Stereoselective Synthesis of Bicyclo[3.1.1]Heptane Derivatives via Intramolecular Photocycloaddition Reaction

Optically active 1,3-bridged cyclobutanes 10 of the bicyclo[3.1.1]heptane ring system and 1,2-bridged cyclobutanes 11 of the bicyclo[3.2.0]heptane ring system were produced by UV irradiation of alpha,beta,gamma,delta-unsaturated esters 9a and 9c-f. The preference of endo-stereochemistry at C-6 bridged head was observed in cross-adducts 10. On the other hand, irradiation of conjugated dienol 9b led via only parallel cycloaddition to 1,2-bridged cyclobutane 11.