Lipase catalyzed acylation of primary alcohols with remotely located stereogenic centres: the resolution of (±)-4,4-dimethyl-3-phenyl-1-pentanol

Article abstract of DOI:10.1016/j.tetasy.2007.07.002

Enantioselective acylation of some (±)-3-alkyl-3-phenyl-1-propanols was performed with enzymes as catalysts. Moderate enantiomeric ratios (E), ranging up to E = 11.6, were obtained. In the resolution, some of the lipases selectively acylated the (+)-enant

Full text of DOI:10.1016/j.tetasy.2007.07.002

Tetrahedron: Asymmetry 18 (2007) 1712–1720
Lipase catalyzed acylation of primary alcohols with
remotely located stereogenic centres: the resolution of
( )-4,4-dimethyl-3-phenyl-1-pentanol
Sunil Sabbani, Erik Hedenstro¨m* and Jimmy Andersson
Chemistry, Department of Natural Sciences, Mid Sweden University, SE-851 70 Sundsvall, Sweden
Received 19 June 2007; accepted 5 July 2007
Abstract—Enantioselective acylation of some ( )-3-alkyl-3-phenyl-1-propanols was performed with enzymes as catalysts. Moderate
enantiomeric ratios (E), ranging up to E = 11.6, were obtained. In the resolution, some of the lipases selectively acylated the (+)-enan-
tiomer while others acylated the (ꢀ)-enantiomer of the c-substituted primary alcohols 1–4. Thus, it is possible to obtain both enantiomers
of the alcohols as remaining substrate with high enantiomeric purity. The resolution of ( )-4,4-dimethyl-3-phenyl-1-pentanol 4 was
extensively studied and screening experiments were conducted to select suitable lipase(s), reaction medium, acyl donor and appropriate
temperature combinations to increase the enantiomeric ratio. Chirazymeꢀ L-6/chloroform/vinyl propionate/38 ꢁC and Chirazymeꢀ
L-7/di-iso-propyl ether/vinyl propionate/0 ꢁC were chosen to obtain both enantiomers, (R)-(+)-4 and (S)-(ꢀ)-4, respectively, via sequen-
tial resolutions in excellent enantiomeric excess (>98%) and in 25% and 22% yield, respectively.
ꢂ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
and from others.12,13 Very few examples are known from
the literature when primary alcohols, with even more remo-
tely located stereogenic centres, are resolved by enzymes in
acylation reactions.7,14–18 Pseudomonas cepacia lipase
(PCL) catalyzed resolution of ( )-3-phenyl-1-butanol 1
has been reported15 with a very low E-value of 2 but the
enantioselective acylation of substrates 2, 3 and 4 via lipase
catalysis has not previously been reported. Herein, we
report a study on the preparation of such alkyl c-substi-
tuted primary alcohols in high enantiomeric purity via
enzymatic resolution.
Enantiopure primary alcohols are valuable intermediates in
asymmetric synthesis of pharmaceuticals,1,2 pheremones3,4
and perfumes.3,5 Enantiomerically pure 3-alkyl-3-phenyl
propanols are used as intermediates in the asymmetric
synthesis of antibacterial agents such as curcuphenol,
curcuphenone and curcuhydroquinone,6 in the synthesis
of bisabolene sesquiterpenes such as turmerone, a natural
cytotoxic agent from Curcuma longa1 and in the synthesis
of the synthetic fragrant florhydral.7 3-Alkyl-3-phenyl
propanols can be oxidized into the corresponding acids
or aldehydes, which are used in the synthesis of some
diazoketone derivatives, which in turn have been used in
Buchner cyclization reactions.8 We have previously
obtained, via lipase catalysis, pure enantiomers of b-alkyl
substituted primary alcohols with good results. However,
the enantioselectivity of lipase catalyzed resolutions of
primary alcohols are known to be considerably lower
compared to when secondary alcohols are used as sub-
strates.9,10 However, some successful examples of lipase
catalyzed resolutions of chiral racemic alcohols with an
alkyl group at the b-position are known from our group11
2. Results and discussion
Racemic alcohols 1 and 2 were prepared via LiAlH4 reduc-
tion of the corresponding acids 5 and 6, which were ob-
tained as described8 from a malonic ester reaction
sequence starting from 1-phenyl-ethanol or 1-phenyl-pro-
panol, respectively (see Scheme 1).
The c-substituted primary alcohols 3 and 4 were obtained
from cinnamic acid via conjugate addition19 of an iso-pro-
pyl or tert-butyl Grignard reagent followed by LiAlH4
reduction of the acids obtained 7 and 8, respectively (see
Scheme 2).
*
Corresponding author. E-mail: erik.hedenstrom@miun.se
0957-4166/$ - see front matter ꢂ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.07.002

S. Sabbani et al. / Tetrahedron: Asymmetry 18 (2007) 1712–1720
1713
R
R
O
R
a, b, c, d
e
OH
OH
OH
R = Me or
R = Et
5: R = Me
6: R = Et
1: R = Me
2: R = Et
Scheme 1. Synthesis of 3-pheny-l-butanol 1 and 3-phenyl-1-pentanol 2. Reagents and conditions: (a) PBr3, pyridine, Et2O; (b) CH2(CO2Et)2, Na(S)/EtOH;
(c) KOH/EtOH, reflux, then HCl; (d) reflux 180–190 ꢁC; (e) (i) LiAlH4/Et2O; (ii) H2O/H3O+.
