Stereodivergent synthesis of all 15-F2 isoprostanes

Article abstract of DOI:10.1021/ja027154u

Isoprostanes, lipid metabolites generated from free radical oxidation of membrane-bound arachidonic acid, have been detected in organisms subjected to oxidative stress; however, the function and cellular targets of the isoprostanes are unclear. As an initial step toward studying the biological role of these molecules, we report the preparation of all known and anticipated 15-F₂ isoprostanes. The stereodivergent strategy to the complete isoprostane library features a ring-opening metathesis to introduce the cis-alkyl side chains that are characteristic of this class of molecules. Resolution to the individual stereoisomers can be accomplished by either a catalytic asymmetric reduction or an auxiliary-based separation protocol. In either case, the individual isomers can be converted to the corresponding 15-F₂ isoprostanes through a straightforward functionalization of the carboxylic acid-containing side chain. The availability of this complete 15-F₂ isoprostane library, containing both known and anticipated lipid metabolites, allows for the first time the side-by-side evaluation of these compounds in a variety of biological assays. Copyright

Full text of DOI:10.1021/ja027154u

Published on Web 08/22/2002
Stereodivergent Synthesis of All 15-F2 Isoprostanes
Thomas O. Schrader and Marc L. Snapper*
Department of Chemistry, Eugene F. Merkert Chemistry Center, Boston College, 2609 Beacon Street,
Chestnut Hill, Massachusetts 02467
Received June 3, 2002
Isoprostanes,1 lipid metabolites generated from free radical
oxidation of membrane-bound arachidonic acid (1),2 have received
considerable attention since their discovery over a decade ago.
Unlike the prostaglandins, these lipid oxidation products are not
necessarily formed under enzymatic control. It is not surprising,
therefore, that a variety of isoprostane stereo- and regioisomers have
been detected in the biological fluids of organisms (including
humans) subjected to oxidative stress.3
critical to develop a better understanding of the biological functions
of the isoprostanes. In a few cases, these lipid metabolites have
been reported to be potent vasoconstrictors,5 smooth muscle growth
factors,6 platelet aggregation factors,7 as well as to possess other
biological activities;8 however, for the most part, the function and
cellular targets of the isoprostanes are unclear. As an initial step
toward studying the biological role of these molecules, we report
herein the synthesis of a complete library of known and anticipated
15-F2 isoprostanes (3-10, Figure 1).9,10
An effective synthetic strategy for accessing all of the 15-F2
isoprostanes requires certain notable features. A stereodivergent
approach is most appropriate where the isoprostane isomers are
generated late from a common isoprostanoid intermediate.11 The
requisite tetra-substituted cyclopentane ring can be accessed rapidly
through a ring-opening metathesis of an appropriately functionalized
bicyclo[3.2.0]heptenyl ring system.12 In this regard, the synthesis
of the 15-F2 isoprostane library commences with the known (()-
TBS-4-hydroxy-2-cyclopentenone 11, which is readily available
from furfuryl alcohol in two steps.13 The [2 + 2] photocycloaddition
between 11 and acetylene gives an inseparable mixture of the
desired bicyclo[3.2.0]heptenones 12 and 13 (Scheme 1);14 these
photoadducts will lead, respectively, to the 15-F2t and 15-F2c series
isoprostanes.
Scheme 1. Preparation of the Ring-Opening Metathesis
Substratesa
a (a) Acetylene, acetone, hν (61% yield @ 83% conv; exo-12:endo-13
) 1.8:1); (b) DiBAl-H, PhCH3, -78 °C; (c) PCC, CH2Cl2; (d) TBSCl,
DMAP, Et3N, CH2Cl2.
DiBAl-H reduction of ketones 12 and 13 (as a mixture) led to a
diastereomeric mixture of 14, 15, 16, and 17, all of which can be
separated by silica gel chromatography. While the reduction of the
cyclobutene 13 led to the formation of desired syn-dihydroxylated
product 17 with acceptable selectivity (17:16 ≈ 9:1), the reduc-
tion of the cyclobutene 12 was less selective (15:14 ) 2.5:1).
Other reducing agents, such as Red-Al, NaBH4, and LAH, provided
more of the undesired anti-dihydroxylated product 14. In either
case, the minor undesired isomer 14 can be recycled back to 12 by
oxidation with PCC, followed by reduction with DiBAl-H to
provide 15 (51%, two steps). The meso-cyclobutenes 18 and 19
Figure 1. The 15-F2 family of isoprostanes.
