Cyclodextrin ketones with the catalytic group at the secondary rim and their effectiveness in enzyme-like epoxidation of stilbenes

Article abstract of DOI:10.1002/ejoc.201001696

Several new cyclodextrin ketones with a ketone attached to the secondary face of the cyclodextrin in the form of a 2,3-O-(2-oxopropane-1,3-diyl) or 2-oxo group are reported. These compounds and a selection of known cyclodextrin ketones having the ketone at the primary face were investigated as epoxidation catalysts for oxidation of stilbenes and styrene. A method for determination of k_cat in these epoxidations is presented, which was used to determine the rate accelerations for the cyclodextrin ketones relative to background reaction. The highest rate acceleration obtained for epoxidation of 4-methoxy-4′-nitro-trans-stilbene was 221. The highest enantioselectivity obtained was 76% ee of (S)-styrene oxide. A cyclodextrin bearing a ketone on the secondary face can catalyze the epoxidation of stilbenes with cavity induced rate increases of 100-200 and stereoselectivity. Copyright

Full text of DOI:10.1002/ejoc.201001696

FULL PAPER
DOI: 10.1002/ejoc.201001696
Cyclodextrin Ketones with the Catalytic Group at the Secondary Rim and
Their Effectiveness in Enzyme-Like Epoxidation of Stilbenes
[a]
[a]
[a]
Thomas Hauch Fenger, Lavinia G. Marinescu, and Mikael Bols*
Keywords: Enzyme models / Chemzymes / Cyclodextrins / Oxidation / Stereoselectivity / Ketones
Several new cyclodextrin ketones with a ketone attached to
the secondary face of the cyclodextrin in the form of a 2,3-O-
presented, which was used to determine the rate accelera-
tions for the cyclodextrin ketones relative to background re-
action. The highest rate acceleration obtained for epoxid-
ation of 4-methoxy-4Ј-nitro-trans-stilbene was 221. The high-
est enantioselectivity obtained was 76% ee of (S)-styrene ox-
ide.
(
2-oxopropane-1,3-diyl) or 2-oxo group are reported. These
compounds and a selection of known cyclodextrin ketones
having the ketone at the primary face were investigated as
epoxidation catalysts for oxidation of stilbenes and styrene.
A method for determination of kcat in these epoxidations is
Introduction
cyclodextrin with pyruvate ester residues attached to the
primary face catalysed epoxidation of 4-chlorostyrene with
Artificial enzymes are molecules built to mimic an exi- up to 40% ee in the presence of oxone. In this study stoi-
[
8]
sting or perceived enzyme active-site as part of a bottom- chiometric amounts of ketone was used.
up approach to understanding enzyme catalysis and/or to
In neither of the above studies were enzyme kinetics dem-
prepare new and selective catalysts.[ Such chemzymes are onstrated and which also made it impossible to determine
typically made by modification of a supramolecular host if rate enhancements due to proximity effects where occur-
capable of substrate binding aiming to turn it into a cata- ring. It was therefore a primary stimulus for this work to
1]
[
2]
lytic machine. Artificial enzymes differ from other cata- attempt to obtain Michaelis–Menten kinetic data for the
lysts in that binding of the substrate by the catalyst before cyclodextrin catalysed epoxidations. To reach this goal we
and during catalysis. This can result in rate-enhancements decided to look at epoxidation of stilbenes since this reac-
due to proximity effects that will not be observed with tion leads to a significant change in UV absorption. In the
other, simpler catalysts. Cyclodextrins have been shown to present work we have also prepared new cyclodextrin
be valuable supramolecular hosts in artificial enzyme mod- ketones 1–4 (Figure 1) with a 2,3-O-(1,3-acetone) group at-
els[ and a number of different reactions are catalysed by tached to the secondary face and investigated their catalysis
cyclodextrin derivatives in aqueous solution in an enzy- of epoxidation of stilbenes. This is compared with the catal-
3]
[
4]
matic manner. But while catalytic epoxidations, particu- ysis by a number of previously reported cyclodextrin
larly asymmetric versions, are immensely popular in chem- ketones, primarily with the ketone attached to the primary
[
5]
istry, artificial enzymes that catalyse epoxidation have re-
ceived comparatively little attention. Previously we pre-
pared several cyclodextrin ketones with a ketone bridge
spanning the primary face.[
6,7]
These compounds catalysed
epoxidation of styrene and indene in water in the presence
of oxone with an enantioselectivity up to 45% ee, and inhi-
bition experiments showed that the cyclodextrin cavity was
involved in the catalysis. Turnover was also established
though 30 mol-% or more of catalyst was required. On the
other hand we were not able to demonstrate Michaelis–
Mentene kinetics and determine the rate acceleration of
these reactions. Subsequently Chan et al. reported that a
[
a] Department of Chemistry, University of Copenhagen,
Universitetsparken 5, 2100 København Ø
E-mail: bols@kemi.ku.dk
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001696.
Figure 1. Structure of novel cyclodextrin ketones 1–4 having a pro-
pan-2-one moiety attached to O2 and O3.
Eur. J. Org. Chem. 2011, 2339–2345
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2339

T. H. Fenger, L. G. Marinescu, M. Bols
FULL PAPER
cyclodextrin face. We find many of the cyclodextrin ketones
do display Michaelis–Menten catalysis of epoxidation, but
the new secondary face ketones are the better catalysts giv-
2
ing a kcat/kuncat over 10 . They are also more stereoselective
than the previous epoxidation giving up to 76% ee.
Results
Epoxidation of stilbene was proposed to be a good reac-
tion to follow by UV because the reaction results in disrup-
tion of the conjugated system and a significant change in
the UV spectrum. Since the substrate is highly absorbing
while the product is not it is however necessary to follow
the reaction with a large excess of cyclodextrin ketone and
a small amount of substrate making sure that saturation is
obtained; under those conditions the k is obtained di-
cat
rectly from the rate.[
9]
Synthesis
Figure 2. Structure of novel cyclodextrin ketone 5 where O2 has
In order to catalyse epoxidation of stilbene, cyclodextrin
derivatives with the ketone on the secondary rim are most
likely to be effective since the secondary face is wider and
been oxidised.
