Discreteness of π-conjugation of 1,6-methano[10]annulene by troponoid fusion at the 3,4-positions

The tropone-fused 1,6-methano[10]annulene 4 was synthesized from 3,4-bis(bromomethyl)-1,6-methano[10]annulene and the protonation of 4 provided the hydroxytropylium ion-fused annulene 5. 1H NMR spectra of 4 and 5 exhibited an unequal shielding effect on the bridge hydrogens from the resulted π ring system, suggesting the existence of the significant homoconjugative transannular interaction on the nonadjacent carbons at the bridge and the discreteness of π-conjugation of the annulene between two bonds at the C7–C8 and C11–C12 to localize the C8–11 dienyl and the remaining parts. The X-ray crystallographic structures of 4 and 5 show relatively short atomic distance between those bridge carbons, and clear bond alternation of the dienyl part and the bond convergence of the remaining part in these compounds, supporting the spectral properties.