Enantioselective synthesis of four isomers of 3-hydroxy-4- methyltetradecanoic acid, the constituent of antifungal cyclodepsipeptides W493 A and B

Article abstract of DOI:10.1271/bbb.69.231

Four possible stereoisomers of 3-hydroxy-4-methyltetradecanoic acid were enantioselectively synthesized by using Sharpless epoxidation and a subsequent epoxide-ring opening reaction with trimethylaluminum as the key steps. The absolute configuration of the β-oxyacid component of antifungal cyclodepsipeptides W493 A and B was consequently determined as 3S,4R.

Full text of DOI:10.1271/bbb.69.231

Biosci. Biotechnol. Biochem., 69 (1), 231–234, 2005
Note
Enantioselective Synthesis of Four Isomers of 3-Hydroxy-4-Methyltetradecanoic
Acid, the Constituent of Antifungal Cyclodepsipeptides W493 A and B
y
1;
Ken-ichi NIHEI,1 Kimiko HASHIMOTO,2 Kazuo MIYAIRI,1 and Toshikatsu OKUNO
1Department of Biochemistry and Biotechnology, Faculty of Agriculture and Life Science,
Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japan
2Department of Applied Chemistry, Faculty of Science and Technology, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
Received August 25, 2004; Accepted October 21, 2004
Four possible stereoisomers of 3-hydroxy-4-methyl-
tetradecanoic acid were enantioselectively synthesized
by using Sharpless epoxidation and a subsequent
epoxide-ring opening reaction with trimethylaluminum
as the key steps. The absolute configuration of the ꢀ-
oxyacid component of antifungal cyclodepsipeptides
W493 A and B was consequently determined as 3S,4R.
Key words: Sharpless epoxidation; W493 A; W493 B;
cyclodepsipeptide;
tetradecanoic acid
3-hydroxy-4-methyl-
By bioassay-guided fractionation, two cyclodepsipep-
tides, W493 A (1) and B (2) (Fig. 1), were isolated from
the culture broth of Fusarium sp. and characterized as
potent antifungal substances with unique activity for
inducing morphological change to hypha in several
fungal species.1,2) Both compounds contain a novel ꢀ-
oxyacid component, 3-hydroxy-4-methyltetradecanoic
acid (HMTA), which may influence their antifungal
potency.3,4)
In order to further investigate the antifungal mode of
action based on derivative synthesis, it was necessary to
stereoselectively synthesize the four HMTA isomers,
(3R,4R)-HMTA (3), (3S,4S)-HMTA (4), (3S,4R)-HMTA
(5), and (3R,4S)-HMTA (6).
The absolute configuration of the naturally occurring
HMTA had previously been determined to be 3S,4R by a
comparison of the 1H-NMR spectrum of the correspond-
ing 2-naphthylmethoxyacetic acid (2NMA) ester5) with
those of the four chemically synthesized stereoisomers.
A synthetic study on HMTAs has previously been
reported by Flippin et al.6) However, no optically pure
HMTAs had been obtained so that its chirality remained
unclear. We describe here details of a convenient
synthesis of the four stereoisomers and their properties.
Epoxy alcohols 7 and 8 were enantioselectively
Fig. 1. Structures of W493 A (1) and B (2).
synthesized in a high yield as previously reported, but
with slight modifications (Scheme 1).7) The ring-open-
ing reaction of 7 by Me3Al proceeded in a regio- and
stereoselective manner to produce 1,2-diol 9.8) After
selectively tosylating the primary hydroxyl group in 9,
the secondary alcohol obtained was converted to ꢀ-
oxyacid 3 via successive steps of cyanide substitution
and alkaline hydrolysis. Likewise, 4 was synthesized
from epoxy alcohol 8.
Selective inversion at C-2 in 9 was needed in order to
furnish ꢀ-oxyacid 5. Accordingly, the primary alcohol
moiety in 9 was protected as a TBS ether and then the
remaining secondary alcohol moiety was tosylated. The
removal of the TBS protecting group with excess TBAF
at 40 ꢀC provided epoxide 10 in one pot. Desired ꢀ-
oxyacid 5 was obtained by cyanide substitution and
hydrolysis of 10. Compound 6 was synthesized from 8
by the same steps as those used for obtaining 5 from 7.
