Efficient syntheses of [11C]zidovudine and its analogs by convenient one-pot palladium(0)-copper(I) co-mediated rapid C-[ 11C]methylation

Article abstract of DOI:10.1002/jlcr.3213

The nucleosides zidovudine (AZT), stavudine (d4T), and telbivudine (LdT) are approved for use in the treatment of human immunodeficiency virus (HIV) and hepatitis B virus (HBV) infections. To promote positron emission tomography (PET) imaging studies on their pharmacokinetics, pharmacodynamics, and applications in cancer diagnosis, a convenient one-pot method for Pd(0)-Cu(I) co-mediated rapid C-C coupling of [¹¹C]methyl iodide with stannyl precursor was successfully established and applied to synthesize the PET tracers [¹¹C]zidovudine, [¹¹C]stavudine, and [ ¹¹C]telbivudine. After HPLC purification and radiopharmaceutical formulation, the desired PET tracers were obtained with high radioactivity (6.4-7.0 GBq) and specific radioactivity (74-147 GBq/μmol) and with high chemical (>99%) and radiochemical (>99.5%) purities. This one-pot Pd(0)-Cu(I) co-mediated rapid C-[¹¹C]methylation also worked well for syntheses of [methyl-¹¹C]thymidine and [methyl-¹¹C]4?- thiothymidine, resulting twice the radioactivity of those prepared by a previous two-pot method. The mechanism of one-pot Pd(0)-Cu(I) co-mediated rapid C-[ ¹¹C]methylation was also discussed. A convenient one-pot method for Pd(0)-Cu(I) co-mediated rapid C-C coupling of [¹¹C]methyl iodide with stannyl precursor was successfully established. Using this method, three antiviral agents were efficiently labeled with ¹¹C to generate the positron emission tomography tracers [¹¹C]zidovudine, [ ¹¹C]stavudine, and [¹¹C]telbivudine. In addition, syntheses of [methyl-¹¹C]thymidine and [methyl-¹¹C]4?- thiothymidine were greatly improved, resulting nearly twice the radioactivity of those obtained by a previous two-pot method. The radiochemical yields (decay-corrected from [¹¹C]carbon dioxide) of these probes reach 53-75%. Copyright

Full text of DOI:10.1002/jlcr.3213

Research Article
Received 08 April 2014,
Revised 28 May 2014,
Accepted 29 May 2014
Published online 3 July 2014 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3213
11
Efficient syntheses of [ C]zidovudine and its
analogs by convenient one-pot palladium(0)–
11
copper(I) co-mediated rapid C-[ C]methylation
a,b
c
a,b
Zhouen Zhang,a,b Hisashi Doi, Hiroko Koyama, Yasuyoshi Watanabe,
*
a,d**
and Masaaki Suzuki
The nucleosides zidovudine (AZT), stavudine (d4T), and telbivudine (LdT) are approved for use in the treatment of human
immunodeficiency virus (HIV) and hepatitis B virus (HBV) infections. To promote positron emission tomography (PET)
imaging studies on their pharmacokinetics, pharmacodynamics, and applications in cancer diagnosis, a convenient one-pot
method for Pd(0)–Cu(I) co-mediated rapid C–C coupling of [11C]methyl iodide with stannyl precursor was successfully
established and applied to synthesize the PET tracers [11C]zidovudine, [11C]stavudine, and [11C]telbivudine. After HPLC
purification and radiopharmaceutical formulation, the desired PET tracers were obtained with high radioactivity (6.4–7.0 GBq)
and specific radioactivity (74–147 GBq/μmol) and with high chemical (>99%) and radiochemical (>99.5%) purities. This
one-pot Pd(0)–Cu(I) co-mediated rapid C-[11C]methylation also worked well for syntheses of [methyl-11C]thymidine and
[methyl-11C]4′-thiothymidine, resulting twice the radioactivity of those prepared by a previous two-pot method. The
mechanism of one-pot Pd(0)–Cu(I) co-mediated rapid C-[11C]methylation was also discussed.
Keywords: [11C]Zidovudine ([11C]AZT); [11C]Telbivudine ([11C]LdT); [11C]Stavudine ([11C]d4T); [Methyl-11C]thymidine; [Methyl-11C]4′-
thiothymidine ([11C]4DST); Rapid C-[11C]methylation
that AZT could also be incorporated into the DNA of cancer cells
Introduction
to terminate the DNA chain, suppressing cancer growth.15 In
Positron emission tomography (PET) has been used for molecular
order to promote PET imaging studies on these anti-HIV and
anti-HBV agents, we present synthesis methods for [11C]
imaging studies in disease diagnosis and as a powerful, non-
zidovudine ([11C]AZT), [11C]stavudine ([11C]d4T), and [11C]
invasive tool for pharmacokinetics and pharmacodynamics
investigations in vivo.1–3 To this end, the PET tracers 2-deoxy-2-
[18F]fluoro-D-glucose ([18F]FDG) and [18F]fluorothymidine ([18F]FLT)
have been developed and used in the clinic for early cancer
diagnosis.1 Some PET tracers derived from therapeutic agents such
as [11C]verapamil,4 [11C]topotecan,5 [18F]fluoroestradiol,6 and [11C]
ketoprofen methyl ester7 have also been synthesized and used
for PET imaging studies on pharmacokinetics, drug resistance,4,5
and cancer characteristics,5,6 as well as on neuroinflammation.8
These studies could help physicians select the therapy and
therapeutic agent best suited for the patient as soon as possible,
resulting in great improvements in disease treatment.1,6,9 It is
probable that PET imaging studies on HIV and HBV infections and
on anti-HIV/anti-HBV agents could promote the development of
personalized therapies to enhance the quality of life for patients with
HIV and HBV infections. However, such PET imaging studies on HIV
telbivudine ([11C]LdT), (Figure 1).
Recently, for syntheses of 11C-labeled PET tracers, four types of
rapid C-[11C]methylation methods (sp–sp3, sp2(alkenyl)–sp3,
sp2(arenyl)–sp3, and sp3–sp3 couplings) were developed, using the
Pd(0)-mediated rapid reaction of [11C]CH3I with organoboron or
organostannyl precursors.16–22 When organostannyl compounds
aDivision of Bio-function Dynamics Imaging, RIKEN Center for Life Science
Technologies (CLST), Kobe, Hyogo 650-0047, Japan
bRIKEN Center for Molecular Imaging Science, Kobe, Hyogo 650-0047, Japan
cDivision of Regeneration and Advanced Medical Science, Graduated School of
Medicine, Gifu University, Gifu 501-1194, Japan
and HBV infections as well as on anti-HIV and anti-HBV agents are dDepartment of Clinical and Experimental Neuroimaging, National Center for
still very limited because of the lack of a suitable PET tracer.10,11
Geriatrics and Gerontology, Obu-shi, Aichi 474-8511, Japan
The synthetic thymidine nucleoside analogs zidovudine (AZT),
*Correspondence to: Zhouen Zhang, Division of Bio-function Dynamics Imaging,
RIKEN Center for Life Science Technologies (CLST), Kobe, Hyogo 650-0047, Japan.
stavudine (d4T),12 and telbivudine (LdT)13 have been approved
for use in the treatment of HIV and HBV infections. These E-mail address: zhang.zhouen@riken.jp
antiviral agents can be phosphorylated by cellular kinases to
**Correspondence to: Masaaki Suzuki, Department of Clinical and Experimental
triphosphate derivatives, resulting in the interruption of HIV or
Neuroimaging, National Center for Geriatrics and Gerontology, Obu-shi, Aichi
HBV replication by the inhibition of both HIV or HBV reverse
474-8511, Japan.
transcriptase and viral DNA synthesis.12–14 Similarly, it was found E-mail:suzukims@ncgg.go.jp
J. Label Compd. Radiopharm 2014, 57 540–549
Copyright © 2014 John Wiley & Sons, Ltd.

