A novel one-pot palladium-mediated synthesis of N-[(2-hydroxyphenyl)methyl]-N-(4-phenoxy-3-pyridinyl) acetamide, the precursor to [11C]PBR28, a PET biomarker for the peripheral benzodiazepine receptor

Article abstract of DOI:10.1016/j.tetlet.2010.04.089

Due to an urgent need to image the peripheral benzodiazepine receptor (PBR) in living human subjects using positron emission tomography imaging, we had cause to prepare N-[(2-hydroxyphenyl)methyl]-N-(4-phenoxy-3-pyridinyl) acetamide (desmethyl-PBR28 (1)), the precursor to [¹¹C]PBR28. Herein, we report a new synthesis of the precursor in which palladium-mediated reduction of the nitro pyridine to the corresponding amino pyridine, and subsequent reductive amination, can be achieved with decaborane in a convenient one-pot procedure. This procedure is operationally simpler than the current alternatives and provides high quality precursor suitable for use in clinical applications.

Full text of DOI:10.1016/j.tetlet.2010.04.089

Tetrahedron Letters 51 (2010) 3353–3355
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A novel one-pot palladium-mediated synthesis of
N-[(2-hydroxyphenyl)methyl]-N-(4-phenoxy-3-pyridinyl) acetamide, the
precursor to [11C]PBR28, a PET biomarker for the peripheral
benzodiazepine receptor
*
Raphaël Hoareau, Peter J. H. Scott
Department of Radiology, University of Michigan, Ann Arbor, MI 48109, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Due to an urgent need to image the peripheral benzodiazepine receptor (PBR) in living human subjects
using positron emission tomography imaging, we had cause to prepare N-[(2-hydroxyphenyl)methyl]-
N-(4-phenoxy-3-pyridinyl) acetamide (desmethyl-PBR28 (1)), the precursor to [11C]PBR28. Herein, we
report a new synthesis of the precursor in which palladium-mediated reduction of the nitro pyridine
to the corresponding amino pyridine, and subsequent reductive amination, can be achieved with decab-
orane in a convenient one-pot procedure. This procedure is operationally simpler than the current alter-
natives and provides high quality precursor suitable for use in clinical applications.
Received 25 March 2010
Revised 20 April 2010
Accepted 21 April 2010
Available online 28 April 2010
Keywords:
[
11C]PBR28
Ó 2010 Elsevier Ltd. All rights reserved.
Positron emission tomography
Radiopharmaceutical synthesis
The use of positron emission tomography (PET) imaging to non-
invasively diagnose disease, monitor patient response to therapy,
and elucidate biochemical mechanisms, all in living human subjects,
is becoming increasingly popular in the global clinical setting.1 In
addition to widespread clinical application, the use of PET imaging
within the pharmaceutical arena is also expanding, where it is
quickly becoming invaluable in drug discovery programs and to
monitor the impact of experimental drugs in clinical trials.2
Presently, there is significant interest in imaging the peripheral
benzodiazepine receptor (PBR), also known as the translocator pro-
tein 18 kDa,3 and a number of PET biomarkers for the PBR have
been reported in recent years.4–8 The PBR is different from the
research groups have concentrated in developing new PET ligands
for PBR and a number of new PET ligands, including DPA 713,5
[
recently reported. In choosing a PET ligand to make available to
clinical investigators at the University of Michigan, we selected
N-(2-[11C]methoxybenzyl)-N-(4-phenoxy-3-pyridinyl) acetamide
([11C]PBR28, 2), developed by Pike and colleagues, because of its
high affinity for PBR (IC50 = 0.658 nm), favorable pharmacokinetics,
and appropriate dosimetry.9
11C]DAA1106,6 18F]FEDAA1106,7 and [11C]PBR28,8 have been
[
The radiosynthesis of [11C]PBR28 is carried out in our laboratory
using well-established radiochemical reactions with carbon-1110
and has been extensively described by both Pike8a and Zheng8c last
year. Briefly, [11C]CO2 is delivered from a General Electric (GE) PET-
Trace cyclotron and converted to [11C]CH4 using a nickel-mediated
reduction in the presence of H2(g) at 350 °C. [11C]CH4 is then con-
verted to [11C]CH3I by reaction with iodine at 720 °C, and the reactiv-
ity of [11C]CH3I is enhanced by passing over a column of AgOTf at
200 °C to provide [11C]CH3OTf. The N-[(2-hydroxyphenyl)methyl]-
N-(4-phenoxy-3-pyridinyl) acetamide (PBR28 precursor, 1) is then
methylated with [11C]CH3OTf to provide [11C]PBR28 which is subse-
quently purified by semi-preparative HPLC. In our laboratory, the
whole process is automated by a GE Tracerlab FXC-Pro synthesis mod-
ule (Scheme 1). In order to provide [11C]PBR28 to physicians, we
needed a supply of high quality precursor 1, suitable for use in clin-
ical research.
