Ionic acylating agents for the enzymatic resolution of sec-alcohols in ionic liquids

Article abstract of DOI:10.1002/ejoc.201000640

Potential acylating agents containing pendant ionic groups have been screened for the enzymatic kinetic resolution of rac-secondary alcohols in ionic liquids with CAL-B as biocatalyst. This study has allowed the identification of the 1-methyl-3-alkylimidazolium cation attached to a carboxylate group through a C₁₀-alkyl chain as an efficient acylating agent for this transformation. This strategy was applied to the resolution of 2-hydroxycyclohexanecarbonitrile in which the 1R,2S enantiomer was isolated in 35 % ee (73 % yield) and the 1S,2R enantiomer in 97 % ee (23 % yield). Potential acylating agents containing pendant ionic groups have been screened for the enzymatic kinetic resolution of rac-secondary alcohols in ionic liquids with CAL-B as biocatalyst. This study has allowed theidentification of the 1-methyl-3-alkylimidazolium cation attached to a carboxylate group through a C₁₀-alkyl chain as an efficient acylating agent for this transformation. Copyright

Full text of DOI:10.1002/ejoc.201000640

FULL PAPER
DOI: 10.1002/ejoc.201000640
Ionic Acylating Agents for the Enzymatic Resolution of sec-Alcohols in Ionic
Liquids
Nuno M. T. Lourenço,*[a] Carlos M. Monteiro,[b] and Carlos A. M. Afonso*[b,c]
Keywords: Kinetic resolution / Ionic liquids / Enzymes / Acylation
Potential acylating agents containing pendant ionic groups
have been screened for the enzymatic kinetic resolution of
rac-secondary alcohols in ionic liquids with CAL-B as bio-
catalyst. This study has allowed the identification of the 1-
methyl-3-alkylimidazolium cation attached to a carboxylate
group through a C10-alkyl chain as an efficient acylating
agent for this transformation. This strategy was applied to
the resolution of 2-hydroxycyclohexanecarbonitrile in which
the 1R,2S enantiomer was isolated in 35% ee (73% yield)
and the 1S,2R enantiomer in 97% ee (23% yield).
or other side-products (depending of the vinyl ester) that
can cause biocatalyst inhibition.[6] The design of new acyl-
ating agents for enzymatic resolution has been an important
topic in this field of research. To overcome this problem
as well as to circumvent difficulties of separation, several
different acyl groups have been reported, namely succinic
anhydride[7] and carbonates.[8] A different approach in-
volves the resolution by simple esterification using free a
carboxylic acid and the continuous removal of water under
vacuum[9] or the use of a dehydrating agent such as molecu-
lar sieves.[10]
The use of vacuum to remove water is restricted to low
volatile substrates and acylating agents such as fatty acids
and is not compatible with common volatile organic sol-
vents in which the biocatalyst is very efficient. Another pos-
sible approach is to perform the enzymatic reaction in the
absence of volatile solvents, which is feasible only for cases
in which the alcohol/acid/ester mixture is liquid under the
operating conditions.[9]
In recent years the use of ionic liquids (ILs) as solvents
for EKR has attracted considerable attention, mainly due
to their peculiar properties such as solubility, almost negli-
gible vapour pressure,[11] increase in biocatalyst activity,
chemical stability[12] and possible combination with a
greener process involving scCO2 extraction and membrane
technology.[13] Many examples of the application of ILs in
biocatalysis resolution have been reported,[14] including hy-
drolysis,[15] esterification[16] and aminolysis.[17]
Introduction
The enzymatic kinetic resolution (EKR) of racemic
alcohols, namely by lipases, is a well-established methodol-
ogy for the preparation of enantiomerically enriched pre-
cursors.[1] Lipases also catalyse chemo-, regio- and stereose-
lective processes such as the EKR of acids, esters or
amides,[2] as well as the desymmetrization of prochiral or
meso compounds.[3] Like other biocatalysts, lipases affect
the rates of reversible reactions in which the equilibrium
reached is determined by thermodynamics. The equilibrium
can be shifted by controlling the concentration of reactants
or products, for example, by controlling the concentration
of water in the reaction media it is possible to perform ester
hydrolysis or synthesis. In cases in which the equilibrium is
not controlled, low yields and the erosion of enantio-
selectivity can be observed. This fact has clearly been illus-
trated for the kinetic resolution of menthol.[4] Several strate-
gies can be applied to shift the equilibrium. For this, vinyl
esters are the most common and efficient acylating agents
used to achieve irreversible transesterifications.[5] However,
there are some limitations associated with the use of this
type of acyl group relating to the formation of acetaldehyde
[a] IBB Institute for Biotechnology and Bioengineering, Centre for
Biological and Chemical Engineering, Instituto Superior
Técnico,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Fax: +351-21-841-9062
E-mail: nmtl@ist.utl.pt
[b] Centro de Química-Física Molecular and Institute of
Nanoscience and Nanotechnology, Instituto Superior Técnico,
1049-001 Lisboa, Portugal
Task-specific ionic liquids (TSILs) containing a hydroxy
group attached to an imidazolium cation have been re-
ported as efficient systems for the resolution of acids by the
enzymatic hydrolysis of the presynthesized ester.[18] The use
of ILs as reaction media is very convenient for the EKR of
free carboxylic acid esterification or transesterification in
which water or low-molecular-weight alcohols can be re-
moved under vacuum due to the non-volatility of the IL
solvent under the operating conditions.
