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Tetracyanoethylene adds to bicyclo[2.1.0]pentenes 1 a, b in a stereospecific endo fashion. Therefore, the formal Diels–Alder reaction, like the [σ2 + π2] cycloaddition with bicyclo[2.1.0]pentane, presumably proceeds in two steps. The reaction with dicyanofumaric and dicyanomaleic esters is nonstereospecific with respect to the olefin components, thus supporting this supposition.
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Klärner et al. (1990) studied this question.
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