Synthesis of 11C-Labelled Symmetrical Ureas via the Rapid Incorporation of [11C]CO2 into Aliphatic and Aromatic Amines

Article abstract of DOI:10.1055/s-0034-1381055

An efficient method to radiolabel symmetrical [¹¹C]ureas using 1,8-diazabicycloundec-7-ene (DBU) and cyclotron-produced [¹¹C]CO₂ has been developed. [¹¹C]urea derivatives were obtained when aliphatic and aromatic amines were used with excellent radiochemical yields (RCYs) of over 70%. The mechanism of the reaction is proposed on the basis of control experiments. This simple and robust methodology provides a powerful tool to prepare ¹¹C-labelled ureas previously inaccessible by existing methods and enable their utilisation for in vivo molecular imaging applications.

Full text of DOI:10.1055/s-0034-1381055

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
© Georg Thieme Verlag Stuttgart · New York
2015, 26, 2257–2260
letter
2257
A. K. Dheere et al.
Letter
Synlett
Synthesis of 11C-Labelled Symmetrical Ureas via the Rapid Incor-
poration of [11C]CO2 into Aliphatic and Aromatic Amines
O
R1
Abdul Karim Haji Dheere
Salvatore Bongarzone
Carlotta Taddei
*
1) DBU, CO2, 1 min
R1
R1
C
*
HN
R2
N
R2
N
R2
2) DBAD, PBu3, 4 min
MeCN, 50 °C
Ran Yan
R1 = alkyl or aromatic group
R2 = H or alkyl group
Antony D. Gee*
RCY = 70–88% (7 entries)
Division of Imaging Sciences and Biomedical Engineering,
King’s College London, 4th Floor Lambeth Wing, London,
SE1 7EH, UK
* indicates 11C radiolabelling position
antony.gee@kcl.ac.uk
Received: 14.05.2015
Accepted after revision: 02.07.2015
Published online: 04.09.2015
We have recently developed a method to radiolabel
asymmetric ureas with 11C using [11C]CO2 directly from the
cyclotron (Scheme 1).5 However, when the method was ap-
plied to the synthesis of symmetrical ureas, low radio-
chemical yields (RCYs) were observed.
DOI: 10.1055/s-0034-1381055; Art ID: st-2015-d0366-l
Abstract An efficient method to radiolabel symmetrical [11C]ureas us-
ing 1,8-diazabicycloundec-7-ene (DBU) and cyclotron-produced
[
11C]CO2 has been developed. [11C]urea derivatives were obtained when
R2
H
N
aliphatic and aromatic amines were used with excellent radiochemical
yields (RCYs) of over 70%. The mechanism of the reaction is proposed
on the basis of control experiments. This simple and robust methodolo-
gy provides a powerful tool to prepare 11C-labelled ureas previously in-
accessible by existing methods and enable their utilisation for in vivo
molecular imaging applications.
*
H
N
1) DBU, CO2, 1 min
N
*
R3
NH2
R1
+
R1
R3
R2
2) DBAD, PBu3
O
* indicates 11C-radiolabelling position
Scheme 1 Synthesis of asymmetric [11C]ureas using [11C]CO2
Key words carbon-11, carbon dioxide, Mitsunobu reaction, symmet-
rical urea, DBU
We hereby report a new development of the previous
methodology that allows the radiolabelling of symmetrical
ureas in moderate to excellent RCYs using a Mitsunobu re-
action.
Positron emission tomography (PET) is a molecular im-
aging technique that is increasingly being used for diagno-
sis, biological research and drug development studies.1
Short-lived carbon-11 (t1/2 = 20.4 min) is one of the most
commonly used radioisotopes for PET imaging. Carbon-11
(11C) is produced by a cyclotron in the form of [11C]CH4 or
[11C]CO2 by the 14N(p,α)11C nuclear reaction.1 From a syn-
thetic perspective, both products are converted into more
reactive secondary synthons such as [11C]methyl iodide,
[11C]methyl tosylate for radiolabelling.2 Although these sec-
ondary synthons are undoubtedly useful they require time-
consuming processing. Therefore, the development of
methods to radiolabel compounds directly and efficiently
with [11C]CO2 is of significant interest.1–3 Due to low CO2 re-
activity, there are only a handful of radiosynthetic methods
available to incorporate [11C]CO2 directly into target mole-
cules of interest.4 A challenge that needs to be addressed
when developing synthetic methods with 11C is that reac-
tions are typically performed with reactants and support-
ing reagents in large excess compared to the 11C synthons
(typically [11C]CO2 is used on a nanomolar scale).
Initially, 11C-radiolabelling of bis(4-methylpiperazin-1-
yl)methanone ([11C]1) was performed in two steps. Cyclo-
tron-produced [11C]CO2 was bubbled in a solution contain-
ing N-methyl piperazine (18.3 μmol), 1,8-diazabicycloun-
dec-7-ene (DBU, 0.8 μmol) in acetonitrile (MeCN, 300 μL) at
room temperature (r.t.). The solution was heated at 50 °C
and Mitsunobu reagents, di-tert-butyl azodicarboxylate
(DBAD, 36.6 μmol) and tributylphosphine (PBu3, 36.6
μmol), dissolved in MeCN (100 μL) were added. The mixture
was stirred for 4 minutes at 50 °C.5 Following this proce-
dure, [11C]1 was obtained with a RCY of 31% (Table 1, entry
1). RCY was determined by radio-HPLC and defined as the
amount of labelled [11C]urea as a percentage of the cyclo-
tron-produced [11C]CO2 trapped in solution.
To optimise the RCY of [11C]1 further, the effects of vary-
ing reaction time, solvent, temperature and reagent con-
centrations were studied.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2257–2260

