PREPARATION OF [C-11] DIETHYL OXALATE AND [C-11] OXALIC-ACID AND DEMONSTRATION OF THEIR USE IN THE SYNTHESIS OF [C-11] 2,3-DIHYDROXYQUINOXALINE

A method for the production of two new carbon-11 labelled difunctional radiolabelling precursors, [C-11]diethyl oxalate, 2, and [C-11]oxalic acid, 3. is described. Methyl chloroformate was reacted with no-carrier-added [C-11]cyanide to generate the intermediate nitrile, methyl [C-11]cyanoformate. Alcoholysis with HCl in ethanol generated 2, which could subsequently be converted to 3 with aqueous acid. The total time of preparation from end-of-trapping of [C-11]cyanide was 6-7 min using combined microwave and thermal treatment or, by exclusively thermal treatment, 15 and 20 min for 2 and 3, respectively. The radiochemical conversion of [C-11]cyanide to 2 and 3 was approximately 80% and approximately 70%, respectively. Both 2 and 3 were used in a model reaction with 1,2-phenylenediamine to synthesize the heterocyclic compound, 2,3-dihydroxyquinoxaline, a basic structural unit in antagonists for the excitatory amino acid receptor system.