Catalyst-free rapid crystallization-induced stereoselective three-component [3 + 2] cycloaddition to polycyclic spirooxindoles under microwave irradiation

[3 + 2] Cycloaddition reactions are powerful tools to build alkaloid skeleton compounds. While maleimides are common dipolarophiles, the successful use of structurally symmetric maleimides in diastereoselectivity remains difficult. A series of polycyclic spirooxindoles were rapidly prepared by microwave-assisted, three-component [3 + 2] cycloaddition of isatins, l-proline and structurally symmetric N-phenylmaleimides with a crystallization-induced diastereomer transformation. Notably, stereoselectivity was well controlled by crystallization-induced diastereomer transformation from the reaction system. Moreover, the target products could be easily obtained by filtration without the need for solvent-intensive column chromatography, with good yields (76–91%) and good diastereoselectivities (> 20:1) in a short time. The gram-scale experiment also demonstrated the practicability of this method.