Fulvene‐Dieters: Synthesis, Structure Elucidation and Thermal BehaviourIn contrast to earlier assumptions, thermal reaction of pure fulvene (1a), 6‐methylfulvene (lb) and 6, 6‐dimethylfuivene (1c) at 227deg; gives oligomeric mixtures consisting mainly of theendo‐[4 + 2]‐eycloaddition products2a,2band2c. Thermal reactivity of the fulvenes decreases strongly in the series1a>1b>1c. While the dimers2band2cequilibrate very easily in solution above room temperature with1band1c, respectively,2aequilibrates with the isomer5a(1, 6‐Dimethyliden3a α, 3bβ, 6a α, 6bβ‐tetrahydro‐1‐H), 6H‐bi (cyclopentadienylen). This surprising rearrangement envolves a formal 1,3‐shift of the 1, 2‐dihydro