Synthesis of carbon-11-labeled aminoalkylindole derivatives as new candidates of cannabinoid receptor radioligands for PET imaging of alcohol abuse

Article abstract of DOI:10.1016/j.bmcl.2014.10.097

Carbon-11-labeled aminoalkylindole derivatives (1-butyl-7-[¹¹C]methoxy-1H-indol-3-yl)(naphthalene-1-yl)methanone ([¹¹C]3), 1-butyl-7-[¹¹C]methoxy-3-(naphthalene-1-ylmethyl)-1H-indole ([¹¹C]5), and 1-butyl-7-[¹¹C]methoxy-3-(naphthalene-2-yl)-1H-indole ([¹¹C]8) were prepared by O-[¹¹C]methylation of their corresponding precursors with [¹¹C]CH₃OTf under basic condition (2 N NaOH) and isolated by a simplified solid-phase extraction (SPE) method in 50-60% radiochemical yields based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 23 min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 185-555 GBq/μmol.

Full text of DOI:10.1016/j.bmcl.2014.10.097

Bioorganic & Medicinal Chemistry Letters 24 (2014) 5581–5586
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of carbon-11-labeled aminoalkylindole derivatives as new
candidates of cannabinoid receptor radioligands for PET imaging of
alcohol abuse
⇑
Mingzhang Gao, Andy Chufan 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
Carbon-11-labeled aminoalkylindole derivatives (1-butyl-7-[11C]methoxy-1H-indol-3-yl)(naphthalene-
1-yl)methanone ([11C]3), 1-butyl-7-[11C]methoxy-3-(naphthalene-1-ylmethyl)-1H-indole ([11C]5), and
1-butyl-7-[11C]methoxy-3-(naphthalene-2-yl)-1H-indole ([11C]8) were prepared by O-[11C]methylation
of their corresponding precursors with [11C]CH3OTf under basic condition (2 N NaOH) and isolated by
a simplified solid-phase extraction (SPE) method in 50–60% radiochemical yields based on [11C]CO2
and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 23 min,
the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was
Article history:
Received 14 October 2014
Revised 28 October 2014
Accepted 30 October 2014
Available online 6 November 2014
Keywords:
Carbon-11-labeled aminoalkylindole
derivatives
185–555 GBq/lmol.
Ó 2014 Elsevier Ltd. All rights reserved.
Radiosynthesis
Positron emission tomography (PET)
Cannabinoid receptor
Alcohol abuse
Alcohol abuse is a major substance abuse, approximately 2.5
million people die from alcohol use every year, and its annual
health- and crime-related costs are estimated around $235 billion.1
There are several therapeutic options available for the treatment of
alcohol abuse, but all existing therapies have only modest efficacy.2
The cause of alcohol abuse is still unknown. However, alcohol
abuse results in chemical and biological changes in the brain,
and it is often associated with substantial psychiatric comorbidity,
brain damage and disease.3 Brain imaging techniques such as pos-
itron emission tomography (PET) enable scientists to better under-
stand alcohol abuse and alcohol-related diseases, for example, with
PET, researchers can compare groups of alcohol-abusing and non-
abusing individuals by quantifying differences in their levels of a
particular neurotransmitter molecule like dopamine or neuro-
transmission component such as a receptor or a transporter.4 We
are interested in PET imaging of alcohol abuse, and we have exten-
sively used [11C]raclopride-PET and [18F]fallypride-PET to study D2
receptor in alcohol abuse, since alcohol abuse significantly affects
the neurotransmitter dopamine and alters its neurotransmission
in the central nervous system (CNS).5–10
offer
a
new therapeutic direction for treatment of alcohol
abuse.11,12 Recently a new class of aminoalkylindole derivatives
has been developed as CBR ligands with dual CB1R antagonist/
CB2R agonist activity with potential for treatment of alcohol
abuse.1 CBR has become an interesting target for PET imaging of
alcohol abuse.13 In our previous works, we have developed a series
of selective CB1R radioligands and CB2R radioligands, as indicated
in Figure 1.14–16 In this ongoing study, we report the design and
synthesis of carbon-11-labeled aminoalkylindole derivatives as
new candidate dual CB1R/CB2R radioligands for PET imaging of
alcohol abuse.
Aminoalkylindole derivatives (1-butyl-7-methoxy-1H-indol-3-yl)
(naphthalene-1-yl)methanone (3), 1-butyl-7-methoxy-3-(naphtha-
lene-1-ylmethyl)-1H-indole (5), and 1-butyl-7-methoxy-3-(naph-
thalene-2-yl)-1H-indole (8) are dual CB1R/CB2R ligands with high
affinity, and the low nanomolar Ki values for CB1R and CB2R are 1.7
and 0.81 nM, 15.4 and 10.9 nM, and 37.3 and 26.5 nM, respectively.1
According to their in vitro Ki values, compound 3 is the most potent
ligand for CB1R and CB2R in comparison with compounds 5 and 8.
