Simplified synthesis of N‐(3‐[18F]fluoropropyl)‐2β‐carbomethoxy‐3β‐(4‐fluorophenyl)nortropane ([18F]β‐CFT‐FP) using [18F]fluoropropyl tosylate as the labelling reagent

A synthesis method has been developed for the labelling of N‐(3‐[¹⁸F]fluoropropyl)‐2β‐carbomethoxy‐3β‐(4‐fluorophenyl)nortropane ([¹⁸F]β‐CFT‐FP), a potential radioligand for visualization of the dopamine transporters by positron emission tomography. The two‐step synthesis includes preparation of [¹⁸F]fluoropropyl tosylate and its use without purification in the fluoroalkylation of 2β‐carbomethoxy‐3β‐(4‐fluorophenyl)nortropane (nor‐β‐CFT). The final product is purified by HPLC. Optimization of the two synthesis steps resulted in a greater than 30% radiochemical yield of [¹⁸F]β‐CFT‐FP (decay corrected to end of bombardment). The synthesis time including HPLC‐purification was approximately 90 min. The radiochemical purity of the final product was higher than 99% and the specific radioactivity at the end of synthesis was typically 20 GBq/µmol. In comparison to alkylation by [¹⁸F]fluoropropyl bromide, the procedure described here results in an improved overall radiochemical yield of [¹⁸F]β‐CFT‐FP in a shorter time. Copyright © 2005 John Wiley & Sons, Ltd.     

Efficient O‐ and N‐(β‐fluoroethylation)s with NCA [18F]β‐fluoroethyl tosylate under microwave‐enhanced conditions

Reactions of no‐carrier‐added (NCA) [¹⁸F]β‐fluoroethyl tosylate with amine, phenol or carboxylic acid to form the corresponding [¹⁸F]N‐(β‐fluoroethyl)amine, [¹⁸F]β‐fluoroethyl ether or [¹⁸F]β‐fluoroethyl ester, were found to be rapid (2–10 min) and efficient (51–89% conversion) under microwave‐enhanced conditions. These conditions allow reactants to be heated rapidly to 150°C in a low boiling point solvent, such as acetonitrile, and avoid the need to use high boiling point solvents, such as DMSO and DMF, to promote reaction. The microwave‐enhanced reactions gave about 20% greater radiochemical yields than thermal reactions performed at similar temperatures and over similar reaction times. With a bi‐functional molecule, such as DL‐pipecolinic acid, [¹⁸F]β‐fluoroethyl tosylate reacts exclusively with the amino group. Copyright © 2004 John Wiley & Sons, Ltd.     

Alternative synthesis for the preparation of 16α‐[18F]fluoroestradiol

We have developed a new precursor, 3,17β‐O‐bis(methoxymethyl)‐16β‐O‐p‐nitrobenzenesulfonylestriol (14c) of 16α‐[¹⁸F]fluoroestradiol ([¹⁸F]FES). Although we could not selectively protect the C17 alcohol in the presence of the C16 alcohol, we were able to prepare and chromatographically isolate the desired C16 TBDMS, C17,C3‐dimethoxymethyl (diMOM) protected estriol derivative and convert into the ultimate fluorination precursor. The MOM protective group proved to be more quickly removed than the cyclic sulfate group. The di‐MOM protective precursor at the C3 and C17 alcohols instead of a cyclic sulfate group shortened hydrolysis time. We prepared three different sulfonate precursors at C16 alcohol. After checking their reactivity in the [¹⁸F]fluorination step and considering the stability of the precursors, we obtained the best results with nosylate precursor 14c.     

Synthesis of N‐(3‐[18F]Fluoropropyl)‐2β‐carbomethoxy‐3β‐(4‐iodophenyl)nortropane ([18F]FP‐β‐CIT)

