First lipase catalysed resolution of epoxy enol esters

Article abstract of DOI:10.1016/j.tetlet.2006.06.009

We report the first enzyme-catalysed kinetic resolution of epoxy enol esters. The lipase-promoted hydrolysis of these compounds provided α-hydroxyketones or α-hydroxyaldehydes (arising from the spontaneous rearrangement of the epoxy enols) and the residual esters with moderate to good enantioselectivity (E up to 100).

Full text of DOI:10.1016/j.tetlet.2006.06.009

Tetrahedron Letters 47 (2006) 6153–6157
First lipase catalysed resolution of epoxy enol esters
a
a
b
a,
*
´
ˆ
Sebastien Gravil, Henri Veschambre, Robert Chenevert and Jean Bolte
a
`
Laboratoire SEESIB UMR CNRS 6504, Universite´ Blaise-Pascal, 63177 Aubiere Cedex, France
bDe´partement de chimie, Faculte´ des Sciences et de Ge´nie, Universite´ Laval, Que´bec, Canada G1K 7P4
Received 28 March 2006; revised 24 May 2006; accepted 2 June 2006
Abstract—We report the first enzyme-catalysed kinetic resolution of epoxy enol esters. The lipase-promoted hydrolysis of these
compounds provided a-hydroxyketones or a-hydroxyaldehydes (arising from the spontaneous rearrangement of the epoxy enols)
and the residual esters with moderate to good enantioselectivity (E up to 100).
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Therefore, the preparation of chiral a-hydroxyaldehydes
remains an issue and we looked for alternative enzy-
matic methods to generate these compounds under mild
conditions. In 1996, Kern and Spiteller5 reported the
synthesis of three racemic long chain aliphatic a-hydr-
oxyaldehydes by a thermal rearrangement of epoxy enol
esters in the presence of a protic acid, followed by
enzymatic hydrolysis of the intermediate a-acetoxyalde-
hydes (Scheme 1). More recently, Shi and co-workers.6
showed that the rearrangement of enol ester epoxides
to a-acyloxy ketones under thermal or acidic conditions
is stereoselective.
a-Hydroxyaldehydes are valuable synthetic intermediates
and are also building blocks for the synthesis of carbohy-
drates and analogues via chemical or enzymatic aldol reac-
tions.1 As part of a research program designed to explore
the utility of transketolase and fructose-1,6-bisphosphate
aldolase in organic synthesis, we required a number of chi-
ral a-hydroxyaldehydes preferably in the enantiomerically
pure form.2 a-Hydroxyaldehydes are generally accessible
by two main routes. First, a general method based on
the ozonolysis of allylic alcohols provides the aldehyde
functionality. Enantiomerically enriched allylic alcohols
can be obtained by enzymatic resolution.3 Alternatively,
a-hydroxyaldehyde acetals are obtained, either by the ring
opening of a 2,3-epoxy-propionaldehyde-diethylacetal by
various nucleophiles or by a Barbier type reaction on a gly-
oxal monoacetal.4 The acetals are then readily hydrolysed
in acidic media. a-Hydroxyaldehydes often present as olig-
omers are very difficult to purify and characterise so that
the last step of their synthesis has to be very efficient and
must not generate by-products. However, in the first meth-
od, the reductive work-up after hydrolysis produces either
triphenylphosphine oxide or dimethylsulfoxide, and in the
second method, the acidic conditions required for acetal
hydrolysis can lead to partial racemisation or isomerisa-
tion into a-hydroxyketones.
It occurred to us that direct enantioselective enzyme
catalysed hydrolysis of epoxy enol acetates could pro-
vide optically pure a-hydroxyaldehydes or a a-hydroxy-
ketones via an unstable hemiketal, and residual epoxy
enol acetates.
Herein, we report which are, to the best of our know-
ledge, the first enzyme-catalysed resolutions of epoxy
enol esters (also called enol ester epoxides) to give
enantiomerically enriched a-hydroxyaldehydes or a-
hydroxyketones.
