Studies towards the total synthesis of an epoxy isoprostane phospholipid, a potent activator of endothelial cells.

Article abstract of DOI:10.1039/b209892j

We report studies toward the total synthesis of an epoxy isoprostane, namely the preparation of compound 9 which is an analogue of the elimination product 7 of the naturally occurring epoxy isoprostane 4 by a straightforward route using a three-component coupling, and have shown by several spectroscopic criteria that it closely resembles the natural material.

Full text of DOI:10.1039/b209892j

Studies towards the total synthesis of an epoxy isoprostane phospholipid,
a potent activator of endothelial cells
Michael E. Jung,*a Annika Kers,a Ganesamoorthy Subbanagounderb and Judith A. Berlinerb
a Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Ave, Los
Angeles, CA 90095, USA. E-mail: jung@chem.ucla.edu; Fax: 310 206-3722; Tel: 310 825-7954
b Department of Medicine, University of California, Los Angeles, 405 Hilgard Ave, Los Angeles, CA 90095,
USA. E-mail: jberliner@mednet.ucla.edu; Fax: 310 267-2163; Tel: 310 825-2436
Received (in Corvallis, OR, USA) 14th October 2002, Accepted 14th November 2002
First published as an Advance Article on the web 17th December 2002
We report studies toward the total synthesis of an epoxy
isoprostane, namely the preparation of compound 9 which is
an analogue of the elimination product 7 of the naturally
occurring epoxy isoprostane 4 by a straightforward route
using a three-component coupling, and have shown by
several spectroscopic criteria that it closely resembles the
natural material.
cyclopentadiene), using deacetylation with electric eel acet-
ylcholine esterase.9 Replacement of the acetyl group by a silyl
group was performed in four steps via 12–14 to give the
monoalcohol 15 which on PCC oxidation gave 16.10
We report herein the total synthesis of the epoxy isoprostane
analogues 8 and 9. Isoprostanes,1 isomers of the well known
prostaglandins, were discovered in 1990,2 and several substitu-
tion patterns in the cyclopentane ring are known (D, E, and F,
1–3).3 They are formed in both biological systems and in vitro,
via a free-radical induced oxidation process, from arachidonic
acid in a pathway independent of cyclooxygenases.4,5 Since the
formation of isoprostanes is not an enzymatic process, a large
number of stereo- and regioisomers are formed with the relative
ratio of the different isomers being dependent on the exact
conditions under which the isoprostanes were formed. We
recently reported the isolation and biological activity of an in
vitro oxidation product of arachidonyl phosphocholine 4, an
oxidized phospholipid with the same biological properties as
minimally modified low density lipoproteins (MM-LDL).6,7
Studies indicate that this MM-LDL is involved in the develop-
ment and progression of atherosclerosis. Based primarily on
mass spectrometry of both the natural compound 4 and its
dehydration product 5 and the isoprostane fatty acid portions 6
and 7, and especially the proton NMR spectra of 7, the
compound was tentatively assigned the structure 4.7 Since the
small amount of material precluded a complete structural
analysis, the assignment of the relative stereochemistry, e.g., the
trans epoxide and the E trisubstituted alkene, is tentative. We
report the synthesis of two close structural analogues of the
isoprostane portion of this interesting phospholipid, namely the
epoxide 8 and its dehydration product 9 and their Z stereoi-
somers, which lends evidence for the correctness of the
structure of 4.
The epoxy aldehyde ester 22 was prepared in five steps from
the ester aldehyde 17 (prepared in one step by ozonolysis of
cyclopentene).10 Wittig reaction using (triphenylphosphor-
anylidene)acetaldehyde 18 gave an E/Z mixture of the aldehyde
19.11 Selective reduction at low temperature gave the alcohol
20. Sharpless asymmetric epoxidation12 of this substrate was
earlier reported to be problematic.13 Using a mild work-up
allowed the desired compound 21 to be isolated in good yield
and ee.14 Although oxidation of this epoxy alcohol 21 with
tetrapropyl perruthenate TPAP/NMO gave low yields and Dess-
Martin periodinane oxidation gave non-reproducible yields,
Swern oxidation afforded the desired epoxyaldehyde 22.
Before attempting the required 3-component coupling15 we
first studied a simpler model, namely the addition of allylcop-
per16 to cyclopentenone 23 in the presence of trimethylsilyl
chloride (TMSCl)17 to give the crude silyl enol ether 24.
Regeneration of the enolate with methyllithium18 and trapping
with the optically active aldehyde 22 afforded in 90% yield a
mixture of four diastereomers 25 (two major and two minor).
Elimination of the mesylate19 and chromatographic separation
gave a 1+1 mixture of two diastereomers of the E-enone 26E in
19% yield and predominantly one diastereomer of the Z-enone
26Z in 6% yield. Analogous trapping of the zinc enolate (made
by the addition of 1 eq of ZnCl2 to the enolate) afforded the
hydroxy ketone 25 as four isomers (in an approximate 1+1+1+1
ratio) in lower yield (33%). Elimination of this mixture gave
26E and 26Z in 22% and 6% yield, respectively. The proton
NMR spectrum of 26E matched generally that of the isolated
acids 6 and 7 but the differences in the structures are so
significant that a close comparison was not expected. However,
the UV and mass spectra were more informative. Compound
26E had a lmax of 255 nm while the lmax of both 6 and 7 were
252 nm. In the ESI-MS (and the APCI-MS), compound 26E
showed an exact mass of 278.1518 and peaks at m/z of 131 and
159 which correspond to the breakdown of the epoxide and
match the peaks of m/z of 115 and 143 seen in the mass spectra
of 6 and 7.
Of the several good methods developed for the preparation of
prostaglandin-like molecules, we chose a modification of
Noyori’s ‘3-component coupling.’8 The enantiomerically en-
riched R-(+)-4-tert-butyldimethylsilyloxy-2-cyclopenten-1-one
16 was prepared from the prochiral diacetate 10 (made from
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CHEM. COMMUN., 2003, 196–197
This journal is © The Royal Society of Chemistry 2003