brain receptors that bind
c-amino butyric acid (GABA) and syn-
thetic benzodiazepines and is found on the outer membrane of
mitochondria in a number of cells, as well as plasma membranes
in erythrocytes. Imaging PBR using PET is of interest to nuclear
medicine physicians because PBR has been implicated in cancer
as well as a range of neurodegenerative diseases and nervous sys-
tem disorders. Moreover, an increase in PBR levels, which could be
quantified by increased uptake of a PBR biomarker, is considered to
be indicative of inflammation. A number of PET biomarkers that al-
low for imaging and evaluation of PBR are known and the most
common to date is [11C]PK11195.4 However, this biomarker is re-
ported to have a number of limitations including low brain pene-
tration and high non-specific binding. Therefore, a number of
A number of different syntheses for the preparation of desmeth-
yl-PBR28 (1) have been reported. They all follow the general sche-
matic illustrated in Figure 1 in which nitro pyridine 5 is first
* Corresponding author. Tel.: +1 734 615 1756; fax: +1 734 615 2557.
E-mail address: pjhscott@umich.edu (P.J.H. Scott).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.089

3354
R. Hoareau, P. J. H. Scott / Tetrahedron Letters 51 (2010) 3353–3355
O
O
by Pike’s group in which the reduction was achieved by treating
with concd HCl in the presence of iron powder, and subsequent
reductive amination, also using sodium borohydride and Dean–
Stark conditions.8a
N
O
N
O
N
N
[
11C]CH3OTf
OH
O11CH3
However, as we considered strategies for the preparation of the
gram quantities of PBR28 precursor necessary to meet our routine
clinical demand, we were excited to read Yoon’s report of a one-pot
palladium/decaborane-mediated reduction of nitroaromatics fol-
lowed by subsequent reductive amination12 and were curious as
to whether it could be adapted for our needs. The use of Pd/C
makes the work-up faster, easier, and sustainable with a low ex-
pected Pd contamination.13 Whilst decaborane is also toxic,14 in
our experience it is easier to remove during work-up than stannous
chloride, and such a synthesis of the PBR28 precursor would be
operationally simpler than those previously reported.
Our novel synthetic approach to the PBR28 precursor is
illustrated in Scheme 2. Initially, commercially available 3-nitro-
pyridin-4-ol 3 was treated with PCl5 and POCl3 to provide 4-
chloro-3-nitropyridine 4 (73% yield) according to Pike’s procedure.8a
With 4 in hand, Wang’s procedure8c was employed to couple it with
phenol, in the presence of potassium carbonate, to provide 5 in 77%
yield without the need for chromatographic purification. Note: 4-
chloro-3-nitropyridine (4) is also commercially available, but the
commercial material was found to be ꢀ80% pure by 1H NMR analy-
sis. Consequently, following coupling with phenol, 5 had to be puri-
fied by flash chromatography and limited recovery from the column
1
2
Scheme 1. Radiosynthesis of [11C]PBR28.
