Derisking the Cu-Mediated 18F-Fluorination of Heterocyclic Positron Emission Tomography Radioligands

Article abstract of DOI:10.1021/jacs.7b03131

Molecules labeled with fluorine-18 (¹⁸F) are used in positron emission tomography to visualize, characterize and measure biological processes in the body. Despite recent advances in the incorporation of ¹⁸F onto arenes, the development of general and efficient approaches to label radioligands necessary for drug discovery programs remains a significant task. This full account describes a derisking approach toward the radiosynthesis of heterocyclic positron emission tomography (PET) radioligands using the copper-mediated ¹⁸F-fluorination of aryl boron reagents with ¹⁸F-fluoride as a model reaction. This approach is based on a study examining how the presence of heterocycles commonly used in drug development affects the efficiency of ¹⁸F-fluorination for a representative aryl boron reagent, and on the labeling of more than 50 (hetero)aryl boronic esters. This set of data allows for the application of this derisking strategy to the successful radiosynthesis of seven structurally complex pharmaceutically relevant heterocycle-containing molecules.

Full text of DOI:10.1021/jacs.7b03131

Subscriber access provided by UNIV OF ARIZONA
Article
De-Risking the Cu-Mediated 18F-
Fluorination of Heterocyclic PET Radioligands
Nicholas Taylor, Enrico Emer, Sean Preshlock, Michael Schedler, Matthew Tredwell,
Stefan Verhoog, Joël Mercier, Christophe Genicot, and Veronique Emilie Gouverneur
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 26 May 2017
Downloaded from http://pubs.acs.org on May 26, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 13
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
De-Risking the Cu-Mediated 18F-Fluorination of Heterocyclic PET
Radioligands
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Nicholas J. Taylor,†ƒ Enrico Emer,†ƒ Sean Preshlock,†§ƒ Michael Schedler,†⊥ Matthew Tredwell,†ǁ
Stefan Verhoog,† Joel Mercier,‡ Christophe Genicot,‡ Véronique Gouverneur†*
† University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, Oxford, OX1 3TA (UK)
‡ UCB Biopharma sprl, 1420 Braine-L'Alleud (Belgium)
ABSTRACT: Molecules labeled with fluorine-18 (18F) are used in positron emission tomography to visualize, characterize
and measure biological processes in the body. Despite recent advances in the incorporation of 18F onto arenes, the
development of general and efficient approaches to label radioligands necessary for drug discovery programs remains a
significant task. This full account describes a de-risking approach towards the radiosynthesis of heterocyclic PET
radioligands using the copper-mediated 18F-fluorination of aryl boron reagents with 18F-fluoride as a model reaction. This
approach is based on a study examining how the presence of heterocycles commonly used in drug development affects
the efficiency of 18F-fluorination for a representative aryl boron reagent, and on the labeling of more than fifty (hetero)aryl
boronic esters. This set of data allows for the application of this de-risking strategy to the successful radiosynthesis of
seven structurally complex pharmaceutically-relevant heterocycle-containing molecules.
INTRODUCTION
synthesis of precursors, which may not be suitable for the
intended 18F-radiolabeling reaction. Chemists have
developed high-throughput screening approaches for the
rapid evaluation of functional group compatibility and
the generality of the selected reaction conditions,
including tolerance to heterocyclic rings. Particularly
noteworthy is the chemical robustness screen developed
by Glorius and co-workers.4 However, no substantial
studies of this kind have been carried out in
radiochemistry.5
Therapeutic innovation is increasingly challenging
due to the complexity of the physiopathology of diseases
where there is a high unmet medical need. For known
and new biological targets, positron emission tomography
(PET) is a quantitative molecular imaging technology that
can provide valuable insight on the in vivo behavior of
drug candidates much earlier in the drug discovery
pipeline. In practice, PET requires molecules labeled with
a positron-emitting radioisotope. For these studies, 18F is
one of the radioisotopes of choice since a significant
proportion of drugs on the market contain a fluorine
substituent, and the half-life of 18F (110 minutes) poses less
constraint for radiosynthesis and imaging time in
comparison with that of 11C (20 minutes).1 Despite the
recent progress made in the field of 18F-radiochemistry,
one outstanding challenge is the labeling of complex
molecules, especially those featuring one or multiple
heterocycles often seen in medicinal chemistry.2 The
molecular properties of these ring systems have benefited
drug discovery due to their unique electronic distribution,
shape and scaffold rigidity.3 These motifs can tailor
Herein, we describe a de-risking strategy towards the
18F-labeling of complex molecules, using the Cu-mediated
oxidative nucleophilic 18F-fluorination of aryl boronic
esters as a model reaction.6 Our approach is based on
screening experiments which provide information
regarding the tolerance of this reaction towards a
selection of heterocycles, in addition to offering insights
on the parameters affecting oxidative nucleophilic
18F-radiofluorination. With this study, we aim to aid
potential users of this reaction in conceiving retro-
radiosynthetic plans that maximize the chances of
successfully radiolabeling complex heterocyclic PET
radioligands (Fig. 1A,B). We exemplify this approach with
the radiosynthesis of seven 18F-labeled medicinally-
significant molecules [18F]1-[18F]7, all featuring one or
more heteroatom-containing ring systems (Fig. 1C).
potency,
physicochemical
and
pharmacokinetic
properties, and are therefore equally important in PET
radioligand design. Today, most studies aimed at defining
the scope of a new 18F-labeling method consider the steric
and electronic properties of the substrate, as well as
functional group tolerance, but lack information on
compatibility with heterocycles. As a result, late stage
18F-incorporation can become a high-risk process for the
labeling of molecules possessing one or more heterocyclic
systems; indeed, significant time is invested in the
RESULTS AND DISCUSSION
Our first study, borne from an academic-industrial
partnership between our laboratory and UCB Biopharma,
showed that aryl boronic esters are amenable to
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 13
nucleophilic 18F-fluorination with potassium 18F-fluoride
1
2
in the presence of [Cu(OTf)2py4] when the reaction is
o
performed in N,N-dimethylformamide (DMF) at 110 C.6
3
4
5
The reaction is compatible with automation, and both
higher Cu:substrate ratio (up to 1.5:1) and the replacement
of DMF with N,N-dimethylacetamide (DMA) improved
6
7
8
9
the
radiochemical
conversions
(RCCs)
for
electron-deficient arenes;7 in addition, [Cu(OTf)2py4]
could be replaced with Cu(OTf)2 and pyridine.7,8 The
influence of heterocycles on the feasibility of this process
is best contextualized with some mechanistic
considerations. Ribas followed by Wang demonstrated
that the Cu-catalyzed nucleophilic fluorination of arenes
proceeds via Cu(III) intermediates in geometrically-
constrained systems.9 The fluorination of aryl iodides
with [Cu(OTf)(tBuCN)2] and AgF was proposed by
Hartwig and co-workers to occur by oxidative addition to
form a Cu(III) intermediate followed by C−F reductive
elimination from an aryl copper(III) fluoride.10 A high
valent Cu(III) species was also invoked by Sanford and co-
workers in the reaction of (hetero)aryltrifluoroborate salts
with KF and Cu(OTf)2.11 These precedents led us to
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
examine the reactivity of
a preformed aryl-Cu(III)
complex derived from azacalix[1]arene[3]pyridine with
[18F]KF in DMF at 110 oC.12 Successful 18F-fluorination
under carrier-added conditions provided experimental
evidence that
a high-valent aryl-Cu(III) species is
amenable to 18F−C bond formation, and is therefore a
plausible intermediate on the pathway converting
pinacol-derived aryl boron reagents into 18F-labeled aryl
fluorides.12 In the 18F-labeling of aryl boronic esters, air is
highly beneficial for 18F-fluorination,12 indicating that the
Cu(III) species could be produced by direct oxidation;
however a Cu(II) into Cu(III)/Cu(I) disproportionation
pathway cannot be discounted. We anticipated that some
heterocycles could influence 18F-fluorination by
facilitating the transmetalation of the aryl group from
boron to copper, or the reductive elimination from
Cu(III)18F intermediates, whilst others with strongly Lewis
basic
coordinating
heteroatoms
could
prevent
18F-fluorination by generating unreactive copper species.
