Synthesis of 4‑Hydroxycarbazole Derivatives by Benzannulation of 3‑Nitroindoles with Alkylidene Azlactones

A general synthesis of 4-hydroxylcarbazoles by domino vinylogous conjugate addition/cyclization/elimination/aromatization of easily prepared 3-nitroindoles with alkylidene azlactones under mild and transition-metal-free conditions has been developed. This method was also applicable to other nitrosubstituted benzofused heterocycles such as 3-nitrobenzothiophene, 2-nitrobenzothiophene, and 2-nitrobenzofuran. The valuable tetracyclic carbazole derivatives, such as 6H-oxazolo­[4,5-c]­carbazole and 3,6-dihydro-2H-oxazolo­[4,5-c]­carbazol-2-one, were readily prepared from the product, demonstrating synthetic utility of this method.