Croconic acid preparation

Infrared-absorbing dyes 88 have been prepared from croconic acid and 4-methyl-chalcogenopyrylium salts. Red shifts of 7 , are noted as the size of the heteroatom increases and range from 845 nm for (88 X = O) to 1081 nm for (88 X = Te) . 

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. 

Acetylene was obtained by Edmund Davy by the action of water on the residue from the preparation of potassium by heating potassium carbonate and carbon (see p. 95), and Gmelin from a product of the same process obtained croconic acid, later recognised as a derivative of benzene (see p. 794). These were also synthetic products. 

Nietzki and Benckiser confirmed the formula C5O5H2 for croconic acid and were able to examine a specimen of the potassium salt which had been prepared by Gmelin and was still in Heidelberg. Since it forms an acid salt,... 

As shown above, di-tert-butoxyethyne 1s the only acetylenic diether prepared so far whose stability allows its use as a synthetic Intermediate. It has been used 1n the synthesis of all the members of the series of monocyclic oxocarbons (deltic, squaric, croconic, and rhodizonic acids), as well as in the synthesis of semisquaric acid, the parent compound of the natural mycotoxins, noniliforndna... 

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