A convenient method to produce [14C]carbon monoxide and its application to the radiosynthesis of [carboxyl-14C]celivarone, [carboxyl-14C]SSR149744

Article abstract of DOI:10.1002/jlcr.3009

[carboxyl-¹⁴C]Celivarone was synthesised from barium [ ¹⁴C]carbonate with overall radiochemical yields in the range 49-53%. The synthetic route involves [¹⁴C]carbonylation methodology, which both decreased the number of synthetic steps and increased the yields obtained from previous synthetic routes. [carboxyl-¹⁴C]Celivarone was synthesised from barium [¹⁴C]carbonate with overall radiochemical yields in the range 49-53%. The synthetic route involves [¹⁴C] carbonylation methodology, which both decreased the number of synthetic steps and increased the yields obtained from previous synthetic routes. Copyright

Full text of DOI:10.1002/jlcr.3009

Research Article
Received 27 September 2012,
Revised 9 November 2012,
Accepted 19 November 2012
Published online 22 January 2013 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3009
A convenient method to produce [14C]carbon
monoxide and its application to the
radiosynthesis of [carboxyl-14C]celivarone,
[carboxyl-14C]SSR149744
b,c
a,c
David M. Whitehead,a,c Sascha Hartmann, Tenzeela Ilyas,
*
Keith R. Taylor,a,c Andrew D. Kohler,a,c and George J. Ellamesa,c
[carboxyl-14C]Celivarone was synthesised from barium [14C]carbonate with overall radiochemical yields in the range 49–53%.
The synthetic route involves [14C]carbonylation methodology, which both decreased the number of synthetic steps and
increased the yields obtained from previous synthetic routes.
Keywords: [carboxyl-14C]celivarone; antiarrhythmic; [14C]carbonylation; lithium [14C]formate
(2),8,9 has a strong antiarrhythmic potential.10 It is structurally
Introduction
The generation, transfer and subsequent reaction of radioactive
related to the marketed Amiodarone11 and Dronedarone.12
gases as a means to introduce carbon-14 into target molecules
remain key processes in the development of radiolabelled phar-
maceuticals. We have recently investigated and developed
Results and discussion
Radiosynthesis of [carboxyl-14C]SSR149744 (1) by [14C]cyanation
methods to generate and capture [14C]carbon monoxide from
relatively non-volatile and stable precursors, providing efficient
routes to the radiolabelling of small molecules. Unlabelled
carbon monoxide remains, after many decades of research,
possibly the most versatile C1 building block available, giving
access to a variety of functionalities.1 [14C]Carbon monoxide is
seldom employed in radiochemical syntheses due to difficulties
with its generation, stability, inherent toxicity and containment;
however, recent advances in the generation and the use of
[14C]carbon monoxide have addressed many of these issues.2–6
The use of [11C]carbon monoxide is also increasingly common in
connection with radiosyntheses for positron emission tomography.7
To this end, we have combined relevant research from the
literature with our own radiochemical expertise and developed a
reproducible and robust carbonylation procedure, which is compati-
ble with standard vacuum transfer equipment. The methodology
has been applied to syntheses within our portfolio. Herein, we report
the application of [14C]carbonylation to the radiosynthesis of the anti-
arrhythmic [carboxyl-14C]Celivarone, [carboxyl-14C]SSR149744 (1).
Celivarone, SSR149744C, or 2-butyl-3-{4-[3-(dibutylamino)-
propyl]benzoyl}-1-benzofuran-5-carboxylate fumarate (Figure 1)
The original approach towards the labelling of [carboxyl-14C]
SSR149744 (1) employed an initial cyanation reaction of the aryl
bromide (3) (Scheme 1). This proceeded cleanly in 42% isolated
yield. Subsequent hydrolysis, using a variety of conditions as
indicated, gave only minor conversion to the corresponding
carboxylic acid. The parent nitrile remained mostly intact and
for this reason, the synthesis was quickly discarded.
