Iodonium ylides for one-step, no-carrier-added radiofluorination of electron rich arenes, exemplified with 4-(([18F]fluorophenoxy)- phenylmethyl)piperidine NET and SERT ligands

Article abstract of DOI:10.1039/c4ra00674g

Iodonium ylide precursors of electron rich arenes, i.e. the NET and SERT ligands 4-((3- and 4-fluorophenoxy)phenylmethyl)piperidine, served as model compounds for the direct substitution with n.c.a. [¹⁸F]fluoride. Good radiochemical yields of a

Full text of DOI:10.1039/c4ra00674g

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PAPER
Iodonium ylides for one-step, no-carrier-added
radiofluorination of electron rich arenes,
exemplified with 4-(([ F]fluorophenoxy)-
Cite this: RSC Adv., 2014, 4, 17293
18
phenylmethyl)piperidine NET and SERT ligands
ab
a
a
a
Jens Cardinale, Johannes Ermert,* Sven Humpert and Heinz H. Coenen
Iodonium ylide precursors of electron rich arenes, i.e. the NET and SERT ligands 4-((3- and 4-
fluorophenoxy)phenylmethyl)piperidine, served as model compounds for the direct substitution with
18
n.c.a. [ F]fluoride. Good radiochemical yields of about 20% were obtained in reaction times of ca. 130
ꢀ1
minutes with a molar activity of the labelled ligands of more than 50 GBq mmol . Those failed as in vivo
probes in first evaluation studies. Several important insights, however, were gained into the reaction of
ylides, e.g. an unexpected formation of regioisomers. The results clearly demonstrate that aryliodonium
ylides are a promising alternative to the well-known diaryliodonium salts for the direct preparation of
Received 23rd January 2014
Accepted 27th March 2014
DOI: 10.1039/c4ra00674g
18
www.rsc.org/advances
complex, electron rich n.c.a. [ F]fluoroarenes.
Recently, Satyamurthy and Barrio reported the successful
Introduction
1
8
radiosynthesis of n.c.a. [ F]uoroarenes by the use of different
8
Fluorine-18 has become one of the most important radionu- iodonium ylides as precursors. Amongst those, the iodonium
clides for imaging probes in positron-emission-tomography ylides derived from Meldrum's acid proved to be the most
1
(
PET) due to its favourable nuclear and chemical properties. In promising ones due to their relative high stability and good
a practical way, however, no-carrier-added (n.c.a.) uorine-18 reactivity. Their general structure is shown in Fig. 1. These
can only be produced in the chemical form of uoride by the compounds are commonly formed by the reaction of Meldrum's
2
,3
hitherto established processes.
demanding a high specic activity have generally to be conditions.
Thus, imaging probes acid with a suitable aryliodine-III compound under basic
9–11
1
8
produced starting from [ F]uoride. While the synthesis of
electron decient n.c.a. [ F]uoroarenes can easily be accom- formed under similar conditions to those generally used for the
plished by nucleophilic substitution, the synthesis of non-acti- radiouorination of iodonium salts. Aer activation of the [ F]-
The reported radiouorination of iodonium ylides was per-
1
8
1
8
1
8
vated or even electron rich [ F]uoroarenes represents still a uoride by a phase transfer catalyst/base activation system (e.g.
challenge which is usually realized by a rather complicate, time Kryptox®2.2.2/potassium carbonate) the iodonium ylide is
1
,4,5
consuming multi-step synthesis.
added in a dipolar aprotic solvent such as DMF and heated to
ꢁ
It is well known that iodonium compounds offer an alter- 110–130 C for 10–15 minutes (Scheme 1), as described by
native route even for the synthesis of electron rich [ F]uo- Satyamurthy and Barrio.
roarenes starting from n.c.a. [ F]uoride. Their application
as precursor for F-radiouorination has mainly been limited pressants. Amongst those are the uorophenoxyethers 4-((3-
to mechanistic studies, and they served in most cases as and 4-uorophenoxy)phenylmethyl)piperidine (3-FPPMP and
precursors for the preparation of versatile, non-activated small 4-FPPMP, Fig. 2) which are potential ligands for the serotonin
F-labelled molecules used as building blocks for further and norepinephrine (reuptake) transporters (SERT and NET).
radiochemical transformations. So far, very little attention has Being electron-rich uoroarenes those offered themselves as
1
8
8
1
8
6,7
In 2003 Orjales et al. published a series of potential antide-
1
8
12
1
8
5
been paid to other hypervalent iodine species for
radiouorination.
a
Institut f u¨ r Neurowissenschaen und Medizin, INM-5: Nuklearchemie,
Forschungszentrum J u¨ lich GmbH, 52425 J u¨ lich, Germany. E-mail: j.ermert@
fz-juelich.de
b
Department of Medical Physics in Radiology, German Cancer Research Center, 69221
Heidelberg, Germany
Fig. 1 Resonance structures of aryliodonium ylides.
