4a-(Z-1-Propenyl)-bicyclo[4.4.0]dec-1(8a)-en-2-one (6) and 4a-(Z-1-propenyl)-bicyclo[4.4.0]deca-1(8a), 7-dien-2-one (17) undergo an intramolecular hydrogen transfer from the methyl group of the propenyl substitutent to the α-carbon atom of the enone group, and cyclization to the [4.4.3]propellane derivatives 9 and 30, respectively, when excited in the π → π* wavelength region. The quantum yield for (Z)-6 → 9 under optimum conditions is 0.29 at 254 nm. These reactions occur specifically from the S₂ (π,π*) state, competing with the S₂ → T decay. The triplet reactions of 6 are E–Z double-bond isomerization, double-bond shift to (E,Z)-8, and rearrangement to (E)-10. Further investigations concern some structural limitations in the scope of the reaction type 6 → 9 and enone S₂ reactivity in general.