Stereodistal Cope rearrangement of trans-1,2-dialkenylcyclobutanes

Although stereoproximity of the ends (C_1 and C_6) of the biallyl system (schematically shown as 1) is required in the ordinary Cope rearrangement, there are a number of cases in which the process occurs despite a stereodistal relationship (2) imposed by the geometry of the reactant. A major mechanistic question is does the stereodistal reaction occur by an indirect mechanism requiring prior epimerization to the stereoproximal reactant, perhaps by way of a diradical intermediate 3, or can it occur directly, for example by closure of 3 at C_1 and C_6, to give Cope rearrangement product 4