reduced to the corresponding amine 6. Amine 6 is initially coupled
with o-salicylaldehyde via reductive amination, and subsequent
acetylation provides PBR28 precursor 1. In the synthesis reported
by Zheng and co-workers in 2009,8c the reduction of the nitro
group was achieved by refluxing with concd HCl in methanol in
the presence of SnCl2, and then a separate sodium borohydride-
mediated reductive amination to provide 1. This approach is effec-
tive and, indeed, we have previously used this strategy to prepare
PBR28 precursor in our laboratory. However, as stannous chloride
is highly toxic,11 there is general reluctance to use such tin com-
pounds during the synthesis of precursors that will be used in
the syntheses of clinical radiopharmaceutical doses. An alternative
and more attractive approach was the two-pot procedure reported
O
N
NO2
O
NH2
N
N
N
Reduction
1. Reductive Amination
2. Acetylation
OH
O
O
5
6
1
Figure 1. General strategy for preparation of PBR28 precursor.
NO2
N
NO2
OH
Phenol
K2CO3, DMF
80 ºC, 1 h
NO2
Cl
O
N
N
PCl5, POCl3
Δ
(73 %)
(77 %)
3
4
5
1. Pd/C, B10H14
MeOH, Δ, 30 min
2. B10H14
,
o-salicylaldehyde
MeOH, rt, 2 h
(25 % - 2 Steps)
O
H
N
N
N
N
1. Ac2O, AcOH
130 ºC, 2 h
2. 5% NaOH in MeOH
rt, 5 h
OH
OH
O
O
(50 % - 2 Steps)
1
7
Scheme 2. Synthesis of N-[(2-hydroxyphenyl)methyl]-N-(4-phenoxy-3-pyridinyl) acetamide (1).

R. Hoareau, P. J. H. Scott / Tetrahedron Letters 51 (2010) 3353–3355
3355
Neurol. Belg. 2002, 102, 127–135; (b) Hashimoto, K.; Inoue, O.; Suzuki, K.;
Yamasaki, T.; Kojima, M. Ann. Nucl. Med. 1989, 3, 63–71.
5. James, M. L.; Fulton, R. R.; Henderson, D. J.; Eberl, S.; Meikle, S. R.; Thomson, S.;
Allan, R. D.; Dolle, F.; Fulham, M. J.; Kassiou, M. Bioorg. Med. Chem. 2005, 13,
6188–6194.
resulted in loweryields (50%)of 5 from this couplingreaction. There-
fore, it is preferential to prepare 4 in house (>90% purity) to avoid
purification, by flash chromatography, of intermediate 5.
With nitropyridine 5 in hand, it was subjected to Yoon’s proce-
dure,12 on gram scale, by dissolving in methanol (50 mL) and treat-
ing with palladium on carbon (0.05 equiv) and decaborane
(0.3 equiv).15 Refluxing for 30 min was sufficient to reduce the ni-
tro group to the corresponding amine and after this time the reac-
tion was cooled to room temperature. Salicylaldehyde (1.2 equiv)
and additional decaborane (0.3 equiv) were added and the reaction
was stirred for a further 2 h at rt. After this time, the reaction was
concentrated in vacuo and methanol (20 mL) was added to the
concentrate to triturate 7 as a white powder (25% yield, >95% pur-
ity by 1H NMR). The somewhat low yield was attributed to loss of
material during purification by trituration, but the loss was
deemed acceptable given the simplicity of the purification. How-
ever, due to the toxicity of decaborane,14 we wished to confirm
that, whilst convenient, trituration was indeed also suitable for re-
moval of residual decaborane from product 7. The proton NMR
spectrum of decaborane is complex due to the caged structure
and both proton–proton and proton–boron coupling interactions.16
Nevertheless, the NMR spectrum for intermediate 7 revealed no
signals attributable to decaborane, confirming residual levels at
least below the NMR limit of detection.
