Preparation of [3H]fluoroethyl tosylate and its use in the labelling of the dopamine transporter radioligand [3H]FE-PE2I

Article abstract of DOI:10.1002/jlcr.3012

[³H]Fluoroethyl tosylate, a novel alkylating tritium labelling agent, was synthesized from tritium gas with high specific activity and with 99% radiochemical purity. [³H]Fluoroethyl tosylate was applied in the tritium labelling of the dopamine transporter radioligand [³H]FE- PE2I. Copyright

Full text of DOI:10.1002/jlcr.3012

Special Issue Article
Received 04 October 2012,
Revised 19 November 2012,
Accepted 23 November 2012
Published online in 21 January 2013 Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3012
Preparation of [3H]fluoroethyl tosylate and its
use in the labelling of the dopamine
transporter radioligand [3H]FE-PE2I†
b
a
Alison R Cochrane,a,b William John Kerr, and Johan Sandell
*
[3H]Fluoroethyl tosylate, a novel alkylating tritium labelling agent, was synthesized from tritium gas with high specific activity
and with 99% radiochemical purity. [3H]Fluoroethyl tosylate was applied in the tritium labelling of the dopamine transporter
radioligand [3H]FE-PE2I.
Keywords: [3H]fluoroethyl tosylate; [3H]FE-PE21; dopamine transporter; PET radioligand
The preparation of the precursor 5 was achieved according to
Introduction
(Scheme 1). Condensation of tosyl chloride with 2,2,2-trifluoroethanol
Fluoroalkyl groups, for example, fluoroethyl and fluoropropyl,
gave 2,2,2-trifluoroethyl tosylate (3), and subsequent elimina-
allow for straightforward incorporation of fluorine-18 in a
tion of 3 in the presence of n-BuLi delivered 2,2-difluorovinyl
molecule.1 These F-18-labelled reagents are typically incorporated
into the target molecule by addition of [18F]fluoroalkyl halides or
tosylates to a nucleophile. The requisite [18F]fluoroalkyl halides or
tosylates can be synthesized in a one-step radiosynthesis starting
from nucleophilic fluorine-18 and are suitable to be produced
by automated synthesizer units. Therefore, fluoroalkyl groups
are desirable structural motifs in positron emission tomography
(PET) radioligands.
tosylate (4) in high yield.5 Subsequent reduction with LiAlH4
furnished the monofluorinated species 5 in a moderate yield.4
A number of hydrogenation catalysts were screened in the
reduction of 5 to [2H]1 under an atmosphere of deuterium gas
(Scheme 1, Table 1). The reaction was monitored by the ultraviolet
(UV) trace on analytical high-performance liquid chromatography
(HPLC), indicating the extent of conversion of the starting
material to product. The values reported refer to the relative values
of UV-areas of [2H]1 and 5 are not corrected for differences in
response factors. The use of palladium(II) oxide, 5% palladium on
carbon in THF or Wilkinson’s catalyst in CH2Cl2 failed to produce
any of the desired product. The reaction employing the iridium-
For cross-species comparative bridging and pre-clinical valida-
tion studies of PET radioligands, compounds labelled with high
specific activity tritium may offer an advantage over those labelled
with fluorine-18. The long half-life of tritium allows for storage of
the compound over an increased period of time, and the
low energy particle emission allows for higher resolution in vitro
imaging than the higher energy positron from F-18 or C-11. A
tritium-labelled precursor such as a nonvolatile [3H]fluoroalkyl
tosylate would offer a convenient direct route to the correspond-
ing tritiated PET radioligand. The location of the tritium label in
the fluoroethyl portion of the molecule allows for better mimicking
of the PET tracer in vivo as the tritium label and the PET label will be
located in the same portion of the molecule and thus produce
similar radiolabeled metabolites.
