A practical chemoenzymatic synthesis of a key intermediate of antifungal agents

Article abstract of DOI:10.1016/S0040-4039(01)00425-7

A novel synthesis of an azole antifungal building block, the optically active diol, is described. The key step involves an enantioselective hydrolysis of a prochiral diester by a lipase.

Full text of DOI:10.1016/S0040-4039(01)00425-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3331–3333
A practical chemoenzymatic synthesis of a key intermediate of
antifungal agents
Yoshihiko Yasohara,a,* Kenji Miyamoto,b Noriyuku Kizaki,a Junzo Hasegawaa and
Takehisa Ohashib
aFine Chemicals Research Laboratories, Kaneka Corporation, 1-8 Miyamae, Takasago, Hyogo 676-8688, Japan
bLifescience Research Laboratories, Kaneka Corporation, 1-8 Miyamae, Takasago, Hyogo 676-8688, Japan
Received 1 February 2001; revised 2 March 2001; accepted 9 March 2001
Abstract—A novel synthesis of an azole antifungal building block, the optically active diol, is described. The key step involves an
enantioselective hydrolysis of a prochiral diester by a lipase. © 2001 Elsevier Science Ltd. All rights reserved.
The search for systemic fungal infections continues to
be important because of the growing population of
immunocompromised patients.1 There is, therefore, a
need for more effective antifungal agents. Two new
antifungal agents, ZD08702 and Sch45450,3 show
potent broad-spectrum antifungal activities compared
to other clinically used antifungal agents. Because these
compounds contain chiral centers comprised of a
difluorobenzene and a triazole group, an optically
active diol (1) is a useful common intermediate for their
synthesis.3–5
ric reaction and thus, application of the reaction for
large scale preparation is difficult.
With a view to plant-scale preparation of the optically
active diol (1) using cheap and readily available materi-
als, we studied a novel approach for enantioselective
preparation of 1 by using an enzymatic stereoselective
hydrolysis of prochiral diesters (4a–e) as a key reaction
(Scheme 1). The prochiral substrate was prepared by
condensing a difluorobenzene group and a C3 unit
using an organometallic technique. Optically active
diols (5a–e) were smoothly converted to 1 without
racemization.
Four synthetic methods have been reported to give 1.
Optically active epoxy alcohol (2) as a precursor of 1
was prepared via optical resolution by hydrolysis using
a porcine pancreatic lipase (PPL)3 or via Sharpless–
Katuski epoxidation for the corresponding allylic alco-
hol.4 Optically active 1 was prepared via Sharpless
asymmetric dihydroxylation for the corresponding
allylic alcohol3 or via optical resolution by esterification
using PPL.6 The theoretical maximum yield of optical
resolution is 50%. Some special reagents are required to
prepare the substrates and the catalyst for the asymmet-
The substrates of stereoselective hydrolysis, 4a–e, were
prepared as follows. A smooth halogen–metal exchange
was realized by the treatment of 1-bromo-2,4-
difluorobenzene with n-butyl lithium in ethyl ether. The
resulting aryllithium was trapped by dichloroacetone
(6) to give the adduct (3) in good yield. The adduct was
a mixture of 2,4-difluoro and 2,6-difluorobenzene
derivatives when THF was used as a solvent. Also, 3
was prepared by treating the Grignard reagent of the
OH
F
N
N
N
N
N
O
N
N
OH
F
OH
N
F
N
N
F
F
O
F
O
N
F
F
N
N
1
F
F
N
ZD0870
Sch45450
* Corresponding author. Fax: +81-794-45-2668; e-mail: yoshihiko.yasohara@kaneka.co.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00425-7

