A 200 mL Erlenmeyer flask equipped with a magnetic stir bar and a rubber septum was charged with 5 grams of dexamethasone (12.7 mmol, 1 equivalent) and 16 mL of anhydrous pyridine. The flask was cooled to 0° C. with an ice bath, and 1.2 mL of methanesulfonyl chloride was added. After 1 hour, 0.8 mL of additional methanesulfonyl chloride was added. At two hours, the solution was poured into 100 mL of cold 1M HCl, forming a precipitate. The precipitate was filtered and redissolved in 250 mL of ethanol, and precipitated into 250 mL of 1M HCl. The precipitate was filtered and recrystallized in ethanol, filtered, and dried under vacuum yielding 4.95 g (82.6%) of dexamethasone mesylate.
