New [11C]phosgene based synthesis of [11C]pyrimidines for positron emission tomography

Article abstract of DOI:10.3987/COM-08-S(F)116

Thymine, 5-FU, and uracil were successfully synthesized through a procedure involving a cyclocondensation of triphosgene with newly developed (α-substituted β-aminoacrylamides intermediates (1a, X= Me; 1b, X= F; 1c, X= H). The radioligands [2-¹¹C]thymine and [2- ¹¹C]5-fluorouracil were synthesized in high radiochemical yields in 16-17 minutes from the end of bombardment by applying the cyclocondensation method with [¹¹C]COCl₂.

Full text of DOI:10.3987/COM-08-S(F)116

HETEROCYCLES, Vol. 77, No. 2, 2009
1307
HETEROCYCLES, Vol. 77, No. 2, 2009, pp. 1307 - 1321. © The Japan Institute of Heterocyclic Chemistry
Received, 15th September, 2008, Accepted, 27th October, 2008, Published online, 28th October, 2008
DOI: 10.3987/COM-08-S(F)116
NEW [11C]PHOSGENE BASED SYNTHESIS OF [11C]PYRIMIDINES FOR
POSITRON EMISSION TOMOGRAPHY
Koh-ichi Seki,a,* Ken-ichi Nishijima,a Kimihito Sanoki,b Yuji Kuge,a
Masayuki Takahashi,b Hiromichi Akizawa,b Nagara Tamaki,a Leonard I.
Wiebe,c and Kazue Ohkurab,*
a Graduate School of Medicine, Hokkaido University, Kita-15, Nishi-7, Kita-ku,
Sapporo 060-8638, Japan
b
Faculty of Pharmaceutical Sciences, Health Sciences University of Hokkaido,
Ishikari-Tobetsu, Hokkaido 061-0293, Japan
c
Faculty of Pharmacy & Pharmaceutical Sciences, University of Alberta, and
Cross Cancer Institute, Edmonton, Alberta, Canada
E-mail: ohkura@hoku-iryo-u.ac.jp
Abstract – Thymine, 5-FU, and uracil were successfully synthesized through a
procedure involving a cyclocondensation of triphosgene with newly developed
α-substituted β-aminoacrylamides intermediates (1a, X= Me; 1b, X= F; 1c, X= H).
The radioligands [2-11C]thymine and [2-11C]5-fluorouracil were synthesized in
high radiochemical yields in 16-17 minutes from the end of bombardment by
applying the cyclocondensation method with [11C]COCl2.
INTRODUCTION
Thymidine phosphorylase (TP; (EC 2.4.2.4)) is an important enzyme which catalyses reversible
deoxyribosylation of thymine (Thy) to thymidine (thymidine + phosphate thymine +
2-deoxy-α-D-ribose 1-phosphate). 5-Substituted uracils (5-XUra) including uracil (Ura) can also be
substrates for TP. TP is also known to activate 5-fluorouracil (5-FU) to 5-fluoro-2’deoxyribonucleoside,
which acts as TP inhibitor.1 Furthermore, it is reported that TP is associated with angiogenesis as a
growth factor, and its expression is strongly associated with the growth of tumors.2 Thus, TP is an
attractive target for imaging and therapy, 3 and many pyrimidine-based radiopharmaceuticals including
thymine have been developed for clinical diagnosis in the field of single photon computed tomography
(SPECT) or positron emission tomography (PET).4 In 1991, Vander Borght et al. synthesized
This paper is dedicated to Professor Emeritus Keiichiro Fukumoto on the occasion of his 75th birthday.

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HETEROCYCLES, Vol. 77, No. 2, 2009
[2-11C]thymine via cyclocondensation of diethyl β-methyl malate with [11C]urea.5 Although other
investigators have attempted to improve this method or develop methodologies involving condensation of
a malate intermediate with the labeling agent [11C]urea derived from phosgene,6 cyanide7 or carbon
dioxide as the key ring closure reactions,8 those condensation reactions were carried out under conditions
as drastic as those employed for 14C labeled thymine synthesis.9 The complexity of the currently available
synthetic routes along with the extended length of preparation time has limited the extensive applicability
of 11C-labeled nucleosides in PET studies.
5-FU, which was originally synthesized in 195710,11 as one of a new class of antitumor fluoroprymidines,
has been also an attractive target as a possible PET ligand. 5-FU-mediated inhibition of thymidylate
12
synthetase was subsequently shown to be one of the major mechanisms responsible for the antitumor
activity of these compounds. Today, almost 50 years later, 5-FU remains front-line therapy, alone or in
combination with other drugs or radiation, for gastric,13,14 colorectal,15,16 and other cancers including
advanced pancreatic cancer.17 Development of a diagnostic PET tracer based on 5-FU would be very
important to assess or predict more successful outcomes in selecting drugs for cancer chemotherapy.
Indeed, it has been demonstrated, for example, that tumor uptake of fluorine-18 labeled 5-FU (5-[18F]FU)
serves a positive prognostic role in selection of patients for 5-FU therapy (Strauss 5-[18F]FU test).18,19
Underutilization of the ‘Strauss 5-[18F]FU test’ may be due, in part, to the proposed need for complex
kinetic modeling rather than simple tumor uptake,20 and/ or to the electrophilic F-18 radiosynthetic
method developed in the early 1970’s.21 The latter method remains the sole method of 5-[18F]FU
18
18
radiosynthesis today, and is thus not popular in units using the O(p, n)18F nuclear reaction on H2 O to
produce aqueous radiofluoride for routine clinical radiofluorinations.
Thus, there are still clinical needs to develop efficient PET tracers directed towards TP for evaluating the
grade of malignancy,22 the proliferative activity of tumor cells,23 and the outcome of cancer
chemotherapy.
Meanwhile, we have recently developed a highly efficient synthesis of [11C]COCl2 with high specific
activity.24 This method has been successfully applied to producing [11C]CGP-12177, a PET tracer for
β-adrenoreceptors which is now supplied for clinical use.25 Application of this [11C]COCl2 to the
synthesis of [11C]pyrimidines would provide a potential procedure for tumor targeting novel PET tracers.
These contexts prompted us to develop a facile and efficient synthesis of [11C]COCl2-based [11C]5-XUra
(X = Me, F, H) by developing a synthetic route to the common precursors, β-aminoacrylamide derivatives
(1a-c) with substituents such as Me (a), F (b), H (c) at the α-position, that is to be subjected to the
cyclization with [11C]COCl2 to form versatile [2-11C]pyrimidine derivatives in the last step (Scheme 1).
We report herein a novel and facile synthesis of [2-11C]thymine and [2-11C]5-fluorouracil through direct
condensation of [11C]COCl2 with the key intermediates β-aminoacrylamides (1a-b).

