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The missing link: The elusive enamine intermediate of nucleophilic proline catalysis was detected and stereochemically characterized by NMR analysis of the aldehyde self-aldolization reaction in dipolar aprotic solvents. NMR exchange spectroscopy (EXSY) was used to observe direct enamine formation from oxazolidinones. Additionally, the stabilization of the intermediate by the appropriate choice of solvent and substitution pattern on the aldehyde is presented.
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Schmid et al. (2010) studied this question.
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