Mechanistic investigations of the conjugate addition/asymmetric protonation/aza-Prins cyclization. Standard conditions: 7c (1.0 equiv), Z-2a-d (1.0 equiv), (R)-3,3’-dibromo-BINOL (20 mol %), ZrCl4 (1.6 equiv), 2,6-dibromophenol (1.0 equiv), TMSCl (1.0 equiv), CH2Cl2, 23 °C.