A new high-yield synthetic route to PET CB1 radioligands [ 11C]OMAR and its analogs

Article abstract of DOI:10.1016/j.bmcl.2012.04.030

OMAR analogs reference standards and their corresponding desmethylated precursors were synthesized from substituted anilines either in 4 and 5 steps with 27-32% and 24-31% yield, or in 3 and 4 steps with 21-30% and 19-28% yield, respectively. [¹¹

Full text of DOI:10.1016/j.bmcl.2012.04.030

Bioorganic & Medicinal Chemistry Letters 22 (2012) 3704–3709
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
A new high-yield synthetic route to PET CB1 radioligands [11C]OMAR
and its analogs
⇑
Mingzhang Gao, Min Wang, Qi-Huang Zheng
Department of Radiology and Imaging Sciences, Indiana University School of Medicine, 1345 West 16th Street, Room 202, Indianapolis, IN 46202, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
OMAR analogs reference standards and their corresponding desmethylated precursors were synthesized
from substituted anilines either in 4 and 5 steps with 27–32% and 24–31% yield, or in 3 and 4 steps with
21–30% and 19–28% yield, respectively. [11C]OMAR and its analog radioligands were prepared from their
desmethylated precursors with [11C]CH3OTf through O-[11C]methylation and isolated by HPLC combined
with solid-phase extraction (SPE) in 50–65% radiochemical yields based on [11C]CO2 and decay corrected
Received 5 March 2012
Revised 2 April 2012
Accepted 5 April 2012
Available online 11 April 2012
to end of bombardment (EOB), with 370–740 GBq/
l
mol specific activity at EOB.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
[
11C]OMAR ([11C]JHU75528)
Cannabinoid receptor 1 (CB1)
Radiosynthesis
Automation
Positron emission tomography (PET)
Brain imaging
The endogenous cannabinoid system consists of cannabinoid
receptors, cannabinoid receptor 1 (CB1) and cannabinoid receptor
2 (CB2), and they are G protein-coupled receptors.1 CB1 is found
predominantly in the brain, CB2 is expressed mainly on immune
tissues and cancer cells, and thus cannabinoid receptors are associ-
ated with various brain, cardiovascular and cancer diseases.2,3
Cannabinoid receptors provide an attractive target for the develop-
ment of therapeutic agents, and many CB1 antagonists and CB2
agonists have been developed and described in the literature.4 Can-
nabinoid receptors also provide a particularly attractive target for
the development of imaging agents, and numerous papers have re-
ported the synthesis and evaluation of radioligands for imaging of
CB1 and CB2 receptors using the biomedical imaging technique
positron emission tomography (PET).5,6 In our previous work, we
have developed a series of PET radioligands for imaging CB2 recep-
tor in cancer.7 In this ongoing study, we turn our effort to the
development of CB1 PET radioligands. CB1 is predominantly ex-
pressed in the central nervous system, and involved in a variety
of brain functions and disorders such as schizophrenia and depres-
sion, obesity, drug addiction and alcoholism, and traumatic brain
injury.8 Many CB1 PET radioligands have been developed, and lat-
est promising candidates progressing to human PET studies are
[
11C]OMAR is originally developed and characterized at the John
Hopkins University.12,13 The importance of this compound as a PET
brain imaging agent is great, and broader research investigation to
fully explore and validate the utility of [11C]OMAR-PET is important.
However, the limited commercial availability, complicated syn-
thetic procedure, and high costs of starting materials and precursor
can present an obstacle to more widespread evaluation of this
intriguing agent. Wishing to study this compound in our PET center,
we decided to make our own material by following the literature
methods.12–16 Although several papers12–16 dealing with the syn-
thesis of [11C]OMAR have appeared, there are gaps in synthetic de-
tail among them, and certain key steps gave poor yields or were
difficult to repeat in our hands. Consequently, we investigated alter-
nate approaches and modifications that eventually resulted in a new
high-yield synthesis of [11C]OMAR and its analog radioligands start-
ing from very beginning materials substituted anilines that was
superior to previous works or addressed more synthetic details to
reveal and explain technical tricks. In this Letter we provide com-
plete experiment procedures, yields, analytical details and new
findings for this new high-yield synthetic route, as well as OMAR
analogs reference standards and desmethyled precursors, and pres-
ent a fully automated radiosynthesis of [11C]OMAR and its analog
radioligands with [11C]methyl triflate ([11C]CH3OTf) including a
new radioligand, 1-(2-bromophenyl)-4-cyano-5-(4-[11C]methoxy-
phenyl)-N-(pyrrolidin-1-yl)-1H-pyrazole-3-carboxamide ([11C]5d).
As outlined in Scheme 1, OMAR analogs including 4-cyano-
1-(2,4-dichlorophenyl)-5-(4-methoxyphenyl)-N-(piperidin-1-yl)-
1H-pyrazole-3-carboxamide (OMAR, 5a), 4-cyano-1-(2,4-dichloro-
[
11C]OMAR ([11C]JHU75528) and [18F]FMPEP-d2,5,9–11 as indicated
in Figure 1.
⇑
Corresponding author. Tel.: +1 317 278 4671.
E-mail address: qzheng@iupui.edu (Q.-H. Zheng).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.04.030

M. Gao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3704–3709
3705
N
O
N
NC
N
H
N
O
N
H
F3C
N
H311CO
Cl
O18FCD2F
Cl
[
18F]FMPEP-d2
[
11C]JHU75528 ([11C]OMAR, [11C]5a)
N
O
N
NC
N
N
O
N
N
O
N
H
NC
N
NC
N
H
H
N
H311CO
Cl
N
N
H311CO
H311CO
Br
Br
Cl
11C]JHU75575 ([11C]5c)
[
11C]5b
[
11C]5d
[
Figure 1. Chemical structure of [18F]FMPEP-d2, [11C]OMAR and its analog radioligands.
