bridge adducts; these, on relatively mild thermolysis, yield dihydrosemibullvalene derivatives by a novel sequence including cheletropic loss of carbon monoxide, electrocyclic ring opening of the immmediate substituted bicyclo[4,2,0] octadienes, and intramolecular [π4a +π2a]cycloaddition of the derived cyclooctatrienes. Other pericyclic reactions of compounds related to those of the title compounds