Synthetic studies of 8-carbamoylimidazo-[5,1-d]-1,2,3,5-tetrazin-4(3H)-one: A key derivative of antitumour drug temozolomide

5-Diazoimidazole-4-carboxamide 4 reacted with trimethylsilyl isocyanate in acetonitrile to afford 8-carbamoylimidazo[5,1-d]-1,2,3,5-tetrazin-4(3H)-one 1, which was undergoing a methylation to give antitumour drug temozolomide 2; while 1,5-dicarbamoyl aminoimidazole 6 failed in an azo-cyclization to give 1 but accomplished a carbon-cyclization to produce 8-carbamoylimidazo[1,5-a] s-triazin-4(3H)-one 7.