C(C1NC=CN=1)(C1NC=CN=1)=O.[C:13]([C:17]1[O:21][C:20]([C:22]([OH:24])=O)=[N:19][CH:18]=1)([CH3:16])([CH3:15])[CH3:14].[S:25]1[CH:29]=[CH:28][CH:27]=[C:26]1[C@@H:30]([NH2:32])[CH3:31].C1CCN2C(=NCCC2)CC1>C1COCC1>[S:25]1[CH:29]=[CH:28][CH:27]=[C:26]1[C@@H:30]([NH:32][C:22]([C:20]1[O:21][C:17]([C:13]([CH3:14])([CH3:15])[CH3:16])=[CH:18][N:19]=1)=[O:24])[CH3:31]
A solution of carbonyl diimidazole (CDI) (2.42 g, 15 mmol) in THF (40 ml) is added to a stirred solution of 5-tert-butyl-oxazole-2-carboxylic acid (2.3 g, 13.6 mmol) in THF (40 ml). The mixture is stirred for 30 min, then (S)-(1-thiophen-2-yl-ethyl)-amine (2.59 g, 20.4 mmol) is added, followed 10 min later by a solution of DBU (2.24 ml, 15 mmol) in THF (20 ml). The mixture is refluxed for 16 h and the THF is then removed by distillation. The residue is dissolved in ethyl acetate and the organic solution is washed with 2N hydrochloric acid and then twice with water. The organic phase is dried over sodium sulfate and evaporated in vacuo. The residue is purified by flash column chromatography (silica gel, petroleum ether/ethyl acetate 7:3 v/v) to give (S)-5-tert-butyl-oxazole-2-carboxylic acid (1-thiophen-2-yl-ethyl)-amide (2.40 g, 63%) as an oil.
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