Intramolecular iron-catalyzed transannulation of furans with O-acetyl oximes: synthesis of functionalized pyrroles

• Published in: Organic chemistry frontiers an international journal of organic chemistry 2021-11, Vol.8 (23), p.6553-6560
• Main Authors: Makarov, Anton S, Fadeev, Alexander A, Uchuskin, Maxim G
• Format: Article
• Language: English
• Subjects: Catalysts; Chromophores; Coordination compounds; Ferric chloride; Furans; Lewis acid; Metal complexes; Nitrogen; Organic chemistry; Oximes; Pesticides; Pyrroles; Ring opening; Telescoping; Transition metal compounds
• ISSN: 2052-4110; 2052-4110
• DOI: 10.1039/d1qo01281a

Furyl-tethered O-acetyl oximes undergo a rearrangement to yield densely substituted pyrroles in the presence of FeCl3·6H2O. The reactivity of a furan ring resembles the behaviour of an activated olefin with a formal leaving group that reacts with electrophilic nitrogen in a 5-exo-trig manner to form a spiropyrroline intermediate followed by aromatization of pyrrole via an intermediate ring opening. In contrast to the common activation modes of O-acyl oximes that comprise homolysis, SET-reduction or oxidative addition of low-valent transition metal complexes across the N–O bond, in the present case the catalyst acts as a Lewis acid activating oxime nitrogen for a nucleophilic attack by the reaction partner. The studied reaction was integrated into a telescoping protocol for the synthesis of a broad range of pyrroles, which could find possible applications as flexible matrices for designing push–pull chromophores and as potential building blocks in organic chemistry.