New Intermediates and Dyes for Synthetic‐polymer Fibres. Benzimidazothioxanthenoisoquinolinone Dioxides and Sulphones of 7H‐Benzimidazo (2, 1‐a) benz(d, e) isoquinolin‐7‐one

Condensation of benzo(k,i)thioxanthene‐3, 4‐dicarboxylic acid‐7, 7‐dioxide with o‐phenylenediamine gives a mixture of the new heterocycles benzimidazo(1, 2‐b) thioxantheno(2, 1, 9‐d, e, f) isoquinoline‐7‐one‐16, 16‐dioxide and benzimidazo(2, 1‐a) thioxantheno(2, 1, 9‐d, e, f) isoquinolin‐8‐one‐5, 5‐dioxide. Separation of these isomers is reported, together with their characterisation by unambiguous synthesis. Similar reaction with substituted o‐phenylenediamines affords other isomer mixes of the above. These new dyes colour polyester fibres in orange hues of excellent fastness to light and sublimation. Comparison of the dyes is made with the uncyclised sulphone derivatives of 7H‐benzimidazo(2, 1‐a) benz(d, e) isoquinoline‐7‐one, which are novel bright yellow dyes of good coloration and fastness properties on polyester.