N-(arenesulfonyl)-aminoalcohols derivedfrom readily available ephedrines or amino acids undergo an intramolecular Friedel–Crafts reaction to afford enantiopure benzosultams bearing two adjacent stereocenters in high yields with fully controlled diastereoselectivity. Low-temperature NMR spectroscopy demonstrated the crucial role played by the conformationally restricted chiral dicationic intermediates.