Improved Method for the Synthesis of Palmitic Acid

Full text of DOI:10.1021/jo9718657

1
348
J . Org. Chem. 1998, 63, 1348-1351
Im p r oved Meth od s for th e Syn th esis of
11 (half-life 20.4 min) is a positron-emitting isotope and
1
1
11
a
C-substituted fatty acid would display predictable
[
ω- C]P a lm itic Acid
1
1
2
kinetics for metabolism of the radiolabel to CO , carbon-
†
‡
11 is the radioisotope of choice. With respect to its
position in the fatty acid structure, a radiolabel placed
at the head of a long-chain fatty acid would be metabo-
lized before the MCAD enzyme is required and, thus,
would provide no useful information. Shorter chain fatty
acids with a headgroup label might circumvent this
problem, but they display poor uptake into the myocar-
dium.6 Therefore, the imaging agent would ideally be a
Eric D. Hostetler, Stephen Fallis,
‡
‡
Timothy J . McCarthy, Michael J . Welch, and
J ohn A. Katzenellenbogen*,†
Department of Chemistry, University of Illinois,
00 South Mathews Avenue, Urbana, Illinois 61801, and
Division of Radiological Sciences,
Washington University School of Medicine,
10 South Kingshighway, St. Louis, Missouri 63110
6
5
long-chain fatty acid labeled at the tail end, such as
Received October 9, 1997
11
[
ω- C]palmitic acid (1). Presented here are improved
methods for the synthesis of 1, using novel tracer-level
techniques.
In tr od u ction
The metabolism of fatty acids provides the major
source of energy for the heart under normal physiological
conditions. This metabolism proceeds via â-oxidation in
two-carbon increments, starting at the carboxyl head and
progressing along the aliphatic tail of the fatty acid.
Each cycle of â-oxidation is catalyzed by a number of
enzymes, including a family of acyl-CoA dehydrogenases,
various members of which are specific for fatty acids of
different chain lengths. A genetic deficiency for the long-
or medium-chain acyl dehydrogenase (LCAD or MCAD)
causes an accumulation of toxic fatty acid metabolic
Resu lts a n d Discu ssion
1
1
Previous methods for the synthesis of [ω- C]palmitic
1
1
acid (1) via incorporation of [ C]CH
3
I possess various
drawbacks.6
,9,10
For example, a previous approach from
this laboratory involves the tedious preparation of a
precursor Grignard reagent that produces large amounts
(ca. 60 mg) of unlabeled nor-acid and utilizes a harsh
intermediates, which are potientially arrythmogenic and
may cause sudden cardiac arrest.1 Currently, there is
6
4
RuO oxidation that generates a number of byproducts.
no clinical method for evaluating cardiac metabolism for
An alternate method described by others uses organo-
cuprate chemistry;10 while mild and general, this ap-
proach requires the use of air-sensitive organometallic
reagents that must be freshly prepared. We sought to
develop a method for the synthesis of 1 that would be
convenient, reliable, and efficient and would produce
minimal amounts of unlabeled byproducts.
or predicting the severity of diseases such as MCAD
deficiency.2
-5
We have previously reported the possibility
of developing a noninvasive technique for the diagnosis
of MCAD deficiency.6 The rate of fatty acid metabolism
could be evaluated by quantification of a positron emis-
sion tomographic (PET) image of the heart generated
upon injection of a suitable radiolabeled fatty acid.7,8 The
rate at which the activity clears from the heart, compared
to an appropriate control, would provide a measure of
the presence and the severity of MCAD deficiency.
Two important issues in developing such a myocardial
imaging agent are the type of radiolabel employed and
its position. Isotopic substitution of a carbon atom would
neither alter the natural structure of the fatty acid nor
interfere with its normal metabolism. Because carbon-
Ishiama et al. have demonstrated the cross-coupling
of a functionalized alkyl borane with a saturated primary
1
1
alkyl iodide via a palladium-catalyzed Suzuki reaction.