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column, equipped with 50 ml sample loops. The detection of
compounds by UV-absorption was carried out with a K-2501
detector (Knauer, Berlin, Germany). When the system was used
for the isolation of products by analytical HPLC the rst valve was
equipped with a 200 ml loop and the column directly connected to
the detectors. Semi-preparative HPLC-runs were conducted on a
HPLC system consisting of a Merck-Hitachi L-6000 pump, a
Rheodyne manual 6-position selector valve in front of the
column, equipped either with a 2 ml or a 1 ml sample loop and a
Phenomenex Luna PFP(2) column (5 mm, 250 ꢂ 100 mm). The
detection of compounds by UV-absorption was carried out with a
S3300 UV-detector (Sykam, F u¨ rstenfeldbruck, Germany).
The detection of the radioactive products was performed
with a NaI(Tl) detector crystal connected with an ACE Mate
signal amplier and an EG&G Ortec Model 276 photomultiplier
base (Ortec, Meerbusch, Germany).
18
Scheme 1 Nucleophilic n.c.a. F-fluorination of arenes using iodo-
nium ylides.
Measurement of the radioactivity of bulk samples was con-
ducted on a Curiementor 2 (PTW, Freiburg, Germany) ionisa-
tion chamber.
Fig. 2 Molecular structures of 3-FPPMP (1) and 4-FPPMP (2), ligands
for the NE- and SE-reuptake transporters, respectively.
Phosphor imager plates used for in vitro autoradiography of
good candidates to test an authentic labelling with uorine-18 brain tissue slices were scanned with a laser phosphor imager
via the new approach. Thus, our objective was to use the radi- BAS 5000 (Fuji, D u¨ sseldorf, Germany) utilizing soware from
1
8
osynthesis of n.c.a. 3- and 4-[ F]FPPMP as model reaction for the vendor (Version 3.14, Raytest, Straubenhardt, Germany).
1
8
the direct F-uorination of corresponding iodonium ylides of
electron-rich arenes as precursors.
Synthesis of precursors and standards
-(Hydroxy(phenyl)methyl)piperidine-1-carboxylic acid tert-
butyl ester (6) was prepared analogous to a procedure described
4
Experimental
General
14
by Ullrich et al. from the Weinreb amide 4 which was prepared
by standard methods (Scheme 2). The reference compounds 3-
FPPMP and 4-FPPMP were prepared from compound 6 as
All chemicals were acquired from Sigma-Aldrich (Tauirchen,
Germany), except for acetonitrile, Kryptox®2.2.2 and potas-
sium carbonate, purchased from VWR-International (Langen-
feld, Germany), and for 3- and 4-uorobenzyloxybenzene,
obtained from Fluorochem (Derbyshire, UK). All compounds
were used without further purication. SepPak C-18 cartridges
were obtained from Waters (Eschborn, Germany) and sodium
sulphate cartridges from Agilent (B ¨o blingen, Germany).
12
reported by Orjales et al. For the preparation of compounds 7–
0 the same procedure was used but without the deprotection
1
step (Scheme 2) as exemplarily described for compound 7. The
spectral data of the previously not described halophenoxy
derivatives 7 and 9 is given below.
The syntheses of 4-benzyloxyphenyliodonium-(5-[2,2-
dimethyl-1,3-dioxane-4,6-dione])ylide (13) and of 4-methox-
yphenyliodonium-(5-[2,2-dimethyl-1,3-dioxane-4,6-dione])ylide
4
-((3-Iodophenoxy)phenylmethyl)piperidine-1-carboxylic acid-
tert-butyl ester (7)
tert-Butyl-4-(hydroxy(phenyl)methyl)piperidine-1-carboxylate
13
(
14) were reported elsewhere. All iodonium ylides were
(1.46 g, 5 mmol) was added portion wise to a stirred suspension
ꢁ
synthesized under exclusion of light and stored at 2–8 C. The
reference compound 2-uorobenzyloxybenzene was obtained by
the reaction of 2-uorophenol with benzylbromide. NMR
spectra were recorded either on a Varian-Inova 400 or a Bruker
DPX Avance 200 spectrometer. The chemical shis are given in
parts per million relative to the solvent signal.
