Total synthesis of the endogenous inflammation resolving lipid resolvin D2 using a common lynchpin

Article abstract of DOI:10.3762/bjoc.9.310

The total synthesis of the endogenous inflammation resolving eicosanoid resolvin D2 (1) is described. The key steps involved a Wittig reaction between aldehyde 5 and the ylide derived from phosphonium salt 6 to give enyne 17 and condensation of the same ylide with aldehyde 7 to afford enyne 11. Desilylation of 11 followed by hydrozirconation and iodination gave the vinyl iodide 4 and Sonogashira coupling between this compound and enyne 3 provided alkyne 18. Acetonide deprotection, partial reduction and ester hydrolysis then gave resolvin D2 (1).

Full text of DOI:10.3762/bjoc.9.310

Total synthesis of the endogenous inflammation
resolving lipid resolvin D2 using a common lynchpin
John Li1, May May Leong1, Alastair Stewart2 and Mark A. Rizzacasa*1
Full Research Paper
Open Access
Address:
School of Chemistry, The Bio21 Institute, The University of
Beilstein J. Org. Chem. 2013, 9, 2762–2766.
1
doi:10.3762/bjoc.9.310
Melbourne, Parkville, Victoria 3010, Australia and 2Department of
Pharmacology and Therapeutics, The University of Melbourne,
Parkville, Victoria, 3010, Australia
Received: 08 October 2013
Accepted: 22 November 2013
Published: 03 December 2013
Email:
Mark A. Rizzacasa* - masr@unimelb.edu.au
This article is part of the Thematic Series "Transition-metal and
organocatalysis in natural product synthesis".
*
Corresponding author
Guest Editors: D. Y.-K. Chen and D. Ma
Keywords:
catalysis; eicosanoid; natural product; resolvin D2; Sonogashira
coupling; total synthesis; Wittig reaction
© 2013 Li et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The total synthesis of the endogenous inflammation resolving eicosanoid resolvin D2 (1) is described. The key steps involved a
Wittig reaction between aldehyde 5 and the ylide derived from phosphonium salt 6 to give enyne 17 and condensation of the same
ylide with aldehyde 7 to afford enyne 11. Desilylation of 11 followed by hydrozirconation and iodination gave the vinyl iodide 4
and Sonogashira coupling between this compound and enyne 3 provided alkyne 18. Acetonide deprotection, partial reduction and
ester hydrolysis then gave resolvin D2 (1).
Introduction
The resolution of inflammation is a tightly governed active tive of DHA (17HDHA) [1]. However, no NMR experiments
process effectively mediated by a range of bioactive polyunsat- were performed due to nanogram quantities isolated and the
urated fatty acids, peptides and proteins. In 2002, a new family stereochemistry was tentatively assigned based on the proposed
of endogenously generated lipid mediators involved in the reso- biosynthesis via lipoxygenase modification of DHA.
lution of inflammation named the resolvins (resolution phase
interaction products) were identified by Serhan and co-workers RvD2 (1) prevents the adherence of polymorphonuclear leuko-
in the inflammatory exudates of aspirin treated mice [1-3]. The cytes (PMN) to the blood vessel wall by promoting the shed-
resolvins are divided into 2 groups, the D-series resolvins D2 ding of L-selectin from PMNs thus preventing binding to E
(
1) and D1 (2) [3], which are derivatives of docosahexaenoic selectin on the endothelial cell lining of the blood vessel [5].
acid (DHA) (Figure 1) and the E-series [4] derived from eicosa- Furthermore, RvD2 (1) promotes the influx and phagocytic
pentaenoic acid. Structural analysis by mass spectrometry (MS) activity of macrophages, facilitating clearance of dead cells and
showed that resolvin D2 (RvD2, 1) was a 17-hydroxy deriva- microbial pathogens, allowing resolution of inflammation and
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Beilstein J. Org. Chem. 2013, 9, 2762–2766.