O
R
O
R
b
a
OH
OH
OH
7: R = Pri
8: R = But
3: R = Pri
4: R = But
Scheme 2. Synthesis of 4-methyl-3-phenyl-1-pentanol 3 and 4,4-dimethyl-3-phenyl-1-pentanol 4. Reagents: (a) (i) RMgCl, THF or Et2O; (ii) H3O+/ice;
(b) (i) LiAlH4/Et2O; (ii) H2O/H3O+.
The procedure (see Scheme 3) for the enantioselective acyl-
ation of the c-substituted primary alcohols 1, 2, 3 or 4 was
started by the addition of an enzyme (stored over dry silica
gel at 4 ꢁC)20 to a stirred solution containing the primary
alcohol, a vinyl ester and an organic solvent all pre-equili-
ent sources were screened in the resolution of 4 at an initial
water activity21,27 of 0.32 with vinyl propionate as the acyl
donor and di-iso-propyl ether or n-hexane as the reaction
medium at room temperature. Most of the enzymes
showed low or very low enantioselectivity while the calcu-
lated enantiomeric ratio22 ranged from 1.1 to 9.3.
brated at a water activity (aw)21 of 0.32 or with 4 A mole-
˚
cular sieves added to obtain dry conditions. All reactions
were carried out under an argon atmosphere at a specific
temperature. In most cases, the reaction was interrupted
below 40% conversion22 when the enzyme was removed
by filtration. The remaining substrate and the product ester
were separated via liquid chromatography (LC) and after
LiAlH4 reduction to the corresponding alcohols, the enan-
tiomeric excesses (ees and eep, respectively) were deter-
mined on a b-Dex 120 chiral GC-column.
However, interestingly when using some enzymes, a prefer-
ence for the (R)-(+)-4 enantiomer was obtained (Lip-PS,
E-2, L-5, Lip-FAP 15, Lip-M, L-8, L-3, L-10, Lip-AK
and L-7 with E = 1.1–9.3) while others showed a preference
for the (S)-(ꢀ)-4 enantiomer (Lip-AS, Nov-435, L-9, Lip-
AL, Lip-TL and L-6 with E = 1.2–6.1). Thus, it is possible
to use two different enzymes to obtain both enantiomers as
the remaining substrate in good enantiomeric purity and in
a tolerable yield (see Table 2 for some of the results).
Chirazymeꢀ L-7 (entry 15, E = 9.3) and Chirazymeꢀ L-6
(entry 16, E = 6.1) with the highest enantioselectivity and
opposite enantiomer selectivity were chosen in further
studies together with Chirazymeꢀ L-10 (entry 14,
E = 3.4) aiming to increase the E-value by altering the reac-
tion conditions (see Table 2).
The absolute configurations of the enantiomers of alcohols
1, 2, 3 and 4 were confirmed by comparing measured spe-
cific rotation values to the ones previously reported (see
Table 1).
Substrate 4 was extensively studied; first we investigated
the enantioselectivity of different enzymes (see Section 3.1
for a list of enzymes tested) for this substrate in the acyl-
ation reaction. In total 15 lipases and 1 esterase from differ-
Different vinyl esters (vinyl acetate, vinyl propionate,
vinyl butyrate, vinyl 2,2-dimethyl-propionate and vinyl
R
R
b
OH
*
*
OCOR´
R
+
(S)-(+)-1 or (R)-(−)-1
(S)-(+)-2 or (R)-(−)-2
(R)-(+)-3 or (S)-(−)-3
(R)-(+)-4 or (S)-(−)-4
a
OH
R
*
OH
1: R = Me
2: R = Et
3: R = Pri c
4: R = But c
(R)-(−)-1 or (S)-(+)-1
(R)-(−)-2 or (S)-(+)-2
(S)-(−)-3 or (R)-(+)-3
(S)-(−)-4 or (R)-(+)-4
Scheme 3. Enantioselective lipase resolution of c-substituted primary alcohols of the type Ph–CH(R)–CH2CH2OH. Reagents: (a) lipase, vinyl ester,
organic solvent, initial aw = 0.32 or ꢁ0; (b) (i) LiAlH4/Et2O; (ii) H2O/H3O+.

1714
S. Sabbani et al. / Tetrahedron: Asymmetry 18 (2007) 1712–1720
Table 1. GC-retention time, sign of specific rotation and configuration for the enantiomers of alcohol 1, 2, 3 and 4
20
a
Substrate Temperature programme
Carrier gas: He pressure (split) t1
Specific rotation ½aꢂD
Configuration
a
t2
1
2
80 ꢁC (3 min), 170 ꢁC (0.3 ꢁC/min) 20 psi (6)
124.08 ꢀ10.8 (c 0.4, CDCl3)
(R)
(S)
25
lit.23 ½aꢂD ¼ ꢀ25:4 (neat)
125.41 +11.5 (c 0.4, CDCl3)
20
lit.24 ½aꢂD ¼ þ25:5 (c 1.52, CHCl3)
80 ꢁC (3 min), 170 ꢁC (0.5 ꢁC/min) 20 psi (6)
80 ꢁC (3 min), 170 ꢁC (0.5 ꢁC/min) 20 psi (6)
102.17 ꢀ2.4 (c 0.8, CDCl3)
(R)
(S)
20
lit.25 ½aꢂD ¼ ꢀ7:9 (c 4.79, CCl4)
103.75 +2.5 (c 0.8, CDCl3)
25
lit.23 ½aꢂD ¼ þ14:2 (neat)
3
4
155.81 ꢀ5.5 (c 4.0, CDCl3)
(S)
20
lit.25 ½aꢂD ¼ ꢀ10:3 (neat)
156.96 +7.3 (c 4.0, CDCl3)
(R)
(R)
100 ꢁC (3 min), 170 ꢁC (1 ꢁC/min)
20 psi (6)
56.69 +11.3 (c 4.0, benzene)
25
lit.26 ½aꢂD ¼ þ7:31 (benzene)
57.12 ꢀ11.0 (c 4.0, benzene)
(S)
a t1 and t2 are the retention times in minutes for the two enantiomers of compounds 1–4, respectively on GC-column b-dex 120.