2. Results and discussion
2.1. Synthesis of substrates
Keywords: Enzyme catalysis; Hydrolases; Kinetic resolution; Hydro-
lysis; Epoxy enol esters.
We prepared racemic enol ester epoxides 1, 2, 3a–c
(Fig. 1) by two different routes. Compounds 1 and 2
can lead to a-hydroxyketones whereas compounds
3a–c are precursors for a-hydroxyaldehydes. Also, it is
*
Corresponding author. Tel.: +33 4 73 40 71 28; fax: +33 4 73 40 77
17; e-mail addresses: Jean.Bolte@univ-bpclermont.fr; jbolte@chimie.
univ-bpclermont.fr
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.06.009

6154
S. Gravil et al. / Tetrahedron Letters 47 (2006) 6153–6157
OAc
d
OH
O
O
R
R
b
OAc
OH
OH
O
a
d
c
OAc
OAc
O
O
O
R
R
R
R
R
R
d
O
OH
Scheme 1. Synthesis of a-hydroxyaldehydes from enol acetate epoxides: (a) epoxidation, (b) thermal or Lewis acid catalysed rearrangement, (c)
protic acid catalysed rearrangement, (d) enzymatic hydrolysis.
AcO
AcO
O
O
O
O
O
OAc
OAc
O
O
OAc
1
2
3a
3b
3c
Figure 1. Epoxy enol ester substrates.
necessary to control the E–Z configuration of acyclic
compounds 2, 3a–c since the presence of the two diaste-
reoisomers will decrease the enantiomeric purity of the
desired hydroxyaldehydes or hydroxyketones.
yield. The isomeric ratio Z/E = 10 was determined by
gas chromatography and a flash column chromatogra-
phy afforded pure Z-4. The epoxidation of this isomer
was carried out with MCPBA (74% yield).
According to a reported procedure,7 1-acetoxy-1,2-
epoxycyclohexane 1 was prepared from cyclohexanone
by acylation with isopropenyl acetate in the presence
of p-toluenesulfonic acid (p-TsOH) as a catalyst fol-
lowed by epoxidation of the intermediate cyclohexanone
enol acetate by m-chloroperbenzoic acid (MCPBA).
1-Acetoxy-1,2-epoxy-1-phenylpropane 2 was obtained
by a similar two-step procedure (Scheme 2). Propiophe-
none enol ester 4 was prepared from propiophenone and
acetic anhydride using p-TsOH as a catalyst with a 43%
We then applied the above method based on the epoxi-
dation of enol esters to the preparation of compounds
3a–c but inseparable mixtures of E–Z isomers were ob-
tained. We therefore examined another way of control-
ling the stereochemistry of the products, starting from
a,b-unsaturated ketones 6a–c (Scheme 2). Compound
6b is commercially available, 6a and 6c were prepared
from aldehydes 5a and 5c by the Wittig reaction with
1-triphenylphosphoranylidene-2-propanone in 58% and
79% yield, respectively. It is generally found that ylides
AcO
O
OAc
O
a
b
Z
- 4
2
OAc
+
E
- 4
H
O
H
H
O
H
O
H
d
c
d
OAc
R
R
R
O
H
H
6b
3a-c
5a and 5c
6a and 6c
5a R = PhCH2
5b R = CH3(CH2)3
5c R = (CH3O)2CHCH2
Scheme 2. Synthesis of 2, 3a–c. Reagents and conditions: Ac2O, p-TsOH, 165 °C, 12 h, Z/E = 10, 9% yield for Z isomer after purification; (b)
MCPBA, CH2Cl2, 1 h, 0 °C, 3 h room temperature, 74% yield; (c) Ph3P@CHCOCH3, THF, room temperature. Compound 6a, 6 days, 58%; 6c, 12 h,
79%; (d) MCPBA, CH2Cl2, room temperature. Compound 6a, 2 days 47%, 6b, 5 days, 68%, 6c, 3 days, 35%.

