Radiolabeling of protected tryptophan with [18F]fluoromethyl tosylate: Formation of [18F]fluoromethyl ester of tryptophan instead of 1-N-[18F] fluoromethyl tryptophan methylester

The reaction of [F-18]fluoromethyl tosylate with methyl(tert-butoxycarbonyl)-L-tryptophanate results in formation the 0-alltylated ester of the tryptophan instead of alkylation of the indole nitrogen of tryptophan as initially anticipated. Treatment of protected tryptophan with NaH in dimethyl formamide (DMF) along with [F-18] fluoromethyl tosylate at 130 degrees C results in the formation of [F-18]fluoromethyl(tert-butoxycarbonyl)-L-tryptophanate. Preferential formation of the O-alkylated product is postulated to be due to the hydrolysis of the ester. Confirmation of the O-alkylation was obtained by synthesizing the [F-19]fluoromethyl(tert-butoxycarbonyl)-Ltryptophanate insitu and examining its NMR characteristics using multiple NMR techniques. Similar results were also obtained when reacting Boc-tryptophan-N-carboxyanhydride precursor with fluoromethyl tosylate.