Furthermore, this method has been shown to efficiently
1
1
3
cross-couple a saturated alkyl borane with [ C]CH I in
good yields in less than 5 min at the tracer level, where
the palladium catalyst is in large excess.12 The reputed
efficiency of this reaction combined with the stability and
simple preparation of the trialkylborane precursor make
application of the Suzuki coupling an attractive strategy
*
To whom correspondence should be addressed. Tel.: (217) 333-
6
310. Fax: (217) 333-7325. E-mail: jkatzene@uiuc.edu.
for the synthesis of tail-labeled fatty acids through
†
11
University of Illinois.
Washington University School of Medicine.
3
incorporation of [ C]CH I, which can be routinely pro-
‡
duced in large amounts (1.0 Ci) through an automated
on-line synthesis.13 In addition, the mild conditions of
the Suzuki coupling permits the fatty acid to be protected
as a tert-butyl ester, allowing facile deprotection with
trifluoroacetic acid to provide the desired radiolabeled
fatty acid directly (cf., Scheme 1). Alternatively, the acid
(1) Van Hove, J . L. K.; Zhang, W.; Kahler, S. G.; Roe, C. R.; Chem,
Y.-T.; Terada, N.; Chace, D. H.; Iafolia, A. K.; Ding, J .-H.; Millington,
D. S. Am. J . Hum. Genet. 1993, 52, 958.
(
2) Kelly, D. P.; Mendelsohn, N. J .; Sobel, B. E.; Bergmann, S. R.
Am. J . Cardiol. 1993, 71, 738.
3) Andresen, B. S.; Bross, P.; Udvari, S.; Kirk, J .; Gray, G.; Kmoch,
(
S.; Chamoles, N.; Knudsen, I.; Winter, V.; Wilcken, B.; Yokota, I.; Hart,
K. Hum. Mol. Genet. 1997, 6, 695.
(
4) Andresen, B. S.; Bross, P.; J ensen, T. G.; Knudsen, I.; Winter,
V.; Kolvraa, S.; Bolund, L.; Gregersen, N. Scand. J . Clin. Lab. Invest.
995, 55, 9.
5) Bhala, A.; Willi, S. M.; Rinaldo, P.; Bennett, M. J .; Schmidtsom-
merfeld, E.; Hale, D. E. J . Pediatr. 1995, 126, 910.
6) Buckman, B. O.; VanBrocklin, H. F.; Dence, C. D.; Bergmann,
S. R.; Welch, M. J .; Katzenellenbogen, J . A. J . Med. Chem. 1994, 37,
481.
7) Bergmann, S. R.; Weinheimer, C. J .; Markham, J .; Herrero, P.
J . Nucl. Med. 1996, 37, 1723.
8) Schwaiger, M.; Hicks, R. J . Nucl. Med. 1991, 32, 565.
(9) Kihlberg, T.; Malmborg, P.; Långstr o¨ m, B. J . Labeled Comp.
Radiopharm. 1991, 30, 151.
(10) Neu, H.; Kihlberg, T.; Långstr o¨ m, B. J . Labeled Comp. Radio-
pharm. 1997, 39, 509.
(11) Ishiyama, T.; Abe, S.; Miyaura, N.; Suzuki, A. Chem. Lett. 1992,
691.
(12) Andersson, Y.; Cheng, A.; Långstr o¨ m, B. Acta Chem. Scand.
1995, 49, 683.
1
(
(
2
(
(13) Larsen, P.; Ulin, J .; Dahlstrom, K.; J ensen, M. Appl. Radiat.
Isot. 1997, 48, 153.
(
S0022-3263(97)01865-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/28/1998

Notes
J . Org. Chem., Vol. 63, No. 4, 1998 1349
parts.15 With this convenient, portable source of ozone,
ozonolysis can be performed on millimole quantities of
substrate in reasonable times (ca. 1 mmol/10 min). This
translates to nearly instantaneous oxidation on the very
small scale (several mg) that is typically used with
radiochemical syntheses. The compact nature of this
device makes it suitable to fit in the confined area of a
shielded hot-cell.
