Lipase catalyzed enantioselective desymmetrization of a prochiral pentane-1,3,5-triol derivative

Article abstract of DOI:10.1016/j.tetasy.2006.10.043

The enantioselective desymmetrization of the prochiral 3-O-silyl protected pentanetriol derivative 3 was carefully investigated. At -10 °C, the bacterial lipase from Burkholderia cepacia immobilized on ceramic particles led to monoacetate (S)-4 in 52% yie

Full text of DOI:10.1016/j.tetasy.2006.10.043

Tetrahedron: Asymmetry 17 (2006) 3091–3099
Lipase catalyzed enantioselective desymmetrization of a
prochiral pentane-1,3,5-triol derivative
Jens Kohler and Bernhard Wunsch*
¨
¨
Institut fu¨r Pharmazeutische und Medizinische Chemie der Westfa¨lischen Wilhelms Universita¨t Mu¨nster,
Hittorfstraße 58-62, D-48149 Mu¨nster, Germany
Received 27 September 2006; accepted 30 October 2006
Available online 28 November 2006
Dedicated to Prof. Dr. Dr. h. c. Gottfried Blaschke on the occasion of his 70th birthday
Abstract—The enantioselective desymmetrization of the prochiral 3-O-silyl protected pentanetriol derivative 3 was carefully investigated.
At ꢀ10 ꢀC, the bacterial lipase from Burkholderia cepacia immobilized on ceramic particles led to monoacetate (S)-4 in 52% yield and
>99.9% ee. At a reaction temperature of ꢀ40 ꢀC the yield and enantioselectivity were even higher, but the reaction time was very long.
Theoretical simulations of the reaction progress indicated an enantioselectivity of 25:1 at ꢀ10 ꢀC and 35:1 at ꢀ40 ꢀC. (S)-4 was converted
into the enantiomerically pure building block 5-azidopentane-1,3-diol (S)-7 in two steps. The absolute configuration of (S)-7 was deter-
mined by exciton-coupled circular dichroism (ECCD) of diester (S)-8.
ꢁ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
pounds leads to enantiomerically pure products in 100%
yield, theoretically. Soriente et al have described the enan-
tioselective hydrolysis of prochiral diacetate II (R =
SiMe2tBu) using the lipase of Pseudomonas fluorescens
(Scheme 1).4 However, only the (R)-configured synthon
(R)-III is available by this method. Therefore, we investi-
gated the direct lipase catalyzed acetylation of a prochiral
diol I to afford the (S)-configured monoacetate (S)-III. In
order to analyze the development of the transformation
The development of pharmacologically interesting drugs is
currently performed only with enantiomerically pure com-
pounds. There are several methods to produce drugs with
high enantiomeric purity. Often the chiral information is
derived from enantiomerically pure building blocks avail-
able from the chiral pool of Nature. These building blocks
are enzymatically synthesized by various organisms. As a
result, enzymes also serve as suitable catalysts to synthesize
enantiomerically pure building blocks in the laboratory,
which are not available from Nature. Whereas Nature
has optimized enzymes for a particular transformation
over millions of years, in the laboratory, the reaction con-
ditions have to be optimized to gain the desired product in
high enantiomeric purity using a given enzyme. The enzy-
matic production of new synthons is the subject of numer-
ous publications.1–3
(S)
HO
OH
HO
OAc
lipase
OR
OR
I
(S)-III
(ref. 4)
Herein, we report the enantioselective desymmetrization of
a prochiral pentane-1,3,5-triol derivative using various lip-
ases. In contrast to the enzymatic resolution of racemates,
the enantioselective desymmetrization of prochiral com-
(R)
AcO
OAc
AcO
OH
lipase (ref. 4)
OR
OR
II
(R)-III
*
Corresponding author. Tel.: +49 251 8333311; fax: +49 251 8332144;
e-mail: wuensch@uni-muenster.de
Scheme 1. Strategy to obtain (S)- and (R)-pentane-1,3,5-triol derivatives.
0957-4166/$ - see front matter ꢁ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.10.043

3092
J. Ko¨hler, B. Wu¨nsch / Tetrahedron: Asymmetry 17 (2006) 3091–3099
and the enantiomeric excess by HPLC the protecting group
R should contain a UV absorbing moiety.
HO
OH
OSiMe2Ph
k1
k2
3
2. Results and discussion
2.1. Synthesis
k6
k5
(R)
(S)
AcO
OH
OSiMe2Ph
HO
OAc
The synthesis was started from prochiral diester 1, which
was silylated with chloro-dimethyl-phenyl-silane and imid-
azole in CH2Cl2.5 The resulting silyl ether 2 was reduced by
LiBH4 to give prochiral pentanediol 3 (Scheme 2).
OSiMe2Ph
(S)-4
(R)-4
k8
k7
Me2PhSiCl
k4
k3
MeO2C
CO2Me
MeO2C
CO2Me
OSiMe2Ph
CH2Cl2
imidazole
OH
AcO
OAc
96%
1
2
OSiMe2Ph
5
LiBH4
Et2O
HO
OH
Scheme 4. Lipase catalyzed acetylation of prochiral diol 3.