O
O
Cl
NC
O
O
N
NH2
R2
H
N
N
R1
N
R1
H3CO
R1
i
ii
iii
R2
R2
1a, R1= R2 = Cl
2a, R1= R2 = Cl
3a, R1= R2 = Cl
1b, R1= Br, R2 =H
2b, R1= Br, R2 = H
3b, R1= Br, R2 = H
iv
R3
N
H
O
R3
O
O
NC
N
NC
NC
OH
H
N
v
N
vi
N
N
N
HO
H3CO
R1
N
R1
H3CO
R1
R2
R2
R2
5a, R1 = R2 = Cl, R3 = piperidinyl
5b, R1 = R2 = Cl, R3 = pyrrolidinyl
5c, R1 = Br, R2 = H, R3 = piperidinyl
5d, R1 = Br, R2 = H, R3 = pyrrolidinyl
6a, R1 = R2 = Cl, R3 = piperidinyl
6b, R1 = R2 = Cl, R3 = pyrrolidinyl
6c, R1 = Br, R2 = H, R3 = piperidinyl
6d, R1 = Br, R2 = H, R3 = pyrrolidinyl
4a, R1= R2 = Cl
4b, R1= Br, R2 = H
Scheme 1. Synthesis of OMAR analogs and their corresponding desmethylated precursors. Reagents and yields: (i) HCl, NaNO2; ethyl 2-chloroacetoacetate, NaOAc, EtOH–
H2O, 90–92%; (ii) 4-methoxybenzoylacetonitrile, Et3N, t-BuOH, 60–63%; (iii) KOH, MeOH, 95–96%; (iv) 4-methoxybenzoylacetonitrile, MeONa/MeOH, 49–56%; (v) oxalyl
chloride, DMF (cat.), 1-aminopiperidine or 1-aminopyrrolidine, DIEPA, CH2Cl2, 53–68%; (vi) LiBr, DMF, reflux, 87–95%.
phenyl)-5-(4-methoxyphenyl)-N-(pyrrolidin-1-yl)-1H-pyrazole-3-
carboxamide (5b), 1-(2-bromophenyl)-4-cyano-5-(4-methoxy-
phenyl)-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide (5c), and
1-(2-bromophenyl)-4-cyano-5-(4-methoxyphenyl)-N-(pyrrolidin-
1-yl)-1H-pyrazole-3-carboxamide (5d); and their corresponding
desmethylated precursors 4-cyano-1-(2,4-dichlorophenyl)-5-(4-
hydroxyphenyl)-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide
(6a), 4-cyano-1-(2,4-dichlorophenyl)-5-(4-hydroxyphenyl)-N-(pyrr-
olidin-1-yl)-1H-pyrazole-3-carboxamide (6b), 1-(2-bromophenyl)-
4-cyano-5-(4-hydroxyphenyl)-N-(piperidin-1-yl)-1H-pyrazole-3-
carboxamide (6c), and 1-(2-bromophenyl)-4-cyano-5-(4-hydroxy-
phenyl)-N-(pyrrolidin-1-yl)-1H-pyrazole-3-carboxamide (6d) were
synthesized from substituted anilines either in 4 and 5 steps with
27–32% and 24–31% yield, or in 3 and 4 steps with 21–30% and
19–28% yield, respectively. Ethyl 2-chloro-2-(2-(2,4-dichloro-
phenyl)hydrazono)acetate (2a) and ethyl 2-(2-(2-bromophe-
nyl)hydrazono)-2-chloroacetate (2b) were obtained in a two-step
reaction by conversion of commercially available anilines (1a,b)
into arenediazonium salts followed by reaction with ethyl
2-chloro-acetoacetate in 90–92% yields. According to the published
method,12–14 the key intermediates 3a,b were firstly prepared
through cycloaddition of 4-methoxybenzoylacetonitrile and
compounds 2a,b in presence of NaOEt at reflux in low yields
(13–19%), because under this reaction condition, the 4-cyano

3706
M. Gao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3704–3709
Table 1
group can easily undergo a side reaction carbamoylation yielding
the corresponding 4-carbamoylpyrazoles 3a0,b0 as major by-
products described elsewhere;13 furthermore, we discovered that
the 3-carboxylate group can also easily undergo the hydrolysis to
yield 3-carboxylic acids 4a,b as other by-products, as shown in
Scheme 2. These side reactions resulted in the poor yields of the
key step cycloaddition reaction. Then other reported methods
{N,N-diisopropylethylamine (DIPEA) as base}15,16 were tested, it
was still to produce by-product of 4-carbamoylpyrazole easily
and low yields (6–26%) of 3a,b. We realized that high reaction tem-
perature is not suitable for this reaction because the cyano group is
easy to convert into carbamoyl group under strong base condition
especially at high reaction temperature, and lower temperature
should be better for this reaction. Subsequently other reaction
conditions including several solvents such as MeOH, EtOH, i-PrOH,
t-BuOH, DMF, MeCN, THF, and 1,4-dioxane; and bases such as Et3N,
DIPEA, NaOH, NaOMe, NaOEt, and NaOiPr were also optimized. We
found the optimized reaction conditions were Et3N as base and
t-BuOH as solvent at room temperature (RT), and key intermedi-
ates 3a,b were obtained in higher yields (60–63%), as shown in
Table 1. Et3N/t-BuOH instead of EtONa/EtOH or DIPEA was used
in cycloaddition reaction to avoid side reactions carbamoylation
and hydrolysis and to significantly improve the yield. The ethyl
esters 3a,b were saponified with KOH at RT to give the carboxylic
acids 4a,b in near quantitative yields (95–96%). The alternative
method for preparation of compounds 4a,b was directly from the
reaction of compounds 2a,b with 4-methoxybenzoylacetonitrile
in NaOMe/MeOH at RT in reasonable yields(49–56%). The idea
was derived from the investigation of the cycloaddition reaction
to prepare compounds 3a,b. This method (NaOMe/MeOH instead
of NaOEt/EtOH) combined two steps cycloaddition and hydrolysis
as a single step to shorten the synthetic steps and to increase the
yield as well. It is a two-step reaction, firstly producing 3a,b
through cycloaddition and secondly affording 4a,b by hydrolysis.
The carboxylic acids 4a,b were converted to the carbonyl chloride
intermediates with oxalyl chloride and tiny amount (two drops) of
DMF as the catalyst followed by coupling with 1-aminopiperidine
or 1-aminopyrrolidine to afford reference standard 5a–d in 53–68%
yields. Oxalyl chloride/DMF/CH2Cl2 instead of SOCl2/toluene was
used in coupling reaction to rise up the yield. Desmethylated pre-
cursors 6a–d were successfully obtained in high yields (87–95%)
through direct demethylation of compounds 5a–d using LiBr as
The representative optimized reaction conditions to produce compounds 3a,b
Reaction conditions
Yield (%)
References and notes
12–14
15,16
36
NaOEt/EtOH, reflux
DIPEA, CH3CN, EtOH or t-BuOH, reflux
Et3N, t-BuOH, RT
13–19
6–26
60–63
demethylation agent in DMF.17,18 Attempts to prepare desmethy-
lated precursors for radiolabeling either via demethylation or by
a direct synthesis were failed, and a similar and parallel synthetic
route (multiple-step protective group approach) to firstly prepare
intermediates containing protective group followed by removal
of protective group was employed in the literature.12–14
Synthesis of the target tracer [11C]OMAR and its analog radioli-
gands [11C]5a–d is indicated in Scheme 3. The precursors 6a–d
were labeled by [11C]methyl triflate ([11C]CH3OTf)19,20 through O-
[
11C]methylation21,22 at 80 °C under basic condition (2 N NaOH)
and isolated by a semi-preparative high performance liquid chro-
matography (HPLC) with a C-18 column and a solid-phase extrac-
tion (SPE) with a disposable C-18 Plus Sep-Pak cartridge (a second
purification or isolation process)23–25 to produce the corresponding
pure radiolabeled compound [11C]5a–d in 50–65% radiochemical
yield, decay corrected to end of bombardment (EOB), based on
[
11C]CO2. [11C]CH3OTf is a proven methylation reagent with greater
reactivity than commonly used [11C]methyl iodide ([11C]CH3I),26
and thus, the radiochemical yield of [11C]5 was relatively high.
