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Stereodivergence in Amine-Catalyzed Regioselective [4 + 2] Cycloadditions of β-Substituted Cyclic Enones and Polyconjugated Malononitriles

Feng, Xin, Zhou, Zhi, Zhou, Rong, Zhou, Qing-Qing, Dong, Lin, Chen, Ying-Chun
Journal of the American Chemical Society 2012 v.134 no.48 pp. 19942-19947
catalytic activity, cycloaddition reactions, enantiomers, hydrogen bonding, octanes, primary amines, quinidine, quinine, salicylic acid
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.