A suspension of the product of Example 52A (1.00 g, 3.445 mmol) in anhydrous methylene chloride (40 mL) was treated with N,O-bis(trimethylsilyl)acetamide (1.77 mL, 7.234 mmol) dropwise, and the resulting orange-colored solution was stirred at room temperature for 30 minutes under a nitrogen atmosphere. Anhydrous pyridine (0.557 mL, 6.89 mmol) was then added, followed by solid 9-fluorenylmethoxycarbonyl chloride (1.114 g, 4.306 mmol) in three portions. The reaction was stirred for 30 minutes, then poured into water (75 mL) and adjusted the pH to 1 with 1N aqueous hydrochloric acid. After stirring for 15 minutes at room temperature, the mixture was transferred to a separatory funnel and extracted with ethyl acetate (500 mL, followed by 2×150 mL). The combined organic extracts were washed with water (2×50 mL) and brine (50 mL), dried over anhydrous sodium sulfate, filtered, and concentrated by rotary evaporation in vacuo. Trituration with methylene chloride provided the title compound as a yellow solid (1.29 g, 73%).
