6
N. Melais et al. / C. R. Chimie xxx (2016) 1e7
lowered in acylation as well as in hydrolysis. For CCL, the
long and narrow site will affect the insertion of the sub-
strate at the active site.
commercially available (Aldrich). The organic solvents were
dried over molecular sieves prior to use.
We observed the affinity of three lipases relative to the
impact of the migrating group of the ester derivatives on
vinyl esters in acylation or on the corresponding racemic
phenylethylesters in hydrolysis. It is interesting to note that
the lipases studied display different selectivities and re-
activities in these two complementary reactions (acylation/
hydrolysis). The results can be correlated to the three-
dimensional shape of each lipase and conditions of each
reaction.
4.2. General procedure for the synthesis of racemic acetates
3ae3f
The acetates 3aeb were synthesized by chemical
acetylation via the corresponding racemic alcohol
(1 mmol), using 1.5 mmol of anhydride acetic, 1.2 mmol of
Et3N, and a catalytic amount of 4 dimethylaminopyridine
(0.1 mmol) in 4 mL of ether. The alkyl esters 3cef were
prepared from 1-phenylethanol (10 mmol) and pyridine
(12 mmol) in dry diethyl ether (10 mL) and cooled (0 ꢀC) in
an ice bath, the acyl chloride (12 mmol) is added dropwise
and the mixture was stirred at room temperature for 24 h.
Next, the reaction mixture was diluted with diethyl ether
and washed with 0.1 M aqueous HCl, saturated NaHCO3,
and finally with brine. The ether solution was dried over
MgSO4. The solvent was evaporated and the crude product
was purified by column chromatography (silica gel, pe-
troleum ether/ethyl acetate, 9:1). The ester derivatives
were obtained with good yields (72% < yields < 99%). The
1H and 13C NMR spectra of these products were in good
agreement with the literature.
3. Conclusion
We compared the structural and environmental pa-
rameters governing the enantioselectivity of three lipases
CAL-B, CCL and PCL for resolving chiral secondary alcohols
by acylation with vinyl esters of different lengths and
shapes, and by hydrolysis of the corresponding racemic
esters for a better understanding of the phenomena
involved in both reactions.
The results show the strong influence of the migrating
group of the acyl donor. There is a significant difference in
reactivity and selectivity of the lipases according to the
structure of the acylating agents. This can be attributed to
their respective paths to the active site. CAL-B is highly
selective in the transesterification of 1-phenyl ethanol with
IA, VA, VD, VL (E > 200), while vinyl benzoate and vinyl
pivalate do not react. With CAL-B, 1-phenyl-ethyl acetate,1-
phenyl-ethyl decanoate and 1-phenyl -ethyl laurate are
hydrolyzed with high selectivities (E > 150).
Comparison of the reactivity and selectivity of lipases
for acylation and hydrolysis shows, for the first time, many
similarities for the two reactions, which depend on the
structure of the migrating group. C. antarctica lipase B is by
far the most effective in both reactions. This enantiocom-
plementarity offers a very interesting additional catalysis
tool; the strategy can be adapted to the desired enantiomer.
4.3. General procedure for enzymatic acylations
The selected enzyme was added to a solution of racemic
1-phenylethanol (1 mmol) and the vinyl ester 2aef
(3 mmol) inꢀdiethyl ether (5 mL), and the mixture was
stirred at 40 C for 24 h. The reaction mixture was filtered
on Celite and concentrated in vacuo. The (S)-1 phenyl-
ethanol and the corresponding (R)-ester were separated by
flash chromatography on silica gel (petroleum ether/ethyl
acetate: 90/10) and analysed by chiral HPLC or GC.
4.4. General procedure for enzymatic hydrolysis of racemic
esters
One mmol of racemic esters of 1-phenylethanol 3aef
was dissolved in 2 mL of ether and added to 4 mL of
phosphate buffer pH 7. The reaction was initiated by the
addition of lipase and the mixture was shaken at 300 rpm
at 40 ꢀC for 24 h. The reaction mixture was filtered on
Celite. The alcohol formed and the remaining ester were
separated by aqueous base-organic solvent liquideliquid
extraction and finely by flash chromatography on silica gel
(petroleum ether/ethyl acetate: 90/10) and analysed by
chiral HPLC or GC.
4. Experimental section
4.1. General
NMR spectra were performed with Bruker spectrome-
ters (300 MHz for 1H, 75 MHz for 13C). Chemical shifts were
reported in d ppm from tetramethylsilane with the solvent
resonance as internal standard for 1H NMR and chloroform-
d (d 77.0 ppm) for 13C NMR. The enantiomeric excesses
(ees) were determined by gas chromatography (Thermo-
Finnigan Trace GC) equipped with an automatic autosam-
pler and using a CHIRALSIL-DEX CB column (25 m;
4.5. Chiral GC analysis and/or Chiral HPLC analysis
0.25 mm; 0.25
m
m), or by a chiral stationary phase HPLC on
Retention times are reported in minutes. The conditions
for the analysis of alcohols (R)-1e9 and acetates (S)-1ae9a
are reported below.
(R,S)-1-Phenylethanol 1: GC (Chiralsil-Dex CB):
tR ¼ 3.9 min, tS ¼ 4.1 min (Tcolumn ¼ 140 ꢀC, flow 1.2 mL/min).
(R,S)-1-Phenylethyl acetate 3a,b: GC (Chiralsil-Dex CB),
tS ¼ 2.9 min, tR ¼ 3.2 min (Tcolumn ¼ 140 ꢀC, flow 1.2 mL/
min). Eluent: petroleum ethereAcOEt: 8/2.
Chiralcel-ODH column using racemic compounds as refer-
ences. Retention times are reported in minutes. C.
antarctica lipase fraction B immobilized on acrylic resin
(CAL-B; LA > 10,000 U/g), P. cepacia lipase (PCL;
LA > 30,000 U/mg), and C. cylindracea lipase (CCL;
LA ¼ 3.85 U/mg) were from Sigma-Aldrich. Vinyl esters
2aef and both enantiomers of 1-phenylethanol 1 are
Please cite this article in press as: N. Melais, et al., The effect of the migrating group structure on enantioselectivity in lipase-