Mild thermolysis of the tetracyclic mesylate (6) in the presence of traces of mineral acid afforded a mixture of the rearrangement products (21) and (22) as well the σ-anti- bishomotetralin (20). On treatment with methanesulfonic acid in dichloromethane, compound (20) gives rise to the [5.4.1] propellene (21) which, on prolonged exposure to these reaction conditions, isomerizes to the mesyloxymethyl compound (22). Under the same conditions, the σ-anti- bishomoindan (4) is converted into the [5.3.1] propellene (3) which, in turn, slowly rearranges to the [4.3.1] propellene (23). A mechanistic rationalization of these observations is advanced which also accounts for the formation of the novel by-product (16) observed during Simmons-Smith cyclopropanation of allylic alcohol (15a).