Our first objective was to categorize heterocycles by
their compatibility with the Cu-mediated 18F-fluorination
of aryl boron reagents. The 18F-fluorination of a model
aryl boronic ester was performed in the presence of one
molar equivalent of heterocycle in order to emulate the
presence of the heterocycle within a target molecule, and
to assess the impact on 18F-labeling efficacy. We selected
4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)benzonitrile 8 for these experiments, a substrate
affording 4-[18F]fluorobenzonitrile [18F]9 in moderate
radiochemical conversion (RCC) (34% ± 15%, n = 122)
upon treatment with [18F]KF and [Cu(OTf)2py4] in DMF
for 20 minutes at 110 °C (Table 1).12 The heterocycles used
in the additive screen are present in a greater than ten-
fold excess with respect to the copper complex (molar
ratio for 8:heterocycle:[Cu] = 11:11:1, Table 1); the effects of
the heterocycles are therefore pronounced. The reactions
were analyzed using radio-TLC/HPLC, providing a
Figure 1. (A) Late-stage 18F-fluorination of complex targets
featuring multiple heterocycles (A, B, C, D) is a high-risk
approach. (B) A rapid screen on the compatibility of the
reaction of a readily available model substrate (M) in the
presence of each heterocycle and/or the target molecule
provides information on feasibility for
18F-radiofluorination. As
result, new high feasibility
retrosynthetic schemes can be designed featuring
a
final step
a
a
post-fluorination step to introduce any incompatible
heterocycle (here A). (C) Seven medicinally-relevant
molecules successfully 18F-labeled using the de-risking
strategy.
ACS Paragon Plus Environment

Page 3 of 13
Journal of the American Chemical Society
Table 1. Use of 8 as a Model Substrate for Studying the Influence of Heterocycles on Radiochemical Conversion
1
2
3
4
[
18F]KF/K222 (20-30 MBq)
[Cu(OTf)2(py)4]
18F
O
B
Key:
RCC < 5%
RCC (No heterocycle):
34% ± 15% (n = 122)
Het
O
5% RCC < 20%
RCC 20%
DMF, 110 °C, 20 min
NC
Minimum RCC: 3%
Maximum RCC: 73%
NC
18F]9
8
1 eq.
5
[
6
7
R1
N
O
S
8
9
N
NR
N
N
R
N
R
R
N
R
R2
10a R = H: 0%, 0%
10b R = Me: 0%, 0%
10c R = Bn: 2% ± 2%[a]
12a R1 = R2 = H: 0%, 0%
12b R1 = Me, R2 = H: 4%, 4%
12c R1 = Bn, R2 = H: 21%, 26%
11a R = H: 0%, 0%
13a R = H: 0%, 0%
14a R = H: 0%, 0%
15a R = H: 0%, 0%
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
11b R = Me: 0%, 0%
11c R = Bn: 4% ± 2%[a]
11d R = Boc: 24%, 61%
13b R = Me: 7%, 15%
13c R = Bn: 39% ± 13%[a]
14b R = Bn: 33%, 41%
15b R = Me: 12%, 26%
10d R = Boc: 40% ± 17%[a]
12d R1 = Bn, R2 = Boc: 54%, 76%
R1
R1
O
O
NR1
NR1
NMe
O
N
O
O
N
N
O
R2
R2
R
N
N
R
R2
N
18a R1 = R2 = H: 0%, 1%
19a R1 = R2 = H: 2%, 9%
20a R = H: 2%, 2%
16a R = H: 1%, 1%
17a R1 = R2 = H: 0%, 0%
21 17%, 34%
18b R1 = H, R2 = Me: 3%, 5% 19b R1 = H, R2 = Me: 12% ± 6%[a] 20b R = Bn: 45%, 59%
18c R1 = R2 = Me: 34%, 36% 19c R1 = R2 = Me: 26%, 35%
16b R = Me: 40%, 44% 17b R1 = H, R2 = Me: 13%, 24%
17c R1 = R2 = Me: 38%, 42%
O
nBu
O
O
S
O
S
26 42% ± 8%[a]
24 37%, 62%
25 13%, 42%
27 61%, 69%
22 31%, 35%
23 27%, 63%
R
N
S
N
N
N
R
O
N
O
N
O
S
O
28 37%, 40%
29a R = H: 19%, 23%
29b R = Ph: 24%, 32%
30
31a R = H: 15%, 21%
31b R = Me: 40%, 44%
32 53% ± 14%[a]
33 42% ± 12%[a]
45%, 58%
N
N
N
N
N
N
H
N
H
N
R
N
R
N
R
R
34 25%, 45%
37% ± 9%[a]
R = H: 0%, 0%
R = H: 0%, 0%
R = H: 0%, 0%
R = H: 0%, 0%
35
36a
36b
37a
38a
39a
R = Me: 74%, 81%
R = Me: 40%, 48%
R = Me: 7%, 7%
R = Me: 29%, 31%
37b
38b
39b
N
N
N
N
N
N
N
N
N
N
N
R
EtO2C
N
N
R
N
R
N
R
R
R
R = H: 0%, 0%
R = H: 0%, 0%
R = H: 0%, 0%
R = Me: 72% ± 13%[a]
R = H: 0%, 0%
R = Me: 62%, 78%
41a R = H: 0%, 0%
41b R = Me: 0%, 0%
41c R = Boc: 7%, 9%
43a
43b
44a
44b
45a
45b
40a
40b
R = H: 0%, 0%
42a
42b
R = Bn: 53%, 68%
R = Me: 44%, 46%
R = Me: 33%, 77%
O
NMe2
N
Me
N
N
N
MeN
N
N
N
N
N
H
N
N
R
R
O
N
N
Me
69% ± 6%[a]
51
R = H: 0%, 0%
46a
46b
R = H: 1%, 1%
47 14%, 22%
48a
48b
49
36%, 53%
50
0%, 0%
R = Bn: 42%, 47%
R = Bn: 17%, 32%
N
N
N
N
N
N
N
N
N
N
57
22%, 28%
52 26%, 28%
56 69%, 72%
53 30%, 39%
54
55
26%, 34%
20%, 30%
F
O
N
N
CO2Et
N
N
N
N
O
N
N
N
N
N
N
N
N
N
NMe2
Me
N
NHR
N
N
S
F
N
59 80% ± 8%[a]
60 43%, 84%
R = H: 0%, 0%
R = CPh3: 17%, 19%
63 1%, 3%
62 0%, 0%
58 45%, 48%
3
61
Conditions: 8 (0.06 mmol), heterocycle (0.06 mmol), [Cu(OTf)2(py)4] (0.0053 mmol), [18F]KF/K222 (20-30 MBq.), DMF (300 ꢀL),
air, 110 °C, 20 mins. In the absence of a heterocyclic additive, 18F-fluorination affords [18F]9 in 34% ± 15% RCC (n = 122).