Radiosynthesis of [carboxyl-14C]SSR149744 (1) by
[
14C]carboxylation
Carboxylation was then pursued as an alternative procedure to
incorporate the 14C-label as outlined in Scheme 2.
Introduction of the radioisotope to afford (4) was achieved by
lithium–halogen exchange of the aryl bromide, followed by
quenching with [14C]carbon dioxide. The [14C]carbon dioxide
aDepartment of Isotope Chemistry and Metabolite Synthesis, Covance Labora-
tories Ltd., Willowburn Avenue, Alnwick, Northumberland, NE66 2JH, UK
bSelcia Radiolabeling, Fyfield Business and Research Park, Fyfield Road, Ongar,
Essex, CM5 0GS, UK
cDepartment of Isotope Chemistry and Metabolite Synthesis, sanofi-aventis,
Willowburn Avenue, Alnwick, Northumberland, NE66 2JH, UK
*Correspondence to: David M. Whitehead, Department of Isotope Chemistry and
Metabolite Synthesis, Covance Laboratories Ltd, Willowburn Avenue, Alnwick,
Northumberland NE66 2JH, UK.
Figure 1. Structure of the potential antiarrythmic Celivarone, SSR149744C.
J. Label Compd. Radiopharm 2013, 56 36–41
E-mail: david.whitehead@covance.com
Copyright © 2013 John Wiley & Sons, Ltd.

D. M. Whitehead et al.
Scheme 1. Attempted radiosynthesis of [carboxyl-14C]SSR149744 (1) by [14C]cyanation.
was generated ex situ, using a standard gas manifold apparatus, manifold by addition of barium [14C]carbonate to concentrated
by the addition of barium [14C]carbonate to concentrated sulfuric acid. The [14C]carbon dioxide produced was subse-
sulfuric acid and subsequent gas transfer under vacuum to the quently gas transferred in vacuo to a second flask and
flask containing the pre-formed frozen lithiated species. The condensed onto a frozen ethereal superhydride solution. This
carboxylic acid (4) was isolated in good yield. Formation of the was subsequently isolated from the manifold system and
isopropyl ester (5) was achieved by reaction of (4) with oxalyl warmed to ꢀ15 ꢁC for 0.5 h before quenching with water to give
chloride and subsequent quenching with propan-2-ol. The radiolabelled lithium [14C]formate in reproducibly quantitative
acid chloride (6) was reacted using Friedel–Crafts conditions radiochemical yield. The impure lithium [14C]formate was
with (5) to afford (7). Reaction of (7) with dibutylamine, in isolated as a white free-flowing amorphous salt following drying
the presence of potassium carbonate and potassium iodide, to constant mass in vacuo over phosphorus pentoxide. Experi-
furnished the title compound [carboxyl-14C]SR149744 (1) in four ence within the group has shown that the residual superhydride
radiochemical steps and 21% overall radiochemical yield.
salts appear to have no detrimental effect on the radiolabelled
formate stability and neither do they affect the subsequent
production of [14C]carbon monoxide. For these reasons, the
lithium [14C]formate, generated for carbonylation use, was not
purified and stored in its crude form at below ꢀ60 ꢁC. A proce-
dure to purify radiolabelled lithium formate by solid-phase
extraction is detailed in the literature.13 This overall process is
summarised in Scheme 3.
Radiosynthesis of [carboxyl-14C]SSR149744 (1) by
[
14C]carbonylation
Subsequently, we considered applying inhouse carbonylation
methodology to the synthesis of [carboxyl-14C]SSR149744 (1).
This methodology is based on a combination of procedures
employed to produce [11C] or [14C]carbon monoxide detailed in
the literature.13,14 [14C]Carbon dioxide was generated on a gas
The dehydration of lithium formate in sulfuric acid generates
carbon monoxide in situ.5,15 This process requires a substantial
Scheme 2. Radiosynthesis of [carboxyl-14C]SSR149744 (1) by [14C]carboxylation.
