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

✍ Scribed by Giuliano Cruciani; Paul Margaretha

• Publisher: John Wiley and Sons
• Year: 1990
• Tongue: German
• Weight: 480 KB
• Volume: 73
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19900730208

✦ Synopsis

Abstract

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[3.3.1.0^2,7^]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.