N‐(3‐[¹⁸F]fluoropropyl)‐2β‐carbomethoxy‐3β‐(4‐iodophenyl)nortropane ([¹⁸F]FP‐β‐CIT) was synthesized in a two‐step reaction sequence. In the first reaction, 1‐bromo‐3‐(nitrobenzene‐4‐sulfonyloxy)‐propane was fluorinated with no‐carrier‐added fluorine‐18. The resulting product, 1‐bromo‐3‐[¹⁸F]‐fluoropropane, was distilled into a cooled reaction vessel containing 2β‐carbomethoxy‐3β‐(4‐iodophenyl)‐nortropane, diisopropylethylamine and potassium iodide. After 30 min, the reaction mixture was subjected to a preparative HPLC purification. The product, [¹⁸F]FP‐β‐CIT, was isolated from the HPLC eluent with solid‐phase extraction and formulated to yield an isotonic, pyrogen‐free and sterile solution of [¹⁸F]FP‐β‐CIT. The overall decay‐corrected radiochemical yield was 25 ± 5%. Radiochemical purity was > 98% and the specific activity was 94 ± 50 GBq/µmol at the end of synthesis. Copyright © 2006 John Wiley & Sons, Ltd.     

Preparation of 3′‐deoxy‐3′‐[18F]fluorothymidine ([18F]FLT) in ionic liquid, [bmim][OTf]

Although 3′‐deoxy‐3′‐[¹⁸F]fluorothymidine ([¹⁸F]FLT) is a prospective radiopharmaceutical for the imaging of proliferating tumor cell, it is difficult to prepare large amount of [¹⁸F]FLT. We herein describe the preparation of [¹⁸F]FLT in an ionic liquid, [bmim][OTf] (1‐butyl‐3‐methyl‐imidazolium trifluoromethanesulfonate). At optimized condition, [¹⁸F]fluorinationin ionic liquid with 5 µl of 1 M KHCO₃ and 5 mg of the precursor yielded 61.5 ± 4.3% (n=10). Total elapsed time was about 70 min including HPLC purification. The rapid synthesis of [¹⁸F]FLT can be achieved by removing all evaporation steps. Overall radiochemical yield and radiochemical purity were 30 ± 5% and >95%, respectively. This method can use a small amount of a nitrobenzenesulfonate precursor and can be adapted for automated production. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis of a phenolic precursor and its efficient O‐[18F]fluoroethylation with purified no‐carrier‐added [18F]2‐fluoroethyl brosylate as the labeling agent

[¹⁸F]2‐Fluoroethyl‐p‐toluenesulfonate also called [¹⁸F]2‐fluoroethyl tosylate has been widely used for labeling radioligands for positron emission tomography (PET). [¹⁸F]2‐Fluoroethyl‐4‐bromobenzenesulfonate, also called [¹⁸F]2‐fluoroethyl brosylate ([¹⁸F]F(CH₂)₂OBs), was used as an alternative radiolabeling agent to prepare [¹⁸F]FEOHOMADAM, a fluoroethoxy derivative of HOMADAM, by O‐fluoroethylating the phenolic precursor. Purified by reverse‐phase HPLC, the no‐carrier‐added [¹⁸F]F(CH₂)₂OBs was obtained in an average radiochemical yield (RCY) of 35%. The reaction of the purified and dried [¹⁸F]F(CH₂)₂OBs with the phenolic precursor was performed by heating in DMF and successfully produced [¹⁸F]FEOHOMADAM, after HPLC purification, in RCY of 21%. Copyright © 2013 John Wiley & Sons, Ltd.     

How to increase the reactivity of [18F]fluoroethyl bromide: [18F]fluoroethylation of amine,phenol and amide functional groups with [18F]FEtBr, [18F]FEtBr/NaI and [18F]FEtOTf

[¹⁸F]Fluoroethyl bromide ([¹⁸F]FEtBr) is a useful synthetic precursor to synthesize ¹⁸F‐labeled compounds. However, the lower reactivity of [¹⁸F]FEtBr with amine, phenol and amide functional groups than that of [¹¹C]CH₃I partly limits its wide application in the synthesis of [¹⁸F]fluoroethylated compounds. The aim of this study was to increase the reactivity of [¹⁸F]FEtBr with various nucleophilic substrates for PET tracers containing amine, phenol and amide moieties. The present strategies included (1) adding NaI into the reaction mixture of [¹⁸F]FEtBr and substrate, where [¹⁸F]FEtI is reversibly formed and becomes more reactive; (2) converting [¹⁸F]FEtBr into much more reactive [¹⁸F]FEtOTf, similar to conversion of [¹¹C]CH₃I into [¹¹C]CH₃OTf. By these efforts, the [¹⁸F]fluoroethylation efficiency of various substrates containing amine, phenol and amide groups with [¹⁸F]FEtBr/NaI and [¹⁸F]FEtOTf was significantly improved, compared with the corresponding reaction efficiency with [¹⁸F]FEtBr. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis,radiofluorination, and preliminary evaluation of the potential 5‐HT2A receptor agonists [18F]Cimbi‐92 and [18F]Cimbi‐150

