Synthesis of furan-2-ones and spiro[furan-2,3'-indoline] derivatives using polyether sulfone sulfamic acid catalysis.

Polyether sulfone sulfamic acid (PES-NHSOH) was prepared by simple sulfonation of a modified polyether sulfone. The number of acidic sites (SOH) was determined to be 4.23 mmol H/g by acid-base titration and 4.