Base-Promoted Annulative Difluoromethylenation of Enaminones with BrCF 2 CO 2 Et toward 2,2-Difluorinated 2,3-Dihydrofurans.

A practical method for the synthesis of 2,2-difluorinated 2,3-dihydrofurans has been established via the [4 + 1] annulation of enaminones and BrCF 2 CO 2 Et with Na 2 CO 3 promotion. This new protocol does not employ any transition metal reagent and enables the annulative difluoromethylation by the partial cleavage of the C═C double bond. In addition, the further treatment with hydrochloric acid in one pot leads to β-keto enoic acids (4-oxo-2-butenoic acids) via a formal enaminone C-N carboxylation.