The use of tetrabutylammonium fluoride to promote N- and O- 11C-methylation reactions with iodo[11C]methane in dimethyl sulfoxide

Article abstract of DOI:10.1002/jlcr.3092

The N- or O-methylation reactions of compounds bearing amide, aniline, or phenol moieties using iodo[¹¹C]methane (1) with the aid of a base are frequently applied to the preparation of ¹¹C-labeled radiopharmaceuticals. Although sodiu

Full text of DOI:10.1002/jlcr.3092

Research Article
Received 21 May 2013,
Revised 14 June 2013,
Accepted 17 June 2013
Published online 16 July 2013 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3092
The use of tetrabutylammonium fluoride to
11
promote N- and O- C-methylation reactions
11
with iodo[ C]methane in dimethyl sulfoxide
Tatsuya Kikuchi,a Katsuyuki Minegishi,a Hiroki Hashimoto,a
Ming-Rong Zhang,a and Koichi Katoa,b
*
The N- or O-methylation reactions of compounds bearing amide, aniline, or phenol moieties using iodo[11C]methane (1) with
the aid of a base are frequently applied to the preparation of 11C-labeled radiopharmaceuticals. Although sodium hydride
and alkaline metal hydroxides are commonly employed as bases in these reactions, their poor solubility properties in
organic solvents and hydrolytic activities have sometimes limited their application and made the associated 11C-methylation
reactions difficult. In contrast to these bases, tetrabutylammonium fluoride (TBAF) is moderately basic, highly soluble in
organic solvents, and weakly nucleophilic. Although it was envisaged that TBAF could be used as the preferred base for
11C-methylation reactions using 1, studies concerning the use of TBAF to promote 11C-methylation reactions are scarce.
Herein, we have evaluated the efficiency of the 11C-methylation reactions of 13 model compounds using TBAF and 1. In most
cases, the N-11C-methylations were efficiently promoted by TBAF in dimethyl sulfoxide at ambient temperature, whereas the
O-11C-methylations required heating in some cases. Comparison studies revealed that the efficiencies of the 11C-methylation
reactions with TBAF were comparable or sometimes greater than those conducted with sodium hydride. Based on these
results, TBAF should be considered as the preferred base for 11C-methylation reactions using 1.
Keywords: isotopic labeling; carbon-11; iodomethane; methylation; tetrabutylammonium fluoride; amide; aniline; phenol
hydroxides are used as the reaction base. A base that overcomes
these technical and chemical issues is therefore desired for
Introduction
A variety of convenient methods have been developed for the
synthesis of 11C-labeled compounds that facilitate the routine
preparation of radiopharmaceuticals for use in positron emission
tomography. The attachment of a 11C-methyl group to a
pendant amino, hydroxy, or sulfenyl moiety represents a
common strategy in the preparation of the 11C-labeled
compounds. Iodo[11C]methane (1) is commonly used as a
methylating agent. This material is introduced through a
nucleophilic substitution reaction, and methods for the
application with synthetic methods using 1.
Tetrabutylammonium fluoride (TBAF) possesses good
solubility properties in a range of organic solvents, as well as
low hydrolytic activity and moderate basicity. Adam et al.1
demonstrated that TBAF could be used as a base for the
preparation of some radiopharmaceuticals using 1. The labeling
efficiencies achieved when TBAF was used as a base were
reported to be comparable with or even higher than those
achieved by sodium hydroxide (NaOH) in the same reactions.
In their report, the TBAF-mediated N- and O-11C-methylation
reactions using 1 were performed at temperatures in the range
of 80–100 °C both in the presence and in the absence of alkaline
metal hydroxides.1 In contrast, we reported a series of TBAF-
promoted α-methylation reactions using 1 where different
automated preparation of
1 are well established. The
nucleophilic substitution reactions of 1 with the NH moieties of
amides and anilines, as well as the OH groups of phenols, require
the addition of a base to generate the corresponding anions,
because the parent functional groups are poorly reactive
towards 1. Suspensions of sodium hydride (NaH) and alkaline
metal hydroxides are typically employed as the bases for these
reactions. Unfortunately, however, these materials are poorly
soluble in organic solvents, and their application to the
automated preparation of radiopharmaceuticals can therefore
be cumbersome. These poor solubility properties can also lead
aMolecular Probe Program, Molecular Imaging Center, National Institute of
Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba263-8555, Japan
to the addition of ambiguous concentrations of the bases to bDepartment of Integrative Brain Imaging, National Center of Neurology and
the precursor solution because the bases are added as
Psychiatry, 4-1-1 Ogawa-higashi, Kodaira, Tokyo 187-8551, Japan
suspensions, and the amount of base present in solution might
not therefore satisfy the requirements of current good
*Correspondence to: Koichi Kato, Department of Integrative Brain Imaging,
National Center of Neurology and Psychiatry, 4-1-1 Ogawa-higashi, Kodaira,
manufacturing practices. In addition, the hydrolysis of 1 can Tokyo 187-8551, Japan.
occur as a considerable side reaction when alkaline metal E-mail: katok@ncnp.go.jp
J. Label Compd. Radiopharm 2013, 56 672–678
Copyright © 2013 John Wiley & Sons, Ltd.