However, the overall biological evaluation including both in vitro
and in vivo studies suggested compounds 5 and 8 appeared to have
the most promise to be CBR ligands for use as treatment of alcohol
abuse.1 These three representative compounds are selected as the
reference standards for radiolabeling.
More and more evidences suggest that endocannabinoid (eCB)
system is another neurotransmitter involved in alcohol abuse,
and cannabinoid (CB) receptor (CBR) antagonist/agonist might
As outlined in Scheme 1, commercially available starting
material 7-methoxyindole (1) was subjected to mild alkylating
⇑
Corresponding author. Tel.: +1 317 278 4671; fax: +1 317 278 9711.
E-mail address: qzheng@iupui.edu (Q.-H. Zheng).
http://dx.doi.org/10.1016/j.bmcl.2014.10.097
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

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M. Gao et al. / Bioorg. Med. Chem. Lett. 24 (2014) 5581–5586
N
O
N
N
O
N
NC
N
NC
N
H
H
N
H311CO
N
H311CO
Cl
Cl
Cl
Cl
[
11C]JHU75528 ([11C]OMAR)
N
O
N
N
O
N
NC
N
NC
N
H
H
N
N
H311CO
H311CO
Br
Br
[
11C]JHU75575
O
CB1R Radioligands
X
O11CH3
O
O
N
H
n
N
H
n
H311CO
N
H
O
H3CO
N
H
OC5H11
n=1, X=H
n=1, X=Cl
n=2, X=H
OC5H11
n=1
O11CH3
X
O
O
N
N
H
n
n
H
H311CO
N
Cl
H3CO
N
OC5H11
Cl
OC5H11
n=1, X=H
n=1, X=Cl
n=2, X=H
n=1
H311CO
O
S
N
O
N
CB2R Radioligands
Figure 1. CB1R and CB2R radioligands.14–16
O
O
i
ii
iii
N
N
H
O
O
N
N
OH
O
1
2
3
4
Scheme 1. Synthesis of aminoalkylindole derivative reference standard 3 and its precursor 4. Reagents, conditions and yields: (i) KOH, 1-bromobutane, DMF, 50 °C, 88%; (ii)
Me2AlCl, 1-naphthoyl chloride, CH2Cl2, 0 °C, 52%; (iii) 48% HBr, HOAc, reflux, 70%.
conditions to give 1-butyl-7-methoxy-1H-indole (2) in 88%
yield. Intermediate 2 was then subjected to Friedel–Crafts like
acylation conditions, using dimethylaluminum chloride and acid
chloride at 0 °C to afford the standard 3 in 52% yield. The desme-
thylation of compound 3 with 48% HBr in the solution of HOAc
provided its desmethylated precursor (1-butyl-7-hydroxy-
1H-indol-3-yl)(naphthalene-1-yl)methanone (4) in 70% yield.
Using protic acid (HBr) instead of the literature method Lewis acid
(BBr3)1 as the desmethylating reagent, it significantly improved the
yield of desmethylation reaction, because protic acid promoted
the desmethylation reaction of phenolic methoxy aminoalkylindole
derivatives to easily form phenolic hydroxyl precursor and
CH3Br.17,18
As shown in Scheme 2, the reduction of compound 3 with LiAlH4
gave the standard 5 in 49% yield. Careful workup by adjusting
appropriate pH to 5 improved the yield of the reduction from 18%
to 49%. Likewise, using 48% HBr in HOAc as the desmethylating
reagent, compound
5 was desmethylated to produce its
desmethylated precursor 1-butyl-7-hydroxy-3-(naphthalene-1-
ylmethyl)-1H-indole (6) in 43% yield. Using Lewis acid (BBr3)1 as
the desmethylating reagent, compound 5 failed to give 6 in
desmethylation reaction.

M. Gao et al. / Bioorg. Med. Chem. Lett. 24 (2014) 5581–5586
5583
O
N
ii
i
N
N
O
O
3
OH
5
6
Scheme 2. Synthesis of aminoalkylindole derivative reference standard 5 and its precursor 6. Reagents, conditions and yields: (i) LiAlH4, AlCl3, THF, 0 °C, 49%; (ii) 48% HBr,
HOAc, reflux, 43%.
I
i
ii
iii
N
N
H
O
O
N
N
1
OH
O
7
9
8
Scheme 3. Synthesis of aminoalkylindole derivative reference standard 8 and its precursor 9. Reagents, conditions and yields: (i) 1-bromobutane, I2, KOH, DMF, NaH, rt, 75%;
(ii) 2-naphthaleneboronic acid, Pd(OAc)2, Cy-JohnPhos, K2CO3, toluene, EtOH, H2O, reflux, 51%; (iii) 48% HBr, HOAc, reflux, 52%.
