432
C. Pilissa˜o, M. G. Nascimento / Tetrahedron: Asymmetry 17 (2006) 428–433
These properties must be considered when explaining
the results and as discussed above. Both enantiomers
can be obtained through an appropriate choice of sol-
vent and enzyme. Furthermore, the ionic liquids can
be readily reused offering another advantage to this
methodology.
4.4. Preparation of (RS)-methyl mandelate
Racemic (RS)-( )-methyl mandelate was prepared by
the esterification of (RS)-( )-mandelic acid with metha-
nol and sulfuric acid. In a typical reaction procedure,
0.005 mol of mandelic acid, 2.5 mol of methanol, and
drops of concentrated sulfuric acid were kept under
reflux for 5 h. The progress of the reaction was moni-
tored by TLC using n-hexane/ethyl acetate (8:2) as the
mobile phase and the reaction was taken to completion.
The racemic mixture of (RS)-methyl mandelate was
separated from the unreacted mandelic acid by dissolv-
ing it in ice-cold ethyl ether and then separating the
insoluble mandelic acid out. Ethyl ether was evaporated
to yield the colorless solid (RS)-methyl mandelate
3. Conclusions
In conclusion, these data show the influence of lipase
source and solvent on the resolution of (RS)-methyl
mandelate
1 via enantioselective aminolysis with
n-butylamine. The corresponding amides were obtained
in high enantiomeric excess (eep >99%) using organic
solvent/ionic liquid mixtures. The solvent also
determined the configuration of the product. Thus, ionic
liquids can be used as solvents for lipase-catalyzed
aminolysis with the advantage of enhancing the enantio-
selectivity, suggesting that they have great potential as
alternative media for biocatalysis and biotrans-
formations.
1
(mp = 53–54 ꢁC). H NMR (CDCl3) d (ppm): 3.75 (s,
1H), 5.17 (s, 1H), 7.25–7.39 (m, 5H).
4.5. General procedure for lipase-catalyzed resolution
of (RS)-1 with n-butylamine
To a solution of (RS)-methyl mandelate 1 (0.15 mmol;
25 mg) and n-butylamine (0.3 mmol; 0.03 mL), the
lipases (25, 50, 100, and 150 mg) free or immobilized
in PEO were added to different pure organic solvents
(25 mL) or to their mixtures with ionic liquids (10:1
v/v) at temperatures of 25, 35, and 45 ꢁC. The mixture
was shaken in a rotary shaker. The reaction progress
and enantiomeric excess values were measured with
a gas chromatograph (GC) equipped with a chiral
column (CP-chirasil-Dex CB, packed b-cyclodextrin,
25 m · 0.25 mm · 0.25 mm, CROMOPACK—Varian).
H2 was used as the carrier gas with a detector, an injec-
tor set at 275 ꢁC, and a column set to temperature ramps
of 80, 140, and 210 ꢁC (5 ꢁC/min and 3 ꢁC/min). The
enantiomeric ratio (E) values were calculated from the
conversion degree and enantiomeric excess of substrate
(ees) and products (eep), according to the Sih, Sharpless,
and Fajans equation.32 Free shareware programs for
the calculation of the enantiomeric ratio can also be
obtained via the Internet.33
4. Experimental
4.1. Materials and methods
Lipases from P. cepacia (PSL) (30,000 U gÀ1 solid),
P. cepacia (PSL-D) (immobilized on diatomaceous
earth, 500 U gÀ1), and P. cepacia (PSL-C) (immobilized
on ceramic particles chemically modified with a meth-
acryl group, 600 U gÀ1) were obtained from Amano
Pharmaceutical Co. (Nagoya, Japan). Lipases from
R. miehei (Lipozyme RM IM 5-6 BAUN gÀ1, RML),
T. lanuginosus (Lipozyme TL IM 250 IUN gÀ1, TLL),
and C. antarctica (NOVOZYM 435—immobilized
lipase type B—10.000 PLU gÀ1, CAL-B) were obtained
from Novozymes Latin America Ltda (Brazil). Poly-
(ethylene oxide) (300.000 g molÀ1) and (RS)-mandelic
acid were purchased from Sigma–Aldrich. n-Hexane,
tert-butanol, chloroform, and n-butylamine were obtained
from Vetec (Rio de Janeiro, Brazil), and all solvents and
other reagents were of analytical grade.
Acknowledgments
This study was supported by the Federal University of
Santa Catarina (UFSC—Brazil), the National Research
Council (CNPq—Brazil), and CAPES (Brazil). We also
thank Amano Pharmaceutical Co. and Novozymes for
their generous donation of lipases and Dr. Jairton
Dupont (IQ-IFRGS—Brazil) for donation of the ionic
liquids.
4.2. Immobilization of lipases on PEO
Enzyme immobilization on PEO was performed by dis-
solving 500 mg of the polymer and 100 mg of the lipases
RML and TLL and 50 mg of PSL, PS-D, and PS-C in
25 mL of water. After stirring for 4 h, the solvent was
evaporated at room temperature in a Petri dish (Teflon)
forming a film, which was then cut into several regular
sections of 3 mm2. Residual water in the film was
removed under vacuum conditions.8
References
1. Henkel, T.; Brunne, R. M.; Reichel, F. Angew. Chem, Int.
Ed. 1999, 38, 643–647.
2. Juaristi, E.; Leon-Romo, J. L.; Reyes, A.; Escalante, J.
Tetrahedron: Asymmetry 1999, 10, 2441–2495.
3. Irimescu, R.; Kato, K. Tetrahedron Lett. 2004, 45, 523–
525.
4.3. General procedure for the synthesis of 1-butyl-3-
methyl imidazolium tetrafluoroborate [BMIm][BF4] and
1-butyl-3-methyl imidazolium hexafluorophosphate
[BMIm][PF6]
These compounds were obtained in 79% and 95% yield,
respectively, as described in the literature.31
4. Faber, K. In Biotransformations in Organic Chemistry;
Springer: Berlin, 1997; Vol. 1, Chapter 1, pp 1–26.