Synthesis of Benzo[a]heptalene

Benzoa]heptalene has been synthesized by two different approaches. The first one follows a pathway to hexahydrobenzoa]heptalenone 19a that has been already described by Wenkert and Kim(Scheme). Indeed, 19a was obtained in a mixture with its double-bond-shifted isomer 19b . Reduction of this mixture to the corresponding secondary alcohols 26a/26b and elimination of H₂O lead to a mixture of the tetrahydrobenzoa]heptalenes 23a-d (Scheme7 and 8). Reaction of 23a-d with 2 equiv. of triphenylmethylium tetrafluoroborate in boiling CHCl₃, followed by treatment with Me₃N in CH₂Cl₂, generated directly 2 , unfortunately in a mixture with Ph₃CH that could not be separated from 2 (Scheme 10 and 11). The second approach via dimethyl benzoa]heptalene-6,7-dicarboxylate ( 30 ) (Scheme 12) that was gradually transformed into the corresponding carbaldehydes 37 and 43 (Scheme 14) both of which, on treatment with the Wilkinson catalyst RhCl(PPh₃)₃] at 130° in toluene, smoothly decarbonylated, finally gave pure 2 as an unstable orange, viscous oil. UV/VIS, NMR, and mass spectra of 2 are discussed in detail (cf. Chapt.3).