Synthesis of 6 H ,8 H ‐isoquino[2,3‐ c ][1,3]benzothiazin‐8‐ones

6 H ,8 H ‐Isoquino‐1,3‐benzothiazin‐8‐ones have been prepared by reaction of 6,7‐dimethoxy‐2 H ‐1,3‐benzo‐thiazines with homophthalic anhydride and by photocyclization of 3‐benzoyl‐4‐methylene‐6,7‐dimethoxy 2 H ‐1,3‐benzothiazine. The compounds are thia analogues of protoberberine alkaloids containing a sulfur atom at C‐5 and a lactam function at C‐8. The mass spectra of the title compounds are discussed.