Total synthesis of 15-D2t- and 15-epi-15-E2t- isoprostanes

Article abstract of DOI:10.1021/jo1000274

(Figure Presented) The first total synthesis of 15-D_2t- isoprostane is described. (-)-(9S,15S)-15-D_2t-IsoP 1 and (+)-(11R,15R)15-epi-15-E_2t-IsoP 2 have been obtained in 15 steps from orthogonally protected enantiopure bicycle 3. Key features include an easy introduction of die cis side chains via ozonolysis, a highly selective enzymatic chemical differentiation of a non-meso-1,5-diol, and the use of a common synthetic intermediate allowing a stereodivergent approach to the target molecules.

Full text of DOI:10.1021/jo1000274

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shown to possess platelet aggregation properties and to act as
Total Synthesis of 15-D2t- and
15-epi-15-E2t-Isoprostanes
smooth muscle growth factors.4 D- and E-IsoPs possess one
carbonyl and one hydroxyl group in the 1,3 position on the
prostane ring. They are less stable than F-isoprostanes and
may undergo further transformations by dehydration to the
corresponding J and A derivatives.5
Yasmin Brinkmann, Camille Oger, Alexandre Guy,
Thierry Durand, and Jean-Marie Galano*
ꢀ
Institut des Biomolecules Max Mousseron, IBMM,
ꢀ
UMR CNRS 5247, Universite Montpellier I, Universite
Montpellier II, Faculte de Pharmacie, 15, avenue Charles
ꢀ
ꢀ
Flahault, 34093 Montpellier Cedex 05, France
jgalano@univ-montp1.fr
Received January 8, 2010
The first total synthesis of 15-D2t-isoprostane is de-
scribed. (-)-(9S,15S)-15-D2t-IsoP 1 and (þ)-(11R,15R)-
15-epi-15-E2t-IsoP 2 have been obtained in 15 steps from
orthogonally protected enantiopure bicycle 3. Key fea-
tures include an easy introduction of the cis side chains via
ozonolysis, a highly selective enzymatic chemical differ-
entiation of a non-meso-1,5-diol, and the use of a com-
mon synthetic intermediate allowing a stereodivergent
approach to the target molecules.
FIGURE 1. Different types of isoprostanes generated by reactive
oxygen species (ROS) from membrane-bound AA: only the 15 series
is represented here for clarity.
The biological activities of E-IsoPs have been examined to
a limited extent.6 Recent reports have shown that the E-type
isoprostanes are potent vasoconstrictors at low nanomolar
concentrations.6a The biological functions of D-IsoPs have
(5) (a) Chen, Y.; Zackert, E. W.; Roberts, J. L., II; Morrow, J. D.
Biochim. Biophys. Acta 1999, 1436, 550. (b) Chen, Y.; Roberts, J. L., II;
Morrow, J. D. J. Biol. Chem. 1999, 274, 10863.
(6) (a) Janssen, L. J. Pulm. Pharmacol. Ther. 2000, 13, 149. (b) Janssen, L.
J. Trends Pharmacol. Sci. 2002, 23, 59. (c) Zhao, M.; Destache, C. J.; Ohia,
S. E.; Opere, C. A. Neurochem. Res. 2009, 34, 2170. (d) Opere, C. A.; Ford,
K.; Zhao, M.; Ohia, S. E. Methods Find. Exp. Clin. Pharmacol. 2008, 30, 697.
(e) Cracowski, J. L.; Devillier, P.; Durand, T.; Stanke-Labesque, F.; Bessard,
G. J. Vasc. Res. 2001, 38, 93.
(7) Total synthesis of IsoPs (selected examples): (a) Hwang, S. W.;
Adiyaman, M.; Khanapure, S.; Schio, L.; Rokach, J. J. Am. Chem. Soc.
1994, 116, 10829. (b) Pudukulathan, Z.; Manna, S.; Hwang, S.-W.;
Khanapure, S. P.; Lawson, J. A.; FitzGerald, G. A.; Rokach, J.
J. Am. Chem. Soc. 1998, 120, 11953. (c) Chang, C.-T.; Patel, P.; Kang, N.;
Lawson, J. A.; Song, W.-L.; Powell, W. S.; FitzGerald, G. A.; Rokach, J.
J. Bioorg. Med. Chem. Lett. 2008, 18, 5523. (d) Taber, D. F.; Xu, M.;
Hartnett, J. C. J. Am. Chem. Soc. 2002, 124, 13121. (e) Taber, D. F.; Jiang, Q.