Z. Zhang et al.
Figure 1. Structures of PET tracers [11C]zidovudine and its analogs.
were used as precursors, copper(I) salt was used in combination to IL, USA). Other chemicals and solvents were purchased from Wako
activate the precursors. A one-pot Pd(0)–Cu(I) co-mediated rapid C- Pure Chemical Industries (Osaka, Japan), Tokyo Kasei Kogyo (Tokyo,
[11C]methylation method has been attempted previously, but the Japan), and Nacala Tesque (Kyoto, Japan).
method demonstrated a low yield and low reproducibility for PET
tracer synthesis.18,20 To overcome this problem, a two-pot process
Synthesis of 5′-O-(tert-butyldimethylsilyl)-2,3′-anhydro-2′-
deoxy-5-iodouridine (3)
was required until now.20,21 In the two-pot process, [11C]CH3I was
trapped in the first reactor containing a solution of Pd2(dba)3 and
P(o-tolyl)3 at room temperature, and then the resulting {[11C]
5-Iodo-2′-deoxyuridine (1) was selectively protected by tert-
butyldimethylsilyl group (TBS) to give 5’-O-(tert-butyldimethylsilyl)-
CH3PdI} complex was transferred to the second reactor containing
2’-deoxy-5-iodouridine (2).24 To a solution of compound 2 (1.81 g,
3.88 mmol) and triphenylphosphine (2.03 g, 7.76 mmol) in DMF
(15 mL) diisopropyl azodicarboxylate (DIAD), (1.56 mL, 7.76 mmol)
was added dropwise. The reaction mixture was kept stirring at room
temperature overnight, then quenched with aq. NH4Cl solution and
extracted with EtOAc. After drying over Na2SO4, the solvent was
removed under vacuum. The crude was purified by silica gel
column chromatography (EtOAc) to give product 3 as a white solid
an organostannyl precursor and copper(I) salt for a C-[11C]
methylation reaction to generate the desired PET tracer [11C]CH3-
R. Recently, we also improved syntheses for the PET tracers
[methyl-11C]thymidine ([11C]Thy) and [methyl-11C]4′-thiothymidine
([11C]4DST) by
two-pot Pd(0)–Cu(I) co-mediated rapid coupling
of [11C]CH3I with 5-tributylstannyl-2′-deoxyuridine and 5-
tributylstannyl-
4′-thio-2′-deoxyuridine, respectively.23 However,
for two-pot Pd(0)–Cu(I) co-mediated C-[11C]methylation,
modifications of the PET tracer synthesizers are required; therefore,
it was not easy to translate it into a general method for clinical use.
Thus, it is very important to establish a much more practical one-
pot Pd(0)–Cu(I) co-mediated rapid C-[11C]methylation method.
Herein, we reported a convenient and temperature-controlled
one-pot Pd(0)–Cu(I) co-mediated C-[11C]methylation process for
the syntheses of [11C]AZT and its analogs.
1
(1.6 g, 91%). H NMR (400 MHz, CDCl3) δ: 0.07 (s, 3H), 0.87 (s, 9H),
2.44–2.49 (m, 1H), 2.67–2.70 (m, 1H), 3.71–3.86 (m, 2H), 4.11–4.31
(m, 1H), 5.23 (s, 1H), 5.53 (d, J= 4.0 Hz, 1H), 7.56 (s, 1H); 13C NMR
(100 MHz, CDCl3 ) δ: À5.46, 18.25, 25.78, 33.48, 61.01, 81.42, 85.96,
88.04, 143.92, 154.13, 167.46; HRMS (ESI) calcd for C15H23IN2O4SiNa
(M + Na)+ 473.0369, found 473.0363.
Synthesis of 5′-O-(tert-butyldimethylsilyl)-3′-azido-3′-deoxy-
5-iodouridine (4)
Materials and methods
A mixture of 3 (900 mg, 2 mmol) and sodium azide (900 mg,
13.8 mmol) in DMF (8 mL) was heated to 110 °C and kept stirring
for 6 h. After cooling at room temperature, the reaction mixture
was quenched with aq. NH4Cl solution and extracted with EtOAc.
After drying over Na2SO4, the solvent was removed under
vacuum. The crude was purified by flash chromatography (Hex/
EtOAc = 3:1) to give product 4 as a white solid (480 mg, 49%).
1H NMR (400 MHz, CDCl3) δ: 0.18 (s, 3H), 0.96 (s, 9H), 2.22–2.27
(m, 1H), 2.47–2.53 (m, 1H), 3.79–4.05 (m, 2H), 4.22–4.26 (m, 1H),
6.16 (t, J = 6.4 Hz, 1H), 8.05 (s, 1H), 8.28 (br s, 1H); 13C NMR
(100 MHz, CDCl3 ) δ: À5.22, 18.49, 26.16, 33.61, 60.77, 63.02,
68.55, 85.04, 85.48, 143.91, 149.64, 159.70; HRMS (ESI) calcd for
C15H24IN5O4SiNa (M + Na)+ 516.0540, found 516.0530.
General remarks
The [11C]methylation was conducted in a lead-shielded hot cell
with remote control of all operations. [11C]Carbon dioxide was
produced by a 14N(p,α)11C reaction using a CYPRIS HM-12S
cyclotron (Sumitomo Heavy Industries, Tokyo, Japan) and then
converted into [11C]CH3I by LiAlH4 reduction followed HI
treatment, using an original automated synthesis system for
11C-labeling in RIKEN CLST.22,23 The obtained [11C]CH3I was used
for the Pd(0)–Cu(I) co-mediated rapid [11C]methylation. The
analytical HPLC system used for the [11C]methylated products,
consisted of a Shimadzu HPLC system with a system controller
(CBM-20A), an online degasser (DGU-20A3), a solvent delivery
unit (LC-20AD), a column oven (CTO-20AC), a photodiode array
detector (SPD-M20A), an Aloka radioanalyzer (RLC-700), and
software (LC-Solution). The radioactivity was quantified with an
ATOMLAB™ 300 dose calibrator (Aloka, Tokyo, Japan).
Synthesis of 5′-O-(tert-butyldimethylsilyl)-3′-azido-2′,3′-
dideoxy-5-(tributylstannyl)-uridine (5)
A solution of 4 (250 mg, 0.5 mmol) in dry THF (3 mL) was treated
with sodium hydride (20 mg, 60% in paraffin) for 10 min at RT.
The reaction mixture was then cooled to À78 °C and treated with
Materials
Tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) and tri 2 equiv. of n-BuLi (1.63 M, 0.61 mL, 1.0 mmol) for 30 min. The
(o-tolyl)phosphine (P(o-tolyl)3) were purchased from Sigma- resulted reaction mixture was added dropwise tributyltin chloride
Aldrich (Tokyo, Japan). Tetrakis(triphenylphoshine)palladium(0) (Pd (0.82 mL, 3.0 mmol) and hexamethylphosphoric triamine (0.54 mL).
