4-HO-TEMPO-Catalyzed Redox Annulation of Cyclopropanols with Oxime Acetates toward Pyridine Derivatives

A 4-HO-TEMPO-catalyzed redox strategy for the synthesis of pyridines through the annulation of cyclopropanols and oxime acetates has been developed. This protocol features good functional group tolerance and high chemoselectivity and also promises to be efficient for the late-stage functionalization of skeletons of drugs and natural products. Mechanism studies indicate that the reaction involves the in situ generated alpha,beta-unsaturated ketones and imines as the key intermediates, which are derived from cyclopropanols and oxime acetates via a TEMPO/TEMPOH redox cycle, respectively. The pyridine products are formed as a result of annulation of enones with imines followed by TEMPO-catalyzed oxidative aromatization by excess oxime acetates. This method not only realizes the TEMPO-catalyzed redox reaction but also broadens the frontiers for TEMPO in catalysis.