OSiMe2Ph
85%
3
Table 1. Investigated lipases
Scheme 2. Synthesis of prochiral diol 3.
Lipase
Product
Origin
A
B
C
D
E
F
G
H
I
Amano AK
Amano PS
Pseudomonas fluorescens
Burkholderia cepacia
Burkholderia cepacia
Candida antarctica B
Candida rugosa
The key step for the introduction of chirality is the enantio-
selective acetylation of prochiral diol 3 using various
lipases. In order to achieve irreversibility, a transesterifica-
tion with isopropenyl acetate (IPA) was carried out
(Scheme 3). Generally, tert-butyl methyl ether (TBME)
was used as the solvent.
Amano PS-CIIa
Chirazyme L-2
Chirazyme L-3 pur.
Chirazyme L-5
Chirazyme L-7
Fluka Lipase
Candida antarctica A
Porcine pancreas
Candida rugosa
Fluka Lipozymea
Mucor miehei
(S)
a Immobilized.
HO
OH
OSiMe2Ph
HO
OAc
OSiMe2Ph
For a better comparison of the catalytic activity and
enantioselectivity of the lipases considered, the esterifica-
tion of 3 was performed using the following standard con-
ditions: in a 50 mL round-bottomed flask, a given amount
of diol 3 was dissolved in 30 mL of a 50:1 TBME/IPA mix-
ture and the given amount of lipase was added (Table 2).
The reactions were carried out at a temperature of
+40 ꢀC and stirred with a magnetic bar. The reaction pro-
gress was controlled by thin layer chromatography (TLC)
and stopped after the given time. For this purpose the
lipase was filtered off and the filtrate was concentrated
in vacuo.
lipase
TBME
3
(S)-4
+
+
CH3
CH2
H3C
O
H3C
OAc
IPA
Scheme 3. Lipase catalyzed enantioselective, irreversible acetylation.
The acetylation of prochiral diol 3 can occur at either of
the OH groups to afford the monoacetates (S)-4 and (R)-
4 or at both OH groups to provide the prochiral diacetate
5 (Scheme 4). In order to obtain monoacetate (S)-4 in a
high yield and enantiomeric purity, the lipase catalyzed
acetylation of diol 3 was carefully optimized.
The composition of the resulting mixture was analyzed
with HPLC methods 1 and 2. HPLC method 1 is an achiral
method (RP-18), which was employed for the determina-
tion of the transformation. The ratio of enantiomers
(S)-4:(R)-4 was analyzed by HPLC method 2 using the
chiral Daicel Chiralpak AD-H stationary phase.
2.2. Lipase screening
The most important feature for the production of mono-
acetate (S)-4 with high enantiomeric purity is the
catalyst (lipase). In order to find the most suitable lipase,
some commercially available preparations from various
sources (four bacterial, four fungal, one mammalian lipase)
were investigated (Table 1).
Table 2 shows the results of the lipase screening. Obviously
the lipase from Candida rugosa (E, H) is not suitable for this
reaction, since even after 144 h or 91 h at 40 ꢀC, the trans-
formation was very low. On the other hand, the lipase from
Candida antarctica B (D) led to a very fast, but not selective
acetylation resulting in large amounts of diacetate 5.

J. Ko¨hler, B. Wu¨nsch / Tetrahedron: Asymmetry 17 (2006) 3091–3099
3093
Table 2. Screening of lipases using standard conditions: 30 mL of TBME/IPA 50:1 at +40 ꢀC
Lipase
m (lipase) [mg]
m (3) [mg]
Time (h)
HPLC method 1
HPLC method 2
n ((S)-4) [%] n ((R)-4) [%]
n (3) [%]
n (4) [%]
n (5) [%]
A
B
C
D
D
E
F
G
H
I
101.1
51.7
157.0
25.3
273.3
265.4
277.6
236.8
258.3
279.8
267.2
248.3
300.0
270.4
51
72
23
18
1
144
25
24
91
21
2.1
48.9
0.2
43.1
47.0
13.3
0.0
15.5
2.5
46.5
37.2
3.6
54.8
4.1
86.5
99.8
73.1
0.1
4.5
3.0
0.1
53.4
98.0
92.0
99.9
—
39.3
—
72.8
37.1
44.3
99.4
2.0
8.0
0.1
—
60.7
—
27.2
62.9
55.7
0.6
0.2
30.1
11.4
97.4
49.0
59.8
96.3
0.2
185.9
164.7
153.1
308.0
122.2
46.3
P
m (X) [mg] = amount of substance [mg]; n (X) [%] = amount of substance [%] = n (X) [mol]/n ( ) [mol].
