Reactions of ketene equivalents illustrate the increasing applications of reagent equivalents to bring about reactions that are not possible with the reagent itself. As the direct route to norbornenones by [4+2]-addition of cyclopentadiene with ketene itself is not possible, a variety of synthons of the type H2C=C(X1)(X2) were created which, after the allowed [4+2]-addition reaction, led to norbornenones by the transformation of the resultant C(X1)(X2) moiety to a carbonyl group. Such syntheses are analysed in the present review. 1. Reactions of α-Acetoxyacrylonitrile 2. Ketene Adducts with α-Chloroacrylonitrile 3. α-Chloroacrylonitrile Adducts via Chlorination of Acrylonitrile Adducts - Acrylonitrile as Ketene Equivalent 4. Norbornenones and Related Systems from [4+2]-Addition Reactions with α-Chloroacryloyl Chloride 5. Nitroethene as Ketene Equivalent 6. Transformation of Acrylic Acid [4+2]-Adducts to Norbornenones 7. Ketene Equivalents in Retrospect and Prospect