Radiolabeling of aromatic compounds using K[*Cl]Cl and OXONE

Article abstract of DOI:10.1002/jlcr.2957

We report a novel labeling method for the rapid radiochlorination of aromatic compounds using no-carrier-added short-half-life radioactive chloride ions ([^38,39Cl]Cl^- and [^34mCl]Cl^-) and OXONE (Sigma-Aldrich, Japan) as an oxidation reagent. The reaction of radioactive chloride ions with anisole in the presence of OXONE gave a mixture of p-[*Cl]chloroanisole and o-[*Cl]-chloroanisole, with a radiochemical conversion yield of more than 85% (not decay corrected) and selectivity of 7:3, within 10 min. Regioselective radioactive chlorination was achieved by chlorodestannylation of tributylstannyl compounds to afford the corresponding radiochlorinated compounds (p-chloroanisole and m-chloroanisole) in satisfactory radiochemical yields (80-95% and 46-65%, respectively). Copyright

Full text of DOI:10.1002/jlcr.2957

Research Article
Received 7 May 2012,
Revised 23 July 2012,
Accepted 26 July 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.2957
Radiolabeling of aromatic compounds using
K[*Cl]Cl and OXONEW
Yuuki Takada,a,b Masayuki Hanyu,a Kotaro Nagatsu,a and
a
*
Toshimitsu Fukumura
We report a novel labeling method for the rapid radiochlorination of aromatic compounds using no-carrier-added short-
half-life radioactive chloride ions ([38,39Cl]ClÀ and [34mCl]ClÀ) and OXONEW (Sigma-Aldrich, Japan) as an oxidation reagent.
The reaction of radioactive chloride ions with anisole in the presence of OXONE gave a mixture of p-[*Cl]chloroanisole and
o-[*Cl]-chloroanisole, with a radiochemical conversion yield of more than 85% (not decay corrected) and selectivity of 7:3, within
10min. Regioselective radioactive chlorination was achieved by chlorodestannylation of tributylstannyl compounds to afford
the corresponding radiochlorinated compounds (p-chloroanisole and m-chloroanisole) in satisfactory radiochemical yields
(80–95% and 46–65%, respectively).
Keywords: chlorination; OXONEW; radioactive chlorine; positron-emission tomography (PET)
to expand the possibility of using 34mCl-labeled compounds as
Introduction
PET tracers.
Organohalogen compounds are useful as pharmaceuticals,
In this paper, we report preliminary studies of a novel labeling
agrochemicals, solvents, and others because their structural
method for the rapid and regioselective chlorination of anisole
variety and range of chemical properties have led to a broad
range of applications. Many halogenated compounds labeled
(1), tributyl(4-methoxyphenyl)stannane (2), and tributyl(3-
methoxyphenyl)stannane (3) with high specific activity using
with positron-emitting radionuclides such as 18F, 76Br, and 124I
are used for clinical diagnosis and molecular imaging by positron-
emission tomography (PET). Recently, our group reported the
efficient production and isolation of 34mCl,1 and the preparation
of 2-[34mCl]chloro-2-deoxy-D-glucose using aliphatic nucleophilic
substitution.2 However, there are few available PET tracers
labeled with radioactive chlorine because of the lack of
appropriate half-life chlorine positron-emitting radionuclides,
with the exception of 34mCl (half-life = 32 min, b+ = 57%), even
though there are a large number of candidates for PET imaging
tracers in pharmaceuticals containing chlorine. Although it
slightly changes their structures and biological activities in some
cases, most PET tracers containing chlorinated aromatics are
labeled with 18F or 11C as an alternative positron-emitting
radionuclide at different positions. Direct labeling of such
compounds with a radioactive chlorine (34mCl) isotope would
reflect their true pharmacokinetics in PET studies.
radioactive chlorine ([*Cl]ClÀ) and commercially available OXONEW
(2 KHSO5ÁKHSO4ÁK2SO4, Sigma-Aldrich, Japan) as an oxidant.
