Broad Scope and High-Yield Access to Unsymmetrical Acyclic [11C]Ureas for Biomedical Imaging from [11C]Carbonyl Difluoride

Article abstract of DOI:10.1002/chem.202100690

Effective methods are needed for labelling acyclic ureas with carbon-11 (t_1/2=20.4 min) as potential radiotracers for biomedical imaging with positron emission tomography (PET). Herein, we describe the rapid and high-yield syntheses of unsymmet

Full text of DOI:10.1002/chem.202100690

Accepted Article
Accepted Article
Title: Broad Scope and High-yield Access to Unsymmetrical Acyclic
[11C]ureas for Biomedical Imaging from [11C]carbonyl Difluoride
Authors: Jimmy Erik Jakobsson, Sanjay Telu, Shuiyu Lu, Susovan
Jana, and Victor W Pike
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202100690
Link to VoR: https://doi.org/10.1002/chem.202100690
01/2020

10.1002/chem.202100690
Chemistry - A European Journal
FULL PAPER
Broad Scope and High-yield Access to Unsymmetrical Acyclic
[11C]ureas for Biomedical Imaging from [11C]carbonyl Difluoride
Jimmy E. Jakobsson, Sanjay Telu, Shuiyu Lu, Susovan Jana and Victor W. Pike*[a]
[a]
Dr. J. E. Jakobsson, Dr. S. Telu, Dr. S. Lu, Dr. S. Jana, Dr. V. W. Pike
Molecular Imaging Branch, National Institute of Mental Health
National Institutes of Health
10 Center Drive, Bethesda, MD, 20892-1003 (USA)
E-mail: pikev@mail.nih.gov
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: Effective methods are needed for labelling acyclic ureas
with carbon-11 (t1/2 = 20.4 min) as potential radiotracers for biomedical
imaging with positron emission tomography (PET). Herein, we
describe the rapid and high-yield syntheses of unsymmetrical acyclic
[11C]ureas under mild conditions (room temperature and within 7
minutes) using no-carrier-added [11C]carbonyl difluoride with aliphatic
and aryl amines. This methodology is compatible with diverse
functionality (e.g., hydroxy, carboxyl, amino, amido, or pyridyl) in the
substrate amines. The labelling process proceeds through putative
[11C]carbamoyl fluorides and for primary amines through isolable
has featured widely for labelling carbonyl functionality.[6] However,
not all potential radiotracers have structures that may be labelled
either efficiently or at all by existing methods.
Unsymmetrical acyclic ureido groups are prevalent in drugs
and drug candidates,[7] and in known and prospective PET
radiotracers.[6b, 6c, 8] Prior methods for labelling ureas with carbon-
11 have been based on coupling amines with [11C]carbon
dioxide,[9] [11C]carbon monoxide,[10] or [11C]phosgene[8] (Figure 1).
Use of cyclotron-produced [11C]carbon dioxide for labelling
unsymmetrical ureas requires the use of a strong base [e.g., 2-
tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-
diazaphosphorine (BEMP)[9a-c] or 1,8-diazabicyclo[5.4.0]undec-7-
[11C]isocyanate intermediates.
Unsymmetrical [11C]ureas are
produced with negligible amounts of unwanted symmetrical [11C]urea
byproducts. Moreover, the overall labelling method tolerates trace
water and the generally moderate to excellent yields show good
reproducibility. [11C]Carbonyl difluoride shows exceptional promise
for application to the synthesis of acyclic [11C]ureas as new
radiotracers for biomedical imaging with PET.
ene (DBU)[9d]] to capture [11C]carbon dioxide in solution.
A
stochiometric amount of phosphoryl chloride relative to the first
amine is also required to dehydrate the derived intermediate
[11C]carbamate to a [11C]isocyanate for reaction with a second
amine. Because of the high reactivity of phosphoryl chloride, the
dehydration step is incompatible with many functional groups and
is vulnerable to moisture. [11C]Carbon dioxide may also be used
to produce either symmetrical[9d] or unsymmetrical[9e] [11C]ureas
Introduction
by use of
a
pair of Mitsunobu reagents, di-tert-butyl
azodicarboxylate and tributylphosphine. One primary amine plus
one secondary amine must be used to achieve acceptable yields.
Moreover, a strong base must still be used for initial [11C]carbon
dioxide capture.[9e] The need for complete removal of the added
base and byproducts from the Mitsunobu reagents from the
labelled product by rapid single pass HPLC can be challenging.
Broad substrate scope has not been shown for this method.
Palladium(II)-mediated conversion of [11C]carbon monoxide
into [11C]ureas requires elevated temperature and produces
mixtures of unsymmetrical and symmetrical [11C]ureas from
dissimilar amines.[10] Agents for trapping [11C]carbon monoxide
need to be present along with a quite large amount of each amine
(e.g., 90 µmol), all of which may exacerbate labelled product
isolation by single pass HPLC. Generally, at least one of the
amines must be a primary amine. Substrate scope is therefore
rather limited. This method has so far been little used for
developing PET radiotracers.