J. Label Compd. Radiopharm 2013, 56 36–41
Copyright © 2013 John Wiley & Sons, Ltd.
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D. M. Whitehead et al.
Scheme 3. Synthetic routes to [14C]carbon monoxide from [14C]lithium formate.
Scheme 4. Formation of the chlorophosphonium cation complex and its reaction
quantity of sulfuric acid which, once contaminated with 14C,
requires disposal. In our hands, the conversion of lithium [14C]
formate to [14C]carbon monoxide using this dehydration metho-
dology was typically around 50%. Prolonged heating was
required to increase these yields. We therefore searched for an
alternative procedure that would liberate [14C]carbon monoxide
from lithium [14C]formate more efficiently. A method employed
in PET chemistry as described by Roeda et al. uses a chloro-
phosphonium cation to rapidly convert lithium [11C]formate
to [11C]formyl chloride.13 The [11C]formyl chloride readily dissoci-
ates at temperatures exceeding ꢀ60 ꢁC to give [11C]carbon
monoxide. By analogy, we found the addition of lithium [14C]for-
mate to the chlorophosphonium cation efficiently liberated [14C]
carbon monoxide in excellent yield (Table 1), quantified by trap-
ping the radioactivity in an ethereal solution of butyl lithium
(Scheme 4).2,16
with [14C]lithium formate.
potassium iodide, gave the arylhalide (3). Using a gas manifold
apparatus, [14C]carbon monoxide was generated and transferred
in vacuo to a flask containing a frozen solution of (3), [1,10-bis
(diphenylphosphino)ferrocene]dichloropalladium(II) and diiso-
propylethylamine suspended in propan-2-ol. The flask was then
isolated from the gas manifold and heated to 70 ꢁC for a period
of 18 h. Following purification, [carboxyl-14C]SSR149744 (1) was
isolated in 49% (from the brominated precursor) and 53% (from
the iodinated precursor) radiochemical yield from barium [14C]
carbonate (Scheme 5).
Conclusion
A reproducible, robust method to produce lithium [14C]formate
and a procedure to subsequently liberate [14C]carbon monoxide
have been developed. The carbonylation reactions attempted
have been successful with yields comparable to those in the
literature. The methodology was applied to the synthesis of
[carboxyl-14C]Celivarone with overall purified radiochemical yields
in the range 49–53% from barium [14C]carbonate.
In a process analogous to the transfer and isolation of [14C]car-
bon dioxide, [14C]carbon monoxide can be condensed using
liquid nitrogen. We found the transfer of the radioactive monox-
ide gas under vacuum relatively efficient at this temperature.
Following [14C]carbon monoxide generation and transfer, the
carbonylation reaction flask was isolated from the manifold
apparatus and heated typically to 70 ꢁC for a period of 18 h.
Incorporation of [14C]carbon monoxide into target molecules is
routinely around 60%. These results are comparable with unla-
belled carbonylation reactions, which typically employ a large
excess of carbon monoxide in pressurised reaction vessels.
Experimental
Barium [14C]carbonate (2200 MBq/mmol) was purchased from American
Following the reaction, the set up is fully purged under Radiolabeled Chemicals, Inc. All steps were carried out at low specific
activity prior to being repeated at high-specific activity. Unless indicated
otherwise, all other reagents and anhydrous solvents were obtained from
Sigma-Aldrich and used without purification. High-specific activity
materials were characterised by comparative TLC (to known standards),
LC-MS and 1H-NMR. Merck silica radio-TLC (RTLC) plates were analysed
by electronic autoradiography using a Packard Instantimager. LC-MS data
were obtained using an Agilent 1200 HPLC system (Agilent Technologies
UK Ltd., Stockport, Cheshire, UK) connected to a Bruker micro-TOF Focus
mass spectrometer operating in ESI mode with sodium formate as
internal calibrant and a Phenomenex Luna C18 column. RP-HPLC analysis
controlled conditions and any unreacted radioactivity is trapped
using butyl lithium scrubbers2 and quantified. The [14C]carbony-
lation procedure described was applied to the synthesis of a
variety of functionalities as exemplified in Table 2.