Radiochemical conversions for the 18F-labeling of 8 in the presence of each heterocycle, using a 11:11:1 molar ratio for
8:heterocycle:[Cu(OTf)2(py)4]. Radiochemical conversions were determined by radio-TLC and radio-HPLC. Screening reactions
were performed twice (n = 2); the radiochemical conversion of [18F]9 for each run is given. [a] Selected screening reactions were
performed eight times (n = 8); in these cases, radiochemical conversions are given in the form mean ± standard deviation.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 13
quantitative assessment of the RCC and therefore the para-tolylboronic acid. This reaction afforded 10% of
1
2
3
4
5
6
7
8
tolerance of the reaction towards each heterocycle.
1-(para-tolyl)-1H-pyrazole along with unreacted phenol; a
similar outcome was observed with complex
[Cu(OTf)2(py)4] in the presence of 4 equivalents of
pyrazole.12 These considerations likely account for the 18F-
When 8 was subjected to labeling conditions in the
presence of heterocycles containing an unprotected cyclic
secondary amine able to act as nucleophile in Chan-Lam
coupling, [18F]9 was not observed. An appropriate
protecting group was important for this class of
molecules. For pyrrolidine and piperidine (pKaH = 11.3 and
11.2, respectively), N-methylation did not influence
basicity to a point that allowed for the 18F-labeling of 8 to
occur in the presence of 10b (N-methyl pyrrolidine, pKaH
= 10.5) or 11b (N-methyl piperidine, pKaH = 10.1).13
N-Protection with an electron-withdrawing group
restored reactivity, as demonstrated with 10d and 11d. For
the less basic piperazine 12a (pKaH1 = 9.4) or morpholine
13a (pKaH = 8.5), N-alkylation was sufficient for these
heterocycles to be compatible with labeling, as
demonstrated with N-benzyl piperazine 12c (pKaH1 = 9.0)
fluorination
benzo[d]imidazole 41a and benzo[d][1,2,3]triazole 44a.
The presence of substituent on particular
intolerance
to
indazole
37a,
a
heteroarene can strongly influence its electronic and
steric properties, and therefore its compatibility with
18F-fluorination. For example, the electron-deficient
imidazole 40b is tolerated without the need for
substitution at C2, in contrast to imidazole 39b for which
the presence of a methyl group at that carbon is essential.
Heteroarenes such as pyridine 50, isoquinoline 56 or
imidazo[1,2-b]pyridazine 59 led to the formation of [18F]9
in RCC higher than observed in their absence. The Lewis
basic ligand pyridine is known to facilitate carbon-
heteroatom coupling at a Cu(III) center and was expected
to favorably influence carbon-fluorine bond formation.17
For example, the radiofluorination of 8 was found to be
very effective in the presence of pyridine (50, pKaH = 5.2,
RCC = 63%), a result contrasting with the stronger Lewis
base N,N-dimethylaminopyridine (DMAP 51, pKaH = 9.6,
RCC = 0%). These data corroborate with the observation
that [EtMe2Cu(DMAP)] is a longer lived intermediate
than [EtMe2Cu(py)], and undergoes reductive elimination
into propane more slowly.18
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
and N-methyl morpholine 13b (pKaH
=
7.4).13,14
Heterocyclic tertiary amines are frequently incorporated
into pharmaceutical agents in order to improve
pharmacokinetic profiles, so the finding that 12c-d, 13b-c,
14b and 15b are tolerated is significant. For amides and
ureas, N-protection was mandatory to minimize
detrimental effects on RCC (16a-21). Substitution of
weakly acidic C−H positions is beneficial, as exemplified
with comparative experiments undertaken with N-methyl
indolin-2-one 17b and the C3-dimethylated indolin-2-one
17c.
Caution should be taken when extrapolating data
secured on single heteroarenes to more complex ring
systems. For example, thiazole 32 and pyridimine 53 are
well-tolerated individually, but the bicyclic 4-(pyrimidin-
4-yl)thiazole 62, a molecule with two nitrogen atoms N3
and N’3 arising from different heteroarenes and ideally
positioned for bidentate Cu(II) coordination, inhibits the
formation of [18F]9. In this respect, the ideal de-risking
experiment is to carry out the 18F-labeling of 8 (or an
alternative model aryl boron reagent) in the presence of
the target molecule itself or a close analogue. Noteworthy,
we found that altering the reaction conditions applied for
screening to the conditions found to be beneficial for
electron-deficient arenes (molar ratio 8:heterocycle:[Cu] =
1:1:1.5)7 could accommodate certain heterocycles that
failed when these are added in large excess with respect
to the copper complex (molar ratio 8:heterocycle:[Cu] =
11:11:1).12
We investigated next the influence of heteroarenes on
the 18F-labeling of 8. Furan 24, benzofuran 25, thiophene
26, benzothiophene 27, oxazole 28, benzo[d]oxazoles 29a-
b, isoxazole 30, benzo[d]isoxazoles 31a-b, thiazole 32,
benzo[d]thiazole 33, pyrrole 34, indole 35, pyrrolo[3,2-
b]pyridine 47, pyridine 50, pyridazine 52, pyrimidine 53,
pyrazine 54, quinoline 55, isoquinoline 56, quinazoline 57,
quinoxaline 58 and imidazo[1,2-b]pyridazine 59 were all
compatible with 18F-fluorination. Heteroarenes containing
a very weakly acidic N−H group such as pyrrole 34 (pKa =
23.0) and indole 35 (pKa = 21.0) are tolerated without the
need for N-protection.15 In contrast, we found that
pyrazole 36a, imidazole 38a, triazoles 43a and 45a, and
tetrazole 46a (pKa = 19.8, 18.6, 13.9 and 8.2, respectively)
all required N-protection to prevent interference with the
formation of [18F]9. When not N-protected, these
heteroarenes are possible Chan-Lam partners in the
coupling with aryl boron reagents, and/or could
coordinate strongly to Cu(II) complexes leading to
The next step was to verify the value of the screening
approach detailed in Table 1 with the 18F-labeling of boron
reagents all containing a heterocyclic system; selected
precursors feature the boronic ester substituent on the
heteroarene itself. We chose scaffolds seen in drug
compounds, including those that would be a challenge for
18F-labeling using conventional methods.2a,3a-b The
(hetero)aryl boron reagents were prepared by subjecting
hetero(aryl) halides to Miyaura borylation conditions.19
This process was generally efficient, although some
heteroaryl boron reagents proved difficult to prepare
and/or purify, for example 2-pyridyl, 5-thiazolyl, 4- and
5-pyrazolyl derivatives. The propensity of some
deactivation.
