To a solution of compound (12b) (125 mg, 0.292 mmol) in 1,4-dioxane (2 mL) were added hydroxylamine hydrochloride (202 mg, 2.92 mmol) and a freshly prepared solution of sodium methoxide in methanol (100 mg, 4.35 mmol of sodium dissolved in 1 mL of methanol) under 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 ethylacetate (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 13b as a solid. HPLC: 86.09% (Rt=12.51 min).