Given the aforementioned background, we considered
that this approach might be applied to the radiosynthesis of
[carboxyl-14C]SSR149744 (1). Reaction of either the iodobenzofuran
or bromo-benzofuran with the acid chloride (6) using Friedel–Crafts
conditions afforded intermediate (8). Subsequent reaction of (8)
with dibutylamine, in the presence of potassium carbonate and of (1) was performed using a Gilson 321 series system (Gilson, Inc.,
Table 1. Liberation of 14CO from Li14CO2H using the chlorophosphonium cation complex
Yield of 14CO
Recovery of
Scale mmole
Complex PPh3Cl
Reaction conditions
production (%)
radioactivity (%)
0.8
0.6
3 equiv.
6 equiv.
THF, ꢀ15 ꢁC, 5 min
72
98
96
99
THF, ꢀ15 ꢁC, 5 min
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D. M. Whitehead et al.
Table 2. The [14C]carbonylation procedure was applied to the synthesis of a range of functionalities
Aryl halide
Nucleophile
ROH
RNH2
H2O
ArSnR3
Product
Esters
Amides
Acids
Ketones
Radiochemical yield (%)
Iodoanisole
Iodoanisole
Iodoanisole
Iodoanisole
Aryliodide
40–60
53
61
67
50–60
44
Triethylsilyl hydride
RNH2
Aldehyde
Isoquinolinone
Arylbromide
Scheme 5. Radiosynthesis of [carboxyl-14C]SSR149744 (1) by [14C]carbonylation.
Wisconsin, USA) and using a Waters XTerra RP8 (Waters Ltd., Elstree, (3 mL). The superhydride solution was frozen (liquid nitrogen bath) under
14C]carbonate (273 mg, 3018 MBq, 1.37 mmol,
Hertfordshire, UK) 3.5 mm (150 ꢂ 4.6 mm) column. Samples were nitrogen. Barium
[
dissolved in methanol prior to analysis. The mobile phase is isocratic 2200 MBq/mmol) was transferred to a side-arm and connected to a
comprising 75% acetonitrile/25% sodium tetraborate aqueous buffer second three-necked flask containing a stirrer bar and concentrated
(2.5 mM Na2B4O7.10H2O adjusted to pH 8.6 with 1 M aq. HCl) at a flow sulfuric acid (5 mL). The second flask was connected to the manifold
rate of 1 mL/min. The run time is 35 min and detection is by UV
absorbance at 210 nm. Under these conditions, SSR149744 elutes at a
retention time of ~15 min. Activities were determined by liquid scin-
apparatus and the acid frozen (dry-ice acetone bath). A vacuum
(3.0ꢂ10-2 mbar) was established within the manifold and the two
pendant flasks. The flask containing the sulfuric acid was defrosted
tillation analysis using a Packard Tri-Carb 1900TR (Packard Instrument and the barium [14C]carbonate added portion wise to the acid. The
Company, Meriden, Connecticut, USA, since integrated into PerkinElmer,
14CO2 produced was gas transferred onto the frozen surface of the
Waltham, Massachusetts, USA). All 1H-NMR data presented were ethereal superhydride solution. Following completion of the gas transfer,
recorded on a Bruker DRX 500 NMR (Bruker Biospin Corporation, Billerica, the super hydride flask was isolated from the manifold system and the
Massachusettes, USA) instrument.