The
structures
of
complexes
[Cu(OTf)2(py)4], [Cu(OTf)2(im)4] and [Cu(OTf)2(pz)4] are
known,16 and show that the distance between the metal
centre and the nitrogen is shorter in the cases of
imidazole (im) (1.991 Å) and pyrazole (pz) (2.006 Å) in
comparison with pyridine (py) (2.026 Å), indicating that
imidazole and pyrazole are more strongly coordinated to
Cu. Radiofluorination of 8 did not proceed when the
reaction was performed using [Cu(OTf)2(pz)4] instead of
[Cu(OTf)2(py)4].12 Complex [Cu(OTf)2(pz)4] is also
ineffective for the coupling of para-tert-butyl phenol and
ACS Paragon Plus Environment

Page 5 of 13
Journal of the American Chemical Society
Table 2. Cu(II)-Mediated 18F-Fluorination of (Hetero)aryl Pinacol-Derived Boronic Esters with [18F]Fluoride
1
2
3
4
[
18F]KF/K222 (20-30 MBq)
[Cu(OTf)2(py)4]
18F
Het
O
B
O
B
Het
18F
or
or
Het
O
O
DMF or DMA
Het
110 °C, 20 min
5
6
7
8
9
O
O
t-BuO2C
N
N
N
N
N
N
N
18F
18F
18F
18F]64: A: 17% ± 14%
18F
18F
18F
18F
[
[
18F]65: A: 0%
[
18F]66: A: 42% ± 21%
[
18F]67: A: 67% ± 5%
[
18F]68: A: 8% ± 5%
B: 47% ± 7%
[
18F]69: A: 20% ± 3%
[
18F]70: A: 80% ± 10%
B: 7% ± 7%
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
18F
BocN
O
N
O
S
18F
O
HN
N
N
N
N
N
N
MeN
18F
18F
18F
18F
18F
Me
[
18F]71: A: 0%
[
18F]72: A: 25% ± 3%
[
18F]73: A: 44% ± 16%
[
18F]74: A: 39% ± 15%
[
18F]75: A: 11% ± 6%
[
18F]76: A: 41% ± 15%
[
18F]77: A: 3% ± 1%
B: 32% ± 2%
B:
16% ± 9%
O
18F
H
18F
18F
18F
18F
S
N
O
18F
CHO
O
O
N
18F
N
O
S
N
Me
Me
[
18F]78: A: 9% ± 9%[a]
B: 28% ± 16%
[
18F]79: A: 53% ± 20%
[
18F]80: A: 22% ± 5%
[
18F]81: A: 20% ± 11%
[
18F]82: A: 20% ± 15%
[
18F]83: A: 68% ± 2%
[
18F]84: A: 50% ± 16%
S
18F
18F
S
18F
18F
18F
N
N
S
S
NMe
N
18F
18F
Ph
O
N
O
N
O
N
O
N
N
O
[
18F]85: A: 8% ± 2%
[
18F]86: A: 24% ± 5%
[
18F]87: A: 27% ± 10%
[
18F]88: A: 6% ± 2%[a]
B: 51% ± 15%
[
18F]89: A: 27% ± 3%
[
18F]90: A: 33% ± 11% 18F]91: A: 21% ± 4%
[
18F
18F
18F
18F
O
N
S
S
N
18F
CO2Et
N
N
18F
N
N
H
H
N
O
H
18F
N
H
H
[
18F]92: A: 36% ± 10%
[
18F]93: A: 18% ± 14%
[
18F]94: A: 38% ± 15%
[
18F]95: A: 52% ± 8%
[
18F]96: A: 30% ± 9%
[
18F]97: A: 27% ± 3%
[
18F]98: A: 32% ± 12%
18F
18F
18F
N
N
N
N
N
N
N
N
N
N
N
N
18F
N
18F
N
18F
18F
N
Ts
Cl
N
Me
[
18F]99: A: 44% ± 12%
[
18F]100: A: 0%[a]
[
18F]101: A: 19% ± 4%
B: 78% ± 6%
[
18F]102: A: 6% ± 5%
B: 12% ± 7%
[
18F]103: A: 54% ± 8%
[
18F]104: A: 9% ± 5%
B: 34% ± 9%
[
18F]105: A: 2% ± 1%
B: 53% ± 8%
B: 0%[a]
18F
N
O
N
O
18F
18F
N
N
N
N
N
N
18F
Me
Me
Cl
N
N
NMe2
Cl
N
18F
N
18F
18F
[
18F]106: A: 1% ± 1%
B: 7% ± 4%
[
18F]107: A: 0%
B: 6% ± 2%
[
18F]108: A: 29% ± 12%
[
18F]109: A: 0%
[
18F]110: A: 5% ± 2%
B: 28% ± 5%
[
18F]111: A: 31% ± 4%
[
18F]112: A: 5% ± 4%
B: 76% ± 4%
B: 1% ± 1%
18F
18F
18F
18F
N
N
18F
N
N
18F
N
NH2
N
N
N
Ph
N
18F]116: A: 0%
B: 47% ± 7%[b]
[
18F]113: A: 52% ± 16%
[
18F]114: A: 5% ± 2%
B: 25% ± 8%
[
18F]115: A: 59% ± 9%
[
18F]117: A: 3% ± 1%
B: 63% ± 11%
[
18F]118: A: 21% ± 5%
[
Conditions A: (Hetero)aryl boronic ester (0.06 mmol), [Cu(OTf)2(py)4] (0.0053 mmol), [18F]KF/K222 (20-30 MBq.), DMF (300 ꢀL),
air, 110 °C, 20 mins; Conditions B: (Hetero)aryl boronic ester (0.02 mmol), [Cu(OTf)2(py)4] (0.03 mmol), [18F]KF/K222 (20-30
MBq.), DMA (300 ꢀL), air, 110 °C, 20 mins. Reactions were performed four times (n = 4); radiochemical conversions were
determined by radio-TLC and radio-HPLC and are given in the form mean ± standard deviation. [a] Reactions were performed
twice (n = 2). [b] Starting from N,N-di-Boc protected starting material; deprotection occurred under reaction conditions.
(hetero)aryl boron reagents to either gain stability or
decompose via protodeboronation in the presence of
Lewis acid additives such as Cu(II) salts has been recently
rationalized.20 A successful radiosynthesis therefore relies
as much on the accessibility of the precursor and its
stability under the reaction conditions as on the
feasibility of the 18F-radiofluorination step itself. In this
study, we subjected more than fifty substrates to
18F-fluorination applying conditions A or B (Table 2).
Conditions A consist of a 11:1 ratio of substrate:Cu(II)
complex in DMF at 110 oC. Under conditions B, a 1:1.5 ratio
o
of substrate:Cu(II) complex was used in DMA at 110 C.
Conditions B were typically required for the more
demanding tertiary amines ([18F]65, [18F]68) and
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 13
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. A comparison of the heterocyclic additive screening and corresponding labeling results for selected substrates. Red
highlight: RCC < 5%. Orange highlight: 5% ≤ RCC < 20%. Green highlight: RCC ≥ 20%. Screening conditions: 8 (0.06 mmol),
heterocycle (0.06 mmol), [Cu(OTf)2(py)4] (0.0053 mmol), [18F]KF/K222 (20-30 MBq.), DMF (300 ꢀL), air, 110 °C, 20 mins.
Labelling conditions (A): (Hetero)aryl boronic ester (0.06 mmol), [Cu(OTf)2(py)4] (0.0053 mmol), [18F]KF/K222 (20-30 MBq.),
DMF (300 ꢀL), air, 110 °C, 20 mins. Radiochemical conversions were determined by radio-TLC and radio-HPLC and are given in
the form mean ± standard deviation. [a] Reactions were performed eight times (n = 8). [b] Reactions were performed four times
(n = 4). [c] Under conditions B, RCC = 7% ± 7% (n = 4). [d] Reactions were performed twice (n = 2). [e] Under conditions B, RCC
= 32% ± 2% (n = 4). [f] Under conditions B, RCC = 47% ± 7% (n = 4). [g] Under conditions B, RCC = 7% ± 4% (n = 4). [h] Under
conditions B, RCC = 53% ± 8% (n = 4).
substrates bearing unprotected N−H functionalities
([18F]77, [18F]78). The use of an excess of Cu(II) complex
was also found beneficial for electron-deficient
heterocycles ([18F]105-[18F]107, [18F]110, [18F]112, [18F]114).