Elemental analyses (EA, microanalyses) were carried out on a
Vario EL cube, elemental analyser (at ZEA-3, Forschungs-
zentrum J u¨ lich).
The products of the labelling reactions were analysed by
radio-HPLC. The HPLC system consisted of a Smartline Pump
1
000, a Luna C-18(2) column (5 mm, 250 ꢂ 4.6 mm) or a Luna
PFP(2) column (5 mm, 250 ꢂ 4.6 mm) (both from Phenomenex,
Scheme 2 Syntheses of the fluorophenoxy- and iodophenoxy deriv-
atives 7–10. (i) Boc O, Na CO , H O–THF (ii) MeNHOMe, i-PrMgCl,
THF, (iii) phenyllithium, THF, (iv) NaBH
Aschaffenburg, Germany). Further, two Rheodyne manual
2
2
3
2
6
-position selector valves (Idex Health and Science, Wertheim-
4
, MeOH, (v) NaH, sodium
ꢁ
Mondfeld, Germany) were positioned in front of and behind the benzoate, fluorohalobenzene (halo ¼ 3-F, 4-F, 3-I, 4-I) DMSO, 65 C.
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of hexane washed NaH (0.25 g, 6.3 mmol, as 60% oil dispersion) (m, 2H), 2.49 (m, 2H), 1.81 (m, 2H), 1.52 (s, 6H), 1.31 (s, 9H),
in 8 ml of anhydrous DMSO. The reaction was stirred at room 1.22–1.08 (m, 3H).
1
3
temperature for 30 min, sodium benzoate (1.45 g) added, and
C NMR (400 MHz, CDCl ) d: 163.6, 159.7, 158.7, 139.0,
3
stirring continued for 30 min. 1-Fluoro-3-iodobenzene (720 ml, 138.1, 131.7, 130.5, 129.8, 128.6, 128.5, 128.1, 127.8, 126.8,
6
.1 mmol) was added, while the reaction temperature was being 126.6, 125.3, 124.8, 119.8, 119.4, 114.9, 114.4, 104.3, 84.4, 74.3,
ꢁ
kept below 20 C by means of a water bath and then the reaction 55.9, 43.0, 28.4, 28.0, 25.8.
mixture heated at 65 C for 15 h. Aer cooling to room
temperature the reaction mixture was diluted with brine and
ꢁ
MS (+ c ESI): m/z ¼ 674.12.
Elemental analysis: calc.: C 54.81%, H 5.39%; found: C
water until all precipitated solids were dissolved again. Subse- 54.81%, H 5.75%.
quently the mixture was extracted with diethylether, the organic
layer dried over MgSO and the solvent removed under reduced
4
(
(
4-((1-Boc-4-piperidyl)phenylmethoxy)phenyl)iodonium-(5-
2,2-dimethyl-1,3-dioxane-4,6-dione))ylide (12)
pressure. Purication by chromatography on silica gel yielded
.12 g (45%) of an orange oil.
1
1
3
H NMR (200 MHz, CDCl ) d: 7.39–7.20 (m, 8H), 6.93–6.74 The synthesis was performed as described for compound 11.
(
(
(
m, 2H), 4.82 (d, 1H, J ¼ 6.7 Hz), 4.25–4.10 (m, 2H), 2.77–2.57
m, 2H), 2.02–1.92 (m, 2H), 1.69 (m, 1H), 1.49 (s, 9H), 1.44–1.29
m, 2H).
Yield: 42% as yellow solid.
1
H NMR (400 MHz, CDCl
3
) d: 7.71 (d, 2H, J ¼ 8.4 Hz, C14–H),
7.33–7.21 (m, 6H), 6.78 (d, 2H), 4.79 (d, 1H), 4.10 (m, 2H),
1
3
C NMR (200 MHz, CDCl ) d: 158.9, 154.8, 139.2, 130.6, 2.60 (m, 2H), 1.91 (m, 4H), 1.63 (s, 6H), 1.41 (s, 9H), 1.33–1.24
3
1
4
29.9, 128.6, 128.0, 126.7, 125.5, 115.0, 94.2, 84.1, 79.4, 43.7, (m, 3H).
1
3
3.4, 28.5, 28.4, 28.1.
C NMR (400 MHz, CDCl ) d: 163.4, 161.5, 154.7, 138.1,
3
MS (+ c ESI): m/z ¼ 494.19.
136.1, 128.8, 128.3, 126.6, 119.2, 104.4, 102.6, 84.6, 79.4, 56.8,
Elemental analysis: calc.: C 55.99%, H 5.72%, N 2.84%; 43.2, 28.4, 25.8.
found: C 56.33%, H 6.21%, N 2.98%.
