Self-sensitized diastereodifferentiating Z-E photoisomerization of 3-, 4-,and 5-benzoyloxycyclooctenes: Intra- versus intermolecular photosensitization

Upon irradiation at 254 nm, (Z)-3-, 4-, and 5-benzoy loxycyclooctenes (1Z-3 Z) efficiently isomerized through intra-and/or intermolecular sensitization to the corresponding diastereomeric E-isomers (1E-3E) with low to moderate diastereomeric ex cesses (de's) of up to 43 %. The major diastereomer prod uced from 1Z was switched from (1R*,3R*)-1E (26% de) to (1R*,3S*;)-1E (-11% de) by simply increasing the substrate concentration from 1 mM to 50 mM. T his unusual switching of product chirality is attributed to the opposite di astereoselectivities for intra- and intermolecular photosensitization. In c ontrast, 2Z and 3Z did not show such switching behavior, but consistently g ave a single diastereomer of up to 9 % and 43 % de, respectively, over a wi de range of substrate concentrations (0.04-50 mM), for which totally differ ent mechanisms are responsible. Thus, the intra- and intermolecular sensiti zations of 2Z exhibit the same diastereoselectivity, while only the intermo lecular sensitization of 3Z can occur as a result of the hindered intramole cular approach of the benzoate substituent to the cyclooctene moiety.