Synthesis of Zincke Aldehydes

1-(2,4-Dinitrophenyl)pyridinium chloride (Zincke salt) reacts with secondary amines to afford the corresponding 5-amino-2,4-pentadienals (Zinke aldehydes).¹⁾ This type of reaction has been applied to the synthesis of various indoles. For example, Vanderwal et al. have reported the total synthesis of norfluorocurarine, a Strychnos alkaloid.²⁾