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

Abstract 2‐Azetidinone‐tethered alkynols and allenols, readily prepared from a propanylidene β‐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 β‐lactam allenols which lead to γ‐lactones, the reaction of β‐lactam alkynols under identical conditions gives pyrroles. The gold‐catalyzed 6‐endo aminocyclization of these allenic γ‐lactones formed fused dihydropyridines. The iron‐catalyzed formation of pyrroles may proceed through a Meyer–Schuster rearrangement followed by β‐lactam ring opening and cyclization by attack of the amino group to the ketone.