MS (+ c ESI): m/z ¼ 674.10.
Elemental analysis: calc.: C 54.81%, H 5.39%, N 2.20%;
found: C 55.3%, H 5.73%, N 1.91%.
4-((4-Iodophenoxy)phenylmethyl)piperidine-1-carboxylic acid-
tert-butyl ester (9)
The synthesis was performed as described for compound 7.
Radiochemistry
Yield: 33% as white solid.
1
8
Fluorine-18 was produced by the irradiation of O-enriched
1
3
H NMR (400 MHz, CDCl ) d: 7.50–7.26 (m, 8H), 6.66–6.58
water with 16.5 MeV protons and a beam current of about 20 mA
using the O(p,n) F nuclear reaction. Irradiations were per-
formed at the baby cyclotron BC1710 (Japan-Steel-Works) at the
Forschungszentrum J u¨ lich using a titanium target. The n.c.a.
F]uoride was separated from the irradiated water by an
(
m, 2H), 8.80 (d, 1H, J ¼ 6.6 Hz), 4.24–4.09 (m, 2H), 2.77–2.57
1
8
18
(m, 2H), 2.09–1.87 (m, 3H), 1.59–1.22 (m, 13H).
1
3
C NMR (400 MHz, CDCl ) d: 158.1, 154.7, 138.0, 128.5,
3
3
127.8, 126.7, 118.3, 84.0, 82.9, 79.3, 43.7 (bs), 43.3, 28.4, 28.0
1
8
[
(
bs).
MS (+ c ESI): m/z ¼ 493.99.
Elemental analysis: calc.: C 55.99%, H 5.72%, N 2.84%;
found: C 56.25%, H 6.2%, N 3.04%.
electrochemical procedure as earlier described by Hamacher
15
et al.
1
8
18
4
-[ F]Fluoro-1-benzyloxybenzene and 4-[ F]uoroanisole
(
(
3-((1-Boc-4-piperidyl)phenylmethoxy)phenyl)iodonium-(5-
2,2-dimethyl-1,3-dioxane-4,6-dione))ylide (11)
Kryptox® 2.2.2 (10 mg) was dissolved in 13.3 ml of a 1 M
aqueous solution of potassium carbonate in a 1.5 ml
In a closed 10 ml reaction vial 640 mg (1.3 mmol) of iodoarene 7 Eppendorf vial with the aqueous solution containing about
1
8
was stirred together with 322 mg of 77% meta-chloroperoxy- 30 MBq n.c.a. [ F]uoride. The solution was diluted with
benzoic acid (1.4 mmol) in 5.2 ml of dichloromethane (DCM) at 900 ml of dry acetonitrile, transferred to a 5 ml V-Vial® with a
ꢁ
ꢁ
40
C for 80 minutes. Aer cooling to ambient temperature silicon septum and the solvent was removed at 80 C and
520 mg of KOH and 240 mg of Meldrum's acid were added and 800 mbar under a gentle argon stream. This azeotropic
the mixture further stirred for 45 minutes. Then the reaction distillation was repeated two times, each by addition of 1 ml
mixture was diluted with DCM, ltered over a paper lter, the of dry acetonitrile. Then the vial was evacuated at a
solids washed with about 50 ml DCM and subsequently ltered pressure between 5 and 20 mbar for 5 minutes and ushed
over cellulose. The solvent of the collected organic phases was with argon. Subsequently, 15 mg of the desired precursor (13
ꢁ
removed under reduced pressure at 30 C until the rst solid or 14) were dissolved in 1 ml of acetonitrile and heated at
ꢁ
precipitated. Then, hexane was slowly added to complete the 130 C for 20 minutes. Aer cooling to room temperature
precipitation. The solid was collected by ltration, washed with an aliquot of the reaction mixture was diluted with HPLC-
hexane and dried in air and in vacuum.
solvent and analysed by radio HPLC (55 : 45 acetonitrile–
1
8
Yield: 292 mg beige solid (33%).
buffer for [ F]uoroanisole and 65 : 35 acetonitrile–buffer
1
18
3
H NMR (400 MHz, CDCl ) d: 7.18–7.11 (m, 8H), 6.94 (t, 1H, for [ F]uorobenzyloxybenzene; buffer: 1% aqueous TEA–
ꢀ
1
J ¼ 4.0 Hz), 6.80 (d, 1H, J ¼ 4.2 Hz), 4.69 (d, 1H, J ¼ 3.2 Hz), 3.98
3 4
H PO , pH 8.0–8.5; Luna C-18; 1 ml min ).