Results and discussion
There are a number of technical issues associated with the synthesis
of compounds labeled with short half-life positron emitters: (1) a
very rapid process from the labeling reaction to purification is
required; (2) the reaction scale is very small, usually nanomolar or
picomolar scale; and (3) the synthetic operation must be performed
with a remote-controlled system to avoid radiation exposure.
In the light of these issues, we examined the chlorination of
aromatic compounds using small-scale cold chlorine, assuming
radioactive substance handling. The chloride source for all cold
experiments was prepared as a 66 mmol/L K2CO3 solution
containing 1.34 mmol/L KCl (0.3 mL), according to the previously
reported procedures.1,2
To date, various methods for the (radio)halogenation of
aromatic compounds have been reported.3,4 However, only Zhang
et al. have evaluated the labeling techniques of short-lived
radioactive chlorine (a mixture of [38Cl]ClÀ and [39Cl]ClÀ) for several
tributylphenylstannanes. The reported specific activity was signifi-
cantly lowered because of the simultaneous production of non-
radioactive compounds as a result of the use of chloramine-T.5
In some cases, such as labeling of highly toxic compounds and
receptor imaging probes, a PET probe labeled by this method
has limitations as a PET tracer because of its low specific activity.
Thus, development of a 34mCl-labeling method for aromatic rings
in a short reaction time and with high specific activity is necessary
aMolecular Probe Program, Molecular Imaging Center, National Institute of
Radiological Science, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
bDepartment of Radiology, School of Medicine, Yokohama City University,
3-9 Fukuura, Kanazawa-ku, Yokohama 236-0004, Japan
*Correspondence to: Toshimitsu Fukumura, Molecular Probe Program, Molecular
Imaging Center, National Institute of Radiological Science, 4-9-1 Anagawa,
Inage-ku, Chiba 263-8555, Japan.
E-mail: t_fukumu@nirs.go.jp
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Copyright © 2012 John Wiley & Sons, Ltd.

Y. Takada et al.
Our initial studies focused on achieving optimal reaction acidic conditions under a stream of N2. When 0.5M H2SO4 solution
conditions for chlorination using appropriately substituted was added to the residue after evaporation of the KCl/K2CO3
aromatic compounds. Anisole (1) was chosen as a model solution, compound 4 was obtained in moderate yield (entry 7).
substrate for performing the chlorination of aromatic compounds This result supported the aforementioned explanation. In an effort
using OXONE (Scheme 1). A KCl/K2CO3 solution or dried KCl to optimize the yield of the desired chlorine addition product 4,
solution was treated with OXONE (25 mmol) and anisole (1mmol) the following processes were necessary: (1) drying of the solution,
in aqueous MeCN for 10 min at 95 ꢀC; however, this reaction did (2) acidification with 0.5M H2SO4, (3) dissolution using an OXONE
not afford chloroanisole (4). This suggested that the synthesis of solution, (4) addition of anisole solution, and (5) heating at 95ꢀC
4 could not be carried out in a mixture of KCl and K2CO3 solution for 10min. Acidification with 0.5M H2SO4/MeCN solution gave
because the oxidative activity (oxidation of chlorine) by OXONE is highly efficient chlorination (entry 9). Investigation of the effects
inhibited by K2CO3. To improve the yield of 4 in chlorination using of the solvent (entries 8 and 10) revealed that dioxane was an
a KCl/K2CO3 solution, the effects of acidic media on the preparation effective solvent, as well as MeCN, but MeOH was not effective
of 4 were examined; the results are shown in Table 1. The addition for this reaction. We therefore concluded that the conditions
of 0.5 M H2SO4 as an acidic medium significantly improved the in entry 9 or 10 could achieve this chlorination most efficiently
oxidative activity of OXONE, resulting in an increase in the yield and rapidly.
