[CH2:1]([O:8][C:9]1[C:14]([C:15]([CH3:18])([CH3:17])[CH3:16])=[CH:13][CH:12]=[CH:11][C:10]=1[C:19]([C:22]1[CH:23]=[C:24]([C:28]2[CH:33]=[CH:32][CH:31]=[CH:30][C:29]=2[O:34][CH3:35])[CH:25]=[CH:26][CH:27]=1)(O)[CH3:20])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.C1(C)C=CC(S(O)(=O)=O)=CC=1>C1(C)C=CC=CC=1.C(OCC)(=O)C>[CH2:1]([O:8][C:9]1[C:14]([C:15]([CH3:17])([CH3:16])[CH3:18])=[CH:13][CH:12]=[CH:11][C:10]=1[C:19]([C:22]1[CH:23]=[C:24]([C:28]2[CH:33]=[CH:32][CH:31]=[CH:30][C:29]=2[O:34][CH3:35])[CH:25]=[CH:26][CH:27]=1)=[CH2:20])[C:2]1[CH:3]=[CH:4][CH:5]=[CH:6][CH:7]=1A solution of crude 27 (5 mmol) in toluene (120 mL) was treated with p-toluene sulfonic acid (0.2 g) and refluxed under a Dean-Stark trap for 2 hr. After cooling the reaction mixture was diluted with ethyl acetate (100 mL) and washed with saturated sodium bicarbonate (80 mL) and water (2×80 mL). The organic layer was dried over sodium sulfate, filtered and the solvent was removed on a rotary evaporator. The crude product was purified by chromatography on silica (80 g) with hexane (4 L). Pure 28 (1.3 g, 58% over 2 steps) was isolated.
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