To a mixture of estradiol (1, 13.6 g, 50 mmol) and hexamethylenetetramine (21.0 g, 150 mmol) was added trifluoroacetic acid (100 mL) at room temperature. The reaction mixture was stirred for 6 h at 100° C., poured into H2O (300 mL), and extracted with Et2O. The combined organic layers were washed with H2O, then with saturated aqueous NaCl, and dried (MgSO4). The desiccant was filtered and the solvent was evaporated at reduced pressure. The residue was dissolved in THF (50 mL) and MeOH (50 mL), potassium carbonate (6.9 g, 50 mmol) was added, and the mixture stirred for 1 h at room temperature. The reaction mixture was then diluted with EtOAc, acidified with 1N HCl, and extracted with EtOAc. The combined organic layers were washed with H2O, then with saturated aqueous NaCl, and dried (MgSO4). The desiccant was filtered and the solvent was evaporated at reduced pressure. The residue was purified by column chromatography (silica gel) using n-hexane:THF (5:1→3:1, v/v) to afford 1.91 g of 2a (13% yield), m.p. 219°-221° C., and 0.552 g of 2b (4% yield), m.p. 146°-147° C.
