A practical method for the preparation of 18F-labeled aromatic amino acids from nucleophilic [18F]fluoride and stannyl precursors for electrophilic radiohalogenation

Article abstract of DOI:10.3390/molecules22122231

In a recent contribution of Scott et al., the substrate scope of Cu-mediated nucleophilic radiofluorination with [¹⁸F]KF for the preparation of ¹⁸F-labeled arenes was extended to aryland vinylstannanes. Based on these findings, the potential of this reaction for the production of clinically relevant positron emission tomography (PET) tracers was investigated. To this end, Cu-mediated radiofluorodestannylation using trimethyl(phenyl)tin as a model substrate was re-evaluated with respect to different reaction parameters. The resulting labeling protocol was applied for ¹⁸F-fluorination of different electron-rich, -neutral and -poor arylstannyl substrates in RCCs of 16-88%. Furthermore, this method was utilized for the synthesis of ¹⁸F-labeled aromatic amino acids from additionally N-Boc protected commercially available stannyl precursors routinely applied for electrophilic radiohalogenation. Finally, an automated synthesis of 6-[¹⁸F]fluoro-L-m-tyrosine (6-[¹⁸F]FMT), 2-[¹⁸F]fluoro-L-tyrosine (2-[¹⁸F]F-Tyr), 6-[¹⁸F]fluoro-L-3, 4-dihydroxyphenylalanine (6-[¹⁸F]FDOPA) and 3-O-methyl-6-[¹⁸F]FDOPA ([¹⁸F]OMFD) was established furnishing these PET probes in isolated radiochemical yields (RCYs) of 32-54% on a preparative scale. Remarkably, the automated radiosynthesis of 6-[¹⁸F]FDOPA afforded an exceptionally high RCY of 54 5% (n = 5).

Full text of DOI:10.3390/molecules22122231

molecules
Article
A Practical Method for the Preparation of 18F-Labeled
Aromatic Amino Acids from Nucleophilic
[18F]Fluoride and Stannyl Precursors for
Electrophilic Radiohalogenation
Fadi Zarrad 1,2, Boris D. Zlatopolskiy 1,2,3,
*
ID , Philipp Krapf 1,2, Johannes Zischler 1,2 and
Bernd Neumaier 1,2,3,
*
ID
1
Institute of Neuroscience and Medicine, Nuclear Chemistry (INM-5), Forschungszentrum Jülich GmbH,
52428 Jülich, Germany; f.zarrad@fz-juelich.de (F.Z.); p.krapf@fz-juelich.de (P.K.);
j.zischler@fz-juelich.de (J.Z.)
Institute of Radiochemistry and Experimental Molecular Imaging, University Clinic Cologne,
50931 Cologne, Germany
Max Planck Institute for Metabolism Research, Cologne 50931, Germany
Correspondence: boris.zlatopolskiy@uk-koeln.de (B.D.Z.); b.neumaier@fz-juelich.de (B.N.);
Tel.: +49-221-4788-2842 (B.D.Z.); +49-246-161-4141 (B.N.)
2
3
*
Received: 6 November 2017; Accepted: 12 December 2017; Published: 15 December 2017
Abstract: In a recent contribution of Scott et al., the substrate scope of Cu-mediated nucleophilic
radiofluorination with [18F]KF for the preparation of 18F-labeled arenes was extended to aryl-
and vinylstannanes. Based on these findings, the potential of this reaction for the production
of clinically relevant positron emission tomography (PET) tracers was investigated. To this
end, Cu-mediated radiofluorodestannylation using trimethyl(phenyl)tin as a model substrate was
re-evaluated with respect to different reaction parameters. The resulting labeling protocol was applied
for 18F-fluorination of different electron-rich, -neutral and -poor arylstannyl substrates in RCCs of
16–88%. Furthermore, this method was utilized for the synthesis of 18F-labeled aromatic amino
acids from additionally N-Boc protected commercially available stannyl precursors routinely applied
for electrophilic radiohalogenation. Finally, an automated synthesis of 6-[18F]fluoro-L-m-tyrosine
(6-[18F]FMT), 2-[18F]fluoro-L-tyrosine (2-[18F]F-Tyr), 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine
(6-[18F]FDOPA) and 3-O-methyl-6-[18F]FDOPA ([18F]OMFD) was established furnishing these PET
probes in isolated radiochemical yields (RCYs) of 32–54% on a preparative scale. Remarkably,
the automated radiosynthesis of 6-[18F]FDOPA afforded an exceptionally high RCY of 54
(n = 5).
±
5%
Keywords: 18F; radiofluorination; destannylation; positron emission tomography
1. Introduction
The enormous clinical potential of PET imaging still remains underexplored owing to the lack
or poor accessibility of suitable molecular probes. Therefore, much effort has been spent in the last
decades towards the development of novel labeling methods with PET nuclides for the preparation
of structurally diverse imaging probes. Undeniably, 18F-labeled ligands play an outstanding role in
PET imaging. The popularity of 18F is mainly due to its easy accessibility at a small cyclotron as well
as its excellent nuclear decay properties like half-life and β
+ energy. Moreover, the half-life of 18F is
sufficiently long to allow shipping of 18F-labeled probes to more distant PET facilities. Additionally, the
relatively long half-life allows the accomplishment of demanding chemical conversions and long-time
Molecules 2017, 22, 2231; doi:10.3390/molecules22122231
www.mdpi.com/journal/molecules

Molecules 2017, 22, 2231
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measurements (up to 6 h). Furthermore, the low
PET images with high resolution.
β
+ energy (0.63 MeV) is ideally suited to acquiring
In the last few years new emerging radiofluorination methods have facilitated access to probes
which had been so far inaccessible or difficult to produce using conventional 18F-labeling procedures.
Particularly, methods for transition metal mediated radiofluorination pioneered by Ritter et al. and
Coenen et al. have the potential to change the paradigm of radiochemistry [
for Cu-mediated 18F-fluorination discovered by Scott et al. and Gouverneur et al. [
developed by inventors and others [
1
–
3
]. Obviously, procedures
] and further
–
12], enabling the preparation of 18F-labeled aromatics and
4–7
8
heteroaromatics regardless of their electronic properties using nucleophilic 18F− have gained special
interest. This is primarily due to the fact that these approaches do not require strictly controlled
conditions (e.g., complete exclusion of oxygen and/or moisture), poorly accessible or extremely
sensitive radiolabeling precursors. Moreover, these methods are easily amenable to automation [13,14]
The latter is a main prerequisite for the implementation of radiolabeling procedures for cGMP
production of clinically relevant PET probes.
.
In the seminal report on Cu-mediated radiofluorination (aryl)(mesityl)iodonium salts were
used as labeling substrates [4]. Nevertheless, these compounds are rather impractical for routine
PET tracer production. Moreover, polyfunctionalized iodonium salts are relatively difficult to
prepare. In many cases these compounds suffer from limited storage capability. Furthermore,
Cu(MeCN)4OTf used in this procedure as a Cu source has only a short shelf life under ambient
conditions. Accordingly, further efforts led to the development of procedures utilizing more stable
and readily available radiolabeling precursors like aryl boronic acids and pinacol boronates [5,6].
Recently, Scott et al. extended Cu-mediated 18F-fluorination to arylstannanes. They produced a
variety of 18F-labeled arenes on a small scale and demonstrated amenability of the novel method
to automation [7]. Arylstannanes can be easily prepared and have a long shelf life. Stannylated
compounds are well known substrates for electrophilic radiohalogenation [15,16]. Fortunately, many
of them are commercially available, including, precursors for 6-[18F]fluoro-L-m-tyrosine (6-[18F]FMT),
2-[18F]fluoro-L-tyrosine (2-[18F]F-Tyr), 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine (6-[18F]FDOPA),
5-[I*]iodo-20-desoxyuridine (5-[*I]IdU) and 5-[*I]iodo-3[2(S)-azetidinylmethoxy]-pyridine ([*I]IAP).
Owing to the known drawbacks of electrophilic radiofluorination (i.e., max. 50% RCY, significantly
lower accessible amounts of [18F]F2 compared to that of 18F−, impracticability of the preparation
of n.c.a tracers, disadvantages of gas vs. liquid target, necessity to handle with F2/Ne gas, etc.)
fluorodestannylation with nucleophilic 18F− could substantially improve the availability of various
PET probes. Unfortunately, the reported protocol is rather impractical for the production of labeled
compounds on a preparative scale due to high losses of 18F− (up to 70%) during 18F-preprocessing
before the radiolabeling step. Additionally, the applied Cu source Cu(OTf)2 is extremely hygroscopic
which may prevent its widespread application for routine radiosyntheses.
Recently, our group demonstrated that 18F-labeled arylstannanes could be obtained by applying
the protocol for alcohol-enhanced Cu-mediated radiofluorination. This approach utilizes not only
bench stable Cu(py)4(OTf)2 but also substantially simplifies the radiosynthesis by obviating time
consuming azeotropic drying steps. However, RCYs obtained with stannyl substrates were found to
be significantly lower than those with pinacol boronate or boronic acid precursors.
These preliminary findings prompted us to investigate Cu-mediated 18F-fluorination of
arylstannanes in more detail. The initial aim of this study was to devise a robust protocol for
radiofluorination of commercially available stannyl precursors. First of all, the newly developed
procedure should be applied for the production of 6-[18F]FDOPA. This tracer is widely applied
for the measurement of integrity and function of the nigrostriatal dopaminergic system, e.g.,
in Parkinson’s disease [17
Numerous protocols for the production of 6-[18F]FDOPA via nucleophilic radiofluorination have been
published [ 13 26 28]. However, the majority of them are cumbersome, poorly reproducible and/or
use insufficiently stable radiolabeling precursors and/or highly corrosive reagents [29]. Moreover,
–21] as well as for the detection and staging of neuroendocrine tumors [22–25].
9
, , –

