Asymmetric construction of spirobenzofuran indolinones via a cascade reaction of 3-hydroxyoxindoles with coumarins†

An interesting asymmetric organocatalytic cascade reaction of 3-hydroxyoxindoles with coumarins was accomplished using a chiral bisguanidinium hemisalt. A series of enantioenriched spirobenzofuran indolinones were afforded in high yields with high diastereo- and enantioselectivities. The reaction was found to involve several steps: an intermolecular Michael reaction, transesterification, a retro-Michael reaction, substitution and decarboxylation, and then an intramolecular Michael reaction to yield the product. The final step of this cascade is found to be the stereo-determining step. In addition, this reaction provides a facile route for the late-stage modification of several drug molecules via the installation of the spirobenzofuran indolinone scaffold.