Given the role of lipid oxidation in illnesses such as atheroscle-
rosis, cancer, diabetes, liver, and neurodegenerative diseases,4 it is
* To whom correspondence should be addressed. E-mail: marc.snapper@
bc.edu.
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J. AM. CHEM. SOC. 2002, 124, 10998-11000
10.1021/ja027154u CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Side-Chain Construction through Ring-Opening
enones (-)-22 and (+)-22. Reduction of (-)-22 with (S)-2-methyl-
CBS-oxazoborolidine (25) produced the 15-R alcohol 26 as the
major diastereomer (Scheme 3). Hydroboration of the reaction
mixture with 9-BBN, followed by oxidative workup, provided the
enantiomerically pure diol 28. Similarly, reduction of enone (+)-
22 with 25, followed by the hydroboration, generates enantiomeri-
cally pure diol 30. The enantiomerically pure C15-epimers, 29 and
31, are prepared by reducing the enantiomerically enriched enones
(-)-22 and (+)-22, respectively, with the opposite enantiopode of
the reduction catalyst, (R)-2-methyl-CBS-oxazoborolidine (32).
Metathesesa
Scheme 4. Completion of the 15-F2t Isoprostanesa
a (a) (IMesH2)(PCy3)Cl2RudCHPh, (24), CH2Cl2, oct-1-en-3-ol; (b) PCC,
CH2Cl2; (c) I2 (1 mol %), CH2Cl2; (d) cat. 24, benzene, oct-1-en-3-one,
80 °C.
can then be prepared in high yield by treatment of alcohols 15 and
17 with TBSCl.
The cis side chains of the isoprostanes can now be introduced
through ring-opening metatheses of the bis-silylated bicyclo[3.2.0]-
heptenediols 18 and 19 (Scheme 2).15 Grubbs’ N-heterocyclic
carbene-containing catalyst 24 with excess octen-3-ol generates the
ring-opened products 20 and 21, as mixtures of isomers in 67%
and 76% yield, respectively. In each case, the Z-olefin isomers could
be converted to the desired E-stereochemistry through a PCC
oxidation, followed by an iodide-catalyzed isomerization to provide
trans-enones 22 and 23.10e Alternatively, cyclobutene 19 could be
converted to enone 23 directly in 55% yield through a ring-opening
metathesis with oct-1-en-3-one. Unfortunately, this enone cross-
metathesis was not successful with cyclobutene 18.
a (a) TEMPO, NCS, Bu4N+Cl-, CH2Cl2/H2O; (b) (Ph3P(CH2)4CO2H)+Br-,
KHMDS, THF; (c) TBAF, THF.
The final steps of synthesis of the 15-F2t isoprostanes are outlined
in Scheme 4. Selective oxidation of the primary alcohol (C6) in
the presence of an allylic alcohol (C15) for diols 28-31 was ac-
complished under phase transfer conditions using catalytic TEMPO
with NCS as the reoxidant.18 Wittig olefination proceeded selec-
tively to produce exclusively the Z-olefin stereochemistry at the
newly formed C5-C6 double bond. Deprotection of the TBS groups
with TBAF in THF then yields the desired 15-F2t isoprostanes
3-6.10a
Scheme 3. Resolution in the 15-F2t Isoprostane Seriesa
The use of the same strategy to resolve the 15-F2c isoprostanes
was met with some difficulty. CBS-reduction of enone (()-23
gave the desired diastereomeric alcohols, however, each in less
than 30% ee. As an alternative, a more classical approach was
pursued (Scheme 5). The diastereomeric allylic alcohols, (()-37
and (()-38, were prepared through a NaBH4 reduction of (()-23
(80% yield). Allylic alcohol (()-37 was acylated with (R)-O-
acetylmandelic acid chloride (47) to give the diastereomeric esters
39 and 40.19 Similarly, esters 41 and 42 were prepared from
(()-38. The esters could be separated by silica gel chromatography
yielding the enantiomerically enriched diastereomers. The acetyl
mandelate esters were then removed under reductive conditions,
and the resulting isomers were hydroborated to give diols 43-46
in an enantiomerically pure fashion.
The synthesis of the 15-F2c isoprostane isomers could then be
completed in a manner similar to the 15-F2t isoprostanes. Selec-
tive oxidation of diols 43-46 to the corresponding hydroxyalde-
hydes, Wittig olefination, and deprotection furnished the individual
15-F2c isoprostanes 7-10 (Scheme 6).10b
The first synthesis of all 15-F2 isoprostane has been ac-
complished. This synthesis includes the preparation of both known,
as well as anticipated, members of this class of lipid metabolites.
