To a suspension of magnesium (0.365 g, 15 mmol) in THF (3.0 mL) was added bromoethane (1.5 mL, 20 mmol) dissolved in THF (3.0 mL) at room temperature. 17-Methyleneestra-1,3,5(10)-trien-3-ol (19, 0.805 g, 3.0 mmol) dissolved in THF (7.5 mL) was added to the reaction mixture, and stirring continued for 30 min. The solvent was removed at reduced pressure, and to the residue were added benzene (35 mL), hexamethylphosphoric triamide (1.4 mL, 7.5 mmol) and paraformaldehyde (0.630 g). Stirring was continued for 6 h at 80° C. After the reaction mixture was cooled to room temperature, 5N HCl (15 mL) was added and the mixture extracted with EtOAc. The combined organic layers were washed with H2O, then with saturated aqueous NaCl, and 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:AcOEt (10:1→3:1, v/v) to afford 0.695 g of 23 (78% yield), m.p. 128°-130° C.