HETEROCYCLES, Vol. 77, No. 2, 2009
1309
O
O
Cl
Cl
X
X
HN
11C
H2N
+
O
11C
O
N
H
H2N
X= Me: 1a
X= F : 1b
X= H : 1c
X= Me : thymine
X= F : 5-FU
X= H : uracil
Scheme 1. Synthetic route of [2-11C]pyrimidine derivatives.
RESULTS AND DISCUSSION
Synthesis of thymine (Thy). The key intermediate, β-aminomethacrylamide (1a), was readily
synthesized from ethyl α-formylpropionate (2a).26 Treatment of 2a with ammonia afforded
diastereomixture of β-aminomethacrylate (3aZ and 3aE). The Z -isomer 3aZ was benzoylated with benzoyl
chloride to give N-benzoylacrylate 4aZ with the desired stereochemistry (Z-form). Treatment of the
resulting 4aZ with ammonia exclusively afforded the desired intermediate (Z)-β-(N-benzoylamino)-
methacrylamide (5aZ) with the stereochemistry maintained in the desired Z-form (Scheme 2). Hydrolysis
of the benzoylated compound (5aZ) failed to give β-aminomethacrylamide (1a). Therefore, 5aZ was
used as a key intermediate for the subsequent ring closure with triphosgene, to Thy. For the synthesis of
non-radio-labeled (cold) Thy, triphosgene was used as a safe and stable replacement for phosgene.
O
O
O
O
Me
Me
Me
Me
CHO
iii
i
ii
EtO
EtO
H2N
EtO
H2N
HN
COPh
HN
COPh
2a
3az, 3aE
5aZ
4aZ
Reagents and Conditions: i) NH3 in MeOH, reflux, 2 h,
ii) PhCOCl, pyridine, CHCl3, 0°C, 2 h, then rt, overnight, iii) NH3 in MeOH,rt, 1 week
Scheme 2. Synthesis of β-(N-benzoylamino) methacrylamide (5aZ)
The key intermediate 5aZ was converted to the alkali metal salts (5aZ-Na or 5aZ-K) with a base.
Addition of triphosgene to the resulting salt under various conditions gave rise to the formation of
benzoylthymine (6a), which was readily hydrolyzed with NH3, or by passing through a short column of
silica gel, to give Thy (Scheme 3).27

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HETEROCYCLES, Vol. 77, No. 2, 2009
OCCl3
OCCl3
O
no reaction
triphosgene
O
CH2Cl2 or THF
reflux
Me
H2N
HN
COPh
5aZ
OCCl3
OCCl3
O
O
O
O
Me
Me
H2O
Me
H2N
HN
HN
base
_
N
rt, 10 min
+
M
O
N
O
N
H
COPh
COPh
6a
Thy
Scheme 3. Synthesis of Thy
As summarized in Table 1, the best result was obtained when the reaction was performed with the sodium
salt (5aZ-Na) in DMF.
Table 1. Yields (%) of Thy
Solvent
THF or CH2Cl2
DMF
Base
--
Mol. eq.
Yield (%)
--
5
5
2
1
ND
99
60
48
56
NaH
DME
NaH
DME
t-BuOK
t-BuOK
DME
ND: not detected.
Synthesis of 5-fluorouracil (5-FU). The key intermediate β-(N-benzoylamino)-α-fluoroacrylamide
(5bE) was synthesized according to the procedure for 5aZ. Sodium ethyl 2-fluoro-3-hydoxyacrylate (2b),
derived from ethyl formate and ethyl fluoroacetate in the presence of sodium methoxide28 was treated
with ammonia and ammonium chloride in methanol to give β-aminoacrylate (3b).29 Benzoylation of the
resulting 3b with benzoyl chloride in pyridine afforded (Z)-ethyl β-benzoylamino-α-fluoroacrylate (4bZ),
wherein the ethoxycarbonyl group and the benzoylamino group occupy the undesired
trans-stereochemistry on the ethylene moiety for the subsequent cyclization with phosgene. In order to
effect geometric isomerization of the Z-isomer 4bZ into the E-isomer 4bE, UV-irradiation of 4bZ with a
high-pressure mercury lamp afforded the equilibrium mixture of 4bZ and 4bE in the ratio of 1 : 9. The
desired E-isomer 4bE was further treated with ammonia to furnish the key intermediate 5bE with the

