Discovery of a dopamine D4 selective PET ligand candidate taking advantage of a click chemistry based REM linker

Article abstract of DOI:10.1016/j.bmcl.2007.12.026

Employing D4 selective azaindoles as lead compounds, a focused library of the carbocyclic arene bioisosteres 1 was synthesized when we took advantage of the click chemistry derived triazolylmethyl acrylate resin 2. Ligand binding assays on monoaminergic GPCRs led to SARs that indicated further lead structure optimizations when the attachment of alkoxy substituents provided both an improvement of the biological properties and the opportunity to introduce ¹⁸F as a radioisotope. Finally, radiosynthesis resulted in formation of the radioligand [¹⁸F]7h that showed optimal log D_7.4 of 2.8 and was determined to be highly stable in human serum. Thus, [¹⁸F]7h represents a promising dopamine D4 selective radioligand for positron emission tomography (PET).

Full text of DOI:10.1016/j.bmcl.2007.12.026

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 983–988
Discovery of a dopamine D4 selective PET ligand candidate
taking advantage of a click chemistry based REM linker
Rainer Tietze,a Stefan Lo¨ber,b Harald Hubner,b Peter Gmeiner,b
¨
Torsten Kuwerta and Olaf Prantea,*
aLaboratory of Molecular Imaging, Clinic of Nuclear Medicine, Schwabachanlage 6, D-91054 Erlangen, Germany
bDepartment of Chemistry and Pharmacy, Emil Fischer Center, Friedrich-Alexander University,
Schuhstraße 19, D-91052 Erlangen, Germany
Received 19 November 2007; revised 6 December 2007; accepted 11 December 2007
Abstract—Employing D4 selective azaindoles as lead compounds, a focused library of the carbocyclic arene bioisosteres 1 was syn-
thesized when we took advantage of the click chemistry derived triazolylmethyl acrylate resin 2. Ligand binding assays on mono-
aminergic GPCRs led to SARs that indicated further lead structure optimizations when the attachment of alkoxy substituents
provided both an improvement of the biological properties and the opportunity to introduce 18F as a radioisotope. Finally, radio-
synthesis resulted in formation of the radioligand [18F]7h that showed optimal logD7.4 of 2.8 and was determined to be highly stable
in human serum. Thus, [18F]7h represents a promising dopamine D4 selective radioligand for positron emission tomography (PET).
ꢀ 2007 Elsevier Ltd. All rights reserved.
The dopamine D4 receptor has attracted considerable
N
N
N
N
attention as a pharmacological target for the treatment
of schizophrenia, Parkinson’s disease, depression, and
attention deficit hyperactivity disorder (ADHD).1
Ongoing efforts have been made to generate selective li-
gands revealing high D4 affinity. Thus, an N-arylpiper-
azine framework proved to be a privileged structural
unit,2 when the second nitrogen atom of the piperazine
moiety is preferably attached to a benzylic CH2 position
of a fused heteroarene moiety (Chart 1). Among the bio-
isosteric elements investigated, azaindole derivatives
including the partial agonists L-745,870,3,4 FAUC
113,5 ABT-724,6 PIP3EA7 and the neutral antagonist
FAUC 2138,9 revealed superior D4 recognition and
selectivity. We were intrigued by the question whether
a bioisosteric replacement of the azaindole regioisomers
by benzene derived carbocyclic analogs could be per-
formed facilitating a regiocontrolled attachment of a
wide range of substituents. As a part of our ongoing
investigations on the discovery of subtype specific
dopaminergic PET candidates,10–13 we herein describe
a two-step procedure involving a click chemistry pro-
Cl
Cl
N
N
N
N
FAUC 113 (pos. 3)
FAUC 213 (Pos. 2)
H
L-745,870
N
N
R
R'
R''
1
N
N
N
N
NH
N
MeO
N
N
N
PIP3EA
ABT-724
Chart 1. Structures of D4 selective lead and target compounds.
moted approach to a focused chemical library and fur-
ther careful lead optimization facilitating a fine tuning
of both selectivity profiles and lipophilicity resulting in
the development of the PET ligand [18F]7h.