T. Kikuchi et al.
structural components, such as the α-carbons of phenyl acetates addition, DMSO exhibited a protective effect against the
as well as Schiff bases bearing alanine analogs, were activated by radiolysis of the [11C]ibuprofen methyl ester under high levels
TBAF, even at ambient temperature.2,3 In general, however, the of radioactivity.3
methylation did not occur as an exclusive reaction, and
undesirable side reactions were also observed, including the
Experimental
radiolysis of the 11C-labeled products. Furthermore, these side
Chemicals
reactions became more pronounced when the reactions were
conducted at higher temperatures. The development of a
method allowing for the 11C-methylation reactions to be
conducted at lower temperatures could therefore provide
greater radiochemical conversions (RCCs) and yields, if the
desired reactions could occur at sufficiently fast rates.
Aqueous 57% hydrogen iodide (HI) solution was purchased from Wako
Pure Chemical Industries Ltd. (Osaka, Japan). A 0.05-M solution of lithium
aluminum hydride (LAH) in THF was prepared by diluting a 1.0-M
solution of the same in material in THF, which was purchased from Sigma
Aldrich (Tokyo, Japan). THF was purchased as the anhydrous grade from
Wako Pure Chemical Industries Ltd. An approximately 1.0-M solution of
TBAF in THF and NaH (60%, dispersion in paraffin liquid) was purchased
from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). The reaction solvents
used in the current study, including DMF and DMSO, were purchased from
Wako Pure Chemical Industries Ltd. as the anhydrous grades. The
acetonitrile (MeCN) and ammonium acetate (AcONH4) (aqueous solution)
used for HPLC analysis in the current study were purchased from Wako Pure
Chemical Industries Ltd. as the HPLC grade. Precursor 2a and nonlabeled
2b (hydrochloride salt), and 4b and 6b–8b were purchased from Sigma
Aldrich. Precursors 3a and 4a, and nonlabeled 3b and 9b were purchased
from ABX GmbH (Radeberg, Germany). The precursors 6a–8a and 12a–14a,
and nonlabeled 12b–14b were purchased from Wako Pure Chemical
Industries Ltd. Precursor 9a was purchased from Pharma Synth AS (Tallinn,
Estonia). The benzyloxy and methoxy benzoyl chlorides, 2-aminomethyl-1-
ethylpyrrolidine, 10% Pd/C, and triethylsilane used for the preparation of
compounds 10a, 11a, 10c, and 11c were purchased from Sigma Aldrich,
Tokyo Chemical Industry Co., Ltd., Wako Pure Chemical Industries Ltd.,
and Shin-Etsu Chemical Co., Ltd. (Tokyo, Japan), respectively. The benzoyl
chloride, methyl 2-amino-2-methylpropanoate hydrochloride and methyl
2-methyl-2-(methylamino)propanoate used for the preparation of 5a and
5c were purchased from Tokyo Chemical Industry Co., Ltd., Sigma Aldrich
Japan, and Enamine Ltd. (Kiev, Ukraine), respectively. All of the other
chemicals used in the current study were purchased as the highest grades
commercially available. All of the reagents in the current study were used
without further purification.
In this context, we have evaluated the TBAF-promoted N- and
O-11C-methylation reaction of amides, anilines, and phenols
11
using 1 with and without heating. The efficiencies of the
C-methylation reactions with TBAF have been compared with
those observed with NaH, which is typically used in both organic
and radio syntheses because of its low nucleophilicity. A series
of model compounds have been selected and evaluated in the
current study for their 11C-methylation reactions, including
the established radiopharmaceuticals, 8-[4-(4-fluorophenyl)-
4-oxobutyl]-3-[11C]methyl-1-phenyl-1,3,8-triazaspiro[4.5]decan-
4-one (3-N-[11C]methylspiperone, 2b), ethyl 8-fluoro-5-[11C]
methyl-6-oxo-5,6-dihydro-4H-benzo[f]imidazo[1,5-a][1,4]
diazepine-3-carboxylate ([11C]flumazenil, 3b), N-butan-2-yl-1-(2-
chlorophenyl)-N-[11C]methylisoquinoline-3-carboxamide ([11C]
PK11195, 4b), and (S)-3,5-dichloro-N-((1-ethylpyrrolidin-2-yl)
methyl)-2-hydroxy-6-[11C]methoxybenzamide ([11C]raclopride,
9b) (Figure 1). With regard to the reaction solvent, dimethyl
sulfoxide (DMSO) was used in the current study to evaluate
the efficiency of the TBAF-promoted N- and O-11C-methylation
reactions, because use of DMSO was necessary for the TBAF-
promoted methylation reactions of Schiff bases bearing
alanine analogs with 1, and these reactions did not occur in
dimethylformamide (DMF), THF, or dichloromethane.2,3 In
Analysis of radiochemical conversions
Radiochemical conversions were analyzed on an HPLC system
consisting of a JASCO PU-2080 pump (JASCO Corporation, Tokyo,
Japan), JASCO UV-2075 UV detector, Rheodyne manual injector (IDEX
Health & Science LLC, Tokyo, Japan) with a 20-μL loop), and a NaI(Tl)
scintillation detector with an ACE Mate Amplifier and BIAS supply
(925-SCINT, ORTEC, Oak Ridge, TN, USA) for radioactivity detection.
