Synthesis, labelling and evaluation of hydantoin-substituted indole carboxylic acids as potential ligands for positron emission tomography imaging of the glycine binding site of the N-methyl- d -aspartate receptor

Article abstract of DOI:10.1002/jlcr.1901

The N-methyl- d-aspartate (NMDA) receptor as a type of ionotropic glutamatergic receptors is essential for physiological processes such as learning, memory and synaptic plasticity. A glutamate-induced overactivation of these receptors, accompanied by increased intracellular calcium concentration, causes cell injury and leads to a large number of acute or chronic neurological disorders, such as stroke, trauma, Parkinson's disease and Alzheimer's disease. In an attempt to visualise the glutamatergic neurotransmission in vivo with positron emission tomography, novel fluoroethoxy- and methoxy-substituted reference compounds based on the lead structure of a hydantoin-substituted indole-2-carboxylic acid were synthesised. The affinities towards the glycine binding site of the NMDA receptor showed K_i values between 322 and 11 nM and the lipophilicities ranged from logD values of 1.51 to 2.53. On the basis of these results, precursor compounds were synthesised containing a phenolic hydroxy moiety to obtain the radiolabelled ligands through an alkylation reaction. Radiosynthesis was achieved by labelling the precursor ethyl 4,6-dichloro-3-((3-(4-hydroxyphenyl)-2,4-dioxoimidazolidin-1-yl)methyl)- indole-2-carboxylate with 2-[¹⁸F]fluoroethyl tosylate or [ ¹¹C]methyl iodide and subsequent cleavage of the ethyl ester moiety. This gave the final products in overall decay-corrected radiochemical yields of 5-7% and 6-9% and specific activities of 24-67 GBq/μmol and 8-26 GBq/μmol, respectively. Copyright

Full text of DOI:10.1002/jlcr.1901

Research Article
Received 29 October 2010,
Revised 30 March 2011,
Accepted 20 April 2011
Published online 29 July 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.1901
Synthesis, labelling and evaluation of
hydantoin‐substituted indole carboxylic acids
as potential ligands for positron emission
tomography imaging of the glycine binding
site of the N‐methyl‐D‐aspartate receptor
a†
a
a
b
b‡
c
*
A. Bauman, M. Piel, S. Höhnemann, A. Krauss, M. Jansen, C. Solbach,
G. Dannhardt,b and F. Röscha
The N‐methyl‐D‐aspartate (NMDA) receptor as a type of ionotropic glutamatergic receptors is essential for physiological
processes such as learning, memory and synaptic plasticity. A glutamate‐induced overactivation of these receptors,
accompanied by increased intracellular calcium concentration, causes cell injury and leads to a large number of acute
or chronic neurological disorders, such as stroke, trauma, Parkinson’s disease and Alzheimer’s disease. In an attempt to
visualise the glutamatergic neurotransmission in vivo with positron emission tomography, novel fluoroethoxy‐ and
methoxy‐substituted reference compounds based on the lead structure of a hydantoin‐substituted indole‐2‐carboxylic
acid were synthesised. The affinities towards the glycine binding site of the NMDA receptor showed Ki values between
322 and 11 nM and the lipophilicities ranged from logD values of 1.51 to 2.53. On the basis of these results, precursor
compounds were synthesised containing a phenolic hydroxy moiety to obtain the radiolabelled ligands through an
alkylation reaction. Radiosynthesis was achieved by labelling the precursor ethyl 4,6‐dichloro‐3‐((3‐(4‐hydroxyphenyl)‐2,4‐
dioxoimidazolidin‐1‐yl)methyl)‐indole‐2‐carboxylate with 2‐[18F]fluoroethyl tosylate or [11C]methyl iodide and sub-
sequent cleavage of the ethyl ester moiety. This gave the final products in overall decay‐corrected radiochemical
yields of 5–7% and 6–9% and specific activities of 24–67 GBq/µmol and 8–26 GBq/µmol, respectively.
Keywords: glutamate; NMDA receptor; glycine binding site; 18F‐fluoroalkylation; 11C‐methylation
binding of glycine and glutamate to the NR1 and NR2 subunits.8
The NR3 subunits also can co‐assemble with NMDA receptors,
Introduction
Glutamate, a major excitatory amino acid in the central nervous
system of mammals, plays an important role not only in
neuronal plasticity and neurotoxicity1–3 but also in neurological
disorders such as Parkinson’s and Alzheimer’s diseases, schizo-
phrenia and Huntington’s chorea.4,5 Receptors responding to
glutamate can be split into two groups: metabotropic glutamate
receptors, which are G‐protein‐coupled receptors that mediate
slow modulatory responses, and ionotropic receptors, which
are gated ion channels that mediate fast neurotransmission.
The ionotropic receptors can be divided into three families,
namely (1) N‐methyl‐D‐aspartate (NMDA), (2) α‐amino‐3‐hydroxy‐
5‐methyl‐4‐isoxazole‐propionate (AMPA) and (3) kainate,6 ac-
cording to their selective agonists. Of these, especially the NMDA
receptor is an important target for medicinal chemistry because
of its high permeability to calcium, which is an important second
messenger in the central nervous system. NMDA receptors have a
tetrameric structure and are assembled from seven known
subunits (NR1, NR2A–NR2D, NR3A–NR3B).7 Functional receptors,
which are composed of at least one NR1 subunit and one or more
although their stoichiometry and function are yet unclear.
Besides the binding sites for glutamate and glycine, functional
NMDA receptors also possess a binding site in the open ion
aInstitute of Nuclear Chemistry, Johannes Gutenberg‐University, Fritz‐Strassmann‐
Weg 2, D‐55128 Mainz, Germany
bInstitute of Pharmacy, Johannes Gutenberg‐University, Staudingerweg 5,
D‐55128 Mainz, Germany
cRadiopharmacy, PET Center, University of Tübingen, Röntgenweg 15, D‐72076
Tübingen, Germany
†Present address: Division of Radiological Chemistry, University Hospital Basel,
Petersgraben 4, 4031 Basel, Switzerland.
‡Present address: Department of Cell Physiology and Molecular Biophysics,
Center for Membrane Protein Research, TTUHSC, 3601 4th Street, MS 6551,
Lubbock, TX 79430, USA.
*Correspondence to: M. Piel, Institute of Nuclear Chemistry, Johannes Gutenberg‐
University, Fritz‐Strassmann‐Weg 2, D‐55128 Mainz, Germany.
of the NR2A–NR2D subunits, can be opened by simultaneous E‐mail: piel@uni‐mainz.de
J. Label Compd. Radiopharm 2011, 54 645–656
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A. Bauman et al.
channel and a binding site for polyamines, thought to be located hydrazone was converted into the corresponding ethyl 4,6‐
on the NR2B subunit.
dichloro‐indole‐2‐carboxylate (5) in a proton‐catalysed Fischer
Over the past years, a variety of positron emission tomog- indole synthesis, which had to be carefully hydrolyzed after the
raphy (PET) and single‐photon emission computed tomogra- reaction. The aldehyde derivative ethyl 4,6‐dichloro‐3‐formyl‐
phy studies with ligands for the different binding sites of the indole‐2‐carboxylate (6) was obtained under the conditions of a
receptor have been performed. They mainly represented high‐ Vilsmeier synthesis using N‐methyl formanilide as a reagent.
affinity open ion channel blockers based on derivatives of Finally, 7 was formed by reductive amination with methyl
MK801,9 phenylcyclidine10 and memantine.11 These ligands glycinate, which had to be pestled to obtain high yields, and
can only bind to their binding site if the receptor is activated sodium triacetoxyborohydride.16
and the ion channel is open. In combination with their typically
The synthesis of the fluoroethyl‐substituted aminophenols
high lipophilicity, which might be accompanied by a high was achieved in two steps from the corresponding nitrophenols
unspecific binding, the obtained data were complicated and 8a–8d (Scheme 2). The fluoroethyl moiety was introduced with
difficult to interpret and did not allow clear pharmacological 1‐bromo‐2‐fluoroethane using K2CO3 in acetone. The workup of
conclusions.
