The thermal [2+ 2] cycloaddition of α-choloracrylonitrile to allene followed by ozonolysis, then elimination of HCl provides access to the new β-cyanocyclobutenone 1 (Scheme 1). Diels-Alder reactions of 1 occur very easily at room temperature. The high reactivity of 1 in [4+ 2] cycloadditions strongly contrasts with the low reactivity of the homologous cyanocyclopentenone 9 (Scheme 4). This is in agreement with calculations (PM3 method).
