Synthesis of 15-(4-[11C]methylphenyl)pentadecanoic acid (MePPA) via Stille cross-coupling reaction

For experimental studies by animal PET [¹¹C]-labeled 15-(4-methylphenyl)pentadecanoic acid (MePPA) is an attractive alternative to the radioiodinated 15-(4-iodophenyl)pentadecanoic acid (IPPA) which has widely been used for imaging of fatty acid metabolism. The important physiological aspect is that the iodine atom and the methyl substituent have similar steric and lipophilic properties. For preparation of [¹¹C]MePPA, Stille cross-coupling reaction was applied since the same tin precursor as for the radiosynthesis of IPPA and readily available [¹¹C]CH₃I can be used. Unsaturated tris(dibenzylideneacetone)dipalladium(0)/tri(o-tolyl)phosphine [Pd₂(dba)₃/P(o-tolyl)₃] was taken as the catalytic system. The reaction conditions were optimized with respect to temperature, time, solvent and amount of precursor. The best radiochemical yields of 73 ± 2.8% (decay corr.) were obtained using 0.525 mg tin precursor in DMF at 80 °C already after a reaction time of 10 min. The labeled methyl ester was hydrolyzed by 1 M NaOH/EtOH at 80 °C within 3 min to give [¹¹C]IPPA in a RCY of 62 ± 3.0%. The radiochemical purity of the product assured by HPLC was >99% and the overall preparation time including HPLC purification and formulation was 40 min.