Fig. 2. Four stereochemical possibilities for the Houk like-List mechanism showing distinct TS with amide hydrogen bonding. Top panel shows the aldol reaction. Panels (A) and (B) represent the Re enamine face that results in the Re–Re, Re–Si prochiral encounters leading to (S,R) and (S,S) products, while panels (C) and (D) depict the Si–Si and Si–Re prochiral encounters that result in (R,S) and (R,R) products. Cahn–Ingold–Prelog conventions are shown for R Phe-NO₂. An enamine derived from N-methyl-prolinamide is shown in all panels.