6
based catalyst [Ir(COD)(PMe2Ph)(IMes)]PF6 in CH2Cl2 resulted in
approximately 5% of [2H]1, whereas a significantly improved
yield of 25% was observed in the presence of the iridium-
based Crabtree’s catalyst, [Ir(COD)(PCy3)(py)]PF6. Employing higher
loadings of Crabtree’s catalyst of 25 and 50 mol% did not further
improve the conversion rate.
aUniversity of Strathclyde, Department of Pure and Applied Chemistry, Glasgow,
United Kingdom
This paper describes the synthesis of [3H]fluoroethyl tosylate
and its application in the labelling of the dopamine transporter
radioligand [3H]FE-PE2I.2,3 This was needed for high resolution
in vitro autoradiographic translational bridging studies prior to
clinical PET studies with [18F]FE-PE2I.
bUniversity of Strathclyde, Pure and Applied Chemistry, 295 Cathedral Street,
Glasgow, Scotland G1 1XL, United Kingdom
cAstraZeneca R&D - Isotope Chemistry, Screening and Profiling Global DMPK IM,
Södertälje, Sweden
*Correspondence to: Johan Sandell, Novandi Chemistry AB, Holmgårdsvägen 27,
SE-141 33 Huddinge, Sweden.
E-mail: Johan.Sandell@novandi.se
Results and discussion
The synthetic route to [3H]fluoroethyl tosylate ([3H]1) was envisaged
to proceed via reduction using tritium gas of previously described
2-fluorovinyl tosylate (5).4 The potential incorporation of two
tritium atoms was expected to provide [3H]fluoroethyl tosylate
([3H]1) with high specific activity.
† This article is published in Journal of Labelled Compounds and Radiopharma-
ceuticals as a special issue on IIS 2012 Heidelberg Conference,edited by Jens Atz-
rodt and Volker Derdau, Isotope Chemistry and Metabolite Synthesis, DSAR-DD,
Sanofi-Aventis Deutschland GmbH, Industriepark Höchst G876, 65926 Frankfurt
am Main, Germany.
J. Label Compd. Radiopharm 2013, 56 447–450
Copyright © 2012 John Wiley & Sons, Ltd.

A. R. Cochrane et al.
F
HO
F
F
O
S
O
S
O
S
NEt3, CH2Cl2
Cl
n-BuLi, THF
93%
O
F
F
O
F
F
87%
O
O
O
F
2
3
4
52%
O
LiAlH4, EtO2
Catalyst, Solvent
2H2
O
S
O
2H
O
S
O
O
2H
F
F
[2H]1
5
Scheme 1. Synthesis of (E)-2-fluorovinyl 4-methylbenzenesulfonate (5) and reduction with deuterium to (E)-2-fluoroethyl 4-methylbenzenesulfonate ([2H]1).
1.6 TBq/mmol and a 98% radiochemical purity. It was found that
Table 1. Screening of hydrogenation catalysts
a high reaction temperature was needed for an acceptable yield.
However, labelling of the PE2I analogue [3H]FE-b-CIT with [3H]1
Catalyst
using conventional heating and the corresponding secondary
amine as precursor resulted in almost complete conversion,
which illustrates a good reactivity of [3H]1 (data not shown).
[3H]7 was stored in absolute ethanol at À18ꢀC at a concentration
of 52 MBq/mL. After 4 months at these storage conditions, the
radiochemical purity had decreased to 96%.
Catalyst
loading (mol%) Solvent % Conversion
PdO
5%Pd/C
Rh(PPh3)3Cl
[Ir(COD)(PMe2Ph)
(IMes)]PF6
Crabtree’s catalyst
Crabtree’s catalyst
Crabtree’s catalyst
70
80
10
10
THF
THF
CH2Cl2
CH2Cl2
0
0
0
5
The specific activity of [3H]1 was not determined directly
because of weak signal strength on mass spectrometry (MS).
However, the specific activity of the carrier free [3H]FE-PE2I should
be directly transferrable to [3H]1 and indicates that, on average, 1.5
tritium atoms were introduced by [3H]1. This was reproducible and
measured from two different batches of [3H]1. Nuclear magnetic
resonance (NMR) analysis of [3H]1 revealed that besides fully
labelled, corresponding to two tritiums, and a small amount of
nonlabelled [3H]1, it was also labelled with one tritium, evenly
distributed over the CHT-O and CHT-F groups. This was in accor-
dance with the MS data on the labelled radioligand.
In conclusion, the preparation of the novel radiolabelling
agent [3H]fluoroethyl tosylate has been achieved with high
specific activity. Its usefulness has been demonstrated by the
preparation of the PET radioligand FE-PE2I in its tritiated form.
