Enzyme-mediated synthesis of EEHP and EMHP, useful pharmaceutical intermediates of PPAR agonists

Article abstract of DOI:10.1016/j.tetasy.2009.10.013

A new scaleable synthetic route to the title compounds has been developed. The reaction pathway is based on the α-chymotrypsin-catalysed hydrolysis of the racemic ethyl 2-ethoxy-3-(p-methoxyphenyl)propanoate or of the racemic ethyl 2-methoxy-3-(p-methoxyphenyl)propanoate to give the corresponding resolved (S)-esters with excellent ee. The acids were easily separated from the (S)-esters by a simple acid-base work-up. The overall yields of 1 and 2 were 16% and 17%, respectively.

Full text of DOI:10.1016/j.tetasy.2009.10.013

Tetrahedron: Asymmetry 20 (2009) 2594–2599
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Enzyme-mediated synthesis of EEHP and EMHP, useful pharmaceutical
intermediates of PPAR agonists
*
Elisabetta Brenna, Claudio Fuganti, Francesco G. Gatti , Fabio Parmeggiani
Dipartimento di Chimica, Materiali ed Ingegneria Chimica ‘G. Natta’, Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A new scaleable synthetic route to the title compounds has been developed. The reaction pathway is
Received 2 September 2009
Accepted 16 October 2009
Available online 10 November 2009
based on the
a-chymotrypsin-catalysed hydrolysis of the racemic ethyl 2-ethoxy-3-(p-methoxy-
phenyl)propanoate or of the racemic ethyl 2-methoxy-3-(p-methoxyphenyl)propanoate to give the cor-
responding resolved (S)-esters with excellent ee. The acids were easily separated from the (S)-esters by a
simple acid–base work-up. The overall yields of 1 and 2 were 16% and 17%, respectively.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
inconsistent with the typical large-scale pharmaceutical produc-
tions. Herein we report the details of our synthesis that has suc-
Both ethyl (S)-2-ethoxy-3-(4-hydroxyphenyl)propanoate (S)-1
(EEHP) and ethyl (S)-2-methoxy-3-(4-hydroxyphenyl)propanoate
(S)-2 (EMHP) are useful intermediates for the synthesis of several
Peroxisome Proliferator-Activated Receptors (PPARs) agonists.1
Among these, the most popular are Tesaglitazar 32 and Navaglita-
zar 43 developed by AstraZeneca and Eli Lilly, respectively (Fig. 1).
These molecules have found their main therapeutic application
in the treatment of type-2 diabetes (T2D), and, although most of
them have recently been discontinued since the risk-benefit bal-
ance was not met,4 (S)-1 and (S)-2 are still important pharmaceu-
tical intermediates for the development of a safer and more
efficient generation of PPAR agonists.5
ceeded in meeting all the above goals.
O
O
OH
OH
O
O
O
O
O
O
O
S
O
O
3, Tesaglitazar
4, Navaglitazar
So far, a large number of synthetic procedures for the prepara-
tion of (S)-16–9 and (S)-2 or related precursors such as the
a
-hydroxyesters10 or the glycidic esters11 have been reported.
However, although asymmetric catalysis has been greatly devel-
oped over the last few decades, the process based on the catalytic
reduction of the a-ethoxycinnamic acid is not sufficiently efficient
O
O
in terms of enantioselectivity (60–92% ee) and practicality (10% in
weight of a Rh-based chiral catalyst).8 Furthermore, the chiral pool
approach, based on the diazotisation of the O-benzylated deriva-
OEt
OEt
OEt
OMe
HO
HO
tives of the naturally occurring and commercially available L-tyro-
sine, seems to be an unappealing route from an industrial point of
view because the overall yield is too low.6a,b Most industrial pro-
cesses for the production of (S)-1 or (S)-2 are based on kinetic res-
olutions, using enzymes7a,c or chiral amines.7b
(S)-1, EEHP
(S)-2, EMHP
O
Thus, keeping this in mind, we have designed a new synthetic
route based on an enzyme-mediated resolution. In addition, the
entire process has been developed in such a way to avoid chro-
matographic purifications and expensive reagents,7d which are
OEt
OH
MeO
5
* Corresponding author. Tel.: +39 02 23993072; fax: + 39 02 23993080.
E-mail address: Francesco.Gatti@polimi.it (F.G. Gatti).
Figure 1. Structures of Tesaglitazar and Navaglitazar, PPAR agonists based on EEHP
and EMHP intermediates, respectively.
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.10.013

E. Brenna et al. / Tetrahedron: Asymmetry 20 (2009) 2594–2599
2595
Next, 5 was submitted to enzymatic resolution screening of
three different lipases (Pseudomonas cepacia, porcine pancreatic,
Candida rugosa) in t-BuOMe, using vinyl acetate as the acyl donor
(Scheme 2). The results are summarised in Table 1. The best ee
was obtained using the P. cepacia lipase (PS-L). Indeed, after 5 days
it was possible to isolate, via column chromatography separation,
the hydroxyester (R)-5 in 50.8% yield and 90.4% ee by HPLC
2. Results and discussion
Initially, we envisaged ethyl 2-hydroxy-3-(4-methoxyphenyl)-
propanoate 5 (Fig. 1) as an ideal intermediate on which to per-
form our preliminary enzyme-mediated resolutions12 for three
main reasons: (i) the lipase-mediated acylations, carried out
on similar substrates such as the phenyl lactic ester,13 often
proceed with excellent enantiomeric excesses (ee); (ii) the alkyl-
ation of the hydroxyl group, which is the divergent step of the
synthesis of the two target molecules, is strategically planned at
the last stage giving clear benefits in terms of number of steps;
(iii) and finally, the aryl methoxy-protecting group is more
convenient, in terms of atom economy, than the O-benzyl ether,
which has been used in almost all previously reported
syntheses.
