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The reaction goes via an enamine, which attacks the aldehyde Re-face to form the R-product. In the transition state the isopropyl group is pseudoequatorial to minimise the 1,3-diaxial repulsions from the axial methyl group alpha to the nitrogen. This forms the R-aldol product.
Benjamin List, Richard A. Lerner and Carlos F. Barbas III, Proline-Catalyzed Direct Asymmeric Aldol Reactions, J. Am. Chem. Soc. 2000, 122, 2395-2396.