Example 025 (4π disrotation)

Photochemical electrocyclic ring-opening of cyclobutene

Photochemical electrocyclic ring-opening of cyclobutene
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MO Theory:  R. B. Woodward and R. Hoffmann, Angew. Chem., Int. Ed., 1969, 8, 781–932
DOI: 10.1002/anie.196907811