Inter- and Intramolecular Carbon-Carbon Bond-Forming Radical Reactions

Free-radical-mediated reactions were studied as a powerful method for constructing the carbon-carbon bond. The intermolecular carbon radical addition to ketimines has not been widely studied. The screening of reactive ketimino radical acceptors showed that ketimines having a 2-phenolic hydroxyl group exhibit good reactivities toward nucleophilic alkyl radicals. The radical addition of these ketimines took place regioselectively at the imino carbon, providing a novel method for the construction of all-substituted sp(3)-hybridized carbon centers. We also found that the pyruvic hydrazone and isatin hydrazone show excellent reactivities to afford the corresponding C-alkylated products. Enantio-or diastereoselective radical addition reaction of ketimines was also tested. The control of stereochemistry in cyclization reactions has been of great importance to radical chemistry. As a cyclization that provides highly functionalized cyclic compounds, the diastereoselective cyclizations of oxime ethers were investigated. The diastereoselective radical cyclizations proceeded through a conformer that minimizes the A(1,3)-strain effect, and the configuration of the alkoxyamino group on the products was highly controlled by the 1,2-stereoinduction. Next, the chiral Lewis acid mediated enantioselective cyclization reactions were studied. Stereocontrol in the radical addition-cyclization-trapping reaction was achieved by the new approach, which employs hydroxamate ester functionality as a chiral Lewis acid coordinating tether between two radical acceptors. The enantioselective cascade reaction of oxime ethers also proceeded smoothly with good enantio-and diastereoselectivities. Recently, we have developed cascade reactions starting from the polarity-mismatched perfluoroalkylation of an electron-deficient acceptor with electrophilic perfluoroalkyl radicals. These cascade reactions provide a novel approach to the asymmetric synthesis of various gamma-lactams.