Total Synthesis and Biological Activity of the Arachidonic Acid Metabolite Hemiketal E2

Article abstract of DOI:10.1021/acs.orglett.8b01578

The total synthesis of hemiketal E₂ (HKE₂) has been accomplished using a gold(I)-mediated cycloisomerization followed by oxidation of the enol ether product to introduce a unique keto-hemiketal, the core structure of HKE₂.

Full text of DOI:10.1021/acs.orglett.8b01578

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Total Synthesis and Biological Activity of the Arachidonic Acid
Metabolite Hemiketal E2
†
§
§
†
§
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Robert E. Boer, Juan Antonio Gimenez-Bastida, Olivier Boutaud, Somnath Jana, Claus Schneider,
and Gary A. Sulikowski*,†,§
†Department of Chemistry, Vanderbilt University, Vanderbilt Institute of Chemical Biology Nashville, Tennessee 37232, United
States
§Department of Pharmacology, Vanderbilt University, Vanderbilt Institute of Chemical Biology Nashville, Tennessee 37232, United
States
S
* Supporting Information
ABSTRACT: The total synthesis of hemiketal E2 (HKE2) has been accomplished using
a gold(I)-mediated cycloisomerization followed by oxidation of the enol ether product to
introduce a unique keto-hemiketal, the core structure of HKE2. Synthetic hemiketal E2
reproduced biosynthetically derived HKE2 in the inhibition of human platelet
aggregation.
icosanoids are produced by enzymatic oxidation of
arachidonic acid (AA) by cyclooxygenases (COX-1,
E
COX-2), lipoxygenases (LOX), and cytochromes P450 in
response to a variety of cellular stimuli such as hormones,
stress, and cytokines.1 These lipid mediators regulate a variety
of biological responses and pathological processes such as
inflammation, cancer, asthma, and autoimmune diseases.2
Studies on eicosanoid biosynthesis, metabolism, and function
have been extensive. The biosynthetic pathways leading to
prostaglandins and leukotrienes are generally believed to
diverge at the point of the initial enzymatic oxygenation of
arachidonic acid by cyclooxygenases and lipoxygenases,
respectively.3 However, in 2011, Schneider and co-workers
described the biosynthesis of eicosanoids HKD2 (3) and HKE2
(4) as products of the consecutive oxygenation of arachidonic
acid by 5-lipoxygenase and cyclooxygenase-2 (AA → 1 → 2 →
3/4, Figure 1).4
Figure 1. Structures of 5S-HETE (1), diendoperoxide (2), hemiketal
E2 (3), and hemiketal D2 (4).\.
D2/E2 are produced enzymatically in small quantities starting
from 5S-HETE and recombinant COX-2.7 Described herein is
the first total synthesis of hemiketal E2 (3).
The discovery of HKD2 and HKE2 resulted from the
hypothesis that 5S-HETE (1), an oxidation product of 5-LOX
and arachidonic acid, may serve as a substrate of the COX
enzyme. In vitro experiments demonstrated 5S-HETE to be
converted by COX-2 (but not COX-1), affording diendoper-
oxide 2 as the enzymatic product.5 The latter underwent
rearrangement to HKD2 and HKE2 in a process reminiscent of
the well-known transformation of intermediate endoperoxide
PGH2 to PGD2 and PGE2; thus, it is the designated
nomenclature for these unusual hemiketals.4 The possible
relevance of the crossover pathway to living systems was
further demonstrated by their ability to induce tubulogenesis
of endothelial cells4 and their biosynthesis following
stimulation of 5-LOX and COX-2 in human leukocytes ex
vivo using lipopolysaccharide (LPS).6 Further study of HKD2
and HKE2 requires their total synthesis, as currently hemiketals
Hemiketals D2 and E2 share common side chains that
incorporate secondary alcohols of (S)-configuration of
reversed attachment to the heterocyclic core (cf. Figure 2,
Rα/Rω). A second difference between the two structures is the
configuration of C9 (9S-HKD2) and C11 (11R-HKE2) as
reflected in the cis and trans relationships between C8−C9
(HKD2) and C11−C12 (HKE2), respectively (3/4, Figure 1).
Based on this structural analysis, we proceeded with a synthetic
strategy whereby the absolute stereochemistry of 9S (HKD2)
and 11R (HKE2) would be introduced in 2-alkoxy(siloxy)-
Received: May 18, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01578
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Our synthesis started with racemic glycidol 7, as derivatives
of glycidol are readily resolved into single enantiomeric
products using the Jacobsen hydrolytic−kinetic resolution.10
For example, PMB ether 8 was readily resolved into (R)-8
using the (S,S)-Co(salen) catalyst.11 After epoxide opening
with trimethylsilyl acetylide, the resulting secondary alcohol
(9)12 was protected as a TBS ether in anticipation of a Felkin−
Anh directed addition to aldehyde 13. Aldehyde 13 was
obtained from 10 by way of the three-step deprotection-
oxidation sequence as shown in Scheme 1. Next, treatment of
