Synthesis of new carbon-11 labeled benzoxazole derivatives for PET imaging of 5-HT3 receptor

Article abstract of DOI:10.1016/j.ejmech.2007.10.017

5-HT₃ receptor is an attractive target for the development of therapeutic agents for use in brain, heart and cancer diseases, and imaging agents for use in biomedical imaging technique PET. Benzoxazole derivatives are a novel class of 5-HT₃ receptor partial agonists with high binding affinity. Carbon-11 labeled benzoxazole derivatives have been synthesized as new potential PET radioligands for imaging 5-HT₃ receptor. The target tracers were prepared by N-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf and isolated by HPLC purification procedure in 40-50% radiochemical yields, which were decay corrected to the end of bombardment (EOB), based on [¹¹C]CO₂. The overall synthesis time was 20-25 min from EOB. The radiochemical purity was >99%, and specific activity was in a range of 74-111 GBq/μmol at the end of synthesis (EOS).

Full text of DOI:10.1016/j.ejmech.2007.10.017

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European Journal of Medicinal Chemistry 43 (2008) 1570e1574
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Laboratory note
Synthesis of new carbon-11 labeled benzoxazole
derivatives for PET imaging of 5-HT3 receptor
*
Mingzhang Gao, Min Wang, Gary D. Hutchins, Qi-Huang Zheng
Department of Radiology, Indiana University School of Medicine, 1345 West 16th Street, L3-208, Indianapolis, IN 46202-2111, USA
Received 19 September 2007; received in revised form 16 October 2007; accepted 17 October 2007
Available online 22 October 2007
Abstract
5-HT3 receptor is an attractive target for the development of therapeutic agents for use in brain, heart and cancer diseases, and imaging agents
for use in biomedical imaging technique PET. Benzoxazole derivatives are a novel class of 5-HT3 receptor partial agonists with high binding
affinity. Carbon-11 labeled benzoxazole derivatives have been synthesized as new potential PET radioligands for imaging 5-HT3 receptor. The
target tracers were prepared by N-[11C]methylation of their corresponding precursors using [11C]CH3OTf and isolated by HPLC purification
procedure in 40e50% radiochemical yields, which were decay corrected to the end of bombardment (EOB), based on [11C]CO2. The overall
synthesis time was 20e25 min from EOB. The radiochemical purity was >99%, and specific activity was in a range of 74e111 GBq/mmol at the
end of synthesis (EOS).
Ó 2007 Elsevier Masson SAS. All rights reserved.
Keywords: Carbon-11; Benzoxazole derivatives; PET; 5-HT3 receptor; Imaging
1. Introduction
antagonist possesses potent antidiarrhetic efficacy, however,
it has several side effects such as constipation. 5-HT3 recep-
tor partial agonists might control gastroenteric motility
without completely blocking 5-HT3-sensitized nerve function
to avoid these side effects with regard to corresponding
5-HT3 antagonists [7]. Four selected title compounds, 5-
chloro-7-methyl-2-(4-methyl-1-piperazinyl)benzoxazole (1b),
5,7-dimethyl-2-(4-methyl-1-piperazinyl)benzoxazole (1d), 5-
chloro-7-methyl-2-(4-methyl-1-homopiperazinyl)benzoxazole
(1f) and 5,7-dimethyl-2-(4-methyl-1-homopiperazinyl)benzo-
xazole (1h), exhibited a high binding affinity to 5-HT3 recep-
tor [7e9]. These 5-HT3 receptor ligands possess N-methyl
position amenable to labeling with a positron emitting radio-
isotope such as carbon-11 as 5-HT3 receptor radioligands.