The stereodiversifying strategy allows for the preparation of the
15-F2 isoprostanes from a common starting material in an efficient
manner. A ring-opening metathesis serves as the key transforma-
tion for introducing the isoprostane side chains. Separation of the
15-F2t stereoisomers was achieved using a catalytic asymmetric
reduction protocol, while separation of the 15-F2c isoprostane
isomers was accomplished using chiral auxiliaries. The availability
a (a) (S)-2-Methyl-CBS-oxazoborolidine (25), catecholborane, toluene,
-78 °C; (b) PCC, CH2Cl2; (c) (R)-2-methyl-CBS-oxazoborolidine (32),
catecholborane, toluene, -78 °C; (d) 9-BBN; NaOOH.
With the lower side chain (C13-C20) in place, the remaining
steps involve installation of the upper side chain and resolution of
the various stereoisomers. Racemic enone (()-22 serves as the
substrate for the resolution. Asymmetric catalytic reduction of
(()-22 using (S)-2-methyl-CBS-oxazoborolidine (25) and cat-
echolborane16 produced enantiomerically enriched diastereomeric
alcohols 26 and 27, which can be separated by silica gel chroma-
tography (Scheme 3).17 The individual alcohols were then reoxidized
with PCC to the corresponding enantiomerically enriched enones
(-)-22 and (+)-22.
Pure 15-F2t isoprostane isomers are now accessible from a second
catalytic asymmetric reduction of the enantiomerically enriched
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C O M M U N I C A T I O N S
Scheme 5. Resolution in the 15-F2c Isoprostane Seriesa
J. D.; Awad, J. A.; Kato, T.; Takahashi, K.; Badr, K. F.; Roberts, L. J.,
II; Burk, R. F. J. Clin. InVest. 1992, 90, 2502.
(2) (a) Morrow, J. D.; Awad, J. A.; Boss, H. J.; Blair, I. A.; Roberts, L. J., II.
Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 10721. (b) Kayganich-Harrison,
K. A.; Rose, D. M.; Murphy, R. C.; Morrow, J. D.; Roberts, L. J., II. J.
Lipid Res. 1993, 34, 1229. A small percentage of isoprostane production
may be attributed to the action of the COX enzymes: (c) Practico´, D.;
Lawson, J. A.; FitzGerald, G. A. J. Biol. Chem. 1995, 270, 9800. (d)
Practico´, D.; FitzGerald, G. A. J. Biol. Chem. 1996, 271, 8919. (e)
Patrignani, P.; Santini, G.; Panara, M. R.; Sciulli, M.; Greco, A.; Rotondo,
M. T.; diGiamberardino, M.; Maclouf, J.; Ciabttoni, G.; Patrono, C. Br.
J. Pharmacol. 1996, 118, 1285.
(3) For lead references on the detection of isoprostane production in vivo,
see: (a) Morrow, J. D.; Roberts, L. J., II. Biochem. Pharmacol. 1996, 51,
1. (b) Lawson, J. A.; Rokach, J.; FitzGerald, G. A. J. Biol. Chem. 1999,
274, 24441.
(4) For example, see: Practico´, D.; Lawson, J. A.; Rokach, J.; FitzGerald,
G. A. Trends Endocrinol. Metab. 2001, 12, 243 and references therein.
(5) (a) Banerjee, M.; Kang, K. H.; Morrow, J. D.; Roberts, L. J., II; Newman,
J. H. Am. J. Physiol. 1992, 263, H660. (b) Takahashi, K.; Nammour, T.
M.; Fukunaga, M.; Ebert, J.; Morrow, J. D.; Roberts, L. J., II; Hoover, R.
L.; Badr, K. F. J. Clin. InVest. 1992, 90, 136.
(6) (a) Fukunaga, M.; Makita, N.; Roberts, L. J., II; Morrow, J. D.; Takahashi,
K.; Badr, K. F. Am. J. Physiol. 1993, 264, C1619. (b) Kunapuli, P.;
Lawson, J. A.; Rokach, J. A.; Meinkoth, J. L.; FitzGerald, G. A. J. Biol.
Chem. 1998, 273, 22442.
a (a) R-Ph(AcO)CHCOCl (47), DMAP, Et3N, CH2Cl2; separate diaster-
(7) (a) Morrow, J. D.; Minton, T. A.; Roberts, L. J. Prostaglandins 1992, 44,
155. (b) Yin, K.; Halushka, P. V.; Yan, Y.-T.; Wong, P. Y. K. J.
