Synthesis of fused indolin-3-one derivatives <i>via</i> (3 + 2)-cycloaddition of <i>in situ</i> generated 3<i>H</i>-indol-3-one with nitrilimines, nitrile oxides and azomethine ylides

A regioselective (3 + 2)-cycloaddition between <i>in situ</i> generated 3<i>H</i>-indol-3-one and three types of 1,3-dipoles leading to three classes of condensed indole derivatives was developed. Two pathways of cycloaddition depending on the structure of the initial dipole were also demonstrated. A wide range of functionalized indol-3-one derivatives (29 examples) were obtained in moderate to good yields (up to 90%) under mild conditions. Among them, four spirocyclic derivatives containing indol-3-one and indol-2-one fragments were obtained.