The synthetic HMTAs, 3, 4, 5 and 6, were converted
y
To whom correspondence should be addressed. Tel/Fax: +81-172-39-3771; E-mail: agokuno@cc.hirosaki-u.ac.jp
Abbreviations: 2NMA, 2-naphthylmethoxyacetic acid; DET, diethyl tartrate; DMAP, 4-dimethylaminopyridine; EDC, 1-ethyl-3-(3-dimethylami-
nopropyl)carbodiimide hydrochloride; HMTA, 3-hydroxy-4-methyltetradecanoic acid; TBAF, tetrabutylammonium fluoride; TBSCl, tert-butyldi-
methylsilyl chloride

232
K. NIHEI et al.
Scheme 1. Synthesis of (3R,4R)-HMTA (3), (3S,4S)-HMTA (4), (3S,4R)-HMTA (5) and (3R,4S)-HMTA (6).
Reagents: (a) H2, 5%Pd/BaSO4, quinoline, MeOH; (b) TBHP, Ti(O-i-Pr)4, D-DET, CH2Cl2; (c) TBHP, Ti(O-i-Pr)4, L-DET, CH2Cl2;
(d) Me3Al, CH2Cl2; (e) TsCl, pyridine/CH2Cl2; (f) NaCN, EtOH/H2O; (g) KOH, EtOH/H2O; (h) TBSCl, imidazole, DMF; (i) TsCl, pyridine;
(j) TBAF, THF.
into the corresponding (S)-2NMA esters, 11, 12, 13 and
1
and (3S,4S)-3-hydroxy-4-methyltetradecanoic acid (4).
To a solution of 9 (0.20 g, 0.87 mmol) in a mixture of
pyridine–CH2Cl2 (1:6, 7 ml) at 0 ꢀC was added TsCl
(0.17 g, 0.89 mmol). After being stirred at 0 ꢀC for 1 h
and at 10 ꢀC for 5 h, the mixture was diluted with
CH2Cl2 (20 ml). The organic layer was successively
washed with a saturated CuSO4 solution and brine, and
then filtration and concentration gave a crude tosylate.
The tosylate in aqueous 40% EtOH (8 ml) was heated to
reflux with NaCN (0.74 g, 15.1 mmol) for 8 h. After
removing most of the EtOH in vacuo, the residue was
extracted with ether. The organic layer was washed with
brine and dried over Na2SO4, filtration and concentra-
tion subsequently affording a crude cyanide. The
cyanide in aqueous 40% EtOH (8 ml) was heated to
reflux with KOH (1.34 g, 23.9 mmol) for 6 h. Most of the
EtOH was removed in vacuo, and the residue was
washed with ether. The aqueous layer was carefully
acidified to pH 2 with concentrated HCl while ice-
cooling under a hood and then extracted with CH2Cl2.
The organic layer was dried over Na2SO4 and concen-
trated in vacuo. The residue was subjected to silica gel
chromatography, eluting with EtOAc–hexane (1:4), and
crystallization from hexane gave 3 (83 mg, 37%) as
14, via their methyl esters. The H-NMR assignment of
13 derived from (3S,4R)-HMTA (5) was in complete
agreement with that of the (S)-2NMA ester from natural
HMTA.1) The absolute configuration of natural HMTA
was consequently determined to be 3S,4R.
Experimental
All melting point (mp) values are uncorrected. Optical
rotation was measured with a Horiba SEPA-300 high-
sensitivity polarimeter. HREI-MS data were measured
with a Jeol JMX-AX500 spectrometer, and 1H- and 13C-
NMR spectra (400 MHz and 100 MHz) were measured
with a Jeol JNM-ꢁ400 spectrometer.
