Synthese und Pyrolyse einer Triafulven‐Vorstufe

Synthesis and Pyrolysis of a Triafulvene Precursor In view of retro‐Diels‐Alder reactions (RDA reactions), the triafulvene precursor 3 has been prepared in a simple three‐step synthesis by dibromocarbene addition at dibenzo‐barrelene (11→12; 44%), halogen‐Li exchange followed by methylation (12→14, 100%) and HBr elimination (14→3, 62%) (Scheme 3). Reactivity of the so far unknown bridged 1,1‐dibromocyclopropane 12 has been explored, including reductions, allylic rearrangements, and ‘carbene dimerizations’ (Scheme 4). First experiments with respect to the thermal behavior of 3 show that RDA reaction, although occurring in most cases, is not the predominant pathway. When 3 is heated in a sealed tube without solvent, two dimers 26 and 27 are isolated in a total yield of 55% (Scheme 6). On the other hand, gas‐phase pyrolysis of 3 at 400° mainly produces rearranged 28 (56%; Scheme 7). It is assumed that bridged trimethylenemethane 29 is an essential intermediate in thermal rearrangements of 3 (Scheme 8).