Exploring the vinylogous reactivity of cyclohexenylidene malononitriles: Switchable regioselectivity in the organocatalytic asymmetric addition to enals giving highly enantioenriched carbabicyclic structures

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