Finally, it was necessary to acetylate 7 to provide desmethyl-
PBR28 1. Initially, we followed Zheng’s room temperature proce-
dure8c (AcCl (2.2 equiv), DMAP (2.5 equiv), DCM, rt) but this reaction
turned out to be poor in our hands and we only acetylated the phenol
function. Therefore, we employed Pike’s acetylation procedure8a
(refluxing mixture of acetic acid and acetic anhydride) to provide
the O- and N-diacetylated species. Subsequent treatment with 5%
sodium hydroxide in methanol at rt was sufficient to deprotect the
phenol, whilst leaving the amide intact, and provided [11C]PBR28
precursor 1 (50% from 7, >95% purity by 1H NMR).17
6. (a) Zhang, M. R.; Kida, T.; Noguchi, J.; Furutsuka, K.; Maeda, J.; Suhara, T.;
Suzuki, K. Nucl. Med. Biol. 2003, 30, 513–519; (b) Maeda, J.; Suhara, T.; Zhang,
M. R.; Okauchi, T.; Yasuno, F.; Ikoma, Y.; Inaji, M.; Nagai, Y.; Takano, A.;
Obayashi, S.; Suzuki, K. Synapse 2004, 52, 283; (c) Probst, K. C.; Izquierdo, D.;
Bird, J. L. E.; Brichard, L.; Franck, D.; Davies, J. R.; Fryer, T. D.; Richards, H. K.;
Clark, J. C.; Davenport, A. P.; Weissberg, P. L.; Warburton, E. A. Nucl. Med. Biol.
2007, 34, 439–446; (d) Ikomo, Y.; Yasuno, F.; Iti, H.; Suhara, T.; Ota, M.; Toyama,
H.; Fujimara, Y.; Takano, A.; Maeda, J.; Zhang, M. R.; Nakao, R.; Suzuki, K. J.
Cereb. Blood Flow Metab. 2007, 27, 173–184.
7. (a) Zhang, M. R.; Maeda, J.; Furutsuka, K.; Yoshida, Y.; Ogawa, M.; Suhara, T.;
Suzuki, K. Bioorg. Med. Chem. Lett. 2003, 13, 201–204; (b) Zhang, M.-R.; Maeda,
J.; Ogawa, M.; Noguchi, J.; Ito, J.; Yoshida, Y.; Okauchi, T.; Obayashi, S.; Suhara,
T.; Suzuki, K. J. Med. Chem. 2004, 47, 2228–2235; (c) Fujimara, Y.; Ikoma, Y.;
Yasuno, F.; Suhara, T.; Ota, M.; Matsumoto, R.; Nozaki, S.; Takano, A.; Kosaka, J.;
Zhang, M.-R.; Nakao, R.; Suzuki, K.; Kato, N.; Ito, H. J. Nucl. Med. 2006, 47, 43–50.
8. (a) Briard, E.; Zoghbi, S. S.; Imaizumi, M.; Gourley, J. P.; Shetty, H. U.; Hong, J.;
Cropley, V.; Fujita, M.; Innis, R. B.; Pike, V. W. J. Med. Chem. 2008, 51, 17–30; (b)
Imaizumi, M.; Kim, H.-J.; Zoghbi, S. S.; Briard, E.; Hong, J.; Musachio, J. L.;
Ruetzler, C.; Chuang, D.-M.; Pike, V. W.; Innis, R. B.; Fujita, M. Neurosci. Lett.
2007, 411, 200–205; (c) 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,
5636–5639; (d) Yasuno, F.; Ota, M.; Kosaka, J.; Ito, H.; Higuchi, M.; Doronbekov,
T. K.; Nozaki, S.; Fujimura, Y.; Koeda, M.; Asada, T.; Suhara, T. Biol. Psychiatry
2008, 64, 835–841; (e) Fujimura, Y.; Ikoma, Y.; Yasuno, F.; Suhara, T.; Ota, M.;
Matsumoto, R.; Nozaki, S.; Takano, A.; Kosaka, J.; Zhang, M. R.; Nakao, R.;
Suzuki, K.; Kato, N.; Ito, H. J. Nucl. Med. 2006, 47, 43–50.
