Synthesis and labeling of a piperazine-based library of 11C-labeled ligands for imaging of the vesicular acetylcholine transporter

Article abstract of DOI:10.1002/jlcr.3208

The cholinergic system is involved in neurodegenerative diseases, and visualization of cholinergic innervations with positron emission tomography (PET) would be a useful tool in understanding these diseases. A ligand for the vesicular acetylcholine transporter (VAChT), acknowledged as a marker for cholinergic neurons, could serve as such a PET tracer. The aim was to find a VAChT PET tracer using a library concept to create a small but diverse library of labeled compounds. From the same precursor and commercially available aryl iodides 6a-f, six potential VAChT PET tracers, [¹¹C]-(±)5a-f, were ¹¹C-labeled by a palladium (0)-mediated aminocarbonylation, utilizing a standard protocol. The labeled compounds [¹¹C]-(±) 5a-f were obtained in radiochemical purities >95% with decay-corrected radiochemical yields and specific radioactivities between 4-25% and 124-597 GBq/μmol, respectively. Autoradiography studies were then conducted to assess the compounds binding selectivity for VAChT. Labeled compounds [ ¹¹C]-(±)5d and [¹¹C]-(±)5e showed specific binding but not enough to permit further preclinical studies. To conclude, a general method for a facile synthesis and labeling of a small piperazine-based library of potential PET tracers for imaging of VAChT was shown, and in upcoming work, another scaffold will be explored using this approach A library of six piperazine-based ligands for the vesicular acetylcholine transporter were synthesized and labeled from a common precursor, commercially available aryl iodides and [¹¹C]carbon monoxide. Labeled compounds were isolated with radiochemical yields ranging from 4% to 25% and with specific radioactivities between 124 and 597 GBq/μmol. As a simple preclinical evaluation of the labeled compounds, their binding potential to VAChT was assessed by autoradiography. Copyright

Full text of DOI:10.1002/jlcr.3208

Research Article
Received 20 March 2014,
Revised 24 April 2014,
Accepted 25 April 2014
Published online 3 July 2014 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3208
Synthesis and labeling of a piperazine-based
1
1
library of C-labeled ligands for imaging of
the vesicular acetylcholine transporter
a
a
a
a
Sara Bergman, Sergio Estrada, Håkan Hall, Rashidur Rahman,
b
c
a
a
Andreas Blomgren, Mats Larhed, Marie Svedberg, Alf Thibblin,
Fredrik Wångsell, and Gunnar Antoni *
b
a
The cholinergic system is involved in neurodegenerative diseases, and visualization of cholinergic innervations with positron
emission tomography (PET) would be a useful tool in understanding these diseases. A ligand for the vesicular acetylcholine
transporter (VAChT), acknowledged as a marker for cholinergic neurons, could serve as such a PET tracer. The aim was to find
a VAChT PET tracer using a library concept to create a small but diverse library of labeled compounds. From the same
1
1
11
precursor and commercially available aryl iodides 6a–f, six potential VAChT PET tracers, [ C]-(±)ꢀa–f, were C-labeled by
1
1
a palladium (0)-mediated aminocarbonylation, utilizing a standard protocol. The labeled compounds [ C]-(±)ꢀa–f were
obtained in radiochemical purities >9ꢀ% with decay-corrected radiochemical yields and specific radioactivities between
4
–2ꢀ% and 124–ꢀ97 GBq/μmol, respectively. Autoradiography studies were then conducted to assess the compounds
11 11
binding selectivity for VAChT. Labeled compounds [ C]-(±)ꢀd and [ C]-(±)ꢀe showed specific binding but not enough to
permit further preclinical studies. To conclude, a general method for a facile synthesis and labeling of a small piperazine-
based library of potential PET tracers for imaging of VAChT was shown, and in upcoming work, another scaffold will be
explored using this approach.
11
Keywords: vesicular acetylcholine transporter; carbonylation; PET; library; C-labeling; vesamicol
Introduction
have been used in biodistribution, single-photon emission
13–23
computed tomography, or PET studies.
Unfortunately, the
The vesicular acetylcholine transporter (VAChT) is acknowledged majority of these compounds display poor kinetic properties,
1
–3
as a marker for cholinergic neurons.
The transporter is limiting their utility as PET tracers (for a review, see Giboureau
12
18
localized presynaptically in cholinergic nerve terminals where it et al.). However, [ F]fluoroethoxybenzo-vesamicol (Figure 1)
transports newly synthesized acetylcholine into synaptic has emerged as a promising PET tracer for VAChT.
23–25
4
,5
vesicles.
In various neurodegenerative diseases, the most
PET tracer candidates are selected, apart from requirements
notable examples being Alzheimer’s disease and Parkinson’s for selectivity and specificity, for ease of radiolabeling. The latter
disease, the loss of cholinergic neurons is highly linked to is challenging and restricts the available structures of substances,
11
disease progression as the reduction of cholinergic neurons in and a typical PET tracer library generally consists of C-labeled
6,7
18
the cortex has been coupled to the loss of cognitive abilities.
or F-labeled alkyl groups – methyl, ethyl, or propyl – attached
Early detection is important as the limited treatment arsenal is to a heteroatom. This gives a few tracer candidates with a limited
predicted to be most beneficial in the early stages of Alzheimer’s variation in properties. Expanding the library requires a unique
8
disease. Thus, a non-invasive method such as positron emission precursor for each labeled compound series and leads to long
tomography (PET) would be of great value in diagnosis as well as iteration times between synthesis and evaluation. In the light
in the study of disease progression and the effect of treatment.
PET measures biochemical and physiological processes in
the body, and a suitable PET tracer for VAChT would
a
Preclinical PET Platform, Department of Medicinal Chemistry, Uppsala
enable the visualization and quantification of brain cholinergic University, Box 574 SE-751 23 Uppsala, Sweden
9
innervations. The search is ongoing, and the compounds
b
Organic Pharmaceutical Chemistry, Department of Medicinal Chemistry,
presented so far are based on 2-(4-phenylpiperidino)cyclohexanol,
Uppsala University, BMC Box 574 SE-751 23 Uppsala, Sweden
10
or vesamicol (Figure 1). Vesamicol binds with high affinity to an
4
c
allosteric site on VAChT and inhibits the uptake of acetylcholine.
