Total synthesis and bioactivity of resolvin E2

Article abstract of DOI:10.1021/ol901350g

Image Presented Resolvin E2 is a potent anti-inflammatory compound, derived from eicosapentaenoic acid. The efficient total synthesis of resolvin E2 by taking advantage of its intrinsic pseudoenantiomeric substructures is reported. The synthetic resolvin

Full text of DOI:10.1021/ol901350g

ORGANIC
LETTERS
2009
Vol. 11, No. 16
3602-3605
Total Synthesis and Bioactivity of
Resolvin E2
Seiji Ogawa, Daisuke Urabe, Yoshiyuki Yokokura, Hiroyuki Arai, Makoto Arita,
and Masayuki Inoue*
Graduate School of Pharmaceutical Sciences, The UniVersity of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
inoue@mol.f.u-tokyo.ac.jp
Received June 16, 2009
ABSTRACT
Resolvin E2 is a potent anti-inflammatory compound, derived from eicosapentaenoic acid. The efficient total synthesis of resolvin E2 by taking
advantage of its intrinsic pseudoenantiomeric substructures is reported. The synthetic resolvin E2 proved to be biologically active in blocking
neutrophil infiltration and reducing proinflammatory cytokines in the acute peritonitis model.
Resolvins are a new family of lipid mediators derived from
omega-3 polyunsaturated fatty acids, namely, eicosapen-
taenoic acid (EPA) and docosahexaenoic acid (DHA), which
are generated during the resolution phase of acute inflam-
mation.1 Resolvin E1 is biosynthesized from EPA via
cyclooxygenase (COX)-2- and 5-lipoxygenase-mediated
conversion and has been shown to possess significant anti-
inflammatory and proresolution properties, thereby protecting
organs from collateral damage.2 Another E series resolvin,
namely, resolvin E2 (1), is formed via reduction of 5S-
hydroperoxy-18R-hydroxy-EPE, an intermediate in the bio-
synthesis of resolvin E1, and exhibits potent anti-inflamma-
tory properties in murine peritonitis.3 It has been hypothesized
that these E series resolvins contribute to the beneficial
actions that have been attributed to EPA in certain human
diseases, particularly those in which inflammation is sus-
pected as a key component in pathogenesis. Motivated by
their therapeutic potential for new treatment of human
disorders associated with aberrant inflammation, we launched
the synthetic studies of resolvins as well as other lipid
mediators. Here we report an efficient total synthesis of
resolvin E24 and its biological activity in reducing neutrophil
infiltration and proinflammatory cytokine productions in vivo.
We planned to simplify the synthetic route to resolvin E2
(1) by taking advantage of its two symmetric substructures
at C5-10 and C13-18 (Scheme 1). Retrosynthetic discon-
nections at C10-11 and C12-13 provided a C11-12 unit
together with pseudo-enantiomeric fragments, 2 and 3, both
of which have the E,Z-conjugated olefin and allylic alcohol
groups. Because of their structural similarity, 2 and 3 would
be prepared from enantiomers of 6 by applying the same
(1) For reviews, see: (a) Serhan, C. N.; Chiang, N.; Van Dyke, T. E.
Nat. ReV. Immunol. 2008, 8, 349–361. (b) Serhan, C. N.; Chiang, N. Br. J.
Pharmacol. 2008, 153, S200–S215.
(2) (a) Arita, M.; Bianchini, F.; Aliberti, J.; Sher, A.; Chiang, N.; Hong,
S.; Yang, R.; Petasis, N. A.; Serhan, C. N. J. Exp. Med. 2005, 201, 713–
722. (b) Schwab, J. M.; Chiang, N.; Arita, M.; Serhan, C. N. Nature 2007,
447, 869–875.
(4) Total syntheses of resolvins E1 were disclosed in ref 2a. Total synthesis
of resolvins D2 and D5 were reported. (a) Rodriguez, A. R.; Spur, B. W.
Tetrahedron Lett. 2004, 45, 8717–8720. (b) Rodriguez, A. R.; Spur, B. W.
Tetrahedron Lett. 2005, 46, 3623–3627. For a review of syntheses of
eicosanoids, see: (c) Nicolaou, K. C.; Ramphal, J. Y.; Petasis, N. A.; Serhan,
C. N. Angew. Chem., Int. Ed. 1991, 30, 1100–1116.
(3) Tjonahan, E.; Oh, S. F.; Siegelman, J.; Elangovan, S.; Percarpio,
K. B.; Hong, S.; Arita, M.; Serhan, C. N. Chem. Biol. 2006, 13, 1193–
1202.
10.1021/ol901350g CCC: $40.75
Published on Web 07/28/2009
 2009 American Chemical Society

