Brønsted Acid-Catalyzed Highly Stereoselective Arene-Ynamide Cyclizations. A Novel Keteniminium Pictet−Spengler Cyclization in Total Syntheses of (±)-Desbromoarborescidines A and C

A Bronsted acid-catalyzed highly stereoselective arene-ynamide cyclization is described. These reactions constitute a keteniminium variant of Pictet−Spengler cyclizations, leading to efficient synthesis of nitrogen heterocycles and related alkaloids. Total syntheses of desbromoarborescidines A and C are illustrated here as first applications of this methodology.