Molecules 2017, 22, 2231
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the production of 6-[18F]FDOPA is frequently used in the literature to demonstrate the potential of
novel 18F-labeling techniques.
Likewise, a broader clinical application of 6-[18F]FMT, a structural analog of 6-[18F]FDOPA with
improved imaging properties, and other radiofluorinated aromatic amino acids is hampered by the
lack of simple production routes [30,31]. Therefore, the labeling method should also be applied to
obtain these compounds in high yields. Finally, the method should be transferred to automated
synthesis modules for cGMP production.
2. Results and Discussion
2.1. Effect of Different Salts on 18F-Recovery from Anion Exchange Resin and 18F-Incorporation
Optimization of radiofluorodestannylation was carried out using trimethyl(phenyl)tin (PhSnMe3)
as a model substrate. First, the 18F-elution capacity of different tetraethylammonium salts in MeOH
was studied (Figure 1). Almost complete radioactivity recovery (95–97%) was achieved with 2.5 µmol
of all four examined salts. 18F-Recovery decreased to 76–89% if only 0.5 µmol salt was used.
Figure 1.
18F-Recovery from anion exchange resin with MeOH solutions of different
tetramethylammonium salts. Conditions: [18F]Fluoride (~50 MBq) was fixed on a QMA-CO3 cartridge
from the male side and the cartridge was rinsed with MeOH (1 mL) in the same direction. Finally,
[
18F]fluoride was eluted with a solution of Et4NX in MeOH (500 µL) from the female side.
Next, in trying to optimize the conditions for alcohol-enhanced Cu-mediated radiofluorination
of aryl stannanes the dependency of radioactivity recovery and 18F-incorporation on different salts
was investigated using solutions of Et4NHCO3, Et4NOTf, KOTf/K2.2.2 and Bu4POMs in nBuOH.
(Table 1). With KOTf/K2.2.2 > 80% of 18F− was eluted from the resin. For other salts radioactivity
recovery amounted to 71–76%. The resulting solutions were diluted with a solution of PhSnMe3
and Cu(py)4(OTf)2 in DMA and heated to give [18F]FPh. Surprisingly, in the case of KOTf/K2.2.2
18F-incorporation did not exceed a RCC of 10%. In contrast, if ammonium or phosphonium salts were
applied, RCCs of 60–69% were achieved [32].
2.2. Dependency of 18F-Recovery and 18F-Incorporation Yields on the Type of Anion Exchange Cartridge
The type of anion exchange cartridge substantially influenced the efficacy of 18F-elution and
especially the subsequent radiolabeling step (Table 2). The highest radioactivity recovery was observed
for Strata X-CO3 followed by QMA-CO3 cartridges (81% and 73%, respectively). However, while using
Strata X-CO3 cartridges only fair RCCs of 24
73 ± 8% if QMA-CO3 cartridges were applied.
±
15% were obtained, 18F-incorporation amounted to

Molecules 2017, 22, 2231
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Table 1. 18F-Recovery and radiochemical conversions (RCCs) of [18F]FPh using different salts in
nBuOH. Conditions: [18F]Fluoride (~50 MBq) was loaded onto a QMA cartridge from the male side.
The cartridge was washed with nBuOH (1 mL) in the same direction and flushed with air (5 mL).
Afterwards 18F– was eluted from the female side with a solution of the respective salt (11
µ
mol) in
nBuOH (300 µL). A solution of PhSnMe3 (14.5 mg, 60 µmol) and Cu(py)4(OTf)2 (20.3 mg, 30 µmol) in
DMA (700 L) under air,
L) was added, the reaction mixture was heated at 100 ◦C for 10 min (300
diluted with H2O (1 mL) and analyzed by HPLC. All experiments were carried out in triplicate.
µ
µ
Et4NOTf
Et4NHCO3
KOTf/K2.2.2 Bu4POMes
Remaining 18F on the cartridge (%)
18F-Recovery (%)
19 ± 1
72 ± 8
69 ± 4
18 ± 2
71 ± 1
63 ± 9
9 ± 2
81 ± 5
9 ± 1
14 ± 4
76 ± 2
60 ± 11
RCC (%)
Table 2. Dependency of 18F-recovery and 18F-incorporation yields on the type of anion exchange
cartridge. Conditions: [18F]Fluoride (~50 MBq) was eluted from the respective anion exchange cartridge
with a solution of Et4NOTf (3.1 mg, 11
µmol) in nBuOH (300
µL) (see legend of Table 1). A solution
of PhSnMe3 (14.5 mg, 60
µ
mol) and C◦u(py)4(OTf)2 (20.3 mg, 30
µ
mol) in DMA (700 L) was added,
µ
the reaction mixture was heated at 100 C for 10 min under air atmosphere, diluted with H2O (1–4 mL)
and analyzed by HPLC. All experiments were carried out in triplicate.
QMA-CO3
Strata X-CO3
Strata X-HCO3
Chromafix PS-HCO3
Remaining 18F on the cartridge (%)
18F-Recovery (%)
19 ± 1
73 ± 8
69 ± 4
14 ± 1
81 ± 1
24 ± 15
18 ± 2
68 ± 9
37 ± 10
35 ± 2
57 ± 3
42 ± 5
RCC (%)
2.3. Influence of Different Alcohols and Water on 18F-Recovery and RCC of [18F]Fluorodestannylation
18F-Recovery as well as RCCs were strongly dependent on the nature of the respective alcohol
(Figure 2). Whereas 18F-recovery was highest for short-chained alcohols (for MeOH, EtOH and
TFE > 85%), RCCs increased if higher alcohols were used (cf. RCCs for nBuOH, tBuOH and
nAmOH > 70%). While TFE allowed efficient 18F-elution from the anion exchange resin, no
18F-incorporation was observed if this alcohol was used as reaction co-solvent. This should be attributed
to the acidic nature of TFE (pKa = 12.4) [33] and even more to a very strong hydrogen bond donor power
of trifluoroethanol [α (TFE) = 1.51] [34]. Consequently, TFE solvates halogenide ions much stronger
than MeOH [35] and, therefore, should strongly decrease nucleophilicity of 18F−. nBuOH and nAmOH
represented a reasonable compromise, between, on the one hand, sufficient 18F-recovery and, on the other
hand, high RCCs. Notably, the reaction was very sensitive to water: 18F-incorporation halved at the water
content of 0.5% (Figure 3). After addition of 15 µL H2O (1.5% final concentration) RCC fell below 10%.
Next, we evaluated the influence of nBuOH content on RCCs (Figure 4). Addition of up to 20–30%
nBuOH was well tolerated and did not cause a significant decrease of RCCs. Further increase of
nBuOH concentration resulted in lower RCCs of 58
±
9 and 42
±
2% in 40% and 50% n-butanolic
solutions, respectively. In contrast to Cu-mediated radiofluorination of arylboronic acids and pinacol
arylboronates where a pronounced increase of RCCs in the presence of nBuOH took place (nBuOH
content of 1–30%), no increase of 18F-incorporation yield was observed for the stannylated precursor.
The remarkable tolerance of Cu-mediated radiofluorination towards alcohols could be presumably
−
F with alcohols, which obviously decreases its basicity. This interaction is,
attributed to solvation of 18
however, not strong enough to significantly affect the nucleophilicity of [18F]fluoride [36,37].
We proposed that the first steps of Cu-mediated radiofluorination of boronate and stannyl precursors
consist of anion metathesis followed by air oxidation of the Cu(II) to the Cu(III) complex stabilized by py, and
in alcohol-containing media by alcoholate ligands (Figure 5) [38]. Thereafter, transmetalation should afford
18F-fluorinated (probably, polynuclear) [39] aryl(III)cuprates with RO and py ligands. Finally, reductive
elimination liberates either the desired [18F]ArF or the ArOR side product from the Chan-Lam coupling [40].

Molecules 2017, 22, 2231
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In our opinion, the beneficial effect of alcohols could be mainly attributed to their capability to stabilize the
transition state of the rate limiting B/Cu(III) transmetalation step by hydrogen bonding interactions between
the hydroxyl hydrogens of alcohol molecules and oxygens of B(OH)2 or BPin groups. This beneficial effect
should be more pronounced for B(OH)2 than for BPin and cannot occur in the case of aryltrialkylstannanes
where no hydrogen bond formation can occur. Indeed, a more distinct beneficial effect was observed for aryl
boronic acids than for aryl pinacol boronates [9]. This effect was absent for arylstannanes since no hydrogen
bond formation can take place. Additionally, in tBuOH medium where the stabilization via hydrogen bond
formation, especially in the case of pinacol boronate substrates, is limited by sterical hindrance of tBu group,
a deleterious and much less pronounced beneficial effect was observed for ArBpin and ArB(OH)2 substrates,
respectively. In contrast, in the case of arylstannane precursors the highest RCCs were observed in the
presence tBuOH in comparison to the other alcohols [36,37].
Figure 2. Effect of alcohol on 18F-recovery and 18F-incorporation. Conditions: 18F− (50–150 MBq) was
eluted from a QMA-CO3 cartridge into the reaction vial with a solution of Et4NOTf (3.1 mg, 11
µmol)
in the corresponding anhydrous alcohol (300 L) (see legend of Table 1); to this solution a solution of
µ
trimethyl(phenyl)tin (14.5 mg, 60
the reaction mixture was heated at 100 C for 10 min under air, diluted with H2O (1 mL) and analyzed by
HPLC. All experiments were carried out in triplicate.
µ
mol) and Cu(py)4(OTf)2 (20.3 mg, 30
µ
mol) in DMA (700
µ
L) was added,
◦
Figure 3. Effect of water on 18F-fluorodestannylation. Conditions: [18F]Fluoride (~50 MBq) was eluted
from a QMA-CO3 cartridge with a solution of Et4NOTf (3.1 mg, 11
legend of Table 1). A solution of PhSnMe3 (14.5 mg, 60 mol) and Cu(py)4(OTf)2 (20.3 mg, 30
in DMA (700 L) containing the respective quantity of H2O was added, the reaction mixture was
µmol) in nBuOH (300
µL) (see
µ
µmol)
µ
heated at 100 ◦C for 10 min under air atmosphere, diluted with H2O (1 mL) and analyzed by HPLC.
All experiments were carried out in triplicate.