Nevertheless, the enzymatic acetylation of diol 3 to afford
diacetate 5 is more convenient than acetylation with acetic
anhydride, since the transformation is nearly quantitative
and the work-up is easier.4 Therefore, the C. antarctica B
lipase will be used for the synthesis of diacetate 5, which
is required for the synthesis of the enantiomeric monoace-
tate (R)-4 by lipase catalyzed enantioselective hydrolysis.
charted in relation to the reaction time to give the dia-
grams depicted on the left side in Figures 1, 2 and 4 top.
The ratio of the enantiomeric monoacetates (S)-4
and (R)-4 was determined by means of HPLC method 2.
The development of the enantiomeric excess during the
reaction time is shown on the right side in Figures 1, 2
and 4 top.
According to Table 2, most of the lipases investigated pro-
vided the (S)-configured monoacetate (S)-4. Only the lip-
ases from C. antarctica B (D), C. rugosa (H) and Porcine
pancreas (G) led to a low excess of the (R)-configured
monoacetate (R)-4.
At first, the progress of the reactions carried out at different
temperatures with 5.0 equiv of the acylating agent IPA was
recorded. A comparison of the reactions carried out at
+20 ꢀC (Fig. 1 top) and at ꢀ10 ꢀC (Fig. 1 bottom) shows
that the enantioselectivity of the lipase was increased by a
reduction in temperature. The yield of monoacetate (S)-4
in sample A (Fig. 1 top, arrow) was 48.3% with 99.53%
ee. Sample B (Fig. 1 bottom, arrow) contained 57.0% yield
of monoacetate (S)-4 with 99.92% ee. However, the trans-
formation at ꢀ10 ꢀC was considerably slower: Point B in
Figure 1 was achieved after only 65 h.
The most promising enantioselectivity was attained using
the bacterial lipases from P. fluorescens (A), B. cepacia
(B, C) and Mucor miehei (I). We decided to further opti-
mize the reaction with Amano PS-CII, a lipase from B.
cepacia (C), which is immobilized on ceramic particles. In
the screening, Amano PS-CII led to the highest enantio-
meric excess. Moreover, immobilization on ceramic parti-
cles allows the direct analysis of the reaction mixture
without cumbersome work-up procedures.
Next the influence of the IPA amount on the reaction pro-
gress was investigated. The lipase catalyzed acetylation was
performed at ꢀ10 ꢀC employing 2.0 equiv (Fig. 2 top) or
15.0 equiv (Fig. 2 bottom) of IPA. Obviously a larger
IPA amount increased the enantiomeric excess: Sample C
taken from the reaction carried out with 2.0 equiv of IPA
at the indicated time (Fig. 2 top, arrow) consisted of
63.7% yield of monoacetate (S)-4 with 99.67% ee. In Figure
2 bottom, the marked position D represents a sample,
which contained a yield of 65.3% monoacetate (S)-4 with
99.84% ee. However, raising the IPA amount to 50 equiv
led to deterioration of the transformation, probably due
to the higher polarity of the solvent.
2.3. Optimization of the reaction conditions using Amano
PS-CII lipase
In order to find the best reaction conditions, the compo-
sition of the reaction mixture and the (S)-4:(R)-4-ratio were
determined periodically. The general procedure was as fol-
lows. In a 50 mL two-necked flask, about 280 mg of diol 3
were dissolved in 30 mL of TBME and 0.50 wt equiv of
Amano PS-CII lipase were added. To avoid damage of
the ceramic particles, the reaction mixture was stirred with
a KPG-stirrer (100 rpm). The reaction temperature was ad-
justed by a cryostat and the reaction was started by the
addition of the respective amount of isopropenyl acetate
(IPA). In order to analyze the transformation, samples of
100 lL taken from the reaction mixture were filtered
through a membrane filter (0.45 lm) and the solvent was
evaporated in a nitrogen stream over 2 min. The residue
was dissolved in 100 lL of acetonitrile for HPLC method
1 and in 100 lL of a n-hexane/propan-2-ol 9:1 mixture
for HPLC method 2. The amount of substances 3, 4 and
5 (n [%]) determined by means of HPLC method 1 was
Small amounts of IPA (2.0 equiv) resulted in a maximum
of the enantiomeric excess (99.82% ee) after 92.6 h. After-
wards, the ee-value decreased due to the partial racemiza-
tion of monoacetate (S)-4. This partial racemization was
caused by lipase catalyzed transesterification of diol 3,
monoacetate 4 and diacetate 5. Thus, in the presence of
low amounts of acylating agent IPA, the lipase uses
monoacetate 4 and diacetate 5 instead of IPA as an acetyl
group donor. This transacetylation leads to a reversibility
of the conversion with a decline in the enantiomeric
excess.

3094
J. Ko¨hler, B. Wu¨nsch / Tetrahedron: Asymmetry 17 (2006) 3091–3099
3
4
5
(S)-4
100
80
60
40
20
0
100
99
98
97
96
95
A
A
n [%]
% ee
0
10
20
30
40
50
0
10
20
30
40
50
time [h]
time [h]
100
80
60
40
20
0
100
99
98
97
96
95
B
B
n [%]
% ee
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
time [h]
time [h]
Figure 1. Progress of the reaction using 5 equiv of IPA; left: amount of compounds 3, 4 and 5 (n [%]); right: enantiomeric excess of (S)-4 (% ee); top:
carried out at +20 ꢀC; Sample A (23.0 h): (S)-4: yield: 48.3%, 99.53% ee; bottom: carried out at ꢀ10 ꢀC; Sample B (65.4 h): (S)-4: yield: 57.0%, 99.92% ee.
