 Acidic hydrolysis then removed the imine.Selective acylation with acetic anhydride (under buffered conditions, the 5-amino group is protonated owing to a considerable difference in p"K"a, 4.2 vs 7.9, preventing acetylation) yields the desired "N"-acetylated product in crystalline form upon extractive workup.Finally, deallylation as above, yielded the freebase of oseltamivir, which was converted to the desired oseltamivir phosphate by treatment with phosphoric acid.