A new approach is described for the synthesis of spiro[piperidine-3,2′-oxindoles] in 35–82 yields with excellent stereoselectivity via the [4 + 2] cycloaddition reaction of donor–acceptor cyclobutanes with iminooxindoles in the presence of 10–30 mol% Sc(OTf)₃ at room temperature. This methodology provides great potential for building spiro-heterocycle compounds from simple building blocks.