Synthesis of new 18F-radiolabeled silicon-based nitroimidazole compounds

Article abstract of DOI:10.1016/j.bmc.2013.04.029

The syntheses of new nitroimidazole compounds using silicon-[ ¹⁸F]fluorine chemistry for the potential detection of tumor hypoxia are described. [¹⁸F]silicon-based compounds were synthesized by coupling 2-nitroimidazole with silyldinaphtyl or silylphenyldi-tert-butyl groups and labeled by fluorolysis or isotopic exchange. Dinaphtyl compounds (6, 10) were labeled in 56-71% yield with a specific activity of 45 GBq/μmol, however these compounds ([¹⁸F]7 and [¹⁸F]11) were not stable in plasma. Phenyldi-tert-butyl compounds were labeled in 70% yield with a specific activity of 3 GBq/μmol by isotopic exchange, or in 81% yield by fluorolysis of siloxanes with a specific activity of 45 GBq/μmol. The labeled compound [¹⁸F]18 was stable in plasma and excreted by the liver and kidneys in vivo. In conclusion, the fluorosilylphenyldi-tert-butyl (SiFA) group is more stable in plasma than fluorosilyldiphenyl moiety. Thus, compound [ ¹⁸F]18 is suitable for further in vivo assessments.

Full text of DOI:10.1016/j.bmc.2013.04.029

Bioorganic & Medicinal Chemistry 21 (2013) 3680–3688
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Synthesis of new 18F-radiolabeled silicon-based nitroimidazole
compounds
Yoann Joyard a,b, Rabah Azzouz a, Laurent Bischoff a, Cyril Papamicaël a, Daniel Labar c, Anne Bol c,
Vanessa Bol c, Pierre Vera b, Vincent Grégoire c, Vincent Levacher a, Pierre Bohn b,
⇑
a UMR 6014, COBRA, CNRS, INSA of Rouen and University of Rouen, rue Tesnière, 76130 Mont-Saint-Aignan, France
b Department of Nuclear Medicine, Henri Becquerel Center and Rouen University Hospital and QuantIF-LITIS (EA4108), 1 rue d’Amiens,76038 Rouen Cedex, France
c Center for Molecular Imaging, Radiotherapy and Oncology, Institute of Experimental and Clinical Research, Université Catholique de Louvain, Brussels, Belgium
a r t i c l e i n f o
a b s t r a c t
The syntheses of new nitroimidazole compounds using silicon–[18F]fluorine chemistry for the potential
detection of tumor hypoxia are described. [18F]silicon-based compounds were synthesized by coupling
2-nitroimidazole with silyldinaphtyl or silylphenyldi-tert-butyl groups and labeled by fluorolysis or iso-
topic exchange. Dinaphtyl compounds (6, 10) were labeled in 56–71% yield with a specific activity of
Article history:
Received 26 March 2013
Accepted 16 April 2013
Available online 22 April 2013
45 GBq/
butyl compounds were labeled in 70% yield with a specific activity of 3 GBq/
or in 81% yield by fluorolysis of siloxanes with a specific activity of 45 GBq/ mol. The labeled compound
18F]18 was stable in plasma and excreted by the liver and kidneys in vivo. In conclusion, the fluor-
l
mol, however these compounds ([18F]7 and [18F]11) were not stable in plasma. Phenyldi-tert-
Keywords:
Fluorine-18
Fluorosilane
PET
l
mol by isotopic exchange,
l
[
osilylphenyldi-tert-butyl (SiFA) group is more stable in plasma than fluorosilyldiphenyl moiety. Thus,
Hypoxia
compound [18F]18 is suitable for further in vivo assessments.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
port on the synthesis of organofluorosilane compounds. The use of
fluorosilanes was primarily introduced by Rosenthal4 and first
Molecular imaging is a fast growing research area, including the
development of new tools, reagents and methods for imaging the
human body. In particular, positron emission tomography (PET)
is a powerful non-invasive molecular-imaging technique providing
physiological and biological information about the distribution of
radiolabeled molecules by 180° coincidence detection of two
simultaneously-emitted photons from positron–electron annihila-
tion. 18F is among the most widely used positron emitters due to its
almost ideal physical properties. With a half-life of 110 min and a
low-energy positron of 640 KeV, 18F can yield PET images with high
resolution. 18F is often incorporated into organic molecules by elec-
trophilic or nucleophilic reactions forming a carbon–[18F]fluorine
bond. Thus, the development of PET followed the need of new tech-
niques for incorporation of radionuclides into biologically active
molecules, such as the use of silicon as a fluorine-accepting agent.
Traditional 18F-labeling requires azeotropically dried [18F]fluo-
ride under basic reaction conditions at high temperature with
the use of a cation-complexing agent, generally Kryptofix [2.2.2],
in order to increase the reactivity of fluoride. An alternative meth-
od to conventional 18F-labeling consists of the creation of Si–18F,
B–18F and Al–18F bonds1–3 instead of a C–18F bond. Herein, we re-
in vivo images showed fast bone uptake. Despite the higher ther-
modynamic bond energy of Si–F compared to the C–F bond, the ki-
netic stability of the Si–F bond against hydrolysis is very low due to
strong bond polarization.5 More recently, Ametamey and Schirrm-
acher6–10 independently reported efficient labeling of organosil-
icon compounds. Schirrmacher et al. described
a rapid and
versatile approach to the synthesis of a 18F-labeled peptide6 based
on simple 19F–18F isotopic exchange from di-tert-butylphenylfluo-
rosilane. This work confirmed the low in vivo stability of the Si–F
bond and overcame this problem by the introduction of steri-
cally-hindered substituents around the silicon atom. Almost paral-
lel to these findings, Ametamey et al. described the synthesis of a
18F-silicon-based building block and employed an approach using
the displacement of a leaving group such as an alkoxy moiety, hy-
droxyl group or hydrogen atom under slightly acidic conditions.9
Again, it was shown that the presence of a bulky substituent was
a key factor to prevent the hydrolysis of fluorosilane.
In recent years, the research activity in our group focused on the
development of new fluorinated tracers for the imaging of hypoxia
by means of positron emitting tomography.11 The presence of hyp-
oxic cells in tumors has long been recognized as a major problem
in radiotherapy and is also a potential problem in the chemother-
apy of cancer. Thus, both identification and quantitative estimation
of tumor hypoxia are important issues in the therapeutic strategy
⇑
Corresponding author. Tel.: +33 2 32 08 29 23; fax: +33 2 32 08 25 50.
E-mail address: pierre.bohn@chb.unicancer.fr (P. Bohn).
0968-0896/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmc.2013.04.029

Y. Joyard et al. / Bioorg. Med. Chem. 21 (2013) 3680–3688
3681
for a better clinical outcome. Nitroimidazoles are known to be in-
volved in metabolic processes which appear under low oxygen
concentration in hypoxic tissues. Various hypoxic tracers contain-
fluorosilanes by comparison with the di-tert-butylphenylfluorosi-
lane prosthetic groups (SiFA) described in the literature16–23
(Fig. 2) will be discussed.
ing nitroimidazole moieties have been synthesized such as 1-
a-D-
(5-[18F]fluoro-5-deoxyarabinofuranosyl)-2-nitroimidazole
2. Results
([18F]fluoroazomycin arabinoside) and 1-(2-nitroimidazolyl)-3-
[
18F]fluoro-2-hydroxypropanol ([18F]fluoromisonidazole; 18F]F-
[
2.1. Chemistry
MISO). However, the interpretation of images is often rendered dif-
ficult due to the low signal to noise ratio depending on the tracer
itself, and is also influenced by the intensity of hypoxia in the
cells.12,13
The design of new silicon-based compounds having a 2-nitro-
imidazole moiety to identify hypoxic cells is reported. In order to
improve their solubilities in water, either an amide function
(Scheme 1) or a polyethylene glycol linker with a 1,2,3-triazole
group was used (Scheme 2).
