A mixture of 2-(4-chlorophenyl)-4-(N-methylaminoacetyl)-3,4-dihydro-2H-1,4-benzothiazine (1.80 g), allyl bromide (0.52 ml), potassium carbonate (2.25 g) and dimethylformamide (20 ml) is stirred at room temperature for 3.5 hours. The reaction mixture is poured into water, and the mixture is extracted with ethyl acetate. After the solvent is distilled off, the residue is purified by silica gel column chromatography (solvent, chloroform:methanol=40:1). The caramel thus obtained is dissolved in ether and thereto is added 1.1 equivalent of oxalic acid. The resulting precipitate is washed with ether and dried to give 4-(N-allyl-N-methylaminoacetyl)-2-(4-chlorophenyl)-3,4-dihydro-2H-1,4-benzothiazine oxalate as powder.
