On the enzymatic hydrolysis of methyl 2-fluoro-2-arylpropionates by lipases

Article abstract of DOI:10.1016/j.tet.2005.06.007

The enzymatic hydrolysis of methyl 2-fluoro-2-arylpropionates was performed using lipases from Candida rugosa and Candida cylindracea (OF-360). A careful analysis of the reaction products revealed that racemic 2-hydroxy-2- arylpropionic acid and traces of 2-arylacrylic acid are formed, in addition to the expected 2-aryl-2-fluoropropionic acid. The presence of powerful electron-releasing groups in the aromatic ring of the substrate increase the amount of 2-hydroxypropionic acid. A mechanistic hypothesis has been formulated according to which the enzyme facilitates the elimination of fluoride ion from the hydrolysed acid with the formation of an α-carboxy-stabilized carbocation which provides 2-hydroxypropionic acids by nucleophilic attack of H₂O and 2-arylacrylic acids by a β-elimination process.

Full text of DOI:10.1016/j.tet.2005.06.007

Tetrahedron 61 (2005) 8005–8012
On the enzymatic hydrolysis of methyl
2-fluoro-2-arylpropionates by lipases
b
b,
Francesca Bellezza,a Antonio Cipiciani,a, Giacomo Ricci and Renzo Ruzziconi
*
*
a
`
‘Centro di Eccellenza Materiali Innovativi Nanostrutturati’ (CEMIN); Universita di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy
b
`
‘Laboratorio composti fluororganici’, Dipartimento di Chimica, Universita di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy
Received 14 April 2005; revised 20 May 2005; accepted 2 June 2005
Available online 29 June 2005
Abstract—The enzymatic hydrolysis of methyl 2-fluoro-2-arylpropionates was performed using lipases from Candida rugosa and Candida
cylindracea (OF-360). A careful analysis of the reaction products revealed that racemic 2-hydroxy-2-arylpropionic acid and traces of
2-arylacrylic acid are formed, in addition to the expected 2-aryl-2-fluoropropionic acid. The presence of powerful electron-releasing groups
in the aromatic ring of the substrate increase the amount of 2-hydroxypropionic acid. A mechanistic hypothesis has been formulated
according to which the enzyme facilitates the elimination of fluoride ion from the hydrolysed acid with the formation of an a-carboxy-
stabilized carbocation which provides 2-hydroxypropionic acids by nucleophilic attack of H2O and 2-arylacrylic acids by a b-elimination
process.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
2-hydrogen of a 2-arylpropionic acid with the quasi-
isosteric fluorine conveys a higher configurational stability
to the chiral carbon,7 thus allowing the drug pharmaco-
dynamics, as well as the stereochemical matching with the
receptor, to be investigated.
The insertion of fluorine atoms into organic molecules can
cause profound and often unexpected effects on their
activities ensuing from the peculiar properties of this
element. Its high electronegativity frequently causes such
dramatic electronic changes that it can also significantly
affect the reactivity, stability and acidity/basicity of the
neighbouring groups.1
Until now, only a limited number of methods have been
reported for the synthesis of side-chain fluorinated
2-arylpropionic acids, and to our knowledge the synthesis
of both (R)- and (S)-2-fluoro-2-(4-isobutylphenyl) propionic
acid (a-fluoroibuprofen) reported by Schlosser and
co-workers in 1996 is the only example of chiral a-fluoro-
profen known to date.8
Besides its ability to mimic the hydroxyl functionality and
to act as a hydrogen-bond acceptor,2 fluorine often causes
remarkable conformational changes in biologically active
compounds resulting in major pharmacological effects such
as an increase in both activity and selectivity. This is why
the synthesis of regio- and stereoselectively fluorinated
molecules is popular with many researchers, especially
during the last few decades.3,4
Although optically active cyanohydrins are readily avail-
able, potentially allowing the stereoselective synthesis of
this compound,9,10 resolution of racemic a-fluoroibuprofen
by enzymatic hydrolysis of its methyl ester was preferred
because it allows the enantiomers to be recovered in
satisfactory yields, as well as the optical purity. Lipase from
Candida cylindracea (OF-360) was found to be the best
catalyst to perform the desired transformation.
In this context, fluorinated non-steroidal anti-inflammatory
drugs (NSAID), especially those belonging to the ‘profen’
family, have received particular attention. Among them,
fluorinated surrogates such as flurbiprofen5 and fluroxa-
profen6 are two of the most successful NSAIDs. Given that
C–F bonds are stronger than C–H bonds, replacing the
During our research on a novel synthetic approach to both
nucleus and side-chain fluorinated 2-arylpropionic acid,11
we needed to resolve the racemic mixture of 2-fluoro-2-
naphthylpropionic acids. Unexpectedly, all attempts to
resolve racemic a-fluoronaproxen by enzymatic hydrolysis
of its methyl ester using different lipases were unsuccessful,
leading to everything but optically pure a-fluoronaproxen.
Keywords: Lipases; Enzymatic hydrolysis; Fluorinated compounds;
Profens; a-hydroxyacids.
Corresponding authors. Tel.: C39 075 5855540, fax: C39 075 5855560;
e-mail: cipan@unipg.it
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.06.007

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F. Bellezza et al. / Tetrahedron 61 (2005) 8005–8012
Nevertheless, we felt that this intriguing process was worthy
of being studied in depth. The results of this investigation
are reported in this paper.