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1
8
18
18
18
3
-[ F]FPPMP ([ F]1) and 4-[ F]FPPMP ([ F]2)
In vitro binding on rat brain slices
1
8
The n.c.a. [ F]uoride was activated with 5 mg Kryptox® 2.2.2 For the in vitro determination of non-specic binding the HPLC-
1
8
18
18
and 6.7 ml 1 M K
uoro-1-benzyloxybenzene and 4-[ F]uoroanisole. Then, ylide was diluted with water, passed through a SepPak C-18 column
1 or 12 (7.5 mg) was dissolved in 0.5 ml of dry acetonitrile, which was subsequently blown dry with a stream of Argon. The
2 3
CO solution and dried as described for 4-[ F]- puried product fraction containing 3-[ F]FPPMP or 4-[ F]FPPMP
1
8

1
1
8
added to the vial containing the dry [ F]uoride complex and product was eluted in 1 ml of a mixture of 90 : 10 (v/v) methanol–
ꢁ
ꢁ
ꢁ
heated to 120 C or 130 C, respectively, for 20 minutes. For acetic acid. The solvent was evaporated at 80 C under reduced
work-up the reaction mixture was cooled in an ice bath for one pressure and a gentle stream of argon and the dry radiotracer dis-
minute, 150 ml of water was added and the whole solution solved in water. Solutions of 3- or 4-FPPMP with concentrations of 1
submitted to semi-preparative HPLC (60 : 40 acetonitrile– nM and 10 mM in 0.5 molar Tris-buffer (pH 7.4) were prepared and
3 4
buffer: 1% aqueous TEA–H PO , pH 8.0–8.5; Luna PFP(2); 4 ml spiked with the corresponding radiotracer. Subsequently rat brain
ꢀ
1
min ). The product fraction was diluted with 15 ml of water slices were incubated for 1 h with those solutions and subsequently
and passed through a SepPak C-18 cartridge. The cartridge was analysed by means of autoradiography using a phosphor imager.
dried by blowing a stream of argon through it and the product
eluted with 1.5 ml of dry DCM via a sodium sulphate cartridge
Results and discussion
Synthesis of precursor and standard compounds
1
8
18
in a 5 ml V-Vial® to yield [ F]8 and [ F]10, respectively. In the
1
8
case of [ F]10 a second HPLC separation was necessary in
order to get a pure product. This was performed by evapora-
tion of the solvent and dissolving the residue in 150 ml HPLC-
solvent on an analytical HPLC-column (55 : 45 acetonitrile–
The syntheses of the iodonium ylides 11 and 12 were accom-
13
plished by a novel one-pot procedure as exemplied in Scheme
3
for ylide 11. By this procedure yields of 33% and 42% were
3 4
buffer: 1% aqueous TEA–H PO , pH 8.0–8.5; Luna PFP(2); 1 ml
achieved, starting from iodoarenes 7 and 9, respectively. This
way the preparation of the ylides was relatively easy and addi-
tionally allowed to avoid the preparation and isolation of cor-
responding aryldiacetoxyiodoarenes which are usually prepared
ꢀ
1
min ). Then, the solvent of the product fractions was
changed to 1.5 ml DCM by means of a C-18 cartridge as
described above.
1
8
18
For deprotection of [ F]8 and [ F]10, 500 ml of triuoro-
acetic acid was added and the whole solvent–acid mixture
16
under acidic conditions. Since latter condition is incompatible
with the Boc-group, the ylide syntheses by common procedures
would have afforded a change of the protection group resulting
in two additional synthetic steps.
ꢁ
removed at 40–50 C under reduced pressure (starting from
8
3
00 mbar) and a gentle argon stream. The crude products
-[ F]FPPMP and 4-[ F]FPPMP were dissolved in 150 ml
1
8
18
HPLC-solvent (60 : 30 : 10 THF–MeOH–buffer: 1% aqueous
TEA–H PO , pH 6.0; Luna C-18; 0.7 ml min ) and puried by
3 4
analytical HPLC.
18
18
Radiosynthesis of 3-[ F]FPPMP ([ F]1)
ꢀ1
In rst labelling experiments the reaction conditions for the
F-exchange on iodonium ylides were chosen as described by
1
8
Identication of the labelled products
The identication of labelled compounds was done by radio
HPLC and coelution with the non-radioactive standard
compounds. The k-values of relevant standard compounds are
summarized in Table 1 together with the corresponding HPLC-
conditions used. The dead time of the HPLC-system equipped
with the C-18 column was determined with thiourea and
ꢀ
1
amounted to 127 s at a ow of 1 ml min and 182 s at a ow of
Scheme 3 Synthesis of iodonium ylide 11 by a one-pot procedure. (i)
(1) mCPBA, CH Cl , (2) KOH, Meldrum's acid.