S. Gravil et al. / Tetrahedron Letters 47 (2006) 6153–6157
6155
containing stabilising groups give E alkenes.8 The E
configuration was confirmed by H NMR analysis of
1-acetoxy-1,2-epoxycyclohexane 1 was chosen as the
model substrate to investigate the kinetic resolution pro-
cess. This compound is unstable in the presence of water
and we operated in diisopropyl ether containing 1% eth-
anol. The resolution was carried out on 1 mmol of sub-
strate, 200 mg of enzyme in 3 ml of solvent. The results
are reported in Scheme 3 and Table 1. The enantioselec-
tive reaction of ( )-1 in the presence of various lipases
gave a-hydroxyketone (R)-8, a small amount of the cor-
responding a-acetoxyketone (R)-9, and the residual
starting material (1S,2S)-1. The progress of the reaction
c (conversion rate) was monitored by gas chromatogra-
phy. Comparison of the specific rotation of chiral non-
racemic 1 with data reported in the literature6 estab-
lished its optical purity and its absolute configuration.
The stereochemistry of the reactions was further con-
firmed by the optical rotation data of known (R)-(+)-
2-hydroxycyclohexanone 8.14 The enantiomeric ratio
E15 was measured using the extent of conversion c and
the optical purity of the starting material. The reaction
mechanism is shown in Scheme 3. Alcohol 8 results from
the rearrangement of hemiacetal 7 provided by the enzy-
matic ester hydrolysis or alcoholysis. The formation of
acetoxycyclohexanone 9 can be due to the esterification
of 8. Indeed lipases are able to acylate alcohols in non-
aqueous solvents in the presence of acyl group donors,
here the substrate 1. During the reaction, an inter-
mediate acetyl-enzyme is formed, which undergoes an
1
the olefinic protons and no Z isomer was detected.
The tandem Baeyer–Villiger oxidation–epoxidation9 of
a,b-unsaturated ketones 6a–c with an excess of m-chloro-
perbenzoic acid provided enol ester epoxides 3a–c (47%,
68% and 35% yield). The Baeyer–Villiger reaction has
been demonstrated to occur with retention of configura-
tion.10 The E (trans) configuration of the oxirane ring
1
was confirmed by H NMR analysis; a small coupling
constant (J 6 1 Hz) is expected for H-1 in the E (trans)
configuration whereas J P 3 Hz is found in compounds
with the Z (cis) configuration.5 Dimethoxypropanal 5c
was obtained by ozonolysis of 4,40-dimethoxybutene.11
2.2. Enzymatic kinetic resolutions
In this preliminary study, in order to demonstrate the
validity of our approach, we studied the hydrolysis of
1, 2, 3a–c in the presence of a few common commercial
enzymes.12
Stereoselective enzymatic hydrolyses of esters can be
carried out in aqueous buffers, in biphasic mixtures of
water and an organic solvent, and in water saturated or-
ganic solvents; an alternative is the transesterification
(alcoholysis) in a solvent of low polarity containing a
simple alcohol.13
EtOAc + Enz-OH
EtOH
AcO
HO
AcO
O
O
O
Enz-OH
+
Enz-OAc
+
O
7
OH
( )-1
(+)-(
S,
S)-1
(+)-(R)-8
O
O
OH
OAc
+
Enz-OH
(+)-(R)-9
(+)-(R)-8
Scheme 3. Enzymatic alcoholysis of 1-acetoxy-1,2-epoxycyclohexane 1.
Table 1. Enzymatic alcoholysis/hydrolysis of 1-acetoxy-1,2-epoxy cyclohexane 1 in diisopropyl ether
Entry
Enzymea
Conditionsb
Time (h)
cc (%)
Product ratio 1/8/9
Residual 1 optical purity (%)d
E
1
2
3
4
5
6
7
CAL
PSL
PSL
CRL
CCL
CCL
CCL
EtOH 1%
EtOH 1%
192
26
48
17.5
96
32
56
53
60
50
59
59
55
44/36/20
47/45/8
40/35/25
50/45/5
41/34/25
40/42/18
45/45/10
71
82
83
90
98
88
92
7.2
15
8.6
40
23
11
22
EtOH 1%
EtOH 1%
H2O satd
7
a CAL: Candida antarctica lipase B; PSL: Pseudomonas sp. lipase; CRL: Candida rugosa lipase; CCL: Candida cylindracea lipase.
b 1 mmol (156 mg) of substrate, 200 mg of lipase in 3 ml of diisopropyl ether.
c After isolation of products.
d Optical purity according to literature data.

