Synthesis of asymmetric [75Se]selenoethers via carbodiimides

Article abstract of DOI:10.1002/jlcr.488

A convenient radiosynthesis of asymmetric [⁷⁵Se]selenoethers was developed using 1,3-disubstituted [⁷⁵Se]selenoureas as intermediates. These were prepared from appropriate carbodiimides and hydrogen [⁷⁵Se]selenide, which c

Full text of DOI:10.1002/jlcr.488

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Labelled Cpd Radiopharm 2001; 44: 587–601.
DOI: 10.1002/jlcr.488
Synthesis of asymmetric [75Se]selenoethers
via carbodiimides
T. Blum, J. Ermert* and H. H. Coenen
Institut fu.r Nuklearchemie, Forschungszentrum Ju.lich GmbH, D-52425 Ju.lich,
Germany
Summary
A convenient radiosynthesis of asymmetric [75Se]selenoethers was developed
using 1,3-disubstituted [75Se]selenoureas as intermediates. These were prepared
from appropriate carbodiimides and hydrogen [75Se]selenide, which could only
be generated from carrier-added (c.a.) [75Se]selenite in aqueous solution using
phosphinic acid as reducing agent. Optimization of this initial labelling step
with dicyclohexylcarbodiimide and polymeric N-cyclohexylcarbodiimide-N0-
methyl polystyrene resulted in radiochemical yields (RCY) of 73 and 55%
(bound on the polymer), respectively, within 45 min. Treatment of [75Se]seleno-
ureas with alkylbromides led to corresponding [75Se]selenouronium salts in
nearly quantitative yields. Hydrolysis under basic conditions provided the
[75Se]selenolates and a second alkylation yielded asymmetric [75Se]selenoethers.
Thus, within 90 min benzylmethyl[75Se]selenide, benzylbutyl[75Se]selenide,
benzylisopropyl[75Se]selenide and 1-phenyl-l-(methyl[75Se]seleno)ethane were
synthesized with respective RCY of about 59, 55, 10 and 60%. Furthermore,
the 75Se-labelled alkylating agent 3-(methyl[75Se]seleno)-1-propanyl p-toluene-
sulfonate and [75Se]selenomethionine were obtained with radiochemical yields
of 51 and 41%, respectively. Copyright # 2001 John Wiley & Sons, Ltd.
Key Words: 75Se-labelling; substituted [75Se]selenoureas; asymmetric [75Se]se-
lenoethers; 75Se-labelled alkylating agent; [75Se]selenomethionine
. .
*Correspondence to: J. Ermert, Institut fur Nuklearchemie, Forschungszentrum Julich GmbH,
.
D-52425 Julich, Germany.
Copyright # 2001 John Wiley & Sons, Ltd.
Received 8 January 2001
Revised 6 April 2001
Accepted 14 April 2001
Published online \ \ \ \

588
T. BLUM ET AL.
Introduction
Selenium-75 (T1=2 ¼120.4 d; 100% EC) has often been used as a
gamma-ray emitting label in various radiopharmaceuticals.1–4 The
decay characteristics of 75Se are not idealfor in vivo application, because
the radiation absorbed dose delivered to the patient in a study is high
and the images with conventional g-cameras are of poor quality.5 The
nuclear properties of 73Se (T1=2 ¼ 7.1 h; 65% b+; 35% EC) suggest that
it may be a more usefulalbelfor
in vivo medicalappilcations using
positron-emission-tomography (PET). This radioisotope can be pro-
duced in sufficient amounts via the 75As(p, 3n)73Se reaction.6 For the
development of radiosyntheses, however, the more easily available
radioisotope 75Se is generally used.7 The most common 75Se-labelled
compounds are selenoethers, as the selenium atom can form two stable
covalent bonds to carbon, where selenium is stabilized in the oxidation
state ꢀ2. A widely used method for the preparation of symmetric
carrier-added (c.a.) [75Se]selenoethers starts with the reduction of
[75Se]selenious acid or elemental 75Se with an excess of NaBH4 to
obtain the [75Se]NaHSe nucleophile. Subsequent alkylation yields the
appropriate 75Se-labelled symmetric selenoethers.8,9 Basmadijan et al.10
suggested the formation of asymmetric c.a. [75Se]selenoethers via
reductive cleavage of 75Se-labelled dialkyl diselenides or by basic
hydrolysis of [75Se]selenouronium salts yielding alkyl[75Se]selenolates,
which can be alkylated in situ.