Keywords: D2-like dopamine receptors; Click chemistry; SPOS; Pos-
itron emission tomography; PET; F-18.
*
Corresponding author. Tel.: +49 9131 8544440; fax: +49 9131
8534325; e-mail: olaf.prante@uk-erlangen.de
0960-894X/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.12.026

984
R. Tietze et al. / Bioorg. Med. Chem. Lett. 18 (2008) 983–988
19
‘Click resins’ enable solid phase supported reactions to
work under nearly perfect conditions fulfilling the
requirements of click chemistry. Utilizing the triazolyl-
methyl acrylate (TMA) linker 2,14 which is readily avail-
able from azidomethyl substituted polystyrene and
propargyl acrylate via copper(I)-catalyzed azide-alkyne
cycloaddition (CuAAC), a regenerative Michael accep-
tor (REM) strategy15–17 was envisaged for a parallel syn-
thesis of bioactive tertiary amines of type 1. In detail, the
resin bound Michael acceptor 2 was treated with the
phenylpiperazines A1-6 to give the intermediates 3a–f
(Scheme 1). N-alkylation with the benzyl bromides B1-
3 followed by Hofmann elimination of the resulting
ammonium ions 4a–r gave final products 1a–r.18 For
all solid phase organic synthesis (SPOS) products inves-
tigated, the yields were satisfying and comparable to
those described for REM-based strategies (Table 1).
LCMS analysis of the unpurified final products 1a–r
employing reversed phase chromatography in combina-
tion with UV (254 nm) and APCI-ion trap detection
indicated an excellent average purity of 95%.
D2short
,
D3,20 and D421 dopamine receptors, and the
porcine D1 subtypes was evaluated in vitro.22 This was
performed in a screening system by measuring the dis-
placement of the radioligands [3H]spiperone for D2,
D3, D4 and [3H]SCH23390 for D1 receptors using
10 lM, 100 nM, and 1 nM concentrations of the test
compounds. The results in Table 1 depicting the dis-
placement data at a final concentration of 100 nM indi-
cate D4 receptor preference for most test compounds.
Comparison of unpolar arenes with the derivatives bear-
ing H-bond accepting groups confirmed our anticipation
that a bioisosteric replacement of the azaindole moiety
requires an electronegative congener that could be repre-
sented by a methoxy or cyano unit. Combining this
pharmacophoric portion with 3,4-dichloro- or 2-meth-
oxyphenylpiperazines via a common methylene linker
led to the most promising D4 ligands 1d,e and 1j,
respectively.
Aiming at the design of 18F-labeled analogs derived
from these lead compounds, our strategy was based on
the formal structural exchange of methoxy groups by
2-fluoroethoxy moieties. Thus, the D4 ligand 1e repre-
sents a ‘starting point’ for further careful lead optimiza-
tion by introducing additional methoxy substituent at
the benzyl system and, thus, optimizing lipophilicity,
while simultaneously varying the position of a fluoroeth-
oxy substituent. Furthermore, we chose 2-methoxy-
phenylpiperazine (A4) and the A2 derived analog
4-chlorophenylpiperazine, which has been successfully
validated as a capable recognition element in vivo,9 as
promising scaffolds in the syntheses of a new series of
test compounds (Scheme 2).
The selection of ‘D4-like’ building blocks for the parallel
synthesis was based on previous SAR studies indicating
that chloro and methoxy groups lead to an increase of
receptor affinity. Taking into account screening data of
our existing in-house library revealing only poor D4
binding
for
N-benzyl-(4-chlorophenyl)piperazine
(Ki = 250 nM), H-bond accepting benzyl substituents
should be considered simulating the electronegative
nitrogen units of the heteroarene based lead compounds.
This type of functionalization was also expected to in-
crease solubility and prevent from putatively high
unspecific, lipophilicity induced membrane binding.
