SYNTHESIS OF [I-123] -METHYL-6-OXO-4H-IMIDAZO[1,5-A][1,4]BENZODIAZEPINE 3-CARBOXYLATE, A POTENTIAL SPECT IMAGING AGENT FOR DIAZEPAM-INSENSITIVE (DI) BENZODIAZEPINE RECEPTORS

[I-123]tert-Butyl 8-iodo-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5-a] [1,4]benzodiazepine 3-carboxylate ([I-123]3], a high affinity and sele ctive radioligand for the diazepam insensitive (DI) benzodiazepine rec eptor was synthesized in 2 steps from tert-butyl -methyl-6-oxo-4H-imid azo[1,5-a][1,4]benzodiazepine 3-carboxylate (4). A key step in the syn thesis of this potential SPECT imaging agent utilized (Ph3P)3Pd mediat ed stannylation of 4 with (Bu3Sn)2 to the corresponding 8-tributyltin derivative 5. Iododestannylation of 5 with no carrier added (I-123]NaI in aqueous ethanolic CH3CO3H furnished [I-123]3 (85%) in greater than 98% radiochemical purity following TLC purification.