To a solution of 3-hydroxy-2-nitroestra-1,3,5(10)-trien-17-one (76b, 1.10 g, 3.5 mmol) in DMF (10 mL) were added imidazole (0.476 g, 7.0 mmol) and tert-butyldimethylchlorosilane (0.690 g, 4.6 mmol) at room temperature. The reaction mixture was stirred for 1 h, and diluted with EtOAc, and washed with H2O, saturated aqueous NaCl, and then dried (Na2SO4). The desiccant was filtered and the solvent was evaporated at reduced pressure. The residue was purified by column chromatography (silica gel) using n-hexane:EtOAc (5:1→2:1, v/v) to afford 1.45 g of 89 (97% yield) mp: 184-185° C.
