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 singlet oxygen reaction with trisubstituted E‐allylic alcohols resulted in the unselective formation of regioisomeric secondary and tertiary allylic hydroperoxides. Increasing the steric demand at the allylic hydroxymethyl group leads to a moderate large‐group effect that can be strongly enhanced by deprotonation of a dimethylated allylic alcohol and even more pronounced by cation complexation. The presence of alkoxide groups did additionally alter the diastereoselectivity of the singlet oxygen ene reactions with chiral allylic alcohols. The molecular combination of Michael ester and enol ether in α‐alkoxy ester substrates made possible the design of chemoselective substrates for singlet oxygen ene versus [2+2] cycloaddition, e. g. a 3,3‐dimethylated Michael ester as a selective substrate for the ene reaction and an adamantyl analogue as a selective 2+2 substrate. The Z/E‐monomethylated substrates as probes for cis regioselectivity revealed that there is no site preference for Z‐alkoxy hydrogen transfer on the way to the corresponding ene product.
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.
The Front Cover shows a cartoon of the hydrogen‐transfer process initiating the singlet oxygen ene reaction with several strong regio‐ and stereodirecting effects following the rule of thumb: If you give singlet oxygen your little finger it will soon have your whole hand (as adapted from a quote by Sigmund Freud). More information can be found in the Article by M. Kleczka et al on page 964 in Issue 11, 2018 (DOI: 10.1002/cptc.201800133).
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