These same properties are often beneficial in a diagnostic ra-
diotracer. 5-HT3 receptor also provides an attractive target for
the in vivo biomedical imaging technique positron emission
tomography (PET) to map 5-HT3 receptor and its related dis-
eases. Benzoxazole derivatives labeled with carbon-11 may
enable non-invasive monitoring of 5-HT3 receptor and its
response to 5-HT3 receptor antagonist and agonist treatment
using PET. To translate therapeutic agent for diagnostic
Serotonin (5-hydroxytryptamine, 5-HT) subtype 3 (5-HT3)
receptor is an important target for the development of thera-
peutic agents including 5-HT3 receptor antagonists and ago-
nists for various neurological and psychiatric disorders,
heart and cancer diseases [1e6], since 5-HT3 receptor is as-
sociated with a variety of biological pathways in central and
peripheral nervous systems. Selective 5-HT3 receptor antago-
nists have been effectively used in antiemetic therapy to pre-
vent the nausea and vomiting for cancer patients who are
undergoing chemotherapy or radiotherapy [6]. Recently
a novel series of benzoxazole derivatives have been devel-
oped as 5-HT3 receptor partial agonists for treatment of diar-
rhea-predominant irritable bowel syndrome [7e9]. A 5-HT3
Abbreviations: PET, Positron emission tomography; 5-HT3, 5-Hydroxy-
tryptamine subtype 3; EOB, End of bombardment; EOS, End of synthesis;
TMS, Tetramethylsilane; RDS, Radionuclide delivery system; INGEN, Indi-
ana Genomics Initiative.
* Corresponding author. Tel.: þ1 317 278 4671; fax: þ1 317 278 9711.
E-mail address: qzheng@iupui.edu (Q.-H. Zheng).
0223-5234/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2007.10.017

M. Gao et al. / European Journal of Medicinal Chemistry 43 (2008) 1570e1574
1571
use, we have designed and synthesized new carbon-11 la-
beled benzoxazole derivatives, 5-chloro-7-methyl-2-(4-[11C]
methyl-1-piperazinyl)benzoxazole ([11C]1b), 5,7-dimethyl-2-
(4-[11C]methyl-1-piperazinyl)benzoxazole ([11C]1d), 5-chloro-
7-methyl-2-(4-[11C]methyl-1-homopiperazinyl)benzoxazole
([11C]1f) and 5,7-dimethyl-2-(4-[11C]methyl-1-homopiperazi-
nyl)benzoxazole ([11C]1h), as potential PET radioligands for
imaging of 5-HT3 receptor.
reference standards 1b, 1d, 1f and 1h in moderate to excellent
yields (60e80%).
2.2. Radiochemistry
Synthesis of the target tracers, carbon-11 labeled benzoxa-
zole derivatives [11C]1b, [11C]1d, [11C]1f and [11C]1h, is out-
lined in Scheme 2. The precursor 1a, 1c, 1e or 1g was labeled
by [11C]methyl triflate ([11C]CH3OTf) [10,11] through N-
[11C]methylation [12] and isolated by HPLC purification pro-
cedure [13] to produce corresponding pure target compound
[11C]1b, [11C]1d, [11C]1f or [11C]1h. The radiochemical
yields were 40e50%, based on [11C]CO2, decay corrected to
end of bombardment (EOB). The radiosynthesis was per-
formed in an automated multi-purpose 11C-radiosynthesis
module, allowing measurement of specific activity during syn-
thesis [14,15]. Overall synthesis time was 20e25 min from
EOB. The specific activity was in the range of 74e111
GBq/mmol at the end of synthesis (EOS). Chemical purity
and radiochemical purity were determined by analytical
HPLC method [16]. The chemical purity of precursors and
reference standards was >95%. The radiochemical purity of
target tracers was >99% and determined by radio-HPLC
through g-ray (NaI) flow detector, and the chemical purity
of target tracers was >95% and determined by reversed-phase
HPLC through UV flow detector.
