Automated synthesis and purification of [18F]fluoro‐[di‐deutero]methyl tosylate

Automated synthetic procedures of [¹⁸F]fluoro‐[di‐deutero]methyl tosylate on a GE TRACERlab FX F‐N module and a non‐commercial synthesis module have been developed. The syntheses included azeotropic drying of the [¹⁸F]fluoride, nucleophilic ¹⁸F‐fluorination of bis(tosyloxy)‐[di‐deutero]methane, HPLC purification and subsequent formulation of the synthesized [¹⁸F]fluoro‐[di‐deutero]methyl tosylate (d₂‐[¹⁸F]FMT) in organic solvents. Automation shortened the total synthesis time to 50 min, resulting in an average radiochemical yield of about 50% and high radiochemical purity (>98%). The possible application of this procedure to commercially available synthesis modules might be of significance for the production of deuterated ¹⁸F‐fluoromethylated imaging probes in the future. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis of 3′‐deoxy‐3′‐[18F]fluoro‐1‐β‐D‐xylofuranosyluracil ([18F]‐FMXU) for PET

The synthesis of a pyrimidine analog, 3′‐deoxy‐3′‐[¹⁸F]‐fluoro‐1‐β‐D ‐xylofuranosyluracil ([¹⁸F]‐FMXU) is reported. 5‐Methyluridine 1 was converted to its di‐methoxytrityl derivatives 2 and 3 as a mixture. After separation the 2′,5′‐di‐methoxytrityluridine 2 was converted to its 3′‐triflate 4 followed by derivatization to the respective N³‐t‐Boc product 5 . The triflate 5 was reacted with tetrabutylammonium[¹⁸F]fluoride to produce 6 , which by acid hydrolysis yielded compound 7 . The crude preparation was purified by HPLC to obtain the desired product [¹⁸F]‐FMXU. The radiochemical yields were 25–40% decay corrected (d. c.) with an average of 33% in four runs. Radiochemical purity was >99% and specific activity was >74 GBq/µmol at the end of synthesis (EOS). The synthesis time was 67–75 min from the end of bombardment (EOB). Copyright © 2005 John Wiley & Sons, Ltd.     

The first enzymatic method for C–18F bond formation: the synthesis of 5′‐[18F]‐fluoro‐5′‐deoxyadenosine for imaging with PET

The use of the key enzyme involved in carbon–fluorine bond formation in Streptomyces cattleya catalysing the formation of 5′‐fluoro‐5′‐deoxyadenosine (5′‐FDA) from fluoride ion and S‐adenosyl‐l‐methionine (SAM) was explored for its potential application in fluorine‐18 labelling of the adenosine derivative. Enzymatic radiolabelling of [¹⁸F]‐5′‐FDA was successfully carried out starting from SAM and [¹⁸F]HF when the concentration of the enzyme preparation was increased from sub‐mg/ml values to mg/ml values. The purity of the enzyme had no measurable effect on the radiochemical yield of the reaction and the radiochemical purity of [¹⁸F]‐5′‐FDA. Copyright © 2003 John Wiley & Sons, Ltd.     

Trifluoromethanesulfonic acid,an alternative solvent medium for the direct electrophilic fluorination of DOPA: new syntheses of 6‐[18F]fluoro‐L‐DOPA and 6‐[18F]fluoro‐D‐DOPA

Previous work from this laboratory has shown that the direct fluorination of 3, 4‐dihydroxy‐phenyl‐L ‐alanine (L ‐DOPA) in anhydrous HF (aHF) or BF₃/HF with F₂ is an efficient method for the synthesis of 6‐fluoro‐L ‐DOPA. Since then, ¹⁸F‐labeled 6‐fluoro‐L ‐DOPA ([¹⁸F]6‐fluoro‐L ‐DOPA) has been used to study presynaptic dopaminergic function in the human brain and to monitor gastrointestinal carcinoid tumors. This work demonstrates that the reactivity and selectivity of F₂ toward L ‐DOPA in CF₃SO₃H is comparable with that in aHF. This new synthetic procedure has led to the production of [¹⁸F]fluoro‐L ‐DOPA and [¹⁸F]fluoro‐D‐DOPA isomers in 17±2% radiochemical yields (decay corrected with respect to [¹⁸F]F₂). The 2‐ and 6‐FDOPA isomers were separated by HPLC and subsequently characterized by ¹⁹F NMR spectroscopy. The corresponding [¹⁸F]‐FDOPA enantiomers have been obtained in clinically useful quantities by a synthetic approach that avoids the use of aHF. Copyright © 2007 John Wiley & Sons, Ltd.     