Data acquisition and interpretation were performed with ChromNAV
(ver. 1.5.2.0, JASCO Corporation). The RCC values were calculated
following the correction of the radiochromatograms for decay. XBridge
C18 (150 × 4.6 mm, 3.5 μm, Waters Corporation, Milford, MA, USA) and
J’sphere ODS-H80 (150 × 4.6 mm, 4 μm, YMC Co. Ltd., Kyoto Japan)
reversed phase analytical columns were used for the RCC analyses with
a mobile phase consisting of a mixture of MeCN and AcONH4 buffer (pH
4.7, 20 mM). The peaks corresponding to the desired 11C-labeled
compounds in the radiochromatogram were confirmed on the basis
of the UV absorptions of the corresponding nonradioactive
compounds, which were coinjected into the HPLC.
The retention times (tR) of 2b and 6b–8b (XBridge C18 column; mobile
phase of MeCN/AcONH4 buffer = 30:70; flow rate of 1 mL/min) were 9.9
(2b), 14.7 (6b), 13.8 (7b), and 8.5 min (8b), respectively, whereas the tR
of iodomethane under these conditions was 7.3 min. The tR values of
3b, 5b, and 9b (J’sphere ODS-H80 column; mobile phase of MeCN/
AcONH4 buffer = 30:70; flow rate of 1 mL/min) were 8.3, 4.8, and
10.5 min, respectively, whereas the tR value of iodomethane under the
same conditions was 12.9 min. The tR of 4b (XBridge C18 column; mobile
phase of MeCN/AcONH4 buffer = 60:40; flow rate of 1 mL/min) was 6.4min,
whereas that of iodomethane under these conditions was 3.1min. The tR
Figure 1. Chemical structures of the model compounds.
J. Label Compd. Radiopharm 2013, 56 672–678
Copyright © 2013 John Wiley & Sons, Ltd.
www.jlcr.org

T. Kikuchi et al.
values of 10b and 11b (J’sphere ODS-H80 column; mobile phase of MeCN/
AcONH4 buffer= 15:85; flow rate of 1 mL/min) were 6.7 and 6.3min,
respectively, whereas the tR value of iodomethane with the same
conditions was 12.5min. The tR values of 12b–14b (J’sphere ODS-H80
column; mobile phase of MeCN/AcONH4 buffer= 30:70; flow rate of
1.5mL/min) were 7.4 (12b), 17.7 (13b), and 16.0min (14b), respectively,
whereas that of iodomethane under these conditions was 8.4min. The
radiochromatograms of the materials were monitored until the
radioactivity in the columns could no longer be detected using a Geiger–
Müller counter (TGS-146B, Hitachi Aloka Medical, Ltd., Tokyo, Japan).
Synthesis of N-(1-ethylpyrrolidin-2-ylmethyl)-hydroxybenzamide
(10a and 11a, precursors for 10b and 11b)
A solution of 3- or 4-benzyloxybenzoyl chloride (1.74 mmol) in dioxane
(15 mL) was added to a stirred solution of K2CO3 (2.89 mmol) and 2-
aminomethyl-1-ethylpyrrolidine (1.56 mmol) in dioxane (15 mL), and the
resulting mixture was stirred for 3 h at ambient temperature. EtOAc
was then added to the reaction, resulting in the formation of a biphasic
solution. The organic phase was collected and washed sequentially with
saturated NaHCO3 and water before being dried over anhydrous MgSO4.
The solvent was then removed in vacuo, and the resulting residue was
purified by column chromatography over Chromatorex NH silica gel (Fuji
Silysia Chemical Ltd., Aichi, Japan) using hexane/EtOAc (2:1, v/v) as the
eluent to give the desired product N-(1-ethylpyrrolidin-2-ylmethyl)-
benzyloxybenzamide (>90%). The benzyl group was removed according
to the following procedure. Triethylsilane (15 mmol) was added to a
mixture of 10% Pd/C (100 mg) and N-(1-ethylpyrrolidin-2-ylmethyl)-
benzyloxybenzamide (1.5mmol) in ethanol (100 mL), and the resulting
mixture was stirred for 3h at ambient temperature. The reaction mixture
was then filtered to remove the Pd/C, and the filtrate was concentrated
under vacuum to give the crude product as a residue, which was purified
by column chromatography over silica gel (Wakogel C-200, Wako Pure
Chemical Ltd.) using EtOAc/2-propanol/28% aqueous NH3 (3:1:0.5, v/v/v)
as the eluent to give the desired product N-(1-ethylpyrrolidin-2-
Production of iodo[11C]methane (1)
[
11C]Carbon dioxide was produced by the 14N(p,α)11C nuclear reaction in
an atmosphere of nitrogen gas containing 0.01% oxygen with 18-MeV
protons using the CYPRIS HM-18 cyclotron (Sumitomo Heavy Industry,
Tokyo, Japan). Following the bombardment process, the [11C]carbon
dioxide was transferred to a reaction vessel containing a 0.05-M solution
of LAH in THF (500 μL) at 0 °C. The THF solvent was then evaporated, and
an aqueous 57% HI solution (400 μL) was added to the vessel. The
resulting mixture was then heated to 150 °C to produce 1. Gaseous 1
was transferred by a N2 gas stream with a flow rate of 30 mL/min and
collected in DMSO or DMF (0.5–1 mL) in a glass vessel at ambient
temperature. The preparation time of 1 was around 7 min following the
end of the bombardment process.
