Carbon-14 radiosynthesis of the benzofuran derivative and β-amyloid plaque neuroimaging positron emission tomography radioligand AZD4694

Article abstract of DOI:10.1002/jlcr.3029

In support of a metabolite study, the β-amyloid plaque neuroimaging positron-emission tomography radioligand AZD4694 was labeled with carbon-14 in 10 radiosynthetic steps starting from radiolabeled carbon dioxide. [ ¹⁴C]AZD4694 was labeled in t

Full text of DOI:10.1002/jlcr.3029

Research Article
Received 14 August 2012,
Revised 19 November 2012,
Accepted 13 January 2013
Published online 16 February 2013 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3029
Carbon-14 radiosynthesis of the benzofuran
derivative and b-amyloid plaque
neuroimaging positron emission tomography
radioligand AZD4694
*
Johan Sandell
In support of a metabolite study, the b-amyloid plaque neuroimaging positron-emission tomography radioligand AZD4694
was labeled with carbon-14 in 10 radiosynthetic steps starting from radiolabeled carbon dioxide. [14C]AZD4694 was labeled
in the benzofuran heterocycle with a specific activity of 2.1 GBq/mmol and with a radiochemical purity of >99%. The
described synthesis constitutes a general method to carbon-14-labeled substituted benzofurans.
Keywords: AZD4694; benzofuran; BOC t-butyloxycarbonyl; PET radioligand; amyloid
A prerequisite for a PET radioligand suitable for sensitive
Introduction
imaging of Ab plaques in a patient with AD is no or at least very
Alzheimer’s disease (AD) was identified more than a century ago on
low amount of radioactive metabolites passing the blood–brain
the basis of histopathologic observations. The diagnosis of AD,
barrier. Radioactive metabolites that enter the brain may obscure
however, is a clinical challenge, and a definite diagnosis can still
brain PET measurements by adding to both specific and nonspe-
only be made post-mortem. Positron emission tomography (PET)
cific binding. Identification of radiolabeled metabolites is an
essential part of the validation of a PET radioligand intended
for use as an imaging biomarker or diagnostic. For this purpose,
AZD4694 was required in its carbon-14-labeled form.
neuroimaging of Ab plaques with radioligands such as the thiofla-
vin derivatives [11C]Pittsburgh compound B and [18F]flutemetamol
and the stilbene derivatives [18F]florbetaben (18F-BAY94-9172)
and [18F]florbetapir (18F-AV-45) is a promising tool that might sup-
port and increase specificity when diagnosing preclinical AD.1–4
Results and discussion
PET neuroimaging of cerebral Ab plaques might also be useful
for the evaluation of new therapies aiming to modify plaque load
and for patient stratification in clinical trials.7,8
In this paper, the radiosynthesis of [14C]AZD4694 is reported. [14C]
AZD4694 was labeled in the benzofuran heterocycle of the molecule
because this label placement should, when incubated with liver
hepatocytes and microsomes, generate a similar profile of radiola-
beled metabolites as to what can be expected from [18F]AZD4694
in vivo. A putative metabolic pathway among fluorine-18-labeled
PET radioligands is in vivo defluorination resulting in free fluorine-
18 in the blood. Identification of radiolabeled defluorinated metabo-
lites of [14C]AZD4694 would suggest this as a potential metabolic
pathway. In vivo high uptake of radioactivity in the skull is indicative
of this metabolic pathway. In the clinical PET studies reported
with [18F]AZD4694, the uptake of radioactivity in the skull has been
insignificant, indicating that defluorination is a nonsignificant meta-
bolic pathway.8
[18F]AZD4694 (2-(2-18F-fluoro-6-(methylamino)-3-pyridyl)ben-
zofuran-5-ol), a pyridinyl benzofuran, is a recently developed
second-generation b-amyloid PET ligand.7 Because of the longer-
lived fluorine-18 radioisotope used for labeling, the radioligand
has potential for wider clinical application than C-11-labeled
agents. Clinical studies suggest that it reaches equilibrium quickly,
permitting scans following a relatively short radiotracer uptake
period, and is associated with relatively high specific to nonspecific
white matter binding, permitting amyloid imaging with high
sensitivity.8 It has been shown ex vivo that after intravenous
administration in aged Tg2576 transgenic mice, the trititiated
isotopologue [3H]AZD4694 binds to b-amyloid deposits in a
reversible manner.7 In brain tissue taken from humans diagnosed
with AD, [3H]AZD4694 selectively labeled b-amyloid deposits in
the gray matter and showed a low level of nondisplaceable
binding in plaque-devoid white matter. Because of its demon-
strated excellent in vitro imaging properties, [3H]AZD4694 is a
A two-step synthesis to substituted carbon-14-labeled [C3-14C]ben-
zofuranone from the corresponding [carbonyl-14C]2-methoxybenzoic
valuable in vitro b-amyloid imaging tool, which creates the possibi- AstraZeneca, Research & Development—Isotope Chemistry, Screening and Profiling
lity of using [3H]AZD4694 in preclinical translational bridging
Global DMPK IM, B212, Södertälje SE-151 85, Sweden
studies to validate clinical [18F]AZD4694 PET imaging protocols.