HETEROCYCLES, Vol. 77, No. 2, 2009
1311
desired stereochemistry (E-form) in quantitative yield (Scheme 4).
O
O
O
F
F
F
i
ii
EtO
EtO
EtO
NH
NH2
ONa
COPh
2b
3b
4bz
iii
O
O
F
F
EtO
iv
H2N
HN
COPh
4bE
HN
COPh
5bE
Reagents and Conditions: i) NH4Cl-MeOH, NH3, rt, 1 week,
ii) PhCOCl, pyridine, CHCl3, 0°C, 2 hr, then rt, overnight, iii) hν (high pressure-Hg-lamp), 4 h
iv) NH3 in MeOH,rt, 1 week
Scheme 4. Synthesis of β-(N-benzoylamino)methacrylamide (5bE)
The sodium salt of the key intermediate 5bE was subjected to cyclocondensation with triphosgene at room
temperature, followed by hydrolysis with ammonia in methanol, resulting in the formation of 5-FU in
high yield (75%, after purification on HPLC) (Scheme 5).
O
O
O
O
F
F
F
F
triphosgne
rt, 12 h
NaH/DME
rt, 3 h
NH3/MeOH
rt, 0.5 h
H2N
H2N
HN
HN
_
N
+
HN
O
N
Na
O
N
H
COPh
COPh
COPh
5bE
5-FU
Scheme 5. Synthesis of 5-FU
Synthesis of uracil (Ura). Ethyl formylacetate (2c), prepared from Meldrum’s acid (2,2-dimethyl-
1,3-dioxane-4,6-dione) and ethyl formate was treated with ammonia to afford β-aminoacrylate (3c), albeit
in an undesired (E)-stereochemistry, which was benzoylated with benzoyl chloride to give benzoyl
aminoacrylate 4cE. Treatment of the resulting 4cE with ammonia afforded benzoyliminopropanamide,
(5cI), instead of giving either the desired Z-β-(N-benzoylamino)acrylamide (5cZ) or
E-β-(N-benzoylamino)acrylamide (5cE). In order to obtain a key intermediate 5cZ in the desired
Z-stereochemisitry, E-β-(N-benzoylamino)acrylate (4cE) was irradiated with a 500 W high-pressure
mercury lamp to give the stereoisomer 4cZ. Treatment of the resulting ester (4cZ) with ammonia, however,

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HETEROCYCLES, Vol. 77, No. 2, 2009
O
O
O
O
O
O
O
O
O
O
H
Me
Me
Me
Me
i
EtO
H
CHO
2c
O
O
O
H
H
H
v
EtO
ii
iii
EtO
H2N
NH2
NH
N
COPh
4cE
COPh
3c
5cI
iv
v
O
O
H
H
EtO
H2N
HN
HN
COPh
4cZ
COPh
5cZ
Reagents and Conditions: i) EtOH, dry benzene, reflux, 90 min, ii) NH4Cl-MeOH, NH3, rt, 1 week, iii) PhCOCl,
pyridine, CHCl3, 0°C, 2 h, then rt, overnight, iv) hν (high pressure-Hg-lamp), 4 h, v) NH3 in MeOH, rt, 1 week
Scheme 6. Synthesis of 5cI
failed to give the desired 5cZ, but afforded the imino tautomer (5cI), which is identical with that obtained
from 4cE via ammonolysis (Scheme 6). The resulting imine (5cI) is the tautomeric isomer of either 5cE or
5cZ, and hence the sodium or potassium salts, if formed sufficiently, can be inter-convertible through the
tautomerism. Therefore, we decided to use 5cI for the subsequent cyclocondensation with triphosgene.
Na+
O-
O
OCCl3
OCCl3
O
NaH(5equiv.)
HN
H2N
5cI
DMF
1 h, rt
5 min, rt
O
N
N
COPh
COPh
O
O
Na+
C-
H2N
HN
N
O
N
H
COPh
Ura
Scheme 7. Synthesis of Ura
Thus, imine 5cI was converted to its sodium salts (5cI-Na) with sodium hydride, and resulting 5cI-Na was