2. Results and discussion
Candida rugosa lipase (CRL) and lipase OF-360 have been
used successfully for kinetic resolutions of various racemic
2-arylpropionic12 and 2-aryloxypropionic acids.13,14 Thus,
we first tried to use these biomaterials for the kinetic
resolution of 2-fluoro-2-(6-methoxynaphth-2-yl)propionic
acid 1 by the lipase-catalyzed hydrolysis of the correspond-
ing methyl ester 2. The latter was prepared in two steps, in
51% overall yield, by adding 6-methoxy-2-naphthyl-
magnesium bromide to methyl pyruvate followed by
fluorination of the resulting methyl 2-hydroxy-2-(6-meth-
oxynaphth-2-yl)propionate 3 with diethylaminosulfur tri-
fluoride (DAST).
Scheme 2.
1 was exclusively converted into a-hydroxyacid 5 after 1 d
(Scheme 2).
The racemic a-fluoroester 2 was hydrolysed by crude lipase
OF-360 (C-OF) and crude CRL (C-CRL) to give mostly, the
racemic 2-hydroxy-2-(6-methoxy-2-naphtyl)propionic acid
5, a small amount of the corresponding acid 1 of very low
optical purity and traces of 2-(6-methoxy-2-naphth-2-
yl)acrylic acid 4 (Scheme 1). The recovered ester 2 had
low optical purity (entries 1 and 2 in Table 1).
Methyl acrylate 6 was partially hydrolysed (47%) to give
the corresponding acid 4 after 1 d. No traces of
a-hydroxyacid were found in the reaction mixture, thus
excluding a lipase-promoted hydration of the acrylic double
bond. The hydrolysis of hydroxyester 3 turned out to be a
very slow process giving 18% of acid 5 (ee 42%) after 14 d,
while 66% of the starting material 3 (ee 9%) was recovered.
No trace of elimination product was detected. As expected,
replacing the fluorine with an OH group completely
inhibited this process.
Since arylacrylic acid 4 was clearly formed by HF
elimination, at first glance, the a-hydroxypropionic acid 5
could have come from either the lipase-catalysed hydration
of the intermediate acrylic ester or by enzyme catalysed
nucleophilic substitution of fluorine by H2O or, even better,
OHK. These results are somewhat intriguing if one
considers that when lipases co-ordinate the substrate in
their active pocket, the hydrolysis or esterification are the
only reactions catalysed.15 In our case, the presence of
fluorine in the a-position of propionate should induce
interactions that cause specific transformations.
All of these observations are in line with the following
mechanistic hypothesis. CRL catalyses the hydrolysis of
racemic a-fluoroester 2 in a relatively fast process, through
the action of the OH group of Ser209 (Scheme 3).15
Afterwards, a nucleophilic substitution reaction of fluoride
takes place leading to the a-hydroxyacid 5 (Scheme 4). In
this step, an acid group present in the active site, probably
the imidazolium moiety of His449, plays an important role in
the removal of the fluoride ion and formation of carbocation
7 that is further stabilized by the a-carboxylate group. The
intermediate 7 may undergo either deprotonation of the
methyl group, catalysed by the imidazole moiety, to give
the elimination product 4; or a nucleophilic attack by H2O,
leading to a-hydroxyacid 5. Of course, the medium polarity
In order to investigate the origin of the above products, the
arylpropionic acid 1, the methyl acrylate 6 and the
a-hydroxyester 3, initially considered probable intermedi-
ates in the whole process, were prepared by standard
procedures and submitted to the action of C-CRL. Under the
same conditions employed in the hydrolysis of ester 2, acid
Scheme 1.
Table 1. CRL and OF-360 lipase-catalysed hydrolysis of racemic methyl 2-fluoro-2-arylpropionate 2 in water at pH 7.2 and 37 8C
Entry
Enzyme
C, %a
Time, d
Product, % (ee)b
a-Fluoro-ester 2
a-Fluoro-acid 1
Acrylic acid 4
a-Hydroxy-acid 5
1
2
C-OF
C-CRL
60
60
3
6
40 (49)
40 (16)
12 (23)
18 (10)
traces
—
48 (0)
42 (0)
a Conversion, % (recovered a-fluoro-ester).
1
b Enantiomeric excess determined by H NMR spectroscopy using Eu(hfc)3 as a chiral shift reagent (substrate/Eu(hfc)3 1/6, CDCl3).

F. Bellezza et al. / Tetrahedron 61 (2005) 8005–8012
8007
Scheme 3.
and mild temperature favour the latter process. In this
particular case, the hypothesis of an E2 HF-elimination
from the starting ester 2 should be discarded, because this
high energy process would require the intervention of a
much stronger base. On the other hand, the direct hydration
of the acrylic double bond giving the a-hydroxyacid 5 is
quite improbable, at least under these mild conditions, since
the Markovnikov addition of water would be strongly
disfavoured by the presence of the carboxylic group.16
The above mechanistic hypothesis would also explain why
the lipase-catalyzed hydrolysis of methyl acrylate 6 stops at
Scheme 4.