Applying both conditions A and B, we labeled fluorine-
containing drug fragments with heterocycles and basic
amines in RCCs up to 80%, including pyrrolidines [18F]64-
[18F]66, piperidines [18F]67-[18F]70, piperazines [18F]71-
[18F]72, morpholines [18F]73-[18F]74, phenothiazine [18F]75,
and tetrahydroisoquinoline [18F]76. The monoprotected
imidazolidin-2-one [18F]77 and tetrahydropyrimidin-
2(1H)-one [18F]78 were formed under conditions B in 32%
and 28% RCC, respectively. Indolinone [18F]79 was formed
in good RCC when both the nitrogen and C3 were
protected, a result corroborating the data presented in
Table 1 for 17a-c. Furans [18F]81-[18F]82, thiophenes
[18F]83-[18F]84 and a range of heterocycles [18F]85-[18F]92
containing multiple heteroatoms (N,O and N,S)
underwent 18F-fluorination in good RCC. Indoles [18F]94-
[18F]98 were labeled at position 4, 5, 6 or 7 in RCC up to
52% without N-protection. For some substrates,
substitution had a strong influence on reactivity. For
example, imidazole [18F]100 could not be labeled under
conditions A or B. In contrast, C2-methylated imidazole
[18F]101 was labeled in 78% RCC under conditions B.
Benzimidazole [18F]102 was radiolabeled in low RCC
(12%). N-Alkylated azoles containing multiple nitrogen
atoms such as indazole [18F]99, triazole [18F]103 and
tetrazole [18F]104 underwent 18F-fluorination in good RCC
(up to 54%). The chloro-substituted pyridines [18F]105-
[18F]107 were labeled on position C4, C5 and C6, as well as
pyrimidines [18F]109-[18F]111, quinolines [18F]112-[18F]113,
isoquinolines [18F]114-[18F]115, quinazolines [18F]116-
[18F]117, and quinoxaline [18F]118. In this series, the 18F-
fluorination was most challenging for pyridines [18F]106-
[18F]107 and pyrimidine [18F]109. For [18F]105-[18F]107, it is
noteworthy that the boronic ester functionality is
displaced with 18F in preference to the chloro
substituent.12
Overall, the data show reasonable agreement between
the screening and 18F-labeling experiments detailed in
Tables 1 and 2, respectively. Figure 2 compares data based
on a larger number of screening experiments (n = 8)
applied to a narrower selection of twelve heterocyclic
systems. Any pair with the same color code indicates
agreement between the screening experiments and
ACS Paragon Plus Environment

Page 7 of 13
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 13
Scheme 1. Final-Stage 18F-Fluorination of Medicinally-Relevant Molecules with [18F]Fluoride
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Heterocycles shown by screening experiment to be tolerated are highlighted in green. 18F-Fluorination conditions: (Hetero)aryl
boronic ester (0.02 mmol), [Cu(OTf)2(py)4] (0.03 mmol), [18F]KF/K222 (20-30 MBq.), DMA (300 ꢀL), air, 110 °C, 20 mins.
Reactions were performed four times; radiochemical conversions were determined by radio-TLC and radio-HPLC and are given
in the form mean ± standard deviation. (A) Gefitinib. (B) IPMICF10.
18F-labeling; this was the case for ureas 19b and [18F]78,
thiophenes 26 and [18F]83, benzo[d]thiazoles 33 and
[18F]90, indoles 35 and [18F]95, and triazoles 45b and
[18F]103. The screening experiments predict correctly that
N-alkylated pyrrolidines 10c and piperidines 11c are highly
challenging substrates (0% RCC and 8% for [18F]65 and
[18F]68, respectively), and that reduced basicity restores
reactivity, as illustrated with the successful labeling of
[18F]66 and [18F]74. For low yielding heterocycles such as
[18F]68, we recommend applying conditions B consisting
of a higher Cu:substrate ratio in the reaction solvent
DMA. These conditions afforded [18F]68 in an improved
RCC of 47%. The comparative data selected in Figure 2
indicate that caution should be exercised when
attempting to extrapolate the result of the screening
experiment to predict feasibility for the labeling of the
heterocycle itself. This is best illustrated with pyridine
and the contrasting data for [18F]108, [18F]106 and [18F]105.
The screening experiment predicts correctly that [18F]108
with a pyridyl substituent can be labeled successfully; this
is in contrast to the outcome for the 18F-labeling on the
pyridine itself. As anticipated, the position of the 18F on
the heterocycle impacts on reactivity considerably; the 3-
fluoropyridine [18F]106 is afforded in very low RCC under
both conditions A (1%) and B (7%), but the pinacol-
derived boronic ester leading to 4-fluoropyridine [18F]105
responded well for 18F-fluorination, but only under
conditions B (RCC = 53%).
substituent. Caution should be exercised if one requires
that the heterocycle itself is 18F-labeled;
2) The radiochemical yield is dependent on the
reaction conditions. We have applied two sets of
conditions A and B in this study, and have demonstrated
that conditions B rescue some of the most challenging
substrates;
3) The reaction is sensitive to steric and electronic
effects, and this should be taken into account in addition
to any compatibility issue with a particular heterocycle;
4) For complex target, we recommend conducting a
screening experiment with the target itself.
When planning for the optimal copper-mediated
radiosynthetic route towards complex targets possessing
one or more heterocyclic sub-motifs, one should consider
several criteria: 1) 18F-incorporation should be late stage,
ideally taking place in the final step; 2) the 18F-
fluorination should be compatible with the functional
groups and any heterocyclic motif(s) present on the aryl
boron precursor; 3) if 18F-fluorination is not the final step
of the radiosynthesis, any post 18F-fluorination
transformation should be rapid and facile; and 4) the aryl
boron reagent should be readily accessible and stable.
With these considerations in mind, our final objective
was to demonstrate that the approach described in Figure
1, combined with the data in Tables 1 and 2, can guide a
low-risk retro-radiosynthesis for molecules featuring one
or more heterocyclic motifs. Our target molecules consist
of seven drugs and radioligands, namely [18F]gefitinib
[18F]1, [18F]IPMICF10 [18F]2, [18F]rufinamide [18F]3,
[18F]flutemetamol [18F]4, [18F]paroxetine [18F]5, [18F]FITM
The results assembled in Table 1, Table 2 and Figure
2 are instructive at various levels:
1) The screening experiments are in good agreement
for 18F-labeling an arene that features the heterocycle as a
ACS Paragon Plus Environment

Page 9 of 13
Journal of the American Chemical Society
Scheme 2. Medicinally-Relevant Molecules Requiring Deprotection Following 18F-Fluorination
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Heterocycles shown by screening experiment to be tolerated are highlighted in green. 18F-Fluorination conditions: Protected
(hetero)aryl boronic ester (0.02 mmol), [Cu(OTf)2(py)4] (0.03 mmol), [18F]KF/K222 (20-30 MBq.), DMA (300 ꢀL), air, 110 °C, 20
mins. Cu(II)-mediated 18F-fluorination reactions were performed four times; radiochemical conversions were determined by
radio-TLC and radio-HPLC and are given in the form mean ± standard deviation. The conversions of the deprotection steps were
determined by radio-HPLC. (A) Rufinamide. (B) Flutemetamol. (C) Paroxetine.
[18F]6 and [18F]risperidone [18F]7 (Fig. 1C). The synthesis of
the pinacol boronic ester precursors is required to access
these molecules in their 18F-labeled form, a process that
may be time-consuming and therefore best initiated if
late-stage 18F-fluorination is likely to be successful.
