Stereoselective ¹¹C Labeling of a “Native” Tetrapeptide by Using Asymmetric Phase-Transfer Catalyzed Alkylation Reactions

The first ¹¹C-labeled unmodified (“native”) peptide is described by alkylation of a tetrapeptide Schiff base, which was achieved by an automated five-step radiochemical reaction. In a proof-of-concept study, [¹¹C]Phe-d-Trp-Lys-Thr was synthesized. This tetrapeptide is the essential pharmacophore of octreotide, an antagonist of somatostatin receptors. The asymmetric alkylation with chiral phase-transfer catalysts enabled direct labeling of a variety of isolated ¹¹C-peptides in a highly stereoselective manner (94 % de) with acceptable radiochemical yields (9–10 %) and practical specific activities (15–35 GBq µmol^–1 or 405–945 mCi µmol^–1) at the end of synthesis. This novel methodology provides a powerful new radiosynthetic method to access novel, stereochemically pure carbon-11-labeled native small peptides ready for in vivo studies.