Addition of NaHCO3 to quench the radiolabeling reaction and to di-
lute the radiolabeling mixture prior to the injection onto the semi-
preparative HPLC column for purification gave better separation of
[
11C]5 from its phenol hydroxyl precursor 6.23–25,27 The radiosyn-
thesis was performed in a home-built automated multi-purpose
11C-radiosynthesis module, allowing measurement of specific
radioactivity during synthesis.28,29 This 11C-radiosynthesis module
includes the overall design of the reaction, purification and
reformulation capabilities of the prototype system. In addition,
11C-tracer specific activity (GBq/mol at EOB) can be automatically
determined prior to product delivery for compounds purified by
the HPLC-portion of the system. Briefly, analysis of the chromato-
graphic data utilized PeakSimple software (SRI Instruments, Las
Vegas, NV). Immediately following elution of the product peak,
the chromatographic data are exported to PeakSimple readable
NH2 O
O
O
NC
O
O
N
N
N
N
H3CO
H3CO
R1
R1
O
Cl
O
O
R2
N
R2
CN
H
N
3a, R1= R2 = Cl
3a', R1= R2 = Cl
H3CO
R1
3b, R1= Br, R2 = H
3b', R1= Br, R2 = H
NaOEt/EtOH
O
R2
NC
OH
2a, R1= R2 = Cl
2b, R1= Br, R2 = H
N
N
H3CO
R1
R2
4a, R1= R2 = Cl
4b, R1= Br, R2 = H
Scheme 2. Cycloaddition reaction and subsequent side reactions carbamoylation and hydrolysis in NaOEt/EtOH at reflux.

M. Gao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3704–3709
3707
O
N
R3
H
R3
O
N
NC
N
NC
N
H
N
N
i
H311CO
HO
R1
R1
R2
R2
6a, R1 = R2 = Cl, R3 = piperidinyl
6b, R1 = R2 = Cl, R3 = pyrrolidinyl
[
[
[
[
11C]5a, R1 = R2 = Cl, R3 = piperidinyl
11C]5b, R1 = R2 = Cl, R3 = pyrrolidinyl
11C]5c, R1 = Br, R2 = H, R3 = piperidinyl
11C]5d, R1 = Br, R2 = H, R3 = pyrrolidinyl
6c, R1 = Br, R2 = H, R3 = piperidinyl
6d, R1 = Br, R2 = H, R3 = pyrrolidinyl
Scheme 3. Synthesis of [11C]OMAR and its analog radioligands. Reagents, conditions and yields: (i) [11C]CH3OTf, CH3CN, 2 N NaOH, 80 °C, 3 min, 50–65%.
files, and the area of the radioactivity peak is converted to mCi at
EOB by comparison to a reference calibration curve previously con-
structed using the same detector, loop and flow rate. The mass
peak from the UV chromatogram (without decay correction) is
similarly compared to a standard curve made at the same UV
wavelength, mobile phase and flow rate. Simple division of the to-
tal EOB radioactivity peak (in mCi) by the total mass peak (in nmo-
od.31–34 Briefly, the chromatographic capacity factors (k0) were
measured by reversed-phase HPLC. k0 = (tRꢀtR0)/tR0, where tR is
the compound’s retention time, and tR0 is retention of an unre-
tained substance determined by injection of an aqueous solution
of potassium nitrite (tR0 = 1.84 min). LogP measurement is based
on the linear relationship, which has been established between
the Log k0 values of most compounds and their LogP values. Four
compounds, benzyl alcohol (LogP 1.16), acetophenone (LogP
1.66), toluene (LogP 2.74) and naphthalene (LogP 3.37), were
chosen as a ‘standard’ calibration mixture for the evaluation of
the LogP’s of unknowns. LogPexp = partition coefficient calculated
from k0 value and calibration curve established by these four
compounds. Calibration equation: LogP = 2.222 Log k0 + 1.915.
Retention times in the analytical HPLC system for [11C]5a–d were
5.11, 4.17, 4.56 and 3.92 min, respectively. The calculation results
showed the LogP values for [11C]5a–d were 2.47, 2.14, 2.29 and
2.03, respectively. The results indicate [11C]OMAR ([11C]5a) is
more lipophilic than [11C]5b, [11C]5c, or [11C]5d; and new com-
pound [11C]5d is less lipophilic than [11C]5a, [11C]5b, or [11C]5c.
The calculated LogP values from ChemDraw Ultra 9.0 (ChemOf-
fice 2005) for [11C]5a–d are 5.14, 4.72, 4.85 and 4.43, respectively.
Obviously, the measured LogP data sequence of [11C]5a–d is con-
sistent with the calculated LogP data sequence of [11C]5a–d. In
general, the LogP values (C18 HPLC) for the compounds expected
to enter the brain readily are in a range of 1–3.35 The data suggest
all compounds [11C]5a–d have suitable lipophilicity. Therefore,
we assume the new tracer [11C]5d has suitable ability to pass
the BBB, and is a promising candidate as potential brain imaging
agent.
The experimental details and characterization data for com-
pounds 2a,b–6a–d and for the tracers [11C]5a–d are given.36
In summary, a new high-yield synthetic route to PET CB1 radio-
ligands [11C]OMAR and its analogs has been developed. This new
synthetic approach provided OMAR analogs and their correspond-
ing desmethylated precursors in high overall chemical yields. An
automated self-designed multi-purpose [11C]-radiosynthesis mod-
ule for the synthesis of [11C]OMAR and its analogs has been built,
featuring the measurement of specific activity by the on-the-fly
technique. The radiosynthesis employed O-[11C]methylation radio-
labeling on oxygen position of the precursor. Radiolabeling proce-
dures incorporated efficiently with the most commonly used
les) gives specific activity at EOB in Ci/lmol. The overall synthesis,
purification and reformulation time was 30–40 min from EOB. The
specific radioactivity was in a range of 370–740 GBq/mol at EOB.
