Synthesis of carbon-11 labeled fluorinated 2-arylbenzothiazoles as novel potential PET cancer imaging agents

Article abstract of DOI:10.1016/j.bmc.2006.08.026

Fluorinated 2-arylbenzothiazoles are new potential antitumor drugs, which show potent and selective inhibitory activity against breast, lung, and colon cancer cell lines. Carbon-11 labeled fluorinated 2-arylbenzothiazoles may serve as novel probes for positron emission tomography (PET) to image tyrosine kinase in cancers. The preparation of 4-fluorinated 2-arylbenzothiazoles 4-fluoro-2-(3-benzloxy-4-methoxyphenyl)benzothiazole (6a) and 4-fluoro-2-(3,4-dimethoxyphenyl)benzothiazole (6b) was achieved by a modification of Jacobson thioanilide radical cyclization chemistry. Hydrogenolytic cleavage of the benzyl ether group of compound 6a using H₂/Pd-C provided the precursor 4-fluoro-2-(3-hydroxy-4-methoxyphenyl)benzothiazole (7) for radiolabeling. Synthesis of radiolabeling precursors and the reference standards 5- and 6-fluorinated arylbenzothiazoles (11c-n) was achieved via the reaction of o-aminothiophenol disulfides with substituted benzaldehydes under reducing conditions. The target radiotracers carbon-11 labeled 4-, 5-, and 6-fluorinated arylbenzothiazoles (3-[¹¹C]6b, 4-[¹¹C]11c, 3-[¹¹C]11c, 5-[¹¹C]11f, 4-[¹¹C]11f, 4-[¹¹C]11i, 3-[¹¹C]11i, 5-[¹¹C]11l, and 4-[¹¹C]11l) were prepared by O-[¹¹C]methylation of the phenolic hydroxyl precursors (7, 11d, 11e, 11g, 11h, 11j, 11k, 11m, and 11n) with [¹¹C]methyl triflate and isolated by solid-phase extraction (SPE) purification in 30-55% radiochemical yields.

Full text of DOI:10.1016/j.bmc.2006.08.026

Bioorganic & Medicinal Chemistry 14 (2006) 8599–8607
Synthesis of carbon-11 labeled fluorinated 2-arylbenzothiazoles
as novel potential PET cancer imaging agents
Min Wang,a Mingzhang Gao,a Bruce H. Mock,a Kathy D. Miller,b George W. Sledge,b
Gary D. Hutchinsa and Qi-Huang Zhenga,*
aDepartment of Radiology, Indiana University School of Medicine, Indianapolis, IN 46202, USA
bDepartment of Medicine, Indiana University School of Medicine, Indianapolis, IN 46202, USA
Received 17 July 2006; revised 17 August 2006; accepted 21 August 2006
Available online 8 September 2006
Abstract—Fluorinated 2-arylbenzothiazoles are new potential antitumor drugs, which show potent and selective inhibitory activity
against breast, lung, and colon cancer cell lines. Carbon-11 labeled fluorinated 2-arylbenzothiazoles may serve as novel probes for
positron emission tomography (PET) to image tyrosine kinase in cancers. The preparation of 4-fluorinated 2-arylbenzothiazoles
4-fluoro-2-(3-benzloxy-4-methoxyphenyl)benzothiazole (6a) and 4-fluoro-2-(3,4-dimethoxyphenyl)benzothiazole (6b) was achieved
by a modification of Jacobson thioanilide radical cyclization chemistry. Hydrogenolytic cleavage of the benzyl ether group of
compound 6a using H2/Pd–C provided the precursor 4-fluoro-2-(3-hydroxy-4-methoxyphenyl)benzothiazole (7) for radiolabeling.
Synthesis of radiolabeling precursors and the reference standards 5- and 6-fluorinated arylbenzothiazoles (11c–n) was achieved
via the reaction of o-aminothiophenol disulfides with substituted benzaldehydes under reducing conditions. The target radiotracers
carbon-11 labeled 4-, 5-, and 6-fluorinated arylbenzothiazoles (3-[11C]6b, 4-[11C]11c, 3-[11C]11c, 5-[11C]11f, 4-[11C]11f, 4-[11C]11i,
3-[11C]11i, 5-[11C]11l, and 4-[11C]11l) were prepared by O-[11C]methylation of the phenolic hydroxyl precursors (7, 11d, 11e, 11g,
11h, 11j, 11k, 11m, and 11n) with [11C]methyl triflate and isolated by solid-phase extraction (SPE) purification in 30–55% radiochem-
ical yields.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
activity against breast, colon, and lung cancer cell lines.5
The lead compound 5-fluoro-2-(3,4-dimethoxyphe-
nyl)benzothiazole (11c) was found to have exquisitely
potent antiproliferative activity with nanomolar GI50
values, <0.1 nM for breast cancer cell lines MCF-7
and MDA 468, and 0.25 nM for colon cancer cell line
HCC2998. These 2-arylbenzothiazole compounds were
screened in the NCI 60 human cancer cell lines, and po-
tent and selective activity was also observed for the com-
pound 11c. Antitumor benzothiazoles have been proven
by Nottingham group to have tyrosine kinase inhibitory
properties and could serve as tyrosine kinase inhibi-
tors.5,6 The enzyme tyrosine kinase is involved in cell
signal transduction processes and associated with the
proliferation, apoptosis, repair, and angiogenesis of can-
cer cells.7 More than two-thirds of human cancers derive
from epithelial tissues and the tyrosine kinase is overex-
pressed in the majority of these cancers.8 Therefore,
numerous selective tyrosine kinase inhibitors with nano-
molar to subnanomolar affinities have been actively
developed as potential anti-cancer agents.9 The overex-
pression of tyrosine kinase in cancers provides an attrac-
tive target for the development of enzyme-based PET
Carbon-11 labeled 2-arylbenzothiazoles have been
developed by Pittsburgh group as noninvasive biomark-
ers for biomedical imaging technique positron emission
tomography (PET) to image brain amyloid in Alzhei-
mer’s disease.1 Pittsburgh Compound-B (PIB), [11C]6-
OH-BTA-1 (Fig. 1), has been investigated in human
study, and the results indicate that detecting amyloid
plaques in the living human brain with amyloid imaging
agents is potentially feasible.2 Similarly, carbon-11 and
fluorine-18 labeled 6-iodo-2-(40-N,N-dimethylamino)-
phenylimidazo[1,2-a]pyridine derivatives3 and benzofu-
ran derivatives4 have been also developed as PET brain
amyloid imaging agents. Recently, a series of new
2-arylbenzothiazoles have been synthesized by
Nottingham group and reported to possess antitumor
activity, which show potent and selective inhibitory
Keywords: Fluorinated 2-arylbenzothiazoles; Carbon-11; Positron
emission tomography (PET); Tyrosine kinase; Cancer imaging.
