A general synthesis of 2′-deoxy-2′-[18F]fluoro-1-β-D- arabinofuranosyluracil and its 5-substituted nucleosides

Article abstract of DOI:10.1002/jlcr.637

Several 2′-deoxy-2′-[¹⁸F]fluoro-1-β-D- arabinofuranosyluracil derivatives have been synthesized. Coupling of 1-bromo-2-deoxy-2-[¹⁸F]fluoro-3,5-di-O-benzoyl-α-D- arabinofuranose 2 with protected uracil derivatives 3a-e followed by hydrolysis and high-performance liquid chromatography purification produced the radiolabeled nucleosides 4a-e in 15-30% yield (d. c.), > 99% radiochemical purity and 55.5-103.6 GBq/μmol specific activities. Copyright

Full text of DOI:10.1002/jlcr.637

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2003; 46: 285–289.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.637
Research Article
A general synthesis of 20-deoxy-20-
[18F]fluoro-1-b-D-arabinofuranosyluracil and
its 5-substituted nucleosides
Mian M. Alauddin, Peter S. Conti* and John D. Fissekis
PET Imaging Science Center, University of Southern California, 2250 Alcazar
Street, # 101, Los Angeles, CA 90033, USA
Summary
Several 20-deoxy-20-[18F]fluoro-1-b-d-arabinofuranosyluracil derivatives have
been synthesized. Coupling of 1-bromo-2-deoxy-2-[18F]fluoro-3,5-di-O-
benzoyl-a-d-arabinofuranose 2 with protected uracil derivatives 3a–e followed
by hydrolysis and high-performance liquid chromatography purification
produced the radiolabeled nucleosides 4a–e in 15–30% yield (d. c.), >99%
radiochemical purity and 55.5–103.6 GBq/mmol specific activities. Copyright
# 2003 John Wiley & Sons, Ltd.
Key Words: fluorine-18; nucleoside
Introduction
Radiolabeled pyrimidine nucleosides such as [11C]-FMAU and [124I]-
FIAU are potential PET imaging agents for tumor proliferation and
gene expression, respectively.1–5 However, the short half-life of 11C and
potential for de-iodination of 124I are less than optimal for imaging
purposes. The [18F]-labeled compounds should be more advantageous.
*Correspondence to: P. S. Conti, PET Imaging Science Center, University of Southern California,
2250 Alcazar Street, # 101, Los Angeles, CA 90033, USA. E-mail: pconti@hsc.usc.edu
Contract/grant sponsor: National Cancer Institute; contract/grant number: CA 72896.
Copyright # 2003 John Wiley & Sons, Ltd.
Received 10 April 2002
Revised 26 April 2002
Accepted 9 July 2002