[18F]IPMICF10 [18F]2 is a tropomyosin receptor kinase
(Trk) inhibitor possessing pyrrolidine and imidazo[1,2-
b]pyridazine sub-motifs; this PET radioligand was
originally labeled on the fluoroethyl chain by SN2 with
[18F]KF,23 but the presence of the fluoroarene motif of
IPMICF10 persuaded us to envisage instead a copper-
mediated 18F-fluorination of the aryl ring. This decision
was reinforced by the observation that imidazo[1,2-
b]pyridazine 59 and 6-(pyrrolidin-1-yl)imidazo[1,2-
b]pyridazine 60 had a beneficial effect on the 18F-labeling
of 8 (Table 1). Accordingly, we obtained [18F]IPMICF10
[18F]2 in 71% RCC applying a final step 18F-fluorination of
boronic ester 120 (Scheme 1B).
Gefitinib 1 is an inhibitor of the epidermal growth
factor receptor–tyrosine kinase (EGFR-TK) that has
shown potent effects in patients carrying specific
EGFR-TK mutations in non-small-cell lung cancer.21
Measurement of intratumoral drug concentration by PET
with [18F]gefitinib can provide insights into the
mechanisms of gefitinib resistance in clinical studies.
[18F]Gefitinib was previously prepared in a three-step
radiosynthesis starting with the [18F]fluorination of 3-
chloro-4-trimethylammonium-nitrobenzene.22 With the
knowledge that quinazoline and N-alkyl morpholine did
not disrupt the 18F-labeling of 8, and are amenable to
labeling themselves ([18F]74, [18F]116, [18F]118), we
envisaged the radiosynthesis of [18F]gefitinib applying a
final step 18F-fluorination of the aryl boron precursor 119
possessing the unprotected diaryl amino functionality. As
planned, [18F]gefitinib [18F]1 was obtained in 22% RCC
applying conditions B (Scheme 1A).
Rufinamide 3 is an anticonvulsant broadly used as an
adjunctive seizure therapy in children and older adults
suffering from Lennox-Gastaut syndrome.24 The
mechanism of action of rufinamide remains unclear;
18F-labeling for PET studies is therefore of interest. The
N-benzylated triazole 43b was not detrimental to the
labeling of 8. Moreover, the results obtained by spiking
experiments with rufinamide itself 3 and N-protected
rufinamide 61 suggest that the synthesis of
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 13
1
2
Scheme 3. Medicinally-Relevant Molecules Requiring a Coupling Reaction Following 18F-Fluorination
3
4
5
6
A FITM
Unsuccessful final-stage fluorination:
[
18F]KF/K222
N
N
O
O
N
N
O
18F
F
N
N
O
[Cu(OTf)2(py)4]
B
N
N
N
H
N
NMe
O
N
NMe
N
DMA
NMe
126
H
H
S
110 °C, 20 min
S
S
7
[
18F]6
6
8
0%
9
Successful radiosynthesis informed by screening
N
N
129 (5 equiv.)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
N
18F]KF/K222
O
N
[
NHMe
N
N
O
18F
N
H
O
O
[Cu(OTf)2(py)4]
S
N
N
O
N
H
O
O
NMe
DMA
B
NaH (6 equiv.)
DMA, r.t., 10 min
O
S
110 °C, 20 min
18F
O
127
[
18F]128
61% ± 13% (n = 4)
[
18F]6
40% ± 13% (n = 4)
over two steps
B Risperidone
Unsuccessful final-stage fluorination:
O
O
N
O
N
N
[
18F]KF/K222
O
O
N
O
O
[Cu(OTf)2(py)4]
18F
18F
B
N
N
N
O
N
DMA
N
110 °C, 20 min
N
N
N
7
130
[
18F]7
0%
Successful radiosynthesis informed by screening
O
N
O
N
[
18F]KF/K222
O
N
[Cu(OTf)2(py)4]
1. TFA, 110 C, 10 min
O
O
18F
B
O
N
18F
DMA
110 °C, 20 min
BocN
O
2.
N
BocN
Cl
N
N
18F]132
42% ± 7% (n = 4)
18F]7
69% conversion
131
[
[
133 (2 equiv.)
N
K2CO3 (2 equiv.), KI (2 equiv.)
DMF, 110 C, 20 min
Heterocycles shown by screening experiment to be tolerated are highlighted in green; fragments not tolerated are highlighted in
red. 18F-Fluorination conditions: (Hetero)aryl boronic ester (0.02 mmol), [Cu(OTf)2(py)4] (0.03 mmol), [18F]KF/K222 (20-30 MBq.),
DMA (300 ꢀL), air, 110 °C, 20 mins. Successful Cu(II)-mediated 18F-fluorination reactions were performed four times.
Radiochemical conversions were determined by radio-TLC and radio-HPLC and are given in the form mean ± standard
deviation. (A) FITM. RCC of [18F]6 was 40% ± 13% (n = 4) over both steps, starting from 127. (B) Risperidone. Conversion of
[18F]132 into [18F]7 was determined by radio-HPLC.
[18F]rufinamide would be within reach from precursor 121
protected at the amide group (Table 1). This was verified
experimentally; N-trityl-[18F]rufinamide [18F]61 was
formed in 68% RCC, and subsequent deprotection with
TFA/i-Pr3SiH in DCE at 80° C afforded [18F]rufinamide
[18F]3 in 98% conversion (Scheme 2A).
Flutemetamol [18F]4 is a PET radiopharmaceutical
used to estimate beta-amyloid neuritic plaque density in
adult patients with cognitive impairment who are being
evaluated for Alzheimer’s disease.25 This radioligand has
been prepared upon treatment of a nitro precursor with
[18F]fluoride,26 and in its protected form, from an
iodonium precursor.27 The results obtained with
fragments 90 and 91 (Table 2) led us to consider the
copper-mediated 18F-fluorination of the O-protected aryl
boronic ester 122 as an alternative route (Scheme 2B).
This reaction led to [18F]123 in 21% RCC; demethylation
with BBr3 in DCE/DCM at 80 °C afforded
[18F]flutemetamol [18F]4 in 43% conversion.
Paroxetine 5 is an antidepressant which selectively
inhibits serotonin reuptake.28 The Ritter group reported
the synthesis of [18F]paroxetine applying a palladium-
mediated 18F-fluorination.29 We started our investigation
with the knowledge that the piperidine motif is best
protected with an electron-withdrawing group (Table 1).
Since the para-nitrophenyl carbamate was an
intermediate in the synthesis of the aryl boronic ester,
this carbamate was chosen for 18F-fluorination. Product
[18F]125 was obtained in 76% RCC and subsequent
treatment with aqueous NaOH for 10 minutes at 110 °C
ACS Paragon Plus Environment

Page 11 of 13
Journal of the American Chemical Society
lacked predictability for heterocyclic radioligands. These
proceeded in 98% conversion to afford [18F]paroxetine
[18F]5 (Scheme 2C).
considerations prompted us to provide radiochemists
with guidelines on the compatibility of the Cu-mediated
18F-radiofluorination with the rings most frequently used
in medicinal chemistry. Our empirical approach consists
of monitoring fluctuation in the RCC of a model reaction
carried out in the presence of heterocyclic additives.
