Application of n.c.a. 4-[18F]fluorophenol in diaryl ether syntheses of 2-(4-[18F]fluorophenoxy)-benzylamines

Article abstract of DOI:10.1002/jlcr.828

The availability of no-carrier-added (n.c.a.) 4-[¹⁸F] fluorophenol offers the possibility of introducing the 4-[¹⁸F] fluorophenoxy moiety into potential radiopharmaceuticals. Besides alkyl-aryl ether synthesis using n.c.a. 4-[¹⁸

Full text of DOI:10.1002/jlcr.828

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2004; 47: 443–455.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.828
Research Article
Application of n.c.a. 4-[18F]fluorophenol in diaryl ether
syntheses of 2-(4-[18F]fluorophenoxy)-benzylamines
Timo Stoll1, Johannes Ermert1, Shunichi Oya2, Hank F. Kung2 and
Heinz H. Coenen1,*
1 Institut fu.r Nuklearchemie, Forschungszentrum Ju.lich GmbH, Ju.lich D-52425,
Germany
2 Departments of Radiology and Pharmacology, University of Pennsylvania,
3700 Market Street, Philadelphia, Pennsylvania 19104, USA
Summary
The availability of no-carrier-added (n.c.a.) 4-[18F]fluorophenol offers the possibility
of introducing the 4-[18F]fluorophenoxy moiety into potential radiopharmaceuticals.
Besides alkyl–aryl ether synthesis using n.c.a. 4-[18F]fluorophenol the diaryl ether
coupling is an attractive synthetic method to enlarge the spectrum of interesting
labelling procedures. As examples the syntheses of n.c.a. 2-(4-[18F]fluorophenoxy)-
N,N-dimethylbenzylamine and n.c.a. 2-(4-[18F]fluorophenoxy)-N-methylbenzylamine
were realized by an Ullmann ether synthesis of corresponding 2-bromobenzoic acid
amides using tetrakis(acetonitrile)copper(I) hexafluorophosphate as catalyst and a
subsequent reduction of the amides formed. The radiochemical yield of the coupling
varied between 5 and 65% based on labelled 4-[18F]fluorophenol. Both compounds
are structural analogues of recently published radiotracers for imaging the serotonin
reuptake transporter sites (SERT). However, in vitro binding assays of both molecules
showed only a low affinity towards monoamine transporters. Copyright # 2004 John
Wiley & Sons, Ltd.
Key Words: diaryl ether synthesis; radiofluorination; n.c.a. 4-[18F]fluorophenol; 2-(4-
[18F]fluorophenoxy)-N,N-dimethylbenzylamine; SERT
Introduction
The increasing use of positron emission tomography (PET) for basic research
and clinical studies demands new labelling strategies using short-lived positron
emitters, especially fluorine-18 (t1=2 ¼ 109:7 min). Several no-carrier-added
18F-labelled aromatic synthons serve as key intermediates in multi-step
. . .
*Correspondence to: H. H. Coenen, Forschungszentrum Julich GmbH, Institut fur Nuklearchemie, Julich
D-52425, Germany. E-mail: h.h.coenen@fz-juelich.de
Copyright # 2004 John Wiley & Sons, Ltd.
Received 21 July 2003
Revised 20 October 2003
Accepted 23 January 2004

444
T. STOLL ET AL.
syntheses of radiopharmaceuticals not available via direct nucleophilic
substitution. Versatile examples are substituted [18F]fluorobenzaldehydes for
the synthesis of aromatic amino acids1 and neurotransmitter agents2,3 or
[18F]fluorohalobenzenes for organometallic (Grignard or lithium)4 and
palladium-mediated coupling reactions.5–7 Also, the 4-[18F]fluorophenoxy
moiety lends itself to potentially extending the variety of radiotracers and
radioligands for PET. It was for example recently shown, that 4-[18F]fluoro-
phenoxyalkanes are readily available via coupling reactions of n.c.a.
4- [18F]fluorophenol with alkylbromides.8 Besides the synthesis of 18F-labelled
alkyl–aryl ethers, however, there is also a need for the preparation of
18F-labelled diaryl ethers.