(½a 2D5
ꢀ
¼ ꢁ8:2 (c 1.50, EtOH)) from the acetoxyester (S)-7 in 43.7%
yield and 96.4% ee by HPLC {½a D25
ꢀ
¼ ꢁ7:8 (c 1.23, EtOH) versus lit.
½
a 2D5
ꢀ
¼ ꢁ8:3 (c 1.50, EtOH)14}.
O
O
Lipase
vinylacetate
OEt
OEt
5
+
OH
OAc
t-BuOMe
MeO
MeO
The preparation of racemic 5 is shown in Scheme 1. Darzens
condensation of anisaldehyde with ethyl chloroacetate in toluene
and in the presence of t-BuOK gave the crude glycidic ester
which, without any purification, was submitted to the Pd/C cat-
alytic hydrogenolysis in EtOH to afford a complex mixture com-
posed mainly of 5 together with other unidentified impurities.
The purification of 5 was achieved by treatment of the latter
mixture with NaOH in EtOH/H2O to give the corresponding acid
6 which could be easily separated, as a white powder, from the
impurities by trituration in EtOAc. Finally, the acid, having a sat-
isfactory purity (over 99% by 1H NMR) was almost quantitatively
reconverted to 5 by standard Fischer esterification and in an
overall yield of 62%.
(S)-7
(R)-5
Scheme 2. Lipase-mediated acetylation of 5. For conversions and ee, see Table 1.
However, although this PS-L-catalysed resolution gave (S)-7
with excellent ee, the use of chromatography makes such proce-
dure not completely ideal for our requirements (efficiency, easy
purification).
Instead, the use of the bovine digestive enzyme
a-chymotrypsin
(
a-CT) could better match our aims. Indeed, it is known that this en-
zyme stereoselectively hydrolyses ethyl a-acetoxy-b-phenylpropio-
nate,15 allowing a column-chromatography-free separation of the
resolved acid from the ester by means of an extremely simple and
practical acid–base work-up. Unfortunately, treatment of 5 with a-
CT at standard conditions (H2O, pH 7.8) gave after a day acid 6 in
an almost quantitative yield and without any appreciable stereose-
lectivity (Scheme 3, Table 1). In addition, the hydrolysis of the acetox-
yester derivative 7, under the same experimental conditions,
furnished after 4 days and a conversion of 42.9% (by GC of the methyl
ester derivative, obtained by treatment with CH2N2 in Et2O) the acet-
oxyacid (S)-8 (38.1% ee by HPLC of the methyl ester) and the acetox-
yester (R)-7 (28.0% ee by HPLC) with poor enantiomeric purities
O
O
H
Cl
+
OEt
MeO
1) t-BuOK, toluene
2) H2, Pd/C, EtOH
(Scheme 3, Table 1). However, it is interesting to note that
a-CT
hydrolyses 5 much quicker than 7, and without stereoselectivity.
Thus, by combining these results, we set up a column-chroma-
tography-free procedure for the separation of (S)-5 from (R)-7
O
O
NaOH
EtOH/H2O
coming from the PS-L-mediated resolution. Indeed, the a-CT-med-
OEt
OH
iated hydrolysis of the latter produced a complex mixture consist-
ing of esters (S)-7 (96.3% ee) and (R)-5 (89.2% ee) in the ratio 93:7,
and acids (R)-6 (95.5% ee) and (S)-8 (97.5% ee) in the ratio of 91:9,
which were separated by the usual acid–base work-up (Scheme 4).
Next, the (S)-7-enriched mixture was hydrolysed (KOH, MeOH/
H2O, 96% yield) to give (S)-6 (82.2% ee) in an overall yield of 37% over
OH
OH
MeO
MeO
5
+ impurities
6
(65% yield)
EtOH
cat. H2SO4
reflux
three steps (PS-L-mediated resolution,
a-CT-mediated hydrolysis
and chemical hydrolysis of the survived fraction). The latter was
esterified in acetone with EtBr, K2CO3 and in the presence of a substo-
ichiometric amount of tetrabutylammonium iodide (TBAI) to give
(S)-7 in a quantitative yield and without loss of enantiomeric purity.
Finally, the hydroxyester (S)-7 was converted to the ethoxyester (S)-9
following a reported procedure (EtBr/EtI, NaH, THF/DMF).6a,6b How-
ever, we obtained (S)-9 with a substantial loss of enantiomeric purity
96%
O
Ac2O
pyridine
OEt
5
OAc
MeO
98%
7
EtBr or MeI
NaH, THF
0°C
{73.1% ee by HPLC, ½a D25
¼ ꢁ14:0 (c 1.17, CHCl3)}. This result was very
ꢀ
disappointing, since such alkylations, carried out on similar/identical
substrates, were described to proceed without any significant loss of
enantiomeric purity and in high yields.
O
OEt
OR
Recently, the alkylation of the hydroxyl group of a similar com-
pound has been extensively studied using different solvents (DMF,
THF/DMF), bases (NaH, NaHMDS, KHMDS, t-AmylONa) and electro-
philes (EtBr, EtI, EtOTs, MeI) at different temperatures, however,
the reagents that gave the lowest epimerisation extent are too
MeO
9, R=Et (78% yield)
10, R=Me (80% yield)
Scheme 1. Preparation of the racemic substrates 5, 7, 9 and 10.

2596
E. Brenna et al. / Tetrahedron: Asymmetry 20 (2009) 2594–2599
Table 1
Enzyme-mediated resolutions of substrates 5, 7, 9 and 10
Entry
Enzyme
Substrate
Time (days)
Conva (%)
Abs. Conf. Transf.
eeb (%) Transf.
eeb (%) Surv.
