NUCLEOPHILIC ENANTIOSELECTIVE SYNTHESIS OF 6-[F-18]FLUORO-L-DOPA VIA 2 CHIRAL AUXILIARIES

Asymmetric nucleophilic synthesis of 6-[F-18]fluoro-L-dopa was investigated in order to reach an enantiomeric excess of close to 100% of the L form of this amino acid. The radiochemical synthesis required [F-18]fluoride as fluorinating agent and regioselective nucleophilic substitution of commercially available 6-nitroveratraldehyde. The [F-18]fluorobenzaldehyde thus obtained was easily converted to the corresponding 2-[F-18]fluoro-4,5-dimethoxybenzyl bromide. This alkylating agent was added to the lithium enolates of 1-(S)-(-)camphor imine of t-butyl glycinate (1) and (S)-(-)- 1 -Boc-2-t-butyl-3-methyl-4-imidazolidinone [(S)- Boc-BMI] (2) in order to compare the enantiomeric excess of the L form obtained in each case with these two chiral inductors. The L-isomer of fluorodopa was isolated after H1 hydrolysis and HPLC purification in 5-10% radiochemical yield (decay corrected). The overall synthesis time was of 110 min. Through this synthetic pathway, the L-isomer of fluorodopa was obtained in 83% e.e with 1 and 96% e.e with 2 respectively, as determined by chiral HPLC. A practical three step preparative scale synthesis of 6-[F-19]fluoro-D,L-dopa is also presented.