Scheme 1
.
Structures of Resolvins and EPA and Retrosynthesis
of Resolvin E2
Scheme 2. Synthesis of Both Enantiomers of 6
Both enantiomers of 6 were prepared from methyl ester
(+)-10 (Scheme 2).9 The critical desymmetrization of meso-7
into (+)-10 was realized using a catalytic amount of the
quinine derivative 9, according to the conditions developed
by Song.10,11 Namely, 1 mol % of 9 and 10 equiv of
methanol were applied to 7 in Et2O to generate (+)-10 in
highly enantioselective fashion (95% yield, 87% ee).12,13
Interestingly, methanolysis of the same 7 using the quinidine
derivative 8, the pseudo-enantiomer of 9, indeed gave the
enantiomeric (-)-10, albeit in lower enantioselectivity (64%
ee). Due to this, we decided to synthesize (-)- and (+)-6
from the same (+)-10 using chemoselective reduction of
either the carboxylic acid or the ester. Lactone (-)-6 was
prepared in three steps: conversion of the carboxylic acid of
(+)-10 into the acid chloride, followed by chemoselective
NaBH4 reduction,14 and subsequent acid-mediated cyclization
of methyl ester 11. The enantiomer (+)-6 was in turn
synthesized by LiEt3BH reduction15 of the methyl ester of
(+)-10 and subsequent cyclization of carboxylic acid 12
under acidic conditions.
strategy. Specifically, the stereocenters at C5 of 4 and C18
of 5 would be generated by substrate-controlled stereose-
lective addition of the corresponding carbon nucleophiles,
while the E,Z-olefins at C6 of 2 and C17 of 3 would be
constructed using a torquoselective thermal electrocyclic ring-
opening reaction5 of cyclobutene aldehydes 4 and 5, respec-
tively.6 Hence, the stereochemistries of the cyclobutane of
(-)- or (+)-6 were envisioned to be transferred to the
stereochemistries of the hydroxy group at C5 or C18 and
the diene at C6 or C17. A pair of optically active six-carbon
units 67 would be obtained from the known achiral meso-
anhydride 78 by enantioselective desymmetrization.
(5) For rationalization of torquoselectivity, see: (a) Rudolf, K.; Spell-
meyer, D. C.; Houk, K. N. J. Org. Chem. 1987, 52, 3708–3710. (b)
Niwayama, S.; Kallel, E. A.; Spellmeyer, D. C.; Sheu, C.; Houk, K. N. J.
Org. Chem. 1996, 61, 2813–2825. (c) Dolvier, W. R., Jr.; Koroniak, H.;
Houk, K. N.; Sheu, C. Acc. Chem. Res. 1996, 29, 471–477.
Synthesis of the C1-10 fragment 2 started with reduction
of (-)-6 by DIBAL-H, followed by addition of Grignard
(6) Wallace demonstrated selective ring opening of cyclobutene alde-
hydes and substrate-controlled stereoselective introductions of various carbon
nucleophiles to disubstituted cyclobutenes: (a) Binns, F.; Hayes, R.; Ingham,
S.; Saengchantara, S. T.; Turner, R. W.; Wallace, T. W. Tetrahedron 1992,
48, 515–530. (b) Hodgetts, K. J.; Wallis, C. J.; Wallace, T. W. Tetrahedron
Lett. 1994, 35, 4645–4648. (c) Hodgetts, K. J.; Wallis, C. J.; Wallace, T. W.
Synlett 1995, 1235. (d) Binns, F.; Hayes, R.; Hodgetts, K. J.; Saengchantara,
S. T.; Wallace, T. W.; Wallis, C. J. Tetrahedron 1996, 52, 3631–3658.
(7) Enantioselective syntheses of (+)- and (-)-6 by enzymatic resolution
were reported: Gourdel-Martin, M.-E.; Comoy, C.; Huet, F. Tetrahedron:
Asymmetry 1999, 10, 403–404. See also ref 6b and 6c.
(9) For a review of synthetic applications of lactone 6 and derivatives,
see: Gauvry, N.; Lescop, C.; Huet, F. Eur. J. Org. Chem. 2006, 520, 7–
5218.
(10) Oh, S. H.; Rho, H. S.; Lee, J. W.; Lee, J. E.; Youk, S. H.; Chin, J.;
Song, C. E. Angew. Chem., Int. Ed. 2008, 47, 7872–7875
.
(11) For a review of asymmetric alcoholysis of cyclic anhydrides, see:
Chen, Y.; McDaid, P.; Deng, L. Chem. ReV. 2003, 103, 2965–2983
.
(12) Enantiomeric excess was determined from 1H NMR spectra of the
MTPA ester of alcohol 11.
(13) Absolute configuration of lactone 6 was deduced from the report
of Wallace and co-workers, see ref 6d.
(14) Fujisawa, T.; Mori, T.; Sato, T. Chem. Lett. 1983, 835–838.
(15) Brown, H. C.; Kim, S. C.; Krishnamurthy, S. J. Org. Chem. 1980,
45, 1–12.
(8) (a) Koltzenburg, G.; Fuss, P. G.; Leitich, J. Tetrahedron Lett. 1966,
29, 3409–3414. (b) Brauman, J. I.; Archie, W. C., Jr. J. Am. Chem. Soc.
1972, 94, 4262–4265. (c) Gauvry, N.; Comoy, C.; Lescop, C.; Huet, F.
Synthesis 1999, 574–576.
Org. Lett., Vol. 11, No. 16, 2009
3603

