Synthesis and properties of methano-bridged tetradehydro-[26]-, -[28]-, -[30]-, -[32]-, -[34]-, and -[38]-annulene

Title annulenes (21)-(28) have been synthesized by a Wittig reaction of 1,6-bis-(4-formylbuta-1,3-dienyl)cyclohepta-1,3,5-triene (5) and the vinylogous dicarbaldehydes (6)-(10) with 3-methylpent-2-en-4-ynyl(triphenyl)phosphonium bromide (11) or triphenyl-(3-t-butylpent-2-en-4-ynyl)phosphonium bromide (12), followed by intramolecular oxidative coupling of the resulting acyclic diacetylenes, and their properties are discussed on the basis of 1H n.m.r. and electronic spectra.