General condition for 11C-methylation
ylmethyl)-hydroxybenzamide (>86%).
N-(1-Ethylpyrrolidin-2-ylmethyl)-3-hydroxybenzamide (10a): 1H-NMR
(300 MHz, CDCl3) δ (ppm): 1.21 (3H, t), 1.71–1.95 (4H, m), 2.27–2.42
(2H, m), 2.72–2.82 (1H, broad m), 2.98–3.05 (1H, m), 3.32–3.43 (2H, m),
3.95–4.04 (1H, m), 6.82–6.85 (1H, dd), 7.25–7.30 (1H, m), 7.45 (1H, d),
7.57 (1H, m) and 7.68 (1H, broad d); 13C-NMR (75 MHz, CDCl3) δ (ppm):
13.04, 22.37, 26.52, 39.25, 48.61, 53.53, 64.23, 114.33, 118.06, 119.21,
129.69, 135.91, 156.60 and 167.70. HRMS (m/z) calcd for C14H20N2O2
[M + H]+: 249.1603; Found: 249.1569.
The DMSO was used as the reaction solvent for the 11C-methylation
reactions using TBAF, whereas DMF was used for the 11C-methylation
reactions using NaH. TBAF (1 eq, 1.0 M in THF, stored in plastic bottle)
or NaH (dispersion in paraffin liquid, 5 eq, 1.0 M in DMF) was added to
a solution of the precursor (1 μmol in 200 μL) 30 or 5 min prior to the
addition of 1, respectively. A DMSO or DMF solution of 1 (~111 MBq,
100 μL) was then added to the reaction mixture, and the resulting
mixture was held at ambient temperature or 80 °C for 3 min. At the end
of reaction, the reaction mixture was cooled on ice and quenched by
the addition of 200 μL of the AcONH4 buffer solution (pH 4.3, 200 mM).
The resulting mixture was then analyzed by HPLC.
N-(1-Ethylpyrrolidin-2-ylmethyl)-4-hydroxybenzamide (11a): 1H-NMR
(300 MHz, CDCl3) δ (ppm): 1.15 (3H, t), 1.66–1.81 (3H, m), 1.88–1.95 (1H, m),
2.26–2.39 (2H, m), 2.79–2.81 (1H, m), 2.86–2.95 (1H, m), 3.23–3.27 (1H, m),
3.36–3.40 (1H, m), 3.74–3.83 (1H, m), 6.84 (2H, d) 7.26 (1H, broad,
overlapped with CDCl3) and 7.72 (2H, d); 13C-NMR (75 MHz, CDCl3) δ
(ppm): 13.32, 22.78, 27.55, 40.33, 48.57, 53.51, 63.43, 115.83, 125.16,
129.07, 160.61 and 168.03. HRMS (m/z) calcd for C14H20N2O2 [M + H]+:
249.1603; Found: 249.1557.
Synthesis of methyl 2-benzamido-2-methylpropanoate (5a,
precursors for 5b) and methyl 2-methyl-2-(N-methylbenzamido)
propanoate (5c)
A solution of benzoyl chloride (2.13 mmol) in dioxane (20 mL) was added
to a stirred solution of K2CO3 (5.86 mmol) and methyl 2-amino-2-
methylpropanoate hydrochloride (1.95 mmol) in water (15 mL), and the
resulting mixture was stirred for 1 h at ambient temperature. EtOAc
was then added to the reaction resulting in the formation of a biphasic
Synthesis of the N-(1-ethylpyrrolidin-2-ylmethyl)-methoxybenzamides
(10c and 11c, analogs of raclopride)
mixture. The organic phase was collected and washed sequentially with The N-(1-ethylpyrrolidin-2-ylmethyl)-methoxybenzamides were prepared
saturated NaHCO3 and water. The organic layer was dried over from the corresponding 3- and 4-methoxybenzoyl chlorides using the
anhydrous MgSO4, and the solvent was removed in vacuo to give a crude procedure described earlier (95%).
product as a residue, which was purified by column chromatography
over silica gel (Wakogel C-200, Wako Pure Chemical Ltd.) using hexane/ (300MHz, CDCl3) δ (ppm): 1.12 (3H, t), 1.63–1.77 (3H, m), 1.86–1.95 (1H, m),
N-(1-Ethylpyrrolidin-2-ylmethyl)-3-methoxybenzamide (10c): 1H-NMR
EtOAc (3:2, v/v) as the eluent to give 5a (88%).
