Overview of Electrocyclic Reactions

An electrocyclic reaction is defined as the formation of a single bond between the terminal atoms of a linear conjugated system of rr electrons and the reverse process. One example is the thermal ring opening of cyclobutenes to butadienes. 

The reason for the stereospecificity is that the groups bonded to the breaking bond rotate in the same sense during the ring-opening process. Such motion, in which all 

The principle of microscopic reversibility (see p. 275) requires that the reverse process, ring closure of a butadiene to a cyclobutene, also be conrotatory. Usually this is thermodynamically unfavorable, but a case in which the ring closure is energetically favorable is conversion of ,Z-l,3-cyclooctadiene to aT-bicyclo[4.2.0]oct-7-ene. The ring closure is favorable in this case because of the strain associated with the U-double bond. The ring closure occurs by a conrotatory process. 

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