Radiosynthesis of 6‐([18F]fluoroacetamido)‐1‐hexanoicanilide ([18F]FAHA) for PET imaging of histone deacetylase (HDAC)

Radiosynthesis of a novel substrate for histone deacetylase (HDAC), 6‐([¹⁸F]fluoroacetamido)‐1‐hexanoicanilide ([¹⁸F]FAHA, [¹⁸F]‐ 3 ) is reported. For precursor synthesis, compound 1 (6‐amino‐1‐hexanoicanilide) was prepared by the reaction of 6‐amino hexanoic acid with thionyl chloride in dichloroethane followed by addition of aniline. Compound 1 was reacted with bromoacetic anhydride in tetrahydrofuran (THF) in the presence of triethylamine to produce the precursor compound 6‐(bromoacetamido)‐1‐hexanoicanilide 2 . Fluorination reactions were performed using tetrabutylammonium fluoride in various solvents at 80°C to prepare the unlabeled reference compound 3 . Radiofluorinations were performed using either n‐Bu₄N¹⁸F or K¹⁸F/kryptofix, and the crude product was purified by high performance liquid chromatography (HPLC). The radiochemical yields were 9–13% decay corrected (d.c.) with an average of 11% using K¹⁸F/kryptofix, and specific activity >2 GBq/µmol at the end of synthesis. The synthesis time was 67–75 min from the end of bombardment (EOB). Copyright © 2006 John Wiley & Sons, Ltd.     

Radiosynthesis of 3‐[18F]fluoropropyl and 4‐[18F]fluorobenzyl triarylphosphonium ions

3‐[¹⁸F]Fluoropropyl‐, 4‐[¹⁸F]fluorobenzyl‐triphenylphosphonium and 4‐[¹⁸F]fluorobenzyltris‐4‐dimethylaminophenylphosphonium cations were synthesized in multi‐step reactions from no carrier added (nca) [¹⁸F]fluoride. The time for synthesis, purification, and formulation was 56, 82, and 79 min with an average radiochemical yield of 12, 6 and 15%, respectively (not corrected for decay). The average specific radioactivity for the three radiolabeled compounds was 14.9 GB q/µmole (403 mCi/µmole) at end of synthesis (EOS). Copyright © 2004 John Wiley & Sons, Ltd.     

A fully automated synthesis of [18F]‐FEAU and [18F]‐FMAU using a novel dual reactor radiosynthesis module

2′‐Deoxy‐2′‐[¹⁸F]fluoro‐5‐substituted‐1‐β‐D ‐arabinofuranosyluracils, including 2′‐deoxy‐2′‐[¹⁸F]fluoro‐5‐methyl‐1‐β‐D ‐arabinofuranosyluracil [¹⁸F]FMAU and [¹⁸F]FEAU are established radiolabeled probes to monitor cellular proliferation and herpes simplex virus type 1 thymidine kinase (HSV1‐tk) reporter gene expression with positron emission tomography. For clinical applications, a fully automated CGMP‐compliant radiosynthesis is necessary for production of these probes. However, due to multiple steps in the synthesis, no such automated synthetic protocols have been developed. We report here a fully automated synthesis of [¹⁸F]‐FEAU and [¹⁸F]‐FMAU on a prototype dual reactor module TRACERlab FX FN. The synthesis was performed by using a computer‐programmed standard operating procedure, and the product was purified on a semipreparative high‐performance liquid chromatography (HPLC) integrated with the synthesis module using 12% EtOH in 50 mM Na₂HPO₄. Finally, the percentage of alcohol was adjusted to 7% by adding Na₂HPO₄ and filtered through a Millipore filter to make dose for human. The radiochemical yield on the fluorination was 40±10% (n=10), and the overall yields were 4±1% (d. c.), from the end of the bombardment; [¹⁸F]FEAU (n=7) and [¹⁸F]FMAU (n=3). The radiochemical purity was >99%, specific activity was 1200–1300 mCi/µmol. The synthesis time was 2.5 h. This automated synthesis should be suitable for production of [¹⁸F]FIAU, [¹⁸F]FFAU, [¹⁸F]FCAU, [¹⁸F]FBAU and other 5‐substitued thymidine analogues. Copyright © 2009 John Wiley & Sons, Ltd.     

