A convenient synthesis of (Z)-4-hydroxy-N-desmethyltamoxifen (endoxifen).

A mixture of the (Z)- and (E)-isomers of 4-hydroxy-N-desmethyltamoxifen was conveniently prepared in four steps. These geometrical isomers were then neatly separated by semi-preparative Reverse Phase High Performance Liquid Chromatography (RP-HPLC) using specified conditions. Additionally, the isolated E-isomer could be equilibrated in aqueous strong acid in acetonitrile or trifluoroacetic acid/dichloromethane to give a clean 1:1 mixture of Z/E isomers that was re-subjected to HPLC separation. In this way, most of the undesired (E)-isomer could be readily converted to the desired (Z)-isomer providing quick access to over 200mg quantities of pure endoxifen (Z-isomer), a potent antiestrogenic metabolite of tamoxifen traditionally used in breast cancer treatment.