*
Corresponding author. Tel.: +317 278 4671; fax: +317 278 9711;
e-mail: qzheng@iupui.edu
0968-0896/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2006.08.026

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M. Wang et al. / Bioorg. Med. Chem. 14 (2006) 8599–8607
R1
3
N
2
4
2
3
5
6
1
R2
F
4
S
6
5
7
1
R3
11CH3
3-[11C]6b: 4-F, R1=O11CH3, R2=OCH3, R3=H
4-[11C]11c: 5-F, R1=OCH3, R2=O11CH3, R3=H
3-[11C]11c: 5-F, R1=O11CH3, R2=OCH3, R3=H
5-[11C]11f: 5-F, R1=OCH3, R2=OCH3, R3=O11CH3
4-[11C]11f: 5-F, R1=OCH3, R2=O11CH3, R3=OCH3
4-[11C]11i: 6-F, R1=OCH3, R2=O11CH3, R3=H
3-[11C]11i: 6-F, R1=O11CH3, R2=OCH3, R3=H
5-[11C]11l: 6-F, R1=OCH3, R2=OCH3, R3=O11CH3
4-[11C]11l: 6-F, R1=OCH3, R2=O11CH3, R3=OCH3
N
N
H
S
HO
[11C]6-OH-BTA-1
Pittsburgh Compound-B, PIB
(Brain amyloid imaging)
Carbon-11 labeled 4-, 5- and 6-fluorinated 2-arylbenzothiazoles
(Cancer tyrosine kinase imaging)
Figure 1. Chemical structures of carbon-11 labeled 2-arylbenzothiazoles.
cancer imaging agents. To translate chemotherapeutic
agents for diagnostic use, we are interested in the design
and synthesis of medical probes for PET imaging of can-
cer. In our previous works, we have developed
[18F]SU1124810 and [11C]Iressa11 as new PET tracers
for imaging cancer tyrosine kinase. However, the in vivo
biological evaluation results including biodistribution
and animal PET imaging with these tracers were disap-
pointing. The specific binding of the tracers to target
enzyme tyrosine kinase in appropriate tumor models
awaits further investigations. In this ongoing study, we
report the synthesis of carbon-11 labeled fluorinated
2-arylbenzothiazoles as novel potential PET cancer
imaging agents as indicated in Figure 1.
61% and 83% yield, respectively. The benzamides 4a
and 4b were converted to their thiobenzamides N-(2-
fluorophenyl)-3-benzloxy-4-methoxythiobenzamide (5a)
and
N-(2-fluorophenyl)-3,4-dimethoxythiobenzamide
(5b) with Lawesson’s reagent in hexamethylphosphora-
mide (HMPA) in 58% and 76% yield, respectively. Cycli-
zation of thiobenzamides 5a and 5b by a modified
method of Jacobson thioanilide radical cyclization
chemistry using potassium ferricyanide (III) and aque-
ous sodium hydroxide14 gave 4-fluorobenzothiazoles 4-
fluoro-2-(3-benzloxy-4-methoxyphenyl)benzothiazole
(6a) and 4-fluoro-2-(3,4-dimethoxyphenyl)benzothiazole
(6b) in 26% and 57% yield, respectively. Hydrogenolytic
cleavage of the benzyl ether group of compound 6a
using H2/Pd–C provided 4-fluoro-2-(3-hydroxy-4-meth-
oxyphenyl)benzothiazole (7) in 73% yield. Compound
7 is the precursor for radiolabeling, and compound 6b
is the reference standard.
2. Results and discussion
2.1. Chemistry
5- and 6-Fluorinated 2-arylbenzothiazoles were synthe-
sized using a modification of the literature procedure.5
The synthetic approach is outlined in Scheme 2. The
reaction of benzoyl chloride with ammonium thiocya-
nate initially gave benzoyl isothiocyanate, which
underwent addition with 3-fluoroaniline to afford 3-flu-
orophenylthiourea (8) in 91% yield. 2-amino-5-fluor-
obenzothiazole (9a) was obtained from the cyclization
of compound 8 with bromine in 64% yield,15 and 2-ami-
no-6-fluorobenzothiazole (9b) was prepared by one-pot
reaction of 4-fluoroaniline with thiocyanogen generated
from bromine and potassium thiocyanate in acetic acid
in 85% yield.16 Compounds 9a,b were treated with aque-
ous potassium hydroxide through hydrolytic cleavage
followed by acidification and air oxidation to convert
to bis(2-amino-4-fluorophenyl)disulfide (10a) and
bis(2-amino-5-fluorophenyl)disulfide (10b) in 76% and
25% yield, respectively.17 Interaction of the disulfides
10a and 10b with a disubstituted or trisubstituted benz-
aldehyde and triphenylphosphine using p-toluenesulfonic
Two approaches were taken to synthesize the precursors
and standard compounds 4-, 5-, and 6-fluorinated
2-phenylbenzothiazoles (Schemes 1, and 2).
Synthesis of 4-fluorinated 2-phenylbenzothiazoles is out-
lined in Scheme 1. Benzylation of starting material 3-hy-
droxy-4-methoxybenzaldehyde was achieved by the
protection of phenolic hydroxyl group with benzyl bro-
mide to provide 3-benzloxy-4-methoxybenzaldehyde (1)
in 94% yield. Oxidation of compound 1 using sodium
chlorite provided 3-benzloxy-4-methoxybenzoic acid
(2) in 93% yield. Compound 2 was reacted with thionyl
chloride to give 3-benzloxy-4-methoxybenzoyl chloride
(3a) in 94% yield.12,13 2-Fluorobenzamides N-(2-fluor-
ophenyl)-3-benzloxy-4-methoxybenzamide (4a) and
N-(2-fluorophenyl)-3,4-dimethoxybenzamide (4b) were
prepared by condensation of aroyl chloride 3a, or
commercially available starting material 3, 4-di-
methoxybenzoyl chloride (3b), with 2-fluoroaniline in

M. Wang et al. / Bioorg. Med. Chem. 14 (2006) 8599–8607
8601
OBn
OCH3
OBn
OH
OCH3
O
HO
O
O
(ii)
(i)
OCH3
H
2
H
1
(iii)
OR
OBn
OCH3
OR
O
OCH3
OCH3
F
F
Cl
NH
(v)
(iv)
NH
3a
3b
OCH3
OCH3
S
5
a: R=Bn
b: R=CH3
O
O
4
a: R=Bn
b: R=CH3
Cl
(vi)
F
F
OR
OCH3
OH
OCH3
N
N
(vii)
S
S
7
a: R=Bn
b: R=CH3
6
Scheme 1. Synthesis of 4-fluorinated 2-arylbenzothiazoles. Reagents and conditions (yields): (i) benzyl bromide, K2CO3, DMF, 65 ꢁC, (94%); (ii)
NaClO2, H2NSO3H, AcOH, 18–20 ꢁC, (93%); (iii) SOCl2, toluene, 100 ꢁC, (94%); (iv) 2-fluoroaniline, Et3N, CH2Cl2, (61% and 83%); (v) Lawesson’s
reagent, HMPA, 100 ꢁC, (58% and 76%); (vi) K3Fe(CN)6, NaOH, 90 ꢁC, (26% and 57%); (vii) H2, 10% Pd–C, MeOH, (73%).