of 4 up to 78.5% (entries 1–4). Additionally, this reaction afforded
We then examined the regioselective reaction of tributyl
p-chloroanisole (4a) and o-chloroanisole (4b) with approximately (4-methoxyphenyl)stannane (2)5 with KCl to afford p-chloroanisole
4:1 selectivity. The preparation of 4 in MeCN/H2O (3/1, v/v) was (4a) using OXONE (Scheme 2). The results are summarized in
successful, but the yield of 4 was scarcely improved (entry 5). In Table 2. Chlorination using the aforementioned procedure for
the case of the reaction in MeCN/H2O (1/1, v/v), the yield was the preparation of 4a was attempted; however, compound 4a
low (entry 6). This might have been a result of the emission of was not obtained (entry 1). Even when 10 min of activation time
volatile chlorine during drying of the KCl/K2CO3 solution under was allowed for oxidation using 4 mmol of OXONE, only a trace
amount of the desired product was obtained (entry 2). The low
Cl incorporation may have been caused by the presence of
tributylstannane groups. We then assessed how the amount of
OXONE might influence the chlorination yield. As the amount
of OXONE increased, the desired product was obtained in
14.9–64.9% yields (entries 3–5). These results indicated that
chlorination of tributylphenylstannanes requires an excess of
OXONE compared with that required for a phenyl group. The
regioselective reaction of 2 with KCl to afford 4a using OXONE
Scheme 1. Chlorination of anisole (1) using OXONE.
was achieved for the first time.
When acetic acid was used as the acidic medium instead of
H2SO4 solution, the reaction resulted in 79.7% yield (entry 6).
When THF, in which tributylphenylstannanes are more soluble
than in MeCN, was used as the solvent, the chlorination of 2
proceeded, depending on the reaction time (entries 7–9).
The meta-selective reaction of tributyl(3-methoxyphenyl)
stannane (3)5 was screened under similar conditions, and the
desired m-chloroanisole (4c) was obtained in yields of 23.4–56.8%
(Scheme 3 and Table 3). Further attempts to obtain the desired
compound in higher yields by changing the reaction conditions
were unsuccessful. We therefore chose the conditions of entry 4
as the optimal reaction conditions.
Table 1. Optimization reactions for the synthesis of chloroa-
nisole (4)*
0.5 M H2SO4
solution (mL)
Solvent
(v/v)
Conversion
yield† (%)
Entry
}
1{
2
3
4
5
20 (H2O)
30 (H2O)
40 (H2O)
60 (H2O)
50 (H2O)
50 (H2O)
50 (H2O)
50 (MeOH)
50 (MeCN)
50 (dioxane)
MeCN/H2O (20/1)
MeCN/H2O (20/1)
MeCN/H2O (20/1)
MeCN/H2O (20/1)
MeCN/H2O (3/1)
MeCN/H2O (1/1)
MeCN/H2O (1/1)
MeOH/ H2O (5/1)
MeCN/H2O (5/1)
dioxane/H2O (5/1)
—
4.8
76.5
78.5
76.5
14.8
55.8
To evaluate this labeling method for tracer-level amounts of
radioactive chloride, anisole (1) and tributylphenylstannanes
(2 and 3) were labeled with two kinds of radioactive chloride,
6
38,39Cl]ClÀ (a mixture of 80% [38Cl]ClÀ and 20% [39Cl]ClÀ) and
34mCl]ClÀ, using our optimal reaction conditions (Table 1, entry
7}
8},‖
9},‖
10},‖
[
[
}
—
9; Table 2, entry 8; Table 3, entry 4) (Scheme 4). The reaction of
anisole (1) with no-carrier-added [38,39Cl]ClÀ using OXONE
proceeded smoothly to give a mixture of p- and o-[38,39Cl]
95.7
100
*Reaction conditions: KCl (0.4 mmol), OXONE (1 mmol), anisole
(10 mmol), 95 ꢀC, 10 min.
†The conversion yield based on HPLC. The products
characterized by GC–MS and HPLC analysis.
{KCl solution was not dried.
}Not detected.
}0.5 M H2SO4 solution was added after drying KCl solution.
‖0.5 M H2SO4 solution was prepared from dilution with
organic solvent, as shown in parentheses.