In this paper, this kind of synthesis is exemplified for the preparation of 2-
(4-[18F]fluorophenoxy)-N,N-dimethylbenzylamine ([18F]-6). This compound
has structural relations to 5-[123I]iodo-2(2-((dimethylamino)methyl)phenoxy-
benzyl alcohol ([123I]ODAM)9 (cf. Scheme 1), which is suitable to visualize the
serotonin reuptake transporter (SERT) sites via single photon emission
tomography (SPET).10 For the non-invasive examination of alterations in the
serotonergic system, radiolabelled selective serotonin reuptake inhibitors
(SSRI) are applied. These pharmaceuticals directly influence this system which
is closely related to psychiatric disorders such as depression and other diseases.
As an example [11C]R(+)McN5652 is applied for in vivo studies of these
neurotransporter sites. However, it has only moderate selectivity for SERT
relative to other monoamine transporters.11–13
Scheme 1. Structures of the target molecules [18F]-5, [18F]-6 and [123I]ODAM
The above-mentioned [123I]ODAM belongs to a new class of SERT
radiotracers, based on the structure of 5-chloro-2-(2-((dimethylamino)methyl)-
phenyl)thio)benzyl alcohol named 403U76. Especially the diaryl ether or
thioether derivatives of N,N-dimethylbenzylamine IDAM,14,15 ADAM16 and
ODAM9,10 possess very high affinity and selectivity for SERT.14 This class of
compounds was labelled with iodine-123 and carbon-11 and proved suitable
for the examination of the serotonin reuptake system with SPET or PET in
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DIARYL ETHER VIA [18F]FLUOROPHENOL
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´
humans.16,17–21 However, they are not optimum, because on the one hand
SPET cannot be quantified and, on the other hand, the regional uptake of
these compounds is too slow to be visualized perfectly with the short-lived
carbon-11. So far, only a few examples of n.c.a. 18F-derivatives of IDAM
directly labelled in an aryl position are reported.22,23 18F-Labelled alterna-
tives are desirable for PET examinations due to favourable attributes
of this radionuclide with regard to the pharmacokinetics of this class of
compounds.
In this paper, the radiosynthesis of 2-(4-[18F]fluorophenoxy)-N-methylben-
zylamine ([18F]-5) and 2-(4-[18F]fluorophenoxy)-N,N-dimethylbenzylamine
([18F]-6) via coupling reactions using the labelling synthon n.c.a.
4-[18F]fluorophenol ([18F]-7) is described. Furthermore, their evaluation as
SSRI is reported.
Experimental
General
All chemicals were purchased from Aldrich (Steinheim, Germany) and Fluka
(Buchs, Switzerland) and used without further purification.
Melting point determinations employed a Mettler FP 61 apparatus and
1
temperatures given are uncorrected. H-, 13C- and 19F-NMR spectra were
recorded on a BRUKER Avance 200 spectrometer with samples dissolved in
CDCl3. All chemical shifts are reported in d ppm using the signals of the
appropriate standard as reference. IR spectra were recorded on a SHIMAD-
ZU IR-460 spectrophotometer. Mass spectra were obtained using an
FINNIGAN Automass Multi mass spectrometer with an electron beam
energy of 70 eV.
Analytical radio-HPLC was performed on a system consisting of a KNAUR
pump 6400 and a KNAUR UV/VIS photometer 3060 with a detector
wavelength of 254 nm. Sample injection was accomplished by an RHEO-
DYNE injector block 7125. For measurement of radioactivity the outlet of the
UV detector was connected to a NaI(Tl) well-type scintillation detector
and the recorded data was processed by the software system RAYTEST
.
Ramona MCS (Nuclear Interface, Munster, Germany). Separation of n.c.a.
2-(4-[18F]fluorophenoxy)-N,N-dimethylbenzylamine and 2-(4-[18F]fluorophe-
noxy)-N-methylbenzylamine was performed using a MULTOSPHERE 120
RP 18 AQ-5 (240 ꢀ 4 mm) column (CS-CHROMATROGRAPHIE SERVICE
GmbH, Langerwehe, Germany) and a mobile phase consisting of methanol/
H2O (60/40) (v/v) at a flow rate of 2.0 ml/min. Radio TLC was performed on
MERCK silica gel plates with solvent system diethyl ether/n-hexane in various
mixtures. Radio TL-chromatograms were measured on an Instant ImagerTM
(PACKARD).
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T. STOLL ET AL.
