Radiosynthesis of 1-iodo-2-[11C]methylpropane and 2-methyl-1-[11C]propanol and its application for alkylation reactions and C―C bond formation

Article abstract of DOI:10.1002/jlcr.3536

The multitude of biologically active compounds requires the availability of a broad spectrum of radiolabeled synthons for the development of positron emission tomography (PET) tracers. The aim of this study was to synthesize 1-iodo-2-[¹¹C]methylpropane and 2-methyl-1-[¹¹C]propanol and investigate the use of these reagents in further radiosynthesis reactions. 2-Methyl-1-[¹¹C]propanol was obtained with an average radiochemical yield of 46?±?6% d.c. and used with fluorobenzene as starting material. High conversion rates of 85?±?4% d.c. could be observed with HPLC, but large precursor amounts (32?mg, 333?μmol) were needed. 1-Iodo-2-[¹¹C]methylpropane was synthesized with a radiochemical yield of 25?±?7% d.c. and with a radiochemical purity of 78?±?7% d.c. The labelling agent 1-iodo-2-[¹¹C]methylpropane was coupled to thiophenol, phenol and phenylmagnesium bromide. Average radiochemical conversions of 83% d.c. for thiophenol, 40% d.c. for phenol, and 60% d.c. for phenylmagnesium bromide were obtained. In addition, [¹¹C]2-methyl-1-propyl phenyl sulphide was isolated with a radiochemical yield of 5?±?1% d.c. and a molar activity of 346?±?113?GBq/μmol at the end of synthesis. Altogether, the syntheses of 1-iodo-2-[¹¹C]methylpropane and 2-methyl-1-[¹¹C]propanol were achieved and applied as proof of their applicability.

Full text of DOI:10.1002/jlcr.3536

Radiosynthesis of 1-iodo-2-[11C]methylpropane and 2-methyl-1-[11C]propanol and its
application for alkylation reactions and C-C bond formation.
Lonneke Rotteveel*1, Alex J. Poot1, Uta Funke1,2, Aleksandra Pekošak1, Ulrike Filp1, Adriaan
A. Lammertsma1 and Albert D. Windhorst1
1Department of Radiology & Nuclear Medicine, VU University Medical Center, Amsterdam,
The Netherlands; 2BV Cyclotron VU, Amsterdam, The Netherlands
*Correspondence to: Lonneke Rotteveel, Radionuclide Center, Department of Radiology &
Nuclear Medicine, VU University Medical Center, De Boelelaan 1085c, 1081 HV
Amsterdam, The Netherlands. E-mail: l.rotteveel@vumc.nl
Keywords:
carbon-11
radiolabelling;
1-iodo-2-[11C]methylpropane;
2-methyl-1-
[11C]propanol; [11C]etherification; [11C]alkylation; positron emission tomography, C-C
bond formation.
Abstract
The multitude of biologically active compounds requires the availability of a broad spectrum
of radiolabeled synthons for the development of positron emission tomography (PET) tracers.
The aim of this study was to synthesize 1-iodo-2-[11C]methylpropane and 2-methyl-1-
[11C]propanol and investigate the use of these reagents in further radiosynthesis reactions. 2-
methyl-1-[11C]propanol was obtained with an average radiochemical yield of 46 ± 6% d.c.
and used with fluorobenzene as starting material. High conversion rates of 85 ± 4 % d.c.
could be observed with HPLC, but large precursor amounts (32 mg, 333 µmol) were needed.
1-iodo-2-[11C]methylpropane was synthesized with a radiochemical yield of 25 ± 7 % d.c.
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doi: 10.1002/jlcr.3536
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and with a radiochemical purity of 78 ± 7 % d.c. The labelling agent 1-iodo-2-
[11C]methylpropane was coupled to thiophenol, phenol and phenylmagnesium bromide.
Average radiochemical conversions of 83 % d.c. for thiophenol, 40 % d.c. for phenol and 60
% d.c. for phenylmagnesium bromide were obtained. In addition, [11C]2-methyl-1-propyl
phenyl sulfide was isolated with a radiochemical yield of 5 ± 1 % d.c. and a molar activity of
346 ± 113 GBq/µmol at end of synthesis. Altogether, the syntheses of 1-iodo-2-
[11C]methylpropane and 2-methyl-1-[11C]propanol was achieved and applied as proof of their
applicability.
Words: 196
Introduction
Positron emission tomography (PET) is a powerful imaging tool, which is used in drug
development, clinical research and patient care to study molecular processes in vivo.[1]
Future progress of this technique is strongly dependent on the development of novel PET
tracers. Carbon is one of the main elements in organic and biologically active molecules, and
replacing carbon-12 by carbon-11 does not change the physicochemical properties nor the
biological profile. Carbon-11 labelled methyl iodide is one of the most frequently used
synthons for the synthesis of PET radiotracers.[2] This is based on the fact that methylation
reactions are relatively simple and because many biologically active compounds contain a
methyl group that can be introduced at a late stage through a radiolabelling reaction with
[11C]methyl iodide.[1-3] However, when the lead structure does not possess a methyl group,
other carbon-11 containing precursors have to be used for radiosynthesis of the tracer.
