[NH:1]1[CH2:6][CH2:5][S:4][CH2:3][C:2]1=[O:7].C([N-]C(C)C)(C)C.[Li+].Cl[C:17]([O:19][CH2:20][CH3:21])=[O:18].Cl>O1CCCC1>[CH2:20]([O:19][C:17]([CH:3]1[S:4][CH2:5][CH2:6][NH:1][C:2]1=[O:7])=[O:18])[CH3:21]A mixture of thiomorpholin-3-one (1 equivalent), tetrahydrofuran (5-40 parts), and lithium diisopropylamide (1-2 equivalents) is stirred at -10° to 50° C. for 1 to 3 hours. To this mixture is added ethyl chloroformate (1 to 2 equivalents). After 1 to 3 hours' stirring, the mixture is acidified with hydrochloric acid, concentrated, diluted with ethyl acetate, washed with water, dried, and concentrated to give 3-oxothiomorpholine-2-carboxylic acid ethyl ester in 80 to 90% yield.
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