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Crosslinked sulfonated polyacrylamide (Cross-PAA-SO 3 H) tethered to nano-Fe 3 O 4  as a superior heterogeneous solid acid catalyst has been used for the preparation of eight spiro-oxindoles by reactions of phenylhydrazine or hydrazine hydrate, isatins, ketoesters and naphthylamine or 2-naphthol under ultrasonic irradiations. The present catalytic procedure is extensible to a wide diversity of substrates for the synthesis of a variety-oriented library of oxindoles. The structures of the prepared oxindoles were fully characterized by  1 H and  13 C NMR spectra, FT-IR spectra, and elemental analysis. Experimental simplicity, wide range of products, excellent yields in short reaction times, reusability of the catalyst, low catalyst loading and use of ultrasonic irradiation, are some of the substantial features of this method. The feasibility of performing one-pot multi-component synthesis under ultrasonic irradiation with a heterogeneous catalyst could improve the reactions rates and shorten the reactions times.

Sonosynthesis of Spiro-Oxindoles Using Crosslinked Sulfonated Polyacrylamide Tethered to nano-Fe 3 O 4 as High Performance Catalyst