Synthesis of triphenylarsonium [¹¹C ]methylide, a new ¹¹C-Precursor. Application in the preparation of [2-¹¹C]indole

The synthesis of the new and highly reactive ¹¹C-precursor triphenylarsonium [¹¹C]methylide 1 and its conversion into [2-¹¹C]indole 6 is described. [¹¹C]Methyltriphenylarsonium iodide 4 was prepared by quaternization of triphenylarsine 2 with [¹¹C]methyl iodide 3 in a THF/DMSO mixture or ethanol. Starting from 3 [¹¹C]methyltriphenylarsonium iodide 4 was obtained in a decay-corrected radiochemical yield of 55%-64% depending on the reaction solvent. The arsonium ylide 1 was prepared in situ by treatment of 4 with butyllithium. Conversion of 1 with o-aminobenzaldehyde 5 in THF/DMSO mixtures yielded [2-¹¹C]indole 6 in radiochemical yields of 23-27% (decay-corrected, in relation to 3). Preparation of 6 was completed after 15 – 20 min (starting from 3). The specific radioactivity of 6 was about 37 GBq/µmol (1 Ci/µmol, related to EOB).