Therefore, multiple alternative alkylation agents have been developed such as [11C]benzyl
iodide [4-8], 1-[11C]-ethyl iodide [9-11], 1-[11C]-propyl iodide [9,11-13], 1-[11C]-butyl iodide
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[9,11], 2-[11C]-iodopropane [10,12] and 1-iodo-2-[11C]methylpropane.[11] These synthons
have been prepared from cyclotron produced [11C]CO2 via Grignard reactions [4-7,11] or
from [11C]CO carbonylation reactions.[9] So far, N- and O-alkylation reactions and
[11C]carbon-carbon bond formation based on alkylation of the -C-atom of amino acetates
have been performed by using these alternative synthons. Interestingly, no palladium
catalysed couplings like Stille reaction or Suzuki reaction have been reported to date,
although these reactions are frequently applied for the radiolabeling reagent [11C]methyl
iodide.[14,15] We were interested in the synthesis of 1-iodo-2-[11C]methylpropane as 2-
methylpropane is an important functional group in medicinal chemistry and drug
development, e.g. for cyclooxygenase-2 (COX-2) inhibitors (figure 1).[16] Radiosynthesis of
1-iodo-2-[11C]methylpropane has been reported by Långström et al. with a decay corrected
radiochemical yield of 20% and a radiochemical purity of 65% for 1-iodo-2-
[11C]methylpropane [11]. In addition, no molar activity has been reported which is an
important parameter in the application of a PET tracer. Therefore, the aim of this study was
to improve the synthesis of 1-iodo-2-[11C]methylpropane and 2-methyl-1-[11C]propanol
(Scheme 1) and to investigate whether these synthons are suited for late stage radiolabelling
reactions. Therefore, we thoroughly investigated the stability of 1-iodo-2-[11C]methylpropane
and 2-methyl-1-[11C]propanol and optimized the reaction parameters with these reagents for
O- and S-alkylation model reactions as well as for model carbon-carbon-bond formations
(Scheme 2). This study resulted in two synthons that can be reliably be produced and applied
in late-stage radiolabelling reactions for PET tracer development.
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Experimental
General
Chemicals were obtained from commercial sources and used without further purification.
Solvents were purchased from Sigma-Aldrich (Zwijndrecht, The Netherlands) and Biosolve
(Valkenswaard, The Netherlands). Tetrahydrofuran (THF) was first distilled from LiAlH4 and
then stored over 3 Å molecular sieves. Dimethylsulfoxide (DMSO), dimethylformamide
(DMF), acetonitrile (MeCN), aceton and 1-Methyl-2-pyrrolidinone (NMP) were dried over 3
Å molecular sieves. Silicon stoppers (20 mm, 27235-U) were obtained from Sigma-Aldrich
(Zwijndrecht, The Netherlands). 10 mL vials (N611568) were obtained from Covidien
(Dublin, Ireland) and crimp-caps (20 mm, 98805) were purchased from Grace (Breda, The
Netherlands). All reactions were performed at room temperature (RT) and under nitrogen
unless otherwise mentioned. Reactions were monitored by thin layer chromatography on pre-
coated silica 60 F254 aluminium plates (Merck, Darmstadt, Germany). Spots were visualized
by UV light 254 nm. Evaporation of solvents was performed under reduced pressure at 40 °C
using a rotary evaporator (Rotavapor ® R II, Flawil, Switzerland), Flash column
chromatography was performed on a Büchi (Flawil, Switzerland), Sepacore system
(comprising a C-620 control unit, C-660 fraction collector, two C-601 gradient pumps and a
C-640 UV detector) equiped with Büchi Sepacore pre-packed flash columns. NMR
1
spectrometry was performed on a Bruker Avance 500 (500.23 MHz for H and 125.78 MHz
13
for C) with chemical shifts (δ) reported in parts per million (ppm) relative to the solvent
1
(CDCl3, H 7.26 ppm, 13C 77.16 ppm). Purity and molar activity were determined by
analytical isocratic HPLC on a Jasco (Easton, MD, USA) PU-1580 station with a Altima C-
18 5 µm (4.6 x 100 mm) column (Grace, Breda, The Netherlands) using a buffer (4 mmol of
sodium formate in water and 4 % DMF) / MeCN (3/7, HPLC method A or 2/8, method B,
v/v) as mobile phase at a flow rate of 1 mL/min, a Jasco UV-2075 Plus UV detector (254 nm)
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and a NaI radioactivity detector (Raytest, Straubenhardt, Germany). Chromatograms were
acquired with Raytest GINA Star software (version 5.8)
Analytical HPLC of the [11C]2-methylpropanoic acid and 2-methyl-1-[11C]propanol were
performed on a Jasco (Easton, MD, USA) PU-1580 station with a Reprogel H+ column 9 µm
(4.6 x 100 mm) column (Dr. Maisch, Ammerbuch-Entringen, Germany) using 9 mmol
sulphuric acid in water at a flow rate of 0.5 mL/min (HPLC method C), a Jasco UV-2075
Plus UV detector (254 nm), a Jasco RI-2031 detector and a NaI radioactivity detector
(Raytest, Straubenhardt, Germany). Chromatograms were acquired with Raytest GINA Star
software (version 5.8)
Semi-preparative HPLC was performed on a Jasco (Easton, MD, USA) UV-2087 Plus station
with a Luna C-18 5 µm (10 x 250 mm) column (Phenomenex, Utrecht, The Netherlands)
using a buffer (4 mmol of sodium formate in water and 4 % DMF) / MeCN (3.5/6.5, HPLC
method D, v/v) as mobile phase at a flow rate of 8 mL/min, a Jasco UV-2075 Plus UV
detector (254 nm) and a homemade NaI radioactivity detector. Chromatograms were acquired
with Jasco ChromNAV CFR software (version 1.14.01).