Molecules 2017, 22, 2231
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Figure 4. Dependency of RCC on nBuOH content. Conditions: [18F]Fluoride (~50 MBq) was eluted
from a QMA-CO3 cartridge with a solution of Et4NOTf (3.1 mg, 11 mol) in MeOH (500 L) (see
captions of Figure 1), MeOH was evaporated at 80 C under a flow of air within 2–3 min. A solution
µ
µ
◦
of PhSnMe3 (14.5 mg, 60
µ
mol) and Cu(py)4(OTf)2 (20.3 mg, 30
µ
mol) in DMA/nBuOH (1 mL) was
◦
added, the reaction mixture was heated at 100 C for 10 min under air atmosphere, diluted with H2O
(1 mL) and analyzed by HPLC. All experiments were carried out in triplicate.
LG
R
LGOTf
Et4N18
F
Et4NOTf
-
LnCuII18
F
18F
LnCuIII18
F
O2
O2
LnCuII(OTf)18
F
LnCuII(OTf)2
R
R
R
L = Py and/or ROH
LG = B(OH)2, Bpin, SnR3
LnCuII
Figure 5. Proposed mechanism of Cu-mediated radiofluorination of aryl pinacol boronates, boronic
acids and stannanes.
2.4. Dependency of RCC on Reaction Solvent
The type of reaction solvent had a significant influence on RCCs (Figure 6). Thus, DMA and
N-methyl-2-pyrrolidone (NMP) afforded the highest RCCs of [18F]FPh of 72% and 73%, respectively.
Figure 6. Dependency of RCC on reaction solvent. Conditions: [18F]Fluoride (~50 MBq) was eluted
from a QMA-CO3 cartridge with a solution of Et4NOTf (3.1 mg, 11
captions of Figure 1), MeOH was evaporated at 80 C under a flow of air within 2–3 min. A solution
µ
mol) in MeOH (500
µ
L) (see
◦
of PhSnMe3 (14.5 mg, 60
µ
mol) and Cu(py)4(OTf)2 (20.3 mg, 30
µ
mol) in DMA/nBuOH (1 mL) was
◦
added, the reaction mixture was heated at 100 C for 10 min under air atmosphere, diluted with H2O
(1 mL) and analyzed by HPLC. All experiments were carried out in triplicate.

Molecules 2017, 22, 2231
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In N,N,N’,N’-tetramethylurea (TMU), DMF and DMSO RCCs amounted to only 28%, 9% and 7%,
respectively. In pyridine, N-methylformamide (NMF) and tBuOH no 18F-incorporation took place.
2.5. Dependency of RCC on Temperature and Time
The dependency of temperature (Figure 7) and time (Figure 8) on RCC revealed rapid reaction
kinetics. Maximal RCCs were achieved already after 5 min incubation at 100 ◦C. The optimal reaction
temperature amounted to 100 ◦C.
Figure 7. Dependency of RCC on temperature. Conditions: [18F]Fluoride (~50 MBq) was eluted from a
QMA-CO3 cartridge with a solution of Et4NOTf (3.1 mg, 11
Table 1). A solution of PhSnMe3 (14.5 mg, 60 mol) and Cu(py)4(OTf)2 (20.3 mg, 30
(700 L) was added, the reaction mixture was heated at different temperatures for 10 min, cooled down,
µmol) in nBuOH (300
µL) (see legend of
µ
µmol) in DMA
µ
diluted with H2O (1 mL) and analyzed by HPLC. All experiments were carried out in triplicate.
Figure 8. Dependency of RCC on reaction time. Conditions: [18F]Fluoride (~50 MBq) was eluted from
a QMA-CO3 cartridge with a solution of Et4NOTf (3.1 mg, 11
µmol) in nBuOH (300
µL) (see legend of
Table 1). A solution of PhSnMe3 (14.5 mg, 60
(700
µ
mol) and Cu(py)4(OTf)2 (20.3 mg, 30
µ
mol) in DMA
◦
µ
L) was added. The reaction mixture was heated at 100 C for different times, cooled down,
diluted with H2O (1 mL) and analyzed by HPLC. All experiments were carried out in triplicate.

Molecules 2017, 22, 2231
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2.6. Dependency of RCC on Precursor Amount and Precursor to Cu(py)4(OTf)2 Ratio
The amount of the stannyl substrate (Figure 9) and Cu(py)4(OTf)2 (Figure 10) was adjusted to
reduce costs and simplify the purification step. If 30–60
mol PhSnMe3 were applied, [18F]FPh was
obtained in RCCs of 70%. At 20 and 10
mol precursor, a decline of 18F-incorporation to 63% and
µ
≥
µ
44%, respectively, was observed. Consequently, all further experiments were performed with 30
of the corresponding stannyl precursor.
µmol
Figure 9. Dependency of RCC on precursor amount. Conditions: [18F]Fluoride (~50 MBq) was eluted
from a QMA-CO3 cartridge with a solution of Et4NOTf (3.1 mg, 11 mol) in nBuOH (300 L) (see
legend of Table 1). A solution of different amounts of PhSnMe3 and Cu(py)4(OTf)2 (20.3 mg, 30 mol)
in DMA (700
L) was added, the mixture was heated under air at 100 ◦C for 10 min, cooled down,
diluted with H2O (1 mL) and analyzed by HPLC. All experiments were carried out in triplicate.
µ
µ
µ
µ
Figure 10. Dependency of Cu(py)4(OTf)2 amount on RCC. Conditions: [18F]Fluoride (~50 MBq) was
eluted from a QMA-CO3 cartridge with a solution of Et4NOTf (3.1 mg, 11 mol) in nBuOH (300 L)
(see legend of Table 1). A solution of PhSnMe3 (7.2 mg, 30 mol) and a given amount of Cu(py)4(OTf)2
in DMA (700
L) was added, the mixture was heated under air at 100 ◦C for 10 min, cooled down,
diluted with H2O (1 mL) and analyzed by HPLC. All experiments were carried out in triplicate.
µ
µ
µ
µ
This precursor amount is higher in comparison to that used by Makaravage et al. which amounted
to 10 mol [ ]. However, owing to the reasonable accessibility of arylstannanes this quantity may be
µ
7
considered as acceptable for the majority of applications. Occasionally, it may be difficult to separate
larger amounts of radiolabeling precursor and/or product of its protodestannylation from a radiolabeled
compound even when using preparative HPLC. Yet, for all PET samples described herein this problem
has not been encountered. The novel protocol for 18F-fluorodestannylation was rather insensitive to the

Molecules 2017, 22, 2231
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stannane/Cu salt ratio. Comparable RCCs were achieved at PhSnMe3/Cu(py)4(OTf)2 ratios of 3:4 to
2:3. A marked decrease of conversion was first observed at a substrate/Cu salt ratio of 3:1.
2.7. Optimized Protocol of 18F-Fluorodestannylation
Based on the optimization study, a novel protocol of radiofluorodestannylation was developed.
In order to obviate the notable loss of radioactivity during 18F-recovery using nBuOH, we modified the
elution procedure. We used Et4NOTf in MeOH for 18F-elution according to Richarz et al. [41
,
42]. After
elution, low boiling methanol was removed within 2–5 min at 100 ◦C, and a solution of arylstannane
precursor and Cu(py)4(OTf)2 (30 mol of each) in pure DMA (1 mL) was added to the residue. Thus,
µ
owing to the absence of the beneficial effect we did not use nBuOH. After that, the reaction mixture
was heated under air at 100 ◦C for 10 min.
The scope of this protocol was evaluated using several model arylstannanes (Figure 11).
The method worked equally well if either SnMe3 or SnBu3 precursors were applied. Substrates
with electron-donating and electron-neutral substituents in m- and p-positions (Figure 11, entries
2, 5 and 6) were radiolabeled in moderate to high RCCs. The introduction of a methoxy group
into o-position (entry 4) resulted in lower RCCs, presumably due to unfavorable interactions of the
substituent with the leaving group, thereby impeding transmetalation. Notably, Scott et al. prepared
o-[18F]fluoroanisole in a RCC of 48
presence of an excess of pyridine [ ]. The excess of pyridine, presumably, can additionally stabilize the
±
4% using Cu(py)4(OTf)2 formed in situ from Cu(OTf)2 in the
7
Cu-complex and thus can overcome the deleterious effect of the o-MeO group. Fair to moderate RCCs
were obtained with precursors with electron-withdrawing substituents (entry 3).
a
Figure 11. Substrate scope of the optimized protocol for 18F-fluorodestannylation. RCC
single experiment was carried out.
±
SD. b RCY,
Finally, 18F-labeled anle186b was successfully prepared for the first time in RCC of 62% and in 48%
isolated RCY. This 3,5-diaryl substituted pyrazole is able to bind to pathological protein aggregates
in
suitable for imaging of such pathologies.
α-synucleinopathies and prion disease [43,
44]. Consequently, [18F]anle186b could be potentially

