A solution of the chlorothiatriazine obtained in Step 3. above (92.3 g) in methylene chloride (250 ml) is added dropwise to a stirred solution of para-nitrophenol (49.4 g) and pyridine (70.2 g) in methylene chloride (500 ml) and refluxed for 3 hours. The reaction mixture is washed with 3N HCl and sodium bicarbonate solution. The methylene chloride layer is dried, filtered, washed and evaporated yielding a solid which is dissolved in hot THF with charcoal, filtered and evaporated. The resulting solid is washed with diethyl ether, dissolved in methylene chloride, filtered through silica gel, washed with methylene chloride, filtered through Celite®, evaporated, and triturated with diethyl ether affording a yellow solid. The solid is recrystallized from hot THF, yielding the desired product as a solid used in the next step without further purification.