As indicated in Scheme 3, a one-pot two-step (N-alkylation and
3-indole iodination) reaction of compound 1 afforded an interme-
diate 1-butyl-3-iodo-7-methoxy-1H-indole (7) in 75% yield. Inter-
mediate 7 was then subjected to Suzuki coupling conditions
utilizing boronic acid to provide the standard 8 in 51% yield, which
was undergone the same desmethylating conditions (48% HBr in
HOAc) to give its desmethylated precursor 1-butyl-7-hydroxy-3-
(naphthalene-2-yl)-1H-indole (9) in 52% yield.
O
N
O
i
N
OH
O11CH3
4
[
11C]3
Synthesis of carbon-11-labeled aminoalkylindole derivatives
(1-butyl-7-[11C]methoxy-1H-indol-3-yl)(naphthalene-1-yl)metha
none ([11C]3), 1-butyl-7-[11C]methoxy-3-(naphthalene-1-ylmethyl)-
1H-indole ([11C]5), and 1-butyl-7-[11C]methoxy-3-(naphthalene-2-
yl)-1H-indole ([11C]8), is indicated in Scheme 4. The desmethylated
precursors 4, 6, and 9 were labeled by a reactive [11C]methylating
i
N
N
agent,
[
11C]methyl triflate ([11C]CH3OTf)19,20 prepared from
O11CH3
OH
11C]CO2, under basic conditions (2 N NaOH) in acetonitrile through
11
[
6
5
C]
[
the O-[11C]methylation and isolated by a simplified solid-phase
extraction (SPE) method21–25 to provide target tracers [11C]3,
[
11C]5, and [11C]9 in 50–60% radiochemical yields, decay corrected
to end of bombardment (EOB), based on [11C]CO2.
11C]CH3OTf is a proven methylation reagent with greater reac-
[
tivity than commonly used [11C]methyl iodide ([11C]CH3I),26 and
thus, the radiochemical yield of [11C]3, [11C]5, and [11C]8 was rela-
tively high. The large polarity difference between the sodium salt
of the phenolic hydroxyl precursor and the corresponding labeled
O-methylated ether product permitted the use of SPE technique
for purification of the labeled product from the radiolabeling
reaction mixture. A C-18 Plus Sep-Pak cartridge was used in SPE
purification technique. The crude reaction mixture was treated
with aqueous NaHCO3 and loaded onto the cartridge by gas pres-
sure. The pH of freshly prepared 0.1 M NaHCO3 might be too low
to effectively deprotonate a phenolic hydroxyl precursor. However,
an excess of 2 N NaOH used in the reaction provided a final pH
after addition of the 0.1 M NaHCO3 high enough to deprotonate
all the phenolic hydroxyl precursor. Any non-reacted precursor
was actually converted to the corresponding sodium salt, and
any non-reacted [11C]CH3OTf was actually hydrolyzed to [11C]
CH3OH, which would not be trapped to the C-18 Sep-Pak. The car-
tridge was washed with water to remove radiolabeled by-product
i
N
N
O11CH3
OH
9
11
8
C]
[
Scheme 4. Synthesis of carbon-11-labeled aminoalkylindole derivative target
tracers [11C]3, [11C]5, and [11C]8. Reagents, conditions and yields: (i) [11C]CH3OTf,
CH3CN, 2 N NaOH, 80 °C, 3 min, 50–60%.
eluted with ethanol (2 Â 2 mL), concentrated by rotary evaporation
and reformulated in saline (10 mL). In our fully automated radio-
synthesis module,27–29 it is difficult to directly elute the labeled
product from a C-18 Sep-Pak to a vial using either 1 Â 1 mL ethanol
or 2 Â 0.5 mL ethanol, due to the back pressure in the C-18 Sep-Pak
and dead volume in the transfer tubing. The high back pressure is
resulted from the eluent (water, ethanol and saline) change. In
order to elute most of the labeled product from the C-18 Sep-Pak,
we need to increase the volume of the eluent ethanol. For the
radiotracer produced for animal study, we used 2 Â 2 mL ethanol
for elution, and rotary evaporation was required before
[
11C]CH3OH, sodium salt of phenolic hydroxyl precursor and
reaction solvent, and total 6 mL (2 Â 3 mL) volume of water was
enough to wash off all impurity. The final labeled product was

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M. Gao et al. / Bioorg. Med. Chem. Lett. 24 (2014) 5581–5586
reformulation. For the radiotracer produced for human study, we
used 2 Â 1 mL ethanol, no evaporation required, and a C-18
Sep-Pak was used for direct reformulation.9,30–32 We have tried
to use a C-18 Light Sep-Pak cartridge instead of a C-18 Plus Sep-
Pak cartridge to allow smaller volume (1 mL) of ethanol and to
avoid laborious rotary evaporation before formulation. However,
there is more serious back pressure in the Light Sep-Pak than in
the Plus Sep-Pak, in addition, dead volume in the transfer tubing
also affects the elution, and thus it is more difficult to efficiently
elute the labeled product from a Light Sep-Pak, which resulted in
the low radiochemical yield. The reason is that our home-built
fully automated module used many PTFE (polytetrafluoroethyl-
ene)/silicone liners (septa) and Teflon tubing, and these materials
cannot afford too high pressure gas (N2) push. The pressure of N2
gas introduced in our module is set at 207 kPa (30 Psi). C-18 Light