Analytical GC of the 2-methyl-1-[11C]propanol and [11C]2-methylprop-1-ene was performed
on a Thermo Scientific trace ultra (Thermo Scientific, The Netherlands) with a thermal
conductivity detector (Thermo Scientific, The Netherlands), HP-PLOT U column 0,32 mm x
30 m (Phenomenex, Utrecht, The Netherlands) and a NaI 588I radiodetector (Teledyne
isotopes, Huntsville, United States). The following program was used: Oven temperature 50
ºC (1 min) and 15 ºC/min to 170 ºC (10 min), inlet temperature 200 ºC, split ratio 50:1 and
flow 3 mL/min. 1 µL of the corresponding sample and 1 µL of air were injected.
Chromatograms were acquired with Raytest GINA Star software (version 5.01)
Radiochemistry was performed with our in-house build synthesis modules equipped with a
VIK-202 ionization-chamber (Comecer, Joure, The Netherlands).[17] The radiochemical
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yields were corrected from the starting amount of [11C]CO2. The HPLC chromatograms were
corrected for decay to obtain decay corrected radiochemical purities and decay corrected
radiochemical conversions.
Synthesis
2-methyl-1-propyl phenyl sulfide 6
2-methylpropane-1-thiol (4.17 mmol, 376 mg, 316 µL) was added to a suspension of NaH
(4.17 mmol, 100 mg) in NMP (5.4 mL) and the mixture was stirred for 10 min. This freshly
prepared solution was added to a solution of fluorobenzene (2.08 mmol, 200 mg, 195 µL) in
NMP (13.5 mL). The reaction was stirred at 100 °C overnight. Water and ethyl acetate were
added, the aqueous layer was separated and the organic layer was washed with water, dried
over Na2SO4, filtered and concentrated. The residue was purified by column chromatography
eluting with ethylacetate/hexane (1:2, v:v). 2-methyl-1-propyl phenyl sulfide was obtained as
a clear oil in 47 % yield (147 mg, 0.979 mmol). 1H NMR (CDCl3) δ 7.35 (m, J=5.00 Hz, 2 H,
CHAr), 7.29 (t, J=7.50 Hz, 2 H, CHAr), 7.18 (t, J=7.00 Hz, 1 H, CHAr), 2.83 (d, J=5.40 Hz, 2
H, PhSCH2), 1.85 - 1.95 (m, 1 H, CH(CH3)2), 1.06 (d, J=5.70 Hz, 6 H, CH(CH3)2) ppm; 13C
NMR (CDCl3), δ 137.09 (SCHAr), 128.53 (3 x CHAr), 125.27 (2 x CHAr), 42.24 (PhOCH2),
27.95 (CH(CH3)2), 21.77 (CH(CH3)2) ppm.
2-methylpropoxybenzene 8
2-methyl-propan-1-ol (5.40 mmol, 400 mg, 498 µL) was added to a suspension of NaH (5.40
mmol, 130 mg) in NMP (5.4 mL) and the mixture was stirred for 10 min. This freshly
prepared solution was added to a solution of fluorobenzene (2.70 mmol, 259 mg, 253 µL) in
NMP (13.5 mL). The reaction was stirred at 100 °C overnight. Water and ethyl acetate were
added, the aqueous layer was separated and the organic layer was washed with water, dried
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over Na2SO4, filtered and concentrated. 2-methylpropoxybenzene was obtained as a clear oil
1
in 70.3 % yield (285 mg, 1.90 mmol) H NMR (CDCl3) δ 7.30 (t, J=7.30 Hz, 2 H, CHAr),
6.90 - 7.00 (m, 3 H, CHAr), 3.74 (d, J=3.80 Hz, 2 H, PhOCH2), 2.02 - 2.16 (m, 1 H,
CH(CH3)2), 1.05 (d, J=3.80 Hz, 6 H, CH(CH3)2) ppm; 13C NMR (CDCl3) δ 159.19 (OCHAr),
129.35 (2 x CHAr), 120.36 (CHAr), 114.45 (2 x CHAr), 74.28 (PhOCH2), 28.28 (CH(CH3)2),
19.31 (CH(CH3)2)
Radiosynthesis
Preparation of isopropylmagnesium chloride solution (4M)
A 10 mL vial was dried in the oven at 150 °C and was capped with a silicone stopper. The
vial was purged with argon and cooled to room temperature. Isopropylmagnesium chloride (2
mmol, 1 mL, 2 M) was added to the capped vial. The solution was concentrated under
vacuum until all the THF was evaporated and a brown/white solid was obtained. Dry THF
(0.5 mL) was added to the vial and the solid was dissolved.
