Drastic enhancement of the enantioselectivity of lipase-catalysed esterification
in organic solvents by the addition of metal ions
Takashi Okamoto and Shinichi Ueji*b
a
a
The Graduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan
Division of Natural Environment and Bioorganic Chemistry, Faculty of Human Development and Sciences, Kobe
b
University, Nada, Kobe 657-8501, Japan. E-mail: ueji@natura.h.kobe-u.ac.jp
Received (in Cambridge, UK) 22nd March 1999, Accepted 12th April 1999
By the addition of metal ion-containing water, a marked
enhancement of the enantioselectivity for the lipase-cata-
lysed esterification of 2-(4-substituted-phenoxy)propionic
For the model reaction (Scheme 1), the R enantiomer of the
butyl ester were preferentially produced in all the substrates
1–5. We investigated the behaviour of the lipase’s enantiose-
lectivity caused by the addition of a small amount of alkaline
metal ion (LiCl, NaCl or KCl)-containing water or a small
acids in Pri
O was observed, the mechanism of which will be
2
discussed briefly on the basis of the initial rate obtained for
each enantiomer of the substrate.
amount of water to the Pri
O of the reaction medium. Fig. 1
2
shows the variation of the E value for the lipase MY-catalysed
esterification of 1 at ca. 40% conversion as a function of the
amount of each additive. As is seen in Fig. 1, when a small
amount of the alkaline metal ion-containing water was added to
the reaction medium, the enantioselectivity was found to be
dramatically enhanced, as compared with addition of water
alone or no additive. In particular, upon addition of 0.5 vol% of
LiCl-containing water, lipase MY displayed the highest E value
(ca. 200), which was 5 and 50 times larger than that for water
alone or no additive, respectively. A drop in the E value,
however, was produced by the addition of a large amount of the
metal ion-containing water (see the bell-shaped plots in Fig. 1).
This can be explained by assuming that an excess of water
molecules hydrated to the ion in the reaction medium causes the
hydrolysis (the reverse reaction) of the corresponding ester
product of 1, thus leading to the loss of enantioselectivity,
probably because the sites of the lipase molecule associated
with the water molecule are restricted. The same trend was also
observed for water alone (Fig. 1). In fact, the ester product of 1
Recently, much attention has been paid to lipase-catalysed
enantioselective transformations in organic solvents, because
lipases display broad substrate specificities with high enantiose-
1
lectivities. For this reason, organic chemists often use lipases
for the synthesis of useful optically active compounds. One of
the most important factors for lipase-catalysed reactions is the
control of their enantioselectivites as a function of reaction
conditions.1
Here, we report a new and a simple method for improving
markedly the enantioselectivity of lipase-catalysed esterifica-
tion of 2-(4-substituted-phenoxy)propionic acids 1–5 in Pri
O
2
as a model reaction by the addition of alkaline metal ion (e.g.
LiCl)-containing water (Scheme 1). This is the first example of
an alkaline metal ion enhancing the enantioselectivity of a
lipase in organic solvents. Furthermore, the mechanism of the
enhanced enantiorecognition will be discussed briefly utilising
data on the initial rate obtained for each enantiomer of substrate
1
in the model reaction.
All the substrates used in the model reactions, racemic
-(4-substituted-phenoxy)propionic acids, were prepared from
was subjct to hydrolysis in Pri
water of 0.75 vol% or above.†
2
O with water or LiCl-containing
2
ethyl 2-bromopropionate and the corresponding 4-substituted-
phenols according to the known method.
In a typical enzymatic reaction, the substrates 1–5 (0.36
In order to investigate the scope of this enhancement effect,
the other substrate 2–5 with a wide variety substituents and the
other enzyme lipase AY were submitted to the model reaction.
From the data summarized in Table 1, the optimum additive
conditions (0.5 vol% of LiCl-containing water) were found to
produce dramatically increased enantioselectivity for all the
substrates 1–5 and both lipases, although lipase AY differs from
lipase MY in its catalytic features.4
2
mmol) and a three-fold molar excess of BuOH (1.08 mmol, 80
mg) were dissolved in 2 ml of Pr1
O. To the solution, a small
2
amount of 2.4 M metal ion-containing water or water (0–0.75
vol%) was added followed by ultrasonic dispersion, and then
powdered lipase MY or lipase AY from Candida rugosa (30
mg) was added. The suspension was shaken (170–200 strokes
2
1
min ) at 37 °C until ca. 40% of the substrate was converted to
the corresponding butyl ester. The enantiomeric ratio (E value)
was calculated from the enantiomeric excess (ee) for the butyl
ester produced, according to the literature.3 The ee was
measured by HPLC on a chiral column [Daicel Chiral Cel
OK].
Fig. 1 The variation of the E value of lipase MY-catalysed esterification of
1
at ca. 40% conversion as a function of the amount of each additive: (a)
Scheme 1
LiCl, (b) NaCl, (c) KCl and (d) water.
Chem. Commun., 1999, 939–940
939