Squaramide-Catalyzed Michael Addition as a Key Step for the Direct Synthesis of GABAergic Drugs

Abstract

Enantioselective organocatalytic Michael additions serve as the key step in syntheses of chiral drugs based on γ-aminobutyric acid. The applicability of various squaramide catalysts for these Michael-type reactions has been assessed. Very good results in terms both activity and enantioselectivity were obtained in the Michael addition of dimethyl malonate to β-nitrostyrenes. On the other hand, a complementary approach, the addition of nitromethane to cinnamaldehydes, worked well with a squaramide catalyst possessing an adjacent pyrrolidine moiety. The corresponding Michael adducts obtained in the best conditions are suitable chiral intermediates for the synthesis of therapeutically useful GABA derivatives. Polymer-immobilized squaramides afforded the Michael adduct in high enantiomeric purity, but yield deterioration was observed between runs. Two different formal total syntheses of baclofen have also been accomplished.