Fax: +351-21-846-4455
E-mail: carlosafonso@ist.utl
[c] iMed. UL, Faculdadade de Farmácia da Universidade
de Lisboa,
Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal
Fax: +351-21-7946476
E-mail: carlosafonso@ff.ul.pt
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000640.
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Enzymatic Resolution of sec-Alcohols in Ionic Liquids
Scheme 1. Enzymatic esterification or transesterification for the resolution of rac-sec-alcohols by ionic an acylating agent in ionic liquid.
Furthermore, the possibility of identifying a new acyl- served (entry 1). These results are also in line with those
ating agent containing one carboxy group (acid or ester) obtained with the ionic acylating agents 8–11 with which
and a pendant stable cation or anion will facilitate product no reaction occurred with rac-1-phenylethanol in acetoni-
separation due to the preferential partition of the formed trile in the presence of CAL-B (Scheme 2). In addition, af-
optically enriched ester to the ionic liquid phase ter 24 h of reaction, destruction of the enzyme support was
(Scheme 1). To investigate this possibility, we performed a observed. In contrast, no destruction of the enzyme support
comprehensive study of EKR in ionic liquids by testing dif- was observed when the reaction was performed with
ferent potential acylating agents.
2,2,2-trifluoroethyl 4-bromobutanoate (7) and high ee val-
ues of the remaining substrate and product were obtained
(Scheme 2).
Results and Discussion
To overcome the lack of reactivity, which could be attrib-
uted to enzyme inhibition of the trimethylalkylammonium
group due to its close proximity to the carboxy reaction
centre, we turned our attention to the use of a possible acyl-
ating agent with a long alkyl chain (C11). Their preparation
started with different ammonium salts, however, our
attempts to prepare and purify these compounds with ac-
ceptable purity and yields failed.
At the same time, a different approach was tested with
the aim of replacing succinic anhydride by other cyclic an-
hydrides, such as compounds 12 and 14, or sulfonate com-
pounds 13 and 15. However, when the EKR was performed
under the same conditions, no ee was observed for the unre-
acted alcohol and in some cases (12 and 14) the alcohol
substrate 1 was not even detected. These negative results
may be due to the acidity of the intermediate species that
is formed when the ring is opened and to the proximity of
ionic part of the acylating agent to the reactive carboxy
group. After these unsuccessful experiments, we decided to
study other possible acylating agents based on the imid-
azolium cation that contain a longer alkyl chain. This type
of acylating agent can be obtained in good yields and puri-
ties simply by nucleophilic substitution of ethyl 11-bromo-
undecanate by methylimidazole (Scheme 3).
Following the strategy previously reported in the litera-
ture of using succinic anhydride as an acylating agent for
the resolution of sec-alcohols,[7] we envisaged an approach
in which both enantiomers could be resolved and separated
only by enzymatic resolution. However, when the enzymatic
resolution of rac-1-phenylethanol was performed in the
presence of CAL-B in 1-butyl-3-methylimidazolium hexa-
fluorophosphate [bmim][PF6] the formation of secondary
products was observed, namely succinic diesters.[19] Several
different conditions were tested to minimize the formation
of secondary products, but with low success. This observa-
tion prompted us to perform a more detailed study for
other substrates, specifically 2-octanol, for which a remark-
able solvent effect on the outcome of the esterification was
observed.[20]
After this first approach, we turned our attention to the
design of new efficient acylating agents specifically com-
posed of two different groups, a carboxy group (ester) and
a non-labile group at which no secondary reactions could
occur.[21] In line with the previous study and to achieve an
easy and fast screening of potential acylating agents, the
occurrence of any EKR was monitored simply by measur-
ing the enantiomeric excess (ee) of the extracted unreacted
alcohol (S)-1 (Table 1). Our investigations started with two
Several imidazolium acylating agents containing an acid
–
different acylating agents, carboxyalkyltrimethylammonium or ester group based upon four different anions [Br–, PF6 ,
–
–
chlorides 2 and 3 (entry 1), although in neither case was an BF4 , N(CN)2 ] were tested under different conditions.
