C[C:2]1[CH:3]=[C:4]([CH:18]=[C:19](C)[CH:20]=1)[O:5][CH2:6][C:7]([NH:9][CH2:10][CH2:11][CH2:12][CH2:13][CH2:14][C:15]([OH:17])=[O:16])=[O:8].[N+:22]([C:25]1[CH:26]=[C:27]([S:31]([CH2:34][CH2:35]O)(=[O:33])=[O:32])[CH:28]=[CH:29][CH:30]=1)([O-:24])=[O:23].O.C1(C)C=CC(S(O)(=O)=O)=CC=1.O>C1C=CC=CC=1>[N+:22]([C:25]1[CH:26]=[C:27]([S:31]([CH2:34][CH2:35][O:17][C:15](=[O:16])[CH2:14][CH2:13][CH2:12][CH2:11][CH2:10][NH:9][C:7](=[O:8])[CH2:6][O:5][C:4]2[CH:3]=[CH:2][CH:20]=[CH:19][CH:18]=2)(=[O:33])=[O:32])[CH:28]=[CH:29][CH:30]=1)([O-:24])=[O:23]A mixture of 6-(2-phenoxyacetylamino)-n-hexanoic acid (made in analogy to Intermediate 15, 3.15 g, 11.9 mmol, 1.1 eq), 2-(m-nitrobenzenesulfonyl)ethanol (2.5 g, 10.8 mmol), and p-toluenesulfonic acid hydrate (0.5 g) in 100 mL of benzene was refluxed overnight with Dean-Stark trap. Thereafter water was added and the mixture was extracted with CH2Cl2. The combined organic layers were washed with saturated NaHCO3 solution, dried over MgSO4, and concentrated under reduced pressure. The residual oil was purified by column chromatography (1% MeOH in CH2Cl2) to give 4.6 g (89%) of 6-(2-phenoxy-acetylamino)-n-hexanoic acid 2-(3-nitrobenzenesulfonyl)ethyl ester.
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