F
NH
NH2
NH2
F
F
NH2
N
S
(i)
(ii)
NH2
NH2
S
8
9a
9b
N
S
(iii)
F
F
R1
NH2
S
N
9a
9b
F
R2
(iv)
(v)
F
S
R3
2
11
c: 5-F, R1=OCH3, R2=OCH3, R3=H
d: 5-F, R1=OCH3, R2=OH, R3=H
e: 5-F, R1=OH, R2=OCH3, R3=H
a: 4-F
b: 5-F
10
f: 5-F, R1=OCH3, R2=OCH3, R3=OCH3
g: 5-F, R1=OCH3, R2=OCH3, R3=OH
h: 5-F, R1=OCH3, R2=OH, R3=OCH3
i: 6-F, R1=OCH3, R2=OCH3, R3=H
j: 6-F, R1=OCH3, R2=OH, R3=H
k: 6-F, R1=OH, R2=OCH3, R3=H
l: 6-F, R1=OCH3, R2=OCH3, R3=OCH3
m: 6-F, R1=OCH3, R2=OCH3, R3=OH
n: 6-F, R1=OCH3, R2=OH, R3=OCH3
Scheme 2. Synthesis of 5- and 6-fluorinated 2-arylbenzothiazoles. Reagents and conditions (yields): (i) NH4SCN, benzoyl chloride, acetone, reflux,
(91%); (ii) Br2, CH2Cl2, < 32 ꢁC, (64%); (iii) KSCN, Br2, AcOH, (85%); (iv) KOH, reflux, (76% and 25%); (v) disubstituted or trisubstituted
benzaldehyde, PPh3, p-TsOH, toluene, reflux, (63–92%).

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M. Wang et al. / Bioorg. Med. Chem. 14 (2006) 8599–8607
acid as catalyst gave corresponding 5- or 6-fluorinated
2-phenylbenzothiazoles 5-fluoro-2-(3,4-dimethoxyphe-
nyl)benzothiazole (11c), 5-fluoro-2-(4-hydroxy-3-meth-
oxyphenyl)benzothiazole (11d), 5-fluoro-2-(3-hydroxy-
11f GI50 1.9 nM for MCF-7 and 2.1 nM for MDA
468, 11i GI50 62 nM for MCF-7 and 5 nM for MDA
468, in comparison with the most potent compound
11c, although their GI50 data are relatively low.5 The
chemical structures of 5-fluorinated 2-arylbenzothiaz-
oles show compound 11f has one more methoxy group
bearing at 5-position of the aryl moiety in comparison
with compound 11c. Likewise, the chemical structures
of 6-fluorinated 2-arylbenzothiazoles show compound
11l has one more methoxy group bearing at 5-position
of the aryl moiety in comparison with compound 11i.
Based on the structure and activity relationship (SAR)
analysis, we can predict compound 11l would likely have
similar antiproliferative activity with compound 11i.
4-methoxyphenyl)benzothiazole
(11e),
5-fluoro-2-
(3,4,5-trimethoxyphenyl)benzothiazole (11f), 5-fluoro-
2-(5-hydroxy-3,4-dimethoxyphenyl)benzothiazole (11g),
5-fluoro-2-(4-hydroxy-3,5-dimethoxyphenyl)benzothiaz-
ole (11h), 6-fluoro-2-(3,4-dimethoxyphenyl)benzothiaz-
ole (11i), 6-fluoro-2-(4-hydroxy-3-methoxyphenyl)ben-
zothiazole (11j), 6-fluoro-2-(3-hydroxy-4-methoxyphe-
nyl)benzothiazole (11k), 6-fluoro-2-(3,4,5-trimethoxy-
phenyl)benzothiazole (11l), 6-fluoro-2-(5-hydroxy-3,4-
dimethoxyphenyl)benzothiazole (11m), and 6-fluoro-2-
(4-hydroxy-3,5-dimethoxyphenyl)benzothiazole (11n) in
63–92% yields. Compounds 11d, 11e, 11g, 11h, 11j,
11k, 11m, and 11n are precursors for radiolabeling,
and compounds 11c, 11f, 11i, and 11l are reference
standards.
The phenolic hydroxyl precursors 7, 11d, 11e, 11g, 11h,
11j, 11k, 11m, and 11n can be directly labeled with a
positron emitting radionuclide carbon-11 at aryl moiety
of 2-arylbenzothiazoles, and 4-, 5-, and 6-fluorinated 2-
phenylbenzothiazoles bearing a fluoro-group at 4-, 5-,
and 6-positions are also the target molecules for radiola-
beling of 2-arylbenzothiazoles with another positron
emitting radionuclide fluorine-18 at benzothiazole moie-
ty, their corresponding precursors would be 4-, 5-, and
6-nitro-2-phenylbenzothiazoles bearing a nitro-group
at 4-, 5-, and 6-positions of benzothiazole moiety, and
fluorine-18 radiolabeling methodology will be exactly
same with the synthesis of the tracer [18F]SU11248
aforementioned.10
The target compounds 11c, 11f, and 11i are the known
compounds,5 and the target compound 11l is a novel
compound. The in vitro assay shows compound 11c
has exquisitely potent antiproliferative activity,
GI50 < 0.1 nM for MCF-7 and MDA 468 breast cancer
cell lines, and potent and selective activity of compound
11c was also observed in the NCI 60 human cancer cell
line panel.5 The reported in vitro data indicate com-
pounds 11f and 11i have similar antitumor activity,
F
11
O
CH 3
N
OCH3
S
3-[11C]6b
11
OCH3
O11CH3
O
CH 3
F
F
N
N
OCH3
S
S
4-[11C]11c
3-[11C]11c
7
OCH3
OCH3
11d, 11e
OCH3
O11CH3
(i)
F
N
11g, 11h
11j, 11k
11m, 11n
F
N
S
S
5-[11C]11f
O11CH3
OCH3
4-[11C]11f
OCH3
11
O
CH3
N
N
O11CH3
OCH3
S
S
F
F
4-[11C]11i
3-[11C]11i
OCH3
O11CH3
OCH3
OCH3
OCH3
O11CH3
N
S
N
S
F
F
4-[11C]11l
5-[11C]11l
Scheme 3. Synthesis of carbon-11 labeled 4-, 5-, and 6-fluorinated 2-arylbenzothiazoles. Reagents (yields): (i) 11CH3OTf, CH3CN, 3 N NaOH,
(30–55%).

M. Wang et al. / Bioorg. Med. Chem. 14 (2006) 8599–8607
8603
2.2. Radiochemistry
obtained in ꢀ20 min from EOB including SPE purifica-
tion and formulation. These chemistry results combined
with the reported in vitro biological data encourage fur-
ther in vivo biological evaluation of carbon-11 labeled
4-, 5-, and 6-fluorinated arylbenzothiazoles as novel po-
tential PET cancer imaging agents for tyrosine kinase.
Synthesis of the target tracers 4-fluoro-2-(3-[11C]meth-
oxy-4-methoxyphenyl)benzothiazole (3-[11C]6b), 5-fluo-
ro-2-(3-methoxy-4-[11C]methoxyphenyl)benzothiazole
(4-[11C]11c), 5-fluoro-2-(3-[11C]methoxy-4-methoxyphe-
nyl)benzothiazole (3-[11C]11c), 5-fluoro-2-(5-[11C]meth-
oxy-3,4-dimethoxyphenyl)benzothiazole
5-fluoro-2-(4-[11C]methoxy-3,5-dimethoxyphenyl)benzo-
(4-[11C]11f),
thiazole 6-fluoro-2-(3-methoxy-4-
(5-[11C]11f),
4. Experimental
4.1. General
[11C]methoxyphenyl)benzothiazole (4-[11C]11i), 6-fluo-
ro-2-(3-[11C]methoxy-4-methoxyphenyl)benzothiazole
(3-[11C]11i), 6-fluoro-2-(5-[11C]methoxy-3,4-dimethoxy-
phenyl)benzothiazole (5-[11C]11l), and 6-fluoro-2-
(4-[11C]methoxy-3,5-dimethoxyphenyl)benzothiazole (4-
[11C]11l) is outlined in Scheme 3. The phenolic hydroxyl
precursor 7, 11d, 11e, 11g, 11h, 11j, 11k, 11m or 11n was
labeled by [11C]methyl triflate (11CH3OTf)18,19 through
O-[11C]methylation20 of hydroxyphenyl position under
basic conditions using 3 N NaOH. The tracer was isolat-
ed by solid-phase extraction (SPE) purification21,22 to
produce pure target radiolabeled compound with
30–55% radiochemical yields, based on 11CO2, decay
corrected to end of bombardment (EOB). The large
polarity difference between the phenolic hydroxyl pre-
cursor and the labeled methylated product permitted
the use of SPE technique for purification of labeled
product from radiolabeling reaction mixture. The reac-
tion mixture was diluted with NaHCO3 and loaded onto
C-18 Sep-Pak cartridge by gas pressure. The cartridge
column was washed with water to remove non-reacted
11CH3OTf and precursor and reaction solvent, and then
final labeled product was eluted with ethanol. Chemical
purity, radiochemical purity, and specific radioactivity
were determined by analytical HPLC method.23 The
chemical purity of precursors and reference standards
was >90%. The radiochemical purity of target tracers
was >95% determined by radio-HPLC through c-ray
(NaI) flow detector, and the chemical purity of target
tracers was > 93% determined by reversed-phase HPLC
through UV flow detector. The average specific radioac-
tivity of target tracers was 1–2 Ci/lmol at end-of-synthe-
sis (EOS).
