[ 123 I]iodomethane, a main product in the synthesis of 5‐[ 123 I]iodo‐6‐nitroquipazine by iododestannylation

Abstract Radioiodinated 5‐iodo‐6‐nitroquipazine (2‐piperazinyl‐5‐iodo‐6‐nitroquinoline), a selective serotonin transporter radiotracer, can be prepared by the reaction of its N‐t‐BOC‐5‐trimethylstannyl precursor with [ 125 I] or [ 123 I]NaI in the presence of chloramine‐T, followed by amine deprotection by heating under acidic conditions. However, the labeling reaction is associated with a large and consistent formation of a volatile radioactive reaction product [37 ± 4% (n = 11] for 123 I and 40 ± 4% (n = 3) for 125 I). The purpose of these experiments was to characterize the volatile radioactive product and to examine the effect of reaction temperature on its formation. The volatile radioactive product was identified as [ 125 I] or [ 123 I]iodomethane by its trapping characteristics and by its retention time in two different HPLC systems. Changing the reaction temperature (22–124°C) did not have a significant effect in the amount of labeled iodomethane produced (n = 13). The conditions chosen for the radioiodination and deprotection were: 50 μg precursor in MeOH, dry Na 123 I, 50 μl 0.02 M chloramine‐T in glacial HOAc/MeOH, heating at 90°C for 15 min, followed by deprotection with CF 3 CO 2 H for 5‐10 min at the same temperature. Purification was achieved by solvent extraction and HPLC. Radiochemical yields for preparations carried out at temperatures between 86°C and 124°C were 24 ± 5% (n = 11) for [ 123 I]‐ and 45 ± 13% (n = 3) for 5‐[ 125 I]iodo‐6‐nitroquipazine; radiochemical purity averaged 95 ± 4% and 99 ± 0% respectively.