[C:1]([Cl:6])(=O)[C:2](Cl)=[O:3].[C:7]([N:11]1C(=O)C(O)=[C:13]([C:18]2[CH:23]=[CH:22][CH:21]=[CH:20][CH:19]=2)[S:12]1(=[O:25])=[O:24])([CH3:10])([CH3:9])[CH3:8].CN(C=O)C>C(Cl)Cl>[C:7]([N:11]1[C:2](=[O:3])[C:1]([Cl:6])=[C:13]([C:18]2[CH:23]=[CH:22][CH:21]=[CH:20][CH:19]=2)[S:12]1(=[O:25])=[O:24])([CH3:10])([CH3:8])[CH3:9]Oxalyl chloride (3.39 g, 2.71 mmol) was added dropwise to a solution of 2-tert-butyl-4-hydroxy-5-phenylisothiazol-3(2H)-one 1,1-dioxide (5 g, 17.77 mol) and DMF (15 mL) in DCM (220 mL) at 0° C. The reaction mixture was then heated at 50° C. for 4 h, concentrated and the residue was diluted with EtOAc and washed with water, then brine and evaporated. The residue was purified by silica gel column chromatography, using a 98:2 mixture of petroleum ether and EtOAc as eluant. The product containing fractions were evaporated, the residue was triturated in petroleum ether and the solvent was decanted to give the title compound (3.9 g, 74%) as a solid; 1H NMR (500 MHz, CDCl3): δ 7.97-7.92 (m, 2H), 7.61-7.51 (m, 3H), 1.75 (s, 9H); 13C NMR (125 MHz, CDCl3): δ 156.5, 142.8, 132.3, 129.3, 128.8, 123.4, 63.0, 27.7.
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