(2S,3R)-3-Methyl-1,2-tridecanediol (9). To a solution
of 77) (0.50 g, 2.33 mmol) in CH2Cl2 (20 ml) at 0 ꢀC was
carefully added 6.50 ml of Me3Al (1.08 M in hexane,
7.02 mmol) under an argon atmosphere. The reaction
mixture was stirred at room temperature for 10 h and
then quenched with 3 M HCl (10 ml) at 0 ꢀC. After
filtering through a pad of Celite, the mixture was
extracted with CH2Cl2. The organic layer was washed
with brine and dried over Na2SO4. Concentration in
vacuo and subsequent silica gel chromatography with
colorless needles, mp 68–71 ꢀC. ½ꢁꢁD +17.20ꢀ (c 1.0,
20
EtOAc–hexane (1:4) gave 9 (0.49 g, 91%) as a colorless
20
oil. ½ꢁꢁD +8.55ꢀ (c 1.0, CHCl3). IR ꢂmax (film) cmꢂ1
:
CHCl3). IR ꢂmax (film) cmꢂ1: 2916, 1705, 1309, 1066.
1H-NMR (CDCl3) ꢃ: 0.88 (3H, t, J ¼ 6:8 Hz), 0.92 (3H,
d, J ¼ 6:8 Hz), 1.26 (18H, m), 1.54 (1H, m), 2.52 (2H,
m), 3.97 (1H, dt, J ¼ 8:0, 4.4 Hz). 13C-NMR (CDCl3) ꢃ:
14.1, 14.2, 22.7, 27.2, 29.4, 29.6, 29.7, 29.9, 31.9, 32.7,
38.1, 38.4, 71.3, 177.3. HREI-MS m=z ðMþ þ HÞ: calcd.
for C15H31O3, 259.2273; found, 259.2287. According to
essentially the same procedure, 4 (83 mg, 33%) was
synthesized from 8 as colorless needles, mp 67–69 ꢀC.
3377, 2924, 1467, 1066, 1014. 1H-NMR (CDCl3) ꢃ: 0.88
(3H, t, J ¼ 6:8 Hz), 0.91 (3H, d, J ¼ 6:8 Hz), 1.26 (18H,
m), 1.53 (1H, m), 2.05 (1H, bs), 2.68 (1H, bs), 3.55 (3H,
m). 13C-NMR (CDCl3) ꢃ: 14.6, 14.6, 22.7, 27.1, 29.4,
29.6, 29.7, 29.9, 31.9, 33.0, 35.8, 65.2, 75.8. HREI-MS
m=z ðMþ{OHÞ: calcd. for C14H29O, 213.2219; found,
213.2185.
(3R,4R)-3-Hydroxy-4-methyltetradecanoic acid (3)

Synthesis of 3-Hydroxy-4-Methyltetradecanoic Acid
233
20
1
½ꢁꢁD ꢂ17:50ꢀ (c 1.0, CHCl3). The H- and 13C-NMR,
HREI-MS, and IR spectral data were in complete
agreement with those of 3.
hexane (1:4), to give the HMTA methyl ester as a
colorless oil, this being used in the next step without
further purification.
(2R)-[(1R)-1-methylundecyl]-oxirane (10). To a solu-
tion of 9 (0.80 g, 3.47 mmol) and imidazole (0.48 g,
7.05 mmol) in DMF (4 ml) at 0 ꢀC was added TBSCl
(0.53 g, 3.52 mmol). After being stirred at 0 ꢀC for 0.5 h
and then at room temperature for 2 h, the reaction
mixture was diluted with CH2Cl2 (6 ml). The organic
layer was washed with brine and dried over Na2SO4,
subsequent filtration and concentration giving a crude
silyl ether. TsCl (0.63 g, 3.30 mmol) was added to a
solution of this silyl ether in pyridine (5 ml) at 0 ꢀC.
