A 1-L round-bottom flask equipped with a magnetic stir bar and rubber septum was purged with nitrogen and charged with 600 ml of USP-grade dry ethanol, 60 ml of anhydrous THF, 4.95 grams (10.5 mmol, 1.05 equivalents) of dexamethasone-mesylate and 10 grams (19.5 mmol, 4.95 equivalents) of spermine. 2-Iminothiolane, 1.38 grams (10 mmol, 1.0 equivalents) in 3 ml of water, was added dropwise to the solution over 5 minutes with vigorous mixing. The solution changed color from clear to clear light yellow. The reaction was monitored by TLC and by analytical HPLC. After three hours at room temperature, the TLC spot at Rt=0 (the charged DS) (Rt=0.7 min DS by analytical HPLC) was maximized, and dexamethasone-mesylate (Rt 0.47, 2.8 min HPLC) spot minimized. The crude reaction mixture was diluted with 15.25 ml (198 mmol, 19.8 equivalents) of trifluoroacetic acid, forming a white precipitate (spermine tetra TFA salt). Solvent was removed with a rotary evaporator and 50 ml of ethanol was added, forming a white precipitate (spermine-4TFA). The pH 3 solution was filtered twice to remove spermine tetratrifluoroacetic acid salt, and ethanol was removed with a rotary evaporator. 50 ml of water was added to the yellow viscous oil, yielding a precipitate (dexamethasone and dexamethasone mesylate), and the solution was filtered through 0.2 μm filter to yield a clear yellow liquid. Water was removed with a lyophilizer over several days to yield dexamethasone-spermine tetratrifluoroacetic acid salt. Yield: 8.228 grams (7.17 mmol, 72%).
