Unexpected Brønsted Acid‐Catalyzed Domino Reaction of 3‐Hydroxyisoindolin‐1‐ones and N‐tert‐Butyl Hydrazones for the Synthesis of 3‐(Hydrazono)isoindolin‐1‐ones

A Bronsted acid-catalyzed aza-Mannich reaction between 3-hydroxyisoindolin-1-ones and N-tert-butyl hydrazones was developed. Under the catalysis of the racemic phosphoric acid or trifluoroacetic acid and mild reaction conditions, this domino reaction allows for efficient synthesis of 3-(hydrazono)isoindolin-1-one derivatives in excellent yields with high chemoselectivities. The E-configuration of the hydrazone products was determined by X-ray single-crystal diffraction of 5w.