Synthesis of Benzoheptalene

Article abstract of DOI:10.1002/hlca.19970800506

Benzoheptalene has been synthesized by two different approaches.The first one follows a pathway to hexahydrobenzoheptalenone 19a that has been already described by Wenkert and Kim (Scheme 1).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 tetrahydrobenzoheptalenes 23a-d (Scheme 7 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 benzoheptalene-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 at 130 deg C 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).