the reaction mixture was realised by extraction with toluene and
An alternative approach focuses on ligands for the glycine 10% NaOH solution at pH 12, and afforded sufficient pure
binding site using derivatives of quinoxaline‐2,3‐diones,12 compounds 9a–9d for the conversion of the nitro group into
2‐quinolones13 and 2‐carboxytetrahydroquinolines.14 They offer the amino group by catalytic hydrogenation with 10% Pd/C to
nanomolar affinities, a moderate lipophilicity and fewer side effects 10a–10d. The benzyl‐protected aminophenols could not be
compared with ligands of the glutamate and ion channel binding obtained in the direct way by refluxing aminophenols with
site. Whereas optimization of the 2‐carboxytetrahydroquinolines benzylchloride and K2CO3 in acetone because N‐benzylation
led to ligands that have high affinities in the nanomolar range (1), was the main reaction route. Therefore, a fast and simple way to
it revealed logD values of about 1.3, resulting in a low brain synthesise these compounds (12a–12d) in a microwave‐assisted
uptake.14 Therefore, indole 2‐carboxylic acid analogues com- one‐step reaction was performed.17
prising a hydantoin moiety were synthesised,5,15 in order to
Compounds 13a–13e were synthesised using bis‐(trichloro-
improve their lipophilicities and to obtain promising candidates methyl) carbonate (triphosgene), which is a less hazardous
for the in vivo visualisation of the NMDA receptor (Figure 1).14
substitute for phosgene (Scheme 3).18 Treatment of the amine
components 10a–10e with triphosgene at 0 °C resulted in the
formation of the corresponding carbamoyl chlorides, which
were then reacted at ambient temperature with 7 to get the
unsymmetrical ureas 13a–13e. To keep the formation of
symmetrical 1,3‐diarylureas low, the initial step was carried out
Results and discussion
Organic syntheses
A series of hydantoin‐containing compounds based on 4,6‐ slowly over a period of 30 min at reduced temperature.19
dichloro‐3‐((2,4‐dioxo‐3‐phenylimidazolidin‐1‐yl)methyl)‐indole‐ Nuclear magnetic resonance (NMR) and mass spectral analyses
2‐carboxylic acid (15) were synthesised for their use as labelling revealed the concomitant ring closure of the ureas 13a–13e to
precursors and potential imaging agents for the glycine‐binding the cyclised hydantoins 14a–14e for up to 20% during the
site of NMDA receptors (Figure 2).
triphosgene coupling (Scheme 4). As this step was next in the
To prepare these compounds, ethyl 4,6‐dichloro‐3‐((2‐ synthesis scheme, the isolation of 13a–13e was not necessary,
methoxy‐2‐oxoethylamino)methyl)‐indole‐2‐carboxylate (7) was and the subsequent cyclisation step was continued by treat-
synthesised as a building block and used for coupling with O‐ ment with freshly prepared sodium ethanolate in ethanol. The
fluoroethylated, O‐methylated and O‐benzylated aminophenols reaction was stopped after 1 h, and because no satisfying
(Scheme 1). The synthesis was started with 4,6‐dichlorohydrazine conditions for the purification were found using silica gel as
(3), which was converted quantitatively into (E/Z)‐ethyl 2‐(2‐ solid phase, the workup was performed by column chromato-
(3,5‐dichlorophenyl)hydrazono)propanoate (4) by refluxing with graphy using basic alumina oxide and THF/hexane or CHCl3/
ethyl pyruvate. The resulting E/Z isomers can be separated by acetone mixtures as mobile phases. In the last step, 14a–14e
column chromatography (hexane/ethyl acetate 10:1 (v/v)), but were saponified with 2 M NaOH and purified by column
typically were used as mixture for the next reaction step. The chromatography to yield the reference compounds 15a–15e.
Figure 1. Lead structure (1) and the methylated and fluoroethylated indole derivatives (2).
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A. Bauman et al.
Figure 2. Lead structure, reference compounds and labelling precursors.
Scheme 1. Synthesis route to 7. Reagents and conditions: (a) ethyl pyruvate, reflux, 16 h; (b) polyphosphoric acid, 120–130 °C; (c) N‐methyl formanilide, POCl3; (d) methyl
glycinate, Na(OAc)3BH.
The reference compounds were obtainable in nine steps using 45%. Benzyloxy anilines 12a and 12c were coupled with 7 to
this reaction procedure in a good overall yield of about 8%.
ureas 16a and 16b, cyclised with sodium ethanolate to
Considering the lipophilicities and in vitro affinities of the 17a, 17b and 17a saponified with 2 M NaOH to 18. With these
reference substances, 15a, 15c and 15e seem to be the most compounds, labelling precursors for a one‐step and a two‐
promising compounds; thus, only their respective precursors step labelling reaction were accessible. Therefore, 17a, 17b and
were synthesised. The reaction scheme for the preparation of 18 were converted into 19a, 19b and 20 by catalytic hydro-
the labelling precursor compounds 19a, 19b and 20 is almost genation with 10% Pd/C. The labelling precursors were ob-
identical to the procedure described above (Scheme 5), except tainable using this reaction procedure in eight steps for 19a
the synthesis of the benzyloxy anilines. In a first attempt, a and 19b and in nine steps for 20 in a moderate overall yield of
reaction of the appropriate aminophenols with benzylchloride about 5–7%.
under basic conditions was examined, resulting in the forma-
tion of the N‐benzylated compounds as main product. There-
fore, an alternative reaction route was used, that is, reacting
In vitro affinity and lipophilicity
both educts under microwave‐assisted conditions, which In vitro binding assays for the glycine binding site of the NMDA
gave the O‐benzylated compounds in moderate yields of about receptor were performed according to reported methods using
J. Label Compd. Radiopharm 2011, 54 645–656
Copyright © 2011 John Wiley & Sons, Ltd.
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A. Bauman et al.
Scheme 2. Synthesis route to 10a–10d and 12a–12d. Reagents and conditions: (a) 1‐bromo‐2‐fluoroethane, K2CO3, acetone, reflux, 48 h; (b) H2, Pd (10%)/C; (c) K2CO3,
KOH, TBABr, microwave (300 W), 40 s.
Scheme 3. Formation of the unsymmetrical ureas using triphosgene as coupling reagent.
rat cortex membranes and [3H]MDL 105,519 as radioligand.20 barrier penetration resulting in a parabolic relationship, with its
Relative affinities of the reference compounds 15a–15e maximum in vivo activity in the middle of the logD range of 1.5–
were determined as IC50 values from displacement curves. The 2.5.22 With these considerations, compounds 15a, 15c and 15e
Ki values were calculated according to Cheng and Prusoff.21 were chosen for further investigations because of their reason-
Within this series of compounds, the inhibition constants of able logD for good brain uptake and similar affinities for the
the 4‐fluoroethoxy, 3‐fluoroethoxy and 4‐methoxy derivatives NMDA receptor (Table 1).
(15a, 15c, 15e) were 81 24 nM, 43 13 nM and 47 14 nM,
respectively. The electron deficient chloro‐derivative 15b had
Radiosyntheses
an inhibition constant of 493 138 nM, the lowest affinity, and
the 2‐fluoroethoxy derivative 15 d had 17 5 nM, the highest Initial synthesis of 4,6‐dichloro‐3‐((3‐(4‐(2‐[18F]fluorethoxy)phenyl)‐
affinity towards the receptor.
2,4‐dioxoimidazolidin‐1‐yl)methyl)‐indole‐2‐carboxylic acid ([18F]
Lipophilicity of the reference compounds 15a–15e was 15a) was performed in a one‐step reaction by labelling the
determined by reversed phase HPLC. Whereas the lipophilicities precursor compound 20 with 2‐[18F]fluoroethyl tosylate ([18F]
of compounds 15a, 15c and 15e range very close together, with FETos) in aprotic solvents and 5 M NaOH as a base. This resulted
logD values of 2.07 to 2.26, the chloro derivative 15b shows in yields of ≤4% for [18F]15a (Scheme 6) owing to the formation
the highest lipophilicity, with a logD value of 2.53, and the of two unknown by‐products. Consequently, a two‐step synthesis
O‐fluoroethoxy derivative 15 d has the lowest value, with a logD route was developed starting from the ethyl‐protected deriva-
of 1.51.
Clearly, the affinity and lipophilicity of the compounds are
tives 19a and 19b.
To identify the most suitable reaction conditions, several
dependent on the electronic properties of the phenyl ring and parameters such as solvent, temperature, base, equivalents of
the position of the fluoroalkyl moiety. In an earlier study, the base, precursor amount and reaction time were examined.
influence of lipophilicity was correlated to the blood‐brain Optimisation of the used base showed that 5 M NaOH gave the
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J. Label Compd. Radiopharm 2011, 54 645–656

A. Bauman et al.
Scheme 4. Synthesis of reference compounds. Reagents and conditions: (a) triphosgene, 10a–10e, DIPEA, DCM; (b) fresh NaOEt/EtOH; (c) 2 M NaOH, THF.
best results, whereas potassium carbonate and lithium diiso- observed. Trials to improve the ester cleavage by using different
propylamide only resulted in unsatisfactory yields. Under base systems as well as by increasing the reaction temperature
optimised conditions, using 3 mg (6.9 mmol) of precursor (19a and shortening the reaction time gave unsatisfactory results.
or 19b) in DMSO with 1.9 equiv. of 5 M NaOH and a reaction Thus, the reaction was terminated after 30 min, and a second
time of 5 min at a temperature of 100 °C, radiochemical yields of purification by semipreparative HPLC was performed. Attempts
30–40% for [18F]14a and 25–30% for [18F]14c were achieved. A to perform this reaction procedure in a one‐pot reaction were
prolongation of the reaction time led in all cases to a drop in not successful because no HPLC conditions were found to
radiochemical yields because of an increasing cleavage of the separate the final product from the by‐products formed during
ester moiety and thermal decomposition.