2. Results and discussion
2.1. Chemistry
Synthesis of precursors and reference standards was ac-
complished using a modification of the previously reported
procedures [7e9]. The synthetic approach is outlined in
Scheme 1. The nitration of compounds 2a,b with nitric acid
in acetic acid provided compounds 3a,b in 88% and 87%
yield, respectively. The hydrogenation of the nitro group of
compounds 3a,b in the presence of H2 with Raney-Ni and
PdeC as catalyst afforded compounds 4a,b in 98% and
97% yield, respectively. Raney-Ni catalyst was more effective
at reducing the nitro group of aromatic compounds with a hal-
ogen substituent. The cyclization of compounds 4a,b with car-
bon disulfide and KOH produced cyclized thiol compounds
5a,b in 91% and 92% yield, respectively. The coupling
reaction of compounds 5a,b with amines piperazine, 1-meth-
ylpiperazine, homopiperazine, and 1-methylhomopiperazine
gave precursors 5-chloro-7-methyl-2-(1-piperazinyl)benzoxa-
zole (1a), 5,7-dimethyl-2-(1-piperazinyl)benzoxazole (1c), 5-
chloro-7-methyl-2-(1-homopiperazinyl)benzoxazole (1e) and
5,7-dimethyl-2-(1-homopiperazinyl)benzoxazole (1g), and
3. Conclusion
An efficient and convenient synthesis of new carbon-11
labeled benzoxazole derivatives has been well-developed.
The synthetic methodology employed classical organic chemistry
R1
R1
NH2
R1
NO2
OH
H2, Raney-Ni, EtOH,
or H2, Pd/C, EtOH
OH
HNO3, HAc
OH
CH3
CH3
CH3
4a, R1=Cl
3a, R1=Cl
2a, R1=Cl
4b, R1=CH3
3b, R1=CH3
2b, R1=CH3
CS2, KOH
EtOH, reflux
R1
R1
(CH2)n
N
O
N
amine, toluene
reflux
R2
N
N
SH
O
CH3
CH3
1a, n=2, R1=Cl, R2=H
5a, R1=Cl
5b, R1=CH3
1b, n=2, R1=Cl, R2=CH3
1c, n=2, R1=CH3, R2=H
1d, n=2, R1=CH3, R2=CH3
1e, n=3, R1=Cl, R2=H
1f, n=3, R1=Cl, R2=CH3
1g, n=3, R1=CH3, R2=H
1h, n=3, R1=CH3, R2=CH3
Scheme 1. Synthesis of benzoxazole derivatives.

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M. Gao et al. / European Journal of Medicinal Chemistry 43 (2008) 1570e1574
R1
R1
(CH2)n
11CH3OTf, CH3CN
3 N NaOH
N
O
(CH2)n
N
O
N
N
H
11CH3
N
N
CH3
CH3
1a, n=2, R1=Cl
1c, n=2, R1=CH3
1e, n=3, R1=Cl
1g, n=3, R1=CH3
[
[
[
[
11C]1b, n=2, R1=Cl
11C]1d, n=2, R1=CH3
11C]1f, n=3, R1=Cl
11C]1h, n=3, R1=CH3
Scheme 2. Synthesis of carbon-11 labeled benzoxazole derivatives.
such as nitration, hydrogenation, reduction, cyclization and
coupling reaction to synthesize unlabeled benzoxazole deriva-
tives. Carbon-11 labeling at nitrogen position of the precursor
through N-[11C]methylation was incorporated efficiently using
[11C]CH3OTf, a signature reaction of carbon-11 radiochemis-
try from our laboratory. Radiosynthesis produced new probes
in amounts and purity suitable for the preclinical application
in animal studies using PET. Labeled product suitable for in-
jection, with the higher specific radioactivity in the range of
74e111 GBq/mmol at EOS, can be obtained within 25 min
from EOB including HPLC purification and formulation.
These chemistry results combined with the reported in vitro bi-
ological data encourage further in vivo biological evaluation of
carbon-11 labeled benzoxazole derivatives as new potential
PET radioligands for imaging of 5-HT3 receptor.
(buffer solution) mobile phase; flow rate 1.5 mL/min and
UV (254 nm) and g-ray (NaI) flow detectors. Semi-preparative
HPLC was performed using a YMC-Pack ODS-A, S-5 mm,
12 nm, 10 ꢀ 250 mm i.d. (Waters) C-18 column; 3:1:3
CH3CN/MeOH/20 mM, pH 6.7 KHPOꢁ4 mobile phase,
5.0 mL/min flow rate, UV (254 nm) and g-ray (NaI) flow de-
tectors. Sterile Millex-GS 0.22 mm vented filter unit was ob-
tained from Millipore Corporation, Bedford, MA.
