1398
Lipase-Catalyzed Kinetic Resolution of ( )-Mandelonitrile under Conventional Condition
J. Braz. Chem. Soc.
the KR with high selectivity, the results showed that the
activity and selectivity were dependent on the solvents.
The reaction in hexane and ethyl ether resulted in minor
selectivity of (S)-2 at 120 h (95% ee, entries 3 and 6,
Table 1). Moreover, the enzymatic reaction conducted in
toluene under orbital shaker resulted in a long period of time
to be completed (168-184 h, entries 10 and 11, Table 1).
In addition, the solvent choice is important for use
in microwave irradiation, as it can influence in the
enantioselectivity of enzymatic reaction. The toluene has
higher boiling point, it appear to be an excellent solvent for
the investigation of microwave-assisted kinetic resolution.10
The kinetic resolution of ( )-mandelonitrile (1) yielded
the (S)-mandelonitrile (2) with good conversion and
optical purity (c 38-40% and 92-93% ee, entries 17-19,
Table 1) under microwave irradiation. The kinetic
resolution under conventional system occurred with a
longer time (c 46-48%, > 98% ee, 168-184 h, entries
10 and 11, Table 1). The temperature effect on the
activity and enantioselectivity of CALB under microwave
minor optical purity (92% ee), but in a shorter time
(8 h) by GC-FID analysis. Surprisingly, the unreacted
mandelonitrile (1), after derivatization with anhydride
acetic and pyridine, was agreed for the (S)-enantiomer (1),
which would be incoherent for a typical KR (GC-FID
analysis).
In accordance to GC-FID and HPLC analyses, it is
concluded that lipase-catalyzed highly enantioselective
kinetic resolution of the ( )-mandelonitrile (1).
Conventionally, the (S)-mandelonitrile (1) was selectively
esterificated to (S)-mandelonitrile acetate (2) in accordance
with Kazlauskas rule, under orbital shaker and microwave
irradiation. However, the (R)-mandelonitrile (1) was
preferentially transformed to benzaldehyde by spontaneous
chemical equilibrium by microwave irradiation and
orbital shaker. For this kinetic resolution, it is suggested
that the rate esterification of (S)-mandelonitrile (1)
(k3, Scheme 1) occurred more slowly than the decomposition
of (R)-mandelonitrile (1) to benzaldehyde (k1, Scheme 1)
under microwave irradiation. Therefore, under microwave
irradiation, the (R)-mandelonitrile (1) that was not acetylated
by lipase was partially converted to benzaldehyde, and
after completed the reaction to 8 h, was enriched with the
(S)-enantiomer 1.
o
irradiation was observed at 60 and 80 C. When the
reaction was realized in toluene at 60 oC, under microwave
irradiation, no product was detected between 1 to 8 h.
o
However, when the reaction was carried out at 80 C,
the collision chances between the enzyme and substrate
molecules increased and this fact might help the formation
of “enzyme-substrate” complex, and then, the reaction
rate was also favored (entries 12-19, Table 1).12 On the
other, the reaction realized in toluene at 32 C applying
conventional condition occurred slowly at 48 and 184 h
(entries 7-11, Table 1).
CALB
organic solvent,
vinyl acetate
OAc
CN
OH
CN
OH
CN
+
k3
orbital shaker
or MW
(R)-1
(S)-2
up to > 98% ee
(S)-1
o
k2
k1
O
Due to the coincidence of the retention times of
mandelonitrile (1) and benzaldehyde in chromatographic
analyses by GC-FID, the reactions were analyzed by
HPLC (see Supplementary Information (SI) section). The
(S)-mandelonitrile acetate (2) was obtained with high
enantiomeric excess (98% ee) and (R)-mandelonitrile (1)
in minor optical purity (54% ee by HPLC; 51% ee by
GC-FID) under conventional condition (SI section).
The poor yield of (R)-1 was suggested by the partially
decomposition of mandelonitrile 1 to benzaldehyde
under microwave irradiation and conventional condition.
HPLC analyses were performed after purification of the
reaction (184 h) with silica flash column chromatography
for kinetic resolution in on orbital shaker. The absolute
configuration of (S)-mandelonitrile acetate (2) was
attributed by comparing the rotation signal measured with
that reported in the literature.15,16 Therefore, it is expected
that the unreacted enantiomer 1 present R-configuration.
When the KR was conduced on microwave irradiation,
the (S)-mandeonitrile acetate (2) was obtained with
H
+
HCN
Scheme 1. Pathway for kinetic enzymatic resolution and the spontaneous
transformation of (R)-mandelonitrile (1) to benzaldehyde.
Finally, the hypothesis described in Scheme 1 was
confirmed by GC-FID and HPLC analyses of the reaction
products under conventional condition and microwave
irradiation (see SI section).
Conclusions
In conclusion, this study investigated the first efficient
kinetic resolution of ( )-mandelonitrile (1) by lipase
from C. antarctica under conventional condition and
microwave irradiation, producing the (S)-mandelonitrile
acetate (2) with high selectivity (up to > 98% ee). The
unreacted (R)-mandelonitrile (1) was partially converted
to benzaldehyde from spontaneous chemical equilibrium
under microwave irradiation and conventional condition.