2-Amino-4,5,6,7-tetrahydrobenzothiazole → 2-methylsulfanyl-4-oxo-5,6,7,8-tetrahydro-4H-9-thia-1,4a-diaza-fluorene-3-carboxylic acid

The suspension of 4,5,6,7-tetrahydro-benzothiazol-2-yl-amine (8 gm, 0.0519 mol) and 5-(bis-ethylsulfanyl-methylene)-2,2-dimethyl-[1,3]dioxane-4,6-dione (14.2 gm, 0.0572 mol) in ethanol (60 ml) was heated at 85-90° C. for 10 hours. Reaction mixture was cooled to room temperature and filtered. The solid, thus obtained, was washed with ethanol (20 ml) followed by with diethyl ether (50 ml) and suck dried to give 7.2 gm of title compound as a solid.