[C:1]([O:5][C:6](=[O:15])[NH:7][C:8]1[S:9][C:10]([C:13]#[CH:14])=[CH:11][N:12]=1)([CH3:4])([CH3:3])[CH3:2].[Li]CCCC.Cl[C:22]([O:24][CH2:25][CH3:26])=[O:23]>C1COCC1>[CH2:25]([O:24][C:22](=[O:23])[C:14]#[C:13][C:10]1[S:9][C:8]([NH:7][C:6]([O:5][C:1]([CH3:4])([CH3:3])[CH3:2])=[O:15])=[N:12][CH:11]=1)[CH3:26](5-Ethynyl-thiazol-2-yl)-carbamic acid tert-butyl ester (1.2 g, 5.350 mMol) was dissolved in THF (25 ml) in inert atmosphere and stirred at −78° C., nBuLi (7.4 ml, 1.6 M solution in hexane, 11.848 mMol) was added drop wise. After complete addition the reaction was stirred for 0.5 hr. followed by addition of ethyl chloroformate (0.775 g, 7.141 mMol) at −78° C. Reaction was allowed to come to 0° C. and quenched with water (50 ml) and extracted with ethyl acetate (25 ml×3). The organic layer was washed with brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure to get (2-tert-butoxycarbonylamino-thiazol-5-yl)-propynoic acid ethyl ester (1.4 g).
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