Stereo‐ and Regioselective [3+3] Annulation Reaction Catalyzed by Ytterbium: Synthesis of Bicyclic 1,4‐Dihydropyridines

An ytterbium catalyzed formal [3+3] cycloaddition of cyclic enamines and α,β‐unsaturated ketones catalyzed is reported. The reaction proceeds with a ‘head to tail’ regioselectivity through a conjugate addition of the enamine moiety followed by an amine‐carbonyl condensation. In addition the use of chiral enamines provided a high degree of stereoselectivity, driven by a possible balance between steric and π‐stacking effects. The resulting bicyclic 1,4‐dihydropyridines were evaluated as antiproliferative agents against A549 (carcinomic human alveolar basal epithelial cell) and SKOV3 (human ovarian carcinoma) human tumor cell lines. Good toxicities were found for some of the compounds against A549 and SKOV3 cell lines, with best IC50 values of 0.89 μM for A549 and 6.69 μM for SKOV3, and a very good selectivity was observed towards MRC5 (non‐malignant) cell lines.