[11C]2-methylpropanoic acid [11C]2
A reaction vial was purged with helium and charged with isopropylmagnesium chloride in
14
THF (600 µmol, 150 µL, 4M). [11C]CO2 was produced by the N (p,α)11C nuclear reaction
performed in a 0.5 % O2/N2 gas mixture using an IBA Cyclone 18/9 cyclotron (IBA,
Louvain-la-neuve, Belgium). The [11C]CO2 was transferred from the target and trapped in a
stainless steel loop dispensed in liquid N2. At 25 °C, the [11C]CO2 was transferred with a He
flow of 10 mL/min into the reaction vessel until the trapped amount of radioactivity was
constant (~2 min). The reaction mixture was quenched with water and analyzed with HPLC
method C. tR = 12.9 min
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2-methyl-1-[11C]propanol [11C]3
[11C]2 was synthesised as described above and instead of adding water, LiAlH4 (100 µmol,
100 µL, 1M) was added and the temperature was increased to 130 °C. At 130 °C, the helium
flow was increased to 25 mL/min and the THF was evaporated (~2 min). The reaction
mixture was cooled to room temperature, quenched with water and analyzed with HPLC
method C. tR = 18.9 min
1-iodo-2-[11C]methylpropane [11C]4
[11C]3 was synthesised as described above and instead of adding water, the reaction mixture
was cooled to 25 °C and HI (4.55 mmol, 600 µL, 57 %) was added. The temperature of the
reaction mixture was increased to 150 °C for 5 min. Product 1-iodo-2-[11C]methylpropane
was distilled by a helium flow of 25 mL/min into a second reaction vessel containing 250-
500 µL of DMSO (25 °C), DMF (0 °C) or THF (0 °C). 1-iodo-2-[11C]methylpropane was
obtained in a decay-corrected radiochemical yield of 25 ± 7 % (n = 10) calculated from
[11C]CO2 with a purity of 78 ± 7 % (n = 10) and an overall synthesis time of approximately
14 min. The identity of the product was confirmed with analytical HPLC method B by co-
injection of 1-iodo-2-methylpropane , tR = 9.3 min.
[11C]2-methyl-1-propyl phenyl sulfide [11C]6; optimization reactions
A reaction vessel was loaded with DMSO (100 µL), thiophenol (12 µmol, 1.3 mg, 1.2 µL)
and K2CO3 (30 µmol, 4.2 mg). 1-iodo-2-[11C]methylpropane was synthesized as described
above and trapped in a reaction vessel with DMSO (500 µL) at 25 °C. 100 µL of this stock
solution was transferred via a syringe to the reaction vessel. The reaction mixture was stirred
for 5 min at 25 °C. The identity of the product was confirmed with analytical HPLC method
B by co-injection of 2-methyl-1-propyl phenyl sulfide, tR = 9.2 min.
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[11C]2-methyl-1-propyl phenyl sulfide [11C]6; full synthesis with LiAlH4
A reaction vessel was loaded with DMSO (400 µL), thiophenol (12 µmol, 2.6 mg, 2.4 µL)
and K2CO3 (60 µmol, 8.4 mg). 1-iodo-2-[11C]methylpropane was synthesized as described
above. The 1-iodo-2-[11C]methylpropane was trapped in the reaction mixture at 25 °C. The
reaction mixture was stirred for 5 min at 25 °C followed by dilution of the reaction mixture
with 1.0 mL of HPLC eluents. The crude product was purified by semi-preparative HPLC
method D. tR = 12.7. A radiochemical yield, decay corrected from the starting amount of
[11C]CO2 (end of cyclotron bombardment) of [11C]CO2 in 11 ± 2% (n = 3) with a
radiochemical purity of > 98 % was obtained in 41 min. The identity of the product was
confirmed with analytical HPLC method B by co-injection of 2-methyl-1-propyl phenyl
sulfide, tR = 9.2 min
[11C]2-methyl-1-propyl phenyl sulfide [11C]6; full synthesis with LiBH4
A reaction vessel was loaded with DMSO (400 µL), thiophenol (12 µmol, 2.6 mg, 2.4 µL)
and K2CO3 (60 µmol, 8.4 mg). 1-iodo-2-[11C]methylpropane was synthesized as described
above but instead of using LiAlH4, LiBH4 (200 µmol, 100 µL, 2M) was used. The 1-iodo-2-
[11C]methylpropane was trapped in the reaction mixture at 25 °C. The reaction mixture was
stirred for 5 min at 25 °C followed by dilution of the reaction mixture with 1.0 mL of HPLC
eluents. The crude product was purified by semi-preparative HPLC method D. tR = 12.7. A
radiochemical yield, decay corrected from the starting amount of [11C]CO2 (end of cyclotron
bombardment) of [11C]CO2 in 5 ± 1 % (n = 4) with a molar activity of 346 ± 113 GBq/µmol
and a radiochemical purity of > 98 % was obtained in 41 min. The identity of the product was
confirmed with analytical HPLC method B by co-injection of 2-methyl-1-propyl phenyl
sulfide, tR = 9.2 min
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[11C]2-methylpropoxybenzene [11C]8 (Approach A)
A reaction vessel was loaded with DMSO (100 µL), phenol (90 µmol, 8.5 mg) and K2CO3
(60 µmol, 8.3 mg). 1-iodo-2-[11C]methylpropane was synthesized as described above and
trapped in a reaction vessel with DMSO (500 µL) at 25 °C. 100 µL of this stock solution was
transferred via a syringe to the reaction vessel. The reaction mixture was stirred for 5 min at
130 °C. The identity of the product was confirmed with analytical HPLC method A by co-
injection of 2-methylpropoxybenzene, tR = 13.5 min.