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F. Bellezza et al. / Tetrahedron 61 (2005) 8005–8012
Scheme 5.
acid 4 and, more importantly, why the lipase-catalyzed
hydrolysis of 2 produces the racemic a-hydroxyacid 5,
while the corresponding a-fluoroacid 1 is recovered with
some optical purity.
and racemic methyl 2-fluoro-2-(4-isopropoxyphenyl)pro-
pionate 14, prepared from the corresponding a-hydroxy
esters, were submitted to the action of C-CRL (Schemes 6
and 7). At 45% conversion, after 20 h, a modest kinetic
resolution was noted for substrate 11 (48% ee for the
remaining ester, and 59% ee for the a-fluoroacid 12), and
the racemic a-hydroxyacid 13 was obtained (14%) (entry 2
in Table 2).
In order to assess the electronic effects of the substituents in
the aromatic ring on the kinetics, as well as on the product
distribution of the enzymatic resolution, methyl 2-fluoro-2-
(naphth-2-yl)propionate 8 was prepared from the corre-
sponding a-hydroxyester by an identical procedure and
submitted to hydrolysis catalyzed by C-CRL. Products 9
(39%; ee 89%) and 10 (13%; ee 0) were obtained after 1 d
analogous to what was observed with ester 2 (Scheme 5).
The unreacted ester 8 exhibited high optical purity (ee 90%)
(Table 2, entry 1).
The hydrolysis rate was practically unchanged, when the
isobutyl group was replaced with an isosteric isopropoxy
group in the a-fluoroibuprofen, while the enantioselectivity
was substantially increased, with the remaining
a-fluoroester 14 exhibiting high optical purity (ee, 95%,
entry 3 in Table 2).
The absence of a methoxy group in the naphthyl ring
increases the rate of hydrolysis and the enantioselectivity of
C-CRL toward 8, leading to the same products, but in
different molar ratios compared to the hydrolysis of 2.
According to the proposed mechanism, electronic effects
substantially affect the product distribution, while leaving
the pattern unchanged. In fact, in this case there was a
substantial decrease of a-hydroxyacid 10 with respect to
a-fluoroacid 9, in contrast to what was observed with
substrate 2. As expected, the absence of an electron-
releasing group makes the removal of the fluoride ion more
difficult.
Moreover, the resulting a-fluoroacid was not detected
because it was rapidly transformed into the corresponding
racemic a-hydroxy-acid 15. In this case, a small amount of
the a-hydroxyester 16 (10%) was also detected. It was
probably formed from a-fluoro-ester 14 in the same way as
a-hydroxyacid 15, according to the above mechanism.
Clearly, the electron-releasing effect of the isopropoxy
group is responsible for the stabilization of the benzylic
carbocation by the carboxylate ion.
The substrate structure seems to play a decisive role in the
lipase-promoted hydrolysis of condensed polycyclic
2-arylpropionates. High enantioselectivity or complete
inhibition of the hydrolytic process may be observed. This
was the case when we attempted to use the above OF-360 or
For comparison purposes, racemic methyl 2-fluoro-2-(4-iso-
butylphenyl)propionate (a-fluoroibuprofen methyl ester) 11
Table 2. CRL-catalysed hydrolysis of racemic methyl 2-fluoro-2-arylpropionate 8, 11 and 14 in water at pH 7.2 and 37 8C
C, %a
Entry
Enzyme
Time, h
Product, % (ee)b
Acrylic acid
a-fluoro-ester
a-fluoro- acid
a-hydroxy- acid a-hydroxy- ester
1
2
3
C-CRL
C-CRL
C-CRL
52
45
60
24
20
18
8 48 (90)
11 55 (48)
14 40 (95)
9 39 (89)
12 31 (59)
—
—
—
—
10 13 (0)
13 14 (0)
15 50 (0)
—
—
16 10 (0)
a Conversion, % (recovered a-fluoro-ester).
1
b Enantiomeric excess determined by H NMR spectroscopy using Eu(hfc)3 as a chiral shift reagent (substrate/Eu(hfc)3 1/6, CDCl3).
Scheme 6.
Scheme 7.

F. Bellezza et al. / Tetrahedron 61 (2005) 8005–8012
8009
CRL lipases in the kinetic resolution of methyl 2-fluoro-2-
(naphth-1-yl)propionate 17.
2-bromo-6-methoxynaphthalene, methyl pyruvate and
other inorganic reagents of the highest purity were used as
purchased. Tetrahydrofuran and diethyl ether were distilled
from potassium hydroxide in the presence of cuprous
chloride and redistilled from sodium wire in the presence of
benzophenone.
C. rugosa lipase (E.C.3.1.1.13 type VII, LOT. 033K0612)
was purchased from Sigma Chemicals Co. and lipase from
C. cylindracea (OF-360) was a gift from Amano Pharma-
ceutical Co.
The starting a-fluoro ester was recovered unchanged after
9 d. Clearly, the success of the enzymatic kinetic resolution
of 2-fluoro-2-naphthylpropionic esters by lipases is strongly
dependent on the position of the propionyl group in the
naphthalene nucleus. This sheds light on the geometry of the
active site of the enzyme, as well as on the structural
requirements of the substrate for the correct binding to this.
