A three-step radiosynthesis of 6-[18F]fluoro-L-meta-tyrosine starting with [18F]fluoride

The radiosynthesis of 6-[F-18]fluoro-L-m-tyrosine has generally been performed by electrophilic radiofluorination, which exhibits several drawbacks. In the present work, a three-step radiochemical synthesis is described starting from [F-18]fluoride. The synthetic sequence, including isotopic exchange, Baeyer-Villiger oxidation, and hydrolysis, were examined comparing four fluorobenzophenone derivatives as labeling precursors. Of those, (2S,5S)-tert-butyl 5-(5-acetyl-2-fluorobenzyl)-2-tert-butyl-3-methyl-4-oxoimidazolidine-1-carboxylate (1a) and (2S,5S)-tert-butyl 2-tert-butyl-5-(2-fluoro-5-(2,2,2-trifluoroacetyl)benzyl)-3-methyl-4-oxoimidazolidine-1-carboxylate (1d) proved to be the most suitable ones. 6-[F-18]Fluoro-L-m-tyrosine was obtained with overall radiochemical yields of 8-13% and an enantiomeric excess of up to 98%.