An agonist PET tracer is of key interest for the imaging of the 5‐HT_2A receptor, as exemplified by the previously reported success of [¹¹C]Cimbi‐36. Fluorine‐18 holds several advantages over carbon‐11, making it the radionuclide of choice for clinical purposes. In this respect, an ¹⁸F‐labelled agonist 5‐HT_2A receptor (5‐HT_2AR) tracer is highly sought after. Herein, we report a 2‐step, 1‐pot labelling methodology of 2 tracer candidates. Both ligands display high in vitro affinities for the 5‐HT_2AR. The compounds were synthesised from easily accessible labelling precursors, and radiolabelled in acceptable radiochemical yields, sufficient for in vivo studies in domestic pigs. PET images partially conformed to the expected brain distribution of the 5‐HT_2AR; a notable exception however being significant uptake in the striatum and thalamus. Additionally, a within‐scan displacement challenge with a 5‐HT_2AR antagonist was unsuccessful, indicating that the tracers cannot be considered optimal for neuroimaging of the 5‐HT_2AR.     

A fast,simple, and reproducible automated synthesis of [18F]FPyKYNE‐c(RGDyK) for αvβ3 receptor positron emission tomography imaging

[¹⁸ F]FPyKYNE‐c(RGDyK) was successfully synthesized by the Cu(I) catalyzed Huisgen 1,3‐dipolar cycloaddition of alkynes to azides using [¹⁸ F]FPyKYNE as a prosthetic group in an overall radiochemical yield of 12%–18% (decay‐corrected) and >99.5% chemical and radiochemical purities in 125 min including quality control. This simple, fully automated two‐step, two‐reactor approach consists of a quick and convenient purification of the prosthetic group using silica gel cartridges and its subsequent use for the labeling of the azido‐c(RGDyK) peptide via click chemistry.     

Chemoenzymatic n.c.a synthesis of the coenzyme UDP‐2‐deoxy‐2‐[18F]fluoro‐α‐D‐glucopyranose as substrate of glycosyltransferases

The development of ¹⁸F‐labelling methods adopted to proteins and bioactive peptides is of great interest in radiopharmaceutical sciences. In order to provide ¹⁸F‐labelled sugars as a polar prosthetic group for an enzymatic ¹⁸F‐labelling procedure, an appropriate nucleotide activated sugar is needed. Here, we present the radiosynthesis of n.c.a. UDP‐2‐deoxy‐2‐[¹⁸F]fluoro‐α‐D‐glucopyranose (UDP‐[¹⁸F]FDG) as a substrate for glycosyltransferases. The MacDonald synthesis of [¹⁸F]FDG‐1‐phosphate was successfully combined with an enzymatic activation to obtain UDP‐[¹⁸F]FDG directly in an aqueous medium located in the void volume of a solid phase cartridge. The radiochemical yield of UDP‐[¹⁸F]FDG was 20% (based on [¹⁸F]fluoride) after a total synthesis time of 110 min. Thus, an intermediate was provided for the enzymatic transfer of [¹⁸F]FDG using UDP‐[¹⁸F]FDG as glycosyl donor making use of a suitable glycosyltransferase. This would represent a highly selective and mild ¹⁸F‐labelling method for glycosylated biomolecules. Copyright © 2007 John Wiley & Sons, Ltd.     

Microwave‐assisted radiosynthesis of [18F]ASEM,a radiolabeled α7‐nicotinic acetylcholine receptor antagonist

An improvement of the original radiochemical synthesis of [¹⁸F]ASEM, an α7‐nicotinic acetylcholinergic receptor radioligand, is reported. The new procedure utilizes microwave‐assisted radiofluorination. In addition, a new preparative HPLC method was developed to eliminate a chemical impurity in the final product. Quality control procedures were also enhanced to improve detection of product with enhanced resolution of potential impurities. [¹⁸F]ASEM was produced in 20.1 ± 8.9% non‐decay corrected (NDC) yield with an average synthesis time of 57 min and an average specific radioactivity of 856 ± 332 GBq/µmol (23 ± 9 Ci/µmol).     

