Highly enantioselective synthesis of no-carrier-added 6-[18F]Fluoro-L-dopa by chiral phase-transfer alkylation

[F-18]Fluoro-L-dopa, an important radiopharmaceutical for positron emission tomography (PET), has been synthesized using a phase-transfer alkylation reaction. A chiral quaternary ammonium salt derived from a Cinchona alkaloid served as phase-transfer catalyst for the enantioselective alkylation of a glycine derivative. The active methylene group of this Schiff-base substrate was deprotonated with cesium hydroxide and rapidly alkylated by the 2-[F-18]fluoro-4,5-dimethoxybenzyl halide (X = Br, I). The reaction proceeded with high yield (> 90%) at 0 degreesC or room temperature in various solvents such as toluene or dichloromethane. Preparation of the [F-18]alkylating agent on a solid support was developed. After labelling, the labeled [F-18]fluoroveratraldehyde was trapped on a (t)C18 cartridge and then converted on the cartridge into the corresponding benzyl halide derivatives by addition of aqueous sodium borohydride and gaseous hydrobromic or -iodic acid. Hydrolysis and purification by preparative HPLC made 6-[F-18]fluoro-L-dopa ready for human injection in a 25-30% decay-corrected radiochemical yield in a synthesis time of 100 min. The product was found to be chemically, radiochemically and enantiomerically pure (ee > 95%). (C) Wiley-VCH Verlag GmbH