A rare but stereochemically unambiguous example of a [π2s + π2a]-cycloaddition is shown on the right. The strain and steric properties of the "trans" double bond enables this generally kinetically unfavorable process. "cis", "trans"-1,5-Cyclooctadiene is also believed to undergo dimerization via this mode. Ketenes are a large class of reactants favoring + 2 cycloaddition with olefins. The MO analysis of ketene cycloaddition is rendered complicated and ambiguous by the simultaneous but independent interaction of the orthogonal orbitals of the ketene but may involve a [π2s + π2a] interaction as well.