[F:1][C:2]([F:28])=[C:3]1[CH2:8][CH2:7][C@H:6]2[C@H:9]3[C@H:18]([CH2:19][CH2:20][C@:4]12[CH3:5])[C:17]1[CH:16]=[CH:15][C:14]([O:21]C2CCCCO2)=[CH:13][C:12]=1[CH2:11][CH2:10]3.C(O)(=O)C(O)=O>CO.O>[F:1][C:2]([F:28])=[C:3]1[CH2:8][CH2:7][C@H:6]2[C@H:9]3[C@H:18]([CH2:19][CH2:20][C@:4]12[CH3:5])[C:17]1[CH:16]=[CH:15][C:14]([OH:21])=[CH:13][C:12]=1[CH2:11][CH2:10]31.3 g of crude 17-difluoromethylene-3-(tetrahydro-pyran-2-yl-oxy)-estra-1,3,5(10)-trien in 30 ml of methanol and 3 ml of water are suspended with 1.3 g of oxalic acid for 1 hour at a bath temperature of 100° C. Then, it is added to water, extracted three times with dichloromethane, washed neutral, dried on sodium sulfate and evaporated to dryness in a vacuum. 1.0 g of crude 17-difluoromethylene-estra-1,3,5(10)-trien-3-ol, which is chromatographed on silica gel with hexane/ethyl acetate, is obtained. 480 mg of pure 17-difluoromethylene-estra-1,3,5(10) -trien-3-ol is obtained as colorless crystals with a melting point of 154-156° C.
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