
(see the Supporting Information). After HPLC separation, the solvent
was removed in vacuo, and the enantiomers of 1a were separated by
chiral HPLC (CHIRALPAK IB column; 250 mm × 4.6 mm; 5 μm;
n-hexane/ethanol = 1:1; flow rate of 1 mL/min; UV detection λ =
280 nm) to afford the two enantiomers (tR 4.95 min and tR 5.45 min)
(Figure 11).The enantiomeric excess was calculated from the peak
areas (Table 7).
130.9 (C-7), 141.1 (C-2), 147.3 (C-3), 147.6 (C-5).
General Procedure for the Oxidative Dimerization of (E)-
and (Z)-7 Using Commercially Available Laccases as Catalysts.
Laccase (28 U) dissolved in sodium acetate buffer (0.1 M, pH 5.0,
1 mL) was added to a solution of 7 (178 mg, 1.00 mmol) in a
cosolvent (2 or 4 mL) and sodium acetate buffer (0.1 M, pH 5.0,
17 mL). The resulting suspension was stirred at room temperature
under O2. NaCl (3 g) was added, and the aqueous phase was extracted
with dichloromethane (80 mL). The phases were separated, and
saturated NaCl solution (20 mL) was added to the aqueous phase.
The aqueous phase was extracted with dichloromethane (80 mL). This
procedure was repeated twice. The combined organic phases were
dried over MgSO4, filtered, and evaporated under reduced pressure.
The crude product was purified by flash chromatography over silica gel
(dichloromethane/cyclohexane = 5:2).
1. Oxidative Dimerization of a Mixture of (E)- and (Z)-7 Using a
Commercially Available Laccase from T. versicolor as the Catalyst.
Reaction of an 8:2 mixture of (E)/(Z)-7 (180 mg, 1.01 mmol) with
laccase (2.6 mg, 28 U) according to the general procedure gave a mixture
of 1a, 1c, and 1d as a white solid in 46% yield (83 mg, 0.23 mmol). The
ratio of 1a:1c:1d after flash chromatography was 6:1:3 (1H NMR).
2. Oxidative Dimerization of (E)-7 Using PdCl2 as the Reagent
According to Chapman et al.4 PdCl2 (90.0 mg, 0.5 mmol) was added
to a solution containing (E)-7 (178 mg, 1.00 mmol) and NaOAc
(670 mg, 8 mmol) in MeOH (17 mL) and H2O (3 mL). The solution
was stirred at 38 °C for 2 h and then allowed to settle for 1 h at
room temperature. The suspension was extracted with diethyl ether
(3 × 30 mL). The combined organic phases were washed with 10%
NaOH solution (2 × 20 mL) and H2O (2 × 20 mL). The organic layer
was dried over MgSO4, filtered, and evaporated under reduced
pressure. The crude product was purified by flash chromatography
over silica gel (dichloromethane/cyclohexane = 5:2) to afford a
mixture of 1a and 1c as a white solid in 42% yield (75 mg, 0.21 mmol).
The ratio of 1a:1c in the crude product was 9:1 (1H NMR). The ratio
of 1a:1c after flash chromatography was 9:1 (1H NMR).
3. Oxidative Dimerization of (E)-7 Using PdCl2 as the Catalyst
According to Poli et al.10 NaOAc (98.4 mg, 1.2 mmol) and PdCl2
(17.74 mg, 0.1 mmol) were added to a solution of (E)-7 (175 mg,
0.98 mmol) in MeOH/H2O = 1:1 (6 mL). The resulting suspension was
stirred at 50 °C for 4 h under O2. The reaction mixture was cooled to
room temperature, and H2O (20 mL) was added. The aqueous phase
was extracted with dichloromethane (3 × 30 mL). The combined organic
phases were dried over Na2SO4, filtered, and evaporated under reduced
pressure. The crude product was purified by flash chromatography over
silica gel (dichloromethane/cyclohexane = 5:2) to afford a mixture of 1a
and 1c as a white solid in 47% (77%)10 yield (81 mg, 0.23 mmol). The
ratio of 1a:1c in the crude product was 9:1 (1H NMR). The ratio of
1a:1c after flash chromatography was 9:1 (1H NMR).
Figure 11. HPLC chromatogram of the separation of the carpanone
enantiomers on an analytical column with a chiral stationary phase.
