3,4-(Hydroxymethyl-ethylenedioxy)thiophene ( 4) and 3,4-(2′-hydroxypropylenedioxy)thiophene ( 4'′) were synthesized from dimethyl 3,4-dihydroxythiophene-2,5-dicarboxylate ( 1), which were isomers and difficult to separate. When they were esterified with tetrathiafulvalene carried carboxylic acid group, only 3,4-(hydroxymethyl-ethylenedioxy)thiophene ( 4) could be esterified. No such selectivity was observed when the isomers were esterified by lauric acid and benzoic acid, respectively. [ABSTRACT FROM AUTHOR]