Table 1 Comparison of 1H NMR Spectra of 7, 9, and 31
Proton
7
Mult.
9
Mult.
31
Mult.
H11
H10
H7
H12
H6
7.53
6.34
6.16
3.65
3.39
2.99
6.0
2.0
1.7
8.4
dd
dd
d
m
dd
m
7.57
6.37
6.14
3.67
3.36
3.01
6.0
2.4
1.8
9.3
dd
dd
d
m
dd
app td
7.47
6.32
5.58
3.40
4.64
2.86
6.0
2.4
1.8
8.4
dd
dd
d
m
dd
app td
H5
J10,11
J11,12
J10,12
J6,7
Having established that the 3-component coupling and
elimination worked even with the sensitive epoxide function-
ality, we then carried out the process on the chiral cyclopente-
none 16. 1,4-Addition of allylcopper to 16 in the presence of
TMSCl afforded the crude silyl enol ether 27. Regeneration of
the enolate and trapping with the aldehyde 22 gave the desired
product 28 in 27% yield together with the enone 29 in 23%
yield, in which elimination of the silyloxy group had occurred.
Elimination of the hydroxy group was carried out as before with
the silyl ether 28 giving the desired analogue of 6, the enone 8,
in 44.5% yield along with the Z isomer 30 in 16.5% yield.
Similar treatment of the enone 29 afforded the analogue of 7, the
dienone 9 in 38% yield along with the Z isomer 31 in 32% yield.
The structural assignment of 8 and 9 was based largely on NOE
data, e.g., NOESY spectra for both compounds showed
J5,6
1.9
1.9
2.0
Thus our studies strongly support the unambiguous structural
assignment of the naturally occurring epoxy isoprostane formed
by free radical-induced oxidation of arachindonyl phosphati-
dylcholine. Furthermore the strategy and methods reported
herein should pave the way for a total synthesis of the epoxy
isoprostane phospholipid 7. We are currently carrying out
biological investigations of the analogues 8, 9, 26EZ, 30 and 31
and the synthesis of the epoxy isoprostanes 6 and 7 and their
derived phospholipids 4 and 5 to further study the endothelial
activation by this novel class of compounds.
The authors would like to thank the National Institutes of
Health for financial support via USPHS grant HL64731.
Notes and references
1 D. F. Taber, J. D. Morrow and L. J. Roberts, Prostaglandins, 1997, 53,
63–7.
2 J. D. Morrow, K. E. Hill, R. F. Burk, T. M. Nammour, K. F. Badr and
L. J. Roberts, Proc. Natl. Acad. Sci., 1990, 87, 9383–7.
3 J. D. Morrow, T. A. Minton, C. R. Mukundan, M. D. Campbell, W. E.
Zackert, V. C. Daniel, K. F. Badr, K. F. ; Blair and L. J. Roberts, J. Biol.
Chem., 1994, 269, 4317–26.
4 J. D. Morrow, J. A. Awad, H. J. Boss, I. A. Blair and L. J. Roberts, Proc.
Natl. Acad. Sci., 1992, 89, 10721–5.
5 J. Rokach, S. P. Khanapure, S. W. Hwang, M. Adiyaman, J. A. Lawson
and G. A. FitzGerald, Prostaglandins, 1997, 54, 823–51.
6 A. D. Watson, N. Leitinger, M. Navab, K. F. Faull, S. Horkko, J. L.
Witztum, W. Palinski, D. Schwenke, R. G. Salomon, W. Sha, G.
Subbanagounder, A. M. Fogelman and J. A. Berliner, J. Biol. Chem.,
1997, 272, 13597–607.
7 A. D. Watson, G. Subbanagounder, D. S. Welsbie, K. F. Faull, M.
Navab, M. E. Jung, A. M. Fogelman and J. A. Berliner, J. Biol. Chem.,
1999, 274, 24787–98.
8 (a) M. Suzuki, T. Kawagishi, T. Suzuki and R. Noyori, Tetrahedron