4.2. 4-Chloro-2-methyl-6-nitrophenol (3a)
Nitric acid (90%, 11.0 g, 157.1 mmol) was slowly added
to
a solution of 4-chloro-2-methylphenol (2a, 18.0 g,
126.2 mmol) in acetic acid (240 mL) at 5 ꢂC. The reaction
mixture was stirred for 40 min. Then ice-water (150 mL)
was added in above mixture, the mixture was filtered, washed
with cold water, and dried in air to give compound 3a as a yel-
low solid (20.88 g, 88%), mp 108e110 ꢂC, Rf ¼ 0.82 (1:3
4. Experimental
1
EtOAc/hexanes). H NMR (DMSO-d6) d: 2.25 (s, 3H, CH3),
4.1. General
7.63 (d, J ¼ 2.6 Hz, 1H, Ph-H), 7.85 (d, J ¼ 3.4 Hz, 1H,
Ph-H), 10.57 (s, 1H, OH).
All commercial reagents and solvents were used without
further purification. [11C]CH3OTf was prepared according to
a literature procedure [10]. Melting points were determined
on a MEL-TEMP II capillary tube apparatus and were uncor-
4.3. 2,4-Dimethyl-6-nitrophenol (3b)
1
rected. H NMR spectra were recorded on a Varian Gemini
According to the procedure for preparation of compound
3a, compound 3b was prepared from 2,4-dimethylphenol
(2b) in 87% yield as brownish solid, mp 64e66 ꢂC,
2000 200 MHz FT-NMR spectrometer using tetramethylsilane
(TMS) as an internal standard. Chemical shift data for the pro-
ton resonances were reported in parts per million (ppm,
d scale) relative to internal standard TMS (d 0.0), and coupling
constants (J ) were reported in hertz (Hz). The low resolution
mass spectra (LRMS) were obtained using a Bruker Biflex III
MALDI-TOF mass spectrometer. Chromatographic solvent
proportions are indicated in a volume/volume ratio. Thin layer
chromatography was run using Analtech silica gel GF uni-
plates (5 ꢀ 10 cm2). Plates were visualized by UV light. Nor-
mal phase flash chromatography was carried out on EM
Science silica gel 60 (230e400 mesh) with a forced flow of
the indicated solvent system in the proportions described be-
low. All moisture- and/or air-sensitive reactions were per-
formed under a positive pressure of nitrogen maintained by
a direct line from a nitrogen source. Analytical HPLC was per-
formed using a Prodigy (Phenomenex) 5 mm C-18 column,
4.6 ꢀ 250 mm; 3:1:3 CH3CN/MeOH/20 mM, pH 6.7 KHPO4ꢁ
1
Rf ¼ 0.79 (1:3 EtOAc/hexanes). H NMR (DMSO-d6) d: 2.21
(s, 3H, CH3), 2.25 (s, 3H, CH3), 7.38 (d, J ¼ 1.6 Hz, 1H,
Ph-H), 7.64 (s, 1H, Ph-H), 10.20 (s, 1H, OH).
4.4. 2-Amino-4-chloro-6-methylphenol (4a)
Compound 3a (8.0 g, 42.6 mmmol) was dissolved in etha-
nol (100 mL), and Raney-Ni (1.0 g, Aldrich) was added to
the solution. The reaction mixture was stirred under a hydrogen
atmosphere (56 psi) for 7 h, and the Raney-Ni was removed by
filtration through celite. The solution was evaporated in vacuo
to afford compound 4a as a brown solid (6.58 g, 98%), mp
131e133 ꢂC, Rf ¼ 0.71 (1:1 EtOAc/hexanes). 1H NMR
(DMSO-d6) d: 2.06 (s, 3H, CH3), 4.77 (s, 2H, NH2), 6.29
(s, 1H, Ph-H), 6.44 (s, 1H, Ph-H), 8.06 (s, 1H, OH).