One‐step reductive etherification of 4‐[18F]fluoro‐benzaldehyde with decaborane

Reductive coupling reactions between 4‐[¹⁸F]fluoro‐benzaldehyde ([¹⁸F] 1 ) and different alcohols by use of decaborane (B₁₀H₁₄) as reducing agent have the potential to synthesize 4‐[¹⁸F]fluoro‐benzylethers in one step. [¹⁸F] 1 was synthesized from 4‐trimethylammonium benzaldehyde (triflate salt) via a standard fluorination procedure (K[¹⁸F]F/Kryptofix^® 222) in dimethylformamide at 90°C for 25 min and purified by solid‐phase extraction. Subsequently, reductive etherifications of [¹⁸F] 1 were performed as one‐step reactions with primary and secondary alcohols, mediated by B₁₀H₁₄ in acetonitrile at 60°C. Various 4‐[¹⁸F]fluorobenzyl ethers (6 examples are shown) were obtained within 1–2 h reaction time in decay‐corrected radiochemical yields of 12–45%. Copyright © 2006 John Wiley & Sons, Ltd.     

Alternative solvents for electrophilic synthesis of 6‐[18F]fluoro‐L‐DOPA

Owing to the ozone layer‐depleting properties of chlorofluorocarbon compounds, alternative solvents for electrophilic fluorination reactions are desirable. Chloroform, dichloromethane, acetone or their deuterated analogues were examined as substitutes for Freon‐11 in the electrophilic synthesis of 6‐[¹⁸F]fluoro‐L ‐DOPA ([¹⁸F]FDOPA). CDCl₃, CD₂Cl₂ and C₃D₆O were found to be suitable solvents in this reaction, with the deuterated solvents providing significantly higher yields than Freon‐11. There were no differences among the solvents in the specific radioactivity, the radiochemical purity, the chemical purity or the microbiological quality of the final product. However, the radiochemical yield of [¹⁸F]FDOPA was increased when acetic acid was added to the precursor solution prior to the fluorination reaction. Copyright © 2009 John Wiley & Sons, Ltd.     

Temperature effects on the stereospecificity of nucleophilic fluorination: formation of trans‐[18F]4‐fluoro‐l‐proline during the synthesis of cis‐[18F]4‐fluoro‐l‐proline

Fluorine‐18 labeled (2S,4S)‐4‐fluoro‐l ‐proline (cis‐[¹⁸F]4‐FPro) has been reported to be a potential positron emission tomography tracer to study abnormal collagen synthesis occurring in pulmonary fibrosis, osteosarcomas, mammary and colon carcinomas. In this paper, we report the stereospecific radiofluorination of (2S,4R)‐N‐tert‐butoxycarbonyl‐4‐(p‐toluenesulfonyloxy) proline methyl ester (at 110°C) to produce diastereomerically pure cis‐[¹⁸F]4‐FPro in 38% radiochemical yield at the end of a 90‐min synthesis. Investigation of the effect of temperature on the stereospecificity of nucleophilic fluorination showed that diasteriomerically pure cis‐[¹⁸F]4‐FPro or trans‐[¹⁸F]4‐FPro was produced at lower temperatures (85°C–110°C) during the fluorination of (2S,4R) or (2S,4S) precursors, respectively. However, at higher temperatures (130°C–145°C), fluorination of (2S,4R) precursor produced a mixture of cis‐[¹⁸F]4‐FPro and trans‐[¹⁸F]4‐FPro diastereomers with cis‐[¹⁸F]4‐FPro as the predominant isomer. Hydrolysis of the purified fluorinated intermediate was carried out either in one step, using 2 m triflic acid at 145°C for 10 min, or in two steps where the intermediate was heated in 1 m HCl at 110°C for 10 min followed by stirring at room temperature in 1 N NaOH for 5 min. The aqueous hydrolysis mixture was loaded onto an anion exchange column (acetate form for one‐step hydrolysis) or an ion retardation column (two‐step hydrolysis) followed by a C₁₈ Sep‐Pak® (Waters Corporation, Milford, MA, USA). Pure cis‐[¹⁸F]4‐FPro was then eluted with sterile water. We also report that epimerization of cis‐[¹⁸F]4‐FPro occurs during the two‐step hydrolysis (H⁺ followed by OH^?) of the intermediate, resulting in 5 ± 3% trans‐[¹⁸F]4‐FPro, whereas the one‐step acid hydrolysis yielded pure cis‐[¹⁸F]4‐FPro in the final product. Copyright © 2011 John Wiley & Sons, Ltd.     

