O[C:2]1[C:7]2[O:8][C:9]3[CH:14]=[CH:13][CH:12]=[CH:11][C:10]=3[C:6]=2[C:5]([CH:15]=[O:16])=[CH:4][CH:3]=1.[C:17](=O)([O-])[O-:18].[K+].[K+].[CH2:23]([O:25][C:26](=[O:29])CBr)[CH3:24]>CN(C=O)C.O>[CH2:23]([O:25][CH2:26][O:29][C:17]([C:2]1[C:7]2[O:8][C:9]3[CH:14]=[CH:13][CH:12]=[CH:11][C:10]=3[C:6]=2[C:5]([CH:15]=[O:16])=[CH:4][CH:3]=1)=[O:18])[CH3:24]Intermediate 19 (500 mg, 2.358 mmol) was dissolved in dry DMF (5 ml). Anhydrous potassium carbonate (650 mg, 4.716 mmol) was added to the above solution and was stirred for 10 min. at 80° C. To this was added ethylbromoacetate (2.0 eq.) and the reaction mixture was stirred for 1 h. The reaction mixture was cooled to room temperature and diluted with water (100 ml) and extracted with ethyl acetate (3×50 ml). The organic extract was washed with water (50 ml) and brine solution (25 ml) and dried over anhydrous sodium sulfate. Removal of solvent gave the product as a white solid (550 mg).
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