Synthesis of an Advanced Precursor of Rivastigmine : Cinchona-derived Quaternary Ammonium Salts as Organocatalysts for Stereoselective Imine Reductions

The enantioselective reduction of ketoimines has been successfully realized, using trichlorosilane as the stoichiometric reducing agent in the presence of catalytic amounts of a Lewis base, specifically a Cinchona derivative. For the first time, a novel class of derivatives was studied, featuring a picolinamide unit bound to the alkaloid scaffold, further functionalized as quaternary ammonium salt at the quinuclidine ring. Excellent yields and from good to high enantioselectivities (up to 92% ee) were obtained in the reduction of ketoimines. The novel catalysts were successfully employed in the synthesis of an enantiomerically pure advanced precursor of the blockbuster drug Rivastigmine.