Simple one-pot process for the bioresolution of tertiary amino ester protic ionic liquids using subtilisin

Article abstract of DOI:10.1016/j.tetasy.2009.09.007

An efficient hydrolase-catalyzed bioresolution of tertiary amino ester protic ionic liquids has been demonstrated. Protic ionic liquids have been prepared in one step from the corresponding tertiary amino alcohols by treatment with butyric anhydride. Afte

Full text of DOI:10.1016/j.tetasy.2009.09.007

Tetrahedron: Asymmetry 20 (2009) 2112–2116
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Simple one-pot process for the bioresolution of tertiary amino ester protic
ionic liquids using subtilisin
*
*
Maude Brossat , Thomas S. Moody , Stephen J. C. Taylor, Jonathan W. Wiffen
Biocatalysis Group, Almac Sciences, David Keir Building, Stanmillis Road, Belfast BT9 5AG, Northern Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient hydrolase-catalyzed bioresolution of tertiary amino ester protic ionic liquids has been dem-
onstrated. Protic ionic liquids have been prepared in one step from the corresponding tertiary amino alco-
hols by treatment with butyric anhydride. After bioresolution, unreacted esters can be easily separated
from the corresponding alcohols by extraction with hexane. Bioresolution of quinuclidin-3-yl butyrate
has been performed with excellent selectivity.
Received 17 June 2009
Accepted 4 September 2009
Available online 6 October 2009
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Several examples have been reported in the literature for the ki-
netic bioresolution of quinuclidin-3-ol. The enantioselective
Enzymatic kinetic resolution of racemic alcohols via acylation
or hydrolysis has been extensively studied and established as one
of the most effective methods for the preparation of optically ac-
tive alcohols. Over the past three decades, novel media for biocata-
lytic processes such as organic solvents,1 solvent-free systems,2
gaseous media,2 supercritical fluids2 and ionic liquids3 have been
subjected to intense research. In particular, ionic liquids have re-
ceived much attention due to their unique physiochemical proper-
ties such as their non-volatility and they have been generally
recognized as green solvents for non-aqueous biocatalysis.4
The use of room temperature ionic liquids (RTILs) as new attrac-
tive reaction media for bioresolutions has been widely discussed.5
The improvement of the enzyme stability and activity, the ease of
product recovery combined with the enhancement of the enanti-
oselectivity make them a powerful tool for biocatalysis.6 In addi-
tion, the introduction of functional groups to ionic liquids by
derivatisation of the cationic component or functionalisation of
the anion resulted in the development of so-called ‘task-specific’
ionic liquids (TSILs).7
hydrolysis of the corresponding butyrate ester of quinuclidin-3-ol
has been demonstrated using horse serum butyrylcholine ester-
ase,9 although the main drawback of this method is the limited
availability of the enzyme. The bioresolution of a tertiary 3-substi-
tuted acetylenic quinuclidinol butyrate ester has been reported
using pig liver esterase in aqueous buffer.10 Nomoto et al. have re-
ported the bioresolution of the butyrate ester using Aspergillus mel-
leus protease.11 The butyrate ester hydrolysis has also been
reported using commercially available subtilisin Carlsberg albeit
under high dilution conditions.12
Herein we report a simple, efficient and inexpensive method for
the bioresolution of tertiary amino ester protic ionic liquids with-
out co-solvent as shown in Scheme 1, which offers processing ben-
efits to the process chemist.
2. Results and discussion
2.1. Preparation of racemic amino ester substrates
Afonso et al. reported the use of a TSIL as an acylating agent for
the one-pot bioresolution-separation of secondary alcohols.8 How-
ever in this case, the use of an additional ionic liquid as a co-sol-
vent was required because of the viscosity of the medium.
Over the course of our studies on the bioresolution of tertiary
amino alcohols, we investigated the feasibility of a cost effective
and time efficient process for bioresolutions, which minimises pro-
cessing time in the pilot manufacturing and uses inexpensive hydro-
lase enzymes.
The butyrate ester of quinuclidin-3-ol 2a was prepared by mixing
the amino alcohol 1 with 1.05 equiv of butyric anhydride (Scheme
2). The formation of the resulting mobile protic ionic liquid 2a is
quantitative and fast (10–30 min). The viscosity of 2a was measured
to be 30 mPa s at 25 °C and is comparable to the viscosity of ethylene
glycol (16.1 mPa s at 25 °C), thereby is suitable for any pilot plant
reactor and is easily stirred and handled at scale.13
2.2. Subtilisin-catalyzed kinetic bioresolution
* Corresponding authors. Tel.: +44 (0) 28 3836 5906; fax: +44 (0) 28 9022 5544
(M.B.).
