The product of Example 320C (400 mg, 0.94 mmol) in N,N-dimethylformamide (10 mL) was reacted with tert-butyl bromoacetate (0.555 mL, 3.76 mmol), potassium carbonate (1.225 g, 3.76 mmol) and tetrabutylammonium iodide (catalytic) at 25° C. for overnight. The reaction mixture was diluted with water and adjusted to pH 7 with glacial acetic acid. The reaction was extracted with ethyl acetate and the organic layer was washed with aqueous sodium bicarbonate, water and brine, dried over anhydrous magnesium sulfate, filtered, and concentrated under reduced pressure. The resulting residue was chromatographed on silica gel eluting with a gradient of 3:1 hexanes:ethyl acetate to 1:1 hexanes:ethyl acetate to give the title compound (195 mg, 38%).