Sch em e 1
To prepare the ester precursor needed for the synthesis
1
1
of [ω- C]palmitic acid (1) by the Suzuki coupling method,
we used commercially available, inexpensive ω-pentade-
calactone (2) (Scheme 1). Ring opening of lactone 2 with
NaOMe afforded the ω-hydroxy methyl ester 3 in high
yield. Conversion of the hydroxy ester 3 to ω-bromo ester
4
3
was achieved using PPh /NBS. Treatment of the bromo
methyl ester 4 with an excess of t-BuOK resulted in HBr
elimination to provide the terminal alkene with concomi-
tant transesterification to give the ω-unsaturated tert-
Sch em e 2
butyl ester 5. Synthesis of the furan precursor (Scheme
6
2
) required only treatment of (ω-bromoalkyl)furan 8 with
t-BuOK to afford terminal alkene 9. The same (ω-
bromoalkyl)furan (8) was used to prepare the precursor
1
1
for the synthesis of [ω- C]palmitic acid via the sulfone.
One-pot conversion (Scheme 3) of bromide 8 to the iodide
followed by displacement of the iodide with sodium
benzenesulfinate furnished sulfone 12.
Treatment of the terminal alkene 5 (Scheme 1) or 9
(Scheme 2) with a slight excess of 9-BBN provided the
required substrate for the radiochemical synthesis of 1.
The resulting trialkylborane 6 or 10, respectively, was
not isolated but rather stored as a stock solution in THF.
Sch em e 3
1
1
[
C]Methyl iodide produced from the automated synthe-
1
3
3 4
sis was collected in 1 mL of THF at 0 °C, and Pd(PPh ) ,
an aliquot of trialkylborane 6 or 10, and NaOH were,
added in succession. Alternate palladium ligands (P(o-
tolyl)
and NaOH was determined to be a superior base to
PO . The reaction mixture was heated at reflux for 4
min, giving incorporation of [ C]CH
tinely between 65 and 75% for both the furan and ester
substrates. Rapid elution of the reaction mixture through
a short column of silica gel affords the Suzuki-coupled
product 7 or 11, respectively, in >99% radiochemical
purity.
3 3 3 4
, AsPh , dppf) were not as effective as Pd(PPh ) ,
K
3
4
1
1
3
I with yields rou-
may be masked as a furan and then revealed by oxidative
cleavage (cf., Scheme 2).
Alkylation of a highly reactive phenyl sulfone dianion14
provides an alternate pathway for the incorporation of
1
1
[
C]CH
3
I. This strategy would require an additional step
to reductively cleave the sulfone following alkylation with
1
1
Deprotection of the ω-labeled ester 7 with neat TFA
at 90 °C for 1 min affords the desired radiopharmaceu-
tical 1 in quantitative radiochemical yield (RCY) and
[
C]methyl iodide, and the process is only compatible
with the acid masked as the furan (cf., Scheme 3).
However, the reductive cleavage can potentially be
combined with the alkylation in a one-pot reaction.
An alternative to the use of the aggressive oxidant
>99% radiochemical purity (RCP), as determined by
HPLC. Likewise, ozonolysis of furan 11 for 2 min using
the pocket ozonolyzer, followed by treatment with per-
acetic acid, provided 1 in >90% RCY and RCP. The
chemical purity of 1 produced by both approaches is also
high (see Experimental Section). The final product is
efficiently synthesized in less than 45 min from ester 6
and in under 55 min from furan 10, from the end of target
bombardment. Preferably, a radiochemical synthesis
should take no longer than 3 half-lives of the radioisotope
being used. These syntheses are within this ideal for
carbon-11 (half-life 20.4 min).
4
RuO for the oxidative cleavage of the furan to the
6
carboxylic acid is highly desirable and could be applied
to both methods. Because of the short half-life of carbon-
1
1, a simple and rapid workup and purification of all
intermediates is required. We envisioned using ozone
for this purpose. Convenient ozone production in a
manner compatible with the facilities of a hospital will
ultimately be required if this radiopharmaceutical (1) is
to be prepared for clinical use by either of the two routes
that involve furan ozonolysis. We sought an alternative
to the large, immobile ozone generators commonly uti-
lized in chemistry laboratories. Thus, we have used a
(15) Beroza, M.; Bierl, B. A. Mikrochim. Acta (Wein) 1969, 720. The
“
pocket ozonolyzer”, which can be conveniently assembled
metal tip of a vacuum leak tester (Tesla coil) is fitted with a glass
sheath containing an inlet for oxygen and an outlet to the reaction.
