Synthesis, Skeletal Rearrangement, and Biological Activities of Spirooxindoles: Exploration of a Stepwise C‐Piancatelli Rearrangement

Based on our previous studies, the scope of the transformation of 2-furylcarbinols into spirofurooxindoles, and also the skeletal rearrangements of spiro[furo-oxindoles] and spiro[thieno-oxindoles] were studied. The spiro[furo-oxindoles] thermally rearranged into spiro[pentenone-oxindoles] by a mechanism involving the conrotatory electrocyclization of the 4π-electron system. The free energy of the electrocyclization step was calculated to interpret the stereochemical outcomes. In contrast, the spiro[thieno-oxindoles] were rearranged under acidic conditions into thieno[2,3-c]quinolin-4-ones, involving an interesting dienone–phenol-like mechanism. The transformation of 2-furylcarbinols into spiro[pentenone-oxindoles] seems to be the first stepwise C-Piancatelli rearrangement. The spirooxindole products were biologically evaluated, and some of them showed promising cytotoxic activities against DU145 and LNCaP tumor cell lines.