For clinical diagnosis it would be desirable to achieve a general
pathway for the rapid radiochemical and fully automated synthesis of
[73Se]selenoethers in order to handle high activities and to minimize the
radiation dose to the operators. An attractive toolfor automated
synthesis is solid-phase chemistry which allows purification of the
products with minimum effort. The first microscale synthesis suitable
for introduction of no-carrier-added (n.c.a.) radioselenium using a
solid-phase support was reported by Schmaljohann.11 For this purpose
polymeric bound n.c.a. [75Se]triphenylphosphineselenide was synthe-
sized, subsequently methylated and then hydrolysed to the methylsele-
nide anion. Nucleophilic substitution on alkyl bromides allowed the
synthesis of asymmetric n.c.a. [75Se]methylselenoethers.
The aim of this study was the preparation of asymmetric [75Se]sele-
noethers via 1,3-disubstituted [75Se]selenoureas as intermediates starting
from [75Se]H2Se and carbodiimides. While the alkylation of [75Se]-
triphenylphosphineselenide is limited to methyltriflate, [75Se]selenoureas
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[
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589
would be expected to make a wide variety of symmetric as well as
asymmetric [75Se]selenoethers accessible, because the alkylation of
selenoureas can be easily accomplished by alkylbromine or -tosylate
derivatives.10 Furthermore, a polymer bound carbodiimide is available
as starting material to fulfil the requirements of a solid-phase synthesis.
Preparation of different [75Se]selenoethers as model compounds
shows the versatility of this method. The radioselenium-labelled amino
acid selenomethionine can be used for the measurement of protein
synthesis in the pancreas12 and possibly even in brain.13 This
radiopharmaceuticalwas previousyl obtained in carrier-added form
by biosynthesis within 30 h with a RCY of 20–40%14 or by a
4
radiochemicalsynthesis within 3 h with a RCY of 80%.
Experimental
General
Unless otherwise mentioned, all chemicals were purchased from Aldrich
(Steinheim, Germany) and Fluka (Buchs, Switzerland) and used
without further purification. N-cyclohexylcarbodiimide-N0-methylpoly-
styrene was obtained from Novabiochem (Schwalbach, Germany),
2-amino-4-bromobutyric acid hydrobromide was obtained from Acros
Organics (Geel, Belgium). Further compounds were synthesized
according to literature methods: benzylmethylselenide,15 benzylbutyl-
selenide,16 benzylisopropylselenide,16 3-(methylseleno)-1-propanol,17
3-(methylseleno)-1-propanyl p-toluenesulfonate11 and ethyl N-tert.-
butoxycarbonyl-2-amino-4-bromobutyrate 3a.11 All selenocompounds
were prepared under an argon atmosphere.
Melting point determinations employed a Mettler FP 61 and are
1
uncorrected. H and 13C NMR spectra were recorded on a Bruker
Avance 200 spectrometer with samples dissolved in CDCl3, CD2Cl2 or
d6-DMSO. All chemical shifts are reported in d ppm using the signals of
the appropriate solvent as a reference. IR spectra were recorded on a
Shimadzu IR-460 spectrophotometer and mass spectra were obtained
using a Finnigan Automass Multi mass spectrometer with an electron
beam energy of 70 eV. High-resolution mass spectra were recorded on a
Finnigan MAT 900 ST apparatus (University of Cologne, Germany).