E (Enant. S)
Yield (%) Transf.
Yield (%) Surv.
1
2
3
4
5
6
7
PS-L
CR-L
PP-L
5
5
5
5
7
5
20
20
1
4
40
40
47.6
27.5
<1
97.8
42.9
53.2
57.1
(S)
(S)
—
96.4
12.4
—
90.4
3.4
—
158
1.3
—
44
—
—
52
–
–
a-CT
a-CT
a-CT
a-CT
—
—
—
—
—
–
(S)
(R)
(R)
38.1
82.1
74.2
28.0
93.0
99.9
2.9
33.6
50.5
42
44
51
54
39
40
9
10
a
By GC (for
By HPLC.
a-CT resolution the analytical samples were treated with CH2N2/Et2O).
b
O
OEt
OR
MeO
O
5, R=H (<1% yield)
(R)-7, R=Ac (54% yield, 28.0% ee)
1)
α
-Chymotrypsin
H2O, buffer pH 7.8
OEt
OR
2) Acid-base work-up
MeO
O
5, R=H
7, R=Ac
OH
OR
MeO
6, R=H (98% yield)
(S)-8, R=Ac (42% yield, 38.1% ee)
Scheme 3.
a-CT-mediated hydrolysis of 5 and 7.
O
O
O
OH
OH
+
OEt
OH
OAc
MeO
MeO
OH
MeO
MeO
(S)-8 (97.5% ee)
1) α-Chymotrypsin
H2O, buffer pH 7.8
(R)-6 (95.5% ee)
Lipase
vinylacetate
(R)-5 (93.7% ee)
91:9
5
+
O
t-BuOMe
2) Acid-base work-up
O
O
OEt
OAc
OEt
OEt
+
OH
(R)-5 (89.2% ee)
OAc
(S)-7 (95.4% ee)
MeO
MeO
(S)-7 (96.3% ee)
93:7
KOH, MeOH/H2O
96%
O
EtBr, TBAI
K2CO3
acetone
O
O
EtBr, NaH
THF, 0°C
OH
OH
OEt
OEt
MeO
OEt
(S)-9 (73.1% ee)
OH
MeO
100%
79%
MeO
(S)-6 (82.2% ee)
overall yield 37%
(S)-5 (82.2% ee)
Scheme 4. Column-chromatography-free synthesis of (S)-6.
expensive to be used on industrial scale.16 Thus, we concluded
that this step has to be carried out before the resolution
process.
substrates 5 and 7 (H2O/MeCN, 95:5, pH 7.8, for solubility reasons
it was necessary to add some MeCN, Scheme 5). After 40 days and a
conversion of 53.2% (by GC), ester (S)-9 {93.0% ee, ½a D25
¼ ꢁ18:1 (c
ꢀ
Accordingly, the racemic ethoxyester 9, obtained by ethylation
1.34, CHCl3)} was separated from acid (R)-11 (82.1% ee by HPLC of
methyl ester derivative) in a yield of 39% and 44%, respectively by
the usual work-up.
of 5 (Scheme 1), was submitted to the
a-CT-catalysed hydrolysis
using experimental conditions similar to those adopted for

E. Brenna et al. / Tetrahedron: Asymmetry 20 (2009) 2594–2599
2597
O
O
NaOH
MeOH/H2O
OEt
OH
OR
OR
96%
MeO
MeO
1) α-Chymotrypsin
H2O/MeCN (95:5)
buffer pH 7.8
(S)-11, R=Et
(S)-12, R=Me
(S)-9, R=Et (93.0% ee)
(S)-10, R=Me (>99.9% ee)
O
OEt
OR
2) Acid-base work-up
1) EtSNa,
DMF, 13 0°C
2) EtOH,
HCl cat., reflux
MeO
O
9, R=Et
10, R=Me
OH
91%
OR
MeO
(R)-11, R=Et (82.1% ee)
(R)-12, R=Me (74.2% ee)
O
OEt
OR
HO
(S)-1, R=Et, overall yield 34%
(S)-2, R=Me, overall yield 35%
Scheme 5. Preparation of 1 and 2 based on the a-CT-catalysed hydrolysis of 9 and 10.
It is important to note that by substituting the acetoxy group
with the less polar ethoxy group, the -CT inverted its stereoselec-
were determined on a Dr. Kernchen Propol digital automatic polar-
imeter. Chiral HPLC analyses were performed on a Merck-Hitachi
L-4250 chromatograph equipped with a Chiralcel OD column and
a UV detector (210 nm); mobile phase: n-hexane/i-PrOH 98:2 for
compounds 5, 7 and the corresponding methyl esters of acids 6,
8, 11; 99:1 for compounds 9, 10 and methyl ester of acid 12; flow
rate: 0.6 mL/min. TLC analyses were performed on Merck Kieselgel
60 F254 plates. All the chromatographic separations were carried
a
tivity [(S) vs (R)] and the reaction rate decreased dramatically
(4 days vs 40 days); it should also be noted that a larger amount
of enzyme was used (about four times more with respect to the
previous hydrolyses).17
Finally, ester (S)-9 was hydrolysed to give acid (S)-11 (½a D25
¼
ꢀ
ꢁ16:6 (c 1.21, CHCl3) versus an authentical sample ½a D25
¼ ꢁ16:8
ꢀ
(c 1.08, CHCl3)18) which in turn was submitted to a two-step se-
quence consisting of the cleavage of the aryl methoxy group fol-
lowed by esterification of the carboxylic group to give (S)-1
(93.0% ee by HPLC of aryl methoxy derivative obtained by treat-
ment with Me2SO4, K2CO3) in an overall yield of 91% (Scheme 5).7b
Analogously the racemic methoxyester 10 was submitted to the
same synthetic sequence to give (S)-2 (>99.9% ee by HPLC) in an
overall yield of 35% (Scheme 5).