[11C]2-methylpropoxybenzene [11C]8 (Approach B)
A reaction vessel was loaded with DMSO (100 µL), fluorobenzene (0.33 mmol, 32 mg, 31
µL) and NaH (0.40 mmol, 9.6 mg). 2-methyl-1-[11C]propanol was synthesized as described
above. The reaction mixture was quenched with HCl in diethyl ether (0.7 mmol, 350 µL, 2
M) or dioxane (1.4 mmol, 350 µL, 4 M), glycerol 400 µL and water 400 µL. The reaction
mixture was heated to 150 °C and the activity was trapped in a reaction vessel with DMSO
(500 µL) at 25 °C. Drying columns were prepared with a diameter of 1.0 cm and a length of
1.5 cm in 3 mL tubes (Sopachem, Nazareth, Belgium). A distillation tube of 10 cm with a
diameter of 1-2 mm was used (Vygon, Valkenswaard, The Netherlands). 100 µL of this stock
solution was transferred via a syringe to another the reaction vessel. The reaction mixture was
stirred for 10 min at 130 °C. The identity of the product was confirmed with analytical HPLC
method A by co-injection of 2-methylpropoxybenzene, tR = 13.5 min.
1-phenyl-2-[11C]methylpropane [11C]11
A reaction vessel was dried, loaded with argon and cooled to room temperature. Co(acac)3
(25 µmol, 8.9 mg), TMEDA (0.30 mmol, 28 mg, 37 µL) and THF (150 µL) were added. The
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1-iodo-2-[11C]methylpropane was synthesized as described above and trapped in the reaction
vessel at 0 °C. The reaction mixture was cooled to -20 °C and phenylmagnesium bromide (50
µmol, 9.1 mg, 50 µL, 1 M) was added. The reaction mixture was heated to 0 °C and stirred
for 10 min. The identity of the product was confirmed with analytical HPLC method B by co-
injection of 1-phenyl-2-methylpropane , tR = 12.1 min.
Results and Discussion
Synthesis of 1-iodo-2-[11C]methylpropane
1-iodo-2-[11C]methylpropane was synthesized in a three-steps one-pot procedure. The first
step was the formation of [11C]2-methylpropanoic acid in a Grignard reaction, by bubbling
cyclotron produced [11C]CO2 through an isopropylmagnesium chloride (1, Table 1) solution.
A trapping efficiency of more than 95% was obtained by passing [11C]CO2 with a flow of 10
mL/min through a solution of isopropylmagnesium chloride in tetrahydrofurane (THF).
Different concentrations of isopropylmagnesium chloride were investigated to obtain the
highest possible conversion, as measured by high-performance liquid chromatography
(HPLC). Increasing the concentration of 1 from 2 M to 4 M resulted in a radiochemical
conversion increase to 82 ± 1 % d.c. instead of 67 ± 5 % d.c., calculated from the starting
amount of [11C]CO2 (table 1, entry 1-3, 6). Increasing the temperature or the reaction time did
not improve the radiochemical conversion (table 1, entry 4 and 5).
The reduction of [11C]2-methylpropanoic acid to 2-methyl-1-[11C]propanol was performed
with 1.0 M LiAlH4. The THF was removed with a helium flow of 25 ml/min at 130 °C and
resulted in a quantitative conversion of [11C]2-methylpropanoic acid to 2-methyl-1-
[11C]propanol (Figure 2). The third step, formation of the 1-iodo-2-[11C]methylpropane,
turned out to be the most challenging one since the addition of hydriodic acid was performed
at temperatures below 60 °C, otherwise the volatile intermediate 2-methyl-1-[11C]propanol
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would evaporate prior to the iodination reaction. The mixture was then heated, observing the
highest radiochemical conversions (Graph 1) after 5 minutes reaction at either 150 or 180 °C
(59 ± 3 and 59 ± 1 % d.c., respectively). When heating the reaction mixture for 5 min at a
temperature of 130 °C, a radiochemical conversion of only 30 ± 13 % d.c. was observed. The
possible rearrangement of the 1-iodo-2-[11C]methylpropane to 2-iodo-3-[11C]butane did not
occur according to HPLC analysis. In addition, the potential side-product [11C]2-methylprop-
1-ene deriving from a β-hydrogen elimination of the iodide, was not observed on radio gas
chromatography as well. Finally, purification of 1-iodo-2-[11C]methylpropane was performed
by distillation of the reaction mixture through a sicapent/NaOH column into a second
reaction vial containing DMSO, DMF or THF as solvents. A radiochemical yield of 25 ± 7 %
d.c. could be achieved, obtaining 1-iodo-2-[11C]methylpropane with a radiochemical purity of
78 ± 7 % d.c. (n = 10) calculated from the starting amount of [11C]CO2 (Figure 3) in 14
minutes time. The stability of 1-iodo-2-[11C]methylpropane (Table 2) was determined at
different temperatures, with different bases and for 5 min, as this might influence the
application and use of this reagent in further radiochemical reactions. Strong bases like
TBAOH and NaOH (aq, 0.06 M) could decompose of 1-iodo-2-[11C]methylpropane into 2-
methyl-1-[11C]propanol and [11C]2-methylprop-1-ene at room temperature. On the other
hand, 1-iodo-2-[11C]methylpropane was stable in the presence of a milder base like K2CO3
(aq, 0.06 M) at temperatures ranging from 25 to 80 °C. However, decomposition of 1-iodo-2-
[11C]methylpropane was observed, even in the presence of these milder bases at higher
temperatures of 100 °C or higher. The water content of the reaction mixtures was kept
constant, as it can induce decomposition of 1-iodo-2-[11C]methylpropane into [11C]2-
methylprop-1-ene and 2-methyl-1-[11C]propanol.