2258
A. K. Dheere et al.
Letter
Synlett
Changing the reaction time from 5 to 15 minutes had no
effect on the RCY (Table 1, entry 2). Next, screening of dif-
ferent solvents was carried out. [11C]1 was not obtained
when toluene, THF, and DMF were used as solvent (Table 1,
entries 3–5) whereas DMSO (Table 1, entry 6) afforded the
desired product with low RCY (24%). MeCN (Table 1, entry
1) produced the highest RCY (31%). Therefore, it was select-
ed as the solvent of choice for further investigations.
We have previously reported that a reaction tempera-
ture of 50 °C is optimal for the formation of asymmetrical
[11C]ureas.5 A similar trend was observed for the synthesis
of symmetrical [11C]ureas whereby the RCY decreased from
31% at 50 °C to 14% at 25 °C (Table 1, entries 1 and 7).
Increasing the concentration of the secondary amine
did not improve the RCY noticeably (Table 1, entry 8). How-
ever, increasing the concentration of DBU from 0.8 μmol to
3.4 μmol (Table 1, entry 9) significantly increased the RCY
from 35% to 84% while trapping the [11C]CO2 efficiently
(95%). The increase in RCY can be explained by the influ-
ence of DBU on the reaction mixture as it promotes the for-
mation of the carbamate anion (intermediate I, Scheme 2).
A further increase in DBU concentration from 0.8 to 6.8
μmol caused a drop of the RCY to 15% (Table 1, entry 10).
The latter may be due to high concentrations of DBU se-
questering protons needed for the protonation of Mitsuno-
bu intermediate I (see Supporting Information, Figure SI2)
resulting in the low RCY.
In attempts to increase the RCY further, DBU was re-
placed with another trapping reagent, 2-tert-butylimino-2-
diethylamino-1,3-dimethylperhydro-1,3,2-diazaphos-
phorine (BEMP).4 High [11C]CO2 trapping was obtained
when BEMP was used however, no [11C]1 was observed (see
Supporting Information, Table SI1 and Table 1, entry 11).
To explore the applicability of the developed method,
the optimised conditions were applied to the radiolabelling
of various symmetrical ureas using a range of aliphatic and
aromatic amines (Table 2). High RCYs (>80%) of the desired
[11C]ureas were observed for primary and secondary
amines such as 1-tetrahydroisoquinoline, benzylamine and
cyclohexylamine (2–4). The less reactive aromatic amine
aniline was also tested and interestingly, 5 was obtained
with a high RCY of 79%. The effect of functional groups on
aniline was studied. Aniline derivatives bearing electron-
Table 2 11C-Radiolabelling of Different Symmetrical Ureas7
O
R1
*
1) DBU, CO2
R1
R1
HN
R2
N
R2
N
R2
11C]2–7
*
2) DBAD, PBu3
[
Table 1 Reaction Optimisation to Radiolabel [11C]16
Compounda
[
11C]Urea
RCY (%)b
88 ± 6
O
O
*
*
1) DBU, CO2
N
N
[
N
*
2
N
N
2) DBAD, PBu3
NH
N
N
11C]1
O
*
Entrya
Time (min)
Trapping reagent
Solvent
RCY (%)b
3
85 ± 5
N
H
N
H
1
2
5
15
5
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
BEMP
–
MeCN
MeCN
toluene
THF
31 ± 8
33 ± 2
0
3
O
*
4
5
6
83 ± 2
79 ± 14
70 ± 2
4
5
0
N
H
N
H
5
5
DMF
0
6
5
DMSO
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
24 ± 12
14 ± 1
35 ± 3
84 ± 4
15 ± 7
0
O
*
7c
8d
9e
10f
11
12
5
N
H
N
H
5
O2N
NO2
5
O
*
5
N
H
N
H
5
5
0
O
O
O
*
a Reaction conditions: [11C]CO2, amine (18.3 μmol), DBU (0.8 μmol), Mitsu-
nobu reagents (36.6 μmol) in MeCN (400 μL) at 50 °C for 5 min.
b RCY was determined by radio-HPLC (n = 3).
7
72 ± 6
N
H
N
H
c Reaction was performed at 25 °C.
a Reaction conditions: [11C]CO2, amine (18.3 μmol), DBU (0.8 μmol), Mitsu-
nobu reagents (36.6 μmol) in MeCN (400 μL) at 50 °C for 5 min.
b RCY determined by radio-HPLC (n = 3).
d Further 18.3 μmol amine was added after trapping.
e Amount of DBU used was 3.4 μmol.
f Amount of DBU used was 6.8 μmol.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2257–2260