J. Org. Chem. 2001, 66, 1876. (f) Taber, D. F.; Kanai, K.; Pina, R. J. Am.
Chem. Soc. 1999, 121, 7773. (g) Schrader, T. O.; Snapper, M. J. Am. Chem.
Soc. 2002, 124, 10998. (h) Pandya, B. A.; Snapper, M. L. J. Org. Chem. 2008,
73, 3754. (i) Quan, L. G.; Cha, J. K. J. Am. Chem. Soc. 2002, 124, 12424.
(j) Lai, S.; Lee, D.; U, J. S.; Cha, J. K. J. Org. Chem. 1999, 64, 7213.
(k) Zanoni, G.; Porta, A.; Vidari, G. J. Org. Chem. 2002, 67, 4346. (l) Spur, B.
W.; Rodriguez, A. R. Tetrahedron Lett. 2002, 43, 4575. (m) Rodrıguez, A. R.;
Spur, B. W. Tetrahedron Lett. 2002, 43, 9249. (n) Nakamura, N.; Sakai, K.
Tetrahedron Lett. 1978, 19, 1549. (o) Pinot, E.; Guy, A.; Fournial, A.; Balas,
L.; Rossi, J. C.; Durand, T. J. Org. Chem. 2008, 73, 3063. (p) Oger, C.;
Brinkmann, Y.; Bouazzaoui, S.; Durand, T.; Galano, J.-M. Org. Lett. 2008,
10, 5087. (q) Jahn, U.; Dinca, E. Chem.;Eur. J. 2009, 15, 58. (r) Elsner, P.;
Isoprostanes,1 isomeric to the enzymatically formed pros-
taglandins, bear their lateral side chains with a cis relation-
ship on the prostane ring. They are formed in vivo as a
racemic mixture by free radical nonenzymatic peroxidation
of membrane-bound arachidonic acid (Figure 1). The path-
way leads to five classes of isoprostanes (F-, D-, E-, A-, and
J-types) that differ in the substitution pattern of the cyclo-
pentane ring. Since their discovery in 1990 by Morrow2 and
co-workers, F-IsoPs that incorporate two hydroxyl groups
in the 1,3 position on their ring system have been studied
extensively. They are used as markers of oxidative stress,3
show biological activity as vasoconstrictors, and have been
(1) For recent reviews, see: (a) Milne, G. L.; Yin, H.; Morrow, J. D.
J. Biol. Chem. 2008, 283, 15533. (b) Jahn, U.; Galano, J.-M.; Durand, T.
Angew. Chem., Int. Ed. 2008, 47, 5894.
(2) Morrow, J. D.; Hill, K. E.; Burk, R. F.; Nammour, T. M.; Badr, K. F.;
Roberts, L. J., II. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 9383.
(3) (a) Musiek, E. S.; Yin, H.; Milne, G. L.; Morrow, J. D. Lipids 2005, 40,
987. (b) Montuschi, P.; Barnes, P. J.; Roberts, L. J., II. FASEB J. 2004, 18,
1791.
(4) (a) Morrow, J. D. Curr. Pharm. Des. 2006, 12, 895. (b) Cracowski,
J. L.; Durand, T. Fundam. Clin. Pharmacol. 2006, 20, 417.
€
Jetter, P.; Brodner, K.; Helmchen, G. Eur. J. Org. Chem. 2008, 2551.
DOI: 10.1021/jo1000274
r
Published on Web 03/10/2010
J. Org. Chem. 2010, 75, 2411–2414 2411
2010 American Chemical Society

Brinkmann et al.
SCHEME 2. Synthesis of Common Synthetic Precursor 3
JOCNote
FIGURE 2. Structure of (-)-(9S,15S)-15-D2t-IsoP 1 and (þ)-
(11R,15R)-15-epi-15-E2t-IsoP 2.
SCHEME 1. Access to 15-D2t-IsoP 1 and to 15-epi-15-E2t-IsoP 2
SCHEME 3. Synthetic Pathway toward 15a and 15b
not yet been determined. For the evaluation of the roles of
these metabolites, synthetic reference material is needed
since isolation from natural sources is scarcely possible.
Furthermore, detailed analytical investigation requires en-
antiopure synthetic compounds; therefore, the development
of stereoselective total synthesis is of great interest. Whereas
several total synthesis of the different types of isoprostanes
has been accomplished,1b,7 to date no total synthesis of
D-IsoPs has been realized. Here we report the first total
synthesis of (-)-(9S,15S)-15-D2t-IsoP 1 as well as the total
synthesis of (þ)-(11R,15R)-15-epi-15-E2t-IsoP 2 (Figure 2)
generated from a common, late-stage synthetic intermediate.