Standards and precursors
2-Bromo-N-methylbenzamide (1) and 2-bromo-N,N-dimethylbenzamide (2) were
synthesized by the means of an earlier described method.24 Colourless oils
were obtained and yields were 89 and 80%, respectively.
2-(4-Fluorophenoxy)-N-methylbenzamide (3) and 2-(4-fluorophenoxy)-N,N-
dimethylbenzamide (4). A mixture of 1.3 g (5 mmol) of 1 or 1.38 g (5 mmol) of
2, respectively, 0.56 g (5 mmol) of 4-fluorophenol, 2.61 g (10 mmol) of caesium
carbonate and 0.15 g (5 mol%) of tetrakis(acetonitrile)copper(I) hexafluoro-
phosphate [Cu(MeCN)4]PF6 in toluene was heated under reflux for 12 h. The
solvent was removed and the residue purified by flash-chromatography with
diethyl ether/n-hexane; 4/1 (v/v) as eluent to give 1.14 g (93%) of colourless
crystals of 3 and 1.12 g (86%) of colourless crystals of 4.
1
3: M.p.: 96–978C; H-NMR (200,13 MHz, CDCl3) in d: 8.21 (bs, 1 H,
(C=O)NH), 7.69–6.83 (m, 8 H, Ar-H), 2.75 (t, 3 H, CH3); 13C-NMR
(50,32 MHz, CDCl3) in d: 166.41 (C(O)N), 155.17 (C-F), 153.15 (C-O-),
153.10 (C-O-), 132.47 (C-H), 130.97 (C-H), 124.17 (C-C(O)N), 121.87 (C-H),
119.01 (C-H, d, J: 6.3 Hz), 117.59 (C-H), 117.13 (C-H, d, J: 19 Hz); 19F-NMR
(188,31 MHz, CDCl3) in d: ꢁ119.93; mass spectrum (m/e): 245 (M+, 100%).
1
4: M.p. 83–858C; H-NMR (200,13 MHz, CDCl3) in d: 7.32 – 6.78 (m, 8 H,
Ar-H), 3.03 (s, 3 H, CH3), 2.93 (s, 3 H, CH3); 13C-NMR (50,32 MHz, CDCl3)
in d: 164.61 (C(O)N), 156.30 (C-F), 155.85 (C-O-), 152.16 (C-O-), 128.99 (C-
H), 126.73 (C-H), 123.22 (C-C(O)N), 121.56 (C-H), 118.10 (d, C-H, J: 7.6 Hz),
116.50 (C-H), 116.23 (d, C-H, 24 Hz); 19F-NMR (188,31 MHz, CDCl3) in d:
ꢁ119.82; mass spectrum (m/e): 260 (M+, 96%), 215 (1 0 0), 185 (17), 157 (11).
2-(4-Fluorophenoxy)-N-methylbenzylamine (5) and 2-(4-fluorophenoxy)-N,N-
dimethylbenzylamine (6). Two millimoles of 3 and 4, respectively, were
dissolved in 10 ml water free THF and cooled to 08C. Ten millilitres of 1 M
BH3 ꢂ THF solution were added slowly till the vigorous reaction ceased. The
solution was then stirred for 2 h and kept under reflux. The reaction was
quenched by adding 1 ml conc. HCl. The solvent was removed under reduced
pressure and the residue taken up in 20 ml water. The solution was heated to
reflux for further 30 min. The acid solution was extracted three times with
10 ml diethyl ether then neutralized with saturated NaHCO3 solution. After
another three extractions with 10 ml diethyl ether the combined organic phases
were dried over Na2SO4. The solvent was removed whereby 0.45 g (1.95 mmol)
of 5 and 0.48 g (1.96 mmol) of 6 were obtained, both of which are colourless
oils. The products started decomposing after the extraction and were therefore
transferred to their hydrochlorides. For this 5 and 6, respectively, were
dissolved in dry diethyl ether and hydrochloric acid (2 M in diethyl ether) was
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added drop wise till no more precipitate formed. The colourless crystals were
filtered and dried under reduced pressure.
