Regiochemical Trends in Intramolecular [2 + 2] Photocycloadditions of 6‐(prop‐2‐enyl)cyclohex‐2‐enones and 5‐(prop‐2‐enyl)cyclopent‐2‐enones

The effect of substituents (X  H, Me, or F at C(6), R  H or Me at C(2′) of the allyl side chain) on the photoisomerization (λ = 350 nm) of 6‐allylcyclohex‐2‐enones 1 in MeCN is studied. Substituents X control the overall efficiency of intramolecular [2 + 2] photocycloadduct formation (Φ: Me > F > H) but do not exercise an influence on the orientation of addition of the exocyclic double bond to the enone CC bond. In contrast, replacement of the prop‐2‐enyl (R  H) by a 2‐methylprop‐2‐enyl (R  Me) side chain causes a change in the tricyclo[3.3.1.02,7]nonan‐6‐one 4 vs. tricyclo[4.2,1.03,8]nonan‐7‐one (5) product ratio from 100:0 (R  H) to roughly 2:1 (R  Me) but has almost no bearing on the relative rates of conversion of 1 to products. For C(6)‐unsubstituted enones 1aa and 1ba (X  H), the efficiency of cyclization becomes low enough so that lumiketone rearrangement to bicyclohexanones 6 and 3‐isopropylcyclopent‐2‐enones 9 becomes competitive. Enones 9 undergo consecutive intramolecular [2 + 2] photocycloaddition to tricyclo[3.2.1.03,6]octan‐2‐ones 7 and to tricyclo[3.2.1.03,6]octan‐7‐ones 8, compounds 8 only being formed when R  Me.