A highly diastereoselective synthesis of tetralin-fused spirooxindole derivatives was described. Treatment of benzylidene oxindoles with a catalytic amount of Sc(OTf)(3) in refluxing hexane afforded the target compounds in good chemical yields with excellent diastereoselectivities (up to >20:1). Detailed investigation of the reaction mechanism revealed that both interconversion of the two diastereomers and their solubility difference in reaction medium were the key to achieving excellent diastereoselectivities.