Table 7. Determination of Enantiomeric Excess of the
Laccase-Catalyzed Oxidations of (E)-7
entry
laccase
enantiomeric excess (1a)
1
2
3
commercialy available; T. versicolor
commercialy available; A. bisporus
recombinant Lccβ; T. versicolor
0.44
0.25
0.43
4. Oxidative Dimerization of (E)-7 Using Co Complex 14 as
Catalyst. (R,R)-(−)-N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexanediaminocobalt(II) (14) (15.5 mg, 0.026 mmol) was
added to a solution of (E)-7 (183 mg, 1.03 mmol) in dichloromethane
(10 mL). The resulting suspension was stirred under O2 at room tem-
perature for 4 h. To remove the complex from the reaction mixture, the
solution was filtrated, and the residue was washed with dichloro-
methane (10 mL). The filtrate was concentrated under reduced
pressure, and the resulting residue was purified by flash chromatog-
raphy over silica gel (dichloromethane/cyclohexane = 5:2) to afford a
mixture of 1a and 1c as a white solid in 79% yield (144 mg,
0.41 mmol). The ratio of 1a:1c in the crude product was 9:1 (1H NMR).
The ratio of 1a:1c after flash chromatography was 9:1 (1H NMR).
5. Oxidative Dimerization of (E)-7 Using Commercially Available
Laccases as Catalysts. Reaction of (E)-7 (182 mg, 1.02 mmol) with
A. bisporus/T. versicolor laccase (49/2.6 mg, 28 U) according to the
general procedure gave a mixture of 1a and 1c as a white solid. The
The enantiomers of 1c were separated by chiral HPLC (CHIRALPAK
IB column; 250 mm × 4.6 mm; 5 μm; n-hexane/ethanol = 1:1; flow rate
of 1 mL/min; UV detection λ = 280 nm) to afford the two enantiomers
(tR 5.07 min and tR 5.57 min) (Figure 12).The enantiomeric excess was
calculated from the peak areas to ee (%) = 1.03.
The enantiomers of 1d were separated by chiral HPLC (CHIRALPAK
IB column; 250 mm × 4.6 mm; 5 μm; n-hexane/ethanol = 1:1; flow rate
of 1 mL/min; UV detection λ = 280 nm) to afford the two enantiomers
(tR 5.50 min and tR 6.18 min) (Figure 13). The enantiomeric excess was
calculated from the peak areas to ee (%) = 0.93.
8. Oxidative Dimerization of (E)-7 Using 4-Methoxy-TEMPO as
the Reagent. 4-Methoxy-TEMPO (50 mg, 0.27 mmol) was added
to a solution of (E)-7 (184 mg, 1.03 mmol) in 10 mL of dry benzene.
The resulting suspension was stirred under O2 at 70 °C for 4 h. The
suspension was concentrated under reduced pressure, and the
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crude product only carpanone (1a) could be detected (1H NMR). The
crude product was purified by column chromatography over silica gel
(dichloromethane/cyclohexane = 5:2) to afford pure carpanone (1a)
(1H NMR) as a white solid in quantitative yield (50 mg, 0.14 mmol).
(3R*)-(2β,7′β,8α,8′β)-1,7-Didehydro-1,2,3,6-tetrahydro-2′,3-
epoxy-3,4:4′,5′-bis(methylenedioxy)-2,7′-cyclolignan-6-one) (1a).
Figure 12. HPLC chromatogram of the separation of the enantiomers
of 1c on an analytical column with a chiral stationary phase.