The successful synthesis of 6 and 7 allowed preparation
of the corresponding ketones: Either 6 or 7 were oxidised
with ozone, which gave ketones 1 and 3, respectively in 69–
a large substrate like stilbene will bind predominately from
this face. Inspired by the work of Fernandez[
10]
who re-
ported 2,3-alkylation of α,β- and γ-cyclodextrin with o-xyl-
ylene dibromide in 28–33% yield, we attempted a similar
alkylation of α-cyclodextrin using methallyl dichloride and
LDA in DMSO (Scheme 1). This reaction gave a 43% yield
of 2,3-alkylated derivative 6. The reaction could also be per-
formed on β-cyclodextrin, but the product 7 was obtained
in a lower yield (27%). The structure of 6 and 7 were deter-
mined by acidic hydrolysis of the compounds, acetylation
and analysis of the acetalyted products using NMR and
MS. This showed the presence of peracetylated glucose 7a
and the modified sugar 6a (Figure 2) as the only products.
70% yield (Scheme 1). Alternatively per-O-methylation of 6
and 7 with MeI/NaH in DMF gave the per-O-methyl al-
kenes 8 and 9 in 80 and 98% yield, respectively. Ozonolysis
of 8 and 9 gave the methylated ketones 2 and 4 in 87 and
95% yield, respectively (Scheme 1).
An alternative secondary rim ketone, 5, was prepared
(Figure 2) where the ketone is situated directly in the carbo-
hydrate backbone through oxidation of the 2-OH. This
compound was made as outlined in Scheme 2: monoallyl-
ation of β-cyclodextrin could be carried out in 29% yield
(Scheme 2) and treatment with MeI and NaH gave the per-
O-methyl 2-O-allyl derivative 10 in 86% yield. Deallylation
with palladium on carbon, MeOH and TsOH gave the
Scheme 1. Synthesis of ketones 1–4.
Scheme 2. Synthesis of ketone 5.
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 2339–2345

Enzyme-Like Epoxidation of Stilbenes
monool 11 in 68% yield (Scheme 2). Oxidation of this com- each case. The rate of the uncatalyzed reaction was also
pound proved difficult. Swern or Jones oxidation did not determined for all substrates allowing kuncat and thus the
give product and PDC only led to 25% conversion. KMnO4 rate acceleration induced by binding to the cyclodextrin de-
oxidation gave decomposition. We believe the difficulties rivative (kcat/kuncat) to be determined. Depending on the
here is caused by a strong hydrogen bond of the 2-OH to catalyst rate-enhancements from 1–221 were obtained.
the 3-OMe of the neighboring glucose residue. The reaction
Table 1. Kinetic data for epoxidation of stilbenes by cyclodextrins.
was eventually achieved with pyridine/SO : When 20 equiv.
of reagents were used 50% conversion was obtained, but
ketone and alcohol were very difficult to seperate. With
3
4
M4ЈNSt = 4-methoxy-4Ј-nitro-trans-stilbene.% ee is the enantio-
meric excess of the product as determined by HPLC (the letters
in parentheses denotes which enantiomer is in excess); n.d.: not
4
0 equiv. of reagents full conversion was achieved but the determined, sel is the selectivity for formation of one enantiomer
yield of 5 was only 40%.
over the other.
The new compounds 1–5 were supplemented with the
Cat.
Substrate
% ee
sel
k
cat
kcat/kun-
cat
[7]
[7]
[6]
[11]
6 –1
known cyclodextrin ketones 12, 13, 14, 15, 16
and
(ϫ10 s )
[11]
1
7.
Compound 12 is formaly a new compound, but is never-
theless a simple analogue of Wong’s ketone.[8] It was made
1
1
1
1
styrene
(Z)-stilbene
(E)-stilbene 56.8 (R,R) 3.6
4M4ЈNSt
n.d.
1.13Ϯ0.09
0.70Ϯ0.02
1.73Ϯ0.02
1.20Ϯ0.01
12
16
47
48
[
12]
n.d.
by derivatisation of benzylated β-cyclodextrin diol
and
hydrogenolysis.
2
2
2
2
styrene
(Z)-stilbene
(Z)-stilbene 52.4 (R,R) 3.2
4M4ЈNSt
75.8 (S)
7.3
1.67Ϯ0.09
0.91Ϯ0.03
3.27Ϯ0.06
2.32Ϯ0.02
17
20
74
93
n.d.
Epoxidation Experiments
3
3
3
3
styrene
(Z)-stilbene
(E)-stilbene 70 (R,R)
4M4ЈNSt
n.d.
4.70Ϯ0.04
3.87Ϯ0.05
3.34Ϯ0.05
2.71Ϯ0.03
49
85
105
109
The ketones were tested for catalysis of epoxidation of
cis- and trans-stilbene, 4-methoxy-4Ј-nitro-trans-stilbene
and styrene. The experiments were conducted in 1:1 aceto-
5.7
n.d.
4
4
4
4
styrene
(Z)-stilbene
(E)-stilbene 37.8 (R,R) 2.2
4M4ЈNSt
n.d.
0.28Ϯ0.12
0.15Ϯ0.001
0.26Ϯ0.01
0.165 Ϯ0.002
3
3
7
7
nitrile/H O at 25 °C with oxone 100 equiv. (2 mm),
2
NaHCO 500 equiv. (10 mm), a low concentration of sub-
3
strate and a large excess of cyclodextrin. This setup has the
advantages that 1) the substrate is dissolved despite its lipo-
philicity, 2) all substrate gets bound to the cyclodextrin
which means that the rate for conversion of substrate equals
Vcat and 3) the reaction can be followed by monitoring the
disappearance of the UV-active substrate since its converted
into less absorbing epoxide. The excess of cyclodextrin de-
rivative was varied from 30–85 equiv. (0.6–1.7 mm) to en-
sure that saturation of binding was reached. This led to
plots such as shown in Figure 3: The epoxidation rate cata-
lysed by the cyclodextrin derivative as a function of cyclo-
dextrin catalyst follows a curve (♦) that approaches a maxi-
mum rate when all substrate is bound in the cavity. For
comparison the rate of epoxidation by a simple ketone, di-
acetoxyacetone, (᭡) shows no saturation. From the rate at
saturation kcat was determined and is listed in Table 1 for
n.d.
5
5
5
styrene
(E)-stilbene 6.5 (S,S)
4M4ЈNSt n.d.
4.1 (S,S)
1.1
1.1
n.d.
3.46Ϯ0.02
2.94Ϯ0.01
n.d.
