Heating 3-chloro-6-(2-allylphenoxy) pyridazine (IIIa) with or without a solvent afforded xanthene (Va) by a novel intramolecular cycloaddition reaction followed by elimination of nitrogen and hydrogen chloride. Formation of a fully aromatized product indicated that elimination of both N₂ and HCl has occurred from the initially formed π⁴+π² cycloadduct (IV). Variously substituted xanthenes were prepared by heating the corresponding 3-chloro-6-(2-allylphenoxy) pyridazines in diethylaniline (DEA). The compound having an allylic group at the para-position of the benzene ring did not give any cyclized product, because the allylic double bond was too far from the pyridazine ring. Similar treatment of 3-chloro-6-(2-propenylphenoxy) pyridazine (XXVI) gave rise to 1-methyldibenzofuran (XXVIII) in a low yield.