5: nmax (KBr): 1502, 1602 (RNH+2 ); H-NMR (200,13 MHz, CDCl3) in d:
1
7.41–6.82 (m, 8 H, Ar-H), 3.83 (s, 2 H, CH2), 2.46 (t, 3 H, CH3), 1.69 (bs, 1 H,
NH); 13C-NMR (50,32 MHz, CDCl3) in d: 155.89 (C-O-), 155.56 (C-F), 154.02
(C-O-), 130.72 (C-H), 130.01 (C-H), 129.58 (C-C(O)N), 124.20 (C-H), 119.45
(d, C-H, J: 5.6 Hz), 116.98 (C-H), 116.33 (d, C-H, J: 19 Hz), 49.68 (CH2), 35.80
(CH3); 19F-NMR (188,31 MHz, CDCl3) in d: ꢁ120.81; mass spectrum (m/e):
231 (M+, 100%), 201 (50).
1
6: nmax (KBr): 1501, 1687 (R2NH+); H-NMR (200,13 MHz, CDCl3) in d:
7.49–6.86 (m, 8 H, Ar-H), 3.53 (s, 2 H, CH2), 2.31 (t, 6 H, CH3); 13C-NMR
(50,32 MHz, CDCl3) in d: 156.43 (CO-), 155.63 (C-F), 154.15 (C-O-), 131.72
(C-H), 130.47 (C-H), 128.78 (C-C(O)N), 124.20 (C-H), 119.71 (d, C-H, J: 8.3),
116.98 (C-H), 116.59 (d, C-H, J: 23 Hz), 57.83 (CH2), 45.90 (2 ꢀ CH3); 19F-
NMR (188,31 MHz, CDCl3) in d: ꢁ121.41; mass spectrum (m/e): 245 (M+,
100%), 214 (39), 111 (18).
Radiosyntheses
4-[18F]Fluorophenol ([18F]-7). The nucleophilic fluorination of (4-(trifluoro-
methyl)phenyl)-benzoyl-40-N,N,N-trimethylammonium triflate with n.c.a.
[18F]fluoride was carried out as previously described by Ludwig et al.8 After
radiofluorination the solvent acetonitrile was removed under reduced pressure
and an argon gas flow from the reaction vessel and the residue was dissolved in
2 ml of acetic acid/acetic anhydride (3/2). In total, 0.8 ml of hydrogen
peroxide/conc. sulphuric acid (2/1) was added and the mixture heated to 908C
for 10 min. The reaction was quenched by transferring the mixture into a
syringe filled with 8 ml of water. This solution was passed through a
conditioned Sep-Pak1 C18 plus-cartridge (WATERS), washed with 3 ml of
water and eluted with 3 ml of diethyl ether. The diethyl ether was removed
under reduced pressure and an argon gas flow. In total, 1.5 ml of 5 N NaOH
and 0.2 ml of methanol were added to the residue and stirred for 10 min at
908C. Then the basic reaction mixture was neutralized with 0.7 ml of
concentrated hydrogen chloride.
2-(4-[18F]Fluorophenoxy)-N,N-dimethylbenzamide ([18F]-4). [18F]-7 was ad-
sorbed on a conditioned Sep-Pak1 C18 plus-cartridge (WATERS) and
washed with 5 ml of water. The cartridge was stuck onto a glass column
(Merck LiChrolut1 65 ꢀ 10 mm) filled with 0.5 g of Na2SO4 and eluted with
2 ml of diethyl ether into a reaction vessel already containing 1.5 ml of stirred
dry toluene. The diethyl ether was carefully removed by an argon gas flow. A
typical amount of 14 MBq (400 mCi) in approximately 0.5 ml of toluene was
transferred to a sealed reaction vial, containing 4.5 mg (20 mmol) of 2-bromo-
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T. STOLL ET AL.
N,N-dimethylbenzamide, 15 mg (40 mmol) of caesium carbonate and an excess
amount of 20 mg of [Cu(MeCN)4]PF6. The reaction mixture was stirred for
25 min at 1108C. The toluene solution containing [18F]-4 and [18F]-7 was
poured on a glass column (15 ꢀ 1.5 cm) filled with LiChrolut1 RP-18 (40–
63 mm) and eluted with diethyl ether/n-hexane; 4/1 (v/v) and monitored via
radio-TLC (diethyl ether/n-hexane; 4/1 (v/v), Rf ¼ 0:55). The solvent was
removed under reduced pressure to yield dry [18F]-4.