HETEROCYCLES, Vol. 77, No. 2, 2009
1313
treated with triphosgene at room temperature for 5 min, to furnish the desired Ura, albeit in poor yield
(2.4%) (Scheme 7). The formation of Ura was confirmed by comparison of the spectroscopic data and
chromatographic behaviors with those of the authentic sample.
[11C] COCl2 based synthesis of [11C]pyrimidines as potential tumor targeting PET tracers. As
described above, pyrimidine rings were formed by the cycloaddition of the developed N-benzoyl
aminoacrylamide intermediates and triphosgene in short reaction times, suggesting that the present
reaction would provide a versatile method for the synthesis of pyrimidine based PET tracers. Then, we
carried out the synthesis of [2-11C]Thy and [2-11C]5-FU, which could serve clinically as useful PET
tracers for the evaluation of the cell proliferation ([2-11C]Thy) and for the assessment and prediction of
outcomes of 5-FU in chemotherapeutic treatment ([2-11C]5-FU).
Synthesis of Synthesis of [2-11C]Thy. 11C-labeled thymine was readily prepared in a similar manner as
described above, using the same automated synthesis system as used for [11C]CGP-12177 production.25
The direct ring closure reaction of [11C]COCl2 with non-activated precursor (5aZ) only restored the
starting material 5aZ. Therefore the precursor (5aZ) was treated with a base to convert it to the alkali
metal salt (5aZ-Na 5aZ-K), into which [11C] COCl2 gas was introduced to give [2-11C]Thy (Scheme 8).
O
O
O
O
N
[11C]COCl2
Me
Me
Me
Me
base
H2O
H2N
HN
11C
H2N
HN
11C
_
N
+
HN
COPh
M
O
N
H
O
COPh
COPh
[2-11C]Thy
6a
5aZ
M = Na: 5aZ-Na
M = K : 5aZ-K
Scheme 8. Synthesis of [2-11C]Thy
Table 2. Yields of [2-11C]Thy
Solvent
DME
DMF
Base
--
Mol. Eq.
Yield (MBq, EOS)
ND
-
NaH
5
5
2
2
1
ND
toluene
DME
DME
DME
NaH
41 (n=2)
NaH
68 ± 65 (n=3)
137 (n=2)
362 ± 53 (n=3)
t-BuOK
t-BuOK
Bombardment was carried out with a 10 μA beam of 18 MeV protons for 10 min.
EOS: end of synthesis. ND: not detected.

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HETEROCYCLES, Vol. 77, No. 2, 2009
The yield of [2-11C]Thy under these conditions was 362 ± 53 MBq at EOS (n=3) (Table 2). The
30
radiochemical yield of [2-11C]thymine was ca. 24% from [11C]COCl2.
[2-11C]Thymine produced was
identical with authentic thymine by comparison of their chromatographic behavior on HPLC. The
formation of [2-11C]thymine was further confirmed by the enzymatic conversion to [2-11C]thymidine.6, 31
Thus, the present cyclocondensation of an appropriate activated N-benzoyl aminoacrylamide intermediate
with highly reactive radiolabeling reagent [11C]COCl2 is a viable method for supplying [2-11C]Thy and
[2-11C]thymidine for clinical PET tracer studies.
Synthesis of [2-11C]5-FU. The key intermediate 5bE, activated as the sodium salt, was subjected to
cyclocondensation with [11C]COCl229 on the same automated synthesis system used for the production of
S-[11C]CGP-12177.25 The total synthesis took 17 minutes from the end of bombardment (EOB) to
isolation of [2-11C]5-FU. The yield of [2-11C]5-FU was 380 MBq at EOS (Scheme 9), for a radiochemical
yield of ca. 25%.
O
O
O
O
F
F
[11C]phosgene
rt, 5 min
F
F
NH3/MeOH
rt, 1 min
NaH/DME
rt, 2.5 h
H2N
H2N
HN
11C
HN
11C
_
N
+
HN
Na
O
N
H
O
N
COPh
COPh
COPh
5bE
5-FU
Scheme 9. Synthesis of [2-11C]5-FU
In all previous reports, labeling of the 2-position of thymine was accomplished by condensation of
[11C]urea and malate at 130 °C in fuming sulfuric acid. Recently, Steel et al reported an improved
method for the preparation of [2-11C]Thy via a multi-step process using [11C]urea derived from
[11C]COCl2. This radiosynthesis of [2-11C]thymine took approximately 30 min from EOB.6 On the
other hand, our strategy involving the cyclocondensation with [11C]COCl2 for the direct production of
[2-11C]thymine is operationally simple, and offers fewer reaction steps at lower temperature. The total
synthesis described herein takes 16 minutes from EOB to isolation of [2-11C]Thy, thus significantly
shortening reaction time, which is a crucial consideration for the preparation of short half-life
radiopharmaceuticals. The success in the synthesis lies in the synthesis of the key precursor 5aZ bearing
proper stereochemistry, and on the application of the highly reactive species, [11C]COCl2, for
cyclocondensation in the final step.
Synthesis of [2-11C]5-FU proceeded in the same way, resulting in the comparable radiochemical yields
with those of [2-11C]thymine using [11C]COCl2.29 Importantly, the radiochemical yields are adequate
for in vivo studies of [2-11C]5-FU uptake in patients, and would appear sufficient for analysis of 1-h
time-activity curves32 using the catenary, three-compartment, five-parameter model developed for