9. Brown, A. K.; Fujita, M.; Fujimura, Y.; Liow, J.-S.; Stabin, M.; Ryu, Y. H.;
Imaizumi, M.; Hong, J.; Pike, V. W.; Innis, R. B. J. Nucl. Med. 2007, 48, 2072–
2079.
10. (a) Scott, P. J. H. Angew. Chem., Int. Ed. 2009, 48, 6001–6004; (b) Miller, P. W.;
Long, N. J.; Vilar, R.; Gee, A. D. Angew. Chem., Int. Ed. 2008, 47, 8998–9033.
11. (a) El-Makawy, A. I.; Girgis, S. M.; Khalil, W. K. B. Mutat. Res. 2008, 657, 105–
110; (b) Silva, C. R.; Oliveira, M. B.; Melo, S. F.; Dantas, F. J.; de Mattos, J. C.;
Bezerra, R. J.; Caldeira-de-Araujo, A.; Duatti, A.; Bernardo-Filho, M. Brain Res.
Bull. 2002, 59, 213–216; (c) Der Muelen, H. C.; Feron, V. J.; Til, H. P. Pathol. Eur.
1974, 9, 185–192.
12. Bae, J. W.; Cho, Y. J.; Lee, S. H.; Yoon, C.-O. M.; Yoon, C. M. Chem. Commun. 2000,
1857–1858.
13. Felpin, F.-X.; Ayad, T.; Mitra, S. Eur. J. Org. Chem. 2006, 2679–2690.
14. Naeger, L. L.; Leibman, K. C. Toxicol. Appl. Pharmacol. 1972, 22, 517–527.
15. Typical experimental procedure using the decaborane-Pd/C system: Under
argon, Pd/C 10% (307.5 mg, 0.29 mmol, 0.06 equiv) was added to a solution of 5
(1.04 g, 4.83 mmol) in MeOH (50 mL). Then, a solution of B10H14 (177 mg,
1.45 mmol, 0.3 equiv) in MeOH (5 mL) was added. The heterogeneous solution
In conclusion,
a simplified synthesis of N-[(2-hydroxy-
phenyl)methyl]-N-(4-phenoxy-3-pyridinyl) acetamide (1), the pre-
cursor for [11C]PBR28, has been developed, employing a one-pot
reduction–reductive amination procedure. This synthetic strategy
is operationally simpler than other reported two-pot procedures
as it eliminates the need to use toxic tin(II) chloride, refluxing
hydrochloric acid, and Dean–Stark reductive amination conditions.
Clinical research with [11C]PBR28 is currently underway and will
be reported in due course.
was refluxed for 30 min and then cooled back to rt.
A solution of
salicylaldehyde (682 mg, 5.58 mmol, 1.2 equiv) in MeOH (5 mL) followed by
an additional solution of B10H14 (180 mg, 1.47 mmol, 0.3 equiv) in MeOH
(5 mL) were added to the reaction mixture. The resulting green solution was
then stirred at rt for 2 h. After this time, the reaction mixture was filtrated off
over a Celite pad, washed with MeOH (200 mL), and concentrated in vacuo. The
yellow crude solid was triturated with MeOH (20 mL) to give compound 7 as a
white solid (358 mg, 25%, >95% purity by 1H NMR). 1H NMR (DMSO-d6,
300 MHz): d 9.63 (s, 1H), 7.85 (s, 1H), 7.70 (d, J = 5.1 Hz, 1H), 7.46 (t, J = 7.5 Hz,
2H), 7.26–7.03 (m, 5H), 6.84–6.71 (m, 2H), 6.53 (d, J = 5.1 Hz, 1H), 5.95 (t,
J = 6.1 Hz, 1H), 4.34 (d, J = 6.1 Hz, 2H). HRMS calcd for C18H17N2O2 (M+H+),
293.1290; found, 293.1287.
