SiFA-modified phenylalanine: A key compound for the efficient synthesis of 18F-labelled peptides

Article abstract of DOI:10.1002/ejic.201100142

Both the racemic and the stereoselective synthesis of the silicon-modified amino acid p-(tBu₂FSi)C₆H₄CH ₂(NH₂)COOH (SiFA-phenylalanine, 6) and its derivatives p-(tBu₂FSi)C₆H₄CH₂(NHBoc)COOH (10) and p-(tBu₂FSi)C₆H₄CH₂(NHFmoc)COOH (11) are reported. The latter two compounds are valuable building blocks for the convenient introduction of silicon-based fluoride acceptors (SiFAs) into peptides by solid-phase peptide syntheses (SPPS). As prove of principle, the Tyr³-octreotate derivatives 12, 13 and 14 were prepared by using standard protocols of the solid-phase peptide synthesis (SPPS). They were labelled with ¹⁸F-fluorine by isotopic exchange in radiochemical yields of up to 70 % (with radiochemical purity ranging between 92 and 99 %). The incorporation of ¹⁸F-fluorine into the SiFA-modified amino acid 6 was studied as the latter may also serve as a tracer in PET. After purification, [¹⁸F]-6 showed a remarkable stability in an isotonic solution. Also reported is the molecular structure, as determined by single-crystal X-ray diffraction analysis, of p-(tBu₂FSi)C ₆H₄CHCH[C(O)OMe]NHC(O)OCH₂Ph (8), which serves as a precursor for the stereoselective synthesis of compound 6.

Full text of DOI:10.1002/ejic.201100142

FULL PAPER
DOI: 10.1002/ejic.201100142
SiFA-Modified Phenylalanine: A Key Compound for the Efficient Synthesis of
18F-Labelled Peptides[‡]
Ljuba Iovkova,[a] Daniel Könning,[a] Björn Wängler,*[b] Ralf Schirrmacher,[c]
Sebastian Schoof,[d,e] Hans-Dieter Arndt,[d,e] and Klaus Jurkschat*[a]
Dedicated to Prof. Dr. Peter Klüfers on the occasion of his 60th birthday
Keywords: Fluorine / Silicon / Radiopharmaceuticals / Synthesis design / Enantioselectivity / Amino acids
Both the racemic and the stereoselective synthesis of the sili-
con-modified amino acid p-(tBu2FSi)C6H4CH2(NH2)COOH
(SiFA-phenylalanine, 6) and its derivatives p-(tBu2FSi)C6H4-
by isotopic exchange in radiochemical yields of up to 70%
(with radiochemical purity ranging between 92 and 99%).
The incorporation of 18F-fluorine into the SiFA-modified
amino acid 6 was studied as the latter may also serve as a
tracer in PET. After purification, [18F]-6 showed a remarkable
stability in an isotonic solution. Also reported is the molecular
CH2(NHBoc)COOH
(10)
and
p-(tBu2FSi)C6H4CH2-
(NHFmoc)COOH (11) are reported. The latter two com-
pounds are valuable building blocks for the convenient intro-
duction of silicon-based fluoride acceptors (SiFAs) into pep- structure, as determined by single-crystal X-ray diffraction
tides by solid-phase peptide syntheses (SPPS). As prove of
principle, the Tyr3-octreotate derivatives 12, 13 and 14 were
prepared by using standard protocols of the solid-phase pep-
tide synthesis (SPPS). They were labelled with 18F-fluorine
analysis,
of
p-(tBu2FSi)C6H4CHCH[C(O)OMe]NHC(O)-
OCH2Ph (8), which serves as a precursor for the stereoselec-
tive synthesis of compound 6.
cancer of various origin has gained widespread interest in
nuclear medicine.[3a–3c] At the same time, there is a great
interest in the synthesis and application of non-proteinog-
enic, organosilicon-modified amino acids that hold poten-
tial in the diagnostic and therapeutic medicine.[3d–3g] The
large number of methods developed for the 18F-labelling of
peptides and proteins still suffer from drawbacks. Multistep
synthetic pathways, the need to use HPLC or distillation
for purification of the 18F-synthon for final peptide cou-
pling, or time-consuming workup often reduce the final
preparative yields. Recently, a direct labelling protocol was
reported for the introduction of 18F into selected biomole-
cules. However, this approach lacks general applicability to
more temperature-sensitive biomolecules.[2d]
Introduction
18F-fluorine is the most commonly used radionuclide for
positron emission tomography (PET).[1] This noninvasive
imaging modality is an important diagnostic tool in medi-
cine because of its ability to locate and assess abnormalities
in neurology, oncology and cardiology.[2a–2c] It enables the
visualization and quantification of complex biochemical
processes and allows the investigation of the in vivo distri-
bution of radiolabelled biomarkers. The application of
radiolabelled amino acids and peptides for the detection of
[‡] This work contains part of the Ph.D. theses of Ljuba Iovkova,
TU Dortmund 2010, and Sebastian Schoof (synthesis of com-
pound 12), TU Dortmund 2010. Parts of it were first presented
as a poster at the 5th European Silicon Days in Vienna, Austria,
September 20–22, 2009, Book of Abstracts p. 133.
[a] Lehrstuhl für Anorganische Chemie II, Technische Universität
Dortmund,
Otto-Hahn-Str. 6, 44221 Dortmund, Germany
[b] Klinik und Poliklinik für Nuklearmedizin, Ludwig-
Maximilians-Universität München,
Therefore, new 18F-acceptors bearing functionalities that
allow the facile conjugation to biomolecules under various
conditions are highly sought after. Because of the short half
life of 18F (110 min), the ideal acceptor should allow the
fast introduction of the radiolabel under mild labelling con-
ditions. Recently, Perrin and co-workers published the car-
rier-added 18F-labelling of trialkoxysilanes as potential
labelling precursors for PET.[4a] In previous publications,[5,6]
we have demonstrated (i) di-tert-butylphenylfluorosilane
(tBu2PhSiF) to exhibit ideal 19F–18F exchange with the 18F-
source and (ii) the resulting 18F-labelled tBu2PhSiF to be
inert against hydrolysis under physiological conditions.[6]
Consequently, the functionalized triorganofluorosilanes
81377 München, Germany
[c] McConnell Brain Imaging Centre, Montreal Neurological
Institute, McGill University,
3801 University Street, Montreal, QC, Canada
[d] Lehrbereich Chemische Biologie, Technische Universität
Dortmund
Dortmund, Germany
[e] Department of Chemical Biology, Max Planck Institute of
Molecular Physiology,
Otto-Hahn-Str. 11, 44227 Dortmund, Germany
2238
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2238–2246

SiFA-Modified Phenylalanine: Efficient Synthesis of 18F-Labelled Peptides
tBu2[p-XC6H4]SiF were prepared as a silicon-based fluorine
acceptor (SiFA) and linked, through the functionality X, to
biomolecules; these were then successfully applied for evalu-
ation in preclinical studies.[7] More recently, we and others
described the in vivo application of tumour-affine peptides
radiolabelled by the SiFA-approach in tumour-bearing
mice.[2d,4d] In continuation of these studies, we report here
the syntheses of SiFA-phenylalanine, the first SiFA-modi-
fied amino acid. Its applicability in solid-phase peptide syn-
theses is demonstrated by incorporation into the peptide
sequence of Tyr3-octreotate, which is a lead compound for
radiotracers in cancer diagnostics.[8a,8b] Furthermore, we
Scheme 2. Preparation of the SiFA-phenylalanine. PCC: pyrid-
iniumchlorochromate.
demonstrate the isotopic exchange of 19F for 18F for the
free amino acid and – as a paradigm for its use in in vivo
imaging – for the synthesized octreotate derivatives.
The SiFA-aldehyde 4 was dissolved in methanol and
treated with an aqueous solution of ammonium chloride
and sodium cyanide to give the amino acid 6 (Scheme 2).
After hydrolysis of the in situ formed cyanohydrine, we un-
fortunately isolated only the methyl ester of the amino acid.
This ester 5 was cleaved by using chymotrypsine,[11] and the
free (S)-SiFA-modified amino acid (S)-6 was obtained in
16% yield with respect to rac-5 (Scheme 2).