4.2. General method to prepare methyl 2-fluoro-2-aryl-
propionates 2, 8, 11, 14 and 17
The suitable arylmagnesium bromide (10 mL, 1.0 M in
THF, 10 mmol), prepared from the corresponding bromo-
arene and magnesium chips by standard procedure) was
added drop-wise to freshly distilled methyl pyruvate
(11 mmol) in THF (5 mL) at K78 8C. The temperature
was allowed to rise at 25 8C, a sat. aq solution of NH4Cl
(30 mL) was added and the mixture was extracted with
diethyl ether (3!25 mL). The collected organic phases
were dried with Na2SO4. After the solvent was evaporated,
chromatography of the crude product on silica gel (eluent,
7:3 petroleum ether/diethyl ether) gave the pure
a-hydroxyester. The following a-hydroxy esters were
prepared and characterized as follows:
3. Conclusion
Lipase from C. rugosa and lipase from C. cylindracea
(OF-360) catalyze the hydrolysis of methyl 2-fluoro-2-
arylpropionates 2, 8, 11 and 14. A careful analysis of the
reaction products revealed that 2-hydroxy-2-arylpropionic
acid and traces of 2-arylacrylic acid are formed in addition
to the expected 2-aryl-2-fluoropropionic acid. Powerful
electron-releasing groups in the aromatic ring of the
substrate increase the amount of 2-hydroxypropionic acid
obtained. A mechanistic hypothesis has been formulated
according to which the enzyme facilitates the elimination of
the fluoride ion from the hydrolysed acid and the formation
of a stable carbocation which produces 2-hydroxypropionic
acid by nucleophilic attack of H2O and 2-arylacrylic acids
by eliminating a proton from the methyl group.
4.2.1. Methyl 2-hydroxy-2-(6-methoxynaphth-2-yl)pro-
1
pionate (3). 76%: mp 86–88 8C; H NMR d 7.94 (d, JZ
1.6 Hz, 1H), 7.74 (d, JZ8.5 Hz, 1H), 7.71 (d, JZ8.2 Hz,
1H), 7.59 (dd, JZ8.7, 1.9 Hz, 1H), 7.14 (dd, JZ8.8, 2.5 Hz,
1H), 7.11 (d, JZ2.4 Hz, 1H), 3.90 (s, 3H), 3.89 (s, 1H), 3.76
(s, 3H), 1.87 (s, 3H); 13C NMR d 176.2, 157.9, 137.7, 133.9,
129.7, 128.4, 126.9, 123.8, 123.8, 119.0, 105.4, 75.8, 55.2,
53.2, 26.6; IR (CHCl3) nmax 3529, 3020, 2958, 2843, 1729,
1606, 1456, 1262, 1289, 1185 cmK1; MS m/z (%) 260 (MC,
35), 242 (1), 201 (100), 185 (12), 159 (18), 144 (14), 43
(52). Anal. Calcd for C15H16O4: C, 69.22; H, 6.20. Found:
C, 69.13; H, 6.11.
The success of the enzymatic kinetic resolution of 2-fluoro-
2-naphthylpropionic esters by lipases is strongly dependent
on the position of the propionyl group in the naphthalene
nucleus. Whereas methyl 2-fluoro-2-(naphth-2-yl)propio-
nate is easily hydrolysed by CRL and lipase OF-360, the
corresponding regioisomer 2-fluoro-2-(naphth-1-yl)propio-
nate is completely inert toward these kinds of lipases.
4.2.2. Methyl 2-hydroxy-2-(naphth-2-yl)propionate.
1
70%: mp 47–48 8C; H NMR d 8.03 (d, JZ1.6 Hz, 1H),
4. Experimental
4.1. General
7.88–7.81 (m, 3H), 7.65 (dd, JZ8.7, 1.9 Hz, 1H), 7.51–7.45
(sym. m, 2H), 3.90 (s, 1H), 3.78 (s, 3H), 1.89 (s, 3H); 13C
NMR d 176.1, 140.0, 133.0, 132.8, 128.3, 128.1, 127.5,
126.2, 126.2, 124.0, 123.4, 75.9, 52.2, 26.6; IR nmax 3498,
3058, 2985–2847, 1732, 1508, 1261, 1246, 1140, 821, 753;
MS m/z (%) 230 (MC, 19), 171 (100), 155 (12), 127 (23), 43
(76). Anal. Calcd for C14H14O3: C, 73.03; H, 6.13. Found:
C, 73.12; H, 6.02.
1
1
If not specified otherwise, H NMR and H-decoupled 13C
NMR spectra were recorded at 400 and 200 MHz,
respectively, in CDCl3 solution using tetramethylsilane as
an internal standard. 19F NMR spectra were recorded at
376 MHz in CDCl3 solution using CFCl3 as a reference
standard. IR spectra were registered in CHCl3 solution in the
4000–625 cmK1 range. Gas-chromatographic analyses were
performed using 30 m!0.32 mm!25 mm capillary col-
umns loaded with two different stationary phases: DB-5 MS
(5% phenyl-methylpolysiloxane) and DB-35 MS (5%
phenyl-methylpolysiloxane) at 70–310 8C. Mass spectra
were obtained at 70 eV. Melting points were corrected after
calibration performed with authentic standards.
4.2.3. Methyl 2-hydroxy-2-(4-isobutylphenyl)propionate.
83%: colourless oil; 1H NMR d 7.43–7.10 (AA0BB0 system,
4H), 3.76 (s, 3H), 3.72 (br s, 1H), 2.44 (d, JZ7.2 Hz, 2H),
1.84 (sept, JZ6.7 Hz, 1H), 1.76 (s, 3H), 0.88 (d, JZ6.7 Hz,
6H); 13C NMR d 176.3, 141.3, 140.0, 129.0, 124.9, 75.7,
53.1, 44.9, 30.1, 26.6, 22.3; IR nmax 3511, 3027, 2955–2848,
1732, 1510, 1606, 1254, 1150 cmK1; MS m/z (%) 236 (MC,
1), 177 (100), 161 (5), 134 (5), 91 (8), 43 (41). Anal. Calcd
for C14H20O3: C, 71.16; H, 8.53. Found: C, 71.37; H, 8.39.