4‐[18F]fluorophenyl ureas via carbamate‐4‐nitrophenyl esters and 4‐[18F]fluoroaniline

Four different no carrier added (n.c.a.) 4‐[¹⁸F]fluorophenylurea derivatives are synthesized as model compounds via two alternative routes. In both cases carbamate‐4‐nitrophenylesters are used as intermediates. Either n.c.a. 4‐[¹⁸F]fluoroaniline reacts with carbamates of several amines, or the carbamate of n.c.a. 4‐[¹⁸F]fluoroaniline is formed at first and an amine is added subsequently to yield the urea derivative. The choice of the appropriate way of reaction depends on the possibilities of precursor synthesis. The radiochemical yields reach up to 80% after 50 min of synthesis time while no radiochemical by‐products can be determined. These high yields were possible due to an optimized preparation of n.c.a. 4‐[¹⁸F]fluoroaniline with a radiochemical yield of up to 90%. From the various ways of its radiosynthesis, the substitution with n.c.a. [¹⁸F]fluoride on dinitrobenzene is chosen, using phosphorous acid and palladium black for reduction of the second nitro group. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis of [18F]‐6‐deoxy‐6‐fluoro‐D‐glucose ([18F]6FDG), a potential tracer of glucose transport

With the goal of developing a PET radioligand for the in vivo assessment of glucose transport, 6-deoxy-6-[¹⁸F]fluoro-D -glucose ([¹⁸F]6FDG) was prepared in two steps from ¹⁸F⁻. Starting with D -glucose, the tosyl- and mesyl-derivatives of 3,5-O-benzylidene-1,2-O-isopropylidene-α-D -glucofuranose were prepared by known methods. Reaction of either of these precursors with ¹⁸F⁻ resulted in the formation of 3,5-O-benzylidene-6-deoxy-6-[¹⁸F]-fluoro-1,2-O-isopropylidene-α-D -glucofuranose in high yield. Subsequent hydrolysis resulted in the production of [¹⁸F]6FDG. Under optimal conditions, [¹⁸F]6FDG is produced 60–70 min after end of bombardment (EOB) in 71 ± 12% yield (decay corrected, based upon fluoride) with a radiochemical purity of ⩾96%. Preliminary experiments have indicated that [¹⁸F]6FDG may be a more representative in vivo tracer for the glucose transporter than 2FDG. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis of [18F]‐labeled N‐3(substituted) thymidine analogues: N‐3([18F]fluorobutyl) thymidine ([18F]‐FBT) and N‐3([18F]fluoropentyl) thymidine ([18F]‐FPT) for PET

Syntheses of N‐3(substituted) analogues of thymidine, N‐3([¹⁸F]fluorobutyl)thymidine ([¹⁸F]‐FBT) and N‐3([¹⁸F]fluoropentyl)thymidine ([¹⁸F]‐FPT) are reported. 1,4‐Butane diol and 1,5 pentane diol were converted to their tosyl derivatives 2 and 3 followed by conversion to benzoate esters 4 and 5, respectively. Protected thymidine 1 was coupled separately with 4 and 5 to produce 6 and 7 , which were hydrolyzed to 8 and 9 , then converted to their mesylates 10 and 11 , respectively. Compounds 10 and 11 were fluorinated with n‐Bu₄N[¹⁸F] to produce 12 and 13 , which by acid hydrolysis yielded 14 and 15 , respectively. The crude products were purified by HPLC to obtain [¹⁸F]‐FBT and [¹⁸F]‐FPT. The radiochemical yields were 58–65% decay corrected (d.c.) for 14 and 46–57% (d.c.) for 15 with an average of 56% in three runs per compound. Radiochemical purity was >99% and specific activity was >74 GBq/µmol at the end of synthesis (EOS). The synthesis time was 65–75 min from the end of bombardment (EOB). Copyright © 2006 John Wiley & Sons, Ltd.     

