The first synthesis of trans-tricyclo[126.96.36.199]oct-4-ene (1), an ethenyl bridged spirohexane, was accomplished in four steps starting from Carpino et al. gem-dichloro ketone 6. An X-ray crystal structure of 1 with one substituent was obtained to provide geometry data on this novel ring system and to confirm the stereochemical assignment of the penultimate synthetic intermediate. Tricyclo[188.8.131.52]oct-4-ene is surprisingly stable. It reacts with glacial acetic acid but only slowly at 145 °C; the products were isolated and identified. A unimolecular rearrangement takes place at elevated temperatures (165 °C and higher), presumably, via a biradical intermediate to afford tricyclo[184.108.40.206]oct-3-ene (23). The structure of this 1,5-bridged bicyclo[2.1.0]pentane derivative was established by NMR and an X-ray crystal structure of its Diels-Alder adduct with isobenzofuran. Tricyclo[220.127.116.11]oct- 4-ene equilibrates with 23, so equilibrium constants and reaction rates were measured over a 20 °C temperature range from 180 °C to 200 °C. The difference in the heats of formation (ΔΔH°(f) (23 - 1)) is -2.1 kcal/mol, which is in good agreement with ab initio (HF and MP2) calculations using the 6-31G(d) basis set (-1.9 (HF) and -1.4 (MP2) kcal/mol). Computations on trans-tricyclo[18.104.22.168]octane and spirohexane also were carried out, and the structures and energies were compared.