4.5- Dihydroxy-4-cyclopentene-1,23-trione

The four-, five-, and six-membered analogs (178,180, and 182) were also obtained from the diprotonation of squaric acid (3,4-dihydroxy-3-cyclobutene-l,2-dione, 177), tri-O-protonation of croconic acid (4,5-dihydroxy-4-cyclopentene-l,2,3-trione, 179), and tetra-O-protonated rhodizonic acid (5,6-dihydroxy-5-cyclohexene-l,2,3,4-tetraone, 181), respectively. These ions were prepared in either Magic Acid (1 1 FSOsH-SbFs) or fluorosulfuric acid at low temperature and characterized by NMR. Ab initio/IGLO calculations showed that di-O-protonated squaric acid (178) is planar and aromatic, whereas the polyprotonated croconic and rhodizonic acids (180 and 182) have more carboxonium ion character, and no indication was obtained for any significant contributing homoaromatic structures. 

Sterically hindered seleno- and telluropyrylium salts 40 react with squaric acid 41 <1993H(35)1149> and also with 4,5-dihydroxy-4-cyclopentene-l,2,3-trione (croconic acid) 42 <2000JOC2236> in the presence of base to afford squarylium and croconate dyes 43 and 44, respectively (Equations 11 and 12). Dyes of this type have important industrial applications (Section 7.11.8.2). 

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