An Aromatic Ion Platform for Enantioselective Brønsted Acid Catalysis

Chiral acid catalysts are useful for the synthesis of enantioenriched small molecules, but the standard catalysts require laborious and expensive preparations. Here, we describe a chiral Brønsted acid prepared in one step from naturally occurring (-)-menthol and readily available 1,2,3,4,5-pentacarbomethoxycyclopentadiene. Aromatic stabilization serves as a key contributing factor to the potent acidity of the resulting compound, which is shown to catalyze both Mukaiyama-Mannich and oxocarbenium aldol reactions with high efficiency and enantioselectivity. Catalyst loadings as low as 0.01 mole percent and preparative scalability (25 grams) are demonstrated. Alternative amide catalysts are also shown to be promising platforms. In addition to proton catalysis, a chiral anion pathway is demonstrated to be viable with this catalyst system.