Table 2. The enantiopreference and enantioselectivity of some lipases when different c-substituted alcohols 1–4 were stirred at different temperatures in di-
iso-propyl ether or chloroform with vinyl propionate, at an initial water activity of 0.32 and under argon atmosphere
Conversion ca (%) eesb (%) eep (%) Ea
Enantiopreference
of the lipase
b
Entry Substrate Lipase
Solvent
Reaction
rate (%/h)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21c
22d
1
1
1
2
2
2
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
L-6
L-7
L-10
L-6
L-7
L-10
L-6
L-7
Chloroform
1.86
1.35
22.1
1.25
1.68
7.75
2.04
2.50
19.3
0.24
5.70
69.0
8.73
1.75
1.29
0.81
0.41
1.20
0.08
0.39
6.70
1.38
26.0
32.5
44.2
30.0
38.7
46.5
49.0
37.5
33.7
13.0
17.0
69.0
26.2
42.1
29.8
38.3
29.3
35.0
12.4
19.0
32.0
33.0
7.0
5.6
20.0
11.0
44.0
27.0
30.0
47.0
19.0
25.0
77.0
12.0
22.7
1.6 (S)-(+)
Di-iso-propyl ether
Di-iso-propyl ether
Chloroform
Di-iso-propyl ether
Di-iso-propyl ether
Chloroform
1.3 (R)-(ꢀ)
3.6 (S)-(+)
1.9 (S)-(+)
2.2 (R)-(ꢀ)
4.1 (S)-(+)
1.7 (R)-(+)
1.9 (R)-(+)
11.3 (R)-(+)
1.3 (R)-(+)
1.6 (R)-(+)
1.0 (S)-(ꢀ)
2.1 (S)-(ꢀ)
3.4 (R)-(+)
9.3 (R)-(+)
6.1 (S)-(ꢀ)
8.2 (S)-(ꢀ)
9.2 (S)-(ꢀ)
11.6 (R)-(+)
11.4 (R)-(+)
6.1 (R)-(+)
7.3 (R)-(+)
35.0
12.0
19.0
40.9
18.4
15.0
39.2
1.8
Di-iso-propyl ether
Di-iso-propyl ether
L-10
Lipas-AS Di-iso-propyl ether
Lipas-AL Di-iso-propyl ether
N-435
L-9
L-10
L-7
L-6
L-6
L-6
L-7
L-7
4.7
2.0
Di-iso-propyl ether
Di-iso-propyl ether
Di-iso-propyl ether
Di-iso-propyl ether
Di-iso-propyl ether
Chloroform
0.90
10.7
31.6
31.5
38.0
29.9
39.0
11.7
19.0
30.0
34.0
30.2
43.4
74.2
61.3
72.0
72.0
82.2
81.0
64.0
68.0
Chloroform 38 ꢁC
Di-iso-propyl ether ꢀ25 ꢁC
Di-iso-propyl ether 0 ꢁC
Di-iso-propyl ether 0 ꢁC
Di-iso-propyl ether 0 ꢁC
L-7
L-7
a Calculated according to the equation c = ees/(ees + eep) and E = ln[1 ꢀ c(1 + eep)]/ln[1 ꢀ c(1 ꢀ eep)].22
b Measured by GC on a b-Dex 120 chiral column.
c
˚
Dry conditions when 266 mg of molecular sieves (4 A)/ml of substrate solution was added.
d
˚
Dry conditions when 10.7 mg of molecular sieves (4 A)/ml of substrate solution was added.
4-tert-butyl-benzoate) were tested as acyl donors28–30 in the
lipase catalyzed acylation of 4. Vinyl propionate gave
somewhat higher E-values together with these lipases and
was used as an acyl donor in further screening experiments
for a suitable reaction media. The two most sterically
demanding acyl donors tested (vinyl 2,2-dimethyl-propio-
nate and vinyl 4-tert-butyl-benzoate) did not react when
used along with Chirazymeꢀ L-7 while vinyl 4-tert-butyl-
benzoate did not react when used with Chirazymeꢀ L-6.
The enantioselectivity of Chirazymesꢀ L-6, L-7 and L-10
were more or less sensitive to changes of reaction
media;31,32 the most influenced was the enantioselectivity
of Chirazymeꢀ L-7. The lipases were tested in organic sol-
vents such as di-iso-propyl ether, tert-butyldimethyl ether,
dichloromethane, THF, n-hexane and chloroform; some
of the results are shown in Table 2. Two combinations of
a solvent and a lipase with the highest and reversed
enantioselectivity together with a useful reaction rate

S. Sabbani et al. / Tetrahedron: Asymmetry 18 (2007) 1712–1720
1715
(Chirazymeꢀ L-6/chloroform, entry 17 E = 8.2, and Chira-
zymeꢀ L-7/di-iso-propyl ether, entry 15 E = 9.3) were then
used to study the lipase catalyzed acylation of 4 at five
different temperatures33 to investigate the effect34 on the
E-value.
analyzing alcohol 4 the (+)-enantiomer is first eluting.
