Tandem Nenitzescu Reaction/Nucleophilic Aromatic Substitution to Form Novel Pyrido Fused Indole Frameworks

5-Hydroxyindoles are privileged structures that form part of various bioactive compounds. The Nenitzescu reaction of quinones and enamines is one of the most powerful methods to obtain 5-hydroxyindoles. In this work, we have applied the Nenitzescu reaction to 2-(2-chloropyrid-3-yl)benzoquinones. Mixtures of regioisomers were obtained that could be separated in the 4- and 6-substituted analogues, and then cyclized separately in a metal-free base-catalyzed reaction, affording novel tetracyclic indole derivatives. These are indeed the first examples reported in the literature of the linear pyrido[3′,2′ : 4,5]furo[3,2-b]indole and angular 1H-pyrido[2′,3′ : 4,5]furo[2,3-c]indole systems. The regioselectivity and the yield of the Nenitzescu reaction were found to be dependent on the N-substituent at the enamine. Furthermore, we analyzed the UV-Vis and PL spectra of the new systems, and this was supported by DFT calculations, allowing us to compare the properties of angular compared to linearly shaped compounds.