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

Add time:08/03/2019    Source:sciencedirect.com

The reaction of [18F]fluoromethyl tosylate with methyl(tert-butoxycarbonyl)-l-tryptophanate results in formation the O-alkylated 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 [18F]fluoromethyl tosylate at 130°C results in the formation of [18F]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 [19F]fluoromethyl(tert-butoxycarbonyl)-l-tryptophanate 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.

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