| Summary: | The intermolecular [2 + 2]-photocycloaddition of the parent flavone molecule (<b>4</b>) as the triplet energy-accepting species and the electron-rich alkene 2, 3-dihydrofuran (<b>5</b>) was performed by visible-light-mediated triplet-sensitization with an iridium-based organometallic sensitizer. The reaction proceeds with high diastereo- and regioselectivity (>98:2 for the regiochemical orientation and with 95% d.s.). In contrast to numerous other ene/enone combinations that are described in the literature and were also performed by us, the reaction between <b>4</b> and <b>5</b> almost solely afforded the <i>cis</i>-<i>syn</i>-<i>cis</i> cyclobutane <b>6,</b> whereas analogous conjugated six- and five-membered cycloalkenones preferentially react to <i>cis</i>-<i>anti</i>-<i>cis</i> cyclobutanes or a mixture of both diastereoisomers (e.g., for the cyclohexanone-derived example <b>9</b>).
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