The sheath is wrapped with aluminum foil, grounded by a copper wire
lead, and insulated with rubber tubing. Ozone is then produced by
turning on the vacuum leak tester and commencing oxygen flow.
from a high-voltage vacuum leak finder and a few simple
(14) Kozikowski, A. P.; Li, C.-S. J . Org. Chem. 1987, 52, 3541.

1
350 J . Org. Chem., Vol. 63, No. 4, 1998
Notes
Application of the sulfone strategy for the synthesis of
ω- C]palmitic acid (1) required treatment of sulfone 12
find further use in radiochemical as well as traditional
organic synthesis.
1
1
[
(
Scheme 3) with BuLi to form the desired geminal
1
1
dianion. [ C]Methyl iodide in THF was added, and
analysis of a quenched aliquot after 2 min showed a
radiochemical incorporation yield of 66%. The anion was
Exp er im en ta l Section
Gen er a l Meth od s. 1H and 13C NMR spectra were obtained
at 400 or 500 MHz. Mass spectra were obtained by one of the
following ionization techniques: fast atom bombardment (FAB),
electron impact (EI), or chemical ionization (CI). Melting points
are uncorrected. HPLC for the radiochemical experiments was
performed on a chromatograph equipped with either a UV
quenched with NH
to reductively cleave the sulfone. After stirring at reflux
for 10 min, quenching with NH Cl afforded alkylfuran
1 in an overall radiochemical yield of 51%. The use of
4 2
Cl, and HMPA and SmI were added
4
1
(
(
operating at 215 nm) or a refractive index detector and a NaI-
Tl) radioactivity detector. All solvents were dried and distilled
a sulfone dianion to incorporate the radiolabel is not as
efficient as the Suzuki coupling, because of the extra step
required for sulfone cleavage. However, the ability to
perform the cleavage in the same reaction vessel as the
alkylation facilitates the use of this strategy. Ozonolysis
2
under N prior to use. Flash chromatography was performed
with Woelm silica gel (0.040-0.063 mm). All reactions except
the Suzuki couplings were performed under a nitrogen atmo-
sphere. Radiolabeled products were analyzed by comparison of
retention times with a standard unlabeled compound via coin-
jection on HPLC (0.46 cm × 25 cm C8 analytical column, 85%
1
1
of furan 11 as before afforded [ω- C]palmitic acid (1) in
0% RCY and >90% RCP in 65 min from end of target
9
3 2
CH CN/15% H O/0.1% TFA, 2 mL/min flow). All radiochemical
bombardment. Material prepared by this approach does
contain some unlabeled nor acid, pentadecanoic acid (see
the Experimental Section).
yields are decay corrected.
Alkylboranes 6 and 10 were prepared according to standard
literature procedure11 and were stored at 0 °C as solutions in
THF. The general workup procedure was as follows: Following
the reaction quench, the aqueous layer was extracted with ether,
and the combined organic layers were washed with water and
We have described three new approaches to the syn-
1
1
thesis of [ω- C]palmitic acid (1) from precursors 6, 10,
and 12 in overall decay corrected yields of 75%, 68%, and
4 4
brine and dried over MgSO . The MgSO was removed by
5
1% in 45, 55, and 65 min, respectively, from the end of
filtration, and the filtrate was evaporated under reduced pres-
sure to afford the crude product.
Meth yl 15-Hyd r oxyp en ta d eca n oa te (3). Sodium metal
target bombardment. Both the alkyl Suzuki and the
sulfone dianion-based strategies present distinct advan-
tages when compared to other methods. The sulfone
strategy does not require tedious preparation of air-
sensitive reagents in advance, rather only the addition
(1.4 g, 62 mmol) was added to MeOH (75 mL) at 0 °C with
stirring. The mixture was warmed to room temperature and
stirred until all of the sodium was consumed. ω-Pentadecalac-
tone (2) (3.0 g, 12.5 mmol) was added with stirring, and the
solution was stirred at 80 °C for 3 h. The reaction was quenched
with 1 N HCl (100 mL) and diluted with water (100 mL).
