One-Flask, Telescoped Synthesis of Pyrano[3,2-b]indol-2-ones from O-Nitroynones and β-Ketoesters: Further Advancement to Functionalized Carbazoles

A new general strategy for accessing pyrano[3,2-b]indol-2-ones, a promising fused hybrid heterocyclic system, has been devised from o-nitroynones and β-ketoesters through a one-flask cascade process involving tandem Michael addition, intramolecular cyclization, and Cadogan-Sundberg reductive cyclization. The utility of this approach has been further amplified by leveraging the cycloaddition proclivity of the α-pyrone moiety in pyrano[3,2-b]indol-2-ones toward a concise entry to carbazoles. Illustrative synthesis of the carbazole natural products hyellazole and chlorohyellazole is also disclosed.