Total synthesis of the epoxy isoprostane phospholipids PEIPC and PECPC

Article abstract of DOI:10.1021/ol051415y

(Chemical Equation Presented) A total synthesis of the naturally occurring hydroxy ketone PEIPC 1, a compound that plays a role in endothelial activation in atherosclerosis, has been completed via a triply convergent preparation of a protected EI derivative 13 from 3,5-diacetoxycyclopentene 7, pentane-1,5-diol, and vinyllithium, using Sharpless epoxidation and enzymatic resolution as key steps. Final coupling with lyso-PC 16 and silyl group deprotection gave PECPC 2 and PEIPC 1, which showed the same activity as natural PECPC and PEIPC.

Full text of DOI:10.1021/ol051415y

ORGANIC
LETTERS
2005
Vol. 7, No. 18
3933-3935
Total Synthesis of the Epoxy
Isoprostane Phospholipids PEIPC and
PECPC
Michael E. Jung,*,† Judith A. Berliner,‡,§ Daniela Angst,† Dawei Yue,†
Lukasz Koroniak,† Andrew D. Watson,‡ and Rongsong Li‡
Departments of Chemistry and Biochemistry, Medicine/Cardiology, and Pathology,
UniVersity of California-Los Angeles, Los Angeles, California 90095
jung@chem.ucla.edu
Received June 17, 2005
ABSTRACT
A total synthesis of the naturally occurring hydroxy ketone PEIPC 1, a compound that plays a role in endothelial activation in atherosclerosis,
has been completed via a triply convergent preparation of a protected EI derivative 13 from 3,5-diacetoxycyclopentene 7, pentane-1,5-diol, and
vinyllithium, using Sharpless epoxidation and enzymatic resolution as key steps. Final coupling with lyso-PC 16 and silyl group deprotection
gave PECPC 2 and PEIPC 1, which showed the same activity as natural PECPC and PEIPC.
We have previously1 isolated, identified, and described the
biological activity of an in vitro oxidation product of
arachidonoyl phosphatidylcholine, 1-palmitoyl-2-(5,6)-
epoxyisoprostane E2-sn-glycero-3-phosphocholine (PEIPC).
This lipid accumulates in oxidized lipoproteins, the mem-
branes of cells exposed to oxidative stress, and in apoptotic
and necrotic cells.2 PEIPC has also been demonstrated to be
present at high levels in atherosclerotic lesions. We have
demonstrated that five HPLC-separable isomers of PEIPC
were formed from the oxidation of 1-palmitoyl-2-arachi-
donoyl-sn-glycero-3-phosphocholine (PAPC). The most ac-
tive isomer 1 was demonstrated to activate several important
inflammatory responses that contribute to atherosclerosis,
including endothelial-monocyte interaction, cAMP respon-
sive element binding protein (CREB) activation, and syn-
thesis of monocyte activators.3,4 On the basis of mass
spectrometry of the natural compound and the proton NMR
spectra of the dehydration product, the compound was
tentatively assigned the structure 1.5 The small amount of
material precluded a complete structural analysis, and the
assignment of the relative stereochemistry, e.g., the trans-
epoxide and the (E)-trisubstituted alkene, was tentative. We
reported recently the development of a three-component
coupling that permitted the total synthesis of two close
structural analogues of the isoprostane portion of this
interesting phospholipid, namely, the epoxide 3 and its
dehydration product 4 and their (Z)-stereoisomers, which
lends evidence for the correctness of the structure of 1
(Scheme 1).6 We report herein the total synthesis of PEIPC
1, which involves the development of a new route for the
synthesis of this class of epoxyisoprostanes utilizing a triply
(3) Subbanagounder, G.; Wong, J. W.; Faull, K. F.; Miller, E.; Witztum,
J. L.; Berliner, J. A. J. Biol. Chem. 2002, 277, 7271-7281.
(4) Cole, A. L.; Subbanagounder, G.; Mukhopadhyay, S.; Berliner, J.
A.; Vora, D. K. Arterioscler. Thromb. Vasc. Biol. 2003, 23, 1384-1390.
(5) Watson, A. D.; Subbanagounder, G.; Welsbie, D. S.; Faull, K. F.;
Navab, M.; Jung, M. E.; Fogelman, A. M.; Berliner, J. A. J. Biol. Chem.
1999, 274, 24787-24798.
† Department of Chemistry and Biochemistry.
‡ Department of Medicine/Cardiology.
§ Department of Pathology.
(1) Watson, A. D.; Leitinger, N.; Navab, M.; Faull, K. F.; Horkko, S.;
Witztum, J. L.; Palinski, W.; Schwenke, D.; Salomon, R. G.; Sha, W.;
Subbanagounder, G.; Fogelman, A. M.; Berliner, J. A. J. Biol. Chem. 1997,
272, 13597-13607.
(6) Jung, M. E.; Kers, A.; Subbanagounder, G.; Berliner, J. A. J. Chem.
Soc. Chem. Commun. 2003, 167.
(2) Berliner, J. A.; Watson, A. D. N. Engl. J. Med. 2005, 353, 8-11.
10.1021/ol051415y CCC: $30.25
© 2005 American Chemical Society
Published on Web 07/30/2005

