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  • Title: Stereodivergence in amine-catalyzed regioselective [4 + 2] cycloadditions of β-substituted cyclic enones and polyconjugated malononitriles.
    Author: Feng X, Zhou Z, Zhou R, Zhou QQ, Dong L, Chen YC.
    Journal: J Am Chem Soc; 2012 Dec 05; 134(48):19942-7. PubMed ID: 23151123.
    Abstract:
    Switchable reaction patterns of β-substituted cyclic enones via amine-based dienamine activation are reported. While γ-regioselective vinylogous Michael addition was observed with alkylidenemalononitriles, a completely different [4 + 2] cycloaddition was obtained with allylidene- or alkynylidenemalononitrile substrates, affording densely substituted bicyclo[2.2.2]octanes or analogous architectures with moderate to excellent diastereo- and enantioselectivity by the catalysis of primary amines from natural quinidine or quinine. Importantly, high diastereodivergence was achieved through unusual hydrogen-bonding interactions of multifunctional primary-amine catalytic systems. Endo cycloadducts were efficiently produced using a combination of 9-amino-9-deoxyepiquinidine and salicylic acid, while exo variants were obtained using 6'-hydroxy-9-amino-9-deoxyepiquinidine. Moreover, we successfully isolated the Michael addition intermediates in some cases, indicating that the above [4 + 2] reaction via dienamine catalysis may proceed by a stepwise Michael-Michael cascade rather than by a concerted Diels-Alder cycloaddition pathway.
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