Dell'Amico, Luca and Rassu, Gloria Maria Rita and Zambrano, Vincenzo and Sartori, Andrea and Curti, Claudio and Battistini, Lucia and Pelosi, Giorgio and Casiraghi, Giovanni and Zanardi, Franca (2014) Exploring the vinylogous reactivity of cyclohexenylidene malononitriles: switchable regioselectivity in the organocatalytic asymmetric addition to enals giving highly enantioenriched carbabicyclic structures. Journal of the American Chemical Society, Vol. 136 (31), p. 11107-11114. eISSN 1520-5126. Article.
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Modulation of the complex reactivity of cyclohexenylidene malononitriles using diverse β-aryl-substituted enals and proper organocatalytic modalities resulted in divergent asymmetric reaction patterns to furnish angularly fused or bridged carbabicyclic frameworks. In particular, use of remotely enolizable dicyanodienes 1, under one-pot sequential amine/NHC catalysis, led to [3 + 2] cycloaddition to afford ε,δ-bonded spiro[4.5]decanone structures 5. Alternatively, modifying the standard amine catalysis by adding a suitable chemical stimulus (p-nitrophenol cocatalyst) switched the reactivity decidedly toward a domino [4 + 2] cycloaddition to afford γ′,δ-bonded bicyclo[2.2.2]octane carbaldehydes 8. Products invariably formed in good yields, with rigorous chemo-, regio-, diastereo-, and enantiocontrol. Experimental evidence, including carbon isotope effects measured by 13C NMR, were indicative of the rate (and stereochemistry) determining step of these transformations and suggested a stepwise mechanism for the [4 + 2] cycloadditive pathway.
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