Structures of oxygenated cyclohexa-1,3-diene-maleic anyhydride cycloadducts. Structural evidence suggests a stepwise retro-Diels-Alder reaction

The structures of a series of cyclohexadiene-maleic anhydride cycloadducts show structural parameters consistent with the early stages of the retro-Diels-Alder (rDA) reaction in the ground state structure. The symmetrical adduct 10 shows effects consistent with a synchronous rDA reaction. An asynchronous rDA reaction is suggested in the structure of 8, and the first evidence suggesting a two-step retro-Diels-Alder mechanism is provided by the deoxygenated adduct 9.