Molecules 2017, 22, 2231
10 of 25
2.8. Preparation of 18F-Labeled Aromatic Amino Acids
Once the optimized protocol for radiofluorination of arylstannanes had been established, we
turned to the production of clinically relevant 18F-labeled aromatic amino acids. Unfortunately,
direct radiolabeling of commercially available N-monoBoc 6-SnMe3 substituted phenylalanine
derivatives afforded radiolabeled intermediates in poor RCCs of 5–6%, presumably, due to concurrent
intramolecular Chan-Lam coupling. This will furnish the respective indolines instead of the desired
radiolabeled products via attack of the intermediate arylcuprate on the amide anion formed by the
proton abstraction with sufficiently basic “naked” fluoride [5].
Consequently, extensive re-optimization studies were carried out using N,O-diBoc protected
3-O-methyl-6-(SnMe3)DOPA OtBu ester (Figure 12). In this case an addition of nBuOH partially
suppressed the undesired cyclization owing to the decrease of basicity of 18F− by hydrogen bonding.
This interaction is, on the other side, not strong enough to significantly affect the nucleophilicity
of [18F]fluoride [36
,
37]. The adjustment of the substrate:Cu(Py)4(OTf)2 ratio to 1:2 allowed to
◦
further improve RCCs to finally 27%. Deprotection using 38% HCl at 100 C for 15 min afforded
,
3-O-methyl-6-[18F]FDOPA ([18F]OMFD) [45 46] in 16% yield. Similarly, 6-[18F]FMT and 6-[18F]FDOPA
were prepared in RCYs of 14% and 9%, respectively [47].
Figure 12. 18F-Fluorodestannylation of N-monoBOC protected [18F]OMFD precursor.
To completely avoid Chan-Lam coupling and improve RCCs, N,N-diBoc protected amino acid
derivatives were synthesized from commercially available N-monoBoc-protected precursors in a
single reaction step in 71–96% yield (Figure 13). Furthermore, N,N,O,O’-tetraBoc-6-(SnMe3)DOPA-OEt
was conveniently prepared from the corresponding N-formyl precursor [48,49] in 72% yield over
3 steps (N-Boc acylation, deformylation with N2H4 followed by the second N-Boc protection).
The application of the fully protected radiolabeling precursors allowed to substantially increase
the 18F-incorporation yield and prepare [18F]OMFD, 2-[18F]FTyr, 6-[18F]FMT and 6-[18F]FDOPA in
RCCs of 37–78% (Figure 14).
Figure 13. Preparation of N,N-diBoc protected radiolabeling precursors.
Finally, the developed radiolabeling protocol was implemented to an automated radiosynthesis
module (Figure 15) [50]. Syntheses starting from 1–40 GBq [18F]fluoride afforded [18F]OMFD,
2-[18F]FTyr, 6-[18F]FMT and 6-[18F]FDOPA in RCYs of 32%, 48
±
7%, 42
±
2%, 54
±
5% (n = 3–5) within

Molecules 2017, 22, 2231
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60–65 min, respectively, as ready-to-use solutions in sodium phosphate buffer. Thus, 14–17 GBq of
6-[18F]FDOPA was produced starting from 37–40 GBq of [18F]fluoride. Molar activities were in the
range of 28–57 GBq/µmol (for 1–7.4 GBq of the corresponding tracer). The tracers prepared by this
method passed all cGMP quality control tests necessary for clinical use, as outlined in the European
Pharmacopeia for 6-[18F]FDOPA [51]. The residual amounts of Cu and Sn in the final solution were
well below the allowed limits specified in the ICH Guidelines (0.07–4.2 and 0.05–0.32
µg/batch vs. 340
and 640
µ
g/day, respectively) [52]. The lower yield in the case of [18F]OMFD is explained by the lower
solubility of the respective protected radiolabeled amino acid which caused losses of the intermediate
during the SPE purification step.
Figure 14. Improved procedure for the preparation of 6-[18F]FDOPA, 6-[18F]FMT, 2-[18F]FTyr and
[
18F]OMFD. a 18F]OMFD, deprotection conditions: 38% HCl, 100 ◦C, 15 min; b RCYs of automated
[
and RCCs of manual (in parentheses) radiosynthesis.
Figure 15. Process flow diagram (PFD) for the automated radiosynthesis of [18F]OMFD, 2-[18F]FTyr,
6-[18F]FMT and 6-[18F]FDOPA. A: MeOH (2 mL); B: Et4NOTf (3.1 mg, 11
µmol) in MeOH (700
µL);
C: Cu(py)4(OTf)2 (20.34 mg, 30 µmol) and radiolabeling precursor (30 µmol) in DMA (1 mL); E: H2O
(1 mL); F: CH2Cl2 (2 mL); G: H2O (9 mL); H: 48% HBr (1 mL) (38% HCl in the case of [18F]OMFD); I:
45% NaOH (300 µL) and 25 mM sodium phosphate buffer (3 mL, pH 4.5).
Remarkably, 6-[18F]FDOPA was obtained in a high RCY of 54
±
5% (n = 5) and in excellent
enantiomeric, chemical and radiochemical purity. To the best of our knowledge, this is the highest
value reported for the synthesis of this important PET tracer. The highest RCYs of n.c.a. 6-[18F]FDOPA
achieved to date according to the protocols for the automated preparation of this tracer reported by
Lemaire et al. [29,53,54] and by Hoepping et al. [29,55] amounted to 4–36 and 19–21%, respectively.

Molecules 2017, 22, 2231
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3. Materials and Methods
3.1. General
Chemicals and solvents were purchased from Sigma-Aldrich GmbH (Steinheim, Germany),
Fluka AG (Buchs, Switzerland), TCI EUROPE N.V. (Zwijndrecht, Belgium), ChemPUR GmbH
(Karlsruhe, Germany), Merck KGaA (Darmstadt, Germany) and ABCR GmbH (Karlsruhe, Germany)
and used as delivered. Anhydrous solvents were purchased from Sigma-Aldrich GmbH
(Steinheim, Germany) and stored under argon. Precursors for electrophilic radiofluorination,
For-6-(SnMe3)DOPA(Boc)2-OEt, Boc-6-(SnMe3)DOPA(Boc)2-OEt, Boc-6-(SnMe3)mTyr(Boc)-OEt,
Boc-2-(SnMe3)Tyr(Boc)-OEt, Boc-4-Boc-3-Me-6-(SnMe3)DOPA-OtBu were purchased from ABX GmbH
(Radeberg, Germany) and used as delivered.
3.2. Nuclear Magnetic Resonance (NMR)
1H-NMR spectra: Bruker DPX Avance 200 (200 MHz), Bruker Avance II 300 (300 MHz) and
Varian INOVA 400 (400 MHz). 1H chemical shifts are reported in ppm relative to residual peaks
of deuterated solvents. The observed signal multiplicities are characterized as follows: s = singlet,
d = doublet, t = triplet, m = multiplet, and br = broad. Coupling constants (J) were reported in Hertz
(Hz). 13C-NMR spectra [additional APT (Attached Proton Test)]: Bruker DPX Avance 200 (50 MHz),
Bruker Avance II 300 (75 MHz) and Varian INOVA 400 (101 MHz). 13C chemical shifts are reported in
ppm relative to residual peaks of deuterated solvents.
19F-NMR spectra: Bruker DPX Avance 200 (188 MHz).
3.3. Mass Spectroscopy
High-resolution mass spectra (HRMS) were measured on LTQ FT Ultra (Thermo Fisher Scientific
Inc., Bremen, Germany). Inductively coupled plasma mass spectra (ICP-MS) were measured on Agilent
7900 ICP-MS (Agilent Technologies, Santa Clara, CA, USA). Low-resolution electrospray ionization
(ESI) positive mode mass spectra were measured on a Thermo Finnigan Surveyor mass spectrometer
(Thermo Fisher Scientific GmbH, Dreieich, Germany).
3.4. Chemistry
All reactions were carried out with magnetic stirring, if not stated otherwise, and, if air or moisture
sensitive, substrates and/or reagents were handled in flame-dried glassware under argon or nitrogen.
Organic extracts were dried with anhydrous MgSO4.
Column chromatography: silica gel technical grade (w/Ca, ~0.1%), 60 Å, 230–400 mesh particle
size from Sigma-Aldrich GmbH (Steinheim, Germany) was applied for the purification of aryl
stannanes. Merck silica gel, grade 60, 230–400 mesh was used for other compounds. Solvent
proportions are indicated in a volume/volume ratio.
Thin layer chromatography (TLC) was performed using aluminum finish ALUGRAM® SIL G/UV254
from Macherey-Nagel GmbH (Düren, Germany) or precoated sheets, 0.25 mm Sil G/UV254 from Merck
KGaA (Dormstadt, Germany). The chromatograms were viewed under UV light (λ = 254 nm).
3.4.1. Tetrakis(pyridine)copper(II) Bis(trifluoromethanesulfonate)
Copper(II) trifluoromethanesulfonate (5 g, 14 mmol) was dissolved in methanol (25 mL). Pyridine
(12 mL, 149 mmol) was added dropwise (exothermic reaction was observed) and the reaction mixture
was stirred for 30 min. The mixture was left at ambient temperature for 1 h and thereafter in fridge (at
5 ◦C) overnight. The blue crystalline precipitate was filtered off, recrystallized from 20% Py in MeOH
and dried under a stream of air affording the desired product [56].