*Correspondence to: Johan Sandell, Novandi Chemistry AB, Holmgårdsvägen 27,
SE-141 33 Huddinge, Sweden.
E-mail: Johan.Sandell@novandi.se
The synthesis of [3H]AZD4694 and [18F]AZD4694 has been
described previously.9
J. Label Compd. Radiopharm 2013, 56 321–324
Copyright © 2013 John Wiley & Sons, Ltd.

J. Sandell
acid has been previously described by Wadsworth et al.10 However, yielded [14C]AZD4694. The demethylation needed strict tempera-
this synthesis utilizes diazomethane in the ring closure step, ture control to avoid substitution of the fluorine by bromine. The
and it was desirable to find a route past this hazardous final product was purified on reversed-phase HPLC to give >99%
and potentially explosive reagent. Instead, a labeling route to radiochemical-purity and UV-purity with a specific activity of
[14C]AZD4694 based on a benzofuranone synthesis described 2.1 GBq/mmol.
by Carvalho et al. was developed, which, as well as the method
The total radiochemical yield calculated from 14CO2 was 5% over
by Wadsworth et al., opens up the possibility to use carbon 10 radiochemical steps. [14C]AZD4694 was stored at ꢀ18 ꢁC as a
dioxide as labeled precursor.11
solid. No signs of decomposition were observed after 10 months
The synthesis of [14C]AZD4694 was accomplished as shown in under these storage conditions.
Scheme 1. The described synthesis constitutes a diazomethane-
free alternative to the method developed by Wadsworth
et al. and is a general method for the synthesis of carbon-
Conclusion
Carbon-14-labeled AZD4694 was synthesized with a total yield of
14-labeled benzofurans.
5% over 10 steps starting from [14C]CO2 to afford the product
with a specific activity of 2.1 GBq/mmol and with a radiochemical
purity of >99%. The described synthesis constitutes a general
method to carbon-14-labeled benzofurans.
The methoxymethyl-protected phenol starting material 2 was
synthesized from the corresponding substituted bromo phenol 1
by alkylation with bromomethyl methyl ether. Lithiation of 2 with
n-BuLi at ꢀ78 ꢁC and condensation with 14CO2 afforded the
specifically carbon-14-labeled acid 3. Esterification and deprotec-
tion of 3 with ethanol in the presence of sulfuric acid gave the ethyl Experimental
salicylate 4.12 Treatment of 4 with ethyl bromoacetate generated
General
513, which was hydrolyzed in basic media to produce the diacid
6.12 Ring closure of 6 in refluxing AcOH/Ac2O in the presence of
All solvents used were of analytical grade and commercially available.