To prepare the first silicon derivative with an amide group
(Scheme 1), (chloropropyl)dinapth-1-ylsilane 1 was synthesized
according to a procedure reported for the introduction of bulky
substituents onto alkoxychlorosilanes.24 The so-obtained chloro
derivative 1 was then reacted with sodium azide to provide the ex-
pected compound 2 in 88% yield. The subsequent reduction of the
azide function to the corresponding amine 3 was achieved by using
hydrogen on 10% Pd/C in 73% yield. Then, the 2-nitroimidazole acid
derivative 5, obtained by a classical alkylation reaction of 2-nitr-
omidazole with tert-butyl-2-bromoacetate followed by the hydro-
lysis of the tert-butyl group, was coupled with amine 3 to afford
precursor 6 in 63% yield.
The second silicon derivative having a polyethylene moiety
with a 1,2,3-triazole group was prepared from 2-(2-chloroeth-
oxy)ethanol (Scheme 2). This compound was reacted with propar-
gyl bromide and sodium hydride in THF to afford compound 8 in
94% yield. The so-obtained chloro derivative 8 was then reacted
with a solution of 2-nitroimidazole in DMF to give the expected
compound 9 in 60% yield. Subsequently, the previously prepared
azide 2 was reacted with alkyne 9 in presence of copper(II) to give
precursor 10 in a moderate 44% yield.
Clinically, we observed low SUVmax (Standardized Uptake Va-
lue) for [18F]F-MISO in NSCLC (non-small-cell lung carcinoma) un-
like FDG uptake, where FDG is a tracer of tumor metabolism.14
These low values may be explained by the heterogeneous distribu-
tion of hypoxia at the cellular level, far below the spatial resolution
of the PET/CT.15 In this case, we looked for a compound with a ra-
pid clearance from healthy tissues and higher retention in hypoxic
cells in order to improve the signal-to-noise ratio independently of
the hypoxia heterogeneity. Whereas the outline of the hypoxic tu-
mor will remain blurred due to limitations of the PET/CT, the over-
all signal will be more intense thus providing easier interpretation
of the images.
The special specifications of these molecules involve striking a
fine balance between the bioreductive trapping in hypoxic cells
on the one hand, and on the other hand assessing a selective deliv-
ery into hypoxic cells. The bioreductive trapping is ensured by the
nitroimidazole moiety, which in anaerobic or hypoxic environment
is reduced to the aminoimidazole. This compound can covalently
bind to proteins, resulting in cellular retention of labeled tracer.
The hypoxic cells-selective delivery is mainly ensured by its lipo-
philicity. Commonly, lipophilicity should be between 0.1
([18F]FETA) and 5.7 ([18F]EF5); lower values leading to rapid clear-
ance from the organism without hypoxic cells uptake; higher val-
ues led to a poor clearance from healthy cells, resulting in a bad
signal-to-noise ratio.12,13
In parallel to the generation of the Si–F bond via siloxane flu-
orolysis, we also developed
a new nitroimidazole-containing
However, the development of a new radiotracer allowing a bet-
ter monitoring of hypoxia is challenging. In previous work, our
group reported on the synthesis of new silicon-based analogues
of [18F]fluoromisonidazole (Fig. 1).11 The labeling step was im-
proved owing to the better affinity of fluorine for silicon than for
carbon. The hydrolytic stability of these new fluorosilanes is, not
surprising, correlated to the steric hindrance at the silicon center.
The best in vivo stability towards hydrolytic cleavage was observed
for a compound having a dinaphtyl substituent. However, the bio-
distribution of this molecule showed that it was mainly retained in
pulmonary capillaries.
compound 18, using di-tert-butylphenylfluorosilane as a building
block. This approach was firstly developed by Schirrmacher et al.
in 2006 and provided superior stability to hydrolysis towards silyl
ether.6 Labeling can be achieved from the precursor 17a via an
isotopic exchange reaction. Finally, radiolabeling can also be per-
formed from 17b using the Ametamey approach based on ex-
change with a hydrogen atom under slightly acidic conditions.9
Precursor 17a and 17b were prepared as reported by Kostikov
(Scheme 3).25,26 Compounds 12a and 12b were synthesized by
nucleophilic substitution of di-tert-butyldifluorosilane and di-
tert-butylchlorosilane respectively, starting from the reaction be-
tween ((3-bromobenzyl)oxy)(tert-butyl)dimethylsilane and tert-
BuLi. Acidic deprotection afforded compounds 13a and 13b in
good yields. Following oxidation of benzyl alcohol moiety (13a)
to benzaldehyde (15), activation with N-hydroxysuccinimide
(16) then coupling with 2-(2-nitro-1H-imidazol-1-yl)ethylamine,
precursor 17a was obtained. After oxidation of benzyl alcohol
moiety (13b) leading benzoic acid (14) then coupling with 2-(2-
In this preliminary work, we report on improvements to the
water solubility of previous dinaphtyl silicon-based compounds.
Moreover, the stability towards hydrolysis of these new labeled
OH
18F
R
R
N
N
N
N
Si
18F
nitro-1H-imidazol-1-yl)ethylamine,27
precursor
17b
was
O2N
O2N
obtained.
[
18F]F-MISO
R = Me, i-Pr, Ph, Napht
2.2. Radiochemistry
Figure 1. [18F]fluoromisonidazole ([18F]F-MISO) compared to its
[
18F]silicon
analogues.
Radiolabeling of 6 and 10 initially proceeded with a common
nucleophilic substitution reaction using CH3CN as solvent, and a
Kryptofix 2.2.2/K2CO3 system to produce naked, highly reactive
18FÀ fluoride anion even in the presence of acetic acid.9 Running
this reaction at room temperature gave a poor conversion. How-
ever, at 75 °C, high 18F incorporation was obtained. An excess of
tBu
Si 18F
tBu
acetic acid (50 lL) is needed for a better radiochemical yield
Figure 2. Silicon–fluoride-acceptor prosthetic group (SiFA).

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Y. Joyard et al. / Bioorg. Med. Chem. 21 (2013) 3680–3688
NO2
N
N R
4: R = CH2COOtBu
5: R = CH2COOH
N
c
1: R = Cl
2: R = N3
3: R = NH2
NO2
a
b
N
6: R = OEt
18F]7: R = [18F]
R
EtO
e
H
N
Si
Si
R
[
d
O
Scheme 1. Reagents and conditions: (a) NaN3, DMF, reflux (88%); (b) H2, 10% Pd/C, EtOH, 25 °C (73%); (c) K2CO3, CH3CN, reflux (98%); (d) EDCI, NEt3, CH2Cl2, 25 °C (63%); (e)
K18F/Kryptofix K2.2.2, AcOH, CH3CN, 70 °C.
O
O
N
O
O
Cl
O
O
Cl
OH
N
a
b
NO2
8
9
c
N
O
N
N
N
N
NO2
d
Si
R
10: R = OEt
18F]11: R = [18F]
[
Scheme 2. Reagents and conditions: (a) NaH, THF, À78 °C then BrCH2CBCH, reflux (94%); (b) 2-nitroimidazole, K2CO3, NaI, DMF, reflux (60%); (c) 2, CuSO4Á5H2O, sodium
ascorbate, dioxane, 25 °C (44%); (d) K18F/Kryptofix K2.2.2, AcOH, CH3CN, 70 °C.
(RCY). Under these conditions (75 °C, KF/K222, CH3CN, AcOH), 71%
and 56% 18F-incorporation can be reached respectively for [18F]7
and [18F]11, after a 30 min reaction time. Following solid phase
extraction (through Waters SEPack silica), [18F]7 and [18F]11 were
easily isolated and re-dissolved in a 0.9% NaCl solution for injection
(Schemes 1 and 2). This solution, suitable for injection contains
ethanol at a concentration of 5%.
