Reactions with methyl 3-hydroxythiophene-2-carboxylate. Part 2. A new route to mono- and di-alkyl ethers of thiotetronic and α-halogenothiotetronic acids
Methyl 2-chloro-3-oxo-2,3-dihydrothiophene-2-carboxylates (2), obtained from methyl 3-hydroxythiophene-2-carboxylate by straightforward halogenation, added alcohols and then lost hydrogen chloride at room temperature to yield thiophene-2,4-diols (4). Successive O-alkylation and alkaline hydrolysis of these compounds gave 3,5-dialkoxythiophene-2-carboxylic acids (6) in nearly quantitative yield. The latter could either be thermally decarboxylated or decarboxylated and partially dealkylated at room temperature by aqueous acid to the ethers of thiotetronic and α-halogenothiotetronic acids in high yield.