mp 189−190 °C (lit.4 mp 185 °C); Rf = 0.10 (dichloromethane/
cyclohexane = 5:2); IR (ATR) ν 2869 (OCH2O), 1674 (CO), 1622
̅
(CC), 1478 (arom CC), 1158 (ArOCH), 1032 (ArOCH), 910
(CCH), 841 (arom CH) cm−1; UV (MeOH) λmax (log ε) 262
1
3
(4.00), 297 (3.66) nm; H NMR (500 MHz, CDCl3) δ 0.71 (d, J
(8-H, 9-H) = 7.7 Hz, 3H, 9-H), 1.15 (d, 3J (8′-H, 9′-H) = 7.0 Hz, 3H,
3
3
3
9′-H), 2.22 (ddq, J (8-H, 9-H) = 7.7 Hz, J (7-H, 8-H) = 5.2 Hz, J
(8-H, 8′-H) = 2.5 Hz, 1H, 8-H), 2.52 (br dq, 3J (8′-H, 9′-H) = 7.2 Hz,
3J (8-H, 8′-H) = 2.5 Hz, 3J (7′-H, 8′-H) = 2.6 Hz, 1H, 8′-H), 3.19 (dt,
4
3J (2-H, 7′-H) = 7.4 Hz, J (2-H, 7-H) = 2.4 Hz, 1H, 2-H), 3.28 (dd,
3J (7′-H, 8′-H) = 2.7 Hz, J (7′-H, 2-H) = 7.2 Hz, 1H, 7′-H), 5.64 (s,
3
2
1H, 10-H), 5.67 (s, 1H, 10-H), 5.70 (s, 1H, 5-H), 5.90 (d, J (10′-H,
10′-H) = 1.4 Hz, 1H, 10′-H), 5.91 (d, 2J (10′-H, 10′-H) = 1.4 Hz, 1H,
3
10′-H), 6.33 (s, 1H, 3′-H), 6.80 (s, 1H, 6′-H), 7.02 (ddd, J (7-H,
8-H) = 5.2 Hz, long-range J = 1.0 Hz, 4J (2-H, 7-H) = 2.5 Hz, 1H, 7-H);
13C NMR (125 MHz, CDCl3) δ 21.2 (C-9), 21.4 (C-9′), 33.5 (C-7′),
35.2 (C-2), 35.5 (C-8′), 36.2 (C-8), 98.8 (C-10), 99.3 (C-3′), 100.1
(C-3), 100.4 (C-5), 101.2 (C-10′), 107.1 (C-6′), 115.3 (C-1′), 126.3
(C-1), 142.5 (C-7), 143.1 (C-5′), 145.1 (C-2′), 146.6 (C-4′), 168.3 (C-
4), 186.9 (C-6); MS (EI, 70 eV) m/z 354 (100) [M]+, 177 (72), 147
(60), 59 (28), 28 (40).
Figure 13. HPLC chromatogram of the separation of the enantiomers
of 1d on an analytical column with a chiral stationary phase.
resulting residue was purified by flash chromatography over silica gel
(dichloromethane/cyclohexane = 5:2) to afford a mixture of 1a and 1c
as a white solid in 79% yield (147 mg, 0.42 mmol). The ratio of 1a:1c
in the crude product was 9:1 (1H NMR). The ratio of 1a:1c after flash
chromatography was 9:1 (1H NMR).
(3S*)-(2α,7′β,8α,8′β)-1,7-Didehydro-1,2,3,6-tetrahydro-2′,3-
epoxy-3,4:4′,5′-bis(methylenedioxy)-2,7′-cyclolignan-6-one) (1c).
9. Oxidative Dimerization of (Z)-7 Using a Commercially
Available Laccase from T. versicolor as the Catalyst. Reaction of
(Z)-7 (184 mg, 1.03 mmol) with laccase (2.6 mg, 28 U) according to
the general procedure gave a mixture of 1a, 1c, and 1d as a white solid
in 40% yield (74 mg, 0.21 mmol). The ratio of 1a:1c:1d after flash
chromatography was 5:1:4 (1H NMR). The compounds 1a, 1c, and 1d
were separated by analytical HPLC (see the Supporting Information).
10. Oxidative Dimerization of (Z)-7 Using Co Complex 14 as
Catalyst. (R,R)-(−)-N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexanediaminocobalt(II) (14) (15.5 mg, 0.026 mmol) was
added to a solution of (Z)-7 (181 mg, 1.02 mmol) in dichloromethane
(10 mL). The resulting suspension was stirred under O2 at room
temperature for 4 h. To remove the complex from the reaction mixture,
the solution was filtrated and the residue was washed with
dichlorometane (10 mL). The filtrate was concentrated under reduced
pressure, and the resulting residue was purified by flash chromatography
over silica gel (dichloromethane/cyclohexane = 5:2) to afford a mixture
of 1a, 1c, and 1d as a white solid in 43% yield (78 mg, 0.22 mmol). The
ratio of 1a:1c:1d after flash chromatography was 5:1:4 (1H NMR).