Dopamine receptor screening of the test compounds
1a–r was performed without further purifications when
The benzaldehyde derivatives 5a–5d, bearing a free hy-
droxyl group in the para-position (5a (vaniline), 5b), in
meta-position (5c), and in ortho-position (5d), respec-
their ability to bind to the cloned human D2long
,
Table 1. Purities,a yields, and screening data for dopaminergic affinitiesb of benzylpiperazines 1a–r
a Determined by LCMS analyses at 254 nm.
b Relative displacement of radioligand at a concentration of the test compound of 100 nM determined as triplicate.

R. Tietze et al. / Bioorg. Med. Chem. Lett. 18 (2008) 983–988
985
piperazine using Na(OAc)3BH readily afforded a series
of 8 target benzyl compounds (7a–7h) in 30–80% yield.24
a
N
N
O
N3
O
N
2
After purification by chromatography, LCMS analysis,
and identification,25 the target compounds 7a–h were
subjected to receptor binding studies to determine the
affinity and selectivity profile with respect to the different
subtypes of dopamine receptors, as described above. In
addition, binding properties to the related serotonin
receptors 5-HT1A and 5-HT2, and to the adrenergic a1
receptor were measured using porcine cortical mem-
branes and the selective radioligands [3H]8-OH-DPAT
or [3H]WAY600135, [3H]ketanserin, and [3H]prazosin,
respectively. For comparison, the affinity constant (Ki)
values of a reference compound (FAUC 213) deter-
mined under the same assay conditions are included in
Table 2.
b
N
N
N
N
O
O
O
O
N
N
c
+
N
N
N
N
R'
R
R
4a-r
3a-f
d
1a-r
All test compounds revealed weak affinity to the D1
receptor, while 7a–d and 7h preferentially bind to the
D4 receptors in the low nanomolar range (1.3–3.7 nM;
Table 2). The 2-methoxyphenyl moiety (7a–d) is benefi-
cial for high D4 receptor affinity, while some of the cor-
HN
HN
N
HN
HN
N
HN
N
F
Cl
Cl
Cl
A1
A2
A3
HN
N
N
N
responding derivatives bearing
a
4-chlorophenyl
substituent (7e–g) revealed decreased D4 affinities.
When comparing 7a–7d among each other, variation
of the fluoroethoxy substitution pattern and introduc-
tion of additional methoxy groups have only marginal
effects on the binding profile, resulting in good D4 affin-
ities, but relatively low D4/D2 subtype selectivities of
only about 30-170. Contrary to the series 7a–d, in the
case of 7e–h the D2 and D4 binding was significantly
influenced by the substitution pattern of the benzyl scaf-
fold (Table 2). In fact, the introduction of a methoxy
group in meta-position led to a markedly increased
D4/D2 subtype-selectivity (7e: 1750) accompanied by
only weak differences in D4 affinity (cf. 7e vs 7f). While
a fluoroethoxy substitution in meta-position turned out
to be less suitable, which is due to decreased D4 affinity
(7g, Table 2), the introduction of additional methoxy
groups and simultaneously maintaining the fluoroeth-
oxy group in the ortho-position turned out to be benefi-
MeO
A4
OMe
A5
A6
Br
Br
Br
MeO
Br
NC
B1
B2
B3
Scheme 1. Reagents and conditions: (a) propargyl acrylate, Cu(I)I,
DMF, THF, DIPEA, 35 ꢁC, 10 h (Ref. 14); (b) A1-6, NMP, rt, 36 h;
(c) B1-3, DMSO, rt, 16 h; (d) TEA, DMF, rt, 4 h.