(PPh3)4) was obtained from Strem Chemicals (Newburyport, MA, After stirring at À78 °C for 1 h, the reaction mixture was warmed-
USA). 2′-Deoxy-L-uridine was brought from Ark Pharm (Libertyville, up at room temperature and kept stirring for another 6 h. Then
J. Label Compd. Radiopharm 2014, 57 540–549
Copyright © 2014 John Wiley & Sons, Ltd.
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Z. Zhang et al.
the reaction solution was quenched with aq. NH4Cl, extracted with Synthesis of 5′-O-(tert-butyldimethylsilyl)-2′,3′-didehyro-2′,3′-
EtOAc. The organic phase was evaporated, and the residue was dideoxy-5-bromouridine (9)
purified by flash chromatography (Hex/EtOAc = 1:4) to give product
5 as a solid (223 mg, 68%). 1H NMR (400MHz, CDCl3) δ: 0.10 (s, 3H),
Compound 8 (0.82 g, 1.7 mmol) was treated with trimethyl
phosphite (7 mL) at 110 °C for 6 h. The reaction mixture was
concentrated under reduced pressure, then purified by flash
0.88–1.64 (m, 36H), 2.25–2.28 (m, 1H), 2.45–2.50 (m, 1H), 3.69–3.99
(m, 3H), 4.21–4.22 (m, 1H), 6.06 (t, J= 6.8 Hz, 1H), 7.12 (s, 1H), 8.14
(br s, 1H); 13C NMR (100 MHz, CDCl3 ) δ: À5.44, À5.32, 9.89, 13.57,
column chromatography (EtOAc/Hex = 1:2) to give product 9 as
a white solid (0.4 g, 58%). 1H NMR (400 MHz, CDCl3) δ: 0.11
13.66, 17.50, 18.30, 25.88, 26.82, 26.93, 27.24, 27.82, 28.82, 28.92,
(s, 6H), 0.90 (s, 9H), 3.84–3.95 (m, 2H), 4.92 (d, J = 3.2 Hz, 1H),
29.02, 37.27, 61.50, 63.22, 84.44, 85.78, 112.85, 143.10, 150.58,
165.95; HRMS (ESI) calcd for C27H51N5O4SiSnNa (M + Na)+
5.89 (d, J = 3.2 Hz, 1H), 6.28 (d, J = 3.2 Hz, 1H), 6.92 (d, J = 3.2Hz,
1H), 7.97 (s, 1H), 8.48 (s, 1H). 13C NMR (100 MHz, CDCl3) δ:
680.2634, found 680.2640.
À5.17, À5.06, 18.72, 26.07, 64.31, 87.67, 90.46, 96.92, 126.14,
134.82, 139.95, 149.70, 158.74. MS (HR-ESI) calcd for
Synthesis of 3′-azido-2′,3′-dideoxy-5-(tributylstannyl)-uridine
(AZT–Sn)
C15H23BrN2O4SiNa (M + Na)+ 427.0490, found 427.0502.
A solution of 5 (60 mg, 0.091 mmol) in THF (1 mL) was cooled to
0 °C and treated with tetra-n-butylammonium fluoride (TBAF)
solution in THF (1 M, 1 mL). After stirring for 1 h, the reaction
mixture was quenched with aq. NH4Cl and extracted with EtOAc.
The organic phase was evaporated, and the residue was purified
by flash chromatography (MeOH/CHCl3 = 1:20) to give product
Synthesis of 5′-O-(tert-butyldimethylsilyl)-2′,3′-didehydro-
2′,3′-dideoxy-5-(tributylstannyl)-uridine (10)
A solution of 9 (200 mg, 0.5 mmol), Pd(PPh3)4 (58 mg, 0.05 mmol),
and bis(tributyltin), (0.5 mL, 1 mmol) in dioxane (10 mL) was
refluxed for 48 h. The reaction mixture was concentrated under
reduced pressure, the residue was purified by a flash column
chromatography (EtOAc/Hex = 1:3) to give product 10 as a clear
1
AZT–Sn as a white solid (45 mg, 91%). H NMR (400 MHz, CDCl3)
δ: 0.88–1.55 (m, 27H), 2.30–2.40 (m, 1H), 2.61–2.67 (m, 1H), 3.80–
3.99 (m, 3H), 4.41–4.43 (m, 1H), 5.98 (t, J = 6.8 Hz, 1H), 7.20 (s, 1H),
8.40 (br s, 1H); 13C NMR (100 MHz, CDCl3 ) δ: 9.89, 13.68, 16.40,
27.05, 27.28, 27.84, 28.94, 37.02, 60.48, 62.36, 84.67, 87.96,
113.30, 145.09, 150.69, 165.76; HRMS (ESI) calcd for
C21H37N5O4SnNa (M + Na)+ 566.1769, found 566.1769.
1
oil (82 mg, 23%). H NMR (400 MHz, CDCl3) δ: 0.07 (s, 3H), 0.08
(s, 3H), 0.88–1.58 (m, 36 H), 3.56–3.79 (m, 1H), 3.79–3.83 (m, 1H),
4.86 (d, J = 3.6 Hz, 1H), 5.86 (d, J = 6 Hz, 1H), 6.44 (d, J = 6 Hz,
1H), 6.99 (q, J = 3.6 Hz, 1H), 7.94 (s, 1H). 13C NMR (100 MHz, CDCl3)
δ: À5.37, À5.27, 9.87, 13.67, 18.32, 25.85, 26.95, 27.24, 27.55,
28.95, 29.05, 65.79, 86.83, 90.09, 113.00, 125.66, 135.30, 143.25,
151.33, 161.12. MS (HR-ESI) calcd for C27H50N2O4SiSnNa
(M + Na)+ 637.2464, found 637.2474.
Synthesis of 5′-O-(tert-butyldimethylsilyl)-5-bromouridine (7)
A solution of 5-bromouridine (6), (1.95 g, 5 mmol) in DMF
(12 mL) was cooled to 0 °C and treated with imidazole (1.0 g,
14.7 mmol) and tert-butyldimethylchlorosilane (TBSCl) (0.945 g,
6.3 mmol). Then, the reaction mixture was warmed at room
temperature and stirred overnight. The reaction was quenched
with aq. NH4Cl solution and extracted with EtOAc. The organic
phase was evaporated, and the residue was purified by silica
chromatography (MeOH/CHCl3 = 1:9) to give product 7 as a
white solid (1.73 g, 66%). 1H NMR (400 MHz, CD3OD) δ: 0.19
(s, 6H), 0.98 (s, 9H), 3.84–4.15 (m, 5H), 5.93 (q, J = 1.6 Hz, 1H),
7.90 (s, 1H), 8.13 (s, 1H). 13C NMR (100 MHz; CD3OD ) δ: À4.85,
19.71, 26.92, 64.43, 71.88, 76.56, 86.95, 90.45, 97.79, 141.26,
Synthesis of 2′,3′-didehydro-2′,3′-dideoxy-5-(tributylstannyl)-
uridine (d4T–Sn)
A solution of 10 (50 mg, 0.07 mmol) in THF (0.5 mL) was cooled
to 0 °C and treated with TBAF solution in THF (1 M, 0.5 mL). After
stirring for 1 h, the reaction mixture was quenched with aq.
