To a solution of the methyl diastereomers (18c) (3.90 g, 8.51 mmol) in 40 mL THF at −78° C., was added LiHMDS (IM in THF, 17.9 mL, 17.9 mmol). The red solution was stirred at −78° C. for 5 min, then a solution of trisyl azide (2.90 g, 9.36 mmol) in 8 mL THF was added. The reaction was stirred at −78° C. for 1.5 h, then was quenched with the addition of AcOH (2.19 mL, 38.3 mmol). The mixture was stirred at rt for 1 h, then was diluted with EtOAc. The organic phase was washed with 1N HCl, H2O and brine, dried (Na2SO4), and concentrated. The product was purified by flash chromatography (35 to 40% EtOAc/hexanes) to afford 2.764 g (65%) of azide 18d. MS (ESI) 500.4 (M+H+), 522.4 (M+Na+).