Positron emission tomography (PET) uses well-designed
radiotracers to image specific biological structures and processes
in living animal and human subjects.[1] Consequently, PET has
become an established molecular imaging modality for
biomedical research,[1-2] medical diagnosis,[3] and drug
development.[4] The short-lived positron-emitter carbon-11 (t1/2
=
20.4 min) is attractive for labelling PET radiotracers, mainly
because this radionuclide may in principle replace non-
radioactive carbon in any organic molecule.[5] Nonetheless,
carbon-11 must always be produced on-site from a cyclotron at
the time of need. Ensuing radiotracer synthesis, purification, and
formulation for intravenous injection must also be completed
rapidly, generally within only two or three half-lives of carbon-11
(i.e., in < 1 hour). The main cyclotron sources of carbon-11, from
which all radiotracer syntheses must begin, are [11C]carbon
dioxide and [11C]methane, each by the 14N(p,α)11C reaction on
nitrogen doped with either oxygen or hydrogen, respectively.[5]
Radiosyntheses must be reproducible and reliable, and amenable
to automation within lead-shielded apparatus for personnel
radiation protection. Nowadays, many secondary labelling agents
may be prepared from the primary cyclotron sources for labelling
various chemotypes.[5] Principal, among these secondary agents,
is [11C]iodomethane for labelling at non-metal heteroatoms (e.g.,
N, O, and S) with [11C]methyl groups.[5c] [11C]Carbon monoxide
[11C]Phosgene was first described in the 1970s[11] and has
since proved to be a useful labelling agent. [8] Despite successive
improvements,[12] all methods for [11C]phosgene synthesis remain
complex, require highly specialized equipment, and must use
hazardous chlorine gas. Therefore, the use of [11C]phosgene has
not been adopted by many laboratories. Moreover, use of
[11C]phosgene
for
[11C]urea
synthesis
has
1
This article is protected by copyright. All rights reserved.

10.1002/chem.202100690
Chemistry - A European Journal
FULL PAPER
mL). We postulated that symmetrical [11C]1,3-diphenylurea
([11C]3) would be formed through successive formation of
[11C]phenylcarbamoyl fluoride ([11C]1) and [11C]phenyl isocyanate
([11C]2). HPLC analysis of reaction mixtures after aqueous
quench indeed revealed the formation of [11C]3 in moderately high
yield (Table 1, entry 1) with only a trace of one of the postulated
intermediates, the isocyanate [11C]2.
By contrast, prompt
analyses of reaction mixtures before any aqueous dilution gave
substantial isolated yields of [11C]2 (39 ± 8%; Table 1, entry 2).
Peak streaking in the radiochromatograms (see Supporting
Information, Figure S2) suggested that this yield was likely
underestimated because of relatively slow hydrolysis of [11C]2
throughout the reversed phase gradient HPLC analysis, which
used an aqueous-organic mobile phase (H2O-MeCN, initially 50:
50 v/v, and ending at 10: 90 v/v). We concluded that the
homocoupling of aniline with [11C]carbonyl difluoride to form the
urea [11C]3 via the isocyanate [11C]2 was mediated rapidly by
water during the reaction quench. This conclusion accords with a
study showing that ureas may be rapidly synthesized from
isocyanates in high yields in cold (4-5 °C) water,[17] and reports of
the promoting effect of bulk[18] or trace water[19] on symmetrical
urea synthesis from isocyanates, including that of [11C]1,3-bis(2-
chloroethyl)urea.[13b]
We surmised that [11C]2 was formed by spontaneous
elimination of hydrogen fluoride from [11C]1. This rapid isocyanate
formation was somewhat unexpected because analogous
isocyanate synthesis from phosgene typically requires heating
and a tertiary amine catalyst.[8a, 14] This observation contrasts with
those on reactions between anilines and excess carbonyl
difluoride in chlorobenzene and dichloromethane, which
exclusively yield the corresponding carbamoyl fluorides in high
yields.[20] However, we did not test labelling reactions in either of
these water-immiscible solvents. Water-miscible solvents are
preferable in carbon-11 chemistry because the reaction mixtures
Figure 1. Comparison of prominent methods for the synthesis of acyclic
[
11C]ureas from amines.
several limitations. Generation of a mixture of the desired
unsymmetrical urea together with the undesired symmetrical urea
often complicates purification and limits the yield. Furthermore,
synthesis of [11C]isocyanates as intermediates for [11C]urea
synthesis from [11C]phosgene and amines is very sensitive to
moisture[13] and often requires heating in the presence of a tertiary
amine catalyst.[8a, 14] Attempts have been made to use alternative
precursors, such as N-organosulfinylamines[13a] or tertiary N-
benzylamines[15], to control the reactivity of phosgene in urea
syntheses, but these approaches have gained limited traction in
PET radiotracer synthesis.
Table 1. Optimization of [11C]carbonyl difluoride trapping with aniline and [11C]3
synthesis.
We recently reported
a straightforward and efficient
synthesis of a new labelling agent, [11C]carbonyl difluoride, from
cyclotron-produced [11C]carbon dioxide by on-line conversion into
[11C]carbon monoxide and then passage over silver(II) fluoride.[16]
In this method, [11C]carbonyl difluoride is produced reliably in
useful overall radiochemical yield and with high no-carrier-added
(NCA) molar activity (> 200 GBq/µmol). This new labelling agent
has been shown to be very effective for 11C-carbonylation
reactions giving intracyclic ureas, as especially exemplified by the
synthesis of a -adrenergic receptor imaging agent, [11C](S)-CGP
12177.[16] Here we aimed to further explore NCA [11C]carbonyl
difluoride as a labelling agent for acyclic ureas, especially
unsymmetrical ureas.
Entry
PhNH2 (µmol)
Additive (µmol)
Yield of [11C]3 (%)a
1
2
10
10
0
0
64 ± 8b
0
(39 ± 8 of [11C]2)c
3
4
5
2
Pyridine (1)
Pyridine (0.1)
Pyridine (1)
43 ± 1
76 ± 1
84 ± 1
10
10
[a] Yields are mean ± range for n = 2. [b] The reaction mixture was quenched
Results and Discussion
with water 1 minute before HPLC analysis. [c] Isolated yield after direct injection
of the crude reaction mixture into HPLC.
hydrolyzed during the analysis. Total synthesis time was 5.5 minutes from the
[
11C]2 was likely continuously
We started our study by investigating the reaction of NCA
[11C]carbonyl difluoride with aniline (10 µmol) in acetonitrile (0.5
2
This article is protected by copyright. All rights reserved.

