New synthesis of fluorine-18-labeled 6-fluoro-L-dopa by cleaving the carbon-silicon bond with fluorine.

A new synthesis of 3,4-dihydroxy-6-[18F]fluoro-L-phenylalanine using 6-trimethylsilyl-3,4-dimethoxy-L-dopa-ethylester as a fluorination substrate is described. The silane is prepared from the corresponding bromo compound by reacting the latter with magnesium and trimethylsilyl chloride. Reaction of the silane with [18F]F2 (0.5% F2 in neon) in a mixture of freon-11/CCl4 (1:1) kept in a dry ice bath, subsequent hydrolysis with concentrated HBr in a bath at 140 degrees C, and simple chromatographic purification yielded 18F-labeled 6-fluoro-L-dopa. A radiochemical yield of about 8% was achieved at the end of the 1-hr synthesis. The specific activity at the end of the synthesis was about 680 mCi/mmol after a 30-min irradiation.