Results and Discussion
Strecker Amino Acid Synthesis of SiFA-Phenylalanine
The Strecker amino acid synthesis is a well-established
method for the preparation of natural and synthetic amino
acids starting from an aldehyde or a ketone.[9,10] The alde-
hyde reacts in a one-pot reaction with ammonia (ammo-
nium chloride) and cyanide (potassium or sodium cyanide)
to form an α-imino nitrile, which is subsequently hydrolyzed
with hydrochloric acid to give the free amino acid. A major
drawback is the use of aqueous reaction medium and the
very poor solubility of many organic aldehydes in water. In
order to evade this problem, alcohols are often added to
the reaction mixture.
The bromo-substituted phenyl acetic acid 1 was con-
verted into compound 2 in two steps. Treatment of the pro-
tected p-bromobenzene derivative 2 with tert-butyllithium
and di-tert-butyldifluorosilane at –78 °C provided the corre-
sponding triorganofluorosilane. Deprotection under acidic
conditions gave the SiFA-modified alcohol 3 in 90% yield
(Scheme 1).
Stereoselective Synthesis of SiFA-Phenylalanine
Because of the low solubility of the alcohol 3 in water
and the low yield of enantiomerically pure amino acid, we
investigated an enantioselective synthesis.[12] The known
SiFA-aldehyde was prepared in good yield as described in
our previous works[5,7] and treated with tetramethylguan-
idine and methyl-2-(benzyloxycarbonylamino)-2-(dimeth-
oxy-phosphoryl)acetate in a Horner–Wadsworth–Emmons
reaction to give compound 8 as a colourless solid
(Scheme 3).
Scheme 3. Synthesis of the amino acid pecursor 8 and enantioselec-
tive synthesis of the SiFA-phenylalanine.
Scheme 1. Synthesis of the SiFA-2-phenylethanol derivative 3 and
its reaction with Swern reagent.
Single crystals of 8 suitable for X-ray diffraction analysis
Oxidation of alcohol 3 with Swern reagent to give the were obtained by recrystallization from n-hexane. The mo-
corresponding aldehyde p-(tBu2FSi)C6H4CH2CHO (4) lecular structure of compound 8 is shown in Figure 1. Se-
failed. Instead, the alkene derivative 4a was produced lected geometric parameters are given in the figure caption.
(Scheme 1). Therefore, PCC was used to effect the necessary
In the solid state, the silicon atom is tetra-coordinated
oxidation of alcohol 3 to aldehyde 4, in good yield and shows distorted tetrahedral configuration with angles
(Scheme 2).
ranging between 116.0(3) and 103.8(3)°. The Si(1)–F(1) dis-
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Wängler, K. Jurkschat et al.
FULL PAPER
Figure 1. Molecular structure of compound 8. Atomic displacement parameters are drawn at the 30% probability level. Selected bond
lengths [Å] and angles [°]: Si(1)–F(1) 1.614(4), Si(1)–C(1) 1.886(4), Si(1)–C(5) 1.884(7), Si(1)–C(9) 1.974(4); F(1)–Si(1)–C(1) 113.4(2),
F(1)–Si(1)–C(5) 103.8(3), F(1)–Si(1)–C(9) 111.6(2), C(1)–Si(1)–C(5) 116.0(3), C(1)–Si(1)–C(9) 105.9(2), C(5)–Si(1)–C(9) 106.0(2).
tance [1.614(4) Å] is rather similar to those in other SiFA- manually to save material (4–5 equiv. are routinely applied
modified compounds.[7] A noteworthy feature is the asym- in an automated synthesis). Cleavage, final deprotection
metric intramolecular N(17)-H···O (29) hydrogen bond that and HPLC-purification of the 10-mer peptide gave the
links two molecules of 8 into a dimer. Such hydrogen bonds labelling precursor 12.
are common for solid-state structures of p-silylarylcarbox-
Likewise, the Tyr3-octreotate derivatives 13 and 14 (Fig-
ure 2) were synthesized on solid supports by using standard
ylic acids and their derivatives.[7]
Hydrogenation of compound 8 in the presence of a chiral 9-fluorenylmethoxycarbonyl (Fmoc) group chemistry pro-
RhI catalyst gave the protected amino acid 9 with an ee tocols.[13] The amino acid d-phenylalanine[1] was replaced
of 98% for the S- and 99% for the R isomer, respectively. by the SiFA-modified R-configured amino acid. A polyeth-
Deprotection took place in concentrated hydrobromic acid ylene glycol linker and aspartic acid were added on the N
solution over a period of 15 min (Scheme 3).
terminus of the peptides, in order to improve the solubility
The SiFA-modified amino acid was treated with Boc2O of the labelling precursors 13 and 14 in water. By using
and Fmoc-Cl under mild conditions in order to introduce
protecting groups that are suitable for the solid-phase pep-
tide synthesis (SPPS) (Scheme 4).
Scheme 4. Introduction of protecting groups for the solid-phase
peptide synthesis.
Solid-Phase Peptide Synthesis (SPPS) with SiFA-Modified
Phenylalanine
The 9-mer precursor peptide was synthesized by auto-
mated solid-phase peptide synthesis by standard 9-fluoren-
ylmethoxycarbonyl (Fmoc) group chemistry protocols. The
resin-bound 9-mer peptide was manually Fmoc-depro-
tected, and the final amino acid (S-SiFA-Phe) was coupled Figure 2. The SiFA-modified Tyr3-octreotate derivatives 12–14.
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Eur. J. Inorg. Chem. 2011, 2238–2246

SiFA-Modified Phenylalanine: Efficient Synthesis of 18F-Labelled Peptides
PEG linkers with different lengths, the hydrophilic proper- target molecule is stable enough to withstand fluoride ion
ties of the peptides should be tailored.
treatment at elevated temperatures for a short period of
time.
Radiolabelling
Conclusions
Previous SiFA-labelling procedures with 18F were mainly
based on the synthesis of a small 18F-label-containing rea-
gent, which is then used to derivatize a peptide or protein
of interest. In order to simplify such a modification, we
have directly introduced a SiFA-modified amino acid 10
into the peptide chain. Thereby, we intended to apply a
more simple and convenient method as described in ref.[4d]
for 18F-radiolabelling of the target peptides, which are func-
tionalized with a fluoride-acceptor already during SPPS.
Such an approach should be flexible and applicable to a
wide range of potential targets, which can easily be purified
before radiolabelling is conducted by a clean 18F/19F ex-
change reaction on the SiFA group, by using easily available
K18F as 18F source. In this fashion, elaborate processing
and purification of the hot 18F-labelled target peptide by
HPLC or other means could be avoided.
In this manuscript we have demonstrated that the di-tert-
butylfluoridosilyl-substituted phenylalanine, p-(tBu2FSi)-
C6H4CH2(NH2)COOH (6), can easily be synthesized both
as a racemic mixture and enantioselectively. Notably, the
SiFA-moiety in compound 6 tolerates the harsh conditions
of standard solid-phase peptide synthesis and simplifies the
preparation of the SiFA-modified octreotate derivatives 12–
14. This strategy for the synthesis of such peptides is an
alternative to the previously reported route according to
which the SiFA-moiety was introduced into amino-oxy pre-
functionalized peptides by using SiFA-aldehyde in a click-
chemistry-type reaction.[5]
The fact that both the amino acid derivative 6 and the
peptides 12–14 can easily be labelled with 18F, that the lab-
elled compounds are stable under physiological conditions
and that, very likely, other SiFA-substituted amino acids
can also be prepared opens new opportunities for the devel-
opment of SiFA-based 18F-labelled radiopharmaceutical
peptides, and potentially proteins too.
To demonstrate the feasibility of this approach, the label-
ling procedure was carried out with 3–5 GBq azeotropically
dried 18F– in 500 μL dimethylsulfoxide (DMSO) or acetoni-
trile at room temperature for 5 min.
In accordance with the amount of labelling precursor
used, the incorporation of 18F-fluorine into SiFA-Phe
reached levels of 40–60%, as determined by radio-HLPC.
The calculated specific activity for [18F]SiFA-Phe was be-
tween 11–31 GBq/μmol (298–919 Ci/mmol). After quick so-
lid-phase extraction (Waters SepPak C18) for purification,
the radiolabelled target [18F]SiFA-Phe was released into iso-
tonic NaCl solution and filtered sterile for direct injection.