[11C]2-methyl-1-propyl phenyl sulfide synthesis by alkylation
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To determine whether 1-iodo-2-[11C]methylpropane can be used for the synthesis of PET
tracers, three model reactions were performed. Two alkylation reactions using thiophenol and
phenol as precursors were investigated. Furthermore, an unprecedented cobalt assisted cross-
coupling reaction using phenylmagnesium bromide as precursor was investigated. [18]
For the synthesis of [11C]6 a high average radiochemical conversion of 83 % d.c. (Table 3,
Entry 1-3) was observed when using 2.5 equivalents of crystalline K2CO3, even at room
temperature for 5 minutes. When aqueous K2CO3 (0.06 M) was used, the formation of 2-
methyl-1-[11C]propanol as side product increased, while unreacted 1-iodo-2-
[11C]methylpropane was still present (Table 3, Entry 5), resulting in an average lower
radiochemical conversions of [11C]6 of 34 % d.c. after 5 minutes. When using one equivalent
of crystalline K2CO3, only 51 % d.cof product [11C]6 was formed, the remainder being 1-
iodo-2-[11C]methylpropane.
Optimized conditions were used to purify [11C]6 by semi-preparative HPLC. [11C]6 could be
isolated with a radiochemical yield of 11 ± 2 % d.c. (n = 3) . However, a large UV impurity,
which could not be separated from [11C]6, was observed in the formulated product. To
overcome this problem, 2.0 M LiBH4 in THF instead of 1.0 M of LiAlH4 as reductant was
used successfully. A radiochemical yield of 5 ± 1 % d.c. (n = 4) and with a molar activity of
346 ± 113 GBq/µmol at end of synthesis was obtained.
[11C]2-methylpropoxybenzene by an alkylation reaction (Approach A)
The model reaction of phenol with 1-iodo-2-[11C]methylpropane resulted in a moderate
average radiochemical conversion of 40 % d.c., applying the same reaction conditions as
those used for the formation of [11C]6 (Table 4, Entry 18). In order to optimize the reaction
conditions to improve the radiochemical conversion towards [11C]8, different parameters
were thoroughly investigated including, solvent, temperature, amount of different bases and
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precursor amount. Unfortunately, none of these opmization conditions improved the
radiochemical conversion (Table 4). Decomposition of 1-iodo-2-[11C]methylpropane in
[11C]2-methylprop-1-ene occurred, in which [11C]8 and [11C]2-methylprop-1-ene were
formed in a 1:1 ratio (Figure 4). An explanation for the lower conversion to the desired
product when using phenol as precursor could be that phenylthiolate anions have a higher
nucleophilicity towards1-iodo-2-[11C]methylpropane resulting in faster reaction kinetics.
Synthesis of 2-methyl-1-[11C]propanol
The synthesis of 2-methyl-1-[11C]propanol was initiated in the same manner as the synthesis
of 1-iodo-2-[11C]methylpropane, but instead of adding hydriodic acid after the reduction
reaction of [11C]2-methylpropanoic acid towards 2-methyl-1-[11C]propanol, different
quenching solvents were added to distill the alcohol out of the reaction mixture at 150 °C.
In the past, multiple quenching solvents have been used to distill alcohols out of the reaction
mixture as purification method for carbon-11 labelled alcohols.[19,20] Several methods were
investigate here as well, but all proved to be unsuccessful, e.g. hydrolysis with hydrogen
chloride in diethyl ether or dioxane did not facilitate the evaporation of 2-methyl-1-
[11C]propanol because all radioactivity remained in the first reaction vial. Glycerol worked
well as quenching solvent to release the 2-methyl-1-[11C]propanol, but its very high viscosity
made practical application of this solvent impossible (Table 5, Entry 1 and 2). In addition to
this, alcoholic compounds from the glycerol were observed in the second reaction vessel as
impurity. Most of the radioactivity was transferred to the second reaction vessel by using
water. In that case, however, a drying column is needed to remove water from the distillate,
as the presence of too much water is unfavourable for most of the follow-up reactions with 2-
methyl-1-[11C]propanol. Therefore, different drying columns were investigated, loaded with
either CaSO4, MgSO4, K2CO3, Na2SO4 or CaO. Most of the columns that were used resulted
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in trapping of the majority of the 2-methyl-1-[11C]propanol, resulting in great losses of
activity. Nevertheless, when Na2SO4 or CaO were used as drying agents, 2-methyl-1-
[11C]propanol could be obtained in isolated radiochemical yields of 33 ± 9 % d.c. and 46 ± 6
% d.c., respectively.