Workup afforded crude 3. Flash chromatography on silica gel
1
1
3
of BuLi minutes before addition of [ C]CH I. More
impressively, the Suzuki coupling utilizes a stable tri-
alkylborane precursor and requires no air-sensitive tech-
niques to incorporate the radiolabel. This reaction does
not rely on a rigorously exact quantity of any reagent,
as does the sulfone method, which requires 2 equiv of
BuLi. Robust preparative methods are especially im-
portant for a radiopharmaceutical synthesis, which must
be highly reproducible. Both reactions operate well when
relatively small amounts of precursor are used, and the
unreacted trialkylborane from the Suzuki coupling is
easily separated by flash chromatography during product
isolation. Both of these factors are critical in minimizing
the quantity of nonradiolabeled byproducts that are
formed in these sequences. Removal of unalkylated
compound created in the sulfone route (i.e., pentadecanoic
acid) would require preparative HPLC, however.
(
2:1 hexanes:EtOAc) afforded 2.98 g (88%) of pure 3 as a white
1
solid: mp 47.0-48.0 °C; H NMR (400 MHz, CDCl
1
2H), 3.58 (t, J ) 7 Hz, 2H), 3.62 (s, 3H); MS (FAB) 273 (M
H, 100). Anal. Calcd for C16
C, 70.20; H, 11.58.
3
) δ 1.20-
.35 (bs, 20H), 1.52 (m, 2H), 1.57 (m, 2H), 2.26 (t, J ) 7 Hz,
+
+
32 3
H O : C, 70.54; H, 11.84. Found:
Meth yl 15-Br om op en ta d eca n oa te (4). To a solution of
alcohol 3 (795 mg, 2.92 mmol) and PPh (1.53 g, 5.83 mmol) in
mL of DMF was added NBS (1.04 g, 5.84 mmol) in portions.
3
5
The reaction was heated at 55 °C for 30 min. Methanol (5 mL)
was added followed by 1 N HCl (50 mL). Workup gave a crude
pink solid. Hexanes were added to dissolve the product, leaving
behind Ph
3
PdO. Flash chromatography on silica gel (20:1
hexanes:EtOAc) afforded 850 mg (87%) of pure bromide 4 as a
1
white solid: mp 38.0-39.0 °C; H NMR (400 MHz, CDCl
.20-1.35 (bs, 18H), 1.38 (m, 2H), 1.58 (m, 2H), 1.82 (m, 2H),
2.26 (t, J ) 7 Hz, 2H), 3.36 (t, J ) 7 Hz, 2H), 3.62 (s, 3H); MS
3
) δ
1
+
81
+ 79
(
FAB) 337 (M ( Br) + H, 69), 335 (M ( Br) + H, 80). Anal.
Calcd for C16 Br: C, 57.33; H, 9.32. Found: C, 57.59; H,
.44.
ter t-Bu tyl 14-P en ta d ecen oa te (5). To a 1.0 M solution of
Although the Suzuki method is more convenient and
efficient than the sulfone route, the sulfone dianion
approach can be used for the incorporation of longer chain
radiolabeled alkyl halides such as [1- C]CH
whereas we have so far been unable to use the Suzuki
reaction for the incorporation of [1- C]CH
lished results).16 Thus, the sulfone dianion approach
would allow the synthesis of [(ω-1)- C]palmitic acid,
whose PET image of the heart would provide an interest-
ing comparison with the image created by uptake of
31 2
H O
9
1
1
3
CH
2
I,
t-BuOK in THF (10 mL) was added bromide 4 (710 mg, 2.12
mmol). After the solution was stirred at room temperature for
1 h, 1 N HCl (50 mL) was added. Workup afforded crude alkene
5. Flash chromatography on silica gel (hexanes) afforded 660
1
1
3 2
CH I (unpub-
mg (75%) of pure 5 as a colorless oil: 1H NMR (400 MHz, CDCl
)
3
1
1
δ 1.20-1.35 (bs, 18H), 1.37 (m, 2H), 1.43 (s, 9H), 2.03 (m, 2H),
2
.19 (t, J ) 7 Hz, 2H), 4.90 (ddt, J ) 10.2, 2.2, 1.3 Hz, 1H), 4.97
(ddt, J ) 17.2, 3.6, 1.6 Hz, 1H), 5.80 (ddt, J ) 17.1, 10.5, 6.6,
1
1
1H); 13C NMR (400 MHz, CDCl ) δ 25.0, 27.9, 28.8, 28.9, 29.0
[ω- C]palmitic acid, due to the differing metabolic fate
3
1
1
29.2, 29.3, 29.4 × 2, 29.5 × 2, 33.7, 35.4, 79.8, 114.0, 139.1, 173.3;
of the ω-1 carbon (ends as [1- C]acetate) vs the ω carbon
+
MS (CI) 241 (M - isobutylene, 100). Anal. Calcd for
1
1
17
(ends as [2- C]acetate). Finally, a convenient method
19 36 2
C H O : C, 76.97; H, 12.24. Found: C, 77.10; H, 12.37.