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

Several 2′‐deoxy‐2′‐[¹⁸F]fluoro‐1‐β‐D‐arabinofuranosyluracil derivatives have been synthesized. Coupling of 1‐bromo‐2‐deoxy‐2‐[¹⁸F]fluoro‐3,5‐di‐O‐benzoyl‐α‐D‐arabinofuranose 2 with protected uracil derivatives 3a–e followed by hydrolysis and high‐performance liquid chromatography purification produced the radiolabeled nucleosides 4a–e in 15–30% yield (d. c.), >99% radiochemical purity and 55.5–103.6 GBq/µmol specific activities. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis of [18F]‐labeled adenosine analogues as potential PET imaging agents

The syntheses of adenosine analogues, 2′‐deoxy‐2′‐[¹⁸F]fluoro‐9‐β‐D ‐arabinofuranosyladenine ([¹⁸F]‐FAA) and 3′‐deoxy‐3′‐[¹⁸F]fluoro‐9‐β‐D ‐xylofuranosyladenine ([¹⁸F]‐FXA) are reported. Adenosine ( 1 ) was converted to its methoxytrityl derivatives 2 and 3 as a mixture. After separation, these derivatives were converted to their respective triflates 4 and 5 . Each triflate was reacted with tetrabutylammonium[¹⁸F]fluoride to produce 6b or 7b , which by acidic hydrolysis yielded compounds 8b and 9b . Crude preparations were purified by HPLC to obtain the desired pure products. The radiochemical yields were 10‐18% decay corrected (d. c.) for 8b and 30‐40% (d. c.) for 9b in 4 and 3 runs, respectively. Radiochemical purity was >99% and specific activity was >74 GBq/μmol at the end of synthesis (EOS). The synthesis time was 90‐95 min from the end of bombardment (EOB). Copyright © 2003 John Wiley & Sons, Ltd.     

N1'‐(p‐[18F]Fluorobenzyl)naltrindole (p‐[18F]BNTI) as a potential PET imaging agent for DOP receptors

The N1'‐(p‐fluorobenzyl)naltrindole 5 has been synthesized by reaction of 3‐O‐benzyl NTI 3 with p‐fluorobenzylbromide under phase transfer catalysis. The subsequent 3‐O‐benzyldeprotection of 4 in HBr/CH₃COOH gave the target compound 5 in three steps from naltrindole 2 . p‐FBNTI 5 is a novel delta opioid receptor antagonist (K_i=0.00312 nM) and antagonizes the delta opioid (DOP) agonist, DPDPE, with a K_e=1.55 nM in the mouse vas deferens preparation. Using the same synthetic strategy the synthesis of p‐[¹⁸F]BNTI 10 was undertaken. The final yield was 4% and the specific activity varied in a range of 250–400 mCi/µmol. Copyright © 2006 John Wiley & Sons, Ltd.     

N3‐Substituted thymidine analogues III: radiosynthesis of N3‐[(4‐[18F]fluoromethyl‐phenyl)butyl]thymidine ([18F]‐FMPBT) and N3‐[(4‐[18F]fluoromethyl‐phenyl)pentyl] thymidine ([18F]‐FMPPT) for PET

Radiosyntheses of two N³‐substituted thymidine analogues, N³‐[(4[¹⁸F]fluoromethyl‐phenyl)butyl]thymidine ([¹⁸F]‐FMPBT) and N³‐[(4[¹⁸F]fluoromethyl‐phenyl)pentyl]thymidine ([¹⁸F]‐FMPPT), are reported. The precursor compounds 9 and 10 were synthesized in six steps and the standard compounds 13 and 14 were synthesized from these precursors. For radiosynthesis, compounds 9 and 10 were fluorinated with n‐Bu₄N[¹⁸F] to produce [¹⁸F]‐ 11 and [¹⁸F]‐ 12 , which by acid hydrolysis yielded [¹⁸F]‐ 13 and [¹⁸F]‐ 14 , respectively. The crude products were purified by high‐performance liquid chromatography to obtain [¹⁸F]‐FMPBT and [¹⁸F]‐FMPPT. The average decay‐corrected radiochemical yield for [¹⁸F]‐ 13 was 15% in five runs, and that for [¹⁸F]‐ 14 was 10% in four runs. The radiochemical purity was >99% and the specific activity was >74 GBq/µmol at the end of synthesis. The synthesis time was 80–90 min from the end of bombardment. Copyright © 2007 John Wiley & Sons, Ltd.     