Commercial 1-bromonaphthalene, 2-bromonaphthalene,

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F. Bellezza et al. / Tetrahedron 61 (2005) 8005–8012
4.2.4. Methyl 2-hydroxy-2-(4-isopropoxyphenyl)propio-
nate (16). 38%: viscous oil; 1H NMR d 7.43–6.81 (AA0BB0
system, 4H), 4.52 (sept, JZ6.1 Hz, 1H), 3.74 (s, 3H), 3.73
(s, 1H), 1.74 (s, 3H), 1.31 (d, JZ6.1 Hz, 6H); 13C NMR d
176.3, 157.4, 134.5, 126.4, 115.3, 75.3, 69.7, 53.1, 26.6,
22.0; IR nmax 3506 (broad), 3041, 2978–2937, 1736, 1609,
1509, 1247, 1119, 955 cmK1; MS, m/z (%) 238 (MC, 4),
223 (1), 179 (76), 137 (100), 121 (15), 43 (68). Anal. Calcd
for C13H14O4: C, 65.53; H, 7.61. Found: C, 65.62; H, 7.55.
27.0 Hz), 142.2, 136.4 (d, JZ22.6 Hz), 129.2, 124.3 (d, JZ
8.1 Hz), 94.7 (d, JZ184.0 Hz), 52.8, 45.0, 30.1, 24.7 (d, JZ
23.8 Hz), 22.3; IR nmax 3035, 2957–2870, 1762, 1745, 1513,
1284, 1263, 1125 cmK1; MS, m/z (%) 238 (MC, 5), 179
(100), 137 (17), 115 (6). Anal. Calcd for C14 H19FO2: C,
70.56; H, 8.04. Found: C, 70.68; H, 8.17.
4.2.9. Methyl 2-fluoro-2-(4-isopropoxyphenyl)propio-
nate (14). 95%. Oil; 1H NMR d 7.39–6.85 (AA0BB0 system,
4H), 4.53 (sept, JZ6.1 Hz, 1H), 3.74 (s, 3H), 1.90 (d, JZ
22.2 Hz, 3H), 1.31 (d, JZ6.1 Hz, 6H); 13C NMR d 171.6 (d,
JZ27.4 Hz), 158.2, 130.7 (d, JZ23.0 Hz), 126.0 (d, JZ
7.8 Hz), 115.4, 94.4 (d, JZ184 Hz), 69.8, 52.7, 24.5 (d, JZ
23.7 Hz), 21.9; 19F NMR d K148.8 (q, JZ22.2 Hz); IR nmax
3028, 2980, 1759, 1744, 1611, 1511, 1247, 1121, 955,
836 cmK1; MS, m/z (%) 240 (MC, 6), 181 (14), 139 (100),
109 (5), 91 (5). Anal. Calcd for C13H17FO3: C, 64.98; 7.13.
Found: C, 65.19; H, 7.20.
4.2.5. Methyl 2-hydroxy-2-(naphth-1-yl)propionate.
76%: mp 80–828C; H NMR d 8.21–8.17 (m, 1H), 7.88–
1
7.83 (m, 2H), 7.65 (dd, JZ7.3, 1.1 Hz, 1H), 7.51–7.43 (m,
3H), 3.67 (s, 3H), 3.66 (s, 1H), 2.01 (s, 3H); 13C NMR d
177.7, 136.7, 134.3, 131.0, 129.5, 129.0, 126.3, 125.5,
124.7, 124.7, 124.1, 76.2, 53.2, 27.2; IR nmax 3535, 3054,
3015, 2956, 1729, 1512, 1259, 1136, 805; MS, m/z (%) 230
(MC, 17), 171 (100), 155 (11), 127 (20), 43 (76). Anal.
Calcd for C14H14O3: C, 73,03; H, 6,13. Found: C, 73.19; H,
6.06.
4.2.10. Methyl 2-fluoro-2-(naphth-1-yl)propionate (17).
1
81%. Mp 75–77 8C; H NMR d 8.22 (dt, JZ8.3, 1.3 Hz,
The above esters were submitted to fluorination with
diethylaminosulfur trifluoride (DAST). DAST (0.17 mL,
0.21 g, 1.28 mmol) was added to a solution of the
hydroxyester (1.0 mmol) in dry dichloromethane (2 mL) at
0 8C under nitrogen atmosphere. The mixture was made to
react for 30 min before water (10 mL) was carefully added.
The organic phase was separated, washed with water
(20 mL) and dried with sodium su1fate. After solvent
evaporation, chromatography of the crude product on silica
gel (100 mL, eluent 4:1 (v/v) petroleum ether/diethyl ether)
allowed the pure a-fluoroesters 2, 8, 11, 14 and 17 to be
obtained.