known vinyl iodide 14,13 with t-butyllithium afforded an
intermediate vinyllithium reagent that was reacted with
dimethylzinc leading to a nonbasic vinyl zincate which was
added to aldehyde 13 to give allylic alcohol 15 in 61% yield
(>95:5 diastereoselectivity). Sonogashira coupling of acetylene
15 with vinyl iodide 1614 gave enyne 17 in 68% yield.
Figure 2. Synthetic strategy to access hemiketals D2 and E2.
aldehydes (cf. 5, Figure 2) and relative stereochemistry (C8−
C9 and C11−C12) would be established in a vinyl metal
addition to 5 directed by the adjacent stereocenter (OP) by
way of either a Felkin−Anh (2-siloxyaldehyde) or chelation-
controlled (2-alkoxyaldehyde) addition.8 The cyclic hemiketal
would be introduced by a two-step process starting with a
metal-mediated cycloisomerization followed by oxidation of
the intermediate enol ether (Figure 2).9 We chose as our first
objective the total synthesis of hemiketal E2 (Scheme 1).
Either a 6-endo or 5-exo-dig mode of cyclization of alkynol
17 was projected to afford the desired hemiketal product
following oxidation of either the pyran or furan product,
respectively.9 After various conditions were examined, it was
determined that gold(I) chloride in tetrahydrofuran proved
most efficient to afford the 5-exo product, furan 18, in 75%
yield.15 While furan 18 could be purified using buffered silica
gel and characterized by NMR analysis, it had a finite lifetime
in benzene-d6 of less than 24 h. Typically, 18 was immediately
epoxidized using dimethyl dioxirane following isolation. The
epoxide product was not isolated, but instead, diol 19, a
product of hydrolysis, was obtained and subsequently oxidized
using IBX in DMSO to give hemiketal 20 in 62% yield from
enol ether 18. Removal of TBS protecting groups was effected
using HF-pyridine in acetonitrile to provide methyl ester 21.
Saponification of ester 21 provided hemiketal E2 (3), identical
Scheme 1. Total Synthesis of HKE2
7
1
by H NMR to HKE2 derived enzymatically.
Hemiketal E2 inhibited human platelet aggregation induced
by the thromboxane receptor agonist U46,619 in a dose-
dependent manner (IC50 < 500 nM). Inhibition was more
potent in platelets isolated from individuals in whom PGE2 at
100 nM concentration was unable to inhibit aggregation.16
Chemically synthesized and enzymatically prepared HKE2 (1
μM) were equally effective at inhibiting platelet aggregation
(Figure 3).
Figure 3. HKE2 inhibits platelet aggregation. Human platelet-rich
plasma was preincubated with vehicle (v) or HKE2 (1 μM) obtained
from chemical synthesis (A) or enzymatic synthesis (B) for 10 min at
37 °C before stimulation of aggregation by the thromboxane receptor
agonist U46,619.17
Unfortunately, efforts directed toward the total synthesis of
hemiketal D2 starting from 9S-5 (P = Bn) proved
unproductive. Problems encountered in effecting the chela-
tion-controlled carbonyl addition included low stereoselectivity
and narrow scope of tolerated functionality in the vinyl metal
reagent (5 to 6, Figure 2). Access to both hemiketals will
B
DOI: 10.1021/acs.orglett.8b01578
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(14) Prepared from methyl 4-(chloroformyl)butyrate in six steps:
(a) Gotz, K.; Liermann, J. C.; Thines, E.; Anke, H.; Opatz, T. Org.
Biomol. Chem. 2010, 8, 2123. (b) Treilhou, M.; Fauve, A.; Pougny, J.
R.; Prome, J. C.; Veschambre, H. J. Org. Chem. 1992, 57, 3203.
(c) Chemin, D.; Linstrumelle, G. Tetrahedron 1992, 48, 1943−1952.
(d) Rodriguez, A. R.; Spur, B. W. Tetrahedron Lett. 2012, 53, 86−89.
(15) (a) Review: Alcaide, B.; Almendros, P.; Alonso, J. M. Org.
Biomol. Chem. 2011, 9, 4405. (b) Harkat, H.; Weibel, J.-M.; Pale, P.
Tetrahedron Lett. 2007, 48, 1439.
(16) Smith, J. P.; Haddad, E. V.; Downey, J. D.; Breyer, R. M.;
Boutaud, O. Arteriosclerosis Thrombosis and Vascular Biology 2010, 30,
E219.
(17) Data are presented as mean SD from four healthy volunteers.
Symbols show differences between the different groups; ***, p <
0.001.
provide opportunity for further biological study of these
interesting and unusual eicosanoids.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01578.
General experimental procedures, characterization data,
and 1H and 13C NMR spectra of new compounds
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: gary.a.sulikowski@vanderbilt.edu.
ORCID
Claus Schneider: 0000-0003-4215-967X
Gary A. Sulikowski: 0000-0002-1067-0767
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Institutes of
Health (5R01 GM115722 to G.A.S. and 5R01 GM076592 to
C.S.). J.A.G.-B. was supported by a postdoctoral award from
the American Heart Association (16POST30690001).
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DOI: 10.1021/acs.orglett.8b01578
Org. Lett. XXXX, XXX, XXX−XXX