Isomerization Experiment with Carpanone (1a). Sodium
acetate buffer (0.1 M, pH 5.0, 4 mL) was added to a solution of
carpanone (1a) (50 mg, 0.14 mmol) in acetone (1.1 mL). A solution
of laccase (8 U) from T. versicolor was dissolved in sodium acetate
buffer (0.1 M, pH 5.0, 1 mL) and added dropwise and with stirring to
the carpanone suspension. The reaction mixture was stirred under O2
at room temperature for 2 h. The reaction mixture was extracted with
dichloromethane (3 × 20 mL). The combined organic phases were dried
over MgSO4, filtered, and evaporated under reduced pressure. In the
Rf = 0.10 (dichloromethane/cyclohexane =5:2); IR (ATR) ν 2924
̅
(OCH2O), 1675 (CO), 1610 (CC), 1477 (arom CC), 1150
(ArOCH), 1037 (ArOCH), 977 (CCH), 894 (arom CH) cm−1; UV
(MeOH) λmax (log ε) 278 (2.77) nm; 1H NMR (500 MHz, CDCl3) δ 1.22
(d, 3J (8-H, 9-H) = 7.5 Hz, 3H, 9-H), 1.30 (d, 3J (8′-H, 9′-H) = 6.6 Hz, 3H,
3
3
3
9′-H), 2.08 (ddq, J (8′-H, 9′-H) = 6.6 Hz, J (7′-H, 8′-H) = 11.6 Hz, J
(8-H, 8′-H) = 3.7 Hz, 1H, 8′-H), 2.40 (ddd, 3J (7′-H, 8′-H) = 11.8 Hz, 3J
(2-H, 7′-H) = 10.4 Hz, 4J (6′-H, 7′-H) = 1.1 Hz, 1H, 7′-H), 2.42 (m, 3J (8-
H, 9-H) = 7.5 Hz, 3J (7-H, 8-H) = 6.4 Hz, 3J (8-H, 8′-H) = 3.7 Hz, 1H, 8-
4
3
H), 2.90 (ddd, J (2-H, 7-H) = 2.7 Hz, J (2-H, 7′-H) = 10.2 Hz, long-
range J = 1.6 Hz, 1H, 2-H), 5.42 (s, 1H, 10-H), 5.55 (s, 1H, 10-H), 5.84
(s, 1H, 5-H), 5.95 (d, J (10′-H, 10′-H) = 1.5 Hz, 1H, 10′-H), 5.97 (d,
2
2J (10′-H, 10′-H) = 1.5 Hz, 1H, 10′-H), 6.60 (s, 1H, 3′-H), 6.88 (d, 4J (6′-H,
7′-H) = 1.2 Hz, 1H, 6′-H), 7.11 (dd, 3J (7-H, 8-H) = 6.4 Hz, 4J (2-H, 7-H)
= 2.7 Hz, 1H, 7-H); 13C NMR (125 MHz, CDCl3) δ 20.2 (C-9), 22.2
(C-9′), 37.6 (C-8′), 40.6 (C-8), 44.3 (C-2), 46.1 (C-7′), 96.0 (C-10), 101.4
(C-10′), 101.6 (C-3), 102.1 (C-3′), 104.5 (C-5), 105.2 (C-6′), 124.0 (C-1′),
132.8 (C-1), 142.8 (C-7), 144.1 (C-5′), 146.2 (C-4′), 147.2 (C-2′), 164.6
(C-4), 183.8 (C-6); MS (EI, 70 eV) m/z 354 (45) [M]+, 324 (22), 309
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(23), 203 (31), 177 (48), 147 (100), 32 (33); HRMS (EI, M+) calcd for
C20H18O6 (354.1104), found 354.1121.