All commercial reagents and solvents were used
without
further
purification
unless
specified. Melting points were determined on
otherwise
a
MEL-TEMP II capillary tube apparatus and are
uncorrected. 1H NMR spectra were recorded on a
Bruker QE 300 FT-NMR spectrometer using tetra-
methylsilane (TMS) as an internal standard. Chemi-
cal shift data for the proton resonances were
reported in parts per million (ppm, d scale) relative
to internal standard TMS (d 0.0), and coupling con-
stants (J) are reported in hertz (Hz). The low reso-
lution mass spectra (LRMS) were obtained using a
Bruker Biflex III MALDI-Tof mass spectrometer,
and the high resolution mass spectra (HRMS) were
obtained using a Kratos MS80 mass spectrometer.
Chromatographic solvent proportions are expressed
on a volume: volume basis. Thin layer chromatogra-
phy was run using Analtech silica gel GF uniplates
(5 · 10 cm2). Plates were visualized by UV light.
Normal phase flash chromatography was carried
out on EM Science silica gel 60 (230–400 mesh)
with a forced flow of the indicated solvent system
in the proportions described below. All moisture-
and/or air-sensitive reactions were performed under
a positive pressure of nitrogen maintained by a
direct line from a nitrogen source.
Analytical HPLC was performed using a Prodigy (Phe-
nomenex) 5 lm C-18 column, 4.6 · 250 mm; 3:1:3
ꢁ
CH3CN/MeOH/20 mM, pH 6.7, KHPO4
mobile
phase, 1.5 mL/min flow rate, UV (254 nm) and c-ray
(NaI) flow detectors. Semi-prep C-18 guard cartridge
column 1 · 1 cm was obtained from E.S. Industries, Ber-
lin, NJ, and part number 300121-C18-BD 10 l. Sterile
vented Millex-GS 0.22 lm filter unit was obtained from
Millipore Corporation, Bedford, MA.
3. Conclusion
An efficient and convenient chemical and radiochemical
synthesis of 4-, 5-, and 6-fluorinated arylbenzothiazole
phenolic hydroxyl precursors, 4-, 5-, and 6-fluorinated
arylbenzothiazole reference standards, and carbon-11
labeled 4-, 5-, and 6-fluorinated arylbenzothiazole target
tracers has been developed. The synthetic methodology
employed classical heterocycle chemistry such as thioani-
lide radical cyclization chemistry. Carbon-11 labeling was
incorporated efficiently using 11CH3OTf, a signature
reaction of carbon-11 radiochemistry from our laborato-
ry. Radiosynthesis produced new probes carbon-11
labeled 4-, 5-, and 6-fluorinated arylbenzothiazoles in
amounts and purity suitable for the preclinical applica-
tion in animal studies by PET imaging techniques.
Labeled product suitable for injection, with the specific
radioactivities in a range of 1–2 Ci/lmol at EOS, can be
4.2. 3-Benzloxy-4-methoxybenzaldehyde (1)
A
suspension of 3-hydroxy-4-methoxybenzaldehyde
(10.0 g, 65.7 mmol), anhydrous K2CO3 (13.6 g,
98.5 mmol), and KI (0.3 g) in anhydrous DMF
(60 mL) was stirred and heated to 65 ꢁC. Benzyl
bromide (9.4 mL, 85.9 mmol) was added dropwise
and the stirred mixture was heated at 65 ꢁC for 22 h.
After cooling to room temperature, water (150 mL)
was added and the mixture was extracted with EtOAc
(2· 100 mL). The combined organic phase was washed
with 1 N NaOH (2· 50 mL) and water (3· 100 mL),
dried over anhydrous Na2SO4, and evaporated at
reduced pressure. A yellowish solid was obtained.

8604
M. Wang et al. / Bioorg. Med. Chem. 14 (2006) 8599–8607
The crude product was purified by column chromatog-
raphy (EtOAc/hexanes, 1:4) to give 1 as a pale yellow
solid (14.9 g, 94%), mp 52–52 ꢁC. 1H NMR (CDCl3)
d: 9.82 (s, 1H, CHO), 7.48–7.26 (m, 7H, Ar-H), 7.00
(d, J = 8.1 Hz, 1H, Ar-H), 5.20 (s, 2H, OCH2), 3.97
(s, 3H, OCH3).
Ar-H), 7.21–7.07 (m, 3H, Ar-H), 6.94 (d, J = 8.1 Hz,
1H, Ar-H), 3.97 (s, 3H, OCH3), 3.96 (s, 3H, OCH3).
4.7. N-(2-Fluorophenyl)-3-benzloxy-4-methoxythiobenz-
amide (5a)
A mixture of compound 4a (3.0 g, 7.74 mmol) and
Lawesson’s reagent (2.2 g, 5.4 mmol) in HMPA (6 mL)
was stirred at 100 ꢁC for 16 h. After cooling, it was
poured into water. The product was extracted into
diethyl ether (3· 100 mL), and the ethereal layer was
washed with water (3· 100 mL), dried over Na2SO4
and evaporated at reduced pressure. The crude product
was purified by column chromatography (EtOAc/hex-
anes, 1:4) to give 5a as a yellow solid (1.8 g, 58%), mp
153–154 ꢁC. 1H NMR (CDCl3) d: 8.94 (br s, 1H,
CSNH), 7.61 (s, 1H, Ar-H), 7.49–7.32 (m, 7H, Ar-H),
7.25–7.16 (m, 3H, Ar-H), 6.90 (d, J = 8.8 Hz, 1H,
Ar-H), 5.21 (s, 2H, OCH2), 3.94 (s, 3H, OCH3).