Radiolabeling of 17a was first carried out under classical condi-
tions of isotopic exchange (room temperature, K[18F]/Kryptofix
2.2.2; CH3CN), but poor conversion to compound [18F]18 was ob-
served. When gradually increasing the temperature from 25 °C to
100 °C, the yield rose from 0% to 70%. The results concerning this
optimization are listed in Table 1, entries 4–9. We observed a rapid
degradation of the precursor 17a in presence of K2CO3. In order to
avoid it, we used acetic acid on K[18F]/Kryptofix 2.2.2 before adding
17a in the reaction mixture. This result highlighted the require-
ment for slightly acidic conditions to obtain good RCY (glacial ace-
2.3. Stability of compound [18F]7, [18F]11 and [18F]18
After purification, radiolabeled compounds were reformulated
in saline (solution for injection). Hydrolytic stabilities were evalu-
ated by radio-HPLC and radio-TLC analysis of [18F]7, [18F]11 and
[
18F]18 aqueous samples at different times (5, 15, 30, 60,
120 min). Dinaphtyl compounds [18F]7, [18F]11 have a half-life of
about 60 min whereas no hydrolysis occurred after 180 min for
compound [18F]18. In vitro stability was also evaluated by incubat-
ing a solution of [18F]7, [18F]11 and [18F]18 in human plasma.
Dinapthyl compounds [18F]7 and [18F]11 were found to be unstable
and rapid radiolysis occurred after a few minutes whereas the SiFA
compound [18F]18 was stable for at least 120 min. After this period
of time, less than 10% of decomposition of the compound [18F]18
occurred in water at pH 7.4. When compounds [18F]7 and [18F]11
were injected in mice, fast bone uptake was observed, demonstrat-
ing a rapid in vivo release of 18F whereas in the case of the di-tert-
butyl compound, [18F]18, only a small amount of bone uptake was
measured after a 90 min acquisition (Table 3).
tic acid: 83 lmol and sodium carbonate: 25 lmol; acid/base molar
ratio was approximately 3.5). High temperature was also necessary
to allow rapid incorporation of the [18F] fluoride anion. The crude
reaction mixture was purified by means of preparative reverse
phase-HPLC. Radiolabeled compound was then isolated in 65%
RCY and >98% radiochemical purity. [18F]18 compound was also
obtained from 17b by direct nucleophilic substitution of the hydro-
gen leaving group.28,29 Previous conditions were applied (0.1 mg;
100 °C; K[18F]/Kryptofix 2.2.2; CH3CN), and led to the formation
of [18F]18 in 24% RCY. A better yield (66%) can be obtained by
increasing the amount of precursor 17b from 0.1 mg to 1 mg. The
yield can also be improved to 81% when conducting the reaction
at 100 °C in DMSO instead of CH3CN. Temperature is an important
parameter, since in DMSO at 60 °C the RCY was only 29%. The
amount of acetic acid also has a dramatic influence on the RCY,
since adding a large quantity decreased the yield (Table 1; entries
10–15).
2.4. Lipophilicities of compounds [18F]7, [18F]11 and [18F]18
The lipophilicities of compounds [18F]7, [18F]11 and [18F]18
were determined under physiological conditions (logD) or by
clogP calculations using ChemDraw 11.0 software. Compound
[
18F]7 was insufficiently stable at pH 7.4, thus the lipophilicity
was measured at pH 4. The low logP value found (0.12) seems
unreliable and might be explained by a fluoride release from the
native compound. The clogP value of [18F]18 is 5.01. Compound
[
18F]18 was sufficiently stable at pH 7.4 to estimate the logD
(2.12), and the logP of compound [18F]11 was estimated by calcu-
lation because of its instability. These results are summarized in
Table 2.

Y. Joyard et al. / Bioorg. Med. Chem. 21 (2013) 3680–3688
3683
OTBS
OTBS
a
Si
X
tBu
tBu
Br
12a: X = F
12b: X = H
b
O
OH
OH
O
d
c
Si
H
Si
X
Si
F
tBu
tBu
tBu
tBu
tBu
e
tBu
13a: X = F
13b: X = H
14
15
O
f
O2N
N
O
N
O
O
O
N
N
H
f
Si
X
Si
F
tBu
tBu
tBu
tBu
16
17a: X = F
17b: X = H
[
g
18F]18: X = 18
F
Scheme 3. Synthesis and radiolabeling of silicon analogue 18 from fluorinated precursor 17a via isotopic exchange and from Si-H precursor 17b by nucleophilic substitution.
(a) 12a: tBuLi, THF, tBu2SiF2, 88%; 12b: tBuLi, THF, tBu2SiClH, 96%; (b) 13a and 13b: HCl (37%), MeOH, 65% and 75%; (c) 14: Jones Reagent, acetone, 71%; (d) 15: PCC, CH2Cl2,
98%; (e) 16: NHS, PhI(OAc)2, AcOEt, 76%; (f) 17a: 2-(2-nitro-1H-imidazol-1-yl)ethanamine, Et3N, CH2Cl2, 50%; 17b: 2-(2-nitro-1H-imidazol-1-yl)ethanamine, DIEA, propane
phosphonic acid anhydride (T3PÒ), CH2Cl2, 55%; (g) KF, Kryptofix 2.2.2, CH3CN, AcOH, 70–90%.
Table 1
Radiosynthesis conditions of compounds [18F]7, [18F]11 and [18F]18; the amount of K2CO3 was always 3.5 mg (25
lmol), experiments were performed in triplicate and the mean
result of RCY was calculated
Entry
Precursor (mg)
AcOH (ll) [
lmol]
Solvent
T (°C)
Time
RCY (%)
1
2
3
4
5
6
7
8
6 (1)
6 (1)
10 (1)
(50) [830]
(5) [83]
(50) [830]
(5) [83]
(5) [83]
(5) [83]
(5) [83]
(5) [83]
(5) [83]
(5) [83]
(5) [83]
(10) [166]
(5) [83]
(5) [83]
(5) [83]
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN/DMSO (1:1)
DMSO
DMSO
75
75
75
25
25
50
80
80
100
100
100
100
100
100
60
30
15
30
5
20
5
71
12
56
0
5
6
49
62
70
24
66
33
82
81
29
17a (0.1)
17a (0.1)
17a (0.1)
17a (0.1)
17a (0.1)
17a (0.1)
17b (0.1)
17b (1)
17b (1)
17b (1)
17b (1)
17b (1)
5
25
15
15
15
15
15
15
15
9
10
11
12
13
14
15
2.5. Preliminary biological evaluation of compound [18F]18
was measured. No significant uptake in heart or muscle was ob-
served (Table 3).
In vivo assessment of [18F]18 was achieved by injection of a
Wistar rat. Dynamic acquisitions under a Mosaic PET camera30
were performed until 90 min after tracer injection. Fluorosilane
3. Discussion
[
18F]18 was distributed in all compartments of the organism, but
The aim of this work was to synthesize new fluorinated nitro-
imidazole compounds in order to further explore their potential
for their in vivo imaging of hypoxia by means of PET. The
most used compound in nuclear medicine for this indication is
predominantly in liver, intestine and bladder after 90 min post
injection. These results indicate an extensive hepatic extraction
paired with renal extraction. A small amount of uptake in bones

3684
Y. Joyard et al. / Bioorg. Med. Chem. 21 (2013) 3680–3688
Table 3
Biodistribution data of fluorosilane [18F]18, 90 min post injection
Organ
Uptake value (% injected dose)
Bones (femur)
Liver
Intestine
Heart
Muscle
Bladder
2.8
7.3
6.5
0.5
0.3
9.5
Table 2
Lipophilicity of radiolabeled compounds [18F]7, [18F]11, [18F]18
[
18F]7 18F]11
0.12 0.03a 5.48b
Due to instability of compound [18F]7 at pH 7.4, measurement was done at
[
[
18F]18
Experimental determination (n = 3)
2.12 0.11c
a
pH = 4.
b
clogP determination.
c
logD7.4
.
Figure 3. Biodistribution of [
18F]18 in rat by small-animal PET (90 min post
injection). A = liver; B = digestive tract; C = bladder; D = bones.
[
18F]F-MISO.12 The drawback of this compound is its bad signal-to-
background ratio. We synthesized nitroimidazole compounds with
a silicon core in order to improve the radiochemical yield. Indeed,
the typical radiochemical yield of [18F]F-MISO is close to 60% as
previously reported in literature with 5 mg of precursor.31,32 Our
method afforded yields ranking from 56% to 71% with 1 mg of pre-
cursor for dinaphtyl compounds and up to 82% for SiFA compound.