Science for Life Laboratory, Department of Medicinal Chemistry, Uppsala
University, BMC Box 574 SE-751 23 Uppsala, Sweden
Vesamicol also has an affinity to σ1,2-receptors, which renders it
unsuitable for use as a marker to study cholinergic neurons, a
*
Correspondence to: Gunnar Antoni, Preclinical PET Platform, Department of
11,12
selectivity problem shared with other VAChT ligands.
A small
Medicinal Chemistry, Uppsala University, Box 574, SE-751 23 Uppsala, Sweden.
E-mail: gunnar.antoni@pet.medchem.uu.se
11
18
125
number of potential VAChT tracers labeled with C, F, or
I
J. Label Compd. Radiopharm 2014, 57 525–532
Copyright © 2014 John Wiley & Sons, Ltd.

S. Bergman et al.
Figure 1. Selected ligands of VAChT.
of this, developing a synthetic method that allows preparation of Scanditronix MC-17 Cyclotron at PET Centre, Uppsala University
11
a library of labeled compounds from a common precursor and Hospital, and obtained as [ C]carbon dioxide. The target gas
commercially available building blocks would enable a fast used was nitrogen (AGA Nitrogen 6.0) containing 0.05% oxygen
11
and efficient screening of a larger library of potential tracers (AGA Oxygen 4.8). [ C]Carbon dioxide was transferred in a
for VAChT with greater variations in properties. With this stainless steel tubing concentrated on silica in a cold trap
11
11
methodology, all promising compounds can, after in vitro (À196 °C). [ C]Carbon dioxide was reduced to [ C]carbon
studies, directly proceed to ex vivo studies and PET studies monoxide over Zn granules at 400 °C and concentrated on silica
without the need for choosing between promising compounds in a cold trap (À196 °C). The aminocarbonylation reactions were
or for different synthetic routes to be developed.
carried out in a 200-μL stainless steel micro-autoclave using a
1
1
30
Utilizing C as the radioactive isotope has an advantage over previously described technique. Reagents were injected with
1
8
11
F when constructing a library of ligands, as C in the form of a Scantec 650 HPLC Pump (Partille, Sweden). Preparative
1
1
11
[
C]carbon monoxide can serve as a linker between two larger purification of compounds [ C]-(±)ꢀa–f was performed on a
11
structures. Åberg et al. nicely demonstrated the utility of [ C] LaPrep VWR International, equipped with LaPrep P206 and
carbon monoxide in the construction of library of P110 pumps and Phenomenex NX C-18 (5 μm, 250 × 10 mm)
a
2
6
acrylamides. The carbonyl motif has recently been explored column using acetonitrile (MeCN)/8.1 mM ammonium carbonate
in VAChT ligands by Tu et al., providing several high-affinity as the mobile phase and flow of 8 mL/min. The identities,
1
3,27,28
ligands (Figure 1) as a result.
By changing a piperidine ring concentration, and radiochemical and chemical purities of the
11
11
to a piperazine ring, transition metal mediated amino C- purified labeled compounds [ C]-(±)ꢀa–f were determined on
carbonylations could be performed for the synthesis of a library of an analytical HPLC equipped with ultraviolet (UV) and
tracers (Figure 1). The piperazine structure has previously been used radioactivity detectors using (±)ꢀa–f as references. The analytical
in VAChT tracers and is best combined with a benzovesamicol system was an Elite LaChrom VWR international, equipped with a
15,29
scaffold rather than a vesamicol scaffold.
The best example is LaPrep P206 pump, an Elite LaChrom L-2400 UV detector in
+
(
À)-5-iodo-2-hydroxy-3-[4-phenylpiperazinyl]tetralin ((À)-DCR140, series with a Bioscan β -flowcount detector, and a Reprosil-Pur
125
Figure 1). This compound was labeled with I and showed both Basic C-18 HD (5 μm, 100 × 4.6 mm) column using MeCN/
high affinity to VAChT and high selectivity towards other receptors, 8.0 mM ammonium carbonate as the mobile phase and flow of
15
together with a high brain permeability in rat.
2 mL/min. Analytical gas chromatography–mass spectrometry
Herein, we present a methodology for a fast and efficient (MS) of compound 1 was performed on a Varian Saturn 3900/
synthesis of a library of potential PET tracers for imaging of 2100 system (Palo Alto, CA, USA) with a Varian Chrompack
VAChT. With a standard protocol, a piperazine-based library of Capillary column CP-SIL 8 CB Low Bleed/MS (30 m × 0.22 mm,
1
1
À1
six C-labeled compounds was synthesized from a common 0.25 μm) and electron impact ionization (70–300 °C, 20 °C min ).
11
precursor, commercially available aryl iodides, and [ C]carbon Analytical reversed-phase HPLC–MS of compounds 2, (±)ꢁ, and
monoxide. As building blocks that differ in both size and (±)ꢀa–f was performed on a Gilson-Finnigan ThermoQuest
lipophilicity from the labeled tracer were used, the purification AQA system equipped with a C18 (Onyx Monolithic C18
of the tracer was facile. Assessment of the binding selectivity (50 × 4.6 mm)) or a C4 (Hichrom ACE C4 (5 μm, 50 × 4.6 mm))
for VAChT was made with autoradiography.
column using MeCN/H O (0.05% HCOOH) as the mobile phase
2
with UV (254 nm) and MS (ESI) detection or on a Dionex Ultimate
3
000 system using MeCN/H O (0.05% HCOOH) as the mobile
2
Materials and methods
phase with MS detection, equipped with a C8 (Phenomenex
Kinetex SB-C8 (4.8 × 50 mm)) column using UV (220 or 254 nm)
detection, and both systems have a flow of 1.5 mL/min.