ee observed. We considered that these poor results could be Even with the knowledge that some of these anions form
related to the well-known negative effect of the chlorine strong hydrogen bonds and present high nucleophilicity
atom on enzyme deactivation.[16,22] To circumvent this pos- consequently reducing the enzyme’s half-life[23] probably by
sibility we decided to use a zwitterionic compound such as changing the enzymeЈs conformation by interacting with
4 and to perform an ionic exchange of the chlorine atom the positively charged sites in the enzyme structure,[24] we
by another enzymatic-friendly ion, namely tetrafluorobor- decided to use these anions to prepare acylating agents.
ate (compounds 5 and 6). However, again, no ee was ob- This was because we have noticed that the negative effect
Eur. J. Org. Chem. 2010, 6938–6943
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6939

N. M. T. Lourenço, C. M. Monteiro, C. A. M. Afonso
FULL PAPER
Table 1. Screening of different potential ionic acylating agents for the EKR of rac-1-phenylethanol with CAL-B in the ionic liquid by
measuring the ee of the free alcohol.[a]
Entry
Acylating agent
Ionic liquid
Vacuum (yes/no)
Time [d]
ee[b] (S)-1 [%]
ee[c] (R)-1 [%]
1
2
3
4
5
6
7
8
2–6
12–15
17
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
–
[bmim][BF4]
[bmim][PF6]
[bmim][BF4]
[bmim][BF4]
[bmim][PF6]
[bmim][PF6]
[bmim][PF6]
[bmim][BF4]
–
no
no
7
7
7
4
7
7
7
4
0
0
10
5
17
77
37
28
77
53
32
22
46
60
–
–
yes[d]
yes[d]
yes[d]
yes[d]
yes[d]
no
97 (24 h)
18
–
17
–
91 (48 h)
72 (48 h)
–
18
18
19
9
20
yes[d]
no
2
4
7
–
–
10
11
12
13
14
21
22
yes[d]
yes[d]
no
93 (48 h)
99 (48 h)
94[e]
22
7
23
30 min
60 min
23
[bmim][PF6]
no
71[f]
[a] All reactions were carried out in 0.25mL of [bmim][PF6] with 0.20 mmol of alcohol, 0.20 mmol of acylating agent and 10 mg of CAL-
B. [b] Determined by HPLC. [c] Determined by HPLC after hydrolysis. [d] 100 Torr. [e] Calculated based on ees and a conversion of
33%. [f] Calculated based on ee values and a conversion of 46%.
Scheme 3. Preparation of acylating agents based on the imid-
Scheme 2. Enzymatic resolution of rac-1-phenylethanol using acyl-
azolium cation. Reagents and conditions: (i) EtOH, H2SO4, tolu-
ene, reflux, Dean–Stark apparatus, 90%; (ii) methyl imidazole,
ating agents based on 4-substituted 2,2,2-trifluoroethyl butanoates.
iPr2O, 70 °C, 98%; (iii) (MX), CH2Cl2 48 h, 87–99%; (iv) NaOH,
H2O, room temp., 83–86%.
of the more denatured anions is not always observed as in
our previous work in which it was possible to resolve the
precursor of indinavir in the presence of [Aliquat]-
The reaction equilibrium can be controlled and shifted
[(N(CN)2)] but not in the presence of [bmim][(N(CN)2)].[25] to product formation by performing the reaction under vac-
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Enzymatic Resolution of sec-Alcohols in Ionic Liquids
uum (100 Torr, 35 °C). Under these conditions minimal to isolate 41% (99% ee) of (R)-1-phenylethanol from the
evaporation of the substrate rac-1-phenylethanol occurs. attached ester and 51% (81% ee) of unreacted alcohol (S)-
The effect of solvent was studied by performing the reaction 1-phenylethanol.