In a previous study we observed that dimethyl, diisopropyl and
diphenyl fluorosilane were unstable in water, when dinaphtylfluo-
rosilane (See Fig. 1; R = Napht) revealed a good stability. However,
this tracer was retained in the pulmonary capillaries maybe due to
its lipophilicity (clogP = 6.47).11 This lipophilicity is outside the lim-
its (0.1–5.7) set by other hypoxic tracers such as EF5 (logP = 5.7).12,13
However all attempts to increase the hydrophilicity of this com-
pound by introducing polar substituents at the naphthalene moiety
failed. Thus, we undertook to tune the hydrophilicity of the linker
between the silicon core and the nitroimidazole.
ers does not require a high specific activity. Indeed, the uptake of a
nitroimidazole compound is not a saturable process and does not
involve a specific receptor but only a redox reaction involving
the nitro group in the hypoxic cell.
Hydrolytic stability of compound [18F]18 is promising (100% in
saline after two hours, and 85% in plasma after two hours). Since
hydrolysis of fluorosilanes can be avoided with bulky groups at
the silicon atom, it is likely that the t-butyl groups provide better
steric hindrance than the naphtyl groups. The silicon atom is thus
protected from nucleophilic attack, and the stability of the silicon–
fluorine bond is improved in water. Nevertheless, in plasma, we
noticed a slight degradation of the labeled compound [18F]18 after
two hours, which might be due to the presence of enzymes such as
hydrolase, or peptidase.
When compounds [18F]7 or [18F]11 were injected in rat, exten-
sive uptake of 18F occurred in bones, demonstrating a rapid release
of fluoride in vivo. These results were in agreement with the
in vitro experiments. When compound [18F]18 was injected in
rat, after 90 min, we observed a biodistribution of the tracer
throughout the organism, including bones (Fig. 3).
Compounds [18F]7 and [18F]11 were not stable in saline (pH
5.5), exhibiting half-lives around 60 min, whereas the first dinaph-
tylfluorosilane synthesized had a half-life of 130 min.11 Moreover,
in basic conditions, compounds [18F]7 and [18F]11 were more
prone to nucleophilic attacks by hydroxyl ion. At 37 °C in plasma,
half-lives of both compounds were below 5 min. We then refo-
cused our efforts to the SiFA (Silicon–fluoride-acceptor) com-
pounds developed by Schirrmacher et al.6–8,16–23 In this case,
silicon is substituted by a phenyl and two t-butyl groups.
Two different radiolabeling pathways can provide access to
compound [18F]18. The first one involves the stable compound
17a where fluorine is isotopically exchanged by fluorine-18. The
second way consists of the addition of fluorine-18 on the silane
17b. We used glacial acetic acid as an additive for this purpose.
Obviously, the specific activity was better when using the second
procedure (respectively, for the first and the second way: 3 GBq/
The stability of compound [18F]18 is within the range found
with other SiFA labeling procedures. Further studies will be needed
to evaluate this radiolabeled compound [18F]18 in mice bearing
hypoxic tumors. Indeed, this tracer has a lipophilicity (logD = 2.12)
that could hamper its clearance from healthy tissues, but is favor-
able with hypoxic tumor uptake. Nevertheless, this study confirms
that t-butyl groups stabilize the silicon–fluorine bond more than
naphtyl groups in vitro and in vivo.
4. Conclusion
We have described the preparation of various [18F]silamisoni-
dazoles derivatives. Only the SiFA derivatized misonidazole is suff-
icently stable in vivo. This compound may be directly labeled by
isotopic exchange or by fluorine addition on the corresponding si-
lane in DMSO or acetonitrile. These mild reaction conditions make
the synthesis of F-MISO analogues a key for the development of
new 18F-radiopharmaceuticals dedicated to the detection of tu-
moral hypoxic domains. Compound [18F]18 will be assessed as a
hypoxic tumor tracer in upcoming studies.
l
mol and ꢀ50 GBq/
lmol), the radiochemical yields being similar.
The specific activity was calculated from a concentration range of
cold fluorinated compound and activities obtained after labeling.
The second way involving the precursor 17b can be performed
both in DMSO and CH3CN and does not require a defined amount
of acetic acid since the precursor is not sensitive to hydrolysis in
basic conditions. We also noted that the labeling from precursor
17a may be achieved directly in CH3CN/H2O (9:1), with a good
RCY (65–75%). Moreover, the mechanism of action of hypoxic trac-

Y. Joyard et al. / Bioorg. Med. Chem. 21 (2013) 3680–3688
3685
(3H, t, J = 5 Hz). 13C NMR (CDCl3) d 137.2; 135.3; 134.0; 133.5;
130.8; 128.9; 128.6; 126.1; 125.6; 125.3; 77.4; 59.6; 45.4; 27.9;
12.8. IR (ATR, cmÀ1) 3053; 2928; 2863; 1501; 1075; 985; 795;
775. HR-MS (ESI+) calcd for C25H28NOSi [M+H]: 386.1940, found:
386.1941.
5. Experimental
5.1. Reagents and instrumentation
All commercially available reagents were purchased from Sig-
ma/Aldrich or Alfa Aesar and used without further purification.
Di-tert-butyldifluorosilane was purchased from Fluorochem. Un-
less otherwise stated, reactions were performed under nitrogen
atmosphere using freshly distilled solvents. All reactions were
monitored by thin-layer chromatography with Merck silica gel
60 F254 pre-coated aluminum plates (0.25 mm). Flash chromatog-
raphy was performed with indicated solvents using silica gel (par-
5.2.1.3. tert-Butyl 2-(2-nitro-1H-imidazol-1-yl)acetate 4.
tert-
Butyl 2-bromoacetate (0.18 g, 0.97 mmol) was added to a solution
of 2-nitroimidazole (0.1 g, 0.88 mmol) and potassium carbonate
(0.12 g, 0.88 mmol) in acetonitrile (2 mL) and reaction mixture
was refluxed for 3 h. The mixture was filtered and solvent removed
under reduced pressure to give the title compound 4 as a white so-
lid (0.19 g, 98%). 1H NMR (CDCl3) d 7.19 (1H, d, J = 1 Hz), 7.06 (1H,
d, J = 1 Hz), 5.00 (2H, s), 1.47 (9H, s). 13C NMR (CDCl3) d 165.1;
128.5; 126.6; 84.4; 51.8; 28.1. IR (ATR, cmÀ1) 3133; 2983; 1741;
1491; 1356; 1145; 830; 775; 750; 650. HR-MS (ESI+) calcd for
C9H14N3O4 [M+H]: 228.0984, found: 228.0983.
ticle size 30–63 lm) purchased from Merck. The SAX cartridges
(Sep-Pak Light (46 mg) Accell Plus QMA Carbonate) were pur-
chased from Waters. 1H, 13C and 19F nuclear magnetic resonance
(NMR) spectra were recorded on a Bruker Advance at 300 MHz
for 1H, 75 MHz for 13C and 282 MHz for 19F with corresponding sol-
vent signals as an internal standard. Chemical shifts are reported in
ppm. Coupling constants J are in Hertz and are reported as d (dou-
blet), t (triplet), q (quartet) and m (multiplet). Low resolution MS
analyses were performed with a Jeol JMS-AX500 spectrometer in
chemical ionization. HR-MS analyses were performed with an
Waters LCT UPremier XE (ESI). Radio TLCs were monitored with a
LabLogic ScanRAM device.
5.2.1.4. 2-(2-Nitro-1H-imidazol-1-yl)acetic acid 5.
tert-Bu-
tyl-(2-nitro-1H-imidazo-1-yl)acetate (0.19 g, 0.86 mmol) 4 was
dissolved in a 30% solution of trifluoroacetic acid in dichlorometh-
ane and stirred for 1 h at room temperature. The solvent was re-
moved in vacuo to give compound 5 as a white solid (0.13 g,
90%) which can be used without further purification;. 1H NMR
(MeOD) d 7.65 (1H, s); 7.22 (1H, s); 5.23 (2H, s). 13C NMR (MeOD)
d 169.9; 129.0; 128.4; 51.8. IR (ATR, cmÀ1) 3148; 2988; 2517;
1746; 1496; 1366; 1145; 775; 705. HRMS (ESI-) calcd for
C5H4N3O4 [MÀH]: 170.0202, found: 170.0195.
