Mechanism:

• Kinetic studies establish the rate limiting step as ...
• the eneamine presenting its re or si π-face
• to the re or si carbonyl π-face
• to form a new C-C bond

Choreography:

• C-C bond formation vs
• Proton transfer?

Catalytic Cycle:

• The proline is regenerated (fast)
• and can be recovered unchanged at the end.