HETEROCYCLES, Vol. 77, No. 2, 2009
1315
5-[18F]FU in vivo.20
We have provided a substantially more useful method for the synthesis of [2-11C]Thy and [2-11C]5-FU.
Because of fewer reaction steps, mild reaction conditions, and reliability of product yield, the present
methodology should find wide application in the preparation of many 11C labeled radiopharmaceuticals.
EXPERIMENTAL
Materials and Analyses
Triphosgene was purchased from Aldrich Chemical Co. Ltd. (St. Louis, MO). All solvents were reagent
grade and distilled using the appropriate methods. Column chromatography was performed using silica
gel 60N (100-210 μm) and Aluminiumoxid 90 active neutral (70-230) Mesh ASTM, 0.063-0.200 mm,
purchased from Merck. Silica gel HPLC was conducted on a Shim-Pack PREP-Sil (H) (250 mm x 20 mm
i.d., Silca gel) using a LC-6A (Shimadzu, Kyoto, Japan) apparatus with monitoring at 254 nm. All
melting points are uncorrected. NMR spectra were measured with a JEOL JNM-EA500 (500 MHz)
spectrometer, and 1H-NMR chemical shift are given on the δ (ppm) scale based on those of the signals of
solvents. MS spectra and high-resolution MS (HRMS) spectra were recorded with JEOL JMS-FABmate
(EI). The elemental analyses (C, H, N) were within ± 0.4% of the theoretical values for C, H and N.
UV-Irradiation was carried out externally with a 500 W high-pressure mercury (h.p. Hg) lamp (Eiko-sha,
Osaka) in a degassed Pyrex tube (> 300 nm) on a merry-go-round apparatus.
Synthesis of Thy
Ethyl α-formylpropionate (2a). A solution of ethyl formate (10.4 mL, 130 mmol) and ethyl propionate
(7.5 mL, 65 mmol) in dry Et2O (80 mL) were added to a suspension of NaH (ca. 1.7 g, ca. 70 mmol) at
0 °C. The reaction mixture was stirred at ambient temperature for 60 h, and then neutralized with
aqueous hydrochloric acid. The reaction mixture was extracted three times with CH2Cl2. After drying
over anhydrous Na2SO4, the solvent was removed under atmospheric pressure to give 2a (3.1 g, 37%) as
oil.
Ethyl β-aminomethacrylate (3aZ, 3aE). To an ethereal solution (5 mL) of 2a (3.1g, 24 mmol) was
added 7M methanolic ammonia (9 mL, 63 mmol). After heating under reflux for 2 h, the solvent was
removed under reduced pressure, to give a mixture of 3aZ and 3aE (3.0 g, 98%) as oil. The Z-isomer
3aZ gradually isomerized itself into 3aE, to afford a mixture of 3aZ and 3aE (1 : 1) when kept at ambient
temperature for 21 h. Thus, the isolation of 3aE was not achieved.
(Z)-Ethyl β-aminomethacrylate (3aZ): H-NMR (CDCl3) δ: 1.25 (3H, t, J =6.9 Hz), 1.68 (3H, s), 4.03 (2H,
br s), 4.14 (2H, q, J =7.1 Hz), 7.43 (1H, t, J =10.3 Hz). EI-MS m/z: 129 [M]+. EI-HRMS m/z: 129.0795
(Calcd for C6H11NO2: 129.0790).