Sep-Pak cartridge works well in manual or semi-automated radio-
synthesis in our lab, because we can easily introduce high pressure
gas push during the purification and reformulation process. Overall
synthesis time was 23 min from EOB, including approximately
11 min for [11C]CH3OTf production, 5 min for O-[11C]methylation
reaction, and 7 min for SPE purification, evaporation and reformu-
lation. SPE technique is fast, efficient and convenient and works
very well for the O-methylated ether tracer purification using the
phenolic hydroxyl precursor for radiolabeling.22,24
& Ziegler Modular Lab C-11 Methyl Iodide/Triflate module, conve-
nient gas phase bromination of [11C]methane and production of
[
method using I2 and ‘wet’ method using LiAlH4 and HI. Our system
will have much less 12C carrier-added in comparison with other
‘dry’ method and ‘wet’ method.20
Chemical purity and radiochemical purity were determined by
analytical HPLC.35 The chemical purity of the precursors and refer-
ence standards was >93%. The radiochemical purity of the target
11C]CH3OTf, a ‘dry’ method using Br2 different with other ‘dry’
tracers was >98% determined by radio-HPLC through
c-ray (PIN
diode) flow detector, and the chemical purity of the target tracers
was >90% determined by reversed-phase HPLC through UV flow
detector.
The experimental details and characterization data for com-
pounds 2–9 and for the tracers [11C]3, [11C]5, and [11C]8 are given.38
In summary, a simple and moderate-to-high-yield synthetic
route to aminoalkylindole derivative precursors (4, 6 and 9) and
reference standards (3, 5 and 8) and carbon-11-labeled aminoalky-
lindole derivative target tracers ([11C]3, [11C]5, [11C]8) has been
developed. An automated self-designed multi-purpose [11C]-radio-
synthesis module for the synthesis of [11C]3, [11C]5, and [11C]8 has
been built. The target tracers were easily prepared by O-[11C]
methylation of their corresponding precursors using a reactive
[
11C]methylating agent, [11C]CH3OTf, and isolated by a simplified
The radiosynthesis was performed in an automated self-
designed multi-purpose 11C-radiosynthesis module, allowing mea-
surement of specific activity at EOB during synthesis.27–29 On line
determination of specific activity at EOB is accurate when
reverse-phase (RP) high performance liquid chromatography
(HPLC) is used as purification method. However, the on-the-fly
technique to determine specific activity at EOB is not applied when
SPE is used as purification method. The specific activity for the
11C-tracers produced in our PET chemistry facility usually ranges
from 370 to 1110 GBq/mol at EOB according to our previous works.
The specific activity of carbon-11-labeled aminoalkylindole deriva-
tives was estimated in a range of 185–555 GBq/mol at the end of
synthesis (EOS) based on other compounds produced in our facility
using the same targetry conditions which have been measured by
the on-the-fly technique or the same SPE purification method.33
The actual measurement of specific activity at EOS was performed
by analytical HPLC34,35 and calculated. The exact values of the
specific activity for the tracers [11C]3, [11C]5, and [11C]8 were
185–555 GBq/mol at EOS, which are in agreement with the esti-
mated values and the ‘on line’ determined values. Specific activity
is defined as the radioactivity per unit mass of a radionuclide or a
SPE purification procedure in high radiochemical yields, short
overall synthesis time, and high specific activity. These methods
are efficient and convenient. It is anticipated that the approaches
for the design, synthesis and automation of new tracer, authentic
standard and radiolabeling precursor, and improvements to
increase radiochemical yield and specific activity of the tracer
described here can be applied with advantages to the synthesis
of other 11C-radiotracers for PET imaging. These chemistry results
combined with the reported in vitro biological data1 encourage fur-
ther in vivo biological evaluation of carbon-11-labeled aminoalky-
lindole derivatives as new candidate PET dual radioligands for
imaging of cannabinoid receptors in alcohol abuse.
Acknowledgments
This work was partially supported by the United States Indiana
State Department of Health (ISDH) Indiana Spinal Cord & Brain
Injury Fund (ISDH EDS-A70-2-079612). 1H NMR and 13C NMR spec-
tra were recorded at 500 and 125 MHz, respectively, on a Bruker
Avance II 500 MHz NMR spectrometer in the Department of Chem-
istry and Chemical Biology at Indiana University Purdue University
Indianapolis (IUPUI), which is supported by the United States
National Science Foundation (NSF) Major Research Instrumenta-
tion Program (MRI) grant CHE-0619254.
