Organocatalyzed [3 + 2] Annulation of Cyclopropenones and β-Ketoesters: An Approach to Substituted Butenolides with a Quaternary Center.

An unprecedented organocatalyzed [3 + 2] annulation of cyclopropenones and β-ketoesters has been developed. This reaction provides a direct approach to highly substituted butenolides with a quaternary center in moderate to good yields. The preliminary mechanism study verified that the enol intermediate is crucial to the reaction outcome and the intermolecular esterification and intramolecular Michael addition process were involved.