3J (1‴b-H, 2‴-H) = 7.4 Hz, 2J (1‴a-H, 1‴b-H) = 13.5 Hz, 1H, 1‴b-H),
3
3
3
2.47 (ddq, J (2-H, 3-H) = 9.8 Hz, J (3-H, 1″-H) = 6.9 Hz, J (3-H,
3
3
4-H) = 4.5 Hz, 1H, 3-H), 2.54 (ddd, J (4-H, 3-H) = 4.6 Hz, J (4-H,
1‴a-H) = 8.5 Hz, 3J (4-H, 1‴b-H) = 4.6 Hz, 1H, 4-H), 4.98 (d,
3J (2-H, 3-H) = 10.0 Hz, 1H, 2-H), 5.89 (s, 2H, OCH2O), 5.90 (s, 2H,
OCH2′O), 6.17 (s, 1H, OH), 6.44 (s, 2H, 8-H and 4′-H), 6.55 (s, 1H, 1′-
H), 6.56 (s, 1H, 5-H); 13C NMR (125 MHz, CDCl3) δ 12.7 (C-2‴), 14.7
(1″-C), 23.8 (C-1‴), 34.4 (C-3), 40.9 (C-4), 79.4 (C-2), 98.4
(C-8), 99.4 (C-4′), 100.9 (OCH2O), 101.2 (OCH2′O), 107.8 (C-1′),
108.6 (C-5), 116.8 (C-6′), 118.5 (C-10), 141.1 (C-2′), 141.8 (C-6), 146.7
(C-7), 147.2 (C-9), 147.9 (C-5′), 149.8 (C-3′); MS (EI, 70 eV) m/z 356
(36) [M]+, 178 (100), 151 (24), 72 (20), 59 (32); HRMS (EI, M+) calcd
for C20H20O6 (356.1260), found 356.1261.
(3S*)-(2α,7′β,8β,8′β)-1,7-Didehydro-1,2,3,6-tetrahydro-2′,3-
epoxy-3,4:4′,5′-bis(methylenedioxy)-2,7′-cyclolignan-6-one) (1d).
(2R*,3R*,4R*)-1,3-Dioxol[6,7-f ]-4-ethyl-3-methyl-2-[(1,3-benzo-
dioxol-5-ol)-yl]-3,4-dihydro-2H-benzopyrane (26b).
Rf = 0.10 (dichloromethane/cyclohexane = 5:2); IR (ATR) ν 2924
̅
(OCH2O), 1674 (CO), 1603 (CC), 1479 (arom CC), 1154
(ArOCH), 1036 (ArOCH), 986 (CCH), 894 (arom CH) cm−1; UV
(MeOH) λmax (log ε) 280 nm (3.83); 1H NMR (500 MHz, CDCl3) δ
3
3
0.99 (d, J (8′-H, 9′-H) = 6.6 Hz, 3H, 9′-H), 1.33 (d, J (8-H, 9-H) =
7.3 Hz, 3H, 9-H), 2.05 (ddd, 3J (7′-H, 8′-H) = 10.0 Hz, 3J (2-H, 7′-H)
= 10.6 Hz, J (6′-H, 7′-H) = 0.9 Hz, 1H, 7′-H), 2.50 (ddq, J (8′-H,
9′-H) = 6.6 Hz, 3J (7′-H, 8′-H) = 10.0 Hz, 3J (8-H, 8′-H) = 9.7 Hz, 1H,
4
3
4
3
8′-H), 2.55 (bdd, J (2-H, 8-H) = 2.4 Hz, J (7-H, 8-H) = 3.8 Hz,
3J (8-H, 9-H) = 7.3 Hz, 3J (8-H, 8′-H) = 9.7 Hz, 1H, 8-H), 2.83 (ddd,
4J (2-H, 7-H) = 2.5 Hz, J (2-H, 7′-H) = 10.9 Hz, long-range J = 1.3
3
1
Rf = 0.2 (dichloromethane/cyclohexane = 5:2); H NMR (500 MHz,
Hz, 1H, 2-H), 5.49 (s, 1H, 10-H), 5.64 (s, 1H, 10-H), 5.83 (s, 1H,
3
5-H), 5.95 (d, 2J (10′-H, 10′-H) = 1.4 Hz, 1H, 10′-H), 5.96 (d, 2J (10′-
CDCl3) δ 0.73 (t, J (1‴-H, 2‴-H) = 7.5 Hz, 3H, 2‴-H), 0.85 (d,
4
3J (1″-H, 3-H) = 7.1 Hz, 3H, 1″-H), 1.79−1.91 (m, 2H, 1‴-H), 2.22
H, 10′-H) = 1.4 Hz, 1H, 10′-H), 6.61 (s, 1H, 3′-H), 6.78 (d, J (6′-H,
7′-H) = 1.2 Hz, 1H, 6′-H), 6.89 ppm (bdd, 3J (7-H, 8-H) = 3.8 Hz, 4J
(2-H, 7-H) = 2.3 Hz, 1H, 7-H); 13C NMR (125 MHz, CDCl3) δ 15.7
(C-9), 17.2 (C-9′), 32.0 (C-8′), 33.6 (C-8), 44.0 (C-2), 46.9 (C-7′),
96.4 (C-10), 101.4 (C-10′), 101.6 (C-3), 101.9 (C-3′), 104.0 (C-5),
104.7 (C-6′), 123.3 (C-1′), 136.3 (C-1), 142.7 (C-7), 144.3 (C-5′),
146.4 (C-4′), 146.5 (C-2′), 165.3 (C-4), 192.3 (C-6); MS (EI, 70 eV)
m/z 354 (40) [M]+, 324 (42), 309 (40), 203 (29), 177 (49), 147
(100), 32 (68); HRMS (EI, M+) calcd for C20H18O6 (354.1104),
found 354.1105.