The described method should be extendable to the simple
preparation of similar high specific activity alkyl tosylates with
tritium gas. Furthermore, as has previously been demonstrated
with [3H]methyl nosylate and [3H]methyl tosylate, the stability
and nonvolatility allows the solvent to be changed and the
reagent to be dispensed in precise quantities.7
10
25
50
CH2Cl2
CH2Cl2
CH2Cl2
25
25
19
The reaction conditions established of 10 mol% of Crabtree’s
catalyst were applied to the reduction of 5 with tritium gas on
a tritium gas manifold system (Scheme 2). The reaction
proceeded without incident, and after purification on normal
phase HPLC, the desired product [3H]1 was obtained with a
radiochemical purity of >99% and in a 42% isolated chemical
yield. [3H]1 was stored either in the HPLC eluent (5% ethyl
acetate in heptanes) or in toluene. After 6 months storage
at À18ꢀC and at a concentration of 240 MBq/mL, the radiochemical
purity was still >99%.
Prior to its application in the labelling of FE-PE2I, an aliquot
of [3H]1 was dispensed, and the solvent was removed by a
stream of nitrogen gas at ambient temperature. The residue
was redissolved in a small volume of CH3CN, which was the
solvent selected for the subsequent labelling reaction.
As depicted in (Scheme 3), treatment of acid precursor 6 with
[3H]1 in MeCN, under microwave heating, furnished the desired
[3H]FE-PE2I ([3H]7) in an 11% isolated radiochemical yield after
normal phase HPLC purification with a specific activity of
Experimental
General methods
All solvents used were of analytical grade and commercially available.
Anhydrous solvents were routinely used for reactions. Reactions were typi-
cally run under an inert atmosphere of nitrogen or argon. Tritium gas was
handled in a tritium gas manifold system (RC TRITEC AG, Teufen Switzerland).
The identities of the final products were established by coelution via HPLC or
thin-layer chromatography (TLC) with authentic nonlabelled material.
Preparative chromatography was run on a Gilson 305 system (Agilent
Technologies Sweden, Kista, Sweden) with a Gilson UV/VIS-151 and a
RAYTEST Ramona detector (Raytest Nordic AB, Höllviken, Sweden).
O
S
O
S
10 mol% Crabtree´s catalyst
3H2, CH2Cl2, 1h, rt
3H
F
O
O
O
O
3H
42%
F
5
[3H]1
Scheme 2. Reduction of (E)-2-fluorovinyl 4-methylbenzenesulfonate (5) to [3H]2-
fluoroethyl 4-methylbenzenesulfonate ([3H]1).
Gas chromatography mass spectrometry were recorded on
GC/Direct Inlet Probe–MS system supplied by Agilent Technologies,
a
www.jlcr.org
Copyright © 2012 John Wiley & Sons, Ltd. J. Label Compd. Radiopharm 2013, 56 447–450

A. R. Cochrane et al.
3H
O
F
I
I
O
O
3H
[3H]1, 5M NaOH, CH3CN
MW 120 °C 30min
11%
OH
N
N
[3H]FE-PE2I, [3H]7
6
Scheme 3. Tritium labelling of [3H]FE-PE2I ([3H]7) with [3H]2-fluoroethyl 4-methylbenzenesulfonate ([3H]1). MW, molecular weight.
consisting of a GC 6890N, G1530N, a G2614A Autosampler, G2613A injector
and a G2589N mass spectrometer. The mass spectrometer was equipped
with an electron impact (EI) ion source. The column used was a DB-5 MS,
inner diameter (ID) 0.18 mm  10 m, 0.18 mm (J&W Scientific). When intro-
duced by GC, a linear temperature gradient was applied starting at 90ꢀC
(hold 0.5min), gradient 40ꢀC/min, ending at 300ꢀC (hold 0.5 min).
3H spectra were recorded on a Bruker DRX600 NMR Spectrometer
(Bruker BioSpin Scandinavia AB, Solna, Sweden), operating at 640 MHz
for tritium and at 600 MHz for proton, equipped with a 5-mm 3H/1H
SEX probe head with Z-gradients. 1H decoupled 3H spectra were
recorded on samples dissolved in CD3OD. For 3H-NMR spectra referen-
cing, a ghost reference was used, as calculated by multiplying the
frequency of internal tetramethylsilane in a 1H spectrum with the Larmor
frequency ratio between 3H and 1H (1.06663975).8 1H-NMR spectra were
recorded on a Bruker Avance III 500 spectrometer.