4.3. 3-Benzloxy-4-methoxybenzoic acid (2)
A suspension of compound 1 (11.6 g, 48.0 mmol) and
sulfamic acid (6.4 g, 64.6 mmol) in 80% AcOH
(90 mL) was stirred and a solution of 80% NaClO2
(5.6 g, 50.0 mmol) in water (23 mL) was added in drop-
wise. The temperature was maintained at 18–20 ꢁC using
an ice-water bath. The yellow slurry was stirred at 20 ꢁC
for 1 h and the reaction mixture was diluted by the addi-
tion of water (80 mL). The mixture was filtered, and the
solid was washed with water and dried to give 2 as a
white solid (11.5 g, 93%), mp 144–145 ꢁC. 1H NMR
(CDCl3) d: 7.78 (dd, J = 2.2, 8.8 Hz, 1H, Ar-H), 7.66
(d, J = 1.5 Hz, 1H, Ar-H), 7.49–7.32 (m, 5H, m,
Ar-H), 6.94 (d, J = 8.8 Hz, 1H, Ar-H), 5.20 (s, 2H,
OCH2), 3.95 (s, 3H, OCH3).
4.8. N-(2-Fluorophenyl)-3,4-dimethoxythiobenzamide
(5b)
Similarly, compound 5b was prepared by the reaction of
compound 4b and Lawesson’s reagent in HMPA as a
yellow solid in 76% yield, mp 106–107 ꢁC. 1H NMR
(CDCl3) d: 9.06 (br s, 1H, CSNH), 7.58 (s, 1H, Ar-H),
7.39 (dd, J = 2.2, 8.8 Hz, 1H, Ar-H), 7.25–7.15 (m,
4H, Ar-H), 6.86 (d, J = 8.1 Hz, 1H, Ar-H), 3.95 (s,
3H, OCH3), 3.93 (s, 3H, OCH3).
4.4. 3-Benzloxy-4-methoxybenzoyl chloride (3a)
A solution of compound 2 (9 g, 34.9 mmol) in toluene
(120 mL) was heated to 90 ꢁC. SOCl2 (3.5 mL,
48.1 mmol) was added dropwise and the solution was
heated at 100 ꢁC for 6 h. After cooling, the solution
was concentrated at reduced pressure to give 3a as a
light brown oil which became a crystal when cooled to
room temperature (9.1 g, 94%), mp 57–58 ꢁC. 1H
NMR (CDCl3) d: 7.83 (dd, J = 2.2, 8.8 Hz, 1H, Ar-H),
7.61 (d, J = 2.2 Hz, 1H, Ar-H), 7.48–7.32 (m, 5H,
Ar-H), 6.94 (d, J = 8.8 Hz, 1H, Ar-H), 5.18 (s, 2H,
OCH2), 3.98 (s, 3H, OCH3).
4.9. 4-Fluoro-2-(3-benzloxy-4-methoxyphenyl)benzo-
thiazole (6a)
A little EtOH, NaOH (1.5 g, 37.5 mmol), and water
(5 mL) were added to compound 5a (1.0 g, 2.7 mmol).
The mixture was added dropwise to a stirred solution
of potassium ferricyanide (III) (3.6 g, 10.9 mmol) in
water (5 mL). The resulting solution was heated at
90 ꢁC for 2 h and then cooled with ice bath. The precip-
itate was collected by filtration, washed with water
(3· 10 mL), then dissolved in EtOAc (20 mL), and insol-
uble solid was removed by filtration. The filtrate was
evaporated at reduced pressure and the crude product
was purified by column chromatography (EtOAc/hex-
anes, 1:4) to give 6a as a pale yellow powder (260 mg,
4.5. N-(2-Fluorophenyl)-3-benzloxy-4-methoxybenzamide
(4a)
Compound 3a (6.0 g, 21.7 mmol) was added to a solu-
tion of 2-fluoroaniline (2.5 mL, 23.9 mmol) and Et3N
(3.0 mL) in CH2Cl2 (100 mL). The reaction mixture
was heated at reflux for 3 h. After cooling, the solution
was washed with water (2· 100 mL), dried over Na2SO4,
and evaporated at reduced pressure. The crude product
was purified by column chromatography (MeOH/
CH2Cl2, 1:99) to give 4a as a white solid (5.1 g, 61%),
mp 170–171 ꢁC. 1H NMR (CDCl3) d: 7.94 (d,
J = 2.9 Hz, 1H, Ar-H), 7.53–7.32 (m, 7H, Ar-H), 7.21–
7.05 (m, 4H, Ar-H), 6.96 (d, J = 8.8 Hz, 1H, Ar-H),
5.23 (s, 2H, OCH2), 3.96 (s, 3H, OCH3).
1
26%), mp 138–139 ꢁC. H NMR (CDCl3) d: 7.78–7.76
(m, 1H, Ar-H), 7.68–7.62 (m, 2H, Ar-H), 7.52 (d,
J = 7.3 Hz, 2H, Ar-H), 7.43–7.27 (m, 4H, Ar-H), 7.20–
7.15 (m, 1H, Ar-H), 6.97 (d, J = 8.1 Hz, 1H, Ar-H),
5.27 (s, 2H, OCH2), 3.96 (s, 3H, OCH3).
4.10. 4-Fluoro-2-(3,4-dimethoxyphenyl)benzothiazole
(6b)
4.6. N-(2-Fluorophenyl)-3, 4-dimethoxybenzamide (4b)
Similarly, compound 6b was prepared by the reaction of
compound 5b and potassium ferricyanide (III) as a pale
yellow powder in 57% yield, mp 127–128 ꢁC (lit.5 mp
Using similar method for the preparation of compound
4a, compound 4b was obtained by the reaction of 2-flu-
oroaniline and 3,4-dimethoxybenzoyl chloride 3b in
CH2Cl2 as a white solid in 83% yield, mp 130–131 ꢁC.
1H NMR (CDCl3) d: 8.02 (d, J = 2.2 Hz, 1H, Ar-H),
7.51 (s, 1H, Ar-H), 7.42 (dd, J = 2.4, 9.5 Hz, 1H,
1
129 ꢁC). H NMR (CDCl3) d: 7.74 (d, J = 2.4 Hz, 1H,
H-7), 7.66–7.60 (m, 2H, H-20, H-60), 7.31 (dt, J = 4.5,
8.1 Hz, 1H, H-6), 7.18 (dd, J = 2.2, 10.3 Hz, 1H, H-5),
6.95 (d, J = 8.8 Hz, 1H, H-50), 4.03 (s, 3H, OCH3),
3.97 (s, 3H, OCH3).

M. Wang et al. / Bioorg. Med. Chem. 14 (2006) 8599–8607
8605
4.11. 4-Fluoro-2-(3-hydroxy-4-methoxyphenyl)benzo-
thiazole (7)
(150 mL) was cooled in an ice bath and stirred, and then
a solution of bromine (5.1 mL, 100.0 mmol) in AcOH
(15 mL) was added dropwise. The reaction mixture
was stirred at room temperature for 2 h, then poured
into cold water (200 mL), basified to pH 11.0 with
NH4OH, and extracted with EtOAc (3· 100 mL). The
combined organic phase was washed with water
(100 mL), brine (100 mL), dried over Na2SO4, filtered,
and evaporated at reduced pressure. A crude yellow
product was obtained, which was purified by column
chromatography (EtOAc/hexanes, 7:3) to give 9b as
white powder (12.1 g, 85%), mp 130–131 ꢁC (lit.16 mp
127–128 ꢁC). 1H NMR (DMSO-d6) d: 7.57 (dd,
J = 2.9, 8.8 Hz, 1H, H-7), 7.45 (s, 2H, NH2), 7.29 (dd,
J = 5.1, 8.8 Hz, 1H, H-4), 7.02 (dt, J = 2.2, 8.8 Hz, 1H,
H-5).
A solution of compound 6a (200 mg, 0.5 mmol) in
MeOH (15 mL) was hydrogenated using 10% Pd–C
(50 mg) as catalyst at atmospheric pressure overnight.