reagent 14 in one pot,16 resulting in stereoselective introduc-
tion of the C5-hydroxy group of 16 (dr ) 6:1, Scheme 3).17
allylic alcohol 20.18 Finally, bromination of the chemically
sensitive allylic alcohol was realized by the action of CBr4
and (CH2PPh2)2 to give the C1-10 fragment 2.19,20
The C13-20 fragment 3 was synthesized from (+)-6
similarly to the C1-10 fragment 2 (Scheme 4). Reduction
Scheme 3. Synthesis of the C1-10 Fragment
Scheme 4. Synthesis of the C13-20 Fragment
of (+)-lactone 6 with DIBAL-H was followed by stereose-
lective addition of ethyl magnesium bromide in the presence
of zinc bromide to afford 21 with the desired C18-
stereochemistry (dr ) 7:1).21 After protecting group ma-
nipulations from 21 to 22 in two steps, Swern oxidation of
the primary alcohol of 22 to aldehyde 5 accelerated the
thermal ring-opening reaction to produce E,Z-diene 23 as a
single isomer. Reduction of the resulting aldehyde of 23 into
alcohol 24, followed by bromination, led to the C13-20
fragment 3.
The high diastereoselectivity is attributable to chelation
between the magnesium alkoxide and the aldehyde and
subsequent nucleophilic attack from the convex face of the
4/7-fused ring system 15.6b,d Next, 1,4-diol 16 was trans-
formed to alcohol 18 by a protection/deprotection procedure:
stepwise introduction of Piv and TBS to the primary and
the secondary alcohols, respectively, and subsequent reduc-
tive removal of the Piv ester from 17.
Final convergent assemblies of the three partial structures
utilized two copper-mediated couplings (Scheme 5). The
bromide of 3 was first displaced with the copper acetylide,
generated from ethynyl magnesium bromide and CuCl,22
delivering the C11-20 fragment 25. Deprotonation of the
23
C11-proton of 25 by n-BuLi in the presence of CuBr·SMe2
at -78 °C then afforded the corresponding copper acetylide,
which was treated with the C1-10 fragment 2, giving rise
to the entire structure 26. Intriguingly, only this particular
condition produced a sufficient amount of the adduct 26. For
instance, use of copper iodide instead of copper bromide for
the coupling only gave a mixture of byproducts, in which
the C6-E,Z-olefins were reacted or isomerized.
Swern oxidation of alcohol 18 at -78 °C generated
aldehyde 4, which underwent the crucial torquoselective
electrocyclic ring-opening reaction even at room temperature
to deliver E,Z-diene 19 as a sole isomer.5 Stereoselective
formation of the diene of 19 would originate from strong
preference of the electron-accepting aldehyde of 4 for the
inward rotation.6 Because of its chemical instability, the
resulting R,ꢀ,γ,δ-unsaturated aldehyde 19 was immediately
subjected to NaBH4 reduction without purification to give
(18) E,Z-Dienes 20 and 24 were easily isomerized into E,E-diene upon
standing at room temperature. Thus, organic solution of the reaction mixture
from the oxidation was directly used for the reduction.
(19) (a) Leblanc, Y.; Futzsimmons, B. J.; Adams, J.; Perez, F.; Rokach,
J. J. Org. Chem. 1986, 51, 789–793. (b) Schmidt, S. P.; Brooks, D. W.
Tetrahedron Lett. 1987, 28, 767–768
(20) Bromination of alcohol 20 with CBr4 and Ph3P gave only trace
amount of the product 2.
.
(16) Wenkert, D.; Ferguson, S. B.; Porter, B.; Qvarnstrom, A.; McPhail,
A. T. J. Org. Chem. 1985, 50, 4114–4119.
(21) Addition of EtMgBr in the presence of Ti(Oi-Pr)3Et gave the desired
product as a single isomer; however, the yield was 23%.
(22) Bloch, R.; Gasparini, G.; Girard, C. Chem. Lett. 1988, 1927–1930.
(23) (a) Kurtz, P. Liebigs Ann. Chem. 1962, 658, 6–20. (b) Normant,
J. F. Synthesis 1972, 63.
(17) Stereochemistries at C5 and C18 positions were unambiguously
determined by the modified Moshers’ method. Ohtani, I.; Kusumi, T.;
Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113, 4092–4096. See
Supporting Information for the detailed procedure.
3604
Org. Lett., Vol. 11, No. 16, 2009

