Enantioselective intermolecular aldol dimerisation catalysed by proline

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Sn2

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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.

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A. K. Sharma and R. B. Sunoj, Angew. Chemie Int. Ed., 2010, 49, 6373–6377.

J. Liu and L. Wang, Synthesis (Stuttg)., 2016, 49, 960–972.