Pharmacol. Exp. Ther. 1994, 270, 1192. (c) Cranshaw, J. H.; Evans, T.
W.; Mitchell, J. A. Br. J. Pharmacol. 2001, 132, 1699.
eomers; (b) DiBAl-H; (c) 9-BBN; NaOOH.
Scheme 6. Completion of the 15-F2c Isoprostanesa
(8) (a) Crankshaw, D. Eur. J. Pharmacol. 1995, 285, 151. (b) Jourdan, K.
B.; Evans, T. W.; Curzen, N. P.; Mitchell, J. A. Br. J. Pharmacol. 1997,
120, 1280.
(9) (a) For a summary of isoprostane nomenclature, see: Taber, D. F.;
Morrow, J. D.; Roberts, L. J., II. Prostaglandins 1997, 53, 63. (b) For an
alternative nomenclature system for the isoprostanes, see: Rokach, J.;
Khanapure, S. P.; Hwang, S. W.; Adiyaman, M.; Lawson, J. A.;
FitzGerald, G. A. Prostaglandins 1997, 54, 853.
(10) For lead references on the syntheses of 15-F2 isoprostanes, see: (a) Corey,
E. J.; Shih, C.; Shih, N.-Y.; Shimoji, K. Tetrahedron Lett. 1984, 25, 5013.
(b) Larock, R. C.; Lee, N. H. J. Am. Chem. Soc. 1991, 113, 7815. (c)
Hwang, S. W.; Adiyaman, M.; Khanapure, S.; Schio, L.; Rokach, J. J.
Am. Chem. Soc. 1994, 116, 10829. (d) Vionnet, J.-P.; Renaud, P. HelV.
Chim. Acta 1994, 77, 1781. (e) Hwang, S.-W.; Adiyaman, M.; Khanapure,
S. P.; Rokach, J. Tetrahedron Lett. 1996, 37, 779. (f) Taber, D. F.; Kanai,
K. Tetrahedron 1998, 54, 11767. (g) Lai, S.; Lee, D.; U, J. S.; Cha, J. K.
J. Org. Chem. 1999, 64, 7213. (h) Durand, T.; Guy, A.; Vidal, J.-P.; Rossi,
J.-C. J. Org. Chem. 2002, 67, 3615.
(11) The advantage of a stereodivergent strategy was recognized and employed
for the preparation of the prostaglandins. Corey, E. J.; Cheng, X.-M. The
Logic of Chemical Synthesis; Wiley Interscience: New York, 1989; pp
76, 250-309.
a (a) TEMPO, NCS, TBACl, CH2Cl2/H2O; (b) (Ph3P(CH2)4CO2H)+Br-,
KHMDS, THF; (c) TBAF, THF.
of this complete 15-F2 isoprostane library allows for the side-by-
side comparison of these lipid metabolites in a variety of biological
assays.
(12) Schrader, T. O.; Snapper, M. L. Tetrahedron Lett. 2000, 41, 9685.
(13) Curran, T. T.; Hay, D. A.; Koegel, C. P.; Evans, J. C. Tetrahedron 1997,
53, 1983.
(14) Sugihara, Y.; Morokoshi, N.; Murata, I. Tetrahedron Lett. 1977, 44, 3887.
(15) Randall, M. L.; Tallarico, J. A.; Snapper, M. L. J. Am. Chem. Soc. 1995,
117, 9610.
Acknowledgment. This work is dedicated to Prof. T. Ross Kelly
on the occasion of his 60th birthday. We are grateful to the National
Institutes of Health (CA-66617) for the financial support for this
project. The ring-opening metathesis reaction was developed
through support from the National Science Foundation (CHE-
0132221).
(16) Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998, 37, 1987.
(17) Stereochemical assignments were made through NMR analyses of the
corresponding Mosher and acetyl mandelate esters and then con-
firmed, when possible, by comparison of the known synthetic isoprostanes
with previously reported compounds. See Supporting Information for
details.
Supporting Information Available: Compound characterization
and experimental procedures (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(18) Einhorn, J.; Einhorn, C.; Ratajczak, F.; Pierre, J.-L. J. Org. Chem. 1996,
61, 7452.
(19) Nowotny, S.; Vettel, S.; Knochel, P. Tetrahedron Lett. 1994, 35, 4539.
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