Total synthesis of E1 and E2 isoprostanes by diastereoselective protonation

Article abstract of DOI:10.1016/S0040-4039(02)02252-9

A short total synthesis of the E-type isoprostanes has been achieved using a two-component coupling process combined with a diastereoselective protonation under reagent control. Mild cleavage of the silyl protective groups with cat. BiBr₃ or HF/Py followed by enzymatic hydrolysis of the methyl ester afforded the free E-type isoprostanes.

Full text of DOI:10.1016/S0040-4039(02)02252-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9249–9253
Total synthesis of E1 and E2 isoprostanes by diastereoselective
protonation
Ana R. Rodr´ıguez and Bernd W. Spur*
Department of Cell Biology, University of Medicine and Dentistry of New Jersey, SOM, Stratford, NJ 08084, USA
Received 23 August 2002; revised 26 September 2002; accepted 30 September 2002
Abstract—A short total synthesis of the E-type isoprostanes has been achieved using a two-component coupling process combined
with a diastereoselective protonation under reagent control. Mild cleavage of the silyl protective groups with cat. BiBr3 or HF/Py
followed by enzymatic hydrolysis of the methyl ester afforded the free E-type isoprostanes. © 2002 Elsevier Science Ltd. All rights
reserved.
In 1990 Roberts et al. reported that phospholipid-
bound arachidonic acid is converted in vivo by a free
radical oxidation to isoprostanes with a cis-arrange-
ment of the two side chains.1 Subsequent investiga-
tions showed that other polyunsaturated fatty acids
undergo similar transformations.2,3 The levels of iso-
prostanes measured in biological fluids were signifi-
cantly higher than the enzymatically produced
prostaglandins. These metabolites are established
markers of oxidative stress in several diseases includ-
ing Alzheimer’s disease.4,5 The release of isoprostanes
during kidney failure and severe liver diseases have
been described.6 Recent reports have shown that the
E-type isoprostanes are potent vasoconstrictors at low
nanomolar concentrations.7
The E-type isoprostanes are rather unstable and tend
to epimerize to the thermodynamically more stable
trans isomers. It has been reported that they can
undergo further elimination to the A-type in either
basic or acidic medium.8 In order to evaluate the bio-
logical and pharmacological properties of these natu-
ral compounds,9 it is necessary to obtain sufficient
quantities by chemical synthesis.10
Figure 1.
Keywords: isoprostanes; protonation; enolates; cuprate; enzyme reactions.
* Corresponding author. Tel.: +1-856-566-7016; fax: +1-856-566-6195; e-mail: spurbw@umdnj.edu
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02252-9

9250
A. R. Rodr´ıguez, B. W. Spur / Tetrahedron Letters 43 (2002) 9249–9253
8-epi-Prostaglandin E1 was isolated in the bioconver-
sion of all-cis-8,11,14-eicosatrienoic acid in low yield.11
8-epi-Prostaglandin E1 in racemic form was obtained
as a by-product in the synthesis of prostaglandins
E1.12,13
11 was converted to 14 with 82% cis-selectivity
(Scheme 1).30
Attention was next turned to the cleavage of the TBS
protective groups avoiding epimerization to the ther-
modynamically more stable trans isomer. Bajwa et al.
introduced bismuth bromide in catalytic amounts in
wet acetonitrile as a mild and effective deprotection
method of alkyl t-butyldimethylsilyl ethers.33 Using
slightly higher amounts of bismuth bromide (30%) a
smooth deprotection of both TBS groups was observed
to give 15 in 62% isolated yield. A faster reaction was
achieved using HF/Py in THF at rt to give 15 in 78%
isolated yield (Scheme 2).32 In both cases no epimeriza-
tion was observed.
