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Asymmetric intermolecular aldol catalysed by S-proline. The reaction goes via a hydroxyacetone enamine, with the -OH group trans to the proline. It attacks the aldehyde on it’s re-facial side to form and anti product. In the chair like transition state the isopropane group is pseudoequatorial to minimise the 1,3-diaxial repulsions from the axial methyl group alpha to the nitrogen. This forms the (S,S)-aldol product, which is anti.