To a mixture of 2-(4-chlorophenyl)-3,4-dihydro2H-1,4-benzothiazine (1.08 g), ethyl acetate (20 ml) and aqueous sodium hydrogen carbonate solution (20 ml) is added dropwise a solution of 4-bromobutyryl chloride (0.97 g) in ethyl acetate (5 ml) with stirring under ice cooling. The ethyl acetate layer is washed, dried and distilled to remove the solvent. The residue is recrystallized from ethyl acetate-n-hexane to give 4-(4-bromobutyryl)-2-(4-chlorophenyl)-3,4-dihyro-2H-1,4-benzothiazine (1.58 g, yield 93%).
