Organocatalyzed Asymmetric Vinylogous Addition of Oxazole-2(3H)-Thiones to α,β-Unsaturated Ketones: An Additive Free Approach for Diversification of Heterocyclic Scaffold

A straightforward organocatalyzed asymmetric addition of oxazole-2(3H)-thiones to α,β-unsaturated ketones is described. This additive-free Michael reaction in the presence of chiral cinchonine-derived primary amines as catalysts has proven to be highly effective for a wide range of cyclic and acyclic enones, leading to the Michael adducts in very good yields and excellent enantioselectivities. The absolute configuration (R) of compound 5j was unambiguously assigned by X-ray diffraction analysis. Furthermore, experimental and theoretical studies were performed and a mechanism is presented and discussed for this novel reaction.