Strategy for synthesis of the isoleucine conjugate of epi-jasmonic acid

Article abstract of DOI:10.1016/j.tetlet.2008.09.144

The TES ether of 2-((1R,2S,3R)-3-hydroxy-2-((Z)-pent-2-enyl)cyclopentyl)acetic acid (5, equal to the reduction product of epi-jasmonic acid) derived from (1R,4S)-4-hydroxycyclopent-2-enyl acetate (19) in 13 steps was activated by using isobutyl chloroformate and was subjected to condensation with isoleucine at room temperature for 48 h. The product was desilylated and oxidized to the isoleucine conjugate of epi-jasmonic acid in 68% yield over three steps. Similarly, allo-isoleucine conjugate of epi-jasmonic acid and three isoleucine conjugates of ent-epi-jasmonic acid, jasmonic acid, and ent-jasmonic acid were synthesized.

Full text of DOI:10.1016/j.tetlet.2008.09.144

Tetrahedron Letters 49 (2008) 7124–7127
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Strategy for synthesis of the isoleucine conjugate of epi-jasmonic acid
*
Narihito Ogawa, Yuichi Kobayashi
Department of Biomolecular Engineering, Tokyo Institute of Technology, B52, Nagatsuta-cho 4259, Midori-ku, Yokohama 226-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The TES ether of 2-((1R,2S,3R)-3-hydroxy-2-((Z)-pent-2-enyl)cyclopentyl)acetic acid (5, equal to the
reduction product of epi-jasmonic acid) derived from (1R,4S)-4-hydroxycyclopent-2-enyl acetate (19)
in 13 steps was activated by using isobutyl chloroformate and was subjected to condensation with iso-
leucine at room temperature for 48 h. The product was desilylated and oxidized to the isoleucine conju-
gate of epi-jasmonic acid in 68% yield over three steps. Similarly, allo-isoleucine conjugate of epi-jasmonic
acid and three isoleucine conjugates of ent-epi-jasmonic acid, jasmonic acid, and ent-jasmonic acid were
synthesized.
Received 5 August 2008
Revised 21 September 2008
Accepted 25 September 2008
Available online 30 September 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Jasmonates regulate stress responses and development in
plants.1,2 Recently, the jasmonate ZIM-domain 1 (JAZ1) protein
was elucidated as the key component of the jasmonate signaling.3
This protein acts to repress transcription of jasmonate-responsive
genes and is degraded through the SCFCOI1 ubiquitin ligase-depen-
dent 26S proteasome pathway. Isoleucine conjugate of ‘jasmonic
acid’4 assists the binding of SCFCOI1 to JAZ1 and the subsequent
degradation. In connection with these findings, structure–activity
relationship of the amino acid part was briefly examined to find
the high activity for the isoleucine conjugate. On the other hand,
structural requirement of the ‘jasmonic acid’ part appears ambigu-
ous because of the use of stereoisomeric mixtures.
S
S
R
S
CO2H
CO2H
7
12
O
O
epi-jasmonic acid (2)
12-oxo-PDA (1)
H
S
H
R
R
CO2H
CONH
CO2H
S
S
S
O
O
OH
The metabolism of linolenic acid has been studied actively to
disclose the functions in plants.1 One important discovery is the
enzyme-promoted production of 12-oxo-PDA (1) with the 9S,13S
chirality (i.e., cis configuration for the two side chains), which
undergoes reduction and subsequent b-oxidations5 (three times)
to produce epi-jasmonic acid (2) (Fig. 1). During the transforma-
tion, the cis configuration is strictly conserved, though the chiral
center next to the carbonyl group is susceptible to epimerization
to the thermodynamically more stable trans isomers. An equilib-
rium ratio of the methyl ester of 2 (i.e., methyl epi-jasmonate)
and the trans isomer (methyl jasmonate) at ambient temperatures
is 5:95,6 which is supported by calculation based on the molecular
mechanics.7 Similar equilibrium ratios are suspected for the other
metabolites. In contrast to the thermodynamic instability,
however, 12-oxo-PDA (1) and tuberonic acid (4) were proven
kinetically quite stable under neutral conditions.8,9 Consequently,
isolation and/or purification appear to involve step(s) that is
responsible for the contamination of the trans isomers.
isoleucine conjugate of
tuberonic acid (4)
epi-jasmonic acid (3)
Figure 1. Some of naturally occurring metabolites of linolenic acid.