Categorization of these ring systems as compatible or not
compatible with our Cu-mediated 18F-fluorination,
combined with the 18F-labeling of more than fifty
1
2
3
4
5
6
7
8
9
Since our screening method decouples the
heterocyclic motifs from the entire structure of the target,
caution should be taken when assessing the steric and/or
electronic impact of multiple heterocyclic fragments on
the reaction center. One representative example is the
labeling of the metabotropic glutamate 1 (mGlu1) receptor
antagonist [18F]FITM [18F]6,30
a molecule currently
accessible by 18F-fluorination of a nitro precursor.31 Our
screening study indicated that copper-mediated
18F-fluorination was compatible with pyrimidine 53 and
thiazole 32 but failed in the presence of 4-(pyrimidin-4-
yl)thiazole 62 (Table 1). Late stage 18F-fluorination is
therefore a high-risk operation, as verified experimentally
by the failure to access [18F]FITM [18F]6 from boronic ester
126 (Scheme 3A). An alternative retrosynthesis featuring
an amide coupling step following 18F-fluorination was
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(hetero)aryl boron reagents, provides
a convenient
starting point to establish low-risk retrosynthetic routes
towards complex molecular radioligands. This was
illustrated with the preparation of seven heterocycle-
containing radioligands. Two radiosyntheses feature the
18F-radiofluorination as the final step, three require a
deprotection step post 18F-fluorination, and two feature a
coupling reaction following 18F-incorporation. These three
scenarios were implemented by design,
a guided
therefore
implemented.
N-(Hydroxysuccimidyl)-
[18F]fluorobenzoate [18F]128 was prepared in 61% RCC and
coupled with heterocyclic amine 129. This two-step
radiosynthesis led to [18F]FITM [18F]6 in 40% RCC over
two steps.
approach that we view as highly advantageous in
comparison with time-consuming and costly trial and
error 18F-experimentation. The approach considered in
this study brings into focus the value of low risk retro-
radiosynthesis, and is geared for practicing radiochemists
who are looking for a rapid route to complex PET
radioligands. Beyond the Cu-mediated 18F-fluorination
selected here as our model reaction, this type of study
applied to other 18F-labeling methods combined with a
rapid assessment of precursor availability should lower
the barrier of adoption of one particular method over
another when a radiolabeled molecule is needed either for
a diagnostic or drug development program.
The last molecule investigated in this study is
risperidone 7, an antipsychotic drug that has never been
labeled with 18F.32 Benzo[d]isoxazoles 31a-b and
N-protected pyrimidin-4(3H)-one 21 are compatible with
18F-labeling, in contrast to N-alkylated piperidines 11b-c
(Table 1). Late stage 18F-fluorination of 130 failed under
our conditions (Scheme 3B); this result was expected
since the 2-methyl-3-(2-(piperidin-1-yl)ethyl)-6,7,8,9-
tetrahydro-4H-pyrido[1,2-a]pyrimidin-4-one fragment 63
is not tolerated (Table 1). An alternative synthetic
approach introducing this motif post 18F-fluorination was
therefore necessary. We started the radiosynthesis by
18F-fluorinating the Boc-protected fragment 131 in 42%
RCC; in situ N-deprotection of [18F]132 with TFA followed
by alkylation under basic conditions afforded
[18F]risperidone [18F]7 in 69% conversion.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures, spectral and analytical data, radio-
HPLC traces and additional 18F-radiolabeling experiments.
This material is available free of charge on the ACS
Publications website at http://pubs.acs.org.
CONCLUSION
The radiosynthesis of complex radioligands is typically
attempted when there is a clinical need, rarely as an
exercise to demonstrate the generality of
radiochemical transformation. As result, the
18F-labeling of complex targets remains
high-risk
endeavor. Since its discovery, the Cu-mediated
AUTHOR INFORMATION
Corresponding Author
a new
a
veronique.gouverneur@chem.ox.ac.uk
a
Present Addresses
18F-radiofluorination of aryl boron reagents has emerged
as an attractive methodology for the labeling of arenes,
and is finding increasing application for the labeling of
imaging agents.33 Post-labeling purification with HPLC
has demonstrated that 18F-fluoroarenes can be isolated in
high radiochemical purity and copper can be sufficiently
removed for clinical applications. However, the reaction
§ National Institute of Mental Health, PET
Radiopharmaceutical Science Section, 9000 Rockville
Pike-MSC 1003, Bethesda, MD 20892 (U.S.A.)
⊥ Covestro Deutschland AG, 51365 Leverkusen (Germany)
ǁ Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-
Platz 1, 45470 Mülheim an der Ruhr (Germany)
Author Contributions
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 13
(d) Zhang, H.; Yao, B.; Zhao, L.; Wang, D.-X.; Xu, B.-Q.; Wang,
ƒ N.J.T., E.E. and S.P. contributed equally to this work.
1
2
3
4
5
6
7
8
M.-W. J. Am. Chem. Soc. 2014, 136, 6326−6332.
(10) (a) Fier, P. S.; Hartwig, J. F. J. Am. Chem. Soc. 2012, 134,
10795-10798. (b) Fier, P. S.; Luo, J.; Hartwig, J. F. J. Am. Chem.
Soc. 2013, 135, 2552-2559.
(11) Ye, Y.; Schimler, S. D.; Hanley, P. S.; Sanford, M. S. J. Am.
Chem. Soc. 2013, 135, 16292-16295.
(12) See the Supporting Information.
(13) Hall, H. J. Jr. Correlation of the base strengths of amines. J.
Am. Chem. Soc. 1957, 7, 5441–5444.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Financial support for this work was provided by UCB, the
EPSRC (EP/L025604/1, NS/A000024/1), BBSRC (BB/K01191X/1)
and CRUK (C5255/A16466).
9
(14) Schep, L. J.; Slaughter, R. J.; Vale, J. A.; Beasley, D. M. G.;
Gee, P. Clinical Toxicology 2011, 49, 131–141.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(15) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-463.
(16) (a) Haynes, J. S.; Rettig, S. J.; Sams, J. R.; Trotter, J.;
Thompson, R. C. Inorg. Chem. 1988, 27, 1237–1241. (b) Liu, S.–J.;
Su, C.–C. J. Coord. Chem. 1995, 36, 95–103. (c) Di Nicola, C.;
Garau, F.; Gazzano, M.; Monari, M.; Pandolfo, L.; Pettinari, C.;
Pettinari, R. Cryst. Growth Des. 2010, 10, 3120–3131.
(17) Qiao, J. X.; Lam, P. Y. S. Synthesis 2011, 6, 829-856.
(18) Bartholomew, E. R.; Bertz, S. H.; Cope, S.; Dorton, D. C.;
Murphy, M.; Ogle, C. A. Chem. Commun. 2008, 1176–1177.
(19) Ishimyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995,
60, 7508-7510.
REFERENCES
(1) Ametamey, S. M.; Honer, M.; Schubiger, P. A. Chem. Rev.
2008, 108, 1501-1516.
(2) (a) Preshlock, S.; Tredwell, M.; Gouverneur, V. Chem. Rev.
2016, 116, 719-766. (b) Hoover, A. J.; Lazari, M.; Ren, H.;
Narayanam, M. K.; Murphy, J. M.; van Dam, R. M.; Hooker, J. M.;
Ritter, T. Organometallics 2016, 35, 1008-1014. (c) Rotstein, B. H.;
Wang, L.; Liu, R. Y.; Patteson, J.; Kwan, E. E.; Vasdev, N.; Liang,
S. H. Chem. Sci. 2016, 7, 4407-4417. (d) Gamache, R. F.;
Waldmann, C.; Murphy, J. M. Org. Lett. 2016, 18, 4522-4525. (e)
Makaravage, K. J.; Brooks, A. F.; Mossine, A. V.; Sanford, M. S.;
Scott, P. J. H. Org. Lett. 2016, 18, 5440-5443.
(3) (a) Vitaku, E.; Smith, D. T.; Njardarson, J. T. J. Med. Chem.