A synthesis of racemic 8-epi-prostaglandin E1 and E2
has been reported by Sakai et al.14 So far only the
methyl ester of 8-epi-prostaglandin E2 has been
obtained in optical active form by Taber et al. using as
the key step a Rh(II)-catalyzed cyclization of a-diazo
ketones.15 A stereoselective synthesis of free acid E-
type isoprostanes (Fig. 1) has not yet been reported.16
The two-component coupling process, developed inde-
pendently by Sih et al.,17 Alvarez et al.18 and Fried et
al.,19 is typically used for the synthesis of the natural
prostaglandins and their analogs20 having the a- and
v-side chains in the thermodynamically more stable
trans configuration. We recently reported that this pro-
cess combined with a diastereoselective protonation
under reagent control afforded the products of kinetic
control, the protected isoprostanes, with the cis
configuration of the two side chains.21
The hydrolysis of esters of the E-type prostaglandins
has been accomplished by lipases and esterases.22,26
The fact that isoprostanes are formed in vivo in
esterified phospholipids and are released by lipases
suggested the use of an enzymatic ester cleavage at
neutral pH to obtain the E-type isoprostanes. Com-
pound 15 was treated with porcine pancreatic lipase
(type II EC 3.1.1.3, Sigma) in THF/H2O at rt afford-
ing crystalline 8-epi-PGE2 (1) in 50% yield after extrac-
tive isolation with ethyl acetate and flash
chromatography. Under the same conditions 14 was
converted to 11-epi-12-epi-PGE2 (2) (Scheme 2).32
In this communication we wish to report the successful
application of this methodology towards a short total
synthesis of the E-type isoprostanes (Fig. 2). The chiral
key intermediates used in the conjugated addition were
easily available on large scale as previously described
(Scheme 1).22–27
In a similar manner starting from 17 and 1821 8-epi-
PGE1 (3) and 11-epi-12-epi-PGE1 (4) were obtained
respectively (Scheme 3).32
The two-component coupling process of the chiral
intermediates 10 and 12 followed by a diastereoselec-
tive protonation28 with methyl acetoacetate as a chelat-
ing proton donor29 gave directly the protected E2
isoprostane 13 with 92% cis-selectivity30 in 54% iso-
lated yield (Scheme 1).31,32 Under the same conditions
Compounds 1, 2, 3, 4 have been identified in vivo.9,34,35
8-epi-PGE1 (3) was identical in all aspects with an
authentic sample (Cayman Chemical Company, Ann
Arbor, MI).
Figure 2.

A. R. Rodr´ıguez, B. W. Spur / Tetrahedron Letters 43 (2002) 9249–9253
9251
Scheme 1. Reagents and conditions: (a) lipase (PPL), vinyl acetate; (b) chromatographic separation; (c) 0.5 M guanidine, MeOH,
0°C; (d) TBSCl, imidazole, Et3N, DMF; (e) 12, n-BuLi, CuCN, MeLi, Et2O, −78°C/methyl acetoacetate, Et2O, −78°C“rt,
CH3COOH.
Scheme 2. Reagents and conditions: (a) HF/Py, THF, 0°C“rt, (or BiBr3 30%, CH3CN, cat. H2O, rt); (b) lipase (PPL), NaCl,
CaCl2, H2O/THF.

9252
A. R. Rodr´ıguez, B. W. Spur / Tetrahedron Letters 43 (2002) 9249–9253
Scheme 3. Reagents and conditions: (a) HF/Py, THF, 0°C“rt, (or BiBr3 30%, CH3CN, cat. H2O, rt); (b) lipase (PPL), NaCl,
CaCl2, H2O/THF.
In conclusion, we have developed a practical synthesis
of isoprostanes of the E-type via the tandem two-com-
ponent coupling process and the diastereoselective pro-
tonation with chelating proton sources under reagent
control. The extension of this methodology towards
other natural products will be reported in due course.
10. Rokach, J.; Khanapure, S. P.; Hwang, S.-W.; Adiyaman,
M.; Schio, L.; FitzGerald, G. A. Synthesis 1998, 569–580.
11. Daniels, E. G.; Krueger, W. C.; Kupiecki, F. P.; Pike, J.
E.; Schneider, W. P. J. Am. Chem. Soc. 1968, 90, 5894–
5895.
12. Schneider, W. P.; Axen, U.; Lincoln, F. H.; Pike, J. E.;
Thompson, J. L. J. Am. Chem. Soc. 1969, 91, 5372–5378.
13. Miyano, M.; Stealey, M. J. Org. Chem. 1975, 40, 1748–
1755.
Acknowledgements
14. Nakamura, N.; Sakai, K. Tetrahedron Lett. 1978, 19,
1549–1552.
15. Taber, D. F.; Hoerrner, R. S. J. Org. Chem. 1992, 57,
441–447.
Financial support of this research in part by USDA
(95-37200-1648) and the Department of Cell Biology
UMDNJ-SOM is gratefully acknowledged.