Regarding the acid part of the isoleucine conjugate, the higher
content of epi-jasmonic acid (32%) than that at equilibrium (5%
by analogy with methyl epi-jasmonate) is determined by GC–MS
for the samples, which were carefully prepared taking account of
the possibility of the epimerization.10a,b The ratio is suggestive of
urgent biological investigation using the isoleucine conjugate of
epi-jasmonic acid, that is, 3.
Previously, condensation of jasmonic acid (trans isomer) with
amino acids had been reported.11 However, no yields are given,
and direct application to epi-jasmonic acid seems difficult due to
possibility of the epimerization.12 As a consequence, we envisioned
a method delineated in Scheme 1 as a secure approach to 3.
Epimerization-free acid 5 was selected as an epi-jasmonic acid pre-
cursor. The acid 5 was prepared as shown in Scheme 1 starting
with 9, which was an intermediate for the synthesis of tuberonic
acid (4).9 Herein, we report results of this investigation and
* Corresponding author. Tel./fax: +81 45 924 5789.
E-mail address: ykobayas@bio.titech.ac.jp (Y. Kobayashi).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.144

N. Ogawa, Y. Kobayashi / Tetrahedron Letters 49 (2008) 7124–7127
7125
Table 1
H
Condensation of a model acid 16a with 6b
CO2H
+
H
H
S
see text
H2N
CO2H
OTES
S
1) 6·Et3N (5 equiv)
H
CO2H
3
7
isoleucine (6)
CO2H
rt, 48 h, THF-H2O
5
CONH
6
2) HCO2H
0 ºC, 30 min
H
OTES
OH
H
CO2H
16 C3-C7, trans
— TES
oxidn
17
CONH
C6-C7, cis/trans
3
Entry
Reagentc (equiv)
Yieldd (%)
OH
OTES
1
2
3
4
5
6
7
8
9
WSC (1.3)
DMT-MM (1.3)
BOPCl (1.3)
HATU (1.3)
HBTU (1.3)
TCBC (1.5)
DCBC (1.5)
Isobutyl chloroformate (1.3)
1-Chloroethyl chloroformate (1.3)
43
51
47
47
51
17
0
8
7
1) Swern oxidn
1) Cy2BH
2) [Ph3PC3H7]+Br—
NaN(TMS)2
60
42
2) H2O2
NaOH
OTES
OH
OTES
OTES
10
a
83%
A diastereomeric mixture from racemic methyl jasmonate.
Condensations with 6ÁEt3N (5 equiv) were carried out in THF and H2O (1:1) at
9
77%
b
room temperature for 48 h.
1) NMO
TPAP (cat.)
c
WSC, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride; DMT-
MM, 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride; BOPCl,
bis(2-oxo-3-oxazolidinyl)phosphinic chloride; HATU, 1-[bis(dimethylamino)meth-
ylene]-1H-1,2,3-triazolo(4,5-b) pyridinium 3-oxide hexafluorophosphate; HBTU,
1-[bis(dimethylamino)methylene]-1H-benzotriazolium 3-oxide hexafluorophos-
phate; TCBC, 2,4,6-trichlorobenzoyl chloride; DCBC, 2,6-dichlorobenzoyl chloride.
5
2) NaClO2
(72% from 11)
OTES
Me2C=CHMe
phosphate buffer
(pH 6.8)
11
d
Isolated yields by chromatography on silica gel.