General
Reagent grade THF was freshly distilled from sodium and Preparative reversed-phase HPLC–MS of compounds (±)ꢁ and
benzophenone under nitrogen atmosphere. Reagents (±)ꢀa–f was performed on a Gilson-Finnigan ThermoQuest
and solvents were obtained commercially from Sigma-Aldrich AQA system equipped with a C8 (Zorbax SB-C8 (5 μm,
St. Louis, MO, USA) and Fisher (Pittsburgh, PA, USA) and used 150 × 21.2 mm)) column using MeCN/H O (0.05% HCOOH) as
a
(
2
without further purification. Flash column chromatography the mobile phase with UV (254 nm) and MS (ESI) detection with
was performed using silica gel 60 (40–63 μm, Merck KGaA, the flow of 5 mL/min or on a Dionex Ultimate 3000 system
1
4
Darmstadt, Germany). Carbon-11 was prepared by the
(
N
equipped with a C8 (Agilent Zorbax SB-C8 (21.2 × 150 mm))
1
1
p,α) C nuclear reaction using 17-MeV protons produced by a column using UV (220 nm) detection and MeCN/H O (0.05%
2
www.jlcr.org
Copyright © 2014 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2014, 57 525–532

S. Bergman et al.
HCOOH) and the flow of 10 mL/min. NMR spectra were recorded separated and washed with brine. The crude mixture was
1
13
on a Varian Mercury plus spectrometer ( H at 399.8 MHz, C at purified with preparative HPLC (20–75% MeCN over 40 min) to
1
1
00.5 MHz) at ambient temperature. Chemical shifts (δ) are give ꢀa as a solid after freeze drying (84 mg, 64%). H NMR
reported in ppm, indirectly referenced to tetramethylsilane via (CD CN) δ: 2.87 (dd, J = 9.8, 5.5 Hz, 1H), 3.02–3.14 (m, 2H), 3.15–
HOD δ 3.31, 3.46 (m, 7H), 3.49–4.02 (m, 3H), 4.05–4.27 (m, 2H), 7.12–7.27
3
1
the residual solvent signal ( H: CHCl
CD
3
δ 7.26, CD
OD δ 49.00, CD
2
1
3
13
2
HCN δ 1.94; C: CDCl
3
δ 77.16, CD
3
3
CN δ 1.32). (m, 6H), 7.48–7.57 (m, 2H); C NMR (CD
3
CN) δ: 27.6, 38.1, 41.6,
8.9, 65.6, 68.0, 115.9 (d, JCF = 22.2 Hz), 127.1, 127.3, 128.3, 128.6,
129.3 (d, JCF = 4.6 Hz), 130.3 (d, JCF = 8.4Hz), 132.4, 133.6, 164.1
2
4
4
3
Chemistry
,2,3,4-Tetrahydronaphthalene-2,3-oxirane (1)
1
+
(
d, JCF = 247.7Hz) 169.4; ESI-MS m/z (M+ H) : 355.3.
1
m-Chloroperoxybenzoic acid (70–75% 3.10 g, 13.0 mmol) was
added dropwise to a stirring solution of 1,4-dihydronaphthalene
(
(±)-Trans-)-3-{4-[(4-bromophenyl)carbonylꢀpiperazin-1-yl}-1,2,3,4-
tetrahydronaphthalen-2-ol (5b)
(1.43 g, 11.0 mmol) in chloroform (55 mL) at 0 °C. The reaction
To a solution of ꢁ (52 mg, 0.22 mmol) in dichloromethane (11 mL)
and ethanol (1 mL), 4b (143 mg, 0.71 mmol), HBTU (153 mg,
0
stirred for 25 h at room temperature. NaOH (0.1 M, 20 mL) was
added to the reaction mixture and extracted with
dichloromethane. The concentrate was dissolved in acetonitrile
and water, and purified by preparative HPLC (5–100% MeCN
mixture was stirred for 20 h while the temperature gradually rose
to room temperature. The reaction mixture was extracted
.40 mmol), and DIPEA (150 μL, 0.86 mmol) were added and
with 5% K
with MgSO
2
CO
3
(3 × 50 mL), and the organic layer was dried
4
, filtered, and concentrated. The crude product
was purified by silica gel chromatography with isohexane : ethyl
acetate 6:1 as eluent to give 1 as a crystal after evaporation of
1
solvent (1.28 g, 79%). H NMR (CDCl
3
) δ: 3.17–3.26 (m, 2H),
over 20 min) to give ꢀb as a solid after freeze drying (59 mg,
3
.29–3.37 (m, 2H), 3.47–3.52 (m, 2H), 7.03–7.08 (m, 2H), 7.13–7.18
1
1
3
6
2
2
7
4
1
4%). H NMR (CD
3
OD) δ: 2.64 (br. s, 1H), 2.72–2.88 (m, 6H),
3
(m, 2H); C NMR (CDCl ) δ: 29.2, 51.5, 126.4, 129.1, 131.4;
+
.90–3.01 (m, 1H), 3.16 (dd, J = 5.66, 16.60 Hz, 1H), 3.48 (br. s,
EI-MS m/z (M) :146.1.
H), 3.74–3.89 (m, 2H), 3.94–4.02 (m, 1H), 7.06–7.09 (m, 4H),
Benzyl 4-[((±)-trans-)-3-hydroxy-1,2,3,4-tetrahydronaphthalen-2-ylꢀ
piperazine-1-carboxylate (2)
13
3
.36 (m, 2H), 7.64 (m, 2H) C NMR (CD OD) δ: 28.9, 39.3, 40.8,
8.3, 67.5, 68.3, 125.3, 127.3, 127.3, 129.8, 129.8, 130.2, 133.1,
+
35.4, 136.1, 136.3, 171.4: ESI-MS m/z (M + H) : 415.15 ESI-MS
1-Cbz-Piperazine (2.1mL, 10.9 mmol) was added to a stirred
+
m/z (M + 2H) : 417.2.
solution of 1 (380 mg, 2.60 mmol) and LiClO (430 mg, 4.04 mmol)
4
in MeCN (13 mL). After refluxing for 22h, the reaction mixture
was cooled to room temperature and concentrated. The crude
product was purified on silica gel chromatography with isohexane:
(
(±)-Trans-)-3-{4-[(4-nitrophenyl)carbonylꢀpiperazin-1-yl}-1,2,3,4-
tetrahydronaphthalen-2-ol (5c)
ethyl acetate 2:1 as eluent to give 2 as a solid after evaporation of
1
To a solution of ꢁ (86 mg, 0.37 mmol) in dichloromethane (4 mL),
4
(
When finished, water was added to the reaction mixture,
and extraction followed. The concentrate was dissolved in
acetonitrile and purified with preparative HPLC (20–75% MeCN
over 40 min) to give ꢀc as a solid after freeze drying (107 mg,
7
solvent (630 mg, 66%). H NMR (CDCl ) δ: 2.54 (brs, 2H), 2.71–2.95
3
c (199 mg, 1.19 mmol), HBTU (209 mg, 0.55 mmol), and DIPEA
190 μL, 1.09 mmol) were added and allowed to react for 2 h.