16. Taber, D. F.; Jiang, Q. Tetrahedron 2000, 56, 5991–5994.
17. Sih, C. J.; Price, P.; Sood, R.; Salomon, R. G.; Peruz-
zotti, G.; Casey, M. J. Am. Chem. Soc. 1972, 94, 3643–
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30. The ratios were determined by HPLC [column: Nucleosil
31.60, 26.61, 24.97, 24.56, 23.21, 22.47, 13.87. [h]2D5=
+60.5 (c 0.67, CHCl3) (lit.14 [h]D=+40.9 (c 0.00075,
MeOH). Compound 16: H NMR (CDCl3, 300 MHz): l
1
5.7–5.6 (dd, J=15.3, 6.3 Hz, 1H), 5.4–5.3 (m, 2H), 5.3
(dd, J=15.3, 9.6 Hz, 1H), 4.3 (m, 1H), 4.1–4.0 (m, 1H),
3.6 (s, 3H), 3.0–2.9 (m, 1H), 2.7 (m, 1H), 2.5 (dd,
J=19.5, 5.7 Hz, 1H), 2.5–2.3 (m, 2H), 2.3 (t, J=7.3 Hz,
2H), 2.1–1.9 (m, 3H), 1.7–1.6 (m, 2H), 1.6–1.2 (m, 8H),
0.9–0.8 (t, J=6.6 Hz, 3H); 13C NMR (CDCl3, 75.5
MHz): l 216.55, 174.35, 137.85, 130.10, 127.84, 126.42,
72.33, 72.18, 51.44, 51.26, 50.64, 44.78, 37.30, 33.35,
31.62, 26.68, 25.00, 24.61, 23.13, 22.46, 13.82. [h]2D5=−56
(c 0.69, CHCl3). Compound 1: 1H NMR (CD3OD, 300
MHz): l 5.7–5.6 (dd, J=15.3, 6.6 Hz, 1H), 5.5–5.3 (m,
2H), 5.3 (dd, J=15.3, 9.9 Hz, 1H), 4.3–4.2 (m, 1H), 4.0
(m, 1H), 3.0 (m, 1H), 2.7–2.6 (m, 1H), 2.6–2.5 (dd,
J=19.2, 5.7 Hz, 1H), 2.5–2.3 (m, 1H), 2.3–2.2 (t, J=7.5
Hz, 2H), 2.3–2.2 (m, 1H), 2.1–1.9 (m, 3H), 1.7–1.6
(quint., J=7.5 Hz, 2H), 1.6–1.2 (m, 8H), 0.9 (t, J=6.6
Hz, 3H); 13C NMR (CD3OD, 75.5 MHz): l 219.70,
177.49, 138.66, 131.12, 129.11, 128.23, 73.32, 72.85, 52.45,
51.74, 45.49, 38.40, 34.34, 32.91, 27.76, 26.17, 25.93,
24.22, 23.64, 14.29. [h]2D5=+87 (c 0.057, MeOH). Com-
100 Silica
(99.75:0.25) u=214 nm].
5
mm, mobile phase: Hexane/i-PrOH
31. To a −78°C solution of 12 (0.69 g, 1.87 mmol) in diethyl
ether (3 ml), in a flame dried flask under argon, was
added a 1.6 M solution of n-butyllithium in hexane (1.3
ml, 2.01 mmol) and stirred at −78°C for 2 h. Copper(I)
cyanide (0.17 g, 1.87 mmol) was placed in a second flame
dried flask and suspended in diethyl ether (6 ml). The
mixture was cooled at −78°C and a 1.4 M solution of
methyllithium in diethyl ether (1.4 ml, 1.90 mmol) was
slowly added. After 20 min at 0°C the clear solution was
cooled to −78°C and the previously prepared vinyllithium
reagent was added via cannula. The reaction was slowly
warmed to −30°C and kept at this temperature for 20
min. After cooling to −78°C compound 10 (0.33 g, 0.94
mmol) in diethyl ether (2 ml) was added. The reaction
mixture was stirred at −78°C (20 min) and at −40°C (10
min) and cooled again to −78°C. The reaction was trans-
ferred into a solution of methyl acetoacetate (1.4 ml, 13.0
mmol) in diethyl ether (25 ml) which was kept at −78 C.
The mixture was slowly warmed to room temperature
and acetic acid (0.55 ml, 9.61 mmol) was added. The
solution was filtered through a pad of celite and washed
with a saturated solution of sodium bicarbonate and
brine. Drying (Na2SO4) and evaporating under vacuo
gave crude 13. The excess of methyl acetoacetate was
removed in high vacuo (0.1 mm) at room temperature.
Purification by flash chromatography [silica gel, hex-
ane:EtOAc (95:5)] afforded 0.30 g (54%) of 13.