tively, were converted to the corresponding fluoroethyl
ethers 6a–6d by alkylation with 2-fluoroethyl tosylate
in the presence of tetrabutylammonium hydroxide, fol-
lowing a procedure described by Wilson and cowork-
ers.23 Subsequent reductive amination of 6a–6d with
commercially available 4-chloro- or 2-methoxyphenyl-
N
N
R4
R3
CHO
R1
R4
R3
CHO
R1
R
a
b
R1
R4
N
R2
R3
R2
R2
HN
R
R= 2-OCH3
or 4-Cl
7a: R= 2-OCH3, R1= H, R2= OCH3, R3= O(CH2)2F, R4= H
7b: R= 2-OCH3, R1= H, R2= H, R3= O(CH2)2F, R4= H
7c: R= 2-OCH3, R1= H, R2= O(CH2)2F, R3= H, R4= H
7d: R= 2-OCH3, R1= O(CH2)2F, R2= H, R3= OCH3, R4= OCH3
7e: R= 4-Cl, R1= H, R2= OCH3, R3= O(CH2)2F, R4= H
7f: R= 4-Cl, R1= H, R2= H, R3= O(CH2)2F, R4= H
5a: R1= H, R2= OCH3,
6a: R1= H, R2= OCH3,
R3= OH, R4= H
R3= O(CH2)2F, R4= H
5b: R1= H, R2= H,
6b: R1= H, R2= H,
R3= OH, R4= H
R3= O(CH2)2F, R4= H
5c: R1= H, R2= OH,
6c: R1= H, R2= O(CH2)2F,
R3= H, R4= H
R3= H, R4= H
7g: R= 4-Cl, R1= H, R2= O(CH2)2F, R3= H, R4= H
7h: R= 4-Cl, R1= O(CH2)2F, R2= H, R3= OCH3, R4= OCH3
5d: R1= OH, R2= H,
6d: R1= O(CH2)2F, R2= H,
R3= OCH3, R4= OCH3
R3= OCH3, R4= OCH3
Scheme 2. Reagents and conditions: (a) 2-fluoroethyl tosylate, N(Bu)4OH, DMF, rt, 24 h; (b) Na(OAc)3BH (4–5 equiv), CH2Cl2, rt, 16 h.

986
R. Tietze et al. / Bioorg. Med. Chem. Lett. 18 (2008) 983–988
Table 2. Receptor binding data of 7a–7h at human D2long, D3, and D4.4 receptors as well as porcine D1, 5-HT1A, 5-HT2, and a1 receptorsa
Compound
pD1
hD2long
hD3
hD4.4
p5-HT1A
p5-HT2
pa1
ClogPb
7a
7b
5400
3400
5200
6800
5700
1400
5100
3500
5500
310
120
1100
77
1.8
3.7
1.6
1.3
12
100
120
2100
5400
1900
280
21
47
3.3
3.5
3.5
3.2
4.1
4.4
4.4
4.1
3.5
7c
7d
57
94
270
330
32
100
36
13
7e
21,000
1100
590
4900
1100
1100
3900
5300
18,000
5200
3300
2800
1700
850
18
7f
7g
50
82
1800
980
160
190
7h
FAUC 213
760
3400
1.7
2.2
100
900
4.4
270
a Ki values in nM are based on the means of 2–4 experiments each done in triplicate.
b Calculated value using the software ClogP (Biobyte Corp.).
N
N
18F
N
N
O
TosO
Cl
Cl
OH
O
O
O
a
18F
O
18F]7h
8
[
Scheme 3. Reagent and condition: (a) NBu4OH, DMF, 110 ꢁC, 3 min.
cial as indicated by Ki values in the low nanomolar
range for D4 (7d and 7h). 7h showed an adequate
balance between D4 affinity (1.7 nM) and exceptional
D4 subtype selectivities (D4/D2 = 450, D4/D3 = 2300,
Table 2).
identity of [18F]7h. [18F]7h was isolated by semiprepara-
tive RP-HPLC followed by solid phase extraction (Sep-
Pak C-18).
The resulting buffered solution (PBS) of [18F]7h was
used to determine experimental logD7.4 values (extrac-
tion in octanol/PBS)29 and for studying the metabolic
stability in human serum. [18F]7h revealed a logD7.4 va-
lue of 2.8 0.2 (n = 6), due to the presence of the partly
protonated piperazine at physiological pH. In addition
to the hydrophilic influence of additional methoxy
groups, the resulting lipophilicity excellently complied
with an optimum. Moreover, [18F]7h was determined
to be highly stable in human serum (>95%, 90 min), as
analyzed by radio-HPLC. These in vitro parameters
suggest advantageous characteristics for in vivo use,
such as adequate blood-brain-barrier penetration com-
bined with high metabolic stability in the blood. This
assumption has to be confirmed by further biodistribu-
tional studies.