M. Gao et al. / European Journal of Medicinal Chemistry 43 (2008) 1570e1574
1573
4.5. 2-Amino-4,6-dimethylphenol (4b)
2.37 (s, 3H, CH3), 2.52 (t, J ¼ 5.2 Hz, 4H, CH2), 3.72
(t, J ¼ 5.2 Hz, 4H, CH2), 6.80 (dd, J ¼ 0.6, 1.8 Hz, 1H, Ph-H),
7.13 (d, J ¼ 1.8 Hz, 1H, Ph-H). MS (ESI): 266 ([M þ H]þ,
100%).
Compound 3b (7.0 g, 41.9 mmmol) was dissolved in etha-
nol (100 mL), and 10% palladiumecarbon (1.0 g, Aldrich)
was added to the solution. The reaction mixture was stirred un-
der a hydrogen atmosphere (57 psi) for 18 h, and the palla-
diumecarbon was removed by filtration through Celite. The
solution was concentrated in vacuo to afford compound 4b
as a brown solid (5.60 g, 97%), mp 127e129 ꢂC, Rf ¼ 0.68
4.8.3. 5,7-Dimethyl-2-(1-piperazinyl)benzoxazole (1c)
Mp 78e80 ꢂC (lit [7], mp 68e70 ꢂC), Rf ¼ 0.27 (1:9
MeOH/CH2Cl2). 1H NMR (CDCl3) d: 2.44 (s, 1H, NH),
2.93 (s, 3H, CH3), 2.94 (s, 3H, CH3), 3.57 (t, J ¼ 5.2 Hz,
4H, CH2), 4.25 (t, J ¼ 5.2 Hz, 4H, CH2), 7.22 (s, 1H, Ph-H),
7.56 (s, 1H, Ph-H). MS (ESI): 232 ([M þ H]þ, 100%).
1
(1:1 EtOAc/hexanes). H NMR (DMSO-d6) d: 2.04 (s, 6H,
2 ꢀ CH3), 4.41 (s, 2H, NH2), 6.10 (d, J ¼ 1.6 Hz, 1H, Ph-H),
6.25 (d, J ¼ 1.6 Hz, 1H, Ph-H), 7.53 (s, 1H, OH).
4.6. 5-Chloro-7-methylbenzoxazole-2-thiol (5a)
4.8.4. 5,7-Dimethyl-2-(4-methyl-1-piperazinyl)
benzoxazole (1d)
1
Compound 4a (5.0 g, 31.7 mmol) was refluxed with potas-
sium hydroxide (4.26 g, 76 mmol) and carbon disulfide
(30 mL) in ethanol (100 mL) for 8 h. The reaction mixture
was evaporated in vacuo, and aqueous hydrochloric acid
(2 N, 38 mL, 76 mmol) was added to the residue. The mixture
was filtered, washed with water, and dried in air to give com-
pound 5a as a brown solid (5.76 g, 91%), mp 230 ꢂC (dec.),
Mp 49e51 ꢂC, Rf ¼ 0.55 (1:9 MeOH/CH2Cl2). H NMR
(CDCl3) d: 2.35 (s, 3H, CH3), 2.36 (s, 3H, CH3), 2.52 (t,
J ¼ 5.2 Hz, 4H, CH2), 3.71 (t, J ¼ 5.2 Hz, 4H, CH2), 6.64 (s,
1H, Ph-H), 6.98 (s, 1H, Ph-H). MS (ESI): 246 ([M þ H]þ,
100%).
4.8.5. 5-Chloro-7-methyl-2-(1-homopiperazinyl)
benzoxazole (1e)
1
Rf ¼ 0.78 (1:1 EtOAc/hexanes). H NMR (DMSO-d6) d: 2.36
(s, 3H, CH3), 3.45 (s, 1H, SH), 7.09 (d, J ¼ 2.2 Hz, 1H, Ph-H),
7.17e7.18 (m, 1H, Ph-H).