According to HPLC analysis, the chemical- and radiochem-
ical purity was between 92 and 99% in all cases. The injec-
tion solution obtained in such a manner was stable over a
period of 45 min in physiological buffer. Hence, our pro-
cedure should be suitable for application in vivo. In a sim-
ilar fashion, isotopic exchange experiments with labelling
precursors 12, 13 and 14 provided the 18F-labelled peptides
in radiochemical yields ranging from 34 (12) to 70% (13).
In order to further simplify the labelling procedure, we
also investigated the labelling step with 18F-fluoride ions in
aqueous medium. No reaction was detected at room tem-
perature even after a reaction time of 15 min. After heating
the reaction mixture to 90 °C, a radiochemical yield of 10%
Experimental Section
General Methods: All solvents used for the synthesis of 6 were puri-
fied by distillation under argon from appropriate drying agents.
Solvents and chemicals used in the labelling experiments were pur-
chased in the highest available grade and used without further puri-
fication. The NMR experiments were carried out with Bruker
DRX 400, Bruker DRX 300 and Varian Mercury 200 spectrome-
ters. The chemical shifts δ are referenced to the solvent peaks with
the usual values calibrated against tetramethylsilane (1H, 13C, 29Si)
and CFCl3 (19F). The high resolution mass spectra were obtained
with a LTQ Orbitrap mass spectrometer (Thermo Electron) with
acetonitrile as the mobile phase. The acetonitrile solutions were
injected by a TriPlus Autosampler onto a DFS-system (Perfluoro-
kerosene as reference), connected with a Trace GC Ultra 2000 sys-
tem [column: DB-5MS (25 m, 0.25 mm ID, film 0.1 μm)]. FT infra-
red spectra were recorded with a Bruker IFS28 spectrometer. Ele-
mental analyses were performed on a LECO-CHNS-932 analyzer.
Crystallography: Crystals of compound 8 suitable for single-crystal
X-ray diffraction analysis were grown by recrystallization from n-
hexane. Crystallographic data are summarized in Table 1. Intensity
was observed after 5 min, and after 15 min, 27% was data were collected with a Bruker AKS single-crystal Smart apex
CCD diffractometer with graphite-monochromated Mo-Kα radia-
tion. The data collections covered almost the whole sphere of the
reciprocal space with 5 sets at different κ angles and 1125 frames
by ω-rotation (Δ/ω = 0.5°) at per frame. Crystal decay was moni-
tored by repeating the initial frames at the end of the data collec-
tion. After analysis of the duplicate reflections, there was no indica-
tion of any decay. The structure was solved by direct methods
(SHELXS97[14]). Refinement applied full-matrix least-squares
methods (SHELXL97[15]). All H atoms were located in the differ-
ence Fourier map, and their positions were isotropically refined
achieved. Further heating did not improve the radiochemi-
cal yields, but several radioactive and nonradioactive decay
products began to form. While certainly more experimenta-
tion must be carried out along these lines, these data show
that the 18F-SiFA radiolabel can be expected to be stable for
diagnostic applications at moderate temperatures in vivo.
Furthermore, they demonstrate that labelling of a SiFA-car-
rying biomolecule target with 18F fluoride ion by thermody-
namically driven 18F/19F exchange up to diagnostically use-
ful levels is feasible in the absence of any catalyst, if the with Uiso constrained at 1.2 times Ueq of the carrier C atom for
Eur. J. Inorg. Chem. 2011, 2238–2246
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B. Wängler, K. Jurkschat et al.
FULL PAPER
non-methyl and 1.5 times Ueq of the carrier C atom for methyl
groups. Atomic scattering factors for neutral atoms and real and
imaginary dispersion terms were taken from International Tables
for X-ray Crystallography.[16] The figure was created by
SHELXTL.[17] CCDC-793484 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
25 mmol) was dissolved in CH2Cl2 and dropped into the reaction
mixture. After stirring the mixture for 22 h at ambient temperature,
it was washed with water (120 mL) and brine (120 mL). The water
phases were extracted with CH2Cl2 (30 mL), the combined organic
phases were dried with MgSO4, and the solvent was evaporated in
vacuo to yield 2 (6.19 g, 20 mmol, 93%) as a colorless liquid. 1H
3
NMR (400.63 MHz, CDCl3): δ = 7.36 (d, JH-H = 8 Hz, 2 H, Hm),
3
3
7.05 (d, JH-H = 8 Hz, 2 H, Ho), 3.75 (t, JH-H = 7 Hz, 2 H, CH2),
3
2.73 (t, JH-H = 7 Hz, 2 H, CH2), 0.83 [s, 9 H, SiC(CH3)3], –0.05
Table 1. Crystallographic data for 8.
[s, 6 H, Si(CH3)2] ppm.
8
2-[4-(Di-tert-butylfluorosilanyl)phenyl]ethanol (3): To a solution of
2 (2.9 g, 9.2 mmol) in diethyl ether (15 mL) tert-butyllithium (1.5 m
in pentane, 12.9 mL, 19.3 mmol, 2.1 equiv.) was slowly added at
–78 °C, and the solution was stirred for another 15 min at this tem-
perature.[21] Di-tert-butyldifluorosilane (1.99 g, 10.1 mmol,
1.2 equiv.) was added, and the solution was stirred for an additional
22 h, during which it was warmed to room temperature. The reac-
tion mixture was hydrolyzed with brine (100 mL). The organic
phase was washed with water (50 mL) and brine (100 mL), and the
combined aqueous phases were extracted with diethyl ether
(3ϫ50 mL). The organic phases were combined, dried with MgSO4
and filtered. The solvent was evaporated to yield 1-[2-(tert-butyl-
dimethylsilanyloxy)ethyl]-4-(di-tert-butylfluorosilanyl)benzene
(3.26 g, 8.2 mmol, 90%), which was used in the next step without
further purification. 1H NMR (200.13 MHz, CDCl3): δ 7.50 (d,
Empirical formula
Formula mass [g/mol]
Crystal system
Crystal size [mm]
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C26H34FNO4Si
471.63
triclinic
0.38ϫ0.35ϫ0.15
P1
7.8992(8)
9.2159(10)
19.301(2)
83.149(3)
79.017(3)
72.545(3)
1312.9(2)
2
¯
γ [°]
V [Å3]
Z
ρcalcd.[mg/m3]
µ [mm–1]
F(000)
1.193
0.127
504
3
3JH-H = 8 Hz, 2 H, Hm), 7.20 (d, JH-H = 8 Hz, 2 H, Ho), 3.81 (t,
θ range [°]
Index ranges
2.15–27.79
–10 Յ h Յ 10
–12 Յ k Յ 12
–25 Յ l Յ 25
15474
99.6
6199/0.038
3845
333
1.028
0.0646
0.2040
3
3JH-H = 7 Hz, 2 H, CH2-O), 2.81 (t, JH-H = 7 Hz, 2 H, Ph-CH2),
4
1.04 [d, JH-F = 1 Hz, 18 H, SiC(CH3)3], 0.82 [s, 9 H, C(CH3)3],
–0.07 [s, 6 H, Si(CH3)2] ppm. 19F{1H} (188.28 MHz, CDCl3): δ =
–189.53 ppm. In a 25-mL one-necked round-bottom flask 1-[2-
(tert-butyldimethylsilanyloxy)ethyl]-4-(di-tert-butylfluorosilanyl)-
benzene (1 g, 2.5 mmol, 1.0 equiv.) was dissolved in methanol
(8 mL).[22] To this magnetically stirred solution, N-iodosuccinimide
(NIS, 30 mg, 0.1 mmol, 0.2 equiv.) was added, and the reaction
mixture was stirred for another 24 h at room temperature. The sol-
vent was evaporated, and the solid material thus obtained was
washed several times with hexane in order to remove residues of
iodine. The resulting residue was dissolved in diethyl ether/hexane
(3:1) and purified by flash chromatography (SiO2, diethyl ether/
hexanes = 3:1) to yield 3 (0.440 mg, 1.6 mmol, 64%) as a white
solid (m.p. 53 °C). 1H NMR (500.13 MHz, CDCl3): δ = 7.53 (d,
No. of reflections collected
Completeness of θmax [%]
No. of independent reflections/Rint.