In order to assess the lipophilicity of the test compounds
as a measure of blood brain barrier permeability, we cal-
culated logP values (Table 2). Within the series of 2-
methoxyphenyl derivatives (7a–7d) ClogP values were
3.2–3.5, being consistent with FAUC 213 as a reference,
whereas the 4-chlorophenyl derivatives (7e–7h) revealed
higher lipophilicity with ClogP of 4.1–4.4. Noteworthy,
within each series of test compounds, the presence of
additional methoxy groups in the benzyl moeity (7d,
7h) clearly reduced lipophilicity to a minimum (Table
2). This effect could help to adequately meet the prereq-
uisites for optimal brain uptake, since radiotracer up-
take into the brain is a function of logP that peaks
between logP of 2 and 3.26
As a proof of concept and due to the promising high D4
receptor affinity and distinct D4 receptor subtype-selec-
tivity of 7h, we aimed at the radiosynthesis of the corre-
sponding 18F-labeled analog. Employing 18F-
fluoroalkylation by means of [18F]fluoroethyl tosylate11
as one of the most commonly used 18F-labeled pros-
thetic groups,27 we prepared the 2-hydroxybenzyl deriv-
ative 8 as a suitable precursor following the general
procedure for reductive amination starting from 5d.24
The intermediate 8 was reacted with tetrabutylammo-
nium hydroxide in DMF at 110ꢁC to form the ortho-
phenoxide in situ before addition of [18F]fluoroethyl tos-
ylate (Scheme 3).28 The decay-corrected radiochemical
yield of [18F]7h was 92 4% after 3 min as determined
by radio-HPLC. The D4 ligand 7h served as authentic
reference in analytical radio-HPLC to confirm chemical
In conclusion, a click chemistry promoted SPOS ap-
proach allowed rapid access to a focused chemical li-
brary when further lead optimization resulted in the
discovery of a series of potential D4 PET ligands. As
exemplified by the 18F-radiosynthesis of [18F]7h, the
use of modern SPOS techniques followed by careful lead
optimization considering the structural requirements for
18F-labeling can facilitate the development of D4 selec-
tive PET ligands.
Acknowledgments
D. Huber and C. Ja¨ger are acknowledged for their con-
tribution to the parallel synthesis. We thank Dr. J.-C.
Schwartz and Dr. P. Sokoloff (INSERM, Paris), Dr. J.

R. Tietze et al. / Bioorg. Med. Chem. Lett. 18 (2008) 983–988
987
After evaporation of the solvent the resulting residue was
dissolved in CH2Cl2 and washed with a saturated solution
of NaHCO3. Evaporation of the solvent afforded the
desired benzylpiperazines 1a–r in 36–86 % yield.
19. Hayes, G.; Biden, T. J.; Selbie, L. A.; Shine, J. Mol.
Endocrinol. 1992, 6, 920.
20. Sokoloff, P.; Andrieux, M.; Besancon, R.; Pilon, C.;
Martres, M.-P.; Giros, B.; Schwartz, J.-C. Eur. J. Phar-
macol. 1992, 225, 331.
Shine (The Garvan Institute of Medical Research, Syd-
ney), and late Dr. H.H.M. Van Tol (Clarke Institute of
Psychiatry, Toronto) for providing D3, D4.4, and D2
receptor expressing cell lines. This research was sup-
ported by a grant of the Deutsche Forschungsgeme-
inschaft (PR 677/2-2).
