[C:1]1([C:3](=[CH:5][CH:6]=[CH:7][CH:8]=1)[OH:4])[OH:2].[CH3:9][C:10]([C:12]1[CH:17]=[CH:16][C:15]([C:18]([F:21])([F:20])[F:19])=[CH:14][CH:13]=1)=O.O.C1(C)C=CC(S(O)(=O)=O)=CC=1.O>C1(C)C=CC=CC=1>[CH3:9][C:10]1([C:12]2[CH:13]=[CH:14][C:15]([C:18]([F:19])([F:20])[F:21])=[CH:16][CH:17]=2)[O:4][C:3]2[CH:5]=[CH:6][CH:7]=[CH:8][C:1]=2[O:2]1A solution of catechol (5.5 g) and 4-(trifluoromethyl)acetophenone (10.3 g) in toluene (150 ml) containing a catalytic amount (0.2 g) of p-toluenesulphonic acid monohydrate was heated under reflux for 3 days. Water produced in the condensation was separated by means of a Dean-Stark trap. After removal of the solvent under reduced pressure, the residue was purified by chromatography on silica gel using methylene chloride as eluant. Pure 2-methyl-2-[4-(trifluoromethyl)phenyl]-1,3-benzodioxole was obtained as a colourless oil (10.8 g, 77%).
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