We envisioned the total synthesis of 15-D2t- and 15-epi-15-
E2t-isoprostanes from enantiopure bicycle 3 incorporating
an orthogonally protected 1,3-diol functionality (Scheme 1).
Ozonolysis of 3 would reveal the 1,2-cis stereochemistry
necessary for side chain functionnalization via HWE and
Wittig reactions. Selective removal of one protecting group
of compounds 4 and 5, ester hydrolysis, and oxidation of the
free hydroxyl to the ketone should afford 6 and 7, respec-
tively. Final deprotection should then provide the target
molecules 1 and 2.
We recently developed a new synthetic strategy that
permits the rapid access to different types of iso-, neuro-,
and phytoprostanes. Following our previously published
results,7p enantiopure bicyclic ketoepoxide 8 (>99% ee)
was obtained from commercially available 1,3-cycloocta-
diene in five steps on a multigram scale.
In order to access orthogonally protected bicycle 3, which
should serve as a common synthetic intermediate for the total
synthesis of 15-D2t- and 15-epi-15-E2t-isoprostanes, the keto
functionality of epoxyketone 8 was reduced chemoselectively
by treatment of 8 with LiAlH4 at low temperature and short
reaction times (LiAlH4 (2.3 equiv per hydride), THF, -78 °C,
20 min, dr 95:5) giving compound 9 (Scheme 2). The observed
diastereoselectivities can be explained by the presence of the
epoxide functionality that leads to an attack of the hydride
source at the sterically less hindered back site of the ketone.
We first protected the alcohol functionality of 9 as TBS and
TPS ethers, but in both cases further reaction with LiAlH4 to
open the epoxide led to deprotection of the silyl ether.
Taking into account these results, we next decided to
introduce an ethoxyethyl ether protection (EE) (ethyl vinyl
ether, CH2Cl2, PPTS cat., rt) affording 10. Epoxide opening
proceeded smoothly giving rise to the exclusive formation
of 11. Hydride attack occurred selectively at one of the two
theoretically possible positions due to the bended structure
of the bicycle. TBS protection of the resulting alcohol
afforded orthogonally protected bicycle 3.
Ozonolysis of 3 followed by NaBH4 reduction gave diol 12
(Scheme 3). Chemical differentiation of the primary alcohols
of 12 under classical protection conditions (TESCl (1 equiv)
or Ac2O (1 equiv)) failed to provide the regioisomerically
pure monoprotected alcohol. Indeed, an approximately 1/1
mixture of isomers was obtained together with recovered
starting material and bisprotected compound. We were
pleased to find that high selectivity was achieved by using
lipase B from Candida antarctica in the presence of vinyl
acetate in THF leading to monoacetylated compound 13
(88%) together with bis-acetylated side product (<10%).
The reaction proceeded with an unprecedented regioselec-
tivity favoring acetylation of the alcohol attached to the
longer side chain.8 The first side-chain introduction was
ꢀ
(8) Oger, C.; Marton, Z.; Brinkmann, Y.; Bultel-Ponce,V.; Durand, T.;
Graber, M.; Galano, J.-M. J. Org. Chem. 2010, in press. The paper describes
the scopes and limitations of this methodology.
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Brinkmann et al.
JOCNote
SCHEME 4. Total Synthesis of 15-D2t-isoprostane
SCHEME 5. Total Synthesis of 15-epi-15-E2t-isoprostane
carried out by DMP oxidation9 of 13 and subsequent HWE
reaction of the resulting aldehyde with the phosphoryl-
stabilized anion of dimethyl (2-oxoheptyl)phosphonate.
Enone 14 was obtained in 82% yield possessing the alkene
functionality with the desired trans stereochemistry. Carbo-
nyl reduction under Luche conditions afforded the corres-
ponding allylic alcohols as an equimolar epimeric mixture.
The epimers were separated by flash chromatography giving
enantiopure 15a and 15b.
formation of undesired elimination side products, decom-
position, or unreacted starting material. Deprotection was
finally achieved by treatment of 6 with BiBr3 (9 equiv,
CH3CN, H2O cat., 0 °C, 30 min) leading to desired com-
pound (-)-1 (48% yield, >99%ee) and minor amounts of
deoxygenated side products.12
For the synthesis of 15-epi-15-E2t-IsoP, allylic alcohol 15b
was protected as EE, and the acetate group was cleaved to
produce alcohol 18. DMP oxidation, Wittig reaction, and
subsequent esterification gave 5 (Scheme 5). Selective re-
moval of the TBS protecting group was realized by reaction
of 5 with TBAF (1.3 equiv, THF, 72%), affording 19.
