Rearrangement of C2-Spirooxindoles: Conversion to the 2-Hydroxyhemi-Indigo and Chromenoindole.

This study demonstrates the rearrangement of C2-spirooxindoles to the 2-hydroxyhemi-indigo and chromenoindole. The N -H-spirooxindole exhibits double proton translocation and its conversion to the ( Z )-2-hydroxyhemi-indigo photoswitch with trifluoroacetic acid, while the N -methyl-spirooxindole undergoes structural rearrangement to the chromenoindole. The mechanism of the reactions was proposed, and the structure of the products was confirmed by one-dimensional (1D) and two-dimensional (2D) NMR spectra and X-ray structure analysis. The photoswitching performance of ( Z )-2-hydroxyhemi-indigo, which allows the stabilization of the E -switched form by intramolecular hydrogen bonding, has been studied in solvents of different polarities. It was found that in the less polar solvents, the E -switched metastable isomer is characterized by high stability.