Chemical purity and radiochemical purity were determined by
analytical HPLC.30 The chemical purity of the precursor 6a–d and
reference standard 5a–d was >96%. The radiochemical purity of
the target tracer [11C]5a–d was >99% determined by radio-HPLC
through c-ray (PIN diode) flow detector, and the chemical purity
of [11C]5a–d was >93% determined by reverse-phase HPLC through
UV flow detector. A C-18 Plus Sep-Pak cartridge was used to signif-
icantly improve the chemical purity of the tracer solution.23–25,30
The chemical purity of the
[
11C]5a–d tracer solution with
Sep-Pak purification was increased higher 10–20% than that with-
out Sep-Pak purification.23–25
The target compounds 5a–c are known compound,12,13 and the
target compound 5d is a new compound. In view of the structure-
activity relationship (SAR) data reported in the literature,13 as
summarized in Table 2, compound 5b (pyrrolidinyl) has better
binding affinity than compound 5a (piperidinyl), and compound
5c (Br) has better binding affinity than compound 5a (Cl). There-
fore we can predict/hypothesize new compound 5d will exhibit
the best binding affinity in this series of compounds 5a–d. Fur-
ther SAR study of the substituent effect is currently under way
to confirm this hypothesis. The measured HPLC lipophilicity coef-
ficient (octanol-water partition coefficient, LogP) is an important
parameter in selecting PET brain radioligand candidates (small or-
ganic molecules) for further evaluations.31–34 The ability of a
brain radioligand to penetrate the blood brain barrier (BBB) could
be due, at least in part, to its lipophilicity, and thus we were
interested in identifying an analog that was significantly lipo-
philic brain tracer. We calculated LogP values of [11C]5a–d based
on their retention times that were measured by C-18 HPLC meth-
[
11C]methylating agent, [11C]CH3OTf, produced by gas-phase pro-
Table 2
The binding affinity (Ki, nM) and lipophilicity (LogP). nd: not determined
duction of [11C]methyl bromide ([11C]CH3Br) from our laboratory.
The target tracers were isolated and purified by a semi-preparative
HPLC combined with SPE procedure in high radiochemical yields,
short overall synthesis time, and high specific activity. These re-
sults facilitate the potential preclinical and clinical PET studies of
Compound Ki
(nM)a
LogP calculated from
ChemOffice
LogP measured by
HPLC
5a
5b
5c
5d
11
7
5.14
2.47
2.14
2.29
2.03
2.0, 3.7 4.72
4.7
nd
[
11C]OMAR and its analog radioligands as brain CB1 imaging agents
4.85
4.43
in animals and humans.
a
Data cited from the literature.13

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M. Gao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3704–3709
an Agilent system, consisting of an 1100 series HPLC connected to a diode
Acknowledgments
array detector and a 1946D mass spectrometer configured for positive-ion/
negative-ion electrospray ionization. The high resolution mass spectra
(HRMS) were obtained using a Waters/Micromass LCT Classic spectrometer.
Chromatographic solvent proportions are indicated as volume: volume ratio.
Thin-layer chromatography (TLC) was run using Analtech silica gel GF
uniplates (5 ꢁ 10 cm2). Plates were visualized under UV light. Normal phase
flash column 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 air-sensitive reactions were
performed under a positive pressure of nitrogen maintained by a direct line
This work was partially supported by Indiana State Department
of Health (ISDH) Indiana Spinal Cord & Brain Injury Fund (ISDH
EDS-A70-2-079612). 1H NMR and 13C NMR spectra were recorded
on a Bruker Avance II 500 MHz NMR spectrometer in the Depart-
ment of Chemistry and Chemical Biology at Indiana University
Purdue University Indianapolis (IUPUI), which is supported by a
NSF-MRI Grant CHE-0619254.
from
(Phenomenex) 5
MeOH/20 mM, pH 6.7 phosphate (buffer solution); flow rate 1.5 mL/min; and
a
nitrogen source. Analytical HPLC was performed using
a Prodigy
lm C-18 column, 4.6 ꢁ 250 mm; mobile phase 3:1:1 CH3CN/
References and notes
UV (254 nm) and
performed using
c
-ray (PIN diode) flow detectors. Semi-preparative HPLC was
Prodigy (Phenomenex) C-18 column, 12 nm,
a
5 lm
1. Miller, L. K.; Devi, L. A. Pharmacol. Rev. 2011, 63, 461.
2. Talwar, R.; Potluri, V. K. CNS Neurol. Disord. Drug Targets 2011, 10, 536.
3. Basu, S.; Dittel, B. N. Immunol. Res. 2011, 51, 26.
10 ꢁ 250 mm; 3:1:1 CH3CN/MeOH/20 mM, pH 6.7 phosphate (buffer
solution) mobile phase; 4.0 mL/min flow rate; UV (254 nm) and -ray (PIN
diode) flow detectors. C18 Plus Sep-Pak cartridges were obtained from Waters
Corporation (Milford, MA). Sterile Millex-FG 0.2 m filter units were obtained
c
4. Pertwee, R. G. Curr. Med. Chem. 2010, 17, 1360.
l
5. Horti, A. G.; Van Laere, K. Curr. Pharm. Des. 2008, 14, 3363.
6. Evens, N.; Bormans, G. M. Curr. Top. Med. Chem. 2010, 10, 1527.
7. Gao, M.; Wang, M.; Miller, K. D.; Hutchins, G. D.; Zheng, Q.-H. Bioorg. Med.
Chem. 2010, 18, 2099.
8. Shohami, E.; Cohen-Yeshurun, A.; Magid, L.; Algali, M.; Mechoulam, R. Br. J.
Pharmacol. 2011, 163, 1402.
9. Wong, D. F.; Kuwabara, H.; Horti, A. G.; Raymont, V.; Brasic, J.; Guevara, M.; Ye,
W.; Dannals, R. F.; Ravert, H. T.; Nandi, A.; Rahmim, A.; Ming, J. E.; Grachev, I.;
Roy, C.; Cascella, N. Neuroimage 2010, 52, 1505.
10. Horti, A. G.; Fan, H.; Kuwabara, H.; Hilton, J.; Ravert, H. T.; Holt, D. P.;
Alexander, M.; Kumar, A.; Rahamim, A.; Scheffel, U.; Wong, D. F.; Dannals, R. F.
J. Nucl. Med. 2006, 47, 1689.
11. Terry, G. E.; Hirvonen, J.; Liow, J.-S.; Zoghbi, S. S.; Gladding, R.; Tauscher, J. T.;
Schaus, J. M.; Phebus, L.; Felder, C. C.; Morse, C. L.; Donohue, S. R.; Pike, V. W.;
Halldin, C.; Innis, R. B. J. Nucl. Med. 2010, 51, 112.