3332
Y. Yasohara et al. / Tetrahedron Letters 42 (2001) 3331–3333
Cl
F
F
O
i
ii
Cl
Cl
OH
Cl
O
Cl
F
F
6
3
iii
10
O
O
O
R
R
O
O
F
O
R
vi
iv
v
F
HO
OH
R
O
O
R
OH
O
OH
OH
O
7a-e
a: R=CH3
b: R=CH2CH3
c: R=CH2CH2CH3
d: R=CH(CH3)2
O
F
8
F
O
4a-e
5a-e
e: R=CH2CH2CH2CH3
vii
O
OH
F
OH
F
O
R
ix
viii
F
OH
O
N
N
OH
OSO2CH3
F
F
N
F
1
2
9a-e
x
Scheme 1. Reagents and conditions: (i) n-butyl lithium, 1-bromo-2,4-difluorobromobenzene, −78°C, 6 (64% from 6); (ii) 20%
KOH, toluene, rt (50%); (iii) AcOK, AcOH, reflux (87%, to 4a); (iv) 8, pyridine, DMAP, acid anhydride, rt (84%); (v) magnesium,
1-bromo-2,4-difluorobromobenzene, −10°C, 4 (86% from 7); (vi) lipase D, acetate buffer, methylcyclohexane, 15°C (90%); (vii)
MsCl, pyridine, AcOEt, 30°C (94%); (viii) 20% KOH, toluene, rt (88%); (ix) 1,2,4-triazole, K2CO3, THF, reflux (40%); (x)
1,2,4-triazole, Na2CO3, MeOH, reflux (83%).
Table 1. Enzymatic hydrolysis of diesters 4a–e by lipase Da
(5a). The stereoselective hydrolysis of a series of diesters
(4a–e) by lipase D is summarized in Table 1. Probably
Substrate
Product (%)b
1,3-rearrangement of the acetyl group caused the low
enantioselectivity for diacetate (4a). By selecting a
proper diester, the monoester was prepared in good
yield and stereoselectivity as high as 97–98% ee.9
Diester
Monoester
Triol
4
4
5
% ee
a
b
c
d
e
27
28
14
26
11
71
69
81
73
71
79
91
97
98
97
2
3
5
1
18
The following procedures for the target compound (1)
were performed without loss of optical activity. Opti-
cally active (R)-monoester (5d) was mesylated and sub-
sequently converted to epoxide (2) in good yield.
1,2,4-Triazole and 2 were reacted to give 1 in 40% yield.
One-pot reaction to give 1 from 9d was possible and a
higher yield was obtained than in the two-step reac-
tion.10 Finally, optically pure (S)-1 was obtained by
recrystallization.11
a Each reaction mixture, comprising 10 mg of substrate, 10 mg of
enzyme in 1 ml of 50 mM acetate buffer (pH 5.0) and 1 ml of
hexane, was stirred at 30°C for 18 h. The mixture was extracted
with ethyl acetate and the organic layer was analyzed with HPLC.
b The yield and the enantiomeric excess of 5a–e were determined by
HPLC analysis using a Chiralpak AD (4.6×250 mm) column (Daicel
Chemicals, Tokyo, Japan) with a hexane/ethanol=9/1 (vol/vol) as a
In summary, we have achieved the enantiomeric prepa-
ration of the key intermediate of chiral azole antifungal
agents by a chemoenzymatic process in only five steps
from inexpensive and readily available starting materi-
als. The chiral monoesters prepared by lipase hydrolysis
may be applicable to the synthesis of a variety of chiral
pharmaceuticals.
mobile phase, flow rate;
detected at 254 nm.
1 ml/min, ambient temperature, and
aryl bromide in THF. Dichoroalcohol (3) was con-
verted to diester (4a) in two steps. Alternatively, 4a–e
were prepared by treating the Grignard reagent with
the corresponding diesters (7a–e) which were easily
prepared from 8, in about 85% yield.7
References
1. Como, J. A. New Engl. J. Med. 1994, 330, 263–272.
2. Peng, T.; Galgiani, J. N. Antimicrob. Agents Chemother.
1993, 37, 2126–2131.
The key hydrolysis was examined by commercially
available enzymes. Among the lipases tested, lipase D
from Rhizopus delemer8 gave the desired (R)-monoester