Analytical radio-HPLC was performed on a system consisting of a
Knauer pump 6400 and a Knauer UV/vis photometer 3060 with a
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T. BLUM ET AL.
detector wavelength of 220 nm. Sample injection was accomplished by a
Rheodyne-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 were processed by the
.
software system Raytest Ramona MCS (Nuclear Interface, Munster,
Germany). Separation of c.a. [75Se]selenomethionine was performed
using a NucleosilTM 100-5 NH2 (250 ꢁ 4 mm) column (CS-Chromato-
graphie Service) with a mobile phase of acetonitrile/0.04 M KH2PO4
(aq.) (77/23) (v/v) at a flow rate of 1.0 ml/min. HPLC of aliquots of all
other labelled products and standards was performed using a m-
Bondapack C 18 (250 ꢁ 4 mm) column (CS-Chromatographie Service
GmbH, Langerwehe, Germany) and a mobile phase consisting of
acetonitrile/H2O (40/60) (v/v), 0.01% trifluoroacetic acid and 0.01%
triethylamine at a flow rate of 1.0 ml/min. Radio-TLC was performed
on Merck silica gel plates with the solvent system diethyl ether/n-hexane
in various concentrations. The developed TL-chromatograms were
measured for radioactivity on an Instant ImagerTM (Packard).
Standards and precursors
1,3-Dicyclohexylselenourea. Hydrogen selenide, generated by adding
sulfuric acid (20 ml, 18 N) to powdered aluminium selenide (1.5 g,
5.2 mmol), was passed into a solution of 1,3-dicyclohexylcarbodiimide
(824 mg, 4 mmol) in diethyl ether (20 ml) for 1 h using a method
developed by Klayman and Shine.18 The solution was stirred at room
temperature for 4 h. The solvent was removed by distillation and the
residue taken up in dichloromethane. After filtration the solvent was
evaporated yielding 1,3-dicyclohexylselenourea as a grey solid. Yield:
1
440 mg (38%); m.p.: 1728C; H-NMR (CD2Cl2): d 5.85 (b, 2H, NH),
3.85 (b, 2H, CH), 1.65 (m, 10H, CH2); 13C-NMR (d6-DMSO): d 173.0
(C¼Se), 54.3 (CH–N), 32.0, 25.0, 24.5 (CH2); mass spectrum: m/e 288
(M+, 25%), 207 (10), 125 (30), 98 (29), 83 (23), 43 (100); HR-MS
calculated for C13H24N2Se: 289.118, found: 289.117.
1-Phenyl-1-(methylseleno)ethane. Selenium powder (450 mg, 5.7 mmol)
was suspended in tetrahydrofuran (20 ml) and methyl lithium in diethyl
ether (4.3 ml, 6.9 mmol) was added while stirring, resulting in a pale
yellow solution. 1-(Bromoethyl)benzene (0.78 ml, 5.7 mmol) was added
and the mixture further stirred for 1 h at room temperature. The
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solution was hydrolysed with water (10 ml), the organic phase
separated, washed with NaHCO3 and brine and dried (Na2SO4)
according to Gulliver et al.17 The solvent was evaporated and the
residue fractionated in vacuo to give a pale yellow oil. Yield: 0.63 g
(55%); nD 20: 1.5783; 1H-NMR (CDCl3): d 7.29 (m, 5 H, Ar–H), 4.16 (q,
1H, CH), 1.90 (s, 3H, Se–CH3), 1.73 (d, 3H, CH–CH3); 13C-NMR
(CDCl3): d 144.7 (Cq), 128.0 ( ¼ CH), 38.2 (Se–CH), 22.5 (CH–CH3), 4.9
(Se–CH3); mass spectrum: m/e 200 (M+, 5%), 105 (100), 95 (24), 77
(82); HR-MS calculated for C9H12Se: 200.0104, found: 200.0101.