Synthesis of [11C]2-methylpropoxybenzene by a nucleophilic aromatic substitution
reaction (Approach B)
The synthesis of [11C]8 could also be accomplished by a nucleophilic aromatic substitution
reaction with fluorobenzene (Table 6) in DMSO at 130 °C for 10 minutes. Entries 1 to 5 in
table 6 show the radiochemical conversion from 2-methyl-1-[11C]propanol to [11C]8 when
distillation was performed with glycerol or water and CaO as a drying agent. The highest
radiochemical conversion 85 ± 4 d.c.% was obtained when using large amounts of
fluorobenzene (32 mg, 332 µmol). These high amounts were necessary, as side reactions with
unidentified alcoholic compounds occurred.
Synthesis of 1-phenyl-2-[11C]methylpropane
We were also interested in exploring the possiblity to perfomrd C-C cross coupling reactions
with 1-iodo-2-[11C]methylpropane. This is in general a challenging reaction since the
neighbouring hydrogen of 1-iodo-2-[11C]methylpropane can give β-hydrogen-elimination,
resulting in a non-reactive side product. It has been described that for non-activated alkyl
halides this competing β-hydrogen elimination is often proceeding faster than the coupling of
the reagents to obtain the desired product. [21] Nevertheless, this cross-coupling reaction was
investigated with good results. The desired product [11C]11 was successfully formed form 1-
iodo-2-[11C]methylpropane as labelling reagent and phenylmagnesium bromide as precursor,
by employing a cobalt assisted carbon-carbon bond formation reaction, with Co(acac)3 or
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CoCl2 and tetramethylethylenediamine (TMEDA) as base. Reaction of 1-iodo-2-
[11C]methylpropane and phenylmagnesium bromide afforded [11C]11, with Co(acac)3
outperforming CoCl2, and β-hydogen elimination was observed. An optimized procedure was
reported by Cahiez et al. by using non-radioactive 2-iodobutane, 2-bromobutane and
phenylmagnesium bromide as starting materials. [18] In the present study, however, using
Cobalt(III)acetylacetonate/ TMEDA in a 1:1 molar ratio in THF at 0 °C for 10 minutes did
not result in any product formation (Table 7, Entry 4). Increasing the amount of TMEDA
gave a average radiochemical conversion of 60 % d.c. (Table 7, Entry 3). Changing the
Cobalt(III)acetylacetonate to Cobalt(II)chloride resulted in only an average radiochemical
conversion of 37 % d.c. of product [11C]11 (Table 7, Entry 2) as half of the 1-iodo-2-
[11C]methylpropane did not react with the phenylmagnesium bromide, and increasing the
reaction temperature did not result in any product formation (Table 7, Entry 1). To our
knowledge this reaction and formation of a C-C bond has never been described within the
field of radiochemistry.
Conclusion
In this study we described the improved radiosynthesis of 1-iodo-2-[11C]methylpropane and
of 2-methyl-1-[11C]propanol. Furthermore, to demonstrate the added value of these reagents
for PET tracer development, these synthons were successfully applied in [11C]etherification,
[11C]alkylation and [11C]cross-coupling reactions, demonstrating the versatile opportunities
with these synthons in radiochemistry. The obtained products displayed high molar activity,
which is particularly important for PET receptor sfdtudies. These two radiolabelled synthons
provide the opportunity to develop novel PET tracers which have been inaccessible so far.
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Acknowledgement
BV Cyclotron VU (Amsterdam) is acknowledged for providing the [11C]CO2.
Conflict of interest
The authors did not report any conflict of interest
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Figure 1: Described COX-2 inhibitors that allow potential radiolabeling with 1-iodo-2-
[11C]methylpropane for PET tracer development.
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Scheme 1: Radiosynthesis of 2-methyl-1-[11C]propanol and 1-iodo-2-[11C]methylpropane
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Scheme 2: Model reactions for the application of 2-methyl-1-[11C]propanol and 1-iodo-2-
[11C]methylpropane for PET tracer development: (A) alkylation of thiols; (B) alkylation of
alcohols; (C) alkylation of the 2-methyl-1-[11C]propanol; (D) Cobalt catalyzed cross-
coupling reaction.
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Table 1: Optimization to synthesize [11C]2-methylpropanoic acid
Entry
isopropylMgCl 1 isopropylMgCl 1 Reaction time
RCCa
n
(M)
(µmol)
(min)
(% d.c. ± SD)
1
2
3
4
5
6
0.5
1.0
2.0
2.0
2.0
4.0
75
0
0
0
1
2
0
56 ± 5
64 ± 8
67 ± 5
67 ± 5
68, 72
82 ± 1
3
3
3
3
2
4
150
300
300
300
600
a RCC = radiochemical conversions based on the HPLC chromatograms
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Graph 1: Yield of 1-iodo-2-[11C]methylpropane as a function of time at 150 °C and 180 °C
(n = 3), determined by radio-HPLC analysis of the crude product.
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19.0
12
10
CPS *1000
AKT
8
6
4
2
0
0
5
10
15
20
min
Figure 2: radioHPLC of 2-methyl-1-[11C]propanol of the crude reaction mixture(HPLC
eluens method C).
