Diastereoselectivity in 1,3-dipolar cycloaddition reactions between indolic nitrones and electron-deficient alkenes

2-Phenyl-3H-indol-3-one N-oxide (2-phenylisatogen), 2-phenyl-3-phenylimino-3H-indole N-oxide and 2-phenyl-3H-indole N-oxide (the tautomeric form of 1-hydroxy-2-phenylindole) reacted in refluxing toluene with maleimides with different substituents at the nitrogen, yielding 1,3-cyclo-addition products. The cycloaddition was diastereoselective for the first two nitrones, whereas in the case of 2-phenyl-3H-indole N-oxide only the formation of the endo cycloadduct was observed. This different behaviour is attributed to the substituent at C-3 of the five-membered ring in the indole nucleus. In 95% ethanol at room temperature, metal cations such as Co-II, Cu-II, Ca-II, Mn-II, Zn-II, and Ni-II catalyse the reaction between 2-phenylisatogen and N-phenylmaleimide without substantially affecting the diastereoselectivity. The crystal structures of endo-3a,10,10a,10b-tetrahydro-10a-phenyl-1H-pyrrolo[3 ' ,4 ' :4,5]isoxazolo[2,3-a]indole-1,3,10-trione (5a), endo-2-methyl-10a-phenyl-3a,10,10a,10b-tetrahydropyrrolo[3 ' ,4 ' :4,5]isoxazolo[2,3-a]indole-1,3-dione (8a), and endo-2, 10a-diphenyl-3a,10,10a,10b-tetrahydropyrrolo[3 ' ,4:4,5]isoxazolo[2,3-a]indole-l,3-dione (8b) are reported.