N-tert.-butoxycarbonylselenomethionine ethyl ester 6. Dry hydrogen
chloride was passed through a suspension of selenomethionine (196 mg,
1 mmol) in ethanol (50 ml) for 2 h with external cooling by ice according
to Deimer et al.19 The solution was evaporated to dryness, the residue
dissolved in ethanol and evaporated to dryness. The procedure was
repeated once more to obtain the crude selenomethionine ethyl ester as
a colourless oil without any trace of hydrochloric acid, which was
dissolved in CH2Cl2 (50 ml). Di-tert.-butyl dicarbonate (218 mg,
1 mmol) and triethylamine (0.28 ml, 2 mmol) were added and the
mixture was stirred for 12 h at room temperature according to Itoh
et al.20 The solvent was removed under vacuum and the crude product
obtained was purified via silica gel column chromatography (hexane/
diethylether: 1/1) to give 207 mg (64%) N-tert.-butoxycarbonylseleno-
methionine ethyl ester as colourless crystals. M.p.: 438C; nmax (KBr):
1748, 1684 (C¼O); 1H-NMR (CDCl3): d 5.15 (d, 1H, NH), 4.40 (q, 1H,
CH), 4.20 (q, 2H, O–CH2), 2.55 (t, 2H, Se–CH2), 2.10 (m, 2H, CH2–
CH2–CH), 2.00 (s, 3H, Se–CH3), 1.45 (s, 9H, C–CH3), 1.25 (t, 3H, CH2–
CH3); 13C-NMR (CDCl3): d 172.6 (C(O)O), 155.7 (C(O)N), 80.3
(C(CH3)3), 61.8 (O–CH2), 54.0 (CH), 33.7 (Se–CH2), 28.7 (C( CH3)3),
20.7 ( CH2–CH2–CH), 14.6 (CH2–CH3), 4.6 (Se–CH3); mass spectrum:
m/e 325 (M+, 9%), 269 (18), 224 (8), 174 (27), 147 (14), 101 (18), 57
(100); HR-MS calculated for C12H23O4NSe: 325.0792, found: 325.0784.
Radiosyntheses
Production of [75Se]selenite. [75Se]Selenium was produced using
20 MeV protons at the compact cyclotron CV-28 at the Forschungszen-
trum Jlich GmbH via the 75As(p,n)75Se reaction on a solid Cu3As-
target. After thermochromatographic separation n.c.a. selenium-75 is
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available in its oxidized form as [75Se]SeO32ꢀ in water.21 All experiments
were performed using 10 mmol natSeO23ꢀ as carrier and were conducted
under argon in a conical5 mlreaction vesselequipped with a magnetic
stirring bar and a teflon rubber septum.
1,3-Dicyclohexyl[75Se]selenourea 2. Reduction of the c.a. [75Se]selenite
solution (0.5 ml, typically containing 370 kBq (10 mCi)) was performed
at the desired temperature (55–958C) with 0.1 mlof 32% hydrochol ric
acid and 0.4 mlof 50% phosphinic acid in water. Via a stream of argon
the generated hydrogen [75Se]selenide 1 was slowly led into a solution of
100 mg 1,3-dicyclohexylcarbodiimide (DCC) in 3 ml of diethyl ether.
For this purpose a device is used (cf. Figure 1), which separates the
reduction from the acid-sensitive DCC. Aliquots were analysed for the
[75Se]selenourea by radio-HPLC under the conditions given above
(k0=5.0) and radio-TLC (diethylether/ n-hexane 3/2 (v/v), Rf=0.52).
Alternatively, 500 mg of N-cyclohexylcarbodiimide-N0-methylpoylsty-
rene were used instead of DCC for polymer-supported formation of
N-cyclohexyl[75Se]selenourea-N0-methylpoyl styrene 8.
Figure 1. Sketch of apparatus for the synthesis and reaction of [75Se]H2Se
[75Se]Selenouronium salts. The [75Se]selenouronium salts could be
prepared in various solvents (0.3 ml) such as acetonitrile, N,N-
dimethylformamide and ethanol by reaction of 2 with the appropriate
alkyl bromide (0.2 mmol), respectively. The alkylation reactions were
monitored by radio-TLC (diethylether/ n-hexane 3/2 (v/v), Rf=0.1–0.2).