18F‐fluoroethylation and saponification. After formulation in
Because of the similar affinity and lipophilicity of 15a and 15c physiological saline solution, [18F]15a and [11C]15e were
and lower labelling yields for [18F]14c, further investigations on obtained in decay‐corrected yields of 5–7% and 6–9% in a
11C labelling chemistry were carried out with 19a only.
synthesis time of about 130 min and 83 min, respectively.
Labelling reactions with [11C]methyl iodide ([11C]CH3I) and
19a were performed in a similar way. Because of the higher
reactivity of [11C]CH3I, the reaction times in the synthesis of
[11C]14e were decreased to 2.5 min. Reducing the reaction tem-
perature to 50 °C or lower led to no significant product for-
Experimental
General
mation. However, the use of a 0.33 M solution of NaOEt in DMSO Solvents were purchased from Fluka, Acros and Fisher Scientific.
instead of the 5 M NaOH solution raised the radiochemical yields (3,5‐Dichlorophenyl)hydrazine (3; ABCR), benzylchloride (Fluka),
of [11C]14e to 25–35%.
1‐bromo‐2‐fluoroethane (Lancaster), potassium carbonate (Fluka)
Purification of the crude products [18F]14a and [11C]14e was and 4‐methoxyaniline (10e; Fluka) were purchased from the
accomplished by semipreparative HPLC. The subsequent cleav- companies shown in brackets; all other chemicals were obtained
age of the ethyl protecting group with 1 M NaOH solution at from Merck. All reagents were used without further purification.
room temperature was incomplete after 30 min, and a decom- For solid‐phase extraction, Sep‐Pak®‐QMA, Sep‐Pak®‐C18 (Waters,
position of the final products [18F]15a and [11C]15e was USA) and Strata X cartridges (1 mL, 30 mg; Phenomenex, USA)
J. Label Compd. Radiopharm 2011, 54 645–656
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A. Bauman et al.
Scheme 5. Synthesis of labelling precursors. Reagents and conditions: (a) triphosgene, 12a, 12c, DIPEA, DCM; (b) fresh NaOEt/EtOH; (c) 2 M NaOH, THF; (d and e) H2,
Pd (10%)/C.
were used. Thin layer chromatography (TLC) was performed using
The logD values were examined according to the OECD
plates from Merck (silica gel 60 F254 and alumina oxide 60 F254). guidelines for testing of chemicals using the HPLC method
1H‐ and 13C‐NMR spectra were recorded using a Bruker AC 300 or (stationary phase: Phenomenex Gemini RP18‐EC, 5 µm,
Bruker AM 400 spectrometer. Chemical shifts were quoted as δ 250 × 4.6 mm; mobile phase: Sörensen buffer (phosphate buf-
values (ppm) downfield from tetramethylsilane. Field desorption fer/MeOH 25:75 (v/v)) at pH 7.4, flow: 1 mL/min).23 To obtain the
(FD) mass spectra were recorded using a Finnigan MAT90 calibration curve, the capacity factors of five compounds with
spectrometer. Analytical and semipreparative HPLC for nonra- reported lipophilicity were determined by measuring the
dioactive syntheses were performed on a system from Dionex retention time tr and dead time t0, and the resulting logk values
consisting of a P680A pump and a UVD 170U UV detector. were plotted versus logD. Determination of the in vitro binding
Analytical HPLC for 18F‐compounds was performed with a system affinity was performed using a [3H]MDL 105,519 binding assay
from Sykam consisting of a S1121 pump, a Knauer K2501 UV according to the method of a previous study.8
detector and a Berthold LB506 radiodetector. Analytical HPLC for
11C‐labelled compounds was performed with a system from
Organic syntheses
Dionex consisting of a P680A pump, a UVD 170U UV detector
(E/Z)‐Ethyl 2‐(2‐(3,5‐dichlorophenyl)hydrazono)propanoate (4)
and a Gabi Star (Raytest) radiodetector. Preparative HPLC was
performed for 18F‐ and 11C‐labelled compounds with a system from
Five grams of (3,5‐dichlorophenyl)hydrazine (28.2 mmol) was
Sykam, consisting of a S1021 pump, a S3200 UV detector and a
NaI(Tl) radioactivity detector, which was integrated in the synthesis
module (11C: automated synthesiser system C‐11 methylation,
GE Medical Systems; 18F: noncommercial synthesis module).
dissolved in 45 mL of EtOH. Three grams (3.11 mL, 28.2 mmol) of
ethyl pyruvate in 10 mL of EtOH was added dropwise, and the
mixture was stirred overnight. The solvent was evaporated and
the residue purified by column chromatography (hexane/ethyl
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J. Label Compd. Radiopharm 2011, 54 645–656

A. Bauman et al.
until an orange solid precipitated. A solution of 7 g (27 mmol) of
5 in 30 mL of 1,2‐dichloroethane was then added, and the
resulting mixture was stirred under reflux for 8 h. Afterwards, it
was poured on a mixture of 29.7 g of NaOAc⋅3H2O and 60 g of
ice and stored overnight in a refrigerator. The precipitate was
filtered off, dried in vacuo and recrystallised using ethyl acetate
to yield 5.9 g (20.6 mmol, 76%) of a yellow solid.
Table 1. IC50, Ki and log D values of the reference compounds
15a–15e
Compound
IC50 [nM]
Ki [nM]
LogD
15a
15b
15c
15 d
15e
81 24*
493 148
43 13
53 16
322 97
2.07
2.53
2.12
1.51
2.26
28
11
31
8
3
9
MS (FD) m/z (% rel. int.): 285.3 (100, [M]+).
17
5
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 13.16 (s, 1H, In‐NH),
10.64 (s, 1H, CHO), 7.53 (d, 1H, Ar‐CH), 7.38 (d, 1H, Ar‐CH), 4.43 (q,
2H, CH2), 1.34 (t, 3H, CH3).
47 14
Ethyl 4,6‐dichloro‐3‐((2‐methoxy‐2‐oxoethylamino)methyl)‐indole‐
2‐carboxylate (7)
acetate 10:1 (v/v); Rf (Z‐isomer): 0.68; Rf (E‐isomer): 0.22) to yield
7.6 g (27.6 mmol, 98%) of E/Z‐isomers as an orange solid.
MS (FD) m/z (% rel. int.): 274.1 (100, [M]+).
To a mixture of 880 mg (7 mmol) of methyl glycinate in 20 mL of
CH2Cl2, 1.06 g (1460 μL, 10.5 mmol) of triethylamine was added
under argon atmosphere. After 10 min, a solution of 2 g (7 mmol)
of 6 was added, and the mixture was stirred for 15 min; 2.4 g
(11 mmol) of Na(OAc)3BH was then added, and the mixture was
stirred for 1.5 h. The reaction was quenched with 20 mL of
saturated NaHCO3 solution and diluted with ethyl acetate. The
layers were separated, and the organic phase was dried with
MgSO4. After purification of the residue by column chromato-
graphy (ethyl acetate/hexane 5:1 (v/v); Rf: 0.44), 1.9 g (5.2 mmol,
75%) of a light brown solid was obtained.
Z‐isomer: 1H‐NMR (300 MHz, DMSO‐d6), δ [ppm]: 11.79
(s, 1H, NH), 7.27 (t, 2H, Ar‐CH), 6.98 (t, 1H, Ar‐CH),
4.23 (q, 2H, CH2), 2.08 (s, 3H, CH3), 1.27 (t, 3H, CH3).
E‐isomer: 1H‐NMR (300 MHz, DMSO‐d6), δ [ppm]: 10.08
(s, 1H, NH), 7.21 (t, 2H, Ar‐CH), 7.02 (t, 1H, Ar‐CH),
4.19 (q, 2H, CH2), 2.05 (s, 3H, CH3), 1.25 (t, 3H, CH3).
Ethyl 4,6‐dichloro‐indole‐2‐carboxylate (5)
To an open 250 mL beaker containing 25 mL of polyphosphoric
acid, 7.6 g (28 mmol) of 4 was added slowly at 70 °C. The
suspension was heated to 120–130 °C for 10 min and then
hydrolyzed by cautious addition of ice. Afterwards, the mixture
was treated with ethyl acetate until the precipitate had
dissolved. The solution was neutralised with NaOH pellets and
extracted with ethyl acetate, and the combined organic phases
were dried with MgSO4. After the solvent was evaporated in
vacuo, the crude product was purified by column chromato-
graphy (hexane/ethyl acetate 3:1 (v/v); Rf: 0.77) to yield 6.6 g
(26 mmol, 93%) of a light brown solid.