2.15–2.29 (2H, m), 2.67 (1H, broad m), 2.81–2.87 (1H, m), 3.17–3.33 (2H, m),
Methyl 2-benzamido-2-methylpropanoate (5a): 1H-NMR (300 MHz, 3.66–3.74 (1H, m), 3.85 (3H, s), 6.85 (1H, broad s), 7.01–7.05 (1H, m) and
CDCl3) δ (ppm): 1.69 (6H, s), 3.79 (3H, s), 6.79 (1H, broad s), 7.40–7.50 7.27–7.39 (3H, m); 13C-NMR (75 MHz, CDCl3) δ (ppm): 14.06, 22.93, 28.19,
(3H, m) and 7.77–7.80 (2H, m); 13C-NMR (75 MHz, CDCl3) δ (ppm): 24.73, 40.75, 48.01, 53.55, 55.39, 62.19, 112.34, 117.37, 118.61, 129.46, 136.32,
52.78, 56.91, 126.92, 128.53, 131.54, 134.50, 166.52 and 175.30. HRMS 159.74 and 167.49. HRMS (m/z) calcd for C15H22N2O2 [M + H]+: 263.1760;
(m/z) calcd for C12H15NO3 [M + H]+: 222.1130; Found: 222.1125.
Compound 5c was synthesized according to the procedure described
Found: 263.1711.
N-(1-Ethylpyrrolidin-2-ylmethyl)-4-methoxybenzamide (11c): 1H-NMR
earlier using methyl 2-methyl-2-(methylamino)propanoate as the starting (300 MHz, CDCl3) δ (ppm): 1.12 (3H, t), 1.60–1.77 (3H, m), 1.84–1.93 (1H,
material (55% yield). m), 2.14–2.30 (2H, m), 2.65 (1H, broad m), 2.78–2.89 (1H, m), 3.17–3.31
Methyl 2-methyl-2-(N-methylbenzamido)propanoate (5c): 1H-NMR (2H, m), 3.65–3.73 (1H, m), 3.85 (3H, s), 6.77 (1H, broad s), 6.93 (2H, d)
(300 MHz, CDCl3) δ (ppm): 1.55 (6H, s), 2.97 (3H, s), 3.73 (3H, s) and
and 7.75 (2H, d); 13C-NMR (75 MHz, CDCl3) δ (ppm): 14.10, 22.91, 28.19,
7.38–7.47 (5H, m); 13C-NMR (75 MHz, CDCl3) δ (ppm): 22.95, 33.44, 52.31, 40.67, 47.96, 53.56, 55.34, 62.21, 113.64, 127.14, 128.61, 161.94 and
60.70, 127.28, 128.35, 129.83, 136.44, 171.83 and 174.57. HRMS (m/z) 167.16. HRMS (m/z) calcd for C15H22N2O2 [M + H]+: 263.1760; Found:
calcd for C13H17NO3 [M + H]+: 236.1287; Found: 236.1292.
263.1801.
www.jlcr.org
Copyright © 2013 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2013, 56 672–678

T. Kikuchi et al.
usefully applied to precursors that are labile to ionic fluoride.
The addition of TBAF to a reaction immediately after the
Results and discussion
The effect of the timing of the TBAF addition to a solution trapping of 1 in a precursor solution could be performed using
containing 2a was evaluated because the duration of the an automatic synthesis module because of the soluble nature
reaction of TBAF with the precursors prior to the addition of 1 of TBAF to organic solvents. We believe that the amenability of
had a significant impact on the success of the methylation TBAF to automated approaches represents
a significant
reaction in THF. As shown in Table 1, the RCC was found to be advantage to the current approach over the use of traditional
independent of the timing of the TBAF addition (Table 1, entries bases, such as NaH.
