[CH3:1][C:2]1[CH:3]=[C:4]([CH:12]=[C:13]([CH3:15])[CH:14]=1)[O:5][CH:6]([CH2:10][CH3:11])[C:7]([OH:9])=[O:8].C(Cl)Cl.[Cl:19][S:20](O)(=[O:22])=[O:21].C([O-])(O)=O.[Na+]>CC(OC)(C)C>[Cl:19][S:20]([C:14]1[C:13]([CH3:15])=[CH:12][C:4]([O:5][CH:6]([CH2:10][CH3:11])[C:7]([OH:9])=[O:8])=[CH:3][C:2]=1[CH3:1])(=[O:22])=[O:21]A 500 mL 3-neck flask was charged with 3,5-dimethylphenoxybutanoic acid (17) (25 g, 120 mmol) and CH2Cl2 (100 mL). This solution was cooled to −27° C. with mechanical stirring then chlorosulfonic acid (42 g, 360 mmol) was added dropwise over 30 min (temperature reached a max. of −20° C.). The material was stirred for an additional 1 hr at −25° C. then NaHCO3 (20 g, 238 mmol) was added to the reaction mixture. The reaction mixture was then added to 150 g of ice. The reaction mixture was then extracted into EtOAc (750 mL), dried over Na2SO4 (10 g), filtered and then concentrated in vacuo. Two triturations of the crude product with MTBE at 5° C. (50 mL then 25 mL) gave 20.0 g 18 (54% yield).
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