Zincke Reaction - Zincke Aldehydes

With secondary amines and not primary amines the Zincke reaction takes on a different shape forming so-called Zincke aldehydes in which the pyridine ring is ring-opened with the terminal iminium group hydrolyzed to an aldehyde :

This variation has been applied in the synthesis of novel indoles :

with cyanogen bromide mediated pyridine activation.