No. of reflections observed with [IϾ2σ(I)]
No. of refined parameters
GoF(F2)
R1(F) [IϾ2σ(I)]
wR2(F2) (all data)
(Δ/σ)max
0.000
0.449/–0.275
Largest difference peak/hole [eÅ–3]
Synthetic Route I
3
3JH-H = 8 Hz, 2 H, Hm), 7.22 (d, JH-H = 8 Hz, 2 H, Ho), 3.88 (dt,
[2-(4-Bromophenyl)ethoxy]-tert-butyldimethylsilane (2): In a 500-
mL three-necked round-bottomed flask LiAlH4 (2.21 g, 5.8 mmol)
was suspended in diethyl ether (70 mL).[18] 2-(4-Bromophenyl)-
acetic acid (10 g, 4.7 mmol) was dissolved in diethyl ether (80 mL)
and slowly dropped into the suspension. After the suspension was
stirred for 15 min at room temperature, the excess LiAlH4 was hy-
drolyzed with water. Sulfuric acid (10% aq., 70 mL) was added to
dissolve the precipitate, and a clear solution resulted. The organic
phase was washed with water (80 mL) and brine (80 mL), and the
combined water phases were extracted with diethyl ether
(3ϫ50 mL). The organic phases were combined, dried with MgSO4
and filtered, and the solvent was evaporated in vacuo to yield 2-(4-
bromophenyl)ethanol (8.71 g, 43 mmol, 92%) as a colourless li-
3
3JH-H = 6 Hz, 3JH-H = 7 Hz, 2 H, CH2-OH), 2.88 (t, JH-H = 7 Hz,
3
4
2 H, Ph-CH2), 1.38 (t, JH-H = 6 Hz, 1 H, OH), 1.05 [d, JH-F
=
1 Hz, 18 H, SiC(CH3)3] ppm. 13C{1H} (100.13 MHz, CDCl3): δ =
142.3 (s, Ci), 134.2 [d, 3J(13C–19F) = 4 Hz, Cm], 129.6 [d,2J(13C–
19F) = 14 Hz, Cp], 128.2 (s, Co), 63.4 (s, Ph-CH2), 39.2 (s, CH2-
OH), 27.3 (s, CH3), 20.1 [d, J(13C–19F) = 12 Hz, CCH3] ppm. 19F
2
(188.28 MHz, CDCl3): δ = –189.4 (s) ppm. C16H27FOSi (282.47 g/
mol): calcd. C 68.0, H 9.6; found C 68.1, H 9.5. HR-GC–MS:
calcd. for C16H27OF28Si 282.1810; found 282.1810.
Transformation of Alcohol 3 with Swern Reagent: A solution of the
alcohol 3 (0.28 g, 1 mmol) in CH2Cl2 (10 mL) was added slowly at
–60 °C to a freshly prepared solution of the oxidizing reagent
1
3
quid. H NMR (400.13 MHz, CDCl3): δ = 7.39 (d, JH-H = 8 Hz, [1 mL, 11 mmol oxalylchloride (COCl)2; 1.7 mL, 22 mmol DMSO,
2 H, Hm), 7.05 (d, JH-H = 8 Hz, 2 H, Ho), 3.75 (t, JH-H = 7 Hz, 25 mL CH2Cl2]. The reaction mixture was stirred for 15 min at this
3
3
3
2 H, CH2-OH), 2.75 (t, JH-H = 7 Hz, 2 H, Ph-CH2), 2.35 (s, 1 H,
temperature during which the solution became cloudy. The reaction
was terminated by the addition of NEt3. By doing so, the solution
OH) ppm. 13C{1H} (100.63 MHz, CDCl3): δ 137.5 (s, Ci) 131.4 (s,
Cm) 130.6 (s, Cp) 120.1 (s, Cm), 63.1 (s, Ph-CH2), 38.3 (s, CH2OH) became clear. After the reaction mixture had been warmed to room
ppm. In a 250-mL two-necked round-bottomed flask 2-(4-bro-
mophenyl)ethanol (4.26 g, 22 mmol) and imidazole (1.73 g,
25 mmol) were dissolved in CH2Cl2 (40 mL) and stirred for 15 min
at room temperature.[19,20] tert-Butylchlorodimethylsilane (3.83 g,
temperature, water (100 mL) was added. The phases were sepa-
rated, and the water phase was extracted with CH2Cl2 (3ϫ50 mL).
The combined organic layers were subsequently washed with a
solution of HCl (2 m, 50 mL) and a saturated NaHCO3 solution
2242
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2238–2246

SiFA-Modified Phenylalanine: Efficient Synthesis of 18F-Labelled Peptides
(50 mL), dried with MgSO4 and filtered. The solvent of the filtrate
was removed in vacuo to give an oily residue. The latter was puri-
fied by chromatography (SiO2, hexane/diethyl ether, 1:1) to give
and the enzyme was removed by centrifugation. The remaining
solution was concentrated and purified on RP-silica gel [CH3CN/
H2O (4:1)]. After evaporation of the solvents, compound 6 (6.5 mg,
19 μmol, 32%) was obtained as a white amorphous solid (m.p. 196–
1
compound 4a as yellow oil (0.29 g, 82%). H NMR (599.83 MHz,
C6D6): δ = 8.71 (s, 1 H, COH), 7.54 [d, 3J(1H-1H) = 7 Hz, 2 H, 198 °C). H NMR (500.13 MHz, [D4]MeOH): δ = 7.58 [d, J(1H-
1
3
Hm], 7.43 [d, 3J(1H-1H) = 7 Hz, 2 H, Ho], 0.98 {s, 18 H, Si[C-
(CH3)3]2} ppm. 13C{1H} (125.77 MHz, C6D6): δ = 182.7 [s, C(Cl)-
OH], 136.2 (s, Ci), 134.4 [d, 3J(13C-19F) = 4 Hz, Cm], 127.8 [d,
1H) = 8 Hz, 2 H, Hm], 7.37 [d, 3J(1H-1H) = 8 Hz, 2 H, Ho], 3.70
[dd, 3J(Hα-Herythro) = 5 Hz, 3J(Hα-Hthreo) = 9 Hz, 1 H, CH2-
CH(NH2)], 3.27 [dd, 3J(Herythro–Hα) = 5 Hz, 2J(Herythro–Hthreo) =
2J(13C-19F) = 14 Hz, Cp], 126.4 (s, Co), 125.4 (s, PhCCl), 27.3 {s, 14 Hz, 1 H, CH2-CH(NH2), Herythro], 2.94 [dd, 3J(Hthreo–Hα) =
Si[C(CH3)3]2}, 20.3 {d, 2J(13C-19F) = 12 Hz, Si[C(CH3)3]2} ppm.
19F (188.28 MHz, [D6]DMSO): δ = –190.2 [s, 1J(19F-29Si) =
298 Hz]. LR-GC-EI-MS: calcd. for ([C16H23NFSiCl2]+ 349.3;
found 349.3.
9 Hz, 2J(Hthreo–Herythro) = 14 Hz, 1 H, CH2-CH(NH2), Hthreo], 1.05
{s, 18 H, Si[C(CH3)3]2} ppm. 13C{1H} (125.77 MHz, [D4]MeOH):
δ = 176.7 (s, COOH), 140.3 (s, Ci), 135.5 [d, 3J(13C-19F) = 4 Hz,
Cm], 133.1 [d, 2J(13C-19F) = 14 Hz, Cp], 129.9 (s, Co), 57.9 (s, CHN),
2
40.1 (s, CH2), 27.9 {s, Si[C(CH3)3]2}, 21.1 {d, J(13C-19F) = 12 Hz,
[4-(Di-tert-butylfluorosilanyl)phenyl]acetaldehyde (4): To a magneti-
cally stirred suspension of pyridinium chlorochromate (PCC,
8.05 g, 37 mmol, 3.7 equiv.) in CH2Cl2 (300 mL)[12b] was added
dropwise a solution of 5 (2.93 g, 10 mmol) in CH2Cl2 (250 mL) at
0 °C. The reaction mixture was stirred for an additional 3 h and
warmed to room temperature. The upper solution was decanted
from the gumlike residue, the residue was washed with diethyl ether
(3ϫ100 mL), and the solvent of the combined organic phases was
evaporated to yield 4 (1.75 g, 6 mmol, 60%) as a yellowish oil,
Si[C(CH3)3]2} ppm. 19F (188.28 MHz, [D4]MeOH): δ = –192.4 [s,
1J(19F-29Si)
=
296 Hz] ppm. HR-LC-ESI-MS: calcd. for
[C17H28O2NF28Si+H]+ 326.1945; found 326.1946. [α]2D0 (ethanol):
–21.82°.