10.1002/chem.202100690
Chemistry - A European Journal
FULL PAPER
release of [
11C]carbon monoxide over silver(II) fluoride. Synthesis and
cryogenic isolation of [11C]carbon monoxide itself required 12 min.
using DMF or DMSO as solvent (see Supporting Information,,
Figures S13 and S14).
can be diluted with water and then purified with rapid single-pass
reversed phase HPLC without first spending time to remove the
solvent. Moreover, an onerous need for automation of the solvent
removal process is obviated.
Table 2. Optimization of [11C]carbonyl difluoride trapping with aniline, followed
by spontaneous [11C]isocyanate formation, and coupling with benzylamine.
Although there is as yet no prominent example, some
symmetrical [11C]ureas could become candidate PET radiotracers,
and efficient methods for their radiosynthesis is therefore of some
interest. Tertiary amines and heterocyclic amines have been
shown to improve the yields of symmetrical diaryl ureas from
isocyanates.[19] Therefore, we investigated whether pyridine
could promote homocoupling of aniline to produce symmetrical
[11C]3. Use of 1 µmol of pyridine and 2 µmol of aniline in
acetonitrile (0.5 mL) gave a moderate yield of [11C]3 (Table 1,
entry 3). The yield increased substantially when using a lower
amount of pyridine (0.1 µmol) and a higher amount of aniline (10
µmol) (Table 1, entry 4). Use of pyridine (1 µmol) and aniline (10
µmol) gave [11C]3 in yet higher yield (84%; Table 1, entry 5).
These few experiments quickly confirmed the viability of reaction
between [11C]carbonyl difluoride and anilines for synthesizing
symmetrical [11C]ureas.
Entry
PhNH2
(µmol)
Solvent
(0.5 mL)
Yield of
11C]3 (%)a
Yield of
11C]4 (%)a
[
[
Our main interest was to develop methodology for the
synthesis of potentially more useful unsymmetrical acyclic
[11C]ureas. We considered that these might be accessed by
treating intermediate isocyanates, generated from [11C]carbonyl
difluoride and one amine, such as [11C]2, with a dissimilar amine.
To test this possibility, we added benzylamine (1 µmol) after
[11C]carbonyl difluoride had been fully introduced into a reaction
mixture containing aniline (i.e., 5.5 minutes from [11C]carbon
monoxide release). The desired unsymmetrical urea [11C]4 was
formed in very high yield from [11C]2 after just 1 minute and
gratifyingly without the symmetrical byproduct [11C]3 (Table 2,
entry 1). A lower amount of aniline (5 µmol) gave a lower practical
yield of [11C]4 (Table 2, entry 2) because of a lower trapping
efficiency for the [11C]carbonyl difluoride. A higher amount of
aniline (20 µmol) did not improve the trapping efficiency (Table 2,
entry 3) and gave some of the undesired [11C]3. Inclusion of 1%
water (5 µL, 0.28 mmol) did not lower the yield of [11C]4 (Table 2,
entry 4), as would be the case with the dehydrative approaches
to [11C]urea synthesis using [11C]carbon dioxide. Therefore, we
considered that high humidity or the use of lower quality non-
anhydrous solvents would be well tolerated. In this regard, a
substantial yield of [11C]4 was obtained even in the presence of a
much higher concentration (5% v/v) of water (added as 25 µL; 1.4
mmol) (Table 2, entry 5). These findings of water tolerance are
important because they indicate the radiosynthetic procedure is
robust and readily amenable to automation for routine PET
radiotracer production.
Use of a different dipolar aprotic non-nucleophilic solvent,
namely THF, produced a high yield of the target unsymmetrical
urea [11C]4 (Table 2, entry 6), but with some accompanying [11C]3.
Two other weakly nucleophilic dipolar aprotic solvents, DMSO
and DMF, gave lower yields of [11C]4 and more of the symmetrical
byproduct [11C]3 (Table 2, entries 7 and 8). DMF is known to react
with carbonyl difluoride to produce carbon dioxide in a difluoro-
decarbonylation reaction.[21] Thus, difluoro-decarbonylation might
be the cause for the lower yield of [11C]4 in DMF. Possibly DMSO
is also reactive towards [11C]carbonyl difluoride. Consistent with
this suggestion, a high proportion of radioactivity was retained in
the reaction mixture as an unidentified polar byproduct when
1
2
3
4
10
5
MeCN
0 ± 0b
84 ± 9b
68 ± 9
82 ± 9b
91 ± 0
0 ± 0
MeCN
MeCN
20
10
2 ± 1b
1 ± 0
H2O/MeCN
(1: 99 v/v)
5
10
H2O/MeCN
(5: 95 v/v)
1 ± 0
62 ± 2
6
7
8
10
10
10
THF
DMSO
DMF
4 ± 1
20 ± 0
7 ± 3
83 ± 2
45 ± 1
39 ± 5
[a] Yields are mean ± range for n = 2, unless otherwise indicated. [b] Yields are
mean ± SD for n = 3. Total synthesis time was 6.5 min from the release of
11C]carbon monoxide.
[
Having established a procedure for the synthesis of [11C]4
in high yield, we then explored whether this procedure might be
applicable to the synthesis of unsymmetrical [11C]ureas from
diverse amines, including some bearing various functional groups
that would be incompatible with earlier methods. Treatment of the
intermediate isocyanate [11C]2 with phenethylamine or
cyclopentylamine, as examples of primary amines, gave excellent
and high yields of the desired unsymmetrical [11C]ureas, [11C]5
and [11C]6, respectively (Table 3), and with no symmetrical
[11C]urea byproduct.
Likewise, the secondary amines, N-
benzylmethylamine and morpholine, gave the unsymmetrical
[11C]ureas [11C]7 and [11C]8 in high and excellent yields,
respectively. Amines having a competing nucleophilic site, such
as the arylamino group in 2-aminobenzylamine or the hydroxyl
group in 2-(piperidin-2-yl)ethan-1-ol, gave the desired ureas,
[11C]9 and [11C]10 in moderate and high yield, respectively, so
demonstrating the preference of the intermediate to react with an
aliphatic amino group over a less nucleophilic arylamino or
hydroxy group. Treatment of the intermediate isocyanate [11C]2
in acetonitrile with the sodium salt of L-phenylalanine in
3
This article is protected by copyright. All rights reserved.

