10-Trifluoroacetyl-11,11a-dihydro-4aH-4a,11a-methanoindeno[1,2-a]azulene 18 and methyl {11,11a-dihydro-4aH-4a,11a-methanoindeno[1,2-a]azulen-10-yl}carboxylate 19 are synthesized starting from the azulene-annulation reaction of 2H-cyclohepta[b]furan-2-one with the pyrrolidine enamine derived from tricyclo[4.3.1.0^1,6]dec-3-en-8-one. The ¹³C NMR spectra at various temperatures reveal that compound 18 exists in a norcaradiene structure, while compound 19 is in equilibrium between norcaradiene and cycloheptatriene structures. The latter compound is the first example of a cycloheptatriene, which experiences a forced shortening of the C6–C11 distance by a three carbon chain.† The facile valence isomerization of compound 19 is ascribed to the interaction of the lowest unoccupied molecular orbital (LUMO) of the Walsh orbital of cyclopropane and the highest occupied molecular orbital (HOMO) of the methoxycarbonylazulene, and the interaction occurs effectively as to weaken the basal C6–C11 bond of the cyclopropane ring, as compared to that in compound 18 involving an azulene nucleus bearing the more electron-withdrawing trifluoroacetyl group. The hydrogen–deuterium exchange reaction of compounds 18 and 19 in MeONa–MeOD occurs stereospecifically to give 12-exo-deuterated products (exo to the bridge methylene), respectively. Marked spectroscopic differences between the anions 30 and 31, which are derived from compounds 18 and 19 by treatment with [²H₅]dimsyl sodium in [²H₆]DMSO, respectively, are also observed: the anionic charge in the former is not delocalized over the trifluoroacetylazulene nucleus, while that in the latter is delocalized over the methoxycarbonylazulene nucleus. The thermal rearrangement of the anion generated from compound 19 is also observed to result in the formation of 11-methoxycarbonylazuleno[1,2-a]azulene. Furthermore, the Diels–Alder reaction of the tricyclo[4.3.1.0^1,6]deca-2,4,7-triene (methanoindeno[1,2-a]azulene) ring system in compounds 18 and 19 has been studied as well.