6156
S. Gravil et al. / Tetrahedron Letters 47 (2006) 6153–6157
hydrolysis, or in the absence of water an alcoholysis by
added ethanol or alcohol 8. When no EtOH was added
(assays 3 and 5), residual water present in the enzyme
preparation and hydroxycyclohexanone 8 were the
nucleophiles competing in the enzyme regeneration
and the yield in 9 increased.
(1 mmol) in a biphasic system buffer/hexane in the pres-
ence of CAL (2 mg) gave the best results (Scheme 4 and
Table 2). In all hydrolysis reactions, the (S,S) enol ester
epoxide was the fast reacting enantiomer yielding the
(S)-hydroxyaldehyde (vide infra) and leaving the
(R,R)-epoxide unreacted in enantiomerically enriched
form. The formation of a-acetoxyaldehydes was not
observed with these substrates. The enantiomeric excess
1-Acetoxy-1,2-epoxy-1-phenylpropane
2 reacts very
1
slowly in organic media and is stable in a phosphate
buffer pH 7. Thus, the hydrolysis was carried out on
1 mmol of substrate in 50 ml of this buffer in the pres-
ence of 27 mg of enzyme, or in a biphasic system hexane
(30 ml)/phosphate buffer (30 ml) in the presence of 2 mg
of enzyme. The results are summarised in Scheme 4 and
Table 2. The enantiomeric excesses and the absolute
configurations were determined by comparison with lit-
erature data; the absolute configurations of residual (+)-
26 and (ꢀ)-2-hydroxy-1-phenylpropan-1-one 1016 are
(1S,2R) and (S), respectively. Lipases from Pseudo-
monas sp. (PSL), Candida cylindracea (CCL) and Can-
dida rugosa (CRL) showed very low enantioselectivity
towards substrate 2. The best results were obtained with
Candida antarctica lipase B (CAL) in phosphate buffer
(E = 34).
of the remaining epoxides were measured by H NMR
spectroscopy using Eu(hfc)3 as a chiral shift reagent
(substrates 3a and 3c) or by chiral gas chromatography
on a chiraldex c-TFA column (substrate 3b). The enan-
tiomeric compositions and absolute configurations of
aldehydes 11a–c were determined by conversion to the
corresponding dimethylacetals 12a–c (in the case of
11c, a transacetalation produced the bis-dimethylacetal)
followed by 19F and 1H analysis17 of the Mosher’s esters
(a-methoxy-a-trifluoromethyl-a-phenyl acetates 13a–c)
(Scheme 5). The absolute configurations of 11a and
11c were further confirmed by correlation with 1,2-diol
14a18 and 14c19 of known absolute configuration.
In conclusion, we have developed the first biocatalytic
kinetic resolution of enol ester epoxides leading to
homochiral enol ester epoxides and a-hydroxyaldehydes
or a-hydroxyketones. Enol ester epoxides are syntheti-
cally useful intermediates. One enantiomer (1R,2R) in
the case of 3a–c can be obtained in a good enantiomeric
excess. The reacting enantiomers (1S,2S) in the case of
Next, we completed some screening experiments in order
to find hydrolases with the ability to resolve enol ester
epoxides 3a–c leading to a-hydroxyaldehydes. Of the en-
zymes and conditions tested, the hydrolysis of 3a–c
AcO
O
AcO
O
O
Enzyme
+
OH
H2O
(-) (S) - 10
( ) - 2
(+) (1
R
S,2R
) - 2
OH
Enzyme
O
O
+
OAc
O
R
OAc
R
biphasic
system
( )-3a-c
(S)-11
(+) (1
R,2
R)-3a-c
a) R = Ph
b) R = (CH2)2CH3
c) R = CH(OCH3)2
Scheme 4. Enzymatic hydrolysis of epoxy enol esters 2 and 3a–c.