out on silica gel columns. The a-CT-mediated hydrolyses were car-
ried out on a Metrohm 718 STAT Titrino pH-stat. Lipase-PS from P.
cepacia (PS-L, Amano Pharmaceuticals Co., Japan), C. rugosa lipase
(CR-L; Sigma, type VII), porcine pancreatic lipase (PP-L, Sigma, type
II) and a-chymotrypsin (a-CT, Sclavo S.p.A.) were employed as pur-
chased. All reagents and solvents were purchased from Fluka or Al-
drich and used without any further purification.
4.2. Preparation of racemic substrates
3. Conclusion
4.2.1. 2-Hydroxy-3-(4-methoxyphenyl)propanoic acid 6
In short, we have disclosed a new synthetic process for the
preparation of EEHP (ee 93%) and EMHP (ee >99%) based on the
To a mechanically stirred solution of anisaldehyde (150 g,
1.1 mol) and ethyl chloroacetate (117 mL, 1.1 mol) in toluene
(500 mL) was added portionwise t-BuOK (63.9 g, 0.57 mol) at such
a rate to keep the temperature below 30 °C by cooling in an ice-
water bath. The complete conversion of the aldehyde was ensured
by the addition of a 10% surplus of chloroacetate and t-BuOK in one
portion. After 2 h at rt, the solution was quenched with ice (450 g)
and NH4Cl solution (satd, 500 mL) and extracted with EtOAc
(3 ꢂ 400 mL). The organic phase was washed with water (3 ꢂ
100 mL) and brine (100 mL). The mixture was dried over Na2SO4
and concentrated under reduced pressure to give a brown oil,
which was dissolved in EtOH (400 mL) and was added to a Parr
reactor containing Pd/C (5% w/w, 70 g). The suspension was shaken
under 70 psi of H2 for 3 days. Afterwards, the mixture was filtered
on a Celite pad and the filtrate was added to an aqueous solution of
NaOH (100 mL, 80 g). After 4 h, most of the EtOH was removed un-
der reduced pressure to give a suspension, which was dissolved in
H2O (500 mL) and washed with Et2O (2 ꢂ 300 mL). The aqueous
solution was acidified with HCl (6 M, 400 mL) and extracted with
EtOAc (3 ꢂ 500 mL). The organic phase was washed with brine
(500 mL) and dried over Na2SO4. Concentration under reduced
pressure and trituration in ice-cold EtOAc (100 mL) gave 6 as a
white powder (140 g, 65% yield, >99% purity by 1H NMR). 1H
a-CT-mediated resolution of racemic materials 9 and 10 in an
overall yield of 16% and 17%, respectively. After the process’s opti-
misation, this new synthetic route might be scaleable on an indus-
trial scale because it avoids the use of expensive reagents and does
not require the utilisation of chromatographic purifications. Fur-
thermore, we have disclosed a new column-chromatography-free
synthesis of the intermediates (R) and (S)-6.
4. Experimental
4.1. General remarks
GC–MS analyses were performed on an Agilent HP 6890 gas-
chromatograph equipped with a 5973 mass-detector, using an Agi-
lent HP-5MS column (30 m ꢂ 0.25 mm ꢂ 0.25
lm). The following
temperature program was employed: 60 °C (1 min)/6 °C/min/
150 °C (1 min)/12 °C/min/280 °C (5 min). 1H and 13C NMR spectra
were recorded on a Bruker ARX 400 spectrometer (400 MHz 1H,
100.6 MHz 13C) in CDCl3 solution at rt, using TMS as internal stan-
dard for 1H and CDCl3 for 13C; chemical shifts d are expressed in
ppm relative to TMS; J values are given in hertz. Optical rotations

2598
E. Brenna et al. / Tetrahedron: Asymmetry 20 (2009) 2594–2599
NMR (400 MHz, CDCl3: d 7.16 (d, J = 8.5, 2H), 6.85 (d, J = 8.5 2H),
4.47 (m, 1H), 3.79 (s, 3H), 3.05 (m, AB syst., 2H). 13C NMR
(100.6 MHz, CDCl3: d 177.7, 158.8, 130.5, 127.7, 114.1, 71.1, 55.3,
39.3.
4.19 (q, J = 7.2, 2H), 3.97 (m, 1H), 3.79 (s, 3H), 3.37 (s, 3H), 2.98
(m, 2H), 1.16 (t, J = 7.2, 3H). 13C NMR (100.6 MHz, CDCl3: d 171.6,
158.1, 129.9, 128.6, 113.4, 81.6, 60.3, 57.7, 54.8, 37.9, 13.8. MS:
m/z (%) 238 [M]+ (5), 206 (30), 165 (17), 121 (100).