General high-performance liquid chromatography (HPLC) con-
ditions: Varian vista 5500, monitoring with ultraviolet detector
(wavelength: 320 nm) and with Bicron frisk-tech radioactivity
detector. Column: Nucleosil 100-5 C18 (150 Â 4.6 mm,
5
l);
1 mL minÀ1, CH3CN/H2O (3:2) or (7:3).
5.2.1.5.
N-(3-(Ethoxydi(naphthalen-1-yl)silyl)propyl)-2-(2-
Compound 3 (0.09 g,
Preparative HPLC: Hypersild Gold C18 (150 Â 10 mm, 5
l
),
3 mL minÀ1, CH3CN/H2O: 3:2).
nitro-1H-imidazol-1-yl)acetamide 6.
0.23 mmol) was added to a solution of 2-(2-nitro-1H-imidazol-1-
yl)acetic acid 5 (0.07 g, 0.25 mmol) in dry dichloromethane with
5.2. Synthesis of radiochemistry precursors and cold standards
5.2.1. Synthesis of analogue 7
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
hydrochloride
(EDCI) (0.04 g, 0.25 mmol). The reaction mixture was stirred at
room temperature overnight. Then, the organic layer was washed
with 10% HCl and saturated Na2CO3 solution. The organic layer
was dried over Na2SO4 and concentrated in vacuo to give a pale
yellow solid. The crude product was purified by flash chromatogra-
phy using ethyl acetate as eluant. Product was purified by prepara-
tive RP-HPLC to remove silanol side product than can be formed
during the synthesis. Compound 6 was obtained as a white solid
(0.08 g, 63%). 1H NMR (CDCl3) d 8.20 (2H, 2d, J = 8 Hz); 7.94 (2H,
d, J = 7 Hz); 7.90 (2H, d, J = 7 Hz); 7.84 (2H, d, J = 8 Hz); 7.50 (2H,
td, J = 8 Hz, J = 3 Hz); 7.42 (2H, td, J = 7 Hz, J = 3 Hz); 7.33 (2H, td,
J = 8 Hz, J = 3 Hz); 7.13 (1H, d, J = 3 Hz); 7.01 (1H, d, J = 3 Hz);
5.62 (1H, t); 4.75 (2H, s); 3.73 (2H, q, J = 5 Hz); 3.23 (2H, q,
J = 6 Hz); 1.60–1.46 (4H, m); 1.20 (3H, t, J = 5 Hz). 13C NMR (CDCl3)
d 164.35; 144.7; 136.7; 135.3; 134.7; 133.3; 130.7; 128.8; 128.2;
127.1; 126.1; 125.6; 125.2; 77.2; 59.6; 51.9; 42.4; 23.4; 18.4. IR
(ATR, cmÀ1) 3283; 2927; 1670; 1491; 1367; 1076; 780; 459. HRMS
(ESI) calcd for C30H30N4O4SiCl [M+Cl]À: 573.1725, found: 573.1727.
5.2.1.1. Ethoxy(3-azidopropyl)dinaphth-1-ylsilane 2.
Eth-
oxy-(3-chloropropyl)dinapht-1-ylsilane 1 was synthesized as re-
ported by Bohn et al.11 Sodium azide (0.02 g, 0.32 mmol) was
added to a solution of compound 1 (0.11 g, 0.27 mmol) in DMF
(2 mL). The reaction mixture was refluxed overnight with a vigor-
ous stirring. Then the reaction was cooled to room temperature
and diethyl ether (5 mL) was added. The Organic layer was ex-
tracted with water (2 Â 2 mL) and brine (2 Â 2 mL), dried over
Na2SO4 and solvent removed under reduced pressure to give the
expected product as a brown oil without further purification
(0.09 g, 84%). 1H NMR (CDCl3) d 8.17 (d, 2H, J = 8 Hz); 7.94 (2H, d,
J = 7 Hz); 7.91 (2H, d, J = 7 Hz); 7.84 (2H, d, J = 8 Hz); 7.49 (2H, td,
J = 8 Hz, J = 7 Hz); 7.41 (2H, td, J = 7 Hz, J = 1 Hz); 7.32 (2H, td,
J = 8 Hz, J = 1 Hz); 3.75 (2H, q, J = 5 Hz); 3.23 (2H, t, J = 5 Hz);
1.71–1.62 (2H, m); 1.59–1.51 (2H, m); 1.25 (3H, t, J = 5 Hz). 13C
NMR (CDCl3) d 137.1; 135.3; 133.6; 133.5; 131.0; 128.9; 128.4;
126.2; 125.7; 125.3; 59.7; 54.3; 23.5; 15.6; 12.9. IR (ATR, cmÀ1
)
3053; 2923; 2092; 1506; 1261; 1075; 800; 770. HR-MS (ESI+) calcd
for C25H25N3ONaSi [M+Na]: 434.1665, found: 434.1679.
5.2.1.6. N-(3-(Fluorodi(naphthalen-1-yl)silyl)propyl)-2-(2-nitro-
1H-imidazol-1-yl)acetamide 7.
Compound
6
(0.010 g,
0.02 mmol) was dissolved in dry THF. Hydrogen fluoride-pyridine
complex was added (1.1 mg, 0.04 mmol) and the mixture was stir-
red for 20 min at room temperature. Then, the solvent was re-
moved under vacuum to give pale a yellow solid in quantitative
yield (0.10 g). 1H NMR (CDCl3) d 8.07 (2H, d, J = 8 Hz), 7.95 (2H,
d, J = 8 Hz), 7.84 (4H, t J = 8 Hz,), 7.54–7.35 (6H, m), 7.11 (1H, s),
7.01 (1H, s), 5.94 (1H, t, J = 6 Hz), 4.82 (2H, s), 3.29 (2H, dd,
J = 13 Hz, J = 7 Hz), 1.71 (2H, dd, J = 13 Hz, J = 7 Hz), 1.52 (2H, dd,
J = 13 Hz, J = 7 Hz). 13C NMR d 164.8; 136.7; 135.2; 135.1; 133.5;
131.9; 131.8; 129.2; 128.6; 128.0; 127.3; 126.8; 126.1; 125.4;
5.2.1.2.
3.
3-(Ethoxydi(naphthalen-1-yl)silyl)propan-1-amine
Compound 2 (0.09 g, 0.23 mmol) was dissolved in ethanol
and passed through 10% mol Pd/C cartridge using hydrogenation
reactor (H-Cube) at room temperature and atmospheric pressure.
After evaporation of solvent, 3-(ethoxydi(naphthalen-1-
yl)silyl)propan-1-amine 3 was obtained as a brown oil and used
without further purification (0.06 g, 73%). 1H NMR (CDCl3) d 8.20
(2H, d, J = 8 Hz); 7.91 (4H, d, J = 7 Hz); 7.84 (2H, d, J = 8 Hz); 7.48
(2H, t, J = 8 Hz); 7.42 (2H, t, J = 7 Hz); 7.33 (2H, t, J = 8 Hz); 3.75
(2H, q, J = 5 Hz); 2.64 (2H, t, J = 5 Hz); 1.48–1.41 (4H, m); 1.22
77.4; 52.3; 42.4; 23.2. 19F NMR (CDCl3) d À163.2. IR (ATR, cmÀ1
)

3686
Y. Joyard et al. / Bioorg. Med. Chem. 21 (2013) 3680–3688
3295; 2933; 1663; 1491; 1367; 910; 779; 720; 447. HRMS (ESI)
calcd for C28H26N4O3FSi [M+H]: 513.1758, found: 573.1752.
129.2; 128.2; 127.9; 127.3; 126.8; 126.1; 125.3; 122.7; 70.7;
69.7; 69.5; 64.6; 52.3; 49.9; 29.8; 24.3. 19F NMR (CDCl3) d
À163.21. IR (ATR, cmÀ1) 2916; 1717; 1468; 1361; 1088; 773;
732; 435. HRMS (ESI) calcd for C33H34N6O4SiF [M+H]: 625.2395,
found: 625.2393.
