A solution of 480 mg of 11h in 7 ml of acetic anhydride and 500 mg of anhydrous sodium acetate was heated at 105° C. for 30 min. The reaction mixture was cooled and treated with 40 ml of ice-water and stirred for 1 hr at ambient temperature to break down excess anhydride. Then, 30 ml of ethyl acetate was added and the stirred mixture was treated with cold conc. aq. NH4OH, to adjust the pH of the reaction mixture to slightly basic. The materials were extracted into ethyl acetate, washed with water, dried and concentrated. The residue was passed over a short silica column, using a gradient of toluene/ethyl acetate as eluent and the purified material thus obtained was treated with diethyl ether, to give 290 mg of 11i as white crystals; Mp 219-220° C.; Rf 0.35 (toluene/ethyl acetate 1/1). MS-ESI: [M+1] 516.04, 518.05.