12. Fan, H.; Ravert, H. T.; Holt, D. P.; Dannals, R. F.; Horti, A. G. J. Labelled Compd.
Radiopharm. 2006, 1021, 49.
13. Fan, H.; Kotsikorou, E.; Hoffman, A. F.; Ravert, H. T.; Holt, D.; Hurst, D. P.;
Lupica, C. R.; Reggio, P. H.; Dannals, R. F.; Horti, A. G. Eur. J. Med. Chem. 2009, 44,
593.
from Millipore Corporation (Bedford, MA).; (b) Ethyl 2-chloro-2-(2-(2,4-
dichlorophenyl)hydrazono)acetate (2a). A mixture of 2,4-dichloroaniline (1a)
(7.29 g, 45 mmol) in 24% HCl (75 mL, 0.52 mol) and water (180 mL) was stirred
for 1.5 h at RT. A solution of sodium nitrite (3.26 g, 47 mmol) in water (30 mL)
was added dropwise to the reaction mixture at 0 °C, and the reaction was
stirred for 45 min at 0 °C. Then the resulting solution was treated with NaOAc
(3.69 g, 45 mmol) and subsequently with ethyl 2-chloro-acetoacetate (7.41 g,
45 mmol) in ethanol (450 mL) at 0 °C. The reaction temperature was keep at
0 °C for 1.5 h and RT for 1.5 h. The resulting precipitate was filtered, washed
with cold water and dried to give 2a (12.3 g, 92%), as a white solid, mp 88–
90 °C. 1H NMR (CDCl3) : 1.41 (t, J = 7.0 Hz, 3H, CH3), 4.39 (q, J = 7.0 Hz, 2H, CH2),
7.25 (ddd, J = 0.5, 2.0, 7.5 Hz, 1H, Ph-H), 7.35 (d, J = 2.0 Hz, 1H, Ph-H), 7.55 (d,
J = 8.5 Hz, 1H, Ph-H), 8.73 (s, 1H, NH). MS (ESI): 295 ([M+H]+, 100%).; (c) Ethyl
2-(2-(2-bromophenyl)hydrazono)-2-chloroacetate (2b). Compound 2b was
prepared from 2-bromoaniline (1b) and ethyl 2-chloro-acetoacetate with the
same procedure described for 2a in 90% yield. White solid, mp 100–102 °C. 1
H
NMR (CDCl3) : 1.41 (t, J = 7.0 Hz, 3H, CH3), 4.39 (q, J = 7.0 Hz, 2H, CH2), 6.90–
6.91 (m, 1H, Ph-H), 7.31–7.34 (m, 1H, Ph-H), 7.49 (dd, J = 1.5, 8.0 Hz, 1H, Ph-H),
7.60 (dd, J = 1.5, 8.0 Hz, 1H, Ph-H), 8.85 (s, 1H, NH). MS (ESI): 305 ([M+H]+,
85%), 307 ([M+3H]+, 100%).; (d) Ethyl 4-cyano-1-(2,4-dichlorophenyl)-
14. Cheng, L.; Jonforsen, M.; Schell, P. WO Patent No. 010217 A1, 2007.
15. Donohue, S. R.; Varnas, K.; Jia, Z.; Gulyas, B.; Pike, V. W.; Halldin, C. Bioorg. Med.
Chem. Lett. 2009, 19, 6209.
16. Donohue, S. R.; Dannals, R. F.; Halldin, C.; Pike, V. W. J. Med. Chem. 2011, 54,
2961.
17. Wang, J.-Q.; Gao, M.; Miller, K. D.; Sledge, G. W.; Zheng, Q.-H. Bioorg. Med.
Chem. Lett. 2006, 16, 4102.
18. Wang, M.; Gao, M.; Miller, K. D.; Sledge, G. W.; Hutchins, G. D.; Zheng, Q.-H. J.
Labelled Compd. Radiopharm. 2008, 51, 6.
19. Jewett, D. M. Int. J. Radiat. Appl. Instrum. A 1992, 43, 1383.
20. Mock, B. H.; Mulholland, G. K.; Vavrek, M. J. Nucl. Med. Biol. 1999, 26, 467.
21. Wang, M.; Yoder, K. K.; Gao, M.; Mock, B. H.; Xu, X.-M.; Saykin, A. J.; Hutchins,
G. D.; Zheng, Q.-H. Bioorg. Med. Chem. Lett. 2009, 19, 5636.
22. Wang, M.; Gao, M.; Hutchins, G. D.; Zheng, Q.-H. Eur. J. Med. Chem. 2009, 44,
2748.
23. Gao, M.; Wang, M.; Mock, B. H.; Glick-Wilson, B. E.; Yoder, K. K.; Hutchins, G.
D.; Zheng, Q.-H. Appl. Radiat. Isot. 2010, 1079, 68.
24. Wang, M.; Gao, M.; Miller, K. D.; Zheng, Q.-H. Steroids 2011, 76, 1331.
25. Wang, M.; Gao, M.; Miller, K. D.; Sledge, G. W.; Zheng, Q.-H. Bioorg. Med. Chem.
Lett. 2012, 22, 1569.
26. Allard, M.; Fouquet, E.; James, D.; Szlosek-Pinaudm, M. Curr. Med. Chem. 2008,
15, 235.
27. Gao, M.; Wang, M.; Hutchins, G. D.; Zheng, Q.-H. Appl. Radiat. Isot. 1891, 2008,
66.
28. Mock, B. H.; Zheng, Q.-H.; DeGrado, T. R. J. Labelled Compd. Radiopharm. 2005,
48, S225.
29. Mock, B. H.; Glick-Wilson, B. E.; Zheng, Q.-H.; DeGrado, T. R. J. Label. Compd.
Radiopharm. 2005, 48, S224.
30. Zheng, Q.-H.; Mock, B. H. Biomed. Chromatogr. 2005, 19, 671.
31. Gao, M.; Wang, M.; Hutchins, G. D.; Zheng, Q.-H. Bioorg. Med. Chem. Lett. 2010,
20, 2529.
32. Haky, J. E.; Young, A. M. J. Liq. Chromatogr. 1984, 7, 675.
33. Fei, X.; Zheng, Q.-H. J. Liq. Chromatogr. Related Technol. 2005, 28, 939.
34. Gao, M.; Mock, B. H.; Hutchins, G. D.; Zheng, Q.-H. Nucl. Med. Biol. 2005, 32, 543.
35. Mathis, C. A.; Wang, Y.; Holt, D. P.; Huang, G.-F.; Debnath, M. L.; Klunk, W. E. J.
Med. Chem. 2003, 46, 2740.
