Process for the preparation of aromatic azo-compounds

A process for the preparation of 3,3′-azo-bis(6-hydroxybenzoic acid) 1

• or a salt or derivative thereof, comprising the step of treating a 5-amino salicyclic acid derivative 2a
• or a salt or derivative thereof, with (i) hydrogen peroxide and acetic acid, followed by (ii) conc. sulphuric acid, to yield a 3,3′-azo-bis(6-hydroxybenzoic acid derivative) 1a
• or a salt or derivative thereof, wherein
• X is OR, SR or N(R)2,
• when X is OR or SR, R is independently an optionally substituted alkyl alkenyl, alkynyl, aryl, arylalkyl, arylalkenyl, arylalkynyl, alkylaryl, alkenylaryl or alkynylaryl group which may include one or more heteroatoms N, O or S in its carbon skeleton, hydrogen, —Si(alkyl)3 or —Sn(alkyl)3, and
• when X is N(R)2, each R is independently an optionally substituted alkyl, alkenyl, alkynyl, aryl, arylalkyl, arylalkenyl, arylalkynyl, alkylaryl, alkenylaryl or alkynylaryl group which may include one or more heteroatoms N, O or S in its carbon skeleton, hydrogen, —SO2-(aryl), —NH2, —NH(alkyl) or —NH(aryl), or both R together form an optionally substituted cycloheteroalkyl, cycloheteroalkenyl or heteroaryl group.