An efficient approach to the synthesis of LTB4 and ω-substituted LTB4 metabolites

Article abstract of DOI:10.1016/S0040-4039(02)01206-6

The first total synthesis of the methyl ester of 20-oxo-LTB₄ 26 is described. The key synthon 6 is an advanced new intermediate which has been used in the synthesis of LTB₄ 1, 20-oxo-LTB₄ methyl ester 26, and 20-hydroxy-LTB₄ 2. The synthetic 26 has been used to study the cytosolic aldehyde dehydrogenase-catalyzed oxidation of LTB₄ to its ω-carboxy metabolite.

Full text of DOI:10.1016/S0040-4039(02)01206-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6063–6066
An efficient approach to the synthesis of LTB4 and v-substituted
LTB4 metabolites
Subhash P. Khanapure,a,† Seongjin Kim,a John F. Penrose,b,‡ K. Frank Austen,b William S. Powellc
and Joshua Rokacha,*
aClaude Pepper Institute and Department of Chemistry, Florida Institute of Technology 150 West University Boulevard,
Melbourne, FL 32901, USA
bDivision of Rheumatology and Immunology, Brigham and Women’s Hospital, Department of Medicine, Harvard Medical School,
Boston, MA 02115, USA
cMeakins-Christie Laboratories, Department of Medicine, McGill University, Montreal, Quebec, Canada H2X, 2P2
Received 16 May 2002; revised 17 June 2002; accepted 18 June 2002
Abstract—The first total synthesis of the methyl ester of 20-oxo-LTB4 26 is described. The key synthon 6 is an advanced new
intermediate which has been used in the synthesis of LTB4 1, 20-oxo-LTB4 methyl ester 26, and 20-hydroxy-LTB4 2. The synthetic
26 has been used to study the cytosolic aldehyde dehydrogenase-catalyzed oxidation of LTB4 to its v-carboxy metabolite. © 2002
Elsevier Science Ltd. All rights reserved.
Leukotriene B4 (LTB4) is a product of the lipoxygenase
pathway of arachidonic acid (AA) metabolism, and is
derived from leukotriene A4 (LTA4) via the enzymatic
addition of water at C-12 of LTA4. LTB4 is a mediator
of inflammation and one of the most potent chemotac-
tic agents produced by human polymorphonuclear
leukocytes.1 It has been implicated in a variety of
human inflammatory diseases and allergic reactions.
The proinflammatory activity of LTB4 is mediated by
the BLT1 receptor.2 It induces chemotaxis,3
aggregation1 and the adhesion4 of inflammatory cells,
especially neutrophils, to endothelial cells. Further-
more, LTB4 synergizes with other chemotactic factors,
to amplify the inflammatory response. Recently, a sec-
ond LTB4 receptor (BLT2 receptor) has been identified
and cloned, its expression is broad and highest in liver,
intestine, spleen and kidney.5 Also it has broader ligand
specificity for various eicosanoids such as 12(S)-
hydroxyeicosatetraenoic acid (12-HETE) and 15(S)-
hydroxyeicosatetraenoic acid (15-HETE). Thus,
LTB4-R2 receptor provides a novel target for anti-
inflammatory therapy and should help us expand our
knowledge of LTB4 function.
A substantial amount of synthetic work on the metabo-
lites of LTB4 has been accomplished.6–8 Earlier on we
reported on the synthesis of two v-metabolites of LTB4
2 and 4 made by neutrophils,9 and confirmed10 that
LTB4-20-hydroxylase (P-450LTB) is the cytochrome P-
450 in the microsomes of human polymorphonuclear
leukocytes that catalyze the v-oxidation of LTB4 to
20-oxo-LTB4. v-Oxidation of LTB4 (Scheme 1) in the
neutrophil is critical because it results in the biological
inactivation of this extremely potent chemoattrac-
Scheme 1. Metabolism of C20-LTB4.
* Corresponding author.
† Present address: NitroMed, Inc., 12 Oak Park Drive, Bedford, MA 01730, USA.
‡ Present address: Norwood Hospital, 800 Washington Street, Norwood, MA 02062, USA.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01206-6

