Rearrangement of C2-Spirooxindoles: Conversion to the 2-Hydroxyhemi-Indigo and Chromenoindole
Kosuke Nakashima1, Dancho Yordanov2,3, Yasuyuki Matsushima1
1Department of Pharmaceutical Chemistry, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji 192-0392, Tokyo, Japan.
Abstract:
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.
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