MS (FD) m/z (% rel. int.): 358.5 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.14 (s, 1H, In‐NH),
7.42 (d, 1H, Ar‐CH), 7.20 (d, 1H, Ar‐CH), 4.35 (q, 2H, CH2), 3.48 (s, 3H,
CH3O), 3.33 (s, 2H, CH2), 3.33 (s, 2H, CH2), 1.98 (s, 1H, NH), 1.35
(t, 3H, CH3).
General procedure I: fluoroethylation of nitrophenols
A mixture of 44 mmol 1‐bromo‐2‐fluoroethane, 40 mmol of the
appropriate nitrophenol and 44 mmol K2CO3 in 40 mL of acetone
was refluxed for 48 h. The solvent was evaporated, and the
residue was taken up in water and extracted twice with 75 mL of
toluene. The combined organic layers were washed three times
with 50 mL of a 10% NaOH solution and dried with Na2SO4, and
the solvent was evaporated in vacuo.
MS (FD) m/z (% rel. int.): 257.0 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm]: 12.41 (s, 1H, In‐NH),
7.43 (t, 1H, Ar‐CH), 7.26 (d, 1H, Ar‐CH), 7.10 (d, 1H, CH), 4.34 (q,
2H, CH2), 1.36 (t, 3H, CH3).
Ethyl 4,6‐dichloro‐3‐formyl‐indole‐2‐carboxylate (6)
1‐(2‐Fluoroethoxy)‐4‐nitrobenzene) (9a)
To 6.1 g (45 mmol) of N‐methyl formanilide was added 3.7 g
(43.2 mmol) of phosphoryl chloride, and the resulting solution Yield: 45%, TLC: Rf (ethyl acetate/hexane 1:1 (v/v)): 0.87.
was stirred at ambient temperature under argon atmosphere
MS (FD) m/z (% rel. int.): 184.5 (100, [M]+).
R1
R1
R1
R2
R2
R2
O
N
N
O
O
N
N
N
N
O
O
Cl
Cl
O
Cl
O
(c)
O
O
O
(a) or (b)
O R3
O R3
R3
Cl
N
N
Cl
N
Cl
H
H
H
19a (R1= OH, R2= H, R3= C2H5)
19b (R1= H, R2= OH, R3= C2H5)
20 (R1= OH, R2= H, R3= H)
[
[
[
18F]14a (R1= O(CH2)218F), R2= H, R3= C2H5)
18F]14c (R1= H, R2= O(CH2)218F, R3= C2H5)
11C]14e (R1= O11CH3, R2= H, R3= C2H5)
[
[
18F]15a (R1= O(CH2)218F, R2= H, R3= H)
11C]15e (R1= O11CH3, R2= H, R3= H)
(a)
Scheme 6. Radioactive labelling of 20 with [18F]FETos yielding [18F]15a in a direct approach and of 19a with [18F]FETos and [11C]CH3I yielding [18F]15a and [11C]15e via
18F]14a and [11C]14e. Reagents and conditions: (a) [18F]FETos, 1.9 equiv. 5 M NaOH, DMSO 100 °C, 5 min; (b) [11C]CH3I, 1.9 equiv. 0.33 M NaOEt, DMSO, 50 °C, 2.5 min; (c)
[
EtOH, aq. 1 M NaOH, 30 min.
J. Label Compd. Radiopharm 2011, 54 645–656
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A. Bauman et al.
1H‐NMR (300 MHz, CDCl3), δ [ppm] = 8.23 (d, 2H, Ar‐CH), 7.01 four equal portions and transferred to four Erlenmeyer flasks
(d, 2H, Ar‐CH), 4.77 (dt, 2H, CH2), 4.28 (dt, 2H, CH2).
(100mL). To each flask, 760 mg (690 μL, 6 mmol) of benzylchloride
was added dropwise. The flasks were covered with a beaker
and carefully heated in a household microwave (360 W) for 40 s.
To prevent the appearance of vapours in the flasks, heating was
interrupted a few times. After cooling the mixtures, they were
combined and extracted twice with 100 mL of ethyl acetate. The
organic layer was washed with saturated Na2CO3 solution, saline
solution, the organic phase dried with MgSO4 and the solvent
evaporated in vacuo. The obtained black residue was purified by
column chromatography.
2‐Chloro‐1‐(2‐fluoroethoxy)‐4‐nitrobenzene (9b)
Yield: 46%, TLC: Rf (ethyl acetate/hexane 3:2 (v/v)): 0.85.
MS (FD) m/z (% rel. int.): 219.2 (100, [M]+).
1H‐NMR (300 MHz, CDCl3), δ [ppm] = 8.29 (d, 1H, Ar‐CH), 8.13
(dd, 1H, Ar‐CH), 6.99 (d, 1H, Ar‐CH), 4.92–4.90 (m, 1H CH2), 4.77–
4.74 (m, 1H CH2), 4.45–4.42 (m, 1H CH2), 4.35–4.32 (m, 1H, CH2).
1‐(2‐Fluoroethoxy)‐3‐nitrobenzene (9c)
Yield: 81%, TLC: Rf (ethyl acetate/hexane 1:1 (v/v)): 0.93.
MS (FD) m/z (% rel. int.): 184.5 (100, [M]+).
4‐(Benzyloxy)aniline (12a)
1H‐NMR (300 MHz, CDCl3), δ [ppm] = 7.84 (dd, 1H, Ar‐CH), 7.73
(t, 1H, Ar‐CH), 7.43 (t, 1H, Ar‐CH), 7.27 (d, 1H, Ar‐CH), 4.86 (m, 1H
CH2), 4.70 (m, 1H CH2), 4.33 (m, 1H CH2), 4.24 (m, 1H CH2).
Yield: 61%, TLC: Rf (ethyl acetate): 0.75.
MS (FD) m/z (% rel. int.): 199.5 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 7.41–7.25 (m, 5H, Ar‐
CH), 6.70 (dt, 2H, Ar‐CH), 6.48 (dt, 2H, Ar‐CH), 4.92 (s, 2H, CH2),
4.62 (s, 2H, NH2).
1‐(2‐Fluoroethoxy)‐2‐nitrobenzene (9d)
Yield: 53%, TLC: Rf (ethyl acetate/CHCl3 1:10 (v/v)): 0.91.
MS (FD) m/z (% rel. int.): 184.5 (100, [M]+).
4‐(Benzyloxy)‐3‐chloroaniline (12b)
1H‐NMR (300 MHz, CDCl3), δ [ppm] = 7.82 (dd, 1H, Ar‐CH), 7.51 (dt,
1H, Ar‐CH), 7.10–7.03 (m, 2H, Ar‐CH), 4.87–4.84 (m, 1H, CH2), 4.71–
4.68 (m, 1H, CH2), 4.42–4.38 (m, 1H CH2), 4.32–4.29 (m, 1H CH2).
Yield: 24%, TLC: Rf (ethyl acetate/hexane 1:4 (v/v)): 0.5.
MS (FD) m/z (% rel. int.): 233.5 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 7.45 (d, 2H, Ar‐CH),
7.40–7.28 (m, 3H, Ar‐CH), 6.81 (d, 1H, Ar‐CH), 6.65 (d, 1H, Ar‐CH),
6.45 (dd, 1H, Ar‐CH), 5.06 (s, 2H, CH2), 5.04 (s, 2H, NH2).
General procedure II: hydrogenation
A mixture of 16 mmol of the appropriate fluoroethylated
nitrophenol and 400 mg of Pd (10%)/C in 80 mL of MeOH was 3‐(Benzyloxy)aniline (12c)
hydrogenated for 4 h at 0 °C and 1 bar pressure. The suspension
was filtered, the solvent evaporated and the residue purified by
column chromatography.
Yield: 35%, TLC: Rf (ethyl acetate/hexane 1:3 (v/v)): 0.48.
MS (FD) m/z (% rel. int.): 199.5 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 7.41–7.27 (m, 5H, Ar‐
CH), 6.88 (t, 1H, Ar‐CH), 6.19 (t, 1H, Ar‐CH), 6.14 (dd, 2H, Ar‐CH),
5.04 (s, 2H, CH2), 4.97 (s, 2H, NH2).
4‐(2‐Fluoroethoxy)aniline (10a)
Yield: 54%, TLC: Rf (ethyl acetate/hexane 1:1 (v/v)): 0.44.
MS (FD) m/z (% rel. int.): 155.1 (100, [M]+).