Synthetic Route II
Methyl Z-2-Benzyloxycarbonylamino-3-[4-(di-tert-butyl-fluoro-sil-
anyl)-phenyl]-acrylate (8): To a –78 °C cooled solution of 2-(benzyl-
oxycarbonylamino)-2-(dimethoxyphosphoryl)acetate
(3.63 g,
1
which solidified on standing. H NMR (300.13 MHz, CDCl3): δ =
10.9 mmol) in THF (50 mL) was added tetramethylguanidine
(1.79 mL, 14.1 mmol), and the reaction mixture was stirred for
30 min. A solution of the SiFA-aldehyde 7 (2.89 g, 10.9 mmol) in
THF (10 mL) was added dropwise, and the reaction mixture was
stirred for 1 h at –78 °C and for an additional 4 h at room tempera-
ture. The reaction mixture was diluted with ethyl acetate and
washed with 1 m HCl, 1 m CuSO4 and saturated NaHCO3 solution.
After drying the organic layer over MgSO4, it was filtered, and the
solvent of the filtrate was evaporated in vacuo. The crude product
thus obtained was purified by column chromatography on silica gel
9.75 (t, 3JH-H = 2 Hz, 1 H, CHO), 7.60 (d, 3JH-H = 8 Hz, 2 H, Hm),
3
3
7.23 (d, JH-H = 8 Hz, 2 H, Ho), 3.68 (d, JH-H = 2 Hz, 2 H, CH2-
CHO), 1.05 [d, JH-F = 1 Hz, 18 H, SiC(CH3)3] ppm. 13C{1H}
4
(100.13 MHz, CDCl3): δ = 189.3 (s, CHO), 138.7 (s, Ci), 135.2 [d,
2
3J(13C-19F) = 4 Hz, Cm], 130.9 [d, J(13C-19F) = 14 Hz, Cp], 128.8
(s, Co), 27.2 (s, CH3), 20.2 [d, 2J(13C-19F) = 12 Hz, CCH3] ppm.
19F (188.28 MHz, CDCl3): δ = –189.9 (s) ppm. HR-GC–MS: calcd.
for C16H25OF28Si 280.1659; found 280.1651. C16H25FOSi
(280.47 g/mol): calcd. C 68.5, H 9.0; found C 68.7, H 9.2.
Methyl rac-2-Amino-3-[4-(di-tert-butyl-fluoro-silanyl)-phenyl]-prop- (hexane/diethyl ether, 3:1) to give 8 (3.29 g, 7.0 mmol, 64%) as a
ionate (5): In a 10-mL glass vessel with a Teflon tap, a saturated
aqueous solution of ammonium chloride (0.25 mL), a concentrated
solution of ammonia (0.27 mL) and sodium cyanide (72 mg in
0.14 mL water) were added.[10] The solution was cooled to 0 °C,
and a methanol solution of 4 (370 mg, 1.3 mmol, 0.27 mL) was
added dropwise. The reaction mixture was stirred for an additional
2 h at room temperature. A concentrated aqueous solution of HCl
(4 mL) was added, and the reaction mixture was heated at reflux
for an additional 12 h. The solvent was evaporated, and the residue
was purified by flash chromatography (SiO2, diethyl ether/hexane
= 1:1, then Et2O, elution of the product with MeOH) to yield race-
colorless oil, which solidified (mp. 34 °C). 1H NMR (400 MHz,
3
C6D6): δ = 7.60 (d, JH,H = 8.0 Hz, 2 H, Ho), 7.41 (s, 1 H, CH),
3
7.45 (d, JH,H = 7.6 Hz, 2 H, Hm), 7.16 (m, 5 H, Hphenyl), 6.24 (s,
1 H, NH), 5.06 (s, 2 H, O-CH2), 3.44 (s, 3 H, COOCH3), 1.10 [s, 18
H, 2ϫSiC(CH3)3] ppm. 13C{1H} (100.63 MHz, C6D6): δ = 165.4 (s,
COOMe), 154.0 (s, NCOO), 136.5 (s, Ci), 135.2 (s, Ci), 134.2 [d,
2
3J(13C-19F) = 4 Hz, Cm], 131.4 [d, J(13C-19F) = 14 Hz, Cp], 129.1
(s, Cm), 128.4 (s, Cp), 128.3 (s, Co), 128.0 (s, Co), 127.6 (s,
HC=CNH), 125.7 (s, HC=CNH), 67.2 (s, PhCH2O), 51.9 (s,
OCH3), 27.2 {s, Si[C(CH3)3]2}, 20.1 {d, 2J(13C-19F) = 12 Hz,
Si[C(CH3)3]2} ppm. 19F (282.38 MHz, C6D6): δ = –189.3 [s, 1J(19F-
29Si) = 297 Hz] ppm. 29Si {1H} (59.63 MHz, C6D6): δ = 14.5 [d,
mic 5 (0.21 g, 62 μmol, 44%) as a white solid (m.p. 78 °C). 1H
3
NMR (400 MHz, CDCl3): δ = 7.57 (d, JH-H = 8 Hz, 2 H, Harom.), 1J(29Si-19F) = 297 Hz] ppm. C26H34FNO4Si (471.64 g/mol): calcd.
3
7.33 (d, JH-H = 8 Hz, 2 H, Harom.), 7.05 (br., 2 H, NH2), 4.61 [dd, C 66.2, H 7.3, N 3.0; found C 66.1, H 7.3, N 2.9. HR-LC-ESI-MS:
3
3JH-H(erythro) = 6 Hz, JH-H(threo) = 7 Hz, 1 H, CH2-CH(NH2)], 3.70 calcd. for [C26H34FNO428Si+H]+ 472.2314; found 472.2310.
3
2
(s, 3 H, OCH3), 3.56 [dd, JH-H = 6 Hz, JH-H(threo) = 14 Hz, 1
(S)-Methyl 2-(Bezyloxycarbonylamino)-3-[4-(di-tert-butylfluorosil-
yl)phenyl]propanoate (S-9): A solution of 8 (3.29 g, 7.0 mmol) and
a catalytic amount of (+)-1,2-bis[(2S,5S)-diethylphospholanolbenz-
enecyclooctadiene]rhodium(I) triflate in dry dichloromethane
(50 mL) was stirred under H2 atmosphere (1 bar) for 24 h. The cat-
alyst was separated by filtration through a short silica gel pad. Af-
ter evaporation of the solvent, compound 9 was obtained as a
colourless oil in quantitative yield (3.32 g, 7.0 mmol). 1H NMR
H, CH2-CH(NH2), Herythro], 3.44 [dd, 3JH-H = 7 Hz, 2JH-H(erythro)
=
14 Hz, 1 H, CH2-CH(NH2), Hthreo], 1.02 [s, 18 H, C(CH3)3] ppm.
13C{1H} (100.13 MHz, CDCl3): δ = 168.9 (s, COOMe), 138.7 (s,
Ci), 135.2 [d, 3J(13C-19F) = 4 Hz, Cm], 130.9 [d, 2J(13C-19F) =
14 Hz, Cp], 128.8 (s, Co), 27.2 {s, Si[C(CH3)3]2}, 20.2 {d, 2J(13C-
19F) = 12 Hz, Si[C(CH3)3]2} ppm. 19F (188.28 MHz, CDCl3): δ =
1
–188.9 [s, J(19F-29Si) = 298 Hz] ppm. HR-LC-ESI-MS: calcd. for
[C18H30O2NF28Si+H]+ 340.2103; found 340.2103.
