[N+](=C)=[N-].[CH3:4][O:5][C:6]([CH:8]1[CH2:12][S:11][CH2:10][C:9]1=[O:13])=[O:7].[CH:14](Cl)(Cl)Cl>CCOCC>[CH3:4][O:5][C:6]([C:8]1[CH:9]([O:13][CH3:14])[CH2:10][S:11][CH:12]=1)=[O:7]15 parts by volume of an 0.5 molar solution of diazomethane in ether are added in the course of about 10 minutes to a solution, cooled to 0° C., of 1.6 parts of 3-oxo-tetrahydrothiophene-4-carboxylic acid methyl ester in 10 parts by volume of dry chloroform. The solution is stirred for 21 hours at 0° C., flushed with nitrogen to remove excess diazomethane and concentrated under reduced pressure. The residue is recrystallized from methylene chloride/petroleum ether. 1.5 parts (86% of theory) of 3-methoxy-dihydrothiophene-4-carboxylic acid methyl ester of melting point 79°-81° C. are obtained.
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