Synthesis, conformation and tropicity of dimethano-bridged tetradehydrothia[21]-, -[23]- and -[25]annulenes

The title thiaanulenes have been synthesized through a double Wittig reaction of bis[(triphenylphosphonio)-methyl]sulfide dibromide with 6-ethynylcyclohepta-1,3,5-triene-1-carbaldehyde and/or its vinylogous aldehyde, followed by intramolecular oxidative coupling of the resulting bisethynyl sulfides. For each compound, two isomers have been isolated which differed in the geometry of their peripheral olefinic moieties. 1H NMR spectra revealed that some of the geometrical isomers consisted of two conformational isomers which differed in the relative disposition of the two methano bridges. One isomer for each of the dimethano-bridged thia[21]- and -[23]annulenes sustained ring current, while the other isomers as well as those for the thia[25]annulene did not.