Anhydrous solvents were routinely used for reactions. Reactions were
lyzed in acidic media to the required 3(2H)-benzofuranone 8.11 typically run under an inert atmosphere of nitrogen or argon. The micro-
sodium acetate afforded compound 7, which was directly hydro-
wave reaction was performed in a Biotage Initiator Eight (Biotage AB,
Uppsala, Sweden). 14CO2 was handled in a 14CO2 manifold system from
RC TRITEC AG, Teufen, Switzerland. Flash column chromatography was
performed using RediSep(R) (Teledyne Isco, Lincoln, Nebraska, USA) silica
gel columns. The identities of appropriate-labeled intermediates as well
as the final product were established by co-elution via HPLC or TLC with
authentic nonlabeled material.
High-performance liquid chromatography radiochemical purity ana-
lyses were performed on an Agilent 1100, HPLC-system with a binary
pump, an auto-injector, a diodide array detector (Agilent Technologies,
Reduction of 3(2H)-benzofuranone 8 with sodium borohydride
and subsequent dehydration with HCl afforded 9 as a solid.14 The
5-methoxybenzofuran 9 was lithiated with n-BuLi at ꢀ78 ꢁC, before
treatment with triisopropylborate to yield the boronic acid 10.
BOC-protected 2-methylamino-5-bromopyridine 16 was synthe-
sized in three steps in high yield from the commercially available
2-amino-5-fluoropyridine 13. Compound 10 was reacted with 16
under Suzuki reaction conditions to afford the 2-pyridylbenzofuran
11. This step also removed the BOC-group of the substituted pyri-
dine. Subsequent demethylation of 11 with boron tribromide Kista, Sweden), and a column oven, coupled in series with a Packard
O
O
*
*
O
Br
O
O
O
CO2H
O
O
Br
O
*
O
a
b
c
d
57%
59%
80%
65%
O
O
OH
OH
O
4
5
O
1
2
3
O
O
e
89%
O
O
OAc
O
O
*
OH
B
OH
O
O
*
O
OH
i
h
g
f
*
*
*
87%
37%
O
g+h yield = 98%
O
O
O
O
OH
6
10
9
8
7
O
Br
F
Br
F
Br
F
O
O
l
O
n
m
j
73%
91%
63%
62%
N
N
N
N
O
F
N
NH2
N
NH2
H
16
15
14
13
*
O
*
HO
H
N
k
H
N
67%
O
N
O
N
F
F
14C]AZD4694
11
[
Scheme 1. Synthesis of [14C]AZD4694. Reagents: (a) bromomethyl methyl ether, NaH, DMF, room temperature, 1 h; (b) 14CO2, BuLi, THF, ꢀ78 ꢁC to room temperature, 1 h;
(c) EtOH, H2SO4, reflux, 2 days; (d) NaI, K2CO3, ethyl bromoacetate, DMF, room temperature, 2 days; (e) NaOH, MeOH, THF, reflux, 2 days; (f) NaOAc, HOAc, Ac2O, reflux, 4 h;
(g) HCl, MeOH, H2O, 75 ꢁC, 1.5 h; (h) i. NaBH4, EtOH, room temperature, 2 h; ii. HCl, room temperature, 1 h; (i) BuLi, THF, triisopropylborate, ꢀ78 ꢁC, 1 h; (j) Pd(PPh3)2Cl2,
aqueous Na2CO3, EtOH, microwave 110 ꢁC, 1 h (k) BBr3, CH2Cl2, 0 ꢁC, 18 h; (l) N-bromo succinimide, MeCN, room temperature, 1 h; (m) sodium bis(trimethylsilyl)amide,
di-tert-butyl dicarbonate, THF, room temperature, overnight; (n) MeI, NaH, DMF, room temperature, 45 min. * denotes position of carbon-14 label.
www.jlcr.org
Copyright © 2013 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2013, 56 321–324

J. Sandell
Radiomatic Flow Scintillator 525TR (Chemical Instruments AB, Lidingö,
Sweden), equipped with a solid scintillator (SolarScint) cell with a volume
of 32 mL. The column used was a Zorbax SB-C18 (2.1 ꢂ 100 mm, 3.5 mm)
(Agilent Technologies Sweden, Kista, Sweden). The column temperature
was set to 40 ꢁC and the flow rate to 0.5 mL/min. A linear gradient was
applied, starting at 100% A (A: 10 mM NH4OAc in 5% MeCN) and ending
at 95% B (B: MeCN), in 13.5 min.