After being stirred at 0 ꢀC for 1 h and then at room
temperature for 5 h, the reaction mixture was diluted
with CH2Cl2 (10 ml). The organic layer was washed
with a saturated CuSO4 solution and dried over Na2SO4,
subsequent filtration and concentration giving a crude
tosylate. To a solution of this tosylate in THF (2 ml) was
dropwise added 5.50 ml of TBAF (1.0 M in THF,
5.50 mmol). The reaction mixture was stirred at 40 ꢀC
for 6 h and then cooled to room temperature. After being
diluted with CH2Cl2, the organic layer was washed with
brine and dried over Na2SO4. Concentration in vacuo
and subsequent silica gel chromatography with EtOAc–
To each HMTA methyl ester, DMAP (0.4 mg,
3.3 mmol) and (S)-2NMA (1.8 mg, 8.3 mmol) dissolved
in 0.3 ml of CH2Cl2 were added EDC (3.0 mg,
15.6 mmol) and Et3N (1.5 mg, 14.8 mmol) at 0 ꢀC. After
stirring for 12 h, the reaction mixture was diluted with
CH2Cl2. The organic layer was successively washed
with a diluted aqueous citric acid solution, saturated
aqueous NaHCO3 solution, water and brine, dried over
Na2SO4, and evaporated. The crude product was purified
by HPLC in a silica gel column (8:0 ꢃ 250 nm, Devel-
osil 60-10, Nomura Chemical Co.), eluting with EtOAc–
hexane (1:4) at 2.0 ml/min to give the (S)-2NMA ester
as a colorless oil. The enantiomeric excess (ee) was
calculated on the basis of the peak area ratio between the
diastereomers in an HPLC analysis at 254 nm.
Methyl (3R; 4R)-3-[(S)-20-naphthylmethoxyacyloxy]-
4-methyltetradecanoate (11). This was obtained in an
85% yield (tR 16.2 min, 92.3% ee). 1H-NMR (CDCl3) ꢃ:
0.87 (3H, d, J ¼ 6:8 Hz), 0.85 (3H, t, J ¼ 6:8 Hz), 1.09–
1.30 (18H, m), 1.71 (1H, m), 2.39 (1H, dd, J ¼ 15:6,
4.4 Hz), 2.47 (1H, dd, J ¼ 15:6, 8.8 Hz), 3.24 (3H, s),
3.46 (3H, s), 4.91 (1H, s), 5.27 (1H, ddd, J ¼ 8:8, 4.4,
4.0 Hz), 7.52 (3H, m), 7.83 (3H, m), 7.90 (1H, s).
Methyl (3S,4S)-3-[(S)-20-naphthylmethoxyacyloxy]-
4-methyltetradecanoate (12). This was obtained in an
80% yield (tR 22.9 min, 85.2% ee). 1H-NMR (CDCl3) ꢃ:
0.69 (3H, d, J ¼ 6:8 Hz), 0.89 (3H, t, J ¼ 6:8 Hz), 0.77–
1.31 (18H, m), 1.52 (1H, m), 2.49 (1H, dd, J ¼ 15:6,
4.4 Hz), 2.53 (1H, dd, J ¼ 15:6, 8.8 Hz), 3.46 (3H, s),
3.60 (3H, s), 4.90 (1H, s), 5.26 (1H, ddd, J ¼ 8:8, 4.4,
4.0 Hz), 7.50 (3H, m), 7.82 (3H, m), 7.90 (1H, s).
Methyl (3S,4R)-3-[(S)-20-naphthylmethoxyacyloxy]-
4-methyltetradecanoate (13). This was obtained in a
73% yield (tR 23.1 min, 77.8% ee). 1H-NMR (CDCl3) ꢃ:
0.62 (3H, d, J ¼ 6:8 Hz), 0.89 (3H, t, J ¼ 6:8 Hz), 0.94–
1.30 (18H, m), 1.56 (1H, m), 2.53 (2H, m) 3.46 (3H, s),
3.58 (3H, s), 4.90 (1H, s), 5.26 (1H, td, J ¼ 6:0, 5.6 Hz),
7.51 (3H, m), 7.83 (3H, m), 7.91 (1H, s).