83
221
1
2
2
(E)-stilbene n.d.
4M4ЈNSt n.d.
0.21 Ϯ0.01
0.086 Ϯ0.002
5
3
1
1
1
3
3
(E)-stilbene n.d.
4M4ЈNSt n.d.
0.29Ϯ0.01
0.36Ϯ0.01
7
15
1
4
4
(E)-stilbene n.d.
4M4ЈNSt n.d.
0.197 Ϯ0.004
0.23Ϯ0.01
4
10
1
1
5
5
(E)-stilbene n.d.
0.085 Ϯ0.003 1.8
0.030 Ϯ0.001 1.2
1
4M4ЈNSt
4M4ЈNSt
4M4ЈNSt
n.d.
1
1
6
7
–
no catalysis
no catalysis
–
–
–
For some of the best (E)-stilbene epoxidations and a styr-
ene epoxidation, the epoxide was isolated upon completed
reaction and the chirality of the product was determined by
HPLC using a Kromasil AmyCoat column. The enantio-
meric excess values for these reactions are listed in Table 1,
as is the enantiomeric selectivity sel for each of these reac-
tions.
Discussion
The highest rate acceleration is obtained with the new
cyclodextrin ketones that have the ketone at the secondary
rim. With ketones 1–3 rate acceleration up to about 100 was
obtained with compound 3, a β-cyclodextrin without
methyl groups, being the best catalyst. The α-cyclodextrin
Figure 3. Plot of the epoxidation rate as a function of equivalents
of catalyst either for catalyst 2 (♦) or diacetoxyacetone (᭡). The
substrate for 2 was 4-methoxy-4Ј-nitrostilbene (with a unit of kcat
–6 –1
of 10 s ), while it was trans-stilbene for diacetoxyacetone (with
unit of 10–5 s ).
–1
Eur. J. Org. Chem. 2011, 2339–2345
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2341

T. H. Fenger, L. G. Marinescu, M. Bols
FULL PAPER
analogue 1 has about half the rate of catalysis, while methyl-
ation of α-cyclodextrin (2) improved catalysis somewhat. A
common observation is that the (E)-stilbenes are by far the
best substrates.
Stilbenes are large substrates and it is to be expected that
β-cyclodextrin derivatives with their larger cavity might be
better suited to these substrates. Indeed 2D NMR experi-
ment show that stilbene bind to β-CD from the secondary
[
13]
face.
markably poorer catalyst. Methylation influence the size
However the methylated β-cyclodextrin 4 is a re-
[
14]
and the dimensions of the cavity particularly its openings
it is possible this account for the lack of activity of 4 by a
change in substrate binding towards unproductive binding.
The 2-ketocyclodextrin 5 gives the best rate accelerations
with 83 for (E)-stilbene and 221 for the substituted (E)-
stilbene. These accelrations are the highest ever observed
for epoxidation by an enzyme model.
The cyclodextrins 12–17 (Figure 4) all gave modest or no
rate-acceleration (Table 1). For 12–15, that have the ketone
on the primary rim, this is not surprising. The stilbenes are
larger than the cyclodextrin cavity and are likely to bind so
that one aromatic ring is inside, while the styryl group is
pointing out through either face. Since the secondary face
is larger it is more likely that the styryl is at this face pre-
dominantly (Scheme 3, center) so that a functional group
at the other rim will be non-functional. Compounds 16 and
1
7 gave no catalysis and we believe that this may be because
[
15]
they are aldehydes – there are no examples of aldehydes
working as catalysts in this reaction in the literature.
Scheme 3. Mechanism of epoxidation of stilbene catalyzed by 2 and
oxone as the stoichiometric reagent.
appears to have a similar preferred binding and catalysis in
all these reactions. On the other hand the ketone 5 gave
very differently 7% ee of (S,S)-stilbene oxide, which reflect
the different structure of this catalyst. This basically means
that 5 can add oxygen to the stilbene equally well from both
sides, while there is a preference for the R-face for 1–4.
The epoxidation of alkenes by dioxiranes is a geometri-
cally demanding reaction and the transition state is believed
to be a spiro structure so that dioxirane and alkene are per-
[
16]
pindicular.
It is possible that the ketone in 5 can better
reach the required transition state for both enantiomers
than is possible for 1–4, and that this is causing the stereo-
selectivity in the latter case. Interestingly 5 is the better cata-
lyst suggesting that the selectivity of 1–4 is obtained by re-
ducing the reaction rate of formation of the (S,S)-stilbene
Figure 4. Other ketones investigated in this work. For detailed
structure see appropriate reference.
As it is seen in Table 1 ketones 1–4 gave an enantiomeric oxide.
excess of the R,R-epoxide of 37–70%. This means these cat-
To get more insight into this question models of the tran-
alysts have a preference for formation of the R,R-isomer sition states for epoxidation of stilbene from the R- and S-
over the S,S-isomer of from 2:1 to 6:1 and overall stilbene faces were made: the catalyst 1 was modelled in chem3D
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 2339–2345

Enzyme-Like Epoxidation of Stilbenes
A
A
based on crystal data of α-cyclodextrin, and stilbene was 2 ,3 -Di-O-(prop-2-ene-1,3-diyl)-α-cyclodextrin (6): Dry α-CD
(
(
1.04 g, 1.07 mmol) was dissolved in DMSO (50 mL) and LDA
2 m, 0.54 mL, 1.07 mmol) was added at room temp. and the reac-
docked inside. MM2 calculation showed that the model of
the R-transition state had a lower energy, which is in ac-
cordance with the observed. This model is shown in Fig-
ure 5. The model shows a snug fit of one phenyl group of
stilbene into the cavity and that the remainder of stilbene
is no longer in plane.
tion was stirred overnight. 3-chloro-2-(chloromethyl)propene
0.12 mL, 1.07 mmol) was added dropwise and the mixture was left
(
stirring overnight. DMSO was removed under reduced pressure
and the residue was purified by flash chromatography, acetonitrile/
H
2
O/NH
4
OH, 10:1:1 Ǟ 6:3:1 giving 0.47 g of the product in 43%
yield. H NMR ([D ]DMSO, 400 MHz): δ = 5.52–5.45 (m, 6 H, 1-
H), 5.01–4.97 (m, 2 H, C=CH ), 4.78 (br. s, 3 H), 4.64–4.50 (br. s,
H), 4.47–4.28 (m, 4 H), 4.00–3.93 (m, 1 H), 3.85–3.43 (m, 33 H),
1
6
2
4
3
1
8
7
1
3
.39–3.16 (m, 11 H) ppm. C NMR ([D
6
]DMSO, 100 MHz): δ =
47.0 (C=CH ), 112.5 (C=CH ), 102.2, 102.0, 100.6, 83.1, 82.9,
2
2
2.5, 82.3, 82.2, 82.1, 79.0, 73.8, 73.6, 72.7, 72.7, 72.5, 72.3, 72.3,
2.2, 72.2, 71.9, 60.2, 59.9 ppm. MALDI-TOF, m/z calcd.