Molecules 2017, 22, 2231
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Yield
8.5 g, 91%
Appearance
Molecular formula
Molar mass
blue solid
C22H20CuF6N4O6S2
678.08042
Calcd for C22H20CuF6N4O6S2: C, 38.97; H, 2.97; N, 8.26. Found: C,
39.1± < 0.1; H, 3.16 ± 0.09; N, 8.33 ± 0.01.
Anal.
3.4.2. 3-(Benzo[d][1,3]dioxol-5-yl)-1-(3-bromophenyl)-3-hydroxyprop-2-en-1-one—General
Procedure 1 (GP1)
To a solution of 1-(benzo[d][1,3]dioxol-5-yl)ethan-1-one (1.5 g, 9.1 mmol), in anhydrous THF
(20 mL) was added 1 m LiHMDS in THF (27.3 mL) and the resulting solution was stirred for 1 h at
◦
◦
−
80 C. The solution was warmed to room temperature and stirred for 2 h. Thereafter, it was cooled to
−
80 C and 3-bromobenzoyl chloride (1.2 mL, 2.0 g, 9.1 mmol) was added dropwise. The solution was
allowed to warm to room temperature and stirred for additional 18 h. Afterwards, a saturated solution
of NH4Cl (50 mL) was added, the pH was adjusted to 7.0 and the mixture was extracted with EtOAc
(3
×
50 mL). The combined organic layers were washed with brine (100 mL), dried and concentrated
under reduced pressure. The residue was purified by column chromatography (Et2O/petroleum ether,
1:4) affording the title compound [57].
Yield
2.82 g, 89%
Appearance
Molecular formula
Molar mass
TLC
yellow solid
C16H11BrO4
347.164
Rf = 0.46 (Et2O/petroleum ether, 1:4)
(200 MHz, CDCl3): δ (ppm) = 7.88 (q, J = 1.7 Hz, 1H), 7.75–7.60 (m, 1H), 7.53–7.34 (m, 2H),
7.31–7.23 (m, 1H), 7.23–7.08 (m, 1H), 6.75–6.61 (m, 1H), 6.54 (s, 1H), 5.87 (s, 2H).
1H-NMR
(50 MHz, CDCl3):
δ (ppm) = 151.34, 147.91, 136.89, 136.87, 134.57, 129.91, 129.50, 125.19, 122.91,
13C-NMR
HRMS
122.42, 107.89, 106.85, 101.61, 92.48.
−
−
m/z: [M
−
H] calcd for C16H10BrO4 : 344.97679; found: 344.97664. Correct isotopic pattern.
3.4.3. 3-(Benzo[d][1,3]dioxol-5-yl)-1-(3-fluorophenyl)-3-hydroxyprop-2-en-1-one
The title compound was synthesized according to GP1 from 1-(benzo[d][1,3]dioxol-5-yl)ethan-
1-one (500 mg, 3 mmol).
Yield
735 mg, 84%
Appearance
Molecular formula
Molar mass
TLC
yellow solid
C16H11FO4
286.2584
Rf = 0.45 (Et2O/petroleum ether, 1:4)
(200 MHz, CDCl3):
δ (ppm) = 16.80 (s, 1H), 7.74 (dd, J = 7.8, 1.1 Hz, 1H), 7.63 (ddd,
1H-NMR
J = 10.0, 6.0, 2.2 Hz, 2H), 7.55–7.37 (m, 2H), 7.32–7.16 (m, 1H), 6.90 (d, J = 8.2 Hz,
1H), 6.71 (s, 1H), 6.07 (s, 2H).
(50 MHz, CDCl3): δ (ppm) = 186.52, 182.20, 165.36, 160.59, 151.79, 148.44, 130.45,
130.29, 130.02, 123.28, 122.76, 122.70, 119.40, 118.98, 114.28, 113.82, 108.41, 107.41,
102.08, 92.88.
13C-NMR
19F-NMR
HRMS
(188 MHz, CDCl3): δ (ppm) = −111.97.
−
−
m/z: [M − H] calcd for C16H10FO4 : 285.05686; found: 285.05685.
3.4.4. 3-(Benzo[d][1,3]dioxol-5-yl)-5-(3-bromophenyl)-1H-pyrazole—General Procedure 2 (GP2)
A solution of 3-(benzo[d][ ]dioxol-5-yl)-1-(3-bromophenyl)-3-hydroxyprop-2-en-1-one (2.4 g,
1,3
6.91 mmol) and hydrazine monohydrate (1 mL, 98%, 13.83 mmol, 2 eq.) in ethanol (30 mL) was
refluxed for 3 h. Water was added to the clear yellow solution and resulting precipitate was collected
by filtration, washed with water and dried under vacuum to provide the title compound [57].

Molecules 2017, 22, 2231
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Yield
2 g, 84%
Appearance
Molecular formula
Molar mass
TLC
colorless solid
C16H11BrN2O2
343.18
Rf = 0.31 (EtOAc/petroleum ether, 1:4)
(200 MHz, DMSO-d6 + DCl):
J = 8.0 Hz, 1H), 7.34 (t, J = 9.2 Hz, 4H), 6.89 (d, J = 8.6 Hz, 1H), 5.96 (s, 2H).
(50 MHz, DMSO-d6 + DCl): δ (ppm) = 148.94, 148.55, 147.18, 146.64, 132.67, 131.93,
131.75, 129.01, 125.66, 123.08, 122.73, 121.20, 109.56, 106.84, 102.31, 101.64, 12.33.
m/z: [M + H]+ calcd for C16H12BrN2O2+: 343.00767; found: 343.00781. Correct
isotopic pattern.
δ (ppm) = 7.97 (s, 1H), 7.79 (d, J = 7.6 Hz, 1H), 7.49 (d,
1H-NMR
13C-NMR
HRMS
3.4.5. 3-(Benzo[d][1,3]dioxol-5-yl)-5-(3-fluorophenyl)-1H-pyrazole
The title compound was synthesized from
fluorophenyl)-3-hydroxyprop-2-en-1-one (335 mg, 1.17 mmol) according to GP2.
3-(benzo[d][1,3]dioxol-5-yl)-1-(3-
Yield
735 mg, 84%
Appearance
Molecular formula
Molar mass
TLC
yellow solid
C16H11FN2O2
282.2744
Rf = 0.32 (EtOAc/petroleum ether, 1:4)
(200 MHz, DMSO-d6 + DCl):
δ (ppm) = 7.60 (s, 2H), 7.41 (d, J = 6.4 Hz, 1H), 7.28 (d,
1H-NMR
J = 11.6 Hz, 3H), 7.14 (d, J = 8.7 Hz, 1H), 6.85 (d, J = 8.3 Hz, 1H), 5.94 (s, 2H).
(50 MHz, DMSO-d6 + DCl): δ (ppm) = 165.50, 160.65, 148.95, 148.50, 147.24, 146.84,
146.78, 131.98, 131.80, 131.59, 131.42, 122.78, 122.61, 121.23, 113.53, 113.07, 109.48,
106.79, 102.28, 101.65.
13C-NMR
19F-NMR
HRMS
(188 MHz, DMSO-d6 + DCl): δ (ppm) = −112.07.
m/z: [M + H]+ calcd for C16H12FN2O2+: 283.08773; found: 283.08775.
3.4.6. 3-(Benzo[d][1,3]dioxol-5-yl)-5-(3-(trimethylstannyl)phenyl)-1H-pyrazole–General Procedure
3 (GP3)
A flame dried flask containing 3-(benzo[d][
(550 mg, 1.6 mmol) and Pd(PPh3)4 (185 mg, 0.16 mmol, 0.1 eq.) were evacuated and purged with argon
(three times). Anhydrous 1,4-dioxane (2 mL) followed by hexamethylditin (830 L, 1.31 g, 4 mmol,
1,3]dioxol-5-yl)-5-(3-bromophenyl)-1H-pyrazole
µ
2.5 eq.) was added, and the reaction mixture was heated to 100 ◦C for 18 h. The black suspension
was filtered through a plug of Celite. 1 m TBAF in THF (2 mL) was added to the filtrate; the mixture
was stirred for 30 min and diluted with EtOAc (50 mL). The resulting solution was washed with
water (50 mL), brine (50 mL), dried and concentrated under reduced pressure. The crude product was
purified by column chromatography and by recrystallization from hexane contained a small amount
of CH2Cl2.
Yield
566 mg, 83%
Appearance
Molecular formula
Molar mass
TLC
colorless solid
C19H20N2O2Sn
427.091
Rf = 0.30 (EtOAc/petroleum ether, 1:4)
(200 MHz, DMSO-d6): δ (ppm) = 13.20 (s, 1H), 7.93 (s, 1H), 7.74 (s, 1H), 7.60–7.22
(m, 5H), 7.12 (s, 1H), 6.99 (d, J = 7.7 Hz, 1H), 6.06 (s, 2H), 0.31 (s, 9H).
1H-NMR
13C-NMR
HRMS
(50 MHz, DMSO-d6):
δ (ppm) = 147.74, 146.86, 132.25, 128.28, 125.03, 118.82, 108.59,
105.60, 101.12, 99.30, −9.29.
m/z: [M + H]+ calcd for C19H21N2O2Sn+: 429.06195; found: 429.06286. Correct
isotopic pattern.

Molecules 2017, 22, 2231
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3.4.7. Methyl 4-Fluorobenzoate
A solution of 4-fluorobenzoyl chloride (500
µ
L, 4.2 mmol) in MeOH (20 mL) was stirred at 40 ◦C
for 2 h and concentrated under reduced pressure affording the crude product [58] which was used
without further purification.
Yield
440 mg, 67%
Appearance
Molecular formula
Molar mass
1H-NMR
colorless oil
C8H7FO2
154.1404
(200 MHz, CDCl3): δ (ppm) = 8.17–7.95 (m, 2H), 7.21–6.97 (m, 2H), 3.92 (s, 3H).
(50 MHz, CDCl3): δ (ppm) = 168.41, 166.27, 163.36, 132.34, 132.16, 126.58, 126.52,
115.85, 115.42, 52.31.
(188 MHz, CDCl3): δ (ppm) = −105.79.
13C-NMR
19F-NMR
3.4.8. Methyl 4-(Trimethylstannyl)benzoate
The title compound [59] was synthesized according to GP3 from methyl 4-iodobenzoate (2 g,
7.6 mmol). The product was purified by column chromatography (Et2O:PE = 1:9).
Yield
1.9 g, 83%
Appearance
Molecular formula
Molar mass
1H-NMR
yellow oil
C11H16O2Sn
298.857
(200 MHz, CDCl3): δ (ppm) = 8.19–7.86 (m, 2H), 7.79–7.37 (m, 2H), 3.92 (s, 3H), 0.33 (s, 9H).
(50 MHz, CDCl3):
13C-NMR
δ
(ppm) = 167.48, 149.69, 135.82, 135.82, 129.86, 128.55, 128.55, 52.08,
−
9.50.
3.4.9. 3-(Trimethylstannyl)benzaldehyde (3)
The title compound [60] was synthesized according to GP3 from 3-bromobenzaldehyde (850 mg,
4.6 mmol) using Pd(PPh3)4 (531 mg, 0.5 mmol, 0.1 eq.) and hexamethylditin (1.9 mL, 9.2 mmol, 2 eq.)
and purified by column chromatography (Et2O:PE = 1:9).
Yield
900 mg, 73%
colorless oil
C10H14O3Sn
268.931
Appearance
Molecular formula
Molar mass
(200 MHz, CDCl3):
δ (ppm) = 10.03 (s, 1H), 8.19–7.89 (m, 1H), 7.89–7.64 (m, 2H), 7.52 (t,
1H-NMR
13C-NMR
J = 7.4 Hz, 1H), 0.34 (s, 9H).
(50 MHz, CDCl3): (ppm) = 193.18, 143.88, 142.05, 137.09, 135.75, 129.87, 128.61,
δ
−
9.33.
3.4.10. (2-Methoxyphenyl)trimethylstannane (4)—General Procedure 4 (GP4)
A solution of 2.5 m nBuLi in hexane (0.52 mL, 1.3 eq.) was added dropwise to a stirring solution
of 2-iodoanisol (131 L, 0.236 g, 1 mmol) in Et2O (4 mL) at
78 ◦C and the mixture was stirred at the
µ
−
same temperature for 30 min. Thereafter, a solution of Me3SnCl (0.24 g, 1.2 mmol, 1.2 eq.) in Et2O
(3 mL) was added dropwise and the reaction mixture was stirred and slowly warmed to ambient
temperature for 2 h. 1 m TBAF in THF (1 mL) was added, the mixture was stirred for 30 min, diluted
with Et2O (50 mL) and washed with 10% NaHCO3 (3
dried and concentrated under reduced pressure. The resulting crude product [61] was directly used
for radiochemical experiments.
×
10 mL), H2O (3
×
10 mL), brine (2
×
10 mL),

