A high-yield and simplified procedure for the synthesis of alpha-[11C]methyl-L-tryptophan

Alpha-[11C]methyl-L-tryptophan (AMT) has been synthesized by stereoselective methylation with [11C]methyl iodide of the lithium-enolate generated by treating dimethyl 2(S), 3a(R), 8a(S)-(+)-hexahydro-8(phenylsulfonyl)pyrrolo [2, 3-b]indole-1,2-dicarboxylate (2) with lithium diisopropyl amide (LDA) at -55 degrees C, followed by ring opening using trifluoroacetic acid and alkaline hydrolysis of the protecting groups. The crude product was purified by a simple reverse-phase C-18 Sep-Pak procedure. The purified product was isolated with an average radiochemical yield of 53 +/- 12% (decay corrected) in 30-35 min from [11C]methyl iodide. At end of synthesis (EOS), 138 +/- 35 mCi (n = 24) of product was collected with a specific activity of ca. 1-1.3 Ci/mumol (EOS) (4-5 Ci/mumol @ EOB) starting from 1.5 Ci (EOB) of [11C]CO2.