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HETEROCYCLES, Vol. 77, No. 2, 2009
(E)-Ethyl β-aminomethacrylate (3aE): 1H-NMR (CDCl3) δ: 1.25 (3H, t, J =6.9 Hz), 1.70 (3H, s), 4.03 (2H,
br s), 4.13 (2H, q, J =6.9 Hz), 6.64 (1H, t, J =11.2 Hz).
Ethyl β-(N-benzoylamino)methacrylate (4aZ , 4aE). A mixture of 3aZ and 3aE (260 mg, 2 mmol)
dissolved in CHCl3 (10 mL) was added to a solution of pyridine (320 μL, 0.4 mmol) and benzoyl chloride
(235 μL, 2 mmol) in CHCl3 (20 mL) at 0 °C and kept overnight at rt. After removal of the solvent, 10%
hydrochloric acid was added to the residual oil and extracted with Et2O. After drying over anhydrous
Na2SO4, the ethereal layer was subjected to silica-gel column chromatography with 10% AcOEt-hexane,
to afford 4aZ (240 mg, 35%).
(Z)-Ethyl β-(N-benzoylamino)methacrylate (4aZ): Colorless crystals, mp 61-62 °C, recrystalized from
AcOEt. 1H-NMR (CDCl3) δ: 1.34 (3H, t, J =6.9 Hz), 1.90 (3H, d, J =1.2), 4.26 (2H, q, J =7.0 Hz), 7.48
(2H, t, J =7.4 Hz), 7.55 (1H, t, J =7.5 Hz), 7.62 (1H, dd, J =1.1, 10.9 Hz), 7.93 (2H, d, J =6.9 Hz), 11.4
(1H, d, J =9.8 Hz). EI-MS m/z: 233 [M]+. EI-HRMS m/z: 233.1053 (Calcd for C13H15NO3: 233.1052).
(E)-Ethyl β-(N-benzoylamino)methacrylate (4aE): 1H-NMR (CDCl3) δ: 1.32 (3H, t, J =7.5 Hz), 2.00 (3H,
s), 4.24 (2H, q, J =7.2 Hz), 7.47 (1H, m), 7.50 (2H, t, J =7.5 Hz), 7.64 (1H, t, J =7.5 Hz), 8.13 (2H, d, J
=8.0 Hz), 8.45 (1H, d, J =1.2 Hz).
(Z)-β-(N-Benzoylamino)methacrylamide (5aZ). A solution of 4aZ in MeOH was added to an excess of
liquid ammonia and allowed to stand at rt for a week. The reaction mixture was subjected to column
chromatography over alumina with 50% AcOEt-hexane, to give 5aZ as colorless crystals. The 1H-NMR
measurement showed the reaction proceeded quantitatively. Colorless crystals, mp 182-184 °C
1
(recrystallized from 50% AcOEt-Hexane). H-NMR (CDCl3) δ: 1.95 (3H, d, J =1.2 Hz), 5.40-5.80 (2H,
br d, NH2), 7.46 (2H, t, J =7.2 Hz), 7.54 (1H, t, J =7.5 Hz), 7.54 (1H, d, J =7.5 Hz), 7.93 (2H, d, J =6.9
Hz), 12.3 (1H, br s). EI-LRMS m/z: 204 [M]+. EI-HRMS m/z: 204.0895 (Calcd for C11H12N2O2:
204.0899). Anal. Calcd for C11H12N2O2: C, 64.69; H, 5.92; N, 13.72. Found: C, 64.50; H, 6.05; N, 13.60.
Non-radio-labeled (cold) Thy. To a solution of 5aZ-Na (100.4 mg, 0.5 mmol) freshly prepared from
NaH in DMF (8 mL), triphosgene (25 mg, 0.08 mmol) dissolved in THF (2.5 mL) was added, and stirred
for 5 min. Then MeOH was added to the reaction mixture and the solvent was removed under reduced
pressure. The residue was dissolved in water and extracted with CHCl3. Aqueous NaOH (10%) was
added to the chloroform layer. After neutralization with 10% hydrochlolic acid, the aqueous solution
was submitted to reverse-phase HPLC with 6% aq. EtOH (3 mL/min), to give Thy quantitatively (32.4
mg).
Synthesis of 5-FU
N-[(1E)-2-carbamoyl-2-fluorovinyl]benzamide (5bE). A solution of ethyl formate (2.2 mL, 27.5
mmol) and ethyl fluoroacetate (2 mL) in dry Et2O (80 mL) was added to a solution of sodium ethoxide

HETEROCYCLES, Vol. 77, No. 2, 2009
1317
(1.4 g, 20 mmol) in benzene (25 mL) at 0 °C. The reaction mixture was stirred overnight at ambient
temperature. The solvent was evaporated under reduced pressure to give crude 2b as oil. Crude 2b
(0.64 mmol, 100 mg) was dissolved in NH4Cl saturated MeOH and 2 M NH3-MeOH and stirred for 3
days at rt. After removal of the solvent, the residue was passed through a short column of alumina by
using Et2O as the eluent to give 3b (69 mg, 0.6 mmol). To a solution of 3b thus obtained in Et2O,
benzoyl chloride (70 μL, 0.6 mmol) and dry pyridine (48.5 μL, 0.6 mmol) were added at -10 °C. The
reaction mixture was allowed to stand overnight at 0 °C, and subjected to HPLC with 10% AcOEt in
hexane, to afford 4bZ-ethyl ester (0.06 mmol, 14.8 mg) and 4bZ-methyl ester (0.08 mmol, 18.4 mg),
respectively.
Ethyl (2Z)-2-fluoro-3-(phenylcarbonylamino)prop-2-enoate (4bZ-ethyl ester): Colorless crystals, mp
101-102 °C (recrystallized from 20% AcOEt in hexane ). 1H-NMR (C6D6) δ: 0.83 (3H, t, J =7.45 Hz,
CH3), 3.88 (2H, q, J =7.45 Hz, CH2), 6.86 (2H, aromatic), 6.99 (1H, aromatic), 7.35 (2H, aromatic), 7.62
(1H, br s, NH), 8.03 (1H, dd, J=11.45 Hz, JH-F =25.75 Hz, CH). EI-LRMS m/z: 237 [M]+. EI-HRMS
m/z: 237.0799 (Calcd for C12H12NO3: 237.0801). Anal. Calcd for C12H12NO3: C, 60.76; H, 5.10; N, 5.90.
Found: C, 60.80; H, 5.10; N, 5.89.
Methyl (2Z)-2-fluoro-3-(phenylcarbonylamino)prop-2-enoate (4bZ-methyl ester): Colorless crystals, mp
142-143 °C (recrystallized from 20% AcOEt in hexane ). 1H-NMR (C6D6) δ: 3.27 (3H, s, CH3), 6.84 (2H,
aromatic), 6.97 (1H, aromatic), 7.26 (2H, aromatic), 7.31 (1H, br s, NH), 7.97(1H, dd, J=11.45 Hz, JH-F
=25.75 Hz, CH). EI-LRMS m/z: 223 [M]+. EI-HRMS m/z: 223.0647 (Calcd for C11H10NO3: 223.0644).
Anal. Calcd for C11H10NO3: C, 59.19; H, 4.52; N, 6.28. Found: C, 59.44; H, 4.59; N, 6.16.
Photochemical isomerization of 4bZ ester. A solution of 4bZ-ethyl ester (50 mg) in MeCN (10 mL)
was irradiated externally in a Pyrex tube at rt for 4 h. The reaction mixture was concentrated in vacuo,
and the residue was submitted to HPLC with 30% AcOEt in hexane to give the E-form 4bE-ethyl ester
and unchanged 4bZ-ethyl ester in 10% and 90% yields, respectively. Similar results were obtained
from 4bZ-methyl ester.
Ethyl (2E)-2-fluoro-3-(phenylcarbonylamino)-2-propenoate (4bE-ethyl ester): Colorless crystals, mp
91-92 °C (recrystallized from 20% AcOEt-hexane ). 1H-NMR (C6D6) δ: 0.83 (3H, t, J =7.45 Hz, CH3),
3.84 (2H, q, J =7.45 Hz, CH2), 6.89 (2H, aromatic), 6.95 (1H, aromatic), 7.79 (2H, aromatic), 7.86 (1H,
dd, JH-F, JH-H =11.45 Hz, CH), 10.2 (1H, br s, D2O exchangeable, NH). EI-LRMS m/z: 237 [M]+.
EI-HRMS m/z: 237.0792 (Calcd for C12H12NO3: 237.0801). Anal. Calcd for C12H12NO3: C, 60.76; H,
5.10; N, 5.90. Found: C, 60.56; H, 5.23; N, 5.87.
Methyl (2E)-2-fluoro-3-(phenylcarbonylamino)prop-2-enoate (4bE-methyl ester): Colorless crystals,
mp 80-81 °C (recrystallized from 20 % AcOEt-hexane ). 1H-NMR (C6D6) δ: 3.19 (3H, s, CH3), 6.89 (2H,
aromatic), 6.94 (1H, aromatic) 7.79 (2H, aromatic), 7.84 (1H, dd, JH-F, JH-H =11.45 Hz, CH), 10.16 (1H,