5-(4-methoxyphenyl)-1H-pyrazole-3-carboxylate (3a).
A
mixture of 4-
methoxybenzoylacetonitrile (1.75 g, 10.0 mmol), compound 2a (3.31 g, 11.2
mmol), Et3N (4.04 g, 40.0 mmol) in t-BuOH (120 mL) was stirred at RT for 22 h.
The resulting precipitate was filtered, washed with solvent t-BuOH, dried in air,
then washed with some water, and dried to give white solid product 3a
(2.13 g). The filtrate was evaporated and residue was extracted with EtOAc
(3 ꢁ 60 mL), washed with brine, dried over Na2SO4, and concentrated. The
crude product was purified by column chromatography on silica gel with
eluent (1:9 EtOAc/hexanes) to afford white solid compound 3a (0.49 g).
Combined two parts of product gave 3a (2.62 g, 63%) as a white solid, Rf = 0.30
(1:3 EtOAc/hexanes), mp 157–159 °C. 1H NMR (CDCl3) : 1.46 (t, J = 7.0 Hz, 3H,
CH3), 3.81 (s, 3H, OCH3), 4.51 (q, J = 7.0 Hz, 2H, CH2), 6.87 (dt, J = 2.0, 9.0 Hz, 2H,
Ph-H), 7.24 (dt, J = 2.0, 9.0 Hz, 2H, Ph-H), 7.35–7.42 (m, 2H, Ph-H), 7.45
(d, J = 2.0 Hz, 1H, Ph-H). MS (ESI): 416 ([M+H]+, 100%).; (e) Ethyl 1-(2-
bromophenyl)-4-cyano-5-(4-methoxyphenyl)-1H-pyrazole-3-carboxylate
(3b). Compound 3b was prepared from compound 2b and 4-
methoxybenzoylacetonitrile with the same procedure described for 3a,
except the reaction time was 45 h, in 60% yield. White solid, Rf = 0.30 (1:3
EtOAc/hexanes), mp 165–167 °C. 1H NMR (CDCl3) : 1.46 (t, J = 7.0 Hz, 3H, CH3),
3.79 (s, 3H, OCH3), 4.51 (q, J = 7.0 Hz, 2H, CH2), 6.85 (dt, J = 2.0, 9.0 Hz, 2H, Ph-
H), 7.26 (dt, J = 2.0, 9.0 Hz, 2H, Ph-H), 7.34–7.37 (m, 1H, Ph-H), 7.41–7.45 (m,
2H, Ph-H), 7.61 (dd, J = 1.0, 8.0 Hz, 1H, Ph-H). MS (ESI): 426 ([M+H]+, 100%).; (f)
4-Cyano-1-(2,4-dichlorophenyl)-5-(4-methoxyphenyl)-1H-pyrazole-3-
carboxylic acid (4a). Method A (from 3a): A mixture of compound 3a (0.83 g,
2.0 mmol) and KOH (0.90 g) in methanol (60 mL) was stirred at RT for 24 h, and
TLC was used to monitor the reaction. After the solvent was removed in vacuo,
the residue was added a little bit of water, and residual aqueous solution was
adjusted to pH 6 with HCl (1 N). The resulting precipitate was filtered, washed
with cold water, and dried to obtain 4a (0.74 g, 96%) as a white solid, Rf = 0.66
(1:3 MeOH/CH2Cl2), mp 280–282 °C. 1H NMR (acetone-d6) : 3.83 (s, 3H, OCH3),
7.00 (dt, J = 2.5, 9.0 Hz, 2H, Ph-H), 7.37 (dt, J = 2.5, 9.0 Hz, 2H, Ph-H), 7.62 (dd,
J = 2.5, 8.5 Hz, 1H, Ph-H), 7.69 (d, J = 2.5 Hz, 1H, Ph-H), 7.85 (d, J = 8.5 Hz, 1H,
Ph-H). MS (ESI): 386 ([MꢀH]ꢀ, 10%), 342 (100%); 388 ([M+H]+, 80%). Method B
(from 2a): 25% NaOMe solution (21.6 g, 100 mmol) was added slowly to a
mixture of 4-methoxybenzoylacetonitrile (1.40 g, 8.0 mmol), compound 2a
(2.65 g, 8.96 mmol) in methanol (100 mL) at RT, and the reaction mixture was
stirred for 16 h. Additional 25% NaOMe (2.16 g, 10 mmol) was added to the
reaction mixture. Then the reaction mixture was stirred at RT for another 16 h.
After the solvent methanol was evaporated under reduced pressure, the
residue was added a little bit of water, and residual aqueous solution was
adjusted to pH 6 with HCl (1 N), extracted with EtOAc (3 ꢁ 80 mL), washed
with brine, dried over MgSO4, and concentrated. The crude product was
purified by column chromatography on silica gel with eluent (1:15 MeOH/
CH2Cl2) to afford 4a (1.52 g, 49%) as a white solid. The analytical data were
same with the data from Method A.; (g) 1-(2-Bromophenyl)-4-cyano-5-(4-
36. (a) General. All commercial reagents and solvents were purchased from
Sigma–Aldrich and Fisher Scientific, and used without further purification.
[
11C]CH3OTf was prepared according to
a literature procedure.20 Melting
points were determined on a MEL-TEMP II capillary tube apparatus and were
uncorrected. 1H NMR and 13C NMR spectra were recorded at 500 and 125 MHz,
respectively, on
a Bruker Avance II 500 MHz NMR spectrometer using
tetramethylsilane (TMS) as an internal standard. Chemical 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 constants (J) were reported in hertz
(Hz). Liquid chromatography-mass spectra (LC–MS) analysis was performed on
methoxyphenyl)-1H-pyrazole-3-carboxylic acid (4b). Method
A (from 3b):

M. Gao et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3704–3709
3709
Compound 4b was prepared from 3b with the same procedure described for 4a
in 95% yield. Yellowish solid, Rf = 0.66 (1:3 MeOH/CH2Cl2), mp 297–299 °C. 1H
NMR (DMSO-d6) : 3.74 (s, 3H, OCH3), 6.95 (dt, J = 2.5, 9.0 Hz, 2H, Ph-H), 7.24
(dt, J = 2.5, 9.0 Hz, 2H, Ph-H), 7.44 (td, J = 1.5, 7.5 Hz, 1H, Ph-H), 7.52 (td, J = 1.5,
7.5 Hz, 1H, Ph-H), 7.71 (dd, J = 1.5, 4.0 Hz, 1H, Ph-H), 7.73 (dd, J = 1.5, 4.0 Hz,
1H, Ph-H). MS (ESI): 398 ([M+H]+, 100%). Method B (from 2b): Compound 4b
was prepared from 2b with the same procedure described for 4a in 56% yield.