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9'3
15
10
5
CPS *1000
ACT
0
0
1.0
2.0
3.0
4.0
5'0
6.0
7.0
8.0
9.0
10.0 11.0 min
Figure 3: radioHPLC chromatogram of 1-iodo-2-[11C]methylpropane (HPLC eluens method
B)
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Table 2: Stability test of 1-iodo-2-[11C]methylpropane under different conditions after 5
minutes
Entry
Basea
Base
(M)
Temp
(°C)
25
purity of [11C]4b
(% d.c.)
0, 0
1
2
3
4
5
6
TBAOH
0.06
0.06
0.06
0.06
0.06
0.06
NaOH (aq)
K2CO3 (aq)
K2CO3 (aq)
K2CO3 (aq)
K2CO3 (aq)
25
53, 27
25
100, 98
98, 95
80
100
130
88, 86
24, 16
a 3 µL of a 4.1 M K2CO3 or NaOH solution were added.
b The results were corrected for the radiochemical purity.
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Table 3: Optimization for thiol alkylation reactions with 1-iodo-2-[11C]methylpropane to
obtain [11C]6
a
Entry
K2CO3
(M)
K2CO3
(mg)
8.3
Temp
(°C)
130
80
RCC
(% d.c.)
84, 85
83, 83
82, 84
82, 20
49, 18
5
5
(M)
0.45
0.06
0.06
0.06
0.06
(mg)
9.9
1.3
1.3
1.3
1.3
1
2
3
4
5
0.30 (s)
0.15 (s)
0.15 (s)
0.06 (s)
0.06 (aq)
4.2
4.2
25
1.7
25
1.7
25
a In Entry 1-4 K2CO3 was added to the reaction vessel as a crystalline base. In Entry 5, 3 µL
of a 4.1 M K2CO3 solution was added to the reaction vessel.
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Table 4: Optimization for alcohol alkylation reactions with 1-iodo-2-[11C]methylpropane to
obtain [11C]8.
Entry
Basea
Base
(M)
Temp
(°C)
80
RCC
(% d.c.)
34, 33
7
(M)
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.90
0.45
1
2
K2CO3 (s)
K2CO3 (s)
K2CO3 (s)
K2CO3 (aq)
K2CO3 (aq)
K2CO3 (aq)
K2CO3 (aq)
K2CO3 (aq)
K2CO3 (aq)
K2CO3 (aq)
NaOH (aq)
KOH (aq)
NaH (aq)
0.15
0.15
0.15
0.12
0.06
0.03
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.30
0.30
b
80
-
c
3
55
-
4
100
100
100
130
80
7, 23
24, 21
27, 26
25, 24
15, 18
-
13, 17d
36, 30
29, 29
27, 21
27, 29
30, 30
19, 27
37, 43
35, 45
5
6
7
8
9
25
10
11
12
13
14
15
16
17
18
100
130
130
130
130
75
Cs2CO3 (aq)
LiHMDS
K2CO3 (sat)
K2CO3 (s)
K2CO3 (s)
130
130
130
a
b
c
d
Bases were dissolved in water and 3 µL thereof was used
Reaction was performed in MeCN
Reaction was performed in Aceton
Reaction was performed in DMF
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a
7.5
4
3
2
1
0
CPS *1000
ACT
10.4
[11C]8
5.2
6.4
9.4
0.0
2.0
4.0
6.0
8.0
10.0
12.0
min
ACT
b
7.2
CPS
600
400
200
0
9.3
4.4
8.2
5.3
0.0
2.0
4.0
6.0
8.0
10.0
12.0
min
c
5.6
mV
2.0
160mV
7.3 Hexane
5.5
0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
min
Figure 4: a) radioHPLC-chromatogram of [11C]8 entry 18, table 4, (HPLC eluens method B).
b) radioHPLC chromatogram of [11C]2-methylprop-1-ene, generated by adding 10 mg of
NaH to 1-iodo-2-[11C]methylpropane. Reaction was done at 130 °C for 5 min in 200 µL
DMSO. (HPLC eluens method B) c) Spiked GC-chromatogram of [11C]2-methylprop-1-ene
generated similar as described in b.
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Table 5: Synthesis of 2-methyl-1-[11C]propanol
Entry
radiochemical
yield
Drying
agent
Tempa
Quenching solvent
n
(°C)
(% d.c. ± SD)
1
2
3
4
Glycerol
80
-
55 ± 21
19
-
Glycerol/DMSO
Water/DMSO
Water
80
80
80
57 ± 12
33 ± 9
46 ± 6
20
7
Na2SO4
10
CaO
a Temperature at which the quenching solvent was added
Table 6: Optimization for alkylation of 2-methyl-1-[11C]propanol to obtain [11C]8
Entry
RCC
n
9
(M)
(% d.c. ± SD)
1
1.65
0.83
0.43
0.32
0.21
0.83
0.43
0.06
0.06
85 ± 4
82, 85
65
4
2
2
3
2
2
2
2
2
2
3
4
61 ± 15
50, 46
72, 41
49, 33
23, 11
3, 3a
5
6
7
8
9
a
0.3 M NaH was used
Entry 1-5: 400 µL of DMSO was used and distillation of 2-methyl-1-[11C]propanol was
performed with glycerol
Entry 6-9: 200 µL of DMSO was used and the distillation of 2-methyl-1-[11C]propanol was
performed with water and CaO as drying column
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