Synthesis of spiro pyrazolone-oxindole and bicyclic pyrazolone derivatives via solvent dependent regioselective aza-1,4/1,6-Michael and intramolecular cycloaddition under catalyst-free conditions

A solvent dependent and highly regioselective [3+2]-cycloaddition reaction of isoxazole-styrenes and azomethine imines is reported under catalyst-free conditions furnishing a library of pyrazolone-spirooxindole hybrids. Regioselectivity for the isomeric structures were achieved in good yields by the reaction of isoxazole-styrene (1) and azomethine imine (2) in different solvents and temperature. The developed method was extended for the synthesis of tri-substituted dinitrogen-fused pyrazolones for the using 1,6-Michael addition reaction. Further, the isoxazole moiety was converted into carboxylic acid as a model study via ring opening