1H), 7.88 (m, 2H), 7.66 (dt, JZ7.3, 1.3 Hz, 1H), 7.54–7.46
(m, 3H), 3.70 (s, 3H), 2.18 (d, JZ22.6 Hz, 3H); 13C NMR d
172.3 (d, JZ26.6 Hz), 134.1 (d, JZ20.2 Hz), 134.0, 130.5,
130.3 (d, JZ1.4 Hz), 128.9, 126.8 (d, JZ1.2 Hz), 125.8,
124.7, 124.6, 124.2 (d, JZ7.8 Hz), 95.0 (d, JZ182 Hz),
52.9, 24.1 (d, JZ24.6 Hz); 19F NMR d K140.9 (q, JZ
22.0 Hz); IR nmax 3034, 2957–2848, 1752, 1602, 1514,
1270, 1124 cmK1; MS m/z (%) 232 (MC, 28), 173 (100) 153
(37). Anal. Calcd for C14H13FO2: C, 72.40; H, 5.64. Found
C, 72.19; H, 5.58.
4.2.11. Methyl 2-(6-methoxynaphth-2-yl)acrylate (6).
The compound was prepared as an authentic specimen by
refluxing methyl 2-hydroxy-2-(6-methoxynaphth-2-yl)-
propionate 3 in toluene for 6 h in the presence of
p-toluensulfonic acid as catalyst while removing water by
a Dean–Stark apparatus. The solvent evaporation at reduced
pressure, after neutralization of the acid catalyst, left an oily
residue which was chromatographed on silica gel (eluent
petroleum ether/diethyl ether 8:2) to give pure methyl
acrylate 6 (30%, 61% of the starting hydroxyester
4.2.6. Methyl 2-fluoro-2-(6-methoxynaphth-2-yl)propio-
nate (2). 68%: mp 95–97 8C; 1H NMR d 7.89 (s, 1H), 7.75
(d, JZ8.9 Hz, 1H), 7.74 (d, JZ8.6 Hz, 1H), 7.55 (dd, JZ
8.6, 1.8 Hz, 1H), 7.17 (dd, JZ8.9, 2.5 Hz, 1H), 7.12 (d, JZ
2.3 Hz, 1H), 3.90 (s, 3H), 3.76 (s, 3H), 2.02 (d, JZ22.3 Hz,
3H); 13C NMR d 171.5 (d, JZ27 Hz), 158.2, 134.4, 134.1
(d, JZ22.6 Hz), 129.8, 128.2, 127.2, 123.6 (d, JZ9.2 Hz),
122.8 (d, JZ7.4 Hz), 119.4, 105.5, 94.8 (d, JZ185 Hz),
55.3, 52.8, 24.6 (d, JZ23.7); 19F NMR d K150.56 (q, JZ
22.3 Hz); IR (CHCl3) nmax 3028, 2956, 2844, 1747, 1608,
1485, 1270, 1128 cmK1; MS m/z (%) 262 (MC, 19), 242 (8),
203 (100), 159 (17). Anal. Calcd for C15H15FO3: C, 68.69;
H, 5.76. Found: C, 68.83; H, 5.69.
1
recovered). Mp 69–71 8C; H NMR d 7.84 (d, JZ1.5 Hz,
1H), 7.73 (m, 2H), 7.49 (dd, JZ8.5, 1.8 Hz, 1H), 7.17–7.12
(m, 2H), 6.40 (d, JZ1.2 Hz, 1H), 5.99 (d, JZ1.2 Hz, 1H),
3.92 (s, 3H), 3.86 (s, 3H); 13C NMR d 167.5, 158.0, 141.2,
134.2, 131.8, 129.7, 128.5, 127.2, 126.5, 126.4, 126.3,
119.0, 105.5, 55.2, 52.2; IR nmax 3062, 3025, 1955–2844,
1719, 1633, 1606, 1264, 1171, 855 cmK1; MS m/z (%) 242
(MC, 100), 199 (9), 183 (61), 168 (13), 139 (27). Anal.
Calcd for C15H14O3: C, 74.36; H, 5.82. Found: C, 74.21; H,
5.76.
4.2.7. Methyl 2-fluoro-2-(naphth-2-yl)propionate (8).
73%. Mp 44–46 8C; H NMR d 7.99 (s, 1H), 7.89–7.81
1
(m, 3H), 7.60 (dd, JZ8.6, 1.8 Hz, 1H), 7.53–7.48 (m, 2H),
3.77 (s, 3H), 2.04 (d, JZ22.3 Hz, 3H); 13C NMR d 171.3 (d,
JZ26.6 Hz), 136.4 (d, JZ22.2 Hz), 133.1, 132.8, 128.4,
128.3, 127.6, 126.6, 126.5, 123.7 (d, JZ9.4 Hz), 122.2 (d,
JZ7.6 Hz), 94.7 (d, JZ185.0 Hz), 52.9, 24.7 (d, JZ23.7);
19F NMR d K151.56 (q, JZ22.3 Hz); IR (CHCl3) nmax
3061, 2998–2874, 1715, 1608, 1266, 855 cmK1; MS, m/z
(%) 232 (MC, 24), 173 (100), 153 (21). Anal. Calcd for
C14H13FO2: C, 72.40; H, 5.64. Found: C, 72.22; H, 5.69.
4.3. General method for enzymatic hydrolysis in
presence of lipase
In a standard experiment (Table 1, entry 1), crude CRL
(120 mg) was suspended in phosphate buffer (0.02 M, pH
7.2, 12 mL). Racemic ester 2 (0.131 g, 0.5 mmol) was added
and the mixture was stirred at 37 8C. After 3 d, the reaction
was terminated by addition of NaCl and then filtered
through Celite. The solution was then acidified with HCl to
pH 2 and extracted with ethyl acetate (3!20 mL).