Y. Yasohara et al. / Tetrahedron Letters 42 (2001) 3331–3333
3333
9. Preparative-scale procedure: diester (4d) (93.5 g, 0.27
mol), lipase D (0.94 g), water (842 ml) and methylcyclo-
hexane (94 ml) were stirred at 15°C for 18 hours. The pH
of the reaction mixture was kept at 5.5 with 10 M NaOH.
The resulting mixture was extracted with ethyl acetate
(200 ml×2) and the organic layer was washed with satu-
rated sodium hydrogencarbonate and brine. The organic
layer was dried over anhydrous sodium carbonate and
concentrated to give 5d (66.6 g) with 90% yield and 96.7%
ee. [h]2D5=−7.04 (c=1.00, MeOH). 1H NMR (CDCl3) l
(ppm) 7.69–7.66 (1H, m), 6.91–6.87 (1H, m), 6.78–6.77
(1H, m), 4.54–4.45 (3H, m), 3.97–3.78 (2H, dd), 2.49–2.42
(1H, m), 1.01–0.99 (6H, m). IR (film) wmax (cm−1) 3400,
1738, 1600, 1500, 968, 850. Calcd for C13H16F2O4: C,
56.9; H, 5.9. Found: C, 56.8; H, 5.9.
10. Procedure: 1,2,4-triazole (5.8 g, 0.085 mol) and sodium
carbonate (13.4 g, 0.127 mol) were added to a solution of
9d (14.89 g, 0.042 mol) in methanol (300 ml) and the
suspension was refluxed for 15 hours under an atmo-
sphere of argon. After cooling and filtration, methanol
was removed from the filtrate in vacuo. Water (50 ml)
was added to the residue and extracted with ethyl acetate
(100 ml). After evaporation, the resulting crystalline solid
was recrystallized from acetonitrile to give optically pure
1 (8.22 g) with 60% yield. [h]2D5=−72.9 (c=1, MeOH), the
reported value for (S)-isomer was [h]2D5=−60 (c=1,
THF).3
3. Muramami, K.; Mochizuki, H. Eur. Pat. Appl. 472392
(1992).
4. Blundell, P.; Ganguly, A.; Girijavallabhan, V. M. Synlett
1994, 263–265.
5. Chung, S.-K.; Lee, K.-W.; Kang, H. I.; Yamashita, C.;
Kudo, M.; Yoshida, Y. Bioorg. Med. Chem. 2000, 8,
2475–2486.
6. Lovey, R. G.; Saksena, A. K.; Girijavallabhan, V. M.
Tetrahedron Lett. 1994, 35, 6047–6050.
7. Typical procedure:
a
solution of 1-bromo-2,4-
difluorobenzene (96.5 g, 0.5 mol) in THF (450 ml) was
added to magnesium (12.8 g, 0.53 mol) at 18°C over 3
hours. After stirring at 5°C for 1 hour, it was slowly
poured into a solution of 7d (103 g, 0.45 mol) in THF
(150 ml) below 15°C over 1 hour. After stirring at 5°C for
1 hour, the reaction was stopped by the addition of a 1.2
M HCl solution (500 ml) below 5°C and the mixture was
extracted with ethyl acetate (500 ml×2). The organic layer
was washed with brine and concentrated to give 4d as an
oil. The crude 4d was distilled to give a pure 4d (bp:
114–119°C/0.5–0.6 mmHg). 1H NMR (CDCl3) l (ppm)
7.73–7.65 (1H, m), 6.95–6.88 (1H, t), 6.85–6.77 (1H, m),
4.54–4.45 (4H, q), 3.90 (1H, s), 2.55–2.47 (2H, m), 1.10–
1.03 (12H, m). IR (film) wmax (cm−1) 3468, 1720, 1618,
1500, 968, 850. Calcd for C17H22F2O5: C, 59.3; H, 6.4.
Found: C, 59.1; H, 6.5.
8. Lipase D was purchased from Amano Enzyme Inc.,
Nagoya, Japan.
11. The enantiomeric excess of 1 was determined by HPLC
as described in Table 1. Detection was 210 nm.
.
.