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