in two different ionic liquids ([bmim][PF6], [bmim][BF4]) or
in the absence of solvent (in the case of the acylating agent
18, which is a liquid at reaction temperature, entry 4). Some
Table 2. EKR of rac-1-phenylethanol in IL by acylating agent
based on imidazolium cation.
enantioselective enzymatic acylation was observed for these
acylating agents under these different conditions (entries 3–
12). The best results were obtained for acylating agents con-
taining the ethyl ester group. In contrast, the poorest results
were obtained for the bromide salt 17 (entries 3 and 5). Un-
der these conditions, ee values of 5–17% were observed for
the extracted unreacted alcohol (S)-1. These data reinforce
the negative effect of halogen atoms on this transformation
described above.[16,22] The ee for the other enantiomer
(97%) was determined after hydrolysis (entry 3). Higher ee
values were obtained for the other acylating agents (en-
tries 6–12). In the case of acylating agent 18, the reaction
was performed under vacuum in two different ILs; the best
result was obtained with [bmim][PF6] as solvent with (S)-1
being obtained with an ee of 77%. Isomer (R)-1 was iso-
lated after hydrolysis with 91% ee (entry 6). For the acylat-
ing agents 19 and 21 no vacuum was applied to the reaction
mixture and moderate ee values were observed (28–53%).
The best result was obtained when the reaction was per-
formed in the presence of compound 20 and [bmim][PF6]
for 2 d. In this case an ee of up to 77% was obtained (en-
try 9). As in previous cases, the best result for acylating
agent 22 was obtained with [bmim][PF6] with an ee of 32%
observed for (S)-1 (entry 11). The results of these screening
tests can be rationalized by the negative effect of the prox-
imity of the permanent ion to the reactive carboxy centre,
the alkyl chain length playing an important role in enzyme
performance. To the best of our knowledge there is at least
one report on the effect of non-ionic acylating alkyl chain
lengths on the EKR of 3-methyl-2-butanol with vinyl esters
and different alkyl chain lengths using CAL-B. The authors
Entry[a] Acylating
agent, X, R
IL
–
Time (S)-1 from i (R)-1 from ii
E[c]
[d] Yield ee[b] Yield
ee
[%]
[%]
[%]
[%]
1
2
18, BF4, Et
18, BF4, Et [bmim][PF6]
2
2
4
4
4
4
4
65
62
69
74
40
51
36
39
62
62
56
78
81
84
19
30
23
19
31
41
35
99[d]
94[d]
99[d]
99[d]
17[e]
99[d]
18[e]
Ͼ200
48
Ͼ200
Ͼ200
2
3
4
5
6
18, BF4, Et [bmim][BF4]
19, BF4, H [bmim][PF6]
20, PF6, Et [bmim][PF6]
21, PF6, H [bmim][PF6]
Ͼ200
2
[a] All reactions were carried out in 0.8 mL of IL with 0.41 mmol
alcohol, 0.41 mmol of acylating agent and 20 mg of CAL-B. [b]
Determined by HPLC. [c] E value calculated according to Equa-
tion (1).[27] [d] Determined by HPLC after enzymatic hydrolysis 1 d,
2.5 equiv. EtOH. [e] Determined by HPLC after enzymatic hydroly-
sis 1 d, 2.5 equiv. H2O.
(1)
Furthermore, because the ionic acylating agent [ethyl
observed that longer alkyl chains gave better ee values than carboxylate [BF4]– 18 (entry 1)] is also liquid at room tem-
shorter alkyl chains.[26] In addition to these encouraging re- perature, it is possible to perform the EKR in the absence
sults there are still some reactivity and selectivity differences of solvent. Under this condition, (S)-1 was isolated in 65%
between vinyl esters and ionic acylating agents. By using yield (39% ee) after 2 d and (R)-1 was isolated in 19% yield
vinyl myristate 23 as acylating agent 1-phenylethanol was (99% ee) after 1 d. To the best of our knowledge this is the
resolved faster and with higher selectivity than with ionic first example of the use of a TSIL at room temperature for
acylating agents (Table 1, entries 13 and 14 vs. entries 3–12). the EKR of alcohols.