Scheme 2. Para-selective chlorination of tributyl(4-methoxyphenyl)stannane (2).
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J. Label Compd. Radiopharm 2012

Y. Takada et al.
Table 2. Regioselective chlorination of tributyl(4-methoxyphenyl)stannane (2)*
Entry
Acidic medium
Solvent OXONE (mmol) Activation time (min) Reaction time (min) Conversion yield† (%)
{
1
2
3
4
5
6
7
8
9
0.5 M H2SO4 /MeCN
0.5 M H2SO4 /MeCN
0.5 M H2SO4 /MeCN
0.5 M H2SO4 /MeCN
0.5 M H2SO4 /MeCN
AcOH
AcOH
AcOH
AcOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
THF
1
4
0
10
10
10
10
10
10
10
10
15
15
15
15
15
15
5
—
Trace
14.9
38.7
64.9
79.7
63.8
81.1
100
10
25
50
50
50
50
50
THF
THF
10
15
*Reaction conditions: KCl (0.4 mmol), acidifying reagent (100 mL), substrate (10 mmol), solvent (150 mL), reaction temperature, r.t.
†The conversion yield based on HPLC. The products characterized by GC–MS and HPLC analysis.
{Not detected.
amount of radiochlorinated 4a and 4b. The production of
radiochlorinated 4a and 4b could have been caused by reaction
with anisole derived from tributyl(3-methoxyphenyl)stannane (3)
by thermal decomposition. The specific activity of [34mCl]4 was
determined to be approximately 100 MBq/mmol (after about
80 min from end of bombardment (EOB)).
Scheme 3. Meta-selective chlorination of tributyl(3-methoxyphenyl)stannane (3).
Experimental
All reagents were obtained commercially and used without further
chloroanisole ([38,39Cl]4a and 4b) with
a radiochemical
purification unless otherwise specified. Analytical high-performance liquid
chromatography (HPLC) was performed using a Shimadzu LC-10 liquid
chromatograph equipped with a Shimadzu SPD-10A UV/Vis detector and
conversion yield of approximately 80% (not decay corrected) and
83:17 selectivity. The treatment of anisole (1) with [34mCl]ClÀ gave
a mixture of [34mCl]4a and [34mCl]4b with a radiochemical a JASCO HPLC system coupled with an NaI (Tl) or BGO scintillation detector
(Ohyo Koken Kogyo Co., ltd, Tokyo, Japan). Gas chromatography/mass
spectrometry (GC–MS) was performed using a GCMS-QP5050A system
(Shimadzu, Kyoto, Japan).
conversion yield of about 95% (not decay corrected) and
selectivity of 7:3. This procedure is therefore particularly effective
for radioactive chlorination of aromatic rings.
In our next attempt, the reaction of tributyl(4-methoxyphenyl)
stannane (2) with [38,39Cl]ClÀ and [34mCl]ClÀ regioselectively Typical experimental procedure for chlorination of anisole
generated the desired [38,39Cl]4a and [34mCl]4a with radiochemical
KCl/K2CO3 solution (0.3 mL; 1.34 mM KCl/66 mM K2CO3) was added to a
conversion yields of approximately 87% and 70% (not decay
sealable vial. The water was removed from the KCl/K2CO3 solution by a
corrected), respectively. On the other hand, when tributyl
(3-methoxyphenyl)stannane (3) was allowed to react under our
optimal reaction conditions, the desired [38,39Cl]4c and [34mCl]4c
were obtained with radiochemical conversion yields of 65% and
N2 flow in an oil bath to give dried KCl/K2CO3. After cooling at 0 ꢀC, the
residue was acidified using 0.5 M H2SO4 in MeCN (50 mL), and OXONE in
aqueous MeCN (50 mL, 1 mmol) and 0.2 M anisole in MeCN (50 mL, 10 mmol)
were added. The mixture was heated at 95 ꢀC for 10 min. The reaction
46% (not decay corrected), respectively, along with a small mixture was cooled to room temperature and diluted with 66 mM
Table 3. Regioselective chlorination of tributyl(3-methoxyphenyl)stannane (3)*
Entry
Solvent
Activation time (min)
Reaction temperature (ꢀC)
Reaction time (min)
Conversion yield† (%)
1
2
3
4
5
6
7
8
THF
THF
THF
THF
THF
THF
THF
MeCN
0
0
5
95
95
95
95
95
95
r.t.
r.t.