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S. P. Khanapure et al. / Tetrahedron Letters 43 (2002) 6063–6066
tant.11–19 Investigation of the steps in the v-oxidation of
LTB4 in the neutrophil will provide clarification on the
balance between the synthesis of LTB4 and its
catabolism. To study the cytosolic aldehyde dehydroge-
nase, which catalyzes the final step in the oxidation of
LTB4 to v-carboxy-LTB4 4 we needed synthetic 20-
oxo-LTB4 3 as well as 20-OH-LTB4 2. Since 3 is
extremely unstable, we decided to use its stable methyl
ester derivative 20-oxo-LTB4 methyl ester 26.
the diol 12 in 93% yield. The lead tetraacetate oxidation
of 12 gave aldehyde 13 in 88% yield. The four carbon
diene units were introduced by performing a Wittig–
Horner reaction using phosphonate 1423 to afford 6 in
83% yield. Synthon 6, which is now the starting point
of many of our LTB4 syntheses, is prepared in multi-
gram scale and is stable for prolonged periods of time
at −20°C.
We are showing in Scheme 4 the detailed synthesis of
26 which has not been reported before. Intermediates 7,
8, and the tetradeutero derivative 9 were prepared
similarly by modifying the Wittig reagent in step b
(Scheme 4). The dithioacetal group in 6 was cleaved
using our recently developed periodic acid method
under anhydrous conditions to yield aldehyde 16 in
87% yield after purification by column chromatography
over silica gel. Aldehyde 16 was reacted with the ylide
generated from phosphonium salt 17 and sodium
bis(trimethylsilyl)amide at −78°C in THF. Flash
column chromatography afforded 18 in 70% yield. The
reduction of ester 18 with DIBAL-H in methylene
chloride at 0°C, followed by aqueous acidic work-up,
gave alcohol 19 in quantitative yield, which was con-
verted to bromide 20 and used in the preparation of
phosphonium salt 21. The Wittig reaction to introduce
the C6ꢀC7 cis double bond was performed using alde-
hyde 2223,24 and sodium hexamethyldisilazide at −98°C
to room temperature to give 23. The minor 6 trans-iso-
mer of 23 (10%, not shown) was easily separated by
flash column chromatography. Note that the high yield
of the cis isomer contrasts with our earlier syntheses of
LTB4 in which the cis:trans ratio is 2:1. The use of the
particular methoxy phosphonium 21 shown here is
responsible for the increase of the cis to trans ratio
from 2:1 to 9:1. This reaction is currently under further
The object of this communication is a new strategy we
developed for a more efficient synthesis of LTB4 deriva-
tives, exemplified by the synthesis of LTB4 1, 20-oxo-
LTB4 methyl ester 3, 20-hydroxy-LTB4 2 (Scheme 2).
The synthesis of 17,17,18,18-tetradeutero-LTB4 10 has
also been approached by the same method.20 Our ear-
lier syntheses of LTB4 were linear, starting from the
v-end of the molecule.21 We were looking for a syn-
thetic strategy which will allow maximal flexibility for
the synthesis of many of the LTB4 analogs planned and
provide a more effective and shortened approach to
LTB4 structural modification (Scheme 2). The advanced
intermediate 6, which will provide the C7ꢀC14 part of
the backbone of the LTB4 was our target and was
prepared first. This new synthetic approach will also
help for the synthesis of LTB4 derivatives for the chem-
ical characterization of the new LTB4 receptor.
The derivatives shown in Scheme 2 were prepared using
the improved synthesis. The synthesis of pivotal syn-
thon 6 was performed as shown in Scheme 3. Com-
pound 5, which contains the would-be 12R-hydroxy of
the target LTB4 derivatives, was prepared as described
previously.21,22 The selective deprotection of acetonide
in 5 was carried out by treatment with trifluoroacetic
acid in tetrahydrofuran at room temperature and gave
Scheme 2. Synthetic approach.
O
O
HO
OH
SEt
HO
O
OH
O
P
CHO
RO
COOMe
RO
RO
RO
OH
MeO
COOMe
b
a
OMe
14
11
L-Arabinose
OH
88%
SEt
SEt
93%
SEt
SEt
SEt
4 steps
c
83%
6
13
5
12
SEt
SEt
R = TBDPS
Scheme 3. Reaction conditions: (a) TFAA, THF, water 50°C 4 days; (b) Pb(OAc)4, Na2CO3, 3 equiv., CH2Cl2 −40 to −30°C 10
min then at −30°C 10 min; (c) phosphonate 14 in THF, cooled to −78°C, LiN(SiMe3)2 added at −78°C, 30 min at −78°C, allowed
to warm to −20°C during 1.5 h then added aldehyde 13 at −20°C, 30 min at −20°C then room temperature overnight.

S. P. Khanapure et al. / Tetrahedron Letters 43 (2002) 6063–6066
6065
Scheme 4. Reaction conditions: (a) ether:THF (4:1) 0°C to room temperature 3 min; (b) phosphonium salt 17 (6 mmol) in THF
(80 mL) cooled to −78°C, n-BuLi (5 mmol), 30 min at −78°C then aldehyde 16, −78°C to room temperature 2 h; (c) DIBAL-H,
2.4 equiv. in CH2Cl2, −60 to −20°C 1 h, then neutralized with cold 10% HCl; (d) DIPHOS, CBr4, CH2Cl2, 0°C 20 min, reaction
mixture filtered through a small silica gel pad and solution of bromide 20 in dry dichloromethane used as such in the next step;
(e) 20 in dry dichloromethane (3-MeOPh)3P room temperature overnight; (f) phosphonium salt 21 in THF cooled to −93°C, 0.95
equiv. NaN(SiMe3)2, 1 min, HMPA 10% added, then aldehyde 22, −93 to −15°C 2 h; (g) TBAF, THF added at 0°C, room
temperature overnight; (h) K2CO3, methanol, 0°C to room temperature 10 min; (i) periodic acid 15, ether:THF (9:1) 0°C 20 min;
(j) PCC/Al2O3/CH2Cl2 room temperature 30 min; (k) ethanethiol, BF3·Et2O room temperature overnight; (l) PPh3 3 equiv.,
acetonitrile reflux 48 h.
investigation and will be described in detail in a future
report. Finally, the pure cis isomer 23 was treated with
tetrabutylammonium fluoride in THF to give the
hydroxy compound 24. The benzoate group was then
removed by treatment with potassium carbonate in
anhydrous methanol and the desired 2625 was prepared
by deblocking of dithioacetal 2526 with periodic acid.27
The analytical HPLC of the crude reaction product
revealed that 26 was obtained in more than 50% yield
from 25. The pure product 26 was isolated in low yield
(14%) after purification by normal phase HPLC.
Grants DK-44730 and HL-69835 (J.R.), AI-31599
(K.F.A.), the NSF for an AMX-360 NMR instrument
(Grant CHE-90-13145), and the MRC of Canada for
Grant MT-6254 (W.S.P.).
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