Asymmetric cinchona alkaloid-catalyzed ketene-N -thioacyl imine cycloadditions afford enantioenriched cis -4,5-disubstituted 1,3-thiazin-6-one heterocycles with high absolute and relative stereocontrol. These cycloadditions enable a strategy for executing catalytic asymmetric direct Mannich additions to enolizable, unactivated imines. The ketene and imine reaction components are both generated in situ from readily available acid chlorides and alpha-amido sulfones, respectively.