1–3). These results suggested that the anions generated by TBAF
Using 2b as a model compound, we also performed a
were relatively stable in DMSO. Adam et al. reported that TBAF preliminary evaluation of time-dependent changes in the activity
can be added at least 20 min prior to the addition of 1,1 whereas of TBAF solutions in storage towards the 11C-methylation
it would not be necessary to consider the timing of TBAF reaction. Once they had been opened, the commercially
addition to
a
DMSO solution containing
a
precursor. available TBAF solutions, which were bottled in plastic and glass,
Furthermore, when TBAF was added to the reaction immediately were stored at ambient temperature without any special
after the trapping of 1 in the precursor solution, the RCCs in the treatment. These TBAF solutions provided a consistent level of
reaction following 3 min at ambient temperature of 2b, 3b, 5b, labeling efficiency for the labeling of 2b over a 1-month period
8b, and 13b were 68 4.9, 89 0.34, 46 2.9, 56 1.4, and (Figure 2). In the current study, we used a TBAF solution
74 6.2%, respectively (n = 3 each), and were comparable with contained in a plastic bottle as a precautionary measure. This
the values shown in Tables 1 (entries 1–3, 7, 15, and 27) and 2 step was taken on the basis of our previous experience, where
(entry 20). These results implied that this reaction could be a significant reduction in the fluoride activity of a TBAF solution
Table 1. Labeling efficiency of the N-11C-methylation reaction using 1
Entry Product Base (eq) Timing of base addition (min)a Reaction temperature (°C)
RCC (%)b
Remained 1 (%)c
1
2
3
4
5
6
7
8
2b
2b
2b
2b
2b
2b
3b
3b
3b
3b
4b
4b
4b
4b
5b
5b
5b
5b
6b
6b
6b
6b
7b
7b
7b
7b
8b
8b
8b
8b
TBAF (1)
TBAF (1)
TBAF (1)
TBAF (1)
NaH (5)
NaH (5)
TBAF (1)
TBAF (1)
NaH (5)
NaH (5)
TBAF (1)
TBAF (1)
NaH (5)
NaH (5)
TBAF (1)
TBAF (1)
NaH (5)
NaH (5)
TBAF (1)
TBAF (1)
NaH (5)
NaH (5)
TBAF (1)
TBAF (1)
NaH (5)
NaH (5)
TBAF (1)
TBAF (1)
NaH (5)
NaH (5)
15
30
60
30
5
rt
rt
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
67 2.8
63 1.9
64 6.6
60 4.9
82 2.1
55 4.1
80 5.7
74 1.9
97 0.62
96 1.2
9.4 0.90
8.6 0.70
3.6 0.020
2.6 0.86
48 2.9
42 3.9
45 0.77
48 1.1
65 7.2
66 1.6
35 7.9
29 5.6
4.5 1.7
5.0 2.4
3.1 2.5
5.7 2.7
59 3.1
63 4.1
27 4.9
29 7.0
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
nd.
1.3 0.89
nd.
nd.
nd.
5
30
30
5
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
5
30
30
5
5
30
30
5
5
30
30
5
5
30
30
5
5
30
30
5
5
rt, room temperature; nd., not detected.
aThe time after the addition of the base to a precursor solution prior to the start of the reaction when 1 was added.
bThe radiochemical conversion (RCC) values were determined using the radiochromatograms from the analytical HPLC following a
decay correction.
cThese reactions were conducted in three independent experiments. The values are expressed as mean standard deviation.
J. Label Compd. Radiopharm 2013, 56 672–678
Copyright © 2013 John Wiley & Sons, Ltd.
www.jlcr.org

T. Kikuchi et al.
Table 2. Labeling efficiency of the O-methylation reaction using 1
Timing of base addition
Reaction temperature
(°C)
RCC
(%)b
Remained
Entry
Product
Base (eq)
(min)a
1 (%)c
1
2
3
4
5
6
7
8
9b
TBAF (1)
TBAF (1)
TBAF (1)
NaH (5)
30
30
30
5
rt
rt
80
rt
80
rt
80
rt
80
rt
80
rt
80
rt
nd.
72 6.3
nd.
nd.
58 19
nd.
nd.
nd.
67 11
nd.
23 9.4
nd.
nd.
10b
10b
10b
10b
11b
11b
11b
11b
12b
12b
12b
12b
12b
84 4.1
73 12
13 0.51
76 0.70
78 12
75 12
8.9 0.87
77 2.8
51 7.7
67 2.2
7.1 3.7
5.8 5.7
23 10
NaH (5)
5
TBAF (1)
TBAF (1)
NaH (5)
30
30
5
9
NaH (5)
5
10
11
12
13
14
TBAF (1)
TBAF (1)
NaH (5)
30
30
5
5
5
NaH (5)
nd.
nd.
NaH (2.5)
(degreased)
NaH (2.5)
(degreased)
NaH (5)
15
16
17
18
19
12b
12b
12b
12b
12b
5
5
5
5
5
80
rt
40 31
2.6 1.1
4.2 6.0
1.2 1.1
0.56 0.55
nd.
nd.
nd.
nd.
nd.
(degreased)
NaH (5)
80
rt
(degreased)
NaH (10)
(degreased)
NaH (10)
(degreased)
TBAF (1)
TBAF (1)
NaH (5)
80
20
21
22
23
24
25
26
27
13b
13b
13b
13b
14b
14b
14b
14b
30
30
5
rt
80
rt
80
rt
80
rt
80
75 7.1
74 1.6
93 3.6
92 1.6
38 3.1
58 4.7
74 0.61
91 2.6
2.8 2.7
nd.
nd.
nd.
27 11
nd.
NaH (5)
5
TBAF (1)
TBAF (1)
NaH (5)
30
30
5
12 7.7
nd.
