Synthesis of chiral 3-substituted phthalides by a sequential organocatalytic enantioselective aldol-lactonization reaction. Three-step synthesis of (S)-(-)-3-butylphthalide

(Chemical Equation Presented) The development of efficient methods for the facile construction of important molecular frameworks is an important goal in organic synthesis. Chiral 3-substituted phthalides are widely distributed in a large collection of natural products with broad, potent, and potentially path-pointing biological activities. In this investigation, we have uncovered an unprecedented organocatalytic asymmetric aldol-lactonization reaction of 2-formylbenzoic esters with ketones/aldehydes for convenient construction of the enantioenriched "privileged" scaffold. As a result of the sensitive nature of substrate structures of an organocatalytic enantioselective aldol reaction, after extensive optimization of reaction conditions, catalyst L-prolinamide alcohol IV is identified as the best promoter. Interestingly, it is found that in this reaction, addition of an acid additive PhCO₂H can significantly enhance reaction efficiency with use of only as low as 2.5 mol % IV for the process. Moreover, due to the sensitivity of reaction conditions toward a sequential aldol-lactonization process without affecting enantioselectivity and racemization, it is essential to remove the catalyst for the subsequent facile lactonization reaction in the presence of K ₂CO₃. The aldol-lactonization processes serve as a powerful approach to the preparation of synthetically and biologically important 3-substitued phthalides with a high level of enantioselectivities. A 3-step catalytic asymmetric synthesis of the natural product of 3-butylphthalide is reported.