Molecules 2017, 22, 2231
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Yield
150 mg, 55%
yellow oil
C10H16OSn
270.947
Appearance
Molecular formula
Molar mass
(200 MHz, CDCl3): δ (ppm) = 7.35 (ddd, J = 9.7, 7.5, 1.7 Hz, 2H), 7.24–6.94 (m, 1H),
6.94–6.72 (m, 1H), 3.80 (s, 3H), 0.27 (s, 9H).
1H-NMR6
3.4.11. (3-Methoxyphenyl)trimethylstannane (5)
The title compound [62] was prepared from 3-iodoanisol (120
to GP4.
µL, 0.236 g, 1 mmol), according
Yield
180 mg, 66%
yellow oil
C10H16OSn
270.947
Appearance
Molecular formula
Molar mass
(200 MHz, CDCl3): δ (ppm) = 7.45–7.18 (m, 1H), 7.18–6.97 (m, 2H), 6.89 (ddd, J = 8.3,
2.7, 1.1 Hz, 1H), 3.85 (s, 3H), 0.33 (s, 9H).
1H-NMR6
3.4.12. (4-Methoxyphenyl)trimethylstannane (6a)
The title compound [7] was prepared from 4-iodoanisol (235 mg, 1 mmol) according to GP4.
Yield
190 mg, 70%
Appearance
Molecular formula
Molar mass
yellow oil
C10H16OSn
270.947
(200 MHz, CDCl3): δ (ppm) = 7.64–7.20 (m, 2H), 6.94 (ddd, J = 6.5, 4.1, 1.9 Hz, 2H),
3.82 (s, 3H), 0.28 (s, 9H).
1H-NMR7
3.4.13. tert-Butyl (S)-2-(Bis(tert-butoxycarbonyl)amino)-3-
{4-[(tert-butoxycarbonyl)oxy]-5-methoxy-2-(trimethylstannyl)phenyl}propanoate
[Boc2-4-Boc-3-Me-6-(SnMe3)DOPA-OtBu]—General Procedure 5 (GP5)
A solution of tert-butyl (S)-2-[(tert-butoxycarbonyl)amino]-3-{4-[(tert-butoxycarbonyl)oxy]5-
methoxy-2-(trimethylstannyl)phenyl}propanoate [Boc-4-Boc-3-Me-6-(SnMe3)DOPA-OtBu] (220 mg,
0.3 mmol) DMAP (17 mg, 0.1 mmol, 0.4 eq.) and di-tert-butyl dicarbonate (229 mg, 1 mmol, 3 eq.) in
anhydrous MeCN (3 mL) was stirred at room temperature for 48 h, and then concentrated under vacuum.
Purification of the residue by column chromatography (Et2O:PE = 1:9) afforded the title compound.
Yield
255 mg, 86%
Appearance
Molecular formula
Molar mass
yellow oil
C32H53NO10Sn
730.483
(200 MHz, CDCl3): δ (ppm) = 7.09 (s, 1H), 6.84–6.68 (m, 1H), 5.29 (s, 3H), 4.88 (dd, J = 9.3,
6.2 Hz, 1H), 3.60–3.36 (m, 2H), 1.53 (s, 9H), 1.48 (s, 9H), 1.38 (s, 18H), 0.30 (s, 9H).
1H-NMR
13C-NMR
HRMS
(50 MHz, CDCl3):
δ (ppm) = 169.01, 152.27, 143.78, 129.41, 114.13, 83.26, 82.84, 81.86, 60.52,
55.74, 28.13, 28.01, 27.77, −8.22.
m/z: [M + Na]+ calcd for C32H53NNaO10Sn+: 754.25836; found: 754.25918. Correct
isotopic pattern.
3.4.14. Ethyl (S)-3-{4,5-Bis[(tert-butoxycarbonyl)oxy]-2-(trimethylstannyl)phenyl}-2-
[bis(tert-butoxycarbonyl)amino]propanoate [Boc2-6-(SnMe3)DOPA(Boc)2-OEt]
The title compound was synthesized according to GP5 from ethyl (S)-3-{4,5-bis[(tert-
butoxycarbonyl)oxy]-2-(trimethylstannyl)phenyl}-2-[(tert-butoxycarbonyl)amino]pro-panoate
[Boc-6-(SnMe3)DOPA(Boc)2-OEt] (200 mg, 0.3 mmol).

Molecules 2017, 22, 2231
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Yield
229 mg, 96%
yellow oil
C34H55NO12Sn
788.519
Appearance
Molecular formula
Molar mass
(200 MHz, CDCl3): δ (ppm) = 7.23 (s, 1H), 7.15–6.97 (m, 1H), 5.03 (dd, J = 9.8, 4.8 Hz,
1H), 4.21 (ddt, J = 10.3, 7.0, 3.5 Hz, 2H), 3.52–3.18 (m, 2H), 1.52 (d, J = 2.5 Hz, 18H), 1.38
(s, 18H), 1.27 (t, J = 5.8 Hz, 3H), 0.33 (s, 9H).
1H-NMR
(50 MHz, CDCl3):
δ (ppm) = 170.17, 152.04, 150.87, 150.72, 143.22, 142.58, 141.37, 140.61,
13C-NMR
HRMS
130.08, 123.94, 83.54, 83.45, 83.19, 61.57, 59.48, 37.93, 29.80, 27.96, 27.72, 14.26, −8.03.
m/z: [M + Na − CH2]+ calcd for C34H55NNaO12Sn+: 812.26384; found: 812.26458.
Correct isotopic pattern.
3.4.15. Ethyl (S)-3-{4,5-Bis[(tert-butoxycarbonyl)oxy]-2-(trimethylstannyl)phenyl}-2-
[bis(tert-butoxycarbonyl)amino]propanoate [Boc2-6-(SnMe3)DOPA(Boc)2-OEt] from
(S)-3-{4,5-Bis[(tert-butoxycarbonyl)oxy]-2-(trimethylstannyl)phenyl}-2-(formylamino)propanoate
[For-6-(SnMe3)DOPA(Boc)2-OEt]
A solution of For-6-(SnMe3)DOPA(Boc)2-OEt (0.765 g, 1.24 mmol), DMAP (17 mg, 0.14 mmol) and
Boc2O (1.08 g, 4.95 mmol) in anhydrous MeCN (4 mL) was incubated for 16 h at ambient temperature.
Thereafter, the reaction mixture was diluted with Et2O (30 mL). N,N-3-(Dimethylamino)-1-propylamine
(0.62 mL, 0.506 g, 4.95 mmol) was added, the mixture was incubated at ambient temperature for 10 min,
washed with 1 m NaHSO4 (3
under reduced pressure to give the crude Boc,For-6-(SnMe3)DOPA(Boc)2-OEt (0.91 g, 100%) which
was immediately used for the next step.
×
10 mL), H2O (3
×
10 mL), brine (2
×
10 mL), dried and concentrated
A solution of N2H4 H2O (140 µL, 140 mg, 2.36 mmol) in MeOH (1 mL) was added dropwise to an
·
ice-cold solution of Boc,For-6-(SnMe3)DOPA(Boc)2-OEt (0.91 g, max. 1.24 mol) in MeOH (7.7 mL) and
the reaction mixture was stirred for 20 min. Et2O (50 mL) was added and the resulting solution was
washed with 1 M NaHSO4 (3
×
10 mL), H2O (3
×
10 mL), brine (2
×
10 mL), dried and concentrated
under reduced pressure. The residue was purified by column chromatography (EtOAc:hexane = 1:3)
to give Boc-6-(SnMe3)DOPA(Boc)2-OEt (0.64 g, 75% over two steps) as a colorless foam which was
immediately used for the next step. Rf = 0.38, EtOAc:hexane = 1:3
Boc2-6-(SnMe3)DOPA(Boc)2-OEt (0.71 g, 72% over three steps) was prepared according to GP4
from Boc-6-(SnMe3)DOPA(Boc)2-OEt (0.64 g, 0.93 mmol) using Boc2O (1.08 g, 4.95 mmol) and DMAP
(16 mg, 0.13 mmol) and purified by column chromatography (EtOAc:hexane = 1:5). Rf = 0.29,
EtOAc:hexane = 1:5.
3.4.16. Ethyl (S)-2-[Bis(tert-butoxycarbonyl)amino]-3{5-[(tert-butoxycarbonyl)oxy]-2
(trimethylstannyl)phenyl}propanoate [Boc-6-(SnMe3)-m-Tyr(Boc)-OEt]
The title compound was synthesized from ethyl (S)-2-[(tert-butoxycarbonyl)amino]-3-{5-[(tert-
butoxycarbonyl)oxy]-2-(trimethylstannyl)phenyl}propanoate (300 mg, 0.5 mmol) according to GP5.
Yield
250 mg, 71%
Appearance
Molecular formula
Molar mass
yellow oil
C29H47NO9Sn
672.403
(200 MHz, CDCl3): δ (ppm) = 7.37 (t, J = 8.4 Hz, 1H), 7.12–6.97 (m, 1H), 6.92 (d,
J = 2.2 Hz, 1H), 5.02 (dd, J = 10.4, 4.5 Hz, 1H), 4.22 (qd, J = 7.1, 2.7 Hz, 2H),
3.59–3.20 (m, 2H), 1.53 (s, 9H), 1.38 (s, 18H), 1.28 (s, 3H), 0.32 (s, 9H).
(50 MHz, CDCl3): δ (ppm) = 170.21, 151.98, 151.90, 151.72, 146.34, 140.06, 137.19,
122.32, 118.86, 83.34, 83.09, 61.58, 59.69, 38.42, 29.82, 27.96, 14.28, −8.10.
m/z: [M + Na]+ calcd. for C29H47NNaO9Sn+: 696.21650; found: 696.21766. Correct
isotopic pattern.
1H-NMR
13C-NMR
HRMS