(
(
(
m, 6H), 3.33 (dd, J = 16.2, 5.7Hz, 1H), 3.53–3.69 (m, 4 H), 3.90
td, J = 9.8, 5.9 Hz, 1 H), 4.13 (brs, 1 H), 5.18 (s, 2H), 7.04–7.23
13
m, 4H), 7.29–7.48 (m, 5H); C NMR (CDCl ) δ: 26.1, 37.6, 44.2,
3
4
8.0, 65.5, 66.3, 67.1, 126.0, 126.1, 127.8, 127.9, 128.4, 128.9, 129.0,
+
133.6, 134.1, 136.5, 155.0; ESI-MS m/z (M+ H) : 367.2.
1
(
(±)-Trans-)-3-(piperazin-1-yl)-1,2,3,4-tetrahydronaphthalen-2-ol (3)
A solution of 2 (138 mg, 0.38 mmol) and Pd/C in 6 mL ethanol/
dichloromethane (2:1) was stirred under H (10 bar) for 5 h. The
6%). H NMR (CD
3
CN) δ: 2.91 (dd, J = 16.0, 10.2 Hz, 1H), 3.02–
3
.18 (m, 2H), 3.19–3.52 (m, 7H), 3.58–4.01 (m, 3H), 4.11–4.27
(
m, 2H), 7.14–7.26 (m, 4H), 7.68–7.76 (m, 2H), 8.29–8.36 (m, 2H);
2
1
3
3
C NMR (CD CN) δ: 28.0, 38.7, 45.2, 49.2, 65.8, 68.4, 125.0,
reaction mixture was then filtered through celite, and the filter
was washed with ethanol. The filtrate was concentrated to give
1
1
27.8, 128.0, 129.5, 130.0, 130.0, 132.7, 134.0, 142.0, 149.8,
+
68.8; ESI-MS m/z (M + H) : 382.
ꢁ
(85 mg, 97%). For use of ꢁ as a precursor in the radiochemistry,
preparative HPLC (0–15% MeCN over 10 min) purification was
performed. The resulting formic acid salt of ꢁ was then basified
with 30% KOH and extracted with dichloromethane to give ꢁ
(
(±)-Trans-)-3-{4-[(naphthalen-2-yl)carbonylꢀpiperazin-1-yl}-1,2,3,4-
1
tetrahydronaphthalen-2-ol (5d)
as a solid after evaporation of solvent (37 mg, 40%). H NMR
(CD
3
OD) δ: 2.54–2.60 (m, 1H), 2.72–2.87 (m, 5H), 2.89–2.99 To a solution of ꢁ (66 mg, 0.28 mmol) in dichloromethane (3 mL),
(
m, 3H), 3.01–3.06 (m, 1H), 3.09–3.19 (m, 3H), 3.89–4.00 (m, 1H), 4d (159 mg, 0.92 mmol), HBTU (178 mg, 0.47 mmol), and DIPEA
1
3
7
6
.03–7.10 (m, 4H); C NMR (CD
8.2, 127.3, 129.8, 130.6, 135.3, 136.1; ESI-MS m/z (M+ H) : 233.0.
3
OD) δ: 29.1, 39.3, 45.9, 48.1, 67.5, (150 μL, 0.86 mmol) were added and allowed to react for 1 h.
+
When finished, 0.1 M NaOH was added to the reaction mixture,
and extraction followed. Purification by silica gel
chromatography eluting with ethyl acetate gave ꢀd as a liquid
(
(±)-Trans-)-3-{4-[(4-fluorophenyl)carbonylꢀpiperazin-1-yl}-1,2,3,4-
tetrahydronaphthalen-2-ol (5a)
1
after evaporation of solvent (72 mg, 67%). H NMR (CDCl ) δ:
3
To a solution of ꢁ (85 mg, 0.37 mmol) in dichloromethane (5 mL) 2.75–3.13 (m, 8H), 3.28 (dd, J = 16.0, 5.5 Hz, 1H), 3.62–4.08
was added 4a (167 mg, 1.19 mmol), O-(benzotriazol-1-yl)-N,N,N′,N (m, 4H), 4.12 (q, J = 7.0 Hz, 1H), 4.35 (brs, 1H), 7.06–7.19 (m, 4H),
13
′
-tetramethyluronium hexafluorophosphate (HBTU) (247 mg, 7.48–7.60 (m, 3H), 7.83–7.97 (m, 4H); C NMR (CDCl ) δ:
3
0
2
.65 mmol), and N,N-diisopropylethylamine (DIPEA) (400 μL, 27.0, 37.8, 41.7, 49.0, 65.9, 66.8, 124.1, 126.3, 126.5, 126.8, 127.1,
.29 mmol); and the solution was stirred overnight in room 127.3, 127.8, 128.4, 128.5, 128.9, 129.0, 132.3, 132.6, 133.2,
+
temperature. Water was added to the reaction mixture and 133.5, 133.7, 170.4; ESI-MS m/z (M + H) : 387.3.
J. Label Compd. Radiopharm 2014, 57 525–532
Copyright © 2014 John Wiley & Sons, Ltd.
www.jlcr.org

S. Bergman et al.
(
(±)-Trans-)-3-{4-[(naphthalen-1-yl)carbonylꢀpiperazin-1-yl}-1,2,3,4- assess the identity, concentration, and radiochemical and chemical
tetrahydronaphthalen-2-ol (5e)
To a solution of ꢁ (39 mg, 0.17 mmol) in dichloromethane (3 mL),
e (104 mg, 0.60 mmol), HBTU (98 mg, 0.26 mmol), and DIPEA
100 μL, 0.57 mmol) were added and stirred for 1 h. When
finished, 0.1 M NaOH was added to the reaction mixture, and
extraction followed. The concentrate was purified with silica gel
purity of the radiotracer. All compounds passed the test for
chemical purity, here defined as no UV absorbing compound
having a peak area exceeding that of the product.
4
(
For determination of the trapping efficiency (TE), defined as
11
conversion of [ C]CO to non-volatile products, the crude reaction
mixture was transferred to a 1-mL glass vial, and the radioactivity
was measured. The THF was evaporated under a stream of helium
for 3–4 min before the radioactivity was measured again.
chromatography with ethyl acetate as eluent to give ꢀe as a
1
solid after evaporation of solvent (47 mg, 71%). H NMR (CDCl )
3
δ: 2.59–3.02 (m, 8H), 3.20–3.37 (m, 3H), 3.82–3.92 (m, 1H),
11
(
(±)-Trans-)-3-{4-[(4-fluorophenyl)[ Cꢀcarbonylꢀpiperazin-1-yl}-1,2,3,4-
3
7
2
1
1
.95–4.09 (m, 2H), 4.13 (q, J = 7.3 Hz, 1H), 7.06–7.18 (m, 4H),
11
tetrahydronaphthalen-2-ol ([ C]5a)
13
.42–7.61 (m, 4H), 7.85–7.93 (m, 3H); C NMR (CDCl ) δ:
3
Preparative HPLC method A and analytical HPLC method A. Product
t = 10.4–11.6 min. 4-Fluoroiodobenzene t = 15.6–16.7 min.
