Synthesis and properties of methano-[12]-, -[18]-, -[20]-, -[22]-, -[24]-annulene and dimethano-[20]- and -[24]-annulene

Reductive couplings of the vinylogues of cyclohepta-1,3,5-triene-1,6-dicarbaldehyde with a low-valent titanium reagent afforded the corresponding monomeric or dimeric products, i.e. singly bridged or doubly bridged annulenes in most cases. These annulenes showed diatropicity or paratropicity, depending on the number of peripheral conjugated π-electrons. The 1H n.m.r. spectra of the methanoannulenes showed a larger temperature-dependency than those of the closely related tetradehydromethanoannulenes, reflecting the lower planarity and the larger conformational mobility of the molecular skeleton in the former compounds.