4.2.8. Methyl 2-fluoro-2-(4-isobutylphenyl)propionate
1
(11). 71%. Oil; H and 19F NMR spectra were identical to
those reported in the literature; 7 13C NMR d 171.5 (d, JZ

F. Bellezza et al. / Tetrahedron 61 (2005) 8005–8012
8011
The combined extracts were treated with an aqueous
solution of NaOH 0.2 M (3!20 mL) and separated. The
organic layer was dried with Na2SO4 and concentrated in
vacuo giving ester 2. The combined aqueous layers were
acidified to pH 2, extracted with ethyl acetate (3!20 mL),
combined and dried with Na2SO4 and then concentrated in
vacuo to give a mixture of acids 1, 4 and 5. The
enantiomeric excess of ester 2 obtained in the enzymatic
hydrolysis was determined by recording 1H NMR spectra in
presence of Eu(hfc)3 as a chiral shift reagent (substrate/
Eu(hfc)3 1/6, CDCl3).
8.6 Hz, 1H), 7.59 (broad d, JZ8.6 Hz, 1H), 7.26 (d, JZ
2.3 Hz, 1H), 7.13 (dd, JZ8.9, 2.3 Hz, 1H), 5.98 (br s, 1H),
3.84 (s, 3H), 1.70 (s, 3H); 13C NMR d 176.6, 157.7, 139.9,
133.8, 130.0, 128.3, 126.7, 124.8, 123.9, 119.0, 106.0, 75.3,
55.5, 27.6; IR (CHCl3) nmax 3495, 3200–2000 (very broad),
3021, 2962–2895, 1718, 1607, 1270, 1132, 1027 cmK1
.
Anal. Calcd for C14H14O4: C, 68.28; H, 5.73. Found: C,
68.39; H, 5.67.
4.4.5. 2-Hydroxy-2-(naphth-2-yl)propionic acid (10).
84%. Mp 155 8C (dec.); 1H NMR d 12.70 (very br s,
1H), 8.02 (br s, 1H), 7.92–7.84 (m, 3H), 7.64 (dd, JZ8.7,
1.9 Hz, 1H), 7.50–7.44 (symmetric m, 2H), 5.96 (br s, 1H),
1.72 (s, 3H); 13C NMR d 176.4, 142.3, 132.9, 132.5, 128.4,
127.8, 127.7, 126.5, 126.3, 124.4, 124.0, 75.4, 27.6; IR
(CHCl3) nmax 3417 (broad), 3200–2000 (very broad), 3031,
2988, 1722, 1599, 1267, 1133, 883 cmK1. Anal. Calcd for
C13H12O3: C, 72.21; H, 5.59. Found: C, 72.38; H, 5.71.
The enantiomeric excess of acids 1 and 5 were determined
on the corresponding methyl esters obtained by treatment
with CH2N2 and separated by silica gel chromatography
(petroleum ether/ethyl acetate 8/2).
4.4. General procedure for the preparation of the 2-aryl-
2-fluoropropionic acids 1, 9, 12, 2-aryl-2-hydroxypro-
pionic acids 5, 10, 13 and 15 and 2-(6-methoxynaphth-2-
yl)acrylic acid 4
4.4.6. 2-Hydroxy-2-(4-isobutylphenyl)propionic acid
(13). 91%. Mp 101–103 8C; 1H NMR d 7.52–7.11 (AA0BB0
system, 4H), 4.81 (br s, 1H), 2.44 (d, JZ7.2 Hz, 2H), 1.83
(sept, JZ6.7 Hz, 1H), 1.71 (s, 3H), 0.86 (d, JZ6.7 Hz, 6H);
13C NMR d 175.8, 141.3, 140.6, 128.6, 125.0, 75.0, 44.6,
30.0, 26.6, 21.7; IR (CHCl3) nmax 3521, 3200–2300 (very
broad), 3031, 2958–2870, 1718, 1510, 1262 cmK1; Anal.
Calcd for C13H18O3: C, 70.24; H, 8.16. Found: C, 70.06; H,
8.18.
The reaction products were identified by their spectroscopic
and analytical properties, by comparison with authentic
specimens prepared as follows:
The suitable ester (1.0 mmol) was made to react for 2 h in
5% methanolic KOH (25 mL) at 20 8C. The solvent was
evaporated, the resu1ting so1id was dissolved in water
(30 mL) and the mixture was extracted with diethyl ether
(2!20 mL). The clear solution was acidified with 5%
aqueous HCl until pH 1, the resulting white suspension was
extracted with ethyl acetate (3!20 mL) and the collected
organic phases were dried with Na2SO4. After solvent
evaporation, the remaining white solid was crystallized
from 1:1 ethanol/hexane to give the pure acid. The acids
were identified on the basis of their spectroscopic and
analytical characteristics.
4.4.7. 2-Hydroxy-2-(4-isopropoxyphenyl)propionic acid
(15). 87%. Mp 103–104 8C; H NMR (acetone-d6) d 7.51–
1
7.47 (sym m, 2H), 6.87–6.83 (sym m, 2H), 4.58 (sept, JZ
6.0 Hz, 1H), 1.70 (s, 3H), 1.26 (d, JZ6.0 Hz, 6H), 13.0–
11.0 (very broad 2H); 13C NMR (acetone-d6) d 176.0, 157.3,
135.6, 126.5, 115.0, 74.8, 69.2, 26.6, 21.4. IR (CHCl3) nmax
3512, 3400–2380 (very broad), 1721, 1500, 1247 cmK1
.