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M. M. ALAUDDIN ET AL.
In addition, other 20-deoxy-20-[18F]fluoro-5-substituted-1-b-d-arabino-
furanosyluracil nucleosides6,7 may be potential imaging agents. Con-
ventional syntheses of such 20-deoxy-20-fluoro-nucleosides are not
suitable for the respective [18F]-labeled products.8–11 We reported
earlier a method suitable for [18F]-FMAU.12–14 Now we report here
syntheses of other 20-deoxy-20-[18F]fluoro-arabinofuranosyluracil analo-
gues by the same process thus providing a general synthetic method for
such of [18F]-nucleosides.
Results and discussion
The scheme for the synthesis of the [18F]-labeled nucleosides 4a–e is
shown in Figure 1. Compound 2 was prepared in two steps as previously
reported.14 In the first step fluorination reaction, ꢀ10 mmol of the
precursor 1, and ꢀ7 mmol of n-Bu4NHCO3 produced a radiochemical
yield of 58–68%. Any variation from this stoichiometry lowers the
yields of the desired product. A lower amount of n-Bu4NHCO3
apparently does not extract the 18F-fluoride efficiently from the reaction
vial. Excess n-Bu4NHCO3 is presumably converted to n-Bu4NOH, and
hydroxide ion competes with fluoride ion reducing the yield.
Compound 2 was recovered by evaporation of the solvent and
excess reagents azeotropically removed with toluene. The amount of
toluene is important, and 1 ml appears to be ideal. A smaller volume
reduces the yield of the coupled product in the subsequent step. A
residual trace of acetic acid converts the compound 2 to the 1-acetoxy
derivative.
O
R
HN
BzO
BzO
O
O
1. 18 F-
3 3a-e
.
18 F
O
N
OBz
Br
HO
-
O
OH
4.
HBr/AcOH
2.
OBz OTf
OBz
OTMS
N
18F
3a R=H
3b R=F
R
2
1
N
3c R=I
3d R=CH3
3e R=C2H5
OH
TMSO
4a-e
Figure 1. Synthetic scheme of 20-deoxy-20-[18F]fluoro-1-b-D-arabinofurano-
syluracil nucleosides
Copyright # 2003 John Wiley & Sons, Ltd.
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RADIOLABELLED NUCLEOSIDES
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Coupling of the 1-bromo-2-[18F]fluorosugar 2 with pyrimidine silyl
ethers 3a–e requires 1 h for optimum yield. The silyl derivative of
thymine (3d) seems to be rather sensitive to the presence of moisture as
indicated by the appearance of solvent turbidity during the addition of
the thymine silyl ether solution to the reaction vessel. The crude reaction
mixture was passed through a Sep-Pak cartridge (silica) and eluted with
10% methanol in dichloromethane to reduce the amount of unreacted
pyrimidine present. After basic hydrolysis, the coupled products
produced the desired nucleosides 4a–e, which were isolated by high-
performance liquid chromatography (HPLC) purification.
Analysis of the products, 4a–e by HPLC showed a single radioactive
peak co-eluting with an authentic standard of the respective
nucleosides; The overall radiochemical yields of this synthesis was
15–30% (d.c.) from the EOB. The radiochemical purity was >99% with
specific activities 55.5–103.6 GBq/mmol. The synthesis time
was 3.5–4.0 h from the EOB. In typical syntheses, 444–666 MBq
of labeled products were obtained from 7400–9620 MBq of [18F]
fluoride.
Experimental
Reagents and instrumentation
2-O-(trifluoromethylsulfonyl)-1,3,5-tri-O-benzoyl-a-d-ribofuranose 1,
1-bromo-2-deoxy-2-[18F]fluoro-3,5-di-O-benzoyl-a-d-arabinofuranose
2 and pyrimidine-2,5-bis-trimethylsilyl ether 3a–e were prepared
following literature methods.9,10,14 Intermediate radiolabeled products
were not isolated but used directly in the subsequent steps. The
corresponding unlabeled compounds were isolated and characterized by
1H and 19F NMR and mass spectrometry.
Proton and 19F NMR spectra were recorded on a Brucker 500 MHz
spectrometer using tetramethylsilane as an internal reference and
hexafluorobenzene as an external reference, respectively. Mass spectra
were obtained on a Finnigan 400 mass spectrometer at the University of
Minnesota using ammonia chemical ionization technique. HPLC was
performed as reported.14 Acetonitrile/water solvent systems, 5.0%
MeCN for FAU (4a), 7.5% for FFAU (4b) and FMAU (4d), and 10%
for FIAU (4c) and FEAU (4e) were used for purification of the
radiolabeled nucleoside and quality control analyses.
Copyright # 2003 John Wiley & Sons, Ltd.
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20-Deoxy-20-[18F]fluoro-5-substitued-1-b-d-arabinofuranosyluracil
derivatives 4a–e
The radiolabeled nucleosides 4a–e were prepared following the reported
method.14 Briefly, compound 2 was heated with freshly prepared 2,4-
bis-O-(trimethylsilyl)uracil derivatives 3a–e (75–85 mmol, 8–9 equiv.) in
1,2-dichloroethane (0.5 ml) at ꢀ1008C for 60 min. The reaction mixture
was cooled, passed through a Sep-Pak cartridge (silica) and eluted with
10% methanol in dichloromethane (2.5 ml). After solvent evaporation
the crude product was heated for 5 min at reflux with sodium methoxide
(1 M solution in methanol, 0.03 ml). The crude product was cooled,
neutralized, diluted with HPLC solvent and purified by HPLC at a flow
of 4.05 ml/min. After solvent evaporation, the pure product was re-
dissolved in saline and filtered through a 0.22 mm filter. An aliquot of the
final product was analyzed by analytical HPLC, and found to be co-
eluting with the authentic compound.
Unlabeled chemical syntheses were performed in the similar manner
1
except in larger scale (40–50 mg of 1). The H NMR spectra were
consistent with the literature.6, 9–11 Additional data was obtained with
19F NMR spectra as presented. Compounds 4a: DMSO-D6: 19F NMR:
d=ꢁ196.6 (dt). 4b: D2O: 19F NMR: d=ꢁ166.8 (d), ꢁ199.6 (dt). 4c:
DMSO-D6: 19F NMR: d=ꢁ196.3 (dt). 4d:13. 4e: DMSO-D6: 19F NMR:
d=ꢁ197.2 (m).
Acknowledgements
The authors wish to thank Mr Allan D. Kershaw for assisting with the
NMR spectra. This work was supported by the National Cancer
Institute Grant CA 72896.
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J Label Compd Radiopharm 2003; 46: 285–289