Acknowledgment
The University of Michigan Cyclotron and Radiochemistry
group gratefully acknowledge funding for this research from the
Office of Biological and Environmental Research (BER) of the Office
of Science (SC), US Department of Energy (DE-FG02-08ER64645).
16. Decaborane: 1H NMR (CDCl3, 500 MHz): d 2.17–4.69 (m, 10H), À0.11 to 1.46
(m, 4H).
17. Acylation procedure: under argon,
a solution of compound 7 (355 mg,
1.21 mmol), DMAP (56 mg, 0.46 mmol, 0.4 equiv), and Ac2O (265 mg,
2.60 mmol, 2.1 equiv) in acetic acid (3 mL) was heated at 130 °C for 2 h.
After cooling back to rt, the reaction mixture was diluted in water (50 mL) and
extracted with ethyl acetate (150 mL). The collected organic layers were
washed with aq satd NaHCO3 (25 mL), brine (25 mL), dried over MgSO4, and
concentrated in vacuo. The crude colorless oil was then re-dissolved in a
solution of NaOH in MeOH (5% w/v, 6 mL) and stirred at rt for 5 h. After
removing the volatiles in vacuo, the residue was diluted in an aq solution of
HCl (pH 5, 50 mL) and extracted with ethyl acetate (150 mL). The combined
organic layers were washed with aq satd NaHCO3 (25 mL), brine (25 mL), dried
over MgSO4, and concentrated in vacuo. The crude white solid was purified by
flash chromatography (hexane/ethyl acetate 1/2.5) to give desmethyl-PBR28
(1) as a white solid (203 mg, 50%, >95% purity by 1H NMR). 1H NMR (CDCl3,
500 MHz): d 9.28 (s, 1H), 8.41 (d, J = 3.4 Hz, 1H), 8.31 (s, 1H), 7.39 (t, J = 7.9 Hz,
2H), 7.28–7.23 (m, 2H), 6.94 (d, J = 8.1 Hz, 1H), 6.77 (d, J = 8.1 Hz, 2H), 6.72 (t,
J = 7.9 Hz, 1H), 6.67–6.62 (m, 2H), 4.82 (s, 2H), 2.02 (s, 3H). HRMS calcd for
C20H18N2NaO3 (M+Na+), 357.1215; found, 357.1200.
References and notes
1. Ametamey, S. M.; Honer, M.; Schubiger, P. A. Chem. Rev. 2008, 108, 1501–1516.
2. Bergström, M.; Hargreaves, R. J.; Burns, H. D.; Goldberg, M. R.; Sciberras, D.;
Reines, S. A.; Petty, K. J.; Ögren, M.; Antoni, G.; Långström, B.; Eskola, O.;
Scheinin, M.; Solin, O.; Majumdar, A. K.; Constanzer, M. L.; Battisti, W. P.;
Bradstreet, T. E.; Gargano, C.; Hietala, J. Biol. Psychiatry 2004, 55, 1007–1012.
3. Recent review of progress in PBR pathology and imaging: (a) Venneti, S.;
Lopresti, B. J.; Wiley, C. A. Prog. Neurobiol. 2006, 80, 308–322; (b) Papadopoulos,
V.; Baraldi, M.; Guilarte, T. R.; Knudsen, T. B.; Lacapere, J.-J.; Lindemann, P.;
Norenberg, M. D.; Nutt, D.; Weizman, A.; Zhang, M.-R.; Gavish, M. Trends
Pharmacol. Sci. 2006, 27, 402–409.
4. (a) Debruyne, J. C.; Van Laere, K. J.; Versijpt, J.; De Vos, F.; Eng, J. K.; Strijckmans,
K.; Santens, P.; Achten, E.; Slegers, G.; Korf, J.; Dierckx, R. A.; De Reuck, J. L. Acta