(400 MHz, C6D6): δ = 7.63 (d, 3JH,H = 7.6 Hz, 2 H, Ho), 7.10–7.25
S-2-Amino-3-[4-(di-tert-butylfluorosilanyl)phenyl]propionic Acid (6): (m, 5 H, Hphenyl), 7.07 (d, 3JH,H = 7.6 Hz, 2 H, Hm), 5.26 (d, JH,H
3
3
A solution of the methyl ester 5 (0.21 g, 62 μmol) in acetonitrile
(2 mL) was added to an aqueous solution of α-chymotrypsine[11]
(5 mg) in phosphate buffer (1 mL, 1 m Na2HPO4, pH = 7.8). The
reaction mixture was kept for 24 h at 30 °C. Acetonitrile was added,
= 8.0 Hz, 1 H, NH), 5.07 (q, 2 H, O-CH2-Ph), 4.77 (d, JH,H
=
7.6 Hz, 1 H, Hc), 3.24 (s, 3 H, COOCH3), 3.04 (dd, 3JH,H = 6.0 Hz,
2JHa-Hb = 13.6 Hz, 1 H, Hb), 2.87 (dd, JH,H = 6.8 Hz, JHa-Hb
=
3
2
13.6 Hz, 1 H, Ha), 1.10 [s, 18 H, 2ϫSiC(CH3)3] ppm. 13C NMR
Eur. J. Inorg. Chem. 2011, 2238–2246
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2243

B. Wängler, K. Jurkschat et al.
FULL PAPER
(400 MHz, C6D6): δ = 171.6 (s, COO), 155.5 (s, NCOO), 138.0 (s,
(0.94 g, 1.3 mL, 12 mmol) and chlorotrimethylsilane (0.35 g,
3
C
i(aromat.)), 136.7 (s, Ci(aromat.)), 134.1 (d, JC,F = 3.8 Hz, Cm(aromat.)), 0.26 mL, 6.4 mmol, 2 equiv.). The reaction mixture was stirred for
131.9 (d, 2JC,F = 13.4 Hz, Cp(aromat.)), 128.7 (s, Cm(aromat.)), 128.3 (s,
p(aromat.)), 128.1 (s, Co(aromat.)), 127.9 (s, Co(aromat.)), 66.6 (s, Ph-
30 min at room temperature followed by addition of 9-fluorenylmeth-
ylchloroformiate (Fmoc-Cl, 0.91 g, 1.1 equiv.); the clear solution
C
CH2-O), 54.9 (s, CH-N), 51.3 (s, O-CH3), 38.1 (s, CH2), 27.2 [s, was stirred for an additional 24 h at room temperature. Water
2
C(CH3)3], 20.1 [d, JC,F
=
12.5 Hz, C(CH3)3] ppm. 19F
(10 mL) was added to the reaction mixture, the organic phase was
separated and the aqueous phase was washed with CH2Cl2 (3ϫ
20 mL). The combined organic layers were dried with MgSO4 and
filtered, and the volatiles of the filtrate were removed in vacuo. The
product was purified by column chromatography (SiO2 acetone/n-
hexane, 3:1). Compound R-11 (1.41 g, 3.1 mmol, 80%) was ob-
tained as a slightly yellow amorphous solid (m.p. 132 °C). 1H NMR
(400.13 MHz, [D6]DMSO): δ = 7.98–7.14 (m, 11 H, Harom.), 4.09
(m, 1 H, H9-Fluorenyl), 4.06 (m, 2 H, HFluorenyl), 3.97 [dd, 3J(Hα-
Herythro) = 5 Hz, 3J(Hα-Hthreo) = 9 Hz 1 H, CH2-CH(NHFmoc)],
(188.28 MHz, CDCl3): δ = –189.9 [s, J(19F-29Si) = 298 Hz] ppm.
29Si{1H } NMR (59.63 MHz, CDCl3): δ = 14.4 [d, 1J(29Si-19F) =
298 Hz] ppm. HR-LC-ESI-MS: calcd. for C26H36FNO428Si
473.2398; found 474.2466 [M + H]+. C26H36FNO4Si (473.64 g/
mol): calcd. C 65.9, H 7.7, N 3.0; found C 65.7, H 7.7, N 2.8.
1
(R)-Methyl 2-(Bezyloxycarbonylamino)-3-[4-(di-tert-butylfluorosil-
yl)phenyl]propanoat (R-9): The preparation of this compound was
performed in analogy to the synthesis of the (S)-isomer. (+)-1,2-
Bis[(2R, 5R)-diethylphospholano]benzolcyclooctadienerhodium(I)
triflate was used as catalyst.
3
2
3.08 [dd, J(Herythro–Hα) = 5 Hz, J(Herythro–Hthreo) = 14 Hz, 1 H,
CH2-CH(NH2), Herythro], 2.85 [dd, 3J(Hthreo–Hα) = 9 Hz, 2J(Hthreo
–
(S)-2-Amino-3-[4-(di-tert-butylfluorosilanyl)phenyl]propionic Acid
(6): Compound S-9 (1.86 g, 3.9 mmol) and an aqueous solution of
HBr (2 mL, 15.7 mmol) were heated for 15 min at 110 °C. The vola-
tiles were removed in vacuo, and the residue was dissolved in di-
ethyl ether. The solution was washed with saturated NaHCO3 solu-
tion. A white solid (insoluble in water and diethyl ether) is formed
at the interface and isolated by filtration. The analytical data for
this compound are identical to the data for the (S)-amino acid ob-
tained by the enzymatic cleavage of the methyl ester. [α]2D0 (ethanol):
–21.89°.
Herythro) = 14 Hz, 1 H, CH2-CH(NH2), Hthreo], 1.02 {s, 18 H,
Si[C(CH3)3]2} ppm. 13C{1H} (100.63 MHz, [D6]DMSO): δ = 174.2
(s, COOH), 153.1 (s, NCOO), 142.2 (s, Carom.), 141.9 (s, Ci), 137.4
(s, Carom.), 136.2 [d, J(13C-19F) = 4 Hz, Cm], 131.0 [d, J(13C-19F)
= 14 Hz, Cp], 129.3 (s, Carom.), 128.1 (s, Carom.), 127.9 (s, Carom.),
127.4 (s, Co), 71.8 (s, OCH2), 62.2 (s, CNH), 37.2 (OCH2CH), 36.6
(s, PhCH2), 27.1 {s, Si[C(CH3)3]2}, 21.1 {d, 2J(13C-19F) = 12 Hz,
Si[C(CH3)3]2} ppm. 19F (188.28 MHz, [D6]DMSO): δ = –188.0 [s,
1J(19F-29Si) = 296 Hz] ppm. 29Si{1H } (59.63 MHz, [D6]DMSO): δ
3
2
1
= 14.1 [d, J(29Si-19F) = 296 Hz] ppm. LR-LC-ESI-MS: calcd. for
[(C32H38NFO428Si)+Na]+ 570.2; found 570.2.
(R)-2-Amino-3-[4-(di-tert-butylfluorosilanyl)phenyl]propionic Acid
(6): The synthesis of this compound was performed in analogy to
the preparation of the (S)-isomer. [α]2D0 (ethanol): 21.96°.
Thr-Cys-Thr-Lys-D-Trp-Tyr-Cys-D-Phe-Ser-L-SiFA-Phe (12): The
9-mer precursor peptide (Thr-Cys-Thr-Lys-d-Trp-Tyr-Cys-d-Phe-
Ser) was synthesized by automated solid-phase peptide synthesis
(CEM peptide synthesizer, Matthews, NC, USA) by using standard
9-fluorenylmethoxycarbonyl (Fmoc) group chemistry protocols.[13]
STr, N-Boc and OtBu were used for side chain protection. 2-
Chloro-trityl resin (0.5 mmol, 0.769 mg) was used as solid support.
