the migration of a divalent group, such as o, s, nr or cr2, which is part of a three-membered ring in a bicyclic molecule, is commonly referred to as a walk rearrangement. this can be formally characterized according to the woodward-hofmann rules as being a (1, n) sigmatropic shift. an example of such a rearrangement is the shift of substituents on tropilidenes (1,3,5-cycloheptatrienes). when heated, the pi-system goes through an electrocyclic ring closing to form bicycle[4,1,0]heptadiene (norcaradiene). thereafter follows a [1,5] alkyl shift and an electrocyclic ring opening.