Mp 88e90 ꢂC (lit [7], mp 68e70 ꢂC), Rf ¼ 0.18 (1:9
MeOH/CH2Cl2). 1H NMR (CDCl3) d: 1.92e2.03 (m, 2H,
CH2CH2CH2), 2.27 (s, 1H, NH), 2.37 (s, 3H, CH3), 2.95 (t,
J ¼ 6.0 Hz, 2H, CH2), 3.07e3.12 (m, 2H, CH2), 3.77e3.84
(m, 4H, CH2), 6.78 (d, J ¼ 1.6 Hz, 1H, Ph-H), 7.12 (d,
J ¼ 2.0 Hz, Ph-H). MS (ESI): 266 ([M þ H]þ, 100%).
4.7. 5,7-Dimethylbenzoxazole-2-thiol (5b)
According to the procedure for preparation of compound
5a, compound 5b was prepared from compound 4b in 92%
yield as brown solid, mp 228 ꢂC (dec.), Rf ¼ 0.80 (1:1
1
EtOAc/hexanes). H NMR (DMSO-d6) d: 2.31 (s, 3H, CH3),
4.8.6. 5-Chloro-7-methyl-2-(4-methyl-1-homopiperazinyl)
benzoxazole (1f)
2.33 (s, 3H, CH3), 3.32 (s, 1H, SH), 6.85 (s, 1H, Ph-H),
6.88 (s, 1H, Ph-H).
Mp 113e115 ꢂC (lit [8], mp 116e117 ꢂC), Rf ¼ 0.46 (1:9
MeOH/CH2Cl2). 1H NMR (CDCl3) d: 1.97e2.09 (m, 2H,
CH2CH2CH2), 2.36 (s, 3H, CH3), 2.39 (s, 3H, CH3), 2.62 (t,
J ¼ 5.2 Hz, CH2), 2.71e2.76 (m, 2H, CH2), 3.75e3.86 (m,
4H, CH2), 6.76 (dd, J ¼ 0.6, 2.0 Hz, 1H, Ph-H), 7.11 (d,
J ¼ 1.6 Hz, 1H, Ph-H). MS (ESI): 280 ([M þ H]þ, 100%).
4.8. General procedure for synthesis of benzoxazole
derivatives (1aeh)
Compound 5 (2 mmol) with amine piperazine, 1-methylpi-
perazine, homopiperazine, or 1-methylhomopiperazine (4 mmol)
were dissolved in dry toluene (25 mL), and mixture was
refluxed for 16 h. The solvent was removed under reduced
pressure, and the residue was purified by flash column chroma-
tography on silica gel with MeOH/CH2Cl2 as eluent (2e8%)
to give compound 1 in 60e80% yield as a brown solid.
4.8.7. 5,7-Dimethyl-2-(1-homopiperazinyl)benzoxazole (1g)
1
Mp 99e101 ꢂC, Rf ¼ 0.21 (1:9 MeOH/CH2Cl2). H NMR
(CDCl3) d: 1.90e2.02 (m, 2H, CH2CH2CH2), 2.03 (s, 1H,
NH), 2.34 (s, 3H, CH3), 2.36 (s, 3H, CH3), 2.92 (t,
J ¼ 5.6 Hz, 2H, CH2), 3.05e3.10 (m, 2H, CH2), 3.73e3.84
(m, 4H, CH2), 6.61 (s, 1H, Ph-H), 6.97 (s, 1H, Ph-H). MS
(ESI): 246 ([M þ H]þ, 100%).
4.8.1. 5-Chloro-7-methyl-2-(1-piperazinyl)benzoxazole (1a)
1
Mp 60e62 ꢂC, Rf ¼ 0.42 (1:9 MeOH/CH2Cl2). H NMR
(CDCl3) d: 2.37 (s, 3H, CH3), 2.98 (t, J ¼ 5.2 Hz, 4H, CH2),
3.67 (t, J ¼ 5.2 Hz, 4H, CH2), 6.80 (dd, J ¼ 0.6, 1.8 Hz, 1H,
Ph-H), 7.13 (d, J ¼ 1.8 Hz, 1H, Ph-H). MS (ESI): 252
([M þ H]þ, 100%).
