Bromo[¹⁸F]fluorination of cyclohexenes: a method for the preparation of [¹⁸F]fluorocyclohexanes

In an earlier report, we described an olefin halofluorination process that was used to label monosubstituted alkenes with fluorine-18 at the no-carrier-added level. In this report, we describe further studies on the radiohalofluorination of more highly substituted and more reactive alkenes, such as the cyclohexenes 1,4-diisopropylcyclohexene (DIPC), 1-methylcyclohexene (MeC) and pregnenolone (3β-hydroxy-5-pregnen-20-one). DIPC, MeC and pregnenolone were labeled with fluorine-18 in 30%, 26% and 35% radiochemical yields, respectively, within 15 min at 0 °C under no-carrier-added conditions. A comparison of the conditions required to obtain optimal yields for fluorine-18 radiohalofluorination with these trisubstituted cyclic alkene systems vs. the monosubstituted alkenes studied earlier, suggests that alkene systems with different degrees of alkyl substitution (and hence different reactivity towards electrophiles) will each require careful optimization of the reaction conditions.