3
4
5
(S)-4
100
80
60
40
20
0
100
C
C
n [%]
99
% ee
98
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
time [h]
time [h]
100
80
60
40
20
0
100
D
D
n [%]
99
98
% ee
0
20
40
60
80
100
120
140
0
20
40
60
80
100
120
140
time [h]
time [h]
Figure 2. Progress of the reaction carried out at ꢀ10 ꢀC; left: amount of compounds 3, 4 and 5 (n [%]); right: enantiomeric excess of (S)-4 (% ee); top:
2.0 equiv of IPA; Sample C (67.2 h): (S)-4: yield: 63.7%, 99.67%; bottom: 15.0 equiv of IPA; Sample D (54.4 h): (S)-4: yield: 65.3%, 99.84% ee. The grey
dotted lines indicates the time (90 h) reactions should be stopped to get the best yield of (S)-4 with >99.9% ee.

J. Ko¨hler, B. Wu¨nsch / Tetrahedron: Asymmetry 17 (2006) 3091–3099
3095
Recently we have published a facile mathematical system
for the simulation of catalytic reactions.6 Based on the the-
oretically possible equilibria (Scheme 4), the development
of the transformation can be calculated using given values
for the rate constants k1 to k8 and the lipase activity a. In
Figure 3 the simulated reaction courses and the corre-
sponding enantiomeric excess are displayed. For these cal-
culations the ratio of rate constants (k1/k5 = k2/k6 = k3/
k7 = k4/k8) was set at 1:10ꢀ3 (Fig. 3 top) and at 1:10ꢀ6
(Fig. 3 bottom) on the product side. The outcome
corresponds to the experimental data shown in Figure 2
and proves the assumption that racemization can be pro-
hibited by suppressing the backward reactions. Hence, a
sufficient excess of the acylating agent IPA has to be
applied to produce monoacetate (S)-4 with high enantio-
meric purity.
the reaction rate was dramatically reduced by lowering
the reaction temperature: sample D was taken after
54.4 h, sample E after 290.5 h. Overall the reaction was
run for about 860 h (about 36 d) at ꢀ40 ꢀC. During the
reactions (S)-4 reached an excellent enantiomeric purity
(>99.99% ee), whereas the yield of (S)-4 was still very high
(>60%) (Fig. 4 top).
At the bottom of Figure 4, the computational simulation of
this reaction is shown. The use of the rate constants given
in Figure 4 resulted in curves, which are nearly identical to
the experimentally determined curves shown at the top of
Figure 4. A crucial feature for the enantioselectivity of
the reaction is the ratio k1/k2 = k4/k3 = k5/k6 = k8/k7,
which is set at 35:1 in the simulation. A comparison of this
ratio at ꢀ40 ꢀC (35:1) (Fig. 4 bottom) with the correspond-
ing ratio for the reaction at ꢀ10 ꢀC of 25:1 (Fig. 3 bottom)
supports the higher enantioselectivity of the lipase at a tem-
perature of ꢀ40 ꢀC. The rate ratio of the transformations
of diol 3 to monoacetate 4 and of monoacetate 4 to diace-
tate 5 was not altered by lowering the reaction temperature.
The simulations shown in Figures 3 and 4 bottom, were
calculated with the ratio k4/k1 = k3/k2 = k8/k5 = k7/
k6 = 4:1.
Finally, the lipase catalyzed acetylation of diol 3 was per-
formed at an extraordinarily low temperature of ꢀ40 ꢀC.
In this experiment, the amount of lipase was raised from
0.5 to 1.0 wt equiv because its activity was strongly reduced
at ꢀ40 ꢀC. It can be seen from Figure 4 that lowering the
temperature from ꢀ10 to ꢀ40 ꢀC further increased the
enantioselectivity of the transformation. Compound (S)-4
in sample E (Fig. 4 top, arrows) has about the same enan-
tiomeric purity (99.86% ee) as (S)-4 in sample D (99.84%
ee) (Fig. 2). However, the yield of monoacetate (S)-4 was
considerably higher for sample E (74.2%) than for sample
D (65.3%). Despite of the increased amount of catalyst,
To the best of our knowledge, this is one of the first exper-
iments,7 which employed an enzyme at ꢀ40 ꢀC. It was sur-
prising that the B. cepacia lipase still worked at such a low
temperature. The increased enantioselectivity and yield
3
4
5
(S)-4
100
80
60
40
20
0
100
n [%]
99
% ee
98
0
2
4
6
8
10
0
2
4
6
8
10
•1000 intervals
•1000 intervals
100
80
60
40
20
0
100
n [%]
99
98
% ee
0
2
4
6
8
10
0
2
4
6
8
10
•1000 intervals
•1000 intervals
Figure 3. Simulated6 progress of the reaction using a constant lipase activity a = 0.0035. The rate constants k1 to k8 are defined in Scheme 4; left: amount
of compounds 3, 4 and 5 (n [%]); right: enantiomeric excess of (S)-4 (% ee); top: k1 = 25, k2 = 1, k3 = 4, k4 = 100, k5 = 25 · 10ꢀ3, k6 = 1 · 10ꢀ3
,
k7 = 4 · 10ꢀ3, k8 = 100 · 10ꢀ3; bottom: k1 = 25, k2 = 1, k3 = 4, k4 = 100, k5 = 25 · 10ꢀ6, k6 = 1 · 10ꢀ6, k7 = 4 · 10ꢀ6, k8 = 100 · 10ꢀ6
.

