6-Nitrodiazo-1,2,4-acid is heated with aqueous sodium sulfite solution in the presence of copper(II) sulfate. The solution is then acidified and heated further to cause desulfonation. The resulting nitrohydroxynaphthalenesulfonic acid is reduced with iron in the presence of acid. This process is preferred to the traditional deamination of compound 175 with ethanol in the presence of a copper catalyst.