4.2.2. Ethyl 2-hydroxy-3-(4-methoxyphenyl)propanoate 5
A solution of 6 (20.0 g, 0.102 mol) and H2SO4 (concd, 1 mL) in
EtOH (abs., 100 ml) was refluxed for 10 h. Then it was concentrated
under reduced pressure, diluted with H2O (50 ml) and extracted
with EtOAc (2 ꢂ 50 mL). The combined organic phase was washed
with NaHCO3 solution (satd, 50 mL), brine (2 ꢂ 50 mL) and dried
over Na2SO4. Removal of the solvent under reduced pressure affor-
ded 5 as a yellow oil (21.9 g, 96% yield, 62% overall yield from anis-
aldehyde, 99.4% purity by GC (tR 21.0 min)). 1H NMR (400 MHz,
CDCl3: d 7.11 (d, J = 8.6, 2H), 6.80 (d, J = 8.6, 2H), 4.36 (dd, J = 6.6,
4.7, 1H), 4.18 (q, J = 7.1, 2H), 3.75 (s, 3H), 2.96 (m, AB syst., 2H),
1.25 (t, J = 7.1, 3H). 13C NMR (100.6 MHz, CDCl3: d 174.2, 158.6,
130.5, 128.5, 113.8, 71.4, 61.5, 55.2, 39.7, 14.1. MS: m/z (%) 224
[M]+ (10), 206 (8), 151 (6), 121 (100), 91 (7), 77 (7).
4.3. Representative procedure for the lipase mediated acylation
A mixture of 5 (5.0 g, 22.2 mmol), vinyl acetate (20 mL), t-
BuOMe (50 mL) and PS-L (2.5 g) was stirred at rt and the reaction
was followed by HPLC. After 5 d the mixture was filtered and con-
centrated under reduced pressure. Column chromatography sepa-
ration (n-hexane/EtOAc, 9:1) gave, in order of elution: (S)-7,
2.60 g, 44% yield, 99.7% purity by GC (tR 22.4 min), 96.4% ee by
HPLC (tR 15.0 min (S), 17.1 min (R)), ½a D25
¼ ꢁ7:8 (c 1.23, EtOH) ver-
ꢀ
sus lit. ½a 2D5
¼ ꢁ8:3 (c 1.50, EtOH), and (R)-5, 2.62 g, 52% yield,
ꢀ
99.8% purity by GC (tR 21.0 min), 90.4% ee by HPLC (tR 26.5 min
(S), 29.1 min (R)), ½a D25
¼ ꢁ8:2 (c 1.50, EtOH).
ꢀ
4.4. General procedure for the
hydrolysis
a-chymotrypsin mediated
4.2.3. Ethyl 2-acetoxy-3-(4-methoxyphenyl)propanoate 7
To an ice-cold stirred solution of 5 (10.0 g, 44.6 mmol) in pyri-
dine/CH2Cl2 (1:1, 100 mL) was added Ac2O (20 mL). After 10 h
the mixture was concentrated under reduced pressure, diluted
with water (100 mL) and extracted with CH2Cl2 (3 ꢂ 100 mL).
The combined organic phase was washed with HCl (1 M, 50 mL),
H2O (50 mL), NaHCO3 solution (satd, 50 mL) and brine (2 ꢂ
50 mL), and dried over Na2SO4. Removal of the solvent under re-
duced pressure gave 7 as a brownish oil (11.6 g, 98% yield, 98.9%
purity by GC (tR 22.4 min). 1H NMR (400 MHz, CDCl3: d 7.13 (d,
J = 8.6, 2H), 6.83 (d, J = 8.6, 2H), 5.15 (dd, J = 4.8, 8.4, 1H), 4.16 (q,
J = 7.1, 2H), 3.78 (s, 3H), 3.06 (m, AB syst., 2H), 2.07 (s, 3H), 1.22
(t, J = 7.1, 3H). 13C NMR (100.6 MHz, CDCl3: d 170.2, 169.7, 158.6,
130.3, 127.9, 113.8, 73.2, 61.3, 55.2, 36.5, 20.5, 14.0. MS: m/z (%)
266 [M]+ (2), 206 (95), 161 (47), 134 (38), 121 (100).
a
-CT (2.0 g) was suspended in aqueous NaCl solution (0.1 M,
200 mL) and Na2HPO4ꢃ2H2O (0.47 g, 3.0 mmol) was added as a buf-
fering agent. The mixture under magnetic stirring was brought to
pH 7.8 by a pH-stat charged with 0.5 M aqueous NaOH. The sub-
strate (20.0 mmol), if necessary dissolved in MeCN (10 mL), was
added and the pH was maintained at 7.8 until about half of the the-
oretical amount of NaOH solution had been consumed. The mix-
ture was then extracted with Et2O (4 ꢂ 100 mL), centrifuging
between the extractions to help break the emulsion. The combined
ether extract was subsequently washed with brine (50 mL), dried
over Na2SO4 and evaporated under reduced pressure to yield the
non-hydrolysed substrate (survived fraction). The aqueous phase
was then brought to pH 2–3 with HCl (1 M). The resulting suspen-
sion was extracted with EtOAc (3 ꢂ 100 mL), centrifuging between
the extractions. The reunited acetate extract was washed with
brine (50 mL), dried over Na2SO4 and evaporated under reduced
pressure to yield the hydrolysed product (transformed fraction).
With substrate 7 (9.0 g): transformed fraction (S)-8, 3.42 g, 42%
yield, 38.1% ee by HPLC of CH2N2 derivative (tR (S) 18.4 min, (R)
20.0 min); survived fraction (R)-7, 4.85 g, 54% yield, 28.0% ee by
HPLC (tR (S) 15.3 min, (R) 17.1 min).
4.2.4. General procedure for the O-alkylation of 5
To a stirred and ice-cold suspension of NaH (60% disp. in min oil,
1.16 g, 28.9 mmol) in dry THF (50 mL) under an N2 atmosphere, a
solution of 5 (5.0 g, 22.3 mmol) in dry THF (20 mL) was added
dropwise. After 15 min a solution of halide (28.9 mmol) in dry
THF (10 mL) was added dropwise. A small surplus of NaH and ha-
lide was added to obtain complete conversion (by TLC). The mix-
ture was quenched with ice (100 g) and NH4Cl solution (satd,
50 mL) and extracted with Et2O (3 ꢂ 150 mL). The combined or-
ganic phase was washed with brine (2 ꢂ 50 mL), dried over Na2SO4
and concentrated under reduced pressure to give a crude oil, which
was used for the next steps without further purification. A sample
was filtered on a silica gel pad in gradient elution (n-hexane?n-
hexane/EtOAc 1:1) to give the corresponding O-alkylated ethyl
ester.