The corresponding amino acid (5 equiv.), O-benzotriazolyl-
(S)-2-tert-Butoxycarbonylamino-3-[4-(di-tert-butylfluorosilanyl)phen-
yl]propionic Acid (10): To a solution of S-6 (1 g, 3.2 mmol) in
CH3CN/CH2Cl2 (1:4, 10 mL) was added a solution of tert-bu-
tyloxycarbonylanhydride [(tBuOCO)2O] (0.64 g, 3.2 mmol) in
CH2Cl2 (10 mL). Triethylamine (NEt3) (0.65 mL) was added to the
reaction mixture, and the latter was stirred for 24 h at room tem-
perature. The mixture was diluted with HCl solution (10 mL,
0.5 m). The organic phase was separated, and the aqueous phase
was extracted with CH2Cl2 (2ϫ 30 mL). The combined organic
phases were dried with MgSO4 and filtered, and the volatiles were
removed in vacuo to give a crude product that was recrystallized
from benzene to provide compound S-10 as an amorphous, white
solid (0.98 g, 2.30 mmol, 75%, m.p. 144–147 °C). 1H NMR
(400.13 MHz, C6D6): δ = 7.65 [d, 3J(1H-1H) = 7 Hz, 2 H, Hm], 7.11
[d, 3J(1H-1H) = 7 Hz, 2 H, Ho], 4.86 (br., 1 H, NH), 4.72 [dd,
3J(Hα-Herythro) = 6 Hz, 3J(Hα-Hthreo) = 8 Hz, 1 H, CH2-CH(NH2)],
N,N,NЈ,NЈ-tetramethyluroniumhexafluorophosphate
(HBTU)
(5 equiv.) and N,N-di-isopropylethylamine (DIPEA) (5 equiv.) in
dimethylformamide (DMF) was used for the couplings. Piperidine
in DMF (20%) was used for deprotection. The N-terminus of the
resin-bound 9-mer peptide was manually Fmoc-deprotected (20%
piperidine in DMF), and the N-terminal amino acid [Boc-protected
S-SiFA-Phe (S-10), 283 mg, 0.664 mmol,1.8 equiv.] was coupled to
the resin-bound peptide by using HBTU (336 mg, 0.886 mmol,
2.4 equiv., 0.5 m in DMF) and DIPEA (183 μL, 1.11 mmol,
3.0 equiv.) for 20 h. The 10-mer peptide was released from the solid
support and deprotected by incubating the resin with excess cleav-
age cocktail [95% trifluoroacetic acid (TFA), 2.5% H2O, 2.5% Et3-
SiH] for 1 h. The resulting solution was concentrated to volume of
2 mL, and the crude peptide was precipitated by adding ice-cold
methyl tert-butyl ether (MTBE, 10 vol.) and isolated by centrifuga-
tion. To ensure complete removal of Boc-derived carbamic acid
residues from tryptophan, the precipitate was resuspended in H2O/
MeOH (1:1, 10 mL). TFA was added (250 μL), and the mixture
was warmed to 40 °C and stirred for 1.5 h. Removal of all volatiles
and lyophilization yielded the crude linear peptide as a colourless
powder (445 mg). Installation of the intramolecular disulfide bridge
was achieved by dissolving the material in MeOH (0.3 m), adding
DIPEA (610 μL, 3.69 mmol, 10 equiv.), and incubating the solution
under O2 atmosphere for 72 h. The crude product was recovered
3
2
3.10 [dd, J(Herythro–Hα) = 6 Hz, J(Herythro–Hthreo) = 14 Hz, 1 H,
CH2-CH(NH2), Herythro], 2.78 [dd, 3J(Hthreo–Hα) = 8 Hz, 2J(Hthreo
–
Herythro) = 14 Hz, 1 H, CH2-CH(NH2), Hthreo], 1.40 [s, 9 H, OC(O)-
C(CH3)3], 1.11 {s, 18 H, Si[C(CH3)3]2} ppm. 13C{1H}
(100.63 MHz, C6D6): δ = 178.7 (s, COOH), 162.4 (s, COOtBu),
141.3 (s, Ci), 135.7 [d, 3J(13C-19F) = 4 Hz, Cm], 130.2 [d, 2J(13C-
19F) = 14 Hz, Cp], 127.4 (s, Co), 73.1 [s, OC(CH3)3], 58.4 (s, CNH),
41.3 (s, CH2), 29.8 [s, OC(CH3)3], 27.9 {s, Si[C(CH3)3]2}, 21.1 {d,
2J(13C-19F)
=
12 Hz, Si[C(CH3)3]2} ppm. 19F (188.28 MHz,
CDCl3): δ = –188.3 [s, 1J(19F-29Si) = 298 Hz] ppm. 29Si{1H}
(59.63 MHz, CDCl3): δ = 14.5 [d, 1J(29Si-19F) = 298 Hz] ppm.
C22H36FNO4Si (425.69 g/mol): calcd. C 62.1, H 8.5, N 3.3; found
C
62.1,
H
8.2,
N
3.5. HR-LC-ESI-MS: calcd. for
[2(C26H36NFO428Si)+H]+ 851.4868; found 851.4870.
(R)-3-[4-(Di-tert-butylfluorosilyl)phenyl]-2-N-[(9H-fluoren-9-yl)meth- by freeze-drying, dissolved in H2O/acetonitrile (3:1), and purified
ylformyl]propionic Acid (11): To a suspension of the R-amino acid by reversed-phase HPLC on Nucleodur C4 column
6 (1 g, 3.2 mmol) in CH2Cl2 (10 mL) were added diethylamine (250ϫ20 mm, Macherey–Nagel, Düren, Germany) at a flow rate
a
2244
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2238–2246

SiFA-Modified Phenylalanine: Efficient Synthesis of 18F-Labelled Peptides
of 20 mL/min by employing a linear 30–45% acetonitrile/water
(0.1% TFA) gradient over 45 min. The pure title compound 12 was
retrieved after lyophylization of suitable fractions as a colourless
powder (102 mg, 0.071 mmol, 19%). M.p. Ͼ240 °C (decomp.). Rf
(95:2.5:2.5). The crude peptides were precipitated with ice-cold di-
ethyl ether, and the solid residue was redissolved in acetonitrile/
water (9:1) and purified in analogy to 12 by reversed-phase HPLC.
19F (188.28 MHz, [D6]DMSO): δ = –187.1 [s, 1J(19F-29Si) = 298 Hz]
1
=
0.03 (SiO2, CHCl3/MeOH/HCOOH, 90:10:2). 19F NMR
ppm. 29Si{1H } (59.63 MHz, [D6]DMSO): δ = 14.2 [d, J(29Si-19F)
1
(188.28 MHz, [D6]DMSO): δ = –186.6 [d, J(19F-29Si) = 297.9 Hz]
ppm. 29Si{1H } NMR (59.63 MHz, [D6]DMSO): δ = 14.5 [d,
= 298 Hz] ppm. LR-LC-ESI-MS: calcd. for [C67H98N12O18FS2Si]+
1469.6; found 1469.8.
1J(29Si-19F) = 297.6Hz] ppm. H NMR (600 MHz, [D6]DMSO): δ
1
Thr-Cys-Thr-Lys-D-Trp-Tyr-Cys-D-SiFA-Phe-(PEG)5-Asp
(14):
= 10.80 (s, 1 H, NHTrp4-indole), 8.98 (d, J = 7.8 Hz, 1 H, NHCys2),
8.77 (d, J = 4.8 Hz, 1 H, NHTrp4), 8.69 (d, J = 6.3 Hz, 1 H, NHSer),
1
19F (188.28 MHz, [D6]DMSO): δ –186.8 [s, J(19F-29Si) = 298 Hz].
29Si{1H} (59.63 MHz, [D6]DMSO): δ 14.1 [d, 1J(29Si-19F) =
298 Hz]. LR-LC-ESI-MS: calcd. for [C67H98N12O18FS2Si]+ 1659.8;
found 1660.0.
8.58–8.52 (m, 2 H, NHTyr3 + NHCys7), 8.35–8.28 (m, 2 H, NHLys5
,
NHThr8), 8.17–8.00 (m, 3 H, NHPhe1, NH2, SiFA-Phe), 7.78–7.63
(m, 2 H, NH2, Lys5-ε), 7.59 (d, J = 9.1 Hz, 1 H, NHThr6), 7.47 (d,
Radiolabelling: Aqueous 18F-fluoride ion (4000–7500 MBq) that
had been produced by the 18O(p,n)18F nuclear reaction on an en-
riched [18O]water (95%) target was loaded onto a Chromabond PS-
HCO3 cartridge (Macherey–Nagel, Düren, Germany) and eluted
with a mixture of acetonitrile (800 μL), water (200 μL), potassium
oxalate solution (1 m, 10 μL) and Kryptofix 2.2.2.® (12.5 mg)
(method 1), or acetonitrile (800 μL), water (200 μL), potassium
carbonate solution (1 m, 10 μL) and Kryptofix 2.2.2.® (12.5 mg),
(method 2) or acetonitrile (800 μL), 75 mm tetrabutyl ammonium
hydrogen carbonate solution (300 μL), (method 3), respectively.