The analytical data were same with the data from Method A.; (h) General
procedure for preparation of OMAR and its analogs (5a–d). DMF (2 drops) and
oxalyl chloride (2.5 mL) were added to a stirring solution of compound 4a
(512 mg, 1.32 mmol) (or 1.32 mmol 4b) in CH2Cl2 (40 mL). After bubbling had
pyrazole-3-carboxamide (6a). Mp 159–161 °C. 1H NMR (acetone-d6) : 1.42 (br
s, 2H, piperidine-H), 1.64–1.68 (m, 4H, piperidine-H), 2.93–2.94 (m, 4H,
piperidine-H), 6.89 (dd, J = 2.0, 8.0 Hz, 2H, Ph-H), 7.27 (dd, J = 2.0, 8.0 Hz, 2H,
Ph-H), 7.63 (dd, J = 2.0, 8.5 Hz, 1H, Ph-H), 7.71 (d, J = 2.5Hz, 1H, Ph-H), 7.82 (d,
J = 8.5 Hz, 1H, Ph-H), 8.59 (1H, OH), 9.08 (s, 1H, CONH). MS (ESI): 456 ([M+H]+,
53%). 4-Cyano-1-(2,4-dichlorophenyl)-5-(4-hydroxyphenyl)-N-(pyrrolidin-1-
yl)-1H-pyrazole-3-carboxamide (6b). Mp 279 °C (decomposed). 1H NMR
(DMSO-d6) : 1.74 (br s, 4H, pyrrolidine-H), 2.90 (br s, 4H, pyrrolidine-H),
6.80 (dt, J = 2.5, 8.5 Hz, 2H, Ph-H), 7.16 (dt, J = 2.5, 8.5 Hz, 2H, Ph-H), 7.65 (dd,
J = 2.0, 8.5 Hz, 1H, Ph-H), 7.85(d, J = 2.0 Hz, 1H, Ph-H), 7.88 (d, J = 8.5 Hz, 1H, Ph-
H), 9.65 (s, 1H, OH), 10.12 (s, 1H, CONH). MS (ESI): 442 ([M+H]+, 50%). 1-(2-
Bromophenyl)-4-cyano-5-(4-hydroxyphenyl)-N-(piperidin-1-yl)-1H-pyrazole-
3-carboxamide (6c). Mp 239-241 °C. 1H NMR (acetone-d6) : 1.41–1.43 (m, 2H,
piperidine-H), 1.62–1.68 (m, 4H, piperidine-H), 2.92 (t, J = 5.5 Hz, 4H,
piperidine-H), 6.87 (dt, J = 2.5, 9.0 Hz, 2H, Ph-H), 7.27 (dt, J = 2.5, 9.0 Hz, 2H,
Ph-H), 7.52 (td, J = 1.5, 7.5 Hz, 1H, Ph-H), 7.60 (td, J = 1.5, 7.5 Hz, 1H, Ph-H), 7.76
(dd, J = 1.5, 3.5 Hz, 1H, Ph-H), 7.77 (dd, J = 1.5, 3.5 Hz, 1H, Ph-H), 8.60 (s, 1H,
OH), 9.05 (br s, 1H, CONH). MS (ESI): 466 ([M+H]+, 100%). 1-(2-Bromophenyl)-
4-cyano-5-(4-hydroxyphenyl)-N-(pyrrolidin-1-yl)-1H-pyrazole-3-carbox-
ceased (about 1–2 h), the reaction mixture was concentrated in vacuo.
A
solution of 1-aminopiperidine (145 mg, 1.45 mmol) (or 1.45 mmol 1-
aminopyrrolidine) and DIEPA (0.43 g, 3.3 mmol) in CH2Cl2 (10 mL) was
added to above freshly prepared acid chloride solution in CH2Cl2 (20 mL) at
0 °C and then stirred at RT for 2.5 h. The volatile compounds were removed in
vacuo and residue was extracted with CH2Cl2 (3 ꢁ 60 mL), washed with brine,
dried over Na2SO4, and concentrated. The crude residue was purified by
column chromatography on silica gel with eluent (1:4 EtOAc/hexanes) to give 5
as
a
white solid in 53–68% yield. 4-Cyano-1-(2,4-dichlorophenyl)-5-(4-
amide (6d). Mp 161–163 °C. 1H NMR (acetone-d6)
: 1.83–1.84 (m, 4H,
methoxyphenyl)-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide
(5a).
pyrrolidine-H), 3.05 (br s, 4H, pyrrolidine-H), 6.87 (dd, J = 2.0, 7.0 Hz, 2H, Ph-
H), 7.27 (dd, J = 2.0, 7.0 Hz, 2H, Ph-H), 7.53 (td, J = 1.5, 8.0 Hz, 1H, Ph-H), 7.61
(td, J = 1.5, 7.5 Hz, 1H, Ph-H), 7.76 (t, J = 1.5 Hz, 1H, Ph-H), 7.77 (t, J = 1.5 Hz, 1H,
Ph-H), 8.59 (s, 1H, OH), 9.00 (s, 1H, CONH). 13C NMR (DMSO-d6) : 23.15, 54.52,
54.59, 93.40, 113.68, 116.61, 117.84, 122.34, 129.71, 131.56, 131.70, 133.00,
134.41, 138.92, 148.42, 152.84, 158.21. MS (ESI): 452 ([M+H]+, 80%). HRMS
(ESI) calcd for C21H19N5O2Br 452.0722 ([M+H]+); found 452.0724.; (j) General
procedure for preparation of [11C]OMAR and its analog radioligands ([11C]5a–
d). [11C]CO2 was produced by the 14N(p,)11C nuclear reaction in the small
volume (9.5 cm3) aluminum gas target provided with the Siemens RDS-111
Eclipse cyclotron. The target gas consisted of 1% oxygen in nitrogen purchased
as a specialty gas from Praxair, Indianapolis, IN. Typical irradiations used for
the development were 50 A beam current and 15 min on target. The
production run produced approximately 25.9 GBq of [11C]CO2 at EOB. In a
small reaction vial (5 mL), the precursor 6 (0.3–0.5 mg) was dissolved in CH3CN
Rf = 0.30 (1:1 EtOAc/hexanes), mp 168–170 °C. 1H NMR (CDCl3) : 1.43 (br s,
2H, piperidine-H), 1.68–1.77 (m, 4H, piperidine-H), 2.90 (br s, 4H, piperidine-
H), 3.80 (s, 3H, OCH3), 6.86 (dt, J = 2.5, 9.0 Hz, 2H, Ph-H), 7.22 (dt, J = 2.5, 9.0 Hz,
2H, Ph-H), 7.36–7.40 (m, 2H, Ph-H), 7.49 (d, J = 1.5 Hz, 1H, Ph-H), 7.53 (s, 1H,
CONH). MS (ESI): 468 ([MꢀH]ꢀ, 100%). 4-Cyano-1-(2,4-dichlorophenyl)-5-(4-
methoxyphenyl)-N-(pyrrolidin-1-yl)-1H-pyrazole-3-carboxamide
(5b).