Anal. Calcd for C12H16O4: C, 64.27; H, 7.19. Found: C,
64.05; H, 7.22.
4.4.1. 2-Fluoro-2-(6-methoxynaphth-2-yl)propionic acid
(1). 78%: mp 119 8C (dec); all spectroscopic and analytical
characteristics were identical to those reported in the
literature.6
4.4.8. 2-(6-Methoxynaphth-2-yl)acrylic acid (4). Mp 172–
174 8C; All spectroscopic and analytical characteristics
were identical to those reported in the literature.17
4.4.2. 2-Fluoro-2-(naphth-2-yl)propionic acid (9). Mp
115 8C (dec.); 1H NMR d 8.07 (br s, 1H), 7.99–7.89 (m, 3H),
7.66 (dd, JZ8.7, 1.9 Hz, 1H), 7.56–7.52 (symmetric m,
2H); 13C NMR d 171.0 (d, JZ27.1 Hz), 137.1 (d, JZ
21.2 Hz), 133.2, 132.9, 128.3, 128.2, 127.5, 126.6, 126.5,
123.7 (d, JZ9.0 Hz), 122.5 (d, JZ7.3 Hz), 94.4 (d, JZ
183 Hz), 23.9 (d, JZ23.9 Hz); 19F NMR d K149.7 (q, JZ
22.1 Hz); IR (CHCl3) nmax 3400–2300 (very broad), 2882,
1706, 1274, 1140, 827 cmK1. Anal. Calcd for C13H11FO2:
C, 71.55; H, 5.08. Found: C, 71.80; H, 5.12.
Acknowledgements
`
Thanks are due to the Ministero dell’Istruzione, Universita e
Ricerca (MIUR) and to the University of Perugia (PRIN
2004, contract n. 2004033322) for financial support.
References and notes
4.4.3. 2-Fluoro-2-(4-isobutylphenyl)propionic acid (12).
86%: mp 69–71 8C; All spectroscopic and analytical
characteristics were identical to those reported in the
literature.6
1. Schlosser, M. The Chemical and Physiological Size of
Fluorine. In Enantiocontrolled Synthesis of Fluoro-Organic
Compounds: Stereochemical Challenges and Biomedical
Targets’; Soloshonok, V. A., Ed.; Wiley: New York, 1999.
2. Schlosser, M.; Michel, D. Tetrahedron 1996, 52, 99–108.
3. Boehm, H.-J.; Banner, D.; Bendels, S.; Kansy, M.; Kuhn, B.;
Mu¨ller, K.; Obst-Sander, U.; Stahl, M. ChemBioChem 2004, 5,
637–643.
4.4.4. 2-Hydroxy-2-(6-methoxynaphth-2-yl)propionic
acid (5). 88%. Mp 170 8C (dec.); H NMR d 12.6 (very br
1
s, 1H), 7.93 (br s, 1H), 7.81 (d, JZ9.0 Hz, 1H), 7.75 (d, JZ

8012
F. Bellezza et al. / Tetrahedron 61 (2005) 8005–8012
4. Ojima, I. ChemBioChem 2004, 5, 628–635.
11. Ricci, G.; Ruzziconi, R. J. Org. Chem. 2005, 70, 611–623.
12. Colton, I. J.; Ahmed, S. N.; Kazlauskas, R. J. J. Org. Chem.
1995, 60, 212–217.
5. Griesbach, R. C.; Hamon, D. P. G.; Kennedy, R. J.
Tetrahedron: Asymmetry 1997, 8, 507–510.
6. Goj, O.; Kotila, S.; Haufe, G. Tetrahedron 1996, 52,
12761–12774.
13. Cipiciani, A.; Cittadini, M.; Fringuelli, F. Tetrahedron 1998,
54, 7883–7890.
7. Rozen, S.; Hagooly, A.; Harduf, R. J. Org. Chem. 2001, 66,
7464–7468.
14. Cipiciani, A.; Bellezza, F.; Fringuelli, F.; Stillitano, M.
Tetrahedron: Asymmetry 1999, 10, 4599–4605.
8. Schlosser, M.; Michel, D.; Zhi-wie, G.; Sih, C. J. Tetrahedron
1996, 52, 8257–8262.
15. Cygler, M.; Grochulski, P.; Kazlauskas, R. J.; Schrag, J. D.;
Bouthillier, F.; Rubin, B.; Serreqi, A. N.; Gupta, A. K. J. Am.
Chem. Soc. 1994, 116, 3180–3186.
9. Belekon, Y. N.; Caveda-Cepas, S.; Green, B.; Ikonnikov,
N. S.; Khrustalev, V. N.; Larichev, V. S.; Moscalenco, M. A.;
North, M.; Orizu, C.; Tararov, V.; Tasinazzo, M.; Timofeeva,
G. I.; Yashkina, L. V. J. Am. Chem. Soc. 1999, 121,
3968–3973.
16. Indeed, no trace of a-hydroxyacid was detected in the lipase-
promoted hydrolysis of the acrylic ester 6. Moreover,
hydration of 6 in alkaline medium gives the corresponding
b-hydroxyester (the anti-Markovnikov addition product).
17. Hiyama, J.; Wakasa, N.; Veda, T.; Kusumoto, T. Bull. Chem.
Soc. Jpn 1990, 63, 640–642.
10. Stelzer, U.; Effenberger, F. Tetrahedron: Asymmetry 1993, 4,
161–164.