Small and Strained Ring Compounds via Unsaturated Carbenes

Unsaturated carbenes, provide a ready means of entry into numerous, diverse small ring systems. They have been employed to prepare strained alkylidenecyclopropanes that open up to the corresponding trimethylenemethanes or in one case m‐xylylene. Triafulvenes, [3]‐radialenes as well as [4]‐radialenes and cyclynes have been prepared by addition of unsaturated carbenes to appropriate π‐systems. They show great potential for the simple preparation of novel small ring heterocycles. Potential formation of allene oxides, allene episulfides, α‐lactams and heterocumulene oxides are discussed.