40
C H64NaO30: 1047.338; found 1046.997.
Hydrolysis and Acetylation of Compound 6: Compound 6 (146 mg,
.14 mmol) was dissolved in 5 mL H O and amberlite IR120H
35 mL) was added, the mixture was heated to 100 °C, for 48 h,
until TLC (acetonitrile/H O/NH OH, 6:3:1) showed no starting
material. The mixture was filtered and washed with water (15 mL),
and the pH adjusted to 7 by NaHCO before evaporation. The
residue was dissolved in pyridine (12 mL) and acetic anhydride
12 mL) and left stirring overnight. The mixture was evaporated
and the compounds purified by flash chromatography EtOAc/tolu-
ene, 1:3 giving 124 mg of the acetylated sugar residues. R = 0.52
, 400 MHz): δ =
.32 (d, J = 3.7 Hz, 5 H, 1-H), 6.25 (d, J = 3.7 Hz, 1 H, 1-H), 5.71
d, J = 8.3 Hz, 6 H, 1-H), 5.59 (d, J = 8.0 Hz, 1 H, 1-H), 5.46 (d,
J = 19.8 Hz, 4 H), 5.24 (t, J = 9.4 Hz, 6 H), 5.16–5.06 (m, 21 H),
.03 (dd, J = 9.8, 5.9 Hz, 4 H), 4.98 (d, J = 4.8 Hz, 2 H), 4.50–4.37
m, 4 H), 4.31–4.22 (m, 16 H), 4.14–4.05 (m, 18 H), 4.02 (dd, J =
1.9, 1.9 Hz, 2 H), 3.95 (ddd, J = 10.2, 4.3, 2.1 Hz, 1 H), 3.83 (ddd,
J = 10.0, 4.5, 2.3 Hz, 7 H), 3.75–3.66 (m, 2 H), 3.56 (dd, J = 9.2,
0
(
2
2
4
3
(
f
1
and 0.47, EtOAc/toluene, 1:2. H NMR (CDCl
6
(
3
5
(
1
Figure 5. Model of the transition state of epoxidation of stilbene
catalysed by 1.
3
2
4
.8 Hz, 1 H), 3.43 (dt, J = 16.7, 8.7 Hz, 2 H), 2.17 (s, 13 H, CH
.15 (s, 3 H, CH ), 2.14 (s, 3 H, CH ), 2.10 (s, 19 H, CH ), 2.09 (s,
H, CH ), 2.08 (s, 14 H, CH ), 2.08 (s, 19 H, CH ), 2.06 (s, 3 H,
), 2.06 (s, 4 H, CH ), 2.03 (s, 13 H, CH ), 2.02–2.02 (m, 33 H,
3
),
3
3
3
3
3
3
Conclusions
CH
3
3
3
It has been shown that cyclodextrins with ketone at the CH ), 2.02 (s, 10 H, CH ), 2.01 (s, 14 H, CH ), 2.00 (s, 18 H, CH )
3
3
3
3
1
3
secondary rim function as epoxidation catalysts in enzyme- ppm. C NMR (CDCl
3
, 100 MHz): δ = C:170.5, 170.4, 170.0,
69.9, 169.5, 169.4, 169.3, 169.2, 169.1, 169.0, 168.8, 168.6 (CO),
46.7, 146.3 (C=CH ), 113.8, 112.4 (C=CH ), 92.0, 91.6, 90.3, 88.9,
1
1
(
6
2
like manner and giving rate enhancements up to 221-fold
and enatioselectivities up to 75% ee. A phenyl group of the
alkene is bound in the cavity of the cyclodextrin and the
epoxidation proceed with preference for the R-face in stilb-
ene.
2
2
C-1) 84.0, 81.5, 81.3, 80.4, 73.3, 73.2, 73.0, 72.7, 72.7, 72.6, 70.1,
9.7, 69.6, 69.1, 67.8, 67.6, 61.8, 61.7, 61.3 (C-2,3,4,5,6), 20.9, 20.7,
0.6, 20.5, 20.4, 20.3 (COCH ) ppm.
3
2A,3 -Di-O-(prop-2-ene-1,3-diyl)-β-cyclodextrin (7): Dry β-CD
A
(
(
1.83 g, 1.61 mmol) was dissolved in DMSO (96 mL) and LDA
2 m, 0.81 mL, 1.61 mmol) was added at room temp. and the reac-
Experimental Section
General: Solvents were distilled under anhydrous conditions. All
reagents were used as purchased without further purification.
Evaporation was carried out in a rotary evaporator. Glassware used
for water-free reactions was dried for 2 h at 130 °C before use. Col-
umns were packed with silica gel 60 (230–400 mesh) as the station-
ary phase. TLC plates (Merck, 60, F254) were visualized by spraying
tion was stirred overnight. 3-chloro-2-(chloromethyl)propen
(0.19 mL, 1.61 mmol) was added dropwise and the mixture was left
stirring overnight. DMSO was removed under reduced pressure
and the residue was purified by flash chromatography, acetonitrile/
H
2
O/NH
4
OH, 10:1:1 Ǟ 6:3:1 giving 0.51 g of the product in 27%
yield. H NMR ([D ]DMSO, 300 MHz): δ = 5.82–5.61 (m, 7 H, 1-
H), 5.01 (m, 2 H, C=CH ), 4.79 (br. s, 4 H), 4.72–4.26 (m, 9 H),
4.15–4.06 (d, J = 10.0 Hz, 1 H), 3.77–3.15 (m, 51 H) ppm.
