5(6)-Nitrofluorescein

5(6)-Nitrofluorescein is the mixture of 5- and 6-nitrofluoresceine isomers. This compound is a useful compound that could be used to synthesize fluorescein 5-isothiocyanate.

A process for the preparation of a mixture (5(6)-Nitrofluorescein) of 3′,6′-dihydroxy-6-nitrospiro[2-benzofuran-3,9′-xanthene]-1-one and 3′,6′-dihydroxy-5-nitrospiro[2-benzofuran-3,9′- xanthene]-1-one comprising the steps of (a) reacting 4-nitrophthalic acid or 4-nitrophthalic anhydride with benzene-1,3-diol in methanesulphonic acid; (b) quenching the reaction in step (a) with a solvent to precipitate product; (c) isolating the precipitate; (d) heating the precipitate in water in order to hydrolyse any methansulphonic acid ester present.