Specifically (π → π*)‐Induced Cyclohexenone Reactions 4a‐( Z ‐1‐Propenyl)‐bicyclo[4.4.0]dec‐1 (8a)‐en‐2‐one and 4a‐ Z ‐1‐Propenyl‐bicyclo[4.4.0]deca‐1(8a), 7‐dien‐2‐one

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 S 2 (π,π*) state, competing with the S 2 → 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 S 2 reactivity in general.