Obviously when a very space demanding and more electron
donating alkyl group, such as tert-butyl, is introduced in
the molecule, the chiral recognition of the lipases and the
chiral column changes. When using Chirazymeꢀ L-6, the
enantiomeric ratio increases (E = 1.6 for alcohol 1, entry
1 to E = 8.2 for alcohol 4, entry 17) when a tert-butyl
group is introduced as the c-substituent. The same observa-
tion is made when using Chirazymeꢀ L-7 (E = 1.3 for alco-
hol 1, entry 2 to E = 9.3 for alcohol 4, entry 15) and in both
cases the change from iso-propyl to tert-butyl seems to be
most important for the enantiopreference of the lipase.
Disappointingly, both Chirazymeꢀ L-6 (entry 7, E = 1.7)
and L-7 (entry 8, E = 1.9) show enantiopreference for the
(+)-enantiomer of alcohol 3 with low selectivity. Conse-
quently, we tested some other lipases to find one with the
reversed selectivity and we found that with both Nov-
ozym-435 (entry 12, E = 1.0) and Chirazymeꢀ L-9 (entry
13, E = 2.1) the (ꢀ)-enantiomer was the faster reacting
enantiomer of this alcohol. For substrate 1, the best results
(E = 3.6) were obtained with lipase Chirazymeꢀ L-10 (en-
try 3) which is somewhat higher than obtained by others
in a similar reaction.15 This lipase seems to show an enanti-
oselectivity of the same magnitude for three of the alcohols
tested 1, 2 (entry 6, E = 4.1) and 4, (entry 14, E = 3.4) inde-
pendent of the alkyl group at the stereogenic centre. Inter-
estingly, when Chirazymeꢀ L-10 was used with substrate 3,
an unexpectedly high E-value value of 11.3 (entry 9) was
obtained with the (+)-enantiomer reacting faster. An in-
crease in the enantiomeric ratios is envisaged for each of
the substrate alcohols 1–3 if a thorough investigation of
the reaction conditions for each of them is undertaken as
for alcohol 4.
Chirazymeꢀ L-6 in CHCl3 gave a higher E-value (entry 18,
E = 9.2) at the higher temperature (38 ꢁC) while Chira-
zymeꢀ L-7 in di-iso-propyl ether gave somewhat higher
enantioselectivity when lowering the temperature to
ꢀ25 ꢁC (entry 19, E = 11.6) (see Table 2 and Fig. 1 for
the results). The water activity is probably changed, at least
when the temperature is lowered far below 0 ꢁC, as the
water might freeze, which probably results in a lower water
activity than expected. Thus, we added molecular sieves
(4 A)35,36 to the reaction at 0 ꢁC to obtain aw ꢁ 0 and inter-
˚
estingly, we found that the enantiomeric ratio was lowered
from 11.4 (entry 20) to 7.3 (entry 22). It was also obvious
that E-value changed with the quantity of molecular sieves
added as addition of more molecular sieves gave an even
lower E-value (E = 6.1, entry 21). The rate of the enzy-
matic reaction in the presence of molecular sieves is higher
than the reactions without molecular sieves (see Table 2,
entry 20, 21 and 22). However, no product was observed
when running the reaction with molecular sieves and with-
out lipase; this might be explained by the fact that the
molecular sieves in conjunction with the lipase are able to
non-selectively catalyze the reaction.
Structurally different c-substituted alcohols 1–4 were tested
as substrates in order to investigate the substrate structure
relationship with the lipase active site (see Table 2). When
alcohols 1, 2 or 3 are used as substrates, the enantioprefer-
ence of Chirazymeꢀ L-6 is the (+)-enantiomer, but when
alcohol 4 is used, the enantiopreference switches to the
(ꢀ)-enantiomer. When using Chirazymeꢀ L-7 the enantio-
preference for alcohols 1 and 2 was the (ꢀ)-enantiomer but
we see a switch in selectivity for alcohols 3 and 4 as now the
(+)-enantiomer reacted faster. A switch is also noticed in
the elution order of the enantiomers from the chiral GC-
column (see Table 1). Thus, when alcohol 1, 2 or 3 are ana-
lyzed, the (ꢀ)-enantiomer is the first eluting one but when
We tried to extend the empirical rule proposed by Kazlaus-
kas et al. and others37 to our substrates 1–4 with c-alkyl
substituents and thus predict the faster reacting enantiomer
in the lipase catalyzed acylation. Our discussion is based on
the assumption that the tert-butyl group is a large group
with a slightly larger volume (calculated in chemprop from
Cambridgesoft) in substrate 4 and the phenyl group is the
large group in substrates 1, 2 and 3. Based on the above
Effect of temperature on E-value
Effect of temperature on reaction rate
Chirazyme® L-7
Chirazyme® L-7
14
12
10
8
Chirazyme® L-6
Chirazyme® L-6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
6
4
2
0
-25
0
rt
38
50
-25
0
rt
38
50
Temperature (ºC)
Temperature (ºC)
Figure 1. The effect of temperature on the E-value and reaction rate when using Chirazymeꢀ L-6/chloroform/vinyl propionate/and Chirazymeꢀ L-7/di-
iso-propyl ether/vinyl propionate at different temperatures in the acylation of substrate 4.

1716
S. Sabbani et al. / Tetrahedron: Asymmetry 18 (2007) 1712–1720
assumptions, we can conclude that if the (+)-enantiomer of
1 is the faster reacting enantiomer for a particular lipase,
then (+)-2, (+)-3 and (ꢀ)-4 should be the faster reacting
enantiomers with the same lipase if the empirical rule is
true for our substrates. Chirazymeꢀ L-6, L-7 and L-10
are lipases, which we tested on substrates 1, 2, 3 and 4.