3096
J. Ko¨hler, B. Wu¨nsch / Tetrahedron: Asymmetry 17 (2006) 3091–3099
3
4
5
(S)-4
100
80
60
40
20
0
100
99
98
97
96
95
E
E
n [%]
% ee
0
200
400
600
800
0
200
400
600
800
time [h]
time [h]
100
80
60
40
20
0
100
99
98
97
96
95
n [%]
% ee
0
2
4
6
8
10
0
2
4
6
8
10
•1000 intervals
•1000 intervals
Figure 4. Measured and simulated6 progress of the reaction at ꢀ40 ꢀC; left: amount of compounds 3, 4 and 5 (n [%]); right: enantiomeric excess of (S)-4
(% ee); top: reaction carried out using 15 equiv of IPA; bottom: Simulation6 of the reaction using a constant lipase activity a = 0.004. The rate constants k1
to k8 are defined in Scheme 4; k1 = 35, k2 = 1, k3 = 4, k4 = 140, k5 = 35 · 10ꢀ6, k6 = 1 · 10ꢀ6, k7 = 4 · 10ꢀ6, k8 = 140 · 10ꢀ6
.
Zn(N3)2
have prompted us to explore further lipase catalyzed trans-
formations at similar unphysiological temperatures.
(S)
(S)
HO
OAc
OSiMe2Ph
N3
OAc
OSiMe2Ph
PPh3/DIAD
toluene
79%
In order to produce large amounts of (S)-4 in a reasonable
period of time, the lipase catalyzed acetylation of diol 3 was
performed at ꢀ10 ꢀC with 15 equiv of IPA. The reaction
was stopped as soon as the ratio of monoacetate 4 to diac-
etate 5 was about 1:1, according to HPLC method 1 (Fig. 2
bottom, grey dotted lines). Upon flash chromatography,
(S)-4 was obtained in >99.9% ee and 52% yield.
(S)-6
(S)-4
(S)
N3
OH
1. CH3OH/K2CO3
2. HCl 0.1M
91%
OH
(S)-7
Scheme 5. Synthesis of (S)-5-azidopentane-1,3-diol (S)-7.
2.4. Absolute configuration
According to our plan the enantiomerically pure monoac-
etate (S)-4 should be transformed into the chiral building
block 5-azidopentane-1,3-diol (S)-7. Diol (S)-7 should also
be used for the determination of the absolute configuration
of (S)-4. Thus, monoacetate (S)-4 was reacted in a Mitsun-
obu reaction8 with Zn(N3)2, PPh3 and diisopropyl azodi-
carboxylate (DIAD) to afford azide (S)-6, which was
hydrolyzed to yield azidodiol (S)-7 (Scheme 5). Enantiome-
rically pure azidodiol (S)-7 represents an interesting build-
ing block, since the positions 1, 3 and 5 of the pentane
scaffold can be modified selectively.
(S)
(S)
N3
OH
N3
O
O
4-BrBzCl
CH2Cl2
62%
OH
O
O
(S)-7
Br
(S)-8
Br
Scheme 6. Synthesis of bis(bromobenzoyl) derivative (S)-8 for determi-
nation of the absolute configuration (4-BrBzCl = 4-bromobenzoyl
chloride).
In order to determine the absolute configuration of mono-
acetate (S)-4, azidodiol (S)-7 was acylated with 4-bro-
mobenzoyl chloride to provide dibenzoate (S)-8 (Scheme
6).
At first the enantiomeric purity of dibenzoate (S)-8 was
investigated with HPLC method 3 using a Daicel Chir-
alpak IB column. According to this analysis, the dibenzo-

J. Ko¨hler, B. Wu¨nsch / Tetrahedron: Asymmetry 17 (2006) 3091–3099
3097
Melting point apparatus SMP 3 (Stuart Scientific), and
are uncorrected; Optical rotation: Polarimeter 341 (Perkin
Elmer), 1.0 dm tube, concentration c (g/100 mL), temper-
ature 20 ꢀC, the unit [deg mL dmꢀ1 gꢀ1] is omitted; IR:
IR spectrophotometer 480Plus FT-ATR-IR (Jasco); 1H
NMR (400 MHz): Unity Mercury Plus 400 NMR spec-
trometer (Varian), d in parts per million related to tetra-
methylsilane; MS: MAT GCQ (Thermo-Finnigan), TSQ
7000 (Thermo-Finnigan), LCQ MAT (Thermo Finnigan),
EI = electron impact; Elemental analysis: Vario EL (Ele-
mentaranalysesysteme GmbH).