Mass spectra were recorded on a Waters LC/MS (Waters Sverige AB,
Sollentuna, Sweden) consisting of an Alliance 2795 (LC), a Waters PDA
2996, and a ZMD single quadrupole mass spectrometer. The mass spec-
trometer was equipped with an electrospray ion source operated in a
positive or negative ion mode. The capillary voltage was 3 kV, and the
cone voltage was 30 V. The mass spectrometer was scanned between
m/z 100–600 with a scan time of 0.7 s. The column temperature was set
to 40 ꢁC. The diode array detector was scanned from 200 to 400 nm.
1H NMR spectra were recorded on a Bruker DRX-600, a Bruker DPX-400,
or a Bruker AVANCE III 500 spectrometer and were referenced to residual
solvent peak (Bruker BioSpin Scandinavia AB, Solna, Sweden).
t-Butyl N-(5-bromo-6-fluoro-2-pyridyl)-N-methyl-carbamate (16)
To a solution of t-butyl 5-bromo-6-fluoropyridin-2-ylcarbamate (15)
(2.0 g, 6.9 mmol) in DMF (30 mL) was added NaH (0.38 g, 9.6 mmol) at
0 ꢁC. The solution was stirred for 10 min at 0 ꢁC after which MeI
(0.55 mL, 8.9 mmol) was added. After stirring for 45 min at room tempera-
ture, aqueous saturated NH4Cl (30 mL) was added, and the resulting mix-
ture was extracted with EtOAc (3 ꢂ 40 mL). The combined organic layers
were washed with brine, dried with Na2SO4, filtered, and evaporated. The
residue was purified by flash chromatography (silica, with 5% EtOAc in
heptane as eluent) to afford the product as a solid (1.90 g, 91% yield).
1H NMR (400 MHz, CDCl3) d: 7.83 (t, J = 8.84 Hz, 1H) 7.69 (dd, J = 8.46,
1.39 Hz, 1H) 3.37 (s, 3H) 1.54 (s, 9H).
[COOH-14C]5-Methoxy-2-(methoxymethoxy)benzoic acid (3)
A
solution of 2-bromo-4-methoxy-1-(methoxymethoxy)benzene (2)
(690 mg, 2.8 mmol) in THF (6 mL) was cooled and stirred under nitrogen
at ꢀ78 ꢁC. BuLi (1.6 M in hexanes, 1.9 mL, 3.1 mmol) was added dropwise,
and the mixture was stirred for an additional 1 h at ꢀ78 ꢁC. The reaction
mixture was transferred to a 14CO2 manifold and evacuated at ꢀ78 ꢁC.
14CO2 (0.145 g, 3.16 mmol, 7 GBq) was charged to the reaction at
ꢀ78 ꢁC, and after warming to room temperature, the reaction mixture
was stirred for 1 h. The mixture was then washed with EtOAc (5 mL),
and concentrated HCl was added until precipitation (pH approximately =
3) of a solid. The slurry was extracted with ethyl acetate (3 ꢂ 10mL), and
the combined organic layers were dried with Na2SO4, filtered, and evapo-
rated. The residue (oil) was used in the next step without further purifica-
tion (344 mg, 58% yield).
Preparative HPLC of [14C]AZD4694 was performed on an LC system
consisting of a Gilson 131 XL sample injector, Gilson prepFC fraction
collector, a Gilson 322 dual head pump, a Gilson UV/VIS 151 diode
array detector set at 254 nm, a Kromasil C8 column 250 ꢂ 20 mm, 5 mm
(Dr. Maisch HPLC GmbH, Germany), and an isocratic elution at 70%MeOH
in 50 mM NH4OAc at a flow rate of 10 mL/min.
