2,3,4,5-Tetrahydro-4-oxo-3-thiophene carboxylic acid, methyl ester (L75a): 25.3 g (1.1 mol) of sodium were implemented in 190 mL abs. methanol to methylate. To methylate solution 70 g (0.360 mol) (L74) was added dropwise at the boiling point and the reaction mixture was heated for 45 min under reflux. The cooling mixture was poured onto a mixture of 500 mL ice and 300 mL water, acidified with 100 mL conc. HCl and the oily precipitating product was brought to crystallize under cooling with ice/sodium chloride bath. The crystals were sucked off, resumed in 100 mL methylene chloride and dried over sodium sulfate and evaporated. There were obtained 16.7 g of pale beige crystals. The aqueous phase was extracted three times with 100 mL of methylene chloride, the organic phase dried over sodium sulfate and evaporated. 30 g of crystalline product Methyl 4-oxotetrahydrothiophene-3-carboxylate was obtained. Yield: 46.7 g of light beige crystals (81 % of theory). mp 29-31 degC. TLC: Bz:MeOH:AcOH = 45:8:4, Rf = 0.75.
