Glyoxalic acids, aryl-, esters

Hinsburg first reported that various a-dicarbonyl compounds (256) condensed with thiodiglycolic esters in the presence of alcoholic sodium ethoxide to give various substituted thiophene-2,5-dicarboxylic esters (257). R1 and R2 in (256) could be H, OH, alkyl, OR, aryl or carboxyl groups o-quinones will also condense. If the condensation is carried out in aqueous alcohol, as is the case when glyoxal (256 R1 = R2 = H) is used, the thiophene-2,5-dicarboxylic acid is isolated directly. Pyruvic acid gives the half-ester of (257 R1 = Me, R2 = OH). The earlier work has been reviewed (52HC(3)l). 

Ester chlorides of dicarboxylic acids, e.g., ethoxalyl chloride afford aryl-glyoxalic esters 550,551... 

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