Palladium-catalyzed carbomethoxylation of benzonorbornadiene (7) yielded dimethyl-1,4-methano-1,2,3,4-tetrahydronaphthalene-2,3-dicarboxylate (9) which was transformed in three steps into 2,3-dimethylene-1,4-methano-1,2,3,4-tetrahydronaphthalene (4) in a high yield. Reaction of 4 with singlet oxygen resulted in the formation of epoxy-endoperoxide (12). X-ray crystallographic analysis demonstrated that epoxide ring is exo as methylene bridge. For the first time, the epoxy-endoperoxide formation from the ''one pot'' reaction of a diene with singlet oxygen was observed. The epoxy-endoperoxide formation mechanism was discussed. CoTPP (Cobalt(II)tetraphenylporphyrin) catalyzed rearrangement of epoxy-endoperoxide gave hydroxy-aldehyde (13) which was oxidized into dialdehyde (14).