labeled compound. Specific activity (MBq/mg) = 3.13 Â 109/A Â t1/2
,
where A is the mass number of the radionuclide, and t1/2 is the
half-life in hours of the radionuclide. For carbon-11, carrier-
free 11C, maximum (theoretical) 11C specific activity = 340,918
GBq/l
mol.36 Actual specific activity of the 11C-tracers in the PET
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N2 atmosphere was added Me2AlCl (69.4 mL, 1 M in hexanes, 69.4 mmol)
dropwise, and the solution was allowed to stir at 0 °C for 40 min, then a solution
of 1-naphthoyl chloride (9.25 g, 48.52 mmol) in CH2Cl2 (60 mL) was added
dropwise. The reaction mixture was stirred for overnight at rt, and carefully
poured into 1 N HCl (70 mL) solution prior to extraction with CH2Cl2
(3 Â 100 mL). The combined organic extracts were washed with NaHCO3, brine,
and dried over anhydrous Na2SO4. The solvent was evaporated under reduced
pressure, and the resulting residue was purified by column chromatography on
silica gel using EtOAc/hexanes (3:97) as eluent to give white solid product 3
(8.04 g, 52%). Rf = 0.38 (1:5 EtOAc/hexanes), mp 108–110 °C. 1H NMR (CDCl3):
d 0.88 (t, J = 7.3 Hz, 3H, CH3), 1.22–1.30 (m, 2H, CH2), 1.72–1.77 (m, 2H, CH2),
3.95 (s, 3H, OCH3), 4.28 (t, J = 7.5 Hz, 2H, CH2), 6.76 (d, J = 8.0 Hz, 1H, Ar-H), 7.20
(s, 1H, Ar-H), 7.24 (t, J = 8.0 Hz, 1H, Ar-H), 7.43–7.51 (m, 3H, Ar-H), 7.62 (dd,
J = 1.0, 7.0 Hz, 1H, Ar-H), 7.89 (d, J = 8.0 Hz, 1H, Ar-H), 7.94 (d, J = 8.0 Hz, 1H,
Ar-H), 8.09 (d, J = 8.0 Hz, 1H, Ar-H), 8.16 (d, J = 8.5 Hz, 1H, Ar-H). MS (ESI): 358
([M+H]+, 100%).
(d) 1-Butyl-7-methoxy-3-(naphthalen-1-ylmethyl)-1H-indole (5): A solution of
AlCl3 (16.0 g, 120 mmol) in THF (80 mL) was dropwise added into a solution of
LiAlH4 (2.0 M in THF, 20 mL, 40 mmol) at 0 °C. After 40 min, compound 3 (3.57 g,
10.0 mmol) in THF (60 mL) was added to the reaction mixture and allowed to
stir at rt for 48 h. Upon completion, the reaction mixture was cooled down in an
ice-bath, quenched with water and acidified with 1 N HCl to pH = 5. The organic
phase was then separated, while the aqueous phase was extracted with EtOAc
(2 Â 100 mL). The combined organic layers were washed with NaHCO3, brine,
and dried over anhydrous Na2SO4. The solvent was evaporated under reduced
pressure, and the resulting residue was purified by column chromatography on
silica gel using EtOAc/hexanes (2:98) as eluent to give white solid product 5
(1.68 g, 49%). Rf = 0.84 (1:2 EtOAc/hexanes), mp 47-49 °C. 1H NMR (CDCl3):
d 0.85 (t, J = 7.3 Hz, 3H, CH3), 1.19–1.24 (m, 2H, CH2), 1.64–1.70 (m, 2H, CH2),
3.92 (s, 3H, OCH3), 4.21 (t, J = 7.3 Hz, 2H, CH2), 4.49 (s, 2H, CH2), 6.48 (s, 1H,
Ar-H), 6.62 (d, J = 8.0 Hz, 1H, Ar-H), 6.98 (t, J = 7.5 Hz, 1H, Ar-H), 7.18 (dd, J = 0.5,
8.0 Hz, 1H, Ar-H), 7.33–7.39 (m, 2H, Ar-H), 7.40–7.47 (m, 2H, Ar-H), 7.72 (d,
J = 8.0 Hz, 1H, Ar-H), 7.84 (dd, J = 1.2, 8.0 Hz, 1H, Ar-H), 8.08 (d, J = 8.0 Hz, 1H,
Ar-H). MS (ESI): 344 ([M+H]+, 100%).
(e) 1-Butyl-3-iodo-7-methoxy-1H-indole (7): A solution of 7-methoxyindole (1,
2.95 g, 20.0 mmol) in DMF (80 mL) was stirred with KOH (1.29 g, 23.0 mmol) at
RT for 40 min, and then treated with I2 (5.23 g, 20.6 mmol). After 30 min, NaH
(60% in mineral oil, 1.04 g, 26.0 mmol) was added portionwise. After an
additional 15 min, 1-bromobutane (3.15 g, 23.0 mmol) was added and the
reaction mixture was stirred for overnight. Upon completion (TLC monitoring),
water was added, the reaction mixture was extracted with EtOAc (3 Â 80 mL).