2-Bromo-4-methoxy-1-(methoxymethoxy)benzene (2)
A
solution of 2-bromo-4-methoxyphenol (1) (2.2 g, 10.8 mmol) in
dimethylformamide (DMF, 11 mL) was stirred and cooled at 0 ꢁC as NaH
(0.52 g, 13 mmol) was added in one portion. After stirring at room
temperature for 20 min, bromomethyl methyl ether (2.0 mL, 22 mmol)
was added in one portion. The reaction was stirred at room temperature
for 1 h. Water (40 mL) was added, and the mixture was extracted with
EtOAc (3 ꢂ 40 mL). Flash chromatography (25 g silica, heptane:CH2Cl2
gradient) afforded the product as an oil (2.68 g, 65% yield).
[Carboxy-14C]ethyl 2-hydroxy-5-methoxy-benzoate (4)
A solution of [14C]5-methoxy-2-(methoxymethoxy)benzoic acid 3 (402 mg,
1,88 mmol) and H2SO4 (95%, 100 mL, 1.9mmol) in EtOH (8mL) was heated
at reflux for 2days. The solution was neutralized with saturated NaHCO3.
Water was added, and the reaction was extracted with EtOAc (5ꢂ 10 mL).
After drying the combined organic layers with Na2SO4, the solution was
filtrated and evaporated to afford a residue that was subjected to flash
chromatography (12 g silica, heptane:CH2Cl2, 3:7) to afford 4 as an oil
(221mg, 59% yield).
1H NMR (500 MHz, DMSO-d6) d: 7.13–7.18 (m, 2H) 6.91 (dd, J = 9.14,
3.15 Hz, 1H) 5.17 (s, 2H) 3.72 (s, 3H) 3.40 (s, 3H).
5-Bromo-6-fluoro-pyridin-2-amine (14)
[Carboxy-14C]ethyl 2-(2-ethoxy-2-oxo-ethoxy)-5-methoxy-
To a stirred solution of 2-amino-5-fluoropyridine (5.37 g, 47.9mmol) in
MeCN (250mL) at 0 ꢁC under nitrogen was added with N-bromosuccini-
mide (9.3g, 52.4mmol), and the reaction was stirred at room temperature
for 1h. The solvent was evaporated, and the residue was redissolved in
EtOAc and water. The separated organic phase was washed with water
and brine. After drying with Na2SO4, filtration and evaporation of the
residue was recrystallized from heptane/EtOAc (6.7g, 73% yield).
1H NMR (500 MHz, CDCl3) d: 7.60 (t, J = 8.51 Hz, 1H) 6.26 (dd, J = 8.35,
1.42 Hz, 1H) 3.81–4.94 (m, 2H).
benzoate (5)
To a stirred solution of NaI (32 mg, 0.21 mmol) and K2CO3 (161mg,
0.17 mmol) in DMF (1.5 mL) was added [14C]ethyl 2-hydroxy-5-methoxy-
benzoate 4 (210mg, 1.06 mmol) dissolved in DMF (1.5mL). The mixture
was stirred for 10 min after which ethyl bromoacetate (0.130 mL, 1.17mmol)
was added in one portion. The solution was stirred at room temperature for
24 h. A second portion of ethyl bromoacetate (0.03 mL, 0.3mmol) was
added, and the reaction was stirred for another 24 h. Water (2mL) was
added, and the mixture was extracted with Et2O (3ꢂ 10 mL). After drying
with Na2SO4, filtration, and evaporation, the residue was subjected to flash
chromatography (4g silica; CH2Cl2:MeOH 100:2) to give the product as an
oil (241mg, 80% yield).