5.2.2. Synthesis of analogue 11
5.2.2.1. 3-(2-(2-Chloroethoxy)ethoxy)prop-1-yne 8.
hydride (0.36 g, 16.06 mmol) was suspended in THF (25 mL) and
stirred at À20 °C. solution of 2-(2-chloroethoxy)ethanol
Sodium
A
5.2.3. Synthesis of analogues 17a and 17b
5.2.3.1. Di-tert-butyl(3-(((tert-butyldimethylsilyl)oxy)methyl)
(0.85 mL, 8.03 mmol) was added. The Reaction mixture was stirred
at À78 °C for 15 min then, a solution of propargyl bromide
(0.83 mL, 9.64 mmol) in THF (5 mL) was added. The mixture was
refluxed for 3 h. The solvent was removed to dryness then the res-
idue was diluted in dichloromethane and washed with water. The
crude product was purified on silica gel with a mixture of ethyl
acetate/dichloromethane as eluant to give compound 8 in 94%
yield. 1H NMR (CDCl3) d 4.11 (2H, d, J = 2 Hz); 3.66 (2H, t,
J = 6 Hz); 3.56 (4H, s); 3.53 (2H, t, J = 6 Hz); 2.40 (1H, t, J = 2 Hz).
13C NMR (CDCl3) d 79.4; 74.6; 71.1; 70.2; 68.8; 58.2; 42.6. IR
(ATR, cmÀ1) 3289; 2862; 1667; 1352; 1296; 1095; 662.
phenyl)fluorosilane 12a.
pentanes (1.7 M, 1.13 mL) was added dropwise to a solution of
((3-bromobenzyl)oxy)(tert-butyl)dimethylsilane (0.26 g,
A solution of tert-butyl lithium in
0.86 mmol) in anhydrous THF (2 mL) over a period of 15 min at
À78 °C. After the solution had been stirred at the same tempera-
ture for additional 15 min, a solution of di-tert-butyldifluorosilane
(0.33 g, 1.80 mmol) in anhydrous THF was added dropwise over a
period of 15 min at À78 °C. The reaction mixture was allowed to
warm to room temperature overnight. The reaction was quenched
by addition of saturated aqueous sodium chloride solution, and the
product was extracted with Et2O (3 Â 5 mL). The combined organic
phase was dried over Na2SO4, filtered, and concentrated in vacuo to
afford 12a as a pale-yellow liquid (0.29 g, 88%) that was carried
forward without further purification. 1H NMR (CDCl3) d 7.57 (1H,
s), 7.48 (1H, d, J = 6 Hz); 7.38–7.31 (2H, m); 4.77 (2H, s); 1.06
(18H, s); 0.94 (9H, s); 0.09 (6H, s). HRMS (EI) calcd for C17H30OS-
i2F [M+.- tBu.]: 325.1819, found: 325.1825.
5.2.2.2.
imidazole 9.
2-Nitro-1-(2-(2-(prop-2-yn-1-yloxy)ethoxy)ethyl)-1H-
Sodium iodide (0.12 g, 0.80 mmol) and potas-
sium carbonate (0.12 g, 0.90 mmol) were added to a solution of
2-nitroimidazole (0.10 g, 0.88 mmol) and compound 8 (0.14 g,
0.90 mmol) in DMF (10 mL). The reaction mixture was stirred for
one night at 110 °C then the solvent was removed under reduced
pressure and residue purified on silica gel (AcOEt/DCM: 1:1). Com-
pound 9 was isolated in good yield (0.11 g, 60%). 1H NMR (CDCl3) d
7.22 (1H, s); 7.05 (1H, s); 4.56 (2H, t, J = 6 Hz); 4.07 (2H, d, J = 2 Hz);
3.79 (2H, t, J = 6 Hz); 3.56 (4H, s); 2.39 (1H, t, J = 2 Hz). 13C NMR
(CDCl3) d 128.0; 127.3; 79.3; 74.7; 70.3; 69.3; 68.9; 58.3; 49.8. IR
(ATR, cmÀ1) 3284; 2923; 2853; 1711; 1541; 1481; 1351; 1095;
830; 645. HR-MS (ESI+) calcd for C10H14N3O4 [M+H]: 240.0984,
found: 240.0985.
5.2.3.2. tert-Butyl((3-(di-tert-butylsilyl)benzyl)oxy)dimethylsi-
lane 12b.
Following the previous procedure, tert-butyl((3-
(di-tert-butylsilyl)benzyl)oxy)dimethyl silane 12b was obtained
as pale-yellow liquid without further purification (96%). 1H NMR
(CDCl3) d 7.53 (1H, s); 7.43 (1H, d, J = 6 Hz); 7.30 (2H, m); 4.775
(2H, s); 3.85 (1H, s); 1.04 (18H, s); 0.93 (9H, s); 0.09 (6H, s). HRMS
(EI) calcd for C17H31OSi2 [M+ÀtBu]: 307.1913, found: 307.1936.
5.2.2.3. 1-(3-(Ethoxydi(naphthalen-1-yl)silyl)propyl)-4-((2-(2-
5.2.3.3. 3-(Di-tert-butylfluorosilyl)benzyl alcohol 13a.
A
(2-nitro-1H-imidazol-1-yl)ethoxy)ethoxy)methyl)-1H-1,2,3-tri-
concentrated aqueous hydrochloric acid solution (37 wt %, 25
lL)
azole 10.
CuSO4Á5H2O (0.02 g, 0.07 mmol) and sodium ascor-
was added dropwise to a solution of crude 12a (0.28 g, 0.73 mmol)
in MeOH (2.5 mL) at 25 °C, and the mixture was stirred at room
temperature overnight. After all volatiles have been removed un-
der reduced pressure, the residue was re-dissolved in ether
(2 mL), and the solution washed with saturated aqueous sodium
bicarbonate solution (2 mL). The aqueous phase was extracted
with ether (3 Â 3 mL), and the combined organic phase was
washed with H2O (3 mL), dried over Na2SO4, filtered, and concen-
trated in vacuo. The residue was purified by flash column chroma-
tography on silica gel (9:1 to 7:1 hexanes/EtOAc) to afford 13a as a
white solid (0.13 g, 65%). 1H NMR (CDCl3) d 7.58 (1H, s); 7.53 (1H,
d, J = 7 Hz); 7.44 (1H, d, J = 7 Hz); 7.38 (1H, t, J = 7 Hz); 4.71 (2H, s);
1.86 (1H, br s); 1.06 (18H, s). 13C NMR (CDCl3) d 139.9; 134.0;
133.3; 132.5; 128.3; 127.8; 65.5; 27.3; 20.2. 19F NMR(CDCl3) d
À188.84. HRMS (ESI): calcd for C15H25FOSiNa [M+Na]: 291.1551,
found: 291.1545.
bate (0.01 g, 0.07 mmol) were added to a solution of compound 9
(0. 10 g, 0.42 mmol) and azide 2 (0.14 g, 0.35 mmol) in dioxane
(20 mL). The reaction mixture was stirred at room temperature
overnight and solvent was removed under reduced pressure. The
residue was purified by flash chromatography (AcOEt/DCM/MeOH:
1:1:0.05) (0.10 g, 44%). 1H NMR (CDCl3) d 8.09 (2H, d, J = 6 Hz);
7.88–7.82 (6H, m); 7.47 (2H, td, J = 6 Hz, J = 2 Hz); 7.40 (2H, td,
J = 6 Hz, J = 2 Hz); 7.30 (2H, td, J = 6 Hz, J = 2 Hz); 7.23 (1H, s);
7.17 (1H, d, J = 1 Hz); 7.04 (1H, d, J = 1 Hz); 4.54 (4H, s); 4.25 (2H,
t, J = 7 Hz); 3.78–3.71 (4H, m); 3.54 (4H, s); 1.94 (2H, m); 1.43
(2H, m); 1.18 (3H, t, J = 7 Hz). 13C NMR (CDCl3) d 144.6; 137.0;
135.3; 134.6; 133.4; 133.2; 131.0; 130.4; 128.9; 128.2; 128.1;
127.3; 126.2; 125.7; 125.3; 122.5; 70.6; 69.5; 69.4; 64.5; 59.6;
52.7; 49.8; 24.8; 18.4; 12.4. IR (ATR, cmÀ1) 3046; 2883; 1721;
1536; 1481; 1361; 1080; 805; 775. HR-MS (ESI+) calcd for
C35H39N6O5Si [M+H]: 651.2751, found: 651.2772.