6.2, 37.6, 42.2, 47.7, 65.6, 66.4, 123.9, 124.7, 125.1, 126.1, 126.3,
26.5, 127.0, 128.4, 129.0, 129.2, 129.2, 129.5, 133.5, 133.6,
R
R
+
33.9, 134.1, 169.4; ESI-MS m/z (M + H) : 387.3.
1
1
(
(±)-Trans-)-3-{4-[(4-bromophenyl)[ Cꢀcarbonylꢀpiperazin-1-yl}-
1
1
(
(±)-Trans-)-3-{4-[(4-methoxyphenyl)carbonylꢀpiperazin-1-yl}-1,2,3,4- 1,2,3,4-tetrahydronaphthalen-2-ol ([ C]5b)
tetrahydronaphthalen-2-ol (5f)
Preparative HPLC method A and analytical HPLC method A. Product
= 13.6–14.7 min. 4-Bromoiodobenzene t = 18.5–19.5 min.
To a solution of ꢁ (70 mg, 0.30 mmol) in dichloromethane (3mL), 4f
t
R
R
(
(
140 mg, 0.92 mmol), HBTU (177 mg, 0.47 mmol), and DIPEA
160 μL, 0.92 mmol) were added and allowed to react for 1 h. When
1
1
(
(±)-Trans-)-3-{4-[(4-nitrophenyl)[ Cꢀcarbonylꢀpiperazin-1-yl}-1,2,3,4-
1
1
tetrahydronaphthalen-2-ol ([ C]5c)
finished, 0.1 M NaOH was added to the reaction mixture, and
extraction followed. The concentrated product was purified
with silica gel chromatography with ethyl acetate as eluent to
Preparative HPLC method A and analytical HPLC method A.
Product t
R
R
= 11.1–11.6 min. 4-Nitroiodobenzene t = 20.6–21.8min.
give ꢀf as a solid after evaporation of solvent (85 mg, 77%).
1
11
3
H NMR (CDCl ) δ: 2.60 (brs, 2H), 2.78–2.96 (m, 6H), 3.32 (dd, ((±)-Trans-)-3-{4-[(naphthalen-2-yl)[ Cꢀcarbonylꢀpiperazin-1-yl}-
1
1
J = 16.0, 5.5 Hz, 1H), 3.59–3.95 (m, 8H), 4.09 (brs, 1H), 6.90–6.96 1,2,3,4-tetrahydronaphthalen-2-ol ([ C]5d)
13
(m, 2H), 7.07–7.18 (m, 4H), 7.38–7.45 (m, 2H); C NMR (CDCl
3
) δ:
Preparative HPLC method B and analytical HPLC method B.
Product t = 15.5–17.2 min. 2-Iodonaphthalene t = 20.2–22.7 min.
2
1
6.3, 37.7, 48.4, 55.3, 65.6, 66.4, 113.7, 126.1, 126.3, 127.5, 129.0,
+
R
R
29.1, 133.6, 134.1, 160.8, 170.3; ESI-MS m/z (M+ H) : 367.3.
1
1
(
(±)-Trans-)-3-{4-[(naphthalen-1-yl)[ Cꢀcarbonylꢀpiperazin-1-yl}-
1
1
Radiochemistry
General procedure for synthesis of compounds [ C]5a–f
Amine precursor ꢁ (3 mg, 13 μmol), Pd(PPh (2 mg, 1.7 μmol),
1,2,3,4-tetrahydronaphthalen-2-ol ([ C]5e)
1
1
Preparative HPLC method A and analytical HPLC method B.
Product t
R
R
= 14.0–15.6 min. 1-Iodonaphthalene t = 19.2–20.0 min.
3 4
)
and aryl iodide 6a–f (11 mg, 39–50 μmol) were dissolved in
00 μL of freshly distilled THF and injected onto a 200-μL
11
(
(±)-Trans-)-3-{4-[(4-methoxyphenyl)[ Cꢀcarbonylꢀpiperazin-1-yl}-1,2,3,4
6
11
tetrahydronaphthalen-2-ol ([ C]5f)
injection loop. Freshly distilled THF was pumped through the
loop using an HPLC pump (30–40 MPa), and the reagents were
there by transferred into a 200-μL, stainless steel micro-autoclave
Preparative HPLC method A and analytical HPLC method A. Product
t = 9.4–11.1 min. 4-Methoxyiodobenzene t = 16.3–17.3 min.
R
R
11
containing no-carrier-added [ C]carbon monoxide (10–100 nmol,
.12–1.0 Pa) in helium (40 mmol, 500 kPa) obtained after an
Autoradiography
0
11
online reduction of [ C]carbon dioxide over zinc granules heated The in vitro binding was investigated in rat and mouse brain
to 400 °C. The reaction was run for 5 min at 150 °C. The crude tissue, and human postmortem brain tissue from Alzheimer
reaction mixture (in 0.2 mL THF) was diluted with 0.8 mL water : patients using autoradiography. Slides were thawed at room
ethanol (1:1) and purified with preparative HPLC. For preparative temperature and pre-incubated in phosphate-buffered saline
purification, either method A or B was used for the purification of (PBS) for 10 min following incubation with the labeled
1
1
compounds [ C]ꢀa–f. Method A: 30–55% acetonitrile over compound at 0.5–3 MBq/mL for 40 min at 37 °C. Non-specific
3 min, 55–100% acetonitrile over 7 min, and 100% acetonitrile binding was determined in the presence of excess (À)vesamicol
over 10 min, all in 8.1 mM ammonium carbonate. Method B: and 1,3-di(2-tolyl)-guanidine or unlabeled reference compound
1
3
5
0–55% acetonitrile over 15 min, 55–100% acetonitrile over (±)ꢀa–f. After incubation, the slides were washed twice for
min, and 100% acetonitrile over 10 min, all in 8.1 mM 3 × 3 min each in ice-cold PBS buffer followed by a brief wash
ammonium carbonate. For analytical HPLC, either method A or in distilled water. The slides were then dried and exposed to
1
1
B was used for identification of compounds [ C]ꢀa–f. Method phosphor imaging plates (Molecular Dynamics Inc., Sunnyvale,
A: 40:60 8.0 mM ammonium carbonate : acetonitrile over 10 min. CA, USA) before scanning in a Phosphor Imager Model 400S
Method B: 50:50 8.0 mM ammonium carbonate : acetonitrile over (Molecular Dynamics Inc.) at a resolution of 100 μm/pixel. For
1
0 min. Retention time on the preparative HPLC for the amine calibration, 20 μL aliquots of the incubation solution were
precursor was 2.0–3.3 min and for the aryl iodides 6a–f 15.6– dropped onto a filter paper and scanned together with the
2.7 min. The radioactivity of the purified product was measured sections. The sections were analyzed by ImageQuant 5.1. The
2
and analytical HPLC equipped with a UV and radioactivity specific binding was defined as subtracting the non-specific
detector together with the reference compound was used to binding from the total binding.