solution warmed to ꢀ15ꢁC and stirred for a period of 15min. The superhy-
dride solution was subsequently frozen and purged with nitrogen venting
through two consecutive 150 mL aqueous 1 M sodium hydroxide scrubbers
to trap any unreacted 14CO2. Water (5mL) was added and the super
Lithium [14C]formate
A three-necked round-bottomed flask, attached to a manifold system and hydride solution defrosted. Both flasks were purged for a further
equipped with a stirrer bar, was loaded with lithium triethylborohydride 30 min with nitrogen, again venting through aqueous sodium hydroxide
(2 equiv., 3 mL, 1 M in THF) and diluted with anhydrous tetrahydrofuran
scrubbers. The ether was removed in vacuo and the crude lithium [14C]
J. Label Compd. Radiopharm 2013, 56 36–41
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D. M. Whitehead et al.
formate completely dissolved in water enabling quantification of [14C] (d, 1H, J= 8.5 Hz), 7.31 (d, 2H, J= 8.5 Hz), 7.56 (dd, 1H, J= 8.5, 2.0 Hz), 7.72
by scintillation counting (98%, 2958 MBq, 1.34mmol). The water was (d, 2H, J= 8.5 Hz), 7.76 (d, 1H, J=1.5Hz). LC-MS (positive ion mode) m/z
subsequently removed under reduced pressure to furnish a white found 574.2551 (100%, [M + H+]).
amorphous free-flowing solid, which was dried to constant mass
(282mg), in vacuo over phosphorus pentoxide. The specific activity of
the impure lithium [14C]formate was measured gravimetrically to be
(5-Bromo-2-butylbenzofuran-3-yl)-[4-(3-
dibutylaminopropyl)phenyl]methanone (3b)
10.5MBq/mg. As 1.34 mmol unlabelled lithium formate has a mass of
94 mg the theoretical chemical purity of the lithium [14C]formate is
estimated to be 33%.
Dibutylamine (1.8mL, 10.7mmol) was added to a stirred suspension of
(5-bromo-2-butylbenzofuran-3-yl)-[4-(3-chloropropyl)phenyl]metha-
none (8b) (1.72 g, 4 mmol) and potassium iodide (0.664 , 4 mmol) in
acetonitrile (10mL). The mixture was heated to reflux (95ꢁC) under
nitrogen for a period of 16h. The solvent was removed in vacuo and the
residue suspended in dichloromethane then filtered to remove the
precipitated salts. The filtrate was concentrated under reduced pressure
and the crude material purified by column chromatography on silica gel
(5% ethyl acetate in hexanes) to furnish the title compound (3b) (1.292 g,
61%). dH (CDCl3): 0.87 (t, 3H, J = 7.0Hz), 0.91 (t, 3H, J = 7.0Hz), 1.27–1.44
and 1.69–1.85 (m, 10H), 2.41 (t, 2H, J = 7.0Hz), 2.46 (t, 2H, J = 7.0Hz), 2.73
(t, 2H, J = 7.5Hz), 2.85 (t, 2H, J = 7.5Hz), 7.31 (d, 2H, J= 8.5Hz), 7.34 (d, 1H,
J = 8.5 Hz) 7.37 (dd, 1H, J= 8.5, 2.0Hz), 7.54 (d, 1H, J = 2.0 Hz), 7.73 (d,
2H, J= 8.5Hz). LC-MS (positive ion mode) m/z found 528.2393 (100%,
[M+ H+]).