5.2.3.4. 3-(Di-tert-butylsilyl)benzyl alcohol 13b.
Following
5.2.2.4.
(2-nitro-1H-imidazol-1-yl)ethoxy)ethoxy)methyl)-1H-1,2,3-tri-
azole 11. Compound 10 (0.01 g, 0.02 mmol) was dissolved in
1-(3-(Fluorodi(naphthalen-1-yl)silyl)propyl)-4-((2-(2-
the previous procedure, tert(3-(Di-tert-butylsilyl)phenyl)methanol
13b was obtained as a colorless liquid without further purification
(75%). 1H NMR (CDCl3) d 7.54 (1H, s); 7.51 (1H, d, J = 5 Hz); 7.47
(1H, d, J = 5 Hz); 7.36 (1H, t, J = 5 Hz); 4.70 (2H, s); 3.86 (1H, s);
1.04 (18H, s). 13C NMR (CDCl3) d 139.9; 136.1; 135.2; 134.6;
127.9; 127.8; 65.8; 29.1; 19.1. IR (ATR, cmÀ1) 3301; 2927; 2850;
2102; 1468; 1017; 803; 702; 459. HRMS (EI) calcd for C15H26OSi
[M+]: 250.1752, found: 250.1747.
dry THF. Hydrogen fluoride–pyridine complex was added
(0.04 mmol) and the mixture was stirred for 20 min at room tem-
perature. Then, solvent was removed under vacuum to give a pale
yellow solid in quantitative yield (0.12 g). 1H NMR (CDCl3) d 8.04
(2H, d, J = 8 Hz); 7.97 (2H, d, J = 8 Hz); 7.87 (2H, d, J = 8 Hz); 7.80
(2H, d, J = 7 Hz); 7.60–7.32 (7H, m); 7.17 (1H, s); 7.06 (1H, s);
4.55 (4H, dd, J = 9 Hz, J = 4 Hz); 4.36 (2H, t, J = 7 Hz); 3.85–3.71
(2H, m); 3.64–3.37 (4H, m); 2.11 (2H, m); 1.59–1.45 (2H, m). 13C
NMR (CDCl3) d 144.8; 136.7; 135.2; 135.1; 134.7; 133.52; 131.9;
5.2.3.5. 3-(Di-tert-butylsilyl)benzoic acid 14.
3-(Di-tert-
butylsilyl)phenyl)methanol 13b (0.12 g, 0.48 mmol) was solubi-
lised in acetone (2.5 mL) and cooled to 0 °C. Jones Reagent (8 M,

Y. Joyard et al. / Bioorg. Med. Chem. 21 (2013) 3680–3688
3687
0.26 mL, 2 mmol) wad added dropwise. The reaction was stirred
for 30 min at 0 °C. Then the reaction was quenched with water
(5 mL) and extracted with ethyl acetate (3 Â 5 mL). The organic
layers were combined, extracted with water, brine, dried over
Na2SO4 and solvent removed under reduced pressure. Crude prod-
uct was purified by flash chromatography on silica gel (Petroleum
ether/AcOEt/AcOH: 90:9:1) to give 14 as a white solid (0.09 g, 71%).
1H NMR (CDCl3) d 8.33 (1H, s); 8.11 (1H, dt, J = 5 Hz; J = 2 Hz); 7.81
(1H, dt, J = 5 Hz, J = 2 Hz); 7.45 (1H, t, J = 5 Hz); 3.93 (1H, s); 1.06
(18H, s). 13C NMR (CDCl3) d 170.9; 141.2; 137.4; 130.8; 127.8;
29.0; 19.1. IR (ATR, cmÀ1) 2928; 2855; 2102; 1677; 1586; 1467;
1428; 1288; 1142; 1142; 941; 817; 744. HRMS (ESI) calcd for
5.2.3.9.
yl)ethyl)benzamide 17b.
3-(Di-tert-butylsilyl)-N-(2-(2-nitro-1H-imidazol-1-
3-(Di-tert-butylsilyl)benzoic acid
(0.05 g, 0.19 mmol) and amine 2-(2-nitro-1H-imidazol-1-yl)ethan-
amine3 (0.05 g, 0.19 mmol) were dissolved in dry dichloromethane
(2 mL). Reaction mixture was cooled to 0 °C, DIEA (0.12 g,
0.95 mmol) was added then a propane phosphonic acid anhydride
solution (50% in DMF) (0.14 g, 0.23 mmol). After 24 h stirring at
room temperature, the crude reaction mixture was washed with
sat. Na2CO3. The aqueous layer was extracted with dichlorometh-
ane (3 Â 2 mL). Organic layers were combined and extracted with
water (2 mL) and brine (2 mL). After drying under Na2SO4 solvent
was removed under reduced pressure and the crude product puri-
fied on silica gel (AcOEt/PE: 8:2). Product was purified by prepara-
tive RP-HPLC (0.04 g, 56%). 1H NMR (CDCl3) d 7.94 (1H, s); 7.73 (2H,
m); 7.39 (1H, t J = 7 Hz); 7.14 (1H, t, J = 5 Hz); 7.06 (1H, s); 6.91 (1H,
s); 4.73 (2H, t, J = 5 Hz); 3.91 (2H, q, J = 5 Hz); 3.88 (1H, s); 1.02
(18H, s). 13C NMR (CDCl3) d 169.2; 139.2; 136.8; 134.5; 132.7;
128.3; 127.9; 127.5; 49.3; 40.4; 29.0; 19.1. IR (ATR, cmÀ1) 2933;
2859; 1654; 1486; 1356; 1287; 1160; 796. HRMS (ESI) calcd for
C15H23O2Si [MÀH]: 263.1467, found: 263.1467.
5.2.3.6. 3-(Di-tert-butylfluorosilyl)benzaldehyde 15.
A solu-
tion of 13a (0.08 g, 0.28 mmol) in anhydrous CH2Cl2 (50 mL) was
added dropwise to a solution of pyridinium chlorochromate
(PCC) (0.15 g, 0.70 mmol) in anhydrous CH2Cl2 (7 mL) at 0 °C. The
reaction mixture was stirred at 0 °C for 30 min and then allowed
to warm to room temperature for 2.5 h, diluted with ether 6 mL,
and filtered. The precipitate was washed with anhydrous Et2O,
and the combined organic phase was concentrated in vacuo. The
residue was purified by flash column chromatography on silica
gel (19:1 hexane/EtOAc) to afford 15 as a yellow oil (0.07 g, 98%).
1H NMR (CDCl3) d 10.05 (1H, s); 8.11 (1H, s); 7.93 (1H, dt,
J = 7 Hz, J = 1 Hz); 7.87 (1H, dt, J = 7 Hz, J = 1 Hz); 1.07 (18H, d,
J = 1 Hz). 13C NMR (CDCl3) d 192.6; 139.8; 135.5; 130.5; 128.3;
27.2; 20.2. 19F NMR (CDCl3) d À188.43. HRMS (ESI+) calcd for
C20H31N4O3Si [M+H]: 403.2165, found: 403.2166.
