Selective synthesis of [2-11C]2-iodopropane and [1-11C]iodoethane using the loop method by reacting methylmagnesium bromide with [11C]carbon dioxide

[2-11C]2-iodopropane ([2-11C]i-PrI) and [1-11C]iodoethane ([1-11C]EtI) were selectively synthesized using the loop method by reacting methylmagnesium bromide (MeMgBr) with [11C]carbon oxide ([11C]CO2), followed by treatment with LiAlH4 and then HI. The loop method, in which a low amount of MeMgBr was used for the Grignard reaction, diminished the formation of non-radioactive iodomethane (MeI) and improved the specific activity of [2-11C]i-PrI and [1-11C]EtI. By examining the reaction temperature and time of MeMgBr with [11C]CO2 in the loop, we determined the optimal respective conditions of forming [2-11C]i-PrI and [1-11C]EtI. Moreover, [2-11C]i-PrI and [1-11C]EtI could be simultaneously synthesized at a designated ratio in one production run. These substances were obtained by gas chromatographic purification as two radiochemically pure products. All the processes from the production of [11C]CO2 to the purification of [11C]RI were automated. When we started from about 37 GBq of [11C]CO2, 3.9–5.3 GBq of [1-11C]EtI or 3.7–4.4 GBq of [2-11C]i-PrI was obtained with a specific activity of 37–99 GBq/μmol at EOS (n=3). This amount of radioactivity is sufficient for the synthesis of [11C]radioligands.