S(O)(O)(=O)=O.CS[C:8](=[NH:10])[NH2:9].[OH-].[K+].[O:13]=[C:14]1[CH:18]([C:19]2[CH:24]=[CH:23][CH:22]=[CH:21][CH:20]=2)[CH2:17][CH2:16][CH:15]1[C:25](OCC)=[O:26]>O>[NH2:9][C:8]1[O:13][C:14]2[CH:18]([C:19]3[CH:24]=[CH:23][CH:22]=[CH:21][CH:20]=3)[CH2:17][CH2:16][C:15]=2[C:25](=[O:26])[N:10]=12-Methyl-2-thiopseudourea sulfate (1.336 g, 9.61 mmol) was dissolved in water (10 mL) and KOH (1.128 g, 20.10 mmol) was added. Under stirring, ethyl 2-oxo-3-phenylcyclopentanecarboxylate (2.03 g, 8.74 mmol) was added and the reaction mixture was stirred at room temperature overnight. The reaction mixture was filtered, washed with water and ether, and dried over anhydrous sodium sulfate to afford 2-amino-7-phenyl-6,7-dihydrocyclopenta[e][1,3]oxazin-4(5H)-one (1.22 g, 5.35 mmol, 61.2% yield) as white solid. LC-MS (M+H)+=229.1. 1H NMR (500 MHz, dimethylsulfoxide-d6) δ ppm 7.57-7.85 (2H, m), 7.08-7.47 (5H, m), 4.25-4.38 (1H, m), 1.72-2.73 (3H, m), 1.09-1.31 (1H, m).
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