How is 7-Acetamido-4-hydroxy-2-naphthalenesulfonic acid (CAS: 6334-97-0) typically synthesized?

7-Acetamido-4-hydroxy-2-naphthalenesulfonic acid is typically synthesized by the sulfonation of 7-acetamido-4-hydroxy-2-naphthoic acid using fuming sulfuric acid in the presence of a catalyst such as aluminum sulfate. The reaction involves the introduction of a sulfonic acid group on the naphthalene ring, followed by the removal of the hydroxyl group to form the sulfonic acid salt. High yield and good selectivity are achieved under controlled conditions.