+ 15
+ 10
+ 5
0
252 (+13.7)
A = (+ 19.1)
CD
Δε
236 (-5.4)
-5
4
2
0
4.2. HPLC analysis
245.1 (39400)
ε ·10-4
UV
Method 1
column:
Merck LiChrospher 100
RP-18e (5 lm); 125—4 mm;
acetonitrile/water 50:50; 1 mL/min;
k = 264 nm for 16 min;
mobile phase:
detection:
200
250
300
350
λ
[nm]
retention times (rt): rt (3) = 1.9 min; rt (4) = 4.1 min;
rt (5) = 12.4 min;
Figure 5. CD and UV spectra of (S)-8 in n-hexane.
scaling factors (sf):
sf (3) = sf (4) = sf (5) = 1.00
[n (%)/area (%)].
ate used for recording the CD spectrum had an enantio-
meric purity of 99.84% ee. The CD spectrum was recorded
in n-hexane and arises from exciton-coupling between the
electric transition moments of the benzoyl chromophores
(Fig. 5). The p ! p* transitions (245 nm) couple with each
other and reveal a bisignate curve: the Cotton effect with
lower energy is positive (252 nm) and the one with higher
energy negative (236 nm), which proves the (S)-configura-
tion of the stereogenic centre.4,9,10
Method 2
column:
Daicel Chiralpak AD-H (5 lm);
250—4.6 mm;
n-hexane/propan-2-ol 51:1;
1 mL/min;
mobile phase:
detection:
k = 264 nm for 40 min;
retention times (rt): rt ((S)-4) = 25.0 min;
rt ((R)-4) = 27.5 min;
3. Conclusions
the rt can be increased
by using n-hexane/propan-2-ol
54:1 to rt ((S)-4) = 27.5 min; rt
((R)-4) = 32.5 min;
sf ((S)-4) = sf ((R)-4) = 1.000
[n (%)/area (%)].
Prochiral diol 3 was enantioselectively monoacetylated by
transesterification with isopropenyl acetate to obtain enan-
tiomerically pure monoacetate (S)-4 (>99.9% ee) using the
immobilized lipase from B. cepacia. The reaction condi-
tions were carefully optimized. Lowering of the reaction
temperature led to an increased enantioselectivity. Even
at ꢀ40 ꢀC, the lipase catalyzed the transesterification but
with a low reaction rate. In order to avoid partial racemi-
zation, 15 equiv of the acylating agent isopropenyl acetate
should be used. Large amounts of monoacetate (S)-4 were
synthesized with 15 equiv of IPA at ꢀ10 ꢀC. The conver-
sion of (S)-4 into enantiomerically pure pentane derived
building blocks was demonstrated by the synthesis of
5-azidopentane-1,3-diol (S)-7.
scaling factors (sf):
Method 3
column:
Daicel Chiralpak IB (5 lm);
250—4.6 mm;
n-hexane/2-propanol 20:1;
1 mL/min;
mobile phase:
detection:
k = 245 nm for 30 min;
retention times (rt): rt ((S)-8) = 17.7 min; rt
((R)-8) = 19.3 min;
4. Experimental
4.1. General
scaling factors (sf):
sf ((S)-8) = sf ((R)-8) = 1.000
[n (%)/area (%)].
Thin layer chromatography (TLC): silica gel 60 F254 (E.
Merck); Flash chromatography (fc): Silica gel 60, 40–
63 lm (E. Merck);11 HPLC equipment: Merck Hitachi,
UV-Detector L-7400, Autosampler L-7200, Pump L-
7100, data acquisition HSM-Software; Melting points:
4.3. Dimethyl 3-(dimethylphenylsilyloxy)glutarate 2
Me2PhSiCl (11.1 g, 65.0 mmol) was dissolved in CH2Cl2
(100 mL) and imidazole (10 g, 147 mmol) was added. A

3098
J. Ko¨hler, B. Wu¨nsch / Tetrahedron: Asymmetry 17 (2006) 3091–3099
solution of dimethyl ester 1 (10.0 g, 56.8 mmol) in CH2Cl2
(50 mL) was added dropwise and the reaction mixture stir-
red for 22 h at room temperature. Afterwards, water
(120 mL) was added, and the aqueous layer was separated
and extracted with CH2Cl2 (2 · 50 mL). The combined or-
ganic layers were washed with water (100 mL), dried over
K2CO3 and concentrated in vacuo. The residue was puri-
fied by fc (B 8 cm; h 20 cm; 7:1 petroleum ether/EtOAc;
fractions 30 mL; Rf 0.30); colourless oil; yield 17.0 g
(96%); IR (ATR, neat): m (cmꢀ1) = 3070 (C–Harom.), 2953
2 · –CH2–O–), 4.51 (t, J = 4.7 Hz, 1H, –CH2–CH–CH2–),
7.47–7.54 (m, 3H, Harom.), 7.66–7.69 (m, 2H, Harom.);
EIMS m/z (rel int.): 296 (M, 11), 219 (MꢀPh, 24), 135
(Me2PhSi, 13), 77 (Ph, 97). Anal. Calcd for C15H24O4Si
(296.4): C 60.78, H 8.16. Found: C 60.41, H 8.42.
