[C:1]([NH:4][C:5]([CH3:12])([CH:9]([CH3:11])[CH3:10])[C:6]([OH:8])=O)(=[O:3])[CH3:2].C(OC(Cl)=O)C.[CH3:19][C:20]1[CH:21]=[CH:22][C:23]([S:26]([NH2:29])(=[O:28])=[O:27])=[CH:24][CH:25]=1.[Na]>C(N(CC)CC)C.C(OCC)C.CN(C)C=O>[C:1]([NH:4][C:5]([CH3:12])([CH:9]([CH3:11])[CH3:10])[C:6]([NH:29][S:26]([C:23]1[CH:24]=[CH:25][C:20]([CH3:19])=[CH:21][CH:22]=1)(=[O:27])=[O:28])=[O:8])(=[O:3])[CH3:2]2-Acetamido-2,3-dimethylbutyric acid (1.5 g) is dissolved in a solution of triethylamine (2.7 g) in diethyl ether (50 mL) and cooled to 0° C.; ethylchloroformate (1.1 g) is added dropwise and the mixture is stirred at 0° to room temperature over one hour. The mixture is filtered, and the filtrate is concentrated in vacuo to afford the oily mixed anhydride (1.5 g). This material is mixed with 3 g of p-toluenesulfonamide, sodium salt, in 25 mL dimethylformamide, and the resulting mixture is heated at 80° C. for 16 hours. The reaction is cooled, concentrated in vacuo, and the residue is partitioned between methylene chloride and saturated, aqueous sodium bicarbonate. Acidification of the aqueous layer with conc. HCl to pH 1, extraction with CH2Cl2, drying, and stripping the organic phase affords the title product (3.3 g).
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