NMR ([D ]DMSO, 100 MHz): 148.1 (C=CH ), 116.7
(C=CH ), 104.5, 104.4, 104.3. 104.2, 103.1 (C-1), 85.5, 84.6, 84.3,
1
6
with cerium sulfate (1%) and molybdic acid (1.5%) in 10% H
2
SO
4
2
and heating until coloured spots appeared. 1H-, C NMR and
COSY experiments were carried out with a Varian Mercury 300
instrument. Monoisotopic mass spectra (MALDI-TOF MS) were
obtained on a Bruker Daltonics mass spectrometer using ditranol
13
13
C
6
δ
=
2
2
84.0, 83.9, 83.8, 83.7, 80.8, 80.7, 76.1, 76.0, 75.9, 75.8, 75.7, 75.5,
75.4, 75.0, 74.9, 74.8, 74.6, 74.4, 74.3, 74.2, 62.6, 62.4, 62.3 ppm.
(1,8-dihydroxyanthron) as matrix. Spectra were calibrated using a
peptide calibration standard solution.
MALDI-TOF, m/z calcd. C46
H
74NaO35: 1209.391; found 1209.533.
Eur. J. Org. Chem. 2011, 2339–2345
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org 2343

T. H. Fenger, L. G. Marinescu, M. Bols
3.35 (m, 18), 3.17–3.11 (m, 5 H) ppm. 13C NMR (CDCl3
FULL PAPER
Hydrolysis and Acetylation of 7: Compound 7 (136 mg, 0.11 mmol)
,
was dissolved in 2 mL H
added, the mixture was heated to 100 °C, for 48 h, until TLC (ace-
tonitrile/H O/NH OH, 6:3:1) showed no starting material. The
mixture was filtered and washed with water (15 mL), and the pH
adjusted to 7 by NaHCO before evaporation. The residue was dis-
2
O and amberlite IR120H (30 mL) was 100 MHz): δ = 210.8 (C=O), 100.9, 100.2, 100.1, 100.0, 99.9, 99.6
(C-1), 86.0, 83.7, 82.4, 82.3, 82.2, 82.1, 81.9, 81.9, 81.2, 81.1, 81.1,
80.1, 77.4, 76.8, 71.4, 71.3, 71.2, 71.1, 71.1, 71.0, 71.0, 70.9, 70.7,
61.7, 61.7, 61.6, 58.8, 58.8, 58.2, 58.0, 57.8, 57.7 ppm. MALDI-
2
4
3
TOF, m/z calcd. C55H94NaO31: 1273.568; found 1273.147.
solved in pyridine (10 mL) and acetic anhydride (10 mL) and left
stirring overnight. The mixture was evaporated and the compounds
purified by flash chromatography EtOAc/toluene, 1:3 giving
2A,3 -Di-O-(prop-2-ene-1,3-diyl)-2B–G,3B–G,6A–G-nonadecakis-O-
methyl-β-cyclodextrin (9): Compound 7 (288 mg, 0.24 mmol) was
dissolved in DMSO (20 mL), NaH (60%, 914 mg, 22.9 mmol) was
A
1
24 mg of the acetylated sugar residues. R
f
= 0.52 and 0.47, EtOAc/
]DMSO, 300 MHz): δ = 6.27 (d, J =
.6 Hz, 6 H, 1-H), 6.19 (d, J = 3.6 Hz, 1 H, 1-H), 5.67 (d, J =
1
2
added and the reaction was stirred for 30 min under N . The mix-
ture was cooled to 0 °C and iodomethane (1.41 mL, 22.9 mL) was
slowly added and the reaction was stirred for 24 h. Water (20 mL)
was added and the mixture was extracted with CHCl (4ϫ30 mL)
3
and the combined org. phases were washed with brine (30 mL).
Concentration and purification by flash chromatography toluene/
toluene, 1:2. H NMR ([D
6
3
8
9
.1 Hz, 9 H, 1-H), 5.54 (d, J = 8.0 Hz, 1 H, 1-H), 5.41 (t, J =
.9 Hz, 6 H), 5.25–5.15 (m, 9 H), 5.13–5.01 (m, 29 H), 5.01–4.91
(
(
m, 8 H), 4.46–4.33 (m, 5 H), 4.20 (dq, J = 9.8, 5.5 Hz, 23 H), 4.04
tt, J = 13.3, 6.8 Hz, 25 H), 3.80 (dd, J = 9.9, 2.2 Hz, 9 H), 3.71–
acetone, 1:1 (R
f
= 0.31), afforded the desired product in 283 mg,
0% yield. H NMR (CDCl , 300 MHz): δ = 5.20 (d, J = 3.8 Hz,
H, 1-H), 5.12 (d, J = 3.7 Hz, 1 H, 1-H), 5.10 (d, J = 3.6 Hz, 1
3
2
2
1
.62 (m, 3 H), 3.52 (dd, J = 9.2, 3.7 Hz, 1 H), 3.47–3.28 (m, 2 H),
.14–2.12 (m, 18 H, CH ), 2.11–2.08 (m, 9 H, CH ), 2.07–2.05 (m,
9 H, CH ), 2.04–2.02 (m, 53 H, CH ), 2.02–1.99 (m, 13 H, CH ),
.99–1.98 (m, 19 H, CH ), 1.96 (s, 48 H,
]DMSO, 75 MHz):
1
8
1
3
3
3
3
3
3
H, 1-H), 5.08 (d, J = 3.5 Hz, 1 H, 1-H), 5.06 (d, J = 3.5 Hz, 1 H,
1-H), 5.05 (d, J = 3.5 Hz, 1 H, 1-H), 5.00 (d, J = 3.6 Hz, 1 H, 1-
H), 4.90 (d, J = 21.6 Hz, 2 H, C=CH
CHH, 4 H), 4.38 (d, J = 13.7 Hz, 1 H, CHH), 4.29 (d, J = 15.1 Hz,
H, CHH), 4.21 (d, J = 13.7 Hz, 1 H, CHH), 3.91–3.83 (m, 5 H),
3
), 1.98 (m, 61 H, CH
3
) ppm. 1 C NMR ([D
3
CH
3
), 1.89 (s, 6 H, CH
3
6
2
), 4.45 (d, J = 15.0 Hz, 1 H,
δ = 170.9, 170.8, 170.7, 170.4, 170.2, 169.8, 169.7, 169.6, 169.5,
1
1
7
6
69.4, 169.2, 169.1, 168.9, (CO) 147.0, 146.6 (C=CH
2
), 114.11,
1
3
3
12.74 (C=CH ), 94.0, 92.3, 91.8, 90.6, 89.2, 84.2, 81.7, 81.6, 80.7,
2
.80–3.68 (m, 9 H), 3.66–3.40 (m, 57 H), 3.35–3.29 (m, 22 H), 3.17–
3.6, 73.5, 73.3, 73.0, 72.9, 72.8, 70.4, 70.0, 69.9, 69.3, 68.0, 67.9,
2.4, 61.9, 61.6 (C-2,3,4,5,6), 21.2, 21.0, 20.9, 20.8, 20.7, 20.6
1
3
.13 (m, 6 H) ppm. C NMR (CDCl
), 110.9 (C=CH ), 102.2, 100.3, 99.0, 98.9, 98.8, 98.6, 98.4
C-1), 84.0, 82.2, 82.0, 81.9, 81.7, 81.6, 81.5, 81.4, 80.7, 80.3, 80.2,
3
, 100 MHz): δ = 148.2
(
(
C=CH
2
2
3
(COCH ) ppm.