NH4Cl solution and extracted with EtOAc. The organic phase
was dried over Na2SO4 and evaporated under vacuum. The
residue was purified by a flash chromatography (MeOH/CHCl3 =
1
1:20) to give product d4T–Sn as a white solid (40 mg, 95%). H
NMR (400 MHz, CDCl3) δ: 0.86–1.57 (m, 27 H), 3.77–3.90 (m, 2H),
4.94 (bs, 1H), 5.88 (d, J = 6, 1H), 6.32 (q, J = 2.8 Hz, 1H), 6.99
(q, J = 3.2Hz, 1H), 7.31 (s, 1 H), 7.97 (s, 1H). 13C NMR (100 MHz,
CDCl3) δ: 9.76, 13.67, 27.23, 28.93, 63.81, 86.97, 90.13, 112.84,
126.63, 134.09, 144.33, 165.89. MS (HR-ESI) calcd for
C21H36N2O4SnNa (M+ Na)+ 523.1599, found 523.1602.
152.07, 161.73. MS (HR-ESI) calcd for
C15H25BrN2O6SiNa
(M + Na)+ 461.0544, found 461.0536.
Synthesis of 5′-O-(tert-butyldimethylsilyl)-5-bromouridine
2′,3′-O-thiocarbonate (8)
A solution of 1,1′-thiocarbonylimidazole (7), (0.63 g, 3.55 mmol)
in dry CH2Cl2 (5 mL) was dropped into a solution of 7 (1.4 g,
3.2 mmol) in dry DMF (5 mL). The reaction mixture was stirred
Synthesis of 3′,5′-bis-O-(tert-butyldimethylsilyl)-2′-deoxy-L-
uridine (12)
for 3 h at room temperature, then diluted with EtOAc (30 mL) To a solution of 2′-deoxy-L-uridine (11), (0.5 g, 2.2 mmol) in DMF
and extracted with H2O. The organic layer was dried over Na2SO4 (5 mL) was added imidazole (0.84 g, 12 mmol), followed by
and reduced under vacuum. The crude product was purified by (0.82 g, 5.5 mmol) TBSCl. The mixture was stirred at room
flash column chromatography (EtOAc/Hex, 25 → 47% ) to give temperature overnight then quenched with aq. NH4Cl solution
product 8 as a white solid (1.33 g, 87%). 1H NMR (400 MHz, CDCl3) and extracted with EtOAc. The organic phase was dried over
δ: 0.09 (s, 6H), 0.89 (s, 9H), 3.84–3.94 (m, 2H), 4.60 (q, J = 4 Hz, 1H), Na2SO4 and evaporated under vacuum. The residue was purified
5.55 (q, J = 4 Hz, 1H), 5.83 (d, J = 2 Hz, 1H), 7.71 (s, 1H), 8.66 (s, 1H). by flash chromatography (EtOAc/Hex, 5 → 30%) to give 12 (1.0 g,
13C NMR (100 MHz, CDCl3) δ: À4.76, 18.96, 26.55, 63.32, 86.01, 100%) as a white foam powder. 1H NMR (400 MHz, CDCl3) δ:
88.14, 89.08, 94.48, 98.30, 140.91, 150.05, 159.49, 189.79. MS 0.08–0.11 (m, 12H), 0.88–0.92 (m, 18H), 2.04–2.09 (m, 1H), 2.29–
(HR-ESI) calcd C16H23BrN2O6SSiNa (M + Na)+ 501.0127, found 2.35 (m, 1H), 3.75–3.92 (m, 3H), 4.40–4.43 (m, 1H), 5.68
501.0128.
(q, J = 5.6Hz, 1H), 6.28 (t, J = 6 Hz, 1H), 7.89 (d, J = 8 Hz, 1H), 8.44
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Z. Zhang et al.
(bs, 1H). 13C NMR (100 MHz, CDCl3) δ: À5.59, À4.88, À4.63, 17.96, reaction solution was mixed with CuCl (0.8 mg, 8 μmol) and
18.33, 25.70, 25.85, 41.83, 62.41, 71.18, 85.18, 87.76, 102.11, K2CO3 (1.4 mg, 10 μmol) and then transferred to the reactor by
140.18, 150.03, 162.96. MS (HR-ESI) calcd for C21H40N2O5Si2Na using a syringe. Then, tributylphenylstannane (3–8 μmol) in
(M + Na)+ 479.2373, found 479.2371.
DMF (50 μL) was added to the reactor and kept at room
temperature while waiting for the [11C]CH3I preparation.
Synthesis of 3′,5′-bis-O-(tert-butyldimethylsilyl)-2′-deoxy-L-5- Subsequently, [11C]CH3I with N2 gas was bubbled into the
bromouridine (13)
reaction mixture by distillation. The resultant reaction mixture
was quickly heated to 65 °C for 5 min. The reaction mixture was
diluted with 50% CH3CN (1 mL). After being passed through a
syringe filter (filter media, PVDF; pore size, 0.2 mm; Whatman
Inc., NJ, USA), the filtrate (20 μL) was sampled for HPLC analysis
(analytical column, COSMOSIL 5C18 MS-II 4.6 ø × 150mm [Nacalai
Tesque]; mobile phase, 65% CH3CN; flow rate, 1.0 mL/min;
detection, UV 254 nm). The retention time of [methyl-11C]toluene
was 5.1 min. The yield of [methyl-11C]toluene (i.e., conversion of
[11C]methyl iodide into the desired product [methyl-11C]toluene)
was determined from the radio-HPLC peak area percentage of the
sum of all radioactive peak areas. Additionally, the side product 1,
1′-biphenyl generated by homocoupling of tributylphenylstannane
was also detected by the aforementioned HPLC with a retention time
of 8.0 min.
A solution of 12 (1.0 g, 2.2 mmol) in dimethoxyethane (20 mL)
was added NaN3 (0.57 g, 8.8 mmol) and N-bromosuccinimide
(0.428 g, 2.42 mmol). The reaction mixture was stirred at room
temperature overnight then diluted with EtOAc and washed
with water. The organic phase was dried over Na2SO4 and
concentrated under reduce pressure. The crude mixture was
purified by a flash chromatograph (EtOAc/Hex, 15 → 30%) to
give product 13 (0.83 g, 70%) as a white solid. 1H NMR (400 MHz,
CDCl3) δ: 0.08–0.16 (m, 12H), 0.89–0.95 (m, 18H), 1.99–2.06
(m, 1H), 2.29–2.35 (m, 1H), 3.76–4.00 (m, 3H), 4.40–4.42 (m, 1H),
6.28 (q, J = 5.6Hz, 1H), 8.08 (s, 1H), 8.29 (s, 1H). 13C NMR (100 MHz,
CDCl3) δ: À5.37,À5.29, À4.85, 17.99, 18.47, 25.72, 26.06, 42.03,
63.04, 72.47, 85.88, 88.47, 96.76, 139.47, 149.23, 158.68. MS (HR-ESI)
calcd for C21H39BrN2O5Si2Na (M + Na)+ 557.1479, found 557.1473.
Method B
Synthesis of 3′,5′-bis-O-(tert-butyldimethylsilyl)-2′-deoxy-L-5-
(tributylstannyl)-uridine (14)
The reaction mixture of Pd2(dba)3, P(o-tolyl)3, CuCl, and K2CO3
was prepared in the same way as in Method A and then
transferred to the reactor by using a syringe. After this, the
reaction mixture was cooled to À10 °C, tributylphenylstannane
(3–8 μmol) in DMF (50 μL) was added and kept below À10 °C
by cooling while waiting for the [11C]CH3I preparation. This
reaction mixture was further cooled to À20 °C to trap [11C]CH3I
gas, followed by the mixture being quickly heated to 65 °C for
5 min. The resulting reaction mixture was diluted with 50%
CH3CN and sampled for radio-HPLC analysis using the procedure
described in Method A.
