Novel ring transformations of condensed [1,2,4]triazolo[4,3-b]pyridazine-6(5H)-one-3(2H)-thiones effected by dialkyl-acetylenedicarboxylates

On heating with dialkyl-acetylenedicarboxylates in DMF condensed [1,2,4]-triazolo[4,3-b] pyridazine-6(5H)-one-3(2H)-thiones undergo unprecedented ring transformations yielding novel tetracyclic 1,3-diazepines and thiazolotriazole derivatives depending on the applied reaction temperature. The observed substrate selectivity was interpreted on the basis of the results of comparative theoretical calculations carried out at semiempirical level (AM1).