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Copyright © 2014 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2014, 57 525–532

S. Bergman et al.
Results and discussion
radiochemical yields and specific activities for the evaluation
Table 1). The different chemical components (amine precursor,
(
1
1
A library of six C-labeled PET compounds was successfully aryl iodide, and labeled amide product) eluted with distinctly
synthesized, and their binding selectivity for VAChT was then different retention times, which facilitated the purification.
evaluated by autoradiography. Taking the library concept to a However, the protocol suited the synthesis of the tracers
radiochemical environment is a promising approach towards differently, which is reflected in the different isolated
experimental tracer development for PET in general and VAChT radiochemical yields and specific activities. Around 65–70% of
11
in particular. Already in the beginning of the project, there is a the [ C]carbon monoxide was trapped as non-volatile products
plan and a possibility to label all interesting compounds apart (TE) during the reaction. From this, the tracers could be obtained
from the possibility to vary the properties of the compounds. with decay-corrected radiochemical yields ranging from 4% to
By using a standard protocol for the labeling and having one 25% with more than 95% radiochemical purity. As an example,
common precursor, variation could easily be introduced into 18.9 GBq of activity was delivered to the hotcell, and the
1
1
the compounds by choosing different and commercially product, [ C]-(±)ꢀd, was isolated in 15.5% radiochemical yield,
available aryl iodides. 98.1% radiochemical purity, and with a specific activity of
Synthesis of the reference compounds (±)ꢀa–f started from 200 GBq/μmol. 2-Naphthylderivative [ C]-(±)ꢀd and electron-
1
1
1
1
1
,4-dihydronaphthalene (Scheme 1). An epoxidation with rich [ C]-(±)ꢀf could be isolated in moderate radiochemical
31
11
meta-chloroperoxybenzoic acid in chloroform yielded 1.
yields, whereas the synthesis of [ C]-(±)ꢀc was rather sluggish,
Opening of the peroxide was performed by benzyl-1-piperazine which was reflected in the low radiochemical yield. A possible
carboxylate with lithium perchlorate acting as a Lewis acid, to explanation for the low radiochemical yield is that the nitro
3
2
give (±)2. Because of the trans-opening of the epoxide, 2 is group is reduced in the presence of carbon monoxide and
obtained as a racemat of the (R,R) and (S,S)-enantiomers. No palladium, reaction otherwise employed to synthesize
separation of the enantiomers were performed, although the aromatic anilines. A sterical influence could be discerned, as
a
38
1
1
(
R,R)-enantiomer often shows higher affinity for VAChT than the 2-naphthyl derivative [ C]-(±)ꢀd could be isolated in higher
13,33–35
11
the (S,S)-enantiomer.
separation was deemed not to be crucial for the synthesis of
At this stage, an enantiomeric radiochemical yield than the 1-naphthyl derivative [ C]-(±)ꢀe.
For assessment of the binding properties to VAChT of the
the library. (±)ꢁ was synthesized by deprotection of (±)2 by a labeled compounds, autoradiography studies were performed.
catalytic dehydrogenation. The reference compounds (±)ꢀa–f Labeled compounds [ C]-(±)ꢀa, [ C]-(±)ꢀc, and [ C]-(±)ꢀf
were made by a peptide coupling, utilizing HBTU and DIPEA in did not show any binding. Labeled compound [ C]-(±)ꢀb
36
11
11
11
1
1
dichloromethane to couple commercially available carboxylic showed an accumulation in the white mass of rat brain, which
3
7
acids 4a–f to (±)ꢁ.
could not be displaced (not shown). This lack of specific binding
1
1
The radiosynthesis of [ C]-(±)ꢀa–f is outlined in Scheme 2. All was surprising as the piperidine analogs of (±)ꢀa, (±)ꢀb, and
reagents (tetrakis(triphenylphosphine)palladium (0), aryl iodide, (±)ꢀc show high affinity for VAChT (2.70 ± 0.40, 0.25 ± 0.02, and
and amine) were dissolved in THF and injected to the micro- 0.48 ± 0.03 nM, respectively). A possible explanation is that
autoclave, which was pre-charged with no-carrier-added [ C] the piperazine ring prevents the compounds from adopting a
carbon monoxide. The aryl iodides were chosen to allow bioactive conformation. In crystal structures of vesamicol and
comparison to Tu et al. compounds and to see how different of a carbonylated benzovesamicol derivative, the phenyl ring
lipophilicities would affect the affinity to VAChT. The use of and the piperidine ring are arranged perpendicular to each
the same reaction parameters and similar conditions for HPLC other.
purification was an important aspect in the creation of the a planar conformation of the piperazine ring and the aryl ring
13
11
13
13,34
11
Compounds [ C]-(±)ꢀa–f would be prone to adopt
1
1
library. Compounds [ C]-(±)ꢀa–f were obtained in sufficient to allow for orbital overlap of the aryl and the amide. This may
Scheme 1. Synthesis of unlabeled reference compounds (±)ꢀa–f. (a) m-CPBA, CHCl
EtOH, CH Cl , 10 bar, rt, 5 h, 97%. (d) HBTU, DIPEA, CH Cl , rt, 1–22 h, 64–77%.
3 4 3 2
, 0 °C ➔ rt, 79%. (b) 1-Cbz-piperazine, LiClO , CH CN, reflux 22 h, 66%. (c) Pd/C, H ,
2
2
2
2
J. Label Compd. Radiopharm 2014, 57 525–532
Copyright © 2014 John Wiley & Sons, Ltd.
www.jlcr.org

S. Bergman et al.
11
3 4
Scheme 2. Radiosynthesis of tracers [ C]-(±)ꢀa–f. (a) Pd(PPh ) , THF, 150 °C micro-autoclave, 5 min.
