Structure of Carminic Acid

Carminic acid was first obtained in crystalline form in 1858 (ref. 197) but its structure proved difficult to elucidate and was not finally established until 1965 through the work of two different groups while stereochemical detail was added in 1981. Some of the earlier work has been summarised (ref.198). 

From classical degradative work on both carminic and kermesic acids (ref. 199, 200) the former was recognized as an anthraqinone resembling the latter and it was formulated (ref.201) as structure (A). This early work led to the surmise 

Thus carminic acid can be represented by structure (D) as 7-(3-D-gluco-pyranosyl-3,5,6,8-tetrahydroxy-1-methylanthra-9,10-quinone-1-carboxylic acid, a formulation substantiated by its H NMR and C NMR spectra (ref. 210). By contrast, the detailed structure of the metal chelates of carminic acid is not known for certain (ref. 211) with regard to the possible competing role of the two chelating functions, namely the salicylic and the alizarin centres. 

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