Referring to scheme 12, compound 41a (13.3 g, 51 mmol) was dissolved in anhydrous dichloromethane (40 mL) and cooled to 0° C. under argon. To the solution were added diisopropyl carbodiimide (6.31 g, 7.7 mL, 50 mmol) and pentafluoro phenol (42, 9.2 g, 50 mmol). After overnight the reaction mixture was evaporated to dryness. To the residue ethyl acetate (100 mL) was added and the filtered to remove diisopropyl urea. The precipitate was washed with ethyl acetate (50 mL). The combined organic layer was washed with saturated sodium bicarbonate and water. The organic layer was dried over sodium sulfate, filtered and evaporated to dryness. Compound 43a (Rf=0.8 in 10% EtOAc/Hexane, 21.65 g, 92%) was obtained, which was directly used for the next step without further purification.
