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 (13)C NMR, were indicative of the rate (and stereochemistry) determining step of these transformations and suggested a stepwise mechanism for the [4 + 2] cycloadditive pathway.
Denmarks chiral bisphosphoramide/silicon tetrachloride system performs as an excellent Lewis base-Lewis acid catalyst for the vinylogous Mukaiyama aldol reaction of pyrrole-and furan-based dienoxy silanes with aromatic and heteroaromatic aldehydes. This asymmetric methodology provides a powerful synthetic entry to a variety of d-hydroxylated g-butenolide-type frameworks with high efficiency and valuable margins of regio-, diastereo-, and enantioselectivity. Notably, the nature of the heteroatom within the vinylogous dienoxy silane donor heavily impacts the diastereocontrol, with syn-configured aldol adducts emerging from pyrroles bearing electron-withdrawing N-protecting groups (Boc, Ts, and Cbz) and anti-configured adducts prevailing when furan-or N-alkyl/alkenylpyrrole donors are involved.
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