Some aspects of the chemistry of pyrimido[1,2-B]pyridazinones

On reacting the 3-aminopyridazines 1a,d,e with dimethyl acetylenedicarboxylate (DMAD), the pyrimido[1,2-b]pyridazin-2-(2H)-ones 2e-g , whereas starting from 1f , the 4(4H)-ones 5a and 3b,d were prepared. In the 2(2H)-one series, the reactions of 2b with various amino compounds resulted in various types of products. The reaction of N-methylaminopyridazines 1g,h with DMAD led to the endo-N-substituted derivatives 8a,b , whereas 1h with diethyl ethoxymethylenemalonate (DEM) gave the exo-N-substituted compound 1k. The constitution of the compounds was proved by spectroscopic and chemical evidences.