Inverse-Diels-Alder reaction of dimethyl 1,2,4,5 -tetrazine-3,6-dicarboxylate with benzene cis-diol gave dihydrodiol coninini 2 the 1,4-dihydropyridazine ring. Attempts at oxidation of the dihydropyridazine ring with PIFA and MnO2, resulted in the formation of 5- and 5,6-dihydroxy-phthalazine derivatives. The oxidation of the dihydropyfidazine ring was achieved by way of photooxygenation. The phthalazine type dihydrodiol is unstable and easily undergoes aromatization. The Diels-Alder reaction of tetrazine with cyclohexadiene acetonide and epoxy-ketal cyclohexene as a dienophile was investigated. These reactions led to the possible carcinogenic phthalazine Type of dihydrodiol and diol epoxide where the hydroxyl groups are protected. (C) 2004 Elsevier Ltd. All rights reserved.