A New Photoreaction of γ,δ-Unsaturated Ketones: The Oxadi-. pi.-ethane (ODPE) Rearrangement. -Only the intramolecular (2 + 2) cycloaddition to give oxetanes of type (II) is hitherto known as a photoreaction of ketones of type (I). However, on triplet sensitization, a photoisomerization to give an isomeric ketone (III) is also possible. On direct irradiation and also on triplet sensitization, the ketone (IV) affords exclusively the isomeric ketone (V). This reaction proceeds through a novel non-allylic 1,3-shift of an acyl group with the formation of a strained ring frame work. The observed photoreaction is influenced by the electronic excitation, by the degree of the alkylation at the ring C atom bearing the acyl group, and by the proximity of the carbonyl C atom to the C=C bond. -(LEITICH, J.; SCHAFFNER, K.; Angew. Chem. 105 (1993) 3, 436-437; MPI Strahlenchem., W-4330 Muelheim/R., Germany; DE)