Our results show that the empirical rule is true only for
the lipase from Pseudomonas species (Chirazymeꢀ L-6).
and 8 as described for other methyl-branched acids
(Scheme 4).38,39 Substrate acids 5 and 6 were esterified
with a modest to slow rate and with a low preference
for the (S)-enantiomer resulting in very low E-values
(E < 2). Although hydrolysis of the methyl ester of 5 with
high E-values has been reported40 no successful resolution
of substrate acid 5 was possible either with immobilized
CRL, Amano PS or CALB (Chirazymeꢀ L-2). No ester
was observed under our conditions for acids 7 and 8, even
when running the reaction for several days.
We have previously successfully resolved several alkanoic
acids, with remotely located methyl-branching, using Can-
dida rugosa lipase as the enantioselective catalyst in esteri-
fication reactions.38,39 Thus, we attempted to esterify
enantioselectively the four racemic substrate acids 5, 6, 7
Finally, in a sequential resolution of ( )-4,4-dimethyl-
3-phenyl-1-pentanol
4 (see Scheme 5), two lipases
(Chirazymeꢀ L-6 and Chirazymeꢀ L-7) with opposite
R
O
R
O
R
O
a
OH
OR´
OH
(S)
(R)
+
5: R = Me
6: R = Et
71: R = Pri
81: R = But
: no reaction
: no reaction
Scheme 4. Enantioselective enzymatic resolution of c-substituted alkanoic acids of the type Ph–CH(R)–CH2COOH. Reagents: (a) lipase, 1-decanol,
isooctane, initial aw = 0.32.
OH
4
a
52% conversion
LC-separation
OH
OCOEt
d
(S)-(−)-4 (66% ee)
(R)-(+)-4 (60% ee)
b
52% conversion
c, d 52% conversion
LC-separation
LC-separation
ester of (R)-(+)-4
(S)-ester of 4
OH
OH
(S)-(−)-4 (22% yield and >98%
ee when aw = 0)
(R)-(+)-4 (25% yield
and >98% ee)
Scheme 5. Lipase catalyzed resolution strategy to obtain >98% ee of both enantiomers of the gamma-substituted primary alcohol ( )-4,4-dimethyl-3-
phenyl-1-pentanol 4. Reagents and conditions: (a) L-7, vinyl propionate, di-iso-propyl ether, initial aw = 0.32, 0 ꢁC; (b) L-7, vinyl propionate, di-iso-propyl
ether, initial aw = 0.32 or aw = 0, 0 ꢁC; (c) L-6, vinyl propionate, chloroform, initial aw = 0.32, 38 ꢁC; (d) (i) LiAlH4/Et2O; (ii) H2O/H3O+.

S. Sabbani et al. / Tetrahedron: Asymmetry 18 (2007) 1712–1720
1717
enantioselectivity were chosen to obtain both the enantio-
mers (S)-(ꢀ)-4 and (R)-(+)-4 enantiomerically pure. Thus,
Chirazymeꢀ L-6/chloroform/vinyl propionate/38 ꢁC/aw =
0.32 and Chirazymeꢀ L-7/di-iso-propyl ether/vinyl propio-
nate/0 ꢁC/aw = 0.32 combinations gave the best E-values at
reasonable reaction rates and were used with an aim to
obtain both (S)-(ꢀ)-4 and (R)-(+)-4 with ee >98%. Conse-
quently, racemic substrate 4 was subjected to lipase cata-
lyzed acylation at an initial water activity of 0.32 in the
presence of vinyl propionate, di-iso-propyl ether and
Chirazymeꢀ L-7 at 0 ꢁC. The reaction was stopped at
52% conversion (see Scheme 5). The product ester of (R)-
(+)-4 (60% ee) was reduced, after LC separation of the
remaining (S)-(ꢀ)-4, with LiAlH4 and the resulting alcohol
(R)-(+)-4 (60% ee) was subjected to Chirazymeꢀ L-6, vinyl
propionate and chloroform at 38 ꢁC. Thus, we obtained
(R)-(+)-4 with >98% ee and in a yield of 25% from the
racemate. The enantiomerically enriched substrate from
above (S)-(ꢀ)-4 (66% ee) was subjected further to lipase
catalysis with Chirazymeꢀ L-7, vinyl propionate, di-iso-
propyl ether at 0 ꢁC, which gave (S)-(ꢀ)-4 with >90% ee.
It was not possible to obtain (S)-(ꢀ)-4 with an ee greater
than 90%, probably due to hydrolysis of the product ester
of (R)-(+)-4 back to substrate (R)-(+)-4 and thereby lower-
ing the ee of remaining substrate (S)-(ꢀ)-4.41 This problem
is more pronounced in polar solvents than in non-polar as
the water content at the same water activity is higher.27 To
obtain (S)-(ꢀ)-4 in high enantiomerically pure form, the
120, 30 m · 0.25 mm · 0.25 lm; He, 20 Psi). Chirazymeꢀ
E-2 (Hogliver esterase), Chirazymeꢀ L-3 (Candida rugosa
Lipase), Chirazymeꢀ L-5 (Candida antarctica lipase A),
Chirazymeꢀ L-6 (Lipase from Pseudomonas species),
Chirazymeꢀ L-7 (Porcine pancreatic lipase), Chirazymeꢀ
L-8 (Thermomyces lanuginosa lipase), Chirazymeꢀ L-9
(Mucor miehei lipase) and Chirazymeꢀ L-10 (Lipase from
Alcaligenes species) were bought from Boehringer Mann-
heim GmbH, Germany but these Chiralzymes are no long-
er commercially availably from the Roche company.