Combined organic layers were washed with water two times, dried over
anhydrous Na2SO4, and concentrated in vacuo. The resulting residue was
purified by column chromatography on silica gel using EtOAc/hexanes
(0.5:99.5) as eluent to afford white solid product 7 (4.94 g, 75%). Rf = 0.67 (1:9
EtOAc/hexanes), mp 42–44 °C. 1H NMR (CDCl3): d 0.92 (t, J = 7.3 Hz, 3H, CH3),
1.27–1.34 (m, 2H, CH2), 1.73–1.79 (m, 2H, CH2), 3.92 (s, 3H, OCH3), 4.35 (t,
J = 7.3 Hz, 2H, CH2), 6.65 (d, J = 7.5 Hz, 1H, Ar-H), 7.01 (dd, J = 0.5, 8.0 Hz, 1H,
Ar-H), 7.05–7.08 (m, 2H, Ar-H). MS (ESI): 329 ([M]+, 20%), 203 (100%).
(f) 1-Butyl-7-methoxy-3-(naphthalen-2-yl)-1H-indole (8): Pd(OAc)2 (45 mg,
34. Gao, M.; Yang, Q.; Wang, M.; Miller, K. D.; Sledge, G. W.; Zheng, Q.-H. Appl.
Radiat. Isot. 2013, 74, 61.
35. Zheng, Q.-H.; Mock, B. H. Biomed. Chromatogr. 2005, 19, 671.
36. Lapi, S. E.; Welch, M. J. Nucl. Med. Biol. 2013, 40, 314.
37. Wang, M.; Gao, M.; Zheng, Q.-H. Bioorg. Med. Chem. Lett. 2013, 23, 5259.
38. (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 literature procedure.20 Melting
a
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 an Agilent system, consisting of an 1100 series HPLC connected to a diode
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. Chro-
matographic solvent proportions are indicated as volume:volume ratio. Thin-
layer chromatography (TLC) was run using Analtech silica gel GF uniplates
(5 Â 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 from a
nitrogen source. Analytical HPLC was performed using a Prodigy (Phenome-
0.2 mmol),
2-(dicyclohexylphosphino)biphenyl
(Cy-JohnPhos)
(88 mg,
0.25 mmol), K2CO3 (2.08 g, 15.0 mmol), 2-naphthaleneboronic acid (1.55 g,
9.0 mmol), and compound 7 (1.65 g, 5.0 mmol) were added into a solution of
toluene (100 mL), EtOH (40 mL), and water (10 mL), and the reaction mixture
was allowed to stir at reflux under N2 overnight. Reaction was monitored via
TLC, and upon completion, the reaction mixture was then concentrated under
vacuo, the residue was extracted with EtOAc (3 Â 100 mL). The combined
organic layers were washed with water, brine, and dried over anhydrous
Na2SO4. The solvent was evaporated under reduced pressure, and the resulting
residue was purified by column chromatography on silica gel using CH2Cl2/
hexanes (2:98) as eluent to afford pinkish oil product 8 (0.84 g, 51%). Rf = 0.68
(1:8 EtOAc/hexanes). 1H NMR (CDCl3): d 0.95 (t, J = 7.5 Hz, 3H, CH3), 1.33–1.41
(m, 2H, CH2), 1.81–1.87 (m, 2H, CH2), 3.95 (s, 3H, OCH3), 4.42 (t, J = 7.2 Hz, 2H,
CH2), 6.68 (d, J = 7.5 Hz, 1H, Ar-H), 7.09 (t, J = 8.0 Hz, 1H, Ar-H), 7.39 (td, J = 1.0,
8.0 Hz, 1H, Ar-H), 7.44 (td, J = 1.0, 7.5 Hz, 1H, Ar-H), 7.62 (d, J = 8.0 Hz, 1H, Ar-H),
7.76 (dd, J = 1.5, 8.5 Hz, 1H, Ar-H), 7.81–7.87 (m, 3H, Ar-H), 8.07 (s, 1H, Ar-H). MS
(ESI): 330 ([M+H]+, 100%).
nex) 5
20 mM phosphate buffer solution (pH 6.7); flow rate 1.0 mL/min; and UV
(254 nm) and -ray (PIN diode) flow detectors. C18 Plus Sep-Pak cartridges
were obtained from Waters Corporation (Milford, MA). Sterile Millex-GS
0.22 m filter units were obtained from Millipore Corporation (Bedford, MA).
(b) 1-Butyl-7-methoxy-1H-indole (2): To suspension of KOH (16.25 g,
lm C-18 column, 4.6 Â 250 mm; mobile phase 3:1:1 CH3CN/MeOH/
(g) General procedure for the preparation of precursors 4,
6 and 9 by O-
c
desmethylation of the indole methyl ether derivatives 3, 5, and 8: A solution of
48% HBr (3 mL) was added into a suspension of the indole derivative 3 (5 or 8)
(1.2 mmol) in 5 mL of 30% HBr in HOAc, and then the reaction mixture was
stirred and heated to reflux for 2–3 h. Upon the completion, the reaction mixture
was cooled down and quenched with NaOH (3 N). After adjusted pH to 7 and
extracted with EtOAc (3 Â 60 mL), the combined organic extracts were dried
over anhydrous Na2SO4, and concentrated under reduced pressure. The result-
ing residue was purified by flash column chromatography on silica gel using
EtOAc/hexanes (4:96) as eluent to give off-brown oil product 4 (6 or 9) in 43-70%
yield.