(2-Butyl-5-iodobenzofuran-3-yl)-[4-(3-chloropropyl)phenyl]
methanone (8a)
A
solution of 4-(3-chloropropyl)benzoic acid (2.35 g, 11.8 mmol
produced in-house) in thionyl chloride (24 mL) was heated to reflux
(100ꢁC, oil bath) for 2h. The thionyl chloride was subsequently removed
in vacuo. The resultant acid chloride was redissolved in anhydrous 1,2-
dichloroethane (20 mL), which was subsequently removed in vacuo. This
process was repeated to ensure that the excess thionyl chloride was
removed. A second flask was loaded with a solution of 2-butyl-5-iodo-
benzofuan (2.30 g, 7.7mmol) in 1,2-dichloroethane (70mL). The solution
was cooled in an ice bath and iron (III) chloride (1.1equiv., 5g, 9.2mmol)
added. The acid chloride (2.50g, 11.5mmol), dissolved in 1,2-dichlor-
oethane (23 mL), was then added dropwise to the cooled iodobenzo-
furan solution over a period of 15 min. The reaction mixture was
warmed to room temperature and stirred for 4 h. Water (30 mL) was
added and the aqueous phase extracted with dichloromethane
(3ꢂ 30 mL). The combined organics were washed further with portions
of water then dried over anhydrous magnesium sulfate. The dichloro-
methane was removed in vacuo. The residue was purified by column
chromatography on silica gel (5% ethyl acetate in hexanes) to afford
8a (2.12 g, 57%). dH (CDCl3): 0.87 (t, 3H, J = 7.4Hz), 1.31 (sextet, 2H,
J = 7.5Hz), 1.72 (quintet, 2H, J = 7.5Hz), 2.15 (quintet, 2H, J = 6.4Hz),
2.84 (t, 2H, J = 7.4Hz), 2.90 (t, 2H, J = 7.3Hz), 3.56 (t, 2H, J= 6.4Hz), 7.25
(d, 1H, J = 8.6 Hz), 7.33 (d, 2H, J= 8.1Hz ), 7.56 (dd, 1H, J = 8.6, 1.8Hz),
[carboxyl-14C]-2-Butyl-3-{4-[3-(dibutylamino)propyl]
benzoyl}-1-benzofuran-5-carboxylate (1)
A three-necked round-bottomed flask was loaded with triphenylphosphine
(3equiv., 660 mg, 2.52 mmol), which was dissolved in anhydrous
tetrahydrofuran (10 mL). The solution was degassed. A second three-necked
round-bottomed flask was loaded with (5-iodo-2-butyl-2,3-dihydrobenzofuran-
3-yl)-[4-(3-dibutylaminopropyl)-phenyl]methanone (3a) (2equiv. 963mg,
1.68 mmol), propan-2-ol (5mL), [1,10-bis(diphenylphosphino)ferrocene]
dichloropalladium(II) (100 mg, 0.12 mmol) and diisopropylethylamine (1 mL)
and subsequently frozen using a liquid nitrogen bath. Perchloroacetone
(1.5 equiv., 191 mL, 1.26 mmol) was added neat to the triphenylphosphine
solution at ꢀ15 ꢁC and stirred under nitrogen for 5 min. The resultant
chlorophosphonium complex was frozen (liquid nitrogen bath) and the
manifold system evacuated to 1 mbar. The chlorophosphonium complex
was warmed to ꢀ15 ꢁC and, with stirring, 1842 MBq lithium [14C]formate
(10.5 MBq/mg, 175 mg, 0.84 mmol for which the estimated purity due
to contamination with superhydride salts is 33%) was added portion
wise. The resultant [14C]formyl chloride dissociated readily producing
14CO. The 14CO was gas transferred to the frozen arylhalide mixture, which
was then isolated from the manifold, defrosted slowly and heated with
stirring to 70 ꢁC for 18 h. Prior to purging the system, both solutions
were frozen using liquid nitrogen baths. Nitrogen was carefully
allowed into the flasks and vented through two in-line butyl lithium
scrubber traps for 60 min whilst the contents of both flasks were
defrosted. Each trap contained 5 mL of 2.5 M butyl lithium in 95 mL
anhydrous THF, cooled to ꢀ78 ꢁC using a dry-ice acetone bath. The
activity recorded in the first trap was 167 MBq, no activity was
recorded in the second trap. The contents of both flasks were diluted
7.72 (d, 1H, J = 1.8Hz), 7.74 (d, 2H, J = 8.2Hz).
LC-MS (positive ion
mode) m/z found 481.0863 (100%, [M + H+]).