Lipase AS (Aspergillus niger), Lipase M 10 (Mucor javani-
cus), Lipase FAP 15 (Rhizopus oryzae), Lipase AK 20
(Pseudomonas fluorescens) and Lipase PS (Burkholderia
cepacia) were obtained from Amano Enzyme Inc., Nagoya,
Japan. Novozym 435 (Candida antarctica lipase B) was
bought from Novo Nordisk (now Novozymes), Denmark.
Lipase AL (Achromobacter sp.) and Lipase TL (Pseudomo-
nas stutzeri) were obtained from Meito Sangyo co., Ltd,
Tokyo, Japan.
3.2. ( )-3-Phenylbutanoic acid 5 and ( )-3-phenylpentanoic
acid 6
The two title acids were prepared according to the malonic
ester procedure8 described for ( )-3-phenylpentanoic acid
6.
3.2.1. ( )-3-Phenylbutanoic acid 5. Acid 5 (7.4 g, 88%
yield) was isolated as a white solid: mp 36–38 ꢁC (lit.42
35–38 ꢁC); MS (EI): m/z: 164 (18) (M+), 147 (3), 131 (2),
119 (5), 118 (29), 105 (100), 104 (11), 91 (10); IR (KBr)
2800–3100, 1707, 1494, 1431, 1320, 1280, 1217, 932, 758,
˚
reaction was repeated in the presence of 4 A molecular
sieves to avoid the presumed hydrolysis of the produced es-
ter of (R)-(+)-4 (see Scheme 5). With this method, we suc-
cessfully obtained (S)-(ꢀ)-4 with an ee >98% although with
a lower E-value in presence of molecular sieves vide supra
(see Table 2, entries 21, 22 and 20).
700 cmꢀ1 1H NMR (CDCl3) d 1.31 (3H, d, J = 7.0),
;
2.54–2.58 (1H, d of d, J = 6.8 and J = 15.5), 2.64–2.68
(1H, d of d, J = 8.3 and J = 15.5), 3.20–3.30 (1H, apparent
sextet J = 7), 7.18–7.22 (5H, m), 11.4–10.8 (1H, br s); 13C
NMR (CDCl3) d 21.9, 36.18, 42.67, 126.57, 126.76,
128.63, 145.48, 179.06. IR data were similar to published
data.42
3. Experimental
3.1. General
The glassware was dried overnight at 150 ꢁC and the reac-
tions were performed under an argon atmosphere. Diethyl
ether used in the chemical syntheses was obtained in a dry
form by distillation using LiAlH4 as a drying agent. Thin
layer chromatography (TLC) was carried out on pre-
coated silica gel plates (Merck 60 F254). Preparative liquid
chromatography (LC) was performed on silica gel (Fluka
Silica Gel 60, 0.040–0.063 mm, 230–400 mesh ASTM)
employing a gradient of pentane/diethyl ether as eluant
(0–100%). Mass spectra were recorded on a Saturn 2000
instrument, operating in the EI mode, coupled to a Varian
3800 GC instrument. NMR spectra were recorded on a
3.2.2. ( )-3-Phenylpentanoic acid 6. Acid 6 (1.6 g, 74%
yield) was isolated as a white solid: mp 48–50 ꢁC (lit.8
47–49 ꢁC); MS (EI): m/z: 178 (20) (M+), 161 (17), 149
(9), 133 (4), 132 (16), 119 (48), 118 (84), 107 (100), 91
(87); IR (KBr) 2500–3100, 1696, 1454, 1405, 1279, 1178,
950, 757, 696; 1H NMR and 13C NMR spectra were similar
to published data.8
3.3. ( )-4-Methyl-3-phenylpentanoic acid 7 and ( )-4,4-
dimethyl-3-phenylpentanoic acid 8
The title acids were prepared from cinnamic acid as de-
1
Bruker DMX 250 (250 MHz H and 62.9 MHz 13C) or a
scribed for acid 7.19
Bruker Digital 500 (500 MHz 1H and 125.8 MHz 13C)
spectrophotometer using CDCl3 as solvent and TMS as
internal reference. Optical rotations were determined using
a Perkin–Elmer 241 and Perkin–Elmer 341 polarimeters
using a 1 dm or a 0.1 dm cell. Gas chromatography on a
Varian 3400 CX was used to monitor the progress of enzy-
matic reactions (Alltech column EC1, 30 m · 0.32 mm; N2,
5.0 bar). Enantiomeric excesses of compounds 1–4 were
determined using GC Varian 3400 (Supelco column b-dex
3.3.1. ( )-4-Methyl-3-phenylpentanoic acid 7. Acid 7
(2.3 g, 75% yield) was isolated as a white solid: mp 44–
46 ꢁC (lit.8 46–48 ꢁC); MS (EI): m/z: 192 (10) (M+), 177
(6), 149 (11), 133 (12), 132 (57), 117 (10), 115 (10), 107
(100), 104 (69), 91 (41); IR (KBr) 3050–2500, 1696, 1495,
1417, 1386, 1295, 950, 780, 748, 703; H NMR (CDCl3) d
0.74–0.75 (3H, d, J = 6.7), 0.92-0.93 (3H, d, J = 6.7),
1.80–1.89 (1H, m), 2.56–2.61 (1H, q, J = 15.6), 2.76–2.80
1

1718
S. Sabbani et al. / Tetrahedron: Asymmetry 18 (2007) 1712–1720
(1H, q, J = 15.6), 2.84–2.88 (1H, m), 7.10–7.30 (5H, m),
9.5–10.5 (1H, br s); 13C NMR (CDCl3) d 20.16, 20.54,
33.08, 38.11, 48.39, 126.41, 128.11, 128.19, 142.53,
3.5. General procedure for obtaining pre-equilibrated water
activity
13
179.04. C NMR spectra were similar to published data.8
One vial containing a solution of 0.26 mmol of the sub-
strate, a c-substituted alcohol 1, 2, 3 or 4 in organic solvent
(1.0 ml), one vial containing the vinyl ester and one vial
containing the organic solvent were equilibrated in a desic-
cator containing a saturated aqueous solution of MgCl2Æ
6H2O (aw = 0.32)41 for 16 h. The enzymes were stored over
dry silica gel at 4 ꢁC and used directly in the enzyme catal-
ysis without any further treatment.