(1-Butyl-7-hydroxy-1H-indol-3-yl)(naphthalen-1-yl)methanone (4): Yield 70%.
Rf = 0.25 (1:5 EtOAc/hexanes). 1H NMR (acetone-d6): d 0.87 (t, J = 7.3 Hz, 3H,
CH3), 1.28–1.32 (m, 2H, CH2), 1.78–1.84 (m, 2H, CH2), 4.44 (t, J = 7.0 Hz, 2H, CH2),
6.77 (d, J = 7.5 Hz, 1H, Ar-H), 7.05 (t, J = 8.0 Hz, 1H, Ar-H), 7.47–7.59 (m, 4H, Ar-
H), 7.67 (dd, J = 1.2, 7.0 Hz, 1H, Ar-H), 7.97–8.03 (m, 3H, Ar-H), 8.11 (d, J = 8.5 Hz,
1H, Ar-H), 8.95 (s, 1H, OH). 13C NMR (acetone-d6): d 13.70, 19.79, 33.83, 50.12,
109.81, 114.79, 117.40, 123.87, 124.68, 125.94, 126.14, 126.26, 126.38, 126.81,
128.25, 129.86, 130.04, 130.94, 133.81, 139.20, 139.75, 143.66, 192.74. MS (ESI):
l
a
290 mmol) in DMF (120 mL) was added 7-methoxyindole (1, 8.54 g, 58 mmol).
After stirring at room temperature (RT) for an hour, 1-bromobutane (11.12 g,
81.2 mmol) was added and the reaction mixture was heated to 50 °C for 12 h.
The resulting mixture was poured into water (200 mL) and extracted with EtOAc
(3 Â 120 mL). Combined organic layers were washed with water two times,
dried over anhydrous Na2SO4, and concentrated in vacuo. The resulting residue
was purified by column chromatography on silica gel using EtOAc/hexanes
(1:99) as eluent to afford colorless oily product 2 (10.36 g, 88%). Rf = 0.90 (1:2
EtOAc/hexanes). 1H NMR (CDCl3): d 0.91 (t, J = 7.5 Hz, 3H, CH3), 1.27–1.34 (m,
2H, CH2), 1.74–1.80 (m, 2H, CH2), 3.91 (s, 3H, OCH3), 4.35 (t, J = 7.5 Hz, 2H, CH2),
6.39 (d, J = 3.0 Hz, 1H, Ar-H), 6.59 (d, J = 8.0 Hz, 1H, Ar-H), 6.94–6.98 (m, 2H,
Ar-H), 7.18 (d, J = 8.0 Hz, 1H, Ar-H). MS (ESI): 204 ([M+H]+, 100%).
(c) (1-Butyl-7-methoxy-1H-indol-3-yl)(naphthalen-1-yl)methanone (3): To
a
solution of compound 2 (8.81 g, 43.32 mmol) in CH2Cl2 (100 mL) at 0 °C under

5586
M. Gao et al. / Bioorg. Med. Chem. Lett. 24 (2014) 5581–5586
344 ([M+H]+, 100%). HRMS (ESI): calcd for C23H21NO2Na ([M+Na]+) 366.1470,
found 366.1472.
consisted of 1% oxygen in nitrogen purchased as a specialty gas from Praxair,
Indianapolis, IN. Typical irradiations used for the development were 50 lA
1-Butyl-3-(naphthalen-1-ylmethyl)-1H-indol-7-ol (6): Yield 43%. Rf = 0.50 (1:5
EtOAc/hexanes). 1H NMR (acetone-d6): d 0.84 (t, J = 7.5 Hz, 3H, CH3), 1.30–1.45
(m, 2H, CH2), 1.73–1.76 (m, 2H, CH2), 4.42 (t, J = 7.5 Hz, 2H, CH2), 4.55 (s, 2H,
CH2), 5.83 (s, 1H, Ar-H), 6.52 (dd, J = 1.0, 8.0 Hz, 1H, Ar-H), 6.72 (t, J = 7.5 Hz, 1H,
Ar-H), 6.85 (dd, J = 1.0, 8.0 Hz, 1H, Ar-H), 7.27 (d, J = 7.0 Hz, 1H, Ar-H), 7.42 (t,
J = 7.5 Hz, 1H, Ar-H), 7.47–7.49 (m, 2H, Ar-H), 7.81 (d, J = 8.5 Hz, 1H, Ar-H), 7.91–
7.93 (m, 1H, Ar-H), 8.09–8.11 (m, 1H, Ar-H), 8.45 (s, 1H, OH). 13C NMR (acetone-
d6): d 14.30, 20.60, 35.20, 37.23, 45.95, 102.71, 107.16, 112.47, 120.27, 124.91,
126.34, 126.44, 126.48, 126.82, 126.95, 127.70, 128.07, 129.47, 131.68, 133.02,
134.89, 135.97, 139.88, 144.67. MS (ESI): 330 ([M+H]+, 18%); MS (ESI): 328
([MÀH]À, 100%). Note: This compound is not so stable.