10.1002/chem.202100690
Chemistry - A European Journal
FULL PAPER
water/acetonitrile (100 µL, 1: 1 v/v) produced the desired urea
[11C]11 in high yield, but with some symmetrical byproduct, most
likely due to water mediated homocoupling. Electron-deficient 4-
iodoaniline instead of aniline for [11C]isocyanate formation and
use of benzylamine as second amine gave a moderate yield of
the urea [11C]12. Use of an arylamine with a very weak (p-fluoro)
or strong (p-nitro) electron-withdrawing substituent for the
[11C]isocyanate formation gave low and very low practical yields
of the ureas, [11C]13 and [11C]14, respectively. These decreases
were primarily due to lower trapping efficiencies for [11C]carbonyl
difluoride. Use of 50 µmol of 4-fluoroaniline gave comparable
yield of [11C]13, albeit with production of some symmetrical
[11C]urea byproduct. Gratifyingly, more reactive electron-rich
anilines, such as 3,5-dimethoxyaniline, p-anisidine, and 3-
aminophenol, gave very high yields of the intermediate
[11C]isocyanates, which reacted with benzylamine to give the
respective [11C]ureas, [11C]15, [11C]16, and [11C]17 in high to
excellent yields and without any symmetrical [11C]urea byproducts.
Also a secondary aniline, N-methylaniline, reacted efficiently with
[11C]carbonyl
difluoride,
presumably
to
form
Table 3. Synthesis of unsymmetrical [11C]ureas by coupling anilines or aniline derivatives and aliphatic amines with [11C]carbonyl difluoride.
[a] L-Phenylalanine in H2O-MeCN (1: 1 v/v; 100 µL) with 1 equiv. of NaOH. [b] 50 µmol 4-fluoroaniline was used, giving a 57: 43 mix of [11C]13 to [11C]1,3-di-4-
fluorophenyl urea. [c] 50 µmol 1,2-diaminobenzene and 1 µmol pyridine were used. [d] HCl salt of hydrazine precursor in H2O-MeCN (1: 1 v/v; 100 µL) and 1 equiv.
of KOH were used. [e] 50 µmol arylamine precursor and 10 µmol pyridine were used. Yields are decay-corrected and based on radioactivity recovered from the
HPLC purification of a reaction aliquot (n = 2). The synthesis time was 6.5 min from release of [11C]carbon monoxide.
4
This article is protected by copyright. All rights reserved.

10.1002/chem.202100690
Chemistry - A European Journal
FULL PAPER
[11C]benzyl(methyl)carbamoyl fluoride, which upon treatment with
a second nucleophilic amine, benzylamine, produced a high yield
of the urea [11C]18. Reaction of [11C]carbonyl difluoride with one
example of an aniline bearing a meta electron withdrawing group,
namely 3-cyanoaniline, gave a moderate yield of the desired urea
[11C]19 after treatment of the labelled intermediate with
challenging because the radiotracer must be built from an
electron-deficient arylamine (benzo[d][1,2,3]thiadiazol-6-amine)
and an aliphatic amine (2-amino-1-(piperidin-2-yl)ethan-1-ol)
holding an amide functionality. The amide functionality would be
incompatible with any [11C]urea synthesis that depends on
phosphoryl trichloride dehydration, as discussed earlier.
benzylamine.
An aniline having a competing nucleophilic
Use of our standard labelling conditions produced a very low
yield (2%) of [11C]23, likely due to the low nucleophilicity of the
bicyclic arylamine. Addition of pyridine (2 µmol) as catalyst
improved the yield to 6%. The symmetrical product was not
formed to any large extent, again reflecting low bicyclic amine
nucleophilicity. A further five-fold increase of arylamine to 50
µmol and of pyridine to 10 µmol, while still retaining the very low
amount of the aliphatic amine nucleophile (1 µmol) gave [11C]23
in a useful yield (44%).
To further explore the use of [11C]carbonyl difluoride for the
synthesis of acyclic [11C]ureas, we attempted to couple two
aliphatic amines. As expected, reaction between [11C]carbonyl
difluoride and benzylamine (10 µmol) in acetonitrile (0.5 mL) did
not stop at formation of the [11C]isocyanate, as with anilines, but
proceeded to give the symmetrical 1,3-dibenzyl urea ([11C]24) in
very high yield (Table 4, entry 1). Even with a lower amount of
precursor (1 µmol), [11C]24 was produced in high yield (Table 4,
entry 2). A further reduction of precursor, to achieve 1: 1
stoichiometry between labelling agent and precursor, is not
feasible in routine NCA radiochemistry because of the variable
and uncontrollable amount of carrier that arises in cyclotron
hydroxyl group, namely 2-(4-phenylamino)-ethan-1-ol, was
readily converted into the desired unsymmetrical urea [11C]20 in
moderate yield.
The synthesis of unsymmetrical [11C]ureas from weakly
nucleophilic anilines can be challenging. Nonetheless, treatment
of the isocyanate [11C]2 in situ with a five-fold excess of 1,2-
diaminobenzene and a catalytic amount of pyridine gave the
desired diaryl unsymmetrical urea [11C]21 in high yield with no
symmetrical byproduct. As a further demonstration of the
versatile reactivity of the [11C]isocyanate intermediates, treatment
of [11C]2 with the free base of 2-chloro-phenylhydrazine
(generated in situ) in water: acetonitrile (100 µL, 1: 1 v/v) gave
[11C]22 in excellent yield and without any symmetrical byproducts.
This reflects the high nucleophilicity of the hydrazine.