A solution of 13 (273 mg, 0.5 mmol) in dioxane (15 mL) was
added Pd(PPh3)4 (58 mg, 0.05 mmol) and bis(tributyltin), (0.5 mL,
1 mmol). The reaction mixture was refluxed for 48 h. The reaction
mixture was concentrated under reduced pressure then purified
by a flash column chromatography (EtOAc/Hex, 10 → 30%) to
1
give product 14 as a clear oil (146 mg, 39%). H NMR (400 MHz,
CDCl3) δ: 0.07–0.09 (m, 12H), 0.87–1.55 (m, 45H), 1.90–2.02(m,
1H), 2.29–2.32(m, 1H), 3.60–3.78 (m, 2H), 3.94 (d, J = 1.2 Hz, 1H ),
4.38 (t, J = 2.8 Hz,1H), 6.23 (t, J = 6 Hz, 1H), 7.16 (s, 1H), 8.16
(s, 1H). 13C NMR (100 MHz, CDCl3 ) δ: À5.70, À5.69, À5.15, 9.54,
13.35, 16.95, 17.66, 18.04, 25.60, 26.43,27.93, 28.60, 40.02, 63.01,
72.20, 85.19, 87.28, 112.14, 142.91, 150.33, 165.65. MS (HR-ESI)
calcd for C33H66N2O5Si2SnNa (M + Na)+ 769.3436, found
769.3414.
Synthesis of [methyl-11C]2-picoline
[Methyl-11C]2-picoline was synthesized following
a similar
process described in Method B for [methyl-11C]toluene. [11C]
CH3I was trapped in the reaction mixtures of Pd2(dba)3 (1.0 mg,
1.1 μmol), P(o-tolyl)3 (5.0 mg, 16 μmol), CuBr (1.0 mg, 7 μmol),
CsF (2.0 mg, 13 μmol), and the precursor 2-tributylsta-
nnylpyridine (3.0 mg, 8.1 μmol) in N-methylpyrrolidone (250 μL)
at À20 °C. The resulting mixture was then quickly heated to
80 °C for 5 min, followed by dilution with 50% CH3CN (1 mL).
After being passed through a syringe filter (filter media, PVDF;
pore size, 0.2 mm; Whatman Inc.), the filtrate (20 μL) was sampled
for HPLC analysis (analytical column, COSMOSIL 5C18 MS-II 4.6
ø × 150 mm [Nacalai Tesque]; mobile phase, 20% CH3CN; flow
rate, 1.0 mL/min; detection, UV 254 nm). The retention time of
[methyl-11C]2-picoline was 5.0 min. From the radio-HPLC peak
area percentage of the sum of all radioactive peak areas, the
yield of [methyl-11C]2-picoline was determined as 89 2%
(n = 2). In addition, the side product pyridine generated via
demetallation of 2-tributylstannylpyridine was also detected by
the aforementioned HPLC method with a retention time of
3.3 min.
Synthesis of 2’-deoxy-L-5-(tributylstannyl)-uridine (LdT-Sn)
Compound 14 (120 mg, 0.16 mmol) in THF (2 mL) was cooled to
0 °C and treated with TBAF in THF (1 M, 1 mL). After stirring for
2 h, the reaction mixture was quenched with aq. NH4Cl solution
and extracted with EtOAc. The organic phase was dried over
Na2SO4 and evaporated under vacuum. The residue was purified
by flash chromatography (MeOH/CHCl3, 5 → 9%) to give the
product LdT–Sn as
a
white solid (80 mg, 97%).1H NMR
(400 MHz, CDCl3) δ: 0.87–1.54 (m, 27H), 2.23–2.53(m, 2H), 3.790–
4.04 (m, 3H), 4.60 (t, J = 3.6 Hz, 1H), 6.12 (t, J = 6.8 Hz,1H), 7.24
(s, 1H), 8.70 (bs, 1H). 13C NMR (100 MHz, CDCl3) δ: 9.85, 13.66,
27.25, 28.92, 39.83, 62.62, 71.92, 87.13, 87.96, 113.05, 145.31,
151.00, 166.30. MS (HR-ESI) calcd for C21H38N2O5SnNa (M + Na)+
541.1704, found 541.1696.
Synthesis of [methyl-11C]toluene by a one-pot Pd(0)–Cu(I)
co-mediated rapid C-[11C]methylation method
Synthesis of [methyl-11C]3-picoline
Method A
Pd2(dba)3 (1.0 mg, 1.1 μmol) and P(o-tolyl)3 (3.4 mg, 11 μmol) [Methyl-11C]3-picoline was synthesized and analyzed following
were dissolved in DMF (250 μL) with sonication. The resultant the same process for [methyl-11C]2-picoline. Precursor 3-
J. Label Compd. Radiopharm 2014, 57 540–549
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Z. Zhang et al.
tributylstannylpyridine (3.0 mg, 8.1 μmol) was used. The desired 5C18 4.6 ø × 150 mm [Phenomenex Inc.]; mobile phase, 5%
tracer [methyl-11C]3-picoline was detected by HPLC with a CH3CN containing 100 mM NH4HCO3; flow rate, 1.0 mL/min;
retention time of 6.0 min. From the radio-HPLC peak area detection, UV 265 nm) with a retention time of 5.4 min.
percentage of the sum of all radioactive peak areas, the yield
of [methyl-11C]3-picoline was determined as 92 2% (n = 2).
Additionally, the side product 3,3′-bipyridine generated via
homocoupling of 3-tributylstannylpyridine was detected by
HPLC with a retention time of 4.9 min.
Synthesis of [11C]Thy
[
11C]Thy was synthesized following the same process for
[11C]AZT. Precursor 5-tributylstannyl-2′-deoxyuridine (5.5 mg,
10.6 μmol) was used. The product was purified by a preparative
HPLC (semipreparative column, Gemini 5C18 21.2 ø × 250 mm
[Phenomenex Inc.]; mobile phase, 4% CH3CN containing 100 mM
NH4HCO3; flow rate, 9.9mL/min; detection, UV 265nm). The
retention time of [11C]Thy was 12.5min. The total time of the
synthesis was 39 min from EOB. The DCY was 75 2% (n = 3) from
[11C]carbon dioxide. Product [11C]Thy was identified by an
analytical HPLC (column, Luna 5C18 4.6 ø × 150mm [Phenomenex
Inc.]; mobile phase, 4% CH3CN containing 100 mM NH4HCO3; flow
rate, 1.0mL/min; detection, UV 265 nm) with a retention time of
6.9min.
Synthesis of 5-[methyl-11C]-3′-azido-2′,3′-dideoxy-thymidine
([11C]AZT)
[11C]AZT was synthesized following the same process described
in Method B for [methyl-11C]toluene. [11C]CH3I was trapped in
the reaction mixtures of Pd2(dba)3 (1.0 mg, 1.1 μmol), P(o-tolyl)3
(3.4 mg, 11 μmol), precursor AZT–Sn (2.0 mg, 3.7 μmol), CuCl
(0.8 mg, 8 μmol), and K2CO3 (1.4 mg, 10 μmol) in DMF (300 μL)
at À20 °C. The resulting mixture was then quickly heated to
80 °C for 5 min, followed by dilution with 10% CH3CN (1 mL).
