Determination by enantioselective synthesis of the absolute configuration of CPE, a potential intermediate in coronatine biosynthesis.

Article abstract of DOI:10.1021/ol0165037

The first enantioselective synthesis of the methyl ester of CPE, a potential intermediate in coronatine (COR) biosynthesis, is described. Comparison of the specific rotation of the synthetic ester with that of the methyl ester of natural CPE established t

Full text of DOI:10.1021/ol0165037

ORGANIC
LETTERS
2001
Vol. 3, No. 19
3045-3047
Determination by Enantioselective
Synthesis of the Absolute Configuration
of CPE, a Potential Intermediate in
Coronatine Biosynthesis
Tao Tao and Ronald J. Parry*
Department of Chemistry MS60, Rice UniVersity, Houston, Texas 77251-1892
parry@rice.edu
Received July 26, 2001
ABSTRACT
The first enantioselective synthesis of the methyl ester of CPE, a potential intermediate in coronatine (COR) biosynthesis, is described. Comparison
of the specific rotation of the synthetic ester with that of the methyl ester of natural CPE established that the latter possesses the (R) configuration.
This configuration is the same as that found at the corresponding asymmetric center of coronatine.
Coronatine (COR) (Figure 1) is a novel phytotoxin produced
by five distinct pathovars of Pseudomonas syringae.1 Infec-
tion of the host plants by these bacteria induces chlorosis
on the leaves due to COR production.1 In addition to
chlorosis, COR also induces hypertrophy, inhibits root
elongation, and stimulates ethylene biosynthesis.1 Recently,
several reports have noted the structural and functional
similarities between COR, jasmonic acid, and 12-oxo-
phytodienoic acid, suggesting that COR may function as a
molecular mimic of the octadecanoid signaling molecules
of higher plants.1
shown that the fermentation broth of several strains of P.
syringae contains small amounts of a cyclopentenone car-
boxylic acid (CPE) (Figure 1) in addition to significant
quantities of COR and CFA. The structure of CPE and the
organization of the genes in the CFA polyketide synthase
suggest that CPE may be an intermediate in CFA biosyn-
thesis.5 Since the release of intermediates in polyketide
biosynthesis is a relatively rare occurrence,6 a more detailed
investigation of the potential role of CPE in CFA biosynthesis
is clearly warranted. We have begun this investigation with
The COR molecule is composed of two building blocks
of distinct biosynthetic origin.2 One of these is the cyclo-
propyl amino acid coronamic acid, which is derived from
L-isoleucine via the intermediacy of L-alloisoleucine.2 The
other component is coronafacic acid (CFA) (Figure 1), which
is a polyketide derived from acetate, butyrate, and a four-
carbon unit derived from R-ketoglutarate.2,3 Mitchell4 has
(1) Bender, C. L.; Alarcon-Chaidez, F.; Gross, D. C. Microbiol. Mol.
Biol. ReV. 1999, 63, 266.
(2) Parry, R. J.; Mhaskar, S. V.; Lin, M.-T.; Walker, A. E.; Mafoti, R.
Can. J. Chem. 1994, 72, 86.
(3) Parry, R. J.; Jiralerspong, S.; Mhaskar, S. V.; Alemany, L.; Willcott,
R. J. Am. Chem. Soc. 1996, 118, 703.
Figure 1. Structures of COR, CFA, and CPE.
10.1021/ol0165037 CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/29/2001