Data for 5: Rf 0.55; colourless oil; yield 1.72 g (43%); IR
(ATR, neat): m (cmꢀ1) = 3070 (C–Harom.), 1737 (C@O);
1H NMR (DMSO-d6): d (ppm) = 0.45 (s, 6H, –Si(CH3)2),
1.74–1.90 (m, 4H, –CH2–CH–CH2–), 2.06 (s, 6H,
2 · CH3–COO–), 4.00–4.16 (m, 5H, 2 · –CH2–O– and
–CH2–CH–CH2–), 7.47–7.55 (m, 3H, Harom.), 7.65–7.68
(m, 2H, Harom.); EIMS m/z (rel int.): 338 (M, 5), 135
(Me2PhSi, 12), 77 (Ph, 94); C17H26O5Si (338.5).
1
(C–H), 1735 (C@O); H NMR (CDCl3): d (ppm) = 0.38
(s, 6H, –Si(CH3)2), 2.53 (d, J = 6.2 Hz, 4H, –CH2–CH–
CH2–), 3.58 (s, 6H, 2 · –CO2CH3), 4.61 (quint,
J = 6.3 Hz, 1H, –CH2–CH–CH2–), 7.31–7.37 (m, 3H,
Harom.), 7.51–7.55 (m, 2H, Harom.); EIMS m/z (rel int.):
310 (M, 12), 233 (MꢀPh, 97), 77 (Ph, 11); C15H22O5Si
(310.4).
4.6. (S)-[5-Azido-3-(dimethylphenylsilyloxy)pentan-1-yl]
acetate (S)-6
4.4. 3-(Dimethylphenylsilyloxy)pentane-1,5-diol 3
Monoacetate (S)-4 (3.04 g, 10.3 mmol; 99.86% ee) was
dissolved in toluene (60 mL). Then Zn(N3)2Æ2Py (2.36 g,
7.7 mmol)8 and triphenylphosphane (5.37 g, 20.5 mmol)
were added. To this suspension, diisopropyl azodicarboxy-
late (4.0 mL, 20.5 mmol) was added dropwise and the reac-
tion mixture was stirred for 22 h at room temperature. The
mixture was transferred to a column without further work-
up and purified by fc (B 8 cm; h 18 cm; 9:1 cyclohexane/
LiBH4 (6.6 g, 300 mmol) was added to a solution of 2
(15.4 g, 50 mmol) in Et2O (700 mL) and the reaction mix-
ture was stirred for 17 h at room temperature. A saturated
solution of NH4Cl (200 mL) was added to destroy excess
LiBH4. The organic layer was separated and the aqueous
layer extracted with CH2Cl2 (3 · 50 mL). The combined
organic layers were dried over K2CO3, concentrated in
vacuo and the residue was purified by fc (B 8 cm; h
20 cm; EtOAc; fractions 30 mL; Rf 0.30); colourless oil;
yield 10.8 g (85%); IR (ATR, neat): m (cmꢀ1) = 3340
(O–H), 3070 (C–Harom.), 2949, 2882 (C–H); 1H NMR
(CDCl3): d (ppm) = 0.44 (s, 6H, –Si(CH3)2), 1.65–1.79
(m, 4H, –CH2–CH–CH2–), 1.93 (s, 2H, 2 · –OH), 3.59–
3.70 (m, 4H, 2 · –CH2–OH), 4.11 (quint, J = 5.8 Hz, 1H,
–CH2–CH–CH2–), 7.36–7.43 (m, 3H, Harom.), 7.58–7.61
(m, 2H, Harom.); EIMS m/z (rel int.): 254 (M, 2), 177
(MꢀPh, 5), 135 (Me2PhSi, 8), 77 (Ph, 68); C13H22O3Si
(254.4).
EtOAc; fractions 30 mL; Rf 0.23); colourless oil; yield
20
2.59 g (79%); ½aꢁD ¼ ꢀ0:2 (c 0.88, CH2Cl2); IR (ATR,
neat): m (cmꢀ1) = 3071 (C–Harom.), 2093 (N@N@N), 1738
(C@O); 1H NMR (DMSO-d6): d (ppm) = 0.46 (s, 3H,
–Si–CH3), 0.47 (s, 3H, –Si–CH3), 1.72–1.85 (m, 4H,
–CH2–CH–CH2–), 2.06 (s, 3H, CH3–COO–), 3.37–3.47
(m, 2H, –CH2–N3), 4.00–4.14 (m, 3H, –CH2–O– and
–CH2–CH–CH2–), 7.48–7.55 (m, 3H, Harom.), 7.66–7.68
(m, 2H, Harom.); EIMS m/z (rel int.): 321 (M, 1), 135
(Me2PhSi, 3), 77 (Ph, 31). Anal. Calcd for C15H23N3O3Si
(321.5): C 56.05, H 7.21, N 13.07. Found: C 55.62, H
7.35, N 12.84.
