CC(C)([O-])C.[K+].[C:7]([O:12]CC)(=O)[CH2:8][CH2:9][CH3:10].[CH:15](OCC)=O.[NH2:20][C:21]([NH2:23])=[S:22]>C1COCC1.C(OCC)C.O.CC(O)=O>[CH2:9]([C:8]1[C:7](=[O:12])[NH:20][C:21](=[S:22])[NH:23][CH:15]=1)[CH3:10]To a suspension of potassium tert-butoxide (12.08 g, 108 mmol) in THF (40 mL) was added a solution of ethyl butyrate (5.69 mL, 43.0 mmol) and ethyl formate (6.93 mL, 86 mmol) in diethyl ether (40.0 mL) dropwise under nitrogen. The mixture was stirred at room temperature for 3 h. After removing the solvent, the residue oil was dissloved in isopropanol (350 mL), and thiourea (6.55 g, 86 mmol) was added. The mixture was stirred at reflux overnight, and then concentrated in vacuo to get a solid. The solid was dissolved in water, adjusted pH to 4 with AcOH, and extracted by DCM. The organic layer was dried over Na2SO4, filtered and concentrated to afford the title compound (5.5 g, 82% yield) as a pink solid.
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