t-Butyl 5-bromo-6-fluoropyridin-2-ylcarbamate (15)
To a solution of 5-bromo-6-fluoropyridin-2-amine (14) (4.1 g, 21.5 mmol)
in THF (60 mL) was slowly added a solution of 1 M sodium bis(trimethyl-
silyl)amide in THF (34.3 mL, 34.3 mmol) at 0 ꢁC. A solution of di-tert-
[Carboxy-14C]2-(carboxymethyloxy)-5-methoxy-benzoic acid (6)
butyl dicarbonate (4.9 mL, 21.5 mmol) in THF (40 mL) was added slowly A solution of [14C]ethyl-2-(2-ethoxy-2-oxoethoxy)-5-methoxybenzoate 5
to the reaction at 0 ꢁC. The reaction was then warmed to room tem- (308 mg, 1.08 mmol) in THF (1 mL) and MeOH (1 mL) was stirred as NaOH
perature and stirred over night. Saturated aqueous NH4Cl (20 mL) was (2.2 mL, 5 M) was added; a white solid started to precipitate. The reaction
added, and the mixture was extracted with ethyl acetate (2 ꢂ 40 mL). mixture was heated at reflux for 4 h during which it turned pink. Water
The combined organic layers were dried with Na2SO4, filtered, and eva- (5 mL) was added, and the mixture was washed with EtOAc (5 ꢂ 5 mL),
porated. The residue was purified by flash chromatography (silica, 7.5% which decolorized the mixture. The reaction was acidified with concen-
ethyl acetate in heptane) to afford the product as a solid (3.9 g, 63% trated HCl and extracted with EtOAc (5 ꢂ 10mL). The combined organic
yield).
extracts were dried with Na2SO4, filtered, and evaporated to give the
1H NMR (400 MHz, CDCl3) d: 7.75 (dd, J = 8.59, 1.26 Hz, 1H) 7.87 product (221 mg, 89% yield), which was used in the next step without
(t, J = 8.59 Hz, 1H) 7.37 (br. s., 1H) 1.52 (s, 9H).
J. Label Compd. Radiopharm 2013, 56 321–324
further purification.
Copyright © 2013 John Wiley & Sons, Ltd.
www.jlcr.org

J. Sandell
[3-14C]5-Methoxy-1-benzofuran-3-yl acetate (7)
[3-14C]2-[2-Fluoro-6-(methylamino)-3-pyridyl]benzofuran-5-ol
[
14C]AZD4694
A solution of 2-(carboxymethoxy)-5-methoxybenzoic acid 6 (110 mg,
0.48 mmol), NaOAc (165 mg, 2.0 mmol), HOAc (495 ml, 8.65 mmol), and
acetic anhydride (825 ml, 8.74 mmol) was stirred under reflux for 11 h.
Water was added, and the mixture was extracted with EtOAc
(3 ꢂ 10 mL). The combined organic layers were dried with Na2SO4, fil-
tered, and evaporated. The residue was subjected to flash chromatogra-
phy (4 g silica, heptane: CH2Cl2 7:3 and then CH2Cl2), which afforded the
product as solid (37 mg, 37% yield).
A solution of [14C]6-fluoro-5-(5-methoxybenzofuran-2-yl)-N-methyl-pyridin-
2-amine (11) (56 mg, 0.20 mmol) in CH2Cl2 (3mL) was cooled at ꢀ78 ꢁC
while BBr3 (400 mL, 4.23 mmol) was added. The reaction was stirred at 0ꢁC
for 18h; after which, it was cooled at ꢀ78 ꢁC, and a small amount of
water was added. Saturated aqueous NaHCO3 was added, and the
mixture was extracted with EtOAc (5ꢂ 5 mL). After drying with Na2SO4,
filtration, and evaporation, the residue was subjected to flash chromatogra-
phy (4g silica, heptane:EtOAc 6:4). Final purification was conducted on
reversed-phase HPLC. Most of the MeOH was evaporated, saturated
aqueous NaHCO3 was added, and the solution was extracted with ethyl
acetate (4 ꢂ 5mL). The combined organic layers were dried with Na2SO4,
filtered, and evaporated to afford the title product as a solid (35.6mg,
67% yield). The radiochemical purity was >99%, and the specific activity
was 2.1GBq/mmol.
[3-14C]5-Methoxybenzofuran-3-one (8)
A solution of 5-methoxybenzofuran-3-yl acetate 7 (73 mg, 0.35 mmol) in
MeOH (0.8 mL) and water (0.2 mL) was stirred as a drop of concentrated
HCl was added, and the reaction was stirred at 75 ꢁC for 1.5 h. The solvent
was evaporated, and the residue was used in the next step without
further purification.