E-mail addresses: maude.brossat@almacgroup.com (M. Brossat), tom.moody
@almacgroup.com (T.S. Moody).
The inexpensive, commercially available liquid form of the en-
zyme subtilisin (from Bacillus licheniformis) was used for bioresolu-
tion studies. The hydrolysis of quinuclidin-3-yl butyrate (RS)-2a
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.09.007

M. Brossat et al. / Tetrahedron: Asymmetry 20 (2009) 2112–2116
2113
O
O
O
OH
R'
O
O
O
N
N
OH
R'
N
+
R
R'
R
O
R
R'
Hydrolase
N
R
O
O
O
.
.
HO
.
HO
HO
Scheme 1. Simple one pot process for the bioresolution of tertiary amino ester protic ionic liquids.
Table 1
O
Subtilisin-catalyzed hydrolysis of (RS)-2a, (RS)-4a to (RS)-6a
OH
O
.
Entry
Substrate
ee substratea (%)
ee productb (%)
Conv.c (%)
Ec
Butyric anhydride
1.05 eq
1
2
3
4
(RS)-2a
(RS)-4a
(RS)-5a
(RS)-6a
97.0
28.9
48.3
40.7
90.4
89.2
3.7
52
25
92
36
79
23
1.5
10
O
N
N
HO
73.8
a
Ee determined by extraction of the ester with hexane and basic hydrolysis in
(RS)-1
(RS)-2a
MeOH using 2 M aqueous NaOH followed by HPLC using CHIRALCEL AD-H column.
b
Scheme 2. Preparation of protic ionic liquid (RS)-2a from (RS)-1.
Ee determined by HPLC using CHIRALCEL AD-H column.
Calculated from eealcohol and eeester after 16 h reaction.
c
was performed by stirring at room temperature over 16 h with
100% v/v of liquid subtilisin as shown in Scheme 3, utilizing the
water present in the enzyme for hydrolysis.
The bioresolution showed high conversion and (S)-selectivity
with good ee for the (R)-butyrate 2a14 and the (S)-alcohol 315 with
a calculated E-value of 79 (Table 1, entry 1).
protic ionic liquid system is the self-buffering of the reaction,
removing the necessity for pH control, which makes the reaction
very easy to process, requiring no specialized equipment. The pro-
file of the reaction is shown in Figure 3. After 16 h, 48% conversion
was reached with 89.1% eealcohol and 95.6% eeester corresponding to
an E-value of 74.
The unreacted (R)-ester was separated from the (S)-alcohol by
extraction with hexane and washing with saturated sodium car-
bonate as shown in Figure 4. The (R)-ester was recovered with
47% yield, 95.6% ee and 99% purity by NMR wt/wt assay.
2.3. Application to other substrates
This methodology was expanded to other tertiary amino ester
protic ionic liquids (RS)-4a to (RS)-6a, as shown in Figure 1.
The subtilisin-catalyzed bioresolution of butyrate ester protic
ionic liquids (RS)-2a, (RS)-4a to (RS)-6a are shown in Table 1. The
corresponding subtilisin catalyzed bioresolution of the free bases
are shown in Table 2. The corresponding free bases are designated
(RS)-2b, (RS)-4b to (RS)-6b and were prepared using a simple basic
extraction from the parent protic ionic liquid.
Figure 2 shows the E-values obtained from the bioresolution of
both the protic ionic liquids and corresponding free bases. For the
bioresolution of quinuclidin-3-ol, the protic ionic liquid shows by
far the best E-value for this series of compounds. An increase in
E-value is obtained for compound (RS)-4 when using the protic io-
nic liquid derivative. Resolution does occur for compounds (RS)-5
and (RS)-6 albeit with lower selectivity. Further work is currently
underway to investigate alternative enzymes that show the de-
sired selectivities for these substrates.
3. Conclusion
In conclusion, the bioresolution of tertiary amino alcohols has
been demonstrated via their corresponding butyrate protic ionic
liquids. This work has demonstrated that hydrolase enzymes, such
as subtilisin, can tolerate high concentrations of ionic mixtures and
still maintain high selectivity in the case of the quinuclidin-3-ol.
