Convenient syntheses of [20,20,20-2H3]-arachidonic acid and [20,20,-2H2]-20-hydroxyeicosatetraenoic acid

Article abstract of DOI:10.1002/jlcr.1039

Deuterated arachidonic acid and 20-HETE were prepared in good overall yields and high stereoselectivities. Key transformations include a trans-specific vinyl dibromide reduction and Suzuki cross-couplings to a lithium borate or a 9-BBN borane. These stand

Full text of DOI:10.1002/jlcr.1039

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2006; 49: 245–252.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.1039
Research Article
Convenient syntheses of [20,20,20-2H3]-arachidonic
acid and [20,20,-2H2]-20-hydroxyeicosatetraenoic acid
J. R. Falck1,*, Siddam Anjaiah1, Venugopal Raju Tuniki1, V. Raj Gopal1
and Jorge H. Capdevila2
1 Department of Biochemistry, University of Texas Southwestern Medical Center,
Dallas, TX 75390, USA
2 Departments of Medicine and Biochemistry, Vanderbilt University School of Medicine,
Nashville, TN 37232, USA
Summary
Deuterated arachidonic acid and 20-HETE were prepared in good overall yields and
high stereoselectivities. Key transformations include a trans-specific vinyl dibromide
reduction and Suzuki cross-couplings to a lithium borate or a 9-BBN borane. These
standards are three and two mass units higher, respectively, than their naturally
occurring counterparts and are useful in mass spectrometry analysis. Copyright #
2006 John Wiley & Sons, Ltd.
Key Words: 2H-labeled standards; Z-vinyl bromide; Suzuki cross-coupling; stereo-
selective
Introduction
Upon release from phospholipid stores, arachidonic acid (AA) is rapidly
oxidized to a plethora of structurally diverse autacoids, inter alia, prostanoids
(PGs), leukotrienes (LTs), epoxyeicosatrienoic acids (EETs), and 20-hydro-
xyeicosatetraenoic acid (20-HETE, also known as 20-OH-AA).1 The latter, a
metabolite of the cytochrome P450 branch of the eicosanoid cascade,2 is a
potent vasoconstrictor, angiogenic factor, and proinflammatory agent.3–5
Mass spectrometry (MS) has been used extensively to quantify AA6 and
20-HETE7–9 and to correlate their levels with normo-10 and pathophysiology11–13
as well as their further metabolic conversions.14–16 The deuterium labeled
standards typically used in such studies are prepared by semi-hydrogenation of
acetylenic intermediates17,18 or by Wittig olefinations.19,20 The complications
*Correspondence to: J. R. Falck, Department of Biochemistry, University of Texas Southwestern Medical
Center, Dallas, TX 75390, USA. E-mail: j.falck@UTSouthwestern.edu
Copyright # 2006 John Wiley & Sons, Ltd.
Received 21 November 2005
Revised 26 November 2005
Accepted 28 November 2005

246
J. R. FALCK ET AL.
associated with these procedures, e.g. label scrambling, over or incomplete
reduction, positional isomerization, and moderate stereoselectivity, generally
necessitate tedious HPLC purification of the products.21 Herein, we describe
an efficient, highly stereoselective route to AA-2H3 ð1Þ and 20-HETE-2H2 ð2Þ
(Figure 1) utilizing inexpensive, commercially available deuterated precursors.
In both cases, the modest level and location of the isotope substitution
minimizes chromatographic differentiation with respect to natural material.22
Results and discussion
Our strategy, summarized in Scheme 1, commenced with the readily available
methyl 14,15-dihydroxyeicosatrienoate23 ð3Þ which was cleaved with lead
tetraacetate at low temperature and the resultant aldehyde was immediately
converted via Corey-Fuchs olefination24 to vinyl dibromide 4 in good overall
yield (Scheme 1). Selective trans-reduction using n-Bu3SnH/Pd(Ph3P)4 led
exclusively to Z-vinyl bromide 5, the key intermediate. The stereochemistry of
the D14,15-olefin in 5 was confirmed by the 7 Hz coupling at 6.17 ppm in the
1H NMR. Subsequent Suzuki cross-coupling of 5 with lithium borate 10 and
borane 13 furnished all-cis 6 and 7, respectively. Free alcohol 8 was smoothly
generated from the 7 via n-Bu4NF desilylation in good yield. Finally,
saponification of 6 and 8 afforded free acids 1 and 2, respectively.