Development of novel PET probes, [18F]BCPP‐EF, [18F]BCPP‐BF,and [11C]BCPP‐EM for mitochondrial complex 1 imaging in the living brain

We developed three novel positron‐emission tomography (PET) probes, 2‐tert‐butyl‐4‐chloro‐5‐{6‐[2‐(2[¹⁸F]fluoroethoxy)‐ethoxy]‐pyridin‐3‐ylmethoxy}‐2H‐pyridazin‐3‐one ([¹⁸F]BCPP‐EF), 2‐tert‐butyl‐4‐chloro‐5‐[6‐(4‐[¹⁸F]fluorobutoxy)‐pyridin‐3‐ylmethoxy]‐2H‐pyridazin‐3‐one ([¹⁸F]BCPP‐BF), and 2‐tert‐butyl‐4‐chloro‐5‐{6‐[2‐(2‐[¹¹C]methoxy‐ethoxy)‐ethoxy]‐pyridin‐3‐ylmethoxy}‐2H‐pyridazin‐3‐one ([¹¹C]BCPP‐EM), for quantitative imaging of mitochondrial complex 1 (MC‐1) activity in vivo. These three PET probes were successfully labeled by nucleophilic [¹⁸F]fluorination or by [¹¹C]methylation of their corresponding precursor with sufficient radioactivity yield, good radiochemical purity, and sufficiently high specific radioactivity for PET measurement. The specificity of these probes for binding to MC‐1 was assessed with rotenone, a specific MC‐1 inhibitor, by a rat brain slice imaging method in vitro. Rat whole‐body imaging by small‐animal PET demonstrated that all probes showed high uptake levels in the brain as well as in the heart sufficient to image them clearly. The rank order of uptake levels in the brain and the heart just after injection was as follows: high in [¹⁸F]BCPP‐BF, intermediate in [¹¹C]BCPP‐EM, and low in [¹⁸F]BCPP‐EF. The kinetics of [¹⁸F]BCPP‐EF and [¹¹C]BCPP‐EM provided a reversible binding pattern, whereas [¹⁸F]BCPP‐BF showed nonreversible accumulation‐type kinetics in the brain and heart. Metabolite analyses indicated that these three compounds were rapidly metabolized in the plasma but relatively stable in the rat brain up to 60 min post‐injection. The present study demonstrated that [¹⁸F]BCPP‐EF could be a useful PET probe for quantitative imaging of MC‐1 activity in the living brain by PET.     

Automated synthesis of 11C‐β‐hydroxybutyrate by enzymatic conversion of 11C‐acetoacetate using β‐hydroxybutyrate dehydrogenase

1‐[¹¹C]‐β‐hydroxybutyrate was produced by conversion from 1‐[¹¹C]‐acetoacetate using (D)‐β‐hydroxybutyrate dehydrogenase in the presence of nicotinamide adenine dinucleotide with purification by ion exchange column chromatography. Radiochemical yield at the end of the synthesis was 10% for a total synthesis time of 36 min. High‐performance liquid chromatography analysis showed ≤4% impurities, principally unconverted acetoacetate. Residual tetrahydrofuran (34±11 ppm) and ethanol (77±30 ppm) were well under the tolerable limits for human studies. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis of [18F]3‐[1‐(3‐fluoropropyl)‐(S)‐pyrrolidin‐2‐ylmethoxy]pyridine ([18F]NicFP): a potential α4β2 nicotinic acetylcholine receptor radioligand for PET

Nicotinic acetylcholine receptors are widely distributed throughout the human brain and are believed to play a role in several neurological and psychiatric disorders. In order to identify an effective PET radioligand for in vivo assessment of the α4β2 subtype of nicotinic receptor, we synthesized [¹⁸F]3‐[1‐(3‐fluoropropyl)‐(S)‐pyrrolidin‐2‐ylmethoxy]pyridine (NicFP). The in vitro K_D of NicFP was determined to be 1.1 nM, and the log P value obtained by HPLC analysis of the unlabelled standard was found to be 2.2. The radiosynthesis of [¹⁸F]NicFP was carried out by a nucleophilic substitution reaction of anhydrous [¹⁸F]fluoride and the corresponding mesylate precursor. After purification by HPLC, the radiochemical yield was determined to be 11.3±2.1% and the specific activity was 0.47±0.18 Ci/μmol (EOS, n = 3). The time of synthesis and purification was 99±2 min. The final product was prepared as a sterile saline solution suitable for in vivo use. Copyright © 2003 John Wiley & Sons, Ltd.     