2A,3 -Di-O-(prop-2-ene-1,3-diyl)-2B–F,3B–F,6A–F-hexadecakis-O-
methyl-α-cyclodextrin (8): Compound 6 (0.51 g, 0.50 mmol) was
dissolved in DMSO (35 mL) and NaH (60%, 1.59 g, 39.7 mmol)
A
7
7
5
9.8, 77.7, 72.9, 72.9, 71.7, 71.4, 71.3, 71.0, 70.9, 70.9, 70.8, 70.7,
0.6, 70.0, 61.5, 61.4, 61.3, 61.2, 61.0, 58.8, 58.7, 58.6, 58.5, 58.3,
8.2, 58.0 ppm. MALDI-TOF, m/z calcd. C65H112NaO35: 1475.688;
2
was added and the reaction was stirred for 30 min under N . The
mixture was cooled to 0 °C and iodomethane (2.47 mL, 39.7 mL)
was slowly added and the reaction was stirred for 24 h. Water
found 1475.401.
2A,3 -Di-O-(2-oxopropane-1,3-diyl)-2B–G,3B–G,6A–G-nonadecakis-O-
A
methyl-β-cyclodextrin (4): Compound 2 (207 mg, 0.14 mmol) was
(
(
(
40 mL) was added and the mixture was extracted with CHCl
3
4ϫ50 mL) and the combined org. phases were washed with brine
40 mL). Concentration and purification by flash chromatography
dissolved in 200 mL CH
through for 10 min, the solution turned blue, then O
through for additional 5 min, S(CH
reaction was left stirring at room temp. overnight. The solvent was
evaporated and the product purified by chromatography toluene/
acetone, 1:1, to afford 180 mg of the desired compound, 87% yield.
H NMR (CDCl3, 400 MHz): δ = 5.14 (d, J = 3.7 Hz, 1 H, 1-H),
.11 (m, 3 H, 1-H), 5.06 (m, 3 H, 1-H), 4.34–4.22 (m, 4 H), 3.94–
3
.32 (m, 93 H), 3.20–3.12 (m, 6 H) ppm. C NMR (CDCl ,
100 MHz): δ = 210.8 (C=O), 99.9, 99.1, 99.1, 98.9, 98.9, 98.8, 98.5
C-1), 86.2, 84.1, 82.0, 81.9, 81.8, 81.7, 81.6, 81.6, 81.5, 81.4, 80.7,
2
Cl
2
and cooled to –78 °C, O
3
was bubbled
was bubbled
2
3
)
2
(2 mL) was added and the
toluene/acetone, 1:1 (R
0
5
f
= 0.26), afforded the desired product in
.61 g, 98% yield. H NMR (CDCl , 400 MHz): δ = 4.98–4.95 (m,
H, 1-H), 4.94 (d, J = 3.54 Hz, 1 H, 1-H), 4.89 (d, J = 23.5 Hz, 2
), 4.45 (d, J = 15.3 Hz, 1 H, CHH), 4.29 (d, J = 13.2 Hz,
H, CHH), 4.24 (d, J = 15.4 Hz, 1 H, CHH), 4.18 (d, J = 13.3 Hz,
1
3
H, C=CH
2
1
1
1
2
5
3
H, CHH), 3.85–3.76 (m, 4 H), 3.74–3.68 (m, 7 H), 3.64–3.55 (m,
2 H), 3.54–3.45 (m, 12 H), 3.44–3.39 (m, 16 H), 3.35–3.26 (m, 18
1
3
13
H), 3.12–3.06 (m, 5 H) ppm. C NMR (CDCl
3
, 75 MHz): δ =
(
8
7
1
9
8
6
48.4 (C=CH ), 111.8 (C=CH ), 101.7, 100.6, 100.4, 100.3, 100.2,
2 2
0.5, 80.4, 80.3, 80.2, 80.1, 77.4, 71.4, 71.1, 71.0, 70.9, 70.8, 70.7,
0.4, 61.5, 61.4, 61.2, 61.1, 59.1, 58.9, 58.8, 58.6, 58.4, 58.3, 58.1
ppm. MALDI-TOF, m/z calcd. C64H110NaO36: 1477.667; found
9.9 (C-1), 83.7, 82.7, 82.5, 82.5, 82.4, 82.4, 82.3, 82.1, 81.8, 81.5,
1.4, 81.3, 81.2, 73.3, 73.0, 71.8, 71.7, 71.5, 71.4, 71.3, 71.3, 70.7,
2.2, 62.1, 62.0, 61.9, 59.2, 59.1, 59.1, 58.4, 58.3, 58.0, 58.0, 57.9
1
477.506.
ppm. MALDI-TOF, m/z calcd. C56H96NaO30: 1271.588; found
1
271.541.