2-(4-[18F]Fluorophenoxy)-N,N-dimethylbenzylamine ([18F]-6). [18F]-4 was
dissolved in 1 ml of THF and transferred by syringe to a sealed reaction
vessel. The solution was treated cautiously with 0.5 ml of 1 M BH3 ꢂ THF
solution at 08C and then stirred for 2 min. Solvent and remaining BH3
were removed under reduced pressure at 808C and the residue dissolved in
methanol/water; 60/40 (v/v). Radio-TLC-analysis of [18F]-6 was performed
with dichloromethane/methanol; 9/1 (v/v) + 0.5% triethylamine as eluent,
Rf ¼ 0:75. The product was purified for further application via radio-HPLC
(LiChrosorb1 RP-18 Select B5 (250 ꢀ 4 mm) (CS-Chromatographie
Service GmbH, Langerwehe, Germany), methanol/water; 60/40 (v/v) +
0.2% CF3COOH; k0 (2-(4-[18F]-fluorophenoxy)-N,N-dimethylbenzylamine:
5.5, k0 (4-[18F]fluorophenol): 15, k0 (2-(4-[18F]fluorophenoxy)-N,N-dimethyl-
benzamide): 34.
2-(4-[18F]Fluorophenoxy)-N-methylbenzamide ([18F]-3) and 2-(4-[18F]fluoro-
phenoxy)-N-methylbenzamine ([18F]-5). Both compounds were prepared as
described above for [18F]-4 and [18F]-6, respectively. Radio-TLC-analysis of
[18F]-3 was performed with diethyl ether/n-hexane; 4/1 (v/v) as eluent,
Rf ¼ 0:43. Radio-TLC-analysis of [18F]-5 was performed with dichloro-
methane/methanol; 9/1 (v/v) + 0.5% triethylamine as eluent, Rf ¼ 0:83.
The product could be purified via radio-HPLC (analogue conditions as above)
k0 (2-(4- [18F]fluorophenoxy)-N-methylbenzylamine): 4.8, k0 (2-(4-[18F]fluoro-
phenoxy)-N-methylbenzamide): 28.
In vitro binding assays
The preparation of membrane homogenates and the performance of binding
assays were carried out as described earlier by Kung et al.15,25
Results and discussion
General
N.c.a. 2-(4-[18F]fluorophenoxy)-N,N-dimethylbenzylamine ([18F]-6) and n.c.a.
2-(4- [18F]fluorophenoxy)-N-methylbenzylamine ([18F]-5) were chosen as target
molecules in order to study the possibilities and limits of diaryl ether syntheses
using n.c.a. 4-[18F]fluorophenol ([18F]-7). Besides the synthetic aspects, these
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compounds were selected because of their structural similarity to the SSRI
[*I]ODAM. Differences to the latter are of course the replacement of
radioiodine by fluorine-18 and the missing methyl hydroxide substituent in
ortho-position to the oxygen-bridge (cf. Scheme 1). In order to examine the
influence of the changed substitution pattern on the biological properties, the
respective affinity to the serotonin, dopamine and norepinephrine transporter
system was also examined.
In general, a diaryl ether synthesis is performed in non-nucleophilic
aprotic pure solvents or in a melt and in presence of a transition metal-
species (e.g. Cu, Pd, Ni) as catalyst. The reaction times vary from 6 to 48 h and
the yields from 5 to nearly 100%, depending on the choice of reaction partners
and catalysts. Relatively mild reactions of electron deficient haloarenes with
electron rich sodium phenolates can be achieved with copper(I)halides as
catalysts (cf. Scheme 2). The copper catalyst forms a complex-intermediate
and coordinates the nucleophilic aromatic substitution of the phenolate on
the haloarene.26 The choice of the catalyst therefore depends on the
substitution pattern of the aromatic compound. In this study a variety of
catalysts was applied.