5.3. Radiochemical synthesis
5.3.1. Production of [18F]FÀ
[
18F]FÀ was produced after a (p, n) reaction from the IBA Cy-
clone 18/9 cyclotron (Université Catholique de Louvain—UCL, Bel-
gium) by irradiating
18O]–H2O with 16.5 MeV protons. The
18F]FÀ was separated from the [18O]–H2O by trapping on an ion
[
[
C15H24FOSi [M+H]: 267.1575, found: 267.1571.
exchange resin (QMA, from Waters). QMA resin was previously
conditioned with sodium bicarbonate (10 mL, 1 M) and water
(20 mL). The [18F]FÀ was eluted by a solution of Kryptofix 2.2.2
5.2.3.7. N-Succinimidyl 3-(di-tert-butylfluorosilyl)benzoate
16. N-Hydroxysuccinimide (0.16 g, 1.40 mmol) and com-
(15 mg, 40 lmol) and K2CO3 (3.5 mg, 25 lmol) in CH3CN/H2O
pound 15 (0.07 g, 0.28 mmol) were added to ethyl acetate (4 mL)
in a 5 mL vial, and the mixture was cooled to 0 °C. Iodobenzene
diacetate (PhI(OAc)2) (0.11 g, 0.35 mmol) was added in one por-
tion, and the reaction mixture was stirred for 15 min at 0 °C and
then allowed to warm to room temperature for 1 h. After dilution
with hexane (4 mL), the crude product was purified by column
chromatography (hexane/ethyl ether 1:1). The N-succinimidyl es-
ter 16 was obtained as an oil that solidified upon standing
(0.08 g, 76%). 1H NMR (CDCl3) d 8.37 (1H, s); 8.19 (1H, d); 7.90
(1H, d); 7.52 (1H, t); 2.90 (4H, s); 1.03 (18H; s). 13C NMR d
169.3; 162.0; 140.2; 135.7; 135.3; 131.6; 128.2; 124.6; 27.2;
25.7; 20.2. 19F NMR (CDCl3) d À188.42. HRMS (ESI+) calculated
for C19H26FNO4SiNa [M+Na]: 402.1507, found: 402.1510.
(9:1, 1 mL). The residual water was removed by coevaporation to
dryness with CH3CN using a stream of helium at 105 °C. This step
was repeated twice more with 0.5 mL CH3CN.
5.3.2. 18F radiolabeling of ethoxydi(naphthalen-1-yl)silyl)propyl
derivatives 6 and 10
After drying, a solution of the precursor in DMSO or CH3CN
(300 lL) with AcOH (5 lL) was added. The reaction mixture was
stirred at 75 °C for 15 min to effect labeling. The crude reaction
mixture was analyzed by analytical reversed phase-HPLC
(CH3CN/Water: 70:30, 1 mL minÀ1, column: Nucleosil 100–5 C18,
150 Â 4.6 mm, 5
l
) and by radio-TLC. The peak of the 18F-labeled
product was confirmed by comparison with the HPLC retention
time of its non-radioactive reference molecule. The retention times
(RT) were 4.30 min and 4.85 min for [18F]7 (N-(3-(fluorodi(naph-
thalen-1-yl)silyl)propyl)-2-(2-nitro-1H-imidazol-1-yl)acetamide)
and [18F]11 (1-(3-(fluorodi(naphthalen-1-yl)silyl)propyl)-4-((2-(2-
(2-nitro-1H-imidazol-1-yl)ethoxy)ethoxy)methyl)-1H-1,2,3-tria-
zole). Subsequently, the reaction mixture was added to water
5.2.3.8. 3-(Di-tert-butylfluorosilyl)-N-(2-(2-nitro-1H-imidazol-
1-yl)ethyl)benzamide 17a.
N-Succinimidyl 3-(di-tert-butyl-
fluorosilyl)benzoate 16 (0.05 g, 0.19 mmol) and 2-(2-nitro-1H-imi-
dazol-1-yl)ethanamine3 (0.05 g, 0.19 mmol) were dissolved in dry
dichloromethane (2 mL). Reaction mixture was cooled to 0 °C and
triethylamine (0.04 g, 0.40 mmol) was added. After 24 h stirring
at room temperature, crude reaction mixture was washed with
satd Na2CO3. Aqueous layer was extracted with dichloromethane
(3 Â 2 mL). Organic layers were combined and extracted with
water (2 mL) and brine (2 mL). After drying under Na2SO4 solvent
was removed under reduced pressure and crude product purified
on silica gel (AcOEt/Petroleum ether: 8:2)(0.05 g, 66%). 1H NMR
(CDCl3) d 7.97 (1H, s); 7.80 (1H, d, J = 7 Hz); 7.73 (1H, d, J = 7 Hz);
7.44 (1H, t, J = 7 Hz); 7.17 (1H, t, J = 5 Hz); 7.07 (1H, s); 6.94 (1H,
s); 4.73 (2H, t, J = 5 Hz); 3.90 (2H, q, J = 5 Hz); 1.03 (18H, s). 13C
NMR (CDCl3) d 168.7; 137.4; 134.7; 132.7; 132.3; 128.3; 128.0;
127.2; 49.1; 40.2; 27.2; 20.1. 19F NMR (CDCl3) d À188.48. IR
(ATR, cmÀ1) 2933; 2859; 1645; 1536; 1490; 1365; 1160; 830;
702. HRMS (ESI) calcd for C20H30N4O3FSi [M+H]: 421.2071, found:
421.2057.
(800 lL) and loaded on a Waters SepPak silica cartridge. The latter
had been preconditioned by subsequent rinsing with ethanol
(5 mL) and water (10 mL). The trapped [18F]ethoxydi(naphthalen-
1-yl)silyl)propyl derivatives 6 or 10 were washed with water
(5 mL), eluted from the cartridge with ethanol (3 Â 2 mL) The sec-
ond ethanol fraction was diluted with physiological saline solution
(0.9%) to give an solution useable for animal experiments. Reverse-
phase HPLC revealed radiochemical purities ranging from 94% (6)
to 97% (10).
5.3.3. 18F radiolabeling SiFA compound 17a and 17b
To a dry [Kryptofix 2.2.2.]/18FÀ complex (7–9 GBq), 17a (0.1 mg)
or 17b (1.0 mg) and glacial acetic acid (5
or CH3CN (300 L) were added. After heating at 110 °C for
15 min, the reaction mixture was diluted with HPLC eluent
lL) in anhydrous DMSO
l

3688
Y. Joyard et al. / Bioorg. Med. Chem. 21 (2013) 3680–3688
2. Ting, R.; Harwig, C. W.; Lo, J.; Li, Y.; Adam, M. J.; Ruth, T. J.; Perrin, D. M. J. Org.
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(RT) of [18F]18 was 8.35 min. The decay corrected radiochemical
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with >98% radiochemical purity. The decay corrected radiochemi-
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Alternative labeling method of [18F]18 from 17a: The [18F]FÀ
(40 mCi) was eluted from the QMA cartridge by a mixture of
Kryptofix 2.2.2 (15 mg, 40
lmol) and K2CO3 (3.5 mg, 25
lmol) in
CH3CN/H2O (9:1; 1 mL). Glacial acetic acid (50
lL) was added to
this mixture before the addition of 17a (0.1 mg). After heating at
100 °C for 15 min in a sealed vial, the reaction mixture was diluted
with HPLC eluent (2 mL, CH3CN/H2O 3:2). This solution was in-
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l), the product peak was identified by
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5.4. Biological evaluation
The wistar rats (300–350 g) were injected with 10 MBq of
[
18F]silafluorinated compound ([18F]7, [18F]11 or [18F]18). Dynamic
acquisitions under Mosaic PET camera (Philips Medical Systems,
Cleveland, OH, USA)30 were performed with 5-min frames during
60 min after [18F]-silafluorinated compound injection. Then, a sta-
tic imaging was performed 90 min after injection. ROIs were drawn
on bones, liver, intestine, heart, bladder and muscle. Standard up-
take values (SUVs) were calculated. All images contained at least
106 true events. All images were reconstructed with a fully 3D iter-
ative algorithm (3D-RAMLA). Before reconstruction, raw data were
corrected for random and scattered coincidences and for system
dead-time. Each reconstructed matrix was composed of 120 trans-
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maintained in a facility approved by the Belgian ministry of agri-
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‘Principles of laboratory animal care’ (NIH publication No. 86–23,
revised 1985) were strictly followed.
Acknowledgments
The authors thank the Association Nationale de la Recherche et
de la Technologie (Grant #543/2010) and la Ligue contre le Cancer
(CD76) for funding Yoann Joyard and the Belgian FNRS (Fonds Na-
tional de la Recherche Scientifique) for the financial support of
radioactive experiments (Grant # FRSM: FC 51070/3.4.605.10 F)
31. Yokell, D. L.; Leece, A. K.; Lebedev, A.; Miraghaie, R.; Ball, C. E.; Zhang, J.; Kolb,
H.; Elizarov, A.; Mahmood, U. Appl. Radiat. Isot. 2012, 70, 2313.
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References and notes
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