Figure 1. Chemical structures of deuterated arachidonic acid and 20-HETE
Scheme 1. Reagents and conditions: (a) Pb(OAc)4, CH2Cl2; (b) CBr4/PPh3; (c)
n-Bu3SnH/Pd(PPh3)4, C6H6; (d) 10, Pd(dppf)Cl2, K3PO4, DMF; (e) 13,
Pd(dppf)Cl2, AsPh3, CsCO3, DMF; (f) n-Bu4NF, THF
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CONVENIENT SYNTHESES OF DEUTERATED ARACHIDONIC ACID AND 20-HETE 247
Scheme 2. Reagents and conditions: (a) NaI, H3CCOCH3; (b) t-BuLi, 9-MeO-
9-BBN
Scheme 3. Reagents and conditions: (a) LiAl2H4; (b) t-BuPh2SiCl; (c) 9-BBN
To access lithium borate 10 (Scheme 2), commercial [5,5,5-2H3]-1-
bromopentane ð9Þ was first converted to the corresponding iodide, metallated
with t-BuLi, and then added to 9-MeO-9-BBN.
Borane 13 was prepared by silylation of the known alcohol25 obtained via
LiAl2H4 reduction of ethyl 4-pentenoate ð11Þ (Scheme 3). Silyl ether 12 was
then subjected to hydroboration with 9-BBN.
Conclusions
Efficient and readily scalable syntheses of deuterated arachidonic acid and
20-HETE were achieved in high isotopic and positional purity. These analogs
are three and two mass units higher, respectively, than their naturally
occurring counterparts and are useful standards for mass spectrometry
analysis.
Experimental
Methyl 15,15-dibromopentadeca-5(Z),8(Z),11(Z),14(Z)-tetraenoate ð4Þ
Lead tetraacetate (1.385 g, 3.125 mmol) was added portionwise over 5 min to
a ꢀ208C solution of methyl 14,15-dihydroxyeicoa-5(Z),8(Z),11(Z)-trienoate23
ð3Þ (1.00 g, 2.841 mmol) in anhydrous dichloromethane (40 ml) under an argon
atmosphere. After 30 min, the reaction mixture was filtered through a small
pad of silica gel (70–230 mesh) and the pad was washed with another portion
of dichloromethane (40 ml). The combined organic filtrate was evaporated
in vacuo and the residue was purified by SiO2 flash column chromatography
using 40% ethyl acetate in hexanes as eluent to give methyl 14-oxotetradeca-
5(Z),8(Z),11(Z)-trienoate (0.672 g, 94%) as a labile, colorless oil that was used
immediately in the next reaction without further purification. 1H NMR
(CDCl3, 250 MHz): d 1.60–1.78 (m, 2H), 2.00–2.15 (m, 2H) 2.30 (t, J ¼ 7:4 Hz,
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J. R. FALCK ET AL.
2H), 2.70–2.85 (m, 4H), 3.18–3.28 (m, 2H), 3.65 (s, 3H), 5.28–5.75 (m, 6H),
9.65 (t, J ¼ 1:8 Hz, 1H).
Triphenylphosphine (2.82 g, 10.75 mmol) and carbon tetrabromide (1.963 g,
5.913 mmol) were stirred together at 08C under an argon atmosphere in
anhydrous dichloromethane (50 ml) for 15 min.24 Into this was cannulated a
solution of the above aldehyde (0.672 g, 2.688 mmol) in dichloromethane
(10 ml). After another 2 h, all volatiles were evaporated and the residue was
purified by SiO2 flash column chromatography using 3% ether in hexanes to
give dibromide 4 (0.870 g, 80%) as a colorless oil. 1H NMR (CDCl3,
400 MHz): d 6.36 (t, J ¼ 7:3 Hz, 1H), 5.54–5.30 (m, 6H), 3.67 (s, 3H),
2.92–2.78 (m, 6H), 2.32 (t, J ¼ 7:3 Hz, 2H), 2.18–2.06 (m, 2H), 1.71 (quintet,
J ¼ 7:3 Hz, 2H); 13C NMR (CDCl3, 75 MHz): 24.89, 25.78, 25.92, 26.71,
31.54, 33.55, 51.62, 89.50, 124.31, 127.63, 128.76, 128.82, 129.19, 130.45,
136.68, 174.11.