(ddq, 3J (2-H, 3-H) = 9.8 Hz, 3J (3-H, 1″-H) = 6.9 Hz, 3J (4-H, 3-H) =
3
3
10.0 Hz, 1H, 3-H), 2.65 (ddd, J (4-H, 3-H) = 10.0 Hz, J (4-H, 1‴a-
H) = 9.1 Hz, 3J (4-H, 1‴b-H) = 4.2 Hz, 1H, 4-H), 4.46 (d, 3J (2-H, 3-
2
H) = 10.0 Hz, 1H, 2-H), 5.89 (d, J = 1.2 Hz, 1H, OCH2O), 5.90 (d,
2J = 1.2 Hz, 1H, OCH2O), 5.91 (d, 1H, J = 1.2 Hz, 1H, OCH2′O),
2
2
5.92 (d, 1H, J = 1.2 Hz, 1H, OCH2′O), 6.44 (s, 1H, 8-H), 6.50 (s,
1H, 4′-H), 6.51 (s, 1H, 1′-H), 6.62 (1H, s, OH), 6.66 (s, 1H, 5-H);
13C NMR (125 MHz, CDCl3) δ 8.3 (C-2‴), 16.8 (C-1″), 24.5 (C-1‴),
34.6 (C-3), 42.5 (C-4), 84.9 (C-2), 99.0 (C-8), 99.6 (C-4′), 101.0
(OCH2O), 101.2 (OCH2′O), 106.6 (C-5), 108.3 (C-1′), 116.0 (C-6′),
118.2 (C-10), 140.9 (C-2′), 143.0 (C-6), 146.0 (C-7), 148.1 (C-3′),
149.6 (C-9), 150.0 (C-5′); MS (EI, 70 eV) m/z 356 (22) [M]+, 178
(100), 151 (13), 28 (20).
(2R*,3R*,4R*)-1,3-Dioxol[6,7-f ]-4-ethyl-3-methyl-2-[(1,3-benzo-
dioxol-5-acetyl)yl]-3,4-dihydro-2H-benzopyrane (27).
Oxidative Cyclization of (E)-7 in Sodium Acetate Buffer (0.1
M, pH 5.0) in the Absence of a Catalyst. A solution of (E)-7 (176
mg, 0.99 mmol) in acetone (4 mL) was added to sodium acetate buffer
(0.1 M, pH 5.0, 18 mL). The resulting suspension was stirred under O2
at room temperature for 19 h. NaCl (3 g) was added to the reaction
mixture, and the aqueous phase was extracted with dichloromethane (80 mL).
Saturated NaCl solution (20 mL) was added to the aqueous phase.
The aqueous phase was extracted with dichloromethane (80 mL). This
procedure was repeated twice. The combined organic phases were
dried over MgSO4, filtered, and evaporated under reduced pressure.
The residue thus obtained was purified by column chromatography
over silica gel (dichloromethane/cyclohexane = 5:2) to afford a mixture
of 26a and 26b as a colorless oil in 73% yield (128 mg 0.36 mmol).
The ratio of 26a:26b in the crude product was 3:2 (1H NMR).
(2S*,3S*,4R*)-1,3-Dioxol[6,7-f ]-4-ethyl-3-methyl-2-[(1,3-
benzodioxol-5-ol)-yl]-3,4-dihydro-2H-benzopyrane (26a).
To a solution of 26b (50 mg, 0.14 mmol) in pyridine (1 mL) were added
acetic acid anhydride (200 μL, 2.1 mmol) and DMAP (2 mg, 0.016
mmol), and the reaction mixture was stirred at room temperature for 4 h.