By-products and adducts were removed by silica gel column chromato-
graphy using diethylether and the purified [ 75Se]selenouronium salts
were eluted from the column with acetone.
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[75Se]Selenoethers. The asymmetric [75Se]selenoethers were prepared
by treatment of the purified [75Se]selenouronium salts with tetrabutyl-
ammonium hydroxide (0.1 mmol) in the presence of appropriate
bromine- or iodine-compounds (0.2 mmol) in 0.3 ml acetonitrile at
room temperature within 1 min. The [75Se]selenoethers obtained were
analysed by radio-HPLC and radio-TLC.
3-(Methyl[75Se]seleno)-1-propanyl p-toluenesulfonate. Alkylation of 2
with 3-bromo-1-propanol(17 ml, 0.2 mmol) in 0.3 ml acetonitrile was
performed within 40 min. After purification, the resulting [75Se]seleno-
uronium salt was treated with a solution of 0.1 mmol tetrabutylammo-
nium hydroxide in 0.3 ml acetonitrile. Subsequently iodomethane (12 ml,
0.2 mmol) was added yielding 3-(methyl[75Se]seleno)-1-propanol. With-
out purification this compound was allowed to react by addition of
toluene-4-sulfonyl chloride (28.5 mg, 0.15 mmol) in 0.1 ml acetonitrile,
triethylamine (30 ml, 0.2 mmol) and trimethylamine hydrochloride
(8 mg, 0.01 mmol) in 0.2 ml acetonitrile at 08C for 1 h. Radio-TLC-
analysis was performed with diethyl ether/n-hexane 1/1 (v/v) as eluent,
Rf=0.6.
[75Se]selenomethionine. The labelled amino acid was prepared as
described for [75Se]selenoethers, starting with alkylation of 1,3-
dicyclohexyl[75Se]selenourea 2 with ethyl N-tert.-butoxycarbonyl-2-
amino-4-bromobutyrate 3a (62 mg, 0.2 mmol) in 0.3 ml acetonitrile.
The resulting [75Se]selenouronium salt was purified according to the
method described above and hydrolysed with tetrabutylammonium
hydroxide (0.1 mmol) in 0.3 ml acetonitrile. The obtained [75Se]seleno-
late was alkylated in situ by iodomethane (12 ml, 0.2 mmol). After ester
hydrolysis and Boc-cleavage using 16% hydrochloric acid (0.3 ml) at 70
and 908C, respectively, [75Se]selenomethionine was obtained and
analysed by radio-HPLC as detailed above (k0=3.16).
Results and discussion
Preparation of 1,3-dicyclohexyl[75Se]selenourea
According to the literature, 1,3-disubstituted selenoureas can be
prepared from carbodiimides and hydrogen selenide in several organic
solvents.22 We investigated the formation of hydrogen [75Se]selenide 1
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Scheme 1. Formation of hydrogen [75Se]selenide 1 and its subsequent conversion
to 1,3-dicyclohexyl[75Se]selenourea 2
Figure 2. Radiochemical yield of 1 as a function of Na2SeO3 carrier concentra-
tion with SnCl2 and H3PO2 as reducing agents. General reaction conditions:
370 kBq n.c.a [75Se]SeO32ꢀ, Na2SeO3-carrier, 45 min, 0.5 mmol DCC, 3ml
diethyl ether; conditions for H3PO2: 3.6 mmol H3PO2, 0.9 ml H2O, 0.1 ml
concentrated HCl, 858C; conditions for SnCl2: 1.5 mmol SnCl2, 1 ml concentrated
H3PO4, 3 008C
under a variety of different reaction conditions by monitoring its
reaction with DCC (Scheme 1). TLC-analyses showed, that all the 75Se-
activity in the DCC-solution existed as 1,3-dicyclohexyl[75Se]selenourea
within 10 s after starting the reduction, i.e. liberation of [75Se]H2Se.