2259
A. K. Dheere et al.
Letter
Synlett
HDBU
H2N
H
H
H
R1
O
*
[
11C]CO2
H
*
DBAD
PBu3
N
O
N
N
*
*
R1
C
N
C
PBu3
R1
C
R1
NH2
R1
R1
O
DBU
O
O
intermediate I
intermediate II
Scheme 2 Proposed route for the synthesis of symmetrical [11C]urea
donating (OMe, 6) or electron-withdrawing (NO2, 7) groups
in para position were radiolabelled in over 70% RCYs. Inter-
estingly, similar RCYs were observed for the synthesis of
[11C]3 when the Mitsunobu reaction time was reduced to
one minute.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1381055.
S
u
p
p
o
nrtIo
g
f
rmoaitn
S
u
p
p
ortiInfogrmoaitn
The reaction for the synthesis of symmetrical [11C]ureas
is proposed to occur in three steps (Scheme 2): (i) formation
of intermediate I from [11C]CO2 and amine in the presence
of DBU; (ii) reaction of I with the Mitsunobu reagents giving
intermediate II; (iii) formation of the urea by nucleophilic
attack of an another amine on II. The formation of interme-
diate I requires the concomitant presence of an amine and
the trapping reagent DBU (see Supporting Information, Ta-
ble SI1 and Table 1, entry 12). These results are in agree-
ment with the previously proposed trapping mechanism
whereby the amine traps the [11C]CO2 forming a zwitterion
which is deprotonated by DBU giving a carbamate anion
(intermediate I).8 When Mitsunobu reagents are added to
the reaction mixture, a phosphine oxonium ion (intermedi-
ate II) is formed (see Supporting Information, Figure SI2). In
the last step, intermediate II reacts with another molecule
of amine producing the [11C]urea.
References and Notes
(1) (a) Miller, P.; Long, N.; Vilar, R.; Gee, A. Angew. Chem. Int. Ed.
2008, 47, 8998. (b) Gee, A. Br. Med. Bull. 2003, 65, 169.
(c) Cunningham, V.; Parker, C.; Rabiner, E.; Gee, A.; Gunn, R.
Drug Discovery Today 2005, 2, 311. (d) Kealey, S.; Turner, E.;
Husbands, S.; Salinas, C.; Jakobsen, S.; Tyacke, R.; Nutt, D.;
Parker, C.; Gee, A. J. Nucl. Med. 2013, 54, 139. (e) Ametamey, S.;
Horner, M.; Schubiger, P. Chem. Rev. 2008, 108, 1501. (f) Antoni,
G. J. Label. Compd. Radiopharm. 2015, 58, 65. (g) Kealey, S.; Gee,
A.; Miller, P. J. Label. Compd. Radiopharm. 2014, 57, 195.
(2) (a) Iwata, R.; Pascali, C.; Bogni, A.; Miyake, Y.; Yanai, K.; Ido, T.
Appl. Radiat. Isot. 2001, 55, 17. (b) Cleij, M.; Clark, J.; Baron, J.;
Aigbirhio, F. J. Label. Compd. Radiopharm. 2007, 50, 19.
(c) Kealey, S.; Husbands, S.; Bennacef, I.; Gee, A.; Passchier, J. J.
Label. Compd. Radiopharm. 2014, 57, 202. (d) Child, C.; Kealey,
S.; Jones, H.; Miller, P.; White, A.; Gee, A.; Long, N. J. Chem. Soc.,
Dalton Trans. 2011, 40, 6210. (e) Hooker, J. M.; Reibel, A. T.; Hill,
S. M.; Schueller, M. J.; Fowler, J. S. Angew. Chem. Int. Ed. 2009, 48,
6001.
In conclusion, a rapid method to radiolabel symmetrical
ureas with 11C has been developed, significantly improving
previously reported routes.5 The method incorporates
[11C]CO2 directly from the cyclotron into primary and sec-
ondary amines to form the corresponding [11C]ureas. Ali-
phatic, benzylic and aromatic molecules were radiolabelled
with high RCYs greater than 70%. The developed method
avoids the need for converting short-lived [11C]CO2 into
other reactive radiolabelling species and enables the oppor-
tunity to study the fate of urea-containing molecules (e.g.
Suramin,9 1,3-dibenzylurea10 and 1,3-dicyclohexylurea11) in
vivo using PET.
(3) (a) van Tilburg, E.; Windhorst, A.; van der Mey, M.; Herscheid, J.
J. Labelled Compd. Radiopharm. 2006, 49, 321. (b) Hooker, J. M.;
Reibel, A. T.; Hill, S. M.; Schueller, M. J.; Fowler, J. S. Angew.
Chem. Int. Ed. 2009, 48, 3482. (c) Wilson, A. A.; Garcia, A.; Houle,
S.; Vasdev, N. Org. Biomol. Chem. 2010, 8, 428. (d) Riss, P.; Lu, S.;
Telu, S.; Aigbirhio, F.; Pike, V. Angew. Chem. Int. Ed. 2012, 51,
2698. (e) Rotstein, B. H.; Hooker, J. M.; Woo, J.; Collier, T. L.;
Brady, T. J.; Liang, S. H.; Vasdev, N. Med. Chem. Lett. 2014, 5, 668.
(4) Rotstein, B. H.; Liang, S. H.; Holland, J. P.; Collier, T. L.; Hooker, J.
M.; Wilson, A. A.; Vasdev, N. Chem. Commun. 2013, 49, 5621.
(5) Haji Dheere, A.; Yusuf, N.; Gee, A. Chem. Commun. 2013, 49,
8193.
(6) See Supporting Information for full experimental details and
compound characterization.
Synthesis of Compound 1: Diisopropylethylamine (194 mg,
1.50 mmol) was added to mixture of 1-methylpiperazine (50
mg, 0.50 mmol), and 4-methylpiperazin-l-ylcarbonyl chloride
(122 mg, 0.75 mmol) in CH2Cl2 (20 mL). The resulting mixture
was stirred at r.t. for 1 h. Then, the mixture was concentrated
and recrystallised from EtOH to give 1 (30 mg, 27%). 1H NMR
(400 MHz, CDCl3): δ = 3.59 (m, 8 H), 2.56 (m, 8 H), 2.90 (s, 6 H).
13C NMR (100 MHz, CD3OD): δ = 163.65, 54.03, 45.04, 43.79.
HRMS: m/z [M + H]+ calcd for C11H22N4O: 227.1794; found:
227.2003.
Acknowledgment
This work was supported by the EPSRC, European Commission, FP7-
PEOPLE-2012-ITN (316882, RADIOMI) and Medical Research Council
(MRC, MR/K022733/1). The authors acknowledge financial support
from the Department of Health through the National Institute for
Health Research (NIHR) comprehensive Biomedical Research Centre
award to Guy’s & St Thomas’ NHS Foundation Trust in partnership
with King’s College London and King’s College Hospital NHS Founda-
tion Trust.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2257–2260