1318
HETEROCYCLES, Vol. 77, No. 2, 2009
br s, D2O exchangeable, NH). EI-LRMS m/z: 223 [M]+. EI-HRMS m/z: 223.0642 (Calcd for C11H10NO3:
223.0644). Anal. Calcd for C11H10NO3: C, 59.19; H, 4.52; N, 6.28. Found: C, 59.29; H, 4.54; N, 6.25.
N-[(1E)-2-carbamoyl-2-fluorovinyl]benzamide (5bE). A solution of 4bE (10 mg, 0.037 mmol) and
excess liq. ammonia in MeOH (1 mL) was allowed to stand in a high-pressure reaction vessel for 4 days
at rt. After evaporation of the solvent in vacuo, the reaction mixture gave N-[(1E)-2-carbamoyl-
2-fluorovinyl]benzamide (5bE) in quantitative yield. 5bE: Colorless crystals, mp 160-161 °C
(recrystallized from 20% AcOEt-hexane). 1H-NMR (CDCl3) δ: 5.68, 6.10 (1H, br s, D2O exchangeable,
NH2), 7.49 (2H, aromatic), 7.57 (1H, aromatic), 7.79(1H, dd, JH-F, JH-H =10.85 Hz, CH), 7.91 (2H,
aromatic), 10.8 (1H, br s, D2O exchangeable, NH). EI-LRMS m/z: 208 [M]+. EI-HRMS m/z: 208.0639
(Calcd for C10H9N2O2: 208.0648). Anal. Calcd for C10H9N2O2: C, 57.69; H, 4.36; N, 13.46. Found: C,
57.65; H, 4.52; N, 13.34.
Non-radio-labeled (cold) 5-FU. A solution of the sodium salts of 5bE, prepared from 5bE (20.0 mg, 0.1
mmol) and NaH, and triphosgene (33 mg, 0.1 mmol, 3 eq) in DME (5mL) was stirred at rt for 16 h.
After addition of 2 M NH3-MeOH (2 mL), the reaction mixture was condenced under reduced pressure.
The residue was submitted to reverse-phase HPLC with 3% MeOH in water (1 mL/min) to give 5-FU in
75 % yield.
Synthesis of uracil (Ura)
Ethyl N-benzoylaminoacrylate (4cE). Ethyl formylacetate (2c) was prepared by refluxing a mixture of
Meldrum’s acid (480 mg, 2.5 mmol) and EtOH (1.2 equv. molar) in benzene for 90 min, according to the
reported procedure.33 A solution of crude 2c (107 mg, 0.92 mmol, 37%), thus prepared, and liquid
ammonia in MeOH (2.0 mL) was allowed to stand for a week at rt, to give 3c. To a solution of crude 3c
(69 mg, 0.6 mmol) and pyridine (48.5 μL, 0.6 mmol) in Et2O (2 mL), benzoyl chloride (70 μL, 0.6 mmol)
dissolved in CHCl3 (20 mL) was added dropwise at -10 °C, and the reaction mixture was kept overnight
at -10 °C. After removal of the solvent, 10% hydrochloric acid was added to the residual oil and
extracted with ether. After drying over anhydrous Na2SO4, the ethereal layer was subjected to silica-gel
column chromatography with 10% AcOEt-hexane as an eluent, to afford ethyl N-benzoylaminoacrylate
(4cE) (14.8 mg, 0.068 mmol, 0.3%). 1H-NMR (CDCl3) δ: 1.27 (3H, t, J =6.9 Hz), 4.17 (2H, q, J =6.9 Hz),
5.64 (1H, d, J =14.3 Hz), 7.45 (2H, t, J =7.5 Hz), 7.55 (1H, t, J =7.5 Hz), 7.85 (1H, d, J = 7.5 Hz), 8.22
(1H, dd, J =11.5, 14.3 Hz), 8.67 (1H, d, J =11.5 Hz, -NH). EI-MS m/z: 219 [M]+.
Photochemical isomerization of E-form 4cE-ethyl ester to Z-form. A solution of E-form 4cE (50 mg)
in MeCN (10 mL) in a Pyrex tube was irradiated externally at rt for 4 h. The reaction mixture was
concentrated in vacuo, and submitted to HPLC with 30% AcOEt in hexane, to give 4cZ in the
desired-Z-form in 30% yield, together with unchanged 4cE in 70% yield.