Facile synthesis of 2‐deoxy‐2‐[18F]fluorosorbitol using sodium borohydride on aluminum oxide

2‐Deoxy‐2‐[¹⁸F]fluorosorbitol (¹⁸F‐FDS) has become increasingly useful in functional renal imaging. FDS is synthesized by the one‐step reduction of 2‐deoxy‐2‐[¹⁸F]fluoroglucose (¹⁸F‐FDG). To develop a more simple and rapid procedure for ¹⁸F‐FDS synthesis, we examined reduction reactions with solid‐supported NaBH₄. Synthetic yields using BH₄–IRA400 (polymer‐based matrix) and NaBH₄–Al₂O₃ (clay‐based matrix) as solid‐supported reagents were compared. NaBH₄–Al₂O₃ was found to be far superior to BH₄–IRA400 in the FDG reduction reaction. IRA 400 was not suitable for this reaction because it adsorbs FDG, in addition to glucose, with no FDS synthesized when using BH₄–IRA400. By contrast, NaBH₄–Al₂O₃ only required a filtration as workup, affording FDS in 90% yield after a total of 10 min. NaBH₄ on alumina was readily consumed in the reaction within 1 min, regardless of the amount used, by simply stirring with a vortex mixer. Complicated procedures, such as microwave irradiation, were not necessary. This simple operation will allow kit formulation and is suitable for radiosynthesis. In conclusion, clay‐supported reagents showed low absorption and were time saving, which are highly compatible with ¹⁸F‐FDS synthesis.     

Simplified and automatic one‐pot synthesis of 16α‐[18F]fluoroestradiol without high‐performance liquid chromatography purification

16α‐[¹⁸F]Fluoroestradiol (16α‐[¹⁸F]FES, 1) is known as a valuable tracer in molecular imaging as estrogen receptor (ER) ligand for investigation of primary and metastatic breast cancer. ER concentration in human breast tumor cells is a significant indicator for the degree of disease and is often monitored by immunoassays or in vitro ligand binding of a tumor biopsy sample. More preferable non‐invasive diagnosis is accessible using 16α‐[¹⁸F]FES (1) as PET tracer. Our aim was to develop a reliable, easy‐to‐use, remotely controlled synthesis for non carrier added (n.c.a.) 16α‐[¹⁸F]FES (1) by nucleophilic substitution using a disposable cassette for GE TRACERlab® MX_FDG. Purification of the crude product using solid phase extraction (SPE) cartridges, Oasis® WAX, HLB Plus, Sep‐Pak® C18 and Light Alumina N, allows abandonment of an HPLC purifying system. Formulation of the final product is included in the automatic synthesis. The experimental conditions for this easy‐to‐use synthesis for routine production of 16α‐[¹⁸F]FES (1) are given in detail. Within 75 min 16α‐[¹⁸F]FES (1) is produced in typically 20% n.c.a., radiochemical yield (non decay corrected). Chemical and radiochemical purity is >95% and >99%, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis of 2‐[18F]fluoroadenosine (2‐[18F]FAD) as potential radiotracer for studying malignancies by PET

2‐[¹⁸F]fluoroadenosine (2‐[¹⁸F]FAD), a potential radioligand for assessment of adenylate metabolism, was synthesized by carrier‐added and no‐carrier‐added procedures via nucleophilic radiofluorination of 2‐fluoroadenosine and 2‐iodoadenosine. The radiochemical yield, specific radioactivity and radiochemical purity of carrier‐added and no‐carrier‐added 2‐[¹⁸F]FAD were 5%, 22–30 mCi/µmol and 99%, and 0.5%, 1200–1700 mCi/µmol and 99%, respectively. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis of 1‐(2′‐deoxy‐2′‐fluoro‐β‐D‐arabinofuranosyl)‐[Methyl‐11C]thymine ([11C]FMAU) via a Stille cross‐coupling reaction with [11C]methyl iodide