ꢀ
1
0.7 ml min
.
2
2
Table 1 k-values of relevant compounds
Compound
k-value
HPLC conditions
3 4
60 : 30 : 10 THF–MeOH–buffer: 1% aqueous TEA–H PO , pH 6.0; Luna C-18; 0.7 ml min
ꢀ1
3
4
2
3
4
2
3
4
-FPPMP
-FPPMP
-Fluoroanisole
-Fluoroanisole
1.95
2.12
2.99
3.43
3.19
4.81
5.83
5.33
10.68
11.04
ꢀ
1
55 : 45 ACN–buffer: 1% aqueous TEA–H
3
PO
4
, pH 8.0–8.5; Luna C-18; 1 ml min
, pH 8.0–8.5; Luna C-18; 1 ml min
-Fluoroanisole
ꢀ1
-Fluorobenzyloxy-benzene (2-FBOB)
-Fluorobenzyloxy-benzene (3-FBOB)
-Fluorobenzyloxy-benzene (4-FBOB)
65 : 35 ACN–buffer: 1% aqueous TEA–H
3
PO
4
18
18
ꢀ1
Boc-3-[1 F]FPPMP ([ F]8)
3 4
ACN–Buffer 60 : 40; buffer: 1% aqueous TEA–H PO pH 8.0–8.5; Luna PFP(2); 4 ml min
8
18
Boc-4-[ F]FPPMP ([ F]10)
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Satyamurthy and Barrio. The substitution reaction was con-
RSC Advances
8
18
Aer the chromatographic separation, product [ F]8 was
1
8
ducted in DMF as solvent delivering the desired product [ F]8 extracted from the eluent with a C-18 cartridge, what could now
in a RCY of 17 ꢃ 2% as shown in Scheme 4. However, the be performed without product losses, and eluted in DCM for
product was accompanied by a high amount of non-polar side deprotection with triuoroacetic acid. As expected, the removal
products hampering the isolation of the product. These side of the Boc-group proved to be quite easy and was accomplished
products are presumably formed by intramolecular rearrange- with quantitative yield. However, another HPLC separation was
16
18
ment and by cleavage as is described in literature.
necessary to obtain n.c.a. 3-[ F]FPPMP in highest purity. This
The side products overloaded the cartridge and caused a was done on an analytical column. The molar radioactivity of
breakthrough of the desired product resulting in losses of more the product was determined in this nal purication and was
ꢀ
1
18
than 50%. Thus, isolation by semi-preparative HPLC became found to be higher than 50 GBq mmol . The RCY of [ F]1,
1
8
mandatory which necessitated a change of the reaction solvent related to starting [ F]uoride, amounted to 20 ꢃ 5% aer a
from DMF to acetonitrile. Surprisingly, this also led to an total synthesis time of about 110 minutes including separation
enhancement of the RCY to 20 ꢃ 5%.
from the HPLC product fraction and solvent change to water.
Since our objective was to demonstrate the preparation and
Aer the reaction in acetonitrile the product mixture was
simply diluted with water and submitted to semi-preparative isolation of a complex radiotracer via an iodonium precursor
HPLC. A too strong dilution below 60 : 40 (v/v) acetonitrile– with a sufficient purity for a rst preclinical evaluation, the
water caused a precipitation of the non-polar components as radiochemical yield was not further optimised. Higher RCYs of
oil, containing also a considerable fraction of the desired up to 45% were observed when higher amounts of precursor in
1
8
18
product Boc-3-[ F]FPPMP ([ F]8), and thus had to be avoided. more solvent (15 mg of 11 in 1 ml of acetonitrile) were used.
1
8
However, the addition of 150 ml water to the reaction mixture However, under these conditions the nal product [ F]1 was
ca. 75 : 25 v/v acetonitrile–water) proved to be sufficient. The usually contaminated with non-radioactive side products
(
chromatogram of the separation is shown in Fig. 3. Although because of adverse effects in the chromatographic separation
there were only small radiochemical impurities, many non- due to more side products and a higher solvent volume.
radioactive side products were detected in the UV-chromato-
gram. Also a large number of less polar side products were
found which eluted later from the HPLC column (not presented
in Fig. 3) and caused the breakthrough of product in the initial
attempts to separate the product by solid phase extraction using
a C-18 cartridge (see above).
