Alpha selective epoxide opening with 18F-: Synthesis of 4-(3-[18F]fluoro-2-hydroxypropoxy)benzaldehyde ([ 18F]FPB) for peptide labeling

Article abstract of DOI:10.1016/j.tetlet.2011.02.064

Strained tricyclic ring systems such as epoxides are rarely used as precursors for the introduction of anionic fluorine-18 into organic compounds intended for positron emission tomography (PET). Here we report the alpha selective ring opening of epoxides

Full text of DOI:10.1016/j.tetlet.2011.02.064

Tetrahedron Letters 52 (2011) 1973–1976
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Alpha selective epoxide opening with 18FÀ: synthesis of 4-(3-[18F]fluoro-2-
hydroxypropoxy)benzaldehyde ([18F]FPB) for peptide labeling
Ralf Schirrmacher a, Philippe Lucas a, Esther Schirrmacher a, Björn Wängler b, Carmen Wängler a,b,
⇑
a Lady Davis Institute for Medical Research, Jewish General Hospital, 3755 Cote Ste-Catherine Road, H3T1E2 Montreal, Quebec, Canada
b Department of Nuclear Medicine, Ludwig Maximilians-University, Marchioninistrasse 15, 81377 Munich, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Strained tricyclic ring systems such as epoxides are rarely used as precursors for the introduction of anio-
nic fluorine-18 into organic compounds intended for positron emission tomography (PET). Here we
report the alpha selective ring opening of epoxides for the introduction of fluorine-18 into small as well
as larger biomolecules via 1- and 2-step protocols. [18F]fluoromisonidazole ([18F]MISO), a tracer for
hypoxia imaging, and the tumor targeting peptide Tyr3-octreotate (TATE) were radiolabeled using epox-
ide opening reactions. In the latter case, the new prosthetic labeling synthon 4-(3-[18F]fluoro-2-hydroxy-
propoxy)benzaldehyde ([18F]FPB) has been used for 18F-introduction.
Received 18 January 2011
Revised 10 February 2011
Accepted 15 February 2011
Available online 19 February 2011
Keywords:
Epoxide opening
Ó 2011 Elsevier Ltd. All rights reserved.
18F-labeling
[
18F]MISO
Peptide labeling
tert-Amyl-alcohol
Fluorine-18 (18F) is one of the most important radioisotopes
used for the labeling of imaging agents utilized in Positron Emis-
sion Tomography as non-invasive imaging tools to detect and
quantify biological processes in vivo. Over the last years, radio-
chemistry has devised numerous methods to introduce 18F into
small as well as large biomolecules by direct fluorination as well
as multistep procedures.1,2 Strained small cyclic ring systems such
as aziridines and epoxides appear to be well-suited precursors for
the introduction of 18FÀ into even complex biomolecules.3,4 One
possible application of this ring opening reaction could be the syn-
thesis of the hypoxia imaging agent [18F]MISO. However, only one
example of a successful application of an epoxide precursor in the
synthesis of [18F]MISO has been described by Welch and co-work-
ers in 1985 so far, yielding the labeled compound in low radio-
chemical yields (RCYs).5 A high yield procedure for [18F]MISO
utilizing an epoxide precursor has been reported in a recent patent
application where the epoxide opening was carried out in a mix-
ture of acetonitrile and 2-methyl-2-butanol (t-amyl-OH) at
100 °C,6 making use of the accelerating effect of tertiary alcohols
in 18F-fluorinations.7,8 Encouraged by these findings, we systemat-
ically investigated the suitability of various epoxide precursors for
the one-step introduction of nucleophilic 18FÀ into model com-
pounds in various solvent and base systems. Furthermore, we ex-
plored the labeling of an epoxide-bearing TATE peptide with 18FÀ
in one step and introduced the labeling synthon [18F]FPB derived
from its epoxide precursor for the 18F-labeling of aminooxy-deriv-
atized TATE.
The opening of epoxides with fluoride, anhydrous hydrogen
fluoride and various other reagents, such as HF-amine complexes,
potassium hydrogen fluoride, and silicon tetrafluoride to name just
a few is an often used procedure in the preparation of fluorohyd-
rines.9,10 In 18F-radiochemistry, however, there are only two 18F-
species available, namely 18FÀ and H[18F]F as a result of the
production process. Since H[18F]F is difficult to handle and unselec-
tive, we focused our investigation on 18FÀ only. We initially studied
the reaction of simple racemic epoxides such as 2-benzyloxirane
(1a) and 2-(phenoxymethyl)-oxirane (1b) in dipolar aprotic sol-
vents, such as acetonitrile, DMF, and DMSO as those are usually
used in nucleophilic fluorinations. Using azeotropically ‘dried’
18FÀ in the form of its K+/Kryptofix2.2.2Ò/18FÀ complex in the pres-
ence of either K2CO3 or potassium oxalate, RCYs between <1% and
30% were obtained for the corresponding 18F-fluorohydrines at
temperatures of 100–160 °C (Table 1). Acetonitrile consistently
yielded the highest RCYs followed by DMF and DMSO (<1%). Know-
ing that tertiary alcohols greatly facilitate nucleophilic reactions
between sulfonic acid esters (e.g., tosylates and mesylates) and
18FÀ,7 we also performed the labeling reactions in t-amyl-OH as
solvent at elevated temperatures using potassium oxalate as the
base. RCYs between 77% and 83% were obtained for both labeled
compounds 2a and 2b after 30 min at 110 °C (Fig. 1). In the radio-
syntheses of 2a–c we found 7–15% higher RCYs using potassium
oxalate as the base compared with K2CO3 and thus used the t-
amyl-OH/oxalate system in all following experiments.
⇑
Corresponding author.
E-mail address: carmen.waengler@med.uni-muenchen.de (C. Wängler).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.064

