Four 1-oxo-cycloalkan-2-ylideneacetic acids, differing in the size of the aliphatic ring and in the nature of the condensed unsaturated cycle, exhibit different reactivities towards hydrazine, leading in only one case to the desired tryciclic pyridazinone. The observed differences in behaviour cannot be ascribed to different configurations at the exocyclic double bond of the four acids: 1H NMR experiments, combined with UV photolysis, have ...