2A,3 -Di-O-(2-oxopropane-1,3-diyl)-β-cyclodextrin (3): Compound
A
2A,3 -Di-O-(2-oxopropane-1,3-diyl)-2B–F,3B–F,6A–F-hexadecakis-O-
methyl-α-cyclodextrin (2): Compound 8 (612 mg, 0.49 mmol) was to 0 °C, O
dissolved in 200 mL CH
through for 10 min, the solution turned blue, then O
through for additional 5 min, S(CH
A
7 (229 mg, 0.14 mmol) was dissolved in 200 mL H
was bubbled through for 7 min, the solution turned
3 2
was bubbled through for additional 5 min, S(CH )
2
O and cooled
3
2
Cl
2
and cooled to –78 °C, O
3
was bubbled
was bubbled
blue, then O
2
2
(2 mL) was added and the reaction mixture was left stirring at room
temp. overnight. The solvent was evaporated and the product puri-
3
)
2
(3 mL) was added and the
reaction was left stirring at room temp. overnight. The solvent was fied by chromatography acetonitrile/H
evaporated and the product purified by chromatography toluene/
2
O/NH
to afford 162 mg of the desired compound, 70% yield. H NMR
([D ]DMSO, 400 MHz): δ = 5.78–5.64 (m, 9 H), 5.09 (d, J =
4
OH, 10:1:1 Ǟ 6:3:1,
1
acetone, 1:1, to afford 582 mg of the desired compound, 95% yield.
H NMR (CDCl , 400 MHz): δ = 5.06 (d, J = 3.49 Hz, 1 H, 1-H) 3.6 Hz, 1 H, 1-H) 4.86 (d, J = 3.4 Hz, 1 H, 1-H), 4.82 (d, J =
3
6
1
5.02 (m, 5 H, 1-H), 4.33 (d, J = 17.6 Hz, 1 H, CHH), 4.30 (d, J = 3.0 Hz, 5 H, 1-H), 4.70 (br. s, 1 H), 4.59–4.44 (m, 9 H), 4.35 (s, 1
1
=
5.3 Hz, 1 H, CHH), 4.24 (d, J = 15.3 Hz, 1 H, CHH), 4.18 (d, J H), 4.19 (d, J = 5.6 Hz, 1 H), 4.15 (d, J = 5.3 Hz, 1 H), 4.05 (d, J
1
3
17.6 Hz, 1 H, CHH), 3.92–3.82 (m, 4 H), 3.80–3.74 (m, 7 H),
= 10.3 Hz, 1 H), 3.97–3.92 (m, 1 H), 3.72–3.19 (m, 41 H) ppm.
C
3.73–3.59 (m, 22 H), 3.57–3.49 (m, 12 H), 3.48–3.44 (m, 16), 3.41–
NMR ([D ]DMSO, 100 MHz): δ = 210.3 (C=O), 102.2, 102.1,
6
2344
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 2339–2345

Enzyme-Like Epoxidation of Stilbenes
1
8
7
02.0, 101.9, 101.8, 101.6, 99.6 (C-1), 85.3, 83.7, 81.9, 81.7, 81.6,
1.5, 81.1, 77.4, 76.9, 76.6, 73.4, 73.3, 73.2 72.7, 72.6, 72.5, 72.4,
2.3, 72.0, 60.3, 60.2, 60.0, 59.9 ppm. MALDI-TOF, m/z calcd.
Determination of Enantiomeric Excess in Epoxides by HPLC Analy-
sis: The epoxide products formed in epoxidation of stilbene and
styrene were analysed by HPLC. The HPLC column used was a
Kromasil® 5-AmyCoat, 4.6ϫ2500 mm, mounted on a UFLC Shi-
madzu HPLC, with degasser DGU-20A, Pump LC-20AD, UV/Vis
detector SPD-20A, and Communication Unit CMB-20A. The sol-
vent used for elution was n-heptane/2-propanol, 90:10, with a flow
rate of 0.5 mL/ min, and a pressure of 34 bar. For sample prepara-
tion the cuvettes were extracted with EtOAc and concentrated prior
to inlet. The enantiomeric excess of the epoxidations were deter-
mined by integration of the elution profile of the sample.
45
C H72NaO36: 1211.370; found 1211.758.
2A,3 -Di-O-(2-oxopropane-1,3-diyl)-α-cyclodextrin (1): Compound
(612 mg, 0.14 mmol) was dissolved in 200 mL H O and cooled
to 0 °C, O was bubbled through for 7 min, the solution turned
blue, then O was bubbled through for additional 5 min, S(CH
2 mL) was added and the reaction mixture was left stirring at room
temp. overnight. The solvent was evaporated and the product puri-
fied by flash chromatography acetonitrile/H O/NH OH, 10:1:1 Ǟ
:3:1, to afford 423 mg of the desired compound, 69% yield. H
A
6
2
3
2
3 2
)
(
2
4
1
6
Acknowledgments
6
NMR ([D ]DMSO, 300 MHz): δ = 5.52 (br. s, 7 H), 5.03 (s, 1 H,
1
4
-H), 4.79 (s, 5 H, 1-H), 4.68–4.44 (m, 9 H), 4.25–4.20 (m, 2 H),
.15 (d, J = 16.5 Hz, 2 H, CH ), 4.06 (d, 2H CH ), 3.85–3.13 (m)
]DMSO, 75 MHz): δ = 210.1 (C=O), 102.2,
01.9, 100.2 (C-1), 84.4, 83.8, 83.1, 82.3, 82.2, 82.0, 78.8, 76.6, 76.5, [1] A. Kirby, F. Hollfelder, From Enzyme Models to Model En-
zymes, Royal Society of Chemistry (RSC), Cambridge, UK,
009.
We thank the Lundbeck Foundation for financial support.
2
2
13
ppm. C NMR ([D
6
1
7
3.6, 73.5, 72.8, 72.4, 72.2, 72.0, 60.5, 60.2, 59.8 ppm. MALDI-
62NaO31: 1049.317; found 1049.200.
2
TOF, m/z calcd. C39
H
[
2] a) R. Breslow, S. D. Dong, Chem. Rev. 1998, 98, 1997; b) J.
Bjerre, C. Rousseau, L. Marinescu, M. Bols, Appl. Microbiol.
Biotechnol. 2008, 81, 1; c) L. Simón, F. M. Muñiz, Á. F. de Ar-
riba, V. Alcázar, C. Raposo, J. R. Morán, Org. Biomol. Chem.
