Di-tert-butyl phosphite (14.8 g, 76.3 mmol, 1.05 eq) was dissolved in 200 ml THF at −78° C. under nitrogen and 72 ml (1.05 eq) of 1.06M Lithium bis(trimethylsilyl)amide in THF was added over 30 min. The mixture was left stirring at −78° C. under nitrogen for 30 min and then added to a solution of the compound of Step 3 (20.2 g, 72.7 mmol, 1 eq) in 200 ml THF at −78° C. The solution was warmed to 0° C. and then poured into 400 ml of half saturated aqueous ammonium acetate. The layers were separated and the aqueous layer was washed with 400 ml isopropyl acetate. The organic layers were combined, dried with sodium sulfate and the solvent removed. The crude solid was then triturated with 15% EtOAc/hexane for 2 hours and 30.4 g (89%) of the title compound was obtained.