1
pound 2: H NMR (CD3OD, 300 MHz): l 5.7–5.6 (dd,
J=15.3, 6.0 Hz, 1H), 5.5–5.3 (m, 2H), 5.3 (dd, J=15.3,
9.6 Hz, 1H), 4.3–4.2 (m, 1H), 4.0 (m, 1H), 3.0 (m, 1H),
2.7–2.6 (m, 1H), 2.6–2.5 (dd, J=19.2, 5.7 Hz, 1H), 2.5–
2.3 (m, 1H), 2.3–2.2 (t, J=7.5 Hz, 2H), 2.2 (m, 1H),
2.1–1.9 (m, 3H), 1.7–1.6 (quint., J=7.5 Hz, 2H), 1.6–1.2
(m, 8H), 0.9 (t, J=6.9 Hz, 3H); 13C NMR (CD3OD, 75.5
MHz): l 219.67, 177.68, 138.61, 131.14, 129.19, 127.75,
73.08, 72.86, 52.26, 51.89, 45.54, 38.42, 34.45, 32.89,
27.75, 26.20, 25.99, 24.16, 23.61, 14.26. [h]2D5=−69 (c 0.13,
1
MeOH). Compound 3: H NMR (CD3OD, 300 MHz): l
32. Satisfactory spectroscopic data were obtained for all
compounds. Selected physical data: Compound 13: 1H
NMR (CDCl3, 300 MHz): l 5.6 (dd, J=15.3, 6.3 Hz,
1H), 5.4–5.3 (m, 2H), 5.1 (dd, J=15.3, 10.2 Hz, 1H), 4.2
(m, 1H), 4.0 (m, 1H), 3.6 (s, 3H), 2.9 (m, 1H), 2.7–2.6 (m,
1H), 2.5–2.4 (m, 1H), 2.4 (dd, J=18.9, 5.1 Hz, 1H), 2.3
(t, J=7.5 Hz, 2H), 2.2 (br. d, J=18.9 Hz, 1H), 2.0 (m,
2H), 1.9–1.7 (m, 1H), 1.7–1.6 (quint., J=7.5 Hz, 2H),
1.6–1.2 (m, 8H), 0.9 (m, 21H), 0.1–0.0 (4s, 12H); 13C
NMR (CDCl3, 75.5 MHz): l 217.57, 173.99, 138.15,
129.92, 127.79, 125.59, 73.18, 72.73, 51.81, 51.34, 50.22,
45.28, 38.21, 33.37, 31.66, 26.65, 25.74 (3C), 25.65 (3C),
24.72, 24.66, 22.81, 22.49, 18.09, 17.91, 13.86, −4.44,
−4.79, −4.95, −5.04. [h]2D5=+49 (c 1.05, CHCl3). Com-
pound 15: 1H NMR (CDCl3, 300 MHz): l 5.7–5.6 (dd,
J=15.3, 6.3 Hz, 1H), 5.4–5.3 (m, 2H), 5.3 (dd, J=15.3,
10.2 Hz, 1H), 4.3 (m, 1H), 4.1–4.0 (m, 1H), 3.6 (s, 3H),
3.0–2.9 (m, 1H), 2.7 (m, 1H), 2.5 (dd, J=19.2, 5.7 Hz,
1H), 2.4–2.2 (m, 2H), 2.3 (t, J=7.3 Hz, 2H), 2.1–1.9 (m,
3H), 1.7–1.6 (quint., J=7.3 Hz, 2H), 1.5–1.2 (m, 8H),
0.9–0.8 (t, J=6.4 Hz, 3H); 13C NMR (CDCl3, 75.5
MHz): l 216.99, 174.34, 137.59, 130.11, 127.63, 126.67,
72.36, 71.99, 51.50, 51.39, 50.65, 44.79, 37.23, 33.32,
5.6 (dd, J=15.3, 6.9 Hz, 1H), 5.3–5.2 (dd, J=15.3, 10.5
Hz, 1H), 4.3–4.2 (m, 1H), 4.0–3.9 (m, 1H), 3.0 (m, 1H),
2.7–2.5 (m, 1H), 2.6–2.5 (dd, J=19.2, 5.4 Hz, 1H), 2.3–
2.2 (t, J=7.5 Hz, 2H), 2.2 (m, 1H), 1.7–1.1 (m, 18H), 0.9
(t, J=6.9 Hz, 3H); 13C NMR (CD3OD, 75.5 MHz): l
220.66, 177.81, 138.47, 128.55, 73.54, 72.93, 52.69, 50.98,
45.51, 38.40, 34.98, 32.94, 30.38, 30.10, 28.30, 26.25,
26.21, 26.05, 23.69, 14.34. [h]2D5=+88 (c 0.33, MeOH).
1
Compound 4: H NMR (CD3OD, 300 MHz): l 5.7–5.6
(dd, J=15.3, 6.0 Hz, 1H), 5.3 (dd, J=15.3, 9.9 Hz, 1H),
4.2 (m, 1H), 4.0 (m, 1H), 3.0 (m, 1H), 2.7–2.5 (m, 1H),
2.5 (dd, J=19.2, 5.4 Hz, 1H), 2.3–2.2 (t, J=7.5 Hz, 2H),
2.3–2.1 (m, 1H), 1.7–1.2 (m, 18H), 0.9 (t, J=6.6 Hz, 3H);
13C NMR (CD3OD, 75.5 MHz): l 220.76, 178.00, 138.42,
127.46, 72.99, 72.88, 52.46, 51.10, 45.56, 38.46, 35.06,
32.94, 30.32, 30.10, 28.34, 26.16, 26.11, 26.09, 23.67,
14.32. [h]2D5=−78 (c 0.095, MeOH).
33. Bajwa, J. S.; Vivelo, J.; Slade, J.; Repic, O.; Blacklock, T.
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