Saponification of the methyl ester followed by DMP oxida-
tion led to 7. Final deprotection was easily achieved by
treatment of 7 with a catalytic amount of PPTS in EtOH/
CH2Cl2 giving rise to (þ)-2 (72% yield, >99% ee, white
solid). Properties of (þ)-(11R,15R)-15-epi-15-E2t-IsoP 2
were identical to published analytical and spectroscopic data
described for the enantiomer except for the sign of the optical
rotation.7m
In summary, the first total synthesis of 15-D2t-IsoP is
described. This opens access to investigation of the not yet
determined biological activities of the D-class isoprostanes.
(-)-(9S,15S)-15-D2t-IsoP 1 and (þ)-(11R,15R)-15-epi-15-
E2t-IsoP 2 have been synthesized in 15 steps from orthogon-
ally protected diol 3. Key features include an easy introduc-
tion of cis side chains via ozonolysis, a highly selective
enzymatic chemical differentiation of a non-meso-1,5-diol,
and the use of a common late-stage synthetic intermediate
allowing a stereodivergent approach to the target molecules.
The relative configuration of the stereogenic center of 15a
and 15b was determined by a variation10a of Mosher’s
method.10b Each epimer was esterified with R-(-)-R- and
S-(þ)-R-acetoxyphenylacetic acid. Analysis of 1H NMR
spectra allowed an unambiguous assignment of the config-
uration at the allylic stereocenter in each of the alcohols.
For the synthesis of 15-D2t-IsoP, 15a was protected as TBS
ether and the acetate group was cleaved to produce alco-
hol 16 (Scheme 4). DMP oxidation and subsequent Wittig
reaction of intermediate aldehyde with (4-carboxybutyl)tri-
phenylphosphonium bromide in the presence of NaHMDS
permitted the introduction of the second side chain. Wittig
olefination proceeded selectively to produce exclusively the
Z-olefin stereochemistry at the newly formed C5-C6 double
bond. For ease of purification, the carboxylic acid function-
ality was esterified with TMSCHN2 affording 4. Selective
removal of the ethoxyethyl ether protecting group was
achieved by treatment of 4 with a catalytic amount of PPTS
in EtOH/CH2Cl2 giving rise to 17 in 70% yield. Ester
hydrolysis and subsequent oxidation of the free hydroxyl
to the ketone afforded 6. All that remained to access to
(9S,15S)-15-D2t-IsoP 1 was the removal of two TBS protect-
ing groups;11 however, this proved to be no simple task.
Attempts to deprotect 6 by TBAF/AcOH, TASF, HF/pyr,
HF aq, HCl (1 N, 0.1 N) under various conditions led to
Experimental Section
(-)-(9S,15S)-15-D2t-IsoP 1. Compound 6 (90 mg, 0.155
mmol, 1 equiv) was dissolved under nitrogen in 5 mL of CH3CN.
The solution was cooled to 0 °C, and BiBr3 (400 mg, 0.89 mmol,
5.7 equiv) was added in one portion under stirring. Then H2O
(40 μL) was added in one portion. After 20 min at the same
temperature another portion of BiBr3 (226 mg, 0.50 mmol, 3.3
equiv) and H2O (30 μL) was added. After 30 min, TLC analysis
indicated the complete disappearance of the starting material.
The reaction was quenched at 0 °C by dropwise addition of a
saturated solution of NaHCO3 until pH = 4 was reached.
Layers were separated, and the aqueous one was extracted three
(9) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277.
(10) (a) Chataigner, I.; Lebreton, J.; Durand, D.; Guingant, A.; Villieras,
J. Tetrahedron Lett. 1998, 39, 1759. (b) Dale, J. A.; Mosher, H. S. J. Am.
Chem. Soc. 1973, 95, 512.
(11) Since 15-D2t-IsoP is extremely labile under basic conditions, it is
absolutely necessary to carry out final cleavage of the protective groups
under mild acidic conditions.
(12) Epimerization of (-)-(9S,15S)-15-D2t-IsoP 1 leading to non-natural
9β-PGD2 is unlikely because enolization of PGD2 gives rise to R,β-unsatu-
rated ketone isomers and/or unstable trienone (deoxygenated compounds).