1974
R. Schirrmacher et al. / Tetrahedron Letters 52 (2011) 1973–1976
Table 1
Optimized conditions for the opening of epoxides with 18FÀ in CH3CN and t-amyl-OH
Epoxide/labeled compound
Solvent/temperature (°C)/time (min)a
Base system
RCY% (%) n = 10
1a/2a
1a/2a
1a/2a
1b/2b
1b/2b
1b/2b
1c/2c
1c/2c
1c/2c
1d/2db
CH3CN/100/30
CH3CN/100/30
t-Amyl-OH/110/30
CH3CN/100/30
CH3CN/100/30
t-Amyl-OH/110
CH3CN/100/30
CH3CN/100/30
t-Amyl-OH/100/30
t-Amyl-OH/100/30
K2CO3
24
30
80
22
28
86
7
10
90
87
2
3
3
4
2
3
3
4
5
7
K2(COO)2
K2(COO)2
K2CO3
K2(COO)2
K2(COO)2
K2CO3
K2(COO)2
K2(COO)2
K2(COO)2
a
The maximum RCY for all reactions was reached at approx. 30 min.
b
The reaction of 1d with 18FÀ was only performed in t-amyl-OH.
by 18FÀ.4 It is likely that the N-terminal epoxide moiety reacts
unselectively with side chain functionalities of the peptide.
To finally obtain [18F]5, the new prosthetic labeling precursor
HO
R
F
non-radioactive
standard compounds
[
18F]FPB ([18F]2d) was synthesized from 1d in high RCYs between
3a-d
160°
DMF
80% and 95% (based on HPLC) and conjugated to 6 in aqueous solu-
tion at pH 4 to form [18F]5 via chemoselective oxime formation in
reliable RCYs of 60–70% as determined by analytical HPLC. The 18F-
labeled product could be isolated in preparative RCYs of 28–32%
after 65–70 min total preparation time (Figs. 1 and 2). The prepara-
tive HPLC purification of [18F]5 was particularly easy since no
radioactive side products were formed during the conjugation
reaction (Fig. 3).
CsF
R
2
2
2
K
/
F
K
HO 18F
18F-/K222/K+
t-amyl-alcohol
T=100-110°
O
R
18
1a-d
]2a-d
[
F
77-95%
The preparation of the secondary labeling precursor [18F]FPB as
well as its conjugation efficiency to aminooxy-derivatized peptides
are thus comparable to the currently, mainly used aldehyde-com-
prising secondary labeling precursor 18F-fluorobenzaldehyde13,14
and identify [18F]FPB as a well-suited secondary labeling precursor
for the radiolabeling of complex aminooxy-derivatized molecules.
a
c
b
d
H
O
R a-d:
N
N
O
NO2
O
In all investigated reactions, the opening was 100% a-selective
with the 18FÀ reacting at the sterically less hindered position which
is in accordance to the findings of Park et al.15 During the epoxide
ring opening reactions, two different enantiomers are formed
which should in general be of no concern for an in vivo application
of labeled biomolecules obtained by this reaction type. For
Figure 1.
a-Selective epoxide opening of model compounds 1a and 1b, the labeling
precursor 1c for the synthesis of [18F]FMISO and 1d yielding [18F]2d for peptide
labeling via oxime formation. Non-radioactive standard compounds 3a–d were
synthesized reacting 1a–d with CsF in DMF at 160 °C.
[
18F]MISO, the formation of enantiomers was postulated not to
Interestingly and in contrast to the reactions performed in dipo-
lar aprotic solvents, in t-amyl-OH no radioactive side products
were detected either by radio-TLC or radio-HPLC. The same reac-
tion conditions were applied to the synthesis of [18F]MISO (2c)
from its epoxide precursor 2-nitro-1-(oxiran-2-ylmethyl)-1H-
imidazole (1c). [18F]MISO was obtained in RCYs of 80–95% in pure
t-amyl-alcohol after 30 min reaction time at 100 °C using K2(COO)2
as base. With this labeling reaction, the [18F]MISO formation
reached a plateau of about 90( 5)% RCY after 30 min. After 5, 10,
and 20 min, the RCYs were about 45( 5)%, 61( 5)%, and 78%( 5),
respectively. The corresponding reaction in CH3CN gave 7–10%
RCY only (Table 1). The preparative RCYs of [18F]MISO after HPLC
purification were 55% after a total synthesis time of 60 min.
In order to validate if those conditions are applicable to the 18F-
labeling of more complex biomolecules such as peptides, Tyr3-oct-
reotate was N-terminally modified with aminooxy-acetic acid11,12
(6, Fig. 2) and conjugated to racemic 4-(oxiran-2-ylmethoxy)benz-
aldehyde (1d) by chemoselective oxime formation yielding peptide
4 (Fig. 2).12 The direct 18F-fluorination of 4 under various condi-
tions did not yield the desired labeled peptide [18F]5. Neither dipo-
lar aprotic solvents nor t-amyl-OH resulted in the formation of
radiolabeled [18F]5. Besides unreacted 18FÀ, different non-radioac-
tive side products could be detected by HPLC (UV channel) which
were not further analyzed. This is in contrast to the results re-
ported for the one-step aziridine opening of complex biomolecules
negatively influence the pharmacologic properties of the radio-
tracer as well.16 Nevertheless, the formation of enantiomers pro-
duced during the epoxide opening may have negative effects on
the pharmacologic properties of small molecules and thus has to
be kept in mind.
Interestingly, the non-radioactive 19F-standard compounds 3a–
d synthesized for analytical purposes could not be obtained using
the corresponding 18F-labeling conditions employing macroscopic
amounts of KF/K222 in t-amyl-OH.
The 19F-fluorinated compounds 3a and 3b could only be synthe-
sized from the epoxide precursors using CsF in DMF at 160 °C for
24 h and analytical data (1H, 13C NMR, mass) were compared with
literature data.17,18 Compound 3c, which was used as non-radioac-
tive standard compound, was purchased from ABX (Radeberg, Ger-
many). The non-radioactive standard compound 3d was obtained
as follows: The racemic epoxide 1d (0.35 g, 1.96 mmol) and CsF
(0.89 g, 5.88 mmol) were dissolved in DMF (7 mL) and reacted at
160 °C for 24 h. Compound 3d was purified via column chromatog-
raphy. Compound 3d: 1H NMR (400 MHz, CDCl3) d (9.84 (s, 1H), 7.8
(d, 2H, J = 8.85 Hz), 6.98 (d, 2H, J = 8.74 Hz), 4.57 (ddd, 2H,
J = 46.97 Hz, 4.63 Hz, 2.31 Hz); 13C NMR (400 MHz, CDCl3)
d
190.80, 132.15, 115.08, 76.80, 69.19, 50.00, 44.72, 29.87, ESI-MS:
m/z 199 ([M+H]+).
The synthesis of aminooxy-derivatized TATE (6) was performed
as previously described.19 The non-radioactive peptide 5 and the