References and notes
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24. General procedure: 6a–6d or 5d (0.7 mmol, 1equiv) and 1-
(2-methoxyphenyl)piperazine or 1-(4-chlorophenyl)piper-
azine (1.3 equiv), respectively, were dissolved in 20 mL dry
CH2Cl2. Na(OAc)3BH (4–5 equiv) was added to the
suspension in one portion. The mixture was stirred
overnight at room temperature. The reaction was
quenched by adding 30 mL saturated NaHCO3. The
product was extracted with ethyl acetate (3 · 20 mL) and
the organic layer was dried over Na2SO4. After evapora-
tion of the solvent, the crude product was purified by silica
gel chromatography using CH2Cl2/MeOH (95:5). 7a–7h
and 8 were obtained in 30–80% yield.
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25. 7a: H NMR (CDCl3, 360 MHz): d = 2.66 (brs, 4H), 3.11
7. Enguehard-Gueiffier, C.; Hubner, H.; El Hakmaoui, A.;
¨
Allouchi, H.; Gmeiner, P.; Argiolas, A.; Melis, M. R.;
(brs, 4H), 3.54 (s, 2H), 3.85 (s, 3H, OCH3), 3.89 (s, 3H,
OCH3), 4.22–4.32 (dt, 2H), 4.69–4.85 (dt, 2H), 6.84–7.01
(m, 7H). LC-MS (APCI): m/z 375.1 [M+H]+. 7b: 1H NMR
(CDCl3, 360 MHz): d = 2.67 (brs, 4H), 3.10 (brs, 4H), 3.55
(s, 2H), 3.86 (s, 3H, OCH3), 4.17–4.27 (dt, 2H), 4.68–4.84
(dt, 2H), 6.84–7.00 (m, 5H), 7.29 (d, 2H). LC-MS (APCI):
m/z 345.0 [M+H]+. 7c: 1H NMR (CDCl3, 360 MHz):
d = 2.68 (brs, 4H), 3.11 (brs, 4H), 3.58 (s, 2H), 3.86 (s, 3H,
OCH3), 4.19–4.29 (dt, 2H), 4.68–4.84 (dt, 2H), 6.83–7.01
(m, 6H), 7.23–7.31 (m, 2H). LC-MS (APCI): m/z 345.1
[M+H]+. 7d: 1H NMR (CDCl3, 360 MHz): d = 2.72 (s,
4H), 3.10 (s, 4H), 3.63 (s, 2H), 3.85 (s, 3H, OCH3), 3.87
(brs, 6H, 2 · OCH3), 4.16–4.26 (dt, 2H), 4.65–4.81 (dt,
2H), 6.54 (s, 1H), 6.83–7.01 (m, 5H). LC-MS (APCI): m/z
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18. General procedure: TMA resin (2) (300 mg, 1,63 mmol/g)
was agitated with a arylpiperazine (A1-6, 10 equiv) in
NMP (6 ml) for 36 h at ambient temperature. The
suspension was filtered through the vessel frit and the
resin was rinsed alternately with MeOH (5 · 10 mL),
CH2Cl2 (5 · 10 mL), and pyridine (5 · 10 mL). Subse-
quent washing with DMSO (2 · 10 mL) was followed by
the addition of a solution of a benzyl bromide (B1-3,
5 equiv) in DMSO (6 mL) and agitation for 16 h at
ambient temperature. After filtration of the solvent the
resin was washed with MeOH (3 · 10 mL) and CH2Cl2
(10 · 5 mL), and TEA (0.6 mL, 14 mmol) in DMF (6 mL)
was added. Stirring at ambient temperature for 6 h was
followed by filtration of the solvent, whereas the resin was
washed with DMF (10 mL) and CH2Cl2 (3 · 10 mL).
