Asymmetric synthesis of L-[3-11C]alanine and L-[3-11C]phenylalanine by a phase-transfer alkylation reaction

Abstract

The asymmetric syntheses of enantiomerically enriched L-[3-¹¹C]alanine and L-[3-¹¹C]phenylalanine (t_½ 20.4 min for ¹¹C) by phase-transfer alkylation of (–)-8-phenylmenthan-3-yl N-(diphenylmethylene) glycenate with [¹¹C]methyl and [α-¹¹C]benzyl iodides, respectively, are presented. The total synthesis times were 35–55 min, from the start of synthesis of the [¹¹C]methyl and [α-¹¹C]benzyl iodides. The products were obtained in 15–40%(decay corrected) radiochemical yields and higher than 98% radiochemical purities. The enantiomeric purities of the amino acids were determined to be 52–55% enantiomeric excess (e.e.). In a typical run 600 MBq of L-[¹¹C]alanine was obtained, starting with 5.2 GBq of [¹¹C]carbon dioxide.