CC[O-].[Na+].C[O:6][C:7](=O)[CH2:8][S:9][CH:10]([CH3:16])[CH2:11][C:12]([O:14][CH3:15])=[O:13].C(O)(=O)C>C(O)C.C(OCC)(=O)C>[CH3:16][CH:10]1[CH:11]([C:12]([O:14][CH3:15])=[O:13])[C:7](=[O:6])[CH2:8][S:9]1A three neck flask was charged with NaOEt (21%, 200 mL, 537 mmol) in ethanol. The solvent was removed under vacuum and to the residue was added toluene (500 mL). The mixture was heated to reflux and methyl 3-(2-methoxy-2-oxoethylthio)butanoate (92 g, 446 mmol) was added dropwise. After addition was complete, the mixture was refluxed for 4 hours. After cooling, the mixture was poured into a mixture of acetic acid (200 g) and crushed ice (200 g). The mixture was stirred overnight and then diluted with ethyl acetate (500 mL). The organic phase was separated and washed with saturated Na2CO3 and brine and then dried. The solvent was removed to afford crude methyl 2-methyl-4-oxo-tetrahydrothiophene-3-carboxylate (77.7 g, 99%). 1H NMR (CDCl3, 400 MHz) δ 11.23-11.15 (d, 1H), 7.27-7.16 (m, 2H), 4.33-4.12 (m, 2H), 3.95-3.68 (m, 2H). 3.60-3.15 (m, 1H), 2.60-2.20 (m, 1H).
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