Scheme 1. An approach to the conjugate 3.
The TES group in the product was removed in HCO2H and the
resulting alcohol 17 was purified by chromatography on silica gel
(CHCl3/EtOAc to CHCl3/EtOAc/HCO2H). Among the reagents, iso-
butyl chloroformate gave a good yield (entry 8). In addition, further
condensation product(s) such as 16-Ile–Ile was not identified.
The above procedure was applied to the optically active acid 5
successfully, and removal of the TES group in HCO2H produced
cyclopentanol 8 in 68% yield. Finally, Jones oxidation (CrO3,
H2SO4) at 0 °C for 30 min gave the isoleucine conjugate 3 quantita-
tively.13 The isomeric purity of 3 over the trans isomer (14 in Fig. 2)
was 96% by 500 MHz 1H NMR spectroscopy.14a The acidic condi-
tions during the oxidation are responsible for the partial (4%) epi-
merization to 14. We repeated the procedure several times with
similar ratios. The conjugate 3 with this purity will be acceptable
for biological study.
Next, we turned our attention to synthesis of the stereoisomers
12–15. Condensation of acid 5 with allo-isoleucine 18 (purchased
from Aldrich) followed by desilylation and subsequent Jones oxida-
tion furnished 12 in 48% with a similar efficiency (Scheme 2). The
epimeric purity at the C7 carbon (next to the carbonyl carbon) was
94% by 1H NMR spectroscopy.
Synthesis of 13, an isoleucine conjugate with the enantiomer of
epi-jasmonic acid, started with 19 (>99% ee by chiral HPLC). As
delineated in Scheme 3, 19 (>99% ee) was converted to 20 by
palladium-catalyzed reaction with methyl malonate followed by
allo-Ile part
(3
S
,7
R
)-isomer
part
H
R
H
H
CO2H
H
CO2H
S
CONH
CONH
R
O
O
allo-Ile conjugate of epi-JA (12)
Ile conjugate of
the enantiomer of epi-JA (13)
(7R)-isomer part
(3S
)-isomer part
S
H
H
H
CO2H
H
CONH
CONH
CO2H
R
O
O
Ile conjugate of JA (14)
Ile conjugate of
the enantiomer of JA (15)
Figure 2. Related compounds of the isoleucine conjugate.
application to synthesis of isomers 12–15 (Fig. 2). Stability of 3 is
also presented.
H
For preliminary study of the condensation, a diastereomeric
mixture of 16 was prepared in good yield from racemic methyl
jasmonate by a sequence of reactions: (1) NaOH, (2) NaBH4, (3)
TESCl, imidazole. The acid 16 was activated by reagents given in
Table 1 and was subjected to condensation with isoleucine (6). In
brief, 16 and the reagent were mixed at room temperature for
3 h and the condensation with the isoleucine and Et3N complex
6ÁEt3N (5 equiv) was examined at room temperature for 48 h.
H
CO2H
H
1) Et3N,
i-BuOCOCl
CONH
+
H
CO2H
5
2) CrO3, H+
H2N
O
allo-isoleucine (18)
12
Scheme 2. Synthesis of allo-Ile conjugate of epi-JA (12).

7126
N. Ogawa, Y. Kobayashi / Tetrahedron Letters 49 (2008) 7124–7127
quite stable at room temperature for three weeks, whereas addi-
1) CH2(CO2Me)2
OAc
t-BuOK, Pd cat.
tion of K2CO3 promoted rapid epimerization to afford a 6:94 mix-
ture of 3 and 14 after 24 h. The rate of the epimerization appears
faster than that of tuberonic acid (4) and much faster than 12-
oxo-PDA (1), though we do not have any reason to explain such
a difference.
CO2Me
HO
HO
2) KI, DMF-H2O
82%
ref 9 and
Scheme 1
19
(> 99% ee)
20
In summary, we developed an access to 3 for the first time, and
the method was successfully applied to synthesis of its epimers
12–15. These compounds will be useful not only for the biological
study at molecular level but also as standards for elucidation of
these compounds from natural sources.
