Accessing Skeletal Diversity under Iron Catalysis using Substrate Control: Formation of Pyrroles versus Lactones

2-Azetidinone-tethered alkynols and allenols, readily prepared from a propanylidene beta-lactam aldehyde, were used as starting materials for divergent ring expansion reactions catalyzed by iron(III) chloride. Worthy of note, in contrast to the iron-catalyzed reactions of beta-lactam allenols which lead to gamma-lactones, the reaction of beta-lactam alkynols under identical conditions gives pyrroles. The gold-catalyzed 6-endo aminocyclization of these allenic gamma-lactones formed fused dihydropyridines. The iron-catalyzed formation of pyrroles may proceed through a Meyer-Schuster rearrangement followed by beta-lactam ring opening and cyclization by attack of the amino group to the ketone.