Total Syntheses of (±)-β-Erythroidine and (±)-8-oxo-β-Erythroidine by an Intramolecular Diels−Alder Cycloaddition of a 2-Amidoacrolein

The total syntheses of (±)-β-erythroidine and (±)-8-oxo-β-erythroidine are described. The tetracyclic ring system of the natural products was quickly assembled by a strategy that features a retrocycloaddition/cycloaddition reaction of an amidodioxin, an intramolecular Heck reaction, and a 6π-electrocyclic ring closure of a dienoic acid.