1) 6·Et3N
CO-isoleucine
i
-BuOCOCl
CO2H
2) HCO2H
50%
X
OTES
ent-5
21, X = H, β-OH
13, X = O
CrO3, H+
Acknowledgments
92%
Racemic methyl jasmonate was kindly provided by Zeon Co.
Ltd, Japan. This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports, and
Culture, Japan.
Scheme 3. Synthesis of Ile conjugate of the enantiomer of epi-JA (13).
decarboxylation in 82% yield.15 Transformations of 20 to ent-9
(structure not shown) and to ent-5 from ent-9 were accomplished
similarly.9 Finally, condensation with isoleucine (6) under the con-
ditions established above followed by desilylation gave 21 in 50%
yield and subsequent oxidation furnished 13 in 92% yield. The dia-
stereomeric ratio of 13 at C7 was 96:4 by 1H NMR spectroscopy.
For synthesis of isoleucine conjugates 14 and 15, optically
active intermediates 22 and the enantiomer ent-22 were obtained
by the kinetic resolution of the racemic alcohol derived from
methyl jasmonate by PPL-assisted acetylation according to the lit-
erature procedure.16 As summarized in Scheme 4, 22 was treated
with aqueous NaOH, and the hydroxy acid was converted to the
TES-protected acid 23. Condensation with 6ÁEt3N followed by
deprotection afforded 24, which was oxidized to 14 in 89% yield.
Diastereomeric purity of 14 over 3 was >99% by 500 MHz. 1H
NMR spectroscopy, indicating slower (if any) deprotonation of 14
at C7 than that of 3 (4%) during the oxidation. The stereochemistry
of the C3 acetamide chain on the cyclopentanone ring might be
responsible for the difference between the two cases. In a similar
manner, ent-22 was transformed to 15 without detectable epimer-
ization by 1H NMR spectroscopy.
References and notes
1. Reviews: (a) Creelman, R. A.; Mullet, J. E. Annu. Rev. Plant Physiol. Plant Mol. Biol.
1997, 48, 355–381; (b) Sembdner, G.; Parthier, B. Annu. Rev. Plant Physiol. Plant
Mol. Biol. 1993, 44, 569–589; (c) Hamberg, M.; Gardner, H. W. Biochim. Biophys.
Acta 1992, 1165, 1–18.
2. Recent results: (a) Xie, D.-X.; Feys, B. F.; James, S.; Nieto-Rostro, M.; Turner, J. G.
Science 1998, 280, 1091–1094; (b) Xiao, S.; Dai, L.; Liu, F.; Wang, Z.; Peng, W.;
Xie, D. Plant Cell 2004, 16, 1132–1142; (c) Staswick, P. E.; Tiryaki, I. Plant Cell
2004, 16, 2117–2127; (d) Taki, N.; Sasaki-Sekimoto, Y.; Obayashi, T.; Kikuta, A.;
Kobayashi, K.; Ainai, T.; Yagi, K.; Sakurai, N.; Suzuki, H.; Masuda, T.; Takamiya,
K.; Shibata, D.; Kobayashi, Y.; Ohta, H. Plant Physiol. 2005, 139, 1268–1283; (e)
Walter, A.; Mazars, C.; Maitrejean, M.; Hopke, J.; Ranjeva, R.; Boland, W.;
Mithöfer, A. Angew. Chem., Int. Ed. 2007, 46, 4783–4785.
3. (a) Thines, B.; Katsir, L.; Melotto, M.; Niu, Y.; Mandaokar, A.; Liu, G.; Nomura, K.;
He, S. Y.; Howe, G. A.; Browse, J. Nature 2007, 448, 661–665; (b) Chini, A.;
Fonseca, S.; Fernández, G.; Adie, B.; Chico, J. M.; Lorenzo, O.; García-Casado, G.;
López-Vidriero, I.; Lozano, F. M.; Ponce, M. R.; Micol, J. L.; Solano, R. Nature
2007, 448, 666–671.
