A novel synthesis of 3-nitroindoles via electrocyclization of 2,3-(dialk-1-enyl)4-nitropyrroles

3-Nitroindoles (10) are prepared in good yields via a thermal 6 pi-electrocyclization of 2,3-(dialkenyl)-4-nitropyrroles (4) in nitrobenzene, a solvent which causes in situ aromatization of the initially formed dihydroindoles (8). The corresponding reaction of 2-alkenyl-3-alkadienyl-4-nitropyrroles (5) also leads to 3-nitroindoles (11), however, now together with 3-nitrotetrahydroindole derivatives (12). The latter compounds are formed by a tandem 6 pi-electrocyclization - intramolecular Diels-Alder reaction, and are the predominant (or only) products when nitrobenzene is replaced by triglyme. (C) 1997, Elsevier Science Ltd. (C) 1998 Elsevier Science Ltd. All rights reserved.