Photochemistry of benzene and quinoxaline fused Δ2-1,2,3-triazolines and their trapping products

The benzene and quinoxaline fused Δ2-1,2,3-triazolines 1a and 1b were synthesized in good yields using Knoevenagel condensation and intramolecular 1,3-dipolar cycloaddition as two of the key reactions. Photolysis (254 nm) of Δ2-1,2,3-triazoline 1a or 1b in acetonitrile led to the homolytic cleavage of nitrogen that generated diethyl diazomalonate 7, highly reactive intermediates aziridines 8a,b, and isoindoles B. The latter two species subsequently underwent rearrangement to give the nitrogen extrusion products 9a,b, and polymers. Furthermore, the reactive intermediates were trapped by dienophiles to give the corresponding cycloadducts. Subsequent rearrangement of the N-bridged cycloadducts gave N-substituted pyrrolo[3,4-b]quinoxalines 12b and 15b in 6% and 9% yields, respectively. Irradiation of 1a in the presence of fumaronitrile led to the isolation of cycloadduct 16a with retention of stereochemistry. Thermal reaction of 1b gave more nitrogen extruded product 9b (58-63% yield) than that by photolysis (5-23% yield), which implied that zwitterionic intermediate might be involved in the former.