Scheme 2. General conditions for an Ullmann ether synthesis (X=electron
releasing substituent; Y=electron withdrawing substituent)
The classical Ullmann ether synthesis has several disadvantages such as
elevated reaction temperatures, enormous purification problems, generally low
yields and the use of stoichiometric quantities of copper. Recent advances are
based on the application of a variety of catalysts, solvents and reaction
conditions.27 A small selection of those procedures was tested in this study for
the purpose of performing the coupling reaction with n.c.a. 4-[18F]fluorophe-
nol ([18F]-7), however, only with limited success. As an example of a classical
coupling reaction pathway28 a solid-state reaction was chosen but the yields
were low and as mentioned above the purification was elaborate. Other
conditions have been tested with copper catalysts like CuCl, Cu2O, CuCl/Cu
in various solvents but necessary reaction times were more than one half-life of
fluorine-18 (>2 h) and yields were rather low. As an alternative to copper
catalysts triphenylphosphate appeared to be attractive29 but it turned out that
it is limited to coupling reactions where heterocycles are involved. More
promising catalysts are palladium compounds and these were successfully used
in the form of a combination of a Pd-catalyst and a ligand30 with respectable
yields of up to 60%. However, with more than 14 h the reaction time was still
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T. STOLL ET AL.
far too long and this alternative could not further be considered for the n.c.a.
radiochemistry of this study. In contrast, the recently suggested catalyst
tetrakis(acetonitrile)copper(I) hexafluorophosphate31 proved very useful for
the anticipated coupling to n.c.a. [18F]fluorodiaryl ethers.
Preparation of n.c.a. 2-(4-[18F]fluorophenoxy)-N-methylbenzamide ([18F]-3)
and n.c.a. 2-(4-[18F]fluorophenoxy)-N,N-dimethylbenzamide ([18F]-4)
Preparation of 4-[18F]fluorophenol ([18F]-7) was performed using earlier
methods8 with slight modifications. An overall radiochemical yield of
65 ꢃ 3% could be achieved. This was determined with radio-TLC and
radio-HPLC after a reaction time of 60 min. The [18F]-7 formed, however, had
to be removed from the aqueous solution, purified and dried carefully in order
to avoid loss of activity. The very basic reaction mixture was neutralised with
an estimated equimolar amount of aqueous hydrogen chloride, the product
separated by solid phase extraction and dried over Na2SO4. The ether was
eluted into a reaction vial already containing 2 ml of dry toluene and removed
by an argon gas flow.
The caesium salt of [18F]-7, as nucleophilic agent, and the 2-bromobenza-
mides, as electron deficient benzoic systems, were not optimum starting
materials for the Ullmann ether synthesis, due to the fact that acid
amides are not strong electron withdrawing groups and also that fluorophe-
nolate is not a good nucleophile (cf. Scheme 2). However, an application
of the water sensitive tetrakis(acetonitrile)copper(I) hexafluorophosphate
catalyst31 was successful, using toluene or xylene as solvent in a sealed
vessel with a reaction temperature of 1108C. This allowed to decrease the
reaction time to 25 min. The catalyst is specific for ortho substituted
benzamides or benzosulphonamides. The strongly variable radiochemical
yields of 5–65% relative to labelled [18F]-7 can be explained by the importance
of dryness during the coupling step. Based on about 25 labelling
experiments an average radiochemical yield of 32ꢃ14% could be obtained.
Very small traces of water could be compensated by an excess amount
of the catalyst. But even small amounts of moisture and the resulting
decomposition of catalyst seemed to disturb the formation of the
reaction-intermediate, which resulted in lower yields. Use of the 2-
chlorobenzamides lead to decreased chemical yields and they were not applied
for radiochemical reactions; 2-iodobenzamides exhibited comparable chemical
and radiochemical yields as the bromo-analogues. Yields have been
determined with radio-TLC and radio-HPLC. Higher reaction temperatures
did not influence the reaction time; on the other hand, lower temperatures
decreased the yield. The purification of the products was performed via
column chromatography and all side products and remaining starting
materials could be removed.
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DIARYL ETHER VIA [18F]FLUOROPHENOL
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Reduction to n.c.a. 2-(4-[18F]fluorophenoxy)-N-methylbenzylamine ([18F]-5)
and n.c.a. 2-(4-[18F]fluorophenoxy)-N,N-dimethylbenzylamine ([18F]-6)
The reduction of the benzamides [18F]-3 and [18F]-4 to n.c.a. 2-(4-
[18F]fluorophenoxy)-N-methylbenzylamine and n.c.a. 2-(4-[18F]fluorophe-
noxy)-N,N-dimethylbenzylamine was carried out using the borane-tetrahy-
drofuran complex after removing the toluene phase. After 5 min reaction time
the reduction was quantitative. For purification the benzylamines were
separated via radio-HPLC. The average achievable radiochemical yield was
20ꢃ5% after less than 2 h reaction time. Starting with a typical amount of
135 MBq [18F]fluoride, 14–15 MBq of [18F]-6 could be obtained after HPLC-
purification. [18F]-5 has been prepared with corresponding yields (Scheme 3).
