An alternative route to avoid the trapping of [18F]fluoride by the C3 carbonyl group in the isatin ring is to protect the C3 carbonyl group. Once protected, it loses the ability for nucleophilic addition by [18F]fluoride. The mesylate precursor 12 for [18F]2 was protected at the C3 position with 1,3-dioxane and this precursor is stable to the basic labeling condition. Under standard labeling conditions, the mesylate group was replaced with [18F]fluoride via nucleophilic substitution to form the 18F labeled intermediate 14 (table 1, entry 5), then 14 can be deprotected with 1 N HCl under microwave irradiation to afford [18F]2. The successful labeling of the C3 carbonyl-protected precursor 12 again indicates the involvement of this carbonyl in the 18F labeling of 2 from the unprotected precursor 7a.