3.3.2. ( )-4,4-Dimethyl-3-phenylpentanoic acid 8. Acid 8
(3.0 g, 55% yield) was isolated as a white solid: mp 104–
105 ꢁC (lit.43 114–116 ꢁC); MS (EI): m/z: 206 (2) (M+),
189 (11), 173 (5), 150 (96), 149 (11), 132 (4), 131 (13), 104
(100), 91 (33), 90 (2); IR (KBr) 3050–2500, 1725, 1635,
1477, 1453, 1269, 1166, 1085, 893, 744, 707; 1H NMR
(CDCl3) d 0.86 (9H, s), 2.68–2.81 (2H, m), 2.91–2.94 (1H,
q, J = 4.4), 7.12–7.25 (5H, m, ArH) 13C NMR spectra were
similar to published data.8
3.6. General procedure for enzyme catalyzed acylation of
the c-substituted alcohols 1, 2, 3 or 4
3.4. ( )-3-Phenyl-1-butanol 1, ( )-3-phenyl-1-pentanol 2,
( )-4-methyl-3-phenyl-1-pentanol 3 and ( )-4,4-dimethyl-3-
phenyl-1-pentanol 4
After the pre-equilibration step as above vinyl ester
(1.0 mmol) was added to the vial containing the substrate
solution and the volume was made up to 1.0 ml by adding
some of the pre-equilibrated organic solvent. Alternatively,
when dry conditions were desired and molecular sieves
The title alcohols were all obtained via LiAlH4 reduction of
the corresponding acid using a published method described
for other types of acids.44
˚
(4 A) were used, the above equilibration step was excluded.
The acylation reaction was started by the addition of 3–
5 mg of enzyme. The reaction was carried out under an ar-
gon atmosphere. After stirring for the appropriate time at a
specific temperature, the reaction was stopped by removing
the enzyme by filtration. The solvent from the filtrate was
evaporated under reduced pressure and the residue was
chromatographed on silica gel (using a gradient of Et2O
in n-pentane as eluant) to separate the unreacted substrate
from the product ester. The product ester obtained was re-
duced44 back to alcohol with LiAlH4 and then the enantio-
meric excesses (ees and eep) were determined using GC
Varian 3400 (column b-dex 120, 30 m · 0.25 mm ·
0.25 lm; N2, 5 Psi; He, 20 Psi). For retention times of sub-
strates 1, 2, 3 and 4 see Table 1.
3.4.1. ( )-3-Phenyl-1-butanol 1. Alcohol 1 (6.5 g, 96%
yield) was obtained as a colourless oil: bp 85 ꢁC/2.0 mbar
(lit.45 117 ꢁC/8 mmHg); MS (EI): m/z: 150 (2) (M+), 133
(6), 132 (36), 118 (8), 117 (72), 105 (100), 104 (12), 91
(40); IR (neat) 3333, 3027, 2959, 2929, 1063, 1494, 1452,
1
1047, 762, 700; H NMR and 13C NMR spectra were sim-
ilar to published data for (3R)-(ꢀ)-3-phenyl-1-butanol.25
3.4.2. ( )-3-Phenyl-1-pentanol 2. Alcohol 2 (1.4 g, 93%
yield) was obtained as a colourless oil: bp 130 ꢁC/5 mbar
(lit.45 108 ꢁC/1 mmHg); MS (EI): m/z: 164 (5) (M+), 147
(37), 146 (93), 131 (6), 119 (22), 117 (100), 105 (87), 104
(15), 91 (28); IR, 1H NMR and 13C NMR data were similar
to published data for the pure enantiomers.25,46
3.6.1. (R)-(ꢀ)- and (S)-(+)-3-phenyl-1-butanol (R)-(ꢀ)-1 and
(S)-(+)-1. ( )-3-Phenyl-1-butanol 1 was acylated follow-
ing the general procedure above using Chirazymeꢀ L-10
in di-iso-propyl ether. The reaction was stopped after 4 h
at 49% conversion to give the remaining substrate (R)-
(ꢀ)-3-phenyl-1-butanol in a yield of 51% and in 43% ee,
3.4.3. ( )-4-Methyl-3-phenyl-1-pentanol 3. Alcohol
3
(2.2 g, 96% yield) was obtained as a colourless oil: bp
170 ꢁC/4 mbar (lit.25 125 ꢁC/0.2 Torr); MS (EI): m/z 178
(2) (M+), 161 (28), 160 (76), 145 (6), 133 (12), 118 ( (13),
117 (57), 105 (100), 104 (9), 91 (4); IR (neat) 3358, 2956,
20
25
½aꢂD ¼ ꢀ10:75 (c 0.4, CDCl3) {lit.23 ½aꢂD ¼ ꢀ25:4 (neat)}
and (S)-(+)-3-phenyl-1-butanol (obtained after reduction
from the produced ester) in a yield of 49% and in 45%
1
1734, 1493, 1453, 1385, 1113, 1045, 743, 701; H NMR
(CDCl3) d 0.73 (3H, d, J = 6.7), 0.97 (3H, d, J = 6.7),
1.2–1.3 [1H, br s, disappears on shaking with D2O), 1.77–
1.86 (2H, m), 2.04–2.11 (1H, m), 2.38–2.42 (1H, m),
3.36–3.50 (2H, m), 7.12–7.29 (5H, m); 13C NMR data
was similar to published data for (3S)-(ꢀ)-4-methyl-3-
phenyl-1-pentanol.47
20
20
ee, ½aꢂD ¼ þ11:5 (c 0.4, CDCl3) {lit.24 ½aꢂD ¼ þ25:5 (c
1.52, CHCl3)}. Analytical data were identical with the data
for the racemic compound 1 above.