1‐(2′‐deoxy‐2′‐fluoro‐β‐D‐arabinofuranosyl)‐[methyl‐¹¹C]thymine ([¹¹C]FMAU) [¹¹C]‐ 1 was synthesised via a palladium‐mediated Stille coupling reaction of 1‐(2′‐deoxy‐2′‐fluoro‐β‐D‐arabinofuranosyl)‐5‐(trimethylstannyl)uracil 2 with [¹¹C]methyl iodide in a one‐pot procedure. The reaction conditions were optimized by screening various catalysts and solvents, and by altering concentrations and reaction temperatures. The highest yield was obtained using Pd₂(dba)₃ and P(o‐tolyl)₃ in DMF at 130°C for 5 min. Under these conditions the title compound [¹¹C]‐ 1 was obtained in 28±5% decay‐corrected radiochemical yield calculated from [¹¹C]methyl iodide (number of experiments=7). The radiochemical purity was >99% and the specific radioactivity was 0.1 GBq/μmol at 25 min after end of bombardment. In a typical experiment 700–800 MBq of [¹¹C]FMAU [¹¹C]‐ 1 was obtained starting from 6–7 GBq of [¹¹C]methyl iodide. A mixed ¹¹C/¹³C synthesis to yield [¹¹C]‐ 1 /(¹³C)‐ 1 followed by ¹³C‐NMR analysis was used to confirm the labelling position. The labelling procedure was found to be suitable for automation. Copyright © 2002 John Wiley & Sons, Ltd.     

Radiosynthesis of 1‐(4‐(2‐[18F]fluoroethoxy)benzenesulfonyl)‐3‐butyl urea: a potential β‐cell imaging agent

Tolbutamide ( 1 ) is a sulfonurea agent used to stimulate insulin secretion in type 2 diabetic patients. Its analogue 1‐(4‐(2‐[¹⁸F]fluoroethoxy)benzenesulfonyl)‐3‐butyl urea ( 3 ) was synthesized in overall radiochemical yields of 45% as a potential β‐cell imaging agent. Compound 3 was synthesized by ¹⁸F‐fluoroalkylation of the corresponding hydroxy precursor ( 2 ) with 2‐[¹⁸F]fluoroethyltosylate in DMF at 120°C for 10 min followed by purification with HPLC in a synthesis time of 50 min. Insulin secretion experiments of the authentic ¹⁹F‐standard compound on rat islets showed that the compound has a similar stimulating effect on insulin secretion as that of tolbutamide ( 1 ). The partition coefficient of compound 3 between octanol/water was determined to be 1.3±0.3 (n=5). Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis of a non‐peptidic PET tracer designed for α5β1 integrin receptor

Arginine–glycine–aspartic acid (RGD)‐containing peptides have been traditionally used as PET probes to noninvasively image angiogenesis, but recently, small selective molecules for α₅β₁ integrin receptor have been developed with promising results. Sixty‐one antagonists were screened, and tert‐butyl (S)‐3‐(2‐((3R,5S)‐1‐(3‐(1‐(2‐fluoroethyl)‐1H‐1,2,3‐triazol‐4‐yl)propanoyl)‐5‐((pyridin‐2‐ylamino)methyl)pyrrolidin‐3‐yloxy)acetamido)‐2‐(2,4,6‐trimethylbenzamido)propanoate (FPMt) was selected for the development of a PET tracer to image the expression of α₅β₁ integrin receptors. An alkynyl precursor (PMt) was initially synthesized in six steps, and its radiolabeling was performed according to the azide–alkyne copper(II)‐catalyzed Huisgen's cycloaddition by using 1‐azido‐2‐[¹⁸F]fluoroethane ([¹⁸F]12). Different reaction conditions between PMt and [¹⁸F]12 were investigated, but all of them afforded [¹⁸F]FPMt in 15 min with similar radiochemical yields (80–83%, decay corrected). Overall, the final radiopharmaceutical ([¹⁸F]FPMt) was obtained after a synthesis time of 60–70 min in 42–44% decay‐corrected radiochemical yield. Copyright © 2014 John Wiley & Sons, Ltd.