for producing ozone has been demonstrated and should
5
-Meth yl-2-(tetr a d ec-13-en -1-yl)fu r a n (9). To a solution
6
of bromide 8 (200 mg, 0.560 mmol) in THF (5 mL) at 0 °C was
added t-BuOK (1.68 mL of 1.0 M in THF, 1.68 mmol). After the
solution was stirred at room temperature for 1 h, 1 N HCl (50
mL) was added. Workup afforded crude alkene 9. Flash
chromatography on silica gel (hexanes) afforded 136 mg (88%)
(
16) Hostetler, E. D.; Fallis, S.; McCarthy, T. J .; Dence, C. S.; Welch,
M. J .; Katzenellenbogen, J . A., unpublished results.
17) Stryer, L., Ed. Biochemistry, 4th ed.; W. H. Freeman and Co.:
New York, 1995; pp 606-612.
(

Notes
J . Org. Chem., Vol. 63, No. 4, 1998 1351
of pure 9 as a colorless oil: 1H NMR (500 MHz, CDCl
) δ 1.20-
Via F u r a n 10. Substituting furan 10 for ester 6, the
procedure follows the synthesis of 1 via 6 through the point of
the silica gel column elution. After the solvent was removed,
3
1
.35 (bs, 18H), 1.61 (m, 2H), 2.04 (m, 2H), 2.26 (s, 3H), 2.56 (t,
J ) 7 Hz, 2H), 4.90 (ddt, J ) 10.2, 2.2, 1.3 Hz, 1H), 4.97 (ddt, J
)
17.2, 3.6, 1.6 Hz, 1H), 5.80 (ddt, J ) 17.1, 10.5, 6.6, 1H), 5.84
2 2
CH Cl (10 mL) was added and the solution was rapidly cooled
1
3
15
(
s, 2H); C NMR (500 MHz, CDCl
3
) 13.5, 28.1, 28.2, 29.0, 29.2
to 0 °C with a dry ice-acetone bath. Ozone was bubbled
through the solution for 2 min, followed by addition of 30%
peracetic acid. The solvent was removed in vacuo over a warm
water bath (60 °C). Water (1 mL) and ether (1 mL) were added,
and the organic layer was analyzed by HPLC. The HPLC traces
for both the radioactivity and mass detectors were identical with
those observed for production of 1 via 6, indicating very high
chemical purity.
(
2 C). 29.4, 29.5, 29.6 (4 C), 33.8, 105.1, 105.7, 114.1, 139.3, 150.0,
+
1
54.8; MS (EI) 276 (M , 8), 95 (100). Anal. Calcd for C19
C, 82.54; H, 11.67. Found: C, 82.65; H, 11.74.