2,2-Difluorovinyl tosylate (4)
To a solution of 2,2,2-trifluoroethyl 4-methylbenzenesulfonate (3) (2.21g,
8.7 mmol) in THF (50 mL) at À78ꢀC was added dropwise 1.6 M n-butyllithium
in hexanes (12.5 mL, 20 mmol). After stirring under a nitrogen atmosphere
at À78ꢀC for 1 h, the solution was neutralized with a mixture of THF/H2O
(1:1, 30mL). Water (~20mL) was added, and the organic phase was
extracted with ethyl acetate (2Â 30 mL), dried over sodium sulfate, filtered,
and evaporated. Purification by flash chromatography on silica gel eluting
with ethyl acetate/heptane (1:1) yielded 1.9g of 4 as a colourless liquid
(8.1mmol, 93%). 1H-NMR (500 MHz, MeOD) d ppm 2.48 (s, 3H), 6.41
(dd, J= 14.98, 3.94Hz, 1H), 7.49 (d, J = 7.88Hz, 2H), 7.84 (d, J = 8.51Hz, 2H).
GC–MS m/z 234 ([M]+).
(E)-2-Fluorovinyl 4-methylbenzenesulfonate (5)
High-performance liquid chromatography radiochemical purity analyses
were performed on an Agilent 1100 HPLC-system with a binary pump,
autoinjector, DAD and column oven, coupled in series with a Packard
Radiomatic Flow Scintillator 525TR (Chemical Instruments AB, Lidingö,
Sweden), equipped with a solid scintillator (SolarScint) cell with a volume
of 32 mL. The column used was an XBridge 3.5mm C18 (150 mm 4.6 mm)
(Waters Sverige AB, Sollentuna, Sweden). The column temperature was
set to 40ꢀC, and the flow rate to 1.0mL/min. A linear gradient was
applied, starting at 100% A (A: 10 mM NH4OAc in 5% CH3CN) and ending
at 95% B (B: CH3CN).
To a solution of 2,2-difluorovinyl 4-methylbenzenesulfonate (4) (1.9 g,
8.1 mmol) in Et2O (50 mL) cooled to À10ꢀC was added LiAlH4 (0.31 g,
8.1 mmol) in one portion. The solution was warmed to room temperature
and stirred for 12 h. After cooling to 0ꢀC, the mixture was neutralized
with an aqueous solution of NaOH (1.0 mL, 0.1 M), then filtered through
a plug of silica, and evaporated. Purification by flash chromatography
on silica gel eluting with a gradient of heptane to heptane/CH2Cl2 (5:1)
yielded 0.91 g of
5
as a colourless oil (8.1 mmol, 52%). 1H-NMR
(500 MHz, MeOD) d ppm 2.48 (s, 3H), 6.91–7.23 (m, 2H), 7.48 (d,
J = 8.20 Hz, 2H), 7.80 (d, J = 8.20 Hz, 2H). GC–MS m/z 216 ([M]+).
Mass spectra were recorded on a Waters LCMS (Waters Sverige AB,
Sollentuna, Sweden) consisting of an Alliance 2795 (LC), Waters 2996
Photodiode Array Detector and
a ZMD single quadrupole mass
General procedure for the D2 reduction of (E)-2-fluorovinyl
4-methylbenzenesulfonate (5)
spectrometer. The mass spectrometer was equipped with an electrospray
ion source operated in a positive or negative ion mode. The capillary
voltage was 3 kV, and cone voltage was 30 V. The mass spectrometer
was scanned between m/z 100–600 with a scan time of 0.7 s. The
column temperature was set to 40ꢀC. The diode array detector was
scanned from 200 to 400 nm. The column used was an XBridge 3.5 mm
C18 (150 mm  4.6 mm). The column temperature was set to 40ꢀC,
and the flow rate to 1.0 mL/min. A linear gradient was applied,
starting at 100% A (A: 10 mM NH4OAc in 5% CH3CN) and ending at
95% B (B: CH3CN).
Liquid scintillation analysis was performed on a PACKARD TRICARB
2900TR (Chemical Instruments AB, Lidingö, Sweden). TLC was performed
on Merck TLC-plates (Silica gel 60F254), and UV light (254 nm) visualized
the spots. Flash column chromatography was performed on Redisep™
(Teledyne Isco, Lincoln, Nebraska, USA) prepacked columns.
