Synthesis of 1,2,3-thiadiazolo[4,5-d] pyridazines,A new heterocyclic ring system

A method was developed for the synthesis of 5-carbethoxy-4-formyl-1,2,3-thiadiazole (I), its isomer (II); 5-benzoyl-4-formyl-1,2,3-thiadiazole (III), and its isomer (IV). It was demonstrated that although compounds I, III and IV with hydrazine gave 7H-1,2,3-thiadiazolo4,5-d]-pyridazin-7-one (XXII), 7-phenyl-1,2,3-thiadiazolo4,5-d]pyridazine (XXIII) and 4-phenyl-1,2,3-thiadiazolo4,5-d]pyridazine (XXV), respectively; however, compound I gave its corresponding hydrazone (XXIV).