Conrotatory ring closure principle

Figure 1 The four stereochemically distinct modes of electrocyclic ring closure wherein the notations inward and outward refer to the axes of rotation as viewed with respect to ( ). The principle of the conservation of orbital symmetry predicts disrotatory (dis) or conrotatory (con) ring closure for the thermally or photochemically induced processes, respectively. The same modes (dis or con) are predicted for the reverse ring opening processes...

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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