The results obtained for the optimized transformation
This strategy was applied to the resolution of a target
are presented in Table 2. The enzymatic reaction was per- secondary alcohol with more potential interest, namely 2-
formed under vacuum using the ionic liquids [bmim][PF6] hydroxycyclohexanecarbonitrile (24), which is a key precur-
and [bmim][BF4] as the best candidates identified in sor of an androgen receptor antagonist that is being devel-
Table 1. With free carboxylic acids as the acylating agent, oped for the treatment of both alopecia and excess sebum
the tetrafluoroborate salt 19 and hexafluorophosphate salt (oily skin).[28]
21 gave moderate optical purity of the attached ester (R)-
With the purpose of identifying the best biocatalyst for
1 (17 and 18%, respectively), allowing the isolation of the the resolution of this substrate, the enzymatic reaction was
remaining unreacted alcohol (S)-1 in higher ee (78 and performed under the same conditions as previously de-
84%, entries 4 and 6, Table 2). However, the acylating scribed using several different enzymes and acylating agent
agents 18 and 20 with [bmim][PF6] or [bmim][BF4] gave a 20 in [bmim][PF6] (Table 3). From the screening of different
much better EKR performance than the free carboxylic enzymes, it was possible to identify CAL-B as the most ap-
acid acylating agents 19 and 21. With the best combination propriate catalyst to resolve the substrate with (1R,2S)-24
of ester 20 in [bmim][PF6] (entry 5, Table 2) it was possible isolated in 70% yield and 29% ee.
Eur. J. Org. Chem. 2010, 6938–6943
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6941

N. M. T. Lourenço, C. M. Monteiro, C. A. M. Afonso
FULL PAPER
Table 3. EKR of 2-hydroxycyclohexanecarbonitrile (24) in duction of pressure on the reaction (entry 4 vs. entry 1). In
[bmim][PF6] with an acylating agent 20 based on the imidazolium
addition, an increase in enzyme loading had no significant
effect on the resolution of 24 (entry 4 vs. entry 5). The ef-
cation using different enzymes.[a]
fects of temperature and reaction time were explored by
performing the reaction for 4 or 5 days and at two different
temperatures (35 and 50 °C).
Under 100 Torr at 35 °C for 4 d, it was possible to isolate
(1R,2S)-24 from i) in 70% yield and 29% ee. Isomer
(1S,2R)-24 was isolated from ii) in 25% yield and 96% ee
after 1 d (entry 1). A longer reaction time (5 d) provided
(1R2S)-24 from i) in 73% yield and 35% ee, and (1S,2R)-
24 from ii) was isolated in 23% yield and 97% ee after 1 d
(entry 3).
At 50 °C under equivalent conditions, it was possible to
isolate (1R,2S)-24 from i) in 58% yield and 52% ee. Isomer
(1S,2R)-24 from ii) was isolated in 37% yield and 90% ee
after 1 d (entry 2). Taking advantage of the nature of the
medium this was reused in a second experiment (entry 2,
results in parentheses): (1R,2S)-24 from i) was isolated in
61% yield and 52% ee and (1S,2R)-24 from ii) was isolated
in 44% yield and 85% ee.
Entry
Lipase[b]
(1R,2S)-24
Yield[c] [%]
ee[d] [%]
1
2
3
4
5
6
7
CAL-B
Lypozyme
CCL
PS amano SD
Amano AS
AYS amano
AK amano
70
69
94
96
99
92
50
29
31
Ͻ1
5.0
Ͻ1
Ͻ1
26
[a] All reactions were carried out in 0.8 mL of [bmim][PF6] with
0.41 mmol rac-2-hydroxycyclohexanecarbonitrile (24), 0.41 mmol
of acylating agent 20 and 20 mg of enzyme. [b] Commercial formu-
lations. [c] The yields were calculated on the basis of the initial rac-
alcohol (0.41 mmol). [d] Determined by GC.
Conclusions
The effects of reaction vacuum, enzyme loading and tem-
perature on the reaction rate were then studied (Table 4).
Because this particular substrate 24 is not volatile under the
operating conditions, we decided to explore the effect of
ethanol evaporation on the course of the reaction by chang-
ing the level of vacuum in the reaction. From our results it
is possible to see that there is no positive effect of the re-
This study has allowed the identification of new ionic
acylating agents based on the imidazolium cation for EKR
in ionic liquids. The strategy opens up new opportunities
for efficient resolutions and easy enantiomer separation
simply by removal of the unreacted enantiomer alcohol by
extraction from the attached reacted opposite enantiomer
that remains in the IL medium. The opposite enantiomer
can be liberated by a second enzymatic reaction in good
yields and ee values, as has been demonstrated for 2-hy-
droxycyclohexanecarbonitrile.