HETEROCYCLES, Vol. 77, No. 2, 2009
1319
1
Ethyl N-benzoylaminoacrylate (4cZ): H-NMR (CDCl3) δ: 1.27 (3H, t, J =6.9 Hz), 4.17 (2H, q, J =6.9
Hz), 5.64 (1H, d, J =14.3 Hz), 7.45 (2H, t, J =7.5 Hz), 7.55 (1H, t, J =7.5 Hz), 7.85 (1H, d, J = 7.5 Hz),
8.22 (1H, dd, J =11.5, 14.3 Hz), 8.67 (1H, d, J =11.5 Hz, -NH). EI-MS m/z: 219 [M]+.
N-[(1E)-2-carbamoylvinyl]benzamide 5cI. A solution of 4cZ (10 mg, 0.046 mmol) and excess liq.
ammonia in MeOH (2.0 mL) was kept at rt for 4 days. After evaporation of the solvent in vacuo, the
residue was submitted to column chromatography on alumina with AcOEt – hexane (1 : 1) to give 5cI in
the yield of 90% (7.5 mg, 0.040 mmol) as colorless crystals.
Similarly 4cE afforded 5cI in 90 % yield. 1H-NMR (CD3OD) δ: 2.66 (2H, d, J =5.7 Hz), 5.15 (1H, bs or t,
J =5.7 Hz), 7.45 (2H, t, J =7.5 Hz), 7.52 (1H, t, J =7.5 Hz), 7.80 (1H, d, J = 7.5 Hz),. EI-MS m/z: 190
[M]+.
Non-radio-labeled (cold) Ura. A solution of 5cI (19.0 mg, 0.1 mmol) and NaH (21.6 mg, 60% oil, 0.5
mmol) in DME was kept at rt for 4 h. Triphosgene (33 mg, 0.1 mmol, 3 eq) was added to the solution,
which was allowed to stand overnight at rt. Then, 7 M ammonia in MeOH (2 mL) was added to the
solution, and the reaction mixture was kept for 2 min at rt. Removal of the solvent under reduced
pressure gave Ura in 2.4% (0.3 mg, 0.0024 mmol) yield.
Synthesis of 2-[11C]Thymine
According to the procedure adopted for non-radio-labeled (cold) Thy, [11C]phosgene was bubbled with
helium carrier into a solution of 5az-Na (0.2 mg) in DME (500 μL) for 1 min at 30 °C . After removal
of the solvent by evaporation, the residue consisting of [2-11C]N-benzoylthymine (6a) was treated with
1.5 M methanolic ammonia for 1 min at rt, and subjected to reverse-phase HPLC (column; μ-Bondapak
C18, 25 cm × 0.39 cm i.d., solvent; 3% EtOH–saline, flow rate; 0.5 mL/min at 40 °C), equipped with a
UV monitor (detected at 254 nm) and a γ counter. The radioactive peak at 11 min was identified as the
desired thymine. Radiochemical purity of [2-11C]thymine was estimated to be 99% by HPLC.
Synthesis of [2-11C]5-FU
According to the procedure for non-radio-labeled (cold) 5-FU, [11C]phosgene was infused with helium
into a solution of 5bE-Na (0.2 mg) in DME (500 μL) for 1 min at 30 °C. After removal of the solvent by
evaporation, the residue consisting of [2-11C]N-benzoyl-5-FU was treated with 1.5 M methanolic
ammonia for 1 min at rt, and subjected to reverse-phase HPLC (column; Inertsil ODS-3, 250 mm x 4.6
mm i.d., solvent; 3% EtOH - Saline, flow rate; 0.5 mL/min at 40 °C), equipped with a UV monitor
(detected at 270 nm) and a γ counter. The radioactive peak at 9.3 min was identified as authentic cold
5-FU.

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HETEROCYCLES, Vol. 77, No. 2, 2009
ACKNOWLEDGMENTS
This work was supported in part by Grants-in-Aid for Scientific Research and High Technology Research
Program from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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