3.6.2. (R)-(ꢀ)- and (S)-(+)-3-Phenyl-1-pentanol (R)-(ꢀ)-2
and (S)-(+)-2. ( )-3-Phenyl-1-pentanol 2 was acylated
following the general procedure above using Chirazymeꢀ
L-10 in di-iso-propyl ether. The reaction was stopped after
5 h at 46.5% conversion to give (R)-(ꢀ)-3-phenyl-1-penta-
nol as remaining substrate in a yield of 54% and in 41%
3.4.4. ( )-4,4-Dimethyl-3-phenyl-1-pentanol 4. Alcohol 448
(2.7 g, 100% yield) was obtained as a white solid: mp 48–
50 ꢁC; MS (EI): m/z 192 (2) (M+), 159 (7), 136 (11), 135
(5), 118 (100), 117 (73), 105 (63), 104 (15), 91 (43); IR
(KBr) 3319, 2951, 1478, 1451, 1231, 1051, 1022, 788, 722,
20
20
ee, ½aꢂD ¼ ꢀ2:4 (c 0.8, CDCl3) {lit.25 ½aꢂD ¼ ꢀ7:9 (c 4.79,
CCl4)} and (S)-(+)-3-phenyl-1-pentanol (obtained after
reduction of the produced ester) in a yield of 47% and in
1
701; H NMR (CDCl3) d 0.88 (9H, s), 1.55 (1H, s), 1.89–
2.08 (2H, m), 2.44–2.47 (1H, d of d, J = 12.2 and
J = 3.0), 3.29–3.45 (2H, m), 7.14–7.27 (5H, m); 13C NMR
(CDCl3) d 28.19, 32.47, 33.67, 52.87, 62.09, 126.16,
127.75, 129.44, 142.33.
25
25
47% ee, ½aꢂD ¼ þ2:5 (c 0.8, CDCl3) {lit.23 ½aꢂD ¼ þ14:2
(neat)}. Analytical data were identical with the data for
the racemic compound 2 above.

S. Sabbani et al. / Tetrahedron: Asymmetry 18 (2007) 1712–1720
1719
3.6.3. (R)-(+)- and (S)-(ꢀ)-4-methyl-3-phenyl-1-pentanol
(R)-(+)-3 and (S)-(ꢀ)-3. ( )-4-Methyl-3-phenyl-1-penta-
nol 3 was acylated following the general procedure as
described above using Chirazymeꢀ L-10 in di-iso-propyl
ether. The reaction was stopped after 1.75 h at 33.7% con-
version to give (S)-(ꢀ)-4-methyl-3-phenyl-1-pentanol as the
remaining substrate in a yield of 66.3% and in 39.2% ee,
the enantiomeric excess of both product and substrate alco-
hols were determined on a b-dex 120 chiral GC-column.
Acknowledgements
We thank Amano Enzyme Inc., Nagoya, Japan for the
kind gift of the lipases AS, Lipase M 10, Lipase FAP 15,
Lipase AK 20 and Lipase PS and we also thank Meito San-
gyo co., Ltd, Tokyo, Japan for the kind gift of the lipases
Lipase AL and Lipase TL. Financial support from Vin-
nova is gratefully acknowledged. We also thank Professor
Anita Maguire and Dr. Francis Harrington for the synthe-
sis of the first batch of ( )-4,4-dimethyl-3-phenylpentanoic
acid.
20
20
½aꢂD ¼ ꢀ5:5 (c 4.0, CDCl3) {lit.25 ½aꢂD ¼ ꢀ10:30 (neat)}
and (R)-(+)-4-methyl-3-phenyl-1-pentanol (obtained after
reduction of the ester produced) in a yield of 33.7% and
25
in 77% ee, ½aꢂD ¼ þ7:25 (c 4.0, CDCl3). Analytical data
were identical with the data for the racemic compound 3
above.
3.6.4. (R)-(+)-4,4-Dimethyl-3-phenyl-1-pentanol (R)-(+)-
4. ( )-4,4-Dimethyl-3-phenyl-1-pentanol 4 was first acyl-
ated following the general procedure above using Chira-
zymeꢀ L-7 in di-iso-propyl ether. The reaction was kept
under continuous stirring for 12 days and the reaction then
stopped at 52% conversion, by filtering off the enzyme. The
filtrate was evaporated and the residue was chromato-
graphed (using a gradient of Et2O in pentane as eluant)
to separate the unreacted substrate (S)-(ꢀ)-4 from the
product ester of (R)-(+)-4. The product ester of (R)-(+)-4
(60% ee) was reduced with LiAlH4 to obtain the enantio-
merically enriched alcohol (R)-(+)-4 (60% ee) and this alco-
hol was subjected to lipase catalyzed acylation as described
above but with Chirazymeꢀ L-6 in chloroform. The reac-
tion was stopped after 50 h at a conversion of 52%. After
general work up as above (R)-(+)-4, 4-dimethyl-3-phenyl-
pentan-1-ol (R)-(+)-4 was obtained in an enantiomeric
excess of >98% and in 25% total yield calculated from
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