Oxidative Ring-Opening Reaction of Cyclopropanone Acetals with Carbonyl Compounds via Photoinduced Electron Transfer. Generation of a β-Carbonyl Radical Species and Its Application to the Synthesis of γ-Hydroxy Ester Derivatives

Article abstract of DOI:10.1021/jo00115a022

Photoinduced electron transfer (PET) reactions of 2-substituted or 2,2-disubstituted cyclopropanone methyl trialkylsilyl acetals 1a-e,g and 1-siloxy-2-oxabicyclo<3.1.0>hexane (1f) with carbonyl compounds 2 (benzophenone (2a), acetophenone (2b), 2-acetonaphthone (2c), 2-acetylpyridine (2d), 4-acetylbenzonitrile (2e), 2,3-butanedione (2f), and benzoyl cyanide (2g)) were examined in the presence of Mg(ClO4)2.Carbon-carbon bond coupling products (γ-hydroxy esters 3 or their derivative butyrolactones 4) between 1 and 2 were formed in good yields.A mechanism is proposed for the product formation which is initiated by the single electron transfer (SET) from 1 to the excited state of 2.The set generates a transient pair of ion radicals, i.e. a ring-opened sec- or tert-β-carbonyl radical from 1 and a ketyl radical ion from 2 stabilized by the Mg salt.This realizes a novel type of carbon-carbon bond formation at the sterically crowded β-position of propanoates.