beam current for 15 min on target. The production run produced approximately
25.9 GBq of [11C]CO2 at EOB. The phenolic hydroxyl precursor 4 (6 or 9) (0.1–
0.3 mg) was dissolved in CH3CN (300 lL). To this solution was added 2 N NaOH
(2 lL). The mixture was transferred to a small reaction vial. No-carrier-added
(high specific activity) [11C]CH3OTf (13.9 GBq) that was produced by the gas-
phase production method20 within 11 min from [11C]CO2 (25.9 GBq) through
[
11C]CH4 (21.8 GBq) and [11C]CH3Br (13.9 GBq) with silver triflate (AgOTf)
column was passed into the reaction vial at rt until radioactivity reached a
maximum (2 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). The reaction vial was connected to a C-18 Plus Sep-Pak cartridge. The
labeled product mixture solution was passed onto the cartridge for SPE
purification by gas pressure. The cartridge was washed with H2O (2 Â 3 mL),
and the aqueous washing was discarded. The product was eluted from the
cartridge with EtOH (2 Â 2 mL), and then passed onto a rotatory evaporator. The
solvent was removed by evaporation (3 min) under vacuum. The final volume of
ethanol after evaporation was ꢀ1 mL. The labeled product was reformulated
1-Butyl-3-(naphthalen-2-yl)-1H-indol-7-ol (9): Yield 52%. Rf = 0.45 (1:5 EtOAc/
hexanes). 1H NMR (acetone-d6): d 0.90 (t, J = 7.0 Hz, 3H, CH3), 1.32–1.41 (m, 2H,
CH2), 1.86–1.91 (m, 2H, CH2), 4.54 (t, J = 7.2 Hz, 2H, CH2), 6.67 (d, J = 7.0 Hz, 1H,
Ar-H), 6.94 (t, J = 8.0 Hz, 1H, Ar-H), 7.41 (td, J = 1.0, 8.0 Hz, 1H, Ar-H), 7.47 (td,
J = 1.0, 8.0 Hz, 1H, Ar-H), 7.55–7.57 (m, 2H, Ar-H), 7.83–7.86 (m, 2H, Ar-H), 7.90–
7.93 (m, 2H, Ar-H), 8.16 (s, 1H, Ar-H), 8.68 (s, 1H, OH). 13C NMR (acetone-d6): d
14.20, 20.55, 35.29, 49.53, 107.98, 112.24, 116.75, 121.42, 125.09, 125.78,
126.90, 127.20, 128.43, 128.52, 128.75, 128.94, 130.77, 134.80, 135.17, 145.53.
MS (ESI): 316 ([M+H]+, 90%). HRMS (ESI): calcd for C22H22NO ([M+H]+) 316.1701,
found 316.1715.
with saline (10 mL), sterile-filtered through a sterile vented Millex-GS 0.22 lm
cellulose acetate membrane and collected into a sterile vial. Total radioactivity
(4.7–7.1 GBq) was assayed and the total volume (10–11 mL) was noted for
tracer dose dispensing. The overall synthesis time including SPE purification and
reformulation was 23 min. The radiochemical yields decay corrected to EOB,
from [11C]CO2, were 50–60%. The same procedure was used to prepare the target
tracers [11C]3, [11C]5 and [11C]8 from their corresponding precursors 4, 6 and 9.
Retention times in the analytical HPLC system were: tR 4 = 3.40 min, tR
(h) General procedure for the preparation of the target tracers (1-butyl-7-[11C]
methoxy-1H-indol-3-yl)(naphthalene-1-yl)methanone ([11C]3), 1-butyl-7-
[
[
11C]methoxy-3-(naphthalene-1-ylmethyl)-1H-indole ([11C]5), and 1-butyl-7-
11C]methoxy-3-(naphthalene-2-yl)-1H-indole ([11C]8): [11C]CO2 was produced
by the 14N(p,
a)
11C nuclear reaction in the small volume (9.5 cm3) aluminum gas
3 = 5.48 min, tR
[
[
11C]3 = 5.61 min; tR 6 = 5.27 min, tR 5 = 7.44 min, tR
11C]8 = 6.64 min.
target provided with the Siemens RDS-111 Eclipse cyclotron. The target gas
11C]5 = 7.51 min; and tR 9 = 3.79 min, tR 8 = 6.56 min, tR
[