4.8.8. 5,7-Dimethyl-2-(4-methyl-1-homopiperazinyl)
benzoxazole (1h)
1
Mp 56e58 ꢂC, Rf ¼ 0.41 (1:9 MeOH/CH2Cl2). H NMR
(CDCl3) d: 1.98e2.10 (m, 2H, CH2CH2CH2), 2.34 (s, 3H,
CH3), 2.36 (s, 3H, CH3), 2.39 (s, 3H, CH3), 2.60e2.65 (m,
2H, CH2), 2.72e2.77 (m, 2H, CH2), 3.76e3.87 (m, 4H,
CH2), 6.61 (t, J ¼ 0.8 Hz, 1H, Ph-H), 6.97 (s, 1H, Ph-H).
MS (ESI): 260 ([M þ H]þ, 100%).
4.8.2. 5-Chloro-7-methyl-2-(4-methyl-1-piperazinyl)
benzoxazole (1b)
Mp 64e66 ꢂC (lit [8], mp 62e64 ꢂC), Rf ¼ 0.57 (1:9
MeOH/CH2Cl2). 1H NMR (CDCl3) d: 2.35 (s, 3H, CH3),

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M. Gao et al. / European Journal of Medicinal Chemistry 43 (2008) 1570e1574
4.9. General method for the preparation of carbon-11
labeled benzoxazole derivatives, 5-chloro-7-methyl-2-
(4-[11C]methyl-1-piperazinyl)benzoxazole ([11C]1b),
5,7-dimethyl-2-(4-[11C]methyl-1-piperazinyl)
benzoxazole ([11C]1d), 5-chloro-7-methyl-2-(4-[11C]
methyl-1-homopiperazinyl)benzoxazole ([11C]1f) and
5,7-dimethyl-2-(4-[11C]methyl-1-
tR [11C]1h ¼ 5.89 min. The radiochemical yields were 40e
50% decay corrected to EOB from [11C]CO2 and 20e25%
at EOS, respectively.
Acknowledgments
homopiperazinyl)benzoxazole ([11C]1h)
This work was partially supported by the Indiana Genomics
Initiative (INGEN) of Indiana University, which is supported
in part by Lilly Endowment Inc. The authors would like to
thank Dr. Bruce H. Mock, and Barbara E. Glick-Wilson for
their assistance in production of [11C]CH3OTf. The referees’
criticisms and editor’s comments for the revision of the man-
uscript are greatly appreciated.
[11C]CO2 was produced by the 14N(p,a)11C nuclear reac-
tion in small volume (9.5 cm3) aluminum gas target (CTI)
from 11 MeV proton cyclotron on research purity nitrogen
(þ1% O2) in a Siemens radionuclide delivery system (Eclipse
RDS-111). The precursor 1a, 1c, 1e or 1g (0.1e0.3 mg, 0.4e
1.1 mM) was dissolved in CH3CN (300 mL). To this solution
was added 3 N NaOH (2 mL, 6 mM). The mixture was trans-
ferred to a small reaction vial. No carrier-added (high specific
activity) [11C]CH3OTf that was produced by the gas-phase
production method [10] from [11C]CO2 through [11C]CH4
and [11C]CH3Br with silver triflate (AgOTf) column was
passed into the reaction vial, which was cooled to 0 ꢂC, until
radioactivity reached a maximum (w2 min), and then the re-
action vial was isolated and heated at 80 ꢂC for 3 min. The
contents of the reaction tube were diluted with NaHCO3
(1 mL, 0.1 M) and water (0.5 mL), and injected onto the
semi-preparative HPLC column with 2 mL injection loop.
The product fraction was collected, the solvent was removed
by rotatory evaporation under vacuum, and the final product
[11C]1b, [11C]1d, [11C]1f or [11C]1h was formulated in saline,
sterile-filtered through a sterile vented Millex-GS 0.22 mm
cellulose acetate membrane and collected into a sterile vial.