The synthesis of a labeled inhibitor of protein arginine
methyltransferase 3, [11C]UNC 2327 ([11C]23)[22], a potential PET
radiotracer, was selected to further show-case the broad utility of
this new methodology. Outside its carbonyl group, 23 does not
have other sites that are amenable to labelling with carbon-11 by
other methodologies. Labelling in the carbonyl group is also very
Table 4. Method development for synthesis of [11C]ureas from aliphatic amines.
n
)
n
)
Entry
R1 in tosylate (µmol)
Additive (µmol)
TE (%)(
R2 in amine (µmol)
R1NH11CONHR2 yield (%)(
Product #
1
2
Bn (10)a
Bn (1)a
Bn (1)b,c
Bn (1)
-
93 ± 3 (2)
-
88 ± 1 (2)
80 (1)
[
11C]24
11C]24
-
85 (1)
-
-
[
3
-
62 ± 5 (2)
65 ± 14 (5)
63 ± 7 (2)
73 ± 15 (2)
74 ± 18 (2)
77 ± 2 (4)
43 (1)
-
NAd
4
-
c-Pen (10)
-
63 (1)
[
11C]25
5
Bn (2)
-
NA
-
6
c-Pen (2)
c-Pen (1)
Bn (3)
-
Bn (10)
Bn (10)
c-Pen (30)
c-Pen (10)
c-Pen (10)
c-Pen (10)
59 ± 13 (2)
62 ± 17 (2)
61 ± 0 (2)
13 (1)
[
[
[
[
[
[
11C]25
11C]25
11C]25
11C]25
11C]25
11C]25
7
-
-
8e
9
Bn (1)
DMAP•TsOH (1)
Pyr (1)
10
11
Bn (1)
72 ± 3 (2)
87 ± 4 (13)
60 ± 1 (2)
83 ± 2 (3)
Bn (1)
Pyr (2)
5
This article is protected by copyright. All rights reserved.

10.1002/chem.202100690
Chemistry - A European Journal
FULL PAPER
[a] 10 µmol free base. [b] HCl salt. [c] Suspension. [d] Product is [11C]benzyl isocyanate. [e] 1.5 mL MeCN. Synthesis time: 5.5 plus 1 min from release of
11C]carbon monoxide. TBAC = t-butylammonium chloride; TE = [11C]carbonyl difluoride trapping efficiency; NA = Not applicable; c-Pen = cyclopentyl; Pyr =
pyridyl. Superscript values in parentheses are numbers of experiments. Yields are mean ± range for n = 2 and mean ± SD for n > 3.
[
carbon-11 production.[16] Instead, we attempted to control the
reactivity of the first amine so that the reaction stopped at the
intermediate [11C]carbamoyl fluoride or [11C]isocyanate. One
approach that has been used with some success when using
[11C]phosgene as labelling agent is to use a salt of the amine in
suspension.[8a] Treatment of benzylammonium chloride (1 µmol)
suspended in acetonitrile (0.5 mL) with [11C]carbonyl difluoride,
produced the intermediate [11C]benzyl isocyanate in the absence
of the symmetrical urea, [11C]24, as confirmed with HPLC analysis
(Table 4, entry 3; also see Supporting Information, Figure S52).
However, alkylammonium chlorides have limited solubility in
ambidently nucleophilic 2-aminobenzylamine to [11C]benzyl
isocyanate intermediate produced high yield of the
a
unsymmetrical urea [11C]26 in which the arylamino group was
retained. 2-aminobenzylamine had to be added in the second
step because the reversed order of addition leads to
intramolecular reaction to produce a cyclic [11C]urea in high
yield.[16] Likewise, addition of ambident secondary amine, 2-
(piperidin-2-yl)ethan-1-ol to [11C]benzyl isocyanate intermediate,
produced the urea [11C]27, with a retained hydroxy group, in high
yield. Pyridinyl and other heterocyclic groups are abundant
among drug molecules. The unsymmetrical urea, [11C]28, which
carries a terminal 3-pyridinyl group, was produced in high yield
acetonitrile.
Moreover, running carbon-11 chemistry in
suspensions is undesirable because the amount of precursor in
solution is unknown and because solids are ill-suited for easy
product separation or analysis with HPLC. We therefore
investigated the use of alkylammonium tosylates as much more
soluble salts for these reactions. Treatment of benzylammonium
tosylate (1 µmol) in acetonitrile with [11C]carbonyl difluoride
resulted in high trapping efficiency for [11C]carbonyl difluoride and
in [11C]benzyl isocyanate as the main product. Addition of
cyclopentylamine in ten-fold excess then produced [11C]25 as the
main product (Table 4, entry 4). Before investigating substrate
scope in more detail, we wanted to improve trapping efficiency
and [11C]isocyanate formation. We found that doubling the
precursor concentration did not improve trapping efficiency (Table
4, entry 5). Reversing the order of amine addition and use of the
tosylate salt of cyclopentylamine as trapping agent gave similar
yield. Addition of 5- or 10-fold excess of benzylamine gave the
unsymmetrical urea [11C]25 with good selectivity (Table 4, entries
6 and 7, respectively).
from [11C]benzyl isocyanate intermediate.
Yield was
progressively lowered by reversed addition of amine components,
by omission of pyridine, and by both. Only primary amines can
form isocyanates. Treatment of benzylammonium tosylate with
[11C]carbonyl difluoride followed by excess N-benzylmethylamine
gave [11C]29 in high yield. Nonetheless, reversed order of amine
component addition, where the secondary ammonium tosylate
salt was used, still gave a moderate yield. An excellent selectivity
was observed in both cases showing that the reaction proceeds
well through either [11C]isocyanate or [11C]carbamoyl fluoride
intermediate. The highest yield of [11C]29 was however obtained
in the presence of pyridine with N-benzylmethylamine as second
amine component.
The synthesis of the unsymmetrical ureas [11C]26‒[11C]29
from primary benzylamine and secondary N-benzylmethylamine
components was tested in the absence of pyridine and gave
generally decreased yields. Yields were decreased yet further if
pyridine was omitted in reversed amine addition mode.