The first total synthesis of (7S,16R,17S)-RvD2 (1) was commu-
nicated by Spur in 2004 [9] but this report did not include an
experimental section although physical data for some com-
pounds was provided. A similar synthesis of RvD2 was utilized
by others for the production of 1 for a biological study [5] but
again, there was no experimental provided. The total synthesis
of resolvin D1 has also been reported [10] along with resolvins
D3 [11], D5 [12], D6 [13] and resolvins E1 [4,14,15], E2
[
16,17] and E3 [18] with full experimental details included for
resolvins D3 [11], E2 [16] and E3 [18]. An improved synthesis
of the C16–C20 fragment of resolvin E1 has also been reported
[
19]. We were interested in accessing amounts of RvD2 (1) for
biological evaluation but without detailed synthetic sequence to
follow and given the very high cost [20] of commercial 1 we
elected to develop an alternative route to provide this important
compound and analogues for further biological evaluation.
Herein we describe a synthesis of RvD2 (1) which includes full
experimental details so that other researchers can produce
useful amounts of this important compound as well as novel
Figure 1: Structures of resolvins D1 (1) and D2 (2).
infection [5]. This successful evaluation of the resolvin series in isomers.
preclinical models of bacterial sepsis has stimulated strong
Results and Discussion
interest in their therapeutic potential, as RvD2 appears to
express the unusual combination of anti-inflammatory and Retrosynthetic analysis
antimicrobial activity. Further interest in inflammation- A retrosynthetic analysis of RvD2 (1) is shown in Scheme 1. It
resolving lipids is stimulated by their inhibitory effects on was envisaged that the target compound 1 could be secured via
inflammatory pain, which are mediated via inhibition of the a Sonogashira coupling to form the C11–C12 bond followed by
activity of TRPV1 and TRPA1 calcium channels on sensory partial reduction. This is similar to the endgame of the reported
nerves [6]. Resolvin D1 (2) [7] has been shown to act directly syntheses of 1 [5,9] but both of these approaches involved for-
on human PMNs and also regulates actin polymerization [8]. mation of the C9–C10 bond as the convergent step. In our ap-
Whilst RvD1 has been shown to act on the FPR2 and GPR32 proach, 1 could arise from enyne 3 and vinyl iodide 4 which
types of G-Protein-coupled receptors, the receptor(s) for RvD2 could both be obtained by Wittig extension using the common
remain to be identified. Identification of the receptors medi- linchpin phosphorus ylide derived from phosphonium salt 6
ating the combined anti-inflammatory and antimicrobial actions [21,22] and each of the homochiral aldehydes 5 and 7 [9].
would facilitate efforts to identify ligands that have better drug-
like properties than RvD2 or its analogues. However, such Synthesis of vinyl iodide 4
efforts have been limited by the lack of availability of suitable The synthesis of fragment 4 began with the production of the
amounts of RvD2.
aldehyde 7 as shown in Scheme 2. A Wittig reaction between
Scheme 1: Retrosynthetic analysis of RvD2 (1).
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Beilstein J. Org. Chem. 2013, 9, 2762–2766.
hemiacetal 8 [23] and the ylide derived from 9 provided the Synthesis of dienyne 3
alkene 10 [9] with excellent stereoselectivity. Oxidation of 10 Our approach to the aldehyde 5 began with the production of
with Dess–Martin periodinane then afforded aldehyde 7. The the known bis-TES ether [9] produced by an alternative proce-
phosphonium salt 6 [21,22] was produced from propargyl bro- dure (Scheme 4) in which the C7 stereochemistry was intro-
mide via silylation of the derived sodium salt with TIPSCl fol- duced via asymmetric dihydroxylation [25,26]. Thus, ester 13
lowed by reaction with triphenylphosphine.
