The synthetic process of 4H-1,4-thiazine 1,1-dioxide (CAS NO. 40263-61-4) contains 3 steps. The synthetic routes are as follows:

A. cis and trans-2,6-Diethoxy-1,4-oxathiane 4,4-dioxide. is passed into a solution of 2,5-dihydrothiophene 1,1-dioxide (30.0 g., 0.254 mole) in 50 ml. of absolute ethanol and 250 ml. of dichloromethane contained in a 1-l., three-necked, round-bottomed flask fitted with a straight glass-inlet tube, a calcium chloride drying tube, and a glass stopper. The solution is cooled in a methanol–dry ice bath and magnetically stirred while the ozone is added. When the solution becomes blue, the addition of ozone is stopped and liquid sulfur dioxide (35 ml., 0.78 mole) is added in portions over a period of 10–15 seconds. After 2 minutes, the cold bath is removed and the reaction solution is allowed to warm to room temperature over a period of 8–16 hours. The resulting dark-brown solution is poured into a 4-l. beaker containing a rapidly stirred mixture of aqueous sodium carbonate (120 g. in 1 l. of cold water) and 200 g. of ice. The reaction flask is rinsed with 50 ml. of water, which is added to the basic mixture. After being stirred for 5 minutes, the basic mixture is poured into a 2-l. separatory funnel and the lower dichloromethane layer is separated and saved. The beaker is rinsed with 200 ml. of dichloromethane and 100 ml. of water, which are then added to the separatory funnel. The contents of the separatory funnel are shaken, and the lower, dichloromethane layer is separated and saved. The aqueous layer is extracted with two more 150-ml. portions of dichloromethane. All of the dichloromethane layers and extracts are combined, and washed with 300 ml. of water and 300 ml. of saturated aqueous sodium chloride. The solution is dried over 3–6 g. of anhydrous magnesium sulfate, filtered, and evaporated with a rotary evaporator at 50–60° in a water bath under aspirator pressure. The residual, cream-colored solid (50–52 g., 88–91%), m.p. 76–118°, is dissolved with magnetic stirring in 850–950 ml. of boiling heptane containing 1–2 g. of activated carbon and filtered hot. The filtrate is cooled to 0° in a refrigerator overnight. The resulting precipitate is filtered, giving cis- and trans-2,6-diethoxy-1,4-oxathiane 4,4-dioxide as a white solid (42–46 g., 74–81%), m.p. 83–117°.

B. 4H-1,4-Thiazine 1,1-dioxide. Caution! This step should be carried out in a hood to avoid exposure to hydrogen chloride gas. A mixture of cis- and trans-2,6-diethoxy-1,4-oxathiane 4,4-dioxide (15.0 g., 0.0669 mole), 3.8 g. (0.071 mole) of ammonium chloride, and 300 ml. of glacial acetic acid is placed in a 500-ml., one-necked, round-bottomed flask fitted with a reflux condenser and a magnetic stirring bar. The mixture is placed in an oil bath preheated to 125–130° and refluxed, with magnetic stirring, for 25–35 minutes, during which the ammonium chloride dissolves, hydrogen chloride is evolved, and the solution becomes brownish yellow in color. The acetic acid is evaporated with a rotary evaporator at 70–80° in a water bath under aspirator pressure. The residual yellow solid is magnetically stirred with a solution of 75 ml. of diethyl ether containing 10 ml. of 2-propanol for 10 minutes. The resulting suspension is filtered, then sucked dry on a Büchner funnel. The yellow solid (8.7–9.2 g.), m.p. 208–212°, is boiled with 225–250 ml. of 2-propanol and filtered hot, removing the residual, greenish-black, insoluble material (0.5–1 g.). The filtrate is cooled to -10° to -5° in a freezer overnight, causing separation of 4.6–5.3 g. (52–60%) of 4H-1,4-thiazine 1,1-dioxide as small yellow needles, m.p. 237–240°, which are filtered. Concentration of the filtrate to 50 ml., followed by filtration and cooling, causes separation of an additional 1.5–2.0 g. (17–23%) of crude yellow solid, m.p. 234–240°.