[11C]30 was obtained in excellent yields from the
intermediate [11C]benzyl isocyanate by treatment with 2-(4-
aminophenyl)ethylamine, again showing the preference of the
intermediate to react with an aliphatic amino group over a less
nucleophilic arylamino group. In the attempted synthesis of
[11C]31, the tosylate salt of the amine precursor in the absence of
pyridine trapped [11C]carbonyl difluoride very inefficiently and
produced only trace product. However, addition of pyridine
increased the trapping efficiency markedly and also the yield of
[11C]31. Likewise, an exceptionally high yield (91%) of the
unsymmetrical urea [11C]32 was obtained as a result of efficient
trapping of [11C]carbonyl difluoride into phenethylammonium
tosylate solution in the presence of pyridine. Yield was much
reduced in the absence of pyridine. Overall, for the syntheses of
[11C]26–[11C]32, selectivity for the desired unsymmetrical
[11C]ureas over the undesired symmetrical [11C]ureas exceeded
90%.
The soluble epoxide hydrolase inhibitor, [11C]AR-9281
([11C]33), has potential for PET imaging in Alzheimer’s and
Parkinson’s disease.[24] The urea carbonyl group of 33 is a
potential site for labelling by using [11C]carbonyl difluoride,
although challenging due to steric hindrance. The tosylate salt of
1-adamantylamine in the presence of pyridine retained 62% of the
radioactivity introduced as [11C]carbonyl difluoride. After adding
1-acetyl-4-aminopiperidine for the usual 1-minute reaction time,
[11C]33 was obtained in only 11% yield, but along with a 45%
isolated yield of [11C]1-adamantyl isocyanate. A duplicate
experiment with 30-minute reaction time gave an improved yield
We hypothesized that the volatility of [11C]carbonyl difluoride
(b.pt., ‒85 °C) limits its time in solution for reaction with the first
amine and that this could be the cause of lower yields from poorly
nucleophilic amines. To increase the residence of [11C]carbonyl
difluoride in solution, we increased the solvent volume three-fold
to 1.5 mL while maintaining the precursor concentration at 2
µmol/mL.
As a result, the trapping efficiency increased
appreciably (Table 4, c.f., entries 8 and 4). However, these
increased amounts of reagents might make purification of a PET
radiotracer product more difficult. Instead, we saw the possibility
of adding an agent that might sequester [11C]carbonyl difluoride
in solution. We considered adding the common acylation catalyst,
4-N,N-dimethylaminopyridine (DMAP), but this would be
expected to deprotonate the alkylammonium salt precursor and
lead to formation of symmetrical products. Therefore, the tosylate
of DMAP was tested, but this gave a low yield of [11C]25 (Table 4,
entry 9). Pyridine is not basic enough to deprotonate aliphatic
amines to a significant extent, whereas it would be expected to
retain [11C]carbonyl difluoride in solution based on its transiently
reversible reaction with acetyl groups.[23] Therefore, pyridine (1
µmol) was added to benzylammonium tosylate precursor and the
[11C]carbonyl difluoride trapping efficiency improved appreciably
(Table 4, entries 10). A further increase in amount of pyridine to
2 µmol increased trapping efficiency yet further and gave
excellent yield of unsymmetrical [11C]25 (Table 4, entry 11).
Having established
a protocol for the synthesis of
unsymmetrical [11C]ureas from aliphatic amines in the presence
of pyridine, we explored substrate scope (Table 5). Addition of
6
This article is protected by copyright. All rights reserved.

10.1002/chem.202100690
Chemistry - A European Journal
FULL PAPER
Table 5. Synthesis of unsymmetrical [11C]ureas by coupling aliphatic amines with [11Cc]arbonyl difluoride.
[a] No pyridine. [b] Reversed order of amine addition. [c] Three times the volume (1.5 mL) was used with retained concentration of precursor and reagents. [d]
Heating for 5 min at 75 °C was used. [e] The second amine was added from H2O: MeCN (1: 1 v/v; 300 µL). Each yield was determined after HPLC purification of
a reaction aliquot.
of [11C]33 (40%), but still with 29% intact [11C]isocyanate (co-
injection in Supporting Informstion, Figure S51). Slow reaction
had also been evident during the preparation of reference
compound 33 (see the SI). Addition of a large quantity of 1-acetyl-
4-aminopiperidine to speed up the reaction was impractical.
Instead, reaction volume was increased to 1.5 mL with retained
concentrations of precursor and reagents. This gave improved
The exploration of [11C]carbonyl difluoride for acyclic [11C]urea
synthesis revealed wide substrate scope and good reproducibility.
This new methodology is compatible with a range of functional
groups and with both aliphatic amines and anilines. Reactions
proceed via [11C]carbamoyl fluoride and for primary amines,
[11C]isocyanate intermediates. Pyridine is effective for enhancing
entrapment and utilization of [11C]carbonyl difluoride when using
trapping efficiency (86%) in the presence of pyridine. Surprisingly, amine tosylates as soluble precursors. This methodology is
heating the reaction mixture at 75 °C for 5 minutes only marginally
improved the yield to 22%. The relatively minor influences of
increased time and temperature suggested that [11C]urea
formation accelerated after the aqueous quench and that water
acts as a catalyst, as alluded to in the synthesis of [11C]3 (Table
1, entry 1). We considered that pyridine might not be an efficient
acylation catalyst in this case and might only be serving to retain
[11C]carbonyl difluoride in solution. Therefore, we excluded
pyridine and added the second amine from water (150 µL) plus
acetonitrile (150 µL). [11C]Carbonyl difluoride trapping efficiency
was maintained. The reaction mixture was heated to 75 °C for 5
minutes and gave the desired urea [11C]33 in high yield (80%) with
only trace unreacted [11C]isocyanate. This again highlights that
water is an effective promoter of [11C]urea formation.
highly promising for complex PET radiotracer production.