Total radioactivity was assayed and total volume was noted.
The overall synthesis, purification and formulation time was
20e25 min from EOB. Retention times in the analytical HPLC
system were: tR 1a ¼ 2.22 min, tR 1b ¼ 2.42 min, tR [11C]1b ¼
2.42 min; tR 1c ¼ 2.13 min, tR 1d ¼ 2.44 min, tR [11C]
1d ¼ 2.44 min; tR 1e ¼ 2.26 min, tR 1f ¼ 3.35 min, tR [11C]
1f ¼ 3.35 min; and tR 1g ¼ 1.89 min, tR 1h ¼ 3.47 min, tR
[11C]1h ¼ 3.47 min. Retention times in the semi-preparative
HPLC system were: tR 1a ¼ 3.30 min, tR 1b ¼ 4.58 min,
tR [11C]1b ¼ 4.58 min; tR 1c ¼ 3.23 min, tR 1d ¼ 4.91 min,
tR [11C]1d ¼ 4.91 min; tR 1e ¼ 3.67 min, tR 1f ¼ 5.17 min, tR
[11C]1f ¼ 5.17 min; and tR 1g ¼ 3.10 min, tR 1h ¼ 5.89 min,
References
[1] R. Suzuki, A. Dickenson, Neurosignals 14 (2005) 175e181.
[2] W. Rahman, R. Suzuki, L.J. Rygh, A.H. Dickenson, Neurosci. Lett. 361
(2004) 229e231.
[3] K. Okamoto, H. Imbe, A. Tashiro, S. Kumabe, E. Senba, Neurosci. Lett.
367 (2004) 259e263.
[4] B. Kopczynska, M. Szereda-Przestaszewska, Life Sci. 75 (2004)
2281e2290.
[5] M. Camilleri, E.A. Mayer, D.A. Drossman, A. Heath, G.E. Dukes,
D. McSorley, S. Kong, A.W. Mangel, A.R. Northcutt, Aliment. Pharma-
col. Ther. 13 (1999) 1149e1159.
[6] R.J. Gralla, R. de Wit, J. Herrstedt, A.D. Carides, J. Ianus, J. Guoguang-
Ma, J.K. Evans, K.J. Horgan, Cancer 104 (2005) 864e868.
[7] S. Yoshida, S. Shiokawa, K.-i. Kawano, T. Ito, H. Murakami, H. Suzuki,
Y. Sato, J. Med. Chem. 48 (2005) 7075e7079.
[8] Y. Sato, M. Yamada, S. Yoshida, T. Soneda, M. Ishikawa, T. Nizato,
K. Suzuki, F. Konno, J. Med. Chem. 41 (1998) 3015e3021.
[9] Y. Sato, M. Yamada, K. Kobayashi, K. Iwamatsu, F. Konno, K. Shudo,
Eur. Pat. Appl. EP 0806419A1, 1997, 25 pp.
[10] B.H. Mock, G.K. Mulholland, M.T. Vavrek, Nucl. Med. Biol. 26 (1999)
467e471.
[11] D.M. Jewett, Appl. Radiat. Isot. 43 (1992) 1383e1385.
[12] M. Wang, G. Lacy, M. Gao, K.D. Miller, G.W. Sledge, Q.-H. Zheng,
Bioorg. Med. Chem. Lett. 17 (2007) 332e336.
[13] X. Fei, J.-Q. Wang, K.D. Miller, G.W. Sledge, G.D. Hutchins,
Q.-H. Zheng, Nucl. Med. Biol. 31 (2004) 1033e1041.
[14] B.H. Mock, Q.-H. Zheng, T.R. DeGrado, J. Labelled Comp. Radiopharm.
48 (2005) S225.
[15] B.H. Mock, B.E. Glick-Wilson, Q.-H. Zheng, T.R. DeGrado, J. Labelled
Comp. Radiopharm. 48 (2005) S224.
[16] Q.-H. Zheng, B.H. Mock, Biomed. Chromatogr. 19 (2005) 671e676.