Performing the bioresolution as the protic ionic liquid reduces pro-
cessing time and removes the need to pH control the reaction and
should be considered when performing other bioresolutions of this
type to reduce costs for the plant chemist.
4. Experimental
4.1. General
2.4. Scale-up
The bioresolution of substrate (RS)-2a was scaled to 10 g using
Chemicals were purchased from Alfa Aesar. Protease enzyme
50% v/v of liquid subtilisin. An additional advantage of using the
subtilisin (from B. licheniformis) was obtained from Enzagen Ltd.
O
O
OH
O
O
Subtilisin
N
+
N
O
O
N
O
.
.
HO
HO
.
HO
(RS)-2a
(R)-2a
(S)-3
Scheme 3. Kinetic bioresolution of the butyrate ester (RS)-2a.

2114
M. Brossat et al. / Tetrahedron: Asymmetry 20 (2009) 2112–2116
OCOC3H7
OCOC3H7
OCOC3H7
N
N
OCOC3H7
N
N
O
Ph
O
O
O
.
.
HO
HO
.
HO
HO
(RS)-4a
.
(RS)-2a
(RS)-6a
(RS)-5a
Figure 1. Tertiary amino ester protic ionic liquids (RS)-2a, (RS)-4a to (RS)-6a.
Table 2
Subtilisin-catalyzed hydrolysis of (RS)-2b, (RS)-4b to (RS)-6b
Quinuclidin-3-ol
Racemic
Entry
Substrate
ee substratea (%)
ee productb (%)
Conv.c (%)
Ec
1
2
3
4
(RS)-2b
(RS)-4b
(RS)-5b
(RS)-6b
55.8
92.8
97.3
98.2
13.2
61.4
62.7
58.9
81
60
61
62
2
13
17
16
Protic ionic
liquid preparation
a
Ee determined by the extraction of the ester with hexane and basic hydrolysis
in MeOH using 2 M aqueous NaOH followed by HPLC using CHIRALCEL AD-H
Quinuclidin-3-yl butyrate
column.14
Racemic
Protic ionic liquid
Ee determined by HPLC using CHIRALCEL AD-H column.15
b
c
Calculated from eealcohol and eeester after 16 h reaction.
Hydrolysis with
Subtilisin
Bioresolution
90
80
70
60
Quinuclidin-3-yl butyrate
and quinuclidin-3-ol
Ionic liquid
50
40
30
20
10
0
Ionic liquids Free bases
Extractive
Work-up
Hexane
sat. NaHCO3
Enantiomerically
enriched
(S)-quinuclidin-3-ol
(RS)-2
(RS)-5
(RS)-4
(RS)-6
Substrates
Enantiomerically
enriched
(R)-quinuclidin-3-yl
butyrate
Figure 2. Comparison of the E-values for the bioresolution of protic ionic liquids
(RS)-2a, (RS)-4a to (RS)-6a and free bases (RS)-2b, (RS)-4b to (RS)-6b.
Figure 4. Facile processing for the bioresolution of racemic quinuclidin-3-ol.
50
40
30
20
10
0
Mass spectra (MS) were recorded on an ThermoQuest Finnigan
LCQ Duo (electrospray) spectrometer.
4.2. General procedure for the synthesis of the butyrate ester
protic ionic liquids, (RS)-2a, (RS)-4a to (RS)-6a
The protic ionic liquids were prepared by reacting the corre-
sponding alcohols with butyric anhydride (1.05 equiv). The mix-
tures were stirred for 30 min at room temperature and used
directly in the bioresolution studies.
0
100 200 300 400 500 600 700 800 900 1000
Time (min)
4.2.1. Quinuclidin-3-yl butyrate. Butyric acid (RS)-2a
Transparent liquid. 1H NMR (CDCl3, 400 MHz) d 4.91–4.95 (m,
1H), 3.37–3.31 (m, 1H), 3.04–2.84 (m, 5H), 2.20 (t, J = 8.0 Hz, 2H),
2.23–2.16 (m, 3H), 2.02–1.95 (m, 1H), 1.87–1.79 (m, 1H), 1.50–
1.75 (m, 6H), 0.98–0.91 (m, 6H). 13C NMR (CDCl3, 100 MHz) d
178.7, 173.1, 68.9, 53.4, 45.7, 44.9, 37.6, 36.2, 24.8, 22.6, 18.9,
18.4, 18.3, 13.9, 13.7. MS (ESI TOF) [M+H]+ = 198. Mp À47 to
À50 °C.
