Chiral Bifunctional Thiosquaramides as Organocatalysts in the Synthesis of Enantioenriched 3,3-Disubstituted Oxindoles

Four novel chiral bifunctional thiosquaramides have been prepared from cyclopentyl dithiosquarates and diamines derived from natural l-Valine and l-tert-Leucine. The novel thiosquaramides have been tested as organocatalyst in the nitro-Michael addition of 3-substituted oxindoles to different beta-aryl-substituted nitroalkenes. The reaction occurred easily in high yields and excellent stereoselectivities, showing that the novel organocatalysts are much more effective than their thioureas and squaramides homologs.