2014, 57, 10257–10274. (b) Taylor, D. R.; MacCoss, M.; Lawson, A.
D. G. J. Med. Chem. 2014, 57, 5845−5859. (c) Roughley, S. D.;
Jordan, A. M. J. Med. Chem. 2011, 54, 3451–3479.
(4) (a) Collins, K. D.; Glorius, F. Nat. Chem. 2013, 5, 597–601.
(b) Malik, H. A.; Taylor, B. L. H.; Kerrigan, J. R.; Grob, J. E.;
Houk, K. N.; Du Bois, J.; Hamann, L. G.; Patterson, A. W. Chem.
Sci. 2014, 5, 2352–2361. (c) Düfert, M. A.; Billingsley, K. L.;
Buchwald, S. L. J. Am. Chem. Soc. 2013, 135, 12877–12885. (d)
Kutchukian, P. S.; Dropinski, J. F.; Dykstra, K. D.; Li, B.; DiRocco,
D. A.; Streckfuss, E. C.; Campeau, L.-C.; Cernak, T.; Vachal, P.;
Davies, I. W.; Krska, S. W.; Dreher, S. D. Chem. Sci. 2016, 7,
2604–2613. (e) Foley, D. J.; Nelson, A.; Marsden, S. P. Angew.
Chem. Int. Ed. 2016, 55, 13650–13657. (f) Sandfort, F.; O’Neill, M.
J.; Cornella, J.; Wimmer, L.; Baran, P. S. Angew. Chem. Int. Ed.
2017, 56, 3319-3323.
(5) The additive screening concept has been applied in a very
limited manner in: Lee, H. G.; Milner, P. J.; Placzek, M. S.;
Buchwald, S. L.; Hooker, J. M. J. Am. Chem. Soc. 2015, 137, 648-
651.
(6) Tredwell, M.; Preshlock, S. M.; Taylor, N. J.; Gruber, S.;
Huiban, M.; Passchier, J.; Mercier, J.; Génicot, C.; Gouverneur, V.
Angew. Chem. Int. Ed. 2014, 53, 7751–7755.
(7) Preshlock, S.; Calderwood, S.; Verhoog, S.; Tredwell, M.;
Huiban, M.; Hienzsch, A.; Gruber, S.; Wilson, T. C.; Taylor, N. J.;
Cailly, T.; Schedler, M.; Collier, T. L.; Passchier, J.; Smits, R.;
Mollitor, J.; Hoepping, A.; Mueller, M.; Genicot, C.; Mercier, J.;
Gouverneur, V. Chem. Commun. 2016, 52, 8361-8364.
(8) Mossine, A. V.; Brooks, A. F.; Makaravage, K. J.; Miller, J.
M.; Ichiishi, N.; Sanford, M. S.; Scott, P. J. H. Org. Lett. 2015, 17,
5780-5783.
(20) Cox, P. A.; Leach, A. G.; Campbell, A. D.; Lloyd-Jones, G.
C. J. Am. Chem. Soc. 2016, 138, 9145–9157.
(21) Anderson, N. G.; Ahmad, T.; Chan, K.; Dobson, R.;
Bundred, N. J. Int. J. Cancer 2001, 94, 774–782.
(22) (a) Seimbille, Y.; Phelps, M. E., Czernin, J.; Silverman, D. S.
J. Labelled Compd. Radiopharm. 2005, 48, 829–843. (b) Läppchen,
T.; Vlaming, M. L. H.; Custers, E.; Lub, J.; Sio, C. F.; DeGroot, J.;
Steinbach, O. C. Appl. Radiat. Isot. 2012, 70, 205–209.
(23) Bernard-Gauthier, V.; Bailey, J. J.; Aliaga, A.; Kostikov, A.;
Rosa-Neto, P.; Wuest, M.; Brodeur, G. M.; Bedell, B. J.; Wuest, F.;
Schirrmacher, R. Med. Chem. Commun. 2015, 6, 2184–2193.
(24) Brodie, M. J.; Rosenfeld, W. E.; Vazquez, B.; Sachdeo, R.;
Perdomo, C.; Mann, A.; Arroyo, S. Epilepsia 2009, 50, 1899–1909.
(25) Vandenberghe, R.; Van Laere, K.; Ivanoiu, A.; Salmon, E.;
Bastin, C.; Triau, E.; Hasselbalch, S.; Law, I.; Andersen, A.;
Korner, A.; Minthon, L.; Garraux, G.; Nelissen, N.; Bormans, G.;
Buckley, C.; Owenius, R.; Thurfjell, L.; Farrar, G.; Brooks, D. J.
Ann. Neurol. 2010, 68, 319–329.
(26) Heurling, K.; Leuzy, A.; Zimmer, E. R.; Lubberink, M.;
Nordberg, A. Eur. J. Nucl. Med. Mol. Imaging 2016, 43, 362–373.
(27) Linstad, E. J.; Vāvere, A. L.; Hu, B.; Kempinger, J. J.;
Snyder, S. E.; DiMagno, S. G. Org. Biomol. Chem. 2017, 15, 2246-
2252.
(28) Lund, J.; Lomholt, B.; Fabricius, J.; Christensen, J. A.;
Bechgaard, E. Acta Pharmacol. Toxicol. 1979, 44, 289–295.
(29) Kamlet, A. S.; Neumann, C. N.; Lee, E.; Carlin, S. M.;
Moseley, C. K.; Stephenson, N.; Hooker, J. M.; Ritter, T. PLOS
ONE 2013, 8, e59187.
(30) Satoh, A. Nagatomi, Y.; Hirata, Y.; Ito, S.; Suzuki, G.;
Kimura, T.; Maehara, S.; Hikichi, H.; Satow, A.; Hata, M.; Ohta,
H.; Kawamoto, H. Bioorg. Med. Chem. Lett. 2009, 19, 5464–5468.
(31) Yamasaki, T.; Fujinaga, M.; Yoshida, Y.; Kumata, K.; Yui, J.;
Kawamura, K.; Hatori, A.; Fukumura, T.; Zhang, M.-R. Bioorg.
Med. Chem. Lett. 2011, 21, 2998–3001.
(32) Kennis, L. E. J. G.; Vandenberk, J.; van Heertum, A. E. M.
T. EP Patent 0,453,042 filed 16 Apr. 1991, and issued 23 Oct. 1991.
(33) (a) Niwa, T.; Ochiai, H.; Watanabe, Y.; Hosoya, T. J. Am.
Chem. Soc. 2015, 137, 14313–14318. (b) Zhang, Z.; Zhang, C.; Lau, J.;
Colpo, N.; Bénard, F.; Lin, K.-S. J. Label. Compd. Radiopharm.
(9) (a) Casitas, A.; Ribas, X. Chem. Sci. 2013, 4, 2301–2318. (b)
Yao, B.; Wang, Z.-L.; Zhang, H.; Wang, D.-X.; Zhao, L.; Wang,
M.-X. J. Org. Chem. 2012, 77, 3336−3340. (c) Yao, B.; Wang, D.–X.;
Huang, Z.–T.; Wang. M.–X. Chem. Commun. 2009, 2899–2901.
ACS Paragon Plus Environment

Page 13 of 13
Journal of the American Chemical Society
2016, 59, 467–471. (c) Zhang, X.; Dunlow, R.; Blackman, B.;
Swenson, R. J. Nucl. Med. 2016, 57, s2, 277. (d) Schäfer, D.; Weiss,
1
P.; Ermert, J.; Castillo Meleán, J.; Zarrad, F.; Neumaier, B. Eur. J.
Org. Chem. 2016, 4621–4628.
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment