ortho-Formylation of estrogens. Synthesis of the anti-cancer agent 2-methoxyestradiol

Several estrogens were mono-formylated using a mixture of paraformaldehyde, MgCl2, and Et3N in refluxing THF. In all cases, the 2-isomer was formed as the major product with high regioselectivity compared to the 4-isomer. Excellent to high yields were obtained in all examples except one. The method was applied for an efficient synthesis of the anti-cancer agent 2-methoxyestradiol.