Synthesis and properties of dimethano-bridged tetradehydro[21]-, -[23]- and -[25]annulenones

Abstract

Syntheses of 7,8,9,10-tetradehydro-1,6:11,16-dimethano[21]annulen-19-one 11, 7,8,9,10-tetradehydro-18-methyl-1,6:11,16-dimethano[21]annulen-19-one 12, 7,8,9,10-tetradehydro-1,6:11,16-dimethano[23]annulen-19-one 13 and 7,8,9,10-tetradehydro-1,6:11,16-dimethano[25]annulen-21-one 14 are described. The influence of an introduction of two methano-bridges upon the structure and tropicity of the tetradehydro-[21]-, -[23]- and -[25]annulenone ring systems is discussed in view of the ¹H NMR and electronic spectra of these annulenones as well as those of the corresponding monocyclic annulenones.