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3.4.17. Ethyl (S)-2-[Bis(tert-butoxycarbonyl)amino]-3-[4-(tert-butoxycarbonyl)oxy]-2-
(trimethylstannyl)phenylpropanoate [Boc2-2-(SnMe3)Tyr(Boc)-OEt]
The title compound was synthesized from ethyl (S)-2-[(tert-butoxycarbonyl)amino]-3-[4-(tert-
butoxycarbonyl)oxy]-2-(trimethylstannyl)phenylpropanoate [Boc2-2-(SnMe3)Tyr(Boc)-OEt] (100 mg,
0.2 mmol) according to GP5.
Yield
117 mg, 85%
Appearance
Molecular formula
Molar mass
yellow oil
C29H47NO9Sn
672.403
(200 MHz, CDCl3): δ (ppm) = 7.17 (d, J = 2.3 Hz, 1H), 7.12–6.84 (m, 1H), 4.98 (dd,
J = 9.7, 5.1 Hz, 1H), 4.21 (qd, J = 7.2, 1.9 Hz, 1H), 3.35 (dd, J = 7.2, 4.2 Hz, 1H), 1.54
(s, 1H), 1.36 (s, 2H), 1.27 (s, 1H), 0.33 (s, 1H).
1H-NMR
(50 MHz, CDCl3): δ (ppm) = 170.19, 151.95, 151.81, 149.48, 144.66, 142.01, 130.20,
128.49, 121.39, 83.39, 83.11, 61.54, 59.89, 37.87, 29.79, 27.97, 27.80, 14.26, −8.10.
m/z: [M + Na]+ calcd for C29H47NNaO9Sn+: 696.21650; found: 696.21700. Correct
isotopic pattern.
13C-NMR
HRMS
3.5. Radiochemistry
3.5.1. General Procedures
All radiosyntheses were carried out using anhydrous DMA and nBuOH stored over molecular
sieves (available from “Acros”, Geel, Belgium, or “Aldrich”). Cu(OTf)2(py)4 was stored under ambient
conditions without any precautions.
[18F]Fluoride was produced by the 18O(p,n)18F reaction by bombardment of enriched [18O]water
with 16.5 MeV protons using a BC1710 cyclotron (The Japan Steel Works Ltd., Shinagawa, Japan) at the
INM-5 (Forschungszentrum Jülich).
All radiolabeling experiments were carried out under ambient or synthetic air. Each radiochemical
experiment was carried out at least in triplicates if not otherwise mentioned. Standard deviations
(SD) were calculated by the least-square method. All experiments were carried out by using one-pot
procedure. Before the determination of radiochemical conversions (RCCs), reaction mixtures were
always diluted with H2O (1–4 mL) to dissolve any 18F-fluoride adsorbed onto the reaction vessel
walls. The loss of radioactivity on the vessel walls did not exceed 13
±
2% from the starting activity
(n > 100). All radiochemical yields (RCYs) are decay corrected and radiochemical purities (RCPs) were
determined after purification.
3.5.2. Processing [18F]Fluoride
Aqueous [18F]fluoride was loaded onto an anion-exchange resin (e.g., QMA cartridge). It should
be noted that aqueous [18F]fluoride was loaded onto the cartridge from the male side, whereas flushing,
washing and 18F− elution were carried out from the female side. If the QMA cartridge had been
loaded, flushed and eluted from the female side only, sometimes a significant amount of [18F]fluoride
remained on the resin (this is probably because QMA-light (46 mg) cartridges have a single frit on the
male side but four frits on the female side).
3.5.3. High-Performance Liquid Chromatography (HPLC)
For manual radiosyntheses the following HPLC system was used:
Ultimate® 3000 HPLC system from Thermo Scientific (Sunnyvale, CA, USA) with Ultimate® 3000
LPG-3400A pump, Ultimate® 3000 VWD-3100 UV/Vis detector and
γ-detector Gabi Star from Raytest
GmbH (Straubenhardt, Germany) were used. The volume of injection was 20 µL.
Columns:

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•
•
Chromolith® SpeedROD RP-18 endcapped 50 × 4.6 mm, Merck KGaA (Darmstadt, Germany).
ProntoSIL C18 ace-EPS 125
(Leonberg, Germany).
×
4.6 mm, Bischoff Analysentechnik und -geräte GmbH
•
•
Gemini® 5 µm C18 110 Å, 250 × 4.6 mm, Phenomenex Inc. (Aschaffenburg, Germany).
Gemini® 5 µm C18 110 Å, 250×10 mm, Phenomenex Inc. (Aschaffenburg, Germany).
For automated syntheses the following system was used:
WellChrom Spectro-photometer K-2501 UV/Vis detector, BlueShadow Pump 80P from KNAUER
Wissenschaftliche Geräte GmbH, Berlin, Germany and AD 1422 PIN-photodiode and scintillator
detector from Eckert & Ziegler Strahlen- und Medizintechnik AG, Berlin, Germany was connected
directly to the automated module.
Columns:
•
•
Synergi™ 4 µm Hydro-RP 80 Å, 250 × 10 mm, Phenomenex Inc. (Aschaffenburg, Germany).
Synergi Hydro-RP 80 Å, 150 21.2 mm, AXIA Phenomenex Inc.
(Aschaffenburg, Germany).
™
4
µm
×
™,
UV and radioactivity detectors were connected in series, giving a time delay of 0.1–0.9 min
depending on the flow rate. 18F-Labeled compounds were identified by co-injection of the unlabeled
reference compounds. The completeness of the radioactivity elution was controlled by analyzing of
the same sample amount choosing a column bypass.
3.5.4. Determination of the Enantiomeric Purity
The enantiomeric purity of radiolabeled amino acids was determined using chiral HPLC.
Conditions: column: CROWNPAK®CR(+) 150
eluent: 0.1 M HClO4 or 5% MeOH in 0.1 M HClO4; flow rate: 1.0 mL/min.
×
4.6 mm 5 µm (Daicel Corporation, Osaka, Japan);
3.5.5. Automated Radiosyntheses
All automated radiosyntheses were carried out in a home-made synthesis module. FFKM valves
(Christian Bürkert GmbH&Co. KG, Ingelfingen, Germany) were applied. All connections between
the valves were made using PTFE tubes and PEEK fittings. The flow scheme for the preparation
of radiolabeled amino acids is depicted in Figures 13 and S1. Synthetic air and He (Westfalen AG,
Muenster, Germany) were used as operating gases.
3.5.6. Miscellaneous Information
Radioactivity was measured with a CRC®-55tR Dose Calibrator from Capintec, Inc. (Florham Park,
NJ, USA) or the Curiementor 2 from PTW GmbH (Freiburg, Germany).
3.5.7. Recovery of 18F− from Anion Exchange Resin with MeOH Solutions of Different
Tetramethylammonium Salts
[18F]Fluoride (~50 MBq) was fixed on QMA-CO3 cartridge from the male side, the cartridge was
washed with MeOH (1 mL) in the same direction. Finally, [18F]fluoride was eluted with a solution of
Et4NX in MeOH (500 µL) from the female side.
3.5.8. 18F-Recovery and RCCs of [18F]FPh Using Different Salts in nBuOH
[18F]Fluoride (~50 MBq) was recovered from QMA-CO3 cartridge with a solution of the respective
salt (11
30 mol) in DMA (700
diluted with H2O (1 mL) and analyzed by HPLC.
µ
mol) in nBuOH (300
µ
L). A solution PhSnMe3 (14.5 mg, 60
µ
mol), Cu(py)4(OTf)2 (20.3 mg,
◦
µ
µL) was added, the reaction mixture was heated at 100 C for 10 min under air,