tert-butyldimethylsilyl (TBS) chloride8 gave the trans-
disubstituted bromoenol ether 9 in 94% yield. The elaboration
of the bottom side chain was easily accomplished by selective
hydroboration-oxidation of the vinyl group at -78 °C to
give the primary alcohol followed by oxidation to the
aldehyde. Wittig reaction with the ylide from hexyl bromide
afforded the desired (Z)-alkene 10 in 84% overall yield.
The crucial coupling was carried out by first forming the
vinyllithium species by treatment of 10 (dried with molecular
sieves) with tert-butyllithium followed by addition of the
epoxyaldehyde 6 to give the allylic alcohol 11 in 75% yield
(Scheme 4). Hydrolysis of the silyl enol ether and dehydra-
Scheme 1
convergent7 preparation of the key protected EI derivative
13 and its coupling to lyso-PC 16. This route also allows
the preparation of the corresponding enone PECPC 2.
The two components for the key coupling were prepared
as follows: monoprotection of pentane-1,5-diol followed by
Swern oxidation, Horner-Emmons reaction, and reduction
gave the (E)-allylic alcohol 5, which was subjected to
Sharpless asymmetric epoxidation and finally oxidized
(Dess-Martin periodinane) to give the epoxide 6 (Scheme
2). The second key component was prepared in the same
Scheme 4
Scheme 2
tion to the enone 12 was effected in 90% yield with formic
acid in aqueous THF. The final three steps involve conver-
sion of the primary PMB ether into the desired acid.
Deprotection of the ether with DDQ, followed by two-stage
oxidation, initial Dess-Martin periodinane and final sodium
chlorite oxidation gave the desired acid, namely, the silyl
ether of EI, 13, in 85% yield over the three steps.
Deprotection of the TBDPS ether proved to be somewhat
difficult due to the sensitivity of the system to both acid and
base. The use of tetrabutylammonium fluoride (TBAF) at
low temperatures gave mostly the trienone 14 (EC), the
product of â-elimination, although by using an acetic acid
buffered solution of TBAF at 23 °C, one could isolate the
desired â-hydroxy enone (EI) 15 in 40% yield along with
the trienone 14 in 53% yield (Scheme 5). Biological testing
of 15 demonstrated its ability to activate CREB using a
promoter-reporter construct.
manner as described earlier,6 namely, conversion of the
diacetate 7 via seven steps into the 2-bromo-4-silyloxy
cyclopentenone 8 in an overall yield of 87% (Scheme 3).
Scheme 3
The final steps in the synthesis of PEIPC involved coupling
to commercially available lyso-PC 16 and final deprotection
(Scheme 6). Although older methods of coupling could be
made to work, e.g., DCC, DMAP,9 a variation of the recent
(7) (a) Suzuki, M.; Kawagishi, T.; Suzuki, T.; Noyori, R. Tetrahedron
Lett. 1982, 4057. (b) Noyori, R.; Suzuki, M. Chemtracts: Org. Chem. 1990,
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(d) Snider, B. B.; Yang, K. J. Org. Chem. 1992, 57, 3615.
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J. Org. Chem. 1989, 4977. (b) Lipshutz, B. H.; Crow, R.; Ellsworth, E. L.;
Dimock, S. H.; Smith, R. A.; Behling, J. R. J. Am. Chem. Soc. 1990, 4063.
We chose to use the very stable tert-butyldiphenylsilyl
(TBDPS) ether as the cyclopentanol protecting group. 1,4-
Addition of vinylcopper to the enone 8 in the presence of
3934
Org. Lett., Vol. 7, No. 18, 2005

Scheme 5
Scheme 6
method of Acharya and Kobayashi provided the best yields.10
Thus, treatment of the acid 13 with 10 equiv of the
Yamaguchi reagent (2,4,6-trichlorobenzoyl chloride) and
DMAP at 23 °C for 1 h in the presence of lyso-PC 16
afforded an 88% yield of the coupled product 17. Removal
of the TBDPS protecting group was effected using TBAF
in THF at 0 °C, which seemed quite clean by TLC analysis
and produced a mixture of the desired compound PEIPC 1
and the elimination product PECPC 2 in 65-70% crude
yield. However, isolation of the water-soluble products
proved to be somewhat difficult. The best procedure was
evaporation to dryness (during which significant â-elimina-
tion to the enone occurred) and then purification by reverse-
phase HPLC, which showed a 3:1 mixture of 2 and 1.
Synthetic PEIPC 1 eluted as an essentially pure single peak
and had an identical HPLC retention time and mass
spectrometric profile as the naturally occurring compound.1,3
Furthermore, negative ion ESI-MS/MS of synthetic EC 14
and naturally occurring EC had identical daughter ion
fragmentation patterns (data not shown).
oxidized phospholipids and inhibit atherogenesis. Further
work in this area is underway.
Acknowledgment. We thank the National Institutes of
Health (HL64731 and HL30568) for generous support. D.A.
thanks the Swiss National Science Foundation and the
Novartis Foundation for fellowships.
Note Added after ASAP Publication. The Acknowledg-
ment was incomplete in the version published ASAP July
30, 2005; the corrected version was published August 4,
2005.
Thus, we have synthesized the important naturally occur-
ring material PEIPC 1 and its dehydration product PECPC
2 by a route that should allow for the preparation of many
structural analogues that may block the activity of these
Supporting Information Available: Experimental pro-
cedures and spectral data for all new compounds, except 1.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(9) Gu, X.; Sun, M.; Gugiu, B.; Hazen, S.; Crabb, J. W.; Salomon, R.
G. J. Org. Chem. 2003, 68, 3749-3761.
(10) Acharya, H. P.; Kobayashi, Y. Angew. Chem., Int. Ed. 2005, 44,
3481-3484.
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