Syntheses and Derivatives of Squaric Acid

Abstract Squaric acid (dihydroxycyclobutenedione), the dianion of which may be regarded as a novel aromatic system, has recently become readily accessible by simple syntheses. The acidic character of squaric acid manifests itself in the formation of salts and of lower alkyl esters, which can be converted into the amides. Squaric acid dichloride shows in its reactions typical acyl chloride behavior. Condensation of squaric acid with pyrrole, phenol, and azulene derivatives yields cyclotrimethine dyes.