J. Org. Chem. Vol. 75, No. 7, 2010 2413

Brinkmann et al.
JOCNote
times with an excess of AcOEt. The combined organic layers
were washed with a saturated solution of NaCl, dried over
MgSO4, and filtered, and the solvent was evaporated under
reduced pressure using a cold water bath. The crude was purified
using demetalated silica gel (solvent: petroleum spirit/AcOEt
3:7, AcOEt-AcOEt/MeOH 9:1) to afford the desired 1 (26 mg,
colorless oil, 48%) and elimination side products 1b and 1c
(14 mg, light yellow-orange oil). 1: Rf 0.64 (AcOEt/MeOH 9:1);
demetalated silica gel (solvent: AcOEt/cyclohexane 8/2 f
AcOEt) giving pure 2 (31.6 mg, white solid, 72%): Rf 0.50
(AcOEt/MeOH 9:1); [R]20
= þ68 (c 10-2, MeOH); 1H
D
NMR (300 MHz, MeOD) δ 5.70 (dd, 1H, J = 15.5 Hz, J =
6.7 Hz), 5.42 (m, 3H), 4.31 (m, 1H), 4.06 (m, 1H), 3.06 (t, 1H,
J = 8.8 Hz), 2.74 (m, 1H), 2.61 (dd, 1H, J = 18.9 Hz, J = 6.1
Hz), 2.48 (m, 1H), 2.34 (t, 2H, J = 7.1 Hz), 2.25-1.96 (m, 4H),
1.72 (m, 2H), 1.54 (m, 2H), 1.36 (m, 6H), 0.95 (m, 3H); 13C NMR
(75 MHz, MeOD) δ 219.5, 177.4, 138.5, 131.0, 129.0, 128.1, 73.3,
72.8, 52.4, 51.7, 45.5, 38.4, 34.4, 32.9, 27.8, 26.2, 26.0, 24.2, 23.7,
14.4.; IR νmax (cm-1) 3394, 2929, 2859, 1728, 1710, 1238, 971;
HRMS (ESI) calcd for C20H31O5 [M - H]- 351.2171, found
351.2169; mp 101.2 °C.
[R]20 = -17.5 (c 0.4 10-2, CDCl3); 1H NMR (300 MHz,
D
CDCl3) δ 5.66 (dd, 1H, J = 15.6 Hz, J = 6.5 Hz), 5.52 (m,
3H), 4.41 (m, 1H), 4.17 (m, 1H), 3.39 (t, 1H and 2 OHs, J = 7.8
Hz), 2.59 (dd, 1H, J = 19.1 Hz, J = 6.5 Hz), 2.30 (m, 5H), 2.11
(m, 2H), 1.80 (m, 1H), 1.67 (m, 2H), 1.52 (m, 2H), 1.29 (m, 6H
and H2O), 0.88 (m, 3H); 13C NMR (75 MHz, CDCl3) δ 215.6,
176.7, 138.3, 131.0, 128.1, 123.7, 73.0, 71.0, 54.0, 50.0, 44.6, 37.0,
32.5, 31.7, 26.2, 25.9, 25.0, 24.3, 22.6, 14.0; IR νmax (cm-1) 3390,
2930, 2860, 1710,1404, 1189, 1013, 972; HRMS (ESI) calcd for
C20H31O5 [M - H]- 351.2171, found 351.2162.
(þ)-(11R,15R)-15-epi-15-E2t-IsoP 2. Compound 7 (61.8 mg,
0.125 mmol) was dissolved in EtOH (4.5 mL) and CH2Cl2 (0.9
mL) under nitrogen, and a catalytic amount of PPTS (10 mol %)
was added in one portion. The resulting reaction mixture
was stirred at room temperature for 1 h 30 min until
complete disappearance of the starting material. Solid NaHCO3
was added, and the solvent was evaporated under reduced
pressure. The crude was purified by flash chromatography using
Acknowledgment. We gratefully acknowledge generous
funding by Prof. Jean-Yves Lallemand (ICSN). We also
thank University Montpellier 1 (Grant No. BQR-2009) for
support. Y.B. thanks the Centre National de la Recherche
Scientifique for postdoctoral fellowships. We thank Prof.
Rita S. Majerle (University of Minnesota) for proofreading.
Supporting Information Available: Experimental proce-
dures and characterization data for all new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
2414 J. Org. Chem. Vol. 75, No. 7, 2010