With substrate 9 (3.0 g): transformed fraction (R)-11, 1.18 g, 44%
yield, 82.1% ee by HPLC of CH2N2 derivative (tR (R) 16.1 min, (S)
17.0 min), ½a 2D5
¼ þ14:6 (c 1.14, CHCl3); survived fraction (S)-9,
ꢀ
1.34 g, 39% yield, 93.0% ee by HPLC (tR (R) 16.3 min, (S) 17.6
min), ½a 2D5
¼ ꢁ18:1 (c 1.34, CHCl3).
ꢀ
With substrate 10 (3.0 g): transformed fraction (R)-12, 1.16 g,
51% yield, 74.2% ee by HPLC of CH2N2 derivative (tR (R) 22.5 min,
(S) 27.4 min) ½a 2D5
¼ þ20:2 (c 1.46, EtOH); survived fraction (S)-10,
ꢀ
1.20 g, 40% yield, 99.9% ee by HPLC (tR (R) 18.9 min, (S) 19.4
Ethyl 2-ethoxy-3-(4-methoxyphenyl)propanoate 9. Halide: EtBr.
4.38 g, 78% yield, 99.5% purity by GC (tR 21.5 min). 1H NMR
(400 MHz, CDCl3: d 7.15 (d, J = 8.7, 2H), 6.81 (d, J = 8.7, 2H), 4.15
(q, J = 7.1, 2H), 3.97 (m, 1H), 3.77 (s, 3H), 3.59 (dq, J = 7.0, 9.2,
1H), 3.55 (dq, J = 7.0, 9.2, 1H), 2.95 (m, 2H), 1.22 (t, J = 7.1, 3H),
1.16 (t, J = 7.0, 3H). 13C NMR (100.6 MHz, CDCl3: d 172.5, 158.4,
130.4, 129.3, 113.7, 80.4, 66.2, 60.7, 55.2, 38.5, 15.0, 14.2. MS: m/
z (%) 252 [M]+ (4), 206 (26), 179 (10), 151 (6), 121 (100), 91 (7).
Ethyl (S)-2-ethoxy-3-(4-methoxyphenyl)propanoate (S)-9. Ha-
lide: EtBr. Starting from 0.50 g of (S)-5 (82.2% ee). 0.44 g, 79% yield,
73.1% ee by HPLC (tR (R) 16.3 min, (S) 17.6 min), 99.5% purity by
min), ½a 2D5
¼ ꢁ16:9 (c 3.74, EtOH).
ꢀ
4.5. Column-chromatography-free preparation of (S)-6 from
racemic 5
Hydroxyester 5 (5.0 g, 22.3 mmol) and vinyl acetate (20 mL)
were dissolved in t-BuOMe (50 mL) and PS-L (2.5 g) was added.
The suspension was stirred at rt until approaching 50% conversion
by HPLC (about 10 days). The reaction mixture was then filtered to
remove the enzyme and concentrated under reduced pressure,
yielding a raw mixture of (S)-7 and (R)-5 in about equal propor-
GC, ½a 2D5
ꢀ
¼ ꢁ14:0 (c 1.17, CHCl3).
tions (5.22 g). Then, to a well-stirred suspension of a-CT (1.05 g)
Ethyl 2-methoxy-3-(4-methoxyphenyl)propanoate 11. Halide:
MeI. 4.24 g, 80% yield, 98.3% purity by GC–MS (tR 20.9 min). 1H
NMR (400 MHz, CDCl3: d 7.16 (d, J = 8.4, 2H), 6.84 (d, J = 8.6, 2H),
in NaCl solution (0.1 M, 180 mL) brought to pH 7.8 by a pH-stat
charged with 0.5 M NaOH solution, was added the mixture of
(R)-5 and (S)-7. After about 50% consumption of the theoretical

E. Brenna et al. / Tetrahedron: Asymmetry 20 (2009) 2594–2599
2599
amount of NaOH (less than 3 days), the mixture was treated
according to the general procedure for the -chymotrypsin-medi-
ated hydrolysis, to give the (S)-7-enriched fraction (2.06 g, ratio
93:7, 96.3% ee of (S)-7, 89.2% ee of (R)-5 by HPLC) and the (R)-6-en-
riched fraction (2.33 g, ratio 91:9, 95.5% ee of (R)-6, 97.5% ee of (S)-
8 by HPLC of CH2N2 derivative).
CHCl3); 1H NMR (400 MHz, CDCl3) d 7.10 (d, J = 8.6, 2H), 6.73 (d,
J = 8.6 Hz, 2H), 4.94 (s, 1H), 4.16 (q, J = 7.1, 2H), 3.97 (dd, J = 6.1,
7.1 Hz, 1H), 3.60 (dq, J = 7.0, 9.2 Hz, 1H), 3.36 (dq, J = 7.0, 9.2 Hz,
1H), 2.94 (m, 2H), 1.23 (t, J = 7.1, 3H), 1.16 (t, J = 7.0, 3H); 13C
NMR (100.6 MHz, CDCl3) d 172.6, 154.4, 130.6, 129.2, 115.1, 80.4,
66.2, 60.8, 38.5, 15.0, 14.2; MS: m/z (%) 238 [M]+ (3), 192 (53),
165 (23), 132 (16), 107 (100).
