[F:1][C:2]1[CH:7]=[CH:6][C:5]([CH:8]([CH2:14][S:15][C:16]2[CH:21]=[CH:20][CH:19]=[CH:18][C:17]=2[F:22])[C:9]([O:11]CC)=[O:10])=[CH:4][CH:3]=1.C(O)=O.CS(O)(=O)=O.O>C(Cl)Cl>[F:1][C:2]1[CH:3]=[CH:4][C:5]([CH:8]([CH2:14][S:15][C:16]2[CH:21]=[CH:20][CH:19]=[CH:18][C:17]=2[F:22])[C:9]([OH:11])=[O:10])=[CH:6][CH:7]=1The crude ester from Step B (5.3 g, 16 mmole) was combined with 20 ml of 88% formic acid and 2.1 ml (33 mmole) of methane sulfonic acid. The emulsion was refluxed for five hours during which time it gradually became homogeneous. After cooling, water and methylene chloride were added and the aqueous phase was separated and extracted twice with methylene chloride. The organic phases were combined, dried (MgSO4) and concentrated. The crude residue was taken up in 4% ethyl acetate/hexane and filtered through a plug of silica gel to remove nonpolar impurities. The product acid was then rinsed from the silica gel with ethyl acetate and the solvent concentrated. The acid was a colorless solid (4.5 g, 95% yield). 1H NMR (CDCl3) δ: 10.05 (1H, br s), 7.20 (5H, m), 7.03 (3H, m), 3.81 (1H, dd, J=6, 8 Hz), 3.57 (1H, m), 3.23 (1H, dd, J= 6, 12 Hz).
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