Chiral Bis-pyridinium Salts as Novel Stereoselective Catalysts for the Metal-Free Diels–Alder Cycloaddition of a,b-Unsaturated Aldehydes

The synthesis of enantiomerically pure C2-symmetric bis-pyridinium salts was realized through a simple condensation of 2-pyridyl-carboxyaldehydes with a chiral diamine. The catalytic properties of such novel compounds were preliminarily studied and the trifluoracetate salts of chiral bis-imines of 2-picolinaldehydes derived from 1,1-binaphthyl-2,2-diamine were shown to catalyze the Diels-Alder reaction between cyclopentadiene and ,-unsaturated aldehydes. Modest exo stereoselectivities and enantioselectivities of up to 55% were obtained.