4.5. (S)-[3-(Dimethylphenylsilyloxy)-5-hydroxypentan-1-yl]
acetate (S)-4 and [3-(Dimethylphenylsilyloxy)pentane-1,5-
diyl] diacetate 5
4.7. (S)-5-Azidopentane-1,3-diol (S)-7
Azide (S)-6 (2.54 g, 7.9 mmol) was dissolved in methanol
(100 mL). K2CO3 (270 mg) was then added and the reac-
tion mixture was stirred for 3 h at room temperature.
Afterwards, HCl (1 M, 7.9 mL) was added and the reaction
mixture was stirred for a further 2 h at room temperature.
Then, it was degassed in an ultrasonic bath, NH3 (25%,
1.6 mL) was added and the mixture was concentrated to
dryness in vacuo. To remove any remaining water, CHCl3
(2 · 70 mL) was added and evaporated in vacuo again. The
residue was then heated at reflux in ethanol (70 mL) for
10 min and then cooled overnight (5 ꢀC). The salts were fil-
tered off, washed with ethanol (70 mL) and the filtrate was
concentrated in vacuo. The residue was purified by fc (B
Diol 3 (3.03 g, 11.9 mmol) was dissolved in TBME
(400 mL) and amano PS-CII lipase (3 g) was added. The
suspension was stirred with a KPG-stirrer and cooled to
ꢀ10 ꢀC. The reaction was started by the addition of isoprop-
enyl acetate (19.5 mL, 179 mmol; ꢀ10 ꢀC). After 42.5 h of
stirring at ꢀ10 ꢀC the KPG-stirrer was removed. The reac-
tion mixture was warmed to room temperature within
about 2 min in a water bath and filtered immediately.
The filtrate was concentrated in vacuo and purified by fc
(B 8 cm; h 20 cm; 1:1 cyclohexane/EtOAc; fractions
30 mL).
6 cm; h 20 cm; EtOAc; fractions 30 mL; Rf 0.22); pale-yel-
20
Data for (S)-4: Rf 0.29; colourless oil; yield 1.84 g (52%);
low oil; yield 1.04 g (91%); ½aꢁD ¼ ꢀ26:7 (c 0.88, CH2Cl2);
20
½aꢁD ¼ þ9:5 (c 0.64, THF); 99.91% ee (HPLC method 2);
IR (ATR, neat): m (cmꢀ1) = 3340 (O–H), 2091 (N@N@N);
1H NMR (CD3OD): d (ppm) = 1.71–1.90 (m, 4H, –CH2–
CH–CH2–), 3.54 (t, J = 7.0 Hz, 2H, –CH2–N3), 3.80 (t,
J = 6.6 Hz, 2H, –CH2–OH), 3.94 (tt, J = 4.7/3.9 Hz, 1H,
–CH2–CH–CH2–); EIMS m/z (rel int.): 146 (MH, 43), 72
IR (ATR, neat): m (cmꢀ1) = 3454 (O–H), 3070 (C–Harom.),
1738 (C@O); 1H NMR (DMSO-d6): d (ppm) = 0.45 (s, 3H,
–Si–CH3), 0.46 (s, 3H, –Si–CH3), 1.67–1.90 (m, 4H, –CH2–
CH–CH2–), 2.06 (s, 3H, CH3–COO–), 4.04–4.16 (m, 4H,

J. Ko¨hler, B. Wu¨nsch / Tetrahedron: Asymmetry 17 (2006) 3091–3099
3099
(C5H12, 100). Anal. Calcd for C5H11N3O2 (145.2): C 41.37,
Acknowledgment
H 7.64, N 28.95. Found: C 41.69, H 7.91, N 28.38.
The authors wish to thank Professor Dr. Hans-Ulrich
Humpf for recording the CD spectrum.
4.8. (S)-(5-Azidopentane-1,3-diyl) bis-4-bromobenzoate
(S)-8
NEt3 (255 lL, 1.84 mmol) and a solution of 4-bro-
mobenzoyl chloride (220 mg, 1.00 mmol) in CH2Cl2
(5 mL) were added to azidodiol (S)-7 (45 mg, 0.31 mmol).
The reaction mixture was stirred for 45 h at room
temperature. Afterwards, a saturated solution of NH4Cl
(50 mL) was added and the mixture was extracted with
CH2Cl2 (2 · 50 mL). The combined organic layers were
dried over Na2SO4, concentrated in vacuo and the residue
was purified by fc (B 3 cm; h 20 cm; 9:1 cyclohexane/
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