Chemoselective Construction of Polycyclic Heterocycles Containing a [6‐6‐5] or [7‐6‐5] Tricyclic Core Skeleton from a 2‐Isocyanophenyl Propargylic Ester

Abstract Functionalized isocyanide chemistry represents an important research area in organic synthesis. A structurally unique 2‐isocyanophenyl propargylic ester has been designed to incorporate the reactivity of isocyanide and propargylic ester functionalities. Thus, the reaction of 2‐isocyanophenyl propargylic ester and 2‐aminoaromatic aldimine facilitates the synthesis of a wide range of polycyclic benzo[b] indolo [3,2‐h][1,6] naphthyridine derivatives. Furthermore, reaction with 2‐hydroxyaromatic aldimine enables the divergent synthesis of both the aforementioned scaffolds and structurally distinct diazabenzo [ f ] naphtho[2,3,4‐ ij ] azulenes featuring a [7‐6‐5] core skeleton. Experimental results and DFT calculations suggest that these transformations likely proceed by the in situ generation of a strained cyclopropenimine species followed by [3+2] cycloaddition. Next, switchable nucleophilic attack/ring expansion/aromatization and nucleophilic addition/ring expansion/elimination account for the observed selectivity.