LC/MS (M + H): 259 (10.2%) 260 (1.6%) 261 (100%) 262 (15.2%) 263
(1.8%). 1H NMR (600 MHz, MeOD-d4) d: 8.02 (dd, J = 9.89, 8.61 Hz, 1H)
7.27 (d, J = 8.68 Hz, 1H) 6.91 (d, J = 2.42 Hz, 1H) 6.78 (d, J = 2.99 Hz, 1H)
6.71 (dd, J = 8.68, 2.42 Hz, 1H) 6.44 (dd, J = 8.40, 1.85 Hz, 1H) 2.89 (s, 3H).
[3-14C]5-Methoxybenzofuran (9)
Acknowledgments
A solution of [14C]5-methoxybenzofuran-3-one (8) (70mg, 0.4mmol) in
EtOH (1 mL) was added to a solution of NaBH4 (31.9mg, 0.84mmol) in
ethanol (1mL), and the reaction was stirred at room temperature for 2 h.
HCl (200 mL, 2M) was added, and the reaction was stirred for 1 h at room
temperature. Water (5mL) was added, and the solution was extracted with
Et2O (5ꢂ 5 mL). After drying with Na2SO4, filtering, and evaporation, the
residue was subjected to flash chromatography (4 g silica, heptane:CH2Cl2
7:3), which afforded the product as solid (62mg, 98% yield from 7).
I would like to thank Stefan Elofsson for the analysis of radiolabeled
compounds, Alexandra Bernlind for the assistance with NMR, and
the isotope chemistry team for the fruitful discussions.
References
[1] W. E. Klunk, H. Engler, A. Nordberg, Y. Wang, G. Blomqvist, D. P. Holt,
M. Bergström, I. Savitcheva, G.-F. Huang, S. Estrada, B. Ausén,
M. L. Debnath, J. Barletta, B, J. C. Price, J. Sandell, B. J. Lopresti,
A. Wall, P. Koivisto, G. Antoni, C. A. Mathis, B. Långström, Ann.
Neurol. 2004, 55, 306–319.
[3-14C](5-Methoxybenzofuran-2-yl)boronic acid (10)
A solution of [14C]methoxybenzofuran 9 (64 mg, 0.43mmol) in THF (1.5mL)
cooled at ꢀ78 ꢁC was stirred as BuLi (1.6M in hexanes, 0.28mL) was added
slowly, and the reaction was stirred for 1 h at ꢀ78ꢁC. Triisopropyl borate
(0.2mL, 0.8mmol) was added, and the reaction was stirred for 1 h at
ꢀ78 ꢁC. HCl (2mL, 2M) was added to the reaction mixture, and the resulting
solution was extracted with EtOAc (4ꢂ 5mL). The combined organic layers
were dried with Na2SO4, filtered, and concentrated to approximately
100 mL. Heptane (approximately 1.5mL) was added upon which a solid
precipitated. The solid was isolated by centrifugation, followed by filtration,
and was dried under vacuum (72mg, 87% yield).
[2] N. Nelissen, K. van Laere, L. Thurfjell, R. Owenius, M. Vandenbulcke,
M. Koole, G. Bormans, D. J. Brooks, R. Vandenberghe, J. Nucl. Med.
2009, 8, 1251–1259.
[3] C. Rowe, C. Ackerman, U. Browne, W. R. Mulligan, K. L. Pike Kerry,
G. O’Keefe, H. Tochon-Danguy, G. Chan, S. U. Berlangieri, G. Jones,
K. L. Dickinson-Rowe, H. P. Kung, W. Zhang, M. P. Kung Mei, D. Skovronsky,
T. Dyrks, G. Holl, S. Krause, M. Friebe, L. Lehman, S. Lindemann,
L. M. Dinkelborg, C. L. Masters, V. L. Villemagne, Lancet Neurol.,
2008, 7, 129–135.