a
(S)-2-Hydroxy-3-(4-methoxyphenyl)propanoic acid (S)-6: To
the (S)-7 enriched fraction from the previous step (2.00 g, 7.5
mmol), dissolved in MeOH (35 mL), an aq solution of KOH (5% w/
w, 35 mL) was added. After completion of the hydrolysis (by
TLC), most of the MeOH has been removed under reduced pressure,
the mixture was acidified with HCl (1 M) and extracted with EtOAc
(3 ꢂ 50 mL). The combined organic phase was washed with brine,
dried over Na2SO4 and evaporated under reduced pressure. The
barely crystalline product obtained was then triturated under
Et2O and filtered under vacuum, yielding hydroxyacid (S)-6
(1.44 g, 98% yield, >99% purity by 1H NMR, 82.2% ee by HPLC of
Ethyl (S)-2-methoxy-3-(4-hydroxyphenyl)propanoate, (S)-2:
white solid; 0.82 g, 92% yield, 99.8% purity by GC (tR 21.51 min);
>99.9% ee by HPLC of Me2SO4 derivative; ½a D25
¼ ꢁ24:2 (c 1.56,
ꢀ
CHCl3); 1H NMR (400 MHz, CDCl3) d 7.07 (d, J = 8.6, 2H), 6.72 (d,
J = 8.6, 2H), 5.47 (s, 1H), 4.18 (q, J = 7.1, 2H), 3.93 (dd, J = 5.7, 7.1,
1H), 3.36 (s, 3H), 2.95 (mAB, 2H), 1.23 (t, J = 7.1, 3H); 13C NMR
(100.6 MHz, CDCl3) d 173.1, 154.4, 130.8, 128.5, 115.7, 82.4, 61.5,
58.5, 38.6, 14.5; MS: m/z (%) 224 [M]+ (3), 192 (40), 151 (20),
119 (12), 107 (100).
CH2N2 derivative) ½a D25
¼ ꢁ9:6 (c 1.03, EtOH).
ꢀ
Acknowledgments
4.6. Ethyl (S)-2-hydroxy-3-(4-methoxyphenyl)-propanoate (S)-5
We thank Dr. M. Hedberg of AstraZeneca for providing a sample
of (S)-11. We thank Dr. D. Tessaro for providing the pH-stat equip-
ment. This work has been supported by E.U. INTENANT project.
To a solution of (S)-6 (1.35 g, 6.9 mmol) in dry acetone (80 mL)
were subsequently added anhydrous K2CO3 (1.10 g, 7.9 mmol),
TBAI (2.00 g, 5.4 mmol) and EtBr (0.59 mL, 7.9 mmol). The white
suspension was stirred at rt until complete consumption of the
acid (by TLC) and was filtered over a Celite pad, washing the filter
cake with Et2O. The still milky filtrate was concentrated under re-
duced pressure, treated with n-hexane/Et2O (9:1, 50 mL) to precip-
itate the remaining TBAI and filtered again (over the same Celite
pad). Evaporation of the solvent afforded the pure hydroxyester
(S)-5 (1.41 g, 100% yield, 99.3 purity by GC, 82.2% ee by HPLC).
References
1. For a review see: Balakumar, P.; Rose, M.; Ganti, S. S.; Krishan, P.; Singh, M.
Pharm. Res. 2007, 56, 91.
2. Anderson, K. PCT Appl. WO 99/62872 (1999 to Astra).
3. Martin, J. A.; Brooks, D. A.; Prieto, L.; Gonzalez, R.; Torrado, A.; Rojo, I.; Lopez de
Uralde, B.; Lamas, C.; Ferrito, M.; Martin-Ortega, M. D.; Agejas, J.; Parra, F.;
Rizzo, J. R.; Rhodes, G. A.; Robey, R. L.; Alt, C. A.; Wendel, S. R.; Zhang, T. Y.;
Reifel-Miller, A.; Montrose-Rafidazeh, C.; Brozinik, J. T.; Hawkins, E.; Misener, E.
A.; Briere, D. A.; Ardecky, R.; Fraser, J. D.; Warshawsky, A. M. Bioorg. Med. Chem.
Lett. 2005, 15, 51–55.
4.7. Hydrolysis of (S)-9 and (S)-10
4. Tesaglitazar has been commercialised under the trade name GalidaÒ by
AstraZeneca, but its production was stopped in 2006, because the benefits
were not worth the risks.
Esters (S)-9 and (S)-10 were hydrolysed according to the proce-
dure reported above for the synthesis of (S)-6.
5. Parmenon, C.; Guillard, J.; Caignard, D.-H.; Hennuyer, N.; Staels, B.; Audinot-
Bouchez, V.; Boutin, J. A.; Dacquet, C.; Ktorza, A.; Viaud-Massuard, M.-C. Bioorg.
Med. Chem. Lett. 2008, 18, 1617.
(S)-2-Ethoxy-3-(4-methoxyphenyl)propanoic acid, (S)-11: white
solid; 1.14 g, 96% yield, >99% purity by 1H NMR; 93.0% ee by HPLC
6. (a) For an approach based on the diazotisation of L-tyrosine derivatives,
of CH2N2 derivative; ½a D25
¼ ꢁ16:6 (c 1.21, CHCl3).
ꢀ
followed by O-alkylation see: Potlapally, R. K.; Siripragada, M. R.; Kotra, N. M.;
Sirisilla, R.; Mamillapalli, R. S.; PCT Appl. WO 02/24625.; (b) Potlapally, R. K.;
Siripragada, M. R.; Mamillapalli, R. S.; Gaddam, O. R.; Jangaigar, T. R.; Velagala,
V. R. M. K. R.; PCT Appl. WO 03/027084.; (c) Zeng, Q. L.; Wang, H. Q.; Liu, Z. R.;
Li, B. G.; Zhao, Y. F. Amino Acids 2007, 33, 536–541.