For comparison, the reducing agents phosphinic acid23 and tin(II)
chloride in strong phosphoric acid24 were studied. As graphically
depicted in Figure 2 a concentration of 10 mmol/l Na2SeO3 carrier
proved to be usefulfor obtaining the highest radiochemicalyiedls of
66 ꢂ 3% of hydrogen [75Se]selenide within 45 min at 858C, using
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phosphinic acid as the obviously more suitable reducing agent. In the
case of SnCl2, the maximum RCY of only 10% was reached at the same
carrier concentration within 45 min at 3008C. Higher concentrations led
to a decrease of the RCY in both cases.
Increasing the temperature led to an almost linear increase of the
radiochemicalyiedl of
1 from about 10% at 558C to about 75%
radiochemical yield (related to [75Se]selenite) obtained at 958C with
phosphinic acid as reducing agent.
Furthermore, Figure 2 shows that the reduction of [75Se]selenite to
[75Se]H2Se is not possible at the n.c.a. level under the given reaction
conditions. All attempts to perform the reduction at the n.c.a. level were
unsuccessful. Even the use of various very strong reducing agents such
as sodium borohydride, lithium aluminium hydride and sodium as well
as of non-isotopic carriers like Na2SO3, Na2S2O3, Na2S, Na2TeO3 and
Na2Te failed. Under all conditions only n.c.a. 75Se0 was obtained.
Obviously, the reduction of selenite to hydrogen selenide does not occur
directly but proceeds via the intermediates Se0 and Se22ꢀ. The first
reduction step from selenite to elemental selenium can be carried out
without any problem at the n.c.a. level. Further reduction from Se0 to
hydrogen selenide most probably requires the interim formation of
diselenide-species. Since these cannot be formed at the n.c.a. level, the
preparation of n.c.a. hydrogen [75Se]selenide does not appear
possible.
Preparation of [75Se]selenouronium salts
Due to the highly polarized carbon–selenium bond an equimolar
reaction of selenourea with various alkyl halides yields the correspond-
ing selenouronium salts as described by Chu and Mautner.25 In order to
apply and optimize this reaction for 75Se-labelled compounds, we chose
the preparation of the [75Se]selenouronium salt 4a starting from 2 and
ethyl N-tert.-butoxycarbonyl-2-amino-4-bromobutyrate 3a, the precur-
sor for [75Se]selenomethionine, as model reaction (Scheme 2).
The formation of 4a was optimized with respect to the effects of
reaction time and solvent. Under otherwise fixed reaction conditions
(cf. Experimental) saturation yields were obtained in all solvents for 4a
after a reaction time of about 40 min. In dipolar aprotic solvents
(acetonitrile, N,N-dimethylformamide) the formation of 4a was nearly
quantitative related to 2, whereas the protic solvent ethanol proved not
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Scheme 2. Formation of the [75Se]selenouronium bromide 4a starting from 1,3-
dicyclohexyl[75Se]selenourea 2 and ethyl N-tert.-butoxycarbonyl-2-amino-4-
bromobutyrate 3a
as suitable since the radiochemical yield of 4a reached only about 80%
(related to 2).
In order to remove by-products and adducts from the [75Se]seleno-
uronium bromide formed, it was separated by silica gel column
chromatography with a recovery rate of about 90% (related to 2).
Under the described optimum reaction conditions further alkyl
bromides were chosen for the formation of [75Se]selenouronium salts in
order to demonstrate the versatility of the reaction sequence described
above. The alkyl bromides, benzylbromide, (1-bromoethyl)benzene and
1-bromo-3-propanolserved as modelcompounds to yiedl the corre-
sponding [75Se]selenouronium salts with a RCY of about 90% (related
to 2) after chromatographic purification.