2‐(Benzyloxy)aniline (12d)
1H‐NMR (200 MHz, CDCl3), δ [ppm] = 6.61 (d, 2H, Ar‐CHa), 6.49 (d,
2H, Ar‐CHb), 4.56 (dt, 2H, CH2), 3.98 (dt, 2H, CH2), 3.43 (br s, 2H, NH2).
Yield: 53%, TLC: Rf (ethyl acetate/hexane 1:2 (v/v)): 0.64.
MS (FD) m/z (% rel. int.): 199.5 (100, [M]+).
1H‐NMR (300 MHz, CDCl3), δ [ppm] = 7.45–7.30 (m, 5H, Ar‐CH),
6.86–6.70 (m, 4H, Ar‐CH), 5.07 (s, 2H, CH2), 4.40–3.00 (br s, 2H, NH2).
3‐Chloro‐4‐(2‐fluoroethoxy)aniline (10b)
Yield: (54%), TLC: Rf (ethyl acetate/hexane/3/2): 0.67.
MS (FD) m/z (% rel. int.): 189.2 (100, [M]+).
General procedure IV: coupling with bis‐(trichloromethyl)
carbonate (triphosgene)
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 6.85 (d, 1H, Ar‐CH),
6.62 (d, 1H, Ar‐CH), 6.46 (dd, 1H, Ar‐CH), 4.94 (s, 2H, NH2), 4.75–
4.72 (m, 1H CH2), 4.59–4.56 (m, 1H CH2), 4.16–4.13 (m, 1H CH2),
4.06–4.03 (m, 1H, CH2).
To a stirred solution of 0.37 mmol triphosgene in 4 mL of dry
CH2Cl2 at 0 °C under argon atmosphere was slowly added, over a
period of 30 min, a solution of 1 mmol of the appropriate amino
compound and 1.1 mmol N‐ethyldiisopropylamine in 4 mL of dry
CH2Cl2. After stirring for 30 min, a solution of 1 mmol 7 and
1.1 mmol N‐ethyldiisopropylamine in 5 mL of dry CH2Cl2 was
quickly added. The mixture was warmed to ambient tempera-
ture and stirred for another 60 min. The solvent was evaporated,
the residue taken up in 30 mL of ethyl acetate and then washed
with 5% HCl solution, saturated NaHCO3 solution and water. The
organic layer was dried, the solvent removed in vacuo and
the residue taken up in 15 mL of diethyl ether. After treating the
suspension for 15 min in an ultrasonic bath, the remaining solid
was filtered off and dried in vacuo.
3‐(2‐Fluoroethoxy)aniline (10c)
Yield: 84%, TLC: Rf (ethyl acetate/hexane 1:1 (v/v)): 0.65.
MS (FD) m/z (% rel. int.): 155.1 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 6.88 (t, 1H, Ar‐CH),
6.17–6.06 (m, 3H, Ar‐CH), 5.05 (s, 2H, NH2), 4.76 (t, 1H, CH2), 4.60
(t, 1H, CH2), 4.13 (t, 1H, CH2), 4.03 (t, 1H, CH2).
2‐(2‐Fluoroethoxy)aniline (10d)
Yield: 68%, TLC: Rf (ethyl acetate/CHCl3 1:10 (v/v)): 0.69.
MS (FD) m/z (% rel. int.): 155.1 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 6.79 (d, 1H, Ar‐CH),
6.74–6.62 (m, 2H, Ar‐CH), 6.52–6.43 (m, 1H, CH2), 4.68 (s, 2H,
NH2), 4.65 (t, 1H, CH2), 4.21 (t, 1H, CH2), 4.11 (t, 1H, CH2).
Ethyl 4,6‐dichloro‐3‐((3‐(4‐(2‐fluoroethoxy)phenyl)‐1‐(2‐methoxy‐2‐
oxoethyl)ureido)methyl)‐indole‐2‐carboxylate (13a)
General procedure III: synthesis of benzyloxy anilines
Yield: 61%, TLC: Rf (Al2O3, CHCl3/acetone 4:1 (v/v)): 0.63.
MS (FD) m/z (% rel. int.): 539.4 (100, [M]+).
A mixture of 5 mmol of the appropriate aminophenol, 20 mmol
K2CO3, 20 mmol fine powdered KOH and 0.5 mmol tributyl-
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.44 (s, 1H, In‐NH),
ammonium bromide was triturated in a mortar, divided into 8.55 (s, 1H, NH), 7.46 (d, 1H, Ar‐CH), 7.26 (m, 2H, Ar‐CH), 7.22 (d,
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J. Label Compd. Radiopharm 2011, 54 645–656

A. Bauman et al.
1H, Ar‐CH), 6.87 (dd, 2H, Ar‐CH), 5.19 (s, 2H, CH2), 4.79 (m, 1H, General procedure V: cyclisation
CH2), 4.62 (m, 1H, CH2), 4.33 (q, 2H, CH2), 4.23 (m, 1H, CH2), 4.12
(m, 1H, CH2), 3.49 (s, 2H, CH2), 3.46 (s, 3H, OCH3) 1.32 (t, 3H, CH3).
To a stirred suspension of 1.0 mmol of the appropriate ureido
compound in 5 mL of EtOH under argon atmosphere, a solution of
0.5 mmol NaOEt in 1 mL of EtOH was added. The mixture was
stirred for 60 min at room temperature, then acidified with 10%
HCl solution and diluted with CH2Cl2. The organic layer was sepa-
rated and dried with MgSO4. The solvent was removed in vacuo,
and the crude product purified by column chromatography.
Ethyl 4,6‐dichloro‐3‐((3‐(3‐chloro‐4‐(2‐fluoroethoxy)phenyl)‐1‐(2‐
methoxy‐2‐oxoethyl)ureido)methyl)‐indole‐2‐carboxylate (13b)
Yield: 70%, TLC: Rf (Al2O3, THF/hexane 1:3 (v/v)): 0.22.
MS (FD) m/z (% rel. int.): 543.1 (100, [M‐MeOH]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.39 (s, 1H, In‐NH),
8.70 (s, 1H, NH),7.65–7.61 (m, 1H, Ar‐CH), 7.47–7.44 (m, 1H, Ar‐
CH) 7.27–7.22 (m, 2H, Ar‐CH), 7.08 (d, 1H, Ar‐CH), 5.18 (s, 2H,
CH2), 4.84–4.80 (m, 1H, CH2), 4.68–4.64 (m, 1H, CH2), 4.41–4.29
(m, 3H), 4.21–4.19 (m, 1H, CH2), 3.82 (s, 2H, CH2), 3.48 (s, 3H,
OCH3), 1.31 (t, 3H, CH3).
Ethyl 4,6‐dichloro‐3‐((3‐(4‐(2‐fluoroethoxy)phenyl)‐2,4‐dioxoimidazolidin‐
1‐yl)methyl)‐indole‐2‐carboxylate (14a)
Yield: 70%, TLC: Rf (Al2O3, CHCl3/acetone 4:1 (v/v)): 0.87.
MS (FD) m/z (% rel. int.): 507.4 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.43 (s, 1H, In‐NH),
7.48 (d, 1H, Ar‐CH), 7.26 (s, 1H, Ar‐CH), 7.23 (d, 2H, Ar‐CH), 7.04 (d,
2H, Ar‐CH), 5.24 (s, 2H, CH2), 4.82 (t, 1H, CH2), 4.66 (t, 1H, CH2),
4.40 (q, 2H, CH2), 4.31 (t, 1H, CH2), 4.21 (t, 1H, CH2), 3.79 (s, 2H,
CH2), 1.37 (t, 3H, CH3).
Ethyl 4,6‐dichloro‐3‐((3‐(3‐(2‐fluoroethoxy)phenyl)‐1‐(2‐methoxy‐2‐
oxoethyl)ureido)methyl)‐indole‐2‐carboxylate (13c)
Yield: 48%, TLC: Rf (Al2O3, CHCl3/acetone 10:1 (v/v)): 0.63.
MS (FD) m/z (% rel. int.): 539.3 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.40 (s, 1H, In‐NH),
8.69 (s, 1H, NH), 7.46 (d, 1H, Ar‐CH), 7.22 (d, 1H, Ar‐CH), 7.20–7.05
(m, 3H, Ar‐CH), 6.55 (dd, 1H, Ar‐CH), 5.20 (s, 2H, CH2), 4.80 (t, 1H,
CH2), 4.64 (t, 1H, CH2), 4.33 (q, 2H, CH2), 4.22 (t, 1H, CH2) 4.11 (t,
1H, CH2), 3.85 (s, 2H, CH2), 3.50 (s, 3H, OCH3), 1.31 (t, 3H, CH3).
