Process for Preparing Dithiine-tetracarboxy-diimides

Process for preparing a compound of formula (I)

• in which
• R1 and R2 are identical or different and are hydrogen;
• C1-C8-alkyl which is optionally substituted with one or more halogen, —OR3, or —COR4;
• C3-C7-cycloalkyl which is optionally substituted with one or more by halogen, C1-C4-alkyl, or C1-C4-haloalkyl;
• or aryl or aryl-(C1-C4-alkyl) each of which is optionally substituted with one or more halogen, C1-C4-alkyl, C1-C4-haloalkyl, —COR4, or sulphonylamino,
• R3 is hydrogen, C1-C4-alkyl, C1-C4-alkylcarbonyl, or aryl, wherein the aryl is optionally substituted with one or more by halogen, C1-C4-alkyl, or C1-C4-haloalkyl,
• R4 is hydroxyl, C1-C4-alkyl, or C1-C4-alkoxy,
• comprising reacting
• dichloromaleimides of formula (VI)
• which R is R1 or R2
• with an inorganic thiosulphate in a solvent or solvent mixture, wherein between 1.1 and 1.8 moles of thiosulphate are used per mole of dichloromaleimide of formula (VI).