18
18
Radiosynthesis of 4-[ F]FPPMP ([ F]2)
1
8
The radiosynthesis of [ F]2 was analogously carried out to the
radiosynthesis of [ F]1, but a slightly higher temperature of
30 C was necessary. The desired product Boc-4-[ F]FPPMP
[ F]10) was formed in about 20% RCY, surprisingly accom-
1
8
ꢁ
18
1
(
1
8
panied by a radioactive side-product which proved to be the
1
8
18
positional isomer Boc-3-[ F]FPPMP ([ F]8) formed in about
10% RCY (Scheme 5).
The resolution of the semi-preparative HPLC was not high
enough to separate these regioisomers from each other.
Therefore, the n.c.a. products were rst isolated together by
semi-preparative HPLC, the eluate concentrated, and the
regioisomers separated on an additional HPLC-system with
an analytical column. Following this, the desired product was
deprotected with a quantitative yield as described above. The
additional steps for the separation of the isomers took about
1
8
18
18
Scheme 4 Radiosynthesis of 3-[ F]FPPMP ([ F]1). (i) n.c.a. [ F]fluo-
ꢁ
ride, Kryptofix®2.2.2., K
CO
2 3
, acetonitrile, 120 C; (ii) CF
3
CO
2
H, DCM.
3
1
0–40 minutes prolonging the total synthesis time to about
40–150 minutes. By the procedure applied here, principally
both regioisomers can be prepared from the same precursor
(
(
compound 12), albeit in different total RCY of about 20%
1
8
18
[
F]2) and 10% ([ F]1), respectively.
In order to further conrm the formation of regioisomers,
the simpler but structurally equal compounds 4-methoxy-
phenyl-(5-[2,2-dimethyl-1,3-dioxane-4,6-dione])ylide 13 and
18
Fig. 3 Radiochromatogram of the separation of [ F]8 by semi-
preparative HPLC. (60 : 40 acetonitrile–buffer: 1% aqueous TEA– Scheme 5 Formation of regioisomers in the synthesis of 4-[ F]10. (i)
18
ꢀ
1
18
ꢁ
3
H PO
4
, pH 8.0–8.5; Luna PFP(2); 4 ml min ).
2 3
n.c.a. [ F]fluoride, Kryptofix®2.2.2., K CO , acetonitrile, 130 C.
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two series of rat brain slices were incubated for 1 h with 1 nM or
10 mM solutions of 3- and 4-FPPMP, respectively, in 0.5 M Tris-
buffer containing the corresponding radiotracer and analysed
by phosphor imaging. In case of both tracers only about 10.3%
and 9.9%, respectively, of the total activity was blocked on the
slices incubated with the 10 mM solution and no anatomical
structures were visible on images taken from the slices incu-
bated with the low concentration of the radiotracer. Hence, 3-
1
8
and 4-[ F]FPPMP appear obviously not suitable for in vivo
imaging of NET or SERT using PET; this is why their further
Scheme 6 Formation of regioisomers in the reaction of ylides 13 and radiopharmacological evaluation seemed not to be indicated.
1
8
18
1
K
4 with n.c.a. [ F]fluoride. (i) n.c.a. [ F]fluoride, Kryptofix®2.2.2.,
ꢁ
2
CO
3
, acetonitrile, 130 C.
Conclusions
4
-benzyloxyphenyliodonium-(5-[2,2-dimethyl-1,3-dioxane-4,6-
1
8
dione])ylide 14 were F-labelled under similar conditions as The one-step n.c.a. radiouorination of more complex electron-
used for the radiosynthesis of Boc-4-[ F]FPPMP 8 (Scheme 6).
rich arenes could be achieved in a reasonable reaction time and
In fact, in both cases the corresponding 3-isomers were also with an overall RCY of about 20% by using suitable iodonium
1
8
1
8
formed in about 4% and 11% RCY, respectively, while the ylide precursors. From those, both compounds, 3- and 4-[ F]-
expected 4-isomers were formed in about 12% and 20%, FPPMP, could quantitatively be obtained by a subsequent
respectively, as identied by comparison of the retention times deprotection. Those ligands, here used as model compounds,
with the respective macroscopic standards. Thus, the formation however, did not prove suitable in rst evaluation tests for use
of regioisomers seems to be a problem with the radiouorination of in vivo imaging.