Methyl 15-bromopentadeca-5(Z),8(Z),11(Z),14(Z)-tetraenoate ð5Þ
The reduction was initiated by addition of the first quarter-portion of
tetrakis(triphenylphosphine)palladium(0) (0.494 g, 0.428 mmol) to a solution
of dibromide 4 (0.870 g, 2.143 mmol) and tributyltin hydride (738 ml,
2.786 mmol) in anhydrous benzene (30 ml) under an argon atmosphere.
Thereafter, another quarter-portion was added every 4 h. After a total of 16 h,
water (10 ml) was added and the mixture was extracted with ether (3 ꢁ 30 ml).
The combined ethereal extracts were washed with water (2 ꢁ 20 ml), brine
(15 ml), dried over Na2SO4, and concentrated in vacuo. The residue was
purified by SiO2 flash column chromatography using 0.5–1.0% ether in
1
hexanes to furnish Z-bromide 5 (0.538 g, 77%) as a colorless oil. H NMR
(CDCl3, 400 MHz): d 6.17 (dt, J ¼ 1:5, 7.0 Hz, 1H), 6.07 (apparent q,
J ¼ 7:0 Hz, 1H), 5.50–5.32 (m, 6H), 3.66 (s, 3H), 2.98 (t, J ¼ 6:4 Hz, 2H), 2.86
(t, J ¼ 6:1 Hz, 2H), 2.81 (t, J ¼ 5:4 Hz, 2H), 2.32 (t, J ¼ 7:3 Hz, 2H),
2.18–2.06 (m, 2H), 1.70 (quintet, J ¼ 7:3 Hz, 2H); 13C NMR (CDCl3,
75 MHz): 24.83, 25.70, 25.84, 26.63, 28.38, 33.46, 51.49, 108.05, 125.54,
127.83, 128.48, 128.85, 129.09, 129.75, 133.02, 173.96.
Methyl [20,20,20-2H3]-arachidonate ð6Þ
A mixture of [5,5,5-2H3]-1-bromopentane ð9Þ (250 mg, 1.62 mmol; methyl-D3
98%, Cambridge Isotopes Laboratories) and NaI (500 mg, 3.33 mmol) were
refluxed for 4 h in dry acetone (10 ml), then cooled to 08C, filtered, and the
filter cake was rinsed with pentane (10 ml). The combined filtrates were
carefully evaporated under reduced pressure and the crude [5,5,5-2H3]-1-
1
iodopentane was used directly in the next step (270 mg, 83%). H NMR
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CONVENIENT SYNTHESES OF DEUTERATED ARACHIDONIC ACID AND 20-HETE 249
(CDCl3, 300 MHz): d 1.30–1.40 (m, 4H), 1.82 (q, J ¼ 7:0 Hz, 2H), 3.18 (t, J =
7:0 Hz, 2H).
t-BuLi (0.82 ml, 1.7 M solution in pentane, 1.07 mmol) was added dropwise
to a ꢀ788C solution of the above [5,5,5-2H3]-1-iodopentane (100 mg,
0.49 mmol) in ether (5 ml). After 3 min, 9-MeO-9-BBN (1.58 ml, 1 M solution
in hexanes, 1.24 mmol) was added followed by THF (5 ml).26 The reaction
mixture was stirred for 10 min at ꢀ788C, brought to room temperature, and
stirred for 1 h. To this reaction mixture was added aqueous 3 M K3PO4 (2 ml).
After 5 min, the crude mixture of lithium borate 10 was used immediately in
the next step.
The solution of lithium borate 10 from above was cannulated into a stirring
mixture of vinyl bromide 5 (55 mg, 0.17 mmol), [1,10-bis(diphenylphosphino)-
ferrocene]palladium(II) chloride [Pd(dppf)Cl2] (13 mg, 0.015 mmol), and AsPh3
(10 mg, 0.032 mmol) in DMF (5 ml). The resultant dark red, heterogeneous
mixture was degassed by sparging with argon for 10 min and then stirred
overnight at RT. The reaction mixture was diluted with ether (100 ml), washed
with water, brine, dried over Na2SO4, and concentrated in vacuo. Purification
of the residue by SiO2 flash chromatography using 2% EtOAc/hexanes
1
provided 6 (40 mg, 74%) as a colorless oil. H NMR (CDCl3, 300 MHz): d
1.24–1.37 (m, 6H), 1.64–1.76 (m, 2H), 2.01–2.14 (m, 4H), 2.31 (t, J ¼ 7:6 Hz,
2H), 2.78–2.85 (m, 6H), 3.56 (s, 3H), 5.25–5.42 (m, 8H); 13C NMR (CDCl3,
75 MHz): d 22.44, 24.90, 25.75, 26.68, 27.35, 29.47, 31.56, 33.57, 51.61, 127.66,
127.77, 128.29, 128.33, 128.70, 129.00, 129.05, 130.63, 174.21.
