Modern valence bond theory, in its spin coupled form. is used to elucidate the electronic rearrangements that take place during the Course of the gas-phase hetero-Diels-Alder cycloaddition reaction of s-cis-acrolein (cis-1-oxabutuadiene or cis-propeneal) and ethene. It is found that the most dramatic changes to the electronic structure occur in a relatively narrow interval of the reaction pathway soon after the transition state and that the system passes through a geometry at which it can be considered to be significantly aromatic. Although concerted. the reaction is markedly asynchronous. with the breaking of the carbon-oxygen pi bond and the formation of the new carbon-oxygen sigma bond. "lagging behind" somewhat the other bond-making and bond-breaking processes.