Preparation of asymmetric [75Se]selenoethers
Treatment of selenouronium salts in the presence of hydroxide yields
the corresponding selenolates.26 Because of rapid autooxidation to
diselenides, selenolates are unstable and in situ alkylation to
selenoethers is recommended. Therefore, iodomethane was added to
the purified [75Se]selenouronium bromide in acetonitrile before adding
the base, so the intermediate 5 could be methylated in situ (Scheme 3).
Using tetrabutylammonium hydroxide (TBAH) as base for hydro-
lysis, 6 was formed in a radiochemical yield of nearly 90% (related to
4a) within 1 min using concentrations of at least 0.7 mol/l iodomethane
and 0.3 mol/l TBAH. In contrast, the use of potassium hydroxide or
pyrrolidine led to unidentified 75Se-labelled by-products. For demon-
stration of the versatility of the above described reaction sequence,
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Scheme 3. Formation of N-tert.-butoxycarbonyl[75Se]selenomethionine ethyl
ester 6 by hydrolysis of the [75Se]selenouronium bromide 4a and in situ
methylation of the [75Se]selenolate 5
Table 1. Radiochemical yields of asymmetric [75Se]selenoethers
Compound
RCY (%)
First/second alkylating agent
Benzylmethyl[75Se]selenide
Benzylbutyl[75Se]selenide
59
55
Benzylbromide/iodomethane
Benzylbromide/butylbromide
Benzylbromide/isopropylbromide
(1-Bromoethyl)benzene/iodomethane
Benzylisopropyl[75Se]selenide 10
1-Phenyl-1-(methyl[75Se]sele- 60
no)-ethane
N-tert.-butoxycarbonyl[75Se]- 59
selenomethionine ethyl ester
57
ethyl N-tert.-butoxycarbonyl-2-amino-4-
bromobutyrate/iodomethane
1-bromo-3-propanol/iodomethane
3-(Methyl[75Se]seleno)-1-
propanol
various asymmetric [75Se]selenoethers were prepared, starting from 2
and appropriate alkylating agents (cf. Table 1) and using the optimum
reaction conditions described above. Labelling of these compounds
resulted in radiochemical yields of 55–60% (related to [75Se]selenite)
with nearly quantitative yields for the two alkylation steps. Further
investigations with stronger alkylating agents, for example iodine
compounds, were therefore not carried out. The relatively low radio-
chemical yield of benzylisopropyl[75Se]selenide was probably caused by
the rapid elimination of isopropylbromide under the given reaction
conditions.
Preparation of [75Se]selenomethionine and 3-(methyl[75Se]seleno)-1-
propanyl p-toluenesulfonate
N-tert.-butoxycarbonyl[75Se]selenomethionine ethyl ester was hydro-
lysed by hydrochloric acid to liberate [75Se]selenomethionine. The
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amino acid was obtained with a maximum radiochemicalyield of about
70%, related to 6, and 41%, related to [75Se]SeO32ꢀ, at a reaction
temperature of 908C within 35 min, whereas at 708C a RCY of about
55% (related to 6) was achieved. Longer reaction times decreased the
RCY due to decomposition reactions occurring especially at the
temperature of 908C.
Preparation of 3-(methyl[75Se]seleno)-1-propanyl p-toluenesulfonate
as a small prosthetic group was studied for possible labelling of suitable
nucleophilic functional groups via [75Se]selenoalkylation. A radio-
chemicalyidel of about 90%, raelted to 3-(methy[ l
75Se]seleno)-1-
propanol, or 51%, related to [75Se]SeO23ꢀ, of this 75Se-labelled alkylating
agent was achieved at 08C in acetonitrile, starting with 3-(methyl[75Se]-
seleno)-1-propanol and toluene-4-sulfonyl chloride as tosylating agent.
Polymer-supported preparation of [75Se]selenoethers
With the aim of developing an attractive tool for rapid and automated
syntheses of radioselenoethers, we synthesized the polymer bound N-
cyclohexyl[75Se]selenourea-N0-methylpoyl styrene 8 which was obtained
by reaction of hydrogen [75Se]selenide 1 with N-cyclohexylcarbodiimide-
N0-methylpoylstyrene
7 (Scheme 4). Using the optimized reaction
conditions for the formation of 2 described above, about 55% (related
to [75Se]selenite) of the 75Se-activity was bound on the polymer.