Figure 3. Reaction profile for the bioresolution of (RS)-2a.
1H NMR spectra were recorded at 400 MHz on a Bruker AV-400
spectrometer, shifts are relative to internal TMS.
The enantiomeric excesses were measured by chiral stationary
phase HPLC on Chiracel AD-H column (250 mm  4.6 mm  10
lm,
Daicel Chemical Industries) with UV detection (k = 254 nm).

M. Brossat et al. / Tetrahedron: Asymmetry 20 (2009) 2112–2116
2115
4.2.2. 1-Methyl-3-piperidinyl butyrate. Butyric acid (RS)-4a
4.4. Large scale bioresolution of quinuclidin-3-ol (RS)-2a
Yellow liquid. 1H NMR (CDCl3, 400 MHz) d 4.96–4.93 (m, 1H),
2.81–2.70 (m, 1H), 2.57–2.45 (m, 3H), 2.36 (s, 3H), 2.30–2.25 (m,
4H), 1.83–1.72 (m, 2H), 1.70–1.54 (m, 5H), 1.52–1.45 (m, 1H),
1.00–0.92 (m, 6H). 13C NMR (CDCl3, 100 MHz) d 178.1, 172.9,
67.8, 57.8, 54.4, 45.2, 36.7, 36.3, 28.2, 21.2, 18.5, 18.4, 13.8, 13.6.
MS (ESI TOF) [M+H]+ = 186.
(RS)-Quinuclidin-3-ol (10 g, 78.7 mmol) was added to butyric
anhydride (13.3 mL, 82.7 mol) at room temperature. The resultant
mixture was stirred at room temperature for 1 h. There is a moder-
ate exotherm as the alcohol dissolves and the ester forms. The for-
mation of the butyrate ester is quantitative and fast and the liquid
product is easily stirred at room temperature (viscosity was mea-
sured to be 30 cP at 25 °C). The protic ionic liquid was used without
work-up or further purification.
Liquid subtilisin (10 mL (50% v/v)) was then added directly in
one portion. The reaction was stirred at 30 °C and ee of the residual
ester monitored by chiral HPLC. After 16 h, the solution was diluted
with 60 mL (6 vols) of saturated sodium carbonate and extracted
five times with 30 mL (3 vols) of hexane to afford, after evaporation
of the solvent, 7.32 g (47% yield) of the (R)-butyrate ester 2a (95.6%
ee) with 99% wt/wt purity by NMR assay.
4.2.3. 1-Benzyl-3-piperidinyl butyrate. Butyric acid (RS)-5a
Yellow liquid. 1H NMR (CDCl3, 400 MHz) d 7.31–7.22 (m, 5H),
4.89–4.85 (m, 1H), 3.64–3.59 (m, 2H), 2.81–2.78 (m, 1H), 2.64–
2.62 (m, 1H), 2.40 (t, J = 5.8 Hz, 2H), 2.38–2.21 (m, 4H), 1.86–1.73
(m, 2H), 1.69–1.55 (m, 5H), 1.45–1.38 (m, 1H), 0.97–0.85 (m,
6H). 13C NMR (CDCl3, 100 MHz) d 179.0, 173.0, 129.7, 129.0,
128.3, 127.5, 68.7, 62.1, 55.9, 52.3, 37.1, 36.3, 29.2, 22.0, 18.5,
17.8, 13.7, 13.4. MS (ESI TOF) [M+H]+ = 262. HPLC analysis: Chiracel
AD-H column, eluent hexane/ethanol/diethylamine (85/15/0.1),
flow rate 1 mL/min, k = 254 nm, t1 = 3.62, t2 = 3.94.
4.5. Derivatisation method with benzoic anhydride
4.2.4. 1-(Dimethylamino)-2-propanyl butyrate. Butyric acid
(RS)-6a
Aminoalcohol (ꢀ20 mg) and benzoic anhydride (ꢀ20 mg) were
placed in a 1 ml HPLC vial and heated at 70 °C for 10 min. After
cooling, 1 mL 2 M HCl was added followed by 1 mL MTBE. The vial
was shaken to partition the amino ester into the acid, and the unre-
acted neutral organics and benzoic acid into the MTBE layer. The
aqueous layer was pipetted into a fresh vial, pH adjusted to 13 with
2 M NaOH and 1 mL MTBE added. The vial was shaken to partition
the derivatised amino ester into the organic phase. Hundred micro-
litres of the MTBE layer were added to 1.5 mL of mobile phase and
the solution dried over MgSO4 before HPLC injection.