R. Schirrmacher et al. / Tetrahedron Letters 52 (2011) 1973–1976
1975
O
O
O
O
O
O
H2N
TATE
various solvents,
100-120°C
6
K+/K222/18F-
18F
N
O
H
O
OH
O
1d
TATE
O
4
N
O
18F
O
O
OH
O
TATE
18F]5
O
K+/K222/18F-
O
O
[
H2N
TATE
60-70%
100°C
6
isolated: 28-32%
t-amyl-OH
H
O
H
O
[
18F]FPB ([18F]2d)
1d
80-95%
HO
TATE
=
HN
NH
O
H
N
H
N
O
NH
O
S
S
O
NH
O
O
HN
NH
O
NH
HO
O
HO
NH2
OH
Figure 2. Synthesis of [18F]5 via oxime formation between [18F]FPB ([18F]2d) and 6 at pH 4. The reaction between 4 and 18FÀ did not yield [18F]5.
(0.2 mg, 0.17
by HPLC.
l
mol) following a published procedure19 and purified
In conclusion, we have demonstrated that nucleophilic a- selec-
tive opening of epoxides with 18FÀ is well-suited to prepare sec-
ondary labeling precursors such as [18F]FPB which in turn can be
used for efficient peptide labeling. The 18F-labeling of epoxides is
strongly facilitated by the use of the tertiary alcohol t-amyl-OH.
Figure 3. Radio-HPLC chromatogram of the crude reaction mixture of
6 and
[
18F]FPB ([18F]2d) yielding 18F]5 (column: ChromolithÒ Performance RP-18
[
Acknowledgments
100Ã4.6 mm, acetonitrile/H2O gradient 100% H2O at t = 0–100% acetonitrile at
t = 30 min, flow: 1.5 mL/min).
This work was supported by the Natural Science and Engineer-
ing Council (NSERC) Canada to R. Schirrmacher.
labeling precursor 4 for the one-step reaction with 18FÀ were syn-
thesized following standard procedures.20,13 Compound 4: MALDI-
TOF-MS: m/z 1281.5 ([M+H]+). Compound 5: MALDI-TOF-MS: m/z
1301.6 ([M+H]+).
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l
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