Scheme 3. Synthesis of n.c.a. 2-(4-[18F]fluorophenoxy)-benzylamines [18F]-5 and
[18F]-6 via an Ullmann ether synthesis starting with n.c.a. 4-[18F]fluorophenol and
corresponding 2-bromobenzamides 1 and 2
Biological evaluation
In vitro binding assays were performed using LLC-PK1 cells over expressing
either one of three different types of monoamine transporters (SERT, DAT or
NET)32 to test the binding selectivity of 2-(4-fluorophenoxy)-N-methylbenzy-
lamine and 2-(4-fluorophenoxy)-N,N-dimethylbenzylamine among monoa-
mine transporters using [125I]N-(3-iodopropene-2-yl)-2b-carbomethoxy-3b-(4-
chlorophenyl) (IPT) as selective ligand for DAT and NET and [125I]IDAM for
SERT. The results are summarized in Table 1. The two compounds displayed
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T. STOLL ET AL.
Table 1. Inhibition constants of 5, 6 and ODAM9 (measured against [*I]IDAM14,15
(SERT) and [*I]IPT (DAT, NET))
Compound
SERT
Ki(nM) DAT
NET
H
N
> 400
> 10; 000
> 10; 000
O
F
5
N
O
100.53ꢃ19.52
3880ꢃ50
550ꢃ50
F
6
HO
N
O
0.12ꢃ0.02
3900ꢃ700
20ꢃ1.9
I
ODAM
only a low inhibition effect on SERT and the other monoamine transporters.
Owing to the disappointing low affinity, biodistribution studies of [18F]-4 and
[18F]-5 were not carried out. The affinities towards other monoamine
transporters are comparable to other compounds of this class10,15,16 and in
comparison to the very closely related ODAM the affinity towards DAT is
nearly the same and it is even less towards NET.
Conclusion
The diaryl ether synthesis using n.c.a. 4-[18F]fluorophenol ([18F]-7) was
exemplified by the synthesis of 2-(4-[18F]fluorophenoxy)-N-methylbenzylamine
([18F]-5) and 2-(4-[18F]fluorophenoxy)-N,N-dimethylbenzylamine ([18F]-6).
The best results were obtained using tetrakis(acetonitrile)copper(I) hexafluor-
ophosphate as catalyst for the Ullmann ether synthesis. The major problem of
this kind of synthesis is the isolation and drying of the formed n.c.a. 4-
[18F]fluorophenol ([18F]-7). The catalyst is very sensitive to moisture and
therefore absolute dryness of starting material and of the solvent used for the
coupling reaction was necessary. Therefore, n.c.a. 4-[18F]fluorophenol was
purified by solid phase extraction, dried over Na2SO4 and the solvent for
elution was removed before commencing the coupling reaction. These steps
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DIARYL ETHER VIA [18F]FLUOROPHENOL
453
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´
were time consuming and not very reliable with the consequence of varying
radiochemical yield of the coupling product. Starting from n.c.a. [18F]fluoride
five reaction steps, three separations by solid phase extraction and a final
HPLC purification were necessary to obtain the product. These conditions
together with the reaction time and the radiochemical yields are summarized in
Scheme 4. Although the present procedure is not yet convenient for routine
production of 18F-labelled radiopharmaceuticals, it was sufficient for
preliminary in vitro evaluation experiments with the radioligands prepared.
Scheme 4. Preparation of n.c.a. 2-(4-[18F]fluorophenoxy)-N,N-dimethylbenzy-
lamine ([18F]-6)
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454
T. STOLL ET AL.
Nevertheless, in principle this method shows the possibility of a diaryl ether
synthesis to prepare certain radiopharmaceuticals containing an n.c.a. 4-
[18F]fluorophenoxy moiety. Furthermore, other catalysts are available for
different substitution patterns with favourably adapted reaction conditions27
and could most likely be applied for other diaryl ether labelling procedures.
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