[27] was treated with AD-mix-α in t-BuOH/H2O to give diol 14
in reasonable yield. The enantioselectivity and absolute con-
figuration of the secondary alcohol was determined by conver-
sion of diol into the bis-(S)-Mosher ester [28,29]. Integration of
the 1H MMR spectrum indicated the e.r. was 93.7:6.3 and
Mosher analysis (See Supporting Information File 2 for details)
confirmed the stereochemistry of the new asymmetric center of
the major enantiomer as S in accord with the predicted outcome
[
25]. Silylation gave bis-TES ether 15 and partial reduction of
the alkyne using P2-Ni as catalyst [30] afforded the alkene 16.
Desilylation of the primary TES group in 16 and concomitant
oxidation to aldehyde 5 was achieved under Swern conditions
as reported by Spur [31].
Scheme 2: Synthesis of aldehyde 7 and phosphonium salt 6.
Treatment of the salt 6 with n-BuLi gave the ylide and conden-
sation with the aldehyde 7 afforded the desired E-enyne 11
along with the Z-isomer in a ratio of 2.2:1 which were easily
separated by flash chromatography (Scheme 3). The minor
Z-isomer could also provide novel stereoisomer analogues of
RvD2 (1). Removal of the TIPS group with TBAF gave
terminal alkyne 12. Alkyne 12 then underwent smooth
hydrozirconation utilizing the procedure reported by Negishi
[
24] were ZrCp2HCl is generated in situ by reduction of
ZrCp2Cl2 with DIBALH in THF. Iodinolysis of the zirconium
species then gave the diene iodide 4 in good yield. Selectivity
for this process was excellent with only a trace of the regio-
isomer formed.
Scheme 4: Synthesis of enyne 3.
Deprotonation of salt 6 with LiHMDS followed by conden-
sation with aldehyde 5 gave the E-enyne and the corresponding
Z-isomer in a 4:1 ratio. The use of LiHMDS as base was crit-
ical for reasonable yields and stereoselectivity in this case.
Global deprotection of 17 with TBAF gave the enyne 3 in good
yield.
Scheme 3: Synthesis of vinyl iodide 4.
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Beilstein J. Org. Chem. 2013, 9, 2762–2766.
Total synthesis of resolvin D2 (1)
through basic alumina immediately prior to use. Spectra for
The completion of the synthesis of RvD2 (1) is shown in RvD2 (1) were always measured for CD3CN. Even with short
Scheme 5. Sonogashira coupling [32,33] between 3 and 4 was exposure to the solvent, we still observed degradation to
very efficient giving the alkyne 18 in good yield. Removal of unidentified compounds. Samples of RvD2 (1) can be stored in
the acetonide was effected by treatment with HCl in MeOH to EtOH or frozen in DMSO solution but should be used immedi-
give the known triol 19 [9]. The final steps to 1 were similar ately upon thawing. Alternatively, the triol 19 proved more
with those previously reported [5,9]. Thus, partial reduction of stable than both RvD2 methyl ester (20) and RvD2 (1) and can
the triple bond using Zn(Cu/Ag) [34] to afforded RvD2 methyl be stored for longer periods prior to conversion to 1 which
ester 20 in 76% yield. A large excess of the Zn reagent was should be used rapidly for biological assessment to avoid de-
required to obtain a good conversion of 19 into 20. The gradation.
1
H NMR spectrum (CDCl3 solvent) of RvD2 methyl ester (20)
compared well to that reported [5]. Final ester hydrolysis and Conclusion
mild acid work-up then gave RvD2 (1).
The total synthesis of RvD2 (1) has been completed using a
common linchpin Wittig reaction. Using this approach, we were
able to prepare sufficient quantities of this important inflamma-
tion resolving compound for further biological evaluation.
Supporting Information
Supporting Information File 1
Experimental.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-310-S1.pdf]
Supporting Information File 2
1
H and 13C NMR spectra of all intermediates and the mass
spectrum of RvD2 (1).
http://www.beilstein-journals.org/bjoc/content/
[
supplementary/1860-5397-9-310-S2.pdf]
Acknowledgements
We thank the ARC Discovery Grants scheme and NHMRC
(
1023185) for financial support.
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