The catalyst was removed by filtration, and the solution
was evaporated at reduced pressure. The crude product
was purified by column chromatography (EtOAc/hex-
anes, 1:4) to give 7 as a pale yellow powder (110 mg,
1
73%), mp 135–136 ꢁC. H NMR (CDCl3) d: 8.05–7.87
(m, 1 H, Ar-H), 7.70–7.62 (m, 2H, Ar-H), 7.49–7.16
(m, 2H, Ar-H), 6.94 (d, J = 8.1 Hz, 1H, Ar-H), 5.76 (s,
1H, OH), 3.97 (s, 3H, OCH3). LRMS (CI): 276.0
([M+H]+, 100%). HRMS (CI): calcd for C14H11O2NFS
276.0489 [M+H]+, found 276.0490.
4.12. 3-Fluorophenylthiourea (8)
4.15. Bis(2-amino-4-fluorophenyl)disulfide (10a)
Benzoyl chloride (17.2 mL, 148.4 mmol) was added
dropwise to a solution of ammonium thiocyanate
(11.3 g, 148.4 mmol) in acetone (20 mL). The suspension
was heated at reflux and 3-fluoroaniline (13.0 mL,
135.0 mmol) was added. The reaction mixture was dilut-
ed with acetone (15 mL) and continued to reflux for 1 h.
Compound 9a (5.0 g, 29.8 mmol) was added to a solu-
tion of KOH (25.0 g, 446.4 mmol) in water (50 mL).
The reaction mixture was heated at reflux for 5 h and
then cooled to room temperature, which was acidified
to pH 6.0 with AcOH. Water (50 mL) was added, and
the resulting mixture was stirred overnight. The precip-
itate was collected, and the crude product was purified
by column chromatography (MeOH/CH2Cl2, 0.5:99.5)
to give 10a as a yellow solid (3.2 g, 76%), mp 74–75 ꢁC
(lit.17 mp 75–76 ꢁC). 1H NMR (CDCl3) d: 7.04 (dd,
J = 6.6, 8.1 Hz, 1H, H-6), 6.41 (dd, J = 2.2, 10.3 Hz,
1H, H-3), 6.29 (dt, J = 2.2, 8.1 Hz, 1H, H-5), 4.48 (s,
2H, NH2).
To this mixture
a
solution of NaOH (16.9 g,
422.5 mmol) in water (100 mL) was added and the yel-
low homogeneous solution was heated at reflux for
1.5 h. Then the reaction mixture was cooled and concen-
trated at reduced pressure to remove the acetone. The
mixture was adjusted pH to 5.0 with concentrated
HCl, and then pH to 11.0 with NH4OH to give a pale
yellow precipitate, which was collected by filtration,
washed with water (3· 30 mL), and dried under vacuum
to yield 8 as a pale yellow solid (19.2 g, 91%), mp 113–
4.16. Bis(2-amino-5-fluorophenyl)disulfide (10b)
1
114 ꢁC (lit.15 mp 114–115 ꢁC). H NMR (DMSO-d6) d:
Using similar method for the preparation of compound
10a, compound 10b was obtained by the reaction of
compound 9b and KOH as a yellow solid in 25% yield,
9.83 (s, 1H, NH), 7.95–7.50 (m, 3H, NH2 and H-2),
7.33 (dt, J = 8.1, 15.5 Hz, 1H, H-5), 7.14 (d,
J = 7.3 Hz, 1H, H-6), 6.91 (dt, J = 2.9, 8.0 Hz, 1H, H-4).
1
mp 75–76 ꢁC. H NMR (DMSO-d6) d: 7.04 (dt, J = 2.9,
8.8 Hz, 1H, H-4), 6.83 (dd, J = 2.9, 8.8 Hz, 1H, H-6),
6.73 (dd, J = 5.2, 8.9 Hz, 1H, H-3), 5.37 (s, 2H, NH2).
4.13. 2-Amino-5-fluorobenzothiazole (9a)
A solution of bromine (4.6 mL, 89.7 mmol) in CH2Cl2
(40 mL) was added dropwise to a solution of compound
8 (14.0 g, 89.7 mmol) in CH2Cl2 (250 mL), maintaining
the temperature below 30 ꢁC. The resulting mixture
was heated at reflux for 3 h and then cooled to room
temperature. The precipitate was collected, suspended
in water (400 mL), basified to pH 11.0 with NH4OH,
and extracted with EtOAc (3· 100 mL). The combined
organic phase was washed with water (100 mL), brine
(100 mL), dried over Na2SO4, filtered, and evaporated
at reduced pressure. A crude yellow product was ob-
tained, which was recrystallized from benzene to yield
9a as a white solid (8.9 g, 64%), mp 182–183 ꢁC (lit.15
mp 181 ꢁC). 1H NMR (DMSO-d6) d: 7.65–7.60 (m,
3H, NH2 and H-7), 7.11 (dd, J = 2.2, 10.3 Hz, 1H,
H-4), 6.83 (dt, J = 3.0, 9.6 Hz, 1H, H-6).
4.17. General method for the preparation of 5- and
6-fluorinated 2-arylbenzothiazoles (11c–n)
Disubstituted or trisubstituted benzaldehyde (2.2 mmol),
p-toluenesulfonic acid (19.0 mg, 0.1 mmol), and triphe-
nylphosphine (262 mg, 1.0 mmol) were added to a solu-
tion of compound 10a or 10b (284 mg, 1.0 mmol) in
toluene (10 mL). The reaction mixture was heated at
reflux for 15 h, and then cooled and evaporated at
reduced pressure. The crude product was purified col-
umn chromatography (MeOH/CH2Cl2, 1:49) to give
substituted 2-arylbenzothiazole in 63–92% yields.
4.17.1. 5-Fluoro-2-(3,4-dimethoxyphenyl)benzothiazole
(11c). The product was obtained from 10a and 3,4-
dimethoxybenzaldehyde as a yellow solid in 90% yield,
1
mp 108–109 ꢁC (lit.5 mp 110 ꢁC). H NMR (DMSO-
4.14. 2-Amino-6-fluorobenzothiazole (9b)
d6) d: 8.13 (dd, J = 5.2, 8.8 Hz, 1H, H-7), 7.85 (dd,
J = 2.2, 9.5 Hz, 1H, H-4), 7.61–7.59 (m, 2H, H-20,
H-60), 7.32 (dt, J = 2.3, 8.9 Hz, 1H, H-6), 7.11
A solution of 4-fluoroaniline (9.5 mL, 100.0 mmol) and
potassium thiocyanate (15.2 g, 200.0 mmol) in AcOH

8606
M. Wang et al. / Bioorg. Med. Chem. 14 (2006) 8599–8607
(d, J = 8.9 Hz, 1H, H-50), 3.87 (s, 3H, OCH3), 3.84 (s,
3H, OCH3).