(5-Bromo-2-butylbenzofuran-3-yl)-[4-(3-chloropropyl)
phenyl]methanone (8b)
The procedure described for the iodo-derivative (8a) was applied starting
from 5-bromo-2-butylbenzofuran (6.19 g, 24.5 mmol), yielding 8b (3.49 g,
33%). dH (CDCl3): 0.88 (t, 3H, J = 7.4 Hz), 1.33 (sextet, 2H, J = 7.5 Hz), 1.74
(quintet, 2H, J = 7.5 Hz), 2.15 (quintet, 2H, J = 6.4 Hz), 2.86 (t, 2H,
J = 7.4 Hz), 2.90 (t, 2H, J = 7.3 Hz), 3.56 (t, 2H, J = 6.4 Hz), 7.33 (d, 2H,
J = 8.6 Hz), 7.34 (d, 1H, J = 8.1 Hz), 7.38 (dd, 1H, J = 8.6, 1.8 Hz ), 7.51
(d, 1H, J = 2.0 Hz), 7.74 (d, 2H, J = 8.2 Hz). LC-MS (positive ion mode) m/z
found 435.0530 (100%, [M + H+]).
(2-Butyl-5-iodobenzofuran-3-yl)-[4-(3-dibutylaminopropyl)
phenyl]methanone (3a)
Dibutylamine (1.25 mL, 7.4 mmol), sodium iodide (0.10 g, 0.7 mmol) with methanol for quantification by scintillation counting. The crude
and (2-butyl-5-iodobenzofuran-3-yl)-[4-(3-chloropropyl)phenyl]methanone product was counted at 1068 MBq (58%). The total activity recovered
(8a) (0.71 g, 1.5mmol) were combined in acetonitrile (5 mL) in a sealed was 1795 MBq (97%). The solvent was removed in vacuo and the crude
10-mL microwave vessel. The reaction mixture was heated using a residue purified by column chromatography on silica gel (10% ethyl
CEM Discover Microwave Reactor (CEM Microwave Technology Ltd., acetate in hexanes with 1% triethylamine) to give (1) (976 MBq,
Buckingham, UK) (300 W, 150 ꢁC, 150 psi) for 1 h then partitioned between 53%). dH (DMSO-d6): 0.79 (t, 3H, J = 7.2 Hz), 0.87 (t, 6H, J= 7.2 Hz), 1.28
diethyl ether (20 mL) and water (20 mL). The ether was extracted with (d, 6H, J = 7.4 Hz), 1.23 (m, 2H), 1.37 (m, 4H), 1.66 (m, 4H), 1.76
further portions of water (2 ꢂ 20 mL) and dried over anhydrous sodium
(m, 2H), 2.44 (m, 6H), 2.70 (t, 2H, J = 7.2 Hz), 2.80 (t, 2H,
sulfate, the solvent was removed in vacuo and the resultant residue purified J = 7.2 Hz), 5.10 (septet, 1H, J = 7.5 Hz), 7.40 (d, 2H, J = 8.5 Hz), 7.70
by column chromatography on silica gel (5% ethyl acetate in hexanes) to (d, 2H, J = 8.5Hz), 7.74 (dd, 2H, J = 8.5, 2.0 Hz), 7.94 (dd,
yield 3a (0.77 g, 90%). dH (CDCl3): 0.86 (t, 3H, J= 7.0 Hz), 0.91 (t, 3H,
1H, J = 8.5, 2 Hz), 8.10 (d, 1H, J = 2Hz). HPLC RT = 15 min. The product
J= 7.0 Hz), 1.23–1.44 and 1.68–1.85 (m, 10H), 2.40 (t, 2H, J= 7.0 Hz), 2.46 was subsequently isolated as the fumaric acid salt and the specific activity
(t, 2H, J= 7.0 Hz), 2.72 (t, 2H, J= 7.5 Hz), 2.83 (t, 2H, J= 7.5 Hz), 7.24 determined gravimetrically to be 3.424 MBq/mg.
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D. M. Whitehead et al.
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The authors did not report any conflict of interest.
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