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0 °C

A cooled (0° C.) solution of compound 55A (510.1 mg, 1.51 mmol) and imidazole (516 mg, 7.58 mmol) in dry DMF (2.6 mL) was treated with 97% tert-butyldimethylsilyl chloride (572 mg, 3.68 mmol), stirred at 0° C. for 5 min then at rt for 24 h. Methanol (3.5 mL) was added and the solution stirred at rt for another 24 h. The mixture was partitioned between 10% citric acid (5.3 mL) and EtOAc (3×50 mL). The combined organic phases were washed with brine, dried (Na2SO4), filtered and concentrated under reduced pressure. The resulting syrup was chromatographed (silica gel; EtOAc/hexane gradient) to yield the title compound (732.3 mg, 100%) as a white solid, mp 123-125° C. HPLC: 3.38 min (retention time) (Conditions: YMC S-5 C-18 (4.6×50 mm), eluting with 0-100% B, 4 min gradient. (A=90% H2O-10% CH3CN-0.1% TFA and B=10% H2O-90% CH3CN-0.1% TFA); Flow rate at 4 mL/min. UV detection at 220 nm. Chiral HPLC: retention time=9.30 min (98.6%); Conditions: AD (4.6×250 mm); Eluted with 20% isopropanol in heptane for 30 min at 1 mL/min. MS (ES) m/z 452 [M+H]+.

1. waitat rt for 24 h
2. stirringthe solution stirred at rt for another 24 h
3. customThe mixture was partitioned between 10% citric acid (5.3 mL) and EtOAc (3×50 mL)
4. washThe combined organic phases were washed with brine
5. dry with materialdried (Na2SO4)
6. filtrationfiltered
7. concentrationconcentrated under reduced pressure
8. customThe resulting syrup was chromatographed (silica gel; EtOAc/hexane gradient)