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ChemInform Abstract Title compounds such as (III) and (IV) are prepared via double Wittig reaction of the dicarbaldehydes (I). Some of these polyenes exist as sole isomers; the geometries of the double bonds can be determined by NMR. In other cases E,Z-mixtures are obtained. The 1H NMR spectra show that the methanothia-(17)-, -(19)-, -(23)-, and -(27)-annulenes sustain ring current, while the remaining title compounds do not. Dynamic NMR analysis of the methylene protons reveals the flipping of the methylene bridge above and below the average plane of the macrocycle.

Methanohomo[15]annulene and methanohomo[19]annulene were synthesized by the Wittig reactions of 1,3-bis(triphenylphosphonio)propane dibromide with 1,6-bis(2-formylvinyl)- and 1,6-bis(4-formyl-1,3-butadienyl)-1,3,5-cycloheptatriene, respectively. Conformational properties of these homoannulenes are discussed. Attempts to convert these homoannulenes into the corresponding 14π- and 18π-methanoannulenyl cations were unsuccessful.