NaH (5)
5
rt, room temperature; nd., not detected.
aThe time after the addition of the base to a precursor solution prior to the start of the reaction when 1 was added.
bThe radiochemical conversion (RCC) values were determined using the radiochromatograms from the analytical HPLC following a
decay correction.
cThese reactions were conducted in three independent experiments. The values are expressed as mean standard deviation.
contained in a glass bottle was observed within 1 month of of 2b and 3b using NaH as a base was highly efficient even at
opening the bottle for the 11C-methylation of phenylacetate ambient temperature (Table 1, entries 5 and 9). Furthermore,
analogs (data not shown). In the previous study, poorly reactive compounds 2a and 3a were successfully 11C-methylated on
polyfluorosilicates could be formed on the silicon atoms of the the presence of TBAF and 1 to give the desired products 2b
glass in the presence of water, and this could lead to a reduction and 3b, respectively (Table 1, entries 2 and 7). When the 11C-
in the fluoride activity of the solution. In contrast, similar losses in methylation reactions of 2a and 3a were conducted at higher
activity have not been observed for TBAF solutions bottled in temperatures, the RCC values of the products 2b and 3b were
plastic, even after several months of storage.
similar to those obtained at ambient temperature (Table 1,
Compounds 2b and 3b are five- and seven-membered cyclic entries 4 and 8). These results suggested that the methylation
amides, respectively. Compounds of this particular type are reactions of the cyclic amides 2b and 3b with 1 were sufficiently
usually prepared by the methylation of the parent amides using fast at ambient temperature when TBAF was used as a base. [11C]
1 in the presence of a metal hydroxide or tetraalkylammonium Methyl trifluoromethanesulfonate ([11C]methyl triflate) is a more
hydroxide at temperatures of 65–80 °C.4–8 Suzuki et al.9,10 reactive methylating agent than 1. This material can be prepared
reported the preparation of 2b and 3b using NaH and 1 under from the substitution of 1 and has been used for the synthesis of
relatively mild conditions (<50 °C). In our hands, the 11C-labeling 2b and 3b with the aid of NaOH.11 The results of the current
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J. Label Compd. Radiopharm 2013, 56 672–678

T. Kikuchi et al.
reactions of 6a and 8a proceeded smoothly at ambient
temperature using NaH or TBAF (Table 1, entries 19–22 and
27–30, respectively). TBAF in particular exhibited a much
higher efficiency compared with the use of lithium nitride
or NaH, even when it was used at ambient temperature for
the 11C-methylation of 8a. In contrast, our attempts to
achieve the 11C-methylation of 7a with NaH or TBAF were
unsuccessful and therefore no better than the use of lithium
nitride (Table 1, entries 23–26). The reason for the lower
efficiency of 11C-labeling of 7b compared with those of 6b
and 8b remains unclear, as mentioned by Cai et al.16
The use of 1 for the 11C-methylation of other functional
groups requiring the assistance of a base represents a significant
target for the TBAF-promoted reaction. Phenolic hydroxy groups
are more acidic than the NH moieties of amides and typically
require the addition of base for the 11C-methylation by 1. A
number of [11C]methyl phenyl ethers have been observed in
the structures of radiopharmaceuticals used for positron
emission tomography. For this reason, the nucleophilic
substitution of the phenolic hydroxy group with 1 was explored
using TBAF (Table 2).
Figure 2. Time-dependent changes in the activity of the stored TBAF solution.
The activity of the stored TBAF solution has been represented by the radiochemical
conversion (RCC) of 2b. Open circle, RCC with TBAF solution bottled in plastic
bottle; closed circle, RCC with TBAF solution bottled in glass bottle.
Compound 9a is the desmethyl labeling precursor of the
study, however, suggested that it was not necessary to heat the dopamine D2 ligand 9b and has a phenolic hydroxy group on
reaction mixture or to transform 1 to [11C]methyl triflate for the its aromatic ring. When the 11C-methylation of 9a was performed
TBAF-promoted 11C-methylation of cyclic amides.
at ambient temperature with 1 and TBAF, the desired product 9b
To expand upon the scope of the current TBAF-promoted 11C- was not obtained, despite the disappearance of the labeling
methylation reaction, we proceeded to investigate the 11C- agent 1 (Table 2, entry 1). On the basis of this result, the use of
methylation of acyclic amides. The treatment of 4a with TBAF a higher reaction temperature for the same reaction was not
and 1 at ambient temperature led to the formation of the 11C- investigated. Relevant to this, it has been reported in the
methylated product 4b, although the RCC of the reaction did literature that TBAF did not promote the 11C-methylation of 9a
not reach a practically useful level (Table 1, entry 11). The extent at 80 °C.1 Furthermore, the use of NaH barely promoted the
of the methylation reaction was improved by the use of a 11C-methylation of 9a, even at 120 °C.17 To investigate the reason
threefold increase in the amount of 4a (3 μmol), which resulted for the poor RCCs observed in these reactions, we explored the
in a moderate RCC for product 4b (33%, n = 2). In these cases, TBAF-promoted 11C-methylation reactions of 10a and 11a that
1 was completely consumed when the reaction was conducted are both analogs of 9a with the hydroxy groups at different
at ambient temperature and could no longer be found in the positions. The reactions of 10a and 11a with TBAF and 1 at
reaction mixture. The use of a higher reaction temperature did ambient temperature led to the efficient formation of 10b and
not therefore lead to an improvement in the RCC value in the 11b, respectively (Table 2, entries 2, 3, 6, and 7). The meta- and
product (Table 1, entry 12). The results of the current study were para-hydroxy moieties therefore represented acceptable
comparable with those reported by Adam et al.1 where the functional groups for the TBAF-promoted 11C-methylation using
methylation reactions were performed at 100 °C. The 11C- 1 in analogs of this type. The pKa values of hydroxy groups
methylation of 4a with NaH and 1 provided 4b with a low RCC, usually differ by a few units, depending on the presence of other
likely because of the reaction solvent used in the current study. substituents on the ring, and ortho-substituted analogs bearing
Although DMF was the solvent in this particular case, the 11C- electron-withdrawing groups are usually the most acidic. TBAF
methylation of 4a has also been reported using 1 and NaH in would therefore be sufficiently basic to generate the phenolate
DMSO.12 In contrast to the synthesis of 4b, TBAF and NaH both anion of 9a, although steric hindrance from the ortho-
promoted the 11C-labeling of 5b with moderate levels of substituent, as well as the hydrogen bond between the
efficiency (Table 1, entries 15–18). These results clearly indicated phenolate anion and amide, may have led to the observed lower
that TBAF could be applied to the 11C-methylation of acyclic reactivity. In contrast to the completion of the 11C-methylation
amides using 1, although the efficiencies of these reactions were reactions of 10a and 11a using TBAF at ambient temperature,
generally lower than those of the cyclic ones.
