Convenient synthesis of 18 F‐radiolabeled R‐(−)‐N‐ n ‐propyl‐2‐(3‐fluoropropanoxy‐11‐hydroxynoraporphine

Aporphines are attractive candidates for imaging D 2 receptor function because, as agonists rather than antagonists, they are selective for the receptor in the high affinity state. In contrast, D 2 antagonists do not distinguish between the high and low affinity states, and in vitro data suggests that this distinction may be important in studying diseases characterized by D 2 dysregulation, such as schizophrenia and Parkinson's disease. Accordingly, MCL‐536 (R‐(−)‐N‐n‐propyl‐2‐(3‐[ 18 F]fluoropropanoxy‐11‐hydroxynoraporphine) was selected for labeling with 18 F based on in vitro data obtained for the non‐radioactive ( 19 F) compound. Fluorine‐18‐labeled MCL‐536 was synthesized in 70% radiochemical yield, >99% radiochemical purity, and specific activity of 167 GBq/µmol (4.5 Ci/µmol) using p ‐toluenesulfonyl (tosyl) both as a novel protecting group for the phenol and a leaving group for the radiofluorination.