[20,20,20-2H3]-Arachidonic acid ð1Þ
LiOH (0.4 ml, 1 M solution in water, 0.37 mmol) was added to a 08C solution
of methyl ester 6 (40 mg, 0.12 mmol) in THF/H2O (5 ml, 4:1) under an argon
atmosphere. After 12 h at RT, the reaction mixture was cooled to 08C,
acidified to pH 4.5 using 1 M aqueous oxalic acid, and extracted with EtOAc
(2 ꢁ 25 ml). The combined organic extracts were washed with water, brine,
dried over Na2SO4, and concentrated in vacuo. Purification of the residue by
SiO2 flash chromatography using 50% EtOAc/hexanes provided 1 (33 mg,
1
88%). H NMR (CDCl3, 300 MHz): d 1.27–1.37 (m, 6H), 1.64–1.76 (m, 2H),
2.03–2.11 (m, 4H), 2.31 (t, J ¼ 7:3 Hz, 2H), 2.78–2.84 (m, 6H), 5.30–5.41
(m, 8H); HRMS (CI, CH4) calculated for C20H30D3O2 (M+1) 308.2669,
found 308.2667.
Methyl [20,20-2H2]-20-(tert-butyldiphenylsilyloxy)eicosa-5(Z),8(Z),11(Z),
14(Z)-tetraenoate ð7Þ
Ethyl 4-pentenoate ð11Þ (500 mg, 3.9 mmol) in ether (5 ml) was added to a
stirring, 08C suspension of LiAl2H4 (81 mg, 1.9 mmol; 98 atom % D, Aldrich
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J. R. FALCK ET AL.
Chem. Co.) in ether (10 ml) under an argon atmosphere. After 45 min, the
reaction mixture was quenched with a saturated aqueous Na2SO4 solution
(5 ml) and then stirred for 30 min at room temperature. The ether layer was
decanted and the aqueous layer was extracted with ether (2 ꢁ 25 ml). The
combined ethereal extracts were washed with water, brine, dried over Na2SO4,
and concentrated in vacuo. The crude [1,1-2H2]-4-penten-1-ol25 (302 mg, 88%)
was used in the next step without further purification.
t-BuPh2SiCl (1.4 g, 5.1 mmol) in CH2Cl2 (5 ml) was added to a 08C mixture
of the above 4-penten-1-ol (300 mg, 3.4 mmol) and imidazole (694 mg,
10.21 mmol) in CH2Cl2 (15 ml) under an argon atmosphere. After 4 h at RT,
the reaction mixture was filtered and the filter cake was rinsed with CH2Cl2
(10 ml). The combined filtrates were concentrated in vacuo and the residue was
purified by SiO2 flash chromatography using 5% EtOAc/hexanes to afford
1
silyl ether 12 (1.04 g, 94%). H NMR (CDCl3, 300 MHz): d 1.04 (s, 9H), 1.64
(t, J ¼ 7:6 Hz, 2H), 2.15 (q, J ¼ 7:0 Hz, 2H), 4.91–5.02 (m, 2H), 5.73–5.86
(m, 1H), 7.34–7.44 (m, 6H), 7.65–7.68 (m, 4H).