1
1
a
Table 1. Products [ C]-(±)ꢀa–f obtained after aminocarbonylation
Trapping
efficiency (%)
Specific radioactivity
Radiochemical
yield (%)
Radioactive
purity (%)
b
c
d
e
Compound
cLogD7.4
(GBq/μmol)
1
1
1
1
1
1
1
1
1
1
1
1
[
[
[
[
[
[
C]-(±)ꢀa
C]-(±)ꢀb
C]-(±)ꢀc
C]-(±)ꢀd
C]-(±)ꢀe
C]-(±)ꢀf
2.52
3.14
2.35
3.37
3.31
2.21
42 ± 10 (2)
65 ± 4 (2)
68 ± 3 (3)
65 ± 9 (2)
70 ± 2 (2)
66 ± 5 (5)
164 ± 46 (3)
337 ± 87 (3)
203 ± 20 (2)
255 ± 130 (3)
124 ± 79 (3)
597 ± 208 (2)
10 ± 0.3 (2)
10 ± 3 (3)
4 ± 3 (2)
25 ± 5 (3)
9 ± 3 (3)
95 ± 4 (2)
97 ± 2 (3)
96 ± 2 (2)
97 ± 0.7 (3)
96 ± 2 (3)
>99 (2)
22 ± 7 (2)
a
Numbers in brackets refer to number of experiments.
b
c
Calculated using Chemaxon Marvin online software.
11
The fraction of [ C]-carbon monoxide converted to non-volatile products after removal of solvent.
d
Calculated with the radioactivity measured at EOS and concentrations determined by analytical HPLC.
e
Isolated and decay corrected. Calculated as a fraction of the radioactivity of the purified product and radioactivity delivered to
the hotcell.
11
11
Figure 2. Autoradiograms of labeled compounds (a) [ C]-(±)ꢀd and (b) [ C]-(±)ꢀe, showing total and non-specific binding in mouse, rat, and human tissue.
1
1
explain the low degree of binding to brain sections by [ C]-(±)ꢀe, the highest specific binding was seen in the mouse
1
1
11
compounds [ C]-(±)ꢀa–c and [ C]-(±)ꢀf.
brain tissue. For neither compound was the specific binding
11
11
Labeled compounds [ C]-(±)ꢀd and [ C]-(±)ꢀe on the other high enough to support further preclinical evaluation. The
hand showed various degrees of specific binding (Figures 2 and 3). lack of high-enough specific binding for these carbonyl-group
For [ C]-(±)ꢀd, the specific binding was highest in the rat containing piperazine ligands with a benzovesamicol scaffold is
brain tissue and lowest in the mouse brain tissue, whereas for in line with the low affinity shown by a carbonyl-group
1
1
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Copyright © 2014 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2014, 57 525–532

S. Bergman et al.
11
11
Figure ꢁ. Quantitative determination of specific binding of labeled compounds (a) [ C]-(±)ꢀd and (b) [ C]-(±)ꢀe.
containing piperazinevesamicol derivative synthesized by Rogers
[5] J. D. Erickson, H. Varoqui, M. K. H. Schäfer, W. Modi, M.-F. Diebler,
3
4
E. Weihe, J. Rand, L. E. Eiden, T. I. Bonner, T. B. Usdin, J. Biol. Chem.
1994, 269, 21929–21932.
et al. but at the same time surprising as the carbonyl piperidine
13
analogs (from Tu et al. ) showed nanomolar affinity for VAChT.
[
6] N. I. Bohnen, D. I. Kaufer, R. Hendrickson, L. S. Ivanco, B. J. Lopresti,
G. M. Constantine, C. A. Mathis, J. G. Davis, R. Y. Moore, S. T. DeKosky,
J. Neurol. 2006, 253, 242–247.
Conclusions
[
[
[
7] K. H. Chen, E. A. Reese, H.-W. Kim, S. I. Rapoport, J. S. Rao,
J. Alzheimers Dis. 2011, 26, 755–766.
8] D. Budd, L. C. Burns, G. L’Italien, P. Lapuerta, Clin. Outcomes Res.
The work presented herein concerns the development of a
methodology to find a suitable VAChT PET tracer. The approach
was based on the synthesis of a small radioligand library of
piperazine-based derivatives. A single amine precursor was
synthesized, the pharmacophore, which next was coupled to a
number of commercially available aromatic iodides though a
2
011, 3, 189–195.
9] N. I. Bohnen, K. A. Frey, Mol. Imaging Biol. 2007, 9, 243–257.
[
[
[
[
[
10] N. Giboureau, K. M. Aumann, C. Beinat, M. Kassiou, Curr. Bioact.
Compd. 2010, 6, 129–155.
11] R. T. Brittain, G. P. Levy, M. B. Tyers, Eur. J. Pharmacol. 1969, 8,
93–99.
12] N. Giboureau, I. Mat Som, A. Boucher-Arnold, D. Guilloteau,
M. Kassiou, Curr. Top. Med. Chem. 2010, 10, 1569–1583.
13] Z. Tu, S. M. N. Efange, J. Xu, S. Li, L. A. Jones, S. M. Parsons, R. H. Mach,
J. Med. Chem. 2009, 52, 1358–1369.
14] Y.-W. Jung, M. E. Van Dort, D. L. Gildersleeve, D. M. Wieland, J. Med.
Chem. 1990, 33, 2065–2068.
11
11
palladium (0)-mediated C-carbonylation reaction using [ C]
11
carbon monoxide to produce a library of potential C-labeled
VAChT tracers. This was followed by a simple preclinical
evaluation of the labeled compounds for assessment of their
binding potential to VAChT, as measured by frozen section
autoradiography studies with sections or brain from different
species. Specific binding in the autoradiography studies was only [15] K. Bando, N. Tomoyoshi, K. Taguchi, Y. Ginoza, Y. Tanaka, K. Koike,
1
1
11
K. Takatoku, Synapse 2000, 38, 27–37.
shown by labeled compounds [ C]-(±)ꢀd and [ C]-(±)ꢀe. The
amount of specific binding however was too low for the
compounds to be interesting for further preclinical evaluation.