Scheme 5. Total Synthesis of Resolvin E2
Figure 1. Synthetic resolvin E2 reduced neutrophil infiltrations and
cytokine productions in zymosan-induced peritonitis.
proinflammatory cytokine productions. In acute peritonitis
with zymosan A, intravenous administration of synthetic
resolvin E2 as low as 0.1 or 1.0 µg significantly blocked
neutrophil infiltrations at 2 h in the inflamed peritoneal cavity
(Figure 1). The potency of resolvin E2’s anti-inflammatory
action was comparable to that of a higher dose of dexam-
ethasone at 10 µg (data not shown). Also 1 (1.0 µg i.V./
mouse) markedly reduced production of proinflammatory
cytokines such as tumor necrosis factor (TNF)-R (123.2 (
15.8 pg vs 56.7 ( 7.3 pg, p < 0.005) and interleukin (IL)-6
(3.40 ( 0.29 ng vs 2.05 ( 0.22 ng, p < 0.02) (see Supporting
Information).
In summary, the efficient total synthesis of resolvin E2
(1) was achieved by utilizing the intrinsic pseudoenantio-
meric nature of the key fragments 2 and 3. Most importantly,
the two stereochemistries of 6 introduced via the desymme-
trization step were effectively transferred to those of the
hydroxy group and the diene for preparing 2 and 3. The
obtained fragments were assembled into 1 in a convergent
fashion. Further synthetic studies and functional analyses of
the resolvins and other lipid mediators are currently underway
in our laboratory.
Four transformations from 26 led to the targeted resolvin
E2 (Scheme 5). Lindlar conditions24 enabled partial reduction
of the alkyne of 26 into alkene 27 without reduction and/or
isomerization of the reactive C6- and C17-E,Z-olefins. Acid-
mediated removal of the ketal of 27 was troublesome because
of the presence of the acid-labile allylic TBS ethers. After
many attempts, we found that Kita’s conditions25 were
effective for selective reaction of the cyclic ketal. Treatment
of 27 with an excess amount of TMSOTf and lutidine
provided aldehyde 28 in high yield. Lastly, NaClO2-mediated
oxidation of the obtained aldehyde 28 into a carboxylic acid
and subsequent desilylation with TBAF gave rise to resolvin
E2 (1).
Acknowledgment. This work was supported by a Grant-
in-Aid for Young Scientists (S) (JSPS) to M.I. and PRESTO
from the Japan Science and Technology Agency (JST) to
M.A. We thank Michiko Kamio (The University of Tokyo)
for technical assistance.
We evaluated the bioactivity of synthetic 1 using the in
vivo inflammation model (Figure 1).3 Zymosan A, a glucan
from the yeast cell wall, was used to induce sterile inflam-
mation characterized by local neutrophil infiltration and
Supporting Information Available: Experimental pro-
cedures and characterization data of newly synthesized
compounds and biological assay data of synthetic 1. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(24) Lindlar, H. HelV. Chim. Acta 1952, 57, 446–450.
(25) Fujioka, H.; Okitsu, T.; Sawama, Y.; Murata, N.; Li, R.; Kita, Y.
J. Am. Chem. Soc. 2006, 128, 5930–5938.
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