Chlorosulfonic acid → 6-[2-(4-Chlorosulfonyl-3,5-dimethyl-phenoxy)acetylamino]hexanoic acid 2-(3-nitro-benzenesulfonyl)ethyl ester

To a cooled solution of 6-[2-(3,5-dimethyl-phenoxy)acetylamino]hexanoic acid 2-(3-nitrobenzenesulfonyl)ethyl ester (Intermediate 16, 4.6 g, 9.1 mmol) in 3 mL of CH2Cl2 was added drop-wise ClSO3H (3 mL, 5 eq., 45.5 mmol)) at 0° C. During the reaction small aliquots of the reaction mixture were taken out as samples, treated with ice, extracted with ethyl acetate, and the ethyl acetate layers were analyzed by thin layer chromatography (TLC). TLC analysis showed that the reaction was complete after 30 min. Then the resulting thick reaction mixture was poured onto the crushed ice with vigorous stirring to give a yellow gummy material mostly in the bottom of the flask. This mixture was extracted with CH2Cl2, the organic layers were dried over anhydrous MgSO4, and concentrated under reduced pressure to give Intermediate 17 (2.2 g, 40%) as a white foam.