Synthesis of [¹²³I]tert‐butyl 8‐iodo‐5,6‐dihydro‐5‐methyl‐6‐oxo‐4H‐imidazo[1,5‐a][1,4]benzodiazepine 3‐carboxylate, a potential SPECT imaging agent for diazepam‐insensitive (DI) benzodiazepine receptors

Abstract [ 123 I] tert ‐Butyl 8‐iodo‐5,6‐dihydro‐5‐methyl‐6‐oxo‐4 H ‐imidazo [1,5‐a][1,4]benzodiazepine 3‐carboxylate ([ 123 I]3), a high affinity and selective radioligand for the diazepam insensitive (DI) benzodiazepine receptor was synthesized in 2 steps from tert ‐butyl 8‐bromo‐5,6‐dihydro‐5‐methyl‐6‐oxo‐4 H ‐imidazo[1,5‐a][1,4]benzodiazepine 3‐carboxylate (4). A key step in the synthesis of this potential SPECT imaging agent utilized (Ph 3 P) 3 Pd mediated stannylation of 4 with (Bu 3 Sn) 2 to the corresponding 8‐tributyltin derivative 5. Iododestannylation of 5 with no carrier added [ 123 ]NaI in aqueous ethanolic CH 3 CO 3 H furnished [ 123 I]3 (85%) in greater than 98% radiochemical purity following TLC purification.