1 could still be detected at levels in excess of 50% of the original
Anilines, especially those bearing electron-withdrawing charge when NaH was used as the base at ambient temperature,
groups, are weak nucleophiles13–15 and require the aid of a base and heating was required to drive the NaH-promoted 11C-
to be effectively methylated using 1. Cai et al.16 recently reported methylations of 10a and 11a towards completion (Table 2,
the 11C-methylation reactions of the nitroanilines 6a–8a at entries 2–9).
ambient temperature over a 10-min period using 1 and lithium
The 11C-methylation reactions of the hydroxybenzoate esters
nitride as a base under ultrasonic conditions. According to the 12a–14a were also explored using the current methodology.
report of Cai et al., the ortho- and para-substituted nitroanilines These compounds were particularly interesting because they
(6a and 8a) proceeded smoothly through the 11C-methylation possess a phenolic hydroxy group without a proximal hydrogen
reaction in 62% and 37% yields, respectively, whereas the meta- bonding donor group. In contrast to the 11C-labeling reactions of
substituted nitroaniline 7a did not provide any of the desired the hydroxybenzamide analogs 10b and 11b, 1 could still be
methylated product.16 In the current study, the 11C-methylation detected in the reaction mixture throughout the 11C-labeling
J. Label Compd. Radiopharm 2013, 56 672–678
Copyright © 2013 John Wiley & Sons, Ltd.
www.jlcr.org

T. Kikuchi et al.
reactions of 12b–14b using TBAF at ambient temperature, and it conditions without any special treatment after they had been
was therefore necessary to heat the reaction to force the TBAF- opened. Based on these results, we believe that TBAF should be
promoted 11C-methylation to completion (Table 2, entries 10, considered as the preferred base for 11C-methylation reactions
11, 20, 21, 24, and 25). A reaction temperature of 80 °C was used using 1 because of the practical advantages that it offers over
in this particular case. It is noteworthy that the reaction the traditional bases.
temperature could be lowered, because the amount of 1
remaining in the reaction mixture when the reaction was
conducted at ambient temperature was only small. The 11C-
methylation of 2-hydroxybenzoate (12a) gave 12b with a good
RCC when TBAF and 1 were used (Table 2, entries 10 and 11).
These results indicated that the presence of the hydrogen bond
led to a significant reduction in the rate of the 11C-methylation of
9a. In contrast, 13a and 14a both underwent the 11C-
methylation with high levels of efficiency at ambient
temperature when NaH was used as a base (Table 2, entries 22,
23, 26, and 27). In this study, we found that 11C-methylation
reactions that were promoted by NaH were generally also
promoted by TBAF and vice versa. Interestingly, however, the
RCC of 12b from the methylation reaction with NaH was lower
than that of the same reaction performed with TBAF, even at
high temperature (Table 2, entries 10–13). Further investigation
of the reaction revealed that the amount of NaH used in the
Acknowledgements
We would like to thank the technical team of the Cyclotron
Section of the National Institute of Radiological Sciences for their
support during the operation of the cyclotron and the
production of the radioisotopes. This work was supported in part
by the Japan Society for the Promotion of Science KAKENHI
(grant no. 24591826).
Conflict of Interest
The authors did not report any conflict of interest.
reaction was critical to the success of the 11C-methylation of References
12b (Table 2, entries 14–19). The RCC of 12b tended to decrease
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base in the N- and O-11C-methylation reactions of amides,
anilines, and phenols using 1. Furthermore, in some cases, TBAF
exhibited higher levels of efficiency in its 11C-methylation
reactions compared with NaH. Based on our results, the N-11C-
methylations of different amides and anilines could be
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J. Label Compd. Radiopharm 2013, 56 672–678