9-BBN (4.5 ml, 0.5 M solution in THF, 2.23 mmol) was added to a stirring,
room temperature solution of 1-(tert-butyldiphenylsilyloxy)pent-4-ene ð12Þ
(594 mg, 1.81 mmol) in THF (5 ml) under an argon atmosphere. After 2 h, the
excess 9-BBN was quenched with water (2 ml). Without purification, the crude
solution of borane 13 was cannulated into a mixture of vinyl bromide 5 (490 mg,
1.51 mmol),
[1,10-bis(diphenylphosphino)ferrocene]palladium(II)
chloride
(123 mg, 0.15 mmol), AsPh3 (92 mg, 0.30 mmol), and CsCO3 (788 mg, 2.42 mmol)
in DMF (5 ml). The reaction mixture was degassed by sparging with argon for
10 min and then allowed to stir overnight at room temperature. Following
dilution with ether (100 ml), the reaction mixture was washed with water, brine,
dried over Na2SO4, and concentrated in vacuo. The residue was purified by SiO2
flash chromatography using 5% EtOAc/hexanes to give 7 (603 mg, 69%) as a
1
colorless oil. H NMR (CDCl3, 300 MHz): d 1.04 (s, 9H), 1.33–1.36 (m, 4H),
1.52–1.55 (m, 2H), 1.67–1.72 (m, 2H), 2.03–2.11 (m, 4H), 2.31 (t, J ¼ 7:6 Hz,
2H), 2.78–2.84 (m, 6H), 3.65 (s, 3H), 5.32–5.42 (m, 8H), 7.36–7.40 (m, 6H),
7.64–7.67 (m, 4H); 13C NMR (CDCl3, 75 MHz): d 19.25, 24.82, 25.49, 25.69,
25.71, 25.73, 26.60, 26.94, 27.29, 29.45, 32.33, 33.39, 51.39, 61.51, 127.65, 127.74,
127.94, 128.21, 128.56, 128.93, 128.95, 129.56, 130.29, 134.15, 135.59, 174.23.
Methyl [20,20-2H2]-20-hydroxyeicosa-5(Z),8(Z),11(Z),14(Z)-tetraenoate ð8Þ
n-Bu4NF (1 ml, 1 M solution in THF, 1.03 mmol) was added to a 08C solution
of methyl ester 7 (540 mg, 0.94 mmol) in THF (10 ml) under an argon
atmosphere. After 3 h at RT, the reaction mixture was diluted with saturated
aqueous NH4Cl (10 ml) and extracted with ether (3 ꢁ 50 ml). The combined
ethereal extracts were washed with water, brine, dried over Na2SO4, and
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CONVENIENT SYNTHESES OF DEUTERATED ARACHIDONIC ACID AND 20-HETE 251
concentrated in vacuo. Purification of the residue by passage through a pad of
silica gel using 30% EtOAc/hexanes provided 8 (261 mg, 82%) as a colorless
1
oil. H NMR (CDCl3, 300 MHz): d 1.27–1.37 (m, 6H), 1.64–1.76 (m, 2H),
2.03–2.11 (m, 4H), 2.31 (t, J ¼ 7:6 Hz, 2H), 2.78–2.84 (m, 6H), 3.66 (s, 3H),
5.30–5.41 (m, 8H); 13C NMR (CDCl3, 75 MHz): d 24.84, 25.47, 25.69, 25.72,
26.62, 27.29, 29.53, 32.54, 33.51, 51.59, 62.21, 127.89, 128.01, 128.23, 128.25,
128.55, 128.96, 129.00, 130.21, 174.21.
[20,20-2H2]-20-Hydroxyeicosa-5(Z),8(Z),11(Z),14(Z)-tetraenoic acid ð2Þ
LiOH (0.5 ml, 1 M solution in water, 0.45 mmol) was added to a 08C solution
of 8 (53 mg, 0.15 mmol) in THF/H2O (5 ml, 4:1) under an argon atmosphere.
After 12 h at RT, the reaction mixture was cooled to at 08C, acidified to pH 4.5
using 1 M aqueous oxalic acid, and extracted with EtOAc (2 ꢁ 25 ml). The
combined organic extracts were washed with water, brine, dried over Na2SO4,
and concentrated in vacuo. The residue was passed through a short pad of
silica gel using 50% EtOAc/hexanes to give 2 (41 mg, 81%). 1H NMR (CDCl3,
300 MHz): d 1.36–1.42 (m, 4H), 1.55 (t, J ¼ 6:7 Hz, 2H), 1.64–1.76 (m, 2H),
2.05–2.16 (m, 2H), 2.34 (t, J ¼ 7:3 Hz, 2H), 5.32–5.45 (m, 8H); HRMS (CI,
CH4) calculated for C20H31D2O3 (M+1) 323.2555, found 323.2556.
Acknowledgements
Financial support provide by the Robert A. Welch Foundation and NIH
(GM31278, DK38226).
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