of iodonium ylides. This surprising observation that the nucle-
The preparation of the ylide precursors succeeded by a
ophilic substitution on ylides is not regiospecic was not repor- comparatively easy one-pot procedure in satisfactory yields of
8
ted by Satyamurthy and Barrio. A similar effect, however, has 33% and 42%. For the radiosynthesis the main challenge
been found by Graskemper et al. in the thermal decomposition of proved to be the purication of the labelled products due to
(
4-methoxyphenyl)(5-methoxy [2.2]paracyclophan-4-yl)-iodonium many non-radioactive side-products formed. Additionally,
hexauorophosphate. There it is suggested, that the strong during the preparation of 4-[ F]FPPMP the surprising forma-
electron donating effect of the methoxy group leads to a tion of two regioisomers was found. The occurrence of posi-
1
8
17
competing uorination via an aryne pathway.
tional isomers was further conrmed by the radiolabelling of
Another interesting point to note is that the amount of non- comparable 4-methoxy- and 4-benzyloxyphenyliodonium ylides,
radioactive side-products in the labelling reactions of ylides 13 demonstrating that the reaction is not regiospecic. Like in this
and 14 was signicantly lower than in case of the more study, separation of isomers might demand additional efforts in
complex ylides 11 and 12. This can either mean, that the given case. In spite of these drawbacks the potential of arylio-
1
8
presence of more functional groups opens alternative reaction donium ylides as precursors for nucleophilic F-labelling of
paths, or, that the bulkier ylides simply tend to decompose via more complex molecules was demonstrated here.
rather unspecic pathways. Due to the high number of side
products formed in case of ylides 11 and 12 (cf. Fig. 3) an
Today the most important alternative route to such
F-labelled radiotracers is a multi-step synthesis in which
1
8
unspecic decomposition is more likely. In the initial labelling either electron withdrawing groups are attached to the benzene
1
8
experiments with ylide 11 several reactions with lower ring to be F-uorinated, or the whole molecule has to be
temperatures were performed, but this only led to a decrease synthesised by a two or more step procedure. In both cases the
of the RCY while no improvement of the amount of side- radiosynthesis gets more complex (e.g. due to additional
products was observed.
reagents) while the necessity of purication by HPLC is also very
18
18
The radiosynthesis of [ F]1 and [ F]2 proved feasible by direct likely, although the total amount of side-products might be
nucleophilic substitution of the corresponding iodonium ylides. signicantly lower. The better approach for a given radiotracer
The isolation of the products, however, was challenging due to the will strongly depend on the individual case.
18
high amount of non-radioactive side-products and, in case of [ F]2,
Since the iodonium ylides represent the more direct
due to the formation of a regioisomer. Here, even a second sepa- approach, their advantage increases the more complex the
ration by HPLC was inevitable resulting in a prolonged synthesis multi-step approach gets. In this respect, the reaction of n.c.a.
1
8
time which might be reduced by further optimisation of the puri- [ F]uoride with iodonium ylides proves to be a powerful
cation process, e.g. by application of a solvent gradient.
alternative to the multi-step synthesis for the preparation of

1
8
electron rich and non-activated [ F]uoroarenes. Additionally,
iodonium ylides also proved to be a better to handle alternative
to the well-known aryliodonium salts. Those ndings warrant
1
8
In vitro binding of 3- and 4-[ F]FPPMP on rat brain slices
1
8
As a rst pharmacological test the non-specic binding of [ F]1 further development of the ylide-method for the direct synthesis
1
8
18
and [ F]2 was examined by a competition experiment. For this, of non-activated or electron rich n.c.a. [ F]uoroarenes.
17298 | RSC Adv., 2014, 4, 17293–17299
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RSC Advances
7
R. Gail, C. Hocke and H. H. Coenen, J. Labelled Compd.
Radiopharm., 1997, 40, 50.
Acknowledgements
We thank Dr Dirk Bier and Annette Schulze for the assistance
with the autoradiographic experiments, Philipp Weiß for labo-
ratory assistance, Dr Marcus Holschbach (all INM-5) and Dr
8 N. Satyamurthy and J. R. Barrio, Patent, WO 2010/117435 A2,
2010.
9 K. Schank and C. Lick, Synthesis, 1983, 292.
Sabine Willbold (ZEA-3), all Forschungszentrum J u¨ lich, for 10 M. Ochiai, Y. Tuchimoto and N. Higashiura, Org. Lett., 2004,
recording the spectroscopic data.
6, 1505.
11 S. R. Goudreau, D. Marcoux and A. B. Charette, J. Org. Chem.,
2
009, 74, 470.
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M. Guillaume, A. Luxen, B. Nebeling, M. Argentini, J. C. Clark 16 G. F. Koser, in The Chemistry of Functional Groups,
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V. W. Pike and F. I. Aigbirhio, J. Chem. Soc., Chem. Commun., 17 J. W. Graskemper, B. Wang, L. Q. Kiel, D. Neumann and
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