[O:1]=[C:2]1[CH:6]([CH3:7])[S:5][CH2:4][CH:3]1[C:8]([O:10][CH3:11])=[O:9].C1(C)C=CC(S(O)(=O)=O)=CC=1.[C:23](OC(C)=C)(=[O:25])[CH3:24]>>[C:23]([O:1][C:2]1[CH:6]([CH3:7])[S:5][CH2:4][C:3]=1[C:8]([O:10][CH3:11])=[O:9])(=[O:25])[CH3:24]Methyl 4-keto-5-methyl-2,3,4,5-tetrahydrothiophene-3-carboxylate (90.83 g) and p-toluenesulphonic acid (1 g) were refluxed for 6 hours in isopropenyl acetate (214 ml) and then stood at room temperature for 22 hours.
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