Process for the production of 5,6-dihydroxyindolines

A process for the production of a 5,6-dihydroxyindoline of formula (I) ##STR3## in which R.sup.1 and R.sup.3 independently of one another represent hydrogen or a C.sub.1-4 alkyl group and R.sup.2 is hydrogen, a C.sub.1-4 alkyl group, or a carboxyl group, comprising the steps of

• A) reacting an idoline ether of formula (II) or a salt thereof ##STR4## in which R.sup.1 and R.sup.3 independently of one another represent hydrogen or a C.sub.1-4 alkyl group, R.sup.4 and R.sup.5 represent C.sub.1-4 alkyl groups or, together with the oxygen atoms to which they are attached, form a C.sub.1-4 alkylenedioxy group, and R.sup.6 is hydrogen, a C.sub.1-4 alkyl group or a group COOR.sup.7 or CONR.sup.7 R.sup.8 and R.sup.7 and R.sup.8 are hydrogen or a C.sub.1-4 alkyl group, with an aqueous solution of hydrobromic acid;
• B) cooling the resulting aqueous reaction mixture to crystallize the compound of formula I therefrom;
• and
• C) isolating the crystallized compound of formula I from the aqueous reaction mixture.