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 CC 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[3.3.1.0 2,7 ]nonan‐6‐one 4 vs. tricyclo[4.2,1.0 3,8 ]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[3.2.1.0 3,6 ]octan‐2‐ones 7 and to tricyclo[3.2.1.0 3,6 ]octan‐7‐ones 8 , compounds 8 only being formed when R Me.