Ketene equivalents

Abstract

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.