Asymmetric organocatalytic anthrone additions to activated alkenes

Asymmetric organocatalytic additions of anthrones to activated alkenes are discussed. The reaction between anthrone or dithranol and α,β-unsaturated aldehydes is catalyzed by diphenylprolinol trimethylsilyl ether in toluene at −40 °C, giving the Michael adducts with good yields and enantioselectivities. Bifunctional amino-thioureas efficiently catalyze the additions of anthrones to both nitroalkenes and maleimides, and high enantioselectivities can be achieved in both instances at room temperature. In the case of nitroalkenes, a Michael addition takes exclusively place. Anthrone generally gives Diels-Alder cycloadducts in the reaction with maleimides, while dithranol affords the Michael adducts. Transition state working models in which the bifunctional catalyst binds simultaneously to the alkene and to the anthrone enolate account for the stereochemical outcome of these additions.

Asymmetric additions of anthrone and dithranol to α,β-unsaturated aldehydes, β-nitrostyrenes and maleimides are efficiently catalyzed by chiral amines (diphenylprolinol trimethylsilyl ether for enals and Takemoto's thiourea for nitrostyrenes and maleimides), affording the Michael or the Diels-Alder adducts (depending on the structures of the reactants) with good yields and enantioselectivities.