= Solution
<acetone> first forms a <proline> <enamine> by reversible iminium formation and alpha-deprotonation. The <enamine> attacks the aromatic <aldehyde> in a hydrogen-bonded arrangement involving the catalyst's carboxylic acid. <hydrolysis> of the resulting iminium releases the beta-hydroxyketone and regenerates <proline>. The usual L-proline arrangement gives the \b[$R$ <secondary alcohol>].
The sterically crowded aryl–<amide> axis is selected at the same bond-forming step: rapid rotation in the <aldehyde> permits the catalyst to react preferentially with the conformer that puts the smaller carbonyl-oxygen side of the <amide> towards the approaching aldol transition structure and the bulky diisopropylamino side away. A chair-like <enamine>–<aldehyde> model minimizes the interaction of the <amide> with the catalyst-side axial <hydrogen>; reversing the <amide> orientation creates the less favourable diastereomeric <transition state>. Replacing planar CHO by tetrahedral CH(OH)CH$_2$COMe then raises the rotation barrier and traps the axis. This is a \b[<dynamic kinetic resolution>], not resolution of two configurationally stable starting atropisomers.
To specify the predicted major axis without relying on a flat bond-line formula, look from the aromatic ipso <carbon> towards the <amide> <carbonyl> <carbon>. Put the higher-priority aryl path, the trimethylsilyl-bearing ortho side, at nine o'clock. The preferred model puts the <carbonyl> <oxygen> at approximately six o'clock and the diisopropylamino group at twelve o'clock. This is \b[$S_a$] using the near-to-far high-priority convention; <oxygen> outranks <nitrogen> at the far end. Coupled with the <alcohol> assignment, the model predicts \b[$(R,S_a)$]. The drawing defines the configuration even if an alternative viewing direction is used. The dynamic-resolution mechanism is established by the primary aldol study; the product-axis assignment here is a stereochemical-model prediction for the printed substrate.<Dynamic kinetic resolution of atropisomeric amides by Vincent Chan and others>[Organic Letters 6, pp. 2051–2053: simultaneous alcohol and amide-axis control]{ref}
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