Although elaborate amine substrates having limited reactivity may
require some optimization of reaction conditions, such as reaction
temperature, number of equivalents, and acylation catalyst or
water addition, high yields are still attainable. These approaches
to optimization are demonstrated here and their careful
consideration should enable fast and trouble-free application.
Moreover, the simplicity of this overall procedure should render it
to be readily transferable to a GMP environment, although this
has yet to be attempted.
Experimental Section
Full details are presented in Supporting Information. The production of
[
[
11C]carbon dioxide and the synthesis of 11C]carbon monoxide,
[
11C]carbonyl difluoride and the typical synthesis of an acyclic [11C]urea
Conclusion
including yield measurements are as follows.
7
This article is protected by copyright. All rights reserved.

10.1002/chem.202100690
Chemistry - A European Journal
FULL PAPER
[4]
[5]
a) S. P. McCluskey, C. Plisson, E. A. Rabiner, O. Howes, Eur. J.
Nucl. Med. Mol. Imaging 2019, 47, 451-489; b) P. M. Matthews, E.
A. Rabiner, J. Passchier, R. N. Gunn, Br. J. Clin. Pharmacol. 2011,
73, 175-186.
a) S. M. Ametamey, M. Honer, P. A. Schubiger, Chem. Rev. 2008,
108, 1501-1516; b) V. W. Pike, Curr. Med. Chem. 2016, 23, 1818-
1869; c) S. Lu, F. B. Simeon, S. Telu, L. Cai, V. W. Pike, The
Chemistry of labelling heterocycles with carbon-11 and fluorine-18
for biomedical imaging, in Advances in Heterocyclic Chemistry,
volume 132, E. F. V. Scriven, C. A. Ramsden (eds), Academic Press,
Cambridge, 2020, p 241-384.
a) B. H. Rotstein, S. H. Liang, M. S. Placzek, J. M. Hooker, A. D.
Gee, F. Dolle, A. A. Wilson, N. Vasdev, Chem. Soc. Rev. 2016, 45,
4708-4726; b) C. Taddei, V. W. Pike, EJNMMI Radiopharm. Chem.
2019, 4, 25; c) J. Eriksson, G. Antoni, B. Långstrom, O. Itsenko, Nucl.
Med. Biol. 2020, 92, 115-137.
A. K. Ghosh, M. Brindisi, J. Med. Chem. 2020, 63, 2751-2788.
a) D. Roeda, F. Dollé, Curr. Top. Med. Chem. 2010, 10, 1680-1700;
b) T. Fukumura, W. Mori, M. Ogawa, M. Fujinaga, M. R. Zhang, Nucl.
Med. Biol. 2021, 92, 138-148.
a) J. W. Hicks, A. A. Wilson, E. A. Rubie, J. R. Woodgett, S. Houle,
N. Vasdev, Bioorg. Med. Chem. Lett. 2012, 22, 2099-2101; b) J. W.
Hicks, J. Parkes, J. Tong, S. Houle, N. Vasdev, A. A. Wilson, Nucl.
Med. Biol. 2014, 41, 688-694; c) A. A. Wilson, A. Garcia, S. Houle,
O. Sadovski, N. Vasdev, Chem. Eur. J. 2011, 17, 259-264; d) A. K.
H. Dheere, S. Bongarzone, C. Taddei, R. Yan, A. D. Gee, Synlett
2015, 26, 2257-2260; e) A. K. H. Dheere, N. Yusuf, A. Gee, Chem.
Commun. 2013, 49, 8193-8195.
a) S. Kealey, S. M. Husbands, I. Bennacef, A. D. Gee, J. Passchier,
J. Label. Compd. Radiopharm. 2014, 57, 202-208; b) S. Roslin, P.
Brandt, P. Nordeman, M. Larhed, L. R. Odell, J. Eriksson, Molecules
2017, 22, 1688.
G. A. Brinkman, I. Hasslisewska, J. T. Veenboer, L. Lindner, Int. J.
Appl. Radiat. Isot. 1978, 29, 701-702.
a) P. Landais, C. Crouzel, Int. J. Rad. Appl. Instrum. A 1987, 38,
297-300; b) Y. Bramoulle, D. Roeda, F. Dollé, Tetrahedron Lett.
2010, 51, 313-316; c) M. Ogawa, Y. Takada, H. Suzuki, K. Nemoto,
T. Fukumura, Nucl. Med. Biol. 2010, 37, 73-76.
[
11C]Carbon Dioxide Production. [11C]carbon dioxide was prepared by
the 14N(p,)11C nuclear reaction by bombarding a nitrogen-1% oxygen gas
target (2.1 MPa) with a beam of protons (16 MeV, 5 µA) from a cyclotron
(PETrace; GE) for 5 min, typically generating 2 to 3 GBq of [11C]carbon
dioxide with a molar activity between 18 and 45 GBq/µmol.
[
11C]Carbon Monoxide Synthesis. Cyclotron-produced 11C]carbon
[
dioxide was converted into [11C]carbon monoxide using a modified Synthia
radiosynthesis apparatus. [11C]Carbon dioxide was first trapped on
molecular sieves (13X) from the released nitrogen-oxygen target gas. The
sieves were then purged with helium to remove residual oxygen. The
molecular sieve trap was then heated to 280 °C while being purged with
helium to release the [11C]carbon dioxide into a small stainless steel cryo-
trap filled with silica immersed in liquid nitrogen (–196 °C). The stainless
steel cryo-trap was then heated to release the trapped [11C]carbon dioxide
into a stream of helium that was passed through a heated molybdenum
column (875 °C). The generated [11C]carbon monoxide was separated
from residual [11C]carbon dioxide by passage over sodium hydroxide
coated on silica (ascarite) and then concentrated within a second stainless
steel cryo-trap filled with silica immersed in liquid nitrogen. The full process
typically required 11 to 12 min from the end of radionuclide production and
gave [11C]carbon monoxide in 71 ± 2% yield (mean ± SD; n = 37), based
on the proportion of the total radioactivity (decay-corrected) that was not
immobilized on ascarite.