For the benzoate ester of quinuclidin-3-ol, eluent hexane/
ethanol/diethylamine (85/15/0.1); flow rate 1 mL/min; typical
retention times were 9.6 min (S)-benzoate ester, 17.9 min (R)-ben-
zoate ester. For substrate 1-methyl-3-piperidinol eluent hexane/
ethanol/diethylamine (85/15/0.1); flow rate 1 mL/min; typical
retention times were for benzoate esters 5.1 min and 5.4 min, 1-
benzyl-3-piperidinol eluent hexane/ethanol/diethylamine (85/15/
0.1); flow rate 1 mL/min; typical retention times for the alcohols
were 5.3 min and 6.0 min, 1-(dimethylamino)-2-propanol eluent
hexane/ethanol/diethylamine (95/5/0.1); flow rate 0.5 mL/min;
typical retention times were for benzoate esters 9.1 min and
9.8 min.
Yellow liquid. 1H NMR (CDCl3, 400 MHz) d 5.13–5.11 (m, 1H),
2.80–2.75 (m, 1H), 2.50 (dd, J = 4.0 Hz, and 11.8 Hz, 1H), 2.37 (s,
6H), 2.27–2.19 (m, 4H), 1.63–1.57 (m, 4H), 1.19 (d, J = 5.0 Hz,
3H), 0.97–0.91 (m, 6H). 13C NMR (CDCl3, 100 MHz) d 177.9,
173.1, 66.7, 62.4, 44.3, 37.0, 35.3, 18.8, 18.6, 18.3, 17.7, 13.9,
13.4. MS (ESI TOF) [M+H]+ = 174.
4.3. General procedure for the synthesis of the free base
butyrate esters, (RS)-2b, (RS)-4b to (RS)-6b
Isolation of the free base esters from the corresponding ionic
liquids was achieved by washing with saturated sodium carbonate
(2 vol) and extracting five times with hexane (1 vol). Evaporation
of the solvent afforded the corresponding ester in 90–95% yield.
To calculate the ee of the ester, it was hydrolysed under basic
conditions with 2 M NaOH in MeOH for 30 min followed by deri-
vatisation with benzoic anhydride as described in Section 4.5.
4.3.1. Quinuclidin-3-yl butyrate (RS)-2b
Transparent liquid. 1H NMR (CDCl3, 400 MHz) d 4.82–4.79 (m,
1H), 3.27–3.22 (m, 1H), 2.91–2.65 (m, 5H), 1.99 (t, J = 5.9 Hz, 2H),
2.00–1.98 (m, 1H), 1.84–1.83 (m, 1H), 1.71–1.63 (m, 3H), 1.57–
1.55 (m, 1H), 1.40–1.37 (m, 1H), 0.97 (m, J = 5.9 Hz, 3H).
The conversion for the hydrolysis of quinuclidin-3-yl butyrate
was determined by GC under the following conditions: column
Chiraldex B, 0.25 mm  30 m, 17 psi He, injection temperature,
150 °C, then 10 °C /min to 210 °C, hold 4 min; retention times were
2.4 min for butyric acid, 4.3 min for quinuclidinol and 6.5 min for
quinuclidinyl butyrate.
4.3.2. 1-Methyl-3-piperidinyl butyrate (RS)-4b
Transparent liquid. 1H NMR (CDCl3, 400 MHz) d 4.90–4.86 (m,
1H), 2.64–2.61 (m, 1H), 2.42–2.39 (m, 1H), 2.30–2.23 (m, 7H),
1.78–1.75 (m, 2H), 1.67–1.57 (m, 3H), 1.45–1.43 (m, 1H), 0.97 (t,
J = 5.9 Hz, 3H).
Acknowledgement
We would like to thank Professor Tony McKervey for reviewing
the manuscript and providing valuable suggestions.