4. The term ‘jasmonic acid’ is used herein to indicate a racemic mixture of
jasmonic acid (major) and epi-jasmonic acid (minor).
5. (a) Li, C.; Schilmiller, A. L.; Liu, G.; Lee, G. I.; Jayanty, S.; Sageman, C.; Vrebalov,
J.; Giovannoni, J. J.; Yagi, K.; Kobayashi, Y.; Howe, G. A. Plant Cell 2005, 17, 971–
986; (b) Koo, A. J. K.; Chung, H. S.; Kobayashi, Y.; Howe, G. A. J. Biol. Chem. 2006,
281, 33511–33520.
6. Seto, H.; Nomura, E.; Fujioka, S.; Koshino, H.; Suenaga, T.; Yoshida, S. Biosci.
Biotechnol. Biochem. 1999, 63, 361–367.
7. Seto, H.; Fujioka, S.; Fujisawa, H.; Goto, K.; Nojiri, H.; Yamane, H.; Yoshida, S.
Biosci. Biotechnol. Biochem. 1996, 60, 1709–1711.
8. Ainai, T.; Matsuumi, M.; Kobayashi, Y. J. Org. Chem. 2003, 68, 7825–7832.
9. Nonaka, H.; Wang, Y.-G.; Kobayashi, Y. Tetrahedron Lett. 2007, 48, 1745–1748.
10. Isolation of the isoleucine conjugate and discussion about the stereochemistry:
(a) Miersch, O.; Brückner, B.; Schmidt, J.; Sembdner, G. Phytochemistry 1992, 31,
3835–3837; (b) Schmidt, J.; Kramell, R.; Brückner, C.; Schneider, G.; Sembdner,
G.; Schreiber, K.; Stach, J.; Jensen, E. Biomed. Environ. Mass Spectrom. 1990, 19,
327–338; Other isolations of the conjugate: (c) Kramell, R.; Miersch, O.; Hause,
B.; Ortel, B.; Parthier, B.; Wasternack, C. FEBS Lett. 1997, 414, 197–202; (d)
Miersch, O.; Bohlmann, H.; Wasternack, C. Phytochemistry 1999, 50, 517–523.
11. (a) Kramell, R.; Schmidt, J.; Schneider, G.; Sembdner, G.; Schreiber, K.
Tetrahedron 1988, 44, 5791–5807; (b) Kramell, R.; Schneider, G.; Miersch, O.
Chromatographia 1997, 45, 104–108.
With the 1H NMR spectra of 3 and its C7 epimer 14 in hand, we
examined chemical stability of 3 in methanol by 1H NMR spectro-
scopy.14b In contrast to the expectation from the partial epimeriza-
tion observed during Jones oxidation, conjugate 3 was proven to be
1) NaOH aq.
CO2H
CO2Me
2) TESCl, imidazole
100%
OTES
23
ODNB
22, 98% ee
12. An attempted condensation of epi-jasmonic acid (2) and isoleucine (6) under
the optimized conditions (Table 1, entry 8) gave a mixture of the desired
product 3 and trans isomer 14 in a 30:70 ratio.
1) 6·Et3N
-BuOCOCl
i
CrO3, H+
89%
CO-isoleucine
13. To a solution of acid 5 (25.0 mg, 0.0766 mmol) in THF (1 mL) were added Et3N
(0.014 mL, 0.099 mmol) and isobutyl chloroformate (0.013 mL, 0.10 mmol).