The mixture was passed through cotton and then syringe filter
(filter media, PVDF; pore size, 0.2 mm; Whatman Inc.). The filtrate
was injected into a preparative HPLC (semipreparative column,
COSMOSIL 5C18 MS-II 20 ø × 250 mm [Nacalai Tesque Inc.];
mobile phase, 13.3% CH3CN; flow rate, 9.9 mL/min; detection,
UV 265 nm). The retention time of [11C]AZT was 12.5 min. The
desired fraction was collected into a flask and concentrated to
remove the organic solvent under reduced pressure. The residue
was then diluted in a mixture of 25% ascorbic acid (200 μL) and
saline (2 mL). The total time of the synthesis was 35 min from end
of bombardment (EOB). The decay-corrected radiochemical yield
(DCY) was 58 6% (n = 3) from [11C]carbon dioxide. Product [11C]
AZT was identified by an analytical HPLC (column, COSMOSIL
5C18 AR-II 4.6 ø × 150 mm [Nacalai Tesque Inc.]; mobile phase,
14% CH3CN; flow rate, 1.0 mL/min; detection, UV 265 nm) with
a retention time of 6.0 min.
Synthesis of [11C]4DST
[11C]4DST was synthesized following the same process for [11C]
AZT. Precursor 5-tributylstannyl-4′-thio-2′-deoxyuridine (4.0 mg,
7.5 μmol) was used. The product was purified by a preparative
HPLC (semipreparative column, Gemini 5C18 21.2 ø × 250 mm
[Phenomenex Inc.]; mobile phase, 7% CH3CN containing
100 mM NH4HCO3; flow rate, 9.9 mL/min; detection, UV
265 nm). The retention time of [11C]4DST was 13.5 min. The total
time of the synthesis was 36 min from EOB. The DCY was 65 4%
(n = 3) from [11C]carbon dioxide. Product [11C]4DST was
identified by an analytical HPLC (column, Luna 5C18 4.6
ø × 150 mm [Phenomenex Inc.]; mobile phase, 7% CH3CN
containing 100 mM NH4HCO3; flow rate, 1.0 mL/min; detection,
UV 265 nm) with a retention time of 7.4 min.
Synthesis of 5-[methyl-11C]-2′,3′-didehydro-2′-deoxy-
Results and discussion
thymidine ([11C]d4T)
Optimization of a one-pot Pd(0)–Cu(I) co-mediated rapid
C-[11C]methylation method of a stannyl precursor
[11C]d4T was synthesized following the same process for [11C]
AZT. Precursor d4T–Sn (2.0 mg, 4.0 μmol) was used. The product
was purified by a preparative HPLC (semipreparative column,
Gemini 5C18 21.2 ø × 250 mm [Phenomenex Inc., CA, USA];
mobile phase, 9% CH3CN; flow rate, 9.9 mL/min; detection, UV
265 nm). The retention time of [11C]d4T was 12.2 min. The total
time of the synthesis was 35 min from EOB. The DCY was
54 3% (n = 3) from [11C]carbon dioxide. Product [11C]d4T was
identified by an analytical HPLC (column, Luna 5C18 4.6
ø × 150 mm [Phenomenex Inc.]; mobile phase, 7% CH3CN; flow
rate, 1.0 ml/min; detection, UV 265 nm) with a retention time of
6.8 min.
In order to optimize the one-pot Pd(0)–Cu(I) co-mediated rapid
C-[11C]methylation method, the stannyl precursor tributyl-
phenylstannane was used as
a model compound for
synthesizing the PET tracer [methyl-11C]toluene. In the previous
one-pot method (Method A), the reaction mixture of Pd2(dba)3,
P(o-tolyl)3, CuCl, K2CO3, and the stannyl precursor (3–4 μmol)
was prepared and kept at room temperature while waiting for
the
[ [
11C]CH3I preparation. Subsequently, 11C]CH3I with
nitrogen gas was bubbled into the reaction mixture by
distillation. The resultant reaction mixture was then heated
5 min at 65 °C for [methyl-11C]toluene generation. We found
that the yield of [methyl-11C]toluene (conversion of [11C]CH3I
into [methyl-11C]toluene) was only around 0.8–12.4% (Figure 2,
Synthesis of 5-[methyl-11C]-2′-deoxy-L-thymidine ([11C]LdT)
[11C]LdT was synthesized following the same process for [11C] Method A). Although the amount of tributylphenylstannane
AZT. Precursor LdT–Sn (4.0 mg, 7.7 μmol) was used. The product was increased to 5–8 μmol, the yield of [methyl-11C]toluene
was purified by a preparative HPLC (semipreparative column, was still less than 50% and much lower than the yield (~90%)
Gemini 5C18 21.2 ø × 250 mm [Phenomenex Inc.]; mobile phase, obtained in an ordinary organic chemical reaction in our
4% CH3CN containing 100 mM NH4HCO3; flow rate, 9.9 mL/min; previous study.17 In the ordinary organic chemical reaction,
detection, UV 265 nm). The retention time of [11C]LdT was after the reaction mixtures of Pd2(dba)3, P(o-tolyl)3),
13.7 min. The total time of the synthesis was 35 min from EOB. tributylphenylstannane, and copper(I) chloride in DMF were
The DCY was 53 3% (n = 3) from [11C]carbon dioxide. Product prepared within 3 min at room temperature, they were
[11C]LdT was identified by an analytical HPLC (column, Luna immediately added to the solution of CH3I in DMF and then
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Z. Zhang et al.
synthesized as a precursor (Figure 3A). 5-Iodo-2′-deoxyuridine (1) was
selectively protected by TBS to give 5′-O-(tert-butyldimethylsilyl)-2′-
deoxy-5-iodouridine (2).24 Compound 2 was treated with
triphenylphosphine and DIAD to yield 5′-O-(tert-butyldim-
ethylsilyl)-2,3′-anhydro-2′-deoxy-5-iodouridine (3), which reacted
with sodium azide to produce 5′-O-(tert-butyldimethylsilyl)-3′-
azido-3′-deoxy-5-iodouridine (4). Subsequently, 4 was treated with
sodium hydride, n-BuLi, and tributyltin chloride to yield 5′-O-(tert-
butyldimethylsilyl)-3′-azido-2′,3′-dideoxy-5-(tributylstannyl)-uridine
(5). Following deprotection of TBS with TBAF, the stannyl precursor
AZT–Sn was obtained. By the optimized one-pot Pd(0)–Cu(I) co-
mediated rapid C-[11C]methylation method, the precursor AZT–Sn
was efficiently coupled with [11C]CH3I to generate the PET tracer
[11C]AZT. After HPLC purification and radiopharmaceutical
formulation, the desired product [11C]AZT was obtained with a
radioactivity of 7.0 0.8 GBq (n=3) and a specific radioactivity in
the range of 74–128 GBq/μmol at the end of synthesis (see Table 1).
The total time of the synthesis was 35 min from EOB. The DCY was
58 6% (n=3) from [11C]carbon dioxide. The chemical and
radiochemical purities of the product were >99% and >99.5%,
respectively. Thus, by using the optimized one-pot Pd(0)–Cu(I) co-
mediated rapid C-[11C]methylation, the PET probe [11C]AZT was
synthesized successfully with sufficient radioactivity and specific
radioactivity and high chemical and radiochemical purities, which
could meet the requirements for preclinical PET studies.
