Patent ID: 7682406

Claim:
A compound of the formula in which D represents a phenyl monosulphonic acid residue, which is unsubstituted or substituted by one or two methyl groups or a nitro group, a phenyl disulphonic acid residue, a salicylic acid residue or a naphthalene mono- or disulphonic acid residue, R 1 and R 2 , independently, represent hydrogen or, a C 1 -C 4 alkyl group, R′ 1 represents hydrogen, hydroxyl, C 1 -C 4 alkyl or C 1 -C 4 hydroxyalkyl, R 3 represents hydrogen, C 1 -C 4 alkyl or phenyl, which is unsubstituted or substituted by one or two groups selected from C 1 -C 4 alkyl, C 1 -C 4 alkoxy, halogen, hydroxyl, —NH 2 , nitro, —SO 3 M and —CO 2 M and M represents hydrogen, an alkaline or alkaline earth metal, ammonium or ammonium that is mono-, di-, tri- or tetrasubstituted by C 1 -C 4 alkyl or C 2 -C 4 hydroxyalkyl or a polyglycolammonium cation.