2260
A. K. Dheere et al.
Letter
Synlett
(7) General Radiolabelling Procedure of
[
11C]Ureas:
[
11C]CO2
(8) (a) Peterson, S.; Stucka, S.; Dinsmore, C. Org. Lett. 2010, 12,
1340. (b) Dinsmore, C.; Mercer, S. Org. Lett. 2004, 6, 2885.
(c) Swamy, K.; Kumar, N.; Balaraman, E.; Kumar, K. Chem. Rev.
2009, 109, 2551. (d) Ma, C.; Pietrucci, F.; Andreoni, W. J. Chem.
Theory Comput. 2015, 11, 3189.
(9) Nunziante, M.; Kehler, C.; Maas, E.; Kassack, M.; Groschup, M.;
Schatzl, H. J. Cell Sci. 2005, 118, 4959.
(10) Sashidhara, K.; Rosaiah, J.; Tyagi, E.; Shukla, R.; Raghubir, R.;
Rajendran, S. Eur. J. Med. Chem. 2009, 44, 432.
from the cyclotron target was bubbled in a stream of helium gas
at a flow rate of 1.4 mL/min post target depressurisation
directly into a reaction vial containing an amine (18.3 μmol)
and DBU (0.13 μL, 0.9 μmol) in MeCN (300 μL). The resulting
solution was stirred and heated at 50 °C for 1 min. In a separate
vial, PBu3 (9 μL, 36.6 μmol) was added to a solution containing
DBAD (8 mg, 36.6 μmol) in MeCN (100 μL) under argon at r.t.
The resulting solution was transferred into the reaction mixture
and stirred for 4 min at 50 °C. The reaction was quenched with
H2O and the crude product was analysed by radio-HPLC.
(11) Hwang, S.; Tsai, H.; Liu, J.; Morisseau, C.; Hammock, B. J. Med.
Chem. 2007, 16, 3825.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2257–2260