4.17.8. 6-Fluoro-2-(4-hydroxy-3-methoxyphenyl)benzo-
thiazole (11j). The product was obtained from 10b and 4-
hydroxy-3-methoxybenzaldehyde as a yellow solid in 86%
1
yield, mp 199–200 ꢁC. H NMR (DMSO-d6) d: 9.86 (s,
4.17.2. 5-Fluoro-2-(4-hydroxy-3-methoxyphenyl)benzo-
thiazole (11d). The product was obtained from 10a and
4-hydroxy-3-methoxybenzaldehyde as a yellow solid in
1H, OH), 8.02–7.97 (m, 2H, H-20, H-4), 7.59
(d, J = 2.2 Hz, 1H, H-60), 7.47 (dd, J = 2.2, 8.9 Hz, 1H,
H-7), 7.36 (dt, J = 2.9, 9.5Hz, 1H, H-5), 6.93 (d,
J = 8.1 Hz, 1H, H-50), 3.88 (s, 3H, OCH3). LRMS (CI):
276.0 ([M+H]+, 78%), 275.0 (M+, 100%). HRMS (CI): calcd
for C14H11O2NFS 276.0489 [M+H]+, found 276.0477.
1
85% yield, mp 157–158 ꢁC (lit.5 mp 156 ꢁC). H NMR
(DMSO-d6) d: 9.91 (s, 1H, OH), 8.11 (dd, J = 5.9,
8.8 Hz, 1H, H-7), 7.83 (dd, J = 2.5, 9.6 Hz, 1H, H-4),
7.60 (s, 1H, H-20), 7.50 (d, J = 8.1 Hz, 1H, H-60), 7.30
(t, J = 8.8 Hz, 1H, H-6), 6.93 (d, J = 8.1 Hz, 1H, H-
50), 3.88 (s, 3H, OCH3).
4.17.9. 6-Fluoro-2-(3-hydroxy-4-methoxyphenyl)benzo-
thiazole (11k). The product was obtained from 10b
and 3-hydroxy-4-methoxybenzaldehyde as a yellow sol-
4.17.3. 5-Fluoro-2-(3-hydroxy-4-methoxyphenyl)benzo-
thiazole (11e). The product was obtained from 10a and
3-hydroxy-4-methoxybenzaldehyde as a yellow solid in
88% yield, mp 170–171 ꢁC (lit.5 mp 169–172 ꢁC). 1H
NMR (DMSO-d6) d: 9.57 (s, 1H, OH), 8.11 (dd,
J = 5.9, 8.8 Hz, 1H, H-7), 7.83 (dd, J = 2.9, 10.3 Hz,
1H, H-4), 7.52–7.48 (m, 2H, H-20, H-60), 7.31 (dt,
J = 2.9, 9.5 Hz, 1H, H-6), 7.07 (d, J = 8.1 Hz, 1H,
H-50), 3.84 (s, 3H, OCH3).
1
id in 80% yield, mp 184–185 ꢁC. H NMR (CDCl3) d:
7.96 (dd, J = 4.4, 8.8 Hz, 1H, H-4), 7.63–7.53 (m, 3H,
H-20, H-60 and H-7), 7.20 (dt, J = 2.9, 9.5 Hz, 1H, H-
5), 6.94 (d, J = 8.1 Hz, 1H, H-50), 5.76 (br s, 1H, OH),
3.97 (s, 3H, OCH3). LRMS (CI): 276.0 ([M+H]+,
83%), 275.0 (M+, 100%). HRMS (CI): calcd for
C14H11O2NFS 276.0489 [M+H]+, found 276.0490.
4.17.10. 6-Fluoro-2-(3,4,5-trimethoxyphenyl)benzothiaz-
ole (11l). The product was obtained from 10b and
3,4,5-trimethoxybenzaldehyde as a yellow solid in 92%
4.17.4. 5-Fluoro-2-(3,4,5-trimethoxyphenyl)benzothiazole
(11f). The product was obtained from 10a and 3,4,5-tri-
methoxybenzaldehyde as a yellow solid in 73% yield, mp
1
yield, mp 151–152 ꢁC. H NMR (CDCl3) d: 7.99 (dd,
J = 5.1, 8.8 Hz, 1H, H-4), 7.57 (dd, J = 2.3, 8.1 Hz,
1H, H-7), 7.29 (s, 2H, H-20, H-60), 7.22 (dt, J = 2.2,
8.8 Hz, 1H, H-5), 3.99 (s, 6H, OCH3), 3.93 (s, 3H,
OCH3). LRMS (CI): 320.0 ([M+H]+, 73%), 319.0 (M+,
100%). HRMS (CI): calcd for C16H15O3NFS 320.0751
[M+H]+, found 320.0729.
1
117–118 ꢁC (lit.5 mp 120–122 ꢁC). H NMR (CDCl3) d:
7.81 (dd, J = 5.2, 8.8 Hz, 1H, H-7), 7.73 (dd, J = 2.9,
9.5 Hz, 1H, H-4), 7.31 (s, 2H, H-20, H-60), 7.16 (dt,
J = 2.3, 8.9 Hz, 1H, H-6), 3.99 (s, 6H, OCH3), 3.93 (s,
3H, OCH3).
4.17.5. 5-Fluoro-2-(5-hydroxy-3,4-dimethoxyphenyl)ben-
zothiazole (11g). The product was obtained from 10a
and 5-hydroxy-3,4-dimethoxybenzaldehyde as a yellow
solid in 63% yield, mp 180–181 ꢁC (lit.4 mp 178–
180 ꢁC). 1H NMR (CDCl3) d: 7.81 (dd, J = 5.1,
8.8 Hz, 1H, H-7), 7.72 (dd, J = 2.2, 9.6 Hz, 1H, H-4),
7.33 (d, J = 2.2 Hz, 1H, H-20), 7.26 (d, J = 2.9 Hz, 1H,
H-60), 7.15 (dt, J = 3.0, 8.8 Hz, 1H, H-6), 5.95 (s, 1H,
OH), 3.40 (s, 3H, OCH3), 3.98 (s, 3H, OCH3).
4.17.11.
6-Fluoro-2-(5-hydroxy-3,4-dimethoxyphenyl)-
benzothiazole (11m). The product was obtained from
10b and 5-hydroxy-3,4-dimethoxybenzaldehyde as a yel-
low solid in 69% yield, mp 174–175 ꢁC. 1H NMR
(CDCl3) d: 7.98 (dd, J = 4.5, 8.9 Hz, 1H, H-4), 7.57
(dd, J = 2.2, 8.0 Hz, 1H, H-7), 7.31 (d, J = 10.4 Hz,
1H, H-20), 7.25–7.18 (m, 2H, H-60, H-5), 5.96 (br s,
1H, OH), 3.99 (s, 3H, OCH3), 3.98 (s, 3H, OCH3).
LRMS (CI): 306.0 ([M+H]+, 88%), 305.0 (M+, 100%).
HRMS (CI): calcd for C15H13O3NFS 306.0595
[M+H]+, found 306.0585.
4.17.6. 5-Fluoro-2-(4-hydroxy-3,5-dimethoxyphenyl)ben-
zothiazole (11h). The product was obtained from 10a
and 4-hydroxy-3,5-dimethoxybenzaldehyde as a yellow
4.17.12. 6-Fluoro-2-(4-hydroxy-3,5-dimethoxyphenyl)-
benzothiazole (11n). The product was obtained from 10b
and 4-hydroxy-3,5-dimethoxybenzaldehyde as a yellow
solid in 78% yield, mp 173–174 ꢁC. H NMR (CDCl3)
d: 7.79 (dd, J = 5.1, 8.8 Hz, 1H, H-4), 7.71 (dd, J = 2.9,
9.5Hz, 1H, H-7), 7.33 (s, 2H, H-20, H-60), 7.14 (dt,
J = 2.2, 8.9 Hz, 1H, H-5), 5.88 (br s, 1H, OH), 4.01 (s,
6H, OCH3). LRMS (CI): 306.0 ([M+H]+, 81%), 305.0
(M+, 100%). HRMS (CI): calcd for C15H13O3NFS
306.0595 [M+H]+, found 306.0584.