Radiosynthesis of the fluorinated sucrose analogue, 1′‐[18F]fluoro‐1′‐deoxysucrose

Fluorinated and deoxysucrose analogues have been proven useful in probing the substrate specificity and roles of sucrose processing enzymes and transporters in plants. To synthesize an ¹⁸F‐labeled fluorodeoxysucrose analogue suitable for in vivo studies, an acyl‐protected, disaccharide‐based radiofluorination precursor (sucrose 1′‐O‐trifluoromethanesulfonyl‐2,3,4,6,3′,4′,6′‐hepta‐O‐acetate; 2) was prepared by regioselective mono‐deacetylation of sucrose octaacetate using a commercial esterase enzyme followed by conversion of the resultant sucrose heptaacetate to the corresponding triflate. Reaction of this triflate precursor with [¹⁸F]fluoride followed by base hydrolysis to remove the acetate groups and HPLC purification gave 1′‐[¹⁸F]fluoro‐1′‐deoxysucrose (4) in an overall synthesis time of 80 min and with a median decay corrected yield of 26% (n = 4). This study demonstrates the use of an enzymatic approach to aid the synthesis of a regiospecific radiofluorination precursor starting from the readily available fully acetylated sugar, thus avoiding the need for a complex classical carbohydrate protection strategy to individually protect each hydroxyl group in the molecule. Copyright © 2012 John Wiley & Sons, Ltd.     

Copper‐mediated reduction of 2‐[18F]fluoroethyl azide to 2‐[18F]fluoroethylamine

¹⁸F‐labelled fluoroalkylamines are attractive reagents for the preparation of positron emission tomography tracers containing amine, amide, and N‐heterocyclic moieties. Herein, we report that 2‐[¹⁸F]fluoroethylamine can be obtained from 2‐[¹⁸F]fluoroethyl azide by reduction with elemental copper under acidic conditions. Azide to amine reduction was achieved in near quantitative analytical yields within 30 min by heating a solution of 2‐[¹⁸F]fluoroethyl azide in the presence of copper wire and aqueous trifluoroacetic acid. Subsequent reaction of 2‐[¹⁸F]fluoroethylamine with benzoyl chloride in the presence of triethylamine provided N‐[¹⁸F]fluoroethyl benzamide in 63% decay‐corrected radiochemical yield from 2‐[¹⁸F]fluoroethyl azide. The utility of the Cu(0)/H⁺ azide reduction method was further exemplified by preparation of the potential GABA_A tracer 9H‐β‐carboline N‐2‐[¹⁸F]fluoroethylamide, which was obtained in 46% decay‐corrected radiochemical yield by reaction of 2‐[¹⁸F]fluoroethylamine with the corresponding 9H‐β‐carboline pentafluorophenyl ester. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis of [18F]‐labeled 2′‐deoxy‐2′‐fluoro‐5‐methyl‐1‐β‐D‐arabinofuranosyluracil ([18F]‐FMAU)

Synthesis of 2′‐deoxy‐2′‐[¹⁸F]fluoro‐5‐methyl‐1‐β‐D‐arabinofuranosyluracil ([¹⁸F]‐FMAU) is reported. 2‐Deoxy‐2‐[¹⁸F]fluoro‐1,3,5‐tri‐O‐benzoyl‐α‐D‐arabinofuranose 2 was prepared by the reaction of the respective triflate 1 with tetrabutylammonium[¹⁸F]fluoride. The fluorosugar 2 was converted to its 1‐bromo‐derivative 3 and coupled with protected thymine 4 . The crude product mixture ( 5a and 5b ) was hydrolyzed in base and purified by HPLC to obtain the radiolabeled FMAU 6a . The radiochemical yield of 6a was 20–30% decay corrected (d.c.) in four steps with an average of 25% in four runs. Radiochemical purity was >99% and average specific activity was 2300 mCi/μmol at the end of synthesis (EOS). The synthesis time was 3.5–4.0 h from the end of bombardment (EOB). Copyright © 2002 John Wiley & Sons, Ltd.