Investigation of the retro-Diels–Alder reaction as a method for the generation of nitroso-olefins

Abstract

N-Methyltetrahydrocarbazole (1) reacts with ethyl 2-nitrosopropenoate and with 3-nitrosobut-3-en-2-one to give the fused oxazines (2) in good yields. These adducts, when heated in xylene, undergo retro-Diels–Alder reaction to give the starting carbazole and the nitroso-olefins, which are intercepted by indole. Thermolysis of the adduct (2a) in pentanol results in the formation of the oximinoketone (6) in good yield. α-Nitrosostyrene also forms an adduct (2c) with N-methyltetrahydrocarbazole, but gives in addition the nitrone (3) and a 2:1 adduct, which is assigned the structure (4). The oxazine (2c) failed to undergo the retro-Diels–Alder reaction when heated in xylene. Attempts to induce a retro-Diels–Alder reaction of the adducts (8) formed from hexamethylbicyclo[2.2.0]hexa-2,5-diene and 3-nitrosobut-3-en-2-one were also unsuccessful.