The mixture was stirred at room temperature for 3 h. The mixture was filtered
off, and the precipitates were washed with THF (3 mL). A solution of isoleucine
(6) (50 mg, 0.38 mmol) and Et3N (0.053 mL, 0.38 mmol) in H2O (4 mL) were
added to the combined filtrates. The mixture was stirred at room temperature
for 48 h, and diluted with saturated NH4Cl. The resulting mixture was
extracted with CHCl3 several times. The combined extracts were dried over
MgSO4 and concentrated. The residue was diluted with HCO2H (2 mL) at 0 °C.
After 30 min, the solution was concentrated to give a residue, which was
purified by chromatography on silica gel (CHCl3/EtOAc to CHCl3/EtOAc/HCO2H)
to give alcohol 8 (17.0 mg, 68%): 1H NMR (300 MHz, CDCl3) d 0.94 (t, J = 7.5 Hz,
3H), 0.95 (d, J = 7.5 Hz, 3H), 0.97 (t, J = 7.5 Hz, 3H), 1.11–1.71 (m, 5H), 1.82–2.31
(m, 9H), 2.55 (dd, J = 13.8, 3.9 Hz, 1H), 4.23 (t, J = 4.8 Hz, 1H), 4.61 (dd, J = 8.4,
4.8 Hz, 1H), 4.0–4.9 (br s, 2H), 5.33–5.50 (m, 2H), 6.10 (d, J = 8.4 Hz, 1H). To a
solution of the above alcohol 8 (7.9 mg, 0.024 mmol) in acetone (1 mL) was
added Jones reagent (2 drops, 4 M solution) at 0 °C. The resulting mixture was
14
2) HCO2H
OH
60%
24
1) 6·Et3N
CO2Me
i
-BuOCOCl
CO2H
15
2) HCO2H
ODNB
ent-22, 99% ee
OTES
ent-23
41% over 2 steps
3) CrO3, H+, 89%
Scheme 4. Synthesis of isoleucine conjugates of jasmonic acid and enantiomer of
jasmonic acid. DNB = 3,5-(NO2)2C6H3C(@O)–.

N. Ogawa, Y. Kobayashi / Tetrahedron Letters 49 (2008) 7124–7127
7127
stirred at 0 °C for 30 min, and i-PrOH was added to quench excess reagent. The
mixture was subjected directly to chromatography on silica gel (CHCl3/EtOAc/
HCO2H) to give the isoleucine conjugate of epi-jasmonic acid 3 (7.9 mg, 100%):
1H NMR (500 MHz, CDCl3) d 0.94 (t, J = 7.5 Hz, 3H), 0.95 (d, J = 7.5 Hz, 3H), 0.96
(t, J = 7.5 Hz, 3H), 1.16–1.38 (m, 2H), 1.44–1.54 (m, 1H), 1.82–2.10 (m, 7H),
2.19–2.30 (m, 2H), 2.34–2.44 (m, 2H), 2.84–2.94 (m, 1H), 4.63 (dd, J = 8.5,
4.5 Hz, 1H), 5.34 (dt, J = 10.5, 7.5 Hz, 1H), 5.45 (dt, J = 10.5, 7.5 Hz, 1H), 6.30 (d,
J = 8.5 Hz, 1H), 6.1–6.8 (br s, 1 H).
14. (a) Compared the signals at d 2.84–2.94 (m) and 2.64–2.71 (dd, J = 14 and 7 Hz)
for 3 and the trans isomer (14) (500 MHz, CDCl3).; (b) Compared the signals at
d 2.76–2.90 (m) and 2.57–2.66 (m) for 3 and the isomer (14) (500 MHz,
methanol-d4).
15. Acharya, H. P.; Kobayashi, Y. Tetrahedron 2006, 62, 3329–3343.
16. (a) Kiyota, H.; Higashi, E.; Koike, T.; Oritani, T. Tetrahedron: Asymmetry 2001, 12,
1035–1038; (b) Asamitsu, Y.; Nakamura, Y.; Ueda, M.; Kuwahara, S.; Kiyota, H.
Chem. Biodivers. 2006, 3, 654–659.