The formation of 6 served again as a model reaction of the alkylation
procedure. N-cyclohexyl[75Se]selenourea-N0-methylpyosltyrene in
acetonitrile was converted with 3a to the corresponding selenouronium
salt. After hydrolysis and reaction with iodomethane, 6 was obtained
with a maximum RCY of 13 ꢂ 5% (related to [75Se]selenite), which is
low in comparison with 59% using non-polymer carbodiimide. Several
attempts to increase the RCY of 6 via the polymer-supported reaction
Scheme 4. Polymer-supported formation of N-cyclohexyl[75Se]selenourea-N0-
methyl polystyrene 8 by reaction of N-cyclohexylcarbodiimide-N0-methyl
polystyrene 7 with hydrogen [75Se]selenide 1
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[
75Se] SELENOETHERS VIA CARBODIIMIDES
599
pathway by change of solvents or variation of reaction time and
temperature remained unsuccessful.
This remarkable difference in the radiochemical yield of 6 could
possibly be explained by a hindered formation of polymer-bound 8.
Obviously, only a small part of the 55% of 75Se-activity detected in the
polymer existed as the appropriate [75Se]selenourea due to poor
permeability of the [75Se]H2Se dissolved in diethyl ether to the
polymer-supported carbodiimide. The larger amount of activity was
possibly bound via adsorption as 75Se0 after oxidation of 1 by some
surface reaction on the resin. Despite the low radiochemical yield of the
polymer-supported pathway, the use of a resin offers the advantage of
separating and purifying 75Se-labelled intermediates and products in a
very convenient way. Therefore, further investigations with different
polymer-bound carbodiimides and their reaction with 1 appear
attractive.
Conclusion
Various asymmetric carrier-added [75Se]selenoethers can be prepared
within a reaction time of 70–90 min in typicalradiochemicalyiedl s of
55–60% (related to [75Se]selenite) via 1,3-disubstituted [75Se]selenoureas
as intermediates. The advantage of the presented reaction sequence is
the possibility to synthesize primary and secondary [75Se]selenoethers
with different alkyl moieties. A previously described method for n.c.a.
75Se-labelling only led to methyl[75Se]selenoethers.11 Furthermore,
compared to earlier methods for c.a. 75Se-labelling not useful for
automated radiosyntheses, improvement of the polymer-supported
pathway appears very attractive for automation.
Due to the probable intermediate formation of diselenides during the
reduction of Se0 to Se2ꢀ, it is not possible to synthesize hydrogen
[75Se]selenide on the n.c.a. level under the applied conditions. The
amount of 10 mmolseel nium carrier has to be added at the beginning of
the radiosynthesis. Thus, the specific radioactivity of 75Se-labelled
products can only reach 370 GBq/mmol when starting with 10 mCi
[75Se]SeO23ꢀ. With this given isotopic dilution, results can directly be
transferred to labelling with 73Se.
However, investigations are under way to synthesize asymmetric
n.c.a. [73Se]selenoethers via [73Se]selenoureas starting from n.c.a. 73Se0.
This appears promising for radiopharmaceuticalresearch, because the
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600
T. BLUM ET AL.
sulphur-atom of various pharmaceutically interesting thioethers (e.g.
homocysteine thiolactone, thio fatty acids) can be substituted by the
positron-emitter selenium-73 in order to obtain longer lived n.c.a. PET-
radiotracers suitable for measurement of slow (patho)physiological
processes.
Acknowledgements
The authors thank Dr M. Schfer (Institut fr Organische Chemie,
Universitt zu Kln) for recording the high-resolution mass spectra, G.
Blessing for preparation of n.c.a. selenium-75 and Dr K. Hamacher for
usefuldiscussions (both FZ-Jilch).
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