OXIDATIVE RING-OPENING REACTION OF CYCLOPROPANONE ACETALS WITH CARBONYL-COMPOUNDS VIA PHOTOINDUCED ELECTRON-TRANSFER - GENERATION OF A BETA-CARBONYL RADICAL SPECIES AND ITS APPLICATION TO THE SYNTHESIS OF GAMMA-HYDROXY ESTER DERIVATIVES

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 (gamma-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-beta-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 beta-position of propanoates.