7353-89-1

7767-60-4

General procedure for the synthesis of 4,6-dihydrothieno[3,4-b]thiophene-2-carboxylic acid methyl ester from 4,5-bis(chloromethyl)thiophene-2-carboxylic acid methyl ester: Methyl ester of 4,5-bis(chloromethyl)thiophene-2-carboxylic acid (9.6 g, 40 mmol) was dissolved in methanol (400 ml). To this solution was added a methanolic solution of anhydrous Na2S (105 ml, 42 mmol, 0.4 M) and the reaction was carried out at 70 °C under reflux. After completion of the reaction, the reaction solution was cooled to room temperature (25°C) and filtered. The filtrate was poured into water to precipitate the solid product, which was purified by recrystallization to give the final methyl 4,6-dihydrothieno[3,4-b]thiophene-2-carboxylate (5.2 g, 66% yield).