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The enamine attacks propanal with the Re face onto propanal's Re face. This causes a Re-Re transition state. This transition state produces an (S,S) iminium, which on hydrolysis forms the (S,S)product also. The (S,S)-3-hydroxy-2-methylpentanal has anti stereochemistry.
Akhilesh K. Sharma and Raghavan B. Sunoj, Enamine versus Oxazolidinone: What Controls Stereoselectivity in Proline-Catalyzed Asymmetric Aldol Reactions?, Angew. Chem. Int. Ed. 2010, 49, 6373-6377, DOI: 10.1002/anie.201001588.