4.3.3. 1-Benzyl-3-piperidinyl butyrate (RS)-5b
Transparent liquid. 1H NMR (CDCl3, 400 MHz) d 7.36–7.28 (m,
5H), 4.91–4.94 (m, 1H), 3.66–3.63 (m, 2H), 2.85–2.83 (m, 1H),
2.67–2.66 (m, 1H), 2.46 (t, J = 5.8 Hz, 2H), 2.36–2.27 (m, 4H),
1.90–1.86 (m, 1H), 1.76–1.63 (m, 2H), 1.45–1.42 (m, 1H), 0.97 (t,
J = 5.9 Hz, 3H). HPLC analysis: Chiracel AD-H column, eluent hex-
ane/ethanol/diethylamine (85/15/0.1), flow rate 1 mL/min,
k = 254 nm, t1 = 3.62, t2 = 3.94.
References
1. (a) Ghanem, A.; Aboul-Enein, H. Y. Tetrahedron: Asymmetry 2004, 15, 3331–
3351; (b) Klibanov, A. M. Nature 2001, 409, 241–246.
2. Hobbs, H. R.; Thomas, N. R. Chem. Rev. 2007, 107, 2786–2820.
3. (a) Cull, S. G.; Holbrey, J. D.; Vargas-Mora, V.; Seddon, K. R.; Lye, G. J. Biotechnol.
Bioeng. 2000, 69, 227–233; (b) Madeira Lau, R.; Van Rantwijk, F.; Seddon, K. R.;
Sheldon, R. A. Org. Lett. 2000, 2, 4189–4191.
4. (a) Yang, Z.; Pan, W. Enzyme Microb. Technol. 2005, 37, 19–28; (b) Sheldon, R. A.;
Lau, R. M.; Sorgedrager, M. J.; van Rantwijk, F.; Seddon, K. R. Green Chem. 2002,
4, 147–151; (c) Lourenco, N. M. T.; Barreiros, S.; Afonso, C. A. M. Green Chem.
2007, 9, 734–736.
5. (a) Domínguez de María, P. Angew. Chem., Int. Ed. 2008, 47, 6960–6968; (b) Van
Rantwijk, F.; Sheldon, R. A. Chem. Rev. 2007, 107, 2757–2785; (c) Baudequin, C.;
4.3.4. 1-(Dimethylamino)-2-propanyl butyrate (RS)-6b
Yellow liquid. 1H NMR (CDCl3, 400 MHz) d 5.07–5.03 (m, 1H),
2.51–2.47 (m, 1H), 2.29–2.25 (m, 9H), 1.67–1.62 (m, 2H), 1.22 (d,
J = 5.0 Hz, 3H), 0.97–0.91 (t, J = 5.9 Hz, 3H).

2116
M. Brossat et al. / Tetrahedron: Asymmetry 20 (2009) 2112–2116
Baudoux, J.; Levillain, J.; Cahard, D.; Gaumont, A.-C.; Plaquevent, J.-C.
11. Nomoto, F.; Hirayama, Y.; Ikunaka, M.; Inoue, T.; Otsuka, K. Tetrahedron:
Asymmetry 2003, 14, 1871–1877.
Tetrahedron: Asymmetry 2003, 14, 3081–3093.
6. (a) Lou, W.-Y.; Zong, M.-H. Chirality 2006, 18, 814–821; (b) Das, S.;
Chandrasekhar, S.; Yadav, J. S.; Rao, A. V. R.; Grée, R. Tetrahedron: Asymmetry
2008, 19, 2543–2545.
7. Fei, Z.; Geldbach, T. J.; Zhao, D.; Dyson, P. J. Chem. Eur. J. 2006, 12,
2122–2130.
12. Muchmore, D. C. (Bend Research, Inc.). U.S. Patent 5,215,918, 1993.
13. (a) Viscosity of the ionic liquids incorporating the imidazolium cation ranges
between 28 and 1110 cP at 20–25 °C.; (b) Huddleston, J. G.; Visser, A. E.;
Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3,
156–164.
8. Afonso, C. A. M.; Lourenço, N. M. T. Angew. Chem., Int. Ed. 2007, 46,
14. The ee of the ester was determined after basic hydrolysis and derivatisation
using benzoic anhydride and compared with a commercial reference.
15. The ee of the alcohol was determined by HPLC after derivatisation using
benzoic anhydride and compared with a commercial reference.
8178–8181.
ˇ ˇ
´
´
9. Primozic, I.; Hrenar, T.; Tomic, S.; Meic, Z. Eur. J. Org. Chem. 2003, 295–301.
10. Coope, J. F.; Main, B. G. Tetrahedron: Asymmetry 1995, 6, 1393–1398.