(S)-2-Methoxy-3-(4-methoxyphenyl)propanoic acid, (S)-12:
white solid; 1.01 g, 95% yield, >99% purity by 1H NMR; >99.9% ee
by HPLC of CH2N2 derivative; ½a D25
¼ ꢁ25:2 (c 1.40, CHCl3).
ꢀ
7. For an approach based on enzymatic/chemical resolution of derivatives of 1 or
2, see: (a) Deussen, H.-J.; Zundel, M.; Valdois, M.; Lehmann, S. V.; Weil, V.;
Hjort, C. M.; Østergaard, P. R.; Marcussen, E.; Ebdrup, S. Org. Process Res. Dev.
2003, 7, 82–88; (b) Linderberg, M. T.; Moge, M.; Sivadasan, S. Org. Process Res.
Dev. 2004, 8, 838–845; (c) Ebdrup, S.; Deussen, H.-J.; Zundel, M.; Bury, P. S.; PCT
Appl. WO 01/11073.
4.8. General procedure for the demethylation and esterification
of (S)-11 and (S)-12
To a suspension of NaH (0.80 g, 60% disp. in min oil, 20.1 mmol)
in DMF (5 mL) was added EtSH (1.49 g, 24.1 mmol) under an N2
atmosphere. After 30 min, a solution of acid (4.0 mmol) in DMF
(5 mL) was added. After 48 h at 130 °C, the reaction mixture was
quenched with a solution of NaHCO3 (satd, 40 mL) and was washed
with CH2Cl2 (3 ꢂ 20 mL). The aq phase was acidified with HCl (1 M)
and extracted with EtOAc (3 ꢂ 20 mL). The combined organic
phase was washed with brine (2 ꢂ 20 mL), dried over Na2SO4 and
concentrated under reduced pressure to about a volume of 5 mL.
Abs. EtOH (10 mL) and HCl (0.2 mL, 12 M) were added to the solu-
tion that was heated at 75 °C distilling off a mixture of EtOAc/
EtOH/H2O, occasionally restoring the volume of EtOH. After 3 h
the reaction mixture was concentrated under reduced pressure, di-
luted with EtOAc (50 mL) and washed with NaHCO3 solution (satd,
50 mL) and brine (2 ꢂ 20 mL). The organic phase was dried over
Na2SO4 and concentrated under reduced pressure to give an oil,
which crystallises on standing.
8. (a) For an asymmetric synthesis of
1 by catalytic reduction of a-
ethoxycinnamic acid, see: Woltering, M.; Bunlakasnanusorn, T.; Gerlach, A.
PCT Appl. US 2007/0149804.; (b) Houpis, I. N.; Patterson, L. E.; Alt, C. A.; Zhang,
T. Y.; Haurez, M. Org. Lett. 2005, 7, 1947–1950.
9. Brenna, E.; Fuganti, C.; Gatti, F. C.; Parmeggiani, F. Tetrahedron: Asymmetry
2009, 20, in press. doi:10.1016/j.tetasy.2009.09.024.
10. (a) Wang, Z.; La, B.; Fortunak, J. M.; Meng, X.-J.; Kabalka, G. W. Tetrahedron Lett.
1998, 39, 5501–5504; (b) Sundholm, O.; Kanerva, L. T. ACH—Models Chem.
1998, 135, 625–640; (c) Baskar, B.; Pandian, N. G.; Priya, K.; Chadha, A.
Tetrahedron 2005, 61, 12296–12306.
11. (a) Gentile, A.; Giordano, C.; Fuganti, C.; Ghirotto, L.; Servi, S. J. Org. Chem. 1992,
57, 6635–6637; (b) Barot, V. K. G.; Kothari, H. M.; Lohray, B. B.; Lohray, V. B. PCT
Appl. WO 2005/019152.; (c) Rizzo, J. R.; Zhang, T. Y. Chimia 2006, 9, 580–583.
12. A similar PS-lipase-mediated resolution has been already carried out on the
methyl ester of 5, Ref. 9b.
13. Yamano, T.; Kikumoto, F.; Yamamoto, S.; Miwa, K.; Kawada, M.; Ito, T.;
Ikemoto, T.; Tomimatsu, K.; Mizuno, Y. Chem. Lett. 2000, 29, 448–450.
14. Koga, K.; Wu, C. C.; Yamada, S.-I. Chem. Pharm. Bull. 1972, 20, 1272.
15. Cohen, S. G.; Weinstein, S. Y. J. Am. Chem. Soc. 1964, 86, 5326–5330.
16. Aikins, J. A.; Haurez, M.; Rizzo, J. R.; Van Hoeck, J.-P.; Brione, W.; Kastemont,
J.-P.; Stevens, C.; Lemair, X.; Stephenson, G. A.; Marlot, E.; Forst, M.; Houpis, I.
N. J. Org. Chem. 2005, 70, 4695–4705.
Ethyl (S)-2-ethoxy-3-(4-hydroxyphenyl)propanoate, (S)-1:
white solid; 0.87 g, 91% yield, 99.9% purity by GC (tR 21.97 min);
17. Ref. 7c describes the
a-CT-catalysed hydrolysis of racemic 1 at pH 7.0, which
furnished (S)-1 with 34% ee and after a conversion of 84%.
18. This sample was provided by AstraZeneca.
93.0% ee by HPLC of Me2SO4 derivative; ½a D25
¼ ꢁ21:3 (c 1.45,
ꢀ