Ethyl 4,6‐dichloro‐3‐((3‐(3‐chloro‐4‐(2‐fluoroethoxy)phenyl)‐2,4‐
dioxoimidazolidin‐1‐yl)methyl)‐indole‐2‐carboxylate (14b)
Yield: 86%, TLC: Rf (Al2O3, THF/hexane 1:3 (v/v)): 0.82.
MS (FD) m/z (% rel. int.): 543.1 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.43 (s, 1H, In‐NH),
7.45 (dd, 2H, Ar‐CH), 7.26 (dd, 3H, Ar‐CH), 5.24 (s, 2H, CH2), 4.86–
4.84 (m, 1H, CH2), 4.70–4.68 (m, 1H, CH2), 4.43–4.36 (m, 3H), 4.33–
4.30 (m, 1H, CH2), 3.79 (s, 2H, CH2), 1.36 (t, 3H, CH3).
Ethyl 4,6‐dichloro‐3‐((3‐(2‐(2‐fluoroethoxy)phenyl)‐1‐(2‐methoxy‐2‐
oxoethyl)ureido)methyl)‐indole‐2‐carboxylate (13d)
Yield: 63%. TLC: Rf (Al2O3, CHCl3/acetone 10:1 (v/v)): 0.47.
MS (FD) m/z (% rel. int.): 538.9 (100, [M]+).
Ethyl 4,6‐dichloro‐3‐((3‐(3‐(2‐fluoroethoxy)phenyl)‐2,4‐dioxoimida-
zolidin‐1‐yl)methyl)‐indole‐2‐carboxylate (14c)
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.42 (s, 1H, In‐NH),
7.88 (s, 1H, NH), 7.79 (dd, 1H, Ar‐CH) 7.46 (dd, 1H, Ar‐CH), 7.23
(dd, 1H, Ar‐CH), 7.05–6.88 (m, 3H, Ar‐CH), 5.19 (s, 2H, CH2), 4.80 (t,
1H, CH2), 4.64 (t, 1H, CH2), 4.35 (q, 2H, CH2), 4.21 (t, 1H, CH2) 4.31
(t, 1H, CH2), 3.87 (s, 2H, CH2), 3.41 (s, 3H, OCH3), 1.32 (t, 3H, CH3).
Yield: 88%, TLC: Rf (Al2O3, CHCl3/acetone 10:1 (v/v)): 0.88.
MS (FD) m/z (% rel. int.): 507.2 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.44 (s, 1H, In‐NH),
7.47 (d, 1H, Ar‐CH), 7.38 (t, 1H, Ar‐CH), 7.25 (d, 1H, Ar‐CH), 7.00–
6.94 (m, 3H, Ar‐CH), 5.25 (s, 2H, CH2), 4.81 (t, 1H, CH2), 4.66 (t, 1H,
CH2), 4.39 (q, 2H, CH2), 4.28 (t, 1H, CH2) 4.17 (t, 1H, CH2), 3.80 (s,
2H, CH2), 1.37 (t, 3H, CH3).
Ethyl 4,6‐dichloro‐3‐((1‐(2‐methoxy‐2‐oxoethyl)‐3‐(4‐methoxyphenyl)
ureido)methyl)‐indole‐2‐carboxylate (13e)
Yield: 67%, TLC: Rf (Al2O3, CHCl3/acetone 10:1 (v/v)): 0.42.
MS (FD) m/z (% rel. int.): 506.9 (100, [M]+).
Ethyl 4,6‐dichloro‐3‐((3‐(2‐(2‐fluoroethoxy)phenyl)‐2,4‐dioxoimidazolidin‐
1‐yl)methyl)‐indole‐2‐carboxylate (14d)
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.38 (s, 1H, In‐NH),
8.52 (s, 1H, NH), 7.46 (d, 1H, Ar‐CH) 7.35–7.30 (m, 2H, Ar‐CH), 7.22
(d, 1H, Ar‐CH), 6.82 (d, 2H, Ar‐CH), 5.18 (s, 2H, CH2), 4.33 (q, 2H,
CH2), 3.82 (s, 2H, CH2), 3.69 (s, 3H, OCH3), 3.48 (s, 3H, OCH3), 1.31
(t, 3H, CH3).
Yield: 69%, TLC: Rf (Al2O3, CHCl3/acetone 10:1 (v/v)): 0.54.
MS (FD) m/z (% rel. int.): 507.2 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.44 (s, 1H, In‐NH),
7.48 (d, 1H, Ar‐CH) 7.42 (dt, 1H, Ar‐CH), 7.25 (d, 1H, Ar‐CH), 7.21
(dt, 2H, Ar‐CH), 7.06 (t, 1H, Ar‐CH), 5.29 (d, 1H, CH2), 5.15 (d, 1H,
CH2) 4.69 (t, 1H, CH2), 4.53 (t, 1H, CH2), 4.40 (q, 2H, CH2), 4.28 (m,
1H, CH2) 4.18 (t, 1H, CH2), 3.88 (d, 1H, CH2), 3.72 (d, 1H, CH2), 1.35
(t, 3H, CH3).
Ethyl 3‐((3‐(4‐(benzyloxy)phenyl)‐1‐(2‐methoxy‐2‐oxoethyl)ureido)
methyl)‐4,6‐dichloro‐indole‐2‐carboxylate (16a)
Yield: 64%, TLC: Rf (ethyl acetate/CHCl3 4:1 (v/v)): 0.75.
MS (FD) m/z (% rel. int.): 551.8 (100, [M]+).
Ethyl 4,6‐dichloro‐3‐((3‐(4‐methoxyphenyl)‐2,4‐dioxoimidazolidin‐1‐yl)
methyl)‐indole‐2‐carboxylate (14e)
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.37 (s, 1H, In‐NH),
7.51–7.31 (m, 9H, Ar‐CH), 6.91 (d, 2H, Ar‐CH), 5.91 (s, 2H, CH2),
5.05 (s, 2H, CH2), 4.33 (q, 2H, CH2), 3.82 (s, 2H, CH2), 3.49 (s, 3H,
OCH3), 1.32 (t, 3H, CH3).
Yield: 77%, TLC: Rf (Al2O3, CHCl3/acetone 10:1 (v/v)): 0.85.
MS (FD) m/z (% rel. int.): 475.9 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.43 (s, 1H, In‐NH),
7.47 (d, 1H, Ar‐CH) 7.25 (d, 2H, Ar‐CH), 7.21 (m, 1H, Ar‐CH), 7.00
(d, 2H, Ar‐CH), 5.24 (s, 2H, CH2), 4.53 (t, 1H, CH2), 4.40 (q, 2H, CH2),
3.78 (s, 2H, CH2), 3.76 (s, 2H, CH2), 1.37 (t, 3H, CH3).
Ethyl 3‐((3‐(3‐(benzyloxy)phenyl)‐1‐(2‐methoxy‐2‐oxoethyl)ureido)
methyl)‐4,6‐dichloro‐indole‐2‐carboxylate (16b)
Yield: 46%, TLC: Rf (Al2O3, CHCl3/acetone 8:1 (v/v)): 0.38.
MS (FD) m/z (% rel. int.): 583.4 (100, [M]+).
Ethyl 3‐((3‐(4‐(benzyloxy)phenyl)‐2,4‐dioxoimidazolidin‐1‐yl)methyl)‐
4,6‐dichloro‐indole‐2‐carboxylate (17a)
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.41 (s, 1H, In‐NH),
7.46 (d, 1H, Ar‐CH), 7.43–7.31 (m, 9H, Ar‐CH), 7.23 (d, 1H, Ar‐CH),
5.20 (s, 2H, CH2), 5.06 (s, 2H, CH2), 4.33 (q, 2H, CH2), 3.85 (s, 2H, Yield: 81%, TLC: Rf (ethyl acetate/hexane 2:1 (v/v)): 0.92.
CH2), 3.49 (s, 3H, OCH3), 1.32 (t, 3H, CH3).
MS (FD) m/z (% rel. int.): 551.8 (100, [M]+).
J. Label Compd. Radiopharm 2011, 54 645–656
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A. Bauman et al.
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.43 (s, 1H, In‐NH),
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.23 (s, 1H, In‐NH),
7.48–7.31 (m, 9H, Ar‐CH), 7.23 (d, 1H, Ar‐CH), 7.08 (d, 1H, Ar‐CH), 7.44 (d, 1H, Ar‐CH), 7.24–7.19 (m, 3H, Ar‐CH), 7.00 (d, 2H, Ar‐CH),
5.24 (s, 2H, CH2), 5.13 (s, 2H, CH2), 4.40 (q, 2H, CH2), 3.78 (s, 2H, 5.27 (s, 2H, CH2), 3.78 (s, 2H, CH2), 3.76 (s, 3H, OCH3).
CH2), 1.36 (t, 3H, CH3).