a determination by enantioselective synthesis of the absolute
configuration of naturally occurring CPE.
Scheme 2a
The structure of CPE suggested that an enantioselective
synthesis of the compound could be accomplished by means
of an alkylation reaction. It was anticipated that the (R)
enantiomer of CPE could be obtained by alkylation of (S)-
4-benzyl-3-butyryl-2-oxazolidinone with a protected halo-
methylcyclopentenol.7 The synthesis of a suitable halo-
methylcyclopentenol derivative is shown in Scheme 1. The
Scheme 1a
a (a) (CH3)3CCOCl, Et3N, THF, -78 to 0 °C; (b) Li salt of 5,
THF, -78 to 0 °C; (c) NaN[Si(CH3)3]2, THF, -78 °C; (d) 4, THF,
-78 °C; (e) H2O2, LiOH, THF-H2O, 0 °C; (f) CH2N2; (g) HF-
C5H5N, 0 °C; (h) DDQ, benzene, 25 °C.
a (a) 1 N HCl, 70 °C, 1 h; (b) (EtO)2P(O)CH2COOEt, K2CO3;
(c) TBDMSCl, imidazole, DMF; (d) DIBAL-H, toluene, CH2Cl2,
-70 °C; (e) EtOOCNdNCOOEt, Ph3P, ZnI2, THF, 0 °C.
iodide 4 gave the desired alkylation product 7 in greater than
90% yield based upon 6. NMR analysis indicated that only
a single diastereomer was produced in the alkylation reaction.
Initial experiments using the TBDPS13 analogue of 4 gave
poorer yields of the desired alkylation product. Furthermore,
removal of the oxazolidinone chiral auxiliary from the
alkylation product was accompanied by some loss of the
TBDPS group. In contrast, the chiral auxiliary could be
cleanly removed from 7 by standard methods14 without loss
of the TBDMS group to produce the free carboxylic acid,
which was then converted to the methyl ester 8 with
diazomethane. Attempts to remove the silyl group from 8
with TBAF produced a mixture of the desired alcohol 9
and the δ-lactone formed between the carboxylic acid and
the freed OH group, with the latter predominating. How-
ever, when ester 8 was treated with HF-pyridine,15 the free
alcohol 9 was produced as the major product, accompanied
by less than 10% of the δ-lactone. (R)-CPE methyl ester
10 was obtained in high yield from 9 by oxidation of
the allylic alcohol group with DDQ in benzene.16 The
synthesis began with the conversion of 2,5-dimethoxytet-
rahydrofuran 1 into 2-carbethoxycyclopenten-1-ol 2.8 At-
tempts to prepare 2 by reduction of 2-carbethoxycyclopenten-
9
1-one
with sodium borohydride and ceric chloride10
produced mostly the saturated alcohol ester. Protection of
the hydroxyl group of 2 with TBDMS chloride followed by
reduction with DIBAL generated the protected hydroxy-
methylcyclopentenol 3. Cyclopentenol 3 was converted into
the iodo derivative 4 by reaction with diethylazodicarboxy-
late, triphenylphosphine, and zinc iodide.11
The remaining stages of the synthesis are shown in Scheme
2. (S)-4-Benzyl-3-butyryl-2-oxazolidinone 6 was synthesized
by reaction of the lithium salt of (S)-4-benzyl-2-oxazolidi-
none 5 with the mixed anhydride formed between butanoic
acid and pivaloyl chloride.12 Treatment of 6 with sodium
hexamethyldisilazide in THF followed by 2.5 equiv of the
(4) Mitchell, R. E.; Young, H.; Liddell, M. J. Tetrahedron Lett. 1995,
36, 3237.
(5) Rangaswamy, V.; Jiralerspong, S.; Parry, R. J.; Bender, C. L. Proc.
Natl. Acad. Sci. U.S.A. 1998, 95, 15469.
(6) Jiralerspong, S.; Rangaswamy, V.; Bender, C. L.; Parry, R. J. Gene
2001, 270, 191.
(7) Evans, D. A.; Ennis, M. D.; Mathre, D. J. J. Am. Chem. Soc. 1982,
104, 1737.
(8) Villierras, J.; Rambaud, M.; Graff, M. Synth. Commun. 1986, 16,
149. Graff, M.; Dilaimi, A.; Sequineau, P.; Rambaud, M.; Villieras, J.
Tetrahedron. Lett. 1986, 27, 1577.
(9) Reich, H. J.; Renga, J. M.; Reich, I. L. J. Am. Chem. Soc. 1975, 97,
5434.
synthetic compound exhibited [R]23 ) -32.9° (c 0.17,
D
CHCl3).
To determine the absolute configuration of naturally
occurring CPE, the compound was isolated as its methyl ester
from the fermentation broth of P. syringae pv. glycinea
(13) Hanessian, S.; Lavallee, P. Can J. Chem. 1975, 53, 2975.
(14) Evans, D. A.; Kim, A. S.; Metternich, R.; Novack, V. J. Am. Chem.
Soc. 1998, 120, 5921.
(10) Luche, J.-L. J. Am. Chem. Soc. 1978, 100, 2226.
(11) Ho, P.-T.; Davies, N. J. Org. Chem. 1984, 49, 3027.
(12) Ho, G.-J.; Mathre, D. J. J. Org. Chem. 1995, 60, 2271.
(15) Trost, B. M.; Caldwell, C. G.; Murayama, E.; Heissler, D. J. Org.
Chem. 1983, 48, 3252.
(16) Walker, D.; Hiebert, J. D. Chem. ReV. 1967, 67, 153.
3046
Org. Lett., Vol. 3, No. 19, 2001

PG4180. The fermentation was carried out in modified
Hoitink-Sinden medium17 at 18 °C.
therefore possesses the (R) configuration. This configuration
is the same as that found at the corresponding asymmetric
center of CFA, an observation that is consistent with the
intermediacy of CPE in CFA biosynthesis. The availability
of synthetic CPE should facilitate additional investigations
of the details of CFA biosynthesis.
HPLC analysis18 indicated that maximum CPE production
occurred after 3 days. CPE methyl ester (3 mg) was isolated
and purified from 24 L of 3-day-old fermentation broth. The
first step in the purification utilized preparative TLC (SiO2,
8% iPrOH in hexane, two developments). This was followed
by normal-phase preparative HPLC purification (8% iPrOH
in hexane, 10 mm × 250 mm SiO2 column). The final stage
of the purification was accomplished by reverse-phase
preparative HPLC (MeOH/H2O, 7:3, 10 mm × 250 mm C18
column). The natural sample of CPE methyl ester purified
in this way displayed chromatographic and spectral properties
identical to those of the synthetic compound, and it exhibited
[R]23D ) -33.0° (c 0.12, CHCl3). Natural CPE methyl ester
Acknowledgment. The authors thank the National Sci-
ence Foundation (MCB-9807774) and the Robert A. Welch
Foundation (C-0729) for financial support. We also thank
Dr. Sao Jiralerspong for preliminary synthetic studies and
Dr. Carol Bender for providing a culture of P. syringae pv.
glycinea PG4180.
Supporting Information Available: Detailed experi-
mental procedures and spectroscopic data for all new
compounds, as well as details for the isolation and purifica-
tion of CPE methyl ester from the fermentation broth of P.
syringae PG4180. This material is available free of charge
via the Internet at http://pubs.acs.org.
(17) Palmer, D. A.; Bender, C. L. Appl. EnViron. Microbiol. 1993, 59,
1619
(18) An aliquot of fermentation broth was acidified to pH 2 and extracted
with ethyl acetate. Solvent was removed from the dried ethyl acetate extract,
and the residue was treated with diazomethane. The resulting mixture of
methyl esters was analyzed at 230 nm on a 4.6 mm × 250 mm SiO2 column
i
using 8% PrOH in hexane.
OL0165037
Org. Lett., Vol. 3, No. 19, 2001
3047