[
16] D. Sorger, M. Scheunemann, J. Vercouillie, U. Grossmann, S. Fischer,
A. Hiller, B. Wenzel, A. Roghani, R. Schliebs, J. Steinbach, P. Brust,
O. Sabri, Nucl. Med. Biol. 2009, 36, 17–27.
This behavior of the compounds strongly suggests that their [17] D. Sorger, M. Scheunemann, U. Grossmann, S. Fischer, J. Vercouille,
A. Hiller, B. Wenzel, A. Roghani, R. Schliebs, P. Brust, O. Sabri,
J. Steinbach, Nucl. Med. Biol. 2008, 35, 185–195.
18] Y. W. Jung, K. A. Frey, G. K. Mulholland, R. del Rosario, P. S. Sherman,
D. M. Raffel, M. E. Van Dort, D. E. Kuhl, D. L. Gildersleeve,
D. M. Wieland, J. Med. Chem. 1996, 39, 3331–3342.
affinity for VAChT is low. However, the synthetic approach using
a single precursor combined with several readily available building
[
11
blocks in a standardized C-carbonylation proved to be an
efficient way to quickly label a small but diverse library of VAChT
ligands. Work is currently underway in utilizing this approach for [19] N. Giboureau, P. Emond, R. R. Fulton, D. J. Henderson, S. Chalon,
L. Garreau, P. Roselt, S. Eberl, S. Mavel, S. Bodard, M. J. Fulham,
investigation of another part of the benzovesamicol scaffold.
D. Guilloteau, M. Kassiou, Synapse 2007, 61, 962–970.
20] M. R. Kilbourn, B. Hockley, L. Lee, P. Sherman, C. Quesada, K. A. Frey,
R. A. Koeppe, Nucl. Med. Biol. 2009, 36, 489–493.
21] K. Shiba, S. Nishiyama, H. Tsukada, K. Ishiwata, K. Kawamura,
K. Ogawa, H. Mori, Synapse 2009, 63, 167–171.
22] S. M. N. Efange, R. H. Michelson, A. B. Khare, J. R. Thomas, J. Med.
Chem. 199ꢁ, 36, 1754–1760.
23] M. Parent, M.-A. Bedard, A. Aliaga, J.-P. Soucy, E. Landry St-Pierre,
M. Cyr, A. Kostikov, E. Schirrmacher, G. Massarweh, P. Rosa-Neto,
Neuroimage 2012, 62, 555–561.
[
[
[
[
Acknowledgements
We gratefully acknowledge the financial support from the
Disciplinary domain of Medicine and Pharmacy, Uppsala University,
and the Swedish Society for Medical Research (SSMF). We thank
Associate Professor Luke Odell for reviewing the manuscript.
[
[
[
24] M. Cyr, M. J. Parent, N. Mechawar, P. Rosa-Neto, J.-P. Soucy, A. Aliaga,
A. Kostikov, D. A. A. Maclaren, S. D. Clark, M.-A. Bedard, Nucl. Med.
Biol. 2014, 41, 96–101.
25] M. Petrou, K. A. Frey, M. R. Kilbourn, P. J. H. Scott, D. M. Raffel,
N. I. Bohnen, M. L. T. M. Müller, R. L. Albin, R. A. Koeppe, J. Nucl.
Med. 2014, 55, 396–404.
Conflict of Interest
The authors declare no conflict of interest.
References
26] O. Åberg, B. Långström, J. Label. Compd. Radiopharm. 2012, 55,
477–483.
[
[
[
[
1] U. Arvidsson, M. Riedl, R. Elde, B. Meister, J. Comp. Neurol. 1997, 378,
454–467.
[27] S. M. N. Efange, A. B. Khare, K. von Hohenberg, R. H. Mach,
S. M. Parsons, Z. Tu, J. Med. Chem. 2010, 53, 2825–2835.
[28] Z. Tu, W. Wang, J. Cui, X. Zhang, X. Lu, J. Xu, S. M. Parsons, Bioorg.
Med. Chem. 2012, 20, 4422–4429.
[29] B. Wenzel, D. Sorger, K. Heinitz, M. Scheunemann, R. Schliebs,
J. Steinbach, O. Sabri, Eur. J. Med. Chem. 200ꢀ, 40,
1197–1205.
2] E. Weihe, J.-H. Tao-Cheng, M. K. Schafer, J. D. Erickson, L. E. Eident,
Proc. Natl. Acad. Sci. 1996, 93, 3547–3552.
3] M. L. Gilmor, N. R. Nash, A. Roghani, R. H. Edwards, Y. Hong,
S. M. Hersch, A. I. Levey, J. Neurosci. 1996, 16, 2179–2190.
4] D. C. Anderson, S. C. King, S. M. Parsons, Mol. Pharmacol. 198ꢁ,
24, 48–54.
J. Label Compd. Radiopharm 2014, 57 525–532
Copyright © 2014 John Wiley & Sons, Ltd.
www.jlcr.org

S. Bergman et al.
[
30] T. Kihlberg, B. Långström, WO02102711A1.pdf. WO 02/102711 A1,
002.
31] L. F. Tietze, A. Heins, M. Soleiman-Beigi, C. Raith, Heterocycles 2009,
7, 1123–1146.
W. D. Kornreich, R. Kaufman, R. S. Jacobs, B. Kirtmant, J. Med. Chem.
1989, 1217–1230.
[35] K. Shiba, K. Ogawa, K. Ishiwata, K. Yajima, H. Mori, Bioorg. Med. Chem.
2006, 14, 2620–2626.
2
[
7
[
[
32] M. Chini, P. Crotti, F. Macchia, Tetrahedron Lett. 1990, 31, 4661–4664. [36] S. Vassiliou, P. Kosikowska, A. Grabowiecka, A. Yiotakis, P. Kafarski,
33] Y. Zea-Ponce, S. Mavel, T. Assaad, S. E. Kruse, S. M. Parsons, P. Emond, L. Berlicki, J. Med. Chem. 2010, 53, 5597–5606.
S. Chalon, N. Giboureau, M. Kassiou, D. Guilloteau, Bioorg. Med. Chem. [37] S. Lauwagie, R. Millet, J. Pommery, P. Depreux, J. Henichart,
2
00ꢀ, 13, 745–753.
Heterocycles 2006, 68, 1149.
[38] A. M. Tafesh, M. Beller, Tetrahedron Lett. 199ꢀ, 36, 9305–9308.
[
34] G. A. Rogers, S. M. Parsons, D. C. Anderson, L. M. Nilsson, B. A. Bahr,
www.jlcr.org
Copyright © 2014 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2014, 57 525–532