Figure 2. The yield of [methyl-11C]toluene generated by Method A (squares) and
Method B (triangles) using various amounts of the precursor tributylphenylstannane
(Ph–Sn). The yield of [methyl-11C]toluene was determined from the radio-HPLC peak
area percentage of the sum of all radioactive peak areas. In Method A, the reaction
mixture of Pd2(dba)3, P(o-tolyl)3, CuCl, K2CO3, and Ph–Sn was prepared and kept at
room temperature while waiting for [11C]CH3I preparation; it was not cooled
while [11C]CH3I gas was bubbling. In Method B, the reaction mixture of Pd2(dba)3, P
(o-tolyl)3, CuCl, K2CO3, and Ph–Sn was controlled below À10 °C before [11C]CH3I
bubbling and further cooled to À20°C during [11C]CH3I gas bubbling. For details,
please see the text.
quickly heated for a coupling reaction. In contrast, during hot
Synthesis of the PET tracer [11C]d4T
PET tracer synthesis,
[
11C]CH3I was synthesized from
cyclotron-generated [11C]CO2 and then bubbled into the
mixture of Pd(0)–Cu(I) and the stannyl precursor by distillation.
Additionally, all reactions had to be carried out in a shielded hot
cell. Thus, the mixture of Pd(0)–Cu(I) and the stannyl precursor
had to be prepared before [11C]CH3I was generated.
[11C]d4T was synthesized previously by Alan J. Fischman and his
colleagues25 by using
a tedious process. In brief, 5′-O-
(tetrahydro-2H-pyran-2-yl)-2′,3′-dideoxy-5-bromouridine was first
treated with n-BuLi at À78 ºC, then reacted with [11C]CH3I, and
finally deprotected by HCl to generate [11C]d4T with
a
According to the HPLC analysis of the reaction mixture, we
found that during the waiting period for the [11C]CH3I preparation,
part of the tributylphenylstannane had been transformed into the
compound 1,1′-biphenyl via homocoupling. Such a side reaction
consumes the stannyl precursor and greatly impedes the
subsequent C-[11C]methylation to generate the desired tracer
[methyl-11C]toluene. In order to inhibit this side reaction, we
optimized the temperature for the hot labeling process, in which
the reaction mixture of the Pd(0)–Cu(I) catalyst and tributyl-
phenylstannane was controlled below À10 °C by cooling before
the [11C]CH3I was generated, further cooled to À20 °C during
[11C]CH3I bubbling, and then followed by quickly heating the
mixture to 65 °C for 5 min. Using this optimized one-pot process
(Method B), the yield of [methyl-11C]toluene was greatly improved
to 93–96% when 4–8 μmol tributylphenylstannane was used
(Figure 2). This optimized one-pot method also worked well for
the C–C coupling of [11C]CH3I with the heteroaryl compounds
2-tributylstannylpyridine and 3-tributylstannylpyridine. From the
radio-HPLC peak area percentage of the sum of all radioactive
peak areas, the yields of [methyl-11C]2-picoline and [methyl-11C]
3-picoline were determined as 89 2% (n = 2) and 92 2%
(n = 2), respectively.
radioactivity of about 0.38 GBq.25 We improved the preparation
process of [11C]d4T by an optimized one-pot Pd(0)–Cu(I) co-
mediated rapid C-[11C]methylation method. d4T–Sn was
designed and prepared as a precursor (see Figure 3B). The
5-bromouridine (6) was protected by TBS to give 5’-O-
tert-butyldimethylsilyl-5-bromouridine (7), which was then
transformed into 5’-O-(tert-butyldimethylsilyl)-2′,3′-didehyro-2′-
3′-dideoxy-5-bromouridine (9) by a Corey–Winter reaction.
Compound 9 then reacted with bis(tributyltin) and deprotected
by TBAF to generate the precursor d4T–Sn. d4T–Sn was then
utilized in the one-pot Pd(0)–Cu(I) co-mediated rapid C-[11C]
methylation method to successfully produce the PET tracer
[11C]d4T. Purification by preparative HPLC followed by
radiopharmaceutical formulation gave the desired product
[11C]d4T with a radioactivity of 6.6 0.3 GBq (n = 3) and a specific
radioactivity in the range of 126–135 GBq/μmol at the end of
synthesis. The total time of the synthesis was 35 min from EOB.
The DCY was 54 3% (n = 3) from [11C]carbon dioxide. The
chemical and radiochemical purities of the product were >99%
and >99.5%, respectively.
Synthesis of the PET tracer [11C]LdT
Synthesis of the PET tracer [11C]AZT
Similarly, by the same one-pot Pd(0)–Cu(I) co-mediated rapid
The aforementioned optimized one-pot Pd(0)–Cu(I) co-mediated C-[11C]methylation method, the anti-HBV agent LdT was also
rapid C-[11C]methylation was successfully applied to label the successfully labeled with the PET nuclide 11C to generate [11C]
anti-HIV agents AZT and d4T and the anti-HBV agent LdT to LdT. As a precursor, LdT–Sn was synthesized following the route
generate the PET tracers [11C]AZT, [11C]d4T, and [11C]LdT, shown in Figure 3C. 2′-Deoxy-L-uridine (11) was protected with
respectively. For synthesis of the PET tracer [11C]AZT, AZT–Sn was TBS to give 3′,5′-bis-O-(tert-butyldimethylsilyl)-2′-deoxy-L-uridine
J. Label Compd. Radiopharm 2014, 57 540–549
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Z. Zhang et al.
Figure 3. Syntheses of [11C]AZT (A), [11C]d4T (B), and [11C]LdT (C). Reagents and conditions: a) TBSCl, imidazole; b) DIAD, PPh3; c) NaN3, 110 °C; d) NaH, n-BuLi, –78 °C, then
Bu3SnCl and HAMP; e) TBAF, 0 °C; f) Pd2(dba)3, P(o-tolyl)3, CuCl, K2CO3, [11C]CH3I trapping at –20 °C, then 80 °C, 5 min; g) 1,1’-thiocarbonyl imidazole; h) trimethyl phosphite,
118 °C; i) Pd(PPh3)4, (Bu3Sn)2, reflux in dioxane; and j) NaN3, NBS.
(12), which was then brominated with N-bromosuccinimide to radioactivity in the range of 75–94 GBq/μmol at the end of
yield 3′,5′-bis-O-(tert-butyldimethylsilyl)-2′-deoxy-L-5-bromouridine synthesis. The total time of the synthesis was 35 min from
(13). Compound 13 was then reacted with bis(tributyltin) in the EOB. The DCY was 53 3% (n = 3) from [11C]carbon dioxide.
presence of the catalyst Pd(PPh3)4 to produce 3′,5′-bis-O-(tert- The chemical and radiochemical purities of the product were
butyldimethylsilyl)-2’-deoxy-L-5-(tributylstannyl)-uridine (14), which >97% and >99.5%, respectively.
was deprotected to give the stannyl precursor LdT–Sn. The
precursor LdT–Sn was coupled with [11C]CH3I to generate the Syntheses of [11C]Thy and [11C]4DST
PET tracer [11C]LdT. Purification by preparative HPLC followed
by radiopharmaceutical formulation gave the product [11C] [11C]Thy and [11C]4DST are effective PET probes for monitoring
LdT with a radioactivity of 6.4 0.3 GBq (n = 3) and a specific cell proliferation and cancer growth.26,27 Recently, we improved
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Z. Zhang et al.
