Tris fulvenes, synthesis

The question of aromaticity versus antiaromaticity and delocalized versus localized double bonds in pentalene (2) dates back to 1922, when Armit and Robinson compared it with naphthalene and postulated that the former might be similarly aromatic [32, 33]. While the first synthesis of a non-fused hexaphenylpentalene (38) [30] provided only some clues as to the non-aromatic reactivity of the pentalene skeleton, the tri-tert-butyl derivative 39, prepared and studied by Hafner et al. in great detail [31], gave a better insight. The ring-proton signals of this alkyl-substituted pentalene 39 are shifted upfield compared to those of fulvene (27) and other cyclic polyenes. This observation led to the conclusion that the pentalene derivative 39 should be an antiaromatic species. However, the results did not permit a distinction... 

Formally, higher oligoquinanes can be formed from fulvene 1 by consecutive addition of etheno bridges (Scheme 1 a). This concept has actually been successfully applied in the synthesis of pentalene 2 but it is unlikely to be practicable for higher members of this family due to the drastical increase in ring strain on going from, e.g., 2 to 3. Therefore all realistic approaches towards compounds 3-7 start from a less unsaturated skeleton and try to introduce the additional double bonds in a stepwise fashion (e. g. 3 8 or 9 10, Scheme 1 b). 

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