A Convenient Synthesis of Spiro[isoxazole-pyrazoloquinoline] Derivatives under Catalyst-Free Conditions.

The same three-component reaction of aromatic aldehyde, 1H-indazol-6-amine and 3-phenylisoxazol-5(4H)-one in refluxing ethanol under catalyst-free conditions gave two entirely different kinds of products depending on the substituents on the benzene ring. The reaction selectively gave 5H-spiro[isoxazole-4,8′-pyrazolo[3,4-f]quinolin]-5-ones with strong electron-withdrawing groups on the aromatic aldehyde, while it resulted in ring-opened pyrazoloquinoline derivatives with other substituents. The former structure was confirmed by X-ray diffraction analysis.