Table 4. EKR of 2-hydroxycyclohexanecarbonitrile (24) in
[bmim][PF6] with the acylating agent 20 based on the imidazolium
cation.[a]
Experimental Section
Procedure for Screening of Task-Specific Ionic Liquids for the Enzy-
matic Resolution of rac-1-Phenylethanol: (Table 1) CAL-B (Novo-
zym 435®; 10 mg) and rac-1-phenylethanol (24.4 mg, 0.20 mmol)
was added to a stirred solution of the acylating agent (0.20 mmol)
in an ionic liquid (0.25 mL) at 35 °C in a thermostatic bath and
the mixture was stirred for 7 days. An aliquot of the reaction mix-
ture (50 μL) was treated by passing through a pipette-sized column
of SiO2 with Et2O (15 mL). The enantiomeric excess of the alcohol
was obtained by HPLC analysis [Chiracel OD, hexane/2-propanol
(95:5), 1 mL/min, λ = 258 nm].
Entry Vacuum Time
(1R,2S)-24from i
Time
(1S,2R)-24from ii
[Torr]
[d] Yield[b] [%] ee[c] [%]
[d] Yield[b] [%] ee[c] [%]
General Procedure for the Enzymatic Kinetic Resolution of rac-1-
Phenylethanol: (Table 2) CAL-B (Novozym 435®; 20 mg) and rac-
1-phenylethanol (50.0 μL, 0.41 mmol) was added to a stirred solu-
tion of the acylating agent (0.41 mmol) in an ionic liquid (0.8 mL).
The reaction mixture was stirred for 4 d under reduced pressure
(100 Torr) at 35 °C in a thermostatic bath. After this time, the reac-
tion mixture was extracted with Et2O (3ϫ7 mL) and the organic
phases were collected and passed through a pipette-sized column
filled with silica and the solvent was evaporated under reduced
pressure to give (S)-1-phenylethanol.
1
100
100
100
15
4
4
5
4
4
70
58 (61)
73
77
80
29
52 (52)
35
30
20
1
1
1
1
1
25
37 (44)
23
23
20
96
90 (85)
97
94
93
2[d]
3
4
5[e]
15
[a] All reactions were carried out in 0.8 mL of [bmim][PF6] with
0.41 mmol of rac-2-hydroxycyclohexanecarbonitrile (24),
0.41 mmol of acylating agent 20 and 20 mg of CAL-B. [b] The
yields were calculated on basis of the initial rac-alcohol
(0.41 mmol). [c] Determined by GC. [d] Reaction was carried out
at 50 °C. The results obtained for one recycling experiment (ionic
liquid, ionic acylating agent and enzyme) are provided in parenthe-
ses. [e] Reaction performed with 52 mg of CAL-B.
The reaction mixture was dried under reduce pressure (20 Torr) for
2 h and after this time 2.5 equiv. of EtOH was added and mixture
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Enzymatic Resolution of sec-Alcohols in Ionic Liquids
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stirred for 1 d at 35 °C in a thermostatic bath. After this time the
reaction mixture was extracted again with Et2O (3ϫ7 mL) and the
organic phases were collected and passed through a pipette-sized
column filled with silica and the solvent was evaporated under re-
duced pressure to give (R)-1-phenylethanol.
[8]
[9]
Enzymatic Kinetic Resolution of rac-2-Hydroxycyclohexanecarbo-
nitrile (24): (Table 3) rac-2-Hydroxycyclohexanecarbonitrile (24;
51.8 mg, 0.41 mmol) was added to a stirred solution of 1-methyl-
3-(11-ethoxycarbonylundecyl)imidazole hexafluorophosphate (20;
0.41 mmol) in [bmim][PF6] (0.8 mL) and CAL-B (Novozym 435®;
20 mg). The reaction mixture was stirred for 5 d under reduced
pressure (100 Torr) at 35 °C in a thermostatic bath. After this time
the reaction mixture was extracted with Et2O (3ϫ7 mL) and the
organic phases were collected and passed through a pipette-sized
column filled with silica and the solvent was evaporated under re-
duced pressure to give (1R,2S)-2-hydroxycyclohexanecarbonitrile
(24; 38.0 mg, 73% yield, 34.7% ee).
[10]
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Acknowledgments
We thank the Fundação para a Ciência e Tecnologia (FCT) (POCI
2010, POCI/QUI/57735/2004; SFRH/BPD/41175/2007, SFRH/BD/
48395/2008), the Fundo Europeu de Desenvolvimento Regional
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PRF#49150-GCI) for financial support. Solchemar, Lda is thanked
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Published Online: November 17, 2010
Eur. J. Org. Chem. 2010, 6938–6943
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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