[4] D. F. Wong, P. B. Rosenberg, Y. Zhou, A. Kumar, V. Raymont,
H. T. Ravert, R. F. Dannals, A. Nandi, J. R. Brasic, W. Ye, J. Hilton,
C. Lyketsos, H. F. Kung, A. D. Joshi, D. M. Skovronsky, M. J. Pontecorvo,
J. Nucl. Med. 2010, 6, 913–920.
[5] C. A. Mathis, B. J. Lopresti, W. E. Klunk, Nucl. Med. Biol. 2007, 34,
809–822.
[3-14C]6-Fluoro-5-(5-methoxybenzofuran-2-yl)-N-methyl-
pyridin-2-amine (11)
[6] J. O. Rinne, D. Brooks, M. Rossor, N. Mortin, N. C. Fox, R. Bullock,
W. E. Klunk, C. A. Mathis, K. Blennow, J. Barakos, A. A. Okello,
S. Rodriguez, M. de Llano, E. Liu, M. Koller, K. M. Gregg, D. Schenk,
R. Black, M. Grundman, Lancet Neurol. 2010, 9, 363–372.
[7] A. Jureus, B.-M. Swahn, J. Sandell, F. Jeppsson, A. E. Johnson, P. John-
ström, J. A. M. Neelissen, D. Sunnemark, L. Farde, S. P. S. Svensson,
J. Neurochem. 2010, 114, 784–794.
[8] Z. Cselényi, M. Eriksdotter Jönhagen, A. Forsberg, C. Halldin, P. Julin,
M. Schou, P. Johnström, K. Varnäs, S. Svensson, L. Farde, J. Nucl. Med.
2012, 53, 415–424.
[9] B.-M. Swahn, J. Sandell, D. Pyring, M. Bergh, F. Jeppsson, A. Juréus,
J. Neelissen, P. Johnström, M. Schou, S. Svensson, Bioorg. Med. Chem.
Lett. 2012, 22, 4332–4337.
[10] A. H. Wadsworth, H. A. Mitchell, I. Fellows, D. R. Sutherland, J. Labelled
Compd. Radiopharm. 1996, 38, 863–871.
A solution of [14C](5-methoxybenzofuran-2-yl)boronic acid 10 (72 mg,
0.37 mmol), tert-butyl 5-bromo-6-fluoropyridin-2-yl(methyl)carbamate
16 (113 mg, 0.37 mmol), 2 M Na2CO3 (0.557 mL, 1.11 mmol) and 1,10-
bis(diphenylphosphino)ferrocene dichloropalladium(II) dichloromethane
complex (13.6 mg, 0.02 mmol) in THF (2.5 mL) was deoxygenated by
bubbling nitrogen through the mixture for 10 min before capping.
The reaction was heated for 1 h at 110 ꢁC by microwaves. Water was
added, and the mixture was extracted with EtOAc (3 ꢂ 10 mL). The
combined organic layers were dried with Na2SO4, filtered, and evapo-
rated. The residue was dissolved in CH2Cl2 (2 mL) and cooled at 0 ꢁC,
and TFA (500 ml, 6.49 mmol) was added. The mixture was stirred at
room temperature for 30 min; after which, it was neutralized with satu-
rated NaHCO3 and extracted with CH2Cl2 (4 ꢂ 5 mL). The combined
organic layers were dried with Na2SO4, filtered, and evaporated. The
residue was dissolved in CH2Cl2 (approximately 2 mL). Heptane
(approximately 3 mL) was added until the product precipitated. The
solid was collected by centrifugation and further triturated with hep-
tane before being dried under vacuum (56 mg, 55% yield).
[11] C. F. Carvalho, M. V. Sargent J. Chem. Soc., Perkin Trans. 1 1984,
1605–1612.
[12] N. Hadj-esfandiari, L. Navidpour, H.Shadnia, M. Amini, N. Samadi, M. A.
Faramarzi, A. Shafiee, Bioorg. Med. Chem. Lett. 2007, 17, 6354–6363.
[13] C. R. Edwards, M. J. Readhead, N. J. Tweddle, Het. Chem. 1987, 24,
495–496.
[14] Y. Kawase, S. Nakamoto Bull. Chem. Soc. Jpn. 1962, 35, 1624–1625.
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