To a solution of compound (12d) (125 mg, 0.276 mmol) in 1,4-dioxane (2 mL) were added hydroxylamine hydrochloride (191 mg, 2.76 mmol) and a freshly prepared solution of sodium methoxide in methanol (95 mg, 4.14 mmol of sodium dissolved in 1 mL of methanol) under a N2 atmosphere. The reaction mixture was stirred at room temperature for 2 h. The reaction mixture was acidified to pH˜6 with 1M HCl and the formed precipitates were filtered off. The filtrate was diluted with ethyl acetate (5 mL) and water (2 mL) and the organic layer was separated. The aqueous layer was washed with ethyl acetate (5 mL) and the combined organic layers were dried on sodium sulfate, filtered and concentrated under reduced pressure. The residue was purified by preparative HPLC to give 13d as a solid. HPLC: =92% (Rt=14.16 min).