A solution of substrate and catalyst in solvent was cooled to À78ꢀC in a
dry ice/acetone slurry bath. The system was evacuated three times, filled
with nitrogen in the first two instances and with deuterium gas in the
third instance. The flask was warmed to room temperature and stirred
for 1 h. A small sample of the crude residue was dissolved in MeOH
and analysed by HPLC.
[3H]2-Fluoroethyl 4-methylbenzenesulfonate ([3H]1)
A solution of (E)-2-fluorovinyl 4-methylbenzenesulfonate (5) (1.6 mg,
7.4 mmol) and Crabtree’s catalyst (0.6 mg, 0.7 mmol) in CH2Cl2 (300 mL)
was stirred under a tritium atmosphere (926 mbar) at room temperature
for 1 h. The solvent was evaporated, and the residue was taken up in
Et2O (~1 mL) prior to filtration through a short plug of silica. The crude
material was purified by preparative HPLC on a Kromasil column
(KR60-10CN, 4.6 Â 250 mm) eluting with 5% EtOAc in heptane at 2 mL/min.
5040 MBq of [3H]1 with a radiochemical purity of 99% was obtained.
3H-NMR (250MBq/mL, 600 MHz, MeOD) d ppm 4.19–4.25 (m, 1T, CHT-O),
4.47–4.56 (m, 1T, CHT-F).
2,2,2-Trifluoroethyltosylate (3)
A solution of 2,2,2-trifluoroethanol (0.73 mL, 10 mmol) and triethylamine
(5 mL, 36 mmol) in CH2Cl2 (10 mL) was cooled to 0ꢀC. p-Toluenesulfonyl
chloride (2.3 g, 12 mmol) was added, and the solution was stirred at
0ꢀC for 1 h, then warmed to room temperature and stirred for a further
12 h. The CH2Cl2 layer was separated and washed with brine (2 mL), dried
over sodium sulfate, filtered, and evaporated. Purification by flash
chromatography on silica gel eluting with CH2Cl2 yielded 2.21 g of 3 as
[3H]FE-PE2I, [3H]N-(3-iodoprop-2E-enyl)-2b-carbofluoroethoxy-
3b-(4-methylphenyl)nortropane ([3H]7)
a colourless solid (8.7mmol, 87%). 1H-NMR (500 MHz, CDCl3) d ppm 2.48 To a solution of [3H]1 (925 MBq) in MeCN (300 mL) was added a solution
(s, 3H) 4.36 (q, J= 7.88 Hz, 2H), 7.40 (d, J= 8.20 Hz, 2H), 7.83 (d, J=8.20Hz, 2H).
of
N-(3-iodoprop-2E-enyl)-2b-carboxy-3b-(4-methylphenyl)nortropane
GC–MS m/z 254 ([M]+).
(6) (0.1 mg, 2.4 mmol) in MeCN (200 mL) and 5 M NaOH (4 mL, 20 mmol).
J. Label Compd. Radiopharm 2013, 56 447–450
Copyright © 2012 John Wiley & Sons, Ltd.
www.jlcr.org

A. R. Cochrane et al.
The solution was heated under microwave irradiation at 120ꢀC for 0.5 h.
TLC analysis (10% MeOH/CH2Cl2) showed almost complete consumption
of [3H]1. The crude reaction mixture was partitioned between CH2Cl2 and
water, and the aqueous layer re-extracted with a further portion of
CH2Cl2. The solvent was evaporated, and the resulting material was pur-
ified by preparative HPLC on a Kromasil column (C8, 5 mm, 10 Â 250 mm)
eluting with 75% MeCN modified with 0.1% NEt3 at 3 mL/min. Fractions
containing the product were combined, concentrated, and dissolved in
absolute ethanol to give 104 MBq of [3H]7 with a specific activity of
1.6 TBq/mmol and a 98% radiochemical purity. The product was stored
in absolute ethanol at a concentration of 52 MBq/mL. LC/MS [(M + H)+]:
458 (8.2%), 459 (1.8%), 460 (55.0%), 461 (12.8%), 462 (100%), 463
(22.3%), 464 (2.8%).
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Conflict of Interest
The authors did not report any conflict of interest.
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J. Label Compd. Radiopharm 2013, 56 447–450