Table 2. Enzymatic hydrolysis of epoxy enol esters 2, 3a–c
Entry
Substrate
Conditions
Enzyme
Time (h)
Ce (%)
ee or (op) of residual substrate (%)
E
1
2
3
4
5
6
7
2
Buffera
CALb
PPLb
CAL
CAL
CAL
PSLb
CAL
17
18
31
1
4
24
11
(92)
(50)
(93)
46
89
88
34c
2
Buffera
9c
21e
15
2
Buffer/hexaned
Buffer/hexaned
Buffer/hexaned
Buffer/hexaned
Buffer/hexaned
56
62
46
47
64
3a
3b
3b
3c
100
75
12
96
a 1 mmol (228 mg) of substrate in 50 ml of phosphate buffer pH 7, 27 mg of enzyme.
b Candida antarctica lipase; PPL: Porcine pancreatic lipase; PSL: Pseudomonas sp. Lipase.
c Calculated from the optical purity of isolated residual substrate and product.
d 1 mmol of substrate (228, 240, 158 and 190 mg of 2, 3a–c) in 30 ml of hexane and 30 ml of phosphate buffer (0.1 M, pH 7) containing 2 mg of
enzyme.
e Determined by GC analysis.

S. Gravil et al. / Tetrahedron Letters 47 (2006) 6153–6157
6157
OH
OH
MeO Ph
A)
B)
MeOH, H+
O
O
R
R
R
OMe
O
R
Mosher's acid chloride
F3C
11
H CH(OMe)2
OMe
O
12
13
OH
OH
NaBH4
R
OH
14a-c
For 12c, 13c : R = CH(OMe)2
11a-c
a) R = Ph
b) R = (CH2)2CH3
c) R = CH(OEt)2
Scheme 5. Determination of enantiomeric excesses and absolute configuration of 11a–c.
8. Kolodiazhnyi, O. I. Phosphorus Ylides, Chemistry and
Applications in Organic Synthesis; Wiley-VCH: Weinheim,
1999.
9. DeBoer, A.; Ellwanger, R. E. J. Org. Chem. 1974, 39, 77–
83.
10. Renz, M.; Meunier, B. Eur. J. Org. Chem. 1999, 737–750.
11. Hoaglin, R. I.; Kubler, D. G.; Montagna, A. E. J. Am.
Chem. Soc. 1958, 80, 5460–5463.
12. We used Candida cylindracea lipase (CCL) from Sigma,
595 U/mg of solid, Candida rugosa lipase (CRL) from
Sigma, 875 U/mg of solid, Porcine pancreas lipase (PPL)
from Sigma, 15 U/mg of solid, Candida antarctica lipase
(CAL) from Novo, 223 U/mg of solid and Pseudomonas
sp. lipase (PSL AK) from Amano, 29 U/mg of solid.
13. (a) Bornscheuer, V. T.; Kazlauskas, R. J. Hydrolases in
Organic Synthesis; Wiley-VCH: Weinheim, 1999; (b) Gais,
H. J.; Theil, F. In Enzyme Catalysis in Organic Synthesis;
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3a–c lead to the (2S) a-hydroxyaldehydes of good enan-
tiomeric excess if the hydrolysis is stopped before 50%
conversion (depending of the E value). On the other
hand, the residual epoxides can be hydrolysed under
mild conditions in the presence of a nonspecific esterase
(like pig liver esterase or pig pancreatic lipase (Table 2))
to lead to (2R) a-hydroxyaldehydes. Moreover, residual
ester enol epoxides can be submitted to a stereoselective
rearrangement with retention or inversion of configura-
tion to provide either enantiomer of a-acyloxy aldehydes
or ketones.6 By this methodology, both isomers of
a-hydroxyaldehydes, which are prone to racemise or
isomerise into hydroxyketone can be obtained in aque-
ous solution at neutral pH. The scope and limitations
of this novel biocatalytic reaction and its synthetic appli-
cation are currently being studied in detail.
14. (a) Bogevig, A.; Sunden, H.; Cordova, A. Angew. Chem.,
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