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3.5.9. Dependence of [18F]Fluoride Recovery and 18F-Incorporation Yields on the Type of an Anion
Exchange Cartridge
[18F]Fluoride (~50 MBq) was eluted from the respective anion exchange cartridge with a solution
of Et4NOTf (3.1 mg, 11
µ
mol) nBuOH (300
µL). A solution PhSnMe3 (14.5 mg, 60 µmol), Cu(py)4(OTf)2
(20.3 mg, 30 mol) in DMA (700
µ
µ
L) was added, the reaction mixture was heated at 100 ◦C for 10 min
under air, diluted with H2O (1 mL) and analyzed by HPLC.
3.5.10. Effect of Alcohol on 18F-Recovery and 18F-Fluorodestannylation
−
18F (50–150 MBq) was eluted into the reaction vial with a solution of Et4NOTf (3.1 mg, 11
µmol)
in the corresponding anhydrous alcohol (300 L); to this solution a solution trimethyl(phenyl)tin
µ
(14.5 mg, 60 µmol), Cu(py)4(OTf)2 (20.3 mg, 30 µmol) in DMA (700 µL) was added, the reaction
mixture was heated at 100 ◦C for 10 min under air, diluted with H2O (1 mL) and analyzed by HPLC.
3.5.11. Effect of Water on [18F]Fluorodestannylation
[18F]Fluoride (~50 MBq) was eluted from the respective anion exchange cartridge with a solution
of Et4NOTf (3.1 mg, 11
µmol) in nBuOH (300
µL). A solution of PhSnMe3 (14.5 mg, 60
µmol),
Cu(py)4(OTf)2 (20.3 mg, 30
µ
mol) in DMA (700
µL) containing H2O was added, the reaction mixture
was heated at 100 ◦C for 10 min under air, diluted with H2O (1 mL) and analyzed by HPLC.
3.5.12. Dependency of RCC on Alcohol Content
[18F]Fluoride (~50 MBq) was eluted from QMA-CO3 with a solution of Et4NOTf (3.1 mg, 11
µmol)
◦
in MeOH (500
µL), MeOH was evaporated at 100 C under a flow of air within 2–3 min. A solution of
PhSnMe3 (14.5 mg, 60
µ
mol), Cu(py)4(OTf)2 (20.3 mg, 30
µ
mol) in DMA/nBuOH (1 mL) was added,
◦
the reaction mixture was heated at 100 C for 10 min under air, diluted with H2O (1 mL) and analyzed
by HPLC.
3.5.13. Optimization of Aprotic Solvent
[18F]Fluoride (~50 MBq) was eluted from QMA-CO3 with a solution of Et4NOTf (3.1 mg, 11
µ
mol)
mol)
in (300
µL) in nBuOH. A solution of PhSnMe3 (14.5 mg, 60
µ
mol) and Cu(py)4(OTf)2 (20.3 mg, 30
µ
◦
in the appropriate solvent was added and the reaction mixture was heated at 100 C for 10 min, cooled
down, diluted with H2O (1 mL) and analyzed by HPLC.
3.5.14. Dependence of RCCs on Temperature and on Time
[18F]Fluoride (~50 MBq) was eluted from QMA-CO3 with a solution of Et4NOTf (3.1 mg, 11
in nBuOH (300 L). A solution of PhSnMe3 (14.5 mg, 60 mol) and Cu(py)4(OTf)2 (20.3 mg, 30
in DMA (700
µ
mol)
µ
µ
µmol)
µ
L) was added the reaction mixture was heated at given temperature for 10 min or at
100 ◦C for given time, cooled down, diluted with H2O (1 mL) and analyzed by HPLC.
3.5.15. Dependence of 18F-Incorporation Rate on the Precursor Amount
[18F]Fluoride (~50 MBq) was eluted from QMA-CO3 with a solution of Et4NOTf (3.1 mg, 11
µmol)
in nBuOH. A solution of given amount of PhSnMe3 and Cu(py)4(OTf)2 (20.3 mg, 30
was heated under air at 100 C for 10 min, cooled down, diluted with H2O (1 mL) and analyzed
by HPLC.
µ
mol), the mixture
◦
3.5.16. Dependence of 18F-Incorporation Rate on the Cu(py)4(OTf)2 Amount
[18F]Fluoride (~50 MBq) was eluted from QMA-CO3 with a solution of Et4NOTf (3.1 mg, 11
in nBuOH. A solution of PhSnMe3 (7.2 mg, 30 mol) and given amount of Cu(py)4(OTf)2 in DMA
µmol)
µ

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◦
(700 µL) was added, the mixture was heated under air at 100 C for 10 min, cooled down, diluted with
H2O (1 mL) and analyzed by HPLC.
3.5.17. Optimized Procedure for 18F-Fluorodestannylation—General Procedure (GP6)
[18F]Fluoride (50–100 MBq) was loaded on an anion exchange cartridge (QMA-CO3,
preconditioned with 1 mL water and dried with air) from the male side. The cartridge was rinsed with
MeOH (1 mL) and dried with air, then [18F]fluoride was eluted with a methanolic solution (500
µ
L) of
mol). Methanol was removed under reduced pressure (600 mBar) in a stream
of argon at 100 ◦C within 3 min. Afterwards, the pressure was reduced to 50 mBar and the reaction
vial was purged with air. A solution of the corresponding precursor (30 mol) and Cu(OTf)2(py)4
(20.3 mg, 30
mol) in DMA (1 mL) was added, the reaction mixture was stirred at 100 ◦C for 10 min
Et4NOTf (2.79 mg, 10
µ
µ
µ
and cooled down to room temperature in an ice bath. The reaction mixture was quenched with water
(4 mL) and analyzed by HPLC.
3.5.18. Manual Synthesis of Radiolabeled Amino Acids—General Procedure 7 (GP7)
[18F]Fluoride (200–300 MBq) was loaded onto an anion exchange cartridge (QMA-CO3
preconditioned with 1 mL water and dried with air) from the male side. The cartridge was washed
with MeOH (1 mL) and dried with air. Thereafter, [18F]fluoride was eluted into the reaction vial using
a solution of Et4NOTf (2.79 mg, 10
pressure (600 mBar) using a stream of air at 100 C within 5 min. A solution of Cu(OTf)2(py)4 (40.7 mg,
µ
mol) in MeOH (500
µ
L). MeOH was removed under reduced
◦
60 µmol) and the corresponding precursor (30 µmol) in DMA (1 mL) was added. The reaction mixture
was stirred at 100 ◦C for 10 min, and cooled down to room temperature in an ice bath. The reaction
mixture was quenched with water (2 mL) and loaded in Sep-Pak C18 Plus light Cartridge. The cartridge
was washed with 5 mL water and the product was eluted with 1 mL EtOH. EtOH was removed under
◦
reduced pressure (600 mBar) using a stream of air at 120 C within 5 min. 48% HBr (1 mL) was added
and the reaction mixture was stirred at 130 ◦C for 10 min. Hydrolysis of the protected [18F]OMFD
was carried out using 38% HCl at 100 ◦C for 10 min. The reaction mixture was cooled down, diluted
−
F loaded onto QMA
with H2O (3 mL) and analyzed by HPLC. RCC was calculated from amount of 18
cartridge, radioactivity amount in the reaction vial after hydrolysis step and HPLC chromatogram.
3.5.19. Automated Synthesis of Radiofluorinated Amino Acids—General Procedure 8 (GP8)
1.
2.
3.
4.
Trapping of [18F]fluoride on an QMA ion exchange cartridge;
Washing of the QMA with MeOH;
Closing air valve (50), and system venting;
Elution of [18F]fluoride from the ion exchange cartridge with a methanolic solution of Et4NOTf
into RV 1;
5.
6.
Open air valve (50) to completely transfer methanolic solution from QMA to RV 1;
Evaporation of MeOH in RV1 at 100 ◦C for 3 min using a flow of synthetic air under
reduced pressure;
7.
Addition of a solution of the radiolabeling precursor (30
DMA (1 mL);
µmol) and Cu(OTf)2(py)4 (40 mg) in
8.
9.
Heating of the reaction mixture in RV1 at 100 ◦C for 10 min;
Cooling of RV1 down to 50 ◦C;
10. Addition of water (1 mL) → in the case of [18F]OMFD: precipitation of precursor;
11. Loading of the mixture onto a SPE cartridge (C18);
12. Rinsing of the SPE cartridge with H2O (9 mL);
13. Elution of the radiolabeled intermediate into RV2 using CH2Cl2 (2.0 mL);
14. Evaporation of CH2Cl2 at 100 ◦C within 3 min using a flow of He under reduced pressure;

Molecules 2017, 22, 2231
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◦
15. Addition of 48% HBr (1 mL) and heating at 130 C for 10 min; in the case of [18F]OMFD: addition
of 38% HCl (1 mL) and heating at 100 ◦C for 10 min;
◦
16. Cooling of RV2 to 55 C and addition of a solution consisting of 45% NaOH (300
Na phosphate buffer (3 mL, pH 4.5);
µL) and 25 mM
17. Loading of the mixture onto the HPLC loop for injection;
18. Injection of the loop content onto the HPLC column and elution with 25 mM sodium phosphate
buffer (pH 4.5) at 8 mL/min;
19. Manual collection of the product fraction in a collection vial (CV1);
20. Transfer the product solution from CV1 into a sterile, filter-vented final product vial via a 0.22
sterile membrane filter using a flow of He.
µm
3.5.20. Molar Activity Calculation
The molar activities (GBq/
product by the amount of the unlabeled tracer determined from the peak area in a UV-HPLC
µ
mol) were calculated by dividing the radioactivity of the 18F-labeled
chromatograms (λ = 225 or 230 nm). The amounts of unlabeled compounds were determined from
the UV absorbance/concentration calibration curves. The molar activities of 6-[18F]FDOPA (7.4 GBq),
6-[18F]FMT (2.5 GBq), 2-[18F]FTyr (1.7 GBq) and [18F]OMFD (1 GBq) were determined to 57, 39, 50 and
27 GBq/µmol, respectively.
3.5.21. Determination of Sn and Cu Content
The contents of tin and copper were determined by Agilent 7900 ICP-MS. Solutions of amino
acid derivatives obtained after HPLC purification were concentrated under reduced pressure and
the residues were taken up in high purity H2O (1 mL). The measured samples were diluted 1:100.
The higher metal content in 6-[18F]FDOPA and 2-[18F]FTyr is explained by the application of 5 mL
instead of 9 mL H2O for Step 12 (GP8) in the respective productions.
4. Conclusions
A high yielding, fast and simple procedure for Cu-mediated radiofluorodestannylation using
[18F]fluoride and easy accessible Sn-precursors was developed. The protocol was successfully
implemented to an automated synthesis module. This allowed for the production of clinically relevant
radiolabeled aromatic amino acids, including 6-[18F]FDOPA, in excellent RCYs in two steps.
1
Supplementary Materials: Supplementary materials are available online. H-, 13C-, 19F-NMR and mass spectra
and radio-HPLC-chromatograms.
Acknowledgments: This work was supported by the DFG grant ZL 65/1-1. The authors thank Austin Craig for
careful proofreading of the manuscript.
Author Contributions: F.Z., B.D.Z., P.K. and J.Z. performed experiments, B.D.Z. and B.N. conceived and designed
experiments, F.Z., B.D.Z., P.K., J.Z. and B.N. wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
References and Notes
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2.
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