[CH:1]([O:8][CH2:9][CH3:10])(OCC)OCC.C(O[C@@H:15]1[C@H:21]2[C@H:22]3[C@H:31]([CH2:32][CH2:33][C@:18]2([CH2:19][CH3:20])[C:17](=[O:35])[CH2:16]1)[C@@H:30]1[C:25](=[CH:26]C(=O)[CH2:28][CH2:29]1)[CH2:24][CH2:23]3)(=O)C.C(NC(C)C)(C)C.O>CCO.C1(C)C=CC(S(O)(=O)=O)=CC=1>[CH2:9]([O:8][C:1]1[CH2:28][CH2:29][C@H:30]2[C:25](=[CH:24][CH2:23][C@@H:22]3[C@@H:31]2[CH2:32][CH2:33][C@@:18]2([CH2:19][CH3:20])[C@H:21]3[CH:15]=[CH:16][C:17]2=[O:35])[CH:26]=1)[CH3:10]
Triethyl orthoformate (5 ml, 30 mmol) and p-Toluenesulfonic acid (50 mg, 0.26 mmol) were added to a suspension of 15α-acetoxy-18-methyl-estr-4-en-3,17-dione (5 g, 14.5 mmol) in EtOH (30 ml). The mixture was stirred at room temperature for 1 h. Diisopropylamine (6.3 ml, 36 mmol) was added, and the mixture was heated under reflux for 2 h, cooled to 10° C. and water (15 ml) was then added. The precipitate was filtered, washed with water and dried. The crude product was purified by flash chromatography, eluting with heptane and ethyl acetate (7:3), crystallized from heptane, and dried under vacuum to afford 3-ethoxy-18-methyl-estra-3,5,15-trien-17-one (3.18 g, yield: 71%).
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