(brs, 4H), 3.21 (brs, 4H), 3.55 (s, 2H), 3.90 (s, 3H, OCH3),
4.23–4.33 (dt, 2H), 4.70–4.86(dt, 2H), 6.84 (d, 2H), 6.86–
6.90 (m, 2H), 7.21 (d, 2H), 7.27(s, 1H). LC-MS (APCI): m/
z 379.2 [M+H]+. 7f: 1H NMR (CDCl3, 360 MHz): d = 2.60
(brs, 4H), 3.16 (brs, 4H), 3.52 (s, 2H), 4.15–4.25 (dt, 2H),
4.66-4.81 (dt, 2H), 6.80 (d, 2H), 6.88 (d, 2H), 7.17 (d, 2H),
1
7.27 (d, 2H). LC-MS (APCI): m/z 349.2 [M+H]+. 7g: H
NMR (CDCl3, 360 MHz): d = 2.68 (brs, 4H), 3.24 (brs,
4H), 3.63 (s, 2H), 4.22–4.31 (dt, 2H), 4.69–4.85 (dt, 2H),
6.84 (d, 2H) 6.86–6.92 (m, 1H), 6.99 (d, 1H) 7.21 (d, 2H),
1
7.25 (m, 2H). LC-MS (APCI): m/z 349.5 [M+H]+. 7h: H
NMR (CDCl3, 360 MHz): d = 2.68 (brs, 4H), 3.19 (brs,
4H), 3.64 (s, 2H), 3.88 (s, 3H, OCH3), 3.89 (s, 3H, OCH3),
4.17–4.27 (dt, 2H), 4.66–4.82 (dt, 2H), 6.55 (s, 1H), 6.83
(d, 2H), 7.20 (d, 2H), 7.27 (s, 1H). LC-MS (APCI): m/z
1
409.2 [M+H]+. 8: H NMR (CDCl3, 360 MHz): d = 2.72
(brs, 4H), 3.21 (brs, 4H), 3.70 (s, 2H), 3.82 (s, 3H), 3.85 (s,
3H), 6.46 (s, 1H), 6.53 (s, 1H), 6.84 (d, 2H), 7.21 (d, 2H).
LC-MS (APCI): m/z 363.1 [M+H]+.
26. Waterhouse, R. N. Mol. Imaging Biol. 2003, 5, 376.
27. For a recent review, see: Schirrmacher, R.; Wa¨ngler, C.;
Schirrmacher, E. Mini-Rev. Org. Chem. 2007, 4, 317.
28. An aqueous solution (1 mL, CH3CN/H2O, 8:2) containing
K2CO3 (15 lmol), Kryptofix 2.2.2 (15 mg), and no-carrier-
added [18F]fluoride (0.5–1.0 GBq, PET Net GmbH,

988
R. Tietze et al. / Bioorg. Med. Chem. Lett. 18 (2008) 983–988
Erlangen) was azeotropicaly dried at 85 ꢁC. As described
1.4 M tetrabutylammonium hydroxide in dry MeOH
(3 min, 110ꢁC). The radiochemical yield of [18F]7h was
92 4% after 3 min (n = 3; radio-TLC, Rf = 0.44; radio-
HPLC, Lichrosorb RP-18, 250 · 4.6 mm, 1 mL/min, 25–
100% acetonitrile in water (0.1% TFA) in 30 min):
tR = 14.1 min). [18F]7h was manually purified by semipre-
parative HPLC (Kromasil C-8, 125 · 8 mm, 4 mL/min,
previously,11 4.5 mg (12 lmol) bistosyloxyethane in
0.5 mL anhydrous acetonitrile was added and the mixture
was stirred for 3 min at 90 ꢁC. After semipreparative
HPLC (Lichrosorb RP18, 125 · 8 mm, 4 mL/min, 40%
acetonitrile in water), [18F]fluoroethyl tosylate was
obtained in a decay-uncorrected radiochemical yield of
about 60%, a molar radioactivity of >330 GBq/lmol and a
radiochemical purity of at least 98%. Subsequently,
750 lL of a solution of [18F]fluoroethyl tosylate in dry
DMF (0.3–0.6 GBq) was added to a preincubated solution
of 8 (2.8 mg, 7.7 lmol) in 250 lL dry DMF and 20 lL of
37.5%
acetonitrile
in
water
(0.1%
TFA),
tR([18F]7h) = 5.7 min, tR(8) = 3.1–4.5 min) followed by
SPE (C-18, 100 mg, Merck).
29. Prante, O.; Hocke, C.; Lo¨ber, S.; Hubner, H.; Gmeiner,
¨
P.; Kuwert, T. Nuklearmedizin 2006, 45, 41.