electrocyclic reaction

A molecular rearrangement that involves the formation of a σ-bond between the termini of a fully conjugated linear π-electron system (or a linear fragment of a π-electron system) and a decrease by one in the number of π-bonds, or the reverse of that process. For example: The stereochemistry of such a process is termed 'conrotatory' or antarafacial if the substituents at the interacting termini of the conjugated system both rotate in the same sense, e.g. or 'disrotatory' (or suprafacial ) if one terminus rotates in a clockwise and the other in a counter-clockwise sense, e.g.