Then, 5% HCl (4 mL) was added, the aqueous layer was extracted with
dichloromethane (3 × 2 mL), and the combined organic layers were
evaporated under reduced pressure. The crude product was purified by
column chromatography over silica gel (dichloromethane/cyclohexane =
5:2) to afford 27 in 20% yield (12 mg, 0.03 mmol): Rf = 0.20 (dichloro-
mp 42−43 °C; Rf = 0.2 (dichloromethane/cyclohexane = 5:2); [α]20
D
= 0 (c 2.35, CHCl3); IR (ATR) ν 3428 (OH), 2876 (OCH2O), 1479
̅
methane/cyclohexane = 5:2); IR (ATR) ν 29626 (OCH2O), 1759 (CO),
̅
(arom CC), 1158 (ArOCH), 1149 (arom OH), 1035 (ArOCH), 857
(arom CH) cm−1; UV (MeOH) λmax (log ε) 236 (2.86), 303 (3.04) nm;
1628 (CC), 1479 (arom CC), 1257 (CO), 1151 (ArOCH), 1035
(ArOCH), 797 (arom CH) cm−1; UV (MeOH) λmax (lg ε) 237 (3.07),
295 (3.09) nm; 1H NMR (500 MHz, CDCl3) δ 0.76 (t, 3J (1‴-H, 2‴-H)
= 7.3 Hz, 3H, 2‴-H), 0.79 (d, 3J (1″-H, 3-H) = 6.6 Hz, 3H, 1″-H), 1.86
(m, 3J (4-H, 1‴-H) = 7.6 Hz, 3J (4-H, 1‴-H) = 4.3 Hz, 2H, 1‴-H), 2.07
3
1H NMR (500 MHz, CDCl3) δ 0.86 (d, J (1″-H, 3-H) = 7.1 Hz, 3H,
1″-H), 1.03 (t, 3J (1‴-H, 2‴-H) = 7.5 Hz, 3H, 2‴-H), 1.51 (ddq, 3J (1‴a-
H, 2‴-H) = 7.4 Hz, 2J (1‴a-H, 1‴b-H) = 13.4 Hz, 3J (4-H,
3
1‴a-H) = 8.6 Hz, 1H, 1‴a-H), 1.77 (ddq, J (4-H, 1‴b-H) = 4.8 Hz,
4541
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The Journal of Organic Chemistry
Featured Article
(ddq, 3J (2-H, 3-H) = 10.1 Hz, 3J (3-H, 1″-H) = 6.6 Hz, 3J (4-H, 3-H) =
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3
10.1 Hz, 1H, 3-H), 2.21 (s, 3H, COCH3), 2.66 (ddd, J (4-H, 3-H) =
9.6 Hz, 3J (4-H, 1‴-H) = 8.5 Hz, 3J (4-H, 1‴-H) = 4.0 Hz,
3
2
1H, 4-H), 4.56 (d, J (2-H, 3-H) = 9.7 Hz, 1H, 2-H), 5.87 (d, J
(a-H, b-H) = 1.5 Hz, 1H, OCH2O), 5.89 (d, 1H, 2J (a-H, b-H) = 1.5 Hz,
OCH2O), 6.00 (s, 2H, OCH2′O), 6.38 (s, 1H, 8-H), 6.61 (s, 1H, 4′-H),
6.66 (s, 1H, 5-H), 6.91 (s, 1H, 1′-H); 13C NMR (125 MHz, CDCl3) δ 8.7
(C-2‴), 15.9 (C-1″), 20.7 (CH3), 24.0 (C-1‴), 35.4 (C-3), 42.5 (C-4),
78.3 (C-2), 98.8 (C-8), 100.9 (OCH2O), 101.9 (OCH2′O), 104.1 (C-4′),
106.4 (C-5), 107.0 (C-1′), 117.2 (C-10), 125.0 (C-6′), 142.1 (C-6), 142.8
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* Supporting Information
Copies of the NMR spectra for all compounds, DFT
calculations, Cartesians, spin work analysis, HPLC separation
methods, and HPLC chromatograms. This information is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ubeifuss@uni-hohenheim.de.
ACKNOWLEDGMENTS
■
This research was supported by SFB 706 (Selective Catalytic
Oxidations of C−H Bonds Using Molecular Oxygen; Stuttgart)
and funded by the German Research Foundation. We thank
Ms. Sabine Mika for recording NMR spectra and Dr. Heiko
Leutbecher and Dipl.-Chem. Hans-Georg Imrich for recording
MS spectra.
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