-Meth yl-2-[14-(ph en ylsu lfon yl)tetr adecyl]fu r an (12). To
H32O:
5
6
a solution of bromide 8 (500 mg, 1.40 mmol) in acetone (5 mL)
was added NaI (231 mg, 1.54 mmol). The solution was stirred
for 1 h, after which time DMF (5 mL) and sodium benzenesulfi-
nate (276 mg, 1.68 mmol) were added. The reaction was heated
to 60 °C and stirred overnight. The reaction was quenched by
addition of 1 N HCl (50 mL). Workup afforded crude sulfone
Via Su lfon e 12. To a solution of 12 (15 mg, 35.8 µmol) and
DMPU (0.1 mL) in THF (0.5 mL) at 0 °C in a 35 mL round-
bottomed flask was added n-BuLi (0.059 mL of 1.22 M in
hexanes, 71.7 µmol). The bright yellow solution was stirred for
1
2. Recrystallization from hexanes afforded 450 mg (77%) of
1
1
1
5 min, after which a mixture of [ C]CH
THF (1.0 mL) was added. The ice bath was removed, and the
reaction was stirred for 2 min. Saturated NH Cl (5 µL) was
added, followed by HMPA (300 µL) and SmI (15 mL of 0.1 M in
THF, 1.5 mmol) via cannula. The purple solution was heated
at 90 °C for 10 min, after which time saturated NH Cl (100 µL)
3 3
I and CH I (0.5 µL) in
pure 12: mp 71.0-72.0 °C; H NMR (400 MHz, CDCl
.40 (bs, 20H), 1.58 (m, 2H), 1.69 (m, 2H), 2.24 (s, 3H), 2.54 (t,
J ) 7 Hz, 2H), 3.07 (m, 2H), 5.82 (s, 2H), 7.56 (m, 2H), 7.63 (m,
3
) δ 1.20-
1
4
+
2
1
H), 7.90 (m, 2H); MS (EI) 418 (M , 9), 95 (100). Anal. Calcd
for C25
H
38
O
3
S: C, 71.73; H, 9.15. Found: C, 71.36; H, 8.90.
4
[1
1
C]Meth yl Iod id e. [ C]Carbon dioxide was produced by
11
was added. The solvent was evaporated, and the residue was
loaded onto a silica gel column (column diameter, 1 cm; column
1
4
11
11
3
the N(p,R) C nuclear reaction and was converted to [ C]CH I
using the PETtrace MeI Microlab (GE Medical Systems, Upp-
sala, Sweden) based on the technique described by Larsen et
height, 7 cm). The column was eluted with 5% Et
(
(
2
O/pentanes
10 mL), the collected eluant was evaporated in vacuo, CH Cl
10 mL) was added, and the solution was rapidly cooled to 0 °C
2
2
1
3
11
al. [ C]Methyl iodide produced in this way was trapped in
THF (1.0 mL) at 0 °C.
with a dry ice-acetone bath. Ozone was bubbled through the
solution for 2 min followed by addition of 30% peracetic acid.
The solvent was removed in vacuo over a warm water bath.
Water (1 mL) and ether (1 mL) were added, and the organic
layer was analyzed by HPLC. A single radioactive peak was
recorded with a retention time identical with that of palmitic
acid. When a UV detector was used in series with the radioac-
tive detector, mass peaks were observed corresponding to
[
ω-11C]P a lm itic Acid (1). Via Ester 6. To a THF solution
11
(
300 µL) of [ C]CH
vessel were added one crystal of Pd(PPh
.075 M, 11.3 µmol), and NaOH (10 µL, 5 M). The reaction
3
I at room temperature in a glass reaction
3 4
)
, ester 6 (150 µL of
0
vessel was capped and heated at 90 °C for 4 min and then cooled
in an ice bath. The solvent was removed in vacuo, and the
residue was loaded onto a silica gel column (column diameter,
1
cm; column height, 8 cm). The column was eluted with 5%
palmitic acid (resulting from the use of carrier CH I) and
3
pentadecanoic acid (resulting from unalkylated starting mater-
ial).
Et O/pentanes (10 mL), and the collected eluant was evaporated
2
in vacuo. Trifluoroacetic acid (200 µL) was added, and the
mixture was heated at 90 °C for 1 min. Water (1 mL) and ether
Ack n ow led gm en t. We wish to thank Bill Margenau
and Dave Ficke for radioisotope production and Keith
Lechner for analytical assistance. This work was sup-
ported by grants from the National Institutes of Health
(1 mL) were added, and the organic layer was analyzed by
HPLC. A single radioactive peak was recorded with a retention
time identical with that of palmitic acid. When a refractive
index detector was used in series with the radioactive detector,
no corresponding mass peaks were observed near the time
window of the radioactive peak, indicating very high chemical
purity.
(R01 CA25863 to J .A.K. and P01 HL13851 to M.J .W.).
J O9718657