Lett., 1982, 4057; (b) R. Noyori and M. Suzuki, Chemtracts: Org.
Chem., 1990, 173; (c) R. Noyori and M. Suzuki, Angew Chem., Int. Ed.,
1984, 847.
9 (a) D. R. Deardorff, C. Q. Windham and C. L. Craney, Org. Synth.,
1996, 73, 25; (b) J.-E. Bäckvall, S. E. Byström and R. E. Nordberg, J.
Org. Chem., 1984, 49, 4619; (c) A. G. Myers, M. Hammond and Y. Wu,
Tetrahedron Lett., 1996, 37, 3083.
10 (a) S. L. Schreiber, R. E. Claus and J. Reagan, Tetrahedron Lett., 1982,
23, 3867; (b) R. E. Claus and S. L. Schreiber, Org. Synth.. Collect. Vol.
VII, 1990, 168.
11 M. A. Ciufolini and S. Zhu, J. Org. Chem., 1998, 63, 1668.
12 Y. Gau, R. M. Hanson, J. M. Klunder, S. Y. Ko, H. Masamune and K.
B. Sharpless, J. Am. Chem. Soc., 1987, 109, 5765.
13 E. J. Corey, S.-I. Hashimoto and A. E. Barton, J. Am. Chem. Soc., 1981,
103, 721.
14 A. M. Kornilov, A. E. Sorochinskii, I. A. Butovich and V. P. Kukhar,
Russ. J. Org. Chem., 1988, 2183.
15 B. B. Snider and K. Yang, J. Org. Chem., 1992, 57, 3615.
16 (a) B. H. Lipshutz, E. L. Ellsworth, S. H. Dimock and R. A. J. Smith, J.
Org. Chem., 1989, 4977; (b) B. H. Lipshutz, R. Crow, E. L. Ellsworth,
S. H. Dimock, R. A. Smith and J. R. Behling, J. Am. Chem. Soc., 1990,
4063.
17 (a) B. H. Lipshutz, E. L. Ellsworth, S. H. Dimock and R. A. J. Smith, J.
Am. Chem. Soc., 1990, 4404; (b) E. J. Corey and N. W. Boaz,
Tetrahedron Lett., 1985, 26, 6015.
interactions between H5 and H7 and, more importantly, between
H6 and H12. It is instructive to compare the spectral data—
NMR, MS, and UV—of the analogues 8 and 9 with the naturally
derived materials 6 and 7. In particular, the proton NMR
spectrum of 9 matched very closely the relevant regions of the
proton NMR spectrum of 7. As shown in Table 1, the peaks for
H11, H10, H7, H12, H6, and H5 were at nearly identical chemical
shift and had virtually identical coupling constants in the two
compounds. The only significant differences were observed in
the olefinic protons of the allyl group and the cis-2-octenyl unit,
as would be expected. This adds compelling evidence to the
assignment of the structure of 7 to the compound derived from
the natural material. By contrast, the proton NMR spectrum of
the stereoisomer 31 does not match the spectral pattern of 7,
especially with regard to protons H7, H6, and H5, all of which
are quite different. In addition, the mass spectrum of 9
underwent an analogous fragmentation to that of 7, showing
mass ions at m/z 147 and 131, which represent cleavages at the
epoxy group that were also seen in 7 (m/z 217 and 115).
Furthermore, the major cleavage in 9 is at the trisubstituted
alkene (m/z 159), which matches a similar peak in 7 (m/z 143).
In addition, the silyl ether 8 showed the expected mass spectral
fragmentation as seen with other compounds in this series.
Finally the ultraviolet spectrum of 9 (lmax of 255) matched very
closely that of 6 and 7 (lmax of 252).
18 O. W. Gooding, C. C. Beard, G. F. Cooper and D. Y. Jackson, J. Org.
Chem., 1993, 58, 3681.
19 M. Suzuki, A. Yanagisawa and R. Noyori, Tetrahedron Lett., 1984, 25,
1383.
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