[F:1][C:2]([F:16])([F:15])[S:3]([O:6][C:7]1[CH:14]=[CH:13][C:10]([CH:11]=[O:12])=[CH:9][CH:8]=1)(=[O:5])=[O:4].[Cr](O[Cr]([O-])(=O)=O)([O-])(=O)=[O:18].[K+].[K+]>S(=O)(=O)(O)O>[F:16][C:2]([F:15])([F:1])[S:3]([O:6][C:7]1[CH:14]=[CH:13][C:10]([C:11]([OH:18])=[O:12])=[CH:9][CH:8]=1)(=[O:4])=[O:5]A suspension of 4-(trifluoromethanesulfonyloxy)benzaldehyde (5.2 g, 20.5 mmol, described in JACS, 1987, 109, 5478) and potassium dichromate (2.5 g) in sulfuric acid (30%, 47 mL) was heated at 65° C. for 1 h. The mixture was allowed to cool to ambient temperature and then poured on ice. The formed precipitate was filtered off and washed with several portions of water. The crude product was recrystallized from water/ethanol (1:1) affording 2.63 g (47% yield) of the title compound: mp 178-180° C.; 1H NMR (400 MHz, DMSO-d6) δ 8.11 (d, 2 H), 7.64 (d, 2 H).
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