hexane (1:39) gave 10 (0.55 g, 75%) as a colorless oil,
:
½ꢁꢁD +3.10ꢀ (c 1.0, CHCl3). IR ꢂmax (film) cmꢂ1
20
1
2927, 931, 877, 829. H-NMR (CDCl3) ꢃ: 0.81 (3H, d,
J ¼ 6:8 Hz), 0.85 (3H, d, J ¼ 6:0 Hz), 1.19 (18H, m),
1.46 (1H, m), 2.40 (1H, t, J ¼ 4:0 Hz), 2.63 (2H, d,
J ¼ 4:0 Hz). 13C-NMR (CDCl3) ꢃ: 14.1, 15.6, 22.7,
26.9, 29.3, 29.6, 29.9, 31.9, 34.6, 36.1, 45.6, 57.0.
HREI-MS m=z (Mþ): calcd. for C14H28O, 212.2140;
Found, 212.2158.
(3S,4R)-3-Hydroxy-4-methyltetradecanoic acid (5)
and (3R,4S)-3-hydroxy-4-methyltetradecanoic acid (6).
According to essentially the same procedure as that used
for the preparation of 3 from 9, except for the tosylation
step, 5 (68 mg, 28%) was synthesized as colorless
20
needles, mp 40–42 ꢀC. ½ꢁꢁD ꢂ3:60ꢀ (c 1.0, CHCl3). IR
ꢂmax (KBr) cmꢂ1: 2928, 1711, 1292, 1045. 1H-NMR
(CDCl3) ꢃ: 0.88 (3H, t, J ¼ 6:8 Hz), 0.90 (3H, d,
J ¼ 6:8 Hz), 1.26 (18H, m), 1.62 (1H, m), 2.47 (1H, dd,
J ¼ 16:4, 9.6 Hz), 2.54 (1H, dd, J ¼ 16:4, 2.8 Hz), 3.90
(1H, ddd, J ¼ 9:6, 5.2, 2.8 Hz). 13C-NMR (CDCl3) ꢃ:
14.1, 14.8, 22.7, 27.1, 29.4, 29.6, 29.6, 29.7, 29.9, 31.9,
32.3, 37.6, 38.2, 71.8, 178.1. HREI-MS m=z ðMþ þ HÞ:
calcd. for C15H31O3, 259.2273; found, 259.2278. Com-
pound 6 (68 mg, 19%) was synthesized from 8 as
Methyl (3S,4S)-3-[(S)-20-naphthylmethoxyacyloxy]-
4-methyltetradecanoate (14). This was obtained in a
70% yield (tR 16.1 min, 75.2% ee). 1H-NMR (CDCl3) ꢃ:
0.85 (3H, d, J ¼ 6:8 Hz), 0.88 (3H, t, J ¼ 6:8 Hz), 1.18–
1.33 (18H, m), 1.81 (1H, m), 2.40 (1H, dd, J ¼ 15:6,
4.4 Hz), 2.45 (1H, dd, J ¼ 15:6, 8.8 Hz), 3.12 (3H, s),
3.45 (3H, s), 4.90 (1H, s), 5.26 (1H, ddd, J ¼ 8:8, 5.2,
4.4 Hz), 7.51 (3H, m), 7.83 (3H, m), 7.90 (1H, s).
20
colorless needles, mp 41–43 ꢀC. ½ꢁꢁD +2.35ꢀ (c 1.0,
CHCl3). The 1H- and 13C-NMR, HREI-MS, and IR
spectral data were in complete agreement with those
of 5.
Acknowledgment
The authors are indebted to Prof. T. Kusumi of
Tokushima University for providing us with chiral
2NMA, and to Dr. E. Fukushi of Hokkaido University
for HREI-MS measurements.
Preparation of the (S)-2NMA esters. Each HMTA
ester (2.0 mg, 7.8 mmol) was dissolved in ether (1 ml),
and ethereal CH2N2 was added until the yellow color
was retained. After evaporating, the residue was purified
by silica gel chromatography, eluting with EtOAc–

234
K. NIHEI et al.
ture-activity studies of the lipophilic and geometric
parameters of polyarylated acyl analogs of ECB. J.
Med. Chem., 38, 3271–3281 (1995).
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