2A-Oxo-2B–G,3A–G,6A–G-Icosakis-O-methyl-β-cyclodextrin (5): Py-
SO (234 mg, 1.41 mmol) was dissolved in DMSO (0.5 mL),
3
B–G,3A–G,6
A–G
2
-O-methyl-β-CD (11) (51 mg, 0.04 mmol) was dis-
solved in DMSO (0.5 mL), the two solutions were mixed and
stirred for 30 min. Et N (0.26 mL, 1.85 mmol) was added and the
mixture was stirred for 1 h. Brine (1 mL) and water (1 mL) were
added, and the mixture was extracted with CH Cl (4ϫ3 mL), the
organic extracts were evaporated and purified by flash chromatog-
raphy (Et O/MeOH, 20:1), giving 21 mg of the desired product as
a colourless foam in 40% yield. H NMR (CDCl
2010, 8, 1763.
[
3] a) F. O. Caballero, C. Rousseau, B. Christensen, T. E. Pedersen,
M. Bols, J. Am. Chem. Soc. 2005, 127, 3238–3239; b) C. Rous-
seau, F. O. Caballero, L. U. Nordstrøm, B. Christensen, T. E.
Pedersen, M. Bols, Chem. Eur. J. 2005, 11, 5094–5101; c) F.
Ortega-Caballero, J. Bjerre, L. S. Laustsen, M. Bols, J. Org.
Chem. 2005, 70, 7217–7226; d) L. Marinescu, M. Mølbach, C.
Rousseau, M. Bols, J. Am. Chem. Soc. 2005, 127, 17578–17579.
4] L. Marinescu, M. Bols, Trends in Glycoscience & Glycotechnol-
ogy 2009, 21, 309.
3
2
2
2
1
3
, 500 MHz): δ =
.85 (d, J = 1.9 Hz, 1 H), 5.65 (d, J = 3.6 Hz, 1 H), 5.60–5.49 (m,
H), 5.35 (d, J = 3.5 Hz, 1 H), 5.15 (d, J = 3.6 Hz, 1 H), 5.13–
.09 (m, 3 H), 5.07 (d, J = 3.6 Hz, 1 H), 5.04 (d, J = 3.1 Hz, 1 H),
.87–4.77 (m, 1 H), 4.62 (d, J = 7.6 Hz, 1 H), 4.28 (d, J = 8.9 Hz,
H), 4.07–3.99 (m, 1 H), 3.96–3.69 (m, 12 H), 3.69–3.44 (m, 51
[
[
5
4
5
4
1
5] a) R. A. Johnson, K. B. Sharpless, Comp. Org. Synth. 1991, 7,
389; b) T. Katsuki, V. S. Martin, Org. React. 1996, 48, 1; c) E.
Rose, B. Andrioletti, S. Zrig, M. Quelquejeu-Ethève, Chem.
Soc. Rev. 2005, 34, 573; d) T. Katsuki, Synlett 2003, 281; e) M.
Frohn, Y. Shi, Synthesis 2000, 14, 1979; f) W. Adam, C. R.
Saha-Möller, C.-G. Zhao, Org. React. 2002, 61, 219.
H), 3.44–3.32 (m, 19 H), 3.32–3.12 (m, 8 H), 3.02 (dd, J = 9.0,
7.7 Hz, 1 H) ppm. MALDI-TOF, m/z calcd: C62H108NaO35,
1
435.657; found 1435.441.
[6] C. Rousseau, B. Christensen, T. E. Pedersen, M. Bols, Org. Bio-
mol. Chem. 2004, 2, 3476.
[7] C. Rousseau, B. Christensen, M. Bols, Eur. J. Org. Chem. 2005,
2734.
Determination of Epoxidation Rates: The epoxidations were moni-
tored on a spectrophotometer Spetronic Genesys 5 by Milton Roy.
For each epoxidation assay 6 samples of 2 mL were made. Four
stock solutions were made, substrate (1 mm, 1:1 acetonitrile:water),
[
[
[
8] W.-K. Chan, W.-Y. Yu, C.-M. Che, M.-K. Wong, J. Org. Chem.
2003, 68, 6576.
9] R. Breslow, J. B. Doherty, G. Guillot, C. Lipsey, J. Am. Chem.
Soc. 1978, 100, 3227–3229.
Oxone (10 mm, 32 mg in 5 mL 1:1 acetonitrile:water), NaHCO
3
(100 mm, 58 mg in 7 mL 1:1 acetonitrile:water), enzyme (6.8 mm,
10] P. Balbuena, D. Lesur, M. J. González Alvarez, F. Mendicuti,
C. O. Mellet, J. M. García Fernández, Chem. Commun. 2007,
1
:1 acetonitrile:water). Each sample contained the following: 20 μL
substrate (20 μm), 230 μL Oxone (2.4 mm), 95 μL NaHCO
9.4 mm), 0 or 90–260 μL enzyme (0.6–1.7 mm), 655 or 395–565 μL
3
31, 3270–3272.
(
[11] T. H. Fenger, J. Bjerre, M. Bols, CHEMBIOCHEM 2009, 10,
solvent (1:1 acetonitrile:water). Four samples contained an increas-
ing concentration of enzyme, and two with none as control. The
reactions were monitored at the following wavelengths: 4-methoxy-
2494–2503.
[12] T. Lecourt, A. Herault, A. J. Pearce, M. Sollogoub, P. Sinaÿ,
Chem. Eur. J. 2004, 10, 2960–2971.
[13] P. Surresh, K. Pitchamni, J. Photochem. Photobiol. A: Chem.
2009, 206, 40–45.
–1
–1
4Ј-nitro-trans-stilbene: 377 nm ε = 24360Ϯ149 m cm ; trans-
–1
–1
stilbene: 308 nm, ε = 25631Ϯ781 m cm ; cis-stilbene: 276 nm, ε
[
[
[
14] S. Immel, F. W. Lichtenthaler, Starch 1996, 48, 225–232.
15] G. Bez, C.-G. Zhao, Tetrahedron Lett. 2003, 43, 7403–7406.
16] K. N. Houk, J. Liu, N. C. DeMello, K. R. Condroski, J. Am.
Chem. Soc. 1997, 119, 10147–10152.
85283Ϯ5023 m–1 cm ; styrene: 248 nm ε = 12191Ϯ197 m cm .
The rate constant was calculated as for the slope of the rate expres-
sion ln([A]/[A ]) = –kt. kcat was determined as the maximum k ob-
–1
–1
–1
=
0
tained at saturation. The ee was calculated using the area of the
first isomer eluting 1 and the last isomer eluting 2; ee = (1–2)/(1+2).
Received: December 16, 2010
Published Online: March 3, 2011
Eur. J. Org. Chem. 2011, 2339–2345
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2345