There is disclosed a process wherein the E isomer of 1,2-diphenyl-1-[4-(2-dimethylaminoethoxy) phenyl]-1-butene is converted to tamoxifen hydrochloride by a process which comprises heating the E isomer of 1,2-diphenyl-1-[4-(2-dimethylaminoethoxy) phenyl]-1-butene, either alone or in admixture with tamoxifen, in hydrochloric acid at a temperature within the range of 35-60 DEG C until substantial conversion of 1,2-diphenyl-1-[4-(2-dimethylaminoethoxy)-phenyl]-1-butene to tamoxifen is accomplished. In this temperature range, i.e., 35-60 DEG C the tamoxifen precipitates from the reaction mixture as the HC1 salt while most of the HC1 salt of the E isomer of 1,2-diphenyl-1-[4-(2-dimethylaminoethoxy)phenyl]-1-butene stays in solution. The tamoxifen HC1 is recovered from the resultant slurry. It can be further purified by conventional procedures, e.g., by chromatography or fractional crystallization.
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