The solvents were removed under reduced pressure (650 mbar) by
co-evaporation by using a stream of helium at 87 °C for 4 min. The
drying step was repeated twice with CH3CN (800 μL, 3 min), and
full vacuum (ca. 10 mbar) was applied in the final drying step
(4 min). The dried 18F complex K (Kryptofix 2.2.2[18F]) was dis-
solved in dry acetonitrile or DMSO (500 μL), respectively, and used
for labelling.
J = 7.8 Hz, 2 H, CHAr-o, SiFA-Phe), 7.42 (d, J = 7.9 Hz, 1 H, CHAr
,
Trp4), 7.33 (d, J = 7.7 Hz, 3 H, CHAr-m, SiFA-Phe and CHAr-Trp4),
7.29 (d, J = 7.4 Hz, 2 H, CHAr-o, Phe1), 7.20–7.15 (m, 2 H,
CHAr-m, Phe1), 7.07 (d, J = 7.5 Hz, 1 H, CHAr, Trp4), 7.05 (s, 1 H,
CHAr-o, Trp4), 7.00 (t, J = 7.4 Hz, 1 H, CHAr, Trp4), 6.91 (d, J =
8.4 Hz, 2 H, CHAr-o, Tyr3), 6.64 (d, J = 8.2 Hz, 2 H, CHAr-m, Tyr3),
5.34 (d, J = 10.5 Hz, 1 H, CHα, Cys7), 5.29 (d, J = 9.7 Hz, 1 H,
CHα, Cys2), 4.87 (dd, J = 10.7, 3.8 Hz, 1 H, CHα, Phe1), 4.59–
4.51 (m, 2 H, CHα, Tyr3, Thr6), 4.39–4.34 (m, 1 H, CHα, Ser),
4.34–4.30 (m, 1 H, CHβ, Thr8), 4.22 (d, J = 2.8 Hz, 1 H, CHα,
Thr8), 4.19 (dd, J = 9.5, 6.0 Hz, 1 H, CHα, Trp4), 4.13–4.07 (m, 1
H, CHα, SiFA-Phe), 4.00–3.92 (m, 2 H, CHα, Lys5, and CHβ,
Thr6), 3.34 (d, J = 7.2 Hz, 1 H, CH2βЈ, Ser), 3.19 (d, J = 10.2 Hz,
2 H, CH2βЈЈ, Ser, and CH2βЈ, Phe1), 3.15 (dd, J = 14.4, 3.0 Hz, 2
H, CH2βЈ, SiFA-Phe, and CH2βЈ, Tyr3), 3.01 (dd, J = 13.9, 9.9 Hz,
1 H, CH2βЈ, Trp4), 2.94–2.72 (m, 8 H, CH2β, Cys2, and CH2β,
Cys7, and CH2βЈЈ, SiFA-Phe, and CH2βЈЈ, Tyr3, and CH2βЈЈ,
Phe1), 2.68 (dd, J = 14.0, 5.4 Hz, 1 H, CH2βЈЈ, Trp4), 2.58–2.52
(m, 2 H, CH2, Lys5-ε), 1.73–1.65 (m, 1 H, CH2βЈ, Lys5), 1.28 (d,
J = 8.0 Hz, 2 H, CH2, Lys5-δ), 1.24 (d, J = 6.4 Hz, 3 H, CH3γ,
Thr8), 1.21–1.17 (m, 1 H, CH2βЈЈ, Lys5), 1.07 (d, J = 6.3 Hz, 3 H,
CH3γ, Thr6), 1.00 (d, J = 8.7 Hz, 18 H, CH3, tBu), 0.72–0.56 (m,
2 H, CH2γ, Lys5) ppm. 13C{1H} NMR (150.8 MHz, [D6]DMSO):
δ = 172.6 (CO, Trp4), 172.0 (CO, Lys5), 171.3 (CO, Thr6), 169.0
(CO, Ser), 156.5 (Cq, Ar-OH, Tyr3), 136.6 (Cq, Ar, Trp4), 134.5
(CH, Arortho, SiFA-Phe), 130.5 (CH, Arortho, Tyr3), 130.2 (CH,
Arortho, Phe1), 130.1 (Cq, Ar, Phe1), 129.5 (CH, Armeta, SiFA-Phe),
Radiosynthesis of [18F]-6, 12, 13 and 14: The SiFA-phenylalanine-
containing peptides 12, 13 and 14 (10–25 nmol, 10–25 μL of a
1 mmol/L stock solution in dry DMSO or acetonitrile) were added
to the solution containing the 18F complex (3–5 GBq) and reacted
at ambient temperature for 5–15 min without stirring. Sub-
sequently, the reaction mixture was added to water (800 μL) and
loaded on a Waters SepPak C-18 light cartridge. The latter had
been preconditioned by subsequent rinsing with ethanol (5 mL)
and water (10 mL). The trapped [18F]SiFA-phenylalanine or [18F]
SiFA-peptides were washed with water for injection (5 mL), eluted
from the cartridge with ethanol (1000 μL) and diluted with isotonic
saline (10 mL). The solution was filtered sterile for further use. Re-
verse-phase HPLC revealed radiochemical purities ranging from 92
(13) to 99% (6). The syntheses yielded 0.31 (6) to 1.2 GBq (14)
of [18F]-6, -12, -13 and -14 starting from 3–5 GBq of the [18F]F-/
Kryptofix 2.2.2.®/K+ complex of the used solution. The products
were obtained in maximum specific activities between 31 (6) and
48 GBq/μmol (14).
127.6 (Cq, Ar, Trp4), 126.9 (CH, Armeta, Phe1), 124.3 (CH, Arortho
,
Phe1), 121.6 (CH, Ar, Trp4), 119.1 (CH, Ar, Trp4), 118.8 (CH, Ar,
Trp4), 115.6 (CH, Armeta, Tyr3), 111.9 (CH, Ar, Trp4), 109.6 (CH,
Ar, Trp4), 67.9 (β-CH, Thr6), 66.8 (β-CH, Thr8), 62.4 (β-CH2, Ser),
58.9 (α-CH, Thr8), 58.8 (α-CH, Thr6), 56.3 (α-CH, Ser), 56.0 (α-
CH, Trp), 54.8 (α-CH, Tyr3), 54.0 (α-CH, Phe1), 53.7 (α-CH, SiFA-
Phe), 53.1 (α-CH, Cys2), 53.0 (α-CH, Lys5), 52.4 (α-CH, Cys7),
45.7 (β-CH2, Cys7), 45.6 (β-CH2, Cys2), 40.0 (β-CH2, Phe1), 39.1
(ε-CH2, Lys5), 37.6 (β-CH2, SiFA-Phe), 37.6 (β-CH2, Tyr3), 31.0
(β-CH2, Lys5), 27.8 (CH3, tBu), 27.1 (δ-CH2, Lys5), 26.5 (β-CH2,
Trp4), 22.4 (γ-CH2, Lys5), 20.9 (γ-CH3, Thr8), 20.3 (Cq, tBu), 20.1
(γ-CH3, Thr6) ppm. HRMS (ESI): calcd. for [C69H96N12O15-
FS2Si]+ 1443.631; found 1443.632.
Acknowledgments
B. W. thanks the Deutsche Forschungsgemeinschaft for financial
support (DFG grant number WA 2132/3-1).
Thr-Cys-Thr-Lys-D-Trp-Tyr-Cys-D-SiFA-Phe-(PEG)1-Asp
(13):
The Tyr3-octreotate derivatives 13 and 14 were synthesized accord-
ing to standard Fmoc-solid-phase peptide synthesis procedures[13]
by using the corresponding amino acid (4 equiv.), HBTU (4 equiv.),
and DIPEA (8 equiv.) in DMF for the couplings and piperidine
(20%) in DMF for Fmoc-deprotection. The deprotection of the
thiol groups (PG = acetamido methyl) and the cyclization were
performed by adding Tl(OCOCF3)3 (4 equiv.). Global deprotection
and cleavage from the resin were performed by using a cleavage
cocktail of trifluoroacetic acid, water and triisopropylsilane
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Received: October 14, 2010
Revised Version Received: February 10, 2011
Published Online: April 1, 2011
2246
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Eur. J. Inorg. Chem. 2011, 2238–2246