Table 1. [11C]zidovudine and its analogs synthesized by one-pot Pd(0)–Cu(I) co-mediated rapid C-[11C]methylation
PET tracers
Radioactivity (GBq)
DCY (%)a
Specific radioactivity (GBq/μmol)
[11C]Zidovudine
[11C]Stavudine
[11C]Telbivudine
[11C]Thy
7.0 0.8
6.6 0.3
6.4 0.3
7.9 0.2
7.6 0.4
58
54
53
75
65
6
3
3
2
4
74–128
126–135
75–94
86–147
85–216
[11C]4DST
Data are expressed as the mean SD (n = 3). All the values were measured at the end of synthesis.
aThe decay-corrected radiochemical yield (DCY) was based on [11C]carbon dioxide.
their preparation by the two-pot Pd(0)–Cu(I) co-mediated rapid of radioactivity adsorbed by the reaction vessels, resulting in a
C-[11C]methylation method with the precursors 5-tributylst- much higher product radiochemical yield than the two-pot
annyl-2’-deoxyuridine and 5-tributylstannyl-4′-thio-2′-deoxyuridine.23 method.
By this two-pot method, after HPLC purification and
radiopharmaceutical formulation, [11C]Thy and [11C]4DST were
Proposed mechanism of the one-pot Pd(0)–Cu(I)
co-mediated C-[11C]methylation with a stannyl precursor
prepared with radioactivities of around 3.7–3.8 GBq and DCY of
around 39–40% (based on the radioactivity of [11C]carbon dioxide).
We previously found that although hot [11C]CH3I gas was
Here, we found that the optimized one-pot method was much
more convenient and efficient than the previous two-pot
method. Using the optimized one-pot method, [11C]Thy was
obtained with a radioactivity of 7.9 0.2 GBq (n = 3) and a
specific radioactivity in the range of 86–147 GBq/μmol at the
end of synthesis. Similarly, [11C]4DST was also effectively
prepared with a radioactivity of 7.6 0.4 GBq (n = 3) and a
specific radioactivity in the range of 85–216 GBq/μmol at the
end of synthesis. The DCYs of [11C]Thy and [11C]4DST were
75 2% and 65 4% (n = 3), respectively. Thus, through the
one-pot method, the yields of [11C]Thy and [11C]4DST were
greatly improved, approximately twice as high as those
obtained by the two-pot process. In this optimized one-pot
method, [11C]CH3I was trapped in the reaction mixture and
cooled to À20 °C, which showed higher [11C] CH3I capture
efficiency than at room temperature. The one-pot method also
shortened the total synthesis time and greatly reduced the loss
trapped in the reaction mixture of the catalyst Pd(0)–Cu(I)
and the stannyl precursor at room temperature, the one-
pot process for Pd(0)–Cu(I) co-mediated rapid C-[11C]
methylation lacked reproducibility.18,20,21 In this study, we
succeeded in solving this problem by cooling the reaction
mixture before [11C]CH3I was introduced. The effect of
cooling could be explained by the mechanism shown in
Figure 4. The transmetallation of the stannyl precursor
(R–SnBu3) with CuCl gave a highly reactive intermediate
organocopper R–Cu and the Lewis acid Bu3SnCl. Bu3SnCl
was neutralized by K2CO3, and the organocopper R–Cu could
react with the {[11C]CH3PdI} complex to generate the desired
PET tracer [11C]CH3–R. During such reaction processes, CuCl
and K2CO3 work synergistically. The neutralization of Bu3SnCl
with K2CO3 could assist the Sn/Cu transmetallation to accelerate
the C-[11C]methylation.17,20,21 However, it should be noted that
Figure 4. The assumed mechanism of the one-pot Pd(0)–Cu(I) co-mediated reaction of [11C]CH3I with a stannyl precursor. L = P(o-tolyl)3.
J. Label Compd. Radiopharm 2014, 57 540–549
Copyright © 2014 John Wiley & Sons, Ltd.
www.jlcr.org

Z. Zhang et al.
the highly reactive organocopper R–Cu could also take part in (homocoupling and demetallation) of R–SnBu3/R–Cu were
side reactions such as demetallation to yield RH and greatly inhibited; and this ensured that in the subsequent
homocoupling to yield R–R. In this context, we previously heating, R–SnBu3/R–Cu could effectively react with the [11C]
demonstrated that the demetallation process could be partly CH3I to generate the desired PET tracer [11C]CH3-R. By this
suppressed by K2CO3 and an excess amount of P(o-tolyl)3.23
optimized one-pot Pd(0)–Cu(I) co-mediated rapid C-[11C]
Here, we observed that tributylphenylstannane and 3- methylation, the anti-HIV agents AZT and d4T and the anti-HBV
tributylstannylpyridine could react with CuCl at room agent LdT were efficiently labeled with 11C to generate the PET
temperature to yield the homocoupling products 1,1′-biphenyl tracers [11C]AZT, [11C]d4T, and [11C]LdT. Furthermore, the
and 3,3′-bipyridine, respectively. When 2-tributylstannylpyridine preparations of the PET tracers [11C]Thy and [11C]4DST were also
reacted with CuCl, the demetallation product pyridine could be greatly improved. All of these obtained PET probes with
detected. We also found that 5-tributylstannyl-2′-deoxyuridine sufficient radioactivity and specific radioactivity and high purity
could react with CuCl to generate 2′-deoxyuridine and 5,5′-bi(2′- could meet the requirements for preclinical PET studies.
deoxyuridine) at room temperature, and this reaction could be
accelerated by heating. Such Cu-mediated homocoupling and
destannylation of stannyl compounds were also reported in
Acknowledgements
several recent papers.28–30 Furthermore, we found that Sn/Cu
transmetallation and R–Cu homocoupling occurred much more
This work was supported by a consignment expense for the
quickly to yield 1,1′-biphenyl when Pd2(dba)3 and P(o-tolyl)3 were
added into the reaction mixtures of tributylphenylstannane and
CuCl. This should be attributed to the oxidation of Pd[P(o-tolyl)3]
Molecular Imaging Research Program on ‘Research Base for
Exploring New Drugs’ from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of Japan. We would like
to thank Mr. Masahiro Kurahashi (Sumitomo Heavy Industry
Accelerator Service Ltd.) for operating the cyclotron.
(Pd0L(n)) by CuCl to yield the active PdCl2[P(o-tolyl)3]n (PdIICl2L
n
(n)), which greatly promoted the homocoupling of the
organocopper R–Cu via substitution and reductive elimination.
Therefore, in the previous one-pot method without cooling, part
of the stannyl precursor (R–SnBu3) would be transformed into
the side products R–R or RH via homocoupling or demetallation
while waiting for the addition of [11C]CH3I. Moreover, when the
hot nitrogen gas containing [11C]CH3I was bubbled into the
reaction mixture without cooling, these side reactions could be
aggravated, even to the extent of exhausting the precursor R–
SnBu3 before C-[11C]methylation. Thus, the previous one-pot
method without cooling gave a low yield and suffered from
low reproducibility.18,20,21 In the two-pot process, the catalyst
Pd0L(n) was present in the first reactor to capture the [11C]
CH3I gas, and the precursor R–SnBu3 in the second reactor
was cooled to 0 °C. In this way, the side reactions of R–Cu/R–
SnBu3 could be well controlled before they met with the
{[11C]CH3PdI} complex. Additionally, this ensured that in the
subsequent heating, R–Cu or R–SnBu3 could efficiently react
with the {[11C]CH3PdI} complex to yield the desired product
Conflict of Interest
The authors did not report any conflict of interest.
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