Access to Bromo-γ-butenolides via Zwitterion-Catalyzed Rearrangement of Cyclopropene Carboxylic Acids.

γ-Butenolides are useful structural motifs in many pharmaceutically relevant compounds. In particular, halogenated γ-butenolides are attractive building blocks because the halogen handles can readily be manipulated to give various functional molecules. In this study, a catalytic synthesis of halogenated γ-butenolides from cyclopropene carboxylic acids was developed using zwitterionic catalysts and N-haloamides as the halogen sources. The catalytic protocol could also be applied to the synthesis of halogenated pyrrolones by using cyclopropene amides as the starting materials.