1
solid in 71% yield, mp 174–175 ꢁC. H NMR (CDCl3)
d: 7.96 (dd, J = 5.2, 8.8 Hz, 1H, H-7), 7.56 (dd,
J = 2.9, 8.1 Hz, 1H, H-4), 7.30 (s, 2H, H-20, H-60),
7.21 (dt, J = 3.0, 8.9 Hz, 1H, H-6), 5.89 (s, 1H, OH),
4.01 (s, 6H, OCH3). LRMS (CI): 306.0 ([M+H]+,
91%), 305.0 (M+, 100%). HRMS (CI): calcd for
C15H13O3NFS 306.0595 [M+H]+, found 306.0596.
1
4.17.7. 6-Fluoro-2-(3,4-dimethoxyphenyl)benzothiazole
(11i). The product was obtained from 10b and 3,4-
dimethoxybenzaldehyde as a yellow solid in 89% yield,
mp 154–155 ꢁC (lit.5 mp 153–155 ꢁC). 1H NMR
(CDCl3) d: 7.97 (dd, J = 4.4, 8.8 Hz, 1H, H-4), 7.68
(d, J = 1.5 Hz,1H, H-20), 7.58–7.54 (m, 2H, H-60, H-
7), 7.21 (dt, J = 2.9, 8.8 Hz, 1H, H-5), 6.95 (d,
J = 8.8 Hz, 1H, H-50), 4.02 (s, 3H, OCH3), 3.96 (s,
3H, OCH3).
4.18. General method for the preparation of carbon-11
labeled 4-, 5-, and 6-fluorinated 2-arylbenzothiazoles
(3-[11C]6b, 4-[11C]11c, 3-[11C]11c, 5-[11C]11f, 4-[11C]11f,
4-[11C]11i, 3-[11C]11i, 5-[11C]11l, and 4-[11C]11l)
11CO2 was produced by the 14N(p,a)11C nuclear reaction
in small volume (9.5 cm3) aluminum gas target (CTI)

M. Wang et al. / Bioorg. Med. Chem. 14 (2006) 8599–8607
8607
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from 11 MeV proton cyclotron on research purity nitro-
gen (+1% O2) in a Siemens radionuclide delivery system
(Eclipse RDS-111). The phenolic hydroxyl precursor 7,
11d, 11e, 11g, 11h, 11j, 11k, 11m or 11n (0.1–0.3 mg)
was dissolved in CH3CN (300 lL). To this solution was
added 3 N NaOH (2–3 lL). The mixture was transferred
to a small reaction vial. 11CH3OTf that was produced by
the gas-phase production method19 from 11CO2 through
11CH4 and 11CH3Br was passed into the reaction vial,
which was cooled to 0 ꢁC, until radioactivity reached a
maximum (ꢀ2 min), and then the reaction vial was isolat-
ed and heated at 80 ꢁC for 3 min. The contents of the reac-
tion vial were diluted with NaHCO3 (1 mL, 0.1 M). This
solution was passed onto a C-18 cartridge by gas pressure.
The cartridge was washed with H2O (2· 3 mL), and the
aqueous washing was discarded. The product was eluted
from the column with EtOH (2· 3 mL) and then passed
onto a rotatory evaporator. The solvent was removed
by evaporation under vacuum. The labeled product
3-[11C]6b, 4-[11C]11c, 3-[11C]11c, 5-[11C]11f, 4-[11C]11f,
4-[11C]11i, 3-[11C]11i, 5-[11C]11l, or 4-[11C]11l was formu-
lated with saline, whose volume was dependent upon the
use of the labeled product in tissue biodistribution studies
(ꢀ6 mL, 3· 2 mL) or in PET imaging studies (1–3 mL) of
tumor-bearing athymic mice,24 sterile-filtered through a
sterile vented Millex-GS 0.22 lm cellulose acetate mem-
brane, and collected into a sterile vial. Total radioactivity
was assayed and total volume was noted. The overall syn-
thesis, purification, and formulation time was ꢀ20 min
from EOB. The decay corrected yield from 11CO2 was 30–
55%. Retention times in the analytical HPLC system were:
tR(7) = 3.39 min, tR(3-[11C]6b) = 4.51 min; tR(11d) = 3.63
min, tR(4-[11C]11c) = 4.74 min; tR(11e) = 3.52 min,
tR(3-[11C]11c) = 4.74 min; tR(11g) = 3.62 min, tR(5-
[11C]11f) = 4.87 min; tR(11h) = 3.42 min, tR(4-[11C]11f) =
4.87 min;
tR(11j) = 3.65 min,
tR(4-[11C]11i) = 4.77 min;
14. Shi, D.-F.; Bradshaw, T. D.; Wrigley, S.; McCall, C. J.;
Lelieveld, P.; Fichtner, I.; Stevens, M. F. G. J. Med.
Chem. 1996, 39, 3375.
15. Schnur, R. C.; Fliri, A. F. J.; Kajiji, S.; Pollack, V. A.
J. Med. Chem. 1991, 34, 914.
tR(11k) = 3.55 min, tR(3-[11C]11i) = 4.77 min; tR(11m) =
3.62 min, tR(5-[11C]11l) = 4.87 min; tR(11n) = 3.43 min,
tR(4-[11C]11l) = 4.87 min.
´
16. Charris, J.; Monasterios, M.; Domınguez, J.; Infante, W.;
Castro, N. D. Heterocycl. Commun. 2002, 8, 275.
17. Hutchinson, I.; Chua, M.-Z.; Browne, H. L.; Trapani, V.;
Bradshaw, T. D.; Westwell, A. D.; Stevens, M. F. G.
J. Med. Chem. 2001, 44, 1446.
18. Jewett, D. M. Appl. Radiat. Isot. 1992, 43, 1383.
19. Mock, B. H.; Mulholland, G. K.; Vavrek, M. T. Nucl.
Med. Biol. 1999, 26, 467.
20. Zheng, Q.-H.; Mulholland, G. K. Nucl. Med. Biol. 1996,
23, 981.
21. Zheng, Q.-H.; Mock, B. H. Biomed. Chromatogr. 2005, 19,
671.
Acknowledgments
This work was partially supported by the Susan G. Ko-
men Breast Cancer Foundation, Breast Cancer Research
Foundation, NIH/NINDS 5P50NS052606 and Indiana
Genomics Initiative (INGEN) of Indiana University,
which is supported in part by Lilly Endowment Inc.
The authors thank Barbara E. Glick-Wilson and Mi-
chael L. Sullivan for their assistance in production of
11CH3OTf. The referees’ criticisms and editor’s com-
ments for the revision of the manuscript are greatly
appreciated.
22. Zheng, Q.-H.; Liu, X.; Fei, X.; Wang, J.-Q.; Ohannesian,
D. W.; Erickson, L. C.; Stone, K. L.; Hutchins, G. D.
Nucl. Med. Biol. 2003, 30, 405.
23. Zheng, Q.-H.; Mulholland, G. K. Synth. Commun. 2000,
30, 333.
24. Zheng, Q.-H.; Stone, K. L.; Mock, B. H.; Miller, K. D.;
Fei, X.; Liu, X.; Wang, J.-Q.; Glick-Wilson, B. E.;
Sledge, G. W.; Hutchins, G. D. Nucl. Med. Biol. 2002,
29, 803.
References and notes
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