Synthesis and properties of methano bridged tetradehydro-[18]annulene, -[20]annulene, -[22]annulene, -[24]annulene, and the related [1,3]cyclophanpolyenediyne compounds

The title annulenes (4a–d) have been synthesized by a Wittig reaction of cyclohepta-1,3,5-triene-1,6-dicarbaldehyde (1a) or its vinylogous dialdehydes (1b–d) with 3-methylpent-2-en-4-ynyltriphenylphosphonium bromide (2), followed by intramolecular oxidative coupling of the resulting acyclic compounds. They proved to be diatropic or paratropic, depending on the number of peripheral π-electrons. The related [1,3]cyclophanpolyenediynes (11), (14), (17), (19), (21), and (23) were prepared by a double Wittig reaction of 3-methylpent-2-en-4-ynal (8) and/or its vinylogous aldehydes (9) and (10), with 1,3-bis(triphenylphosphoniomethyl)benzene dibromide (7), followed by intramolecular oxidative coupling of the resulting acyclic compounds. Their properties are compared, on the basis of 1H n.m.r. and electronic spectral results, with those of tetradehydromethanoannulenes.