PBr3-Mediated [5 + 1] Annulation of α-Alkenoyl-α-carbamoyl Ketene-S,S-acetals: Access to Substituted Pyridine-2,6(1H,3H)-diones.

A facile and efficient synthesis of substituted pyridine-2,6(1H,3H)-diones via an intramolecular [5 + 1] annulation of readily available α-alkenoyl-α-carbamoyl ketene-S,S-acetals mediated by phosphorus bromide (PBr3) under very mild conditions is described.