3‐((3‐(4‐(Benzyloxy)phenyl)‐2,4‐dioxoimidazolidin‐1‐yl)methyl)‐4,6‐
Ethyl 3‐((3‐(3‐(benzyloxy)phenyl)‐2,4‐dioxoimidazolidin‐1‐yl)methyl)‐ dichloro‐indole‐2‐carboxylic acid (18)
4,6‐dichloro‐indole‐2‐carboxylate (17b)
Yield: 87%, TLC: Rf (CHCl3/acetone 1:1 (v/v) +3% HCOOH): 0.60.
Yield: 61%, TLC: Rf (Al2O3, CHCl3/acetone 8:1 (v/v)): 0.52.
MS (FD) m/z (% rel. int.): 550.8 (100, [M]+).
MS (FD) m/z (% rel. int.): 523.8 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.27 (s, 1H, In‐NH),
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.44 (s, 1H, In‐NH), 7.48–7.31 (m, 8H, Ar‐CH), 7.18 (d, 1H, Ar‐CH), 6.90 (d, 2H, Ar‐CH),
7.47–7.34 (m, 7H, Ar‐CH), 7.25 (d, 1H, Ar‐CH), 7.05–6.99 (m, 2H, 5.18 (s, 2H, CH2), 5.04 (s, 2H, CH2), 3.74 (s, 2H, CH2).
Ar‐CH), 6.93 (d, 1H, Ar‐CH), 5.24 (s, 2H, CH2), 5.09 (s, 2H, CH2), 4.39
(q, 2H, CH2), 3.80 (s, 2H, CH2), 1.36 (t, 3H, CH3).
General procedure VII: deprotection of benzyl groups
The benzyl‐protected compound (0.31 mmol) was dissolved in
200 mL of MeOH at 60 °C. Pd (10%)/C (30 mg) was added and
General procedure VI: saponification
To a solution of 0.5 mmol of the appropriate ester compound in
9 mL of THF, 1 mL of H2O and 400 μL of 2 M NaOH solution were
added. The mixture was stirred at room temperature till completion
(TLC‐control) and then diluted with 10 mL of ethyl acetate and
10 mL of H2O. The layers were separated, and the aqueous phase
was acidified with 2 M HCl. The suspension was stored in the refrig-
erator overnight, and the precipitate filtered off and dried in vacuo.
hydrogenated at 1 bar pressure until the reduction was
complete (TLC‐control). The catalyst was removed by filtration,
the solvent removed in vacuo and the crude product purified
by column chromatography.
4,6‐Dichloro‐3‐((3‐(4‐hydroxyphenyl)‐2,4‐dioxoimidazolidin‐1‐yl)
methyl)‐indole‐2‐carboxylic acid (20)
Yield: 52%, TLC: Rf (CHCl3/acetone 4:1 (v/v) + 3% HCOOH): 0.31.
MS (FD) m/z (% rel. int.): 433.7 (100, [M]+).
4,6‐Dichloro‐3‐((3‐(4‐(2‐fluoroethoxy)phenyl)‐2,4‐dioxoimidazolidin‐
1‐yl)methyl)‐indole‐2‐carboxylic acid (15a)
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.22 (s, 1H, In‐NH), 9.62
(s, 1H, OH), 7.40 (d, 1H, Ar‐CH), 7.17 (d, 1H, Ar‐CH), 7.04 (d, 2H, Ar‐CH),
6.76 (d, 2H, Ar‐CH), 5.21 (s, 2H, CH2), 3.73 (s, 2H, CH2). 13C‐NMR
(100.6 MHz, DMSO‐d6), δ [ppm] = 169.7 (C = O), 162.8 (C = O), 157.2
(C‐OH), 155.2 (C = O), 137.4 (C‐quart.), 130.3 (C‐quart.), 128.9 (C‐quart.),
128.4 (2 CH.), 127.3 (C‐quart.), 123.6 (C‐quart.), 122.9 (C‐quart.), 121.2
(CH), 115.5 (2 CH), 113.2 (C‐quart.), 111.6 (CH), 48.8 (CH2), 36.3 (CH2).
Yield: 79%, TLC: Rf (CHCl3/acetone 4:1 (v/v) + 3% HCOOH): 0.49.
MS (FD) m/z (% rel. int.): 480.0 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 11.91 (s, 1H, In‐NH),
7.42 (d, 2H, Ar‐CH), 7.40 (s, 1H, Ar‐CH), 7.12 (s, 1H, Ar‐CH), 6.82 (d,
2H, Ar‐CH), 5.22 (s, 2H, CH2), 4.78 (t, 1H, CH2), 4.62 (t, 1H, CH2),
4.20 (t, 1H, CH2), 4.10 (t, 1H, CH2), 3.79 (s, 2H, CH2).
4,6‐Dichloro‐3‐((3‐(3‐chloro‐4‐(2‐fluoroethoxy)phenyl)‐2,4‐
dioxoimidazolidin‐1‐yl)methyl)‐indole‐2‐carboxylic acid (15b)
Ethyl 4,6‐dichloro‐3‐((3‐(4‐hydroxyphenyl)‐2,4‐dioxoimidazolidin‐
1‐yl)methyl)‐indole‐2‐carboxylate (19a)
Yield: 46%, TLC: Rf (CHCl3/acetone 4:1 (v/v) + 3% HCOOH): 0.36.
MS (FD) m/z (% rel. int.): 514.7 (100, [M]+).
Yield: 63%, TLC: Rf (ethyl acetate/hexane 2:1 (v/v)): 0.75.
MS (FD) m/z (% rel. int.): 461.0 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.11 (s, 1H, In‐NH),
7.44 (s, 2H, Ar‐CH), 7.31–7.23 (m, 2H, Ar‐CH), 7.15 (d, 1H, Ar‐CH),
5.31 (s, 2H, CH2), 4.87–4.82 (m, 1H, CH2), 4.70–4.68 (m, 1H, CH2),
4.44–4.38 (m, 1H, CH2), 4.33–4.29 (m, 1H, CH2), 3.82 (s, 2H, CH2).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.46 (s, 1H, In‐NH), 9.73
(s, 1H, OH), 7.48 (d, 1H, Ar‐CH), 7.26 (d, 1H, Ar‐CH), 7.08 (d, 2H, Ar‐CH),
6.81 (d, 2H, Ar‐CH) 5.23 (s, 2H, CH2), 4.39 (q, 2H, CH2), 3.77 (s, 2H, CH2),
1.37 (t, 3H, CH3). 13C‐NMR (100.6 MHz, DMSO‐d6), δ [ppm] = 169.7
(C = O), 159.7 (C = O), 157.2 (C‐OH) 155.2 (C = O), 137.7 (C‐quart.),
129.4 (C‐quart.), 128.4 (C‐quart.), 128.3 (2 CH), 127.5 (C‐quart.), 123.6
(C‐quart.), 122.8 (C‐quart.), 121.7 (CH), 115.6 (2 CH), 114.2 (C‐quart.),
111.8 (CH), 61.5 (CH2), 48.8 (CH2), 36.3 (CH2), 14.5 (CH3).
4,6‐Dichloro‐3‐((3‐(3‐(2‐fluoroethoxy)phenyl)‐2,4‐dioxoimidazolidin‐
1‐yl)methyl)‐indole‐2‐carboxylic acid (15c)
Yield: 77%, TLC: Rf (CHCl3/acetone 4:1 (v/v) + 3% HCOOH): 0.41.
MS (FD) m/z (% rel. int.): 479.3 (100, [M]+).
Ethyl 4,6‐dichloro‐3‐((3‐(3‐hydroxyphenyl)‐2,4‐dioxoimidazolidin‐
1‐yl)methyl)‐indole‐2‐carboxylate (19b)
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.32 (s, 1H, In‐NH),
7.44 (d, 1H, Ar‐CH), 7.38 (t, 1H, Ar‐CH), 7.21 (d, 1H, Ar‐CH), 7.00–
6.94 (m, 3H, Ar‐CH), 5.27 (s, 2H, CH2), 4.81 (t, 1H, CH2), 4.65 (t, 1H,
CH2), 4.28 (t, 1H, CH2) 4.17 (t, 1H, CH2), 3.81 (s, 2H, CH2).
Yield: 53%, TLC: Rf (ethyl acetate/hexane 1:1 (v/v)): 0.63.
