C3- or C4-hydroxyalkylated phenols are highly reactive towards peroxidation with oxone, which results in the formation of tertiary C3 hydroperoxides. This reaction can also be performed with photochemically generated singlet oxygen. However, other characteristic singlet oxygen reactions do not proceed with caroate. The initially formed hydroperoxides cyclize in the presence of a Lewis acid catalyst based on boron, indium, or iron to give spiroannulated peroxides. These exhibit restricted ring inversion whereas larger nine-membered-ring peroxides are thermally less stable and show higher ring flexibility (according to NMR analysis).

The photosensitized [2 + 2]-cycloaddition of chalcones, conjugated cyclopentenones, and cyclohexenones with electron-rich alkenes such as cyclic enolethers and polymethylated alkenes was investigated. While cyclic enones showed high regio- and stereoselectivity, acyclic enones resulted in a more complex product mixture containing dimers as well as four dominant regio- and diastereoisomers. This complex product mixture can be controlled by adjusting the reaction conditions such as sensitizer, solvents, or additives.

The intermolecular [2 + 2]-photocycloaddition of the parent flavone molecule (4) as the triplet energy-accepting species and the electron-rich alkene 2, 3-dihydrofuran (5) 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 4 and 5 almost solely afforded the cis-syn-cis cyclobutane 6, whereas analogous conjugated six- and five-membered cycloalkenones preferentially react to cis-anti-cis cyclobutanes or a mixture of both diastereoisomers (e.g., for the cyclohexanone-derived example 9).

C-3-oder C-4-hydroxyalkylierte Phenole sind hochreaktive Substrate fürd ie Hydroperoxierung durch Oxone und führen zur Bildung von tertiären C-4-Hydroperoxiden. Diese Reaktion kann auchm it photochemisch erzeugtem Singulett-Sauerstoff durchgeführt werden. Andere charakteristischeS ingulett-Sauerstoffreaktionen verlaufen jedochn ichtm it Caroat. Die zunächst gebildeten Hydroperoxide werden unter Lewis-Säure-Katalyse mit Bor-, Indiumoder Eisen-basierten Katalysatoren zu spiroverknüpften Peroxiden 6, 9 cyclisiert, die eine eingeschränkte Ringinversion aufweisen, während die grçßeren neungliedrigen Peroxide 12 thermisch weniger stabil sind und -mçglicherweise korreliertauche ine hçhere Ringflexibilitäta ufweisen (durchN MR-Analyse).

Photocatalytic [2+2]-cycloadditions between cyclic enones and electron-rich cyclic enol ethers are initiated by triplet-triplet energy transfer from an excited iridium photocatalyst to the enone acceptor. The composition of the resulting...