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

Brønsted-acid-catalyzed selective Friedel-Crafts monoalkylation of isatins with indolizines in water

The controlled mono-addition of indolizines to isatins under very mild conditions is described. The reaction occurs in water using diphenylphosphate as the catalyst, which is dramatically accelerated by surfactant addition. 3-Hydroxy-3-indolizinyl-2-oxindole scaffolds were synthesized in up to >99% yield. Notably, in organic solvents only bis-addition products were observed.

One-pot synthesis of 3-hydroxy-2-oxindoles via acyloin rearrangements of 2-hydroxy-indolin-3-ones generated in situ from 2-alkynyl arylazides

A novel one-pot method to prepare 3-hydroxy-2-oxindoles via acyloin rearrangements of 2-hydroxy-indolin-3-ones generated in situ from 2-alkynyl arylazides has been described.