5
10
5
10
5
10
15
15
25.0
38.9
23.4
42.0
33.2
36.5
56.8
42.0
5
10
10
10
10
*Reaction conditions: KCl (0.4 mmol), AcOH (100 mL), OXONE (50 mmol), substrate (10 mmol), solvent (150 mL).
†The conversion yield based on HPLC analysis. The products characterized by GC–MS and HPLC analysis.
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Y. Takada et al.
Conclusion
In conclusion, we have developed a novel radiolabeling method
for the rapid and regioselective chlorination of anisole and
tributylphenylstannanes as model compounds using no-
carrier-added [*Cl]ClÀ with OXONE as an efficient oxidation
reagent. This labeling technique should increase the usefulness
of 34mCl-labeled compounds as PET probes because this method
incorporates radioactive chlorine on aromatic rings with high
specific activity. Furthermore, the results obtained in the present
study should extend to further studies aimed at the preparation
of probes labeled with other radioactive halogens such as 76Br
and 124I.
Acknowledgements
We thank the staff of the Cyclotron Operation Section and the
Department of Molecular Probes of the National Institute of
Radiological Sciences (NIRS) for their support with the operation
of the cyclotron and the production of radioisotopes.
Scheme 4. Reaction of anisole (1) and tributylphenylstannanes (2 and 3) with [*Cl]ClÀ.
Conflict of Interest
K2CO3. Aliquots of this mixture were analyzed by reversed-phase HPLC
onto an Inertsil ODS-3 column (4.6 Â 150 mm, 5 mm) using a mobile
phase of MeCN/H2O (1:1) at a flow rate of 1.0 mL/min, for monitoring
UV absorption at 254 nm. The residual solution was extracted with ether.
After drying the organic layer, GC–MS analysis was performed.
The authors did not report any conflict of interest.
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Dried KCl was dissolved in acetic acid (100 mL), and 0.5 M OXONE (100 mL,
50 mmol) was added. After the mixture was allowed to stand for 5 min at
room temperature, 0.2 M tributylphenylstannane in THF (50 mL, 10 mmol)
was added. The reaction mixture was heated at 95 ꢀC for 10 min. The
reaction mixture was cooled to room temperature and then diluted
with 66 mM K2CO3. Aliquots of this mixture were analyzed by HPLC and
GC–MS.
Radiolabeling procedure
Production of radioactive chlorine was carried out using an AVF-930
cyclotron (Sumitomo Heavy Industry, Tokyo, Japan). Radioactive chloride
[
38,39Cl]ClÀ, a mixture of 80% [38Cl]ClÀ and 20% [39Cl]ClÀ, was simulta-
neously produced by the 40Ar(p, 2pn)38Cl and 40Ar(p, 2p)39Cl nuclear
reaction using a proton beam (40 MeV),6 and [34mCl]ClÀ was generated
by the 32S(a, d)34mCl nuclear reaction using a beam of a-particles
(65 MeV).1 Separations of radioactive chlorides from target materials
were performed according to published methods.1,2,5 Each [*Cl]ClÀ
solution was passed through an anion-exchange cartridge (Sep-Pak Light
Accell QMA, Waters, Milford, MA, USA) preconditioned with K2CO3 (0.1 M,
10 mL) and H2O (30 mL) to trap [*Cl]ClÀ from water. The trapped [*Cl]ClÀ
was eluted with K2CO3 solution (66 mM, 0.3 mL).
The radiolabeling was carried out according to the cold experimental
procedure mentioned previously. The characterizations of the *Cl-labeled
compounds were performed by analytical HPLC, using authentic samples
as references.
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