MS (FD) m/z (% rel. int.): 461.4 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.42 (s, 1H, In‐NH), 9.69
(s, 1H, OH), 7.46 (d, 1H, Ar‐CH), 7.23 (d, 2H, Ar‐CH), 6.77–6.73 (m, 3H,
Ar‐CH), 5.23 (s, 2H, CH2), 4.39 (q, 2H, CH2), 3.78 (s, 2H, CH2), 1.37 (t,
3H, CH3). 13C‐NMR (100.6MHz, DMSO‐d6), δ [ppm] = 169.3 (C= O),
161.1 (C= O), 157.8 (C‐OH) 154.7 (C= O), 137.8 (C‐quart.), 133.4 (C‐
quart.), 129.6 (C‐quart.), 129.3 (CH), 128.6 (C‐quart.), 127.4 (C‐quart.),
122.8 (C‐quart.), 121.6 (CH), 117.3 (CH), 115.0 (CH), 114.0 (CH), 113.9
(C‐quart.), 111.8 (CH), 61.5 (CH2), 48.8 (CH2), 36.3 (CH2), 14.5 (CH3).
4,6‐Dichloro‐3‐((3‐(2‐(2‐fluoroethoxy)phenyl)‐2,4‐dioxoimidazolidin‐
1‐yl)methyl)‐indole‐2‐carboxylic acid (15d)
Yield: 53%, TLC: Rf (CHCl3/acetone 4:1 (v/v) + 3% HCOOH): 0.36.
MS (FD) m/z (% rel. int.): 479.2 (100, [M]+).
1H‐NMR (300 MHz, DMSO‐d6), δ [ppm] = 12.32 (s, 1H, In‐NH),
7.44 (d, 1H, Ar‐CH) 7.41–7.38 (m, 1H, Ar‐CH), 7.26–7.17 (m, 3H, Ar‐
CH), 7.05 (dt, 1H, Ar‐CH), 5.24 (q, 2H, CH2), 4.69 (t, 1H, CH2), 4.53
(t, 1H, CH2), 4.28 (m, 1H, CH2), 4.17 (m, 1H, CH2), 3.80 (q, 2H, CH2).
Radiochemistry
4,6‐Dichloro‐3‐((3‐(4‐methoxyphenyl)‐2,4‐dioxoimidazolidin‐1‐yl)
methyl)‐indole‐2‐carboxylic acid (15e)
2‐[18F]Fluoroethyl tosylate ([18F]FETos)
Yield: 54%, TLC: Rf (CHCl3/acetone 4:1 (v/v) + 3% HCOOH): 0.42.
MS (FD) m/z (% rel. int.): 447.7 (100, [M]+).
A solution of aqueous [18F]F‐ (500–8000 MBq) produced by the
18O(p,n)18F nuclear reaction on an isotopically enriched [18O]H2O
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Copyright © 2011 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2011, 54 645–656

A. Bauman et al.
target was added to a solution of 15 μL of 1 M K2CO3 and 10– (stationary phase: Luna 5 µmC18(2), 250 × 10 mm; mobile phase:
15 mg of Kryptofix® 222 in 0.8 mL of MeCN. The water was MeCN/0.25 M NH4OAc (pH 5) 70:30 (v/v), flow rate: 6 mL/min, tr:
removed using a stream of N2 at 80 °C and co‐evaporated to 7.1min). After diluting the product‐containing HPLC fraction with
dryness with MeCN (2 × 1 mL). To the dried K/[18F]F‐/Kryptofix® 50 mL of H2O, it was loaded on a solid‐phase extraction column
222/carbonate complex, 4 mg (15 mmol) of 1,2‐ditosyloxyethane (Strata X, Phenomenex; conditioned with 10 mL EtOH and 10 mL
in 1 mL of MeCN was added, and the mixture was heated at water), washed with 10 mL of H2O, dried with He and eluted in a
85 °C for 3 min. Purification of the crude product was accom- second reaction vessel with 1.5 mL of EtOH. After addition of 100
plished using semipreparative HPLC under isocratic condi- μL of a 1 M NaOH solution, the reaction was stirred at ambient
tions (column: Lichrospher RP18‐EC5, 250 × 10 mm, eluent: temperature for 30 min, diluted with 4 mL of H2O and again
MeCN/H2O 50:50 (v/v), flow rate: 5 mL/min, Rt: 8 min). After purified by semipreparative radio‐HPLC (stationary phase: Luna
diluting the HPLC fraction containing [18F]FETos with 40 mL 5 µm C18(2), 250 × 10 mm; mobile phase: MeCN/0.25 M NH4OAc
of H2O, the product was loaded on a Sep‐Pak C18‐column, (pH 5) 40:60 (v/v), flow rate: 6 mL/min, tr: 7.3 min). The isolated
dried in a stream of N2 and eluted with 1 mL of DMSO to yield product fraction was transferred into a vessel containing 50 mL
[18F]FETos after a total synthesis time of 50 min, with a radio- of H2O, passed on a Strata X solid‐phase extraction column,
chemical yield of about 70%.
washed with 10 mL of H2O, dried with He and eluted with 1.5mL of
EtOH. The solvent was removed in vacuo and the residue taken
up in physiological saline solution to yield 130–220 MBq of [11C]15e
[11C]Methyl iodide ([11C]CH3I)
Carbon‐11 was produced at the PET Center Tübingen as [11C]CO2 in a radiochemical yield of 6–9% and specific activities between
with a PETtrace cyclotron (GE Medical Systems, Uppsala, Sweden) 8 and 26 GBq/µmol after a synthesis time of about 85 min.
by irradiation of N2 (0.5% O2) using the 14N(p,α)11C nuclear reac-
tion with 16.5MeV protons. [11C]CH3I was obtained by an auto-
Conclusion
mated synthesis module (MeI MicroLab, GE Medical Systems). First,
[11C]CO2 was trapped on a molecular sieve (4Å) and converted
On the basis of a hydantoin‐substituted indole 2‐carboxylic acid lead
structure, five compounds for the glycine binding site of the NMDA
to [11C]CH4 in the presence of a Ni catalyst and H2 at 360 °C, and
then [11C]CH4 was reacted with iodine at 760°C to obtain [11C]CH3I.
receptor were synthesised. From these reference compounds, the
in vitro affinity and lipophilicity were determined. They exhibit IC50
values ranging from 17 to 453 nM and have logD values from 1.51
Automated synthesis of 4,6‐dichloro‐3‐((3‐(4‐(2‐[18F]fluorethoxy)
phenyl)‐2,4‐dioxoimidazolidin‐1‐yl)methyl)‐indole‐2‐carboxylic acid
to 2.53, depending on the substitution pattern and the electronic
([18F]15a)
properties. With these results, labelling precursors for 18F‐ and 11C‐
derivatives were synthesised for a two‐step synthesis route using
the secondary labelling precursors [18F]FETos and [11C]CH3I. The
radiosynthesis of [18F]15a and [11C]15e was hampered to a high
degree by side reactions and partial decomposition of the final
product during the ester cleavage. Decay‐corrected radiochemical
yields of 5–7% and 6–9% were obtained for the final compounds.
Therefore, future efforts will focus on improvement of the
radiochemical yields of the compounds, for example by examina-
tion of different protecting groups for carboxylic acids and their
biological evaluation.
A solution of 3 mg (6.9 µmol) of 19a and 2.55 μL of 5 M NaOH in
1 mL of DMSO was transferred to a reaction vessel of an
automated synthesis module and tempered for 5 min at 100 °C.
After addition of 2.5 to 3 GBq of [18F]FETos in 1 mL of DMSO, the
reaction was stirred for 5 min at 100 °C, then diluted with 4 mL of
eluent and purified with HPLC (MeCN/0.25 M NH4OAc (pH 5)
60:40 (v/v), flow rate: 6 mL/min, tr: 14.6 min). After diluting the
HPLC fraction containing the product with 50 mL of H2O, it was
loaded on a solid‐phase column (Strata X, Phenomenex), washed
with 10 mL of H2O, dried with N2 and eluted in a second reaction
vessel with 1 mL of EtOH. After addition of 100 μL of a 1 M NaOH
solution, the reaction was stirred at ambient temperature for
30 min, diluted with 4 mL of H2O and again purified by HPLC
(MeCN/0.25 M NH4OAc (pH 5) 40:60 (v/v), flow rate: 6 mL/min, tr:
Acknowledgements
We are grateful to Dr Fabian Debus from the group of Prof H.
7.3 min). The isolated product fraction was transferred into a Lüddens, Department of Psychiatry, University of Mainz, for
supporting the autoradiography experiment. This work was
financially supported by DFG grant Ro 985/20‐1.
vessel containing 50 mL of H2O, passed on a Strata X solid‐phase
extraction column, washed with 10 mL of H2O, dried with N2 and
eluted with 1.5mL of EtOH. The solvent was removed in vacuo,
and the residue taken up in physiological saline solution to yield
55–90 MBq of [18F]15a in a decay‐corrected radiochemical yield
of 5–7% and specific activities between 24 and 67 GBq/µmol after
a synthesis time of about 130 min.
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J. Label Compd. Radiopharm 2011, 54 645–656