The singlet oxygen reactivities and regioselectivities of the model compounds 1b-d were compared with those of the geminal (gem) selectivity model ethyl tiglate (1a). The kinetic cis effect is k(E)/k(Z) = 5.2 for the tiglate/angelate system 1a/1a' without a change in the high gem regioselectivity. Further conjugation to vinyl groups enabled mode-selective processes, namely, [4 + 2] cycloadditions versus ene reactions. The site-specific effects of methylation on the mode selectivity and the regioselectivity of the ene reaction were studied for dienes 1e-g. A vinylogous gem effect was observed for the γ,δ-dimethylated and α,γ,δ-trimethylated substrates 1h and 1i, respectively. The corresponding phenylated substrates 1j-l showed similar mode selectivity, as monomethylated 1j exhibited exclusively [4 + 2] reactivity while the tandem products 12 and 14 were isolated from the di- and trimethylated substrates 1k and 1l, respectively. The vinylogous gem effect favors the formation of 1,3-dienes from the substrates, and thus, secondary singlet oxygen addition was observed to give hydroperoxy-1,2-dioxenes 19 and 20 in an ene-diene transmissive cycloaddition sequence. These products were reduced to give alcohols (16, 17, and 18) or furans (24 and 25), respectively, or treated with titanium(IV) alkoxides to give the epoxy alcohols 26 and 27. The vinylogous gem effect is rationalized by DFT calculations showing that biradicals are the low-energy intermediates and that no reaction path bifurcations compete.

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 primary chemical reactions of singlet molecular oxygen with polyunsaturated carotenoids are the focus of this research report. Model compounds that exhibit electronic properties and substituent pattern similar to natural carotenes, xanthophylls or apocarotenoids, respectively, were investigated with regard to photooxygenation reactivity. For dienes and trienes as substrates, high tandem reactivity was observed and hydroperoxy-endoperoxides were isolated as the secondary products of singlet oxygen reaction. The electronic gem-effect on the regioselectivity of the ene reaction is conserved also in vinylogous positions and thus appears to originate from a radical-stabilizing effect. In an attempt to combine different peroxide groups derived from natural products as a tool for new pharmaceutically active products, a dyade synthesis of an artemisinine-safranol with subsequent singlet oxygen addition was realized.

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).

Die Dichotomie zwischen medizinischen und explosiven Eigenschaften mittelgroßer cyclischer Peroxide wird von A. G. Griesbeck et al. in ihrer Zuschrift auf S. 13966 ff. erklärt. Der Gewinn an Stabilisierung der spiroverknüpften und ringanellierten cyclischen Peroxide (bestimmt aus der Gibbs‐Energiedifferenz zwischen Ausgangsverbindung und Produkt) nimmt mit zunehmender Ringgröße ab. Der Grund hierfür sind die gegenläufigen Effekte stabilisierender Natural‐Bond‐Orbital(NBO)‐Wechselwirkungen und destabilisierender Ringspannung.