Rf = 0.25 (1:1 EtOAc/hexanes), mp 125–127 °C. 1H NMR (CDCl3) : 1.89–1.92
(m, 4H, pyrrolidine-H), 3.05 (br s, 4H, pyrrolidine-H), 3.80 (s, 3H, OCH3), 6.86
(dt, J = 2.5, 9.0 Hz, 2H, Ph-H), 7.22 (dt, J = 2.5, 9.0 Hz, 2H, Ph-H), 7.36–7.40 (m,
2H, Ph-H), 7.48 (dd, J = 0.5, 2.0 Hz, 1H, Ph-H), 7.54 (br s, 1H, CONH). MS (ESI):
456 ([M+H]+, 60%). 1-(2-Bromophenyl)-4-cyano-5-(4-methoxyphenyl)-N-
(piperidin-1-yl)-1H-pyrazole-3-carboxamide (5c). Rf = 0.29 (1:1 EtOAc/
Hexanes), mp 254–256 °C. 1H NMR (CDCl3) : 1.43 (br s, 2H, piperidine-H),
1.73-1.77 (m, 4H, piperidine-H), 2.90 (br s, 4H, piperidine-H), 3.79 (s, 3H,
OCH3), 6.83 (dt, J = 2.5, 9.0 Hz, 2H, Ph-H), 7.24 (dt, J = 2.5, 9.0 Hz, 2H, Ph-H),
7.36–7.44 (m, 3H, Ph-H), 7.57 (br s, 1H, CONH), 7.65 (dd, J = 1.5, 8.0 Hz, 1H, Ph-
H). MS (ESI): 480 ([M+H]+, 80%), 482 ([M+3H]+, 100%); 478 ([MꢀH]ꢀ, 100%). 1-
(2-Bromophenyl)-4-cyano-5-(4-methoxyphenyl)-N-(pyrrolidin-1-yl)-1H-
pyrazole-3-carboxamide (5d). Rf = 0.23 (1:1 EtOAc/hexanes), mp 229–
231 °C. 1H NMR (DMSO-d6) : 1.72 (br s, 4H, pyrrolidine-H), 2.89 (br s, 4H,
pyrrolidine-H), 3.73 (s, 3H, OCH3), 6.97 (dd, J = 2.0, 7.0 Hz, 2H, Ph-H), 7.28
(dd, J = 2.0, 7.0 Hz, 2H, Ph-H), 7.46 (td, J = 1.5, 8.0 Hz, 1H, Ph-H), 7.54 (td,
J = 1.5, 8.0 Hz, 1H, Ph-H), 7.73 (dd, J = 1.5, 8.0 Hz, 1H, Ph-H), 7.84 (dd,
J = 1.5, 8.0 Hz, 1H, Ph-H), 9.63 (s, 1H, CONH). 13C NMR (DMSO-d6) : 21.91,
53.34, 55.30, 92.11, 113.18, 114.40, 117.37, 121.22, 128.83, 130.50,
130.88, 132.44, 133.22, 137.09, 147.34, 151.35, 157.02, 160.75. MS
(ESI): 466 ([M+H]+, 62%). HRMS (ESI) calcd for C22H21N5O2Br 466.0879
([M+H]+); found 466.0863.; (i) General procedure for preparation of the
desmethylated precursors of OMAR and its analogs (6a–d). Lithium
(400 L). To this solution was added 2 N NaOH (2 lL). No carrier-added (high
specific activity) [11C]CH3OTf that was produced by the gas-phase production
method20 from [11C]CO2 through [11C]CH4 and [11C]CH3Br with silver triflate
(AgOTf) column was passed into the reaction vial at RT, until radioactivity
reached a maximum (ꢂ2 min), and then the reaction vial was isolated and
heated at 80 °C for 3 min. The contents of the reaction vial were diluted with
NaHCO3 (0.1 M, 1 mL), and injected onto the semi-preparative HPLC column
with 3 mL injection loop for purification. The product fraction was collected in
a recovery vial containing 30 mL water. The diluted tracer solution was then
passed through
(5 mL ꢁ 4). The cartridge was eluted with EtOH (1 mL ꢁ 2), followed by 10
mL saline, to release
11C]5. The eluted product was then sterile-filtered
a C-18 Sep-Pak Plus cartridge, and washed with water
[
through a Millex-FG 0.2 m membrane into a sterile vial. Total radioactivity was
assayed and total volume was noted for tracer dose dispensing. Retention
times in the semi-preparative HPLC system were: tR 6a = 6.53 min, tR
bromide (0.86 g, 10 mmol) was added to
a
solution of compound
5
5a = 9.81 min, tR
11C]5b = 8.43 min; tR 6c = 6.02 min, tR 5c = 8.78 min, tR
and tR 6d = 5.13 min, tR 5d = 7.83 min, tR
11C]5d = 7.83 min. Retention times in
the analytical HPLC system were: tR 6a = 3.12 min, tR 5a = 5.11 min, tR
[
11C]5a = 9.81 min; tR 6b = 5.78 min, tR 5b = 8.43 min, tR
(0.5 mmol) in DMF (10 mL) under nitrogen, then the reaction mixture was
refluxed for 44 h. The reaction mixture was cooled down to RT, added
some water, extracted with EtOAc (3 ꢁ 80 mL), washed with brine, and
dried over MgSO4. The organic layer was evaporated under reduced
pressure to give crude product, which was purified by column
chromatography on silica gel with eluent (2:98 MeOH/CH2Cl2) to obtain
6 as a white solid in 87–95% yield, Rf = 0.21–0.28 (4:96 MeOH/CH2Cl2). 4-
Cyano-1-(2,4-dichlorophenyl)-5-(4-hydroxyphenyl)-N-(piperidin-1-yl)-1H-
[
[
11C]5c = 8.78 min;
[
[
t
11C]5a = 5.11 min; tR 6b = 2.83 min, tR 5b = 4.17 min, tR
R 6c = 2.95 min, tR 5c = 4.56 min, tR
11C]5c = 4.56 min; and tR 6d = 2.45 min, tR
11C]5d = 3.92 min. The decay corrected radiochemical yields
[
11C]5b = 4.17 min;
[
5d = 3.92 min, tR
[
of [11C]5a–d from [11C]CO2 were 50–65%.