[6]
[7]
[8]
[9]
[10]
Synthesis of
[
11C]1-Benzyl-3-phenylurea ([11C]4).
[
11C]Carbon
[11]
[12]
monoxide was passed over solid silver(II) fluoride and bubbled (5 mL/min)
through a solution of aniline (10 µmol) in acetonitrile (0.5 mL) for 5.5 min.
Benzylamine (1 µmol) in acetonitrile (0.1 mL) was added to the reaction
mixture.
[13]
a) G. D. Brown, D. Henderson, C. Steel, S. Luthra, P. M. Price, F.
Brady, Nucl. Med. Biol. 2001, 28, 991-998; b) T. Conway, M. Diksic,
J. Nucl. Med. 1988, 29, 1957-1960; c) F. Dollé, L. Martarello, Y.
Bramoulle, M. Bottlaender, A. D. Gee, J. Label. Compd.
Radiopharm. 2005, 48, 501-513.
Radio-HPLC Analysis and Yield Measurement. An aliquot of the
reaction mixture was diluted with water and injected onto HPLC. The
amount of isolated radioactive product was compared to that of the decay-
corrected amount of radioactivity injected onto HPLC to give the
radiochemical purity of the labelled compound in the reaction mixture. This
purity value was multiplied by the efficiency of the initial trapping of
radioactivity in the reaction vial (decay-corrected and assumed to be all
[14]
[15]
H. Babad, A. G. Zeiler, Chem. Rev. 1973, 73, 75-91.
L. Lemoucheux, J. Rouden, M. Ibazizene, F. Sobrio, M. C. Lasne, J.
Org. Chem. 2003, 68, 7289-7297.
J. E. Jakobsson, S. Lu, S. Telu, V. W. Pike, Angew. Chem. Int. Ed.
2020, 59, 7256-7260, ; Angew. Chem. 2020, 132, 7323-7327.
M. Mane, R. Balaskar, S. Gavade, P. Pabrekar, D. Mane, Arab. J.
Chem. 2013, 6, 423-427.
J. L. J. Blanco, C. S. Barría, J. M. Benito, C. O. Mellet, J. Fuentes,
F. Santoyo-González, J. M. G. Fernández, Synthesis 1999, 1907-
1914.
S. Perveen, S. M. A. Hai, R. A. Khan, K. M. Khan, N. Afza, T. B.
Sarfaraz, Synthetic Commun. 2005, 35, 1663-1674.
a) N. Zhang, X. M. Zhou, H. D. Quan, A. Sekiya, J. Fluor. Chem.
2015, 178, 208-213; b) H. D. Quan, N. Zhang, X. M. Zhou, H. Qian,
A. Sekiya, J. Fluor. Chem. 2015, 176, 26-30.
[16]
[17]
[18]
[
11C]carbonyl difluoride), thus giving a measure of the radiochemical yield
of the labelled compound based on [11C]carbon monoxide. Only negligible
amounts (< 0.1%) of radioactivity adhered to needles, stainless steel lines,
and the silver(II) fluoride column. The full process from end of cyclotron
irradiation to HPLC injection takes less than 20 minutes.
[19]
[20]
Acknowledgements
[21]
[22]
F. S. Fawcett, D. D. Coffman, C. W. Tullock, J. Am. Chem. Soc.
1962, 84, 4275-4285.
F. Liu, F. Li, A. Ma, E. Dobrovetsky, A. Dong, C. Gao, I. Korboukh,
J. Liu, D. Smil, P. J. Brown, S. V. Frye, C. H. Arrowsmith, M.
Schapira, M. Vedadi, J. Jin, J. Med. Chem. 2013, 56, 2110-2124.
A. R. Fersht, W. P. Jencks, J. Am. Chem. Soc. 1970, 92, 5432-5422.
a) Y. Du, I. Minn, C. Foss, W. G. Lesniak, F. Hu, R. F. Dannals, M.
G. Pomper, A. G. Horti, EJNMMI Res. 2020, 10, 67; b) P. D. Jones,
H. J. Tsai, Z. N. Do, C. Morisseau, B. D. Hammock, Bioorg. Med.
Chem. Lett. 2006, 16, 5212-5216.
The authors acknowledge support from the Intramural Research
Program of the National Institutes of Health (NIMH: Project
number ZIA-MH002793). The authors are grateful to the NIH
Clinical Center PET Department (Chief: Dr. P. Herscovitch) for
cyclotron production of carbon-11.
[23]
[24]
Keywords: carbon-11 • [11C]carbonyl difluoride • ureas •
positron emission tomography • radiochemistry
[1]
[2]
M. E. Phelps, Proc. Natl. Acad. Sci. USA 2000, 97, 9226-9233.
a) S. Chua, A. Groves, Biomedical imaging: applications and
advances, Elsevier, Cambridge; Waltham, 2014, p. 3-40; ; b) K.
Heurling, A. Leuzy, M. Jonasson, A. Frick, E. R. Zimmer, A.
Nordberg, M. Lubberink, Brain Res. 2017, 1670, 220-234; c) V. L.
Villemagne, V. Doré, S. C. Burnham, C. L. Masters, C. C. Rowe, Nat.
Rev. Neurol. 2018, 14, 225-236; d) M. Slifstein, A. Abi-Dargham,
Semin. Nucl. Med. 2017, 47, 54-63.
[3]
a) K. Herholz, W. D. Heiss, Mol. Imaging Biol. 2004, 6, 239-269; b)
A. Gallamini, C. Zwarthoed, A. Borra, Cancers (Basel) 2014, 6,
1821-1889.
8
This article is protected by copyright. All rights reserved.

10.1002/chem.202100690
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
Broad scope synthesis of [11C]ureas from [11C]carbonyl difluoride and aliphatic or aromatic amines.
9
This article is protected by copyright. All rights reserved.