The synthesis of the title compound 12 is described. This diazoketone, distinguished by its Cs-symmetry and by a built-in cisoid diene unit, gives 5-methyltricyclo[3.3.0.0²˙⁸]oct-6-ene-3-one (13) when subjected to thermal or catalytic denitrogenation with rhodium (II) acetate or copper (II) acetylacetonate. Direct irradiation of 12 at 350 nm causes Wolff rearrangement, whereas the benzophenone-sensitized photolysis gives again ketone 13. A 1,4-carbene or carbenoid addition was never observed under the conditions described. The ketone 13 equilibrates thermally or under base catalysis with 1-methyltricyclo[3.3.0.0²˙⁸]oct-6-ene-3-one (17). The key step of this apparent CH₃ migration is shown to be a Cope rearrangement of the corresponding enols (hydroxysemibullvalenes).